Anti-Arthritic Activities of Supercritical Carbon Dioxide ...
SUPERCRITICAL CARBON DIOXIDE TREATMENT OF …
Transcript of SUPERCRITICAL CARBON DIOXIDE TREATMENT OF …
SUPERCRITICAL CARBON DIOXIDE TREATMENT OF
PHOTORESISTS AND PLASMA-DAMAGED
NANOPOROUS LOW-k FILMS
by
GANGADHARAN SIVARAMAN BE
A THESIS
IN
ELECTRICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
December 2003
ACKNOWLEDGEMENTS
Firsdy I thank my advisors Dr Richard Gale and Dr Shubhra
Gangopadhyay for all the help guidance and support they have provided not only
for this work but throughout my term as a graduate student at Texas Tech
University
I would like to thank Bashar I Lahlouh a PhD Student from the Physics
Department TTU and Jorge Lubguban Post-Doctoral research associate Jack
Maddox lab for helping me with learning the different things required to
accomplish this work I will be failing in my duties if I fail to thank Dr Laurie
Williams and Dr Jerry King of the Los Alamos National Lab (LANL) for their
help during the course this work
I acknowledge the encouragement that I have received from all my
friends They have been really supportive during difficult times Finally I thank
my parents for providing me with a good education The support encouragement
and love I have received from my family has been a real motivation for me and
has always inspired me to do my best
11
IV
Table of contents
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES v
CHAPTER
I INTRODUCTION 1
II PHOTORESISTS H
III SUPERCRITICAL FLUIDS 19
rV INSTRUMENTATION 27
41 Fourier fransform infrared spectroscopy 27
42 Prism coupler 38
V EXPERIMENTS AND RESULTS 41
51 Photoresist removal using SCCO2 43
52 Treatment of low-k dielectric materials with SCCO2 52
VI CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 65
61 Future Work 66
REFERENCES 67
APPENDIX 69
A - PREPARATION OF PIRANHA SOLUTION 69
B - CALCULATION OF AMOUNT OF CO-SOLVENT 70
m
LIST OF TABLES
31 Physical data for gaseous supercritical fluid and liquid states 23
41 Electromagnetic spectrum 29
51 Results of SCCO2 Acetone treatment 47
52 Results of SCCO2 PCO3 freatanent 48
53 Experimental conditions for correlating PR removal with cross-linking 50
54 Results for correlating cross-linking and photo resist removal 51
55 TEL samples used in these experiments 53
56 Contact angle measurements of HMDS treated TEL films 59
57 Contact angle measurements of TCMS freated TEL films 62
IV
LIST OF FIGURES
11 A coupling model for 3 adjacent interconnects 4
12 Schematic illustration of rinse liquid stored between resist patterns 7
21 Photolysis and Subsequent reactions of diazo naptho quinone upon 13 UV exposure
22 Three level dissolution scheme for a commercial DNQnovolak 14 resist
23 Contrast curve of an ideal positive resist 15
31 The phase diagram of a single substance 21
32 Pressure-temperature-density surface for CO2 23
33 Diffusivity of supercritical CO2 24
41 Light as an electromagnetic wave 30
42 Comparison between Absorption and the Transmission spectra 31
43 Illusfration of Beers law 32
44 Fundamental molecular vibrations possible for a molecule 34
45 Michelson interferometer 36
46 Schematic of a prism coupler 39
51 Schematic of supercritical carbon dioxide system 41
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
ACKNOWLEDGEMENTS
Firsdy I thank my advisors Dr Richard Gale and Dr Shubhra
Gangopadhyay for all the help guidance and support they have provided not only
for this work but throughout my term as a graduate student at Texas Tech
University
I would like to thank Bashar I Lahlouh a PhD Student from the Physics
Department TTU and Jorge Lubguban Post-Doctoral research associate Jack
Maddox lab for helping me with learning the different things required to
accomplish this work I will be failing in my duties if I fail to thank Dr Laurie
Williams and Dr Jerry King of the Los Alamos National Lab (LANL) for their
help during the course this work
I acknowledge the encouragement that I have received from all my
friends They have been really supportive during difficult times Finally I thank
my parents for providing me with a good education The support encouragement
and love I have received from my family has been a real motivation for me and
has always inspired me to do my best
11
IV
Table of contents
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES v
CHAPTER
I INTRODUCTION 1
II PHOTORESISTS H
III SUPERCRITICAL FLUIDS 19
rV INSTRUMENTATION 27
41 Fourier fransform infrared spectroscopy 27
42 Prism coupler 38
V EXPERIMENTS AND RESULTS 41
51 Photoresist removal using SCCO2 43
52 Treatment of low-k dielectric materials with SCCO2 52
VI CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 65
61 Future Work 66
REFERENCES 67
APPENDIX 69
A - PREPARATION OF PIRANHA SOLUTION 69
B - CALCULATION OF AMOUNT OF CO-SOLVENT 70
m
LIST OF TABLES
31 Physical data for gaseous supercritical fluid and liquid states 23
41 Electromagnetic spectrum 29
51 Results of SCCO2 Acetone treatment 47
52 Results of SCCO2 PCO3 freatanent 48
53 Experimental conditions for correlating PR removal with cross-linking 50
54 Results for correlating cross-linking and photo resist removal 51
55 TEL samples used in these experiments 53
56 Contact angle measurements of HMDS treated TEL films 59
57 Contact angle measurements of TCMS freated TEL films 62
IV
LIST OF FIGURES
11 A coupling model for 3 adjacent interconnects 4
12 Schematic illustration of rinse liquid stored between resist patterns 7
21 Photolysis and Subsequent reactions of diazo naptho quinone upon 13 UV exposure
22 Three level dissolution scheme for a commercial DNQnovolak 14 resist
23 Contrast curve of an ideal positive resist 15
31 The phase diagram of a single substance 21
32 Pressure-temperature-density surface for CO2 23
33 Diffusivity of supercritical CO2 24
41 Light as an electromagnetic wave 30
42 Comparison between Absorption and the Transmission spectra 31
43 Illusfration of Beers law 32
44 Fundamental molecular vibrations possible for a molecule 34
45 Michelson interferometer 36
46 Schematic of a prism coupler 39
51 Schematic of supercritical carbon dioxide system 41
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
11
IV
Table of contents
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES v
CHAPTER
I INTRODUCTION 1
II PHOTORESISTS H
III SUPERCRITICAL FLUIDS 19
rV INSTRUMENTATION 27
41 Fourier fransform infrared spectroscopy 27
42 Prism coupler 38
V EXPERIMENTS AND RESULTS 41
51 Photoresist removal using SCCO2 43
52 Treatment of low-k dielectric materials with SCCO2 52
VI CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 65
61 Future Work 66
REFERENCES 67
APPENDIX 69
A - PREPARATION OF PIRANHA SOLUTION 69
B - CALCULATION OF AMOUNT OF CO-SOLVENT 70
m
LIST OF TABLES
31 Physical data for gaseous supercritical fluid and liquid states 23
41 Electromagnetic spectrum 29
51 Results of SCCO2 Acetone treatment 47
52 Results of SCCO2 PCO3 freatanent 48
53 Experimental conditions for correlating PR removal with cross-linking 50
54 Results for correlating cross-linking and photo resist removal 51
55 TEL samples used in these experiments 53
56 Contact angle measurements of HMDS treated TEL films 59
57 Contact angle measurements of TCMS freated TEL films 62
IV
LIST OF FIGURES
11 A coupling model for 3 adjacent interconnects 4
12 Schematic illustration of rinse liquid stored between resist patterns 7
21 Photolysis and Subsequent reactions of diazo naptho quinone upon 13 UV exposure
22 Three level dissolution scheme for a commercial DNQnovolak 14 resist
23 Contrast curve of an ideal positive resist 15
31 The phase diagram of a single substance 21
32 Pressure-temperature-density surface for CO2 23
33 Diffusivity of supercritical CO2 24
41 Light as an electromagnetic wave 30
42 Comparison between Absorption and the Transmission spectra 31
43 Illusfration of Beers law 32
44 Fundamental molecular vibrations possible for a molecule 34
45 Michelson interferometer 36
46 Schematic of a prism coupler 39
51 Schematic of supercritical carbon dioxide system 41
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
LIST OF TABLES
31 Physical data for gaseous supercritical fluid and liquid states 23
41 Electromagnetic spectrum 29
51 Results of SCCO2 Acetone treatment 47
52 Results of SCCO2 PCO3 freatanent 48
53 Experimental conditions for correlating PR removal with cross-linking 50
54 Results for correlating cross-linking and photo resist removal 51
55 TEL samples used in these experiments 53
56 Contact angle measurements of HMDS treated TEL films 59
57 Contact angle measurements of TCMS freated TEL films 62
IV
LIST OF FIGURES
11 A coupling model for 3 adjacent interconnects 4
12 Schematic illustration of rinse liquid stored between resist patterns 7
21 Photolysis and Subsequent reactions of diazo naptho quinone upon 13 UV exposure
22 Three level dissolution scheme for a commercial DNQnovolak 14 resist
23 Contrast curve of an ideal positive resist 15
31 The phase diagram of a single substance 21
32 Pressure-temperature-density surface for CO2 23
33 Diffusivity of supercritical CO2 24
41 Light as an electromagnetic wave 30
42 Comparison between Absorption and the Transmission spectra 31
43 Illusfration of Beers law 32
44 Fundamental molecular vibrations possible for a molecule 34
45 Michelson interferometer 36
46 Schematic of a prism coupler 39
51 Schematic of supercritical carbon dioxide system 41
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
LIST OF FIGURES
11 A coupling model for 3 adjacent interconnects 4
12 Schematic illustration of rinse liquid stored between resist patterns 7
21 Photolysis and Subsequent reactions of diazo naptho quinone upon 13 UV exposure
22 Three level dissolution scheme for a commercial DNQnovolak 14 resist
23 Contrast curve of an ideal positive resist 15
31 The phase diagram of a single substance 21
32 Pressure-temperature-density surface for CO2 23
33 Diffusivity of supercritical CO2 24
41 Light as an electromagnetic wave 30
42 Comparison between Absorption and the Transmission spectra 31
43 Illusfration of Beers law 32
44 Fundamental molecular vibrations possible for a molecule 34
45 Michelson interferometer 36
46 Schematic of a prism coupler 39
51 Schematic of supercritical carbon dioxide system 41
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
52 CN - absorption band of exposed and unexposed photoresist 43
53 Effect of heat in cross-linking DNQ photoresist 44
54 C=0 - absorption band DQN photoresist 45
55 Experimental conditions for removing PR using acetone as co- 46 solvent
56 SEM images depicting photo resist removal from SiTi surface 49
57 Photographs showing removal of cross-linked and uncross-linked 51 PR
58 Treatment of plasma damaged MSSQ with HMDS 52
59 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 54 Treated old way
510 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS 55 SCCO2 HMDS treatment
511 SCCO2HMDS treatment of plasma damaged MSSQ with HMDS - 56 CO2 HMDS treatment
512 SCCO2HMDS freatment of plasma damaged MSSQ with HMDS - 57 HMDS vapor freatment
513 CH-absorption band of HMDS feated film 1 58
514 2 step pulsing with TCMS 60
515 1 step pulsing with TCMS 61
516 CH Absorption band of TCMS freated Film 1 62
517 SEM micrographs for filml freated with TCMS 63
VI
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
CHAPTER I
INTRODUCTION
In semiconductor wafer manufacturing the removal of photoresist and
photoresist residue was until recently considered as a commodity process [1]
The removal of photoresists is basically a two-step process consisting of an ashing
step in which the bulk of the photoresist is removed and a wet cleaning step
during which the hard to remove highly cross linked photoresist is removed The
semiconductor industry is facing a number of challenges in this second step the
wet cleaning step Current solvents used particularly sulfuric acid and hydrogen
peroxide have proven to be costiy in terms of money safety to workers and
envfronmental contamination
The semiconductor industry should be able to meet the demand of all
modem luxuries we expect such as telecommurucations and computers To do this
