Diffusion Stop-Layers for Superconducting Integrated Circuits and Qubits With Nb-Based Josephson Junctions

摘要

New technology for superconductor integrated circuits has been developed and is presented. It employs diffusion stop-layers (DSLs) to protect Josephson junctions (JJs) from interlayer migration of impurities, improve JJ critical current $(I_{rm c})$ targeting and reproducibility, eliminate aging, and eliminate pattern-dependent effects in $I_{rm c}$ and tunneling characteristics of ${rm Nb/Al/AlO}_{rm x}/{rm Nb}$ junctions in integrated circuits. The latter effects were recently found in Nb-based JJs integrated into multilayered digital circuits. E.g., it was found that Josephson critical current density $(J_{rm c})$ may depend on the JJ's environment, on the type and size of metal layers making contact to niobium base (BE) and counter electrodes (CE) of the junction, and also change with time. Such $J_{rm c}$ variations within a circuit reduce circuit performance and yield, and restrict integration scale. This variability of JJs is explained as caused by hydrogen contamination of Nb layers during wafer processing, which changes the height and structural properties of ${rm AlO}_{rm x}$ tunnel barrier. Redistribution of hydrogen impurities between JJ electrodes and other circuit layers by diffusion along Nb wires and through contacts between layers causes long-term drift of $J_{rm c}$.