A method includes forming a gate structure across a channel region from a top view, the gate structure comprising a work function metal and a gate dielectric layer wrapping around the work function metal, the gate dielectric layer having a U-shaped cross-sectional profile; performing a first plasma etching process, by using a chlorine-containing reactant, on the gate structure; performing a second plasma etching process, by using a bromine-containing, reactant on the gate structure.