m.e vlsi technology unit i ppt
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UNIT I
CRYSTAL GROWTH, WAFER
PREPARATION, EPITAXY ANDOXIDATION
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Advantages of Siover Ge
Sihas a larger bandgap(1.1 eVfor Siversus 0.66 eVfor Ge)
Sidevices can operate at a higher temperature (150oC vs100oC)
Intrinsic resistivityis higher (2.3 x 105-cm vs47 -cm)
SiO2is more stable than GeO2which is also water soluble
Siis less costly.
The processing characteristics and some material properties of silicon
wafers depend on its orientation.
The planes have the highest density of atoms on the surface, so
crystals grow most easily on these planes and oxidation occurs at a higher
pace when compared to other crystal planes.
Traditionally, bipolar devices are fabricated in oriented crystalswhereas materials are preferred for MOS devices.
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Defects
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Any non-silicon atoms
incorporated into the
lattice at either a
substitutionalorinterstitial site are
considered point
defects
Point defects are important in the kinetics of diffusion and oxidation.
Moreover, to be electrically active, dopantsmust occupy substitutionalsites
in order to introduce an energy level in the bandgap.
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Dislocations are line defects.
Dislocations in a lattice are dynamic
defects. That is, they can diffuse under
applied stress, dissociate into two or
more dislocations, or combine with
other dislocations.
Dislocations in devices are generally
undesirable, because they act as sinks
for metallic impurities and alter
diffusion profiles.
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Electronic Grade Silicon
Electronic-grade silicon (EGS), a polycrystalline material of high purity, is the
starting material for the preparation of single crystal silicon. EGS is made from
metallurgical-grade silicon (MGS) which in turn is made from quartzite, whichis a relatively pure form of sand. MGS is purified by the following reaction:
Si(solid) + 3HCl (gas) SiHCl3 (gas) + H2 (gas) + heat
The boiling point of trichlorosilane(SiHCl3) is 32oC and can be readily
purified using fractional distillation. EGS is formed by reacting
trichlorosilanewith hydrogen:
2SiHCl3 (gas) + 2H2 (gas) 2Si (solid) + 6HCl (gas)
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CzochralskiCrystal Growth
The Czochralski(CZ) process, which
accounts for 80% to 90% of worldwide
silicon consumption, consists of dipping a
small single-crystal seed into molten
silicon and slowly withdrawing the seed
while rotating it simultaneously.The
crucible is usually made of quartz or
graphite with a fused silica lining. After
the seed is dipped into the EGS melt, thecrystal is pulled at a rate that minimizes
defects and yields a constant ingot
diameter.
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Oxygen in Silicon
Oxygen forms a thermal donor in silicon
Oxygen increases the mechanical strength ofsilicon
Oxygen precipitates provide getteringsites forunintentional impurities
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Thermal Donors
Thermal donors are formed by thepolymerization of Siand O into complexes suchas SiO4in interstitial sites at 400oC to 500oC
Careful quenching of the crystal annihilates
these donors
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Wafer Preparation
Gross crystalline imperfections are detected visually and defective crystals
are cut from the boule. More subtle defects such as dislocations can be
disclosed by preferential chemical etching
Chemical information can be acquired employing wet analytical techniques
or more sophisticated solid-state and surface analytical methods
Silicon, albeit brittle, is a hard material. The most suitable material for
shaping and cutting silicon is industrial-grade diamond. Conversion of
silicon ingots into polished wafers requires several machining, chemical,
and polishing operations
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After grinding to fix the diameter, one or
more flats are grounded along the length of the
ingot. The largest flat, called the "major" or
"primary" flat, is usually relative to a specific
crystal orientation. The flat is located by x-raydiffraction techniques.
The primary flat serves as a mechanical
locator in automated processing equipment to
position the wafer, and also serves to orient the
IC device relative to the crystal. Other smallerflats are called "secondary" flats that serve to
identify the orientation and conductivity type of
the wafer.
The drawback of these flats is the reduction
of the usable area on the wafer. For some200 mm and 300 mm diameter wafers, only a
small notch is cut from the wafer to enable
lithographic alignment but no dopanttype or
crystal orientation information is conveyed.
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Slicing determines four wafer parameters:
Surface orientation (e.g., or )
Thickness (e.g., 0.5 0.7 mm, depending on
wafer diameter) Taper, which is the wafer thickness variations
from one end to another
Bow, which is the surface curvature of thewafer measured from the centerof the waferto its edge
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Wafers
The wafer as cut varies enough in thickness to warrant an
additional lapping operation that is performed under pressure using
a mixture of Al2O3and glycerine.
