VLSI Fab - Lectures

8/8/2019 VLSI Fab - Lectures

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S. Go alan Amrita Viswa Vid a eetam 24.01.05

VLSI Fabrication

Semiconductor materialsCrystal structuresDefects in crystals


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Classification of Materials Based on mechanical / physical properties:

Metals

Ceramics (compound of metal and non-metal) Polymers (chains of carbon-carbon and carbon-hydrogen bond)

Based on electrical properties:

Conductors ( = 10-4

10-6

ohm.cm) Insulators ( = 10 10 ohm.cm) Semiconductors ( = 10 10 10 -4 ohm.cm) Superconductors

Based on ordering of atoms: Crystalline Amorphous Partially crystalline

Based onatomic natureand bonding


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Periodic TableSemiconducting region


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Ternary compounds: Al xGa 1-xAsElectronics : Si

Optoelectronics : binary (GaAs), and ternary compounds


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(a) Metallic bonding (b) Ionic bonding

Chemical bonding in solids

In sodium chloride (NaCl), Nabeing highly electropositive, givesone electron to chlorine (which ishighly electronegative).

Positive ions in a sea ofelectrons.


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Chemical bonding in solids

In Covalent bonding, the atomsinvolved share the electrons inthe outermost shell equally e.g.

Si, CH4, diamond, etc

In Van der waalls bonding ,coulumbic forces hold moleculesor layers of atoms together.


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Crystalline: ordered arrangement of atoms throughoutthe solidAmorphous: random arrangement of atoms in lattice(no order)Partially crystalline: partial short range order (e.g.some polymers)

Grain boundary


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Crystalline structure can be of two types:

single crystal - planes of atom oriented in thesame direction throughout the solid

Polycrystalline - planes of atoms in adjacent

regions (grains) oriented along different directions Silicon substrates used for VLSI Circuits: single

crystal silicon Gate electrode for MOSFETs:Polysilicon


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y

x

z

c

ba

Structure of crystalline solids

A UNIT CELLis the is the smallest cell in a lattice, whichwhen replicated by transalation, rotation etc., yields theentire soild.

Lattice: 3-dimensional array of points in space

Unit cell is characterized by latticeparameters

a, b, c : edge lengths along x, y, andz directions

, , : inter-axial angles between x-y, y-z, and z-x respectively.


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Structure of crystalline solids

Seven crystal systems exist based on geometry (latticeparameters) of unit cell:

Cubic : a = b = c; = = = 90

Hexagonal : a = b = c; = = 90 ; = 120

Tetragonal : a = b c; = = = 90

Rhombohedral : a = b = c; = = 90

Orthorhombic : a b c; = = = 90 Monoclinic : a b c; = = 90

Triclinic: a b c; 90


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1. Fix coordinate system and choose vector along direction in thelattice.

2. Determine components of vector along axes.3. Reduce to lowest set of integers.4. Equivalent [100] directions indicated as .

Miller Indices for Directions


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Miller Indices for Planes

Take reciprocals of intercepts and reduce to lowest set of integers, unless intercept is fraction of unitcell edge.

Choose coordinate systemwith origin at any latticepoint and orient axes alongedges of cube.

Determine intercepts of

planes with axes in multiples of unit cell edges.


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{ } : used to denote family of planes. {100} : (100), (010), (001), (100), (010), (001)

Equivalent sets of (100) planes by

rotation of the unit cell within the cubiclattice: e.g. {100} planes.


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Basic Crystal Structures

Cubic structures

Hexagonal Close Packed (HCP)


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Crystal structures of Si and GaAs

SiGaAs

Diamond structure

For Si, one silicon atom occupies all FCC positions + one siliconatom occupies tetrahedral sites formed by 1 corner atom and 3adjacent face centered atoms

Can be thought of as two interpenetrating FCC structures

Zinc Blende (ZnS) structure


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Simple Cubic (SC)

Cell parameters

Latticeconstant a

Atomic radius r

Number of atoms per unit cell: 8 corner atoms each shared by 8 adjacentunit cells 8 x (1/8) = 1 atom/unit cell lattice parameter in terms of atomic radius: a = 2r, where r is the radiusof atom.

atomic density = number of atoms per unit cell / unit cell volume = 1/(a3

) Number of nearest neighbors : Each corner atom is in contact with 6adjacent corner atoms

Atomic packing fraction (APF) = Volume of atoms in unit cell/ unit cell vol


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Crystal structures of Si and GaAs

a

43

crystal lattice is face centered cubic (FCC), with two atombasis [at (0,0,0) and (1/4, 1/4, 1/4) ]

- two interpenetrating FCC lattices- lattice constant a: cube side length

silicon (rm temp): 5.43 ; GaAs: 5.65 nearest neighbor distance dn =

Atoms/unit cell:

4 atoms inside cube6 atoms half inside at face centers8 atoms 1/8 inside at cornerstotal of 8 atoms unit cell

atomic density : 8 / a3


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Defects in Crystals

Point defects : missing atoms, extra atoms,impurity atoms

Line defects : Edge dislocation, screw dislocation

(1-Dimensional)

Area defects : stacking faults, etc (2-D)

Volume defects : precipitates of impurities (3-D)


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Point defects vacancyvacancy substitutionalimpuritysubstitutionalimpurity

interstitialimpurityinterstitialimpurity

selfinterstitialselfinterstitial Frenkel defectFrenkel defect

Schottky: cation-anion vacancy (e.g. Na+ and Cl-)

Impurities can eitheroccupy substitutional orinterstitial sites

Interstitials can haveatoms of same type (selfinterstitials) or impurity

atoms


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Defect concentrations

n Natomic

e E 2 k T vacancy-interstitial pair:Frenkel defect

Eformation ~ 1 eV T = 300K:n ~ 1014

T = 1300K:n ~ 1021

n Natomic e E k T

Eformation ~ 2 eV T = 300K:n ~ 0

T = 1300K:n ~ 1015

isolated vacancy:


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Rate constants andArrhenius plots

thermally activated process i.e., process must

thermally overcome anenergy barrier

plot log(y) vs 1/Tif process has the simplethermally-activated behavioryou will get a straight line!

kTE

o

A

eyy

=

0.000 2

0.000 4

0.000 6

0.000 8

400 600 800 1x10 +003 1.2x10 +003 1.4x10 +003

temperature

y EA

= 1eV

EA = 0.5eV

0.001 0.001 5 0.002 0.002 5 0.003

1x10 +005

1x10 +010

1x10 +015

1/[temperature]

log[y] EA = 1eV

EA = 0.5eV

27C250C500C1000C


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Line Defects:

Edge dislocation

Edge dislocation: extra half plane of atoms in lattice Formation energy is high, concentration usually low

Dislocation line is to plane of paper Above dislocation line bonds are stretched Below dislocation line, bonds are compressed Dislocation can move (in this case, left or right) the plane

on which the dislocation moves is called slip-plane

extra plane of atoms

dislocation


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Line Defects:

Screw dislocation

screw dislocations are most commonly formed duringcrystal growth

Burgers vector indicates the magnitude and direction ofdislocation: in this case it is parallel to dislocation line.

