Piezoelectric micromachined ultrasonic transducers with rectangular diaphragms for dual-frequency applications

Piezoelectric micromachined ultrasonic transducer (pMUT) consisting of a piezoelectric capacitor built on a micromachined silicon membrane has been used for ultrasound transmitting and sensing. It typically operates at the fundamental flexural vibration frequency, where maximum sensitivity can be achieved, and is insensitive at higher order of resonances because the harmonic signals generated from different parts of the diaphragm tend to cancel with each other. This leads to a very narrow bandwidth. In this paper, rectangular-shaped pMUTs for dual-frequency application are proposed and fabricated using piezoelectric P(VDF-TrFE) copolymer coating and silicon micromachining technologies. The electrode patterns are properly designed to efficiently make use of both (0, 0) and (2, 0) vibration modes of the rectangular membrane. By adjusting the length and width of the diaphragm, the ratio of the two frequencies can be varied on demand within a wide range. The corresponding design principles are also useful for other micromachined devices such as surface acoustic wave devices, delay lines, resonators, etc.