Abstract

In this paper we discuss the stress and sorbed moisture in mixed SiO₂-TiO₂ thin films and the dependence of these on deposition temperature and pressure, chemical composition of the film, and annealing conditions. SiO₂ and TiO₂ commonly used visible range optical thin film materials in gradient-index coatings. Films deposited by evaporative techniques have a porous microstructure. This microstructure and the sorption of moisture in the pores give rise to mechanical stress, which can cause catastrophic failure of the coating. Therefore, techniques to eliminate moisture sorption and to minimize stress are highly desired. It was observed that sorption of water causes compressive stress in SiO₂ rich films, indicating different bonding arrangements of sorbed water at the grain surfaces in these two materials. Deposition conditions that promote a denser film microstructure, such as high substrate temperature, low background pressure, or energetic ion bombardment tend to reduce sorbed water and the tensile stress of TiO₂ rich films. For a given set of deposition conditions the stress becomes more tensile as the TiO₂ content increases. However, this composition dependence is sensitive to the method of mixing, being linear with composition in alternating layered mixtures and nonlinear in codeposited mixtures. Annealing reduces water in the films and increases tensile stress as the film undergoes densification. High temperature annealing also causes segregation and crystallization of TiO₂. The grain size dependence on composition and annealing time have been determined for the entire range of compositions.

© 1989 Optical Society of America