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Dry Etching, General Principles
Dr. Marc Madou, Winter 2011
Class 4
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Content
Dry etching: definition
Pressure units and modes of gasflow
Plasmas or discharges How to create a vacuum
Plasmas: DC and AC
Paschen curve
Dry etching mechanisms Dry etching types and equipment Etching profiles:
Sputtering Chemical
Ion-enhanced
Ion-enhanced inhibitor
Etching profiles in physical etching
Faceting
Ditching
Redeposition
Comparing wet with DryEtching
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Dry etching: definition
Dry etching techniques are those that useplasmas (hot ionized gases) to drive chemicalreactions or employ energetic ion beams toremove material. Dry-etching processes yieldfiner patterns than wet etching (surface
tension !). These techniques also offersgreater safety as large quantities of corrosiveacids or bases are not required.
Within a dry etching reaction chamber thewafers lie directly in the plasma glow (alsocalled a discharge), where reactive ions areaccelerated towards the wafer (often biased).
The ions are a species likely to attack thesubstrate material chemically with or withoutselectivity.
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Definitions of Vacuum Regimes:1) Rough Vacuum: ~0.1- 0 torr (atmospheric pressure is 0 torr)2) Medium Vacuum:~ 0.1 to 10-4 torr3) High Vacuum: ~ 10 - to 10-4 torr
4) Ultrahigh Vacuum: 10-
torr 2 modes of gas flow:
Viscous Flow regime:gas density (pressure) is high enough, manymoleculemolecule collisions occur and dominate the flow process (onemolecule pushes another). Collisions with walls play a secondary rolein limiting the gas flow.
Molecular flow regime: gas density (pressure) is very low, few molecule-molecule collisions occur and molecule- chamber wall collisions dominate the
flow process (molecules are held back by walls). See further below formathematical expressions for these two regimes.
Pressure units
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How to create a vacuum
Visit on your own timehttp: et.nmsu.edu ETCLASSES vlsi files ARTICLE.HTM on vacuum pumping (take the quiz at the end).
The first thing that all basic systems have is a rough-pumping system. It is used to reduce the pressure from
atmospheric pressure in the chamber to a lower pressurelevel that other low-pressure systems can use. Then therehas to be a fine-pumping system that must be able toattain sufficient pumping speed to handle the outgassingfrom the work produced in the chamber of the vessel.There must also be vacuum gauges that determine thepressure at certain points of the system.
Pumps: Diffusion pumps operate from 10-4 Torr to 5x10-11 Torr.
Diffusion pumps operate by boiling a fluid, oftenhydrocarbon oil, and angling the dense vapor stream in adownward conical direction back into the pump boiler. Gasmolecules from the system that enter the oil curtain arepushed toward the boiler by momentum transfer from the
large fluid molecules.
Diffusion pump
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Mechanical pump. Pump operation is based on bulk flow of gas; hence the pump worksin the viscous flow regime. Used for obtaining "rough" vacuum (10 -3 Torr), which is thelower limit of the viscous flow regime
Simplest plasma chamber is 2 parallel plate electrode set (anode and cathode) in alow pressure Argon filled chamber (e.g. 0.001 to 1 Torr). The two electrodes arepositioned parallel to each other, with the top electrode and chamber wallselectrically grounded while the lower electrode and substrate holder are connectedthrough a dc-blocking capacitor and matching network to a 13.5 MHz F generator(AC plasma case)
Plasmas : DC and AC
Principle of mechanical operation:(1) begin expansion cycle
(2) seal off expanded volume(3) compress gas out exhaust
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Apply 1.5 kV over 15 cm--field is 100-V cm. Breakdown ofArgon when electrons transfer a kinetic energy of 15. eV to theArgon gas.
These energetic collisions generate a second electron and apositive ion for each successful strike.
If the two electrons reenergize creating an avalanche of ions andelectrons we get a glow or plasma.
At the start of a sustained gas breakdown a current starts flowingand the voltage drops to about 150 V.
To sustain a plasma, a mechanism must exist to generateadditional free electrons after the plasma generating ones havebeen captured at the anode.
The additional electrons are formed by ions of sufficient energystriking the cathode (emitting secondary Auger electrons).
This continuous generation provides a steady supply of electronsand a stable plasma.
Plates too close: no ionizing collisions (not enough energy), toofar too many inelastic collisons in which ions loose energy.
Plasmas: DC and AC
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Plasmas : DC and AC
Plasma dark spaces: dark because the higher energyelectrons cause ionization rather than light-generatingexcitation.
Plasma is always positive, this follows from kinetics: fora random velocity distribution the flux of ions and
electrons upon a surface is given by:
where n is a density and v> an average velocity. Ions
are 4000 to 100,000 times more heavy than electrons sothe average velocity of electrons is much larger. Electronflux to surrounding surfaces is larger resulting in apositive charge on the plasma.
Assymetry of voltage distribution: electrons move fasteraway from the cathode than positive ions are accelerated
towards it larger space charge (also the dark space islarger at the cathode).
j i,e !n i,e vi, e
4
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Plasmas : DC and AC
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The largest voltage drop is in front ofthe cathode where charged particles willexperience their largest acceleration.The cathode gets etched the anode doesnot !! Substrates to be etched are laid
down on the cathode. Efficiency or strength of a particular
plasma is evaluated by the average electron energy (temperature)
ion energy (temperature) electron density (e.g. 10 and 1012 cm-3)
ion density (e.g. 10 to 1012
cm-3
) neutral species density (e.g. 1015 to 10 1
cm -3)
ion current (e.g. 1 to 10 mA cm 2).
ve ! kT e (e.g. 110eV)
v i!
kT i (e.g. 0.04 eV)
Plasmas:DCandAC
The ratio between ionized speciesand neutral gas species is 10- to10-4.
