EBB 323 Semiconductor Fabrication Technology

Oxidation

Dr Khairunisak Abdul RazakRoom 2.03School of Material and Mineral Resources EngineeringUniversiti Sains Malaysiakhairunisak@eng.usm.my

OutcomesBy the end of this topic, students

should be able to:• List principle uses of silicon dioxide (SiO2) layer in silicon

devices• Describe the mechanism of thermal oxidation• Draw a flow diagram of a typical oxidation process• Describe the relationship of process time, pressure, and

temperature on the thickness of a thermally grown SiO2 layer

• Explain the kinetics of oxidation process• Describe the principle uses of rapid thermal, high

pressure and anodic oxidation

Uses of dielectric films in Semiconductor technology

Principle uses of Si dioxide (SiO2) layer in Si wafer devicesSurface passivationDoping barrierSurface dielectricDevice dielectric

What is oxidation?Formation of oxide layer on wafer

High temperatureO2 environment

1. Surface passivationSiO2 layer protect semiconductor devices from contamination(ganda):

i. Physical protection of the sample and underlying devices

Dense and hard SiO2 layer act as contamination barrier Hardness of the SiO2 layer protect the surface from scratches during fabrication process

Si Si

SiO2 passivation layer

Cont..ii. Chemical in nature

Avoid contamination from electrically active contaminants (mobile ionic contaminants) of the electrically active surface

e.g. early days, MOS device fabrication was performed on oxidised Si remove oxide layer to get rid of the unwanted ionic contamination surface before further processing

2. Doping barrierIn doping need to create holes in a surface layer in which specific dopants are introduced into the exposed wafer surface through diffusion or ion implantation

SiO2 on Si wafer block the dopants from reaching Si surface

All dopants have slower rate of movement in SiO2 compared to Si

Relatively thin layer of SiO2 is required to block the dopants from reaching SiO2

Cont..SiO2 possesses a similar thermal expansion coefficient with Si

At high temperature oxidation process, diffusion doping etc, wafer expands and contracts when it is heated and cooled close thermal expansion coefficient, wafer

does not warp

Si

Dopants

SiO2 layer as dopant barrier

3. Surface dielectricSiO2 is a dielectric does not conduct electricity under normal circumstances

SiO2 layer prevents shorting of metal layer to underlying metalOxide layer

MUST 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 ‘field oxide’

Wafer

Oxide layer

Metal layer

Dielectric use of SiO2 layer

S D

Field oxide MOS gate

source Drain

4. Device dielectric• In MOS application

– Grow thin layer SiO2 in the gate region

– Oxide function as dielectric in which the thickness is chosen specifically to allow induction of a charge in the gate region under the oxide

• Thermally grown oxides is also used as dielectric layer in capacitors– Between Si wafer and conduction layer

Types of oxidation

1. Thermal oxidation

2. High pressure oxidation

3. Anodic oxidation

Device oxide thicknesses•Most applications of semiconductor are dependent on their oxide thicknesses

Silicon dioxide thickness, Å

Applications

60-100 Tunneling gates

150-500 Gates oxides, capacitor dielectrics

200-500 LOCOS pad oxide

2000-5000 Masking oxides, surface passivation

3000-10000 Field oxides

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

SiO2 growth stages

Si wafer

Si wafer

Si wafer

Initial

Linear

Parabolic

Oxygen atoms combine readily with Si atoms

Linear- oxide grows in equal amounts for each time

Around 500Å thick

In a furnace with O2 gas environment

Above 500Å, in order for oxide layer to keep growing, oxygen and Si atoms must be in contact

SiO2 layer separate the oxygen in the chamber from the wafer surface

Si must migrate through the grown oxide layer to the oxygen in the vapor

oxygen must migrate to the wafer surface

Three dimension view of SiO2 growth by thermal oxidation

Si substrate

SiO2

SiO2 surfaceOriginal SiO2 surface

Linear oxidation

Parabolic oxidation of silicon

where X = oxide thickness, B = parabolic rate constant, B/A = linear rate constant, 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

Cont..• Implication of parabolic relationship:

– Thicker oxides need longer time to grow than thinner oxides

• 2000Å, 1200C in dry O2 = 6 minutes

• 4000Å, 1200C in dry O2 = 220 minutes (36 times longer)

• Long oxidation time required:– Dry O2

– Low temperature

Dependence of silicon oxidation rate constants on temperature

Oxide thickness vs oxidation time for silicon oxidation in dry oxygen at various temperatures

Oxide thickness vs oxidation time for silicon oxidation in pyrogenic steam (~ 640 Torr) at various temperatures

