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Advancing Ultraviolet Observations through Innovative Coating Technologies

Astrophysical studies conducted at ultraviolet (UV) wavelengths provide critical insights into the universe’s most dynamic phenomena. However, the high energy associated with ultraviolet photons results in diminished efficiency in their interaction with conventional observation instruments, leading to lower throughput. Innovative advancements in thin film coating technology are addressing this limitation by modifying instrument surfaces at the atomic level.

Researchers at the NASA Jet Propulsion Laboratory (JPL) are pioneering new coating methodologies for UV measurement instruments by employing atomic layer deposition (ALD) and atomic layer etching (ALE). Traditional optical coatings have predominantly relied on physical vapor deposition (PVD), a method where the coating layer is formed by vaporizing material and condensing it onto a prepared surface. In contrast, ALD and ALE utilize a cyclical process involving self-limiting chemical reactions to apply or remove material one atomic layer at a time. This self-regulating feature allows for conformal coatings over complex geometries with an exact layer thickness based on the number of ALD or ALE cycles executed.

These techniques have found frequent application in the semiconductor sector, particularly for the fabrication of high-performance transistors. Their adoption as optical coatings, particularly in the UV spectrum, has been limited. This is largely due to the restricted availability of optical coating materials; metal fluorides are favored over commonly used metal oxides, as the higher optical band energy of fluoride materials minimizes absorption losses. Through a co-reaction process involving hydrogen fluoride, JPL’s team has successfully developed a range of fluoride-based ALD and ALE methods.

Additionally, aluminum layers are often employed to create ultraviolet reflective mirrors and bandpass filters. While aluminum boasts superior UV reflectivity, it is prone to forming a surface oxide that absorbs UV light. Hence, the metal fluoride coatings play a crucial role in safeguarding the aluminum from oxidation while ensuring sufficient transparency for efficient light reflection.

The initial applications of ALD in this domain are evident in the optics development for two Small Satellite (SmallSat) astrophysics missions focused on UV observation: the Supernova Remnants and Proxies for ReIonization Testbed Experiment (SPRITE) CubeSat mission and the Aspera mission. The reflective coatings on the mirrors for these missions incorporate aluminum safeguarded by lithium fluoride, supported by a novel PVD process developed at NASA’s Goddard Space Flight Center, along with an ultra-thin layer of magnesium fluoride deposited using ALD.

The inclusion of lithium fluoride allows SPRITE and Aspera to achieve greater UV sensitivity compared to missions like the Hubble Space Telescope, which relies solely on magnesium fluoride for mirror protection. However, lithium fluoride’s susceptibility to moisture presents a challenge, potentially compromising coating performance before launch. To mitigate this risk, a thin magnesium fluoride layer (approximately 1.5 nanometers) has been applied via ALD atop the lithium fluoride on the SPRITE and Aspera mirrors. This thin layer is designed to maintain optical performance while providing additional stability against humidity prior to launch. Similar strategies are being contemplated for future NASA flagship projects, such as the Habitable Worlds Observatory (HWO).

Structures comprising multilayer aluminum and metal fluorides can also serve as bandpass filters, enabling the selective passage of signals within specific wavelength ranges in the UV domain. The repeatability and precision of thickness control afforded by ALD present significant advantages in this context. While a suitable ALD process for aluminum deposition remains under development, the JPL team is creating a custom vacuum coating chamber that integrates PVD aluminum and ALD fluoride techniques. This innovative system has already yielded UV bandpass filters that can be directly applied to imaging sensors like silicon CCDs. These coatings enhance sensor performance in the UV spectrum while minimizing sensitivity to longer-wavelength visible light that might introduce unwanted background noise to observations.

Recently, multilayer aluminum and metal fluoride coatings have been delivered as part of a UV camera for the Star-Planet Activity Research CubeSat mission (SPARCS), led by Evgenya Shkolnik at Arizona State University. The camera, developed by JPL, utilizes a delta-doped Si CCD integrated with the ALD/PVD filter coatings in the far ultraviolet channel, achieving high efficiency in a wavelength band centered around 160 nm while suppressing out-of-band light response.

Looking ahead, the JPL team is aiming to implement a similar bandpass filter on an array of larger-format silicon Complementary Metal-Oxide-Semiconductor (CMOS) sensors for the recently selected NASA Medium-Class Explorer (MIDEX) UltraViolet EXplorer (UVEX) mission, spearheaded by Fiona Harrison at the California Institute of Technology, which is slated for launch in the early 2030s.

For more in-depth information about this project, visit the NASA TechPort entry.

Project Lead: Dr. John Hennessy, Jet Propulsion Laboratory (JPL)

Source
science.nasa.gov