Simultaneous high-Q confinement and selective direct piezoelectric excitation of flexural and extensional lateral vibrations in a silicon phononic crystal slab resonator

The complete phononic band gap (PnBG) of a phononic crystal (PnC) slab is used to efficiently and simultaneously confine both flexural and extensional (longitudinal) elastic vibrations in a resonator. The PnC structure is fabricated by etching a hexagonal (honeycomb) array of air holes in a silicon (Si) slab. Flexural and extensional acoustic modes in the structure are directly excited by a piezoelectric transducer stack fabricated directly on top of the resonant structure to excite the desired modes. Using a frequency sweep analysis, it is shown that the modes of the structure can be selectively excited by modification of the transducer. The fabricated PnC resonator resonates at 120 MHz and 129 MHz, which correspond to a flexural and an extensional mode, respectively. The measured resonance frequencies are in very good compliance with the eigen frequency and frequency sweep finite element analyses. By fitting a Butterworth-Van Dyke model, the electrical characteristics of the resonator in air are extracted. As a result, input impedances of 1200 Ω and 5000 Ω, and quality factors (Qs) of 3600 and 10,000 for the flexural and the extensional modes are obtained, respectively. The Q of 10,000 is, to our knowledge, the largest value reported for any PnC resonator to date.