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Surface acoustic wave (SAW) resonators are key components of mobile communication systems, and the development of wideband SAW resonators is especially required because next-generation mobile communication systems require high-speed data transmission using wide frequency bands. In this study, two layered structures—(i) a c-axis-tilted ScAlN bilayer film/silicon substrate and (ii) a c-axis-zigzag ScAlN film/silicon substrate—are proposed for highly coupled SAW resonators, and the electromechanical coupling coefficient K2 of the non-leaky Rayleigh-mode SAW propagating in these structures is investigated theoretically. With a proper c-axis tilt angle and thickness selection of the ScAlN film, the first-mode Rayleigh SAW in the c-axis-tilted ScAlN bilayer film/silicon substrate and c-axis-zigzag ScAlN film/silicon substrate offers high K2. The maximum K2 was 9.56%, obtained in the c-axis-zigzag ScAlN film/silicon substrate structure, which was 2.5 times the maximum K2 in the c-axis-tilted ScAlN monolayer film/silicon substrate structure; this is because the c-axis-zigzag ScAlN films increase the shear vertical component of the SAW particle displacement, most of which is concentrated in the c-axis-zigzag ScAlN film on the silicon substrate. It is also important that the second-mode SAW (Sezawa wave) in the c-axis-zigzag ScAlN film/silicon substrate structure is not excited when the K2 value of the first-mode SAW is maximized. The techniques for fabricating a c-axis-tilted ScAlN film and a c-axis-zigzag ScAlN film have been reported, and well-established micromachining technology related to silicon semiconductors offers significant advantages in the manufacturing and processing of SAW resonators. Therefore, the c-axis-zigzag ScAlN film/silicon substrate structure has significant potential for SAW device applications. [ABSTRACT FROM AUTHOR]

Spontaneous polarization of tetragonal ferroelectrics cannot be inverted unless the applied electric field is greater than the coercive field. In the case of the cubic phase, on the other hand, polarization and piezoelectricity can be induced merely by applying an electric field. In this study, we proposed polarity-inverted cubic/tetragonal multilayer film resonators which allow switching between the fundamental and high-order mode resonances through the independent control of the polarization of the cubic layer. Frequency switching in bulk acoustic wave (BAW) resonators based on all-epitaxial cubic 0.95Pb(Mg1/3Nb2/3)O3 (PMN)-0.05PbTiO3 (PTO)/tetragonal Pb(Zr,Ti)O3 (PZT) bilayer film stacks is demonstrated theoretically and experimentally. Under a negative voltage application, which is less than the coercive field of the tetragonal PZT layers, a fundamental mode resonance (327 MHz) is observed, whereas a second-mode resonance (779 MHz) is observed under a positive voltage application in BAW resonators. A theoretical simulation based on Mason's equivalent circuit model, taking account of the polarity-inverted bilayer structure, shows good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

A fingerprint reader based on epitaxial PbTiO3 (PTO) transducer array operating thickness extensional mode in GHz range was fabricated. The device consisted of nine square transducers with 0.1 mm per side arranged 1.3 mm apart in a 3 × 3 array. Minimum conversion loss of the fabricated transducer was 2.5 dB at 0.8 GHz, and electromechanical coupling coefficient kt2 was estimated to be 28.9%. In contrast to MHz range ultrasonic fingerprint readers such as those based on piezoelectric micromachined ultrasonic transducers using a piezoelectric transducer and ScAlN, a GHz range transducer enables imaging in higher spatial resolution. Furthermore, PTO transducers have a high dielectric constant and electromechanical coupling coefficient kt2. A high dielectric constant realizes 50 Ω impedance matching with small electrode area. The small acoustic source is expected to improve the spatial resolution of a fingerprint reader. We achieved the fingerprint imaging by evaluating the acoustic reflectance of the medium/transducer interface. Furthermore, piezostage that allows mechanical movement in the 100 nm order was introduced to image a 12 × 12 μm2 area with total of 3600 data points. [ABSTRACT FROM AUTHOR]