A method for forming a semiconductor device includes providing a substrate, forming an oxide layer over the substrate, forming a plurality of first gate oxide layers by etching the oxide layer, forming a second gate oxide layer between adjacent first gate oxide layers, forming a silicon layer over the plurality of first gate oxide layers and the second gate oxide layer, and etching the plurality of first gate oxide layers, the silicon layer, and the second gate oxide layer to expose the substrate, thereby forming a plurality of gate structures. The first gate oxide layer of the plurality of first gate oxide layers has sloped sidewalls. A thickness of the second gate oxide layer is less than a thickness of the first gate oxide layer. Each gate structure includes an etched first oxide layer, a portion of the second gate oxide layer, and a portion of the silicon layer.