Development of mechanical stress in fluoride multilayers for UV applications

Abstract

Since excimer laser applications extend to deep and vacuum UV wavelengths at 193 nm and 157 nm, renewed research interest has recently arisen on fluoride thin films due to their unrivaled position as wide-band-gap material for the vacuum UV (VUV). In order to evaluate the development of mechanical stress in all dielectric fluoride mirrors which causes difficulties to grow the layer stacks on fused silica substrates with a sufficient large number of quarter-wave pairs of a low (L) and of a high index (H) fluoride material, a systematic study was performed on evaporated quarter-wave stacks of LaF₃/MgF₂ and LaF₃/AlF₃ with a growing number of LH-pairs. The samples deposited onto fused silica and silicon substrates by a low-loss evaporation technology in a BAK 640 coating plant were investigated by means of complex ex - situ mechanical stress analysis including temperature dependence of stress, optical measurements, infrared measurements, evaluation of structural and morphological parameters by AFM and XRD. When deposited at high substrate temperature of about 300°C, the LaF₃/MgF₂ tends show high tensile stress due to the thermal stress component arise from the large thermal expansion coefficient difference between the substrate and the film materials resulting in micro crack formation already starting after deposition of about 10 layer pairs. LaF₃/AlF₃ appear to have a larger crack resistance due to lower stress which can be correlated to the higher water content in these kind of stacks. By adjusting the deposition temperature, mirror stacks with high reflectance at 193nm can be grown.