Growth and vacuum ultraviolet characteristics of magnesium fluoride protected aluminum reflective coatings prepared by electron-beam evaporation

Magnesium fluoride protected aluminum (MgF₂/Al) vacuum ultraviolet (VUV) coatings were prepared by electron-beam evaporation process with excellent controllability. The effects of preparation conditions on the microstructure and VUV reflectivity of MgF₂/Al coatings were studied. The experimental results showed that crystalline MgF₂ layers were grown at 250 °C and were beneficial to improving the VUV reflectivity of mirrors. However, the temperature rise led to the growth of Al nanowires on the mirror surfaces with relatively thick Al films, and the Al nanowires continued to grow with the extension of baking time. It was found that reducing the thickness of Al film could not only suppress the generation of Al nanowires, but also decrease the surface roughness of mirrors. The negative impacts of Al nanowires on the performance of VUV mirror were analyzed. The physical mechanism of suppressing the generation of Al nanowires was discussed. This work would provide a reference for the growth of VUV grade Al film by electron-beam evaporation, and also valuable experience for the study of thermal stability and surface roughness control of Al-based VUV coatings.