Patent Publication Number: US-2023155565-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application No. 63/052,141 filed on Jul. 15, 2020 and is a Continuation application of PCT Application No. PCT/JP2021/025977 filed on Jul. 9, 2021. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an acoustic wave device. 
     2. Description of the Related Art 
     An acoustic wave device using plate waves that propagate through a piezoelectric film made of LiNbO 3  or LiTaO 3  is known in the related art. For example, Japanese Unexamined Patent Application Publication No. 2012-257019 discloses an acoustic wave device that uses Lamb waves as plate waves. In the acoustic wave device, a piezoelectric substrate is provided on a support body. The piezoelectric substrate is made of LiNbO 3  or LiTaO 3 . An interdigital transducer (IDT) electrode is provided on the upper surface of the piezoelectric substrate. A voltage is applied between a plurality of electrode fingers connected to one potential of the IDT electrode and a plurality of electrode fingers connected to the other potential of the IDT electrode. Lamb waves are thereby excited. The IDT electrode is provided with reflectors on both sides thereof. An acoustic wave resonator using plate waves is thus provided. 
     Japanese Unexamined Patent Application Publication No. 2011-182096 discloses an example of a ladder filter. In the ladder filter, a plurality of acoustic wave devices are connected to each other by a plurality of wires. The plurality of wires include a wire connected to a hot potential and a wire connected to a ground potential. The wire connected to the hot potential and the wire connected to the ground potential face each other. 
     In such an acoustic wave resonator described in Japanese Unexamined Patent Application Publication No. 2012-257019, unwanted bulk waves may be excited. The bulk waves propagate in the thickness direction of the piezoelectric substrate. The bulk waves thus may be reflected by the support body. In a case where, as in Japanese Unexamined Patent Application Publication No. 2011-182096, wires that are connected to different potentials face each other, a signal of unwanted bulk waves may be extracted by one of the wires. The signal of the unwanted bulk waves also may be extracted by one of mutually facing busbars. In these cases, there is a likelihood of ripples being generated in frequency characteristics of the acoustic wave device. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide acoustic wave devices that are each able to reduce or prevent ripples in frequency characteristics. 
     An acoustic wave device according to a preferred embodiment of the present invention includes a support substrate, a piezoelectric layer on the support substrate, a functional electrode on the piezoelectric layer, a first electrode film and a second electrode film on the piezoelectric layer, facing each other, and having different electric potentials from each other, and a dielectric film between at least one of at least a portion of the first electrode film and the piezoelectric layer and at least a portion of the second electrode film and the piezoelectric layer. 
     According to preferred embodiments of the present invention, it is possible to provide acoustic wave devices that are each able to reduce or prevent ripples in frequency characteristics. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plan view of an acoustic wave device according to a first preferred embodiment of the present invention. 
         FIG.  2    is a sectional view along line I-I in  FIG.  1   . 
         FIG.  3    is a sectional view along line II-II in  FIG.  1   . 
         FIG.  4    is a graph showing reflection characteristics of the first preferred embodiment of the present invention and a comparative example. 
         FIG.  5    is a sectional view illustrating an example of propagation of unwanted bulk waves in a comparative example. 
         FIG.  6    is a sectional view illustrating a portion of an acoustic wave device according to a first modification of the first preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
         FIG.  7    is a plan view of an acoustic wave device according to a second modification of the first preferred embodiment of the present invention. 
         FIG.  8    is a sectional view illustrating a portion of an acoustic wave device according to a second preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  2   . 
         FIG.  9    is a sectional view illustrating a portion of an acoustic wave device according to a third preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
         FIG.  10    is a sectional view illustrating a portion of an acoustic wave device according to a modification of the third preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
         FIG.  11    is a sectional view illustrating a portion of an acoustic wave device according to a fourth preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
         FIG.  12    is a sectional view illustrating a portion of an acoustic wave device according to a modification of the fourth preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
         FIG.  13    is a schematic plan view of an acoustic wave device according to a fifth preferred embodiment of the present invention. 
         FIG.  14    is a sectional view along line in  FIG.  13   . 
         FIG.  15    is a circuit diagram of the acoustic wave device according to the third preferred embodiment of the present invention. 
         FIG.  16    is a plan view of the acoustic wave device according to the fourth preferred embodiment of the present invention. 
         FIG.  17    is a sectional view along line IV-IV in  FIG.  16   . 
         FIG.  18 A  is a schematic perspective view illustrating an acoustic wave device that uses bulk waves of a thickness shear mode, and  FIG.  18 B  is a plan view illustrating an electrode structure on a piezoelectric layer. 
