Krypton-sputtered tantalum films for scalable high-performance quantum devices

TL;DR

This study introduces krypton-sputtered tantalum films grown at lower temperatures, enabling scalable fabrication of high-performance superconducting quantum devices with improved microwave properties and qubit quality factors.

Contribution

It demonstrates that krypton gas promotes BCC tantalum growth at lower temperatures, compatible with standard semiconductor processes, and validates high-quality quantum device performance.

Findings

Krypton sputtering enables BCC Ta growth at 200°C.

Films show higher electronic conductivity and superconductivity.

Qubits achieve median quality factors up to 14 million.

Abstract

Superconducting qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and qubits. This makes Ta an attractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.