Development of mechanical stress in fluoride multilayers for UV applications

Since excimer laser applications extend to 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 critical development of mechanical stress in all dielectric UV mirrors a systematic study was performed on LaF 3/MgF2 and LaF3/AlF3 stacks on silica and silicon substrates. The samples, were investigated by means of complex ex - situ mechanical stress analysis including temperature dependence of stress, optical measurements, evaluation of structural and morphological parameters by AFM and XRD. When deposited at high substrate temperature of about 300°C, LaF3/MgF2 tends to show high tensile stress due to the large thermal expansion coefficient difference between the substrate and the film materials. It results in micro crack formation already starting after deposition of about 10 layer pairs. LaF3/AlF3 appears 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.