Oxidation and diffusion

In thermal diffusion, which pair correctly gives the profile shapes for diffusion with a constant surface concentration and for diffusion of a fixed amount of impurity?

  1. AConstant surface concentration: complementary error function (erfc); fixed amount: Gaussian
  2. BBoth are linear profiles
  3. CConstant surface concentration: Gaussian; fixed amount: complementary error function
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Answer: A. Constant surface concentration: complementary error function (erfc); fixed amount: Gaussian

In predeposition, where the surface concentration is held constant, the profile has the shape of a complementary error function (erfc). In drive-in, where a fixed amount of introduced impurity is pushed deeper, the total stays constant as it spreads, giving a Gaussian profile.

  • A. Correct. Constant surface concentration gives erfc; a fixed amount gives a Gaussian.
  • B. Diffusion profiles are not linear.
  • C. The pairing is reversed.
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