Development of 90 GHz polarization-sensitive LEKIDs for CMB experiments

Kinetic inductance detectors (KIDs) are an emerging, powerful technology for precision CosmIC MICROWAVE Background (CMB) measurements, offering high sensitivity, ease of large-scale integration, and simplified readout architectures. In particular, polarization-sensitive KIDs operating at 90 GHz achieve an optimal balance between atmospheric transparency, foreground contamination, and CMB sensitivity for ground-based observations. Here, we approach the specific problem of measuring the faint polarized signal of the CMB by means of simple and scalable detector arrays. We present simulations and preliminary measurements on a prototype 90 GHz polarization-sensitive horn-coupled lumped element KID array, designed for groundbased CMB polarimeters. The detector array is made of a Ti-Al 10+30 nm thick bilayer, deposited on a Si substrate, designed to be operated at 150 mK. The KID layout has been optimized for absorption efficiency and polarization purity, as well as for high quality factor and multiplexing factor. The laboratory characterization, performed in a testbed dilution cryostat, includes the measurement of the critical temperature of the superconducting film, the dark responsivity and noise, and the quasi-particle lifetime.