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Physics Procedia

Sub-Micron SNIS Josephson Junctions for Metrological Application

Natascia De Leo, Matteo Fretto, A. Sosso, Emanuele Enrico, Luca Boarino, V. Lacquaniti

Published Jan 1, 2012• 4 citations
Publisher

Abstract

The overdamped Nb/Al-AlOx/Nb superconductor-normal metal-insulator-superconductor SNIS Josephson junctions recently developed at INRIM exhibit high values of critical current density and characteristic voltage, making these devices appealing both for metrology and digital electronics. High current densities could allow reduction of junction size, increasing the integration level. In this work authors present first results concerning the realization of sub-μm SNIS including a nanolithographic technique (Electron Beam Lithography, EBL) into the conventional process, to reduce junction dimensions. Fabrication and electrical parameters have been evaluated considering the scaling of the areas of junctions, furthermore RF and temperature behaviour have been studied.

DOI
10.1016/j.phpro.2012.06.054

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Physics Procedia•Jan 1, 2012•4 citations
The overdamped Nb/Al-AlOx/Nb superconductor-normal metal-insulator-superconductor SNIS Josephson junctions recently developed at INRIM exhibit high values of critical current density and characteristic voltage, making these devices appealing both for metrology and digital electronics. High current densities could allow reduction of junction size, increasing the integration level. In this work authors present first results concerning the realization of sub-μm SNIS including a nanolithographic technique (Electron Beam Lithography, EBL) into the conventional process, to reduce junction dimensions. Fabrication and electrical parameters have been evaluated considering the scaling of the areas of junctions, furthermore RF and temperature behaviour have been studied.
This PhD Thesis is focused on the realization of superconducting quantum devices based on overdamped Nb/Al-AlOx/Nb SNIS (Superconductor - Normal metal - Insulator - Superconductor) tunneling Josephson junctions, interesting for several application fields (voltage metrology, digital electronics, radiation sensors, nanoSQUID, etc.). The challenges faced by quantum electronics and metrology are directing the new generations of devices towards smaller dimensions and higher levels of integration. Taking into account this requirement, the fabrication of SNIS-based devices has been addressed on downscaling the junction dimensions form the micro to the nanoscale. In particular, the effective area of junctions has been reduced exploiting three different lithographic techniques: the optical lithography to realize SNISs for a micrometer resolution, the Electron Beam Lithography (EBL) at the subµm and, finally, the Focused Ion Beam (FIB) sculpting method to achieve nanometer sizes. Specifically, prototypes have been realized exploiting the thin film technology, to guarantee a good control of electrical parameters of junctions, a higher reproducibility of their current-voltage I-V characteristics, and an accurate dimension control. Reliable and simpler fabrication processes have been implemented and validated, and the device downscaling was pursued without affecting the fundamental properties of SNIS junctions such as the non hysteretic I-V response and the skill on generating quantised voltage steps under radiofrequency irradiation. This work has thereby led to the definition and validation of a new generation of devices and processes down to the nanometer scale, and these approaches represent precious experiences of nanofabrication valuable for new research activities and projects
An interesting feature of Superconductor Normal metal Superconductor - Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor Normal metal Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with RnIc values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of fabrication parameters, stable and reproducible nonhysteretic curent-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2and characteristic voltages up to 450 μV at 4.2 K, overdamped junctions are suitable for many aplications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.
Superconductor-normal metal-superconductor (SNS) junctions are well-suited for large-scale integrated superconducting circuits. Potential applications are programmable voltage standards, digital-to-analog converters and large-scale RSFQ circuits. The application of SNS junctions provides some advantages, e.g. a simplified fabrication technology and reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages I/sub C/R/sub N/. In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-Al technology. Nb-Ti-Nb Josephson junctions and junction arrays were fabricated down to the submicrometer range with I/sub C/R/sub N/ values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent. Shapiro steps are observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed and successfully tested in 10 V SIS circuits.
Niobium-based junctions are still the best candidates for many small-scale applications of superconductivity and in particular, for the design and the fabrication of programmable voltage standards and D/A converters of fundamental accuracy. Non-hysteretic junctions with a large current density JC, and a reasonable characteristic voltage Vc are required, while, in order to make these circuits available to a wider market, operation at temperatures above 4.2 K would be important. In this work we report our results achieved on Nb/Al-AlOX/Nb junctions whose properties are studied in order to optimize the aforementioned features. Their behavior is analyzed showing how the overdamped characteristic is obtained with a less critical structure compared to other type of junctions. Moreover, using the scattering matrix approach, we have calculated a deviation of superconducting current-phase relation in such a SNIS Josephson junction from sinusoidal one and its effect on the current amplitudes of Shapiro steps as function of temperature. Measurement of the step dependence on temperature are also reported, where we considered the possibility of using bothn= 1 andn= 2 RF step above 4.2 K, for a possible reduction of the number of junctions in array circuits.
Device-modeling physics of four-layered Nb/Al-ultra-thin AlO x barrier-Nb Josephson structures is discussed, empha sizing practical applications. The temperature effect on the minigap induced in a proximized Al layer and that on a critical voltage across the SNIS junction is numerically simulated. The McCumber-Stewart parameter is found to decreasefromabout2at1.7Ktovalues nearunityat4.2K, becauseofthe temperature suppressedsubgap resistance.Due to the single-valued current-voltage dependence and comparatively high critical voltage values, the overdamped SNIS junctionshave demonstrated well-developed rf-induced featuresinthe current-voltage characteristicsevenfarabove4.3 K, appealing properties for programmable Josephson voltage standard applications.