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IEEE Transactions on Applied Superconductivity

Stability of a Quasi-Optical Superconducting NbTiN Hot-Electron Bolometer Mixer at 1.5 THz Frequency Band

Hiroyuki Maezawa, T. Yamakura, T. Shiino, Satoshi Yamamoto, S. Shiba, Nami Sakai, Yoshihisa Irimajiri, Ling Jiang, Naomasa Nakai, Masumichi Seta, Akira Mizuno, Tomoo Nagahama, Yasuo Fukui

Published Jan 24, 2011• 6 citations
Publisher

Abstract

We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of$\pm$1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.

DOI
10.1109/tasc.2010.2100350

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We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of±\pm±1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.
We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.
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.
In this paper, the performance of a quasi-optical NbN superconducting hot-electron bolometer (HEB) mixer, cryogenically cooled by a close-cycled 4-K refrigerator, is thoroughly investigated at 300, 500, and 850 GHz. The lowest receiver noise temperatures measured at the respective three frequencies are 1400, 900, and 1350 K, which can go down to 659, 413, and 529 K, respectively, after correcting the loss and associated noise contribution of the quasi-optical system before the measured superconducting HEB mixer. The stability of the quasi-optical superconducting HEB mixer is also investigated here. The Allan variance time measured with a local oscillator pumping at 500 GHz and an IF bandwidth of 110 MHz is 1.5 s at the dc-bias voltage exhibiting the lowest noise temperature and increases to 2.5 s at a dc bias twice that voltage.
In this paper, we present the development of the waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixers, cryogenically cooled by a 4 K close-cycled refrigerator. The NbTiN thin film is formed on a crystalline quartz substrate by sputtering an NbTi target with the Ar and N2gas at room temperature. The HEB mixer element is fabricated by using the 12 nm NbTiN film, and is mounted on a waveguide block. Measurement of a Fourier transform spectrometer shows that the response of the mixer is centered near 810 GHz with a bandwidth of about 500 GHz. The uncorrected DSB receiver noise temperature is measured to be 500 K, and the noise bandwidth is to be 1.4 GHz at 810 GHz. The present result shows that a good noise performance can be obtained for the NbTiN HEB mixer even with a relatively thick film (12 nm) fabricated at the room temperature.
Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device is a 0.15 - 0.3 micrometer microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20 - 100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa2Cu3O7-(delta ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.
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.