High-SPL Air-Coupled Piezoelectric Micromachined Ultrasonic Transducers Based on 36% ScAlN Thin-Film

In this paper, air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) using 36% scandiumdoped aluminum nitride (ScAlN) thin-film are presented. ScAlN is known to exhibit higher piezoelectric properties compared to pure AlN leading to significant performance improvements in various piezoelectric micro-electromechanical systems (MEMS) applications including PMUTs. Here, the concentration of Sc in the actual sputtered 1-μm-thick ScAlN film was 36%, which is slightly below the maximum at the phase boundary. The ScAlN PMUTs were fabricated from an SOI wafer, where the dry etching of ScAlN film was optimized. The frequency response and displacement sensitivity of the PMUTs were characterized in the air using laser Doppler vibrometry confirming 2× higher transverse piezoelectric coefficient than AlN. The acoustic transmission and reception of the PMUTs were evaluated from a high-sensitivity microphone and pulse-echo measurements. The PMUTs were designed to operate below 100 kHz in order to mitigate the absorption loss, which resulted in a high transmit pressure of 105-dB sound pressure level (SPL) at 10 cm and only 30-dB attenuation at the 2-m range. Through implementing 36% ScAlN film, the presented PMUTs exhibited a large displacement and consequently, a high SPL compared to the state-of-the-art PMUTs and the bulk transducer considering the size and the excitation voltage.