Patent ID: 8298858

Claim:
A method for manufacturing a semiconductor device comprising the steps of: forming a gate electrode layer over a substrate; forming a gate insulating layer over the gate electrode layer; forming a conductive film over the gate insulating layer; etching the conductive film to form source and drain electrode layers; forming a first oxide semiconductor film over the gate insulating layer and the source and drain electrode layers by a sputtering method; forming a second oxide semiconductor film over the first oxide semiconductor film by a sputtering method; and etching the first oxide semiconductor film and the second oxide semiconductor film to form an oxide semiconductor layer and a semiconductor layer, wherein the oxide semiconductor layer is provided to be in contact with the gate insulating layer and side face portions of the source and drain electrode layers, wherein the semiconductor layer is provided over the oxide semiconductor layer, and wherein a proportion of a flow rate of an oxygen gas to the whole film-formation gas for forming the second oxide semiconductor film is made lower than a proportion of a flow rate of an oxygen gas to the whole film-formation gas for forming the first oxide semiconductor film.