In this paper, the effect of chemical-mechanical polishing (CMP) of silicon nitride films was investigated using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Silicon nitride films with different deposition methods (PECVD and LPCVD) and two different silica-based slurries were studied. The surface chemical structure of as-deposited LPCVD him is slightly better than that of as-deposited PECVD film, and the surface morphologies (RMS roughness similar to 0.3 nm for LPCVD him, and 1.9 nm for PECVD film) are much different. After CMP, the LPCVD film was more dramatically oxidized, and the surface became rougher for the process conditions used. The PECVD film was less dramatically oxidized, a subsilicon nitride (SiNx) appeared on the surface, and the surface became smoother. These results suggest that the quality of as-deposited nitride firm, as well as the CMP process conditions, impact significantly on the quality of the polished surface.