Piezoelectric Micromachined Ultrasonic Transducers (pMUT) with Free Boundary

This work presents theoretical analysis and finite element simulations of a novel piezoelectric micromachined ultrasonic transducer (pMUT) design. A pMUT with free boundary condition is introduced for the first time and compared with conventional pMUTs of fixed boundary. For the same transducer dimensions, the proposed design has more than 2.5x electromechanical coupling coefficient compared to that from traditional clamped boundary design due to increased mechanical capacitance by the free edge. The cavity underneath the disc behaves as an acoustic resonator. By matching the frequencies of mechanical and acoustical resonances, the fractional bandwidth of the free boundary pMUT has 13x more fractional bandwidth compared to that of the fixed boundary design. In addition, the internal stresses created during microfabrication are expected to be released with the free edges. These results indicate that free pMUTs have the potential for high performance ultrasonic transducer applications.