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Superconductor Science and Technology

Nb-Al oxide-Nb junctions for 3 mm SIS receivers

T Lehnert, C Grassl, K. H. Gundlach, J. Blondel

Published Sep 1, 1991• 14 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/4/9/010

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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.>

IEEE Transactions on Magnetics•Mar 1, 1989•17 citations
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.
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.
High-quality small-area Nb-AlO/sub x/-Nb junctions have been fabricated by the selective niobium anodization process (SNAP). The influence of the preparation conditions on the junction properties were investigated. Numerical calculations based on I-V (current-voltage) characteristics measured by a data acquisition system were used to simulate the mixer performance. It has been shown that the knee-shaped structure on the I-V curve of refractory material junctions significantly affects the signal properties of the mixer. The signal and noise properties for different types of integrated SIS (superconductor-insulator-superconductor) mixing elements have been investigated experimentally in the frequency range of 37-53 GHz; the mixer conversion loss was as low as 4 dB at 45 GHz. The mixing elements consist of an odd number of SIS junctions connected in series for RF and in parallel for DC biasing. The DC bias wiring and IF (intermediate frequency) filters of these structures were designed to provide tuning out of junction capacitance and thin-film-electrode inductance.>
A fabrication process for submicron Nb-Al-oxide-Nb tunnel junctions using photoresist lines and optical lithography is described. With 0.6 mu m wide resist lines, junction areas of about 0.4 mu m 2 were obtained. At present the maximum current density obtainable is 8500 A cm -2 , and for this the best V m parameters of the submicron junctions are 47 mV at 4.2 K and 150 mV at 2.5 K. Compared with standard photoresist pads for junction definition, photoresist lines are more stable and result in junction areas closer to the areas on the mask. A weak increase of the gap voltage with decreasing junction size is observed. Finally, the performance of a 345 GHz receiver incorporating a two-junction array with integrated filter and tuning circuit is discussed.
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 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.>