Superconductor Electronics Fabrication Process with MoNx Kinetic Inductors and Self-Shunted Josephson Junctions

Recent progress in superconductor electronics fabrication has enabled single-flux-quantum (SFQ) digital circuits with close to one million Josephson junctions (JJs) on ${\text{1}}\hbox{-}{\text{cm}}^{2}$ 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- $J_{{\rm{c}}}$ JJs and compact thin-film MoN_x kinetic inductors instead of geometrical inductors. We present fabrication details and properties of ${\text{MoN}}_{x}$ films with a wide range of $T_{{\rm{c}}}$, including residual stress, electrical resistivity, critical current, and magnetic field penetration depth $\lambda _{0}$. As kinetic inductors, we implemented Mo₂ N films with $T_{{\rm{c}}}$ about 8 K, $\lambda _{0}$ about 0.51 μm, 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 $J_{{\rm{c}}}= {\text{0.6}}\,{\text{mA}}{/}\mu{\text{m}}^{2}$ , and with 10-nm thick Si_1−xNb_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−xNb _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.