A new ingredient for quantum computing: krypton gas

To commercialize quantum computing, manufacturers need high-quality superconducting materials for microchips, but they also require a reliable, sustainable nanofabrication process.

Tantalum is a corrosion-resistant metal that meets the first criteria but not the second. That's because it has to be deposited on a substrate at temperatures that typically exceed 400 degrees Celsius - too hot for many semiconductor foundries' current tools.

Cornell researchers have developed a method that uses krypton gas to slash that deposition temperature to 200 degrees while depositing on silicon, a standard high-quality substrate. The process resulted in thin films that also have substantially higher electronic conductivity.

"Tantalum as a material has been shown to be very exciting from a device performance perspective, but its manufacturability had some question marks because of integration challenges such as required process temperatures," said Valla Fatemi, assistant professor and Aref and Manon Lahham Faculty Fellow in the Cornell Duffield College of Engineering, who led the project. "We figured out a relatively simple change, by using some physical and materials insights, to bring that temperature down into a zone that is translatable to nanofabrication systems in industry, while showing that in our academic context we can have leading-edge performance of these devices."

The findings published Aug. 18 in Nature Materials. The study's lead author is postdoctoral researcher Maciej Olszewski, Ph.D. '26

Quantum computing promises to handle faster, more complex computations by leveraging the unique quantum mechanical properties of superconductors, which carry current with little to no energy loss. The basic building blocks of the hardware are quantum bits, or qubits, which can store and process massive quantities of information. But correctly combining high-performance materials and nanofabrication techniques has proved to be one of the primary bottlenecks slowing the technology's commercial development.

Fatemi's lab explores the intersection of experimental condensed matter physics and quantum devices. Recently, his team developed the characterization and nanofabrication methods to achieve high-end performance for niobium-based materials. In that work, niobium was bombarded with ions of the noble gas argon, which knocked off nobium's atoms so they deposited on a substrate - a process called sputtering - forming a thin film.

"That work got the ball rolling for us in understanding a lot of the surface science and how that correlates with improving performance," Fatemi said.

The researchers swapped niobium for tantalum, a transition metal with more stable surface properties and high resistance to corrosion - and a leading material for many superconducting components. But tantalum is not without challenges. When deposited at lower temperatures, the material is in a crystal phase with undesirable properties. That problem that can only be remedied by heating it more than 400 degrees Celsius for deposition or by seeding the surface with other materials to alter its performance. At the same time, if tantalum gets too hot, it can mix with the silicon substrate and form a thick layer that leads to information loss and lowers the chip's performance.

The new solution: Olszewski hypothesized that using krypton as the ionized gas instead of argon would transfer more momentum and kick off the tantalum atoms with greater energy, thereby stabilizing the targeted crystal phase on the silicon substrate - all at a much lower temperature.

"There's this whole set of tooling and fabrication lines that don't really go above 400 Celsius, and they're built for that. And tantalum on silicon, when you use the old method, was right on the border of that," Olszewski said. "There was little margin to do things reliably. Using krypton brought that threshold down to 200 Celsius. So you now have this big window to be able to do reliable fabrication."

A crucial final step of the process: adding what is known as a Josephson junction, an overlap of two metals separated by an electrical insulator that enables the quantum tunneling of electrons that creates qubits.

"The performance of the devices in our lab are very sensitive to that step now. Historically, that was not necessarily obvious. But our devices are at a level of performance that we're able to see big differences based on subtle changes that we make to the formation of the Josephson junction," Fatemi said. "So we're entering a new domain for what matters or doesn't matter."

The researchers found that the resulting thin film produced qubits of incredibly high quality and greatly boosted device performance.

"We're right at the world-leading edge," Fatemi said. "This is a big step forward not only for our group but also Cornell's efforts in superconducting quantum information devices."

Co-authors include David Muller, the Samuel B. Eckert Professor of Engineering in Duffield Engineering and co-director of the Kavli Institute at Cornell for Nanoscale Science; doctoral students Lingda Kong, Daniel Tong, Haoran Lu, Saswata Roy and Luojia Zhang; postdoctoral researchers Simon Reinhardt and Xinyi Du; Gabriele Di Gianluca of University of Florida, Gainesville and the Cornell NanoScale Science and Technology Facility Research Experience for Undergraduates (CNF REU) program; and Aleksandra Biedron of the New York Center for Research, Economic Advancement, Technology, Engineering and Science (NY CREATES).

The research was supported by the Microelectronics Commons Program, a DoW initiative, and the U.S. Air Force Office of Scientific Research. The researchers made use of CNF, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation, and the Cornell Center for Materials Research.