AMAT

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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 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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Dry 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

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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
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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
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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
DD0exp(-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
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Origin of Substrate Orientation Effect
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Substrate Orientation Effect - Oxidation Charts
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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 30C
• 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)
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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 (950-
  1200C) results in wafer strain