Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF2 layers

Brian I. Johnson; Tahereh G. Avval; Grant T. Hodges; Victoria Carver; Karen Membreno; David D. Allred; Matthew R. Linford

doi:10.1117/12.2529893

9 September 2019 Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF₂ layers

Brian I. Johnson, Tahereh G. Avval, Grant T. Hodges, Victoria Carver, Karen Membreno, David D. Allred, Matthew R. Linford

Author Affiliations +

Proceedings Volume 11116, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems II; 111160O (2019) https://doi.org/10.1117/12.2529893
Event: SPIE Optical Engineering + Applications, 2019, San Diego, California, United States

Abstract

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.

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Brian I. Johnson, Tahereh G. Avval, Grant T. Hodges, Victoria Carver, Karen Membreno, David D. Allred, and Matthew R. Linford "Using ellipsometry and x-ray photoelectron spectroscopy for real-time monitoring of the oxidation of aluminum mirrors protected by ultrathin MgF₂ layers", Proc. SPIE 11116, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems II, 111160O (9 September 2019); https://doi.org/10.1117/12.2529893

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