Fabrication of surface-micromachined circular piezoelectric micromachined ultrasonic transducers with various etching holes using XeF₂ and simulation of their vibrational characteristics

Piezoelectric micromachined ultrasonic transducers (pMUTs) are of great interest for numerous applications, including fingerprint imaging, precise acoustic sensing, and ultrasonic medical imaging, due to their properties of high design freedom and low power consumption when miniaturized for a transducer array. However, micromachined transducers tend to have relatively low acoustic intensity compared with conventional ceramic-based transducers. Therefore, it is preferable and recommended to achieve high acoustic intensity and enhanced displacement of the pMUTs. This paper presents circular pMUTs with various clamped membranes and explores the displacement and mode shape of membranes at resonance frequencies that are critical for enhanced acoustic intensity. Each circular clamped membrane had four etched holes at the edge of the membrane to control the boundary condition, which affected the resonance frequencies and membrane displacement by controlling the effective stiffness. The displacement and effective area as a function of etching hole size were simulated using COMSOL software and verified experimentally. The maximum displacement of a membrane 60 µm in diameter exhibited a 25% improvement when the etching hole angle was 20° compared with the fully clamped membrane. The effective area for acoustic intensity increased from 54.2% to 61.7% at 60° when the mode shape changed from circular to square. The experimental and simulation results were in reasonable agreement, and the results helped clarify how the geometric design of the suspended membrane of the pMUT affects its vibrational characteristics.