4EPC-05 1 Controlling the interface properties of submicrometric Nb/Al-AlO x -Nb Josephson junctions Vincenzo Lacquaniti, Natascia De Leo, Matteo Fretto, Cristina Cassiago, Roberto Rocci, Andrea Sosso, and Mikhail Belogolovskii Abstract—Submicrometric Josephson devices are of a particular interest to the emerging field of quantum information, nanosized superconducting quantum interference devices (nano-SQUID) fabrication, AC voltage synthesis circuits for quantum metrology, etc. In this contribution, we report the development of our technology for producing non-hysteretic Nb/Al-AlOx-Nb four-layered junctions to the range of transverse dimensions less than micrometric ones, based on the focused ion beam (FIB) sculpting technique. We observed essential modifications of the main electrical characteristics as a result of the shrinking of the junction area, and relate them to the changes of the buried Nb/Al interface. We display that the critical current-versus- temperature dependence can provide information on the quality of the junction interfaces. This aspect could be useful to better control the entire fabrication process, allowing a further large- scale integration of Josephson nanodevices with improved transport characteristics. Index Terms—Focused Ion Beam, Josephson junctions, temperature dependence, asymmetric junctions. I. INTRODUCTION HE DYNAMICS of a Josephson junction is usually described by the RCSJ (resistively and capacitively shunted junction) model introduced by Stewart [1] and McCumber [2]. This approach applies to junctions whose dimensions transverse to the current flow direction is less than the Josephson penetration depth J, which is typically of the order of several micrometers. Thus, submicrometric junctions can be definitely described by the RCSJ model with the only variable quantity, the Josephson phase . The dynamics of a Josephson junction under bias current can be understood within the equivalent picture of a particle exposed to friction moving along the coordinate  in a tilted washboard potential U (φ) [3]. If the bias current I is less than the critical value Ic, the “particle” is trapped in the zero- voltage state. (Corresponding author: Matteo Fretto) V. Lacquaniti, N. De Leo, M. Fretto, C. Cassiago, A. Sosso, R. Rocci are with Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, I-10141, Italy, e-mail: v.lacquaniti@inrim.it; n.deleo@inrim.it. m.fretto@inrim.it; c.cassiago@inrim.it; a.sosso@inrim.it; r.rocci@inrim.it. M. Belogolovskii is with Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, 72 Rosa Luxemburg Str., 83114 Donetsk, Ukraine , e-mail: bel@fti.dn.ua For I > Ic it starts rolling down the potential with ‹φ› ≠ 0, which corresponds to a voltage drop over the junction. When the bias current is going back below Ic and then down to zero, the potential barrier is gradually increasing, so that the particle eventually stops its movement and returns back to the zero- voltage state. Whether it happens right below Ic or only close to I = 0 depends on the relation of the friction to the “particle” mass. A very clear and commonly used measure for the damping in a Josephson junction is the McCumber-Stewart damping parameter ȕc, a product of the characteristic Josephson frequency ωc = 2eVc/ħ, where the characteristic voltage Vc = IcR, and the decay time RC = RC, R is the device resistance in the operating voltage range, C is its capacitance . (1) For ȕc >> 1, the junction is in the underdamped regime, so that the “particle” will continue rolling until I ≈ 0 is reached. It is equivalent to a hysteresis in the current-voltage (I-V) curve. Inversely, for ȕc << 1, the overdamped regime, the particle will stop its movement after the occurrence of the first potential barrier at I ≈ Ic without any hysteresis in the I-V characteristic. Josephson junctions with overdamped current-voltage curves are showing wide operative margins and represent an area of intense interest for different applications, from fast superconductive (S) electronics to AC voltage synthesis circuits for quantum metrology. Many efforts have been dedicated to the development of fabrication processes of the overdamped devices. Up to now, for this aim externally shunted SIS, with an insulating (I) barrier, and internally shunted SNS, with a normal (N) conducting film, tri-layers have been used. At the same time, some novel technologies which are better suited to meet as many requirements imposed by practical applications as possible are evidently needed. These requirements include a large range of electrical parameters like Ic and Vc, an extended stability relating the temperature effect, possibility to realize circuits with reduced dimensions and suitably high Vc values for RSFQ logic and programmable arrays in order to achieve the same maximum voltage output with a less number of junctions and to use higher order steps in the I-V curves under external radiation. Besides it, it is desirable to base the fabrication process on already existing Nb technology and to work at temperatures above that of liquid helium, using cryocoolers of 10 K family. In our previous papers [4]-[7] we developed such type of overdamped Josephson junctions, namely, Nb/Al-AlOx-Nb four-layered structures with comparatively thick (30-120 nm) T