07b.oxidation.web

7/28/2019 07b.oxidation.web

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OXIDATION- Overview

Process Types

Details of Thermal OxidationModels

Relevant Issues


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Uses

As a part of a structure

e.g. Gate Oxide

For hard masks

e.g. In Nitride Etch, implant mask ...

Protecting the silicon surface (Passivation ) Insulator (ILD/IMD)

As part of mild etch (oxidation / removal cycles)

Whether useful or not, automatically forms in ambient Native Oxide ( ~ 20 A thick)

except H-terminated Si (111)


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Processes

Thermal Oxidation (Heating)

Dry vs Wet

Electrochemical Oxidation (Anodization)

Oxide (and nitride)

adhere well to the silicon

good insulator

Breakdown voltage 10 MV/cm

==> Can make a very thin gate


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Structure

Tetrahedral Structure each Si to four O

each O to two Si

Single crystal quartz (density 2.6 g/cm3)

Fused silica (density 2.2 g/cm

3

)

Reaction with water

Time Domain CVD

2 0Si O Si H Si OH Si OH

Si-OH termination is stable

structure is more porous than Si-O-Si


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Thermal Oxidation

Dry oxidation

2 2Si O SiO 2 2 22 2Si H O SiO H

Dense oxide formed

(good quality, low diffusion)

slow growth rate

NEED TO KEEPWATER OUT OF THE

SYSTEM

Wet oxidation

Overall reaction

Relatively porous oxide formed

(lower quality, species diffuse faster)

Still good quality compared t

electrochem oxidation, forexample

faster growth rate

Wet oxide for masking

Dry oxide for gate ox


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Wet Oxidation Proposed Mechanism

2Si O Si H O SiOH SiOH

22 2Si OH Si Si Si O Si H

Hydration near Silicon/ Silicon oxide interface

Oxidation of silicon

Hydrogen rapidly diffuses out

Some hydrogen may form hydroxyl group

2

1

2Si O H SiOH


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Diffusivities in Oxide

Oxygen diffuses faster (compared to water)

Sodium and Hydrogen diffuse very fast

Water

Oxygen

Hydrogen

Sodium

1/T

Diffusivity(logscale)


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Oxide Growth (Thermal)

SiOxide

Original Si surface

To obtain 1 unit of oxide,

almost half unit of silicon is

consumed (0.44)

Oxidation occurs at the

Si/SiO2 interface

i.e. Oxidizing species has

to diffuse through alreadyexisting silicon oxide


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Oxide Growth (Thermal)

SiliconOxideAir (BL)At any pointof time, amount

of oxide is

variable x

Usually,concentration of

oxidizing species

(H2O or O2) is

sufficiently high

in gas phase

==> Saturated

in the oxide

interface xDistance

Concentration

o

i


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Oxidation Kinetics

At steady state diffusion through oxide = reaction rate at the Si/SiO2

interface

Oxygen diffuses faster than Water

However, water solubility is very high (1000 times)

==> Effectively water concentration at the interface is

higher

==> wet oxidation fasterdN

J D

dx

( )

o iN N

D

x

iRate k N o

i

NN D

kx D

At steady state

Diffusion

Reaction


11/19

Oxidation Kinetics

Oxide Growth Rate

oDN

JDx

k

Flux at

steady state

dxdt

= Flux/ # oxidizing species per unit volume (of SiO2)

n = 2.2 1022 cm-3 for O2 = 4.4 1022 cm-3 for H2O

J

n

oDNdx

Ddt x k

0i

x x at t Eqn

Initial Condition

6.023x1023 molecules

=1 mol of oxide = x g of

oxide

= y cm3 of oxide (from

density)

2.2 x 1022 molecules/cm3 One O2 per SiO2

Two H20 per SiO2


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Deal-Grove Model

2 022 ( )DND

x x tk n

Solution

2

i ix x

BBA

2DA

k

2o

DNB

n

where

2x x

tBB

A

OR

is the time needed to grow the initial oxide

A and B depend on diffusivity D, solubility and #

oxidizing species per unit volume n

A and B will be different for Dry and Wet oxidation

Bruce Deal & Andy Grove


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Linear & Parabolic Regimes

Linear vs Parabolic Regimes

Kinetic Controlled vs Mass Transfer Controlled

( )B

x tA

Very short Time

2 ( )x B t

Longer Time

If one starts with thin oxide (or bare silicon)

12

2

40.5 1 ( ) 1

B

x A tA

2

( ) 4

A

tB

2

4

At

B

2

4

At

B


14/19

Exponential Regime

Hypothesis 1

Charged species forms

holes diffuse faster / set up electrical field

diffusion + drift ==> effective diffusivity high space charge regime controls

length = 15 nm for oxygen, 0.5 nm for water

==> wet oxidation not affected

For dry oxidation, one finds that is not zero in the model

fit

A corresponding to an initial thickness of 25 nm provides

good fit

Initial growth at very high rate

Approximated by exponential curve

If one starts with bare oxide


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Exponential Regime Hypothesis 2

In dry oxidation, many open areas exist

oxygen diffuses fast in silicon

hence more initial growth rate

once covered by silicon di oxide, slow diffusion

Hypothesis 3

Even before reaction (at high temp), oxygen dissolved in

silicon (reasonable diffusion)

once temp is increased, 5 nm quick oxide formation


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Temp Variation of

Linear/Parabolic Coeff

Linear [B/A] Parabolic [B]Solubility and Diffusion function of temp

May & Sze


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Effect of Doping Doping increases oxidation rate Segregation

ratio of dopant in silicon / dopant in oxide

e.g. Boron

incorporated in oxide;

more porous oxide

more diffusion parabolic rate

constant is higher

P not incorporated inoxide

no significant

change in parabolic

rate constant

Ma & Sze


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Issues

Na diffuses fast in oxide

Use Cl during oxidation

helps trap Na

helps create volatile compounds of heavy metals

(contaminant from furnace etc)

use 3% HCl or Tri chloro ethylene (TCE)

Ref: VLSI Fabrication Principles by S.K. Ghandhi


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Electrochemical

Use neutral solution and apply potential

Pt as counter electrode (Hydrogen evolution)

Use Ammonium hydrogen Phosphate or Phosphoric acid or

ammonia solution

Silicon diffuses out and forms oxide Increase in oxide thickness ==> increase in potential needed

self limiting

Oxide quality poor

Used to oxidize controlled amount and strip for diagnosis

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