Spinning deposition of silica and silica-titania optical coatings: A round robin test

A round robin test has been performed on sol-gel processing for the deposition of silica and silica-titania films on silicon substrates by spin-coating. Three solution preparation processes for silica coatings and three for silica-titania coatings were used to prepare samples at each of the participating laboratories. The films have been characterized mainly by thickness (profilometry and ellipsometry measurements), refractive index, porosity, and optical scattering. Different processes gave different thicknesses. Thickness differences were found in films prepared by the same process and by the same deposition parameters, but in different laboratories, when heat-treated at 500°C. Variations were reduced in samples annealed at 1000°C. Refractive index and porosity measurements suggest that variations were due to structural differences, particularly porosity. Furthermore, films heat-treated at 500°C were not completely stabilized, and showed index and porosity variations after six months.