Screw dislocation: aportion of crystal is shiftedby a lattice distance byshear stress


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VLSI Fabrication

Defects in crystals

Si wafer fabricationZone refining

28.01.05


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Topics of last class

Crystalline and amorphous materials Single crystal and poly crystal structures

Semiconductor materials Basic crystal structures Crystal parameters and Miller indices Structure of Si and GaAs Defects in crystals

Point, line, area and volume defects

Point defects and line defects Vacancy, interstitial, substitutional impurity, frenkel, schottky Edge and screw dislocations


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Line Defects:Edge dislocation

Edge dislocation: extra half plane of atoms in lattice Formation energy is high, concentration usually low

Dislocation line is to plane of paper Above dislocation line bonds are stretched Below dislocation line, bonds are compressed

Dislocation can move (in this case, left or right) the planeon which the dislocation moves is called slip-plane

extra plane of atoms

dislocation


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Line Defects:Screw dislocation

screw dislocations are most commonly formed duringcrystal growth Burgers vector indicates the magnitude and direction of

dislocation: in this case it is parallel to dislocation line.

Screw dislocation: aportion of crystal is shiftedby a lattice distance by

shear stress


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B B B B

B B B B

C C C C

C CCC

A A A A A

A A A A A

A A A A A

B B B B

B B B B

C C C C

C CCC

A A A A A

A A A A A

A A A A A

B B B B

B B B B B

BBBBB

B

B

If atoms in a particular layer arearranged in positions A, thenthe following layer of atoms cango to position B or position C.

After B, the next layer can beeither C or A

Stacking sequences in crystals


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Area defects: stacking faults

Stacking sequence example :

A B C A B C A B C .. A B A B A B A B

Missing or extra plane causes a 2-D defect or stacking fault A B C A B C A B A B C (missing plane C) A B A B A B C A B (extra plane C)


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Volume defects: precipitates or 2 nd phase

The crystal structure of precipitate could be very differentfrom the original lattice

undesirable in active region of wafer

O 2 precipitates in inactive regions are sometimes beneficialfor gettering (removal of defects)

If impurity atoms in aparticular region getclustered together, then a

2nd phase is formed.


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Gettering Precipitates tend to act as trap sites for dislocations, and other

defects. By having a highly strained regions (such as scratching the

back side of wafer), extrinsic gettering can be achieved. By having O2 precipitates away from the active region (in the

substrate), you can reduce defects in channel region by pull

them away called intrinsic gettering. Denuded zone depth in wafer below which precipitates are

present (20-30m)

Denuded zone depth needs to be optimum.

p-type siliconp-type silicon

poly (gate)poly (gate)

oxide (channel insulator)oxide (channel insulator)channelchannelchannel

n-type silicon n-type siliconsource drain

n-type silicon n-type siliconsource drain

Denuded zone


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Fabrication Starting Point: Step1: Metallurgical grade Si from SiO2 (quartzite)

Quartzite is heated with coke, charcoal, etc in an electricarc furnace to give 98% pure Si

SiO2 (s) + 2C (s) Si (l) + 2CO

Step 2: Pulverized Si is treated with anhydrous HCl at300C to form tri-chloro Silane (SiHCl3)

Si + 3HCl = SiHCl3 + H2

Electrode

Liquid Si


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Fabrication Starting Point:

Step 3: Fractional distillation of SiHCl3 to removeunwanted impurities

SiHCl3 is a liquid at room temperature with a boiling

point of 32C Step 4: Reduction of SiHCl3 in Hydrogen to form

Electronic Grade Si (EGS)

SiHCl3 + H2 = Si + 3HCl Reaction takes place in a reactor containing a

resistance heated Si rod which serves as anucleation point for deposition of Si

Impurity in ppb range

Polycrystalline Si obtained


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Step 5: Czochralski Bulk Crystal Growth

Insert a single crystal Si seed into melt Pull crystal SLOWLY (~ 4 mm/minute) while rotating (for

uniformity).

Container-less process. Results in very few defects.

seed

Grown crystal

silica crucible

Graphite susceptor

meltRF coil

Anticlockwise rotation

Molten Si at 1412 C.

For obtaining single cryustal Si from EGS


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Add dopants to melt, but

incorporation governed bydistribution coefficient or segregation

coefficient , k d = C S /C L. Common impurities areC and O from crucible.

Cs and C L are equilibrium concentrations of dopant insolid and liquid near interface


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Diameter increases as pull rate reduced. Industry growslarge 300 mm diameter boules or ingots .

As molten Si solidifies, it imitates the structure of theseed crystal and hence the process results in a hugesingle crystal

Czochralski Bulk Crystal Growth

images from Mitsubishi Materials Siliconhttp://www.egg.or.jp/MSIL/english/msilhist0-e.html

pull directionseed

rotation

M d li CZ h


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dm/dt = amount freezing per unit time = density Vpull = pull rate A = cross sectional area

Modeling CZ growth

latent heat of fusion (L): heat flux (power) released is

( ) pullvALtd

xdAL

td

mdL ==


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critical factor is heat flow from liquid to solid heat flux (power) balance

( )2

solid1

liquid pull

xd

TdA

xd

TdAvAL =+

Heat released

on solidification

thermal diffusion in liquidfrom hot liquid towards

solidification interface+

thermal diffusion in solid from solidification interfacetowards cooler sides/end of boule

=

Thermal conductivity of liquid Thermal conductivity of solid


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interface between liquid and solid should be an isotherm temperature fluctuations cause problems!

