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IEEE Transactions on Applied Superconductivity

A THz SIS Mixer With a NbTiN-Ground Plane and SIS Microtrilayers Directly Grown on a Quartz Substrate

Akira Endo, Takashi Noguchi, M. Kroug, S. V. Shitov, Wenlei Shan, T. Tamura, Takafumi Kojima, Yoshinori Uzawa, T. Sakai, Hisato Inoue, Kenta Kohno

Published Jun 1, 2009• 4 citations
Publisher

Abstract

A new structure and a fabrication process for multi-material THz-SIS mixers are proposed. In this design, both the micrometer-sized SIS trilayers (MTLs: microtrilayers) and the ground plane are deposited directly onto the substrate. This structure is expected to possess a number of unique features, e.g., (1) the quality of the SIS junction is not affected by the physical nature of the ground plane film; (2) the heat can escape directly from the junction into the substrate. The influence of the MTL-structure on the junction quality and circuit characteristics have been investigated. Numerical calculation suggests that the extra rf loss around the junction can be kept small if the offset between the junction and the ground plane is less than 1 mum. MTL-SIS mixers have been fabricated using Nb/Al-AlN(or Al-AlOx)/Nb SIS junctions and NbTiN/Al microstrip lines. The leakage current of the SIS junction can be made as small as that of the best all-Nb devices. The MTL-SIS structure will be useful in the development of future THz SIS mixers. The abstract goes here.

DOI
10.1109/tasc.2009.2019053

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Abstract Millimeter (mm) and sub-mm superconducting structures, such as waveguides and mixers on substrate chips, are pivotal in sensing applications but often suffer from dielectric losses in the substrate at these wavelengths. To address this challenge, we introduce a method for fabricating Superconductor Insulator Superconductor (SIS) mixers that significantly mitigates theses losses by employing micromachined metallic substrates. This approach not only broadens the applicability of SIS devices, including THz mixers, but also demonstrates a novel broadband SIS mixer design for the band of 211–375 GHz. Our design features an electroplated Au substrate for effective waveguide-to-substrate transition and twin SIS junctions using Nb–Al/AlN–Nb technology We validate the viability and integrity of the SIS junction on metallic substrate with cryogenic transport measurements. Our results confirm the stability and quality of the junctions across junction sizes ranging from 0.5 µ m 2 to 3.8 µ m 2 highlighting the potential of our method to enhance the performances and reliability of SIS mixers for THz applications. This work proves the feasibility of our novel fabrication approach and lays the groundwork for future advancements in superconducting technology.

Superconductor Science and Technology•Jun 27, 2025•1 citation
A new structure and a fabrication process for multi-material THz-SIS mixers are proposed. In this design, both the micrometer-sized SIS trilayers (MTLs: microtrilayers) and the ground plane are deposited directly onto the substrate. This structure is expected to possess a number of unique features, e.g., (1) the quality of the SIS junction is not affected by the physical nature of the ground plane film; (2) the heat can escape directly from the junction into the substrate. The influence of the MTL-structure on the junction quality and circuit characteristics have been investigated. Numerical calculation suggests that the extra rf loss around the junction can be kept small if the offset between the junction and the ground plane is less than 1 mum. MTL-SIS mixers have been fabricated using Nb/Al-AlN(or Al-AlOx)/Nb SIS junctions and NbTiN/Al microstrip lines. The leakage current of the SIS junction can be made as small as that of the best all-Nb devices. The MTL-SIS structure will be useful in the development of future THz SIS mixers. The abstract goes here.
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.
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.
SIS mixers incorporating two-junction, NbTiN tuning-circuits have been designed and fabricated using two different types of tunnel-junctions. The first type of tunnel junction-NbTiN/MgO/NbTiN--has the advantage of a large gap voltage (5 mV), but has a relatively soft I-V characteristic and high specific capacitance as compared to Nb/Al-Ox/Nb tunnel-junctions. The second type of junction--Nb/Al-AlNx/NbTiN, is a hybrid structure, which has many of the advantages of Nb junctions, with a slightly larger energy gap voltage (3.5 mV) and more robust thermal properties. A detailed description of the deposition techniques used in making the AlNx devices will be given in a separate paper. In this paper, we discuss the deposition of high-quality NbTiN films, and the trade-off between stress and quality in these films. We also discuss the deposition details for the NbTiN/MgO/NbTiN junctions. We measured the magnetic-penetration-depth of our NbTiN films with SQUID circuits. Using long resonators coupled to Josephson junctions, we measured propagation velocities for our microstrip-line circuits; this measurement gave us an independent estimate of the NbTiN magnetic-penetration-depth and a qualitative measure of RF-losses in the microstrip-lines.
A process of trilayer deposition to obtain small Nb/Al-AlO x /Nb junctions (1 mu m 2 ) with excellent I-V characteristics, good reliability and high current densities is described. To optimize this process, it has been useful to carry out several studies. The authors found that it was necessary to heat sink the quartz substrate during all trilayer fabrication steps. Anodization spectroscopy and RIE etching have been used to verify the quality of the tunnel barrier which is directly correlated with the I-V characteristics. An oxygen plasma is used to reduce the resist diameter and thus to expose the junction perimeter to the SiO layer, so as to obtain small junction areas with small leakage currents. The first results obtained with these junctions integrated in a 380 GHz receiver are promising: the DSB receiver noise temperature is 310 K at 374 GHz with a mixer noise temperature of about 155 K.
Abstract Millimeter (mm) and sub-mm superconducting structures, such as waveguides and mixers on substrate chips, are pivotal in sensing applications but often suffer from dielectric losses in the substrate at these wavelengths. To address this challenge, we introduce a method for fabricating Superconductor Insulator Superconductor (SIS) mixers that significantly mitigates theses losses by employing micromachined metallic substrates. This approach not only broadens the applicability of SIS devices, including THz mixers, but also demonstrates a novel broadband SIS mixer design for the band of 211–375 GHz. Our design features an electroplated Au substrate for effective waveguide-to-substrate transition and twin SIS junctions using Nb–Al/AlN–Nb technology We validate the viability and integrity of the SIS junction on metallic substrate with cryogenic transport measurements. Our results confirm the stability and quality of the junctions across junction sizes ranging from 0.5 µ m 2 to 3.8 µ m 2 highlighting the potential of our method to enhance the performances and reliability of SIS mixers for THz applications. This work proves the feasibility of our novel fabrication approach and lays the groundwork for future advancements in superconducting technology.