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Softwaretechnik-Trends

Wideband AlN-based SIS devices for frequencies around 700 GHz

C. F. J. Lodewijk, T. Zijlstra, Denis Loudkov, T. M. Klapwijk, Fausto Patricio Mena, Andrey M. Baryshev

Published Jan 1, 2007• 6 citations
OpenAlex

Abstract

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.

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We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.

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The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical observatory that is being built on a 5000 m altitude plateau in Chile to perform heterodyne measurements in the frequency range of 30 to 950 GHz. These measurements will reveal the presence of characteristic molecules that tell us more about the evolution of the early universe and the formation of stars and planetary systems. Because of the limited atmospherical transmission of radiation at these frequencies, ALMA is divided into 10 frequency bands with a sufficiently high transmission. This thesis describes the research that has been done to develop superconductor - insulator - superconductor (SIS) tunneljunctions as detectors for Band 9 of ALMA (602 - 720 GHz). To realize these devices, a tuning circuit has been developed that optimizes the coupling of electromagnetic radiation from the antenna to the SIS junction. Further, a new method has been developed to create SIS junctions with aluminum nitride (AlN) as the insulating material. This method uses a remote plasma from an inductively coupled plasma source. Junctions that have been grown with this new method show a much better uniformity in transmissivity of the tunneling barrier at high critical current densities. Using the new method, SIS devices with AlN tunnel barriers have been developed that have a record low noise temperature over the full bandwidth of Band 9.

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NbTiN Based Tuning Structures for Broadband Nb-Al 2 O >3 -Nb SIS Mixers from 640 GHz-800 GHz

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We present measurements with a prototype waveguide SIS mixer for band 2 (640 GHz – 800 GHz) of HIFI (Heterodyne Instrument for FIRST). The Nb/Al2O3/Nb junctions have an RnA product from 12–18 Ωμm. These junctions are embedded in microstrip circuits tuning out the device capacitance using a NbTiN ground layer and various top conductor materials such as NbTiN, Nb and Al. The NbTiN films are fabricated by DC magnetron sputtering which is optimized for a minimum value of the DC resistivity and for a high critical temperature by varying the sputter pressure, sputter power and Ar:N2 flow. Typical NbTiN film parameters for device fabrication are ρ = 120 μΩcm and Tc = 14.5 K. The best uncorrected noise temperatures we measured so far are for devices with NbTiN/SiO2/Nb tuning. At 4.2 K bath temperature, a standard Yfactor measurement yields uncorrected receiver noise temperatures of 350 K at 642 GHz and 500 K at 810 GHz for 1.5 GHz intermediate frequency. We discuss the effect of the various tuning circuit materials on the mixer performance. In addition, measurements at IF’s between 4 GHz and 8 GHz will be presented.

Softwaretechnik-Trends•Dec 1, 2001•0 citations
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.
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
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 Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical observatory that is being built on a 5000 m altitude plateau in Chile to perform heterodyne measurements in the frequency range of 30 to 950 GHz. These measurements will reveal the presence of characteristic molecules that tell us more about the evolution of the early universe and the formation of stars and planetary systems. Because of the limited atmospherical transmission of radiation at these frequencies, ALMA is divided into 10 frequency bands with a sufficiently high transmission. This thesis describes the research that has been done to develop superconductor - insulator - superconductor (SIS) tunneljunctions as detectors for Band 9 of ALMA (602 - 720 GHz). To realize these devices, a tuning circuit has been developed that optimizes the coupling of electromagnetic radiation from the antenna to the SIS junction. Further, a new method has been developed to create SIS junctions with aluminum nitride (AlN) as the insulating material. This method uses a remote plasma from an inductively coupled plasma source. Junctions that have been grown with this new method show a much better uniformity in transmissivity of the tunneling barrier at high critical current densities. Using the new method, SIS devices with AlN tunnel barriers have been developed that have a record low noise temperature over the full bandwidth of Band 9.
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.
Abstract We report a Nb/Al-AlN/Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron cyclotron resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with R n A down to ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 25 at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance C s versus R n A is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted C s .
We present measurements with a prototype waveguide SIS mixer for band 2 (640 GHz – 800 GHz) of HIFI (Heterodyne Instrument for FIRST). The Nb/Al2O3/Nb junctions have an RnA product from 12–18 Ωμm. These junctions are embedded in microstrip circuits tuning out the device capacitance using a NbTiN ground layer and various top conductor materials such as NbTiN, Nb and Al. The NbTiN films are fabricated by DC magnetron sputtering which is optimized for a minimum value of the DC resistivity and for a high critical temperature by varying the sputter pressure, sputter power and Ar:N2 flow. Typical NbTiN film parameters for device fabrication are ρ = 120 μΩcm and Tc = 14.5 K. The best uncorrected noise temperatures we measured so far are for devices with NbTiN/SiO2/Nb tuning. At 4.2 K bath temperature, a standard Yfactor measurement yields uncorrected receiver noise temperatures of 350 K at 642 GHz and 500 K at 810 GHz for 1.5 GHz intermediate frequency. We discuss the effect of the various tuning circuit materials on the mixer performance. In addition, measurements at IF’s between 4 GHz and 8 GHz will be presented.