Oxidation and diffusion

In LOCOS (local oxidation), what is the phenomenon called in which oxide encroaches like a bird's beak under the edge of the nitride mask?

  1. ADishing
  2. BNotching
  3. CBird's beak
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Answer: C. Bird's beak

In LOCOS, areas covered with nitride are not oxidized, but oxidant diffuses sideways under the mask edge and oxidation proceeds there, so the oxide encroaches in a shape like a bird's beak. This reduces the usable device area, so LOCOS was replaced by STI as scaling progressed.

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