Patent ID: 8217443

Claim:
A semiconductor device, comprising: a semiconductor layer; a trench dug downward from the surface of the semiconductor layer, the trench having a bottom surface and a side surface; a source region formed in a surface layer portion of the semiconductor layer adjacently to a first side of the trench in a prescribed direction; a drain region formed in the surface layer portion of the semiconductor layer adjacently to a second side of the trench opposite to the first side in the prescribed direction; a first insulating film formed on the bottom surface and the side surface of the trench; a floating gate stacked on the first insulating film and opposed to the bottom surface and the side surface of the trench through the first insulating film, the floating gate extending over the source region and the drain region so as to cover only a portion of the source region and only a portion of the drain region; a second insulating film formed on the floating gate such that the second insulating film has a shape that substantially copies that of the first insulating film; and a control gate that is at least partially embedded in the trench, so that a portion embedded in the trench is opposed to the floating gate through the second insulating film, and that has a planarized top surface, wherein the first insulating film has a thin portion, having a relatively small thickness, in contact with the drain region, and a thick portion, having a relatively large thickness, formed by the remaining portion of the first insulating film other than the thin portion, the thick portion covering the entire bottom surface of the trench.