Beam and Cantilever Supported PMUTs for Enhanced Transmit Sensitivity and Linearity

This paper introduces curved-beam (CB) and curved-cantilever (CC) piezoelectric micromachined ultrasonic transducers (PMUTs) using 30% scandium-doped aluminum nitride (ScAlN), comparing their performance with conventional (C-PMUT) in the 45–75 kHz range. The CC-PMUT and CB-PMUT designs incorporate 2 μm edge slits to achieve piston-mode vibration, significantly enhancing the effective vibration area to 65% and 60%, respectively, compared to 33% for C-PMUT. This structural design improves linearity, with CC-PMUT demonstrating minimal frequency sensitivity to DC bias (±40 V) at 2.5 Hz/V, and CB-PMUT (7.5 Hz/V) both outperforming C-PMUT (100 Hz/V). Under increasing AC voltage excitation, CC-PMUT exhibit exceptional linearity with minimal resonance variation, while C-PMUTs and CB-PMUTs show nonlinear behavior with increasing frequency and stiffening effects. C-PMUT display the highest non-linearity, with a duffing coefficient 2.5 times larger than CB-PMUT. The CB-PMUT exhibits the highest displacement sensitivity (1025 nm/V) compared to C-PMUT (825 nm/V) and CC-PMUT (610 nm/V). Despite its four-fold smaller area, the CB-PMUT generates comparable absolute pressure to the C-PMUT. Its exceptional normalized transmission pressure sensitivity of 77 dB SPL/V/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> surpasses both C-PMUT (65 dB) and CC-PMUT (67 dB), making it the highest reported among PZT and ScAlN counterparts. C-PMUT demonstrate the highest receive sensitivity at 8.7 mV/Pa, followed by CB-PMUT at 4.2 mV/Pa and CC-PMUT at 3.3 mV/Pa. In pitch-catch experiments, CB-PMUT and C-PMUT detect signals up to 3.8 m and 4 m, respectively, while CC-PMUT reach 2.7 m (10 V<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub>, 12 dB SNR threshold), demonstrating their potential for ultrasonic range-finding.