A Multi-frequency-based Broadband ScAlN PMUT Array

Our research develops the piezoelectric micromachined ultrasound transducer (PMUT) as a new probe for medical imaging, which solves the limitation of a single operating frequency of traditional ultrasound probes and adjusts the spatial resolution and required depth of penetration through different operating frequencies. Using the biosafe and CMOS-compatible piezoelectric material scandium aluminum nitride (ScAlN), we designed multifrequency ScAlN PMUT arrays. This design can transcend the limitations of traditional ultrasound technology and provide operating frequencies suitable for various diagnostic applications, demonstrating the potential to improve the diagnostic flexibility of medical ultrasound imaging.We developed a multi-frequency PMUT array using the biosafe and CMOS-compatible piezoelectric material ScAlN, which has a higher piezoelectric coefficient compared to materials such as AIN and PZT, which can enhance the overall performance. First, we apply the optimized ensemble total parameter model for theoretical analysis and use finite element modeling to optimize the geometric dimensions and material properties of the designed device. In addition, we fabricated a PMUT array with different cavity sizes through an advanced cavity SOI (CSOI) process, which facilitates switching between multiple operating frequencies. The operating frequency and performance of the PMUT array were verified using an impedance analyzer and digital holographic microscope.We successfully fabricated a ScAlN-based 16 mm × 1.3 mm PMUT array with a total of 192 PMUT elements using CSOI technology. The chip contains 3 linear arrays of 64 elements each with a single center frequency. Impedance analyzer testing shows that the array has selectable operating frequencies of 5.0 MHz, 8.0 MHz and 14.4 MHz, and the corresponding mechanical resonance is also 90nm/V, 68nm/ V,20 nm/V. These three frequencies are suitable for abdominal imaging that requires high ultrasound depth, cardiac imaging that requires ultrasound depth and resolution, and high-resolution vascular imaging. Additionally, because these center frequencies approximate the fundamental, second, and third harmonic frequencies, they can be used for harmonic or subharmonic imaging. Our designed ScAlN PMUT array that can switch multiple operating frequencies is expected to be customized to meet various ultrasound imaging requirements, thereby increasing the flexibility of clinical medical diagnosis.