Aging of lithium fluoride thin films according humidity and temperature

Future flagship NASA missions call for the ability to observe in the far ultraviolet (FUV) range of the electromagnetic spectrum. The best material to be the reflective surface of the mirrors for these space-based telescopes is thin-film aluminum (Al). Unfortunately, Al readily oxidizes in Earth's atmosphere, and the oxide layer that forms is opaque to FUV radiation, so a protective layer is needed to preserve an Al mirror's reflectance. Thin films of lithium fluoride (LiF) are being investigated to be such a protective barrier due to it having the highest band gap of any solid material. Unfortunately, LiF is hygroscopic, which can potentially degrade a LiF thin film’s integrity. Surprisingly little is known about LiF's hygroscopicity, thus we investigate this property of LiF thin films. LiF thin films were deposited onto cut silicon (Si) wafers via thermal evaporation and stored in environments controlled for humidity and temperature. Films were characterized using atomic force microscopy (AFM). Samples stored in cold environments (about 3° C) degraded over time for all humidities tested. Samples stored in hot environments (about 60° C) did not degrade, and in some cases became more smooth over time, except in the case of a completely saturated environment. It was also found that calcium sulfate anhydrous (CaSO4, brand name Drierite), the desiccant used to control humidity, absorbed more moisture than anticipated, making the hot environments extremely dry.