Feed aggregator

Several medications now calm painful inflammation in the intestines. Diets free of ultraprocessed foods also help

In the April issue of SciAm, follow a man’s journey to a diagnosis, learn about exciting new schizophrenia treatments, and more

Letters to the editors for the December 2024 issue of Scientific American

California is experiencing wider swings between wet and dry spells

Writers, artists, photographers and researchers share the stories behind the stories

Science in meter and verse

Artificial Intelligence uses too much energy. Developers need to find better ways to power it or risk adding to the climate crisis

Despite a brief panic, asteroid 2024 YR4 isn't likely to hit us anytime soon, but we can still study the asteroid up close to help us understand these space rocks.

5 min read

Atomic Layer Processing Coating Techniques Enable Missions to See Further into the Ultraviolet

Astrophysics observations at ultraviolet (UV) wavelengths often probe the most dynamic aspects of the universe. However, the high energy of ultraviolet photons means that their interaction with the materials that make up an observing instrument are less efficient, resulting in low overall throughput. New approaches in the development of thin film coatings are addressing this shortcoming by engineering the coatings of instrument structures at the atomic scale.

Researchers at the NASA Jet Propulsion Laboratory (JPL) are employing atomic layer deposition (ALD) and atomic layer etching (ALE) to enable new coating technologies for instruments measuring ultraviolet light. Conventional optical coatings largely rely on physical vapor deposition (PVD) methods like evaporation, where the coating layer is formed by vaporizing the source material and then condensing it onto the intended substrate. In contrast, ALD and ALE rely on a cyclic series of self-limiting chemical reactions that result in the deposition (or removal) of material one atomic layer at a time. This self-limiting characteristic results in a coating or etchings that are conformal over arbitrary shapes with precisely controlled layer thickness determined by the number of ALD or ALE cycles performed.

The ALD and ALE techniques are common in the semiconductor industry where they are used to fabricate high-performance transistors. Their use as an optical coating method is less common, particularly at ultraviolet wavelengths where the choice of optical coating material is largely restricted to metal fluorides instead of more common metal oxides, due to the larger optical band energy of fluoride materials, which minimizes absorption losses in the coatings. Using an approach based on co-reaction with hydrogen fluoride, the team at JPL has developed a variety of fluoride-based ALD and ALE processes.

(left) The Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE) CubeSat primary mirror inside the ALD coating facility at JPL, the mirror is 18 cm on the long and is the largest optic coated in this chamber to-date. (right) Flight optic coating inside JPL ALD chamber for Pioneers Aspera Mission. Like SPRITE, the Aspera coating combines a lithium fluoride process developed at NASA GSFC with thin ALD encapsulation of magnesium fluoride at JPL. Image Credit: NASA-JPL

In addition to these metal-fluoride materials, layers of aluminum are often used to construct structures like reflective mirrors and bandpass filters for instruments operating in the UV.  Although aluminum has high intrinsic UV reflectance, it also readily forms a surface oxide that strongly absorbs UV light. The role of the metal fluoride coating is then to protect the aluminum surface from oxidation while maintaining enough transparency to create a mirror with high reflectance.

The use of ALD in this context has initially been pursued in the development of telescope optics for two SmallSat astrophysics missions that will operate in the UV: the Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE) CubeSat mission led by Brian Fleming at the University of Colorado Boulder, and the Aspera mission led by Carlos Vargas at the University of Arizona. The mirrors for SPRITE and Aspera have reflective coatings that utilize aluminum protected by lithium fluoride using a novel PVD processes developed at NASA Goddard Space Flight Center, and an additional very thin top coating of magnesium fluoride deposited via ALD.

Team member John Hennessy prepares to load a sample wafer in the ALD coating chamber at JPL. Image Credit: NASA JPL

The use of lithium fluoride enables SPRITE and Aspera to “see” further into the UV than other missions like NASA’s Hubble Space Telescope, which uses only magnesium fluoride to protect its aluminum mirror surfaces. However, a drawback of lithium fluoride is its sensitivity to moisture, which in some cases can cause the performance of these mirror coatings to degrade on the ground prior to launch. To circumvent this issue, very thin layers (~1.5 nanometers) of magnesium fluoride were deposited by ALD on top of the lithium fluoride on the SPRITE and Aspera mirrors. The magnesium fluoride layers are thin enough to not strongly impact the performance of the mirror at the shortest wavelengths, but thick enough to enhance the stability against humidity during ground phases of the missions. Similar approaches are being considered for the mirror coatings of the future NASA flagship Habitable Worlds Observatory (HWO).

