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IEEE Transactions on Applied Superconductivity

Controlling the Interface Properties of Submicrometric Nb/A– $\hbox{AlO}_{\rm x}$–Nb Josephson Junctions

V. Lacquaniti, Natascia De Leo, Matteo Fretto, Cristina Cassiago, Roberto Rocci, A. Sosso, Mikhail A. Belogolovskii

Published Nov 7, 2014• 4 citations
Publisher

Abstract

Submicron 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 paper, we report on the development of our technology for producing non-hysteretic Nb/A-AlOx-Nb four-layered junctions with a diverse range of submicron dimensions, 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 show that the electrical parameters-versus-temperature dependence can provide information on the quality of the junction interfaces. This aspect could be useful to better control of the entire fabrication process, allowing a further large-scale integration of Josephson nanodevices with improved transport characteristics.

DOI
10.1109/tasc.2014.2367232

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Controlling the Interface Properties of Submicrometric Nb/A– AlOx\hbox{AlO}_{\rm x}AlOx​–Nb Josephson Junctions
Submicron 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 paper, we report on the development of our technology for producing non-hysteretic Nb/A-AlOx-Nb four-layered junctions with a diverse range of submicron dimensions, 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 show that the electrical parameters-versus-temperature dependence can provide information on the quality of the junction interfaces. This aspect could be useful to better control of the entire fabrication process, allowing a further large-scale integration of Josephson nanodevices with improved transport characteristics.
We have studied low-temperature electrical transport properties of nanoscale Nb∕(Al–)Al2O3∕Nb and NbN∕AlN∕NbN Josephson junctions (JJs) fabricated by focused-ion-beam (FIB) milling. This FIB fabrication process yields high-quality JJs whose superconducting gap energy agrees with the bulk value. In this paper, we report the improvement in the precision of the fabrication technology by employing a weaker ion beam current and by introducing a step of XeF2-gas-assisted milling, which allowed us to reduce the anodization voltage. For Nb JJs, we measured the current-voltage (I-V) characteristics of single-electron transistors (SETs) and examined the current observed within the superconducting energy gap. At finite voltages, we observed current peaks due to the Josephson-quasiparticle (JQP) cycle. Based on the JQP-peak positions, we estimated the charging energy Ec of the SETs. For all SETs, Ec∕kB is larger than 1K, which is consistent with 0.1×0.1μm2 junctions. We also analyzed the superconducting critical current at zero voltage. The ratio Ic∕Ic0 depended on the junction size, where Ic is the measured critical current and Ic0 is the theoretical one. The I-V curves of NbN single Josephson junctions also depended on the junction size, and they were qualitatively different, especially around the energy gap. We will discuss the possible origins for the junction-size dependence.
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
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.
Fabrication And Characterization Of High-Quality Nb/Al-AlOx_xx​/Nb Cross-Type Josephson Tunnel Junctions Utilising CMP-based Planarisation Techniques
Josephson tunnel junctions (JJs) are the fundamental building blocks of today's most advanced superconducting electronic components, such as qubits and superconducting quantum interference devices (SQUIDs). Given the ongoing demand for scalability of these devices on the wafer-scale, it is crucial to control the fabrication process as precisely as possible to ensure uniform quality and reproducibility. Today, window-type JJs are often used in the production of DC SQUIDs and while they are reliable and reproducible, they suffer from alignment inaccuracies caused by photolithography and unavoidable parasitic capacitances, thus limiting the energy sensitivity of DC SQUIDs. These problems can be circumvented by producing JJs with cross-type geometry, which allows for smaller junction areas and eliminates parasitic capacitances. Here we discuss the fabrication of Nb/Al-AlOx/Nb-based cross-type JJs, where the trilayer is embedded in sputter-deposited SiO2_22​ to allow for planarisation of the structures and to ensure the reliable insulation of the sidewalls of the bottom electrode. Instead of lift-off processes that require a lot of time and potentially leave unwanted wings along the edges of microstructures behind that might compromise subsequent layers, we use chemical-mechanical polishing (CMP) for the removal of excess SiO2_22​. This is not only much faster, but produces smooth and uniform surfaces, which in turn effectively improves the reliability of our JJ production process resulting in a high yield of over 90 %90\,\%90% on wafer-scale. In addition, we discuss the influence of fabrication details on the quality and electrical properties of our JJs with different junction areas down to 1 μm×1 μm1\,\mathrm{μm} \times 1\,\mathrm{μm}1μm×1μm distributed across the wafer and extract the junction specific quality parameters from their IV-characteristics and their Fraunhofer patterns.
Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.