         FIG.  19    is a sectional view of a portion along line A-A in  FIG.  18 A . 
         FIG.  20 A  is a schematic elevational cross-sectional view for describing Lamb waves propagating through a piezoelectric film of an acoustic wave device, and  FIG.  20 B  is a schematic elevational cross-sectional view for describing bulk waves of a thickness shear mode propagating through a piezoelectric film in an acoustic wave device. 
         FIG.  21    illustrates an amplitude direction of bulk waves of a thickness shear mode. 
         FIG.  22    is a graph showing resonance characteristics of an acoustic wave device that uses bulk waves of a thickness shear mode. 
         FIG.  23    is a graph showing a relationship between a fractional bandwidth as a resonator and d/2p where p is a distance between centers of mutually adjacent electrodes and d is a thickness of a piezoelectric layer. 
         FIG.  24    is a plan view of an acoustic wave device that uses bulk waves of a thickness shear mode. 
         FIG.  25    is a graph showing a map of the fractional bandwidth with respect to Euler angles (0°, θ, ψ) of LiNbO 3  when d/p is approximated to zero as closely as possible. 
         FIG.  26    is a partially omitted perspective view for describing an acoustic wave device that uses Lamb waves. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be clarified by describing preferred embodiments of the present invention with reference to the drawings. 
     Each preferred embodiment described in the present description is exemplary, and components can be partially replaced or combined together among different preferred embodiments. 
       FIG.  1    is a plan view of an acoustic wave device according to a first preferred embodiment of the present invention.  FIG.  2    is a sectional view along line I-I in  FIG.  1   .  FIG.  3    is a sectional view along line II-II in  FIG.  1   . 
     As illustrated in  FIG.  1   , an acoustic wave device  10  includes a piezoelectric substrate  12  and an IDT electrode  11 . As illustrated in  FIG.  2   , the piezoelectric substrate  12  includes a support member  13  and a piezoelectric layer  14 . In the present preferred embodiment, the support member  13  includes only a support substrate. However, the support member  13  may be, for example, a multilayer body including a support substrate and an electrically insulating layer. 
     The support member  13  includes a through hole  13   a  as a cavity portion. The piezoelectric layer  14  covers the through hole  13   a  of the support member  13 . 
     The piezoelectric layer  14  includes a first main surface  14   a  and a second main surface  14   b . The first main surface  14   a  and the second main surface  14   b  face each other. In the first main surface  14   a  and the second main surface  14   b , the second main surface  14   b  is a main surface on the side of the support member  13 . The piezoelectric layer  14  is, for example, a lithium niobate layer in the present preferred embodiment. More specifically, the piezoelectric layer  14  is, for example, a LiNbO 3  layer. However, the piezoelectric layer  14  may be, for example, a lithium tantalate layer, such as a LiTaO 3  layer. 
     The support substrate is, for example, a silicon substrate in the present preferred embodiment. The material of the support substrate is, however, not limited to this material. 
     As shown in  FIG.  1   , the IDT electrode  11  is provided on the piezoelectric layer  14 . The IDT electrode  11  includes a first busbar  16 , a second busbar  17 , a plurality of first electrode fingers  18 , and a plurality of second electrode fingers  19 . The first busbar  16  corresponds to the first electrode film. The second busbar  17  corresponds to the second electrode film. As illustrated in  FIG.  3   , the first busbar  16  and the second busbar  17  face each other. 
     A dielectric film  15  is provided between the first busbar  16  and the piezoelectric layer  14  and between the second busbar  17  and the piezoelectric layer  14 . Meanwhile, the dielectric film  15  is not provided, on the piezoelectric layer  14 , at a portion between the first busbar  16  and the second busbar  17 . As a material of the dielectric film  15 , for example, silicon oxide, silicon nitride, resin, or the like may be used. 
     The first busbar  16  and the second busbar  17  are connected to electric potentials that differ from each other. In the present preferred embodiment, the first busbar  16  is connected to a hot potential, and the second busbar  17  is connected to a ground potential. The electric potentials to which the first busbar  16  and the second busbar  17  are connected are, however, not limited to the above. For example, the first busbar  16  may be connected to the ground potential while the second busbar  17  is connected to the hot potential. 
