Programmable Voltage Standards Research Articles

Cryogen-Free Operation of SNIS for AC Quantum Voltage Standards

We report on our recent and ongoing activities on helium-free operation with ac Josephson standards based on programmable superconductor-normal conductor-insulator-superconductor (SNIS) arrays. Liquid helium-free operation provides ease of use, which is a wider number of applications and users. Moreover, it allows to reduce cable loading, which is crucial to overcome frequency-related limitations to the accuracy. Thermalization problems not faced with helium cooling are still challenging, in particular with programmable standards. To identify the effect of He-free cooling on electrical behavior, we compared the array steps in cryocooler a liquid helium. Results show that SNIS arrays can be operated in cryocooler and are interesting for application to programmable voltage standard.

Temperature Stability of SNIS Josephson Arrays Between 4.2 K and Critical Temperature in Cryocooler

Cryogen-free operation is essential to expand applications of superconductivity, is unavoidable in some special cases and, in perspective, provides a solution to the expected shortages of liquid helium. In electrical metrology applications, high-temperature operation to reduce the refrigerator size and complexity is not yet possible since arrays of Josephson junctions for voltage standard applications made with high-temperature superconductors are not presently available. The superconductor-normal metal-insulator-superconductor (SNIS) technology developed at INRIM uses low-temperature superconductors but allows operation at temperatures close to 6 K. Thus, it is interesting for the cryocooler operation of a programmable voltage standard. We report on measurements of SNIS devices cooled with a closed-cycle refrigerator and in liquid helium to test the electrical behavior and its dependence on specific fabrication parameters that can be used to optimize the temperature stability.

High precision comparison between a programmable and a pulse-driven Josephson voltage standard

Over the last 15 years, research in ac Josephson voltage metrology has focused on two fundamentally different systems: the programmable and the pulse-driven Josephson voltage standards (JVSs). This paper reports the first high precision comparison between the two types of JVS. The METAS programmable voltage standard was moved from Switzerland to the Netherlands to be compared with the Dutch pulse-driven system during four days in November 2010. After a careful investigation of the systematic sources of errors, the comparison was made at a frequency of 500 Hz and an rms amplitude of 104 mV. At that level, the voltage difference measured between the fundamental frequency components of the two standards was −0.18 ± 0.26 µV V−1 (k = 2), showing an excellent agreement between the two systems.

1 V programmable voltage standards based on SNIS Josephson junction seriesarrays

Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards inclose cooperation between INRIM and PTB. The arrays, consisting of 8192 overdampedNb/Al–AlOxNb SNIS Josephson junctions, were successfully operated at microwave frequenciesaround 70 GHz. While the characteristic voltage of the junctions ranged from0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro stepn = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristicvoltage, demonstrating the small parameter spread of the SNIS junctions. Operation attemperatures up to 6–7 K has also been verified, measuring quantized voltages up to 1.25 V on then = 1 step.

Engineering Overdamped Niobium-Based Josephson Junctions for Operation Above 4.2 K

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 J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , 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-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> /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 both <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> = 1 and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> = 2 RF step above 4.2 K, for a possible reduction of the number of junctions in array circuits.

Temperature behavior of SNS-like Nb/Al–AlOx/Nb Josephson junctions

Overdamped Nb/Al–AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. Stable and reproducible non-hysteretic current–voltage characteristics have been obtained with a proper choice of the fabrication parameters, featuring critical current densities Jc up to 25kA/cm2 and characteristic voltages up to 450μV. While these values make the junctions interesting for RSFQ electronic circuits, their response to an RF signal at 70GHz has demonstrated their suitability for both programmable and ac voltage standard. In these work we analyse the temperature behavior of these junctions up to T/Tc=1, Tc being the niobium critical temperature, which gives relevant information on the junction structure and, especially, on the oxide insulator/metallic film barrier, which is the key for the reproducible transition from an hysteretic to a non-hysteretic behavior. The results are also compared with other data of hysteretic and overdamped junctions.

