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

  1. Lert Chayanun,
  2. Janka Biznárová,
  3. Lunjie Zeng,
  4. Per Malmberg,
  5. Andreas Nylander,
  6. Amr Osman,
  7. Marcus Rommel,
  8. Pui Lam Tam,
  9. Eva Olsson,
  10. August Yurgens,
  11. Jonas Bylander,
  12. and Anita Fadavi Roudsari
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 the 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 (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM).

Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

  1. Janka Biznárová,
  2. Amr Osman,
  3. Emil Rehnman,
  4. Lert Chayanun,
  5. Christian Križan,
  6. Per Malmberg,
  7. Marcus Rommel,
  8. Christopher Warren,
  9. Per Delsing,
  10. August Yurgens,
  11. Jonas Bylander,
  12. and Anita Fadavi Roudsari
We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270μs, corresponding to Q = 5 million, and a highest observed value
of 501μs. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.