BULLKID: Monolithic array of particle absorbers sensed by Kinetic Inductance Detectors

We introduce BULLKID, an innovative phonon detector consisting of an array of dices acting as particle absorbers sensed by multiplexed Kinetic Inductance Detectors (KIDs). The dices are carved in a thick crystalline wafer and form a monolithic structure. The carvings leave a thin common disk intact in the wafer, acting both as holder for the dices and as substrate for the KID lithography. The prototype presented consists of an array of 64 dices of 5.4x5.4x5 mm$^3$ carved in a 3" diameter, 5 mm thick silicon wafer, with a common disk 0.5 mm thick hosting a 60 nm patterned aluminum layer. The resulting array is highly segmented but avoids the use of dedicated holding structures for each unit. Despite the fact that the uniformity of the KID electrical response across the array needs optimization, the operation of 8 units with similar features shows, on average, a baseline energy resolution of $26\pm7$ eV. This makes it a suitable detector for low-energy processes such as direct interactions of dark matter and coherent elastic neutrino-nucleus scattering.