Development of Phonon-Mediated Lumped Element Kinetic Inductance Detectors With Surface Activated Bonding

We have proposed novel phonon-mediated lumped element kinetic inductance detectors (LEKIDs) that exploit the anisotropic phonon propagation properties of stoichiometric lithium niobate (sLN). Because of its strong piezoelectricity introducing significant microwave loss, we adopted surface activated bonding (SAB) technology to spatially separate the LEKID electrodes from the sLN substrate, enabling high internal quality factors ($Q_{i}$). Simulations and experiments confirmed that this separation significantly suppresses$Q_{i}$degradation. LEKIDs were fabricated on the front side of a sapphire substrate, with sLN bonded to the backside using SAB. We achieved an improvement of approximately 1,000 times in$Q_{i}$compared to devices on sLN. We also observed phonon responses from$\alpha$-ray irradiation using an$\bf ^{241}$Am radioactive source. These results demonstrate the potential of phonon-mediated detectors utilizing SAB.