Effects of deposition temperature on optical properties of MgF₂ over-coated Al mirrors in the VUV

Both long-term environmental durability and high reflectance of protected-Al mirrors are of great importance for developing the optical instruments in the vacuum ultraviolet (VUV) applications. In this paper, the dependence of spectral property and environmental durability of MgF₂ over-coated Al mirrors using a 3-step method on deposition temperature of the outermost MgF₂ layer are investigated in detail. Optics (reflectance), structure (surface morphology and crystalline), and environmental durability (humidity test) are characterized and discussed. The results show that both optical and moisture-resistant properties of MgF₂ over-coated Al mirrors are dependent on MgF₂ deposition temperature, and the optimal deposition temperature for the outermost MgF₂ layer should be between 250 °C and 300 °C for MgF₂ over-coated Al mirrors to have both reasonably high reflectance in the VUV spectral range and high moisture resistance for long lifetime applications. (paper)