Electron–Phonon Coupling in Ti/TiN MKIDs Multilayer Microresonator

Over the last few years, there has been a growing interest toward the use of superconducting microwave microresonators operated in quasi-thermal equilibrium mode, especially applied to single particle detection. Indeed, previous devices designed and tested by our group with X-ray sources in the keV range evidenced that several issues arise from the attempt of detection through athermal quasiparticles produced within direct strikes of X-rays in the superconductor material of the resonator. In order to prevent issues related to quasiparticles self-recombination and to avoid exchange of athermal phonons with the substrate, our group focused on the development of thermal superconducting microresonators. In this configuration, resonators composed of multilayer films of Ti/TiN sense the temperature of an absorbing material. To maximize the thermal response, low-critical-temperature films are preferable. By lowering the critical temperature, though, the maximum probing power bearable by the resonators decreases abruptly because of the weakening of the electron–phonon coupling. A proper compromise between the value of critical temperature (and hence sensitivity to energy deposition) and readout power bearable by the device has to be found in order to avoid signal-to-noise ratio degradation. In this contribution, we report the latest measurement of the electron–phonon coupling.