A method for manufacturing a semiconductor device includes forming a dummy gate structure on a semiconductor fin; forming a plurality of gate spacers on opposite sidewalls of the dummy gate structure; removing the dummy gate structure from the semiconductor fin; forming a gate structure on the semiconductor fin and between the gate spacers, wherein the gate structure comprises a gate dielectric layer and a work function metal over the gate dielectric layer; performing a first plasma etching process by using a first reactant to etch back the gate structure performing a second plasma etching process by using a second reactant on the etched-back gate structure, wherein the first plasma etching process has a first removal rate of the gate dielectric layer, the second plasma etching process has a second removal rate of the gate dielectric layer, and the second removal rate is greater than the first removal rate.