DENSIFIED GATE SPACERS AND FORMATION THEREOF

A method includes following steps. Fins are formed over a substrate. A dummy gate structure is across the fins. A spacer layer is deposited over the dummy gate structure. The spacer layer has a first portion in a void of the dummy gate structure and a second portion outside the void of the dummy gate structure. The second portion of the spacer layer is treated to have a different material composition than the first portion of the spacer layer, and is then etched to form gate spacers on sidewalls of the dummy gate structure. An etching process is performed on the dummy gate structure to form a gate trench between the gate spacers. The etching process etches the first portion of the spacer layer at a faster etch rate than etching the gate spacers. A gate structure is formed in the gate trench.