Bistable Bimorph PMUT with Large Switching Displacement and Acoustic Output

This paper reports, for the first time, bistability in a bimorph Piezoelectric Micromachined Ultrasonic Transducer (PMUT) composed of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Sc}_{0.3} \text{Al}_{0.7}\ \mathrm{N}$</tex> layers, with a total device thickness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.65 \mu\ \mathrm{m}$</tex>. The device exhibits large switching displacement exceeding <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$11 \mu\ \mathrm{m}$</tex> and peak-to-peak velocity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$16\ \mathrm{m} / \mathrm{s}$</tex> under bipolar square-wave excitation. The switching process excites a higher-order <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(0,3)$</tex>-like mode that dominates the dynamic response. Sound pressure level (SPL) of 74 dB at 88 kHz was measured at 5 cm. Across the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20-100 \text{kHz}$</tex> input frequency range, the total SPL is maintained above 75 dB. The normalized SPL when compared with state-of-the-art PMUT results, demonstrates strong acoustic transmission capability of thin bistable bimorph PMUTs.