Patent ID: 7709320

Claim:
A method comprising: forming a trench in a semiconductor substrate, a top surface of said substrate defining a horizontal direction, said trench having contiguous upper, middle, lower, and bottom regions, said middle region between said upper and lower regions, said lower region abutting said bottom region, said trench extending from said top surface of said substrate into said substrate in a vertical direction perpendicular to said top surface of said semiconductor substrate, said upper region of said trench adjacent to said top surface of said semiconductor substrate and having a vertical sidewall profile and a first width in said horizontal direction at a first junction of said upper region and said middle region, said middle region of said trench tapering from said first width to a second width in said horizontal direction at a juncture of said middle region and said lower region, said first width being greater than said second width, said lower region having a vertical sidewall profile, said bottom region being bulbous and having a maximum width measured in said horizontal direction that is greater than said second width.