Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device comprising the steps of:

• forming a gate electrode layer;
• forming a gate insulating layer over the gate electrode layer;
• performing heat treatment on the gate insulating layer to reduce hydrogen in the gate insulating layer;
• forming an oxide semiconductor layer overlapping with the gate electrode layer with the gate insulating layer interposed therebetween;
• forming a source electrode layer and a drain electrode layer which are electrically connected to the oxide semiconductor layer:
• forming an insulating layer over the source electrode layer and the drain electrode layer and in contact with the oxide semiconductor layer;
• forming an aluminum film in contact with the insulating layer;
• and
• performing an oxygen doping treatment on the insulating layer and the aluminum film, so that at least a portion of the aluminum film is oxidized to be an aluminum oxide film.