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IEEE Transactions on Magnetics

High-quality sub-micron Nb trilayer tunnel junctions for a 100 GHz SIS receiver

A. Hodge Worsham, D. E. Prober, J.H. Kang, John X. Przybysz, Michael J. Rooks

Published Mar 1, 1991• 17 citations
Publisher

Abstract

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.

DOI
10.1109/20.133883

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IEEE Transactions on Magnetics•Mar 1, 1987•7 citations
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.
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.
The authors describe a junction fabrication process that produces high-quality Nb edge junctions of areas less than 0.2 mu m/sup 2/ on thin quartz (0.10 nm) substrates. This process utilizes a 10:1 projection wafer stepper for high-resolution and high-accuracy layer-to-layer registration. Large numbers of junctions were fabricated reliably with high-quality I-V (current-voltage) characteristics and with impedances suitable for use in SIS (superconductor-insulator-superconductor) mixers for submillimeter astronomy. Junctions produced by this process can be stored on the shelf for more than two years with no special precautions taken, and with an impedance change of less than 10%. No failures have been observed after thermal cycling. Currently these junctions are being used in an astronomical receiver that demonstrates state-of-the-art performance in the atmospheric window centered at 345 GHz. With some improvements to the process, it is possible to make junctions with areas sufficiently small for omega RC approximately=3 at 800 GHz.>
We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.
The authors describe their fabrication process of Nb-Al oxide/Nb tunnel junctions with integrated filter structure. The process includes the lift-off stencil technique and liquid anodization. The quality of the junctions with areas down to about 1 mu m 2 is discussed and the trilayers are analysed by anodization spectroscopy. The occurrence of proximity effect structures for trilayers with thick (160 AA) Al films and for trilayers with poor anodization profiles is demonstrated. Finally the mixer performance of a two-junction array is outlined in the range around 100 GHz.
The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have been fabricated with V/sub m/ values as high as 65 mV and areas down to 0.1 mu m/sup 2/. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported.>
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.