Stacked Nb-MoSi/sub 2/-Nb Josephson junctions for AC voltage standards

Superconductor-normal metal-superconductor (SNS) Josephson junctions have proven to be a critical technology for voltage standards. NIST has used SNS junctions for both dc and ac programmable voltage standards. Previous devices have used primarily PdAu as a normal-metal barrier material. In this paper we present measurements of circuits having MoSi/sub 2/ barriers. Stacking enables the junctions to be packed more densely, thus increasing design flexibility and margins for the microwave circuits. In this work, measurements are presented from two- and three-junction stacks for application to ac Josephson voltage standards, which show output voltage and distortion near our best previously published results.

Operation of a NbN-based programmable Josephson voltage standard chip with a compact refrigeration system

A refrigeration system was designed and constructed for realizing a liquid-He-free programmable Josephson voltage standard. The system is equipped with a two-stage Gifford-McMahon cooler, a thermal-radiation shield, a magnetic-field shield and semi-rigid coaxial cables to supply microwave power to a chip. The performance of the system was examined by use of a NbN-based 8-bit digital-to-analog converter (DAC) chip designed as a 1 V programmable voltage standard. When operated at 8.5 K on the cryocooler, constant-voltage steps with amplitudes greater than 1 mA were observed for every segment of junction arrays on the chip.

Nb/Al/Nb junctions with a wide range of characteristic voltages for superconducting electronic applications

Josephson devices for superconducting electronic applications, such as RSFQ logic circuits and programmable voltage standards, require nonhysteretic junctions with a high speed or a high voltage resolution and hence with characteristic voltages spanning over several orders of magnitude. We present here our recent results on Nb/Al/Nb junctions where, by changing some fabrication parameters such as the thickness and deposition rate of the Al barrier, it is possible to obtain junctions with very different electrical properties. These junctions have characteristic voltages varying from a few tens of /spl mu/V up to more than 1 mV, with critical current densities from 10/sup 3/ up to 10/sup 6/ A/cm/sup 2/.

Stacked SNS Josephson junction arrays for quantum voltage standards

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.

Error estimation in recording I–V characteristics of Josephson junctions

A method for characterizing a current total step in the I-V characteristic of a Josephson junction array is considered. In this method, an appropriately selected approximating curve is statistically fitted to the experimentally found curve. A self-calibration algorithm for an array of junctions incorporated into programmable voltage standards is suggested.

Improved 1 V programmable Josephson voltage standard using SINIS junctions

A programmable voltage standard of fundamental accuracy has been realized with a binary sequence of nonhysteretic junctions. The series arrays of intrinsically shunted superconductor–insulator–normal conductor–insulator–superconductor (SINIS) Josephson junctions have been fabricated using the reliable Nb–Al/AlO x technology. The arrays of 8192 junctions (14 bit) are arranged in a new, improved design allowing a large Shapiro step width of 1 mA to be achieved.

NbN / TiN x / NbN / TiN x / NbN double-barrier junction arrays for programmable voltage standards

A series array of NbN/TiNx/NbN/TiNx/NbN double-barrier junctions was fabricated on Si wafers, and their current–voltage (I–V) characteristics were measured with and without microwave power in order to investigate their performance for programmable voltage standards. By adjusting the thickness of the TiNx barrier, nearly identical critical currents were obtained for 128 lower and 128 upper junctions in an array. When applying 8 GHz microwave power to an array, a large constant-voltage step (∼3 mA) appeared on the I–V curve. The zero-voltage critical current and the height of the first (n=1) constant-voltage step showed microwave-power dependences coincident with theoretical prediction.

10 V SINIS Josephson junction series arrays for programmable voltage standards

A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al/sub 2/O/sub 3/ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed.

SNS junction on Nb-Ti base for microwave circuits

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.

