Design of a Superconducting Planar Orthomode Transducer for the 84-116 GHz Band

We describe the design of a planar orthomode transducer (OMT) for 84-116 GHz (3 mm band). The OMT consists of a chip that integrates a superconducting planar circuit and a mechanical assembly with input and output waveguide ports. The chip is based on a 250 $\mu \mathrm{m}$ thick silicon (Si) substrate and on $8 \mu \mathrm{m}$ thick Silicon Nitride $^{(Si_{3}{N}_{4})}$, membranes that sustain independent thin-film Niobium (Nb) circuitries for the two polarization channels. The chip has internal cut-outs to prevent spurious propagation and in-band resonances. The microfabrication of the chip and of the mechanical assembly are carried out at IRAM. The OMT mechanical design features two parallel waveguide outputs that are also parallel to the waveguide input. The OMT is suitable for integration in cryogenic focal plane arrays operating at 4 K.