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Microelectronics Processing Oxidation
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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
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Thermal SiO2 Properties
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Thermal SiO2 Properties (cont.)
(7) Amorphous material
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Oxidation Techniques
• Thermal Oxidation
• Rapid Thermal Oxidation
Oxidation Process
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
- 900-1,200C 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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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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The goal of oxidation is to grow a high quality oxide layer on a silicon substrate
Goal of Oxidation Process
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Passivation
• Physically protects wafers from scratches and particle ..contamination
• Traps mobile ions in oxide layer
Functions of Oxide Layers (1)
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Masking
• During Diffusion, Ion Implantation, and Etching
Function of Oxide Layers (2)
SiO2
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Insulating Material
• Gate region- Thin layer of oxide- Allows an inductive charge to pass between gate metal and silicon
Function of Oxide Layers (3)
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Dielectric Material
• Insulating material between metal layers- Field Oxide
Function of Oxide Layers (4)
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Dielectric Material
• Tunneling oxide- Allows electrons to pass through oxide without resistance
Function of Oxide Layers (5)
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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 DRAMShipment
1997 1999 2003 2006 2009 2012Minimum Feature Size (nm) 250 180 130 100 70 50DRAM Bits/Chip 256M 1G 4G 16G 64G 256GMinimum Supply Voltage(volts)
1.8-2.5 1.5-1.8 1.2-1.5 0.9-1.2 0.6-0.9 0.5-0.6
Gate Oxide Tox Equivalent(nm)
4-5 3-4 2-3 1.5-2 <1.5 <1.0
Thickness Control (% 3 σ) ± 4 ± 4 ± 4-6 ± 4-8 ± 4-8 ± 4-8Equivalent Maximum E-field(MV cm-1 )
4-5 5 5 >5 >5 >5
Gate Oxide Leakage (DRAM)(pA μm-2)
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01
Tunnel Oxide (nm) 8.5 8 7.5 7 6.5 6Maximum Wiring Levels 6 6-7 7 7-8 8-9 9Dielectric Constant, K forIntermetal Insulator
3.0-4.1 2.5-3.0 1.5-2.0 1.5-2.0 <1.5 <1.5
*NTRS- National Technology Roadmap for Semiconductors
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Oxidation occurs in tube furnace- Vertical Tube Furnace- Horizontal Tube Furnace
Thermal Oxidation Equipment
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Bubbler
Wet Thermal Oxidation Techniques
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Flash System
Wet Thermal Oxidation Techniques
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Dryox System
Wet Thermal Oxidation Techniques
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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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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
Oxide Growth Mechanism (1)
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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
Oxide Growth Mechanism (2)
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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 stepb) 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 relationshipof 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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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
Effect of High Pressure Oxidation
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Effect of High Pressure Oxidation
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High Pressure Oxidation
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Chlorine species - Anhydrous chloride (CI2)- Anhydrous hydrogen chloride (HCI)- Trichloroethylene – TCE- Trichloroethane – TCA
• Oxide growth rate increases
• Oxide cleaner
• Device performance is improved
Chlorine added with Oxidants
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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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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
Dopant Redistribution During Thermal Oxidation (2)
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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) boronb) boron withhydrogen ambientc) Phosphorusd) 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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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
Additional (Chemical) Oxidation Processes
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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
Additional Oxidation Processes
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Rapid Thermal Oxidation Equipment
Additional Oxidation Processes
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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
Additional Processes - Thermal Nitridation
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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- 1200°C) results in wafer strain
Additional Oxidation Processes
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