Development of Self-Shunted Josephson Junctions For a Ten-Superconductor-Layer Fabrication Process: Nb/NbN$_x$/Nb Junctions

Abstract: To increase integration scale of superconductor electronics, we are developing a new, SFQ7ee, node of the fabrication process at MIT Lincoln Laboratory. In comparison to the existing SFQ5ee node, we increased the number of fully planarized superconducting layers to ten and utilized NbN and NbN/Nb kinetic inductors to increase the inductor number density above 100 million per cm$^2$. Increasing the Josephson junction (JJ) number density to the same level requires implementing self-shunted high-$J_c$ JJs. We investigated properties of Nb/NbN$_x$/Nb trilayer JJs as a potential replacement of high-$J_c$ Nb/Al-AlO$_x$/Nb JJs, where NbN$_x$ is a disordered, nonsuperconducting nitride deposited by reactive sputtering. Dependences of the $I_cR_n$ product and Josephson critical current density, $J_c$ on the NbN$_x$ barrier thickness and on temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated JJs can be described by the microscopic theory of SNS junctions, assuming no suppression of the energy gap in Nb electrodes near the NbN$_x$ interfaces and a Cooper pair decay length in the NbN$_x$ barrier of about 2.3 nm. Current-voltage characteristics of the JJs are well described by the RCSJ model. In the studied range $J_c$ < 10 mA/$\mu$$m^2$, the Nb/NbN$_x$/Nb JJs have lower specific resistance $R_nA$, lower $I_cR_n$ product, and a stronger dependence of the $I_cR_n$ on temperature than self-shunted or critically damped externally shunted Nb/Al-AlO$_x$/Nb JJs with the same critical current density; here $A$ is the JJ area, $R_n$ the JJ effective shunting resistance.