Patent ID: 8431449

Claim:
A manufacturing method of a semiconductor device comprising the steps of: forming a first insulating film over a substrate; forming a first mask over the first insulating film; performing a slimming process on the first mask, so that a second mask is formed; performing an etching process on the first insulating film by using the second mask, so that a second insulating film is formed; forming a first conductive film over the second insulating film and the substrate; performing a polishing process on the first conductive film and the second insulating film, so that a third insulating film, a source electrode, and a drain electrode are formed; forming an oxide semiconductor film over the third insulating film, the source electrode, and the drain electrode; forming a gate insulating film over the oxide semiconductor film; and forming a gate electrode in a region which is over the gate insulating film and overlaps with the third insulating film.