Wideband and Highly Sensitive Micromachined PZT Film-Based Ultrasonic Microphone with Parylene Film and Flexible Helmholtz Resonator Enhancement

The innovative piezoelectric micromachined ultrasonic microphone fabricated by a bottom-up scheme with wideband operation frequency and sensitivity higher than a commercial MEMS microphone is presented for the first time. The device is constructed of a freestanding triangular cantilever beam array fabricated by titanium foil and quality hydrothermal PZT film of residual stress less than 60 MPa. A unique method is applied to deposit parylene film, fully covering the surface of triangular cantilever beam array and sealing the gaps between beams for enhancing the performance of the ultrasonic microphone. The deposited 0.5 μm-thick parylene film increases 3 dB sensitivity compared to that without parylene film sealing. The sensitivity further increases after the fabricated microphone integrated with a parylene flexible Helmholtz resonator cavity.