US Patent 8076735 Semiconductor device with trench of various widths

A semiconductor device and a method for fabricating the same are described. A polysilicon layer is formed on a substrate. The polysilicon layer is doped with an N-type dopant. A portion of the polysilicon layer is then removed to form a plurality of dummy patterns. Each dummy pattern has a top, a bottom, and a neck arranged between the top and the bottom, where the width of the neck is narrower than that of the top. A dielectric layer is formed on the substrate to cover the substrate disposed between adjacent dummy patterns, and the top of each dummy pattern is exposed. Thereafter, the dummy patterns are removed to form a plurality of trenches in the dielectric layer. A plurality of gate structures is formed in the trenches, respectively.