Patent ID: 8207025

Claim:
A manufacturing method of a semiconductor device comprising the steps of: forming a first insulating film over a surface; 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 using the second mask, so that a second insulating film is formed; forming a first conductive film over the second insulating film; performing a polishing process on the first conductive film and the second insulating film, so that a second conductive film and a third insulating film having equal thicknesses are formed; etching the second conductive film, so that a source electrode and a drain electrode which are thinner than the second conductive film are formed; forming an oxide semiconductor film in contact with 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.