AMAT

1
Microelectronics Processing Oxidation
2
Content

• Properties of SiO2
• Oxidation Process
• Functions of SiO2
• Equipment for Si Oxidation
• Mechanism of Si Oxidation

• Factors affecting oxidation
• Doping
• Substrate Orientation
• Pressure
• Chlorine addition
• Dopant Redistribution
• Polysilicon Oxidation
• Additional Oxidation Processes

3
Thermal SiO2 Properties
4
Thermal SiO2 Properties (cont.)
(7) Amorphous material
5
Oxidation Process

• Oxidation Techniques
• Thermal Oxidation
• Rapid Thermal Oxidation

• Thermal Oxidation Techniques
• Wet Oxidation
• Si (solid) H20
  SiO2 (solid) 2H2
• Dry Oxidation
• Si (solid) O2 (gas)
  SiO2(solid)

6
Conceptual Si Oxidation System

• Thermal Oxidation
• Heat is added to the oxidation tube during the
  reaction ..between oxidants and silicon -
  900-1,200?C temperature range - Oxide growth
  rate increases as a result of heat
• Used to grow oxides between 60-10,000Å

7
Thermal Oxidation Process

• Wafers are placed in wafer load station
• Dry nitrogen is introduced into chamber -
  Nitrogen prevents oxidation from occurring
• Nitrogen gas flow shut off and oxygen added to
  chamber - Occurs when furnace has reached
  maximum temperature - Oxygen can be in a dry gas
  or in a water vapor state
• Nitrogen gas reintroduced into chamber - Stops
  oxidation process
• Wafers are removed from furnace and inspected

• Dry Thermal Oxidation Characteristics
• Oxidant is dry oxygen
• Used to grow oxides less than 1000Å thick
• Slow process - 140 - 250Å / hour

8
Dry Thermal Oxidation Process

• Thin Oxide Growth
• Thin oxides grown (lt150Å) for features smaller
  than 1 ..micrometer - MOS transistors, MOS
  gates, and dielectric components
• Additional of chemical species to oxygen
  decreases ..oxide growth rate (only in special
  cases)
• - Hydrochloric acid (HCI) -
  Trichloroethylene (TCE) - Trichloroethane (TCA)
• Decreasing pressure slows down oxide growth rate

9
Wet Thermal Oxidation

• Wet Thermal Oxidation Characteristics
• Oxidant is water vapor
• Fast oxidation rate - Oxide growth rate is
  1000-1200Å / hour
• Preferred oxidation process for growth of thick
  oxides

10
Goal of Oxidation Process
The goal of oxidation is to grow a high quality
oxide layer on a silicon substrate
11
Functions of Oxide Layers (1)

• Passivation
• Physically protects wafers from scratches and
  particle ..contamination
• Traps mobile ions in oxide layer

12
Function of Oxide Layers (2)

• Masking
• During Diffusion, Ion Implantation, and Etching

SiO2
13
Function of Oxide Layers (3)

• Insulating Material
• Gate region - Thin layer of oxide - Allows an
  inductive charge to pass between gate
  metal and silicon

14
Function of Oxide Layers (4)

• Dielectric Material
• Insulating material between metal layers - Field
  Oxide

15
Function of Oxide Layers (5)

• Dielectric Material
• Tunneling oxide - Allows electrons to pass
  through oxide without resistance

16
Functions and Thickness of Oxide Layers
17
Projections for Si Technology
18
Thermal Oxidation Equipment
Oxidation occurs in tube furnace - Vertical Tube
Furnace - Horizontal Tube Furnace
19
Wet Thermal Oxidation Techniques
Bubbler
20
Wet Thermal Oxidation Techniques
Flash System
21
Wet Thermal Oxidation Techniques
Dryox System
22
Thickness of Si consumed during oxidation
23
Kinetics of Si02 Growth - Oxide Growth Mechanism

• Oxidant (O2) reacts with silicon atoms
• Silicon atoms are consumed by reaction
• Layer of oxide forms on silicon surface

24
Oxide Growth Mechanism (1)

• Linear Parabolic Model
• Linear (first) Stage of Oxidation - Chemical
  reaction between silicon and oxidants at wafer
  surface - Reaction limited by number of silicon
  atoms available to react with oxidants -
  During the first 500Å of oxide growth, the oxide
  grows linearly with time - Growth rate begins
  to slow down as oxide layer grows

25
Oxide Growth Mechanism (2)

• Linear Parabolic Model
• Parabolic Stage - Begins when 1,000Å of oxide
  has been grown on silicon - Silicon atoms are
  no longer exposed directly to oxidants -
  Oxidants diffuse through oxide to reach
  silicon - Reaction limited by diffusion rate of
  oxidant

