Title: AMAT
1 Microelectronics Processing Oxidation
2Content
- 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
3Thermal SiO2 Properties
4Thermal SiO2 Properties (cont.)
(7) Amorphous material
5Oxidation 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)
6Conceptual 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
8Dry Thermal Oxidation Process
- Thin Oxide Growth
- Thin oxides grown (lt150Å) for features smaller
than 1 ..million - 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
9Wet 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
10Goal of Oxidation Process
The goal of oxidation is to grow a high quality
oxide layer on a silicon substrate
11Functions of Oxide Layers (1)
- Passivation
- Physically protects wafers from scratches and
particle ..contamination - Traps mobile ions in oxide layer
12Function of Oxide Layers (2)
- Masking
- During Diffusion, Ion Implantation, and Etching
SiO2
13Function of Oxide Layers (3)
- Insulating Material
- Gate region - Thin layer of oxide - Allows an
inductive charge to pass between gate
metal and silicon
14Function of Oxide Layers (4)
- Dielectric Material
- Insulating material between metal layers - Field
Oxide
15Function of Oxide Layers (5)
- Dielectric Material
- Tunneling oxide - Allows electrons to pass
through oxide without resistance
16Functions and Thickness of Oxide Layers
17Projections for Si Technology
18Thermal Oxidation Equipment
Oxidation occurs in tube furnace - Vertical Tube
Furnace - Horizontal Tube Furnace
19Wet Thermal Oxidation Techniques
Bubbler
20Wet Thermal Oxidation Techniques
Flash System
21Wet Thermal Oxidation Techniques
Dryox System
22Thickness of Si consumed during oxidation
23Kinetics 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
24Oxide 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
25Oxide 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
26Deal-Grove Model (1)
27Deal-Grove Model (2)
28Deal-Grove Model (3)
29Deal-Grove Model (4)
30Deal-Grove Model (5)
31Deal-Grove Model (6)
32Deal-Grove Model (7)
33Deal-Grove Model (8)
34Deal-Grove Model (9)
35Limiting cases in Si oxidation
36Deal-Grove Model Parameters
37Deal-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)
38Values for the coefficients Do and EA
Each of the coefficients B, and B/A has an
Arrhenius relationship of the type
DD0exp(-EA/kT)
39Diffusivities of some materials in silicon glass
40Examples
41Effect of Xi on Wafer Topography (1)
42Effect of Xi on Wafer Topography (2)
43Factors that Affect Oxidation
44High 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
45High Doping concentration effect
46Growth Rate Dependence on Si Substrate Orientation
47Origin of Substrate Orientation Effect
48Substrate Orientation Effect - Oxidation Charts
49Effect 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
50Effect of High Pressure Oxidation
51High Pressure Oxidation
52Chlorine 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
53Oxidation With Cl Containing Gas
54Effect of HCl on Oxidation Rate
55Local Oxidation of Si (LOCOS)
56Local Oxidation
57Dopant Redistribution During Thermal Oxidation (1)
58Dopant 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
59Dopant Redistribution During Thermal Oxidation (3)
60Dopant Redistribution During Thermal Oxidation (4)
61Dopant Redistribution During Thermal Oxidation (5)
a) boron b) boron with hydrogen ambient c)
Phosphorus d) gallium
62Thin Oxide Growth
63Structure of SiO2-Si Interface
64Thin Oxide Tunneling Current Comparison
65Polycrystalline Si Oxidation
66Polysilicon Oxidation
67Oxide inspection techniques
Surface Inspection Oxide Thickness Oxide
Cleanliness
68Additional (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
69Additional 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
70Additional Oxidation Processes
Rapid Thermal Oxidation Equipment
71Additional 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
72Additional 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