Multi-layer polymer-metal structures for acoustic impedance matching in high-frequency broadband ultrasonic transducers design

Although high-frequency (≥ 100 MHz) ultrasound has demonstrated its capability in a variety of applications, the fabrication of high frequency ultrasonic transducers with both high sensitivity and broad bandwidth remains challenging. One main reason is the mismatch of acoustic impedance between piezoelectric materials and the loading medium. Due to reliance on both specific acoustic impedance of matching materials and precise thickness control, the conventional quarter wavelength (1/4 λ) matching layer design is impractical for high-frequency transducers. Based on the transmission line theory and Mason model, we interfaced polymer-metal-polymer matching layers for transducers over 100 MHz. The modeling result comparison between transducers without matching layer and those with two conventional matching layers demonstrated that the matching performance of polymer-metal-polymer matching layers could be as good as the one of conventional matching layers. Meanwhile, unlike conventional 1/4 λ matching layer design, our design is independent on materials with specific acoustic impedance, while precise thickness control of polymer and metal can be achieved by deposition. The polymer-metal-polymer matching layers scheme paves the way to high frequency ultrasonic transducers.