Oxidation of aluminum thin films protected by ultrathin MgF 2 layers measured using spectroscopic ellipsometry and X-ray photoelectron spectroscopy

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF2, are used to protect aluminum mirrors against oxidation. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometric (SE) studies of aluminum oxidation as a function of MgF2 overlayer thickness (thickness 0-5 nm). Dynamic SE tracks the extent of oxide growth every ca. 2s over a period of several hours after the evaporated Al + MgF2 bilayer is removed from the deposition chamber. Aluminum oxidation changes under the fluoride layer were quantitatively verified with XPS. Changes in chemical state from Al metal to Al oxide were directly observed. Oxide growth is computed from relative XPS peak areas as corrected for electron attenuation through the MgF2 overlayer. An empirical formula fits time-dependent data for aluminum surfaces protected by MgF2 as a function of MgF2 layer thickness: aluminum-oxide thickness = kSE*log(t)+bSE. The slope depends only on MgF2 thickness, decreasing monotonically with increasing MgF2 thickness. This method of employing SE coupled with XPS can be extendable to the study of other metal/overlayer combinations.