Subsequent chemical etching removes any remaining damaged and
contaminated regions.Polishing is the final step. Its purpose is to
provide a smooth, specularsurface on which device features can be
photoengraved.
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Typical Specifications for Silicon Wafers
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Epitaxial Growth
Deposition of a layer on asubstrate which matchesthe crystalline order of thesubstrate
Homoepitaxy Growth of a layer of the same
material as the substrate
Si on Si
Heteroepitaxy Growth of a layer of a
different material than thesubstrate
GaAs on Si
Ordered,crystallinegrowth; NOTepitaxial
Epitaxial
growth:
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General Epitaxial DepositionRequirements
Surface preparation Clean surface needed Defects of surface duplicated in epitaxial layer Hydrogen passivation of surface with water/HF
Surface mobility High temperature requiredheated substrate Epitaxial temperature exists, above which deposition is ordered Species need to be able to move into correct crystallographic
location
Relatively slow growth rates result Ex. ~0.4 to 4 nm/min., SiGe on Si
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General Scheme
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Thermodynamics
Specific thermodynamics varies by process Chemical potentials
Driving force
High temperature process is mass transport controlled, not very sensitiveto temperature changes
Steady state Close enough to equilibrium that chemical forces that drive growth are
minimized to avoid creation of defects and allow for correct ordering
Sufficient energy and time for adsorbed species to reach their lowestenergy state, duplicating the crystal lattice structure
Thermodynamic calculations allow the determination of solid compositionbased on growth temperature and source composition
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Kinetics
Growth rate controlled by kineticconsiderations Mass transport of reactants to surface
Reactions in liquid or gas Reactions at surface Physical processes on surface
Nature and motion of step growth Controlling factor in ordering
Specific reactions depend greatly on methodemployed
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Kinetics Example
Atoms can bond to flat surface, steps,or kinks. On surface requires some critical radius Easier at steps Easiest at kinks
As-rich GaAs surface
As only forms two bonds to underlyingGa Very high energy
Reconstructs by forming As dimers Lowers energy Causes kinks and steps on surface
Results in motion of steps on surface
If start with flat surface, create steponce first group has bonded Growth continues in same way
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Vapor Phase Epitaxy
Specific form of chemical vapor deposition (CVD) Reactants introduced as gases Material to be deposited bound to ligands Ligands dissociate, allowing desired chemistry to reach
surface Some desorption, but most adsorbed atoms find proper
crystallographic position Example: Deposition of silicon
SiCl4 introduced with hydrogen Forms silicon and HCl gas
Alternatively, SiHCl3, SiH2Cl2 SiH4 breaks via thermal decomposition
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Precursors for VPE
Must be sufficiently volatile to allowacceptable growth rates
Heating to desired T must result in pyrolysis Less hazardous chemicals preferable
Arsine highly toxic; use t-butyl arsine instead
VPE techniques distinguished by precursorsused
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Varieties of VPE
Chloride VPE Chlorides of group III and V elements
Hydride VPE Chlorides of group III element
Group III hydrides desirable, but too unstable
Hydrides of group V element
Organometallic VPE Organometallic group III compound
Hydride or organometallic of group V element
Not quite that simple Combinations of ligands in order to optimize
deposition or improve compound stability
Ex. trimethylaminealane gives less carboncontamination than trimethylalluminum
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Other Methods
Liquid Phase Epitaxy
Reactants are dissolved in amolten solvent at hightemperature
Substrate dipped intosolution while thetemperature is held constant
Example: SiGe on Si
Bismuth used as solvent
Temperature held at 800C High quality layer
Fast, inexpensive
Not ideal for large area layersor abrupt interfaces
Thermodynamic driving force
relatively very low Molecular Beam Epitaxy
Very promising technique
Elemental vapor phasemethod
Beams created byevaporating solid source inUHV
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Molecular Beam Epitaxy
Source: William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, W25
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Molecular Beam Epitaxy: Idea !
Objective: To deposit single crystal thin films !
Inventors: J.R. Arthur and Alfred Y. Chuo (Bell Labs, 1960)
Very/Ultra high vacuum (10
-8
Pa)
Important aspect: slow deposition rate (1 micron/hour)
Slow deposition rates require proportionally better vacuum.
