Disclosed is a fabrication process of a highly reliable semiconductor device formed by stacking and pattering a polycrystalline silicon film, a tungsten nitride film and a tungsten film over a gate insulator film on a semiconductor substrate, thereby forming gate electrodes. Then, a conductive plasma processing is performed using an ammonia gas at a temperature for the semiconductor substrate of 500 C. or lower, thereby nitriding the side wall for the gate electrode to form a nitride film, and then conducting plasma processing by using an oxygen gas in a state at a temperature for the semiconductor substrate of 500 C. or lower thereby restoring damages or defects in the silicon oxide film present in the surface portion of the semiconductor substrate at the periphery of the gate electrode.