     The plurality of first electrode fingers  18  and the plurality of second electrode fingers  19  illustrated in  FIG.  1    correspond to the at least one pair of electrodes. The at least one pair of electrodes face each other. The plurality of first electrode fingers  18  are periodically disposed. Each of one ends of the plurality of first electrode fingers  18  is connected to the first busbar  16 . The plurality of second electrode fingers  19  are periodically disposed. Each of one ends of the plurality of second electrode fingers  19  is connected to the second busbar  17 . The plurality of first electrode fingers  18  and the plurality of second electrode fingers  19  are interdigitated with each other. Hereinafter, the first electrode fingers  18  and the second electrode fingers  19  may be simply referred to as the electrode fingers. The IDT electrode  11  may include a single layer of a metal film or may include a multilayer metal film. 
     In the acoustic wave device  10 , the plurality of electrode fingers as the at least one pair of electrodes correspond to the functional electrode. An alternating-current voltage is applied to the functional electrode of the IDT electrode  11 , and acoustic waves are thus excited. In the present preferred embodiment, the acoustic wave device  10  uses, as principal waves, bulk waves of a thickness shear mode. More specifically, the acoustic wave device  10  uses, as the principal waves, bulk waves of a thickness shear primary mode. The acoustic wave device  10 , however, may be an acoustic wave device that uses, as the principal waves, plate waves such as Lamb waves. In the present preferred embodiment, shear horizontal (SH) waves are excited as unwanted bulk waves. 
     In a plan view, a direction in which the first electrode fingers  18  and the second electrode fingers  19  face each other is referred to as an electrode finger facing direction. The plan view is a view direction from the upper side in, for example,  FIG.  2    or  FIG.  3   . A region in which mutually adjacent electrode fingers overlap each other when viewed in the electrode finger facing direction is an intersection region D. The intersection region D is a region including the electrode finger at one end of the IDT electrode  11  in the electrode finger facing direction to the electrode finger at the other end thereof. More specifically, the intersection region D includes an edge portion of the electrode finger at the one end on the outer side in the electrode finger facing direction to an edge portion of the electrode finger at the other end on the outer side in the electrode finger facing direction. 
     In addition, the acoustic wave device  10  includes a plurality of excitation regions C. The excitation regions C are also regions in which mutually adjacent electrode fingers overlap each other when viewed in the electrode finger facing direction. The excitation regions C are each a region between one pair of the electrode fingers. In more detail, each excitation region C is a region from the center of one of the electrode fingers in the electrode finger facing direction to the center of the other one of the electrode fingers in the electrode finger facing direction. The intersection region D thus includes the plurality of excitation regions C. Bulk waves of the thickness shear mode are excited in the excitation regions C. When the acoustic wave device  10  uses plate waves, the intersection region D is an excitation region. 
     One of the unique features of the present preferred embodiment is that the dielectric film  15  is provided between the first busbar  16  as the first electrode film and the piezoelectric layer  14  and between the second busbar  17  as the second electrode film and the piezoelectric layer  14 . Consequently, it is possible in the acoustic wave device  10  to reduce or prevent an influence of unwanted bulk waves on frequency characteristics and possible to reduce or prevent ripples in the frequency characteristics. This is described below by comparing the present preferred embodiment and a comparative example. 
     The comparative example differs from the first preferred embodiment in that the dielectric film is not included. In the first preferred embodiment, reflection characteristics as the frequency characteristics between the first busbar and the second busbar are measured. Similarly, in the comparative example, reflection characteristics between the first busbar and the second busbar are measured. 
       FIG.  4    shows reflection characteristics in the first preferred embodiment and the comparative example. The reflection characteristics shown in  FIG.  4    are relationships between S 11  and frequencies.  FIG.  5    is a sectional view illustrating an example of propagation of unwanted bulk waves in the comparative example. The arrow B in  FIG.  5    indicates a portion of the unwanted bulk waves. 
       FIG.  4    shows that ripples are generated in all of the frequency bands indicated in  FIG.  4    in the reflection characteristics in the comparative example. As illustrated in  FIG.  5   , unwanted bulk waves propagated from, for example, the first busbar  16  are reflected by the support substrate in the comparative example. The signal of the unwanted bulk waves is extracted by the second busbar  17 . Therefore, the ripples shown in  FIG.  4    are generated. In contrast, it is shown that ripples are reduced or prevented in the reflection characteristics of the first preferred embodiment. 
     As illustrated in  FIG.  3   , the dielectric film  15  is provided between the first busbar  16  and the piezoelectric layer  14  and between the second busbar  17  and the piezoelectric layer  14  in the present preferred embodiment. Due to the dielectric film  15 , the electromechanical coupling coefficient varies compared with a case where the dielectric film  15  is not provided. Consequently, it is possible to reduce or prevent an influence of the unwanted bulk waves on the reflection characteristics. Accordingly, it is possible to reduce or prevent ripples in the reflection characteristics as frequency characteristics. 
