ISAT 436 Micro-/Nanofabrication and Applications Thermal Oxidation David J. Lawrence Spring 2004

Thermal Oxidation of Silicon H Upon exposure to oxygen, the surface of a silicon wafer oxidizes to form silicon dioxide (SiO 2 ). H This silicon dioxide is in the form of a thin film. H It is a high-quality electrical insulator. H It can also be used as a barrier to block impurity diffusion. H These properties of SiO 2 are largely responsible for the dominant role of silicon in the microelectronics industry.

Thermal Oxidation of Silicon H Elevated temperatures speed up the oxidation process. H This thermal oxidation process is usually carried out at 900 to 1200°C … H … in an atmosphere of either oxygen or water vapor. H See Jaeger, Chapter 3, page 43.

Thermal Oxidation of Silicon H Silicon is consumed as the oxide grows. H As a result, approximately 54% of the final oxide film is above the original surface of the silicon wafer, and... H … approximately 46% of the final oxide film is below the original surface of the silicon wafer. H See Jaeger, Chapter 3, page 44.

Thermal Oxidation of Silicon H The oxidation reaction occurs at the silicon wafer surface. H Oxygen or water vapor must diffuse through the growing SiO 2 film in order to reach silicon, with which it can combine to form more SiO 2. H As the oxide grows, oxygen (or water vapor) must diffuse through more and more SiO 2 before it can reach unoxidized silicon. H Therefore, the oxide growth rate decreases as time goes on. H See Jaeger, Chapter 3, pages 44 to 46.

Thermal Oxidation of Silicon H The silicon crystal “orientation” affects the number of bonds/cm 2 available at the wafer surface. H The most common wafer orientations are denoted (100) and (111). (These identifications are called “Miller indices”.) H The oxide growth rate and the quality of the Si- SiO 2 interface (boundary) depend on the wafer orientation.

Thermal Oxidation of Silicon H The SiO 2 thickness for a given wafer orientation and given oxidation conditions (temperature and oxidant (O 2 or H 2 O) ) can be calculated … H … or the SiO 2 thickness can be determined from a graph. H See graphs in Jaeger, Chapter 3, page 49.

Thermal Oxidation of Silicon H SiO 2 can also be used as a barrier to block impurity diffusion. H When used in this way, the SiO 2 layer is called a “diffusion mask”. H In the diagram below, phosphorus was diffused through the “window” (hole) in the SiO 2 layer. H The “masking properties” of SiO 2 are described on pages (See graph on page 53.) Cross section: p-type substrate SiO 2 “window” n-type