A stack structure for forming a gate of a MOS transistor includes a substrate including a plurality of shallow trench isolations therein; a dielectric layer, a conductive layer and a hard mask layer formed on the substrate in sequence; and a tri-layer stack comprising a top photo resist layer, a silicon-containing photo resist layer and a bottom anti-reflective coating (BARC) on the hard mask layer, wherein the silicon-containing photo resist layer comprises 10-30% silicon and the hard mask layer has a high etching selectivity ratio to the conductive layer.