Transcript AMAT - Dynamic Vapor Generation | RASIRC

Microelectronics Processing
Oxidation
Microelectronics Processing Course - J. Salzman - Jan. 2002
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Content
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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
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Thermal SiO2 Properties
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Thermal SiO2 Properties (cont.)
(7) Amorphous material
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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)
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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
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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
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Processing
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Dry Thermal Oxidation Process
Thin Oxide Growth
• Thin oxides grown (<150Å) 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
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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
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Goal of Oxidation Process
The goal of oxidation is to grow a high quality
oxide layer on a silicon substrate
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Functions of Oxide Layers (1)
Passivation
• Physically protects wafers from scratches and particle
..contamination
• Traps mobile ions in oxide layer
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Function of Oxide Layers (2)
Masking
• During Diffusion, Ion Implantation, and Etching
SiO2
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Function of Oxide Layers (3)
Insulating Material
• Gate region
- Thin layer of oxide
- Allows an inductive charge to pass between gate
metal and silicon
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Function of Oxide Layers (4)
Dielectric Material
• Insulating material between metal layers
- Field Oxide
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Function of Oxide Layers (5)
Dielectric Material
• Tunneling oxide
- Allows electrons to pass through oxide without
resistance
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Functions and Thickness of Oxide Layers
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Projections for Si Technology
Future projections for silicon technology taken from the SIA NTRS*
Year of First DRAM
ShipmentFeature Size (nm)
Minimum
DRAM Bits/Chip
Minimum Supply Voltage
(volts)
Gate Oxide Tox Equivalent
(nm)
Thickness Control (% 3 σ)
Equivalent Maximum E-field
(MV cm-1 )
Gate Oxide Leakage (DRAM)
(pA μm-2)
Tunnel Oxide (nm)
Maximum Wiring Levels
Dielectric Constant, K for
Intermetal Insulator
*
1997
1999
2003
2006
2009
2012
250
256M
1.8-2.5
180
1G
1.5-1.8
130
4G
1.2-1.5
100
16G
0.9-1.2
70
64G
0.6-0.9
50
256G
0.5-0.6
4-5
3-4
2-3
1.5-2
<1.5
<1.0
±4
4-5
±4
5
± 4-6
5
± 4-8
>5
± 4-8
>5
± 4-8
>5
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
8.5
6
3.0-4.1
8
6-7
2.5-3.0
7.5
7
1.5-2.0
7
7-8
1.5-2.0
6.5
8-9
<1.5
6
9
<1.5
NTRS- National Technology Roadmap for Semiconductors
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Thermal Oxidation Equipment
Oxidation occurs in tube furnace
- Vertical Tube Furnace
- Horizontal Tube Furnace
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Wet Thermal Oxidation Techniques
Bubbler
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Wet Thermal Oxidation Techniques
Flash System
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Wet Thermal Oxidation Techniques
Dryox System
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Thickness of Si consumed during
oxidation
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Kinetics of Si02 Growth - Oxide Growth
Mechanism
1. Oxidant (O2) reacts with silicon atoms
2. Silicon atoms are consumed by reaction
3. Layer of oxide forms on silicon surface
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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
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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
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Deal-Grove Model (1)
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Deal-Grove Model (2)
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Deal-Grove Model (3)
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Deal-Grove Model (4)
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Deal-Grove Model (5)
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Deal-Grove Model (6)
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Deal-Grove Model (7)
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Deal-Grove Model (8)
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Deal-Grove Model (9)
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Limiting cases in Si oxidation
(a)
(b)
a) Interface reaction is the rate limiting step
b) Limited by oxidant transport through the SiO2 rate
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Deal-Grove Model Parameters
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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)
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Values for the coefficients Do and EA
Each of the coefficients B, and B/A has an Arrhenius relationship
of the type: D=D0exp(-EA/kT)
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Diffusivities of some materials in
silicon glass
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Examples
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Effect of Xi on Wafer Topography (1)
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Effect of Xi on Wafer Topography (2)
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Factors that Affect
Oxidation
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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
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High Doping concentration effect
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Growth Rate Dependence on Si
Substrate Orientation
Wafer Orientation
• Oxide grows faster on <111>
wafers
- more silicon atoms available
to react with oxidant
• Affects oxide growth rate during
Linear Stage
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Origin of Substrate Orientation Effect
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Substrate Orientation Effect - Oxidation
Charts
(a)
(b)
Growth of SiO2 on <100> and <111> oriented
Si wafers:
(a) dry oxygen; (b) steam.
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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 30°C
•Dry Thermal oxidation
- Pressure in oxidation tube increased
• Wet Thermal oxidation
- Steam pressure introduced into oxidation tube
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Effect of High Pressure Oxidation
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High Pressure Oxidation
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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
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Oxidation With Cl Containing Gas
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Effect of HCl on Oxidation Rate
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Local Oxidation of Si (LOCOS)
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Local Oxidation
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Dopant Redistribution During Thermal
Oxidation (1)
Dopant concentration
Dopant concentration
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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
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Dopant Redistribution During Thermal
Oxidation (3)
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Dopant Redistribution During Thermal
Oxidation (4)
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Dopant Redistribution During Thermal
Oxidation (5)
a) boron
b) boron with
hydrogen ambient
c) Phosphorus
d) gallium
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Thin Oxide Growth
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Structure of SiO2-Si Interface
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Thin Oxide Tunneling Current
Comparison
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Polycrystalline Si Oxidation
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Polysilicon Oxidation
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Oxide inspection techniques
Surface Inspection
Oxide Thickness
Oxide Cleanliness
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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
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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
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Additional Oxidation Processes
Rapid Thermal Oxidation Equipment
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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
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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 (9501200°C) results in wafer strain
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