A method includes forming a gate structure and an interlayer dielectric (ILD) layer over a substrate; selectively forming an inhibitor over the gate structure; performing an atomic layer deposition (ALD) process to form a dielectric layer over the ILD layer, wherein in the ALD process the dielectric layer has greater growing rate on the ILD than on the inhibitor; and performing an atomic layer etching (ALE) process to etch the dielectric layer until a top surface of the inhibitor is exposed, in which a portion of the dielectric layer remains on the ILD layer after the ALE process is complete.