0

xd

Td

1

=

( )2

pull solid

d T L A v A

d xthermal current

= 14424 43

2

pull

solid

d T Lv

d x

= 2

pull

solid

d T Lv

d x

= or


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most of the heat is lost via radiation from the sidesof the boule

thermal current still proportional to cross sectionalarea A (diameter) 2 and v pull

if the heat sink is from sides of boule: thermal resistance inversely proportional to

perimeter diameter,

temperature change (voltage) = I thermal R thermal

( )[ ] pull pull vdiamdiam

constant vdiamconstant T = 2

( ) T vdiam pull 1


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F th ifi ti Fl t Z P


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Further purification: Float Zone Process Also called Zone Refining process

Used to grow Si with lower contaminations than obtained fromCzochralski technique

Start with a solid Si bar with a seedattached to the bottom. An RF coil isused to keep a small region molten.The RF coil is progressively moved

up.

segregation effects used intentionallyto purify semiconductor material

As float zone moves up, the liquidbecome more richer, while impuritiesare removed from the solid

The process is done in ancontrolled ambient using Ar


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Further Steps in (100) Si wafer fabrication:1. Grind boule into cylinder and put notch on {110}orientation.

2. Saw into wafers, and grinding/ polishing of damage.

3. Chamfer edges and chemical-mechanical polish front.


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desire is to keep number of chips (die) per wafer high, even as die size increases Several challenges with non-uniformities with

larger wafer diameter

1970 1975 1980 1985 1990 19950

50

100

150

200

250

300

W a f e r d i a m e t e r ( m m )

Year

Wafer diameter trends


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VLSI FabricationCzochralski process

Liquid Encapsulated CzochralskiBridgman processWafer specification

31.01.05


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2


Area and volume defects Stacking faults and precipitates

Gettering Extraction of Electronic Grade Silicon from

quartzite

Czochralski crystal growth Relationship between pull velocity and temp

gradient in solid

Relationship between pull velocity and crystaldiameter

Float Zone process

G tt i g


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Gettering Process by which defects (e.g. metal atoms) diffuse through

the crystal and get trapped in a gettering site

O2 precipitates intentionally used in inactive region toremove defects away from active region.

Excess oxygen is trapped by rapid cooling. O2 precipitates

are formed when the supersaturated solution is annealed athigh temperatures

bulk wafer

device region

back side damage

bulk faults

mobile impurities

bulk wafer

device region

back side damage

bulk faults

mobile impurities


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Gettering

Precipitate shape and direction depends ontemperature of annealing 650C: Rod shaped along [110] direction in (100)

plane 800C: square precipitates on (100) planes with

[111] rounded edges

1000C: octahedra shaped precipitates

actual starting material oxygen concentrationand process determined by trial device faband performance evaluation.


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S 5 C h l ki B lk C l G h


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Step 5: Czochralski Bulk Crystal Growth

For obtaining single crystal Sifrom EGS

Molten Si at 1412 C.

Insert a single crystal Si seedinto melt and pull while rotatingin anticlockwise direction

Container-less process.Results in very few defects.

pull directionseed

rotation

pull directionseed

rotation

( ) T vdiam pull 1

As molten Si solidifies, it imitates the structure of theseed crystal and hence the process results in a hugesingle crystal

Diameter increases as pullrate reduced

C h l ki th i i l


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Initially, when seed comes in contact with liquid, there are alot of defects created at the interface due to thermal stress.In order to prevent these from agglomerating in the crystal,the initial pull rate is high (small dia neck minimizesdislocation).

Later, the pull rate is decreased to get desired diameter.This technique results in highly perfect crystal.

Boule and liquid container are rotated in opposite directionsto minimize temperature gradient in liquid.

Czochralski growth principles

I iti d i C h l ki th


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Furnace evacuated initially and then back filled with inertgas to maintain strict control of ambient.

Impurity redistribution at solid-liquid interface governed

by distribution coefficient or segregation coefficient ,kd = C S /C L.

Common impurities are C and O from crucible.

Most of oxygen escapes as SiO (g). Magnetic field commonly used to reduce concentration of

defects: the Lorentz force (qVxB) will keep the ionizedimpurities away from S-L interface ( magneticallyconfined CZ ).

Mag. field can be axial or transverse to boule.

Impurities during Czochralski growth


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Oxygen in CZ Silicon

concentrations typically in 10 16 - 10 18 cm -3 range segregation coefficient k ~ 1.25

more in solid than liquid

contact area between crucible and melt decreases asgrowth procedes

oxygen content decreases from seed to tang end

effects of oxygen in silicon ~ 95% interstitial; increases yield strength of silicon via

"solution hardening" effect as-grown crystal is usually supersaturated (occursabove about 6 x 10 17 )

I i i i h lid (C ) i b


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Impurity concentration in the solid (C s) at any point can beobtained as a function of initial liquid concentration C o,distribution coefficient k as:

where X is the fraction of liquid solidified

C S = k C o 1 X( )k 1

This assumes well-mixed liquid

In reality, the liquid is not well mixed dueto existence of re-circulation cells.

There is a region near the S/L interface,where very little mixing occurs called

boundary layer (b)

Recirculation cells

b


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The impurities entering solid must diffuse through

this region. Taking the effect of boundary layer into account, kcan be replaced by an effective segregation

coefficient k e

( ) DVbe ek k k

k += 1

Where V is the pull velocity, b is the boundary layer thickness, and D is impurity diffusivity


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Fl Z


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Float Zone process segregation effects used

intentionally to purifysemiconductor material

zone refining consists of

repeated passes through thesolid by a liquid zone

Where L is the length of fusionzone

float zone silicon used for highresistivity

Zone Refining k = 0.5

1E+16

1E+17

1E+18

position x (L=0.1)

C o n c e n

t r a t i o n

( # / c m 3

)

Pass 1

Pass 3

M difi i i FZ


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Modifications in FZ processDisadvantage of FZ process: doping concentration isnot uniform

Core doping

Start with doped polysilicon rod and deposit undopedpoly rod on top to get desired concentration (processcan be repeated)

Pill doping Dopant inserted through small holes drilled on top

Gas doping Gases such as PH3 or AsCl3 injected in the molten

zone

Transmutation doping (only for n-type doping) Isotope changed through exposure to neutrons

Ch ll i d i h h f G A


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Challenges associated with growth of GaAs:

Vapor pressure of Ga is 0.001atm while that of As is ~ 10atm at melting point of Si (1238C).