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Plasmas : DC and AC
An important quantity to describe aplasma is the ratio of electrical fieldover pressure (Equation I). Withincreasing fields the velocity of freeelectrons or ions increases (~E) but an
increase in pressure decreases theelectron or ion mean free path(~1 P).The mean free path (P) is givenby Equation (II) where nv is the numberof molecules per unit volume,
The number of molecules per unitvolume, nv, can be determined from
Avogadro's number and the ideal gaslaw, leading to Equation (III)
The bombarding flux of ions on thecathode is given by Equation (VI):
kT i , e ~E
P (I)
(II)
(Equation III)
i ! qn iQ i E
(Equation IV)
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Plasmas : DC and AC
d ! lnm
i
Tim
ma ! d
! T
Ema
!
AC plasmas for etching insulating surfaces.
Capacitor makes voltage distribution assymetric in this case.
A DC self bias results.
Etching energy:
Plasma energy:
Self bias:
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AC Plasmas
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Paschen curve
Pasc e C rve i irBreak o
oltage
Press re x ista ceP X ( mm Hg-mm)
t 1 atmosp ere = 760 mm Hg
0
0
200
400
600
800
1000
1200
1400
10 2 0 30 4 02
2.6 m 13. 16 m
Ne Physics aChemistry
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Dry chemical etching mechanisms
Reactive species generation (1)
Diffuse to the solid (2)
Adsorption at the surface (3)
Reaction at the surface (4)
Reactive cluster desorption (5)
Diffusion away from the surface( )
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Continuous dry-etching spectrum
low pressure 100 millitorr: physicalsputtering unselective
directional radiation damage
100 millitorr range:RIE
physical and chemical
directional
more selective than sputtering
higher pressures:plasma etching
chemical (10-1000 times faster) --see extreme example, gas phaseetching with XeF2 (not really aplasma)
isotropic
more selective
least damage
Dry chemical etching mechanisms
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XeF2 Gas Phase Etching (highpressure, chemical only)
no plasma ( ust pump)
10 m min
no damage
isotropic
very selective (Si over Al,photoresist, oxide and nitride)
CM S compatible
Dry chemical etching mechanisms: purely chemical
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Dry chemical etching mechanisms: Physical-chemical etching: Energy-driven anisotropy
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Dry chemical etching mechanisms: Physical-chemical etching: Inhibitor-driven anisotropy
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Dry etching types and equipment
Dry Etching
Glow dischargemethods-DiodeSet-up
Ion beam methods-Triode Set-ups
Plasmaetching
Reactiveion etchingsputtering
Sputteretching
Ionmilling
Reactiveion beametching
hysica etching
Ion beamassistedchem.etch
eacti egas
p asma
eacti egas
p asma
nertgas
nert gasi n
nert gasi n
eacti e gasi n eam
0.2 - 2Torr 0.01-0.2
Torr
10-4-10-3Torr
energy
m ard.
Highenergy
Highenergy
N reacti eneutra s
eacti eneutra s
mereacti e
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Dry etching types and equipment
RIE chamber with load lock
Substrate hol er for io etchi g
Substrate hol er for epositio
o e
Catho e
acuum chamber all
Matchi g et ork
13.56 MHz
F Ge
erator,1-2 kW
( F electro e ith target)
Ground shield
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CAIBE RIBE IBE MIE MERIE RIE Barrel
Etchi ng
PE
Pressure
(Torr)~10-4 ~10-4 ~10-4 10-3-10-2 10-3-10-2 10-3-
10-1
10-1-100 10-1-101
Etch
Mechanism
chem/
phy
chem/
phy
phy phy chem/
phy
chem/
phy
chem chem
Selectivity good good poor poor good good excellent goodProfile anis or
iso
anis anis anis anis iso or
anis
iso iso or
anis
Dry etching types and equipment
Reactivegas Inertgas
+++++++ +++++++Plasmasource
RIBE CAIBEReactivegas
Vacuum pump
Substrate Substrate
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Acronym/Technique E lanation
AI E Chemically assistedion beam etching
ERIE agnetically enhancedreactive ion etching
IE agnetically enhanced ionetching
PE lasma etchingRI E eactive ion beam
etchingRIE eactive ion etching
Dry etching types and equipment
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Dry etching types and equipment : RIBE vs.CAIBE
CAIBE is RIE in a triodesystem (e.g. 10, 000 min)
RIBE ion is reactive and etches
(e.g. 100 min)
Re cti e s I ert s
+ + + + + + + + + + + + + +l s s rce
RIB CAIBRe cti e s
V c
str te str te
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Etching profiles in dry etching
Sputtering: directional, physical.
Chemical: non-directional(diffusion).
Ion-enhanced energetic:
directional. Ion-enhanced inhibitor: directional.
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Etching profiles in physical etching
Faceting: angle of preferentialetching
Ditching (trenching): sometimescaused by faceting
Redeposition: rotational stagemight reduce this effect.
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Homework
1 How is a DC plasma created and how does an RF plasma differ? Why is aplasma always positive with respect to the reactor vessel walls? In whichetching setup would you prefer to etch an insulator? Is space positivelycharged?
2 Detail the different dry etching profiles available and how you obtain them.
3. Explain the DC breakdown voltage versus electrode distance curve(Paschens law) and how it is relevant to dry etching. How isminiaturization of an electrode set equivalent to creating a local vacuum?
4. Discuss the etch profiles in physical etching. Also draw profiles exhibitingfaceting, ditching, and redeposition.
5. Design a process to fabricate a polyimide post 100 m high and 10 m in
diameter on a Si cantilever. The Si cantilever must be able to move up anddown over a couple of microns.
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