Semiconductor device and method for manufacturing the same

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

• forming an oxide insulating film;
• forming an oxide semiconductor film over the oxide insulating film;
• forming a gate insulating film over the oxide semiconductor film;
• forming a gate electrode layer and an insulating film overlapping with the oxide semiconductor film over the gate insulating film;
• selectively introducing a dopant into the oxide semiconductor film using the gate electrode layer and the insulating film as masks;
• forming sidewall insulating layers covering side surfaces of the gate electrode layer and side surfaces of the insulating film over the gate insulating film;
• forming a conductive film over the oxide semiconductor film, the gate insulating film, the gate electrode layer, the insulating film, and the sidewall insulating layers;
• forming an interlayer insulating film over the conductive film;
• and
• removing the interlayer insulating film and the conductive film by a chemical mechanical polishing method to expose the insulating film over the gate electrode layer and form a source electrode layer and a drain electrode layer.

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

• forming an oxide insulating film;
• forming an oxide semiconductor film over the oxide insulating film;
• forming a gate insulating film over the oxide semiconductor film;
• forming a gate electrode layer and an insulating film overlapping with the oxide semiconductor film over the gate insulating film;
• selectively introducing a dopant into the oxide semiconductor film using the gate electrode layer and the insulating film as masks;
• forming sidewall insulating layers covering side surfaces of the gate electrode layer and side surfaces of the insulating film over the gate insulating film;
• forming a conductive film over the oxide semiconductor film, the gate insulating film, the gate electrode layer, the insulating film, and the sidewall insulating layers;
• forming an interlayer insulating film over the conductive film;
• and
• removing the interlayer insulating film and the conductive film by a chemical mechanical polishing method to expose the gate electrode layer and form a source electrode layer and a drain electrode layer.