they are required to use the best technology that is available [2] This technology
involves using a very high volume of chemicals and water because the fabrication
of wafers is a series of chemical steps and process Up to 20 of all process steps
are wafer surface cleaning steps and require consumption of huge quantities of
chemicals and ulfrapure water In 1992 it was estimated that a 55-gallon drum of
organic solvent from purchase to disposal cost about $5000 Since the solvents
have only limited lifetime and needs to be changed frequently these costs can add
up quickly In addition to the use of these chemicals there is a need to rinse
wafers with deionized water The industry also needs to freat the contaminated
water and dispose off the hazardous wastes With this we can see the total cost
1
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
rising rapidly The cost of operation of a deionized water system was estimated to
be $130000 per year in 1997 In comparison Smith and Huse (1998) showed that
the cash flow needed to sustain a supercritical carbon dioxide system performing
the same function after seven years of operation would be less than half of that of
system using deionized water [2]
Another major drawback of the wet stripping method is that these
methods of photoresist removal introduce hazardous chemicals into the
atmosphere causing health issues with the workers working with these chemicals
in the semiconductor industry Although most of the industries have implemented
stringent engineering and administrative controls to reduce the social costs
associated with these chemicals employee exposure to these chemicals continues
to be a great problem This has driven the industry to seek for alternative methods
of photoresist removal [2]
Low-K dielectric materials and Copper interconnects are needed to
address problems with power consumption signal propagation delays and
crosstalk between interconnects The semiconductor industry is now trying to
introduce these materials as the size of the ICs continues to reduce The use of
low-K dielectrics and Copper interconnect technology is inevitable and Copper
and low-K technology challenge the conventional stripping and cleaning
technologies There are four key reasons for this challenge [1]
1 The move from contact (oxide) Via (Oxide) and line (metal)
etch technology to single or dual damascene (oxide only) etch
technology
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
2 The change in metal from Aluminum (Al) to Copper (Cu)
3 The infroduction of low-K materials
4 The reduction in critical dimensions with the OlSfim
technology being in production right now and the 013 jam
technology will be completely in production by fourth quarter
2004 The 01-micron era could span 2005 to 2007 and 007-
micron technology may take the industry out to 2010 [3]
With the transition to porous low-K dielectrics combined with the copper
metallization the preferred approach would certainly be dual damascene
technology In this method only a few processing steps are needed and hence the
manufacturing cost would be lowered However the dual damascene method
would requfre an additional cleaning step when compared to conventional reactive
ion etching technology since the wafers need to be cleaned (ash and wet clean)
after the via etch (in case of the via first approach) and once again cleaned after
the trench etch
When changing the metallization scheme from Aluminum to Copper
special attention needs to be given to possible copper contamination of the wafer
backside particularly in case of wet benches special attention needs to be paid so
that the copper is not carried to the back side of the wafer This will probably
requfre a change in the way the cleaning is done and may also require a change in
cleaning chemistry The biggest cleaning issue with copper is preventing it from
getting into the front-end processing where it can desfroy the integrity of the gate
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
oxide creating leakage currents Back-to-front-end contamination can occur when
copper flakes off the back of a wafer in solution during wet processing
Continuing improvement of microprocessor performance involves
decreasing device size This allows an increase in device packing density and an
increase in the number of functions that can reside on a single chip Higher
packing density requires a much larger increase in the number of interconnects
These enhancements have led to a reduction in the chip area dedicated to
interconnect and an increase in the number of levels The reduction in area of
intercormects increases the interconnect resistance The reduction in spacing
between intercormects increases the capacitance between interconnects
Crosstalk a phenomenon of noise induced in one signal line by a signal switching
on the neighboring line (vertical or lateral) is mainly caused by the coupling
capacitance between neighboring interconnects The following diagram illustrates
the crosstalk between interconnects
^
ltgtgt^
Co
Figure 11 A coupling model for 3 adjacent intercormects
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
Figure 11 [5] shows a general coupling model for 3 adjacent interconnects
running in parallel Co is the line to ground capacitance and Cm is the inter wire
capacitance The effective capacitance of the center wire Ceff is given equation
(11)
Y -Y~ Y -Y Ceff=C^+Cbdquo - ^ mdash ^ +Cbdquo mdash^ Equation (11)
_ poundi J _ h
Vi represent voltage deviation of wfre i E is the power supply voltage
When the center wire switches alone and both its neighbors are inactive then Ceff
= Co + 2 Cm If all the three wfres switch simultaneously in the same direction
then Ceff = Co and the RC delay for the center wire actually decreases from the
case of solitary transition If the center wire and the neighboring wfres switch
simultaneously in opposite directions then Ceff = Co + 4Cm this yields the worst
case wfre delay in the center wire In order to reduce this crosstalk we need to
reduce the capacitance between intercormects which essentially depends on the
dielectric constant of the separating insulator Currently two types of materials
seem to be the market leaders Spin on polymers (Dow Chemical Company) and
CVD deposited carbon containing Si02 dielectrics (C-Si02) (offered by
Novellus Applied Materials Mattson and others) Eventually the porous versions
of these materials with even lower k will be required These types of low-k
materials requfre a re-thinking of the way in which cleaning is done
Photoresists and photoresist residues are usually removed using Piranha
solution in the front end processing and by a combination of dry ashing and wet
cleaning m the backend processmg With 018^m technology in production right
now and 013|4m technologies being introduced by the end of the fourth-quarter 5
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
2004 cleaning will become more and more of a challenge for conventional
techniques A primary challenge in front end cleans is the continuous reduction in
the defect levels As a rule a killer defect is less than half the size of the device
hne width For example at 018 pm geometries 009 iim particles must be
removed [6] The issue is that smaller particles are physically more difficult to
remove because it is harder to deliver the necessary force to minuscule
dimensions If one considers that aspect ratios (heightwidth) for contacts in
DRAMs are afready approaching 101 one can imagine that cleaning technology
and in particular the wet bench industry will face increasing challenges in the
coming years
Another major drawback of the increase in aspect ratios (heightwidth) is
that the resist pattern collapse occurs when the rinse liquid is dried off and this is
due to the capillary force of the rinse liquid The resist pattern collapse depends
on the aspect ratio of the resist patterns The tolerable height of resist pattern is
lower for finer structures Resist pattern collapse is a serious problem m
lithography because its one limit of the critical dimensions [7] After the resist is
immersed in developer the resist surface acqufres hydrophilic property and the
surface of rinse liquid stored between the resist patterns is concave as shown in
Figure 12
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
Resist Pattern
^sy Rinse Liquid
fiffi^-jon - S]xUlaquotiatlaquo
Figure 12 Schematic illustration of rinse liquid stored between resist patterns
A negative pressure P in the rinse liquid is produced and the source of this
pressure is the surface tension of the rinse liquid The pressure P can be expressed
as
P = ^ R
a is the sxirface tension of the rinse liquid
R is the radius of curvature of the rinse liquid
The resist pattern peeling force is given by
Equation (12)
F= PxA
P is the pressure exerted by the rinse hquid
A is the aspect ratio of the resist pattern
Equation (13)
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
This equation clearly shows that the peeling force is directly proportional
to the aspect ratio of the resist pattern and the surface tension of the rinse liquid
and this force increase as the aspect ratio increases [7] In comparison if we use
super critical carbon dioxide for removing photoresist and photoresist residue
supercritical carbon dioxide almost has zero surface tension and hence the peeling
force on the resist patterns will be very minimal
Considering the issues and concerns which were discussed above most of
the semiconductor industries are looking for an alternate and an effective way to
remove the photoresist and photoresist residue Improvements are being made to
the existing cleaning technologies and also new cleaning technologies have
emerged over the last few years few examples of new cleaning technologies
include [1]
1 Cleaning wafers using dense fluid technology
2 Cleaning wafers using sulfur trioxide
3 Cleaning wafers using Supercritical carbon dioxide (SCCO2) and
surfactants
4 Cleaning wafers using SCCO2 and co-solvents
The study of gases under high pressure was a major topic 125 years ago it
was found that highly compressed gases were good solvents and that their ability
to dissolve substances was dependent on pressure and density of compressed
gases and it can be greatiy influenced by slight changes in pressure [4]
Supercritical fluid technology has been widely used in industry for extraction and
purification process and over the past few years it has been considered as a
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
possible alternative in areas where there are very few environmentally benign
alternatives are available such as photoresist and photoresist residue stripping
When semiconductor manufacturers begin to implement low-k materials
with a dielectric constant below 30 they enter a new dimension of manufacturing
challenges that begins witii choosing the material itself Any choice of these
materials appears to have downsides The manufacturers are wondering which
material shortcomings they are going to choose to integrate around Issues of
adhesion via poisoning resistance to plasma etching and various other issues
plague different materials Low-k dielectric integration in a dual-damascene
structure requires film compatibility with all etching stripping CMP lithography
and metallization processes
As-deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the film becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination In this work we have also tried to
treat the etched nanoporous organosilicate films with supercritical carbon dioxide
and a suitable co-solvent so that the methyl groups lost during the plasma
treatment are reintroduced and the film becomes hydrophobic making it less
susceptible to moisture contamination
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
In this work I will discuss the experiments which we have done using
SCCO2 and Propylene Carbonate (co-solvent) to remove photoresist and
photoresist residue We have also shown that the amount of co-solvent required to
remove the photoresist is considerably less when compared the amount of
chemicals and ulfra pure water used in the industries
