Nano-Scale AlN-based piezoelectric micromachined ultrasonic transducer for biomedical applications

Abstract

Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) have shown great potential in biomedical applications, yet conventional micro-scale PMUTs face challenges due to thick diaphragms and large membrane dimensions, which limit electromechanical coupling and reduce acoustic output. Also, full-area electrodes often excite unwanted higher-order vibration modes, while small electrodes limit the effective coupling. Thus, this research offers a solution to these problems by proposing a Nano-Scale Circular Double-Lamination AlN-based PMUT design. The proposed design reduces the diaphragm radius to 500 nm, improving strain transfer and electromechanical coupling. Additionally, this design adopts a partial electrode configuration of 156.25 nm with a top central electrode to enhance the electric field, while eliminating higher-order vibration modes and maximizing displacement at the diaphragm. Moreover, PZT's complex perovskite crystal structure and polarization mechanisms cause a high dielectric constant and dielectric losses, which restrict its use in implantable devices. Thus, this design uses lead-free Aluminum Nitride (AlN) with a wurtzite crystal structure, which provides low dielectric loss and ensures efficient energy conversion even at high frequencies. COMSOL Multiphysics simulations demonstrate a resonant frequency of 18.883 MHz, a broad bandwidth of 10 MHz, an energy density of 1.8 J/m³, and high central displacement, making the device ideal for high-frequency superficial clinical imaging and wearable biomedical applications.