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TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

Fabrication of 3 THz Quasi-Optical NbN Hot Electron Bolometer Mixers

Akira KAWAKAMI, Yoshihisa Irimajiri, Shukichi Tanaka, Satoshi OCHIAI, Iwao Hosako

Published Jan 1, 2014• 0 citations
Publisher

Abstract

Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.

DOI
10.2221/jcsj.49.433

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Hot electron bolometer mixers (HEBM) are expected to be used as low-noise heterodyne mixers in the terahertz frequency range. We are developing quasi-optical HEBMs for applications such as atmospheric remote sensing and radio astronomy in the terahertz frequency range. Therefore, we developed a fabrication method using niobium nitride (NbN) ultrathin films deposited on a silicone (Si) substrate for terahertz applications. First, a magnesium oxide buffer layer 50 nm thick was deposited on a Si substrate at approximately 525 °C. After deposition of this layer, NbN ultrathin films were deposited at ambient temperature. Using the buffer layers, we found the transition temperature of the 3.5-nm-thick NbN thin films to be approximately 10 K. The same method was applied to fabricate NbN-HEBMs on a Si substrate. The fabricated quasi-optical mixers were designed to operate at 3.1 THz. The receiver noise temperature was found to be approximately 1930 K (DSB) at 3.1 THz.
Low noise receivers using quasi-optical superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixers have been designed, fabricated and measured in the terahertz (THz) frequency range for THz applications in astronomy and cosmology. The NbN HEB mixers consist of a planar complementary antenna and several nanometer (nm) thick NbN bridge connecting across the antenna's inner terminals on high-resistivity Si substrates. A double sideband (DSB) receiver noise temperature of 664 K at 0.76 THz, 920 K at 1.6 THz, 1630 K at 2.5 THz and 1710 K at 3.1 THz has been obtained without corrections and no anti-reflection (AR) coating on the surface of the lens.
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 paper reports the latest development and the measured results of NbTiN hot electron bolometer mixers at THz frequencies. The devices are based on 4-7 nm thin NbTiN films, which were deposited by reactive magnetron sputtering of NbTi target in an Ar/N2 atmosphere, on heated high-resistivity Si substrates over a 20 nm thick AlN buffer layer. The quality parameters of the film are transition temperature and normal-state resistivity, which were optimized by varying the sputtering parameters. The resistivity and the critical temperature of the films are about 400 μΩcm and 9 K, respectively. Bolometers (4 μm wide and 0.7 μm long), integrated with logarithmic spiral antennas are fabricated. Mixer noise performance is tested in a quasi-optical receiver in 1.5-4 GHz IF band. The DSB receiver noise temperature of 700 K, 1100 K and 3000 K is obtained at 0.7, 1.6 and 2.6 THz LO frequencies, respectively.
To put space-based and airborne heterodyne instruments into operation at frequencies above 1 THz the superconducting NbN hot-electron bolometer (HEB) will be incorporated into heterodyne receiver as a mixer. At frequencies above 1.3 THz the sensitivity of the NbN HEB mixers outperform the one of the Schottky diodes and SIS-mixers, and the receiver noise temperature of the NbN HEB mixers increase with frequency. In this paper we present the results of the noise temperature measurements within one batch of NbN HEB mixers based on 3.5 mn thick superconducting NbN film grown on Si substrate with MgO buffer layer at the LO frequencies 2.5 THz and 3.8 THz.
Superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixers at terahertz (THz) have been designed, fabricated and measured for applications in astronomy and cosmology. The NbN HEB mixer consists of a planar antenna and an NbN bridge connecting across the antenna's inner terminals on high-resistivity Si substrates. Double sideband (DSB) receiver noise temperatures of 698 K at 0.65 THz, 904 K at 1.6 THz, 1026 K at 2.5 THz and 1386 K at 3.1 THz have been obtained at 4.2 K without corrections and using changeable input local oscillation (LO) power to eliminate the affects of direct detection and instability of the LO power. The excess quantum noise factor beta of about 4 has been estimated using a quantum noise model.