The remaining chapters in this thesis are arranged as follows Chapter 2
describes photoresists and their use in semiconductor industry Chapter 3
describes the supercritical fluids and their application to photoresist and
photoresist residue removal Chapter 4 describes briefly the instrumentation used
in this project Chapter 5 describes the various experiments that were done and the
results Chapter 6 has the Conclusion
10
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
CHAPTER II
PHOTORESISTS
Integrated circuits are the most important products of the modem
elecfronics industry They are built up from various arrangements of transistors
diodes capacitors resistors and by metallization of the paths linking the active
circuit elements The pattems defining these regions and the linking pathways
must first be drawn by a lithographic process on a layer of resist material and
then fransferred onto the semiconductor substrate by an etching process
Lithography is the art of making precise designs on thin films of resist
material by exposing them to a suitable form of patterned radiation eg UV ion
beam or X-ray with the formation of a latent image on the resist that can
subsequendy be developed by treatment with solvents Photoresist is usually a
multi component material The active ingredient in the resist which is the
photoactive compound undergoes a chemical reaction upon exposure to light
There are 2 types of photoresist namely negative and positive photoresist
Negative photoresist upon exposure to light becomes less soluble in a developer
solution whereas positive photoresist becomes more soluble after exposure
Many polymers are altered on exposure to ultraviolet radiation and this
has led to the development of photolithographic techniques using conventional
UV radiation from a mercury vapor lamp with an emission spectmm in the near-
UV wavelength range of 430 405 and 365 nm The photoresists used for
integrated circuit manufacturing normally have three components a resin or base
11
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
material a photoactive compound and a solvent that controls the mechanical
properties such as the viscosity of the base keeping the photoresist in liquid state
[8] In this work I have used Shipley 1813 (S1813) positive photoresist and I
would like to discuss more about this photoresist
SI813 is a diazo-naptho-quinonenovolak (DNQnovolak) based positive
photoresist SI813 has 3 main ingredients a) diazo-naptho-quinone which is the
photo active compound b) novolak resin is the binder matrix of the photoresist
meta cresol is the novolak resin in this photoresist and c) a solvent propylene
glycol monomethyl ether acetate in this photoresist In positive photoresist the
photoactive compound acts as dissolution inhibitor before exposure and hence
slowing the rate at which resist will dissolve when placed in a developing
solution Upon exposure to light a chemical process occurs by which the inhibitor
becomes a sensitizer increasing the dissolution rate of the resist Ideally the
inhibitor will completely prevent any dissolution of the resist and the enhancer
would produce infinite dissolution rate this cannot be achieved in practice The
following reaction shows the effect of light on positive DNQnovolak
photoresists[8]
12
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
0
+ Light
-N
Diazo Naptho Quinone
(Dissclution Inhibitor)
o R
Wolff leaiTangement Q
C-OH
Water
J^ Dissohitiou Enhancer
Figure 21 Photolysis and Subsequent reactions of diazo naptho quinone
upon UV exposure
The nitrogen molecule in the photoactive compound is weakly bonded and
as shown in Figure 21 the addition of UV light will free the nitrogen molecule
from the carbon ring leaving behind a highly reactive carbon site One way to
stabilize the structure is to move one of the carbon outside the ring The oxygen
atom is then covalently bonded to the external carbon atom and this process is
knovra as Wolff rearrangement The resultant molecule is called a ketene In
presence of water a final rearrangement occurs in which the double bond to the
external carbon atom is replaced with a single bond and an OH group and this
final product is a carboxylic acid This process works as a photoactive process
because the starting material will not dissolve in the base solution and carboxyUc
acid on the other hand readily reacts with the base developer solution and
dissolves in it This dissolution occurs for two reasons The resincarboxylic acid 13
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
mixture will rapidly take up water the nitrogen released during the reaction also
foams the resist further assisting the dissolution [8] The chemical reaction that
occurs during this process is the breakdown of carboxylic acid into water soluble
amines such as aniline and salts of sodium or potassium (depending on the
developer) and this process continues until all the exposed resist is removed
Figure 22 clearly shows that unexposed DNQnovolak photoresist has a
non-zero but very small dissolution rate and the exposed resist has a higher
dissolution rate Modem resists show a dissolution ratio of well over three orders
of magnitudes between exposed and exposed resist regions [9]
D I s s o L u I I o N
R A I E (lun Sec)
1000 _
100
10
1
Dl
NovoWiResiii
Novolak resin photoh-si5 products
Novolak ieiii diazo naptho quinone
Figure 22 Three level dissolution scheme for a commercial DNQnovolak resist
Three important measures of performance of a photoresist is the contrast
of the photoresist The contrast of the resist is measured as follows Coat a wafer
with a layer of photoresist and measure the thickness The resist is given a
uniform exposure of light for a small period of time the exposure dose is tiien the
light intensity (in Mwcm^) multiplied by the exposure time Now the wafer is 14
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
unmersed in a developer solution for a fixed period of time and then remove it
from developer and measure the thickness If the light intensity was not too large
then very little of the photoactive compound would have changed from
dissolution inhibitor to dissolution enhancer and hence thiclaiess of the
photoresist would be about the same as the original thickness The experiment is
then repeated for increasingly large doses of exposure If we plot the normalized
remaining resist thickness versus logarithm of incident dose a contrast curve will
be obtained as shown in Figure 23 [8]
Fraction
o r Resist
Remaining
10
08 -
06 -^
04 -
02 -
0
D
f bull ^
1 1
Dioo
1 bull
01 10 100
Exposure Dose (mj cm-)
Figure 23 Confrast curve of an ideal positive resist
The contrast curve has three regions low exposure where almost all the
resist remains high exposure where all the resist is removed and the transition
region between these two extremes In order to derive a numerical value of
contrast of a photoresist we first approximate the steeply sloped portion of the
curve by a straight line The line extends from the lowest energy dose for which
all of the resist is removed We name the energy density at this point as Dioo The
dose at which the line has a Y value of 1 is approximately the lowest energy
15
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
needed to begin to drive the photochemistry The energy density at this point is
called Do The contrast (v) is defined as
1 V - bull Equation (21)
logioCAooZ-Do)
Contrast can be thought as the measure of ability of a resist to distinguish
between the light and dark portions of the mask The resist contrast is a very
convenient measure of the resolving power of the photoresist Higher the contrast
value of a resist higher is the resolving power Ideally a photoresist with
infinitely high contrast (a step function as a contrast curve) could resolve images
of structures as long as there still is a finite intensity difference in the diffracted
light (aerial image) Although it is not immediately apparent from its definition
the contrast value depends on the resist thickness there is an approximately linear
decrease in contrast value with increasing resist thickness and many lithographers
use a rule of thumb that fihn thickness multipHed with resist contrast is a constant
number
In this section I am briefly explaining the processing steps on using
photoresists The first step is to spin coat the photoresist on clean silicon wafers
from the point of view of photoresist manufacturer spin coating is the most
pleasurable step because in this step ahnost 90 of the photoresist flies off the
wafer and into the receptacle It has been attempted numerous times to reclaim
this material but all these attempts have failed since it has been proven impossible
to meet the exacting particle and contamination standards of photoresists The
second step is the pre-baking step this is also known as soft bake or pre-exposure
bake This is a physical process of converting the liquid cast film into a solid 16
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requfrements for a
masters degree at Texas Tech University or Texas Tech University Health Sciences
Center I agree that the Library and my major department shall make it freely
available for research purposes Permission to copy this thesis for scholarly purposes
may be granted by the Dfrector of the Library or my major professor It is
understood that any copying or publication of this thesis for financial gain shall not
be allowed without my further written permission and that any user may be liable for
copyright infringement
film Typical pre-baking conditions for DNQnovolak resist are 1 - 2 minutes in a
hot plate at 100 - 120 deg C or 30 minutes at 70 - 90 deg C in forced afr ovens After
this step the photoresist is exposed to UV light using an appropriate mask
Optical exposure apparatus have evolved from its simple begirmings (contact
printing) to todays multi-million dollar stepper tools Its complexity has grown
correspondingly I do not want to cover the theory behind optical lithography in
my thesis instead I will keep my attention focused on key practical aspects of
photoresist processing
A thermal treatment of exposed but undeveloped photoresist usually
called the post exposure bake is the next step Post exposure bakes are reported to
enhance sensitivity and process latitudes for a number of resists The most
stunning effect of post-exposure bake is the disappearance of standing waves
When a resist coatmg on a flat wafer surface is frradiated the tight which is not
absorbed by the resist is reflected with high efficiency by the wafer surface The
incoming and outgoing light waves interfere and form a standing wave pattem
which is fransferred into the resist [9] The next step would be the development
step there are number of developers in use for DNQnovolak photoresists The
most important two classes are buffered metal ion containmg and the metal ion
free developers A typical buffered system is sodium metasilicate which offers an
advantage of not intioducing an additional anion The metal ion free developers
are aqueous solutions of tetra methyl ammonium hydroxide The buffered system
can be used at lower pH for the same normality and thus offer better
discrimination and contrast However there is widespread concern about
17
contamination by sodium ions which may cause failure of finished IC devices
Metal ion fi-ee developers are gaining ground nowadays since their use allows
process engineers to exclude development as a source of sodium contamination
The next step is the post development bake this is also called as post-bake
or hard bake The finished resist images are subject to thermal treatment Besides
removal of residual solvent or water the remaining DNQ molecules are
decomposed quickly at temperatures above 110 deg C In the absence of water the
multifunctional DNQ reacts with novolak hydroxyl group to cause cross-linking
thus further increasing the thermal stability of the resist structures Even without
the DNQ molecules the novolak itself also cross-links at elevated temperatures
Post-baking also increases the adhesion of resist to substrate in part by removal
