Rational design of CMUTs with annular electrodes for high ultrasonic emission via ESSE enabled stiffness adjustment

Capacitive micromechanical ultrasonic transducers (CMUTs) with high transmitting acoustic pressure are in urgent demand in the rapidly growing field of air-coupled and therapeutic ultras ound. However, most current CMUTs can rarely balance the performance improvement and batch fabrication capacity, which severely impedes their practical applications. This paper proposes novel CMUTs with annular electrodes that can implement significant improvement in multiple performances while featuring a simple structure and batch fabrication feasibility. The annular electrode configurated between the membrane edge and center areas can effectively soften the corresponding-area membrane stiffness through electrostatic spring softening effects, and leave the stiffness of the membrane central area unchanged, finally enabling the membrane to produce a piston-like deformation, thus improving average displacement and output acoustic pressure. A finite element method was employed to analyze the effect of annular electrodes on the CMUT main performance. The results demonstrated that the novel structure could achieve prominent enhancement in multiple performances, such as an average to maximum displacement rate of 0.46 (about 0.32 for conventional CMUTs), maximum improvements of 300%, 255%, and 11% in average displacement, acoustic pressure, and electromechanical coupling coefficients compared to those of conventional ones. Deep analyses of the variation of the main performances including acoustic pressure, receiving sensitivity, and collapse voltage suggested that an optimal electrode coverage range of 36% ∼ 55% can be used to achieve relatively high comprehensive performances. Meanwhile, the proposed CMUTs feature a simpler structure and fabrication process in comparison with previous ones, showing great promise in air-coupled and therapeutic ultrasound applications.