Broadband and High-Pressure Output PMUT Array Based on Lead-Free KNN Thin Film

High-performance lead-free piezoelectric ultrasound transducers have been highly desirable for applications in implantable, bio-compatible medical devices and environment-friendly consumer electronics. In this paper, we demonstrated design, fabrication, and characterization of lead-free (K, Na)NbO
3
(KNN) based piezoelectric micromechanical ultrasonic transducer (PMUT) arrays with center frequency 4.7 MHz in liquid and 5.85 MHz in air. High-quality actuator type of KNN thin film (FWHM of 0.32°, e
31
,
f
= 12 C/m
²
, ϵ
r
= 1200) was deposited via physical vapor deposition (PVD) and patterned using an optimized wet etching process with an oxide layer as a mask. Moreover, a -6dB fractional bandwidth of 95.7% is achieved via the optimization of layer stacks and acoustic impedance. A high transmitting sensitivity of 3.8 kPa/V @ 3 cm from one PMUT super-pixel (area 0.278 mm
²
, 3×12 PMUTs) was obtained, which is comparable to previous PZT (lead-included) film based PMUTs.