John Hennessy (Alphabet Chairman) – Atomic Layer Deposition for Astronomical Imaging (Mar 2023)

Chapters

Abstract

Revolutionizing Space Exploration: The Pivotal Role of ALD in Ultraviolet Coatings and NASA’s Quest for the Cosmos

The world of space exploration is poised for a revolutionary transformation, primarily driven by advancements in ultraviolet (UV) coating technology. A key player in this transformation is atomic layer deposition (ALD), a technique that has proven pivotal in enhancing the performance of space equipment. This article explores the significant contributions of John Hennessey, an electrical engineer turned ALD expert, and his instrumental role at NASA’s Jet Propulsion Lab (JPL) in developing advanced UV coatings for astronomical imaging.

Hennessey’s Journey with ALD at NASA’s Jet Propulsion Lab

John Hennessey’s introduction to ALD began during his graduate studies at MIT. While working on germanium-based devices, he recognized the potential of ALD for gate dielectric deposition. Collaboration with the Roy Gordon Group at Harvard led to the acquisition of a small R&D ALD tool at MIT. After completing his studies, John pursued research opportunities at JPL due to family reasons.

The novelty and magic of ALD’s self-limiting nature appealed to John’s curiosity. The ability to achieve sub-angstrom uniformity and conformal deposition fascinated him. At JPL, he joined a group led by Sholeh Nixad, focused on UV detector development and back-surface modification of silicon sensors. The goal was to make devices with high UV quantum efficiency and improve anti-reflection filter coatings.

NASA’s Jet Propulsion Laboratory and Ultraviolet Coatings

JPL, a federally funded research and development center managed by Caltech for NASA, is uniquely positioned to foster innovative research. Although it is a NASA center, JPL employees are not civil servants but employees of Caltech. This organizational structure provides more freedom and less bureaucracy, enabling innovative research like Hennessey’s work in ALD. JPL’s focus is on planetary exploration, astronomy, and space-based Earth science.

The Advancements in UV Coatings for Space Missions

Hennessey’s work has been pivotal in advancing UV coatings, a technology that first gained prominence with its use in the Hubble Space Telescope. Since then, advancements in ALD techniques have led to improved UV coatings with better uniformity, lower defect densities, and enhanced durability. Current UV coatings can withstand the harsh radiation and temperature extremes encountered in space. These coatings are critical for capturing images of distant objects in the universe, enhancing the sensitivity of telescopes by reducing reflections and protecting delicate components from harsh space environments. UV coatings enable the study of celestial objects emitting ultraviolet radiation, such as hot stars and active galaxies.

The Unique Relationship Between JPL and NASA

JPL is somewhat insulated from congressional budget issues and government shutdowns due to its unique funding structure. Funding comes from NASA, mostly for large-scale missions and flagship projects, and also via soft-competed funding. The speaker has been working on ultraviolet coatings and ALD at JPL for 10 years. This research is particularly interesting and elegant due to its applications in space.

The Significance of ALD in Space Technology

ALD stands out for its capability to achieve uniform depositions with sub-angstrom uniformity, a feature Hennessey finds particularly fascinating. This precision is critical in space technology, especially in the development of UV coatings for optical systems, where maximizing efficiency and minimizing losses are paramount. JPL moved from conventional optical coating materials like oxides to fluoride materials for shorter wavelengths. ALD emerged as the preferred method for depositing these coatings without damaging underlying detectors. Prior methods like sputtering or E-beam evaporation had negative side effects. Lithium fluoride has a higher band energy than other fluoride materials, enabling effective coatings closer to 100 nanometers. This is important for astronomers and astrophysicists as it allows them to observe more spectral diagnostic lines, especially hydrogen transition lines.

The Application of ALD in UV Detector and Mirror Coatings

Hennessey’s involvement extends to the development of UV detector coatings and the exploration of fluoride materials for UV coatings, crucial for shorter wavelength applications. This aspect of his work is significant as it crosses key spectral diagnostic lines in astrophysics and planetary science. ALD fluoride coatings are used in telescope systems like James Webb, with numerous mirrors and image sensors. They help enhance UV sensitivity and image quality.

