This paper presents three-dimensional (3D) models of high-frequency piezoelectric micromachined ultrasonic transducers (PMUTs) based on the finite element method (FEM). These models are verified with fabricated aluminum nitride (AlN)-based PMUT arrays. The 3D numerical model consists of a sandwiched piezoelectric structure, a silicon passive layer, and a silicon substrate with a cavity. Two types of parameters are simulated with periodic boundary conditions: (1) the resonant frequencies and mode shapes of PMUT, and (2) the electrical impedance and acoustic field of PMUT loaded with air and water. The resonant frequencies and mode shapes of an electrically connected PMUT array are obtained with a laser Doppler vibrometer (LDV). The first resonant frequency difference between 3D FEM simulation and the measurement for a 16-MHz PMUT is reasonably within 6%, which is just one-third of that between the analytical method and the measurement. The electrical impedance of the PMUT array measured in air and water is consistent with the simulation results. The 3D model is suitable for predicting electrical and acoustic performance and, thus, optimizing the structure of high-frequency PMUTs. It also has good potential to analyze the transmission and reception performances of a PMUT array for future compact ultrasonic systems.

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.

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.

Micro-machined ultrasonic transducer (MUT) arrays have their structure and fabrication technology fundamentally different to those of thickness mode piezoelectric transducers. Their performance needs to be modelled and optimized before MEMS fabrication process is engaged. A 3-dimentional finite element modeling (3-D FEM) analysis is established for the simulation of a 2-D 7×7 piezoelectric micromachined ultrasonic transducer (PMUT) array in its cavity and buffer structure. The main characteristics and parameters of the PMUT are calculated in frequency domain, in terms of the electrical impedance, resonant modes patterns, stress distribution, and in particular the cross talk between array elements, The method provides an efficient tool for the design and optimization of PMUT structures. A novel structure with double active layers PMUT is proposed and simulated with the FEM.

Aluminum nitride (AlN) is widely used in SAW/FBAR devices, energy harvesting, biosensors, and ultrasonic transducers. The preparation of high-quality AlN film plays the key role on the process integration with CMOS circuits. Here, we report an AlN thin film prepared by RF reactive sputtering at room temperature on substrate with different treatments, which is more suitable and economical for CMOS-compatible MEMS fabrication compared with previous works. The AlN thin film was deposited on Si/SiO₂/Si₃N₄ with different surface roughness, which were commonly employed as insulating layers or device layers in CMOS circuits. The morphological characterization by AFM and SEM showed that the AlN thin film was uniform and compact with low roughness and fine grain. The bottom Mo electrode was prepared by DC sputtering at room temperature. In addition, AlN seed layer from the same sputtering process increased the adhesion between the electrode and the substrate, thereby improving the stability of subsequent processes. In summary, we prepared uniform AlN with few defects by a simple and controllable approach. This strategy is potentially applied to CMOS-compatible optical and acoustic devices.

For high-resolution ultrasonic imaging, an 18 MHz lead zirconate titanate (PZT) based two-dimensional (2D) piezoelectric micromachined ultrasonic transducers (pMUTs) array with polydimethylsiloxane (PDMS) waveguide are developed. The array is composed of 8 × 16 elements with a pitch of 75 µm and each element can be independently electrically excited. The structure is fabricated by micromachining. The resonance frequency obtained in air is 18.3 MHz and 18 MHz with and without PDMS waveguide filling, respectively. The displacement responses show a sensitivity of 2.6 nm/V and a bandwidth of 14% with PDMS waveguide.