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Molecular Beam Epitaxy: ProcessUltra-pure elements are heated in separate quasi-knudson effusion cells (e.g., Ga and As) until theybegin to slowly sublimate.
Gaseous elements then condense on the wafer,where they may react with each other (e.g., GaAs).
The term beam means the evaporated atoms donot interact with each other or with other vacuumchamber gases until they reach the wafer.
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Molecular Beam
A collection of gas molecules moving in the same direction.
Simplest way to generate: Effusion cell or Knudsen cell
Test Chamber
SampleOrifice
Oven
Pump
Knudson cell effusion beam system28
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Molecular beamOven contains the material to make the beam.
Oven is connected to a vacuum system through ahole.
The substrate is located with a line-of-sight to theoven aperture.
From kinetic theory, the flow through the aperture issimply the molecular impingement rate on thearea of the orifice.
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Molecular Beam
Impingement rate is:
The total flux through the hole will thus be:
The spatial distribution of molecules from the orifice of aknudsen cell is normally a cosine distribution:
m
kT
kT
pvnI
8
4
1
4
1
mkT
rpIAQ
2
2
cos
4
1' vnI
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Molecular Beam
The intensity drops off as the square of the distance from the
orifice.
High velocity, greater probability; the appropriate distribution:
cos2
,
1cos
2
2
L
r
mkT
pI
or
LIAI
sub
sub
mkTwhere
dvvv
n
dnv
/2
exp22
2
4
3
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Molecular Beam
Integrating the equation gives:
as the mean translational energy of themolecules
kTEtr 2
I
# Intensity is maximum in the
direction normal to the orifice and
decreases with increasing, which
causes problems.
# Use collimator, a barrier with a
small hole; it intercepts all of the
flow except for that traveling towards the sample.32
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MBE: In-situ process diagnostics
RHEED (Reflection High Energy Electron Diffraction) is used to monitor thegrowth of the crystal layers.
Computer controlled shutters of each furnace allows precise control of thethickness of each layer, down to a single layer of atoms.
Intricate structures of layers of different materials can be fabricated this waye.g., semiconductor lasers, LEDs.
Systems requiring substrates to be cooled: Cryopumps and Cryopanels are usedusing liquid nitrogen.
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ATG Instability
Ataro-Tiller-Grinfeld (ATG) Instability: Often encountered during MBE.
If there is a l attice mismatch between the substrate and the growing film, elastic energy is accumulated in the growing film.
At some critical film thickness, the film may break/crack to lower the free energy of the film.
The critical film thickness depends on the Youngs moduli, mismatch size, and surface tensions.
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Principle uses of Si dioxide (SiO2) layer in Siwafer devicesSurface passivationDoping barrier
Surface dielectricDevice dielectric
OXIDATION
What is oxidation?Formation of oxide layer on waferHigh temperatureO2 environment
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Doping barrier
In doping need to create holes in a surfacelayer in which specific dopants are introducedinto the exposed wafer surface through diffusionor ion implantation
SiO2 on Si wafer block the dopants from reachingSi surface
All dopants have slower rate of movement in SiO2
compared to SiRelatively thin layer of SiO2 is required to block thedopants from reaching SiO2
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Cont..
