  Microwave loss in superconducting titanium nitride (TiN) films is 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. However, while ALE processes
for TiN have been reported, they either employ HF vapor, incurring practical
complications; or the etch rate lacks the desired control. Further, the
superconducting characteristics of the etched films have not been
characterized. Here, we report an isotropic plasma-thermal TiN ALE process
consisting 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 selective
etching 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 that
the superconducting critical temperature of the etched film does not decrease
beyond that expected from the decrease in film thickness, highlighting the
low-damage nature of the process. These findings have relevance for
applications of TiN in microwave kinetic inductance detectors and
superconducting qubits.