Arsenic evaporates and maintaining stoichiometry willbe difficult.

The thermal conductivity of GaAs (0.07W/cm-K)

is 1/3 rd of that of silicon (0.21W/cm-K) Heat dissipation is more difficult

Critical resolved shear stress for creatingdislocation is very small (1/4 th of silicon) at mp Very easy to create dislocations in GaAs

GaAs is typically grown by LEC or Bridgman methods

Li id E l t d C h l ki (LEC)


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Liquid Encapsulated Czochralski (LEC) A sealant material such as B2O3 is

used on top of GaAs to prevent outdiffusion of Arsenic.

B2O3 melts at ~400C and seals GaAs.

Seed crystal is inserted throughsealant on to GaAs.

Crystal growth occurs usually at~20atm (high pressre LEC).

Pull rates around 1cm/hr.

GaAs

B2O3

A slight excess of As isused to compensatefor some out diffusion

Bridgman Growth


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Bridgman Growth Solid Ga and As are fused into a fused silica ampoule, which

is later sealed. Smaller temperature gradients result in lower dislocation densities

Separate As chamber sometimes included in ampoule with

small orifice to maintain stoichiometry Tube furnace is made to pass through trough containing

ampoule (ampoule kept stationary to minimize disturbance).

Molten GaAs crystallizes at bottom. Seed can be used if necessary.

Crystal diameter typically 1 2. Growth of larger crystal

requires greater process control.

Ampoule

Seed

Comparing LEC and Bridgman methods for GaAs


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Comparing LEC and Bridgman methods for GaAs

Higher defect densities of >10 4 /cm 2 due to verticaltemperature gradient

Alloying with Indium (0.1%)can reduce defects, butmakes wafers more brittle

Used only for small diawafers.

Resistivity higher thanBridgman (100Mohm-cm)

Lowest dislocation density(< 10 3 /cm 2)

Large diameter possible Problem - low resistivitywafers Vertical bridgman or vertical

giant freeze methods

LEC Bridgman

Wafer preparation


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Wafer preparation Boule characterized for resistivity and crystal perfection Mechanically trimmed into proper diameter Wafer slicing

within 0.5

, within 2 5 off axis lapping grind both sides, flatness ~2-3 mm ~20 mm per side removed

edge profiling etching

chemical etch to remove surface damaged layer ~20 mm per side removed

polishing chemical-mechanical polish, SiO2 / NaOH slurry ~25 mm per polished side removed gives wafers a mirror finish

cleaning and inspection


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W f ifi ti


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Wafer specifications

Flats used to identify and orient wafers Large flat perpendicular to [110] direction

Used to align the wafers during lithography

Secondary flat used to identify doping type

P-type Primary

Secondary

(111) (100)

n-type Primary

Secondary45deg

Secondary180deg


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VLSI FabricationOxidation of SiliconProperties of SiO2

Mechanism of oxidation

09.02.05


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SiO 2 is stable down to 10 -9 Torr , T > 1000C

SiO2

can be etched with HF which leaves Si unaffected

SiO 2 is a diffusion barrier for B, P, As

SiO 2 is good insulator , > 10 16 cm, Eg = 9 eV!

SiO 2 has high dielectric breakdown field , 10 7 V/cm

SiO 2 growth on Si high-quality Si / SiO 2 interface

The beneficial properties of SiO 2 and the superior Si/SiO

2interface are believed to be the principle

reasons for the success of semiconductor industry.

Types of SiO used in devices


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Types of SiO 2 used in devices

Poly Si

Substrate

Gate oxide

FOX

LTO

Metal

MOSFET

Field OxideGate Oxide

LowTemperatureOxide

Device Isolation (Field oxide) Insulator for MOS device (Gate oxide) Inter-metal dielectric (Low-temperature oxide)

Mask and Pad oxide

D


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The growth rate, quality, and properties of SiO2

depend on the intended application:

Gate oxide: Very high quality ultra thin oxides (currently 1-2nm) High dielectric constant preferred High density

Amorphous structure required

Inter-metal dielectric:

Low density desired Low dielectric constant desired (to have reduced RC-delay) Quality not as critical as gate oxide


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The growth rate, quality, and properties of SiO2

also depend on the oxidation/depositiontechnique:

Gate oxide : Processed at high temperature Growth rate depends on ambient, temp., etc. (e.g. wet vs. dry)

N-incorporation preferred to get higher K

Inter-metal dielectric:

Processed at lower temperatures Fluorine incorporation preferred Deposition rates not critical

S f Sili Di id


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Structure of Silicon Dioxide

When the tetrahedral elements are linked in a structuredway, we get a crystalline material

When the tetrahedral units are linked to each other randomly, we get amorphous material Usually a more open structure with lower density

The basic unit of SiO2 is aTetrahedral structure

Each bond makes 109.5with others

S f Sili Di id


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Thermally grown SiO2 is usually amorphous. The larger the fraction of bridging to non-bridging, the stronger the oxide

(e.g. dry vs. wet). Common impurities include water related complexes, B, P, Na, K, etc. B and P are network formers : reduce the bridging to non-bridging ratio

(by substituting for Si). Na, K are common network modifiers (interstitials).

network former

hydroxyl group

network modifier

silicon

bridging oxygen

non-bridging oxygen


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SiO f ti


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SiO 2 formation

Oxidation of Si Thermal oxidation (wet and dry)

Anodization

Deposition Chemical vapor deposition (CVD or MOCVD) Physical vapor deposition (PVD)

Evaporation

Methods of Oxidation


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Essentially involves heating Si wafers at high temperatures (usually 900C 1050C) in an oxidizing ambient.

