AVS 65th International Symposium & Exhibition
    Thin Films Division Thursday Sessions
       Session TF+AS+EL+PS-ThM

Paper TF+AS+EL+PS-ThM5
Using Ellipsometry and XPS to Understand the Degradation of Thin-film Aluminum Mirrors Protected by Ultrathin Fluorides

Thursday, October 25, 2018, 9:20 am, Room 102A

Session: In-situ Characterization and Modeling of Thin Film Processes
Presenter: Brian I. Johnson, Brigham Young University
Authors: M.R. Linford, Brigham Young University
B.I. Johnson, Brigham Young University
R.S. Turley, Brigham Young University
D.D. Allred, Brigham Young University
Correspondent: Click to Email
The LUVOIR (Large, UV-optical-IR) telescope is a potential NASA flagship space-based observatory of the 2020’s or 30’s. It will utilize the largest mirrors ever put into space. The reflective coating for the mirrors will be aluminum, since there is no material with comparable reflectance at shorter wavelengths. However, to achieve high reflectance over the broadest energy range, the top surfaces of such Al mirrors must be protected against the formation of oxide layers that form quickly in air using wide-bandgap fluoride coatings, traditionally about 25 nm of MgF2 . Researchers have been endeavoring to use fluorides which are transparent further into the VUV (vacuum ultraviolet) like LiF and AlF3, and to make these barriers more continuous by depositing them on heated surfaces and making the barriers thinner. However, when the barriers are thinner and when materials like LiF are exposed to moist air, degradation of VUV reflectance is observed. Thus, studying fluoride barrier-coated mirrors is vital. We have recently reported on the time dependent growth of apparent aluminum oxide thickness for two Al mirrors protected by ultrathin fluoride layers. These measurements were based on variable-angle, spectroscopic ellipsometric (VASE) measurements. (Allred, Thomas, Willett, Greenburg, & Perry, 2017) (Miles, 2017) . VASE, however, does not provide chemical composition data. An independent analytical technique which is sensitive to surface composition is required. We have undertaken such investigations using X-ray photoelectron spectroscopy (XPS), and now report on correlations between optical properties and XPS for fluoride-coated aluminum mirror test structures.