Microstructural Analysis of Corrosion Inhibition in sub-100-nm-Scale Josephson Circuits

We have studied nanobridge Josephson junctions (nJJs) and nJJ-based circuits using microstructural analysis and measurements of electron transport properties to reveal the possible origins of a spread in superconducting parameters (Tc,Ic, etc.) and long-term stability. The structures were prepared by dc magnetron sputtering from Nb, Ti and TiN targets, with electron beam exposure of HSQ resist and reactive ion etching in pure SF6gas. Microstructural characterization was performed using aberration-corrected scanning transmission electron microscopy imaging and elemental mapping using energy-dispersive X-ray spectroscopy. The distributions of elements in nanostructures based on Ti-Nb-Ti heterostructures and TiN films were compared. Oxygen-free TiN nJJs are of interest for the realization of corrosion-resistant superconducting circuits, including qubits with operating temperatures down to 10 mK. Ti-Nb-Ti heterostructures contain oxygen that has been chemisorbed by the Ti layers and are intended primarily for operation at 4.2 K. Superconducting through-silicon vias between circuits on opposite sides of a wafer will also be realized by using direct writing of superconducting current leads with focused-ion-beam-induced deposition of superconducting films. Particular emphasis will be paid to the inhibition of oxygenation and corrosion on the nanometer scale by using new materials and methods, which promise to bring superconducting chip manufacture closer to circular-economy-related objectives. Our work helps to realize the large-scale integration of superconducting circuits that have long-term stability, including nanoSQUIDs, qubits and classical superconducting digital circuits, such as Single Flux Quantum based circuits.