Semiconductor device and method of forming the same

A method of forming a semiconductor device includes performing a first pre-amorphous implantation process on a substrate, where the substrate has a gate stack. The method further includes forming a first stress film over the substrate. The method also includes performing a first annealing process on the substrate and the first stress film. The method further includes performing a second pre-amorphous implantation process on the annealed substrate, forming a second stress film over the substrate, and performing a second annealing process on the substrate and the second stress film.