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IEEE Transactions on Magnetics

A 100 GHz SIS mixer of Nb/Al-AlOx/Nb junctions

Junji Inatani, Takashi Kasuga, Akira Sakamoto, Hiroyuki Iwashita, Satoshi Kodaira

Published Mar 1, 1987• 19 citations
Publisher

Abstract

We have made an all-refractory SIS mixer with four Nb/Al-AlOx/Nb junctions connected in series. The mixer is designed to operate at 80-120 GHz. The area of each junction is 2.5 × 2.5 μm2. Both these SIS junctions and a mm-wave choke filter are fabricated on a crystalline quartz substrate. These Nb/Al-AlOx/Nb junctions have shown ideal dc-characteristics and clear photon-assisted tunneling steps. RF measurements were made with a mixer mount which had two mechanical tuners. The conversion loss was less than 9 dB for 75-95 GHz. The minimum value was 8 dB. This value will be drastically improved by optimizing the normal resistance of the SIS junctions.

DOI
10.1109/tmag.1987.1065164

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•Jan 1, 1993•0 citations
We have made an all-refractory SIS mixer with four Nb/Al-AlOx/Nb junctions connected in series. The mixer is designed to operate at 80-120 GHz. The area of each junction is 2.5 × 2.5 μm2. Both these SIS junctions and a mm-wave choke filter are fabricated on a crystalline quartz substrate. These Nb/Al-AlOx/Nb junctions have shown ideal dc-characteristics and clear photon-assisted tunneling steps. RF measurements were made with a mixer mount which had two mechanical tuners. The conversion loss was less than 9 dB for 75-95 GHz. The minimum value was 8 dB. This value will be drastically improved by optimizing the normal resistance of the SIS junctions.
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
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.
This paper describes the first experimental results for a waveguide-type all-NbN superconductor-insulator-superconductor (SIS) heterodyne mixer on an MgO substrate designed to operate over the gap frequency of Nb. The mixer consists of an NbN/MgO/NbN junction, which has a length of one wavelength at 880GHz as a tuning circuit, an NbN/MgO/NbN microstrip as a λ/4 impedance transformer, and an RF choke filter. The mixer chip was designed using a high-frequency-structure simulator. Its return-loss and embedding-impedance characteristics were examined using a 180-times-scaled mixer model. By optimizing the cutting and polishing processes for the MgO substrate, we were able to fabricate the mixer chip with an accuracy of less than 5 μm. We succeeded in mounting the chip on a mixer block and in estimating the receiver noise temperature. The uncorrected minimum double-sideband receiver noise temperature was 740 K at 824 GHz. A comparison of the receiver noise temperature in a quasi-optical SIS mixer fabricated on the same wafer as the waveguide mixer showed that input noise was the major contributor to receiver noise in the waveguide mixer.
An array of four Nb-Al-Al 2 O 3 -Nb SIS junctions in series is used as the mixing element in a waveguide mixer at 345 GHz. The area of one junction is 4 mu m 2 and its normal state resistance is about 12 Omega , yielding at 345 GHz, omega RC approximately 5. The receiver noise temperature below 550 K is between 330 and 375 GHz with best results around 350 K measured with a hot- and cold-load calibration. The conversion loss calculated from the hot/cold measurements ranges from 10.5 to 12.5 dB. The mixer noise temperature is around 200 K, with an instantaneous bandwidth of 2 GHz.
Parallel connected for dc and IF, but in series for rf current SIS junction arrays are under investigation since 1982. Novel conception of the improved matching for both input and output mixer ports have been developed and confirmed successfully with experimental receivers in 40-180 GHz frequency range. Parallel and parallel-series combinations integrated up to 11 Nb-AlOx-Nb type SIS junctions have been tested in full height waveguide mixers. The receivers' noise temperatures Tr(DSB) I50K have been achieved in the laboratory tests across frequency range 40-150GHz. The Tr as low as 35K have been carefully measured at 145GHz. The parallel biased SIS arrays for integrated on a chip SIS receiver have been designed and tested successfully. The integrated receiver noise temperature as low as 85K was achieved at 140GHz. New SIS array design for mixer in frequency range 500-1000 GHz is presented.