Some embodiments of the present disclosure provide a semiconductor device including a semiconductive substrate, a metal gate including a metallic layer proximal to the semiconductive substrate. A dielectric layer surrounds the metal gate. The dielectric layer includes a first surface facing the semiconductive substrate and a second surface opposite to the first surface. A sidewall spacer surrounds the metallic layer with a greater longitudinal height. The sidewall spacer is disposed between the metallic layer and the dielectric layer. An etch stop layer over the metal gate comprises a surface substantially coplanar with the second surface of the dielectric layer. The etch stop layer has a higher resistance to etchant than the dielectric layer. A portion of the etch stop layer is over the sidewall spacer.