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By NASA
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)
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Last Updated Mar 18, 2025 Related Terms
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By NASA
5 Min Read Reinventing the Clock: NASA’s New Tech for Space Timekeeping
The Optical Atomic Strontium Ion Clock is a higher-precision atomic clock that is small enough to fit on a spacecraft. Credits: NASA/Matthew Kaufman Here on Earth, it might not matter if your wristwatch runs a few seconds slow. But crucial spacecraft functions need accuracy down to one billionth of a second or less. Navigating with GPS, for example, relies on precise timing signals from satellites to pinpoint locations. Three teams at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, are at work to push timekeeping for space exploration to new levels of precision.
One team develops highly precise quantum clock synchronization techniques to aid essential spacecraft communication and navigation. Another Goddard team is working to employ the technique of clock synchronization in space-based platforms to enable telescopes to function as one enormous observatory. The third team is developing an atomic clock for spacecraft based on strontium, a metallic chemical element, to enable scientific observations not possible with current technology. The need for increasingly accurate timekeeping is why these teams at NASA Goddard, supported by the center’s Internal Research and Development program, hone clock precision and synchronization with innovative technologies like quantum and optical communications.
Syncing Up Across the Solar System
“Society requires clock synchronization for many crucial functions like power grid management, stock market openings, financial transactions, and much more,” said Alejandro Rodriguez Perez, a NASA Goddard researcher. “NASA uses clock synchronization to determine the position of spacecraft and set navigation parameters.”
If you line up two clocks and sync them together, you might expect that they will tick at the same rate forever. In reality, the more time passes, the more out of sync the clocks become, especially if those clocks are on spacecraft traveling at tens of thousands of miles per hour. Rodriguez Perez seeks to develop a new way of precisely synchronizing such clocks and keeping them synced using quantum technology.
Work on the quantum clock synchronization protocol takes place in this lab at NASA’s Goddard Space Flight Center in Greenbelt, Md.NASA/Matthew Kaufman In quantum physics, two particles are entangled when they behave like a single object and occupy two states at once. For clocks, applying quantum protocols to entangled photons could allow for a precise and secure way to sync clocks across long distances.
The heart of the synchronization protocol is called spontaneous parametric down conversion, which is when one photon breaks apart and two new photons form. Two detectors will each analyze when the new photons appear, and the devices will apply mathematical functions to determine the offset in time between the two photons, thus synchronizing the clocks.
While clock synchronization is currently done using GPS, this protocol could make it possible to precisely synchronize clocks in places where GPS access is limited, like the Moon or deep space.
Syncing Clocks, Linking Telescopes to See More than Ever Before
When it comes to astronomy, the usual rule of thumb is the bigger the telescope, the better its imagery.
“If we could hypothetically have a telescope as big as Earth, we would have incredibly high-resolution images of space, but that’s obviously not practical,” said Guan Yang, an optical physicist at NASA Goddard. “What we can do, however, is have multiple telescopes in various locations and have each telescope record the signal with high time precision. Then we can stich their observations together and produce an ultra-high-res image.”
The idea of linking together the observations of a network of smaller telescopes to affect the power of a larger one is called very long baseline interferometry, or VLBI.
For VLBI to produce a whole greater than the sum of its parts, the telescopes need high-precision clocks. The telescopes record data alongside timestamps of when the data was recorded. High-powered computers assemble all the data together into one complete observation with greater detail than any one of the telescopes could achieve on its own. This technique is what allowed the Event Horizon Telescope’s network of observatories to produce the first image of a black hole at the center of our galaxy.
The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks.EHT Collaboration Yang’s team is developing a clock technology that could be useful for missions looking to take the technique from Earth into space which could unlock many more discoveries.
An Optical Atomic Clock Built for Space Travel
Spacecraft navigation systems currently rely on onboard atomic clocks to obtain the most accurate time possible. Holly Leopardi, a physicist at NASA Goddard, is researching optical atomic clocks, a more precise type of atomic clock.
While optical atomic clocks exist in laboratory settings, Leopardi and her team seek to develop a spacecraft-ready version that will provide more precision.
The team works on OASIC, which stands for Optical Atomic Strontium Ion Clock. While current spacecraft utilize microwave frequencies, OASIC uses optical frequencies.
The Optical Atomic Strontium Ion Clock is a higher-precision atomic clock that is small enough to fit on a spacecraft.NASA/Matthew Kaufman “Optical frequencies oscillate much faster than microwave frequencies, so we can have a much finer resolution of counts and more precise timekeeping,” Leopardi said.
The OASIC technology is about 100 times more precise than the previous state-of-the-art in spacecraft atomic clocks. The enhanced accuracy could enable new types of science that were not previously possible.
“When you use these ultra-high precision clocks, you can start looking at the fundamental physics changes that occur in space,” Leopardi said, “and that can help us better understand the mechanisms of our universe.”
The timekeeping technologies unlocked by these teams, could enable new discoveries in our solar system and beyond.
More on cutting-edge technology development at NASA Goddard By Matthew Kaufman, with additional contributions from Avery Truman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Sep 18, 2024 EditorRob GarnerContactRob Garnerrob.garner@nasa.govLocationGoddard Space Flight Center Related Terms
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By Space Force
Secretary of the Air Force Frank Kendall has begun traveling through the U.S. European Command area of responsibility beginning Aug. 19, 2024, to strengthen international partnerships.
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By Space Force
Secretary of the Air Force Frank Kendall has begun traveling through the U.S. European Command area of responsibility beginning Aug. 19, 2024, to strengthen international partnerships.
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By European Space Agency
ESA, on behalf of the European Commission, has signed a €12 million contract with Leonardo S.p.A (Italy) and Istituto Nazionale di Ricerca Metrologica to design and develop a new ultra-precise atomic clock technology for Galileo.
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