Effect of ion milling on superconducting Aluminum resonators

Major challenges with Aluminum based qubits fabrication process arises from ion milling that induces increased surface roughness leading to higher TLS losses and reduced Internal Quality factor of resonators (Qi) . Qubits measured using dispersive coupling via these resonators are subjected to lower coherence time and faster resonator decays through increased single photon energy loss .

This work demonstrates a comprehensive study on the effect of ion milling using Argon in the loadlock of the deposition tool during fabrication. The resonators are part of a subtractive etch process where qubits and resonators are fabricated in a single step process, superior to the traditional lift-off process.[1] Varying milling time, power has seen to directly affect the loaded and internal quality factors of the resonators which is accredited to rougher top surface increasing TLS coupling and increased metal-air loss boundary. A wide range of resonators are studied at their single photon limit which are undercoupled, overcoupled and critically coupled regimes. An improved mill-less process using Niobium is also studied for high Qi single photon resonators to compare with this work.

1. Stehli et.al Coherent superconducting qubits from a subtractive junction fabrication process. Appl. Phys. Lett. 21 September 2020; 117 (12): 124005.