Mechanical stress, optical and surface properties of high temperature annealed HfO2, Sc2O3 and Al2O3 binary mixture thin films deposited by ion beam sputtering

Abstract

High compressive stress is one of the main drawbacks of ion beam sputtered coatings by deteriorating the flatness of optical components. Mixtures of high refractive index metal oxides with SiO2 allow one to increase the laser induced damage threshold of multilayer stacks. Study of optical, surface roughness and stress properties of HfO2–Al2O3, Sc2O3–Al2O3, HfO2–Sc2O3 binary mixtures using a broad range of post-deposition thermal annealing up to 900°C is presented. Admixing Al2O3 in moderate concentrations to HfO2 and Sc2O3 allows one to sustain a low surface roughness, to decrease the extinction of layers during thermal treatment, while obtaining –360…–560 MPa tensile stress after annealing to 500°C, depending on the particular mixture. The obtained data allow one to point out possible candidates – HfO2(56%)–Al2O3(44%), Sc2O3(70%)–Al2O3(30%), HfO2(~70%)–Sc2O3(~30%) – and the 500–600°C annealing temperature range for the design of stress compensated multilayer coatings for the UV spectral range with potentially increased laser induced damage threshold.