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International Kharkiv Symposium on Physics and Engineering of Microwaves, Millimeter and Submillimeter Waves

Study on SIS Mixers for "ALMA" Band-10

S. V. Shitov, Oleg V. Koryukin, Andrey V. Uvarov, Maksim A. Bukovski, Yoshinori Uzawa, Takashi Noguchi, Mahoto Takeda, Z. Wang, M. Krough, Alexander N. Vystavkin

Published Jun 1, 2007• 0 citations
Publisher

Abstract

This paper studies superconductor insulator superconductor (SIS) mixers for ALMA Band-10 project. The possible scheme of the ALMA Band-10 cartridge with balanced lens-antenna mixers is presented. The new balanced full-height waveguide SIS mixer comprises two equal mixer chips fabricated on quartz substrates. Each chip contains an RF circuit with hybrid twin-SIS junction NbTiN/Nb-AlOx-Nb/Al. The preliminary experimental study of Band-10 receiver with a double-slot antenna SIS mixer mounted with a 10-mm immersion silicon lens is done to define an optimum layout of the tuning circuit and a combination of materials leading to the best performance of the SIS mixer.

DOI
10.1109/msmw.2007.4294614

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This paper studies superconductor insulator superconductor (SIS) mixers for ALMA Band-10 project. The possible scheme of the ALMA Band-10 cartridge with balanced lens-antenna mixers is presented. The new balanced full-height waveguide SIS mixer comprises two equal mixer chips fabricated on quartz substrates. Each chip contains an RF circuit with hybrid twin-SIS junction NbTiN/Nb-AlOx-Nb/Al. The preliminary experimental study of Band-10 receiver with a double-slot antenna SIS mixer mounted with a 10-mm immersion silicon lens is done to define an optimum layout of the tuning circuit and a combination of materials leading to the best performance of the SIS mixer.
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.
A fixed-tuned SIS(Superconductor-Insulator-Superconductor) mixer across 120∼180 GHz band has been developed. This mixer employs an SIS chip fabricated by Nobeyama radio observatory which consists of a series array of 6 Nb/Al-AlO/Nb junctions in a microstrip line on a fused quartz substrate. The SIS chip is placed at the center of the half-height waveguide mixer mount to have a good incoming signal coupling over the whole frequency band. No mechanical tuner was used in the SIS mixer and the RF signal and local oscillator power are injected to the mixer via a cooled cross-guide coupler. In order to prevent the IF signal loss, the If output impedance of the SIS mixer was matched to the 50 input impedance of the IF chain. Measured double sideband noise temperatures of a receiver using the SIS mixer are 32∼131 K over 120∼180 GHz band. The developed SIS mixer is now in use for radio astronomical observations on the TRAO 14 m radio telescope.
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.
down to only 2-3 p,W for a two polarized SIS receiver, if submicron SIS junctions are used in the balanced mixer configuration. There are a few options for waveguide balanced and sideband separating mixers employing the straightforward connection of the DSB mixers using hybrid couplers [2]. However, the low-loss operation of the waveguide-based connections and the required balancing of the mixers both are of a great concern in the THz frequency range. This is a good reason to look for the more compact design of the RF circuit for waveguide or/and consider an integrated quasioptical SIS mixer design. It is known that the conversion gain of a SIS junction is reducing essentially above the gap frequency, and it turns negligibly low above twice the gap frequency. The ALMA Band-10 DSB receiver specification for TRx is 230 K within 80% of the band 787-950 GHz and 345 K at any frequency of the band. The IF range of 4-12 GHz is required for the DSB receiver with ripples below 4 dB within each 2 GHz segment. All these make the accurate design of the mixer's RF and IF circuits very important. Since the Q-factor of a SIS junction at submillimeter wavelength is high (Q>>1), the twin-SIS junction [3]-[5] is a good solution providing about twice RF bandwidth of a single junction. However, it is known that two junctions in the twin-SIS are unequally coupled at RF across the band, thus providing less gain than one can expect from the equivalent junction of the same R. This effect is difficult to estimate experimentally, since two junctions are connected in parallel at DC. The growing importance of such accurate analysis in THz range can be motivated with concern of the essential drop of conversion gain of a Nb-based SIS mixer closer to 1 THz. To analyze the effect of unequal pump and dynamic resistance for a two-junction SIS mixer, the Tucker's 3-port model can be used yet being expanded to multi-junction mixing arrays. Abstract-A few concepts of a wide-band balanced SIS mixer employing submicron-sized SIS junctions are under development for 787-950 GHz frequency range. A quasioptical DSB balanced mixer with integrated cross-slot antenna is considered as the less laborious and cheaper option. The silicon lens-antenna beam efficiency is expected above 80 % across the whole band with first-order sidelobe below -16 dB. To use the conservative horn antenna solution, a singlechamber waveguide DSB balanced mixer is developed. Two equal probe-type SIS chips are inserted into a full-height waveguide through its opposite broad walls; these two mixers are driven by the signal waveguide in series. The LO current is transferred to the mixers in parallel via a capacitive probe inserted through the narrow wall of the signal waveguide from the neighboring LO waveguide. The HFSS model demonstrated the LO power coupling efficiency above -3 dB, almost perfect signal transfer and the LO cross talk below 30 dB that take into account misalignment (misbalance) of the chips. It is demonstrated numerically using Tucker's 3-port model that unequal pump of junctions of a twin-SIS mixer can lead, in spite of the perfect signal coupling, to degradation of the gain performance up to -3 dB, especially at the top of the ALMA Band-10.
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.
We present the design of an integrated dual-polarisation superconductor-insulator-superconductor (SIS) mixer operating at ALMA Band 10 frequency range. All the RF components, including the orthomode transducer (OMT), the quadrature hybrid and the SIS mixer circuits, are designed as superconducting planar circuits to form a single planar mixer chip. The mixer circuits will be fabricated using standard thin film deposition technology on an ultra-thin silicon-on-insulator (Sol) substrate. In this paper, we discuss in detail the design of the various RF components that make up the complete dual-polarisation SIS mixer chip, and provide the predicted performance using rigorous electromagnetic modelling. The optimised designs of these components are then integrated in a superconducting quantum mixer circuit modelling (SuperMix) environment for assessing the performance of the heterodyne parameters of the mixer such as mixer gain and noise temperature.