UCSC thin-film expertise to help develop NASA’s next generation of space telescopes

Nobuhiko Kobayashi
Nobuhiko Kobayashi
Nobuhiko Kobayashi, professor of electrical and computer engineering at UC Santa Cruz, began his career developing thin-film transistors required for controlling liquid crystal displays. Thirty years later, his expertise in thin films could improve the performance of NASA’s next generation of space telescopes that employ optical mirrors based on complex thin-film structures.

Kobayashi’s Nanostructured Energy Conversion Technology and Research (NECTAR) lab at UC Santa Cruz has received funding from NASA and will work with teams at the Jet Propulsion Laboratory in Pasadena and the University of Colorado, Boulder, to develop high performance, durable, and large-scale protected aluminum-based mirrors for the spectral range covering the far ultraviolet (FUV) and the near infrared (NIR), using distinctive atomic layer deposition capabilities established at NECTAR.

Kobayashi never expected to be working with telescopes, let alone ones that would orbit planet Earth.

“This all started as a skunkworks project we did with the UC Observatories,” Kobayashi said. “Six years ago, Prof. Joseph Miller, a former director of UC Observatories, and Prof. Andrew Phillips approached me saying they were interested in improving the performance of very large and thick mirrors based on silver for their ground-based astronomical telescopes.”

Silver-based mirrors can degrade and are vulnerable to a range of structural degradations when exposed to the environment in which ground-based astronomical telescopes operate. Traditional protective coatings are difficult to apply without changing a mirror’s reflective qualities. Kobayashi realized he could use thin-film manufacturing processes to improve a mirror’s optical performance and long-term endurance.

Kobayashi implemented a technique borrowed from the microelectronics industry called atomic layer deposition (ALD) to apply a thin film of aluminum oxide (a sort of “sapphire”), creating an extremely uniform layer only a few atoms thick. Silver surfaces coated with this method were protected from the operational environments, with very little distortion.

John Hennessy, a technologist at NASA’s Jet Propulsion Laboratory, heard Kobayashi describe the technique at an International Society for Optics and Photonics conference in San Diego. Hennessy told Kobayashi that NASA was developing aluminum-based mirrors for the next generation of space-based astronomical telescopes, and he wondered whether it would be possible to scale up ALD to develop a protective coating for an aluminum mirror.

An aluminum-based mirror has both advantages and disadvantages, Kobayashi said. “An aluminum-based mirror, in contrast to silver-based mirrors, can go all the way into the far ultraviolet. However, aluminum is a tricky material to work with because it gets oxidized so easily,” he said.

Creating a mirror that would be able to function in orbit was another hurdle.

“In space, there’s all kinds of ionizing radiation that can damage mirrors, so the surface of aluminum-based mirrors needs to be protected by a coating,” Kobayashi said. “The biggest challenge is developing a robust protective coating transparent to the FUV spectral range without hampering the optical performance of the mirrors.”

Not many materials are transparent to the FUV spectral range. The aluminum oxide protective coating used for silver-based mirrors wouldn’t work in this instance, so Kobayashi’s team will begin exploring compounds that contain fluorine.

“Unlike such metal oxides as aluminum oxide, for which well-established ALD processes exist, metal fluorides need to be extensively investigated in terms of choosing fluorine-containing precursors and designing ALD processes appropriate for mirrors used in astronomical telescopes,” Kobayashi said.

The project is unique in the sense that it brings together multiple disciplines at UC Santa Cruz: astronomy, materials science, and engineering.

“I am grateful for strong endorsement and continuous support by Brian Dupraw, Chris Ratliff, Prof. Andrew Phillips, Prof. Michael Bolte, and Prof. Claire Max of the UC Observatories,” Kobayashi said. "I would also like to thank Dr. David Fryauf, a former Ph.D. student and postdoctoral fellow now at Intel, for his great contribution to the preliminary work."