Enhanced far-UV reflectance of Al mirrors protected with hot-deposited MgF2

Mirrors based on Al protected with a MgF₂ film provide high reflectance over a broad spectral range down to the wavelength of 120 nm in the Far UV (FUV). After more than 50 years since the development of this technology, a significant FUV reflectance enhancement has been obtained in the last years. Such enhancement originates mostly in the higher transparency of the MgF₂ protective layer deposited on a hot Al-coated substrate. Research has been conducted at GOLD to measure the dependence of the FUV reflectance enhancement with MgF₂ deposition temperature. A reflectance enhancement was found for freshly-prepared samples; moreover, the reflectance degradation over time of Al films protected with hot-deposited MgF₂ was also smaller than for the coatings deposited at room temperature. A reflectance as high as 90% was measured at 121.6 nm (hydrogen Lyman α line) for aged samples. A FUV reflectance enhancement was also obtained on samples fully deposited at room temperature and later annealed in vacuum. The reflectance of Al mirrors as a function of MgF₂ deposition temperature, as well as of post-deposition annealed mirrors, and their stability over time is presented. Structural data on film roughness, density, and main crystal orientations for mirrors with a MgF₂ film deposited both at room temperature and at 250°C are also presented.