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Superconductor Science and Technology

Nb/Al-AlOx/Nb junctions for a 380 GHz SIS receiver

Philippe Feautrier, Martin Hanuš, Pascal Febvre

Published Sep 1, 1992• 8 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/5/9/008

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IEEE Transactions on Applied Superconductivity•Jun 1, 2009•4 citations
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.
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.
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 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.>
NbN tunnel junctions are of great interest for THz heterodyne receivers because their large gap voltage of 5 mV yields an upper frequency limit of 4/spl Delta//h=2.4 THz for quasiparticle mixing. AC losses in NbN films, however, imply that a NbN matching circuit can be used at most to the NbN gap frequency, 2/spl Delta//h=1.2 THz. Another issue is the poor thermal conductivity of NbN films which complicates heat extraction from the junction. One solution to these problems are hybride mixer chips in which NbN junctions are integrated into appropriate superconducting or normal metal matching circuits. Both for waveguide mixers and quasioptical mixers quartz substrates are the first choice. We report on a room-temperature process for NbN-MgO-NbN junction fabrication which permits the realization of hybride mixer chips on quartz substrates. Appropriate plasma conditions for NbN film deposition are created by employing a second Nb target as a selective nitrogen pump. NbN films on quartz substrates without and with intermediate layers of Nb, Al, Al/SiO/sub 2/, and Nb/SiO/sub 2/ have critical temperatures above 15 K and normal state resistivities from 100 to 130 /spl mu//spl Omega/cm. In the first step, NbN junctions with nominal areas of 0.36 /spl mu/m/sup 2/ and current densities of 10 kA/cm/sup 2/ at 5.5 mV were integrated into Nb films forming the tuning circuit and a dipole antenna. The lowest receiver noise temperature in the 350 GHz range was 245 K double side band.
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.
A new structure and a fabrication process for multi-material THz-SIS mixers are proposed. In this design, both the micrometer-sized SIS trilayers (MTLs: microtrilayers) and the ground plane are deposited directly onto the substrate. This structure is expected to possess a number of unique features, e.g., (1) the quality of the SIS junction is not affected by the physical nature of the ground plane film; (2) the heat can escape directly from the junction into the substrate. The influence of the MTL-structure on the junction quality and circuit characteristics have been investigated. Numerical calculation suggests that the extra rf loss around the junction can be kept small if the offset between the junction and the ground plane is less than 1 mum. MTL-SIS mixers have been fabricated using Nb/Al-AlN(or Al-AlOx)/Nb SIS junctions and NbTiN/Al microstrip lines. The leakage current of the SIS junction can be made as small as that of the best all-Nb devices. The MTL-SIS structure will be useful in the development of future THz SIS mixers. The abstract goes here.