(a) LEKID 3-D stack. In red, the superconducting film (thickness not to scale). The micromachined AR was made using only silicon substrates. The backshort lid is superconducting to avoid affecting the quality factor of the resonators. (b) The two kinds of planar LEKID designs adopted: the classical (CL) meander or the Hilbert (H) dual-polarisation structure. Bottom panel: picture of the final prototypes at 3 mm.