Piezoelectric Micromachined Ultrasonic Transducer (PMUT) Based on Bilayer X-Cut Lithium Niobate

This work demonstrates, for the first time, PMUTs based on bilayer X-cut lithium niobate (LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>). Different figures of merit (FoMs) for PMUT materials were first analyzed, highlighting LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> as a promising and well-balanced alternative. To leverage its superior material properties, PMUTs were designed based on bilayer X-cut LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> to fully harnesses the in-plane stress associated with the bending of the structure, thereby enhancing transduction efficiency. The fabricated devices show high electromechanical coupling <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\ k_{t}^{2})$</tex> of 4.6%, albeit significantly lower than the simulated value due to parasitic effects. A comparison with PMUTs based on other materials is provided. Our devices show comparable performance to PZT-based PMUTs, and significant improvements with respect to ScAlN-based PMUTs in terms of static displacement sensitivity, by a factor of 5, and radiated acoustic power, by a factor of 7. These results showcase the strong potential of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LiNbO}_{3}$</tex> for high-performance PMUTs.