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ISAT 436 Micro-/Nanofabrication and Applications

ISAT 436 Micro-/Nanofabrication and Applications. Thermal Oxidation David J. Lawrence Spring 2004. Thermal Oxidation of Silicon. Upon exposure to oxygen, the surface of a silicon wafer oxidizes to form silicon dioxide ( SiO 2 ). This silicon dioxide is in the form of a thin film .

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ISAT 436 Micro-/Nanofabrication and Applications

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

  2. Thermal Oxidation of Silicon • Upon 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.

  3. Thermal Oxidation of Silicon • Elevated 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. • See Jaeger, Chapter 3, page 43.

  4. Thermal 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.

  5. Thermal Oxidation of Silicon • The 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.

  6. Thermal Oxidation of Silicon • The 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.

  7. Thermal 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.

  8. “window” SiO2 Cross section: n-type p-type substrate Thermal Oxidation of Silicon • SiO2 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.)

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