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Superconductor Science and Technology

Fabrication of Josephson superconductor-normal metal-superconductor series arrays using a focused ion beam

Richard Moseley, Anthony J. Bennett, W.E. Booij, Edward J. Tarte, Mark G. Blamire

Published Nov 1, 1999• 2 citations
Publisher

Abstract

A new fabrication method for superconductor-normal metal-superconductor junctions employing focused ion beam patterning of a superconductor-normal metal bilayer has been used to make series junction arrays. We investigated their current-voltage (I-V) characteristic and examined their response to microwaves at 4.2 K. Multiple integer Shapiro steps were observed in the I-V response of an array to microwaves. In addition, we found that the properties of each junction in a double-junction series array did not change in the junction spacing range studied (0.2-8 µm). Modelling the I-V characteristic of large arrays showed the junction variation of the critical current was 20%.

DOI
10.1088/0953-2048/12/11/353

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Applied Physics Letters•Oct 4, 1993•54 citations
A new fabrication method for superconductor-normal metal-superconductor junctions employing focused ion beam patterning of a superconductor-normal metal bilayer has been used to make series junction arrays. We investigated their current-voltage (I-V) characteristic and examined their response to microwaves at 4.2 K. Multiple integer Shapiro steps were observed in the I-V response of an array to microwaves. In addition, we found that the properties of each junction in a double-junction series array did not change in the junction spacing range studied (0.2-8 µm). Modelling the I-V characteristic of large arrays showed the junction variation of the critical current was 20%.
We have applied our existing focused ion beam based nanoscale planar SNS bridge junction technology to the fabrication of series arrays, with a view to lumped array applications. Single junctions are created in a micrometre-width bilayer track (125 nm Nb on 75 nm Cu) by milling a trench 50 nm wide in the upper superconducting layer. Individual junctions with non-hysteretic, resistively-shunted current-voltage characteristics, critical current I C ~1 mA and characteristic voltage I C R N ~50 µV at 4.2 K can be fabricated routinely. The characteristics of 10-junction series arrays at junction spacings of 0.2 µm to 1.6 µm have been studied at 4.2 K and above. Locking of all the junctions in the array under an applied microwave field is observed as the transition temperature is approached. This effect is achieved at a lower temperature for shorter junction spacings, suggesting a penetration depth-dependent electromagnetic coupling mechanism. Measurements of differential resistance versus current reveal the I C distribution of individual junctions within the array. Spreads of critical current δ I C = I Cmax / I Cmin ~1.5 at 4.2 K are typically observed. The spread in normal state resistance, R N between junctions is negligible in comparison, as the unbroken normal metal layer shunts all of the junctions in the array. This allows locking to be achieved in spite of the appreciable spread in I C .
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.
Single nonhysteretic superconductor-metal-superconductor Josephson junctions consisting of Nb-Ti-Nb were prepared using the well-known anodization process for the isolation of the junctions. The 4×4 μm2 junctions show overdamped Josephson current-voltage characteristics at 4.2 K. The characteristic voltage IcRn is about 8.0 μV at a critical current density jc of about 29 kA/cm2. Under the influence of 10.5 GHz microwave irradiation the constant-voltage steps of n=0 and ±1 are observed with current amplitudes of about 3 mA.
We demonstrate a direct writing method for the fabrication of planar Josephson junctions from high-quality superconducting niobium titanium nitride (NbTiN) thin films, by creating local disorder using focused He ion beam irradiation. We control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, enabling us to successfully fabricate Josephson junctions with highly tunable weak links ranging from the metallic to the insulating phase, utilizing the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both superconductor-normal metal-superconductor (SNS) and superconductor-insulator-superconductor (SIS) type of devices, and show that we can achieve an exceptionally wide range ( ∼ 5 orders of magnitude) of critical current densities and junction resistances. The SNS-type junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages up to ∼ 1.5 mV , and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that the junctions are suitable for a wide range of applications in superconducting electronics because of the excellent mechanical, electrical, and microwave properties of NbTiN. In particular, as NbTiN has previously been used to fabricate high-quality factor microwave resonators, we see the method as a promising and simple way to realize high-frequency superconducting qubits and other quantum devices using only a single superconducting film.
NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.
We have fabricated step edge superconductor-normal metal-superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current-normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor-normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current-voltage characteristics with a 760 GHz beam from a far infrared laser.