Development of Through-Substrate via Process for Silicon-Based Monolithic Microwave Integrated Circuits SIS Mixer

We designed and fabricated superconducting monolithic microwave integrated circuits (MMICs) at 2 mm wavelengths. Balanced superconductor-insulator-superconductor (SIS) mixers and silicon membrane-based planar orthomode transducers are incorporated into the MMICs, which is used to develop compact heterodyne array receivers for radio astronomical observations. Because of substrate cavity modes, crosstalk between the outputs of balanced mixers was observed at intermediate frequencies, which deteriorated the noise rejection ratio. Three-dimensional electromagnetic simulation proved that the cavity modes can be effectively suppressed by introducing conducting through-substrate vias (TSVs). We have been developing a TSV fabrication process compatible with that for the superconducting MMICs containing silicon membranes by using non-Bosch deep silicon etching and using sputtered Nb films. The fabricated TSVs were characterized by scanning electron microscopy and R - T measurement.