Self-aligned differential oxidation in trenches by ion implantation

In accordance with the present invention, a trench MOSFET is formed by creating a trench in a semiconductor substrate. A portion of either a side wall of the trench or the bottom of the trench is implanted with an implant species. An insulating layer is then grown overlying the bottom and side wall of the trench. The implant species is selected such that the insulating layer grows more quickly on the bottom of the trench than on the side wall of the trench, resulting in a thicker insulating layer in the bottom of the trench than on the trench side walls.