Supercritical NF₃ Surface Passivation for Quantum Devices

Surfaces of superconducting qubits have been demonstrated to contribute dissipation and decoherence correlated with oxide growth.  Currently, the primary strategy for lossy surfaces is to etch devices with an oxide-removing, HF-based etchant followed by quickly moving devices into the vacuum space of a dilution refrigerator for characterization.  Other strategies utilize a capping layer to prevent oxide growth via materials such as noble metals and MgO, but this approach fails to cap dry-etched device edges which are locations of high electric field and thus the most undesirable location for oxides.  In order to inhibit oxide growth on all surfaces of a qubit, we propose a supercritical NF₃ treatment for the passivation of superconducting surfaces and resulting stabilization of device performance against air exposure.  We present our methodology for accomplishing the NF₃ treatment and discuss the first phase of this research involving the passivation of Ta films on Si substrates.