Superconductor Electronics Fabrication Process with MoN$_x$ Kinetic Inductors and Self-Shunted Josephson Junctions

TL;DR

This paper introduces a new superconductor electronics fabrication process utilizing MoN$_x$ kinetic inductors and self-shunted Josephson junctions to enable higher integration density in SFQ circuits.

Contribution

Development of a fabrication process with self-shunted high-J$_c$ JJs and compact MoN$_x$ kinetic inductors, reducing circuit area and improving integration scale.

Findings

MoN$_x$ films with T$_c$ up to 8 K and tunable inductance were characterized.

Nb-based self-shunted JJs with various tunnel barriers were fabricated and tested.

Electron transport in Si$_{1-x}$Nb$_x$ barriers varies with x, affecting junction performance.

Abstract

Recent progress in superconductor electronics fabrication has enabled single-flux-quantum (SFQ) digital circuits with close to one million Josephson junctions (JJs) on 1-cm$^2$ chips. Increasing the integration scale further is challenging because of the large area of SFQ logic cells, mainly determined by the area of resistively shunted Nb/AlO$_x$-Al/Nb JJs and geometrical inductors utilizing multiple layers of Nb. To overcome these challenges, we are developing a fabrication process with self-shunted high-J$_c$ JJs and compact thin-film MoN$_x$ kinetic inductors instead of geometrical inductors. We present fabrication details and properties of MoN$_x$ films with a wide range of T$_c$, including residual stress, electrical resistivity, critical current, and magnetic field penetration depth {\lambda}$_0$. As kinetic inductors, we implemented Mo$_2$N films with T$_c$ about 8 K, {\lambda}$_0$ about 0.51 {\mu}m, and inductance adjustable in the range from 2 to 8 pH/sq. We also present data on fabrication and electrical characterization of Nb-based self-shunted JJs with AlO$_x$ tunnel barriers and J$_c$ = 0.6 mA/{\mu}m$^2$, and with 10-nm thick Si$_{1-x}$Nb$_x$ barriers, with x from 0.03 to 0.15, fabricated on 200-mm wafers by co-sputtering. We demonstrate that the electron transport mechanism in Si$_{1-x}$Nb$_x$ barriers at x < 0.08 is inelastic resonant tunneling via chains of multiple localized states. At larger x, their Josephson characteristics are strongly dependent on x and residual stress in Nb electrodes, and in general are inferior to AlO$_x$ tunnel barriers.