  Providing a background discrimination tool is crucial for enhancing the
sensitivity of next-generation experiments searching for neutrinoless double-
beta decay. The development of high-sensitivity (< 20 eV RMS) cryogenic light
detectors allows simultaneous read-out of the light and heat signals and
enables background suppression through particle identification. The Cryogenic
wide- Area Light Detector with Excellent Resolution (CALDER) R&D already proved
the potential of this technique using the phonon-mediated Kinetic Inductance
Detectors (KIDs) approach. The first array prototype with 4 Aluminum KIDs on a
2 $\times$ 2 cm2 Silicon substrate showed a baseline resolution of 154 $\pm$ 7
eV RMS. Improving the design and the readout of the resonator, the next CALDER
prototype featured an energy resolution of 82 $\pm$ 4 eV, by sampling the same
substrate with a single Aluminum KID.
