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

Abstract

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF 2 , 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 MgF 2 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 + MgF 2 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 MgF 2 overlayer. An empirical formula fits time-dependent data for aluminum surfaces protected by MgF 2 as a function of MgF 2 layer thickness: aluminum-oxide thickness = k SE *log(t)+b SE . The slope depends only on MgF 2 thickness, decreasing monotonically with increasing MgF 2 thickness. This method of employing SE coupled with XPS can be extendable to the study of other metal/overlayer combinations.