11F-3 Performance of Flexure-Mode pMUT 2D Arrays

We report on the performance characteristics of 2D pMUT arrays. 2D arrays suitable for diagnostic ultrasound imaging containing 81 elements were bulk micromachined in silicon substrates. The devices consisted of suspended PZT thin film membranes formed by deep reactive ion etching. Membrane widths ranged from 50 to 200 mum, with element pitch from 100 to 300 mum. Center frequency, fractional bandwidth, acoustic pressure output and pulse-echo insertion loss were measured in a de-ionized water tank. Pulse-echo images were made using the Duke University T5 Phased Array Scanner. Measured resonance frequencies for the flexure-mode pMUT elements in transmit mode were in the range of 4-13 MHz, and different vibrational modes were observed. Larger membrane widths tended to operate with standard plate-mode vibration, whereas smaller membranes were relatively thickness independent and operated in a transverse resonant mode proportional to membrane length. Acoustic pressure output was 322 Pa/V for a 75 mum pMUT single element measured at 8.6 MHz by a calibrated hydrophone at 22 mm range. This compared to 382 Pa/V for a commercial 2D bulk ceramic array with 350 mum elements operating at 3.5 MHz at the same distance. Pulse-echo insertion loss measured by reflection from an aluminum block at 22 mm range in the water tank was -106 to -124 dB per element for 75 mum elements. It was observed that the pMUTs were more efficient in transmit than receive mode, as transmit insertion loss was -38 dB per element. Receive sensitivity was increased by applying a dc bias to the pMUT elements prior to receiving acoustic reflection. Because flexure-mode pMUTs operate above the coercive voltage in transmit mode, receive bias increased the polarization of the piezoelectric film prior to transducer receive. Receive sensitivity increased by as much as 18 dB with up to 20 V dc bias. Pulse- echo B-mode images were produced with 9x9 element (1.1 mm x 1.1 mm) pMUT arrays operating at 7 MHz, producing axial resolution of 1 mm at 10 mm depth in a tissue phantom.