Kavli Affiliate: Austin J. Minnich
| First 5 Authors: Azmain A. Hossain, Sela Murphy, David S. Catherall, Anthony J. Ardizzi, Austin J. Minnich
| Summary:
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 mechanisms such as 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 O$_2$ plasma and H$_2$/SF$_6$ plasma. Exposure to O$_2$
plasma rather than O$_2$ gas yields a greater fraction of Nb in the +5
oxidation state, which is then volatilized by NbF$_5$ formation with exposure
to an H$_2$/SF$_6$ plasma. The SF$_6$:H$_2$ flow rate ratio is chosen to
produce selective etching of Nb$_2$O$_5$ over NbN, enabling self-limiting
etching within a cycle. An etch rate of 1.77 r{A}/cycle was measured at 125
$^circ$C using ex-situ ellipsometry. The $T_c$ of the ALE-etched film is
higher than that of an RIE-etched film of a 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.
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