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IEEE Transactions on Magnetics

High quality Ta/PbBi tunnel junctions for 85-110 GHz SIS mixer experiments

G.J. Cui, Dean W. Face, Elie K. Track, D. E. Prober, Antti V. Räisänen, Denis Crété, P. L. Richards

Published Mar 1, 1987• 7 citations
Publisher

Abstract

We report on the fabrication of small area (1-6 μm2), high critical current density (103-104A/cm2) Ta/Ta oxide/ Pb0.9Bi0.1SIS tunnel junctions and six junction series arrays designed for use as broadband SIS quasiparticle mixers as 85-110 GHz. These junctions have small subgap leakage currents (1 to 5%), and a "sharp" current rise of width ΔV ∼30-50 μV at the sum gap voltage. An RF filter is fabricated on the substrate along with the junctions. The impedance of this filter combined with the junction capacitance is designed to provide a broadband RF match to the mixer over the entire frequency range. A sliding backshort is the only mechanical tuning element. The single junctions as well as arrays are fabricated with a window geometry on fused quartz substrates. The fused quartz substrates require the deposition of a thin amorphous Ge layer which is conducting as room temperature to avoid charging effects during the ion-beam processing steps. Preliminary measurements at 85- 100 GHz show a relatively low gain (-6.9 dB) compared to the performance expected for junctions of this quality. Model calculations suggest that the low gain arises from errors in the implementation of the RF filter design; this can be improved in future work.

