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
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 , but novel materials could be implemented for HEB fabrication to improve their parameters. Magnesium diboride (MgB 2 ) discovered in 2001  has the highest critical temperature (T c = 39 K) among intermetallic compounds. "
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.
- "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. The work on NbN technology has produced mixers with noise temperature below 1000 K. "
Conference Paper: Development of hot-electron THz bolometric mixers using MgB 2 thin films[Show abstract] [Hide abstract]
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.
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- "Superconducting hot-electron bolometer mixers [1~3] are so far the most sensitive heterodyne detectors at THz frequency, already demonstrating good use in ground-based telescopes  and Herschel Space Observatory . The advantages of high sensitivity and low LO power requirement make HEB mixers a good choice for astronomical telescope. "
ABSTRACT: In this paper, we report on the measured and simulated far-field beam-patterns of a quasi-optical NbN superconducting hot electron bolometer (HEB) mixer at 600GHz. This superconducting HEB mixer consists of an extended hemispherical lens with a diameter of 12.7mm and an extension length of 2.45mm, a twin-slot planar antenna (two slots measuring 148.5μm × 10.4μm with a separation of 78.98μm) and a 5.5-nm thick NbN thin-film micro-bridge with an area of 2μm × 0.2μm. The far-field beam pattern of this mixer is measured by a direct-detection technique with a dynamic range of nearly 25dB, showing an FWHM beam angle of 2.7° and -18dB level of the first side-lobe. The measured beam of the quasi-optical mixer is nearly collimated and has good Gaussian beam efficiency. In addition, the far-field beam-pattern is measured at different DC bias voltages of the superconducting HEB mixer and at different bath temperatures. The measured results are compared with the ones simulated by two different methods. Detailed measurement and simulation results will be presented.