A method of manufacturing a semiconductor device, the method including providing a semiconductor substrate; forming a gate pattern on the semiconductor substrate such that the gate pattern includes a gate dielectric layer and a sacrificial gate electrode; forming an etch stop layer and a dielectric layer on the semiconductor substrate and the gate pattern; removing portions of the dielectric layer to expose the etch stop layer; performing an etch-back process on the etch stop layer to expose the sacrificial gate electrode; removing the sacrificial gate electrode to form a trench; forming a metal layer on the semiconductor substrate including the trench; removing portions of the metal layer to expose the dielectric layer; and performing an etch-back process on the metal layer to a predetermined target.