ebb 323 semiconductor fabrication technology oxidation dr khairunisak abdul razak room 2.03 school...
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EBB 323 Semiconductor Fabrication Technology
Oxidation
Dr Khairunisak Abdul RazakRoom 2.03School of Material and Mineral Resources EngineeringUniversiti Sains [email protected]
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:
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