RESUMEN
A new model in finite element method to study round-trip performance of piezoelectric micromachined ultrasonic transducers (pMUTs) is established. Most studies on the performance of pMUT are based only on the transmission sensibility, but the reception capacity is as much important as the transmission one, and is quite different from this latter. In this work, the round-trip sensitivity of pMUT is defined as the product of the frequency response of transmitted far field pressure to source voltage excitation and that of reception output to return wave pressure. Based on this sensitivity characteristic, firstly, a multi-parameter optimization for a cavity pMUT is performed using the sensitivity-bandwidth product parameter SBW as criterion. The radii of the electrode and the piezoelectric layer, the thicknesses of the piezoelectric layer and the vibration diaphragm are adjusted to maximize the performance. Secondly, an acoustic matching method is proposed and applied to pMUTs for the first time. As a result, the round-trip sensitivity can be evaluated and the pulse-echo response of wide-band excitation can be simulated, giving the most quantitative and intuitive feedback for pMUT design. The optimization enhances the sensitivity-bandwidth product by 52% when the top electrode and piezoelectric layer are both etched to 75% radius of the cavity beneath; the introduction of an acoustic matching layer shows significant bandwidth expansion in both the transmitting and receiving process.
RESUMEN
Piezoelectric micromachined ultrasonic transducers (PMUT) are promising elements to fabricate a two-dimensional (2D) array with a pitch small enough (approximately half wavelength) to form and receive arbitrary acoustic beams for medical imaging. However, PMUT arrays have so far failed to combine the wide, high-frequency bandwidth needed to achieve a high axial resolution. In this paper, a polydimethylsiloxane (PDMS) backing structure is introduced into the PMUTs to improve the device bandwidth while keeping a sub-wavelength (λ) pitch. We implement this backing on a 16 × 8 array with 75 µm pitch (3λ/4) with a 15 MHz working frequency. Adding the backing nearly doubles the bandwidth to 92% (-6 dB) and has little influence on the impulse response sensitivity. By widening the transducer bandwidth, this backing may enable using PMUT ultrasonic arrays for high-resolution 3D imaging.
