Oxidation and diffusion

Which correctly describes the thermal oxidation of heavily doped polycrystalline silicon (polysilicon)?

  1. AIt does not oxidize
  2. BBecause of grain boundaries and high doping, it often oxidizes faster than single crystal, and the oxide quality tends to be inferior
  3. CIt always oxidizes at the same rate as single-crystal silicon
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Answer: B. Because of grain boundaries and high doping, it often oxidizes faster than single crystal, and the oxide quality tends to be inferior

Polysilicon has randomly oriented grains and many grain boundaries, and with high concentrations of dopants such as phosphorus it oxidizes faster. Oxidation proceeding along grain boundaries tends to roughen the interface, and the oxide on top tends to be inferior, for example in breakdown strength, to thermal oxide on single crystal.

  • A. Polysilicon does oxidize.
  • B. Correct. It tends to oxidize faster and give lower-quality oxide.
  • C. Grain boundaries and impurities make the rate differ from single crystal.
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More Oxidation and diffusion questions

  1. Which correctly compares dry oxidation (O2) and wet oxidation (H2O) in the thermal oxidation of silicon?
  2. When an SiO2 film of thickness x is formed by thermally oxidizing silicon, approximately how much silicon is consumed?
  3. How does the diffusion coefficient of dopants in silicon depend on temperature?
  4. Which method is mainly used for device isolation in scaled LSIs?
  5. Once the oxide has become sufficiently thick in thermal oxidation, how does its thickness relate to oxidation time?