1 microelectronics processing course - j. salzman - jan. 2002 microelectronics processing oxidation

1Microelectronics Processing Course - J. Salzman - Jan. 2002

Microelectronics Processing Oxidation

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

Thermal SiO2 Properties

Thermal SiO2 Properties (cont.)

(7) Amorphous material

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)

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

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

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

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

The goal of oxidation is to grow a high quality oxide layer on a silicon substrate

Goal of Oxidation Process

Passivation

• Physically protects wafers from scratches and particle ..contamination

• Traps mobile ions in oxide layer

Functions of Oxide Layers (1)

Masking

• During Diffusion, Ion Implantation, and Etching

Function of Oxide Layers (2)

Insulating Material

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

Dielectric Material

• Insulating material between metal layers- Field Oxide

Dielectric Material

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

Functions and Thickness of Oxide Layers

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

Oxidation occurs in tube furnace- Vertical Tube Furnace- Horizontal Tube Furnace

Thermal Oxidation Equipment

Bubbler

Wet Thermal Oxidation Techniques

Flash System

Dryox System

Thickness of Si consumed during oxidation

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

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)

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)

Deal-Grove Model (1)

Limiting cases in Si oxidation

(a) (b)a) Interface reaction is the rate limiting stepb) Limited by oxidant transport through the SiO2 rate

Deal-Grove Model Parameters

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)

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)

Diffusivities of some materials in silicon glass

Examples

Effect of Xi on Wafer Topography (1)

Effect of Xi on Wafer Topography (2)

Factors that Affect Oxidation

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

High Doping concentration effect

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

Origin of Substrate Orientation Effect

Substrate Orientation Effect - Oxidation Charts

(a) (b)Growth of SiO2 on <100> and <111> oriented Si wafers:(a) dry oxygen; (b) steam.

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

High Pressure Oxidation

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

Oxidation With Cl Containing Gas

Effect of HCl on Oxidation Rate

Local Oxidation of Si (LOCOS)

Local Oxidation

Dopant Redistribution During Thermal Oxidation (1)

Dopant concentration Dopant concentration

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

a) boronb) boron withhydrogen ambientc) Phosphorusd) gallium

Thin Oxide Growth

Structure of SiO2-Si Interface

Thin Oxide Tunneling Current Comparison

Polycrystalline Si Oxidation

Polysilicon Oxidation

Oxide inspection techniques

Surface Inspection

Oxide Thickness

Oxide Cleanliness

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

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

Rapid Thermal Oxidation Equipment

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

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

1 microelectronics processing course - j. salzman - jan. 2002 microelectronics processing oxidation

oxidation tube

dry thermal oxidation

dry oxidation si solid

thermal sio

solid slide

gas sio

dry gas

dry oxygen

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