A method for manufacturing a semiconductor device, includes: forming a dummy gate structure on a semiconductor substrate; forming a plurality of gate spacers on opposite sidewalls of the dummy gate structure; removing the dummy gate structure from the semiconductor substrate; forming a metal gate electrode on the semiconductor substrate and between the gate spacers; and performing a plasma etching process to the metal gate electrode, wherein the plasma etching process comprises performing in sequence a first non-zero bias etching step and a first zero bias etching step.