Realization of Superconducting Quantum Devices from Micro to Nano Scale at INRIM

In the last century, the International System of Units (SI) changed its perspectives, evolving from an artifact-based system to a system mainly based on physical processes at the atomic level and fundamental constants. In particular, the discovery of the Josephson effect made available a quantum voltage standard. As an consequence, in the last decade the worldwide consistency of the representation and maintenance of the electrical units and the electrical measurements based on it has improved a hundredfold. The Josephson effect, together with the quantum Hall effect for the definition of a modern resistance standard, will certainly play a major role in the next modernization of the SI, when the kilogram, the last base unit still based on a physical artifact, will be linked to fundamental constants. Furthermore, the definition of a new SI is strictly linked to the technological issues for the realisation of new devices capable to reproduce the electrical units with quantum accuracy. The challenges faced by modern metrology, sensing and quantum electronics are directing the new generations of devices towards smaller dimensions and higher levels of integration. Taking into account this requirement, at INRIM we fabricate and characterize superconducting quantum devices for a wide range of applications. The devices are based on the overdamped Nb/Al-AlOx/Nb SNIS (Superconductor-Normal metal-Insulator-Superconductor) Josephson junctions and exploiting thin film technology to guarantee a good control of electrical parameters of junctions, a higher reproducibility of their currentvoltage response, and increasingly an accurate dimension control. The main objective of the current experimental activity is addressed on downscaling the junction size from the micro to the nanoscale, exploiting three different lithographic techniques: optical lithography to realize SNISs with a size resolution of the order of the microme- ter, electron beam lithography (EBL) at the submicrometer range and, lastly, a recently proposed mesoscopic tunnel junction nano-fabrication technique, based on focused ion beam (FIB). We report the fabrication process and the electrical characterisation of a 1 Volt binary-divider array based on 8192 overdamped SNIS Josephson junctions for the programmable Josephson Voltage Standard (PJVS) [1] and of a 3D nano-SQUID (Super- conducting Quantum Interference Device) [2] as a sensor for the detection of magnetic nanoparticles. Preliminary results on Nb-based nano-constrictions fabricated by FIB and anodisation will also be presented [3]. The effect of FIB exposure and anodization voltage on the superconducting transition temperature and on the temperature dependence of the resistance of the nano-constrictions has been investigated, as well as the behaviour of these constrictions as overdamped SNS-like Josephson junctions. [1] V. Lacquaniti, N. De Leo, M. Fretto, A. Sosso, F. Mueller, J. Kohlmann, Superconductor Sci. Technol. 24 (2011). [2] C. Granata, A. Vettoliere, R. Russo, M. Fretto, N. De Leo, V. Lacquaniti, Appl. Phys. Lett. 103 (2013). [3] N. De Leo, M. Fretto, V. Lacquaniti, C. Cassiago, L. D'Ortenzi, L. Boarino, S. Maggi, accepted for publication in IEEE Trans. Appl. Supercond. (2016).