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IEEE Transactions on Applied Superconductivity

Bandwidth Limitations of Nb/AlN/Nb SIS Mixers Around 700 GHz

C. F. J. Lodewijk, T. Zijlstra, Shaojiang Zhu, Fausto Patricio Mena, Andrey M. Baryshev, T. M. Klapwijk

Published Jun 1, 2009• 10 citations
Publisher

Abstract

We study, using niobium-technology, the bandwidth of SIS mixers operating at frequencies close to the energy-gap frequency. Microstriplines of niobium-silicon dioxide-niobium have different properties for the top and bottom superconductor, which we find to depend on the used fabrication process. Replacing the AlOxtunnel barrier by AlN, the bandwidth increases by 53%. The measurements, using a Fourier Transform Spectrometer (FTS) and performed in ambient air, demonstrate that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. Excellent noise temperatures are found over a full band of 600 to 720 GHz.

DOI
10.1109/tasc.2009.2018231

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This thesis describes the development, modeling, and optimization of Nb superconducting microstrip tuning circuits for Superconductor-Insulator-Superconductor mixers. The ultimate goal is to extend the quantum limited noise performance of Nb/AlOx/Nb SIS mixers to above the Nb gap frequency (680 GHz). The mixers are intended to be used in the Atacama Large Millimeter Array (ALMA) band 9 frequency range (600-720 GHz). The behavior of Nb microstrip tuning circuits especially above the gap frequency is modeled using the complex conductivity given by the Mattis-Bardeen theory of the anomalous skin effect [72]. A computer code utilizing two-port network theory is utilized to predict and optimize the RF coupling efficiency and bandwidth of the tuning circuits and embedding impedances. The effect of material and geometrical variations on the coupling has been evaluated. The installment of the devices in the waveguide has been found to be a crucial factor explaining the observed scattering in measurement results for nominally identical devices. Despite parameter variations, modeling of the RF coupling efficiency proved to be successful within the limitations set by scattering of the measured results. Using this model, the tuning circuit geometry has been optimized for bandwidth and maximum coupling efficiency. During the work several SIS devices have been fabricated and Fourier transform spectroscopy measurements have been made for RF coupling evaluation. Finally, using simulations, it has been found that application of Nb/AlN/Nb junctions would increase the bandwidth to an extent which should dramatically decrease the sensitivity to parameter variations.

