Manufacturing method of oxide semiconductor device

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

• forming a source electrode and a drain electrode on an insulating surface;
• forming an oxide semiconductor film over the source electrode and the drain electrode;
• forming a gate insulating film over the oxide semiconductor film;
• forming a gate electrode over the gate insulating film;
• reducing a resistance of part of the oxide semiconductor film to form a low-resistance region;
• forming an interlayer insulating film covering the oxide semiconductor film, the gate insulating film, and the gate electrode;
• forming an opening in the interlayer insulating film reaching the oxide semiconductor film;
• performing nitrogen plasma treatment on the oxide semiconductor film through the opening in the interlayer insulating film;
• and
• forming a metal film in the opening and over the interlayer insulating film.