SiO2 possesses a similar thermal expansioncoefficient with Si
At high temperature oxidation process, diffusion
doping etc, wafer expands and contracts when it isheated and cooled
close thermal expansion coefficient, wafer does notwarp
Si
Dopants
SiO2
layer as dopant barrier
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Surface dielectric
SiO2 is a dielectric does not conduct electricityunder normal circumstancesSiO2 layer prevents shorting of metal layer tounderlying metal
Oxide layerMUST BE continuous; no holes or voidsThick enough to prevent induction
If too thin SiO2 layer, electrical charge in metal layer cause a
build-up charge in the wafer surface cause shorting!!Thick enough oxide layer to avoid induction called fieldoxide
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Wafer
Oxide layer
Metal layer
Dielectric use of SiO2 layer
S D
Field oxide MOS gate
source Drain
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Types of oxidation
1. Thermal oxidation2. High pressure oxidation
3. Anodic oxidation
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Device oxide thicknesses
Most applications of semiconductor aredependent on their oxide thicknesses
Silicon dioxidethickness,
Applications
60-100 Tunneling gates
150-500 Gates oxides, capacitordielectrics
200-500 LOCOS pad oxide
2000-5000 Masking oxides, surfacepassivation
3000-10000 Field oxides
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Thermal oxidation mechanisms
Chemical reaction of thermal oxide growth
Si (solid) + O2 (gas) SiO2 (solid)
Oxidation temperature 900-1200C Oxidation: Si wafer placed in a heated
chamber exposed to oxygen gas
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SiO2 growth stages
Si wafer
Si wafer
Si wafer
Initial
Linear
Parabolic
Oxygen atoms combine readily with Si atomsLinear- oxide grows in equal amounts for each time
Around 500 thick
In a furnace with O2
gas environment
Above 500, in order for oxide layer to keep growing, oxygenand Si atoms must be in contact
SiO2 layer separate the oxygen in the chamber from the wafersurface
Si must migrate through the grown oxide layer to theoxygen in the vapor
oxygen must migrate to the wafer surface
Three dimension view of SiO2 growth by thermal
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Three dimension view of SiO2 growth by thermaloxidation
Si substrate
SiO2
SiO2 surfaceOriginal SiO2 surface
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Linear oxidation
Parabolic oxidation of silicon
where X = oxide thickness, B = parabolic rate constant, B/A = linear rateconstant, t = oxidation time
Parabolic relationship of SiO2 growth parameters
where R = SiO2 growth rate, X = oxide thickness, t = oxidation time
tA
BX
BtX
2
t
XR
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Cont..
Implication of parabolic relationship: Thicker oxides need longer time to grow than thinner
oxides
2000, 1200C in dry O2 = 6 minutes 4000, 1200C in dry O2 = 220 minutes (36 times longer)
Long oxidation time required:
Dry O2
Low temperature
D d f ili id ti t t t
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Dependence of silicon oxidation rate constants ontemperature
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Kinetics of growth
Si is oxidised by oxygen or steam at high temperatureaccording to the following chemical reactions:Si (solid) + O2 (gas) SiO2 (solid) (dry oxidation)
OrSi (solid) + 2H2O (gas) SiO2 (solid) + 2H2(gas) (wet oxidation)
Also called steam oxidation, wet oxidation, pyrogenic steamFaster oxidation OH- hydroxyl ions diffuses faster in oxide layerthan dry oxygen2H2 on the right side of the equation shows H2 are trapped inSiO2 layer
Layer less dense than oxide layer in dry oxidationCan be eliminated by heat treatment in an inertatmosphere e.g. N2
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2 mechanisms influence the growth rate of the oxide1. Actual chemical reaction rate between Si and O2
2. Diffusion rate of the oxidising species through an already grown oxidelayer
No or little oxide on Si the oxidising agent easily reach the Si surface
Factor that determine the growth rate is kinetics of the silicon-oxidechemical reaction
Si is already covered by a sufficiently thick layer of oxide
Oxidation process is mass-transport limited Diffusion rate of O2 and H2O through the oxide limit the growth rate is
diffusion of O2 and H2O through the oxide A steam ambient is preferred for the growth of thick oxides:H2O
molecules smaller than O2 thus, easier diffuse through SiO2 that causehigh oxidation rates
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Si oxidation
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Mass transport of O2 molecules from gas ambient towards theSi through a layer of already grown oxide
Flux of O2 molecules is proportional to the differential in O2concentration between the ambient (C*) and oxide surface (C0)
Where h is the mass transport coefficient for O2 in the gas phase
Diffusion of O2 through the oxide is proportional to thedifference of oxygen concentration between the oxide surfaceand the Si/SiO2 interface. The flux of oxygen through the oxide,F2 becomes
Where,
Ci = oxygen concentration at theSi/SiO2 interface
D = diffusion coefficient of O2 or H2O in oxide
tox = oxide thickness
2.5...................02ox
i
tCCDF
1.5.....).........( 0*
1 CChF
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Kinetics of the chemical reaction betweensilicon and oxygen is characterised by reaction
constant, k:
In steady state, all flux terms are equal: F1 = F2= F3. Eliminating C0 from the flux equations,we can obtain:
4.5...................
1
*
D
tk
h
k
CC
oxssi
3.5.................3 is
CkF
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If N0x is a constant representing thenumber of oxidising gas moleculesnecessary to grow a unit thickness ofoxide, one can write:
The solution to this differential equationis:
5.5.......
