Patent ID: 6846721

Claim:
A method of manufacturing a semiconductor device, comprising the steps of: forming a mask layer having a first aperture on a main surface of a semiconductor substrate including silicon; selectively removing a part of said semiconductor substrate along said first aperture by using said mask layer as a mask to form a recess defined by a side surface and a bottom surface in said semiconductor substrate; oxidizing the side surface and the and the bottom surface of said recess to form a first silicon oxide film having a side portion and a bottom portion; selectively removing the bottom portion of said first silicon oxide film and a part of said semiconductor substrate by using said mask layer as a mask to form a trench; and thermal oxidizing a surface of said trench, the method further comprising the steps of: forming a second silicon oxide film on a main surface of said semiconductor substrate prior to the step of forming said mask layer, wherein the step of forming said recess includes the step of removing a cart of said second silicon oxide film to form a second aperture continuing from said first aperture in said second silicon oxide film and the step of removing a part of said semiconductor substrate to form said recess continuing from said second aperture; and forming a polysilicon layer between said second silicon oxide film and said mask layer, wherein the step of forming said recess includes the step of removing a part of said polysilicon layer to form a third aperture continuing from said first aperture in said polysilicon layer and the step of forming a second aperture and said recess continuing from said third aperture, and wherein the step of forming said first silicon oxide film includes the step of oxidizing the portion of said polysilicon layer that defines said third aperture to form said first silicon oxide film.