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AlN Barrier SIS Junctions in Submm Heterodyne Receivers: Operational Aspects

Bout, J., A. M. Baryshev, J. Barkhof, Fausto Patricio Mena, Ronald Hesper, Brian D. Jackson, W. Wild, C. F. J. Lodewijk, D. Ludkov, T. Zijlstra, Elfi van Zeijl, T. M. Klapwijk, W. Wild

Published Apr 1, 2008• 0 citations

Abstract

(ALMA) [1] sit e is an o bservatory which consists of more than fifty 12 meter diameter sub-mm telescopes, locat ed at an altitude of 5000 m in t he Atacama desert in Chile. It covers the 30-950 GHz frequency range which is divided into ten bands following the atmospheric transmission w indows. ALMA fro nt-ends w ill be u sing Superconductor-Insulato r-Superconductor (SIS) heterodyne mixers as the key sensitive elements for a ll of its high frequency bands. The ALMA band 9 receiver which covers 600-720 GHz is being developed in t he Neth erlands. Th e first eight rec eivers h ave already been built base d on AlOx barrier SIS junctions mixers. These mixers have excellent noise temperature but show limited RF and IF ba ndwidth. Using new AlN bar rier highe r c urrent density SIS junctions, it is possible to improve on the RF and IF bandwidth of SIS mixe rs thus makin g this te chnology significantly more attra ctive f or th e u se in l arge s eries of S IS receivers. Currently, a good sensitivit y and band coverage has already b een ach ieved for AlN mixers [2]. Ho wever, performance of AlN mixe rs at highe r fr equencies de pends highly on Josephso n noise suppression and i s only optimal if a precise magnetic field is applied on the barrier. Present routines for Jose phson noise suppr ession in AlO x b arrier mixe rs were not s ufficient. I n th is p aper th ese rou tines will b e d iscussed in detail and its a daptation to AlN barrier mix er operation will be reported. I.

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(ALMA) [1] sit e is an o bservatory which consists of more than fifty 12 meter diameter sub-mm telescopes, locat ed at an altitude of 5000 m in t he Atacama desert in Chile. It covers the 30-950 GHz frequency range which is divided into ten bands following the atmospheric transmission w indows. ALMA fro nt-ends w ill be u sing Superconductor-Insulato r-Superconductor (SIS) heterodyne mixers as the key sensitive elements for a ll of its high frequency bands. The ALMA band 9 receiver which covers 600-720 GHz is being developed in t he Neth erlands. Th e first eight rec eivers h ave already been built base d on AlOx barrier SIS junctions mixers. These mixers have excellent noise temperature but show limited RF and IF ba ndwidth. Using new AlN bar rier highe r c urrent density SIS junctions, it is possible to improve on the RF and IF bandwidth of SIS mixe rs thus makin g this te chnology significantly more attra ctive f or th e u se in l arge s eries of S IS receivers. Currently, a good sensitivit y and band coverage has already b een ach ieved for AlN mixers [2]. Ho wever, performance of AlN mixe rs at highe r fr equencies de pends highly on Josephso n noise suppression and i s only optimal if a precise magnetic field is applied on the barrier. Present routines for Jose phson noise suppr ession in AlO x b arrier mixe rs were not s ufficient. I n th is p aper th ese rou tines will b e d iscussed in detail and its a daptation to AlN barrier mix er operation will be reported. I.

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.

We present the design and preliminary experimental results of superconductor-insulator-superconductor (SIS) mixers for Atacama large millimeter array (ALMA) band 10 (780-950GHz) receivers. To achieve wide frequency coverage as well as low noise temperature, we integrated high current density (10 kA/cm2) and small-sized (one micrometer in diameter) Nb/AlOx/Nb SIS junctions with a tuning microstrip line using NbTiN ground plane and Al wiring layers. High quality NbTiN films with Tc reaching 14.5 K and Al films with residual resistivity ratio as large as 15 have been achieved. To minimize the waveguide transmission loss and facilitate the fabrication of mixer block, a full height waveguide was adopted. The waveguide probe was optimized to achieve low input impedance in the frequency range of 780-950 GHz while adopting a backshort cavity of zero depth. Preliminary RF testing with a Nb/Al microstrip tuning circuit demonstrated a DSB receiver noise temperature about 400 K at 832.5 GHz, which promises the accommodation of receiver performance in ALMA specification with low loss NbTiN/Al microstrip lines.

Superconductor-insulator-superconductor (SIS) mixers are the most sensitive heterodyne receivers for mm and submm wavelengths. SIS mixers are installed on all state of the art submm observatories such as the Atacama Large Millimeter array (ALMA) and the Atacama Pathfinder Experiment (APEX). Recently, an improved type of Nb/AlN/NbN SIS junction has become available that combines higher gap voltage with high current density. This opens new parametric space for further improvement of the SIS mixer's performance and bandwidth both for frequencies higher and lower than the gap frequency of Nb (700 GHz). In this contribution we will describe designs of SIS mixers based on a new type of SIS junction, both for high (~900 GHz) and low (~200 GHz) frequencies.

The Atacama Large Millimeter Array (ALMA) is the largest radio astronomical enterprise ever proposed. When completed, each of its 64 constituting radio-telescopes will be able to hold 10 heterodyne receivers covering the spectroscopic windows allowed by the atmospheric transmission at the construction site, the altiplanos of the northern Chilean Andes. In contrast to the sideband-separating (2SB) receivers being developed at low frequencies, double-side-band (DSB) receivers are being developed for the highest two spectroscopic windows (bands 9 and 10). Despite of the well known advantages of 2SB mixers over their DSB counterparts, they have not been implemented at the highest-frequency bands as the involved dimensions for some of the radio frequency components are prohibitory small. However, the current state-of-the-art micromachining technology has proved that the structures necessary for this development are attainable. Here we report the design, modeling, realization, and characterization of a 2SB mixer for band 9 of ALMA (600 to 720 GHz). At the heart of the mixer, two superconductor-insulator-superconductor (SIS) junctions are used as mixing elements. The constructed instrument presents an excellent performance as shown by two important figures of merit: noise temperature of the system and side band ratio, both of them within ALMA specifications.

Band10 of the Atacama Large Millimeter Array (ALMA) is a planned heterodyne receiver covering the frequency range 790 GHz-950 GHz. The waveguide mixers are superconductor insulator superconductor (SIS) Nb/Al-AlOx/Nb junctions integrated with NbTiN/SiO2/Al tuning circuits. Our junction definition process has been greatly simplified by introducing an inductively-coupled-plasma (ICP) etcher to the fabrication. The etching process, which employs a low-pressure CF4/O2plasma, allows to remove the entire Nb/Al-AlOx/Nb tri-layer in asinglestep. SIS mixers with current densities up to 10 kA/cm2and quality factor above 15 have been fabricated with good reproducibility. Minimum receiver noise temperature, corrected for optical losses, is 210 K at 860 GHz. With the goal to further improve the performance, optimization of circuit design and fabrication technology are discussed.

In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA/cm/sup 2/ and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. In addition, a superconducting magnet fixed onto the compact mixer block to provide efficient magnetic field coupling is designed. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 108 K at the band center and temperatures of less than 167 K over the whole band (for an intermediate-frequency range of 4-8 GHz).