Manufacturing method of semiconductor device

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

• forming a base insulating film;
• forming an oxide semiconductor layer over and in contact with the base insulating film;
• forming a source electrode layer and a drain electrode layer over the oxide semiconductor layer;
• forming a gate insulating film over and in contact with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer;
• forming a gate electrode layer over the gate insulating film in a region overlapping with the oxide semiconductor layer;
• forming an aluminum oxide film over and in contact with the gate insulating film and the gate electrode layer;
• and
• performing heat treatment on the oxide semiconductor layer,
• wherein a thickness of the aluminum oxide film is greater than 50 nm and less than or equal to 500 nm.

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

• manufacturing a transistor by a method comprising the steps of:
• forming a base insulating film;
• forming an oxide semiconductor layer over and in contact with the base insulating film;
• forming a source electrode layer and a drain electrode layer over the oxide semiconductor layer;
• forming a gate insulating film over and in contact with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer;
• forming a gate electrode layer over the gate insulating film in a region overlapping with the oxide semiconductor layer;
• and
• forming an aluminum oxide film over and in contact with the gate insulating film and the gate electrode layer;
• and
• performing heat treatment on the transistor,
• wherein a thickness of the aluminum oxide film is greater than 50 nm and less than or equal to 500 nm.