Semiconductor Device and Manufacturing Method of the Same

A method for manufacturing a semiconductor device including a field-effect transistor, comprising steps of:

• (a) forming a trench in a semiconductor substrate;
• (b) forming a first insulating film in the trench;
• (c) forming an intrinsic polycrystalline silicon film over the first insulating film;
• (d) introducing impurities of a first conductivity type into the intrinsic polycrystalline silicon film to form a first conductive film;
• (e) after the step (d), etching the first conductive film so as to provide a portion of the first conductive film in a lower part of the trench, thereby to form a first gate electrode of the field-effect transistor;
• (f) forming a second insulating film in the trench, wherein the second insulating film is disposed above the first insulating film and the first gate electrode;
• (g) forming a doped polycrystalline silicon film of the first conductivity type, having higher impurity concentration than the first gate electrode, over the second insulating film;
• and
• (h) etching the doped polycrystalline silicon film so as to provide a portion of the doped polycrystalline silicon film in an upper part of the trench, thereby to form a second gate electrode of the field-effect transistor.