New far-ultraviolet reflectivity measurements from ALD-deposited mirror coatings

We describe new results from the ALD application of fluoride mirror coatings that offer high performance over the UV bandpass (90 -235 nm). Such coatings provide an alternative to PVD and have broad applications in the UV, particularly for UV astronomy. The fabrication process is carried out at the JPL Microdevices Lab, beginning with an optically thick layer of evaporated Al which is capped with an ALD protective fluoride layer, such as LiF. Capping the Al is necessary because it otherwise oxidizes quickly and loses significant reflectance. Immediately before applying the ALD protective thin film we use atomic layer etching (ALE) to remove any native oxides that manifested after Al deposition. FUV reflectivity measurements are conducted in the vacuum ultraviolet space hardware characterization facilities at the University of Colorado at Boulder. We present the results of reflectivity testing of five samples from 90 to 230 nm; samples include Al+LiF (3), Al+LiF+MgF2 (1), and Al+LiF+LiF3 (1). We vary the number of ALE and ALD cycles across several Al+LiF samples to begin exploring the optimal amount of etching and film thickness. Preliminary results confirm <80% reflectivity at 1200 A with one Al+LiF sample yielding 86 ± 2%. Future work will include testing how layer thickness and ALD deposition temperature impact FUV reflectivity with a focus on optimizing reflectivity below Lyβ.