Superconductor Science and Technology

Submicron Nb-Al-oxide-Nb junctions for frequency mixers

M. Voß, Alexandre Karpov, K. H. Gundlach

Published May 1, 1993• 12 citations

Abstract

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.

DOI: 10.1088/0953-2048/6/5/011

Related papers

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

A. Hodge Worsham, D. E. Prober, J.H. Kang +2 more

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.

IEEE Transactions on MagneticsMar 1, 199117 citations

Fabrication of high-quality submicron Nb/Al-AlO x /Nb tunnel junctions

Haifeng Yu, Cao Wen-Hui, Zhu Xiao-Bo +6 more

Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.

Chinese Physics BAug 1, 20087 citations

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

T Lehnert, C Grassl, K. H. Gundlach +1 more

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.

Superconductor Science and TechnologySep 1, 199114 citations

Fabrication of Nb/Al-N/sub x//NbTiN junctions for SIS mixer applications

B. Bumble, Henry George LeDuc, Jeff A. Stern +1 more

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.

IEEE Transactions on Applied SuperconductivityMar 1, 200153 citations

High-current-density small-area SIS junctions for high-frequency radio astronomy

Eric E. Bloemhof

At the Smithsonian Astrophysical Observatory, we have been developing expertise in the construction of high-frequency heterodyne receivers using SIS junctions as their first mixing elements. This research has been carried out in support of the Submillimeter Array (SMA), a radio interferometer soon to be built on Mauna Kea that will require multiple reciever systems at a number of bands between 177 GHz and 920 GHz. In this paper, we describe research on the fabrication of all-refractory SIS junctions. To be useful as high-frequency mixers, SIS junctions must have small area and high critical current density. At the lower SMA frequencies, Nb/Al-Oxide/Nb ahs proven to be an ideal material system, and optical photolithography has sufficient resolution for junction definition. At the higher frequencies, NbN/MgO/NbN may enjoy an advantage due to its larger energy gap, and electron-beam lithography may be necessary for defining junctions much smaller than one micron in size. We report good initial results for Nb/Al-Oxide/Nb junctions, fabricated using a self-aligned optical technique, with area $AP 0.5 micrometers 2 and critical current density $AP 10,000 Amp/cm2.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIEJul 2, 19941 citation

Fabrication and characterization of high current-density, submicron, NbN/MgO/NbN tunnel junctions

Jeff A. Stern, Henry George LeDuc, A. Judas

At near-millimeter wavelengths, heterodyne receivers based on SIS tunnel junctions are the most sensitive available. However, in order to scale these results to submillimeter wavelengths, certain device properties should be scaled. The tunnel-junction's current density should be increased to reduce the RC product. The device's area should be reduced to efficiently couple power from the antenna to the mixer. Finally, the superconductor used should have a large energy gap to minimize RF losses. Most SIS mixers use Nb or Pb-alloy tunnel junctions; the gap frequency for these materials is approximately 725 GHz. Above the gap frequency, these materials exhibit losses similar to those in a normal metal. The gap frequency in NbN films is as-large-as 1440 GHz. Therefore, we have developed a process to fabricate small area (down to 0.13 sq microns), high current density, NbN/MgO/NbN tunnel junctions.

Jan 1, 199212 citations