Patent ID: 8883558

Claim:
A method for manufacturing a semiconductor device comprising: forming a gate electrode over a substrate; forming a gate insulating layer over the gate electrode; forming a conductive film over the gate insulating layer; etching the conductive film to form a source electrode and a drain electrode; forming a first oxide semiconductor film over the gate insulating layer, the source electrode and the drain electrode by a sputtering method; forming a second oxide semiconductor film over the first oxide semiconductor film by a sputtering method; forming a mask over the second oxide semiconductor film; and etching the first oxide semiconductor film and the second oxide semiconductor film by using the mask to form a first oxide semiconductor region and a second oxide semiconductor region, wherein the first oxide semiconductor region is provided so that part of the first oxide semiconductor region is in contact with the gate insulating layer and side surface portions of the source electrode and the drain electrode, and wherein a proportion of a flow rate of an oxygen gas in a film-forming gas for forming the second oxide semiconductor film is made higher than a proportion of a flow rate of an oxygen gas in a film-forming gas for forming the first oxide semiconductor film.