Kinetics of growth• Si is oxidised by oxygen or steam at high

temperature according to the following chemical reactions:

Si (solid) + O2 (gas) SiO2 (solid) (dry oxidation)Or

Si (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 layer than dry oxygen2H2 on the right side of the equation shows H2 are trapped in SiO2 layer

Layer less dense than oxide layer in dry oxidationCan be eliminated by heat treatment in an inert atmosphere e.g. N2

• 2 mechanisms influence the growth rate of the oxide

1. Actual chemical reaction rate between Si and O2

2. Diffusion rate of the oxidising species through an already grown oxide layer

• 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-oxide chemical 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 cause high oxidation rates

Si oxidation

Oxygen concentration profile during oxidation

•Mass transport of O2 molecules from gas ambient towards the Si through a layer of already grown oxide

•Flux of O2 molecules is proportional to the differential in O2 concentration 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 the difference of oxygen concentration between the oxide surface and 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...................02

ox

i

t

CCDF

1.5.....).........( 0*

1 CChF

•Kinetics of the chemical reaction between silicon 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

*

Dtk

hkC

Coxss

i

3.5.................3 isCkF

•If N0x is a constant representing the number of oxidising gas molecules necessary to grow a unit thickness of oxide, one can write:

•The solution to this differential equation is:

5.5.......1

*

Dtk

hk

CkNCkNFN

dt

dt

oxss

soxisoxox

ox

6.5..........1

00*

t

ox

t

sox

oxss

dtdtCkNDtk

hk

ox

•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........02

*2

dtCNth

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/

)(1

2 12

BA

tAtox

is introduced to take into account the possible presence of an oxide layer on the 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 of chemical 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

•When thick oxides are formed, the growth rate is limited by the diffusion rate of oxygen through the oxide. Eq 5.12 becomes:

• The coefficient B is called “parabolic growth coefficient” and is independent on crystal 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 point defects 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 of dopants. Some dopants diffuse faster when annealed in oxidising ambient than in neutral gas such at N2

14.5..............)( BttBtox

Oxidation rateControlled by:

1. Wafer orientation

2. Wafer dopant

3. Impurities

4. Oxidation of polysilicon layers

1. Wafer orientationLarge no of atoms allows faster oxide growth

<111> plane have more Si atoms than <100> plane

• Faster oxide growth in <111> Si• More obvious in linear growth stage and at low

temperature

Crystal structure of silicon

<100> plane

<111> plane

Dependence of oxidation linear rate constant and oxide fixed charge density on silicon orientation

2. Wafer dopant(s) distributionOxidised Si surface always has dopants; N-type or P-type

Dopant may also present on the Si surface from diffusion or ion implantation

Oxidation growth rate is influenced by dopant element 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 the undoped oxidation region

• Doping induced oxidation effects are more obvious in the linear stage oxidation

Schematic illustration of dopant distribution as a function of position is the SiO2/Si structure indicating the redistribution and segregation of dopants during silicon thermal oxidation

Distribution of dopant atoms in Si after oxidation is completed

During thermal oxidation, oxide layer grows down into Si wafer- behavior depends on conductivity type of dopant

N-type: higher solubility in Si than SiO2, move down to wafer. Interface consists of high concentration N-type doping

P-type: opposite effect occurs e.g Boron doping in Si move to SiO2 surface causes B pile up in SiO2 layer and depletion in Si wafer change electrical properties

3. Oxide impurities

Certain impurities may influence oxidation rate

e.g. chlorine from HCl from oxidation atmosphere increase growth rate 1-5%

4. Oxidation of polysilicon

Oxidation of polysilicon is essential for polysilicon conductors and gates in MOS devices and circuits

Oxidation of polysilicon is dependent on

Polisilicon deposition method

Deposition temperature

Deposition pressure

The type and concentration of doping

Grain structure of polysilicon

Thermal oxidation methodThermal oxidation energy is supplied by heating a wafer

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 applied during oxidation

2 atmospheric techniques1.Tube furnace

2.Rapid thermal system

Oxidation methodsThermal oxidation

Atmospheric pressure

Tube furnace Dry oxygen

Wet oxygen

Rapid thermal Dry oxygen

High pressure Tube furnace Dry or wet oxygen

Chemical oxidation

Anodic oxidation

Electrolytic cell Chemical

Horizontal tube furnace• Quartz reaction tube – reaction

chamber for oxidation• Muffle – heat sink, more even

heat distributing along quartz tube

• Thermocouple – placed close to quartz tube. Send temp to band controller

• Controller – send power to coil to heat the reaction tube by radiation/conduction