Dry oxidation - Si (s) + O 2 (g) SiO 2 (s)

Wet oxidation - Si (s) + 2H 2O(g) SiO 2 + 2H 2 (g)

proposed process:

1) H2O + Si-O-Si Si-OH + Si-OH2) diffusion of hydroxyl complex to SiO2 -Si interface

Wet oxidation usually results in a more open structure and hence the oxidehas lower density ( SiO2 = 2.15 gm/cm 3 ) than dry oxide ( SiO2 = 2.25 gm/cm 3)

900 1200C

900 1200C

Thermal Oxidation

Si - OH Si - O - Si+ Si - Si + H 2

Si - O H Si - O - Si

Thermal Oxidation Furnace


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Wafer loading end

The temperature ramp rates, ambient flow

rate etc, are microprocessor controlled.

CVD systems are very similar:i.e. Gases react on wafer surface todeposit thin films

Horizontal furnace



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Methods of Oxidation Rapid Thermal Oxidation (RTO)

Heat wafer rapidly to high temperatures and keep at hightemperatures for a very short period of time (< 2minutes) in oxidizingambient (O2, NO, N2O, etc)

Ramp rates are very high Room Temp to 900-1000C in < 1minute Typically these furnaces can process only one wafer at a time

Used for high-quality thin oxides

Reaction similar to the one for dry oxidation (for O 2 ambient)

N2

Wafer

RTO Schematic

O2

Heating lamp

Deposition techniques for SiO2


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Deposition techniques for SiO2

Chemical Vapor DepositionIn CVD, two gases are introduced onto a furnace. Thegases react close to the heated wafer surface and theproduct is deposited on the wafers.For different temperature regimes, different chemicalreactions are used e.g. Silane reacting with Oxygen in atmospheric pressure or low pressure

(LPCVD) at temperatures between 300C and 500C

SiH4 (g) + O2 (g) SiO2 + 2H2 (g)used for inter-metal dielectric due to low dielectric constant and low

deposition temperature

For 500-800C, decomposition of TEOS is used

Si (OC2H5)4 SiO2 + by products

450C

700C

Deposition techniques


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Deposition techniques

Decomposition of TEOS has more conformal films due to higher deposition temperatures

Deposition rate is given by: exp (- E a / kT)

Activation energy for decomposition of TEOS is ~1.9eV while for reaction of Silane is 0.6eV

For higher temperatures (900C), SiO 2 is formed by reactingdichloro silane with nitrous oxide (N2O).

SiCl 2H2 + 2N 2O SiO 2 + 2N 2 +2HCl900C



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Physical Vapor Deposition- sputter atoms from a Si target using O 2 as oxidation species.

(Electric field ionizes Ar gas into ions and electrons. Theseions impinge on target to knock off atoms which react withoxidizing species. The product is accelerated by an electricfield to reach wafer)

PlasmaAr + and e -

Ar

Wafer

Cathode

PVD Schematic

O2

SiO2 Thickness


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Currently gate oxide thickness required is ~1nm


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Growth of SiO2


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X 0.44X

When Si oxidizes, there is volume expansion (~2.2X) Diffusivity of Si in SiO2 is several orders of magnitude

smaller than diffusivity of O2 O2 is believed to diffuse through the oxide to react with Si

at interface (tracer studies).

Growth of SiO2


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20


Overall reaction for dry oxidation.

Pathway increases with Si vacancies.

High doping increases charged vacancies and hence linear oxidation rate

At room temperature, O2 and Si are not mobile enough in SiO2hence reaction stops after a while.


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21


Next class

Deal-Grove Model for Thermal oxidation

Rate constants Effect of impurities on oxidation

b


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VLSI Fabrication

Oxidation

12.02.05

Topics covered in last class


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2


p

Types of SiO2 in devices Gate oxide, field oxide, LTO

Oxidation / deposition techniques Thermal oxidation (wet and dry) Chemical Vapor Deposition Physical Vapor Deosition

Properties and structure of SiO2 Bridging and non-bridging oxygen Network formers and network modifiers

Topics for today


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3


Mechanism of oxidation Deal Grove model for thermal oxidation Linear and Parabolic rate constants

Growth models for thin oxides

p y

Growth of SiO2


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4


X 0.44X

When Si oxidizes, there is volume expansion (~2.2X) Diffusivity of Si in SiO 2 is several orders of magnitude

smaller than diffusivity of O 2 O 2 is believed to diffuse through the oxide to react with Si

at interface (tracer studies).

Original Sisurface

Deal Grove model for thermal oxidation


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5


C represents the concentration of oxygen (atoms per unitvolume) at each position

J represents the oxygen flux moving through the cross-section: atoms per unit area per unit time

C g

J 1

J 2

J 3

C s Co C i

SiSiO2StagnantGas layer



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6


Oxygen transport across stagnantgas layer is given by

Where h g is mass transport coefficient.

. ( 1 )

Oxygen transport across oxide layer is governed by diffusion asgiven by Ficks law:

Where D O is diffusivity of O 2 in the oxide and X o is thickness of oxide.

.. ( 2 )

J1

= hg

(Cg C

s)

J 2 = D o (C o C i) / X o



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7


In equilibrium,

combining equations 1, 2, and 3, we have three unknowns, Cs, Co,and Ci.but the concentration of oxygen in oxide is given by Henrys law:

Where H is Henrys gas constant and Ps is obtained from PV = nRT

Flux due to oxygen reacting with Si atinterface is given by:

Where Ks is the reaction rate constant

J 3 = k sC i .. ( 3 )

J 1 = J 2 = J 3 .. ( 4 )

Co = HP s = HkTC s .. ( 5 )


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Oxidation Rate


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9


===n j

t d xd

R

The growth rate is given by J/N 1, where N 1 is the number molecules of oxygen per unit volume of SiO 2,(For oxidation with O 2, N 1 has a typical value of 2.2 x 10 22

cm-3

)

Integrating the above equation assuming that X i is thethickness at time t = 0, we get:

.. ( 7 )

Xo Where C * = HP gConcentration in bulk of

oxide

Where


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10


Where

and

Equation 7 can be rewritten as:

Where

.. ( 8 )

Initial thickness of oxide

Rate constants


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11


For very thick oxides (or long times) ,

Xo2 = B (t + ) dependence is parabolic: (thickness)2 time

characteristic of a diffusion limited process hence B is called parabolic rate constant

- growth rate is diffusion controlled B = 2DC*

/N 1

t + >> A 2 4 B

( ) ( ) +=+ t B B A

t At X o 42 2

Rate constants


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For thin oxides (or short times),

