Characterization of process-related interfacial dielectric loss in aluminum-on-silicon by resonator microwave measurements, materials analysis, and imaging

We systematically investigate the influence of the fabrication process on dielectric loss in aluminum-on-silicon superconducting coplanar waveguide resonators with internal quality factors (Qi) of about one million at the single-photon level. These devices are essential components in superconducting quantum processors; they also serve as proxies for understanding the energy loss of superconducting qubits. By systematically varying several fabrication steps, we identify the relative importance of reducing loss at the substrate–metal and substrate–air interfaces. We find that it is essential to clean the silicon substrate in hydrogen fluoride (HF) prior to aluminum deposition. A post-fabrication removal of the oxides on the surface of the silicon substrate and the aluminum film by immersion in HF further improves the Qi. We observe a small, but noticeable, adverse effect on the loss by omitting either standard cleaning (SC1), pre-deposition heating of the substrate to 300 °C, or in situ post-deposition oxidation of the film’s top surface. We find no improvement due to excessive pumping meant to reach a background pressure below 6 × 10−8 mbar. We correlate the measured loss with microscopic properties of the substrate–metal interface through characterization with x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, transmission electron microscopy, energy-dispersive x-ray spectroscopy, and atomic force microscopy.