A method for forming a semiconductor device includes forming a dielectric layer over a substrate. The method further includes forming a select gate layer over the dielectric layer. The method further includes etching the select gate layer at a first etch rate to form a first portion of a sidewall of a select gate, wherein the step of etching the select gate layer at the first etch rate includes using an oxidizing agent to oxidize at least a top portion of the substrate underlying the dielectric layer to form an oxide layer. The method further includes etching the select gate layer at a second etch rate lower than the first etch rate to form a second portion of the sidewall of the select gate, wherein the step of etching the select gate layer at the second etch rate includes removing only a top portion of the dielectric layer.