Microstrip resonators with internal quality factors over one million made from MBE-Al films capped with an in-situ deposited oxide layer

Superconducting resonators are used for dispersive readout on superconducting qubit chips for quantum computing. The internal quality factor (Q_i) of a resonator is a measure of the ability to store microwave energy and is closely related to the coherence time of qubits. Substantial efforts have been made to improve the Q_i of resonators, which includes improving material quality by enhancing the crystallinity of the film and optimizing the fabrication process. However, most reported Q_i’s for Al resonators patterned on sapphire are lower than 1x10⁶.

Here, we present high-quality Al resonators fabricated on sapphire. The single-crystal Al films were grown by molecular beam epitaxy (MBE) and were immediately capped with a stoichiometric Al₂O₃ layer which was in-situ deposited on the Al film using e-beam evaporation. This unique approach is appealing since it provides an oxide layer with uniform and controllable thickness to protect the underlying superconducting film, which cannot be achieved by the conventional oxidation method. The high-quality Al₂O₃ layer suppressed the formation of two-level systems and thus reduced the energy relaxation channels. With this approach and an optimized fabrication process, we demonstrated resonators with Q_i exceeding one million at 5.1 GHz in a single photon limit.