of solvent but also by a hot melt effect by which the contact surface between
the resist and the surface subsfi-ate is maximized For dry etching post baking is
requfred for all but for the mildest etch processes When usmg fluorine plasma for
etching siUcon dioxide a post bake of 125-130 deg C may be adequate but while
using chlorine plasma to etch aluminum post bakes up to 160 C may be required
tins is needed to yield high etch resistance [9]
CHAPTER III
SUPERCRITICAL FLUIDS
When two molecules approach each other in a fluid at a temperature
where their relative speed is likely to be low their mutually attractive forces will
bring a temporary association between them If there is a sufficient density of
molecules there is a possibility of condensation to a liquid This may lead to
surface tension in a fluid because of attraction between the molecules that make
up the liquid In the absence of other forces this mutual attraction of the
molecules causes the liquid to coalesce to form spherical droplets This can be
seen for example when rain falls on freshly polished metal surfaces
As a general rule the greater the proportion of polar groups (eg OH
groups) m a molecule the stronger the atfractive forces between them Strong
attractive forces give rise to a high surface tension and a tendency to form discrete
droplets on a surface rather than wet it evenly The large proportion of OH groups
in water is responsible for its high surface tension whereas alcohols with then
smaller proportion of OH groups have lower surface tensions
Surface tension can be thought of as the force that holds a liquid together
In the depths of a volume of liquid each molecule is surrounded on all sides by
other molecules the forces between them balance out and the entire mass is in
equilibrium The situation is different at the surface of the liquid At a liquid-afr
interface for example the molecules at the surface are bemg attracted by the
surrounding liquid but not by the air The forces are imbalanced and consequendy
19
the liquid behaves as if had a sfretched skin Surface tension can therefore be
quantified in terms of the forces acting on a unit length at the liquid-air interface
Micro Electro Mechanical System (MEMS) devices are made using a
combination of masking plasma etching of polysilicon film deposited on the
wafer and wet etching done in liquid phase solution such as hydrofluoric acid
The final HF etch is followed by water rinse Silicon is practical MEMS material
that is capable of great amount of flexibility before fracturing However the
compliant nature of silicon makes it susceptible to fabrication problems A
significant problem in the fabrication of MEMS components is the sticking of
released structures to the substrate during the conventional rinsing and drying
step A major reason for this stiction is liquid bridging [14] Liquid bridging is
due to the surface tension effects of frapped capillary liquids upon drying The
liquid usually water used to rinse the nfrcrostructure is trapped in the narrow gap
between the suspended structures and the silicon substrate And obviously
reduction or elimination of surface tension will lessen or eliminate surface stiction
due to liquid bridging
On the other hand if the temperature and the relative speeds between
these molecules are high the attractive forces between these molecules will be too
weak to have more than slight effect on molecular velocities and condensation
cannot occur however high the molecular density It is reasonable to expect on
basis of molecular behavior that for every substance there is a temperature below
which condensation to a liquid and evaporation to a gas is possible but above
which these processes cannot occur There is a temperature above which a single
20
substance can only exist as fluid or supercritical fluid and not as either liquid or
gas [10] This temperature is called as the critical temperature
This phenomenon can be explained using Figure 31 [10] which is a phase
diagram of a single substance The diagram is a schematic and the pressure axis is
not linear the solid phase at high temperature occurs at very high pressures The
areas where the substance exists as a single solid liquid and gas are labeled the
triple point is where the three phases coexist The curves represent coexistence
between two phases
^ni eiciiacjaii FMtl
Critical point
TEMPERATURE
Figure 31 The phase diagram of a single substance
If we move upwards along the gas-liquid coexistence curve both the
temperattire and pressure increase The liquid becomes less dense because of
thermal expansion and the gas becomes denser because of increase in pressure
Eventtially the densities of the two phases become identical the distinction
21
between gas and liquid disappears and the curve comes to an end at the critical
point The substance is now described as a supercritical fluid The temperature
and pressure corresponding to the critical point are known as the critical
temperature and critical pressure respectively
The supercritical state of any material is attained when the temperature
and pressure are raised above the critical point (critical temperature (Tc) and
critical pressure (Pc)) The critical temperature (Tc) can be defined as the highest
temperature at which the gas can be converted to liquid by increasing the
pressure The critical pressure can also be defined as the highest pressure at which
a liquid can be converted to a gas by increasing the temperature
Today the semiconductor manufacturing industry is faced with daunting
task The manufacturing width of individual lines of a cfrcuit has reached 025
micron one-quarter the size it was in 1990 [3] Considering these line widths and
aspect ratios (heightwidth) of 7 and above we do not know whether it is possible
to get the cleaning liquids mto these kinds of dimensions The attractiveness of
super critical fluids as process solvents arises from then unique combination of
liquid-like and gas-like properties Super critical fluids exhibit gas-like transport
properties of difftisivity and viscosity yet possess liquid-like mechanical
properties such as density rapid wetting and excellent penetration characteristics
[1] Table 31 shown below has a comparison of the physical data for the gaseous
supercritical and liquid states [1] It is clearly seen that the physical properties of
the supercritical state are in between those of liquids and gases
22
Table 31 Physical data for gaseous supercritical fluid and liquid states
State
Gas (ambient)
Supercritical fluid
Liquid (ambient)
Density
(gmL)
00006-0002
02-05
06-16
Dynamic Viscosity
(gcm-sec)
00001-0003
00001-00003
0002-003
Diffusion
Coefficient
(cm^sec)
01-04
00007
0000002-000002
The critical values of temperature and pressure are unique for every
compound The critical temperature of CO2 is 311 degC and the critical pressure is
738 bar (1072 psi) Figure 32 [11] shows the pressure-temperature-density
surface of pure CO2 As can be seen m Figure 32 the density of supercritical
CO2 can vary over a wide range from gas-like values of 0002 gcm^ (STP) to
liquid-like values of over 10 gcm [11]
6000 4000
2000 _ l o S M
Figure 32 Pressure-temperattire-density surface for CO2 23
This reflects a density change of several orders of magnitude by relatively
modest variations of temperadjre and pressure which we were able to achieve in
the supercritical CO2 system that was built in the Jack Maddox lab Texas Tech
University [Chapter 5]
A black dark circle shows the critical point of C02 and it can be seen that
relatively small changes in temperature or pressure near the critical point result
in large changes in density It is this tunability of density and therefore tunability
of solvent power makes upper critical CO2 more attractive for cleaning
apptications In addition to the high liquid-like densities achievable in
supercritical CO2 it also possesses gas-like difftisivity shown in Figure 33 [11]
10-V
0i-DirfusivTES or S0-tJTrs Jji IN tiDHMAL LIOIJiS
bullao s o 80
TEMPCTiATUr-T f C i
Figure 33 Difftisivity of supercritical CO2 [ 11 ]
24
Use of supercritical fluids especially supercritical CO2 as alternatives for
photoresist removal offers many potential advantages over conventional organic
solvents CO2 is in general chemically inert with respect to inorganic materials
and is therefore compatible with a variety of substrates CO2 is non polar and
hence forces of attraction between thefr molecules would be very small as
described earlier in this chapter Low surface tension results in reduced stiction in
MEMS devices and makes the submicron devices less vulnerable to pattem
collapse CO2 is noncombustible readily available in high purity inexpensive and
its critical conditions are easily achievable CO2 also has extensive transportation
infrastructure as virtually all restaurants serve carbonated drinks and this requires
the use of pressurized CO2 cylinders After the cleaning step carbon dioxide is
easily separated from the extracteddissolved poljoners resulting in streams of
concentrated polymer (or polymer residue) and pure reusable CO2 Also
supercritical CO2 evaporates completely when depressurized leaving no residue
and hence subsequent aqueous rinsing and drying steps are also not requfred
This unique combination of physical economical and chemical properties of
supercritical CO2 has prompted an evaluation of its use as a replacement for
environmentally threatening chemicals currently used in semiconductor
manufacturing [12]
In the supercritical state carbon dioxide behaves as a classical non-polar
organic solvent Consequentiy pure super critical carbon dioxide is good for
removing non-polar materials such as oils and greases To remove polar
substances such as water it is common to add modifiers which increase the polar
25
nattire of the fluid Common modifiers are methanol and acetone [13] It has been
found that pure liquid propylene carbonate is an effective low toxicity
replacement for methylene chloride and methyl chloroform in the de bonding of a
negative Poly Methyl Methacrylate (PMMA)-based photoresist [13] This finding
and the fact that super critical carbon dioxidepropylene carbonate based
photoresist removal is being tried by a number of research organizations
prompted us to investigate the use of propylene carbonate as a SCC02 modifier
for its ability to remove hard-baked photoresists Propylene carbonate is an
environmentally fiiendly solvent having no Personal Exposure Limit (PEL) It is
non-flammable non-toxic and biodegradable [13]
The mechanism by which the supercritical carbon dioxideco-solvent
freatment removes the photoresist has not yet been determined However it is
well known that polymeric materials can be made to swell by diffirsion of CO2
molecules and it is likely that such a swelling occurs effectively softernng the
resist At the same time the reactive ester groups of the propylene carbonate acts
to degrade the photoresist reducing its molecular weight [13] Such a reduction of
molecular weight promotes solubility of the photoresist in the supercritical fluid
promoting its removal
26
CHAPTER IV
INSTRUMENTATION
In this chapter I will discuss about the different instmmentation
techniques which were used to characterize the all the experiments done The
following characterization techniques are discussed in this chapter
41 Fourier Transform Infrared specfroscopy and
42 Prism coupler
41 -Fourier Transform Infrared spectroscopv
Spectroscopy is a type of chemical analysis done by shining light on a
sample to determine what is inside the sample Chemists commonly measure the
absorbance how much light is absorbed by the sample or the transnuttance how
much light passes through the sample An analogy of how spectroscopy woks is
that if you imagine that light as food and the sample is a room full of people a
complete spectrum of light would be like giving all the food in a grocery store to
the people in the room Imagine that you knew there was only one person who
would eat asparagus If all of the asparagus came through the room uneaten that
person could not be in the room but if some were missing you would know that