Xo ~ (B/A) (t + );

thickness is linearly increasing with timehence B/A is called linear rate constant

- growth rate is controlled by reaction at interfaceB/A = ksC * /N 1

( ) ( )

+=

++ t

A B

B At At X o 14

12 22

1

t +


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13


Dependent on

reaction rate betweenoxidizer and silicon (k) Temperature Si orientation Pressure Oxidizing ambient

solid solubility of oxidizer inoxide (N 0) H 2O: 3 x 10 19 cm -3

O 2: 5 x 10 16 cm -3

B/A = ksC s /N 1

Parabolic rate constant


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14


B = 2DC s /N 1

Dependent on diffusivity of oxidizer in oxide (D)

AND solid solubility of oxidizer in oxide

(N0) temperature dependence mainly

from diffusivity is NOT orientation dependent IS oxidizer dependent


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Pressure dependence


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16


Higher pressures result in increased growth rates

(increased rate constants)

Halogenic Oxidation


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17


g

Addition of 1 3% Chlorine to oxygen increases the growth rate than pure O2

helps remove metallic contaminants in the form of volatile chlorides Results in better interface with Si

Better electrical characteristics (Vth, mobility) Better dielectric strength

HCl used as halogen source (corrosive) Trichloro ethylene (TCE) carcinogenic Trichloro ethane (TCA) forms toxic COCl 2

Effect of HCl on parabolic rate constant


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18


p

Effect of HCl on linear rate constant


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19


Thin Oxides


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D-G model fits data for broad range of thicknesses But for very thin oxides (


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VLSI Fabrication

Oxidation

14.02.05

Topics covered in last class


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2


Deal Grove model for oxidation Oxide growth rate from flux of oxygen atoms

Linear and Parabolic Rate constants Oxide growth as a function of time for long and

short times (thick or thin films)

Factors affecting rate constant (temperature,pressure, etc..)

Halogenic oxidation Effect of Cl in O2 on the growth rate and rateconstants

Topics for today


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3


Growth models for thin oxides

Effect of dopants on oxidation rates

Oxide characterization


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Problems with Deal Grove model


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5


Although D-G model predicts the thickness well thickoxides, it does not correctly model thin oxide growthFor the first 200A, the actual oxide growth rate is muchhigher than predicted.

Dry oxidation growth curves do not extrapolate back tozero oxide thickness at zero time.

To compensate for this discrepancy value can beadjusted.

Even then, below 300A, this model fails

Various models have been proposed.

0.01

0.1

1

0.1 1 10

D r y

O x i

d a

t i o n T

h i c k n e s s

( m

)

time (hours)

1100 C

1000 C

900 C

(100) Si

0.01

0.1

1

0.1 1 10

D r y

O x i

d a

t i o n T

h i c k n e s s

( m

)

time (hours)

1100 C

1000 C

0.01

0.1

1

0.1 1 10

D r y

O x i

d a

t i o n T

h i c k n e s s

( m

)

time (hours)

1100 C

1000 C

900 C

(100) Si

Models for thin oxide growth


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( I ) Enhanced arrival of oxidation species at interface:1. Deal-Grove suggested presence of Electric field enhances

motion of diffusing species to interface Issue: this model requires that the diffusing species must be ionic

2. Existence of holes or micro-channels in oxide enhancesoxygen diffusion to interface

Issue: this model cannot explain uniform oxide thickness acrosswafer

3. Difference between thermal expansion coefficientbetween Si and SiO2 causes stress which enhances

oxygen diffusionProblem with these model: For the thin oxide regime, theoxidation is reaction rate limited, not diffusion limited. The linear

rate constant (B/A) is independent of diffusivity.



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( II ) Increased solid solubility of O 2

in oxide: Causes greater reaction with interface Not well accepted

Henrys law not true for thin oxides (as it assumes that the

adsorbed oxygen does not dissociate not recombine)

( III ) Oxygen reaction at interface occurs over some

finite distance (Massoud et. al. ref. 13, Ch 4) Oxygen diffuses some distance into silicon (through defects) and

reacts Shown to be true for very low temperatures

L is the characteristic distance over which the reaction occurs, and C is const


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Experimental data agrees well with Reisman, Massoud, and Han &Helms models. None of the models are widely accepted. Since Massoud et. al. model is an extension of Deal-Grove, this

model is used in process simulators.

Effects of Dopants during oxidation


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Substrate usually doped prior to oxidation During oxidation, impurity redistributes between oxideand silicon according to segregation coefficient, k

If k > 1, oxide rejects impurity Dopant accumulates in silicon under oxide reaching maximum at

iinterface If impurity diffuses rapidly in SiO2, dopant rapidly removed from

interface

If k < 1, oxide takes up dopants Impurity concentration at substrate decreases near interface

2SiO

Si

C

C k =


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11


Effect of dopants during oxidation


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12


Boron Weakens structure, and reduces viscosity For heavily doped substrate (C B > 10 20 cm -3), diffusivity

of oxygen enhanced Increase in parabolic rate constant

Phosphorous k = 10 Phosphorous pile up at interface causes increased

reactivity with oxygen Rapid increase in linear rate constant Parabolic rate constant shows only small increase

Problems with thermal oxidation: OSF


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13


Oxidation induced stacking faults (OSF) caused bycreation of large number of silicon self-interstitials

OSF usually lies on {111} and found close to Si-SiO2interface

Act as gettering sites for heavy metal impurities Cause excess leakage of device

Length of stacking fault is linearly proportional tooxidation time

density can vary from ~0 to 10 7 / cm 2

Any process that injects Si vacancies inhibitformation of OSF: e.g. nitridation


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14


Next class


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15


Next class

Characterization of Oxides Thickness Dielectric strength

Si-SiO2 interface

Diffusion


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16


VLSI Fabrication


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VLSI Fabrication

Oxide characterizationPhoto Lithography

21.02.05



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2

Problems with DG oxidation model Thin oxide growth models

Effect of dopants during oxidation Oxidation induced stacking faults

Oxide characterization methods


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3

Thickness characterization Optical method

Electrical method Interface characterization

Interface state density

Charge traps Photo Lithography

Steps in lithography

Mask making Pattern transfer

Microscopic techniques


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4

Create a step in the oxide

Use lithography to create a step by etching awayportions of the oxide

For thickness > 1000A, use SEM For thickness < 1000A, use TEM

Surface profilometer

Mechanically scan wafer with a needle stylus Deflection of needle is measured and amplified as afunction of position

