CALDER: The Second-Generation Light Detectors

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 detectors 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 detector, superconductors with higher kinetic inductance, such as the substoichiometric titanium nitride (TiNx), were developed. TiNx is deposited with reactive dc magnetron sputtering. Thus, the fabrication process is subtractive and consists of EBL patterning through negative tone resist AR-N 7700 and SF6 etch using a Deep Reactive Ion Etching - Inductively Coupled Plasma. Critical temperature of TiNx samples was measured using the 4-point probe geometry.