A semiconductor device structure and methods of forming the same are described. The structure includes a gate dielectric layer disposed over a substrate, a gate electrode layer disposed over the gate dielectric layer, and a first gate spacer disposed adjacent the gate dielectric layer. The first gate spacer includes an inner surface facing the gate dielectric layer and an outer surface opposite the inner surface, and the first gate spacer includes a fluorine concentration that decreases from the inner surface and the outer surface towards a center of the first gate spacer. The structure further includes a second gate spacer disposed on the outer surface of the first gate spacer, and the second gate spacer includes a fluorine concentration that decreases from an outer surface towards an inner surface.