Lossy Materials in the UV

All optical materials exhibit some degree of loss in the UV spectrum. Mirrors, in particular, have limited reflectivity in the UV. Aluminum is the primary choice for UV mirrors, but it is susceptible to oxidation.

Protective Coatings

Transparent protective coatings are applied to aluminum mirrors to prevent oxidation and improve performance. Common protective coatings include magnesium fluoride and lithium fluoride.

Solid State Detectors

Solid state detectors can achieve 100% internal quantum efficiency. Anti-reflection coatings or specialty filter coatings are used to minimize reflection loss.

Beam Splitters

Beam splitters in telescopes are often made of dielectric mirror coatings. Fluoride thin film materials are commonly used in these coatings.

Efficiency

The primary goal of UV coatings is to maximize efficiency. This is especially important for telescopes like James Webb and systems operating in the visible or infrared spectrum.

The Future of Space Exploration: Challenges and Prospects

Looking ahead, the future of space exploration is bright, with the potential for significant advancements in mirror coating technologies. Challenges remain in achieving the desired uniformity in mirror reflectance, essential for future telescopes aiming to perform visible wavelength coronagraphy. The technology readiness levels (TRLs), on-orbit validation, and laboratory testing are critical steps in assessing and advancing the readiness of these technologies for space missions. CubeSat missions like the Sprite CubeSat mission, a collaboration between JPL and the University of Colorado Boulder, serve as essential platforms for demonstrating and validating new technologies. The decadal survey process also plays a vital role in shaping the direction of space exploration and technology development, with flagship mission concepts such as HabEx and LUVOIR being considered.

Hennessey’s Continued Passion for ALD and Space Exploration

Hennessey’s enthusiasm for ALD and its application in space technology remains unwavering. His ongoing efforts to advance mirror coatings and explore ALD’s role in various aspects of UV instrumentation, including detector coatings and beam splitters, are a testament to his commitment to this field.

A New Era in Space Exploration

In conclusion, the advancements in ALD and UV coating technology represent a significant leap forward in space exploration. The work of experts like John Hennessey at NASA’s JPL is not only enhancing our understanding of the universe but also paving the way for future missions that will further unravel the mysteries of the cosmos. The next few years promise exciting developments, with the potential for groundbreaking contributions to space telescopes and missions, heralding a new era in our quest for the stars.

Supplementations:

CubeSat Mission Lifetimes:

CubeSats typically have mission durations of several months to a year, up to two years in some cases. De-orbiting within a specific threshold time (around a year) is required for CubeSats in low Earth orbit.

NASA Technology Programs and Sounding Rockets:

Sounding rocket missions can take one to two years to assemble before launch, while CubeSat programs may take three to four years. Researchers like Kevin France and Brian Fleming propose science cases and work with NASA technology programs to secure funding for their missions.

Sprite CubeSat Mission:

Sprite is a CubeSat mission focused on studying mirror coatings. It has primary mirrors and optics coated with lithium fluoride and a separate calibration channel with mag fluoride protected aluminum mirror coatings. This design allows for a direct comparison of coating degradation on orbit.

Decadal Survey Mission Concepts:

Decadal surveys are conducted by major science branches of NASA and the National Science Foundation to determine future directions for large projects. The astrophysics decadal survey, delayed to 2021 due to COVID-19, resulted in the recommendation to develop a space telescope for detecting Earth-like planets orbiting Sun-like stars.

HabEx and LUVOIR Mission Concepts:

The Habitable Worlds Observatory (HabEx) and Large Ultraviolet Optical Infrared Surveyor (LUVOIR) were among the UV space telescope concepts considered in the decadal survey. The eventual mission design will combine elements from both HabEx and LUVOIR, with technology development taking place over the next decade.

Community Involvement in Mission Direction:

The decadal survey process involves input from the entire astrophysics community, ensuring a collaborative approach to setting priorities for future space missions.

Upcoming Milestones in John Hennessy’s Work:

Hennessy anticipates exciting developments in the coming years, including the launch of the Sprite CubeSat mission and the continued advancement of mirror coating technologies.

Notes by: BraveBaryon