Patent ID: 8093152

Claim:
A trench forming method, comprising: a substrate preparation step of preparing an SOI substrate in which an insulating layer and a silicon semiconductor layer are laid one over another on a support substrate; an oxide film forming step of forming an oxide film on a surface of said silicon semiconductor layer; an oxide film removing step of removing a portion of said oxide film; and an etching step of etching, using at least two types of etching gases that have different compositions to each other, said surface of said silicon semiconductor layer with said oxide film as a mask to form a trench that reaches said insulating layer and has a constant cross-sectional area along its entire depth, wherein said etching step includes at least three etching steps, each etching said silicon semiconductor layer with a respective depth to form a respective portion of the trench by using, as an etching gas, one of the at least two types of etching gases.