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
Outcomes
• • • • • •
By the end of this topic, students should be able to: List principle uses of silicon dioxide (SiO 2 ) 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 SiO 2 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 O 2 environment Principle uses of Si dioxide (SiO 2 ) layer in Si wafer devices Surface passivation Doping barrier Surface dielectric Device dielectric
1. Surface passivation
i.
SiO 2 layer protect semiconductor devices from contamination: Physical protection of the sample and underlying devices Dense and hard SiO 2 layer act as contamination barrier Hardness of the SiO 2 layer protect the surface from scratches during fabrication process
SiO 2 layer passivation 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 SiO 2 on Si wafer block the dopants from reaching Si surface All dopants have slower rate of movement in SiO 2 compared to Si Relatively thin layer of SiO 2 is required to block the dopants from reaching SiO 2
Cont..
SiO 2 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 SiO 2 layer as dopant barrier
3. Surface dielectric
SiO 2 is a dielectric
does not conduct electricity under normal circumstances SiO 2 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 SiO 2 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 Field oxide Dielectric use of SiO 2 layer source S MOS gate Drain D
• •
4. Device dielectric
In MOS application
–
Grow thin layer SiO 2
–
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 60-100 150-500 200-500 2000-5000 3000-10000 Applications Tunneling gates Gates oxides, capacitor dielectrics LOCOS pad oxide Masking oxides, surface passivation Field oxides
• • •
Thermal oxidation mechanisms Chemical reaction of thermal oxide growth Si (solid) + O 2 (gas)
SiO 2 (solid) Oxidation temperature 900-1200
C Oxidation: Si wafer
heated chamber
placed in a exposed to oxygen gas
SiO
2
growth stages
Initial Linear Parabolic Si wafer Si wafer Si wafer In a furnace with O 2 gas environment Oxygen atoms combine readily with Si atoms Linear- oxide grows in equal amounts for each time Around 500 Å thick Above 500 in contact Å, in order for oxide layer to keep growing, oxygen and Si atoms must be SiO 2 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 SiO 2 growth by thermal oxidation
SiO 2 surface Original SiO 2 surface SiO 2 Si substrate
Linear oxidation
X
B A t
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 SiO 2 growth parameters
R
X
2
t
where R = SiO 2 time growth rate, X = oxide thickness, t = oxidation
• •
Cont..
Implication of parabolic relationship:
–
Thicker oxides need longer time to grow than thinner oxides
• •
2000 Å, 1200
C in dry O 2 4000 Å, 1200
C in dry O 2 times longer) = 6 minutes = 220 minutes (36 Long oxidation time required:
–
Dry O 2
–
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) + O 2 (gas)
SiO 2 (solid) (dry oxidation) Or Si (solid) + 2H 2 O (gas)
SiO 2 (solid) + 2H 2 (gas) (wet oxidation) Also called steam oxidation, wet oxidation, pyrogenic steam Faster oxidation – OH hydroxyl ions diffuses faster in oxide layer than dry oxygen 2H 2 on the right side of the equation shows H 2 trapped in SiO 2 layer are Layer less dense than oxide layer in dry oxidation Can be eliminated by heat treatment in an inert atmosphere e.g. N 2
• • • •
2 mechanisms influence the growth rate of the oxide 1. Actual chemical reaction rate between Si and O 2 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 O 2 and H 2 O through the oxide limit the growth rate is diffusion of O 2 and H 2 O through the oxide A steam ambient is preferred for the growth of thick oxides:H 2 O molecules smaller than O 2 thus, easier diffuse through SiO 2 that cause high oxidation rates
Si oxidation
Oxygen concentration profile during oxidation
•
Mass transport of O 2 molecules from gas ambient towards the Si through a layer of already grown oxide
•
Flux of O 2 molecules is proportional to the differential in O 2 concentration between the ambient (C * ) and oxide surface (C 0 )
F
h
(
C
*
C
).........
.....
