Acoustic matching strategy of piezoelectric micromachined ultrasound transducer towards medical imaging application

Abstract Piezoelectric micromachined ultrasonic transducers (PMUTs) play a crucial role in the advancement of portable and wearable ultrasound imaging devices. Matching strategy of conventional thickness mode transducer for acoustic propagation is quite well developed yet. However, PMUTs which employ thin film’s flexural mode vibration lack systematic acoustic matching guidelines critical for optimizing performance. This study introduces the first comprehensive design framework for PMUT matching layers, integrating theoretical modeling, finite element method (FEM) analysis and ultrasound characterization as well as phased-array ultrasound imaging validation. Being different from conventional thickness mode devices, the results indicate that a matching layer cannot simultaneously improve PMUT’s sensitivity and bandwidth (BW). Employing the matching layer thickness to be a quarter ultrasound wavelength, materials having an acoustic impedance above 1.5 MRayl enhance sensitivity, whereas those with an impedance below 1.5 MRayl improve BW. For our developed PMUT having center frequency of 6.3MHz, a PDMS with an acoustic impedance of 1 MRayl was selected as matching layer, and it increases the PMUT’s BW from 20% to 39% while resulting in a 41% reduction in sensitivity. The results align closely with the theoretical predictions. To enhance PMUT’s sensitivity, matching materials like polybutadiene and thermoplastic polyurethane are more effective. These guidelines provide a foundation for rapid material selection and dimensional optimization, facilitating the broader application of miniaturized PMUTs in medical imaging and industrial sensing.