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Microwave loss in superconducting titanium nitride (TiN) films is attributedto two-level systems in various interfaces arising in part from oxidation andmicrofabrication-induced damage. Atomic layer etching (ALE) is an emergingsubtractive fabrication method which is capable of etching with Angstrom-scaleetch depth control and potentially less damage. However, while ALE processesfor TiN have been reported, they either employ HF vapor, incurring practicalcomplications; or the etch rate lacks the desired control. Further, thesuperconducting characteristics of the etched films have not beencharacterized. Here, we report an isotropic plasma-thermal TiN ALE processconsisting of sequential exposures to molecular oxygen and an SF$_6$/H$_2$plasma. For certain ratios of SF$_6$:H$_2$ flow rates, we observe selectiveetching of TiO$_2$ over TiN, enabling self-limiting etching within a cycle.Etch rates were measured to vary from 1.1 \r{A}/cycle at 150 $^\circ$C to 3.2\r{A}/cycle at 350 $^\circ$C using ex-situ ellipsometry. We demonstrate thatthe superconducting critical temperature of the etched film does not decreasebeyond that expected from the decrease in film thickness, highlighting thelow-damage nature of the process. These findings have relevance forapplications of TiN in microwave kinetic inductance detectors andsuperconducting qubits.

Isotropic plasma-thermal atomic layer etching of superconducting TiN films using sequential exposures of molecular oxygen and SF$_6/$H$_2$ plasma