Bottom-up micromachined PZT film-based ultrasonic microphone with compressible parylene tube

Abstract

This paper reports on a micromachined ultrasonic microphone using a bottom-up fabrication scheme. Starting with a 4μm-thick titanium foil as the substrate, each functional film and key element was added to the foil substrate to complete the ultrasonic microphone. The piezoelectric PZT film hydrothermally grown on the patterned substrate with low residual stress effectively deflected the unimorph-sensing cantilever array of the microphone under ultrasound pressure. The created cantilever array structure secured on a 250μm-thick SU8 hollow plate formed an ultrasonic microphone plate that was tested with a sensitivity of −60 dBV/Pa at 21 kHz (with 0 dB gain amplification) and an operation bandwidth of 5 to 55 kHz. Different thicknesses of parylene films ranging from 0.5 to 2 μm overlaid over the entire sensing region and converted the cantilever-to-diaphragm-structured microphone for further investigation. An enhanced result was observed when the deposited parylene film thickness was in the submicron range. The sensitivity of the microphone can be further enhanced by up to 33% by adding a parylene-film-made compressible tube to act as a Helmholtz resonator. The Helmholtz resonator model was discussed and compared with the experimental results. The output amplitude of the developed microphone assembled with the compressible tube demonstrates a 15 dB increase compared to that of a commercial capacitive MEMS ultrasonic microphone.