Evidence that evaporated Al/AlF3 bilayer thin films stored in a 327 K oven for over 2500 hours have not degraded

Four evaporated, thin-film Al samples protected by a thin (29±2 nm) aluminum fluoride (AlF3) overcoat stored in dry (dew point 276K ), 327 K air over a period of 2500 hours exhibited no significant changes in the thickness of the protective AlF3 layer nor growth in aluminum oxide as observed by variable-angled, spectroscopic ellipsometry. Two of the samples had AlF3 evaporated at T>200°C, two without substrate heating. No difference in aging was noted amongst the samples. Since many months may elapse between fabrication and launch of the completed observatory, this result contributes to understanding the boundaries in temperature and humidity separating negligible changes in fluoride-containing optical components from unacceptable degradation. While negligible changes in thicknesses were observed, there were changes in the ellipsometric data, psi and delta, with time. In this study, we also present our use of an effective medium approximation model in understanding changes in the fluoride layer with aging. The observed changes in SE parameters are here interpreted as changes in void fraction, though the presence of some water was not ruled out. Apparent void fraction fell by a factor of two by the end of the 2500 hours. The decreasing void fraction suggests that the films might be becoming more compact with time. Other surface sensitive techniques such as AFM are needed to narrow down possible explanations for observed changes.