Sealed Silicon Cavities (SSC) Equips Piezoelectric Micromachined Ultrasonic Transducer Arrays with an Ultra-High Fill Factor of 95.5%

This work presents, for the first time, a 95.5% ultra-high fill factor sealed silicon cavity- piezoelectric micromachined ultrasonic transducer (SSC-PMUT) array. A 31×26 array is designed, fabricated, and characterized. By leveraging silicon migration technology optimized for PMUT arrays, the element spacing has been reduced to 1.4 μm. The fabricated array has an electromechanical coupling coefficient of 2.4% at 5.8 MHz and an air-coupled displacement sensitivity of 33.2 nm/V. The 31×26 array exhibits a transmission sensitivity of 4.1 kPa/V at 6.25 mm, featuring a symmetrical radiation pattern and a main lobe width of 19°. Directivity measurements show that the ultra-high fill factor enables the SSC-PMUT array to have a wider main lobe width and lower side lobe levels with a smaller array size. Experimental results demonstrate that the 95.5% fill factor SSC-PMUT array could be a promising platform for wide-angle and high-resolution ultrasound sensing/imaging applications.