Patent ID: 8105903

Claim:
A method for making a trench MOSFET with a thick trench bottom oxide, comprising: forming a plurality of gate trenches in an epitaxial layer of a first conductivity doping type; forming a first insulation layer along an inner surface of said gate trenches and a top surface of said epitaxial layer; depositing a layer of un-doped poly silicon or amorphous silicon onto said first insulation layer; depositing a nitride layer onto said un-doped poly silicon or amorphous silicon and carrying out a nitride anisotropic etching to leave said nitride layer only on sidewalls of said gate trenches; oxidizing said un-doped poly silicon or amorphous silicon on bottoms of said gate trenches to form said thick trench bottom oxide having thickness greater than said first insulation layer along the sidewalls of said gate trenches; and removing said nitride layer from the sidewalls of said gate trenches.