person was in the room Furthermore no matter how much broccoli you put in the
room the asparagus-lover would never eat any and so you could never know if
he or she was in the room or not [20]
27
Similarly in a chemical analysis many different kinds (wavelengths or
energies) of light (a spectmm) are shone through a sample Some of the light is
absorbed By knowing what wavelengths of light are absorbed by the sample we
know what is inside But if we are looldng for a specific molecule or
characteristic and shine the wrong wavelengths of light through a sample no
matter how much light we put through we will never leam anything about the
sample
Infrared spectroscopy measures the vibrations of molecules Each
functional group or stmctural characteristic of a molecule has a unique
vibrational frequency that can be used to determine what functional groups are in
a sample When the effects of all the different functional groups are taken
together the result is a unique molecular fmgerprint that can be used to confirm
the identity of a sample
Fourier transform infrared (FTIR) spectroscopy is a characterization
technique widely used in physics chemistry and biology It is an easy way to
identify the presence of certain functional groups in a molecule Infrared
specfroscopy is a technique based on the interaction of infrared radiation with the
vibrations and rotations of the atoms of a molecule An infrared absorption
spectium can be obtained by passmg radiation through a sample and detennining
what fraction of the incident radiation is absorbed at a particular energy The
energy at which any peak in an absorption spectmm appears corresponds to the
frequency of a vibration of a chemical bond of a sample molecule [21]
28
To understand how infrared radiation interacts with material we need first
to have a quick look at the properties of light First of all light can have both
wavelike and particles like properties In the wave-like picttire light is an
elecfromagnetic wave as shown in Figure 41 It is a wave with both electric and
magnetic fields perpendicular to each other The IR radiation occurs in the long
wavelength side of tire electromagnetic spectinm as shown in Table 41 In IR
specti-oscopy wavenumbers (m units of cm) are usually used The wavenumber
is the number of elecfromagnetic waves in a length of one centimeter
Equation (41)
where k is the wave number c is the speed of light A is the wavelength and v is
the frequency
In some cases wave number is defined in units of radians per centimeter
in that case wave number is given by k = 2 pi gt
X c
Table 41 Electromagnetic spectrum
Radiation
Type
Wave
number
(cm-)
Visible UV
and
X-rays
gt 14000
Near
Infrared
14000 to
4000
Mid
Infrared
4000 to
400
Far
Infrared
400 to
4
Microwaves
Radio Waves
lt 4
29
Electromagnetic Wave bull Magnetic Field (B)
Electric mdash Field (E)
Wavelength (A)
Propagation Direction
Figure 41 Light as an electromagnetic wave
IR radiation is energetic enough to excite molecular vibration or rotation
to higher energy levels IR specfra usually have sharp features that are
characteristic of specific types of molecular vibrations making the spectra useful
for identification purposes Different molecules absorb infrared radiation at
different wavelengths Thus infrared spectmm contains both qualitative and
quantitative information of the sample material
The IR spectmm can be plotted in different ways but the most popular
ways are the absorption and fransmission specfra Figure 42 [21] shows a
comparison between these two specfra the IR absorption (transmission) intensity
is plotted as a function of the IR wavenumber Absorbance is defined as follows
[24]
A = logi-^) Equation (42)
30
i 0 b 2 0 0 D laquo 0 0 2 D D Q
W a v e n u m b e r ( c m )
pound raquo Q 0 4 D 0 0 4 - 0 0 0
T 0 0 t o o o o t i o f O ( i s o o o
W a v e n u m b e r ( c m ]
3 S 0 O 4 0 0 0 4 ^ 0 0
Figure 42 Comparison between Absorption and the Transmission spectra [21]
Where A is the absorbance is the light intensity with the sample in the IR
beam (sample spectmm) and lo is the light intensity measured with no sample
(background spectrum) The purpose of measuring Ig is to measure all the
contributions from sources other than the sample (contributions of the
spectrometer and the envfronment to the sample)
31
Transmission can be defined as follows [22]
I Equation (43)
Where T is the fransmittance lo and I have the same meaning as discussed
above From equations (42) and (43) we get
^ = log(i) Equation (44)
The basic law in absorption spectroscopy for quantitative analysis is
Beers law Beers law relates the concentration of the sample to the measured
absorbance of the sample spectrum
Consider a radiation of intensity bdquo is entering a sample of length b as
shown m Figure 43 Because some of the light will be absorbed the fransmitted
light intensity is less than lo
I
bull4 bull
b
Figure 43 Illustration of Beers law
Beers law is commonly expressed as [21]
^ = log (^ ) = log (^ )=abc
32
Equation (45)
where
lo = the radiation intensity entering the sample
1 = the radiation intensity that has passed through the sample
A = the absorbance
T = the fransmittance
a = the absorptivity in Lmol cm
b = the optical path length in cm
c = the sample concentration mol L
The absorptivity a is a specific to each molecule at certain wavenumber
that characterizes the capacity of that molecule to absorb infrared radiation The
value of a varies from one molecule to another and from one wavelength to
another but is constant for a given molecule at a given wavelength The quantity
b is the optical path length that is the distance the infrared radiation beam
traverses in the sample The quantity c indicates the concentration of the required
sample molecules in the entire sample If the optical path length is held constant
Beers law states that the absorbance is directly proportional to the concenfration
of the sample at a given wavelength
There must be a change in a dipole moment during the vibration in order
for a molecule to absorb infrared radiation If the dipole moment does not change
vibration is infrared inactive For the same reason the homo nuclear diatomic
molecules (N2 O2 H2 and CI2) and noble gases do not absorb IR radiation When
a molecule absorbs IR radiation it vibrates in different ways The bonds can
33
stretch contract and bend Figure 44 [23] shows the fundamental vibrations that
can be observed for a molecule [26]
L-J v l J Stretching vibration (v)
Ph (7gt (7^ In plane bend (8) ^mdash ^-^ -h Out of the page
Into the page
^ ( ] Out of plane bend (y)
ti-
Rock in plane bend (p)
+
+
Wag out of plane bend (x)
Twist (x)
Bend symmetrical
Figure 44 Fundamental molecular vibrations possible for a molecule
34
Fourier Transform Infrared (FTIR) fransmission spectra for all the samples
were recorded with a Perkin-Elmer spectirometer with 16 cm resolution The
FTIR specfra of the samples were obtained by subtracting the spectmm of the
silicon substi-ate from the spectmm of the silicon substrate with the
photoresistnanoporous film The absorption spectmm a((i)) was determined using
the Lambert-Beers law [21]
t
t is the film thickness in cm and To(Gi) is the fitted baseline corresponding
to zero absorption The concentration of the bonds contributing to the IR-active
bands (TV) is proportional to the integrated absorption of the band It can be
described using the following equation [21]
N=c^^dco^CI
Where C is a proportionality constant which varies as the inverse of the
oscillator sfrength a is the absorption coefficient and is the integrated
absorption Since the oscillator strength of the various bonds used m the present
study was not available to us and since the integrated absorption is dfrectly
proportional to the concentration the integrated absorption (7) values were used to
measure the concentiation of the bonds We beheve that this assumption is valid
for the qualitative analysis of the data
A Fourier transform infrared spectroscope is a device based upon the
Michelson interferometer The development of interferometry was initiated in
1880 when Dr Albert A Michelson invented his mterferometer to study the speed
35
of light and to fix the standard meter with the wavelength of a known spectral line
[23] FTIR method is based on the old idea of the interference of two radiation
beams to yield an interferogram An interferogram is a signal produced by two
radiation beams Interferogram is the interference intensity as a function of the
change of optical path difference The two domains of distance and frequency are
inter convertible by the mathematical Fourier transformation Figure 45 shows a
schematic of a Michelson interferometer
Movable Mirror
^ bull
Fi-ced Mirror
Figure 45 Michelson interferometer
Light that enters a Michelson interferometer is split into two beams Each
beam completes its own path and then the beams are recombined The
recombmed beam enters a power detector All of this is accomplished using a
beam splitter (a semi-silvered mirror) and two nurrors one fixed and one
36
movable Assuming that the incident light is monochromatic (of a single color)
and coherent (all of the photons are in phase) the intensity of the recombined
beam is dependent entirely on the path length difference between the two possible
paths through the interferometer If the path lengths differ by an integer multiple
of the optical wavelength for example the two beams will interfere
constructively (producing a maximum intensity output) If one path length is
changed by half a wavelength then the beams will interfere destmctively
resulting in a minimum (zero theoretically) intensity at the output
The Michelson interferometer modulates the incoming optical radiation by
changing the optical path difference (OPD) between the two possible paths in the
interferometer in a smooth continuous fashion A change in path difference
(called retardation) is accomplished by moving one of the two mirrors at a
constant velocity over a fixed distance When the mirror has traveled the required
distance which is governed by the required spectral resolution ft is quickly
returned to the start position to begm the next scan During the motion of the
moving mirror each wavelength of the collected radiation is modulated at a
unique frequency that is a function of the wavelength of the radiation and the
velocity of the movmg mirror The signal generated would be a suie wave of
constant amplitude and constant frequency assuming a broadband source such as
a blackbody taking into account all the wavelengths which make up the target
radiation and adding together all these sinusoids produces what is called an
interferogram Therefore the interferogram is a coded representation of tiie target
37
spectmm The Fourier Transform or decoding of the interferogram provides the
spectmm of the target radiation
Michelson interferometers provide a significant sensitivity advantage over
grating prism and circular variable filter spectrometers [22] There are two
significant reasons for the sensitivity advantage The first can be described as a
multiplex advantage The Michelson interferometers single detector views all the
wavelengths simultaneously throughout the entire measurement This effectively
lets the detector dwell on each wavelength for the entire measurement time
measuring more photons This improvement is called the multiplex advantage
and in effect increases the integration time
The second advantage is due to the light gathering capability or larger
tiiroughput The interferometer is not limited in aperture (sht width or height) as
severely as dispersive or cfrcular variable filter instmments This translates into a
much higher throughput or light gathering capability Both of these advantages
enable the Michelson FTIR to provide superior sensitivity over otiier
spectrometers over the uifrared portion of the spectmm
42-Prism Coupler
During the course of this work a Metricon 2010 prism coupler was used
to measure the thickness of the photoresist Prism Coupler utilizes advanced
optical wave guiding techniques to rapidly and accurately measure both the
thickness and the refractive index of dielectric and polymer films The technique
