Electrode effects on frequency spectra and electromechanical coupling factors of HBAR

In this paper, effects of electrodes on the spectra of high-overtone bulk acoustic resonator are analyzed phenomenologically by numerical simulations. Acoustic impedance ratio of the substrate to piezo-film, Z/sub ab//Z/sub 0/, and acoustic impedance ratio of the electrode to piezo-film, Z/sub e//Z/sub 0/, are taken as the category parameters for different combinations of three kinds of substrates, three kinds of piezo-films, arid seven kinds of electrodes. The numerical simulations for all of the combinations are carried out. The results indicate that the acoustic impedance ratio of the substrate to piezo-film is a key factor for the variations of the space of parallel resonance frequency and the effective coupling factor versus the mode order. The mechanical effects of the electrodes have significant contribution as well. Several approximate formulae are given to evaluate the effective coupling factor for different combinations.