36% Scandium-Doped Aluminum Nitride Piezoelectric Micromachined Ultrasonic Transducers

Piezoelectric micromachined ultrasonic transducers (PMUTs) using 36% Scandium-doped aluminum nitride (ScAlN) thin-film are presented in this work. ScAlN is known to exhibit higher piezoelectric properties compared to pure AlN leading to significant improvements in various MEMS applications including PMUTs. Here, the concentration of Sc in the actual sputtered 1 um thick ScAlN film was 36% that is slightly below the phase boundary. The piezoelectric coefficient is close to maximum at this high concentration leading to significant improvement in the PMUT performance. The ScAlN film quality was verified via X-Ray Diffraction (XRD), electron probe microanalysis (EPMA), and d33 meter. The operation frequency was designed to around 80 kHz to achieve long range detection, where the air absorption loss is low. The displacement sensitivity of 1 mm diameter ScAlN PMUT was 806 nm/V at 84 kHz resonant frequency. A large displacement of 4.8 um was achieved at 20 Vpp input at the fixed frequency of 84 kHz. This represents a factor of 9 increase in transmit amplitude compared to prior art air-coupled AlN PMUTs.