Characterization of fabrication methods to reach high coherence superconducting quantum circuits

The fabrication of superconducting qubits and resonators with long coherence times and high quality factors is an important milestone on the way towards useful quantum processors. Although significant improvements in coherence time have been made over the last years, reaching qubit lifetimes well beyond 100 µs involves careful investigation of all fabrication steps. Here, we demonstrate that such high device qualities can be achieved by a combination of substrate cleaning, etching optimization and post-process sample cleaning. We optimize ion milling, ozone descumming and metal interconnecting processes. Thereby, we reach quality factors well above 3x10⁶ for thin-film niobium CPW resonators and qubit lifetimes of over 150 µs . Additionally we present our results on surface passivation methods and etching techniques from which we expect further improvements in the quality of superconducting quantum circuits.