High Voltage Excitation and Nonlinear Transmission of a 16 MHz AlN-Based Piezoelectric Micro-Machined Ultrasonic Transducer

This paper reports the performance of a 16 MHz AlN-based piezoelectric micro-machined ultrasonic transducer (PMUT) under high voltage excitation and determines its limit of power transmission capability. The proposed PMUT array consists of 44×39 elements fabricated on a silicon-on-insulator (SOI) substrate with 1 μm AlN thin film on 5 μm silicon membrane. With all the array elements electrically connected together, the electrical impedance of the PMUT was 11.6-j30 Ohm at 15.8 MHz resonant frequency in air and 4.72-j25 Ohm at 13.9 MHz when the device was water charged. The LDV (Laser Doppler Vibrometer) measurements showed that the transmission efficiency and the bandwidth for free vibration were 1.7 nm/V and 2.1%, respectively. The resonance modes were specifically associated with the substrate and the cavity structure but they are 10~15 dB less in amplitude than the fundamental resonance of the array. Nonlinear behavior was observed when increasing the burst amplitude and the PMUT array broke down when the input mean power applied on the single element exceeded 5.2 mW. This strategy is potentially applied to CMOS-compatible high-resolution ultrasonography.