• Source zone- heating zone

Place the sample

Horizontal tube furnaceIntegrated system of a tube furnace consists of several sections:

1. Reaction chamber

2. Temperature control system

3. Furnace section

4. Source cabinet

5. Wafer cleaning station

6. Wafer load station

7. Process automation

Vertical tube furnacesSmall footprint

Maybe placed outside the cleanroom with only a load station door opening into the cleanroom

Disadvantage: expensive

Rapid Thermal ProcessingBased on radiation principle heatingUseful for thin oxides used in MOS gatesTrend in device miniaturisation requires reduction in thickness of thermally grown gate oxides

< 100Å thin gate oxide

Hard to control thin film in conventional tube furnace

Problem: quick supply and remove O2 from the system

RTP system: able to heat and cool the wafer temperature VERY rapidly

RTP used for oxidation is known as Rapid Thermal Oxidation (RTO)

Have O2 atmosphere

Other processes use RTP system:Wet oxide (steam) growth

Localised oxide growth

Source/ drain activation after ion implantation

LPCVD polysilicon, amorphous silicon, tungsten, silicide contacts

LPCVD nitrides

LPCVD oxides

RTP design

e.g. RTP time/temperature curve

High Pressure OxidationProblems in high temperature oxidation

Growth of dislocations in the bulk of the wafer dislocations cause device performance problems

Growth of hydrogen-induced dislocations along the edge of opening surface dislocations cause electrical leakage along the surface or the degradation of silicon layers grown on the wafer for bipolar circuits

Solve: low temperature oxidation BUT require a longer oxidation time

High pressure system similar to conventional horizontal tube furnace with several features:

Sealed tube

Oxidant is pumped into the tube at pressure 10-25 atm

The use of a high pressure requires encasing the quartz tube in a stainless steel jacket to prevent it from cracking

High pressure oxidation results in faster oxidation rate

Rule of thumb: 1 atm causes temperature drop of 30CIn high pressure system, temperature drop of 300-750C

This reduction is sufficient to minimise the growth of dislocations in and on the wafers

Advantage of high pressure oxidationDrop the oxidation temperature

Reduce oxidation time

• Thin oxide produced using high pressure oxidation higher dielectric strength than oxides grown at atmospheric pressure

High pressure oxidation

Oxidant sources1. Dry oxygen

2. Water vapor sourcesa) Bubblers/ flash

b) Dry oxidation

c) Chlorine added oxidation

1. Dry oxygen• Oxygen gas must dry not

contaminated by water vapor

• If water present in the oxygen:– Increase oxidation rate– Oxide layer out of specification

• Dry oxygen is preferred for growing very thin gate oxides ~ 1000Å

2a. Bubblers• Bubbler liquid – DI water heated close to boiling

point 98-99C – create a water vapor in the space above liquid

• When carrier gas is bubbled through the water and passes through the vapor saturated with water

• Influence of elevated temp inside tube water vapor becomes steam and results in oxidation of Si surface

• Problem: contamination of tube and oxide layer from dirty water and flask

2b. Dry oxidation (dryox)• O2 and H2 are introduced directly into oxidation

tube mixes• High temperature in tube forms steam wet

oxidation in steam• Advantage:

– Controllable: gas flow can be controlled by flow controllers

– Clean: can purchase gases in a very clean and dry state

• Disadvantage: explosive property of H2 overcome by flow in excess O2

2c. Chlorine added oxidation• Chlorine addition:

– Reduce mobile ionic charges in the oxide layer– Reduce structural defects in oxide and Si

surface

– Reduce charges at Si-SiO2 interface

• Chlorine sources: – Gas: anhydrous chlorine (Cl2), anhydrous

hydrogen chloride– Liquid: trichloroethylene (TCE), trichloroethane

(TCA)

• TCA is preferred source for safety and ease of delivery

Post-oxidation evaluation• Surface inspection

– quick check of the wafer surface using UV light (surface particulates, irregularities, stains)

• Oxide thickness – several techniques such as color comparison, fringe counting,

interference, ellipsometers, stylus apparatus, scanning electron microscope

• Oxide and furnace cleanliness– Ensure oxide consists of minimum number of mobile ionic

contaminants. Use capacitance/voltage (C/V) evaluation: detect total number of mobile ionic contaminants NOT the origin of contaminants

Thermal nitridation• < 100Å SiO2 film possesses poor

quality and difficult to control

• Silicon nitride (Si3N4)

– Denser than SiO2 less pin holes in thin film ranges

– Good diffusion barrier

• Growth control of thin film is enhanced by a flat growth mechanism (after an initial rapid growth)

Nitridation of <100> silicon