In an embodiment, a method of forming a semiconductor device includes forming a dummy gate stack over a substrate; forming a first spacer layer over the dummy gate stack; oxidizing a surface of the first spacer layer to form a sacrificial liner; forming one or more second spacer layers over the sacrificial liner; forming a third spacer layer over the one or more second spacer layers; forming an inter-layer dielectric (ILD) layer over the third spacer layer; etching at least a portion of the one or more second spacer layers to form an air gap, the air gap being interposed between the third spacer layer and the first spacer layer; and forming a refill layer to fill an upper portion of the air gap.