consists of measuring the angles at which a prism wiU couple tight from a laser
38
beam into the sample film The thickness and refractive index of the film are
calculated from the measured angles [25]
Figure 46 [26] shows the schematics of a prism coupler The sample to be
measured is brought into contact with the base of a prism by means of a
pneumatically-operated coupling head creating a small air gap between the film
and the prism
Laser Source detector
Coupling head
Figure 46 Schematic of a prism coupler
A laser beam stiikes the base of the prism and is normally totally
reflected at the prism base onto a photo detector At certain discrete values of the
incident angle 9 called mode angles photons can ttmnel across the afr gap into
the film and enter into a guided optical propagation mode causmg a sharp drop m
the intensity of light reaching the detector Measurements are made by loading the
sample against the prism and rotating the laser tight until coupling occurs
Coupling is indicated by a minimum in the output from the photo detector The
angle is measured and the rotation continued until the next coupling mode is
39
observed When coupling occurs a part of the incident energy ttmnels through the
low-index gap into the film It is then altemately reflected at the film-substrate
and at the film-gap interfaces so that a zigzag propagation along the guide results
Because the reflection at the film-gap interface is not total but is in tum perturbed
by the presence of the prism the guide is leaky and part of the energy m the film
escapes back into the prism [25] The coupling phenomenon described here is
shnilar to that of the tunneling of electrons through a barrier hence the term
optical tunneling has been used
To a rough approximation the angular location of the first mode
determines fihn index while the angular difference between the modes
determines the thickness allowing thickness and index to be measured completely
independently Measurements are made using a computer-driven rotary table
which varies the incident angle 9 and locates each of the film propagation modes
As soon as two of the mode angles are found film thickness and index can be
calculated The entfre measurement process is fully automated and requires
approximately twenty seconds
40
CHAPTER V
EXPERIMENTS AND RESULTS
In this chapter I will describe the different experiments that we did for
photoresist removal and surface modification treatment of nanoporous
organosilicate films using supercritical carbon dioxide and suitable co-solvents
Figure 51 shows the schematics of the supercritical carbon dioxide (SCCO2)
system that we used for our experiments
Liquid injection unit
High pressiu-e gauges C
TC
High purity CO2 at 600 psi
Exhaust
Vi
pump
V High pressure filter
Drive air High pressme y^ waives
Tc Thermocouple
Figure 51 Schematic of supercritical carbon dioxide system
This system consists of a stainless steel chamber designed to handle 16000
pounds per square inch (PSI) a high pressure air driven Haskel booster pump
[15] is used to pressurize this chamber and a Jasco liquid injection pump [16]
(operate at pressures up to 7000 PSI) is used for co-solvent injection The
chamber assembly is placed inside an oven which can be heated up to 360degC The
41
inlet and the outlet lines of this system are heated to improve the solubility of the
co-solvents in supercritical carbon dioxide This is expected to keep the lines
clean and free of co-solvents after the process is complete A thermocouple (TC)
is placed in the chamber so that we can sense the temperature of the chamber
Two high pressure gauges are provided one at the inlet and one near the chamber
to sense the pressure in the chamber The outlet is connected to exhaust through a
separation chamber The separation chamber is used to collect the co-solvents and
particles recovering the reactants for reuse The system can also be evacuated
using a mechanical vacuum pump to minimize atmospheric contamination in the
system The temperature and pressure of the high-pressure chamber could be
controlled to suit the experimental needs
In all experiments the high-pressure vessel was pre-flushed with CO2
(between 100-700 PSI) five thnes Pre-flushing helps reduce any contamination
from atmosphere At the end of each freattnent the vessel was also post-flushed
five times with supercritical carbon dioxide (between treatment pressure and 1500
PSI) Post-flushing helps hi removing the remaining co-solvent and other
particulate matter from the system Fourier Transform Infrared (FTIR)
spectioscopy contact angle measurements prism coupler optical mterferometer
and Scanning Election Microscope (SEM) were used for characterizing different
experiments done The theory behind these characterization techniques was
explained in Chapter 4
42
51-Photoresist removal using SCCO
Before starting any photoresist removal experiments we wanted to
characterize the photoresist In order get a good understanding of FTIR
spectroscopy I started characterizing the S-1813 photoresist for different
exposure times and different cross-linking temperatures When we expose the
photoresist to ultraviolet (UV) light nitrogen is released from the photoresist
[Chapter 2 Figure 21] This can be verified by taking the FTIR spectmm of the
photoresist before and after exposing to the UV and analyzing the 2100 cm band
in the FTIR spectmm which represents the CN-absorption band of the
photoresist Figure 52 shows the CN absorption band of the exposed and
unexposed photoresist
pound 2D0
Photoresist not exposed to U V - Photoresist ocposed to U V
1900 1950 2000 2050 2100 2150 2200 2250
Wave number in cm
Figure 52 CN-absorption band of exposed and unexposed photoresist
The photoresist in both cases was spun on pfranha cleaned [Appendix A]
silicon wafer at 6000 rpm for 30 seconds yielding post bake thickness of
approxunately 12 microns One of them was exposed to UV for 15 seconds and
the other was not exposed to UV light It is cleariy seen from Figure 53 that the 43
photoresist exposed to UV light has very littie nitrogen and the photoresist which
was not exposed has substantial amount of nitrogen in it The ratio of the areas of
these curves is 18979 This also verifies the fact that nitrogen is liberated when
we expose the SI813 photoresist to UV light
We then went on to characterize the cross linking of the SI813
photoresist Figure 53 shows the effect of heat on cross linking the DNQ based
positive photoresist
0 II
c
Meta Cresol Novolak
Figure 53 Effect of heat in cross-linking DNQ photoresist
When we heat DNQ based photoresist the nitrogen molecule in the
photoactive compound which is weakly bonded is liberated and results in
formation of an intermediate compound called ketene If we further continue to
44
heat the photoresist ketene reacts with meta cresol novolak resin and starts to
cross link If we observe Figure 53 carefully we can see that the C=0 bond
changes to C-O C=0 is an IR active but the C-0 is not IR active Consequentiy
with cross linking of the photoresist we should be able to see a reduction in the
C=0 peak when we take FTIR spectmm of photoresists cross linked at different
temperatures Figure 514 shows the FTIR spectmm of photoresists cross-linked
at 60 80 100 and 120 degree Celsius respectively it can be clearly seen that as
we cross-link the photoresist at higher temperatures the intensity of C=0 bond
(1700 cm) reduces
FTIR Spectrum of photo resist crosslinked at different temperatures
1400 -
1200 -
- 1000
i 800 4 o
o 600-1 o ^ 400 -
o pound 200 4
- 2 0 0 -i 1 i 1 1 T 1640 1660 1680 1700 1720 1740 1760 1780 1800
Wave number in cm
Figure 54 C=0 - absorption band DQN photoresist
45
S1813 photoresist was spin-coated on silicon wafers at 6000 rpm for 30
seconds thickness of the photoresist was measured using a Metricon 2010 [17]
prism coupler and thickness In all our experiments photoresists spun under these
conditions was used
We used acetone as a co-solvent in supercritical carbon dioxide to remove
this photoresist Figure 55 schematically represents the photoresist removal
experiments done with acetone
7000
Pressure in PSI
1500
Injection of acetone
Figure 55 Experimental conditions for removing PR using acetone as co-solvent
Three pulses of 30 acetone per pulse [Appendix B] (Ihr 30 minutes 30
minutes) was used with supercritical carbon dioxide to remove the photoresist the
treatment temperature was 85degC This treatment was not effective in removing the
photoresist from the silicon substrate Thickness of photoresist was measured
using a dektak we were not able to use the prism coupler Table 51 shows the
thickness of photoresist before and after treatment
46
Table 51 Results of SCCO^Acetone treatment
Photoresist cross linking
Temperature (^C)
60
80
100
120
Thickness in microns
(Before treatment)
133
126
121
119
Thickness in microns
(After treatment)
0 3-0 4
0 6-0 7
0 2-03
0 4-0 5
At this point we stopped using acetone as a co-solvent and started to use
propylene carbonate (PCO3) as a co solvent We wanted to verify whether the co-
solvent with SCCO2 or SCCO2 alone was responsible for the photoresist removal
Photoresist was treated with only SCCO2 no co-solvent was used and we found
that this treatment has no effect on photoresist removal Photoresist was then
freated with SCCO2 and 30 PCO3 [Appendix B] three pulses (30 minutes each)
of SCCO2PCO3 was used The system was pulsed between 1500 and 3500 PSI
The photoresist was completely removed from the sihcon substrate This
experiment confirmed that co-solvent was responsible for the photoresist removal
and SCCO2 alone has very little effect on photoresist removal We then wanted to
reduce the amount of PCO3 to optimize for complete removal of photoresist We
also wanted to show that the freattnent would be effective in removing the less
cross linked photoresist than highly cross-linked photoresist We decided to take
only two samples of photoresist one which is barely cross-linked (baked at 60degC
for 5 minutes represented by O) and the second sample which was sttirdily
47
cross-linked (baked at 100degC for 90 minutes represented by X) Table 52
shows the list of experiments that were done to optimize the photoresist removal
All the experiments were done using PCO3 as co-solvent and we pulsed the
system between three times between 1500 and 3500 PSI the co-solvent was
injected at 1500 PSI and then the pressure was raised to 3500 PSI and it was
retained at this pressure for 30 minutes The chamber temperature was kept
around 85C
Table 52 Results of SCCO2 PCO3 treatment
Exp
1
2
3
4
Experimental conditions
343 ) co-solvent (098 ml per pulse)
68 co-solvent (195 ml per pulse)
136 co-solvent (39 ml per pulse)
171 co-solvent (4875 ml per pulse)
Result
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
Only a small amount of
photoresist residue was
left behind
The photoresist was
completely removed
Our industrial contacts tell us that 171 of PC03 is too much to be
considered for a production process When we used 343 PCO3 we had only a
very little amount of photoresist residue left behind We tiiought of using 5
PCO3 [Appendix B] (142 ml per pulse) for 3 pulses and 5 [Appendix B] (214
ml) acetone in the fourth pulse too see if acetone can dissolve this residue We
48
were eventually successful in removing the photoresist completely from the
silicon substrate We wanted to see the effect of this treatment on metal substrate
1000 Angstroms of titanium was deposited on a clean silicon substrate using the
electron beam evaporation system SI813 photoresist was spun over this titanium
substrate and was cross linked at 100degC for 90 minutes We tried to remove the
photoresist using the above-mentioned recipe We were able to remove the
photoresist completely Figure 56-a shows the SEM image before treatment and
56-b shows the SEM image after treattnent
Figure 56 SEM images depicting photoresist removal from SiTi surface
a - before treatment b - after tteatment
We also wanted to verify whether cross-linking of photoresist had any
effect on its removal from the silicon substrate For the removal to be effective its
