DENSIFIED GATE SPACERS AND FORMATION THEREOF

A semiconductor device includes a semiconductor fin, a gate structure, a source epitaxial structure, a drain epitaxial structure, a first gate spacer, and a second gate spacer. The semiconductor fin is over a substrate. The gate structure extends across the semiconductor fin. The source epitaxial structure and the drain epitaxial structure are on opposite sides of the gate structure, respectively. The first gate spacer separates the source epitaxial structure from the gate structure. The second gate spacer separates the drain epitaxial structure from the gate structure. The first and second gate spacers are made of an organosilicate glass material having a dielectric constant greater than a dielectric constant of silicon oxide.