Plasma Thermal Atomic Layer Etching of Niobium Nitride using Sequential Exposures of O₂ Plasma and H₂/SF₆ Plasma

Microwave loss in thin film superconducting niobium nitride (NbN) films has been attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with angstrom-scale etch depth control and potentially less damage. To the best of our knowledge, we report the first NbN ALE process consisting of sequential exposures to oxygen plasma and an SF₆/H₂ plasma. For certain ratios of SF₆⁠:H₂ flow rates, we observe selective etching of NbO_x over NbN, enabling self-limiting etching within a cycle. Etch rates were measured at 2.1 Å/cycle at 125°C using ex-situ ellipsometry. We also investigated the effects of the ALE process on the surface chemistry of thin film NbN and its cryogenic electrical properties. These findings have relevance for applications of NbN in superconducting quantum devices.