necessary to achieve some amount of sorption of the SCCO2 (andor
SCCO2PCO3) into the polymer Cross-linking lessens the amount of free volume
in the polymer (photoresist) available for CO2 sorption This is not so difficult to
appreciate when we consider that cross-linked polymers also tend to be more
impervious to any type of solvent And with the increased difficulty in creating 49
this sorption of SCCO2 we also limit the amount of swelling which can be
effected upon the photoresist This mechanism promotes photoresist removal [12
13] We used the following experimental conditions to verify this fact The system
was pulsed three times between 1500 and 3500 PSI the co-solvent was injected at
1500 PSI and then the pressure was raised to 3500 PSI and it was retained at this
pressure for 30 minutes The chamber temperature was kept around 85degC
Table 53 Experimental conditions for correlating PR removal with cross-linking
Exp
1
Experimental conditions
21 PCO3 (058 ml per pulse)
Result
More residue was left in
the cross-linked
photoresist than the un
cross-linked photoresist
Figure 57-a shows the picture of stripped photoresist which was highly
cross-linked (X) and 57-b shows the picture of stripped photoresist which was
barely cross linked (O) Figures 57-c and 57-d shows the picture of the substrate
and substrate coated with photoresist respectively
50
Figure57 Photographs showing removal of cross-linked and uncross-linked PR
Since Figure 57-c and 57-d has no white spots I assume that the white
spots seen in Figure 57-a and 57-b are the residual remain of photoresist that has
not been removed We counted the total number of white points in figure a and
figure b and the results are summarized in Table 54
Table 54 - Results for correlating cross-linking and photoresist removal
Sample
Highly cross-linked photoresist
Barely cross-linked photoresist
Number of white points
111706
10470
From Table 54 it is evident that the number of white points in the cross-
linked photoresist after treatment is greater than the number of white points in the
barely cross-linked photoresist after treatment This indicates that more 51
photoresist residue is left over in the photoresist which was highly cross-linked
than the barely cross-linked counterpart
52-Treatment of low-k dielectric materials with SCC02
As deposited nanoporous organosilicate films are hydrophobic and
etching stripping and cleaning processes must be precisely tuned to keep the
material hydrophobic Plasma treatment of this low-k film removes methyl group
from the fihn and replaces it with polar silanol group If methyl groups are
removed the fihn becomes hydrophilic and thereby increasing k value and
making it subject to moisture contamination We tried to tteat the plasma-etched
nanoporous organosilicate films with supercritical carbon dioxide and a suitable
co-solvent so that the methyl groups lost during the plasma tteatment are
reinttoduced and the film becomes hydrophobic making it less susceptible to
moisture contamuiation Figure 58 explams the sequence of reaction
schematically
-
Si mdashCH3
Si-l-CH
M S S Q
imdashOH
Plasma mdash-^imdashOH
Treatment O
-Si -T-OH
Plasma damaged M S S Q
CH3 Y CH3
+ CH3-S-N-S1-CH3
-SimdashOH c k CH3
HMDS
Plasma damaged
MSSQ
CH
^ 1 - 0 mdash S 1 - C H 3
- CH3 bullOH
Treated MSSQ
Figure 58 Treatment of plasma damaged MSSQ with HMDS 52
The nanoporous organosilicate samples used in these experiments (Table
55) were provided by Tokyo Electron America The etching and ashing processes
damaged the dielectric properties of the nanoporous films These processes
removed the methyl group and replaced it with the highly polar silanol group
HMDS and TCMS dissolved in Supercritical CO2 (SCCO2) were used at several
temperatures and pressures to treat the damaged samples The goal of these
experiments was to determine if the methyl group could replace silanol group in
the film FTIR specttoscopy and contact angle measurements were used to
characterize these tteatments 2800 cm to 3000 cm band in the FTIR spectmm
which represents the CH-absorption band of the low-k film is used for
characterizing the CH content before and after tteatment
Table 55 TEL samples used in these experiments
Sample
1
2
3
4
5
Description
Etched and ashed
Undamaged
Undamaged
Etched and ashed
Etched and ashed
Substrate
P-type silicon
P-type silicon
P^-type silicon
P^-type sihcon
P-type silicon with extta under-layer of Si02 of 2OOA
Used for
FTIR amp Contact angle
FTIR amp Contact angle
Dielectric
Dielectric
FTIR and CV
Dielectric Constant
225 plusmn01
30 plusmn01
53
For the SCCO2HMDS treatment four experiments were performed In the
first experiment the required amount of HMDS was directly added to the vessel
The vessel was then pre-flushed as mentioned earlier After the last pre-flush the
vessel pressure was kept at 700 PSI and the vessel-heating process was started
When the vessel temperature reached 150degC the pressure was increased to 3000
PSI for 30 minutes The vessel was then post-flushed as mentioned before After
the last post-flush the vessel was depressurized to 700 PSI and it was left to cool
down to room temperature The vessel was then finally depressurized to
atmospheric pressure Figure 59 schematically represents the sequence of this
experiment This is the way our group tteated the low k samples from IBM All
these freatments on IBM samples were done in the chemical enghieering
department We wanted to verify whether this treatment would work on our
system m the Maddox lab We name this experiment as Treated old way
i
Pre ssure (p
si)
(
^OO psi
^
H]n)S fliiec Aildedtoves
Treated Old way
5000 i)si
1 30 minutes
l O (les C 1 sel
Time in minutes
S
0 mpere tu
re
a
0
Figure 59 SCCO2HMDS freatment of plasma damaged MSSQ witii HMDS - Treated old way
54
At 700 PSI CO2 is still a gas and as the temperature of the vessel is
increased the added HMDS starts to evaporate This fact led us to do the second
experiment where we repeated the same procedure as above with one major
difference In this experiment instead of leaving the vessel pressure at 700 PSI
during the vessel-heating process the pressure in the vessel was raised to
supercritical pressure (3000PSI) as the temperature reached 35degC This prevents
the HMDS from evaporating and only the SCCO2HMDS effects are the dominant
freatment mechanism The vessel temperature was then increased to 150degC After
30 minutes at this temperature the vessel was post-flushed and allowed to cool
down as mentioned above In this experiment we attempted to isolate the SCC02
tteatment from any possible HMDS vapor treatment In this experiment the
vessel was kept at supercritical presstire from start to end the pressure was
increased to 3000 PSI at 35degC to insure a supercritical state and to prevent HMDS
evaporation during the whole experiment Figure 510 schematically represents
the sequence of this experiment
i
Pressu
re (p
si)
k
SCCO2 HAroS Treatment
3000 psi
0 minutes I
150 (lee C
hO^l)
bull mpera tu
ra
Q Cft M
s I
nmgt^ directly Time in minutes
Adileltl to ressel
Figure 510 SCCO2HMDS treattnent of plasma damaged MSSQ with HMDS SCCO2 HMDS tteatment
55
In the third experiment the same procedure as in the first experiment was
repeated but the pressure was never raised beyond 700 PSI The vessel was then
heated to 150degC In this experiment the effects of HMDS vapor mixed in 700 PSI
CO2 were studied The post flushing was performed between 700 and 100 PSI in
this experiment Figure 511 schematically represents the sequence of this
experiment
C02 IDVroS Treatment
CD CA CO
c CD
bulla
00 psi
30 minutes
150 deg C
I
c
a
HMDS duectly Added to vessel
Time in minutes
Figure 511 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -CO2 HMDS treatment
Finally in the fourth experiment the vessel was used to perform a pure
HMDS vapor treatment at 150degC for 30 minutes The same pre-flushmg and post-
flushing procedure was used but no CO2 was used during the heating process
Figure 512 schematically represents the sequence of this experiment
56
HMDS ^al)or treatment
CO cgt CO
c CD
bull o CO
30 niiiiiites
15(f rlesr C
1
- bull
Q 9 9k
laquo ^
HMDS directly Added to vevsel
Time in minutes
Figure 512 SCCO2HMDS tteatment of plasma damaged MSSQ with HMDS -HMDS vapor tteatment
Figure 513 shows the effects of the four HMDS treatments on the CH
content of the damaged film (fihnl) This tteatment as usual mcreased the CH
content to the same level as the undamaged film but deeper understandmg for this
process is requfred to optimize the tteatment In the second experiment FTIR
analysis shows no increase in the CH content after the SCCO2 treattnent hi this
experiment no HMDS vapor was present The third and the fourth experiments
were designed to further study the heathig procedure used m the old experiments
In both experiments the CH content was recovered to same level as m the
undamaged film (exactiy as in the fnst experiment) The common factor in the
fnst third and fourth experiments is that the HMDS was allowed to evaporate
during the heating process hi the first experiment the HMDS was added dfrectiy
to the vessel then the vessel was pre-flushed with CO2 and it was left at 700 PSI
until the vessel was heated to 150degC During the heating process HMDS will 57
evaporate and this will cause a surface modification to of the sample After the
temperature reaches 150C the pressure was increased to 3000PSI to complete
the tteatment In the thfrd experiment the CO2 pressure was kept at 700PS1 until
the temperature of the vessel reached 150degC and it was left there for 30 minutes
In the fourth experiment the HMDS was added to the vessel and no CO2 was
used The temperature of the vessel was then increased to 150degC for 30 minutes
As can be seen from the FTIR analysis in Figure 513 these three experiments
were able to recover the CH content in the damaged film while the second
experiment where no HMDS vapor was allowed was not able to recover the CH
content These facts indicate that the HMDS vapor and not the SCCO2 is the
main factor in recovering the CH content of the damaged films
350 -
300 -
C 2 30 -
i t 200 ^ lt]gt o
Z bullbulli 100 -
o
lt ffi 50-J
0 -
-50
-U n d a m g e d
S C C 0 2 H M D S t r e a t e d ( e x p 2 )
T r e a t e d o l d w a y ( e x p P l )
-D a m a g e d
CO H M D S 7 0 O p s ( e x p 3 )
H M D S v a p o r in t h e
S C C 0 2 s y s t e m ( e x p 4 )
- r 1 r 2750 2800 2850 2900 2950 3000 3050
W a v e n u m b e r ( c m )
Figure 513 CH-absorption band of HMDS treated film 1
58
Table 56 Contact angle measurements of HMDS treated TEL films
Exp
1
2
3
4
5
6
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
Treated old way
SCC02 HMDS
HMDS vapor treatment
C02 HMDS at 700 PSI
Contact angle in degrees
89
28
80
102
101
105
Table 56 shows the results of contact angle measurement on these
samples before and after the HMDS tteatmentThe contact angle measurements
mdicate that the surface which was rendered hydrophihc by the plasma tteattnent
has been recovered and made hydrophobic by the SCCO2HMDS tteatment [19]
The recovery of the CH content in ttie HMDS vapor treatment can be attributed to
die formation of a thm layer on the surface of the damaged film Y S Mor etal
also assumed the formation of this layer [18] in thefr work on shnilar plasma
damage treatment
A second set of experiments was aimed at dying alternative chemicals for
this treattnent One atttactive choice was ttichloromethylsilane (TCMS) This
molecule has only one methyl group attached to Si in comparison to the three-
methyl groups in case of the HMDS Also the size of the TCMS molecule is
59
relatively smatier than that of the HMDS so it might be more efficient in
diffusing into the nanoporous organosilicate matrix
In one experiment after pre-flushing the vessel 10 TCMS [Appendix B]
was used in two-steps pulsed SCC02 treatment In the first step three pulses
(1500 -7000 PSI) for 30 min each at 70degC were used to treat the damaged
samples In the second step the temperature was increased to 160degC and then
similar pulsing procedure as in the first step was applied at this temperature
Post-flushing depressurization and cooling down were done in the same way as
