Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device comprising:

• forming a gate electrode;
• stacking a gate insulating film, a semiconductor film, an impurity semiconductor film, and a conductive film in this order to cover the gate electrode;
• processing the conductive film to form a source electrode and a drain electrode;
• processing an upper part of the semiconductor film to form a source region, a drain region, and a semiconductor layer, the upper part of a portion of which does not overlap with the source region and the drain region is removed, while the impurity semiconductor film is divided;
• forming a passivation film over the gate insulating film, the semiconductor layer, the source region, the drain region, the source electrode, and the drain electrode;
• forming an etching mask over the passivation film;
• processing the passivation film, the semiconductor layer, and the gate insulating film to have an island shape, while an opening reaching the source electrode or the drain electrode is formed, with the use of the etching mask;
• removing the etching mask;
• and
• forming an electrode over the gate insulating film and the passivation film,
• wherein before the semiconductor film is formed, a surface of the gate insulating film is exposed to plasma generated using a gas containing oxygen.