BYU Authors: Brian I. Johnson, Tahereh G. Avval, Grant T. Hodges, Victoria Carver, Karen Membreno, David D. Allred
, and Matthew R. Linford, published in Proc. SPIE
To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF2, 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 MgF2 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 + MgF2 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 MgF2 layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF2 as a function of MgF2 layer thickness. Here, k is a factor which depends only on MgF2 thickness, and decreases with increasing MgF2 thickness. The techniques developed can illuminate other protected mirror systems.