1
*
D
tk
h
k
CkNCkNFN
dt
dt
oxss
soxisoxox
ox
6.5..........1
00
* t
ox
t
sox
oxss
dtdtCkND
tk
h
kox
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If tox=0 when t=0, th eintegration yields:
Or
Defining new constant A and B in terms of D, ks, Nox and C*:
We can obtain:
From which we find tox :
7.5........0
2
*2
dtCNt
h
D
k
Dtoxox
s
ox
8.5............211
2 *2 tCDNthk
Dt oxoxs
ox
10.5................2
9.5............11
2
*
ox
s
NDCB
and
hkDA
11.5.....................2 BtAtt ox
12.5.................4/
)(
12 12
BA
tA
tox
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is introduced to take into account the possible presence of an oxide layer onthe Si before thermal oxide growth being carry out
Oxide layer can be a native oxide layer due to oxidation of bare Si by ambient air
or thermally grown oxide produced during a prior oxidation step=0 if the thickness of the initial oxide is equal to zero
When thin oxides are formed the growth rate is limited by the kinetics ofchemical reaction between Si and O2.
Eq. 5.12 becomes:
Which is linear with time.
The ratio is called linear growth coefficient, and is dependent on crystal
orientation of Si
13.5........... tA
Btox
A
B
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When thick oxides are formed, the growth rate is limited by the diffusion rate ofoxygen through the oxide. Eq 5.12 becomes:
The coefficient B is called parabolic growth coefficient and is independent oncrystal orientation of Si.
The parabolic growth coefficient can be increased:
Increase the pressure of the ambient oxygen up to 10-20 atm (high pressure
oxidation)The linear growth coefficient can be increased:
Si consists of high concentration of impurities e.g. phosphorous: increase pointdefects in the crystal which increase the oxidation reaction rate at the Si/SiO2 interface
Oxidation process also generate point defects in Si which enhance diffusion ofdopants. Some dopants diffuse faster when annealed in oxidising ambient than in
neutral gas such at N2
14.5..............)( BttBtox
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Oxidation rate
Controlled by:1. Wafer orientation2. Wafer dopant3. Impurities
4. Oxidation of polysilicon layers1. Wafer orientation
Large no of atoms allows faster oxide growth plane have more Si atoms than plane Faster oxide growth in Si More obvious in linear growth stage and at low
temperature
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2. Wafer dopant(s) distribution
Oxidised Si surface always has dopants; N-type or
P-typeDopant may also present on the Si surface fromdiffusion or ion implantation
Oxidation growth rate is influenced by dopantelement used and their concentration e.g. Phosphorus-doped oxide: less dense and etch faster
Higher doped region oxidise faster than lesser doped
region e.g. high P doping can oxidise 2-5 times theundoped oxidation region
Doping induced oxidation effects are more obvious inthe linear stage oxidation
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Schematic illustration of dopant distribution as a function of position is the SiO2/Si structure indicatingthe redistribution and segregation of dopants during silicon thermal oxidation
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Distribution of dopant atoms in Si afteroxidation is completed
During thermal oxidation, oxide layer grows down
into Si wafer- behavior depends on conductivitytype of dopantN-type: higher solubility in Si than SiO2, move down towafer. Interface consists of high concentration N-typedoping
P-type: opposite effect occurs e.g Boron doping in Simove to SiO2 surface causes B pile up in SiO2 layer anddepletion in Si wafer change electrical properties
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3. Oxide impurities
Certain impurities may influence oxidationrate
e.g. chlorine from HCl from oxidationatmosphere increase growth rate 1-5%
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4. Oxidation of polysiliconOxidation of polysilicon is essential forpolysilicon conductors and gates in MOS
devices and circuitsOxidation of polysilicon is dependent on
Polisilicon deposition method
Deposition temperature
Deposition pressureThe type and concentration of doping
Grain structure of polysilicon
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Thermal oxidation method
Thermal oxidation energy is supplied by heating awafer
SiO2 layer are grown:Atmospheric pressure oxidation oxidation without
intentional pressure control (auto-generated pressure);also called atmospheric technique
High pressure oxidation high pressure is appliedduring oxidation
2 atmospheric techniques1.Tube furnace
2.Rapid thermal system
Oxidation methods
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Oxidation methods
Thermal oxidation
Atmosphericpressure
Tube furnace Dry oxygen
Wet oxygen
Rapid thermal Dry oxygen
High pressure Tube furnace Dry or wetoxygen
Chemical oxidationAnodicoxidation
Electrolytic cell Chemical