Deposition of Enhanced Protected Aluminum for Broadband Applications

The objective of this project is to optimize the design and fabrication of high reflectance /low absorption broadband UV coating for space telescope mirrors.  This is a technology maturation for risk reduction effort. The innovative elements of our approach include: (1) deposit aluminum (Al) with an overcoat of lithium fluoride (LiF) using thermal resistive evaporation, (2) increasing the fluorine efficiency of the films, (3) overcoat the treated coating with a few monolayers of magnesium fluoride (MgF₂) and aluminum tri-fluoride (AlF₃).

Pure aluminum will achieve high reflectance at the proposed spectral range, however, it becomes oxidized immediately after deposition, thus requires an overcoat to protect from oxidation. LiF is usually the material of choice to use as an overcoat of aluminum due to its high band gap energy and low absorption properties in the FUV, however, its performance is severely hindered due to its hygroscopic nature as its RMS surface roughness increases when exposed to atmospheric conditions. A thin overcoat of a more durable and space flight proven material such as MgF₂ or AlF₃ layer will be applied to the LiF layer to protect the material while minimizing the absorption effects on the FUV optical properties.