A semiconductor MOS device includes a semiconductor substrate; a gate oxide layer disposed on the semiconductor substrate; a fully silicided gate electrode disposed on the gate oxide layer; a composite thin film interposed between the fully silicided gate electrode and the gate oxide layer; a spacer on sidewall of the fully silicided gate electrode; and a source/drain region implanted into the semiconductor substrate next to the spacer. A method for forming the semiconductor MOS device is disclosed.