High-Frequency and Broadband PZT-Based PMUT Array with Negative Poling for Enhanced Coupling Efficiency

High-frequency piezoelectric micromachined ultrasonic transducers (PMUTs) face inherent trade-offs between bandwidth and sensitivity, limiting their resolution in medical imaging and industrial sensing applications. To address this challenge, this study presents a multi-frequency PMUT array design integrated with negative poling treatment. The array combines resonators of varying radii <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(18-22 \ \mu \mathrm{m})$</tex> within a single channel <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(3\times 40$</tex> elements), which utilizes frequency superposition to broaden the operational bandwidth. Experimental results demonstrate a center frequency of 18.1 MHz and a −6 dB bandwidth of 15.5 MHz in Fluorinert FC-3283, which triples the bandwidth of conventional single-frequency designs (4.9 MHz) while retaining 84% sensitivity. Negative poling treatment (-10 V DC for 5 minutes) further enhances the acoustic pressure output to 6.25 kPa/V by optimizing the domain alignment of PZT films. Experimentally verified, with a pulse duration of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.26 \ \mu \mathrm{s}$</tex>, the resulting axial resolution improves to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$147 \ \mu \mathrm{m}$</tex> (from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$206 \ \mu \mathrm{m}$</tex> in single-frequency arrays), confirming the potential for high-resolution imaging and microstructural defect detection. This work provides a scalable framework for balancing bandwidth and efficiency in next-generation ultrasonic transducers.