New Piezoelectric Thin Film Resonator Promising for Use in Very High Frequency Communication Systems

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The purpose of this study is to investigate a new piezoelectric thin film resonator using acoustic λ/4 multilayers, which is promising for use in a very high frequency range from 1 to 20GHz and can be incorporated within integrated circuits. The main results of this study are summarized below.1. The λ/2 mode configuration of piezoelectric thin film resonator using acoustic λ/4 multilayers, where the piezoelectric film is λ/2 thick, was analyzed in consideration of propagation loss, and then electrical quality factor Q and effective electromechanical coupling factor K were calculated as functions of the acoustic impedance ratio of multilayers and the layer number.2. The quality of c-axis oriented piezoelectric ZnO films grown by ECR-assisted MBE were evaluated with RHEED and X-ray diffraction. It has been confirmed that high quality ZnO single crystal films can be grown.3. A new simple evaluation method for acoustic properties, such as density and stiffness, of thin films was proposed. In this method, the acoustic properties are evaluated from the measured resonance frequency change of high overtones of a piezoelectric thickness-mode resonator due to deposition of a test film on the resonator surface.4. Dispersion characteristics of elastic waves which propagate along the surface being trapped in a piezoelectric film were analyzed. It has been shown that a vibration trapped in the finite electroded region can be occurred.5. A 3-GHz piezoelectric ZnO film resonator with five λ/4 acoustic multilayers was fabricated on a Z-cut LiTaO_3 substrate and this type of resonators were shown to be promising for use in a very high frequency range from 1 to 20GHz.