A semiconductor device and its manufacturing method, wherein the NMOS device is covered by a layer of silicon nitride film having a high ultraviolet light absorption coefficient through PECVD, said silicon nitride film can well absorb ultraviolet light when being subject to the stimulated laser surface anneal so as to achieve a good dehydrogenization effect, and after dehydrogenization, the silicon nitride film will have a high tensile stress; since the silicon nitride film has a high ultraviolet light absorption coefficient, there is no need to heat the substrate, thus avoiding the adverse influences to the device caused by heating the substrate to dehydrogenize, and maintaining the heat budget brought about by the PECVD process.