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

S. C. Shi, M. J. Wang, Takashi Noguchi

Published Mar 1, 2002• 0 citations
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Abstract

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.

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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.
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.
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.
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.
The heterodyne mixing performance of three respective distributed junction arrays, i.e., a number of superconductor-insulator-superconductor (SIS) junctions distributed along a thin-film transmission line involving two, five, and ten junctions are measured and compared to their Fourier transform spectroscopy (FTS) detection responses. It has been found that distributed junction arrays have a rather large bandwidth in comparison to conventional SIS junction devices, while still keeping a quantum-limited noise performance. Detailed experimental results are presented.