Self- and Mutual Inductance of NbN and Bilayer NbN/Nb Inductors in Planarized Fabrication Process With Nb Ground Planes

TL;DR

This paper reports on measurements of self- and mutual inductance in NbN and bilayer NbN/Nb inductors with Nb ground planes, demonstrating their potential for high-density superconducting digital electronics.

Contribution

It introduces a fabrication process for NbN and bilayer NbN/Nb inductors with tunable inductance and shows that mutual inductance is geometry-dependent, not superconducting property-dependent.

Findings

Mutual inductance is independent of superconducting properties within studied range.

Measured magnetic penetration depth is approximately 491 nm for NbN films.

Kinetic inductance indicates reduced superfluid density, likely due to carrier localization.

Abstract

We present measurements of the self- and mutual inductance of NbN and bilayer NbN/Nb inductors with Nb ground plane(s) fabricated in an advanced process for superconductor electronics developed at MIT Lincoln Laboratory. In this process, the signal traces of logic cell inductors are made either of a 200-nm NbN layer with $T_c$=15 K or of an in-situ deposited NbN/Nb bilayer, replacing a 200-nm Nb layer M6 in the standard SFQ5ee process with nine superconducting layers. Nb ground planes were preserved to maintain a high level of interlayer shielding and low intralayer mutual coupling. A two-step patterning of the top Nb and the bottom NbN layers of the NbN/Nb bilayer allows to create inductors in a very wide range of linear inductance values, from low values ~ 0.4 pH/$\mu$m typical for Nb geometrical inductors to ~ 35 pH/$\mu$m typical to thin-film kinetic inductors. Mutual inductance of NbN and Nb inductors, of NbN inductors, and of bilayer inductors is the same as between two Nb inductors with the same geometry and placement between the ground planes, i.e., mutual inductance does not depend on superconducting properties of the signal traces in the studied range of linewidths. We measured magnetic field penetration depth and kinetic inductance of NbN films with thickness t=200 nm to be $\lambda$ = 491+/-5 nm and 1.51 pH/sq, and 2.06 pH/sq at t=150 nm. The kinetic inductance was found to be larger than that expected for superconductors with short mean free path, indicating a reduction in the superfluid density, likely due to carrier localization effects. Kinetic inductance associated with right-angled bends of the NbN inductors is negligible at linewidths $w<{\lambda}^2/t$, indicating a very small current crowding in structures with superconducting ground plane(s). Implementation of NbN and NbN/Nb inductors can significantly increase integration scale of superconductor digital electronics.