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IEEE Transactions on Applied Superconductivity

IF bandwidth and noise temperature measurements of NbN HEB mixers on MgO substrates

Shigehito Miki, Yoshinori Uzawa, Akira Kawakami, Zhen Wang

Published Mar 1, 2001• 20 citations
Publisher

Abstract

We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.

DOI
10.1109/77.919313

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IEEE Transactions on Applied Superconductivity•Jun 1, 2003•29 citations
We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.
We present the results of our studies of NbN phonon-cooled HEB mixers at terahertz frequencies. The mixers were fabricated from NbN film deposited on a high-resistivity Si substrate with an MgO buffer layer. The mixer element was integrated with a log-periodic spiral antenna. The noise temperature measurements were performed at 2.5 THz and at 3.8 THz local oscillator frequencies for the 3 x 0.2 μm2 active area devices. The best uncorrected receiver noise temperatures found for these frequencies are 1300 K and 3100 K, respectively. A water vapour discharge laser was used as the LO source. The largest gain bandwidth of 5.2 GHz was achieved for a mixer based on 2 nm thick NbN film deposited on MgO layer over Si substrate. The gain bandwidth of the mixer based on 3.5 nm NbN film deposited on Si with MgO is 4.2 GHz and the noise bandwidth for the same device amounts to 5 GHz. We also present the results of our research into decrease of the direct detection contribution to the measured Y-factor and a possible error of noise temperature calculation. The use of a square nickel cell mesh as an IR-filter enabled us to avoid the effect of direct detection and measure apparent value of the noise temperature which was 16% less than that obtained using conventional black polyethylene IR-filter.
Experimental work on the development of a quasi-optical heterodyne receiver for the THz frequency range is reported. The mixing device is a superconducting hot electron bolometer (HEB) made from ultrathin, d = 3 - 5 nm, niobium nitride (NbN) films. Work described in this thesis comprises: The design of a prototype receiver using quasi-optical coupling. Device fabrication involving reactive magnetron sputtering of NbN films and lithographic definition of the antenna integrated mixer. An investigation of the receiver performance at frequencies between 0.6 THz and 2.5 THz. The best performance is obtained with HEB mixers made from NbN films with a thickness d = 3-3.5 nm and a critical temperature T c = 9 - 9.5 K. Double-side band receiver noise temperatures are 530 K at 0.6 THz, 650 K at 1.6 THz and 1100 K at 2.5 THz. Corrected for losses in the receiver input optics, these values amount to only 10 times the theoretical quantum limit. MgO as the substrate material has been shown to provide improved acoustic coupling between NbN film and the substrate compared to other materials such as Si or sapphire. A maximum IF gain bandwidth of 4.5 GHz has been measured under the same operating conditions which yield lowest noise temperature. The local oscilltor power requirement is ~250 nW absorbed in the bolometer. Quasi-optically coupled HEB mixers have become the device-of-choice for radio astronomical applications in the frequency regime at and above 1 THz.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The best results of the DSB noise temperature at 1.5 GHz IF frequency obtained with one device are: 1300 K at 650 GHz, 4700 K at 2.5 THz and 10000 K at 3.12 THz. The measurements were performed at 4.5 K ambient temperature. The amount of local oscillator (LO) power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain measured at 650 GHz is -9 dB, the total conversion gain is -14 dB.
The performance of a hot-electron bolometric (HEB) mixer based on NbN from 0.9 to 2.5 THz was investigated using a quasi-optical receiver configuration. An HEB mixer is an ultra-thin superconducting NbN strip located at the feed point of a thick normal conducting Au spiral antenna on a high-resistivity Si substrate. The active area of the mixer was 3 nm thick, 0.4 μm long and 4 μm wide. The quasi-optics consisted of an MgO hyperhemisphere with anti-reflection caps made of Kapton-JP polyimide film and an offset parabola to reduce input losses. The frequency dependence of the double-sideband receiver noise temperature was investigated at several frequencies by using a backward wave oscillator or an optically pumped far-infrared laser as the local oscillator. Results demonstrated low-noise and wide-band characteristics, below 1 K GHz −1 over the measured frequency range. At 917 GHz, the measured receiver noise temperature was 550 K across a 500 MHz intermediate-frequency bandwidth centred at 1.5 GHz, which is slightly better than that of other HEB mixers at around this frequency. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.
Hot-electron bolometer (HEB) mixers are used in many low noise heterodyne radio astronomical receivers. Their noise temperature is at the level of 10–15 times the quantum limit, and their gain bandwidth is from 2 GHz to slightly more than 5 GHz depending on the type of substrate material used for a device fabrication. The HEB mixers presented in this Letter are a complex multilayer thin-film structure containing an ultra-thin superconducting film of NbN as a detecting element, and a thick normal metal layer as an antenna structure. The overall performance of the device has been improved by optimisation of the HEB mixer fabrication steps starting with ultra-thin NbN films, followed by NbN/Au bi-layers for antenna structures. Patterning was achieved using electron-beam lithography and lift-off processes. The noise temperature of the HEB mixer fabricated with the improved technology is about 800 K, as measured in a cryogenic-free system with a quantum cascade laser as a local oscillator.
In this paper, we present the results of IF bandwidth measurements on 3-4 nm thick NbN hot electron bolometer waveguide mixers, which have been fabricated on a 200-nm thick MgO buffer layer deposited on a crystalline quartz substrate. The 3-dB IF bandwidth, measured at an LO frequency of 0.81 THz, is 3.7 GHz at the optimal bias point for low noise receiver operation. We have also made measurements of the IF dynamic impedance, which allow us to evaluate the intrinsic electron temperature relaxation time and self-heating parameters at different bias conditions.