A semiconductor device comprises at least one gate structure disposed on a substrate; a first dielectric layer disposed on the substrate and contacting an outer sidewall of the at least one gate structure; a second dielectric layer having a L shape disposed on the first dielectric layer and contacting the outer sidewall of the at least one gate structure; an etch stop layer contacting the second dielectric layer, the first dielectric layer and the substrate, wherein the second dielectric layer has an upper portion and a lower portion contacting the upper portion, the upper portion extends along the outer sidewall, the lower portion extends from the outer sidewall to the etch stop layer; and an air gap between the second dielectric layer and the etch stop layer; wherein the first dielectric layer and the lower portion of the second dielectric layer have a same width.