Stability of lithium fluoride thin films as a function of humidity and temperature

Aluminum mirrors protected by metal fluoride overcoats are crucial for FUV observations. Many contemplated missions specify optics elements with high reflectance down to 103 nm (Lyman beta). Lithium fluoride (LiF) has the highest band gap of any solid material and thus finds applications in FUV optics. However, LiF is difficult to work with because of its hygroscopic nature. The instability of these films was investigated by evaporating LiF onto silicon wafers and aging in environments with different relative humidities and temperatures. Samples were characterized using variable-angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM). From these methods we found that storing LiF in a hot environment improved sample surface stability, and that in dry, hot environments, surfaces became more smooth after many hours in storage.