A method includes forming a gate structure on a semiconductor substrate; depositing a carbon-containing seal layer over the gate structure; depositing a nitrogen-containing seal layer over the carbon-containing seal layer; introducing an oxygen-containing precursor on the nitrogen-containing seal layer; heating the substrate to dissociate the oxygen-containing precursor into an oxygen radical to dope into the nitrogen-containing seal layer; after heating the substrate, etching the nitrogen-containing seal layer and the carbon-containing seal layer, such that a remainder of the nitrogen-containing seal layer and the carbon-containing seal layer remains on a sidewall of the gate structure as a gate spacer.