High-resistivity superconductor-normal-superconductor junctions for an AC Josephson voltage standard

Abstract We report here our results on the development of Nb/Al/Nb Josephson junctions for a programmable voltage standard. The Al barrier is 5–120 nm thick. Its properties are quite different with respect to the single Al films because of diffusion between Al and the Nb counter-electrode. The resistivity of the Al barrier ranges from hundreds of microhm centimetres to a few milliohm centimetres, probably depending on the surface roughness of the Al layer, measured by atomie force microscopy (AFM). The junctions, with areas between 25 and 200 μm2, have critical currents I C in the milliampere range, associated with normal resistances R N of about 1 Ω. The very high I C R N product of these junctions allows us to use a microwave source of 70 GHz and above, as in present dc voltage standards. Preliminary rf measurements on these devices are reported. Structural investigations by AFM and X-ray diffraction were also carried out to relate the electrical properties of the junctions to the Nb-Al interfaces.

SNS and SIS Josephson junctions with dimensions down to the submicron region prepared by a unified technology

Several applications of Josephson junctions (JJs) in low-temperature superconducting circuits such as programmable voltage standards and SFQ logical circuits or SQUID sensors require JJs with sub-µm dimensions. A method for preparing superconductor-isolator-superconductor (SIS) JJs as well as superconductor-normal metal-superconductor (SNS) JJs was developed which allows the preparation of both types of junctions by nearly the same technological process. Titanium is used as the normal metal for the SNS JJ. Ti can be anodically oxidized and a technology similar to the well-tried Nb-Al technology for SIS JJs can be applied to the SNS system, too. By electron-beam exposure of a negative deep UV photoresist (ma-N 2400) a stable resist mask is generated which has to withstand the following three self-aligned process steps: etching of the Nb counterelectrode by RIE, isolation by anodization and lifting of a SiO planarization layer. By this process SNS (Nb-Ti-Nb) and SIS (Nb-Al/AlOx-Nb) Josephson junctions with smallest junction areas of 0.49 µm2 were prepared and characterized by dc measurements and under the influence of microwave irradiation. The Nb-Ti-Nb junctions have IcRn products of 87 µV. They can be used for programmable Josephson voltage standard circuits at frequencies of 70 GHz.

Josephson arrays at 70 GHz for conventional and programmable voltage standards

This paper describes the performance under real calibration conditions of improved conventional Nb/Al/AlO/sub x//Nb 10 V Josephson junction series arrays fabricated in the last two years at PTB. A new type of Nb/Al/AlO/sub x//Al/AlO/sub x//Al/Nb array is also described that is useful for programmable voltage standards in the frequency range from 70-90 GHz. A direct comparison with a conventional Josephson junction series array was carried out at 1.018 V. The results agree within an uncertainty of better than 1 nV.

SNS programmable voltage standard

Superconductor-Normal-Superconductor (SNS) junctions have been used in the design and fabrication of a 1-V rapidly programmable voltage standard. The superconducting circuit is a series array of 32 768 Nb-PdAu-Nb junctions with taps that divide the array into a binary sequence of smaller array segments with a minimum segment size of 128 junctions. The 16-GHz drive frequency is set by the characteristic frequency of the junctions. A computer-controlled 8-channel bias system controls the current in each segment and allows the rapid selection of any one of 513 discrete voltage levels. The system is designed for fast dc measurements and the synthesis of precise ac waveforms.

Shapiro steps in large-area metallic-barrier Josephson junctions

The current amplitudes of Shapiro steps in large-area metallic-barrier Josephson junctions, both with and without a ground plane, are investigated with the goal of optimizing junction parameters for programmable voltage standards. Using the resistively shunted junction model without capacitance, we calculate maximum step amplitudes as a function of reduced frequency and junction dimension for both one- and two-dimensional junctions. For junctions without a ground plane, we conclude that step amplitudes of order 10 mA are practical, but significantly larger amplitudes require excessive microwave power.