Proceedings Article | 9 September 2019
Proc. SPIE. 11116, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems II
KEYWORDS: Oxides, Mirrors, Statistical analysis, Magnesium fluoride, Data modeling, Metals, Silicon, Aluminum, Statistical modeling, Oxidation
To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF<sub>2</sub>, are used to protect the of aluminum mirrors against oxidation since aluminum oxide absorbs short wavelength light. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and ellipsometric (SE) studies of aluminum oxidation as a function of MgF<sub>2</sub> over a range of layer thickness (0-6 nm). We also show for the first time, dynamic SE data which, with appropriate modeling, tracks the extent of oxide growth every few seconds over a period of several hours after the evaporated Al + MgF<sub>2</sub> bilayer is removed from the deposition chamber, exposing it to the air. For each SE data set, because the optical constants of ultrathin metals films depend strongly on deposition conditions and their thickness, the optical constants for Al, as well as the Al and Al<sub>2</sub>O<sub>3</sub> thicknesses, were fit. SE trends were confirmed by X-ray photoelectron spectroscopy. There is a chemical shift in the Al 2s electron emission peak toward higher binding energy as the metal oxidizes to Al<sup>+3</sup>. The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF<sub>2</sub> layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF<sub>2</sub> as a function of MgF<sub>2</sub> layer thickness. Here, k is a factor which depends only on MgF<sub>2</sub> thickness, and decreases with increasing MgF<sub>2</sub> thickness. The techniques developed can illuminate other protected mirror systems.
