MOISTURE ADSORPTION AND OPTICAL INSTABILITY IN THIN FILM COATINGS.

Abstract

Materials in the form of thin films that have been deposited from the vapor phase are significantly different from similar bulk materials, both optically and mechanically, because of their columnar structure and consequent porosity. Their porosity have been verified in different ways. The effects of the pores on optical and mechanical performance and, in particular, the influence of water adsorption, have also been demonstrated. Three techniques used for investigating optical instabilities in thin films are given. They all involve sharp resonances. The resonances are associated either with surface plasmons, metal-dielectric narrowband filters, or all dielectric narrowband filters. These resonances are very sensitive functions of layer properties and hence can be used to detect and measure changes in the layers, particularly those that are induced by adsorption of moisture. Moisture adsorption in thin films is a complex process that occurs unevenly in patches. Using resonance techniques, the adsorption isotherms of change in refractive index, of growth rate in patch size, and of peak wavelength shift, which are all important in characterizing the porosity of films, have been measured. Some effects that locally increase film porosity and create central pores that permit water to penetrate into multilayer structures have been investigated. Based on these results, some suggestions for preventing water adsorption in films are then made. Moisture penetration into thin film structures is the major source of optical coating instability and it is therefore very important that the mechanisms of penetration by understood. Some deductions of the mechanisms are made from the experimental results.