The demand for high-performance lead-free piezoelectric ultrasound transducers has grown significantly, driven by their applications in implantable, biocompatible medical devices and environmentally friendly consumer electronics. In this study, we present the design, fabrication, and characterization of arrays of lead-free (K, Na)NbO

$_{3}$

(KNN)-based piezoelectric micromechanical ultrasonic transducers (PMUTs) with a center frequency of 4.7 MHz in liquid and 5.85 MHz in air. High-quality KNN thin film (FWHM of 0.32

$^{\circ}$

$e_{\mathrm{31,}f}=$

$-$

12 C/m

$^{2}$

$\epsilon _{r}$

$=$

1200) was deposited via physical vapor deposition (PVD) and patterned using an optimized wet etching process with an oxide layer as a mask. Additionally, we obtained a

$-$

6 dB fractional bandwidth of 95.7% through optimizing layer stacks and transducers mutual acoustic impedance based on finite element model (FEM) and lumped element model (LEM) methods. We achieved high transmitting performance of 3.8 kPa/V at 3 cm away from a PMUT super-pixel (with an area of 0.278 mm

$^{2}$

, consisting of

$3\times 12$

PMUTs). The measured transducer performance is comparable to previous PMUTs based on PZT (lead-included) thin films and demonstrates the potential of KNN-based PMUTs in future advanced applications. 2024-0005

Chong Yang

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