Illuminating the degradation of lithium fluoride mirror coatings in humid environments

Abstract

Lithium fluoride (LiF) is difficult to work with because of its hygroscopic nature (it pulls water out of air). The stability limits of LiF thin films and the nature of their failure when exposed to humid air are poorly understood. We show that LiF films undergo irreversible changes in optical properties and microstructure as determined by ellipsometry and SEM when exposed to dew points greater than 6 C. On the other hand, samples stored at a dew point of -22 C (4% RH at room temperature), showed only small changes in ellipsometric parameters. The ones stored at intermediate humidity 6 C (21% RH at room temperature) showed larger changes in ellipsometric parameters. SEM shows that deliquescence as well as efflorescence is important in LiF thin films. In situ spectroscopic ellipsometric measurements using a controlled variable humidity environment illuminates the changes in LiF thin films moving from moisture absorption to complete deliquescence.