A method for forming a semiconductor device is provided. A plurality of trenches are formed in the substrate. An isolation oxide layer is formed in the trenches and on the substrate. A shield polysilicon is deposited in the trenches and on the isolation oxide layer on the substrate. A first etching process is performed to remove a first portion of the shield polysilicon. A first removal process is performed to remove a first portion of the isolation oxide layer. A second etching process is performed to remove a second portion of the shield polysilicon. A second removal process is performed to remove a second portion of the isolation oxide layer. An inter-poly oxide layer is formed on the remaining shield polysilicon and the remaining isolation oxide layer, wherein the inter-poly oxide layer has a concave top surface.