Making Materials for Quantum Technologies: Laser Technique Unlocks Extreme Conditions for Thin Film Synthesis

Thin films might not come up in conversation every day, but they are all around us. Take the metallic plastic films of chip bags, for example, or the anti-reflective coatings on eyeglasses. Even the coatings on pills that make them easier to swallow are thin films. Depositing extremely thin layers of materials in a consistent and uniform way is also crucial to the production of semiconductors, which are the foundation of modern electronics.

Not all materials can be easily deposited in such thin layers, such as materials with very high melting points. Now, Caltech researchers led by Austin Minnich, professor of mechanical engineering and applied physics, and deputy chair of the Division of Engineering and Applied Science, have demonstrated a laser-based method for generating thin films of materials, such as niobium. The work could directly impact superconducting electronics used in quantum computers.

The team recently described the work in a paper in the journal Applied Physics Letters.

A common route to producing thin films is to heat the source material up to a high enough temperature that it produces vapor, which then condenses on the growth surface to produce a thin film. But materials that contain certain elements that scientists refer to as "ultra-refractory" will only melt above a scorching 3,000 Kelvin (2726.85°C).

"If you think of your coffee cup with vapor coming off, it's the same idea except, let's say, you're doing it with tungsten. Now your coffee would be at something like 3,600 Kelvin," Minnich says. "What kind of cup is strong enough to withstand 3,600 Kelvin?"

The answer is there isn't one. That is why it has been extremely challenging to create thin films of such ultra-refractory materials through the typical thermal evaporation route. But thanks to developments in laser technology for metal welding and cutting, a workaround is now possible: skip the cup.

With special funding from a Caltech De Logi Science and Technology Grant, the scientists custom built an instrument to enable the approach, marking the first time it has been used in the United States. This process involves obtaining a pellet of the desired material and focusing a high-power laser on it such that only the very center melts while the pellet overall remains solid, serving as its own "cup." The melted portion of the pellet generates enough vapor to condense and deposit on a substrate above.

In the new paper, the authors use their technique to deposit ultrathin nickel films using a 1-kilowatt fiber laser narrowly focused on a nickel target within a vacuum chamber. Close to nickel's melting point of 1728 Kelvin (1454.85°C), the heat vaporized enough material to produce a thin film. Measurements of the electrical conductivity of the resulting film indicated that the film was of equal or better quality to films deposited using other methods.

"This work highlights how technological developments in apparently unrelated industries like metal cutting can have a large impact in other fields," Minnich says. "TLE has a lot of interesting implications for quantum technology because many superconducting quantum computers use ultra-refractory materials such as niobium, tantalum, and their alloys."

He emphasizes the importance of the flexible De Logi funding, which provides grants of up to $1 million to Caltech professorial and research faculty for projects that are likely to have a strong long-term impact on science and/or technology.

The lead author of the paper, "Characterization of ultrathin nickel films deposited by thermal laser evaporation," is David S. Catherall (Ph.D. '26). Additional authors include current Caltech graduate students Yifei Yan, Finley B. Donachie, and Azmain A. Hossain (Ph.D. '26). The work was supported by the Air Force Office of Scientific Research, as well as the De Logi Grant and an Explorer Grant from Caltech's Resnick Sustainability Institute. The current study was based upon work supported by the National Science Foundation Graduate Research Fellowship Program. Critical support and infrastructure was provided by the Kavli Nanoscience Institute and the Molecular Materials Research Center of the Beckman Institute at Caltech.