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Journal of Physics Conference Series

Fabrication of aluminum nitride barrier SIS mixer devices using nitrogen plasma diluted by noble gases

Akira Endo, Hisato Inoue, Shin’ichiro Asayama, Takashi Noguchi, M. Kroug, T. Tamura

Published Feb 1, 2008• 2 citations
Publisher

Abstract

Nb/Al–AlN x /Nb SIS junctions have been fabricated by various RF plasma nitridation techniques. The reproducibility of the R N A product from batch to batch was improved by diluting the N 2 plasma with He or Ar. For the He+N 2 process, a power-law relation between the R N A product and the nitridation time was found to hold along 10 0 -10 6 Ω μm 2 , which implies that the thickness of the barrier grows logarithmically during the time it is exposed to the plasma. By applying a pulse-time-modulated RF power, we were able to slow down the nitridation significantly. Finally, we present the results of preliminary RF measurements. The Nb/Al–AlN x /Nb SIS mixers were tested at around 150GHz and 350GHz, and showed noise performances comparable to those of Nb/Al–AlO x /Nb SIS mixers.

DOI
10.1088/1742-6596/97/1/012251

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Nb/Al–AlN x /Nb SIS junctions have been fabricated by various RF plasma nitridation techniques. The reproducibility of the R N A product from batch to batch was improved by diluting the N 2 plasma with He or Ar. For the He+N 2 process, a power-law relation between the R N A product and the nitridation time was found to hold along 10 0 -10 6 Ω μm 2 , which implies that the thickness of the barrier grows logarithmically during the time it is exposed to the plasma. By applying a pulse-time-modulated RF power, we were able to slow down the nitridation significantly. Finally, we present the results of preliminary RF measurements. The Nb/Al–AlN x /Nb SIS mixers were tested at around 150GHz and 350GHz, and showed noise performances comparable to those of Nb/Al–AlO x /Nb SIS mixers.
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.
We investigate the influence of substrate bias on the plasma nitridation process to form tunnel barriers in superconducting tunnel junctions, especially focusing on the bias effects on the barrier transparency. In addition, we assess the role of substrate bias in the nitridation process in the enhancement of the reproducibility of AlN-SIS fabrication process. We find that this parameter is critical to optimize the fabrication of high-performance tunnel-junctions for superconducting mixer technology.
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We report results on SIS tunnel junctions based on AlN tunnel barriers grown with plasma nitridation from a remote inductively coupled plasma source. Results for the noise temperature are shown and compared to AlOx results in a ALMA Band 9 mixer. The parameters for AlOx devices are RnA=25 Ωμm with a normal state resistance of 25 Ω and an optimized tuning circuit. For the AlN devices we have RnA = 2.9 Ωμm (Jc=71kA/cm), A=0.36 μm, Vgap=2.77 mV, and Rsg/Rn = 14. The data for AlN devices with a not yet optimized tuning circuit show comparable noise temperatures and a flat noise response.