Semiconductor device having inter-poly oxide layer between gate polysilicon and shield polysilicon and method of forming a semiconductor device

A method of forming a semiconductor device, the method comprising:

• forming trenches in a substrate;
• disposing an isolation oxide layer in the trenches;
• disposing a shield polysilicon in the trenches so that the shield polysilicon is partially surrounded by the isolation oxide layer;
• etching the isolation oxide layer;
• disposing an inter-poly oxide (IPO) layer on an upper surface of the shield polysilicon so that the IPO layer is laterally surrounded in the trenches by the isolation oxide layer;
• etching the IPO layer and the isolation oxide layer;
• lining at least upper sidewalls of the trenches with a gate oxide layer;
• and
• disposing a gate polysilicon on an upper surface of the IPO layer and on an upper surface of the isolation oxide layer, so that the gate polysilicon is laterally surrounded by the gate oxide layer;
• wherein the step of etching the IPO layer and the isolation oxide layer and the step of disposing the gate polysilicon are performed so that the contact surface of the gate polysilicon with the IPO layer and the isolation oxide layer is substantially flat, and the contact surface has a curvature measured along a diameter of the contact surface of less than 5 degrees of its area;
• wherein the step of etching the isolation oxide layer is performed so that a thickness of the isolation oxide layer tapers from a greater thickness near the shield polysilicon to a smaller thickness near a top opening of the trench so that the surface of the isolation oxide layer facing towards the center of the trench has a convex shape;
• and
• wherein the step of etching the IPO layer and the isolation oxide layer is performed at least until silicon is exposed at upper sidewalls of the trenches.