Modeling and characterization of a silicon-epoxy 2-2 composite material

This paper presents modeling and characterization of a 2-2 silicon-epoxy composite used as matching layer for high frequency transducers. The composite was fabricated using Deep Reactive Ion Etching (DRIE), common in the MEMS industry, to form deep trenches into a silicon wafer, and fill them with epoxy resin. This composite was used as acoustic matching layer in an air-backed 15 MHz transducer and characterized by electrical impedance measurements in air. The effective acoustic properties of the composite, i.e., speed of sound, acoustic impedance and mechanical loss tangent, were deduced from the measured electrical impedances. The estimated parameters were compared with results from analytical and FEM models. The models show that the first lateral resonance in the silicon-epoxy 2-2 composite is primarily defined by the composite period, not by the epoxy kerf, and no switching between the two lowest modes is seen near the “interaction zone” in the dispersion curves, where the two lowest branches are close to each other. Higher loss was also observed in coarser composite structures, probably due to dispersion. The simulation results were verified by pulse-echo measurements on two transducers with the composite matching layer period 20 μm and 40 μm, which are just below and just above the interaction zone. The measurements show good agreement with the theoretical calculations.