Atomic layer etching of niobium nitride using sequential exposures of O₂ and H₂/SF₆ plasmas Available to Purchase

Niobium nitride (NbN) is a metallic superconductor that is widely used for superconducting electronics due to its high transition temperature (⁠ $T c$⁠) and kinetic inductance. Processing-induced damage negatively affects the performance of these devices by increasing the microwave surface loss. Atomic layer etching (ALE), with its ability to etch with angstrom-scale control and low damage, has the potential to address these issues, but no ALE process is known for NbN. Here, we report such a process consisting of sequential exposures of O₂ plasma and H₂/SF₆ plasma. Exposure to O₂ plasma rather than O₂ gas yields a greater fraction of Nb in the +5 oxidation state, which is then volatilized by NbF₅ formation with exposure to an H₂/SF₆ plasma. The SF₆:H₂ flow rate ratio is chosen to produce selective etching of Nb₂O₅ over NbN, enabling self-limiting etching within a cycle. An etch rate of 1.77 Å/cycle was measured at 125  $°$C using ex situ ellipsometry. The $T c$ of the ALE-treated film is higher than that of a reactive ion etching-treated film of similar thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of NbN in single-photon detectors and superconducting microresonators.