10 Oxidation &10

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Oxidization Furnace

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Native and deposited films

Native films are grown out of the semiconductor itself. Thermal or electrochemical means Eg. SiO2.

Films can be deposited by chemical or physicalmethods.

Native films have better quality and excellent interfacewith the substrate.

But all films cannot be grown .

Thicker films are to be deposited.

Film thickness and quality is determined by growth/deposition pressure and temperature.

Used as gate oxides and for passivation ,masking,interconnects etc.

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Thermal oxidation of Si

Slice of Si is always covered with 15-20Ao of SiO2 (upper limit 40oA)

when it is exposed to atmosphere.

Patchy in nature and of no technological value.

Thicker and better quality SiO2 layers can be grown if the slice issubjected to oxidizing ambient at elevated temperature.

SiO2 helps to reduce the surface state density of Si by tying danglingbonds.

Dry oxidation

Slice is subjected to dry oxygen. growth rate less, but of goodquality.

wet oxidation Subjected to water vapor. Thicker films but of poor quality.

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Thermal oxidation of Si

44% of Si is cosumed.(2.27 times thickness of the Si

consumed).

Oxidizing species should move through the growing

oxide layer for further oxidation of Si.

Growth rate decreases as growth progresses.

Chemistry of oxidation.

222

22

22 HSiOOHSiSiOOSi

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Kinetics of oxide growth Assume a Si slice brought in contact with the oxidant

with a gas phase concentration of Ng. High mass transfer coefficient is assumed so that

surface concentration No is determined by the solidsolubility of reactants.

Solid solubility of dry oxygen in SiO2 is 5.2 x 1016

molecules cm-3 and the corresponding value for watervapor is 3 x 10 19 cm-3.

No

N1

The flux of reactants arriving at

the Si-SiO2

interface is

x

NND

x

NDj o 1

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Kinetics of oxide growth

Assuming linear reaction, the flux of reaction

products is given by

Equating both fluxes

The growth rate of oxide layer is given by

1kNj

k

Dx

DNj

0

k

Dx

nDN

n

j

dt

dx

/

0

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Solving this equation with the boundary value that at t=0at x=0

For small values of t

For large t

tn

DNx

k

Dx 02

22

12021

2

1

DntkN

kDx

tn

kNx

0

2

12

1

02

t

n

DNx

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In the initial phases the growth is kinetic

controlled and linear.

At later phases the growth is diffusion

limited and proportional to the square root

of time.

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Linear & Parabolic rate constants

BtAxx 2Oxidation of Si can be represented by

For small t

tABx

For large t

Btx

B/A is the linear rate constant

B is the parabolic rate constant.

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KT

E

KT

E

eCA

B

eCB

2

2

1

1

Mathematical relationships for the rate constants.

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.00001

{111} Si

{100} Si

{111} Si

{100} Si

H2O

Dry O2

Linearr

ateconstantB/A

m

/hr

10

1

0.1

0.01

.001

.0001

0.6 0.7 0.8 0.9 1 1.1 1000/T K-1

EA

=2.0eV

EA=2.05eV

H2O

Dry O2

{111} Si

{111} Si

{100} Si

{100} Si

Log of Linear rate constant falls

off with 1/T, with a slope of

2.0 eV/molecule for

dry oxidation.

2.05eV/molecule for

wet oxidation.

comparable to the

energy required to break Si-Si

bonds (1.83 eV/molecule).

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0.6 0.7 0.8 0.9 1.0

1000/T K-1

1

0.1

0.01

0.001

Dry O2 EA=1.23eV

H2O EA=0.78eV

Independent of orientation

ParabolicrateconstantBm2/hr

Log of parabolic rate constant

falls off with 1/T, with a slope of

1.23 eV/molecule for dry

oxidation.0.78 eV/molecule for wet

oxidation.

comparable to the activation

energy of Dry O2(1.18

eV/molecule) and H2O (0.79

eV/molecule) in SiO2

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Orientation dependent effects.

Linear rate constant

Growth rate constant higher for {111} surface.

Greater by 1.68:1 against {100} surface. Depends on reaction at the surface.

In-turn depends on the surface atomic density.

Atomic density is higher for {111} surface compared to {100}

surface.

Parabolic rate constant. Depends on diffusivity of species.

Independent of orientation.

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Rate constants (growth rate) higher for wet oxidation,compared to dry oxidation.

Water vapor has greater solid solubility compared to dry O2 , inSiO2.

