Development of High-Performance Piezoelectric Micromachined Transducers for Near Ultrasound
Near-ultrasound refers to sound with frequencies just above the range of human hearing, from about 18 to 40 kHz. This band is rarely used for typical ultrasound applications and is ignored for all except the most demanding audio applications. We highlight the advantages of using this band and present a design study on the development of high-efficiency, resonant transducers for near-ultrasound. Piezoelectric Micromachined Ultrasound Transducers, or PMUTs, are MEMS resonators that are used to generate and receive ultrasound and acoustic waves. They are fabricated as multilayered diaphragms consisting of a passive structural layer coated with a piezoelectric material sandwiched between metal films. In this dissertation, we report the realization of a novel near-ultrasound PMUT system especially designed for Data-over-Sound (DoS) applications. This realization includes investigation of new transducer designs, innovation in fabrication processes, and a significant advance in acoustics and electronics integration. We use analytical and coupled finite element models of clamped circular plates with in-plane stresses to generate design maps for PMUTs. Residual tensile stresses generated during fabrication processes have the effect of stiffening the diaphragms and increasing their resonant frequencies. We experimentally estimate the magnitude of these stresses in sol-gel PZT-coated SOI wafers and fabricate transducers with dimensions optimized for near-ultrasound. The transducers are 50 times smaller and 20 times more efficient than conventional electrodynamic micro speakers in the near-ultrasound range. We then present a novel design for PMUTs with “bossed” diaphragms that allows further reduction in device footprint and power consumption while improving sensitivity and efficiency. The dimensions of the central boss structure are optimized using simulations. The fabricated devices are found to be up to 10 times smaller than conventional PMUTs for the same frequencies, and less sensitive to variations in residual stress. We have studied and optimized the effects of packaging and the acoustic environment on the performance of the transducers using finite element and boundary element acoustic simulations. The devices are packaged with 3D-printed acoustic resonators and horns designed to boost sensitivity, improve bandwidth, and widen the directivity of the transducers. The results of the simulations are experimentally verified by scanning the acoustic field of the transducers. The transducers are finally integrated into battery- and solar-powered DoS beacons and wireless sensor nodes, complete with a low-power microcontroller for modulation/demodulation, a low Q-current amplifier, a MEMS microphone, an acoustic resonator, and the near-ultrasound transducer — all in a compact package with a transmission range of up to 30 meters and a battery reserve of up to 4 weeks.