DOI
10.1109/tmag.1987.1065137

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We report on the fabrication of small area (1-6 μm2), high critical current density (103-104A/cm2) Ta/Ta oxide/ Pb0.9Bi0.1SIS tunnel junctions and six junction series arrays designed for use as broadband SIS quasiparticle mixers as 85-110 GHz. These junctions have small subgap leakage currents (1 to 5%), and a "sharp" current rise of width ΔV ∼30-50 μV at the sum gap voltage. An RF filter is fabricated on the substrate along with the junctions. The impedance of this filter combined with the junction capacitance is designed to provide a broadband RF match to the mixer over the entire frequency range. A sliding backshort is the only mechanical tuning element. The single junctions as well as arrays are fabricated with a window geometry on fused quartz substrates. The fused quartz substrates require the deposition of a thin amorphous Ge layer which is conducting as room temperature to avoid charging effects during the ion-beam processing steps. Preliminary measurements at 85- 100 GHz show a relatively low gain (-6.9 dB) compared to the performance expected for junctions of this quality. Model calculations suggest that the low gain arises from errors in the implementation of the RF filter design; this can be improved in future work.
We report on a novel step-defined process used to produce small area (1–6 μm2), high critical current density (102–104 A/cm2) superconducting tunnel junctions with Ta base electrodes and Pb0.9Bi0.1 counterelectrodes. These junctions have extremely small subgap leakage currents, a ‘‘sharp’’ current rise of width ΔV∼30 μV at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. Accurate measurements at 34.5 GHz with a 1.5 GHz intermediate frequency (IF) give single sideband mixer gains up to G=1.1±0.1 and mixer noise temperatures as low as TM=3.8±1.0 K. This value of TM is close to the quantum limit TQ=ℏω/k ln 2=2.5 K. The lowest receiver noise temperature measured at the 1.3 K input to the mixer block, TR=24±1 K, is the best value reported for any heterodyne receiver. Infinite available gain is obtained for a low (25 MHz) IF and a coupled gain as large as 1.6±0.4 is observed.
A modified SNIP process was used to fabricate high-quality 0.5, 2, and 4 mu m/sup 2/ small-area Nb/AlO/sub x//Nb trilayer tunnel junctions with current densities as large as 5000 A/cm/sup 2/. The average junction quality factors for the junctions at 4.4 K were V/sub m/(2 mV)=39 mV for J/sub c/=3000 A/cm/sup 2/ and V/sub m/ (2 mV)=27 mV for J/sub c/=5000 A/cm/sup 2/. The best values of V/sub m/ obtained were 50 mV for J/sub c/=3000 A/cm/sup 2/ and 41 mV for J/sub c/=5000 A/cm/sup 2/. These devices were designed and fabricated for use in a W-band mixer receiver. The substrate was 50- mu m-thick fused or crystal quartz. Special methods were developed for handling such thin insulating substrates and patterning films. The fabrication process was self-aligned and used SiO/sub 2/ instead of anodized Nb as the thick insulator. SiO/sub 2/ isolated the junction area and defined the opening for contact to the Nb wiring layer. The authors have fabricated series arrays of up to 12 junctions, with individual junction areas of 0.5 mu m/sup 2/. The array I-V quality was not degraded compared to that of an individual junction.
We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.
Small area (<or=1- mu m/sup 2/), high-current-density NbN-MgO-NbN tunnel junctions with I-V characteristics suitable for high-frequency mixers have been fabricated. Mesa-geometry junctions with an area of about 1 mu m/sup 2/ and critical current density of 5-10 kA/cm/sup 2/ are integrated with superconducting microstrip lines designed to resonate out the junction capacitance. Accurate values of junction capacitance and magnetic penetration depth are required for the proper design of the microstrip. A study was made of the mixer gain and noise performance near 205 GHz as a function of the inductance provided by the microstrip line. This has confirmed, at a high millimeter-wave frequency, values of junction capacitance of 85 fF/ mu m/sup 2/ and recently measured values of a magnetic penetration depth of 380 nm. Mixer noise temperatures as low as 134 K at 1.5 K have been obtained for properly tuned junctions. A significant improvement in mixer performance on cooling from 4.2 K to 1.5 K was observed. Edge-geometry junctions with an area of 0.3 mu m/sup 3/ and critical current density of 18-25 kA/cm/sup 2/ have also been fabricated. These junctions give a mixer noise temperature of 145 K at 4.2 K without the use of integrated tuning elements. These are the best results ever achieved for NbN-based SIS mixers.
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.
Today superconducting tunnel junctions are the best mixer elements in the 0.7 to 3 mm wavelength range. This open structure mixer, built on the basis of such a junction approaches the best waveguide systems (1). Its particular merrits are low cost, easy assembly, and convenient tuning by one single element. The mixer element is a series array of 3 PbBi junctions deposited on a fused-quartz substrate. The junctions are positioned in the middle of a planar dipole structure. In order to isolate the bias circuit and the IF amplifier band stop filters were deposited on both ends of the dipole. The usual perodic high-low impedance structure was used for these filters. A substrate holder presses the substrate with moderate force towards an elliptical immersion lens made from crystal quartz. This concept was pioneered by (2). Special features of our design are (i) the junction composition, providing a gap voltage of 10mV for the array, (ii) the use of a resonant dipole feed with choked bias connections, (iii) the imaging by one single aspheric lens of crystal quartz. The lens-junction assembly is mounted in a copper block which also carries the IF connector. A 3 mm screw in the substrate holder provides a movable backplane behind the dipole feed. On both sides of the mixer block superconducting coils with 6000 turns each provide the necessary magnetic field. The results below were measured at 100 mA corresponding to 110 Gauβ at the junction. At this flux density the Josephson effect is at its first minimum. Mixer, coils, and a low noise preamplifier are mounted in a ℓHe bath dewar which has a quiescent hold time of 50 hours. An uncoated 1.05 mm crystal quartz window on the 77 K radiation shield rejects 10 μm radiation and has its resonant transmission at 340 GHz. A 0.25 mm Mylar window, resonant at 350 GHz, seals the vacuum. The local oscillator, (4) is combined with the signal by a 35 μm Mylar foil transmitting nominally 96% of the signal and 4% of the ℒO power. Apart from the immersion lens in the mixer block there is no further lens in the signal path. The ℒO source is focused onto the mixer by a plastic lens between the beam combiner and the ℒO output. The receiver noise was measured for a number of frequencies (fig 1) using the Y-factor method where an absorber at 290 K and a wet absorber at 77 K stored in ℓN2 were brought into the signal path. The best value of 214 K at 342 GHz, measured with a bath temperature of 1.5K, coincides with the resonance of the IR-filter. The pumped and unpumped i/v curve together with the calibration curves for hot and cold load are given in fig.2. Receiver noise as a function of IF frequency (fig 3) was measured in 50 MHz bandwidth and coincides with the noise curve of the preamplifier. The angular diagram (fig 4) was measured using a chopped cold load with a 2 degree aperture moved in 1 degree steps. It meets the design goal of a clean, symmetrical f/10 beam and would match the Cassegrain beam of e.g. the 30 m IRAM telescope on Pico Veleta without additional optics.