A rapid initial growth phase is found in the case of dry O2compared to wet oxidation. The molecular oxygen reaching the oxide surface dissociates

into O2-and O2

2- and one or two holes. These holes have higher

mobility compared to the oxygen ions. This creates a spacecharge at the gas oxide interface which enhances the transportof oxygen into the silicon surface by providing an additional driftcomponent.

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Rapid initial growth phase

Deal Grove model is found to be applicable for

thicknesses involving more than 20-25nm.

In order to include this rapid initial growth ,the model is

re-written as

x2+Ax =B (t + t i)

Where ti is the parameter for accounting the initial

thickness xi.

B

Axxt i

i

i

2

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Dependence on doping

Heavily doped Si oxidizes faster. Surface reaction rate more due to increased defects.

Diffusion through the oxide increases as segregationof the impurities into the oxide will weaken its

structure. Growth behavior ofphosphorous doped Silicon

Has large segregation constant and it is a slow diffuser in theoxide. Hence it piles up at the interface on Si side.

Reaction rate at the interface is enhanced ,so linear rate

constant increases with doping. Since only a small amount of phosphorous is incorporated

into the oxide its structure is not much affected. Parabolicrate constant is not affected by doping.

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Dependence on doping

Boron doping.

Large amount of boron is incorporated into the oxide,

weakening its structure. Hence diffusion is enhancedand parabolic rate constant increases with doping.

There is a depletion of boron at the surface, so

reaction rate and linear rate constant is not affected.

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Behavior of rate constants with doping

phosphorous doping

Significant change

No significant change

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Oxidization Graph

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Chemical properties of SiO2

Amorphous orvitreous state at lower

temperatures.

At high temperatures returns to its crystalline

form. Rate ofdevitrification can be neglected below

1000oC.

Can cause cracks in the oxide film, thus quality is

degraded.

Melting point 1732oC.

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Pure (Intrinsic) Silica

Polyhedra of oxygen ions with centers

occupied by Si ions.

Crystalline quartz

amorphous

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Pure (Intrinsic) Silica

Bridging and non bridging oxygen ions.

Bridging

oxygen atom

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Extrinsic Silica glass

Presence ofimpurities greatly modifies thestructure and properties of SiO

2. Impurities can be substitutional.

Substitutes for a Si ion.

Eg. B 3+ and P 5+

These are called network formers since they can exist invitreous state like SiO2.

Interstitial impurities. Weakens the network.

Na+ , K+

Called network modifiers, as they do not form glasses bythemselves.

Na is a very important contaminant.

NaSiOOSiSiOSiONa 22

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Extrinsic Silica glass

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Oxygen and water vapor in SiO2

Oxygen is more free to move in the network.

The water vapor enters into reaction with

Bridging oxygen ions to form pairs of stable non-

bridging hydroxyl groups. Weaken the structure.

SiOHOHSiSiOSiOH 2

OHSiSiOH

HSiOSiSiOHSi

2

2

2

1

2

1

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Impurity redistribution during Oxide

growth

Oxidation ofdoped Si.

Impurities will redistribute at the interface. Determined by

Segregation of impurities into the oxide.

Diffusion of impurities in the oxide.

Rate at which the interface moves with respect to thediffusion rate.

Segregation constant m.

Ratio of equilibrium concentration of the impurity in Sito that in SiO2.

Function of temperature and orientation.

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growth

CSI= concentration in Si

C= concentration in the oxide at the interface

Diffusion in oxide

slow (boron)

m

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growth

Diffusion in oxide slow(phosphorous)

m>1

C/CSI

x

SiO2 Si

Diffusion in oxide fast

(Gallium)

m>1

C/CSI

x

SiO2 Si

CSI= concentration in Si

C= concentration in the oxide at the interface

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Oxide Charges

Si-SiO2 is a transition layer between crystalline Siliconand amorphous silica.

Charges and traps are present in thermally oxidized Si.

Most of the charges are at the interface.

A charge at the interface can induce a charge ofopposite polarity in the underlying Silicon

Affect the threshold voltage of a MOS.

Interface charge is represented by

N = Q/q N = number of charges per unit area.Q = net effective charge per unit area.

q = electronic charge

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Interface charge

Mobile ions Qm -

Interface trapped charges (interface states) sudden termination of

semiconductor lattice at the interface Qit

Fixed oxide charge Qfun completed bonds (SiOx) as well as ionic Si.

Oxide trapped charges due to imperfections in SiO2 and radiations

Qot. Net oxide charge is taken as positive

Negative voltage is to be applied to get flat band conditions.

motitfox QQQQQ

ox

oxmsFB

C

QV

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