Development of Self-Shunted Josephson Junctions for a Ten-Superconductor-Layer Fabrication Process: Nb/NbN_x/Nb Junctions

To increase the integration scale of superconductor electronics, we are developing a new node, titled SFQ7ee, 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 possible inductor number density above a hundred million inductors per cm ² . Increasing the Josephson junction number density to the same level requires implementing self-shunted high- J _c Josephson junctions. We investigated the properties of Nb/NbNx/Nb trilayer junctions as a potential replacement of high- J _c Nb/Al-AlO _x /Nb junctions, where NbN _x is a disordered, overstoichiometric, nonsuperconducting nitride deposited by reactive sputtering. Dependences of the I_cR_n product and Josephson critical current density, Jc on the NbNx barrier thickness and temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated junctions can be well 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 NbNx barrier, ξ _n T _c of about 2.3 nm. Current-voltage characteristics of the junctions are well described by the resistively and capacitively shunted junction (RCSJ) model without excess current. In the studied range J _c < 10 mA/μm ² , the Nb/NbN _x /Nb junctions have lower values of the specific resistance R_nA , lower IcRn products, and a stronger dependence of the I_cR_n on temperature than the self-shunted or critically damped externally shunted Nb/Al-AlO _x /Nb junctions with the same critical current density; A is the junction area, I _c the junction critical current, and R _n the effective shunting resistance.