Superconducting Coplanar Waveguide Filters for Submillimeter Wave On-Chip Filterbank Spectrometers

J Low Temp Phys DOI 10.1007/s10909-016-1579-8 Superconducting Coplanar Waveguide Filters for Submillimeter Wave On-Chip Filterbank Spectrometers A. Endo1,2 · S. J. C. Yates3 · J. Bueno4 · D. J. Thoen1 · V. Murugesan4 · A. M. Baryshev3,5 · T. M. Klapwijk2,6 · P. P. van der Werf7 · J. J. A. Baselmans1,4 Received: 29 September 2015 / Accepted: 2 March 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract We show the first experimental results which prove that superconducting NbTiN coplanar–waveguide resonators can achieve a loaded Q factor in excess of 800 in the 350 GHz band. These resonators can be used as narrow band pass filters for on-chip filter bank spectrometers for astronomy. Moreover, the low-loss coplanar waveguide technology provides an interesting alternative to microstrip lines for constructing large scale submillimeter wave electronics in general. Keywords Spectroscopy · Filters · Submillimeter wave · Astronomical instrumentation · Microwave kinetic inductance detectors B A. Endo 1 Department of Microelectronics, Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands 2 Kavli Institute of Nanoscience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands 3 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands 4 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 5 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands 6 Physics Department, Moscow State Pedagogical University, 119991 Moscow, Russia 7 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands 123 J Low Temp Phys 1 Introduction On-chip filterbank spectrometers that use superconducting resonators as narrow band pass filters are becoming more popular as the design for realizing nextgeneration low-resolution millimeter–submillimeter (mm-submm) wave (100–1000 GHz) spectrometers for astronomy [1–3]. The concept relies on the availability of superconducting microresonators with sufficiently high Q factors to achieve the required frequency resolution, and a transmission line with low enough losses to carry the signal from the antenna to the far end of the filterbank. In cases where transmission line resonators are used as the band pass filters, the two requirements are related; the internal (unloaded) Q i of the resonator is associated to the transmission loss of the line through [4] Qi = π , αλ (1) where α is the attenuation constant and λ is the wavelength in the resonator. (Note that Eq. 1 holds only if Q i is limited by the nominal transmission loss of the line, and not if losses at the ends of the resonator dominate.) For example, the DESHIMA spectrometer [1,2] in development requires filters with a loaded Q l = 500, equal to the designed frequency resolution of F/F = 500, at 326–905 GHz. Because transmission lines that carry the signal from one element to the next are the most fundamental building blocks for high frequency electronics, there are many applications that would grossly benefit from a transmission line technology with low losses in the mm-submm band; among those are superconductor–insulator–superconductor (SIS) mixer devices [5], traveling wave kinetic inductance parametric amplifiers [6], and near-field microscopes [7]. Coplanar waveguides (CPWs) are one of the most widely used kinds of transmission lines for superconducting mm-submm electronics. The advantages of CPWs include: (1) it can be made with a metal film deposited directly on a crystalline dielectric substrate, thereby eliminating the presence of amorphous dielectric materials that can be lossy [8], (2) it is trivial to make a short to the ground, making it easy to realize λ/4 resonators. Another advantage of CPWs that is often quoted is the ease of fabrication because it is a ‘single layer’ structure, but this holds less for long lines that require airbridges [9,10] to suppress the odd-mode excitation. Although the intended evenmode of the CPW is less radiative, the radiation loss per unit length increases rapidly as a function of frequency F; in the case of a perfect conductor with no losses and no kinetic inductance, the attenuation constant is approximately proportional [11] to F 3 . This has been the main reason that previous attempts to develop an on-chip direct detection spectrometer have adopted microstrip lines and not CPWs [2,3,8] for their resonant filters, though microstrips have their own challenge to minimize material losses, especially in the higher-frequency submillimeter band [8]. In this paper, we revisit the use of CPWs as narrow band pass filters for on-chip filterbank spectrometers. In order to suppress radiation loss, we fabricate sub-micron lines using electron-beam lithography [12]. We also take advantage of the fact that the kinetic inductance of superconducting films suppresses radiation loss [13], because 123 J Low Temp Phys the fraction of energy carried as the kinetic energy of Cooper pairs is not radiative. We experimentally prove that it is possible to achieve a loaded Q l in excess of 500 required for the 350 GHz band of DESHIMA, indicating that the intrinsic (unloaded) Q i is even higher. 2 Device Design and Fabrication Micrographs of one channel of the filterbank are presented in Fig. 1A–G. An equivalent-circuit representation of the filterbank is included in Fig. 1H. Each channel is a combination of a filter, and a NbTiN/Al hybrid MKID [14]. The filter is a λ/4 resonator with one side open and the other side short circuited. The filter, as well as the ∼30 mm long signal line that carries the signal from the antenna to the filter bank, are made of a NbTiN CPW with a central line width of S = 0.6 µm and a slot width of W = 1.0 µm. The shorted end of the filter runs in parallel to the signal line, and the open end runs in parallel to the MKID. The filter transmission has been simulated using a commercial software Sonnet EM, to achieve a loaded Q i of 560–615. After making a 90◦ turn on each side, the submm signal is guided to CPWs that have an Al center line to have the signal absorbed therein. The antenna is a double-slot antenna similar to the one adopted by Janssen et al. [14], backed with a Si lens with a diameter of 8 mm. The device is fabricated on a 350 µm-thick c-plane sapphire substrate. After the wafer was cleaned in 85 vol% phosphoric acid at 110 ◦ C for 30 min, 350 nm of NbTiN was deposited by dc reactive sputtering of a NbTi target in an Ar and N2 plasma [15]. The pattern in the NbTiN, including the filter and signal line, was defined using electron-beam writing on PMMA resist, followed by an SF6 + O2 capacitivelycoupled plasma etch and an O2 plasma cleaning. The next step was the creation of the support (...truncated)