Transcript 5- Oxidation - USM :: Universiti Sains Malaysia

EBB 323 Semiconductor
Fabrication Technology
Oxidation
Dr Khairunisak Abdul Razak
Room 2.03
School of Material and Mineral Resources Engineering
Universiti Sains Malaysia
[email protected]
Outcomes
By the end of this topic, students
should be able to:
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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
What is oxidation?
Formation of oxide layer on wafer
High temperature
O2 environment
Principle uses of Si dioxide (SiO2) layer
in Si wafer devices
Surface passivation
Doping barrier
Surface dielectric
Device dielectric
1. Surface passivation
SiO2 layer protect semiconductor
devices from contamination:
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
SiO2 passivation
layer
Si
Si
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 barrier
In 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
Dopants
Si
SiO2 layer as dopant barrier
3. Surface dielectric
SiO2 is a dielectric  does not
conduct electricity under normal
circumstances
SiO2 layer prevents shorting of metal
layer to underlying metal
Oxide layer
MUST BE continuous; no holes or voids
Thick 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’
Metal layer
Oxide layer
Wafer
Dielectric use of SiO2 layer
source
Drain
S
Field oxide
D
MOS gate
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 900-1200C
• Oxidation: Si wafer  placed in a
heated chamber  exposed to
oxygen gas
SiO2 growth stages
Initial
Linear
Parabolic
Si wafer
Si wafer
Si wafer
In a furnace with O2 gas environment
Oxygen atoms combine readily with Si
atoms
Linear- oxide grows in equal amounts for
each time
Around 500Å thick
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
Original SiO2
surface
SiO2 surface
SiO2
Si substrate
Linear oxidation
X 
B
t
A
Parabolic oxidation of silicon
X 
Bt
where X = oxide thickness, B = parabolic rate constant, B/A =
linear rate constant, t = oxidation time
Parabolic relationship of SiO2 growth parameters
X
R
t
2
where R = SiO2 growth rate, X = oxide thickness, t = oxidation
time
Cont..
• Implication of parabolic relationship:
– Thicker oxides need longer time to grow
than thinner oxides
• 2000Å, 1200C in dry O2 = 6 minutes
• 4000Å, 1200C 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
steam
Faster oxidation – OH- hydroxyl ions diffuses faster in
oxide layer than dry oxygen
2H2 on the right side of the equation shows H2 are
trapped in SiO2 layer
Layer less dense than oxide layer in dry oxidation
Can be eliminated by heat treatment in an inert
atmosphere e.g. N2
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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)
F1  h(C*  C0 )..............5.1
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,
C0  Ci
F2  D
...................5.2
tox
Ci = oxygen concentration at theSi/SiO2 interface
D = diffusion coefficient of O2 or H2O in oxide
tox = oxide thickness
•Kinetics of the chemical reaction between
silicon and oxygen is characterised by
reaction constant, k:
F3  ksCi .................5.3
In steady state, all flux terms are equal: F1 =
F2 = F3. Eliminating C0 from the flux
equations, we can obtain:
Ci 
C*
k kt
1  s  s ox
h
D
...................5.4
•If N0x is a constant representing the
number of oxidising gas molecules
necessary to grow a unit thickness of
oxide, one can write:
dtox
N ox k sC *
 N ox F  N ox k sCi 
.......5.5
k s k s tox
dt
1 
h
D
•The solution to this differential equation
is:
tox

0
k s k s tox
t

h
D dt  dt..........5.6
ox
0
N ox k sC *
1
•If tox=0 when t=0, th eintegration yields:
tox2  D D 
   tox  N oxC *dt  0........5.7
2  ks h 
Or
 1 1
tox2  2 D  tox  2 DNoxC *t............5.8
 ks h 
Defining new constant A and B in terms of D, ks, Nox and
C*:
 1 1
A  2 D   ............5.9
 ks h 
and
B  2 DC* N ox ................5.10
We can obtain:
t 2  Atox  Bt.....................5.11
From which we find tox :
tox 
A
(t   ) 
 1 2
.................5.12
1 

2
A / 4B 
• 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:
B
t   ...........5.13
A
Which is linear with time.
B
•The A ratio is called “linear growth coefficient”, and is dependent
on crystal orientation of Si
tox 
•When thick oxides are formed, the growth rate is limited by the
diffusion rate of oxygen through the oxide. Eq 5.12 becomes:
tox  B(t   )  Bt..............5.14
• 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
Oxidation rate
Controlled by:
1.
2.
3.
4.
Wafer orientation
Wafer dopant
Impurities
Oxidation of polysilicon layers
1. Wafer orientation
Large no of atoms allows faster oxide growth
<111> plane have more Si atoms than <100>
plane
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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) distribution
Oxidised 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.
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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 method
Thermal 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 techniques
1.Tube furnace
2.Rapid thermal system
Oxidation methods
Thermal oxidation
Atmospheric
pressure
High pressure
Tube furnace
Dry oxygen
Wet oxygen
Rapid thermal
Dry oxygen
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 furnace
Integrated system of a tube furnace
consists of several sections:
1.
2.
3.
4.
5.
6.
7.
Reaction chamber
Temperature control system
Furnace section
Source cabinet
Wafer cleaning station
Wafer load station
Process automation
Vertical tube furnaces
Small footprint
Maybe placed outside
the cleanroom with
only a load station
door opening into the
cleanroom
Disadvantage:
expensive
Rapid Thermal Processing
Based on radiation principle heating
Useful for thin oxides used in MOS
gates
Trend 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 Oxidation
Problems 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 30C
In high pressure system, temperature drop of 300-750C
 This reduction is sufficient to minimise the growth of
dislocations in and on the wafers
Advantage of high pressure oxidation
Drop 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 sources
1. Dry oxygen
2. Water vapor sources
a) 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 98-99C
– 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