Development of an \(8\times 8\) CPW Microwave Kinetic Inductance Detector (MKID) Array at 0.35 THz

Journal of Low Temperature Physics July 2016, Volume 184, Issue 1–2, pp 103–107 | Cite as Development of an \(8\times 8\) CPW Microwave Kinetic Inductance Detector (MKID) Array at 0.35 THz AuthorsAuthors and affiliations Jing LiJin-Ping YangZhen-Hui LinDong LiuSheng-Cai ShiS. MimaN. FurukawaC. Otani Open Access Article First Online: 29 December 2015 990 Downloads 1 Citations Abstract Microwave kinetic inductance detectors (MKIDs) are promising for THz direct detector arrays of large size, particularly with simple frequency-division multiplexing. Purple Mountain Observatory is developing a terahertz superconducting imaging array (TeSIA) for the DATE5 telescope to be constructed at Dome A, Antarctica. Here we report on the development of a prototype array for the TeSIA, namely an \(8\times 8\) CPW MKID array at 0.35 THz. The resonance frequencies of the MKIDs span the 4–5.575 GHz band with an interval of 25 MHz. Each detector is integrated with a twin-slot antenna centered at 0.5 THz and with a relative bandwidth of 10 %, while the whole MKID array with a micro-lens array. Detailed design and measurement results will be presented. KeywordsMicrowave kinetic inductance detectors (MKIDs) CPW  Superconducting resonator TiN TeSIA  1 Introduction China is planning to construct an observatory at Dome A, Antarctica, which has been found to be an excellent site (with low precipitable water vapor and low atmospheric boundary layer) on the earth for THz and Optical/IR astronomy. The 5-m THz telescope (DATE5 [1]), mainly targeting at the 350 and 200 \(\upmu \)m atmospheric windows, is one of the two telescopes to be built there. A science case for the DATE5 is to observe extreme starburst galaxies at different redshifts to better understand the nature and evolution of these enigmatic and important objects. To meet this requirement, a superconducting imaging camera named TeSIA is proposed [2]. TeSIA operating at 350 \(\upmu \)m has an array size of \(32\times 32\) pixels and requires a background-limited sensitivity as high as 10\(^{-16}\) W/Hz\(^{0.5}\). A long-wavelength (850 \(\upmu \)m or 0.35 THz) prototype with a smaller size of \(8\times 8\) pixels is being developed with both transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs) [3]. Here we mainly report on the development of an \(8\times 8\) CPW MKID array at 0.35 THz. As is well known, microwave kinetic inductance detectors (MKIDs) use frequency-domain multiplexing (FDM) that allows thousands of pixels to be read out over a single microwave transmission line followed by a cryogenically cooled low-noise amplifier [4]. In addition, a large number of MKIDs can be integrated with a filter bank to realize on-chip spectrometers such as DESHIMA and SuperSpec [5, 6]. The MKID array we are developing makes use of TiN superconducting films [7] with a critical temperature of approximately 4.5 K. Such a superconducting film is just suitable for both the operating temperature of 0.3 K and the frequency of 0.35 THz for our \(8\times 8\) CPW MKID array. The \(8\times 8\) TiN MKID array will be integrated with an \(8\times 8\) micro-lens array of 0.95-mm-diameter hyper-spherical Si lens. The FDM readout for this MKID array is similar to those used by other groups, but makes use of a commercial arbitrary wave-function generator to generate 64-tone input signal with 45-dB SNR. 2 MKID Design and Fabrication We adopted the coplanar-waveguide (CPW) type resonator to design our \(8\times 8\) TiN MKID array because it has a relatively simple architecture of only one thin-film layer on the substrate. This kind of resonator has a quarter-wavelength transmission line with one end capacitively coupled to a microwave feed line and the other matched to a planar antenna (twin-slot antenna, for example) [8]. Figure 1 shows the overall view of a fabricated MKIDs that consist of a coupler, a resonator, and a planar antenna, as well as the enlarged parts of the coupler and the antenna. Open image in new window Fig. 1 Overall view of an MKIDs as well as its enlarged parts of the coupler and the antenna We first designed the coupler, which is an elbow structure at the end of the resonator near the microwave feed line, just as shown in Fig. 1. With this layout, the feed line and the coupler share the same metal section as the ground. The CPW feed line was designed to have a 10 \(\upmu \)m center strip width and 6 \(\upmu \)m gaps, corresponding to a characteristic impedance of 50 \(\Omega \). The CPW line for the coupler was designed to have a 3 \(\upmu \)m center stripe and 2 \(\upmu \)m gaps. The ground space between the feed line and the coupler was set at 2 \(\upmu \)m. Based on this structure, we can have different coupling strength by changing the coupler length only. Using an electromagnetic microwave simulator [9], we simulated the dependences of the coupling strength upon the coupling quality factor (\(Q_\mathrm{c}\)), the resonance frequency (\(f_{0}\)), and the coupler length (\(L_\mathrm{c}\)). As is well known, \(Q_\mathrm{c}\) can be calculated directly from \(S_{13}\), i.e., the transmission from the feed line to the coupler, according to \(Q_\mathrm{c} ={\pi }/2\left| {\text{ S }_{13}} \right| ^2\) [10]. Note that our simulations cover a resonance frequency range from 3 to 7 GHz and \(L_\mathrm{c}\) from 5 to 1005 \(\upmu \)m, and that the metal film is assumed to be lossless. The designed \(8\times 8\) TiN MKIDs have coupling quality factors ranging from 50 to 1000 k and resonance frequencies between 4 and 5.575 GHz with an interval of 25 MHz. The lengths of the resonators were calculated straightforwardly according to an effective quarter-wavelength with zero penetration depth. The twin-slot antenna was designed simply at a center frequency of 0.35 THz and with a relative bandwidth of 10 %. Note that the impact of the high resistivity of TiN films will be taken into account for further simulation. The \(8\times 8\) MKID array was fabricated in the clean room of RIKEN Center for Advanced Photonics (Japan). As introduced before, we chose TiN superconducting films for this MKID detector array [11]. Its \(T_\mathrm{c}\) can be controlled between 0 and 5 K by the components of Ti and N\(_{2}\) [7]. Firstly, a 100-nm-thick TiN film was deposited on a high resistivity Si wafer in a DC magnetron sputtering system. Secondly, the CPW lines were defined in contact lithography by a mask aligner. Thirdly, the etching course was done in an ICP machine. The details of the process are summarized in Table 1. The fabricated MKIDs, as shown in Fig. 1, have a \(T_\mathrm{c}\) approximately equal to 4.5 K. Table 1 Process parameters for TiN MKIDs Parameter Value Si substrate thickness \(\sim \)380 \(\upmu \)m Si substrate resistivity \(\ge \)1 k\(\Omega \) cm Si substrate size 3-inch Ti target size 6-inch Target-substrate distance 130 mm Gas fl (...truncated)