The effect of first contact polymer storage on xenon difluoride passivated lithium fluoride on aluminum films

Xenon difluoride passivated aluminum with a lithium fluoride overcoat (Al+XeLiF) mirror coatings are promising candidates for future space telescope missions due to their high reflectance down to 100nm. The XeLiF mirror coating blocks aluminum oxidation. Aluminum oxide is undesirable since it significantly reduces the far UV (100-to-190nm) reflectance. Cleaning techniques for this ideal coating require the balance of traditional surface cleaning with careful handling of the hygroscopic LiF. Photonic Cleaning Technologies’ First Contact Polymers (FCP) are proven to clean and protect optical surfaces effectively. A specialized FCP formulation may be required for Al+XeLiF. We monitored Al+XeLiF changes under repeated application of four FCP formulations using variable angle spectroscopic ellipsometry (VASE), reflectance, and Atomic Force Microscopy (AFM). We also applied multiple FCP formulations to both Al+XeLiF and Al+LiF samples and stored them in different humidity environments to monitor potential protective qualities of each formulation. We observed that two FCP formulations can effectively protect Al+XeLiF in 40% RH without Al loss over half a year.