•Aug 15, 2006•0 citations
We study, using niobium-technology, the bandwidth of SIS mixers operating at frequencies close to the energy-gap frequency. Microstriplines of niobium-silicon dioxide-niobium have different properties for the top and bottom superconductor, which we find to depend on the used fabrication process. Replacing the AlOxtunnel barrier by AlN, the bandwidth increases by 53%. The measurements, using a Fourier Transform Spectrometer (FTS) and performed in ambient air, demonstrate that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. Excellent noise temperatures are found over a full band of 600 to 720 GHz.
Abstract — We have developed a new fabrication process for AlN barriers with excellent reproducibility and control compared to previous work, enabling the use of these barriers in SIS mixers. We report results with niobium-aluminum nitride-niobium superconductor-isolator-superconductor (SIS) mixers for 700 GHz, demonstrating a wide bandwidth behavior with Fourier Transform Spectrometer (FTS) measurements. These measurements have been performed in air and reveal that the bandwidth is no longer limited by the tuning circuit but by the atmospheric absorption of radiation. We also present noise temperature measurements of AlN SIS mixers that complement the FTS results. I.
We have designed, fabricated, and tested superconductor-insulator-superconductor (SIS) mixers based on Nb/AlN/NbN twin tunnel junctions for waveguide receivers operating in a frequency range of 790-950 GHz. Electromagnetic simulations and measurement results of the mixer performance are presented. The junctions have a high gap voltage of 3.15 mV and a high current density of about 30 kA/cm2, providing a wide receiver band, which was confirmed by Fourier transform spectrometer (FTS) and noise temperature measurements. The corrected receiver noise temperature varies from 240 K at low frequencies to 550 K at the high end of the band. The influence of the SIS junction heating on its characteristics has been studied and compared to another similar high current density technologies. The heating does not have a critical impact on the mixer performance.
The bandwidth and noise temperature of superconducting-insulating-superconducting (SIS) mixers are strongly determined by the matching between antenna and tunnel-junction. Optimal scientific benefit from Band 9 (600 to 720 GHz) of the Atacama large millimeter array requires a proper design of the matching circuit by taking into account the energy gap of the niobium (680 GHz). In preliminary experiments we have found that tapered structures lead to significant improvements, which suggests that a systematic analysis of optimized geometrical shapes might be beneficial. The tuning circuits are analysed using a model, which includes the Mattis-Bardeen theory for the surface impedance of Nb and which allows arbitrary shapes. We will present simulations and experimental results focusing on the transmission efficiency, which can conveniently be evaluated experimentally by using a Fourier transform spectrometer. We will show how the shape influences the transmission efficiency and relate it to observed noise temperatures of the mixers.
Nb-based superconductor - insulator - superconductor (SIS) mixers with Nb tuning circuits have demonstrated good results up to the Nb gap frequency. Above the gap frequency the performance is expected to degrade quickly because RF loss in Nb becomes significant. In this paper we present the results of an effort to extend Nb-based SIS mixers to THz frequencies by employing lower-loss normal-metal Al wiring and tuning structures. The SIS mixer has two Nb/Al-oxide/Nb junctions connected by an Al microstrip inductor. The direct detection response of the device was measured by a Fourier transform spectrometer. A double-side-band receiver noise temperature of 840 K was obtained at 1042 GHz when the device was operated at 2.5 K.
A 600-700GHz SIS mixer with the twin-junction tuning circuit has been designed and fabricated [1] The surface impedance of thin-film superconducting microstrip lines, based on Mattis-Bardeen theory [2], is included in the optimization of RF impedance matching. The receiver noise temperature measured in the frequency range of 630-660GHz is below 200K and the lowest receiver noise temperature of 181K is achieved at 656GHz. The FTS response of the SIS mixer shows a good RF coupling from 600GHz to 700GHz. Both the noise performance and the FTS response can be quantitatively described by numerical results using the quantum theory of mixing. Some detail considerations on the mixer model calculation, such as spreading inductance around the junction tuning structure and mixer's embedding impedance, are discussed. Introduction Nb-based superconductor-insulator-superconductor (SIS) mixers have been used very successfully in sub-mm wavelength detection. Their receiver noise temperature has reached to three times the quantum limit, —3 hfrkB, below the junction's gap frequency. However, at high frequency bands, Nb SIS mixers need to be designed carefully because the photon energy is close to or higher than the gap energy of Nb, A-1.4meV. When the frequency band covers the gap frequency of Nb, —670GHz, the RF loss of Nb superconducting transmission lines increases dramatically and the surface impedance has strong frequency dependence. Usually, the RF properties of a superconducting transmission line can be described by the Mattis-Bardeen theory [2] in good accuracy. From results reported so far [4-9], the best receiver noise temperature is close to 5 hi7kB in the band of 600-700GHz. Despite of the input loss contributed by the experimental setup, some other properties of SIS mixers should be taken into account. In this paper, we characterize the performance of a 600-700GHz mixer by comparing the experimental and simulated results.
This thesis describes the development, modeling, and optimization of Nb superconducting microstrip tuning circuits for Superconductor-Insulator-Superconductor mixers. The ultimate goal is to extend the quantum limited noise performance of Nb/AlOx/Nb SIS mixers to above the Nb gap frequency (680 GHz). The mixers are intended to be used in the Atacama Large Millimeter Array (ALMA) band 9 frequency range (600-720 GHz). The behavior of Nb microstrip tuning circuits especially above the gap frequency is modeled using the complex conductivity given by the Mattis-Bardeen theory of the anomalous skin effect [72]. A computer code utilizing two-port network theory is utilized to predict and optimize the RF coupling efficiency and bandwidth of the tuning circuits and embedding impedances. The effect of material and geometrical variations on the coupling has been evaluated. The installment of the devices in the waveguide has been found to be a crucial factor explaining the observed scattering in measurement results for nominally identical devices. Despite parameter variations, modeling of the RF coupling efficiency proved to be successful within the limitations set by scattering of the measured results. Using this model, the tuning circuit geometry has been optimized for bandwidth and maximum coupling efficiency. During the work several SIS devices have been fabricated and Fourier transform spectroscopy measurements have been made for RF coupling evaluation. Finally, using simulations, it has been found that application of Nb/AlN/Nb junctions would increase the bandwidth to an extent which should dramatically decrease the sensitivity to parameter variations.