in the SCC02HMDS experiments Figure 514 schematically represents the
sequence of this experiment
2 step pulsing with TCMS
CD CO
CD
CO
000 psi
30 minutes
lez Time in minutes
Injection of TCMS
Figure 514 2 step pulsing with TCMS
hi another experiment only the 160degC pulsing treatment (see the previous
experiment description) was done This experiment was performed to differentiate
between the effects of the high-temperature and the low-temperattire effects on
this treatment Table 57 gives the results of contact angle measurement on these
60
samples before and after the TCMS treatment Figure 515 schematically
represents the sequence of this experiment
Time in mimites Injection of TCv IS
Figure 515 1 step pulsmg with TCMS
Figure 516 shows die effects of the SCCO2TCMS tteatment on the
damaged film (flhnl) on CH and Si-CHn bands As it can be seen clearly from
this flgure the pulsed tteatment was successful to recover and add to the CH
content of the damaged films The inset of this figure also shows the increase in
the Si-CHs content of this fihn which is consistent with the increase of the CH
content
61
Q 2 0 0 -
o
t t a p p u l laquo l n g vIHi T C M S i lt p laquo l 1
- 1 11raquop p u l l i n g VI t i l T C M S IS s p S i I
- U n - d a m a g e d
2 S D ZSOCI raquoS ( I
bull M bullbulllaquobull n um b 9 r fo m i
Figure 516CH Absorption band of TCMS tteated Fihn 1
Table 57 Contact angle measurements of TCMS tteated TEL films
Exp
1
2
3
4
Treatment
TEL Sample undamaged
(As given)
TEL Sample (Plasma
etched)
2 step pulsing with
TCMS
1 step pulsing with
TCMS
Contact angle in degrees
89
28
91
82
Table 57 gives the results of contact angle measurement on these samples
before and after the TCMS treatment
62
J
Film 1 tteated with TCMS Magnified Image
Figure 517 SEM micrographs for filml treated with TCMS
SEM nucrograph of fihnl after the SCCO2TCMS tteatment is shown in
Figure 517 This figure shows that the SCCO2TCMS treattnent left a deposit on
the surfaces of the treated fihns The formation of deposits explains the enhanced
CH and Si-CHa intensities The deposit formation was verified by taking multiple
SEM micrographs for each sample as can be seen in Figure 517 In addition to
the deposit formation TCMS is also very corrosive and it is extremely reactive
with water These facts led us to abandon usmg ttiis material as an alternative for
the HMDS
The SCCO2HMDS treatment used earlier to treat the MSQl (JSR4) and
MS02 (JSR6) is in fact HMDS vapor treatment The set of experiments expl
through exp4 discussed above indicates clearly that the HMDS vapor is the
only factor that causes the increase of the methyl concenttation in the treated
63
films For the SCCO2TCMS tteatment deposits formed on the film surface
These deposits enhance the methyl concentration considerably The OH content
of these films reduced Due to the corrosive nature and the high water reactivity
of the TCMS this material was abandoned
64
CHAPTER VI
CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK
Supercritical fluids are receiving wide attention in manufacturing
processes as cleaning solvents and reaction media The use of supercritical fluids
for the exttaction of organic compounds has been a commercial process for many
years In this work we have investigated the use of supercritical CO2 and co-
solvents to remove photoresist from silicon substrate and to cure the plasma
induced damage in low-k dielectric films
Results of photoresist removal experiments indicate significant promise in
the use of supercritical fluids for replacement of organic liquid solvents removing
photoresist from silicon substrate We were able to remove photoresist from
silicon subsfrate by sung 3 pulses of 5 propylene carbonate (142 ml per pulse)
and 1 pulse of 5 acetone (214 ml) This accounts to 64 ml of co-solvents this
is much lesser than that used by the industry for removing photoresist an
photoresist residue from the sihcon subsfrate
Due to the attractive physical properties of SCCO2 it was used as a carrier
for HMDS and TCMS to cure the plasma-damaged porous low-k films The
HMDS treatment turned out to be a vapor tteatment and it was found that HMDS
vapor was indeed responsible for the recovery of methyl content in the plasma
damaged porous low k films In the SCC02TCMS treatment deposfts formed on
the film surface These deposits enhance the methyl concenttation considerably
The OH content of these films reduced TCMS was found to be extremely
65
reactive with water and extremely corrosive Due to the corrosive nature and the
high water reactivity of the TCMS this material was abandoned
61-Future Work
Pulsed SCCO2 co-solvent treatment for removing photoresist from silicon
subsfrate needs to be optimized The process temperature and pressure needs to be
optimized to the lowest possible values to make the process more practical for
industry The conttol of all the valves needs to be changed from manual control to
computer confrol where a better control can be achieved for the pressurization
and depressurization process A recycling system for recycling the CO2 needs to
be designed by doing so I believe tat we can reuse the CO2 for at least five or six
times and there saving some money
66
REFERENCES
1 Maximilian A Biberger amp Paul Schilling Supercritical Systems Inc Fremont CA USA Don Frye amp Michael E Mills The Dow Chemical Company Midland MI USA Photoresist and Photoresist Residue Removal with Supercritical CO2 (Journals Edition 12 Semiconductor Fabtech Published July 2000)
2 Brian D Knutson Risk Control MS The use of Supercritical CO2 based solvents as a cost effective and environmentally sound alternative to current photoresist stripping solvents ND
3 International Technology Roadmap for Semiconductors 1999 roadmap (httppublicittsnet^
4 Spall WD Supercritical carbon dioxide precision cleaning for solvent and waste reduction International J Environmentally conscious design and manufacture 281 (1993)
5 Hsinwei Chou and Shengkai Chiu Cross talk reduction and tolerance in deep sub-micron intercormects Dept of ECE University of Wisconsin Madison ND
6 Trends in wafer cleaning - Ruth DeJule Associate Editor mdash Semiconductor International 811998 (httpwww^e-insitenetsemi conductorindexasplayout=articleamparticleid=CAl 63977)
7 Toshihiko Tanaka Mitsuaki Morigami Nobufumi Atoda SORTEC Corporation 16-1 Wadai Tsukuba Ibaraki 300-42 Japan Mechanism of Resist Pattem collapse ND
8 Steven A Campbell The Science and Engineering of Microelectronic Fabrication Second Edition (Oxford series in Electrical Engineering) Chapter 8
9 R R Dammel Diazonaphthoquinone Based Photoresists SPIE Optical Eng Press Bellingham WA 1993
10 Tony Clifford Fundamentals of Supercritical Fluids Oxford University Press Oxford 1998
11 Laurie L Williams Removal of Polymer Coating with Supercritical Carbon Dioxide Doctoral dissertation Dept of Mechanical Engineering Colorado State University Fall 2001
67
12 J Rubin LB Davenhall J Barton CMV Taylor and K Tiefert A comparison of chilled DI water Ozone and C02 - Based Supercritical fluids as replacements for photoresist - stiipping solvents lEEECMPT international electtonics manufacturing technology symposium 1998
13 L T Taylor Supercritical Fluid Extraction Wiley New York (1996)
14 Edward M Russick Carol L J Adkins and Christopher W Dyck Super critical carbon dioxide extraction of solvent from micro machined structures Supercritical Fluids Extraction and Pollution Prevention Edited by Martin A Abraham and Aydin A Sunol oxford university press 1997
15 Haskel booster pump httpwwM^haskelcom
16 Jasco liquid injection pump httpwv^wjascoinccom
17 Metricon prism coupler httpv^^wwmetriconcom
18 Y S Mor et al Effective repafr to ultra-low-k dielectric material (k~2) by Hexamethyldisilazane tteatment J Vacuum Science technology B 20(4) JulyAug 2002
19 RS Ward Y Tian Z Chen and G Somorjai Environmentally Induced Surface Rearrangement of Polyurethanes using SFG AFM XPS and Contact Angle Goniometry 25th Annual Meeting of the Society for Biomaterials April 29 - May 1 1999 Providence Rhode Island USA
20 httpwwwwoostereduchemistrvisbrubakerdefaulthtm
21 Bashar I Lahlouh Plasma-Enhanced Chemical Vapor Deposition of Low Dielectric Constant Materials Doctoral dissertation of Submitted to Texas Tech University Spring 2003
22 B Smith Infrared Spectral interpretation CRC Press Boca Raton FL
1998
23 Theory of FTIR Specfroscopy Temet Instruments Oy - Pulttitie Helsinki Finland (httpwwwgasmetfiTechnical DataTheory ofFT-IP_gpgrtroscopvPDF)
24 E Meloan Elementaiy Infrared Spectroscopy The Macmillan Company New York 1963
75 bttpsairontechcom
76 wwwhghtutorontocatsargentecel465sllbppt
68
APPENDIX A
PREPARATION OF PIRANHA SOLUTION
A piranha solution is used to remove organic residues from substrates
Piranha solution is a 51 mixture of concentrated sulfuric acid (H2SO4) with
hydrogen peroxide (H2O2) Piranha solutions are extremely energetic and may
result in explosion or skin bums if not handled with extreme caution When
preparing the pfranha solution its advisable to always add the hydrogen peroxide
to the sulfuric acid Piranha solution is very energetic and can explode if it reacts
with organic compounds like acetone It is very likely to become hot more than
100 degrees C When preparing piranha solution we should follow the safety
precautions such as wearing goggles to protect our eyes and gloves to protect our
hands
69
APPENDIX B
CALCULATION OF AMOUNT OF CO-SOLVENT
An ideal gas is defined as one in which all collisions between atoms or
molecules are perfectiy elastic and in which there are no intermolecular attractive
forces One can visualize it as a collection of perfectiy hard spheres which collide
but which otherwise do not interact with each other An ideal gas can be
characterized by three state variables absolute pressure (P) volume (V) and
absolute temperature (T) The relationship between them may be deduced from
kinetic theory and is called the ideal gas law
PV^nRT Equation (Bl)
where n = the number of moles of the gas m consideration
R = the universal gas constant = 8315 Joules mole Kelvin
P = the pressure of the gas
V = the volume occupied by the gas
T = the temperature of the gas in Kelvin
Johannes Van der Waals was interested in the kinetic theory of gases and
fluids and his primary work was to develop an equation which apphed to real
gases unlike that of ideal gas which assumes that there are no attractive forces
between molecule and that molecules have zero volume In reality molecules
have a small volume and atttactive forces exist between them Van der Waals
70
introduced these properties into the theory by means of two constants which were
specific to each gas Van der Waals law states that
P + a[mdashf [V - nb] = nRT Equation (B2)
Where a and b are constant for a particular gas For Carbon dioxide a=
0365 (J mVmole^) and b = 427x10^ (mVmole) Van der Waals was awarded a
Nobel Prize in 1910 for his work on the equation of state of gases and hquids The
weak electtostatic attractions between atoms were named Van der Waals forces
in his honor
In this work to calculate the amount of co-solvent required to be added I
used the Van der Waals equation of state [equation B2] If we plug in the values
for pressure temperature volume and R a and b beuig constants the Van der
Waals equation becomes a cubic equation with n (number of moles of CO2 m this
case) as a variable to be solved This equation can be solved for n A mole is the
quantity of a substance whose weight fri grams in equal to the molecular weight of
the substance
So when we get the number of moles of CO2 if we multiply that by the
molecular weight of CO2 we will be able to get the amount of CO2 in grams This
is the amount of CO2 hi the high-pressure vessel at a give pressure and
temperature Let Y be the percent of co-solvent required be added The amount of
co-solvent in grams will be given by
[amount of CO in grams] Amount of co-solvent = Yx^ ^ grams
71
Amount of co - solvent (grams) The amount of co-solvents m (ml) -
Density of co - solvent (gramscm^)
72
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