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

Superconducting qubits and microwave resonators based on tantalum thin films have recently demonstrated large increases in performance. This makes Ta an attractive material for superconducting quantum computing applications, but so far direct deposition has largely relied on high substrate temperatures exceeding 400 °C to achieve the cubic (bcc) phase of tantalum. Here we show that changing the sputter gas from argon to krypton promotes synthesis of bcc tantalum films on silicon at temperatures as low as 200 °C. This provides a wide process window compatible with back-end-of-line fabrication standards. The microwave performance of coplanar-waveguide resonators fabricated from krypton-sputtered films shows an excellent tight performance distribution. Higher-temperature-grown films exhibit higher losses, in correlation with the degree of tantalum-silicon intermixing. Finally, we demonstrate with these films transmon qubits with a compact, 20-μm capacitor gap, achieving quality factors up to 16.9 million.