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IEEE Transactions on Applied Superconductivity

Tuning Circuit Material for Mass-Produced Terahertz SIS Receivers

Yoshinori Uzawa, Y. Fujii, Álvaro González, Keiko Kaneko, M. Kroug, Takafumi Kojima, Akihira Miyachi, Kazumasa Makise, Shingo Saito, Hirotaka Terai, Z. Wang

Published Dec 25, 2014• 24 citations
Publisher

Abstract

A total of 73 dual polarization superconductor–insulator–superconductor (SIS) receivers for the Atacama Large Millimeter/submillimeter Array (ALMA) have been successfully produced to observe astronomical signals in the frequency band of 787–950 GHz. For the mass production of waveguide SIS mixer chips used in the receivers, we have tested two different compositions of$\mbox{Nb}_{1-x}\mbox{Ti}_{x}\mbox{N} $thin films for use in the mixer's superconductor low-loss transmission lines. These components are being fabricated for mass-produced waveguide SIS mixer chips in receivers. One composition was prepared by employing a sputtering target with 20 wt.% Ti and 80 wt.% Nb$(x=0.2) $, and the other was prepared with a target composed of 30 wt.% Ti and 70 wt.% Nb$(x=0.3) $. Both films are deposited using dc magnetron sputtering on quartz substrates. We found that the ratio between the superconducting energy gap and the critical temperature$(2{\Delta}/{k}_\mathrm{B}{T}_\mathrm{C})$of the Nb0.8Ti0.2N films is larger than that of the Nb0.7Ti0.3N films. Since we obtained better sensitivities in the SIS mixers using the Nb0.8Ti0.2N films, they were used to produce all the receivers. These showed excellent performance complying with the stringent ALMA specifications.

DOI
10.1109/tasc.2014.2386211

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IEEE Transactions on Applied Superconductivity•Jun 1, 2005•27 citations
A total of 73 dual polarization superconductor–insulator–superconductor (SIS) receivers for the Atacama Large Millimeter/submillimeter Array (ALMA) have been successfully produced to observe astronomical signals in the frequency band of 787–950 GHz. For the mass production of waveguide SIS mixer chips used in the receivers, we have tested two different compositions of\mboxNb1−x\mboxTix\mboxN\mbox{Nb}_{1-x}\mbox{Ti}_{x}\mbox{N} \mboxNb1−x​\mboxTix​\mboxNthin films for use in the mixer's superconductor low-loss transmission lines. These components are being fabricated for mass-produced waveguide SIS mixer chips in receivers. One composition was prepared by employing a sputtering target with 20 wt.% Ti and 80 wt.% Nb(x=0.2)(x=0.2) (x=0.2), and the other was prepared with a target composed of 30 wt.% Ti and 70 wt.% Nb(x=0.3)(x=0.3) (x=0.3). Both films are deposited using dc magnetron sputtering on quartz substrates. We found that the ratio between the superconducting energy gap and the critical temperature(2Δ/kBTC)(2{\Delta}/{k}_\mathrm{B}{T}_\mathrm{C})(2Δ/kB​TC​)of the Nb0.8Ti0.2N films is larger than that of the Nb0.7Ti0.3N films. Since we obtained better sensitivities in the SIS mixers using the Nb0.8Ti0.2N films, they were used to produce all the receivers. These showed excellent performance complying with the stringent ALMA specifications.
A total of 73 dual polarization superconductor-insulator-superconductor (SIS) receivers for the Atacama Large Millimeter/submillimeter Array (ALMA) have been successfully produced to observe astronomical signals in the 787-950 GHz frequency band. The receivers use two double sideband (DSB) waveguide mixers with Nb/AlOx/Nb tunnel junctions and NbTiN/SiO2/Al microstrip tuning circuits on quartz substrate. The best achieved DSB receiver noise temperature is 125 K, corresponding to about 3 hf/kBfor 4 K operation.
We present the design and preliminary experimental results of superconductor-insulator-superconductor (SIS) mixers for Atacama large millimeter array (ALMA) band 10 (780-950GHz) receivers. To achieve wide frequency coverage as well as low noise temperature, we integrated high current density (10 kA/cm2) and small-sized (one micrometer in diameter) Nb/AlOx/Nb SIS junctions with a tuning microstrip line using NbTiN ground plane and Al wiring layers. High quality NbTiN films with Tc reaching 14.5 K and Al films with residual resistivity ratio as large as 15 have been achieved. To minimize the waveguide transmission loss and facilitate the fabrication of mixer block, a full height waveguide was adopted. The waveguide probe was optimized to achieve low input impedance in the frequency range of 780-950 GHz while adopting a backshort cavity of zero depth. Preliminary RF testing with a Nb/Al microstrip tuning circuit demonstrated a DSB receiver noise temperature about 400 K at 832.5 GHz, which promises the accommodation of receiver performance in ALMA specification with low loss NbTiN/Al microstrip lines.
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.
We have developed a single polarization engineering model of the Atacama Large Millimeter/Submillimeter Array (ALMA) Band 10 (0.78–0.95 THz) receivers. The front-end optics comprises a pair of ellipsoidal mirrors and a corrugated feed horn. A waveguide mixer block is attached to the feed horn in which an NbTiN-based superconductor–insulator–superconductor (SIS) mixer chip, which uses a quartz substrate, is mounted onto a WR-1.2 full-height waveguide. The SIS mixer employs two Nb / AlO x / Nb junctions and an NbTiN/SiO 2 /Al microstrip tuning circuit. A very wide intermediate frequency (IF) system with a bandwidth of 4–12 GHz is employed. The receiver demonstrated double-sideband (DSB) noise temperatures of below 240 K at local oscillator (LO) frequencies ranging from 792 to 945 GHz, without any correction for loss in front of the receiver. The lowest DSB receiver noise temperature, 179 K, was obtained at the center frequency of the designed band, which corresponds to about 4 quanta. These results represent state-of-the-art sensitivity for a receiver at an operating physical temperature of 4 K.
Band10 of the Atacama Large Millimeter Array (ALMA) is a planned heterodyne receiver covering the frequency range 790 GHz-950 GHz. The waveguide mixers are superconductor insulator superconductor (SIS) Nb/Al-AlOx/Nb junctions integrated with NbTiN/SiO2/Al tuning circuits. Our junction definition process has been greatly simplified by introducing an inductively-coupled-plasma (ICP) etcher to the fabrication. The etching process, which employs a low-pressure CF4/O2plasma, allows to remove the entire Nb/Al-AlOx/Nb tri-layer in asinglestep. SIS mixers with current densities up to 10 kA/cm2and quality factor above 15 have been fabricated with good reproducibility. Minimum receiver noise temperature, corrected for optical losses, is 210 K at 860 GHz. With the goal to further improve the performance, optimization of circuit design and fabrication technology are discussed.
In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA/cm/sup 2/ and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. In addition, a superconducting magnet fixed onto the compact mixer block to provide efficient magnetic field coupling is designed. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 108 K at the band center and temperatures of less than 167 K over the whole band (for an intermediate-frequency range of 4-8 GHz).