Patent ID: 8236635

Claim:
A method for manufacturing a semiconductor device, comprising: forming a gate electrode layer over a substrate having an insulating surface; stacking a gate insulating layer, an oxide semiconductor film, and a conductive film over the gate electrode layer; forming a first mask layer over the gate insulating layer, the oxide semiconductor film, and the conductive film; performing a first etching with the first mask layer to etch the oxide semiconductor film and the conductive film so that an oxide semiconductor layer and a conductive layer are formed; forming a second mask layer by ashing the first mask layer; and performing a second etching with the second mask layer to etch the oxide semiconductor layer and the conductive layer so that an oxide semiconductor layer having a depression, a source electrode layer, and a drain electrode layer are formed, wherein the first mask layer is formed using a light-exposure mask, wherein each of the first etching and the second etching is dry etching with use of an etching gas, wherein the oxide semiconductor layer having the depression includes a region with a smaller thickness than a region overlapping with the source electrode layer or the drain electrode layer, wherein first portions of the oxide semiconductor layer extend beyond outer side edges of the source electrode layer and the drain electrode layer, and wherein the first portions of the oxide semiconductor layer are thinner than second portions of the oxide semiconductor layer, the second portions being below the source electrode layer and the drain electrode layer.