1. Field of the Invention
This invention generally relates to filters, and more particularly, to a filter having multiple piezoelectric thin-film resonators.
2. Description of the Related Art
Recently, there has been an increasing demand for filters with compact and light resonators and filters configured by combining these filters as wireless communication equipment that may be typically cellular phones has spread rapidly. In the past, dielectric filters or surface acoustic wave (SAW) filters were mainly used. Nowadays, piezoelectric thin film resonators and filters using these resonators are attracting attention, in which the piezoelectric thin film resonators are characterized in that they gave good high-frequency performance and are compact and producible as a monolithic device.
The piezoelectric thin-film resonators may be categorized into an FBAR (Film Bulk Acoustic Resonator) type and an SMR (Solidly Mounted Resonator) type. Japanese Patent Application Publication No. 60-189307 and Japanese Patent Application Publication No. 2004-200843 disclose piezoelectric thin-film resonators of FBAR type. FIG. 1 show a conventional FBAR type resonator and a conventional SMR type resonator in parts (a) and (b), respectively. A lower electrode 13, a piezoelectric film 14 and an upper electrode 15 are laminated in this order on a substrate 11 having a cavity 16 or an acoustic multilayer film. The cavity 16 or an alternative acoustic multilayer film is formed below an overlapping portion in which the upper electrode 15 and the lower electrode 13 overlap each other across the piezoelectric film 14. Such a portion is referred to as a resonance portion 23. The cavity 16 in the FBAR may be formed by dry or wet etching from the backside of the substrate 11, which may be made of silicon. The cavity 16 may be formed at a position between the lower electrode 13 and the substrate 11 by removing a sacrificed layer provided on the surface of the silicon substrate 11 by wet etching. In this case, the cavity 16 may be called a gap. The acoustic multilayer film in the SMR has first layers having a relatively low acoustic impedance and second layers having a relatively high acoustic impedance so as to be laminated alternately to a thickness equal to λ/4 where λ is the wavelength of an acoustic wave of the resonator.
The upper and lower electrodes may be made of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir) or the like. The piezoelectric thin film may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), lead titanate (PbTiO3) or the like. The substrate 11 may be made of glass other than silicon.
The piezoelectric thin-film resonator of FBAR or SMR type has a loss caused by leak acoustic waves 30 propagated outwards from the resonance portion 23, as shown in parts (a) and (b) of FIG. 1. Portions that are located further out than the resonance portion 23 are referred to as non-resonance portions. The leak acoustic waves 30 propagated through the non-resonance portions are not converted into electric signals and cause a loss. This phenomenon is called lateral leakage of the acoustic waves 30. The filters with the piezoelectric thin-film resonators are required to have restrained lateral leakage and reduced loss. Further, the filters of this type are required to have high resistance to electrostatic discharge (ESD) damage.