Vacuum ultraviolet (VUV) reflective coatings play an important role in many high-tech fields including cosmic physics, space research, life science and synchrotron radiation. In this research, the emphasis are focused on the aging effect of 135.6nm high reflective coatings. The coatings were deposited by resistive heating method based on Lanthanum fluoride (LaF3) and Aluminum fluoride (AlF3). Optical property, surface morphology and roughness, and composition were characterized in different period after deposited. Due to the porous structure and the worse stability of lanthanum fluoride, the content of C and O element increased in LaF3 thin films during aging process. On one hand, the content of C and O are the hydrocarbon contamination from environment and packing boxes. On the other hand, due to the oxidation of film materials, the fluorides will turn into oxyfluoride, which will increase O content. As a result, there will be an increase of absorption and a decrease of reflectance of the 135.6nm high reflective coating. The surface roughness decreased which led to the reduction of scattering and the rise of reflectivity. There will be LaF3-AlF3 mixed layers between interfaces because of interface diffusion, which will further reduce the film performance.
The damage resistance of 355nm laser can be improved for fluoride mirrors with an SiO2 overcoat layer. Three kinds of samples are deposited with the overcoat layer of different thicknesses. Calculated with Stoney equation, the residual stress of the film can be altered to the compressive stress with the increase of the overcoat layer. Through the investigation of the damage pits, we find the damages are the thermal-mechanical coupled and induced by the nodules buried in the fluoride multilayers. The surface morphologies around the damage pits are analyzed from the two respects: Cracks and surface ablation. The two characters of the damage morphologies can be suppressed with the addition of the SiO2 overcoat layer.