Diffusion stop-layers for superconducting integrated circuits and qubits with Nb-based Josephson junctions

TL;DR

This paper introduces diffusion stop-layers (DSLs) made of thin aluminum layers to protect Nb-based Josephson junctions in superconducting circuits, significantly improving their uniformity, reproducibility, and stability by preventing impurity diffusion effects.

Contribution

It presents a novel implementation of in-situ deposited Al diffusion stop-layers that enhance JJ performance and reliability in superconducting integrated circuits.

Findings

Improved Jc uniformity across 150-mm wafers.

Enhanced reproducibility of Josephson junction critical currents.

Elimination of aging and pattern-dependent effects in Nb-based JJs.

Abstract

New technology for superconductor integrated circuits has been developed and is presented. It employs diffusion stoplayers (DSLs) to protect Josephson junctions (JJs) from interlayer migration of impurities, improve JJ critical current (Ic) targeting and reproducibility, eliminate aging, and eliminate pattern-dependent effects in Ic and tunneling characteristics of Nb/Al/AlOx/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 (Jc) 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 Jc 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 AlOx 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 Jc. At least two DSLs are required to completely protect JJs from impurity diffusion effects - right below the junction BE and right above the junction CE. The simplest and the most technologically convenient DSLs we have found are thin (from 3 nm to 10 nm) layers of Al. They were deposited in-situ under the BE layer, thus forming an Al/Nb/Al/AlOx/Nb penta-layer, and under the first wiring layer to junctions' CE, thus forming an Al/Nb wiring bi-layer. A significant improvement of Jc uniformity on 150-mm wafer has also been obtained along with large improvements in Jc targeting and run-to-run reproducibility.