This disclosure relates to a spacer structure of a field effect transistor. An exemplary structure for a field effect transistor includes a substrate; a gate structure that has a sidewall overlying the substrate; a silicide region in the substrate on one side of the gate structure having an inner edge closest to the gate structure; a first oxygen-sealing layer adjoining the sidewall of the gate structure; an oxygen-containing layer adjoining the first oxygen-sealing layer on the sidewall and further including a portion extending over the substrate; and a second oxygen-sealing layer adjoining the oxygen-containing layer and extending over the portion of the oxygen-containing layer over the substrate, wherein an outer edge of the second oxygen-sealing layer is offset from the inner edge of the silicide region.