Developing techniques to form functional films at low temperatures is crucial for enabling their application on materials with limited heat resistance. In this study, silica films were prepared by irradiating a perhydropolysilazane (PHPS) precursor, which contains reactive Si–H and N–H groups and is free from organics, with vacuum ultraviolet (VUV) light under a controlled atmosphere. To investigate the quality of the resulting films and mechanisms behind photochemical conversion, infrared and electron spectroscopic measurements were performed. The results revealed that VUV irradiation converted the top surface of the film into silica, irrespective of the atmospheric conditions. However, differences in the oxygen concentration affected both the VUV light intensity penetrating the film and the oxygen diffusion into it, leading to the formation of silica or silicon oxynitride within the film. Under irradiation, PHPS was converted to silica not only from the surface but also from the interface with the substrate. In addition, silica was formed even at long irradiation distances, in which case VUV light barely reached the PHPS surface owing to absorption by atmospheric oxygen. These findings demonstrate that the silica-film formation involved two processes: breaking of chemical bonds induced by VUV photons and oxidation due to oxygen diffusion from the surface. The spectroscopic measurements and X-ray reflectivity results indicated that the composition, chemical state, Auger parameter, and density of the prepared silica film were comparable to those of a silica film prepared through heat treatment at 500°C. Further, the prepared silica film using VUV irradiation had a smoother surface than that of the heat-treated film. The thickness of the prepared film remained unchanged before and after VUV irradiation, indicating that the formation process using VUV irradiation suppressed the shrinkage of the film during the conversion of PHPS to silica.