Characterization of fabrication methods to reach high coherence superconducting quantum circuits

The fabrication of highly coherent superconducting qubits 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, deposition optimization and post-process sample cleaning. We apply various characterization methods such as x-ray diffractometry, surface characterization and residual resistance measurements to optimize the quality of superconducting thin films. We also optimize ion milling, ozone descumming and metal interconnecting processes. Thereby, we reach quality factors well above 10^6 for thin-film aluminium and niobium CPW resonators and qubit lifetimes of at least 150 µs.