Oxidation and diffusion

In LOCOS, what is the phenomenon called in which a thin nitride forms on the silicon surface under the nitride mask, later causing the gate oxide there to be locally thin or defective?

  1. ADishing
  2. BBird's beak
  3. CKooi effect (white ribbon)
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Answer: C. Kooi effect (white ribbon)

During field oxidation, the nitride reacts with steam to form ammonia and the like, which can nitride the silicon surface under the edge of the nitride mask. This nitride makes the oxide thinner there during later gate oxidation, causing breakdown failures. It is called the Kooi effect and is countered by growing and removing a sacrificial oxide.

  • A. Dishing is the recessing of a line's center during CMP.
  • B. Bird's beak is the oxide encroaching under the edge of the nitride mask.
  • C. Correct. A nitride locally degrades the gate oxide.
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More Oxidation and diffusion questions

  1. Why does the compressive stress in thermally grown oxide become smaller when oxidation is done above about 950 °C?
  2. Why can SiO2 serve as a diffusion mask against boron and phosphorus but works poorly as a mask against gallium and aluminum?
  3. In regions where the impurity concentration far exceeds the intrinsic carrier density, the diffusion coefficient of arsenic and others varies with concentration. Which statement about this is correct?
  4. Which correctly describes the thermal oxidation of heavily doped polycrystalline silicon (polysilicon)?
  5. Which correctly compares dry oxidation (O2) and wet oxidation (H2O) in the thermal oxidation of silicon?