ISAT 436ISAT 436Micro-/Nanofabrication Micro-/Nanofabrication

and Applicationsand Applications

Thermal OxidationThermal OxidationDavid J. Lawrence

Spring 2004

Thermal Oxidation of SiliconThermal Oxidation of SiliconUpon exposure to oxygen, the surface of a

silicon wafer oxidizes to form silicon dioxide (SiO2).

This silicon dioxide is in the form of a thin film. It is a high-quality electrical insulator. It can also be used as a barrier to block impurity

diffusion.These properties of SiO2 are largely responsible

for the dominant role of silicon in the microelectronics industry.

Thermal Oxidation of SiliconThermal Oxidation of SiliconElevated temperatures speed up the oxidation

process.This thermal oxidation process is usually carried

out at 900 to 1200°C …… in an atmosphere of either oxygen or water

vapor.

22 SiOOSi

222 H2SiOOH2Si See Jaeger, Chapter 3, page 43.

Thermal Oxidation of SiliconThermal Oxidation of Silicon

Silicon is consumed as the oxide grows.As a result, approximately 54% of the final oxide

film is above the original surface of the silicon wafer, and ...

… approximately 46% of the final oxide film is below the original surface of the silicon wafer.

See Jaeger, Chapter 3, page 44.

Thermal Oxidation of SiliconThermal Oxidation of SiliconThe oxidation reaction occurs at the silicon

wafer surface.Oxygen or water vapor must diffuse through the

growing SiO2 film in order to reach silicon, with which it can combine to form more SiO2 .

As the oxide grows, oxygen (or water vapor) must diffuse through more and more SiO2 before it can reach unoxidized silicon.

Therefore, the oxide growth rate decreases as time goes on.

See Jaeger, Chapter 3, pages 44 to 46.

Thermal Oxidation of SiliconThermal Oxidation of SiliconThe silicon crystal “orientation” affects the

number of bonds/cm2 available at the wafer surface.

The most common wafer orientations are denoted (100) and (111). (These identifications are called “Miller indices”.)

The oxide growth rate and the quality of the Si- SiO2 interface (boundary) depend on the wafer orientation.

Thermal Oxidation of SiliconThermal Oxidation of Silicon

The SiO2 thickness for a given wafer orientation and given oxidation conditions (temperature and oxidant (O2 or H2O)) can be calculated … … or the SiO2 thickness can be determined from a graph.See graphs in Jaeger, Chapter 3, page 49.

Thermal Oxidation of SiliconThermal Oxidation of SiliconSiO2 can also be used as a barrier to block impurity diffusion.When used in this way, the SiO2 layer is called a “diffusion mask”.In the diagram below, phosphorus was diffused through the “window” (hole) in the SiO2 layer.

The “masking properties” of SiO2 are described on pages 51-53. (See graph on page 53.)

Cross section: p-type substrate

SiO2“window”

n-type