Mitigation of Surface Dielectric Loss in Superconducting Quantum Devices via Combined Atomic Layer Etching and Deposition

We present a post-fabrication surface treatment combining atomic layer etching and deposition (ALE and ALD) to reduce the loss from the exposed surfaces of microwave superconducting quantum devices. This approach involves removing the metal native oxide through ALE and encapsulating the device surfaces in-situ with a thin dielectric layer using ALD. When applied to aluminum-based coplanar waveguide resonators fabricated on Silicon substrates and operating in the 5-6 GHz band, this method is found to decrease the loss associated with two-level system absorption at single-photon power levels by a factor of 2. Similarly, when applied to transmon qubits with transition frequencies covering a 2.5-5.5 GHz band, the ALE+ALD process yields an increase in the mean Q-factor from 1.75 to 4.5 million, with some devices achieving Q-factors in excess of 9 million. Material analysis reveals that the surfaces of the treated devices have less residual contamination induced by the fabrication processes and a thinner, more aluminum-rich aluminum oxide layer. Ongoing work is focused on further optimizing the process to include removal of the Silicon substrate native oxide, and to develop a better understanding of the respective impact of the ALE versus ALD steps on device performance.