Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF_2 layers

摘要

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF_2, 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_2 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_2 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 Al2O3 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~(+3). The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF_2 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_2 as a function of MgF_2 layer thickness. Here, k is a factor which depends only on MgF_2 thickness, and decreases with increasing MgF_2 thickness. The techniques developed can illuminate other protected mirror systems.