A semiconductor device includes a gate stack, an epitaxy structure, a first spacer, a second spacer, and a dielectric residue. The gate stack is over a substrate. The epitaxy structure is formed raised above the substrate. The first spacer is on a sidewall of the gate stack. The first spacer and the epitaxy structure define a void therebetween. The second spacer seals the void between the first spacer and the epitaxy structure. The dielectric residue is in the void and has an upper portion and a lower portion under the upper portion. The upper portion of the dielectric residue has a silicon-to-nitrogen atomic ratio higher than a silicon-to-nitrogen atomic ratio of the lower portion of the dielectric residue.