Stress compensation in hafnia/silica optical coatings by inclusion of alumina layers

Abstract

Hafnium dioxide films deposited using electron-beam evaporation tend to exhibit high tensile stresses, particularly when deposited on low-thermal-expansion substrates for use in a low-relative-humidity environment. Hafnia has been shown to be a critical material, however, for use in high-peak-power laser coatings, providing exceptional deposition control and laser-damage resistance. To correct for tensile thin-film stresses in hafnia/silica multilayer coatings, alumina compensation layers were incorporated in the multilayer design. Determination of the stresses resulting from alumina layers in different coating designs has led to the realization of the influence of water diffusion and the diffusion-barrier properties of alumina that must be considered. The inclusion of alumina layers in a hafnia/silica multilayer provides the ability to produce low-compressive-stress, high-laser-damage-threshold coatings.