5 .
1 0
Where h is the mass transport coefficient for O 2
1
in the gas phase
•
Diffusion of O 2 through the oxide is proportional to the difference of oxygen concentration between the oxide surface and the Si/SiO 2 oxygen through the oxide, F 2 interface. The flux of becomes
F
2
D C
0
t ox C i
..........
.........
5 .
2
Where, C i = oxygen concentration at theSi/SiO 2 interface D = diffusion coefficient of O 2 or H 2 O in oxide t ox = oxide thickness
•
Kinetics of the chemical reaction between silicon and oxygen is characterised by reaction constant, k:
F
3
k s C i
..........
.......
5 .
3
In steady state, all flux terms are equal: F 1 F 2 = F 3 . Eliminating C 0 from the flux equations, we can obtain: =
C i
1
k s h C
*
k s t ox D
..........
.........
5 .
4
•
If N 0x is a constant representing the number of oxidising gas molecules necessary to grow a unit thickness of oxide, one can write:
dt ox dt
N ox F
N ox k s C i
1
N ox k s C
*
k s
k s t ox h D
.......
5 .
5 •
The solution to this differential equation is:
t o x
0 1
k s
k s t h N ox k s C
*
D ox dt ox
t
0
dt
..........
5 .
6
•
If t ox =0 when t=0, th eintegration yields:
t
2
ox
2
D k s
D h
t ox
N ox C
*
dt
0 ........
5 .
7
Or
t
2
ox
2
D
1
k s
1
h
t ox
2
DN ox C
*
t
..........
..
5 .
8
Defining new constant A and B in terms of D, k s , N ox C * :
A
2
D
k
1
s
1
h
..........
..
5 .
9
and
and B
2
DC
*
N ox
..........
......
5 .
10
We can obtain:
t
2
At ox
Bt
..........
..........
.
5 .
11
From which we find t ox
t ox
A
2
:
1 (
t A
2 / ) 4
B
1 ..........
.......
5 .
12
•
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 O 2 .
Eq. 5.12 becomes:
t ox
B A
t
..........
.
5 .
13
Which is linear with time.
•
The ratio is called “linear growth coefficient”, and is dependent
A
on crystal orientation of Si
•
When thick oxides are formed, the growth rate is limited by the diffusion rate of oxygen through the oxide. Eq 5.12 becomes:
t ox
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/SiO 2
–
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 N 2
Oxidation rate
Controlled by: 1. Wafer orientation 2. Wafer dopant 3. Impurities 4. Oxidation of polysilicon layers 1. Wafer orientation Large 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) 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.
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 SiO 2 /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 SiO 2 , 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 SiO 2 surface causes B pile up in SiO 2
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 SiO 2 layer are grown: Atmospheric pressure oxidation
oxidation without intentional pressure control (auto-generated pressure); also called atmospheric technique High pressure oxidation
applied during oxidation high pressure is 2 atmospheric techniques 1.Tube furnace 2.Rapid thermal system
Oxidation methods
Thermal oxidation Atmospheric pressure Tube furnace Dry oxygen Wet oxygen High pressure Anodic oxidation Rapid thermal Dry oxygen Tube furnace Chemical oxidation Dry or wet oxygen 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
1.
2.
3.
4.
5.
6.
7.
Integrated system of a tube furnace consists of several sections: 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 < 100 Å thin gate oxide Hard to control thin film in conventional tube furnace Problem: quick supply and remove O 2 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 O 2 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
e.g. RTP time/temperature curve RTP design
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 ~ 1000 Å
• • • •
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)
O 2 and H 2 tube
are introduced directly into oxidation 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 H 2
overcome by flow in excess O 2
• • •
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-SiO 2 interface Chlorine sources:
–
Gas: anhydrous chlorine (Cl 2 ), 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
< 100 Å SiO 2 film possesses poor quality and difficult to control Silicon nitride (Si 3 N 4 )
–
Denser than SiO 2
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