Very good resolution ( ~ few angstroms)



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5

Optical method 1 Dip part of wafer in dilute HF to remove oxidecompletely

A gradation of thickness exists between etched andunetched oxides Thickness obtained from color sequence

Optical method 2 - ellipsometry Polarized coherent light is reflected off the oxidesurface at some angle

Reflected light and intensity measured as a functionof polarization angle

Comparing intensities of incident & reflected light andchange in polarization angle, film thickness and index

of refraction can be determined



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Optical method 3 interference method Incident light almost normal to wafer Intensity of reflected light measured as a function of

Optical maximum when incoming and outgoing wavesinterfere constructively for some wavelength

Destructive interference minima Between maximum and minimum measured

n is the index of refraction of oxide Thickness down to a few hundred angstroms

tSiO2 = / 2n ox

Thickness by Electrical method Vg


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Breakdown voltage Use metal electrode on top of oxide Apply continuous voltage to electrode and measure

current Initially current increases slowly and Suddenly current starts increasing rapidly

breakdown From breakdown voltage and breakdown field

(12MV/cm for SiO2), thickness can be determined.

Si

ESiO2 = VBD / tox Ig

VgVbd


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Charges in oxide and Si-SiO2 interface


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Q ot due to defects in SiO2

Qm due to processing

Q it due to suddentermination of silicon lattice atSi-SiO2 interface

Q f due to presence of ionicSi and dangling Si bonds

All these charges affect theelectrical characteristics of the oxide

Electrical characterization of oxides

TDDB (ti d d t di l t i b kd )


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TDDB (time dependent dielectric breakdown)

Constant voltage (other techniques - constant current, or rampedvoltage techniques)

Apply constant voltage for extended period of time, and monitor current through oxide

Current decreases due to electron trapping in oxide bulk Breakdown due to accumulated trapped positive charge near

interface Area under I t curve gives total charge to breakdown

I (A)

t (sec)

I (A)

t (sec)

Less

trappedcharge

more

trappedcharge

Qm


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11

Cause a shift in the CV curve laterally Can be determined from the Vg Qm determined from bias temperature stressing

CTCT

CoxCoxCox

before BTSbefore BTSafter BTSafter BTS Qm / Cox Qm / Cox

p-typesubstrate

voltage (metal wrt substrate)

c a p a c

i t a n

c e

p-typesubstrate

voltage (metal wrt substrate)

c a p a c

i t a n

c e

measure C-V curvesbefore and after BTS stress- heat sample to100C and apply electricfield for 10-20min

Qm Cox x Vt Q

m / t

oxA

capq

Interface trapped charge


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Due to unsatisfied bonds at interface Measurement difficult Decreases the slope of C-V profile

Determined by comparing actual CV with theoreticalCV (obtained from oxide thickness andsemiconductor work function, and doping levels)

High temperature annealing can reduce interfacetrapped charge

C

Vg-2V +2V

Due to interfacestates


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13

PHOTO LITHOGRAPHY

Lithography

Process of transferring patterns of geometric shapes


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Process of transferring patterns of geometric shapeson a mask to a thin layer of radiation sensitivematerial (called photo resist) covering the surface of wafer

Two step process Transfer pattern from mask on to photo resist (PR) Transfer of pattern from PR to wafer by etching

Device layout is broken into several layers of information

Each layer is a map for the location of one film on IC

Steps in Mask fabricationDefine chip function


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Define chip function

Breakdown into sub-functions

Layout of sub-functions on floor plan usingdesign rules

Construct high-level model to testfunctionality and performance

Make adjustments to design

Transfer design to pattern generator

Mask


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Same size as finished chip or an integral factor (5x or 10x) of final chip During exposure, the image size is reduced.

Typically 150mm square Made of fused silica Essential properties

High degree of optical transparency Small thermal expansion coefficient Flat and polished surface

Resistant to scratches Chromium is used as opaque layer Typically 15-20 masks are used in a process sequence


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Electron beam used to create the pattern on the mask dueto its high precision The quartz is first covered with chrome followed by PR E-beam is raster scanned on to PR

Un-wanted PR is removed and chromium is etched

VLSI Fabrication


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Photo Lithography

23.02.05



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2

Characterization of oxides Thickness

Breakdown Interface states

Charges in SiO2 and Si-SiO2 interface Photolithography Steps in mask fabrication



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3

Define chip function







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4

The quartz is first covered with chrome followed by PR Electron beam used to create the pattern on the mask dueto its high precision Computer driven e-beam is raster scanned on to PR



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Steps in standard lithography processDehydration bakeWafer with film


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y

Adhesion promoter application

Resist application

Soft bake or pre-bake

Aligning and Light Exposure

Develop

Hard bake or Post-bake Etching

Photo Resist (PR) Is a radiation sensitive material which changes chemically


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on exposure to light Usually a carbon based organic molecule Two types of resist:

Positive Regions of resist exposed to light dissolve quickly in developer Unexposed regions remain unchanged and are not removed by

developer Negative

Regions exposed to light are hard to remove by developer Unexposed regions are easily removed by developer

Positive resists result in better resolution than negativeresist

film to be patternedsubstrate (with topography!)

film to be patternedsubstrate (with topography!) Photo resist

Exposed regions

Positive and Negative resist


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maskmask

exposingradiationexposingradiation

made insolublemade insoluble made solublemade soluble

film to be patterned

mask blank: transparent,mechanically rigid

masking layer:opaque,patternable

masking layer:opaque,patternable

develop

etch

NEGATIVE POSITIVE

photoresistphotoresist

Components of PR


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Matrix material or resin binder provides mechanical properties such as adhesion and

etch resistance Inert to incident radiation

Sensitizer or inhibiter or PAC

Inhibits dissolution in developer Photo active compound absorbs light (visible or UV)and causes photo-chemical change

Solvent Keeps the photo resist as a liquid

Characteristics of PR


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Photoresist is an organic molecule Aromatic rings (closed chain hydrocarbons) long chain polymers

Sensitivity Amount of light energy required to create a chemical

change Higher sensitivity results in quicker developing

Resolution

Smallest feature size that can be reproduced on PRwithout distortion

Basic pattern transfer techniques


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11

contact

gap

mask

photoresist

optical imaging system

proximity

Imaging/Projection

Usually 4Xor 5XReduction

1:1 Exposure Systems


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12

Contact printing is capable of high resolution but has unacceptable defectdensities. Proximity printing cannot easily print features below a few mm (except for

x-ray systems). Projection printing provides high resolution and low defect densities anddominates today. Typical projection systems use reduction optics (2X - 5X), step and repeat

or step and scan mechanical systems, print 50 wafers/hour and cost $5 -10M.

