Patent ID: 8637347

Claim:
A method for manufacturing a semiconductor device, the method comprising the steps of: forming a gate electrode layer over a substrate having an insulating surface; forming a gate insulating layer over the gate electrode layer; forming an oxide semiconductor layer over the gate insulating layer; forming a metal film over and in contact with the oxide semiconductor layer; etching the metal film to form an exposed region in the oxide semiconductor layer and to form a source electrode layer and a drain electrode layer over and in contact with the oxide semiconductor layer; performing plasma treatment on the exposed region in the presence of a gas containing an oxygen element; and forming an oxide insulating film over and in contact with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer, wherein the etching is performed so that the exposed region of the oxide semiconductor layer has a smaller thickness than a region of the oxide semiconductor layer which is covered with the source electrode layer or the drain electrode layer.