1. EBB 323 Semiconductor Fabrication Technology
Oxidation
Dr Khairunisak Abdul Razak
Room 2.03
School of Material and Mineral Resources Engineering
Universiti Sains Malaysia

2. Outcomes By 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

3. Uses of dielectric films in Semiconductor technology

4. Principle uses of Si dioxide (SiO2) layer in Si wafer devices
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

5. 1. Surface passivation
SiO2 layer protect semiconductor devices from contamination:
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

6. 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

7. 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

8. 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

9. 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’

10. Metal layer
Oxide layer
Wafer
Dielectric use of SiO2 layer
source
Drain S D
MOS gate
Field oxide

11. 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

12. Types of oxidation Thermal oxidation High pressure oxidation
Anodic oxidation

13. Device oxide thicknesses
Most applications of semiconductor are dependent on their oxide thicknesses
Silicon dioxide thickness, Å
Applications
60-100
Tunneling gates
Gates oxides, capacitor dielectrics
LOCOS pad oxide
Masking oxides, surface passivation
Field oxides

14. Thermal oxidation mechanisms
Chemical reaction of thermal oxide growth
Si (solid) + O2 (gas)  SiO2 (solid) 
Oxidation temperature C
Oxidation: Si wafer  placed in a heated chamber  exposed to oxygen gas

15. SiO2 growth stages Si wafer Initial
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
Si wafer
Linear
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
Si wafer
Parabolic

16. Three dimension view of SiO2 growth by thermal oxidation
SiO2 surface
Original SiO2 surface
SiO2
Si substrate

17. Parabolic relationship of SiO2 growth parameters
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

18. 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

19. Dependence of silicon oxidation rate constants on temperature

20. Oxide thickness vs oxidation time for silicon oxidation in dry oxygen at various temperatures

21. Oxide thickness vs oxidation time for silicon oxidation in pyrogenic steam (~ 640 Torr) at various temperatures

22. 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

23. 2 mechanisms influence the growth rate of the oxide
Actual chemical reaction rate between Si and O2
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

24. Si oxidation
Oxygen concentration profile during oxidation

25. 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

26. 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:

27. 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:

28. 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 :

29. Which is linear with time.
 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 becomes:
Which is linear with time.
The ratio is called “linear growth coefficient”, and is dependent on crystal orientation of Si

30. The parabolic growth coefficient can be increased:
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 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

31. Oxidation rate Controlled by: Wafer orientation Wafer dopant
Impurities
Oxidation of polysilicon layers
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

32. Crystal structure of silicon
<100> plane
<111> plane

33. Dependence of oxidation linear rate constant and oxide fixed charge density on silicon orientation

34. 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

35. 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

36. 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

37. Oxide impurities
Certain impurities may influence oxidation rate
e.g. chlorine from HCl from oxidation atmosphere  increase growth rate 1-5%

38. 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

39. 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
Tube furnace
Rapid thermal system

40. Oxidation methods Thermal oxidation Atmospheric pressure Tube furnace
Dry oxygen
Wet oxygen
Rapid thermal
High pressure
Dry or wet oxygen
Chemical oxidation
Anodic oxidation
Electrolytic cell
Chemical

41. 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

42. Horizontal tube furnace
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

43. Vertical tube furnaces
Small footprint
Maybe placed outside the cleanroom with only a load station door opening into the cleanroom
Disadvantage: expensive

44. 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

45. 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

46. RTP design
e.g. RTP time/temperature curve

47. 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

48. High pressure oxidation results in faster oxidation rate
High pressure system  similar to conventional horizontal tube furnace with several features:
Sealed tube
Oxidant is pumped into the tube at pressure 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 C
 This reduction is sufficient to minimise the growth of dislocations in and on the wafers

49. 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

50. Oxidant sources Dry oxygen Water vapor sources Bubblers/ flash
Dry oxidation
Chlorine added oxidation

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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)

57. Nitridation of <100> silicon