M.E VLSI technology UNIT I ppt

7/28/2019 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

3

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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4

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

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M.E VLSI technology UNIT I ppt