Using Ellipsometry and XPS to Understand the Degradation of Thin-Film Aluminum Mirrors Protected by Ultrathin Fluorides
(Room Room 104-C)
29 Apr 19
9:40 AM
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10:00 AM
Reflector mirrors utilized by NASA employ aluminum for reflective coatings as no other material has comparable reflectance at short wavelengths. However, formation of oxide layers on top of the aluminum reflectors absorb the desired high energy radiation. To overcome this, protective wide-bandgap fluoride coatings (such as MgF2) that do not interfere with desired broadband reflectance are traditionally deposited on aluminum mirrors. The goal of depositing thin fluoride based layers onto the aluminum mirrors is to create a barrier that prevents oxidation of the aluminum while allowing adequate reflection at lower wavelengths. MgF2 Barrier layers are usually 25 nm thick, but prevent reflectance below 122 nm. In our research, novel fluoride multilayer coatings were applied to aluminum surfaces to act as dielectric mirrors capable of reflecting energies between the 91 and 103 nm range independent from the aluminum. Meanwhile the aluminum surface is protected from oxidation and is able to reflect above 122 nm. In this study, we measured oxidation build-up on aluminum surfaces with spectroscopic ellipsometry and X-Ray photoelectron spectroscopy as a function of fluoride multi-layer composition and thickness. Here, fluoride thicknesses needed to protect vacuum ultra violet optical surfaces can be assessed.