Characterization of fabrication methods to reach high coherence superconducting quantum circuits based on niobium

Abstract: 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. Additionally we optimize our dry etching, ozone descumming and metal interconnecting processes. Thereby, we reach quality factors well above 3x10^6 for thin-film niobium CPW resonators and qubit life- and coherence times of over 300 µs for transmon qubits.