Real-Time Monitoring of Aluminum Oxidation Through Wide Band Gap MgF2 Layers for Protection of Space Mirrors

Room # EB402

Because of its extraordinary and broad reflectivity, aluminum is the only logical candidate for advanced space mirrors that operate deep into the UV. However, aluminum oxidizes rapidly in the air, and even a small amount of oxide (as little as a nanometer) can have a noticeable, detrimental impact on its reflectivity at short wavelengths. Thin films of wide band gap materials like MgF2 have previously been used to protect aluminum surfaces. Here we report the first real-time, spectroscopic ellipsometry (SE) study of aluminum oxidation as a function of MgF2 over layer thickness, which ranged from 0 – 6 nm. SE data analysis was performed vis-à-vis a multilayer optical model that included a thick silicon nitride layer. The optical constants for evaporated aluminum were initially determined using a multi-sample analysis (MSA) of SE data from MgF2 protected and bare Al surfaces. Two models were then considered for analyzing the real-time data obtained from Al/MgF2 stacks. The first used the optical constants of aluminum obtained in the MSA with two adjustable parameters: the thicknesses of the aluminum and aluminum oxide layers. The thicknesses obtained from this model showed the expected trends (increasing Al2O3 layer thickness and decreasing Al layer thickness with time), but some of the Al2O3 thicknesses were unphysical (negative). Because the optical constants of very thin metals films depend strongly on their structures and deposition conditions, a second, more advanced model was employed that fit the optical constants for Al, and also the Al and Al2O3 thicknesses, for each data set. In particular, the Al and Al2O3 thicknesses and optical constants of Al were determined in an MSA for each of 50 evenly spaced analyses in each four-hour dynamic run performed. The resulting optical constants for Al were then fixed for that sample and the thicknesses of the Al and Al2O3 layers were determined. While the first and second models yielded similar Al and Al2O3 thickness vs. time trends, the film thicknesses obtained in this manner were more physically reasonable. Thicker MgF2 layers slow the oxidation rate of aluminum. The results from this work should prove useful in protecting space mirrors prior to launch.