A method for fabricating a semiconductor device is disclosed. In an embodiment, the method may include providing a semiconductor substrate; forming gate material layers over the semiconductor substrate; forming a hard mask layer over the gate material layers; patterning the hard mask layer to from a hard mask pattern; forming a spacer layer over the hard mask pattern; etching back the spacer layer to form spacers over sidewalls of the hard mask pattern; etching the gate material layers by using the spacers and the hard mask pattern as an etching mask to form a gate structure; and performing a tilt-angle ion implantation process to the semiconductor substrate.