A method of making a semiconductor device includes forming a gate stack on a substrate. The method further includes depositing a first spacer layer on a sidewall of the gate stack. The first spacer layer includes silicon and carbon. The method includes performing a first nitrogen plasma treatment process on the first spacer layer to increase a density of the first spacer layer. The method further includes depositing a second spacer layer on the first spacer layer. The second spacer layer includes silicon, carbon, and nitrogen.