Resolution of Imaging Systems


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contact shadow formation, no

diffraction

gapl 2

3min

proximity some diffraction, sharp filter

cut-off, flat response in

passband

projection:- low pass filter, smooth

decrease in passband

contact

projection

illumination, intensity I o , wavelength

position

i n t e n s i

t y

Io

position

i n t e n s i

t y

IoIo

proximity

lmin (g )

Optics Basics of Diffraction Ray tracing (assuming light travels in straight lines as particle ) works


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14

well as long as the dimensions are large compared to . At smaller dimensions, diffraction effects dominate ( light treated as a

wave ). Diffraction is bending of light waves around corners. If the aperture is on the order of l, the light spreads out after passing

through the aperture. (The smaller the aperture, the more it spreadsout.)

If we want to image the aperture on an image plane (resist), we cancollect the light using a lens and focus it on the image plane.


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15

But the finite diameter of the lens means some information is lost(higher frequency components).

A simple example is the image formed by a small circular aperture(Airy disk).


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Note that a point image is formed only if 0, f 0 or d . Diffraction is usually described in terms of two limiting cases:

Fresnel diffraction - near field. Fraunhofer diffraction - far field.


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Characterization of oxides Thickness

Breakdown Interface states

Charges in SiO2 and Si-SiO2 interface Photolithography Steps in mask fabrication



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The quartz is first covered with chrome followed by PR Electron beam used to create the pattern on the mask dueto its high precision Computer driven e-beam is raster scanned on to PR


Each successive layer has to be aligned with the previous

Aligning using Masks


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layer (for e.g. the gate electrode has to come on top of gate oxide accurately)

Each mask contains alignment marks which help inaligning the layers on top of each other.

Important alignment features: Resolution:

ability of PR to accurately transfer patterns on to filmunderneath

Is the minimum feature size that can be transferred with

minimal tolerance Measured in terms of 3-sigma (standard deviation of minimumfeature size)

Important alignment features:

Aligning using Masks


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Registration: Measure of overlay accuracy from layer to layer Measured in terms of 3-sigma

Throughput: Number of wafers processed per hour For industry, this number has to be sufficiently high while

maintaining good resolution and registration

Usually alignments are automated

Overlay errors between two patterns


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goal: align two identical patterns one on top of theother

level 1level 2

: pure registration error : distortion error

overlay error: sum of all errors

really a statistical quantity rule of thumb: total overlay error not more than 1/3 to 1/5

of minimum feature size

what can go wrong??

Basic pattern transfer techniques


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8

contact

gap

mask

photoresist

optical imaging system

proximity

Imaging/Projection

Usually 4Xor 5XReduction

1:1 Exposure Systems


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9

Contact printing is capable of high resolution but has unacceptable defectdensities. Proximity printing cannot easily print features below a few mm (except for

x-ray systems). Projection printing provides high resolution and low defect densities anddominates today. Typical projection systems use reduction optics (2X - 5X), step and repeat

or step and scan mechanical systems, print 50 wafers/hour and cost $5 -10M.

Resolution of Imaging Systems


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contact shadow formation, no

diffraction

gapl 2

3min

proximity some diffraction, sharp filter

cut-off, flat response in

passband

projection:- low pass filter, smooth

decrease in passband

contact

projection

illumination, intensity I o , wavelength

position

i n t e n s i

t y

Io

position

i n t e n s i

t y

IoIo

proximity

lmin (g )

Optics Basics of Diffraction Ray tracing (assuming light travels in straight lines as particle ) works


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11

well as long as the dimensions are large compared to . At smaller dimensions, diffraction effects dominate ( light treated as a

wave ). Diffraction is bending of light waves around corners. If the aperture is on the order of l, the light spreads out after passing

through the aperture. (The smaller the aperture, the more it spreadsout.)

If we want to image the aperture on an image plane (resist), we cancollect the light using a lens and focus it on the image plane.


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12

But the finite diameter of the lens means some information is lost(higher frequency components).

A simple example is the image formed by a small circular aperture(Airy disk).


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13

Note that a point image is formed only if 0, f 0 or d . Diffraction is usually described in terms of two limiting cases:

Fresnel diffraction - near field. Fraunhofer diffraction - far field.

Contact and Proximity Systems( Fresnel Diffraction)


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14

Contact printing systems operate in the near field or Fresnel diffractionregime.

There is always some gap g between the mask and resist.

The aerial image can be constructed by imagining point sources withinthe aperture, each radiating spherical waves (Huygens wavelets).

Fresnel diffraction


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Interference effects and diffraction result in ringing andspreading outside the aperture.

Fresnel diffraction applies when

Within this range, the minimum resolvable feature size is

For e.g.

Wmin


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Projection Systems (Fraunhofer Diffraction) Resolution is given by Raleighs criterion: Wmin =


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17

k1 is an experimental parameter which depends on the lithographysystem and resist properties and is 0.6 - 0.8.

Obviously resolution canbe increased by:

decreasing

increasing NA (bigger lenses)

Projection Systems( Fraunhofer Diffraction)


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However, higher NA lenses also decrease the depth of focus :

k2 is usually experimentally determined. Usually, it is better to decrease wavelength of light. Another useful concept is the modulation transfer

function or MTF , defined as shown below:

Can be thought of as a measure of the optical contrast of arealimage

Projection Systems (Fraunhofer Diffraction)


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Higher the MTF, better the contrast

MTF dependent ondiffraction grating

Spatial Coherence Finally, another basic concept is the spatial coherence of


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the light source.

The spatial coherence of the system is defined as:

VLSI Fab - Lectures

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