Semiconductor device having field plate electrode and method for manufacturing the same

A method for manufacturing a semiconductor device, comprising:

• forming a second semiconductor layer of a first conductivity type on a first semiconductor layer of the first conductivity type;
• forming a trench in the second semiconductor layer;
• forming a first conductive film in the trench through a first insulating film so as to bury the trench;
• removing the first insulating film of the upper portion of the trench on an opening side of the trench;
• oxidizing the side surface of the upper portion of the exposed trench and the entire upper portion of the exposed first conductive film so as to form a gate insulating film and a second insulating film;
• forming a gate electrode between the gate insulating film and the second insulating film;
• forming a third semiconductor layer of the second conductivity type on the upper portion of the second semiconductor layer;
• and
• forming a fourth semiconductor layer of the first conductivity type on the upper portion of the third semiconductor layer.