In our previous study, a-Si thin films were suggested as a passivation layer to prevent the degradation of Si-O-N-F films that were phase shift materials for 157 nm optical lithography. However few studies about the optical properties of a-Si in the deep ultraviolet (DUV) region have been reported. a-Si thin films having thickness of less than 25 nm were deposited using dc magnetron sputtering with a mesh to ensure the uniformity. The thickness, composition and optical properties of a-Si were analyzed with SEM, AES, UV spectroscopy and a VUV spectroscopic ellipsometer. Using the results of spectroscopic ellipsometry (SE) analysis, optical properties, including absorbance, transmittance, optical band gap and optical constants (n, k), of a-Si films with different thickness (11 to approximately 25 nm) were analyzed using the model of EMA(a-Si+SiO)/a-Si (EMA:effective medium approximation). From the SE analysis of a-Si films, we onfirmed that a-Si film with high transmittance in DUV could be used as a passivation layer of Si-O-N-F films.