Patent ID: 8778729

Claim:
A manufacturing method of a semiconductor device comprising the steps of: forming an insulating film over a surface of a substrate; forming a first mask over the insulating film; performing a slimming process on the first mask, so that a second mask is formed; performing an etching process on the insulating film using the second mask, so that an insulating layer is formed; forming an oxide semiconductor layer covering the insulating layer; forming a conductive film covering the oxide semiconductor layer; performing a polishing process on the conductive film, so that a surface of the conductive film is flattened; performing an etching process on the conductive film, so that a conductive layer is formed and a surface of the conductive layer is lower than a surface of an uppermost part of the oxide semiconductor layer; forming a gate insulating film in contact with the conductive layer and the oxide semiconductor layer; and forming a gate electrode in a region which is over the gate insulating film and overlaps with the insulating layer.