Multilayer structures of aluminum and metal fluorides can also function as bandpass filters (filters that allow only signals within a selected range of wavelengths to pass through to be recorded) in the UV. Here, ALD is an attractive option due to the inherent repeatability and precise thickness control of the process. There is currently no suitable ALD process to deposit aluminum, and so additional work by the JPL team has explored the development of a custom vacuum coating chamber that combines the PVD aluminum and ALD fluoride processes described above. This system has been used to develop UV bandpass filters that can be deposited directly onto imaging sensors like silicon (Si) CCDs. These coatings can enable such sensors to operate with high UV efficiency, but low sensitivity to longer wavelength visible photons that would otherwise add background noise to the UV observations.

Structures composed of multilayer aluminum and metal fluoride coatings have recently been delivered as part of a UV camera to the Star-Planet Activity Research CubeSat (SPARCS) mission led by Evgenya Shkolnik at Arizona State University. The JPL-developed camera incorporates a delta-doped Si CCD with the ALD/PVD filter coating on the far ultraviolet channel, yielding a sensor with high efficiency in a band centered near 160 nm with low response to out-of-band light.

A prototype of a back-illuminated CCD incorporating a multi-layer metal-dielectric bandpass filter coating deposited by a combination of thermal evaporation and ALD. This coating combined with JPL back surface passivation approaches enable the Si CCD to operate with high UV efficiency while rejecting longer wavelength light. Image credit: NASA JPL

Next, the JPL team that developed these coating processes plans to focus on implementing a similar bandpass filter on an array of larger-format Si Complementary Metal-Oxide-Semiconductor (CMOS) sensors for the recently selected NASA Medium-Class Explorer (MIDEX) UltraViolet EXplorer (UVEX) mission led by Fiona Harrison at the California Institute of Technology, which is targeted to launch in the early 2030s. 

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

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

Share

• 
  
  
• 
  
  
• 
  
  
• 
  
  

Details

Last Updated

Mar 18, 2025

Related Terms

• Technology Highlights
• Astrophysics
• Astrophysics Division
• Jet Propulsion Laboratory
• Science-enabling Technology

Explore More

5 min read NASA’s Webb Images Young, Giant Exoplanets, Detects Carbon Dioxide

Article


1 day ago

2 min read Hubble Sees a Spiral and a Star

Article


4 days ago

4 min read Discovery Alert: ‘Super-Earth’ Swings from Super-Heated to Super-Chill

Article


7 days ago

NASA’s Perseverance rover recorded unusual sounds as a Martian dust devil passed directly over the robotic vehicle in 2021, and we now know they came from electrical activity in the storm

NASA’s Perseverance rover recorded unusual sounds as a Martian dust devil passed directly over the robotic vehicle in 2021, and we now know they came from electrical activity in the storm

Analyzing the metaphors we use to talk about ChatGPT can reveal how we’re thinking about the new tech

A personal quest and progress in brain science finally put a name on baffling behaviors

Play this crossword inspired by the April 2025 issue of Scientific American

Finish the cycle of numbers in this math puzzle

As a complex picture of schizophrenia emerges, so do new ways to treat the disorder

Dark matter could be an entire dark sector of the universe, with its own particles and forces

The U.S. Army Corps of Engineers has uncharacteristically been working with nature instead of bulldozing it into submission. Will this enlightened approach prevail?

Studies of the horns, spikes, plates and clubs of dinosaurs could help settle a long-standing debate over their function

It was the opening scene of "Apollo 13" and was recreated for the Neil Armstrong biopic "First Man." It was also included in director Mark Craig's "Last Man on the Moon," inspiring "Apollo 1."