At NASA's Jet Propulsion Laboratory (JPL), researchers are leveraging two precise methods-atomic layer deposition (ALD) and atomic layer etching (ALE)-to create next-generation UV instrument coatings. Unlike traditional physical vapor deposition (PVD), where material is vaporized and condensed onto surfaces, ALD and ALE use carefully controlled chemical reactions to deposit or remove material layer by atomic layer. This technique allows uniform coatings on complex shapes and offers precise control over thickness.

ALD and ALE are well-established in semiconductor manufacturing, particularly in crafting advanced transistors. Their application to UV optics, however, is relatively novel. UV optical coatings often require metal fluorides instead of metal oxides, due to their higher optical bandgap, which reduces unwanted light absorption. JPL scientists have developed several fluoride-based ALD and ALE processes through chemical reactions with hydrogen fluoride.

Aluminum is frequently used in UV instruments for mirrors and filters because of its high UV reflectivity. However, it oxidizes easily, which diminishes performance. Metal fluoride coatings serve to shield aluminum surfaces from oxidation while preserving reflectance.

This ALD-based approach has been integrated into the telescope optics for two upcoming SmallSat missions focused on UV astronomy: the Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE), led by Brian Fleming at the University of Colorado Boulder, and Aspera, under Carlos Vargas at the University of Arizona. These missions utilize mirrors coated with aluminum and protected by lithium fluoride via innovative PVD methods from NASA Goddard Space Flight Center, capped with an ultra-thin magnesium fluoride layer applied through ALD.

Lithium fluoride coatings allow SPRITE and Aspera to detect UV wavelengths beyond those accessible to the Hubble Space Telescope, which employs only magnesium fluoride protection. However, lithium fluoride's moisture sensitivity poses challenges before launch. To counter this, SPRITE and Aspera mirrors received an ALD-applied magnesium fluoride layer approximately 1.5 nanometers thick-thin enough to maintain performance at short UV wavelengths, yet robust enough to resist humidity-induced degradation. Similar methods are under consideration for NASA's future Habitable Worlds Observatory (HWO).

Beyond mirrors, multilayer aluminum and metal fluoride stacks function as UV bandpass filters, which transmit only specific wavelength ranges. ALD's precision and consistency make it ideal for constructing these filters. Although aluminum cannot yet be deposited via ALD, JPL has created a specialized vacuum chamber that combines PVD aluminum with ALD fluoride coatings. This hybrid system has been used to apply UV filters directly to imaging sensors like silicon CCDs, enhancing their UV sensitivity while reducing interference from visible light.

Such coated structures were recently delivered as part of a UV camera for the Star-Planet Activity Research CubeSat (SPARCS), led by Evgenya Shkolnik at Arizona State University. The camera employs a delta-doped Si CCD with ALD/PVD-coated filters in its far-UV channel, achieving strong sensitivity around 160 nm and minimal detection of unwanted wavelengths.

Looking ahead, the JPL team aims to adapt these bandpass filters for larger silicon CMOS sensor arrays for NASA's upcoming Medium-Class Explorer (MIDEX) mission, the UltraViolet EXplorer (UVEX), spearheaded by Fiona Harrison at Caltech. Scheduled for launch in the early 2030s, UVEX will benefit from these advanced coating technologies.

For additional details, see the entry for this project on NASA TechPort

Research Report:Enhanced Mirror Coatings Will Enable Future NASA Observatory

Related Links
Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE)
Understanding Time and Space