Method for producing semiconductor device

The present invention provides a method for producing a semiconductor device using a self-aligned shallow trench isolation process isolating elements formed so as to be self-aligned to a gate structure, the method comprising the steps of: providing a first polysilicon layer overlying a gate insulator layer on a substrate; forming a trench through the first polysilicon layer, and into the substrate; providing an oxide layer overlying the substrate including the trench such that a top surface of the oxide layer within the trench is higher than a bottom surface of the first polysilicon layer; providing a second polysilicon layer overlying the oxide layer such that a top surface of the second polysilicon layer within the trench is lower than the top surface of the first polysilicon layer; and planarizing the second polysilicon layer, the oxide layer, and the first polysilicon layer, while stopping the step of planarizing at the top surface of the second polysilicon layer within the trench.