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19 th International Symposium on Space Terahertz Technology A novel THz SIS mixer with a NbTiN-groundplane and SIS micro-trylayers directly grown on a quartz substrate A. Endo 1,2,3 , T. Noguchi 3 , M. Kroug 3 , S. V. Shitov 3,4 , W. Shan 5 , T. Tamura 3 , T. Kojima 3,6 , Y. Uzawa 3 , T. Sakai 3 , H. Inoue 1 , K. Muraoka 1,2 , K. Kohno 1 1 IoA, Univ. of Tokyo; 2 JSPS Research Fellow; 3 NAOJ; 4 IREE; 5 PMO; 6 Osaka Pref. Univ. 3-3 It is common to adopt superconductors with large gap energies (e.g., NbTiN, NbN) or normal metals with low resistivity (e.g., Al) as materials for the transmission lines of SIS mixers that operate above the gap frequency of Nb (~0.7THz). However, these materials are not necessarily the best materials for fabricating good quality SIS junctions. Therefore, a combination of SIS junctions and transmission lines made of different materials is a potent candidate in designing a THz-SIS mixer. For example, a combination of Nb SIS junctions with a NbTiN/Al microstripline can show good performance at above 1THz[1]. Usually, these mixers are fabricated by depositing an SIS-trilayer on top of the groundplane. In such a configuration, the requirements on the film quality of the grounplane tend to be relatively tight, if both good quality SIS junctions and a low-loss transmission line are to be realized. There is also a potential problem of heat trapping in the junction placed on a superconductor with a higher gap energy[2]. We propose an alternative structure and fabrication process for such multi-material SIS mixers. In this design, both the µm-sized SIS trilayers and the groundplane are deposited directly onto the quartz substrate, or possibly with a buffer layer in between (Fig1). This structure is expected to possess a number of unique features, for example: • The quality of the SIS junction is less affected by the physical nature of the groundplane film. • The heat can escape directly from the junction into the substrate. We have investigated the influence of this structure on the junction quality and circuit characteristics, by means of numerical calculation and experiment. Numerical calculation suggests that the effect of the extra resistance around the junction is tolerable in terms of noise performance, if the width of the gap is ≤0.5µm. SIS mixers based on this structure have been fabricated using Nb/Al-AlOx(or AlNx)/Nb SIS junctions and NbTiN/Al microstriplines (Fig. 2). The junction quality is comparable to that of all-Nb devices (Fig. 3), suggesting that the junction quality is less affected by the groundplane properties. These SIS mixers will be tested for possible application in the Band 10 receiver (787-950 GHz) of ALMA (Atacama Large Millimeter and submillimeter Array)[3], as well as the upcoming 800GHz-band dual polarization receiver (AERO) for the ASTE (Atacama Submillimeter Telescope Experiment). FIG. 1. Cross sectional geometry of the SIS mixer with a micro-trylayer. FIG. 2. SEM micrograph of a twin-junction. The diameter of the junction is 1µm. FIG. 3. dc I(V) curve of an Nb/Al-AlN x /Nb SIS junction with NbTiN/Al microstripline [1] Jackson et al., APL 97, 113904 (2005) [2] Leone et al., APL 76, 780 (2000) [3] Shan et al., IEEE Trans. Appl. Supercond. 17, 363 (2007) 32
19 th International Symposium on Space Terahertz Technology 3-4 Design and Performance of Waveguide Mixers with All NbN tunnel junctions on MgO substrates Wenlei Shan 1 , Masanori Takeda 2 , Kojima Takafumi 3,4 , Yoshinori Uzawa 3 , Shengcai Shi 1 , and Zhen Wang 2 1. Purple Mountain Observatory, 2 West Beijing Road, Nanjing 210008, China 2. Kansai Advanced research Center, National Institute of Information and Communications Technology, Iwaoka 588-2, Iwaoka-cho, Nishi-ku, Kobe, 651-2492, Japan 3. National Astronomical Observatory of Japan, Mitaka, Tokyo, 181-8588, Japan 4. Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-Cho, Sakai, Osaka, 599-8531, Japan Low noise waveguide mixers with all NbN tunnel junctions on MgO substrate have been developed approaching (0.78-0.95THz). For the first time, such waveguide mixers were experimentally verified to be as good as their quasi-optical counterparts in sensitivity. Improvements are likely in virtue of high quality NbN film as well as an effective waveguide probe design that has following properties. First, the waveguide probe works in a full height waveguide to minimize the waveguide loss. Second, the design makes use of a resonance choke filter on MgO substrate with a thickness of 35 micrometers desirable for convenient and accurate mounting, which can effectively block the signal from leaking to IF port. Third, the low impedance of such a probe is close to that of the SIS junction resulting reduced loss by not using a quarter-wavelength impedance transformer. As for the mixing element, both half-wavelength self-compensating distributed SIS junction and parallel-connected twin-junction (PCTJ) are designed and evaluated. Measurement results show that the PCTJ mixer is superior in both gain and noise likely due to smaller loss in the tuning circuit. 33
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