Patent ID: 8878177

Claim:
A method for manufacturing a semiconductor device comprising the steps of: forming an island-shaped oxide semiconductor layer; forming a first conductive film covering the island-shaped oxide semiconductor layer; removing the first conductive film in a region overlapping with the island-shaped oxide semiconductor layer by a chemical mechanical polishing method, and forming an opening in the first conductive film; processing the first conductive film having the opening, and forming a source electrode layer and a drain electrode layer; forming a gate insulating layer over the island-shaped oxide semiconductor layer, the source electrode layer, and the drain electrode layer; forming a gate electrode layer over the gate insulating layer, wherein the gate electrode layer overlaps with the island-shaped oxide semiconductor layer; introducing an impurity into the island-shaped oxide semiconductor layer using the gate electrode layer as a mask, and forming a first impurity region and a second impurity region in the island-shaped oxide semiconductor layer; forming a second conductive film over the gate insulating layer and the gate electrode layer; forming an insulating layer over the second conductive film; processing the insulating layer, and forming a sidewall insulating layer in contact with a side surface of the second conductive film, wherein the sidewall insulating layer is not in contact with the gate electrode layer; and etching the second conductive film using the sidewall insulating layer as a mask, and forming a conductive layer having a first portion in contact with a side surface of the gate electrode layer and a second portion in contact with an upper surface of the gate insulating layer.