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

Area of Science:

• Materials Science
• Optics
• Plasma Physics

Background:

• High-purity aluminum exhibits excellent far-ultraviolet (FUV) reflectance, crucial for astrophysical instrumentation.
• Atmospheric oxidation of aluminum forms native oxide, degrading FUV optical properties.
• Protective coatings are essential to maintain aluminum's FUV reflectance.

Purpose of the Study:

• To develop and optimize a hybrid plasma-enhanced atomic layer deposition (PEALD) system for passivating aluminum mirrors.
• To investigate the use of in situ electron beam (e-beam) plasma for creating metal fluoride films.
• To establish effective pretreatment methods for subsequent fluoride layer deposition.

Main Methods:

• A modified atomic layer deposition (ALD) reactor was used, combining inductively coupled plasma (ICP) and e-beam driven plasma sources.
• The effects of sample grounding, SF₆/Ar flow, and temperature on AlF₃ film properties were studied.
• In situ e-beam plasma processing and XeF₂ passivation were explored as seed layer pretreatments.

Main Results:

• Optimal conditions yielded AlF₃ coatings with high FUV reflectivity (92% at 121 nm, 42% at 103 nm).
• The performance of the developed coatings is comparable to state-of-the-art AlF₃ passivation layers.
• Both in situ e-beam plasma and XeF₂ passivation proved effective for preparing unoxidized aluminum surfaces.

Conclusions:

• The hybrid PEALD system offers a viable method for creating high-performance FUV mirror coatings.
• Optimized in situ e-beam processing provides a promising pathway for depositing various fluoride-based layers (e.g., MgF₂, LiF) using ALD/PEALD.
• This technique enhances the utility of aluminum mirrors in FUV applications, particularly in space-based instrumentation.