IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 28, NO. 8, DECEMBER 2018 2400403 CALDER: The Second-Generation Light Detectors Ivan Colantoni , Laura Cardani, Nicola Casali, Angelo Cruciani, Fabio Bellini, Maria Gabriella Castellano, Carlo Cosmelli, Antonio D.’ Addabbo, Sergio Di Domizio, Maria Martinez , Claudia Tomei, and Marco Vignati Abstract—The main aim of the cryogenic wide-area light detectors with excellent resolution project is the development of cryogenic light detectors with large active area (∼50 mm × 50 mm) and noise energy resolution smaller than 20-eV RMS. Such de- tectors will be used to discriminate the background in next generation large-mass bolometric experiments, such as cryogenic underground observatory for rare events. In this paper, we present the fabrication process of the phonon-mediated kinetic inductance detectors (KIDs). In the first part of the project, Al KIDs have been developed. Thin film Al (40 nm) were evaporated on high quality, high resistivity (>10 kΩ·cm) Si(1 0 0) wafers using a high vacuum electron beam evaporator. Detectors were patterned by direct-write Electron Beam Lithography (EBL) using positive tone resist AR-P 669.06. To improve the energy resolution of our detec- tor, superconductors with higher kinetic inductance, such as the substoichiometric titanium nitride (TiN x ), were developed. TiN x is deposited with reactive dc magnetron sputtering. Thus, the fabrica- tion process is subtractive and consists of EBL patterning through negative tone resist AR-N 7700 and SF 6 etch using a Deep Reactive Ion Etching—Inductively Coupled Plasma. Critical temperature of TiN x samples was measured using the 4-point probe geometry. Index Terms—Critical temperature, KID, sputtering, substoi- chiometric titanium nitride. I. INTRODUCTION T HE goal of CALDER project is to develop and operate KIDs with large active areas (50 mm × 50 mm) and Manuscript received February 18, 2018; revised April 22, 2018; accepted May 9, 2018. Date of publication May 29, 2018; date of current version July 16, 2018. This work was supported in part by the European Research Council (FP7/2007–2013) under Contract CALDER 335359 and in part by the Italian Ministry of Research under the FIRB Contract RBFR1269SL. The work of I. Colantoni was supported by the Science Foundation Ire- land under Grant 15/IA/2880 for supporting his actual contract. This paper was recommended by Associate Editor R. Cristiano. (Corresponding author: Ivan Colantoni.) I. Colantoni was with CNR IFN, Roma 00156, Italy. He is now with the Dublin Institute for Advanced Studies, School of Cosmic Physics, Dublin D04, Ireland (e-mail:, ivan@cp.dias.ie). L. Cardani, N. Casali, A. Cruciani, C. Tomei, and M. Vignati are with INFN– Sezione di Roma, Roma 00185, Italy. M. Martinez is with the Dipartimento di Fisica, Sapienza Universit` a di Roma, Roma 00185, Italy. F. Bellini and C. Cosmelli are with INFN–Sezione di Roma, Roma 00185, Italy, and also with the Dipartimento di Fisica, Sapienza Universit` a di Roma, Roma 00185, Italy. M. G. Castellano is with CNR IFN, Roma 00156, Italy. A. D’Addabbo is with the INFN Laboratori Nazionali del Gran Sasso, Assergi 67010, Italy. S. Di Domizio is with the Dipartimento di Fisica, Universit` a degli Studi di Genova, Genova 16146, Italy, and also with the INFN - Sezione di Genova, Genova 16146, Italy. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2018.2841925 noise energy resolution smaller than 20-eV RMS. Large active areas could be obtained using the so-called phonon-mediated approach [1]. The photons are coupled to the KIDs through a large insulating substrate, which converts them into phonons. The a-thermal phonons that are not thermalized or lost through the substrate supports or edges reach the superconductor and break cooper pairs (CPs). KIDs base their working principle on the kinetic inductance, a property of superconductors cooled below the critical tem- perature. When an ac bias is applied to a superconductor, the CPs, because of their mass, exhibit a kinetic inertia to the field variation. This inertia acts as an inductance and depends on the number of CPs, which can be modified by energy releases in the superconductor. A KID is obtained by including the su- perconductor in a high-quality factor LC circuit excited at the resonant frequency. When a radiation is absorbed, a fraction of CPs is broken into quasi-particles causing changes in the transfer function of the resonant circuit. The signal is extracted by monitoring the phase and amplitude modulation of the wave traveling through the circuit. The key feature of KIDs is that dif- ferent resonators can be coupled to the same transmission line by making them resonate at slightly different frequencies. The resonant frequency of each sensor can be easily tuned by means of small modifications of the layout of the capacitor and/or in- ductor of the circuit. With this technique, multiplexing factors of the order of thousands have been already demonstrated [2]. Bolometric experiments searching for rare events aim to run at very low background, by suppressing all the radioactive con- taminations, including the natural radioactivity of the materials that constitute the whole experimental setup. Those experiments could play a key role in our understanding of neutrino physics and dark matter nature. Bolometers are large crystals coupled with temperature sensors and operated as calorimeters at about 10 mK. They are particularly suitable for 0ν DBD searches because they can reach high efficiency (larger than 75%), ex- cellent energy resolution (0.1% from a few keV to a few MeV), and low intrinsic background. The lack of an active background rejection tool can be compensated using a light detector, which identifies the nature of the interacting particle exploiting the different light emissions. Experiments which could benefit from the potentiality of cryogenic light detectors are CUORE [3] and CUPID-0 [4], CUORE is running at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. CUORE searches for the 130 Te 0ν DBD using 988 TeO 2 cubic crystals (50 mm × 50 mm × 50 mm). The CUORE sensitivity to 0ν DBD is currently limited by the background produced by α decays originating on the surface of the inert materials facing the detector. The electrons 1051-8223 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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