Passivation Strategies for Far-Ultraviolet Al Mirrors Using Plasma-Based AlF₃ Processing

High purity aluminum in its bulk form has intrinsically high reflectance in the far-ultraviolet (FUV) regime and finds utility in astrophysical instrumentation applications. However, bulk Al oxidizes rapidly in the atmosphere, and its native oxide strongly absorbs and severely degrades the observed FUV properties relative to bare Al. Various techniques have been investigated to produce coatings that inhibit aluminum oxide formation and lead to high FUV mirror reflectance. This work examines the development and use of a uniquely modified, hybrid plasma-enhanced atomic layer deposition (PEALD) system to passivate aluminum mirrors with metal fluoride films. This system combines two plasma sources in a commercial atomic layer deposition (ALD) reactor. The first is a conventional inductively coupled plasma (ICP) source operated as a remote plasma, and the second is an electron beam (e-beam) driven plasma near the mirror surface. To establish the operating conditions for the in situ e-beam plasma source, the effects of sample grounding, SF₆/Ar flow, and sample temperature on resulting AlF₃ films were investigated. Optimal operating conditions produced mirrors with excellent FUV reflectivity, 92% at 121 nm and 42% at 103 nm wavelengths, which is comparable to state-of-the-art AlF₃-based passivation coatings and matches that of previously reported ex situ e-beam plasma-processed mirrors. This optimized in situ e-beam process, along with XeF₂ passivation, is then explored to produce a clean seed layer (unoxidized Al surface) for subsequent PEALD of AlF₃. Both approaches are demonstrated as valid pretreatments before PEALD of AlF₃, showing a promising pathway for the deposition of other fluoride-based layers, such as MgF₂ or LiF, with ALD or PEALD.