Patent ID: 7332398

Claim:
A method of manufacturing a trench-gate silicon semiconductor device, the method including forming trenches in a semiconductor body in an active transistor cell area of the device, the trenches each having a trench bottom and trench sidewalls, and providing silicon oxide gate insulation in the trenches such that the gate insulation at the trench bottoms is thicker than the gate insulation at the trench sidewalls, wherein the method includes, after forming the trenches the steps of: forming a silicon oxide layer at the trench bottoms and trench sidewalls; depositing a layer of doped polysilicon adjacent the trench bottoms and trench side walls; forming silicon nitride spacers on the doped polysilicon adjacent the trench sidewalls leaving the doped polysilicon exposed at the trench bottoms; thermally oxidising the exposed doped polysilicon to grow said thicker gate insulation at the trench bottoms; removing the silicon nitride spacers; and providing gate conductive material within the trenches.