Patent ID: 6905944

Claim:
A method of defining an upper portion and a lower portion of a trench etched into a semiconductor substrate to permit said lower portion to be processed differently from said upper portion, comprising: etching a trench into a semiconductor substrate; forming a nitride layer over a sidewall of said trench; forming an oxide layer over said nitride layer; depositing and recessing a filler material to cover said oxide layer in a lower portion of said trench; removing said oxide layer to expose said nitride layer in an upper portion of said trench above said filler material; removing said filler material from said trench while leaving said oxide layer and said nitride layer in said lower portion of said trench; depositing silicon selectively on said exposed nitride layer in said upper portion of said trench; removing said oxide layer and said nitride layer from said lower portion of said trench; processing said lower portion of said trench selectively to nitride; and subsequently processing said upper portion of said trench.