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 andis presented. It employs diffusion stoplayers (DSLs) to protect Josephsonjunctions (JJs) from interlayer migration of impurities, improve JJ criticalcurrent (Ic) targeting and reproducibility, eliminate aging, and eliminatepattern-dependent effects in Ic and tunneling characteristics of Nb/Al/AlOx/Nbjunctions in integrated circuits. The latter effects were recently found inNb-based JJs integrated into multilayered digital circuits. E.g., it was foundthat Josephson critical current density (Jc) may depend on the JJ'senvironment, on the type and size of metal layers making contact to niobiumbase (BE) and counter electrodes (CE) of the junction, and also change withtime. Such Jc variations within a circuit reduce circuit performance and yield,and restrict integration scale. This variability of JJs is explained as causedby hydrogen contamination of Nb layers during wafer processing, which changesthe height and structural properties of AlOx tunnel barrier. Redistribution ofhydrogen impurities between JJ electrodes and other circuit layers by diffusionalong Nb wires and through contacts between layers causes long-term drift ofJc. At least two DSLs are required to completely protect JJs from impuritydiffusion effects - right below the junction BE and right above the junctionCE. The simplest and the most technologically convenient DSLs we have found arethin (from 3 nm to 10 nm) layers of Al. They were deposited in-situ under theBE layer, thus forming an Al/Nb/Al/AlOx/Nb penta-layer, and under the firstwiring layer to junctions' CE, thus forming an Al/Nb wiring bi-layer. Asignificant improvement of Jc uniformity on 150-mm wafer has also been obtainedalong with large improvements in Jc targeting and run-to-run reproducibility.