26
Deal-Grove Model (1)
27
Deal-Grove Model (2)
28
Deal-Grove Model (3)
29
Deal-Grove Model (4)
30
Deal-Grove Model (5)
31
Deal-Grove Model (6)
32
Deal-Grove Model (7)
33
Deal-Grove Model (8)
34
Deal-Grove Model (9)
35
Limiting cases in Si oxidation
36
Deal-Grove Model Parameters
37
Deal-Grove model (10) - Effect of temperature on
the rate constants B, and B/A
B(T)Boexp(-EA/kT) (B/A)(T)(B/A)oexp(-EA/kT)
38
Values for the coefficients Do and EA
Each of the coefficients B, and B/A has an
Arrhenius relationship of the type
DD0exp(-EA/kT)
39
Diffusivities of some materials in silicon glass
40
Examples
41
Effect of Xi on Wafer Topography (1)
42
Effect of Xi on Wafer Topography (2)
43
Factors that Affect Oxidation
44
High Doping concentration effect

• Dopants in silicon
• Dopants increase oxide growth rate - During
  Linear Stage of oxidation N-type dopants increase
  growth rate
• Dopants cause differential oxidation - Results
  in the formation of steps - Affects etching
  process

45
High Doping concentration effect
46
Growth Rate Dependence on Si Substrate Orientation
47
Origin of Substrate Orientation Effect
48
Substrate Orientation Effect - Oxidation Charts
49
Effect of High Pressure Oxidation

• Atmospheric pressure - Slow oxide growth rate
• An increase in pressure increase oxide growth
  rate
• Increasing pressure allows temperature to be
  ..decreased - Oxide growth rate remains the
  same - For every 10atm of pressure the
  temperature can be reduced 30C
• Dry Thermal oxidation - Pressure in oxidation
  tube increased
• Wet Thermal oxidation - Steam pressure
  introduced into oxidation tube

50
Effect of High Pressure Oxidation
51
High Pressure Oxidation
52
Chlorine added with Oxidants

• Chlorine species - Anhydrous chloride (CI2) -
  Anhydrous hydrogen chloride (HCI) -
  Trichloroethylene TCE - Trichloroethane TCA
• Oxide growth rate increases
• Oxide cleaner
• Device performance is improved

53
Oxidation With Cl Containing Gas
54
Effect of HCl on Oxidation Rate
55
Local Oxidation of Si (LOCOS)
56
Local Oxidation
57
Dopant Redistribution During Thermal Oxidation (1)
58
Dopant Redistribution During Thermal Oxidation (2)
Dopants affect device performance - The change
in dopant location and concentration during
oxidation can affect the device operation -
N-type dopants move deeper into silicon so high
concentration at the silicon/silicon dioxide
interface - P-type dopants move into the silicon
dioxide and deplete the silicon layer
59
Dopant Redistribution During Thermal Oxidation (3)
60
Dopant Redistribution During Thermal Oxidation (4)
61
Dopant Redistribution During Thermal Oxidation (5)
a) boron b) boron with hydrogen ambient c)
Phosphorus d) gallium
62
Thin Oxide Growth
63
Structure of SiO2-Si Interface
64
Thin Oxide Tunneling Current Comparison
65
Polycrystalline Si Oxidation
66
Polysilicon Oxidation
67
Oxide inspection techniques
Surface Inspection Oxide Thickness Oxide
Cleanliness
68
Additional (Chemical) Oxidation Processes

• Anodic Oxidation Process
• Wafer is attached to a positive electrode
• Wafer is immersed in bath of potassium nitrate
  ..(KNO3)
• Immersion tank contains a negative electrode
• Oxygen produced when current is applied
• Reaction between silicon and oxygen occurs

69
Additional Oxidation Processes

• Anodic Oxidation Characteristics
• Oxidation reaction occurs at the surface of the
  oxide - Silicon atoms move to top of oxide layer
  during oxidation
• Used to grow oxide on wafers that will be tested
  for ..dopant location and concentration

70
Additional Oxidation Processes
Rapid Thermal Oxidation Equipment
71
Additional Processes - Thermal Nitridation

• Thermal Nitridation Characteristics
• Alternative method to Oxidation
• Oxidant is nitrogen - Pure ammonia gas (NH3) -
  Ammonia plasma
• Reaction produces silicon nitride (Si3N4) -
  Reaction occurs at the gas/silicon nitride
  interface - Silicon atoms diffuse through
  silicon nitride layer during process
• Silicon nitride is a good substitute for silicon
  dioxide - Silicon nitride is denser than silicon
  dioxide - Silicon nitride has a higher
  dielectric rating

72
Additional Oxidation Processes

• Thermal Nitridation Disadvantage
• Process puts high level of strain on wafer -
  Thermal expansion rate of silicon nitride is 2
  times greater than silicon dioxide - High
  temperature processing techniques (950-
  1200C) results in wafer strain