Bi-layer Kinetic Inductance Detectors for space observations between 80-120 GHz

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We have developed Lumped Element Kinetic Inductance Detectors (LEKID) sensitive in the frequency band from 80 to 120 GHz. In this work, we take advantage of the so-called proximity effect to reduce the superconducting gap of Aluminium, otherwise strongly suppressing the LEKID response for frequencies smaller than 100 GHz. We have designed, produced and optically tested various fully multiplexed arrays based on multi-layers combinations of Aluminium (Al) and Titanium (Ti). Their sensitivities have been measured using a dedicated closed-circle 100 mK dilution cryostat and a sky simulator allowing to reproduce realistic observation conditions. The spectral response has been characterised with a Martin-Puplett interferometer up to THz frequencies, and with a resolution of 3 GHz. We demonstrate that Ti-Al LEKID can reach an optical sensitivity of about 1.4 10⁻¹⁷ W/Hz⁰.⁵ (best pixel), or 2.2 10⁻¹⁷ W/Hz⁰.⁵ when averaged over the whole array. The optical background was set to roughly 0.4 pW per pixel, typical for future space observatories in this particular band. The performance is close to a sensitivity of twice the CMB photon noise limit at 100 GHz which drove the design of the Planck HFI instrument. This figure remains the baseline for the next generation of millimetre-wave space satellites.