Piezoelectric thin-film resonator and filter using the same

摘要

A piezoelectric thin-film resonator includes: a substrate (10); a lower electrode (20) that is formed on the substrate and made of a material having a Poisson's ratio of 1/3 or lower; a piezoelectric film (30) that is formed on the lower electrode (20) and the substrate (10); and an upper electrode that is formed on the piezoelectric film, so that the piezoelectric film is partially sandwiched between the upper and lower electrodes in a resonator portion (60) where the lower and upper electrodes overlap to face each other. The substrate (10) is located below the resonator portion (60), and has a void (50). An end of the lower electrode (20) is located inside the void (50), and the void (50) is wider than the resonator portion (60). In this piezoelectric thin-film resonator, an additional film (90) is formed on an extraction region of the upper electrode (40) remote from the lower electrode (20) and the additional film extends up to the vicinity of said resonator portion. Furthermore the outer periphery of the piezoelectric film (30) interposed between the lower and upper electrodes (20, 40) coincides with the outer periphery of the resonator portion except at the extraction region of the upper electrode (40), the outer periphery of the piezoelectric film (30) coincides with the outer periphery of the upper electrode (40) in the resonator portion (60) and the outer periphery of the piezoelectric film (30) that coincides with the outer periphery of the upper electrode (40) in the resonator portion (60) is located above the void so as to suppress horizontal leakage of surface acoustic waves in the structure.