Measuring the dielectric loss tangent of aluminum oxide with superconducting resonators

Recent demonstrations of millisecond-scale coherence with tantalum on silicon transmon qubits showed that improving the fabrication of the Al/AlOx/Al Josephson junction using high purity, ultrahigh vacuum deposition was critical for achieving lifetime-limited coherence [1], and that the performance of such qubits is limited by two-level system (TLS) absorption at the device surfaces. Here, we measure the linear absorption due to TLSs in aluminum oxide by measuring the power and temperature dependence of superconducting aluminum resonators, as has been previously demonstrated for tantalum [2]. We then vary film and surface properties to correlate surface losses using materials characterization techniques. We find that losses in aluminum superconducting devices are primarily limited by surface losses, and more specifically by amorphous aluminum oxides. Treatment with 49% HF removes surface aluminum oxides completely; however, the rapid oxide regrowth rate leads to under 2 nm fresh AlOx surface oxide. This treatment successfully reduces surface losses in aluminum resonators by 40%.

[1] Bland, M., Bahrami, F. et al. arXiv:2503.14798 (2025)

[2] Crowley, K., et al. Phys. Rev. X 13, 041005 (2023)