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Superconductor Science and Technology

Simulation of the performance of a five-junction array for 780 950 GHz

S. C. Shi, Ming-Jye Wang, Takashi Noguchi

Published Nov 21, 2002• 3 citations
Publisher

Abstract

The performance of a five-junction (Nb/AlOx/Nb) array is characterized in the frequency range of 780-950 GHz for different wiring and ground layers of the thin-film microstrip lines of the junction tuning inductance and the associated impedance transformer. Three kinds of thin films (i.e., Nb, Al and NbTiN) are taken for this investigation. The individual SIS junctions of the five-junction array have an area of 1 mum(2) and a critical current density of 10 kA cm(-2). The performance of parallel-connected twin junctions, which have the same junction parameters as those of the five-junction array, is also studied for comparison.

DOI
10.1088/0953-2048/15/12/317

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SIMULATION OF THE PERFORMANCE OF A 5-JUNCTION ARRAY FOR 780-950GHz

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IEEE Transactions on Applied Superconductivity•Nov 28, 2016•12 citations
The performance of a five-junction (Nb/AlOx/Nb) array is characterized in the frequency range of 780-950 GHz for different wiring and ground layers of the thin-film microstrip lines of the junction tuning inductance and the associated impedance transformer. Three kinds of thin films (i.e., Nb, Al and NbTiN) are taken for this investigation. The individual SIS junctions of the five-junction array have an area of 1 mum(2) and a critical current density of 10 kA cm(-2). The performance of parallel-connected twin junctions, which have the same junction parameters as those of the five-junction array, is also studied for comparison.
Abstract: The performance of a five-junction (Nb/A10xNb) array is characterized in the frequency range of 780-950GHz for different wiring and ground layers of the thin-film tnicrostrip lines of the junction tuning inductance and the associated impedance transformer. Three kinds of thin films (i.e., Nb, Al, and NbTiN) are taken for this investigation. The individual SIS junctions of the five-junction array have an area of 1,tni2 and a critical current density of 10kA/cm2. The performance of parallel-connected twin junctions, which have the same junction parameters as those of the five-junction array, is also studied for comparison. I.
Two years ago, we have fabricated and tested superconductor-insulator-superconductor (SIS) mixers based on Nb/AlN/NbN twin tunnel junctions for waveguide receiver operating in frequency range of 790 - 950 GHz, which demonstrated noise temperature from 250 K at low frequencies to 500 K at the high end of the band. Due to high current density of the junctions (up to 30 kA/cm2 ), a wideband design was realized. Based on result for twin junction, a new mixer design with single SIS junction was proposed and produced. The single junction design is expected to have narrower frequency range than twin-based one, but still wide enough, and provide better coupling in the middle of the band. In addition, the critical current suppression will be much better, than for twin junction, which has an intrinsic problem of difference between areas of individual junctions. The SIS junction in new design is made by Nb/AlN/NbN technology and incorporated in a microstrip line consisting of a 300 nm thick bottom electrode (ground plane) made of NbTiN and a 500 nm thick top electrode made of Al. The top Al electrode is passivated by the SiO2 layer for protection. The DC tects of the fabricated wafer shows, that optimization of the technology allowed to realize gap voltage of 3.2 mV even for junctions of 0.5 square micron area fabricated on the NbTiN bottom layer. The FTS and noise temperature measurement of new mixers will be done in the nearest future and will be presented in comparison with the twin junction design results at the conference.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AlOx/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region. Considering the bandwidth and output power of local-oscillator (LO) signal sources are quite limited around 1THz, we firstly develop a waveguide NbN-based SIS mixer at 0.5THz. Three types of SIS junctions, i.e., long junction, parallel-connected tunnel junction (PCTJ) and distributed junction array (DJ) are investigated. They are all comprised of NbN-AlN-NbN tri-layer fabricated on an MgO substrate and have the same current density (Jc) of 10kA/cm2. In this paper, we describe their design, fabrication and preliminary experimental results.
Twin SIS (superconductor-insulator-superconductor) junctions, with a simple structure, yet large bandwidth and low noise temperature, have been widely used in millimeter- and submillimeter-wave heterodyne mixers. With increasing frequency, however, the length of the tuning inductance connecting the two individual SIS junctions becomes short while the junction area remains fixed. With a relatively short tuning inductance, the effect of the junction's spreading inductance becomes non-negligible, and device fabrication becomes more difficult. By adopting vertically stacked SIS junctions (VSJs), which have an equivalent geometric capacitance inversely proportional to the junction number, it becomes feasible to increase the junction tuning inductance. In this paper, we report on the design, fabrication, and characterization of twin Nb/Al-AlOx/Nb/Al-AlOx/Nb VSJs for the 500-GHz frequency band.
We have designed superconductor-insulator-superconductor (SIS) mixers using Nb/AlN/Nb tunnel junctions integrated with NbTiN/SiO2/NbTiN microstrip tuning circuits for the Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 (0.78-0.95 THz). In order to design the tuning circuits, we used terahertz time domain spectroscopy to experimentally derive the complex conductivity of relatively thick NbTiN films deposited on quartz substrate directly as a ground plane and on RF-sputtered SiO2as a microstrip. Results showed that the conductivities of both films can be described by a modified Mattis-Bardeen theory. We also investigated the junction heating effects originating from quasi-particle trapping in the Nb electrodes embedded in NbTiN circuits by varying the volume of the electrodes. It was confirmed that current-voltage curves of Nb/AlN/Nb tunnel junctions with thick (~200 nm) counter electrodes contacting NbTiN wirings showed higher gap voltages as well as reduced back-bending than those with thin (~50 nm) electrodes. Making use of these findings, our designed SIS mixers employing Nb/AlN/Nb junctions with a current density of 15 kA/cm2and all-NbTiN tuning circuits may cover the Band 10 with good sensitivity.