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

Niobium nitride (NbN) is a metallic superconductor that is widely used for superconducting electronics due to its high transition temperature (Tc) 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 Tc 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.