Hot-electron bolometer terahertz mixers for the Herschel Space Observatory

Department of Microtechnology and Nanoscience, Physical Electronics Laboratory, Chalmers University of Technology, SE-41296 Göteborg, Sweden.
Review of Scientific Instruments (Impact Factor: 1.61). 04/2008; 79(3):034501. DOI: 10.1063/1.2890099
Source: PubMed


We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.

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    • "HEBs could be used at higher frequencies than SIS mixers (1.3 THz upper limit). Typically, phonon-cooled HEBs are made from ultrathin films of NbN [7], but novel materials could be implemented for HEB fabrication to improve their parameters. Magnesium diboride (MgB 2 ) discovered in 2001 [8] has the highest critical temperature (T c = 39 K) among intermetallic compounds. "
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    ABSTRACT: In this paper we compare the performance of MgB2 Hot-Electron Bolometer Mixers operating at Local Oscillator frequencies of 0.6 THz and 1.63 THz. The minimum noise temperatures that were obtained are 700 K and 1150 K for 0.6 THz and 1.63THz respectively. The receiver noise bandwidth is of the order of 2.2-3GHz for 10nm thick HEB devices with a Tc of 8.5K. Sub-micrometer size HEBs were also fabricated with no degradation of the initial film quality when a 20nm MgB2 film with a Tc of 22K was used. In the direct detection mode, the maximum voltage responsivity is in the range of 1-2kV/W at 1.63THz and the optimal bias current is around 1/4-1/3 of the Ic at 4.2K.
    IEEE Transactions on Applied Superconductivity 10/2014; 25(3). DOI:10.1109/TASC.2014.2365134 · 1.24 Impact Factor
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    • "NbN superconducting thin films have been optimized for the application of HEB mixers and as a result have been chosen for use on such space missions as Herschel HIFI[2]. The work on NbN technology has produced mixers with noise temperature below 1000 K[3]. "
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    ABSTRACT: Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
    • "Previously, the noise temperature of NbN HEB mixers was reported to be sensitive to the bath temperature [3], [25], increasing almost immediately as the bath temperature rises. Similar behavior has been observed for a MgB mixer with a low [20]. "
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    ABSTRACT: A noise bandwidth (NBW) of 6-7 GHz was obtained for hot-electron bolometer (HEB) mixers made of 10 nm MgB2 films. A systematic investigation of the (IF) gain bandwidth as a function of the MgB2 film thickness (30, 15, and 10 nm) is also presented. The gain bandwidth (GBW) of 3.4 GHz was measured for a 10 nm film, corresponding to a mixer time constant of 47 ps. For 10 nm films a reduction of the GBW was observed with the reduction of the critical temperature (T-c). Experimental data were analyzed using the two-temperature model. From the theoretical analysis, the electron-phonon time (tau(e-ph)) -, the phonon escape time (tau(esc)) and the electron and phonon specific heats (c(e), c(ph)) were extrapolated giving the first model for HEB mixers of MgB2 films.
    IEEE Transactions on Terahertz Science and Technology 07/2013; 3(4):409-415. DOI:10.1109/TTHZ.2013.2252266 · 2.18 Impact Factor
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