Semiconductor device for power transistor

A semiconductor device manufacturing method comprising the steps of:

• (a):
• preparing a semiconductor substrate over which a drift layer is formed;
• (b) forming a channel layer over the drift layer;
• (c) forming a source region over a surface of the channel layer;
• (d) forming a trench which passes through the source region and the channel layer and reaches the drift layer;
• (e) forming a conductor film over a region from an inner wall of the trench to the source region;
• (f) forming an insulating film over the conductor film;
• (g) performing anisotropic etching on the insulating film;
• (h) after the step (g), oxidizing the conductor film which is exposed from the insulating film and thereby forming a second insulating film;
• (i) after the step (h), removing the insulating film;
• (j) after the step (i), removing the conductor film which is not yet oxidized;
• (k) after the step (j), forming a first insulating film over the exposed inner wall of the trench;
• (l) after the step (k), forming a third insulating film over a region from the first insulating film to the second insulating film;
• (m) after the step (l), forming a gate insulating film which includes the first insulating film, the second insulating film and the third insulating film by performing patterning on the third insulating film;
• and
• (n) after the step (m), forming a gate electrode which is in contact with the gate insulating film.