Conversion of Solution-Derived Perhydropolysilazane Thin Films into Silica in Basic Humid Atmosphere at Room Temperature

Spin‐on perhydropolysilazane (PHPS) thin films were converted into mechanically hard silica thin films by an exposure to the vapor from aqueous ammonia. Infrared absorption and X‐ray photoelectron spectroscopic analyses were conducted to clarify the details of the PHPS‐to‐silica conversion and the nature of the silica thin film products. The PHPS‐to‐silica conversion was found to proceed rapidly between 2 and 3 h of exposure via a reaction‐limited process, where the refractive index and the pencil hardness greatly decreased and increased, respectively. Finally, the O/Si mole ratio close to 2 was achieved, which has never been realized in literature for PHPS‐derived silica thin films. It was also found that the condensation of Si–OH groups proceeds in films immediately after PHPS hydrolysis, which is similar to the base‐catalyzed hydrolysis–condensation reaction of silicon alkoxides. Although the silica thin films obtained had refractive indices similar to that of silica glass, high pencil hardness over 9H on Si(100) substrates, and O/Si mole ratios close to 2, it was concluded that they are nonidentical to silica glass, containing trace amounts of Si–OH and Si–H groups.