Piezoelectric resonator, filter, and electronic communication device

Abstract

A piezoelectric resonator includes a substrate, a vibration unit disposed on the substrate and having a structure in which at least one pair of an upper electrode and a lower electrode opposed to each other, the upper and lower electrodes sandwiching the upper and lower surfaces of an internal thin-film portion including at least one layer of a piezoelectric thin-film, and an external thin-film portion provided under the lower electrode and including at least one layer of a piezoelectric thin-film or a dielectric thin-film, the vibration unit being vibrated in an n-th harmonic (n is an integer of 2 or more), the upper electrode and the lower electrode being provided substantially in the positions of the loops of the n-th harmonic.

What is claimed is:  
     
         1 . A piezoelectric resonator comprising: 
 a substrate;    a vibration unit disposed on the substrate and having a structure in which at least one pair of an upper electrode and a lower electrode are opposed to each other, the upper and lower electrodes sandwiching the upper and lower surfaces of an internal thin-film portion including at least one layer of a piezoelectric thin-film; and    an external thin-film portion provided under the lower electrode and including at least one layer of one of a piezoelectric thin-film and a dielectric thin-film; wherein    the vibration unit is vibrated in an n-th harmonic, where n is an integer of 2 or greater, the upper electrode and the lower electrode are located substantially in the positions of the loops of the n-th harmonic.    
     
     
         2 . A piezoelectric resonator according to  claim 1 , wherein the n-th harmonic is a second order harmonic, and the film thickness ratio r=t o /t i  in which t o  represents the thickness of the external thin-film, and t i  represents the thickness of the internal thin-film is set at a value at which the resonance frequency temperature coefficient of the entire piezoelectric resonator is nearly zero.  
     
     
         3 . A piezoelectric resonator according to  claim 1 , wherein the respective thin-films of at least one of the internal thin-film portion and the external thin-film portion are combined in such a manner as to have different resonance frequency temperature coefficients.  
     
     
         4 . A piezoelectric resonator according to  claim 1 , wherein the external thin-film portion includes at least one of a thin-film having an SiO 2  thin-film as a major component, a thin-film including an SiN thin-film as a major component, and a thin-film including an Al 2 O 3  thin-film as a major component.  
     
     
         5 . A piezoelectric resonator according to  claim 1 , wherein the internal thin-film portion includes a thin-film having ZnO as a major component, a thin-film including AlN as a major component, a thin-film including lead titanate zirconate as a major component, a thin-film including lead titanate as a major component, and a thin-film including barium titanate as a major component.  
     
     
         6 . A piezoelectric resonator according to  claim 1 , wherein the substrate has one of a hole and a concavity, and the vibration unit is disposed above the one of the hole and the concavity.  
     
     
         7 . A piezoelectric resonator according to  claim 1 , wherein the piezoelectric resonator is a thickness-longitudinal vibration type resonator.  
     
     
         8 . A piezoelectric resonator according to  claim 1 , wherein the substrate has a hole that passes through a top surface to a bottom surface of the substrate and the vibration unit is disposed above the hole.  
     
     
         9 . A piezoelectric resonator according to  claim 8 , further comprising a diaphragm that is arranged to cover the hole.  
     
     
         10 . A piezoelectric resonator according to  claim 1 , wherein the opposed electrodes are made of aluminum and the internal thin-film portion is made of zinc oxide.  
     
     
         11 . A piezoelectric resonator according to  claim 1 , wherein the n-th harmonic is a second order harmonic and nodes of the second harmonic exist in the internal and external thin-film portions.  
     
     
         12 . A piezoelectric resonator according to  claim 11 , wherein the film thickness ratio r=t o /t i  in which t o  represents the thickness of the external thin-film, and t i  represents the thickness of the internal thin-film is set at a value at which the resonance frequency temperature coefficient of the entire piezoelectric resonator is nearly zero.  
     
     
         13 . A piezoelectric resonator according to  claim 12 , wherein the film-thickness ratio r is preferably in the range of about 0.6 to about 1.3.  
     
     
         14 . A piezoelectric resonator according to  claim 1 , wherein the resonance frequency temperature coefficient TCF is about +10 ppm/° C. to about −10 ppm/° C.  
     
     
         15 . A piezoelectric-resonator according to  claim 1 , wherein the external thin-film portion includes two layers having different resonance frequency temperature coefficients.  
     
     
         17 . A filter including a plurality of the piezoelectric resonators according to  claim 1  and a filter circuit, wherein the electrodes of the piezoelectric resonators are connected to the filter circuit.  
     
     
         18 . A filter including a plurality of the piezoelectric resonators according to  claim 1 , wherein the piezoelectric resonators are connected in a ladder arrangement.  
     
     
         19 . A duplexer including the filter according to  claim 17 .  
     
     
         20 . A duplexer including the filter according to  claim 18 .  
     
     
         21 . An electronic communication device including at least one piezoelectric resonator according to  claim 1.