Enhancement of the Transmission of Piezoelectric Micromachined Ultrasonic Transducer With an Isolation Trench

A new piezoelectric micromachined ultrasonic transducer (pMUT) with an isolation trench between cells was proposed to improve the output pressure. A 2-D finite-element model was utilized to evaluate and compare the performance of the conventional design with fully clamped boundary and the trench design. It shows that the trench design can improve the membrane displacement or output pressure of pMUTs without a significant change in the resonant frequency; 8 × 8 aluminum nitride (AlN)-based pMUTs arrays with fully clamped boundary design and the isolation trench were fabricated and characterized. An impulse response of the pMUTs array was first employed to determine the resonant frequency. A 200-cycle burst at the resonant frequency was then delivered to pMUTs and the acoustic output pressure was measured by a hydrophone. The trench design could increase the output pressure by ~76% with a shift of its center frequency by only 0.03 MHz. The nonlinear relationship between pressure output and applied high voltage still exists in the trench design. The presence of residual stress in the membrane, and substrate during fabrication was found to have little impact on the displacement and resonant frequency of pMUT. In summary, the presence of isolation trench can reduce the deflection-induced tensile stress on the edge of the membrane and subsequently improve the performance of pMUTs.