Krypton Gas Helps Cornell Team Solve a Manufacturing Bottleneck in Quantum Chips

To build quantum computers, manufacturers need high-quality superconducting materials for microchips as well as a manufacturing process that fits existing factory tools. Tantalum, a corrosion-resistant metal, performs well in quantum devices but is difficult to manufacture: it must normally be deposited onto a silicon substrate at temperatures above 400 degrees Celsius (752 Fahrenheit), hotter than many semiconductor factories' tools can handle.

A Cornell University team led by assistant professor Valla Fatemi developed a method that lowers this deposition temperature to 200 degrees Celsius (392 Fahrenheit) while also producing tantalum films with substantially higher electronic conductivity, according to a study published in Nature Materials. The team, whose lead author is postdoctoral researcher Maciej Olszewski, achieved this by using krypton, a noble gas, instead of argon during a process called sputtering, in which the gas's ions knock tantalum atoms free so they can settle onto the silicon substrate.

At low temperatures, tantalum ordinarily forms in a crystal structure with poor performance; correcting that has previously required either heating above 400 degrees Celsius or seeding the surface with other materials. But if tantalum gets too hot, it mixes with the silicon substrate in a way that degrades chip performance. Because krypton atoms are heavier than argon atoms, they transfer more momentum when knocking tantalum free, stabilizing the desired crystal structure at a much lower temperature and widening the range in which manufacturers can reliably work.

The resulting thin films produced qubits, the basic information-processing units of quantum computers, of very high quality, according to the researchers. Fatemi said the work builds on the team's earlier research using argon-sputtered niobium films, and that the tantalum devices are now sensitive enough that even subtle changes to their fabrication produce measurable differences in performance.