     In the acoustic wave device  10 , the thickness of the dielectric film  15  provided between the first busbar  16  and the piezoelectric layer  14  is the same or substantially the same as the thickness of the dielectric film  15  provided between the second busbar  17  and the piezoelectric layer  14 . The thicknesses of the dielectric film  15 , however, may differ from each other. 
     The dielectric film  15  provided between the first busbar  16  and the piezoelectric layer  14  and the dielectric film  15  provided between the second busbar  17  and the piezoelectric layer  14  are integrally provided. However, the dielectric film  15  provided between the first busbar  16  and the piezoelectric layer  14  and the dielectric film  15  provided between the second busbar  17  and the piezoelectric layer  14  may be provided as separate dielectric films. 
     Referring back to  FIG.  1   , the support member  13  includes only the support substrate in the first preferred embodiment. The support member  13 , however, may be a multilayer body including the support substrate and an electrically insulating layer. In this case, the piezoelectric layer  14  is provided on the electrically insulating layer. As a material of the electrically insulating layer, for example, silicon oxide, silicon nitride, or tantalum oxide is usable. 
     The cavity portion is not limited to the through hole. The cavity portion may be, for example, a hollow portion. The hollow portion is defined by, for example, a recessed portion provided on the support member. More specifically, the hollow portion is defined by the recessed portion being sealed by the piezoelectric layer  14  or the like. Alternatively, a recessed portion that opens on the side of the support member  13  may be provided on the piezoelectric layer  14 . The cavity portion may be defined by this structure. In this case, the support member  13  does not necessarily include a recessed portion or a through hole. 
     It is sufficient that the dielectric film  15  is provided between at least one of at least a portion of the first busbar  16  as the first electrode film and the piezoelectric layer  14  and at least a portion of the second busbar  17  as the second electrode film and the piezoelectric layer  14 . For example, the dielectric film  15  is not provided between the second busbar  17  and the piezoelectric layer  14  in a first modification of the first preferred embodiment illustrated in  FIG.  6   . The dielectric film  15  is also not provided, on the piezoelectric layer  14 , at a portion between the first busbar  16  and the second busbar  17 . Meanwhile, the dielectric film  15  is provided between the first busbar  16  and the piezoelectric layer  14 . In this case, it is also possible to reduce or prevent ripples in the frequency characteristics. 
     The dielectric film  15  is, however, preferably provided between the piezoelectric layer  14  and the entireties or substantially the entireties of the first busbar  16  and the second busbar. Consequently, it is possible to effectively reduce or prevent ripples in the frequency characteristics more reliably. 
       FIG.  7    illustrates, as a second modification of the first preferred embodiment, an example in which the acoustic wave device uses plate waves. As illustrated in  FIG.  7   , one pair of a reflector  22 A and a reflector  22 B are provided on both sides of the IDT electrode  11  in the electrode finger facing direction on the piezoelectric layer  14  in a plan view. Consequently, it is possible to suitably improve the resonance characteristics in a case where plate waves are used. In the present modification, the IDT electrode  11  is structured similarly to the first preferred embodiment. The dielectric film  15  is also provided similarly to the first preferred embodiment. It is thus possible to reduce or prevent ripples in the frequency characteristics. 
       FIG.  8    is a sectional view illustrating a portion of an acoustic wave device according to a second preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  2   . 
     The present preferred embodiment differs from the first preferred embodiment in that a dielectric film  35  is provided, on the piezoelectric layer  14 , at a portion between the first busbar  16  as the first electrode film and the second busbar  17  as the second electrode film. The present preferred embodiment also differs from the first preferred embodiment in that the dielectric film  35  is a multilayer film. In terms of features other than the above-described features, the acoustic wave device in the present preferred embodiment has a configuration the same as or similar to that of the acoustic wave device  10  in the first preferred embodiment. 
     The dielectric film  35  includes a first layer  35   a  and a second layer  35   b . The second layer  35   b  is provided on the first main surface  14   a  of the piezoelectric layer  14 . The first layer  35   a  is laminated on the second layer  35   b . In the present preferred embodiment, the material of the first layer  35   a  and the material of the second layer  35   b  differ from each other. The material of the first layer  35   a  and the material of the second layer  35   b , however, may be the same as each other. 
     The first layer  35   a  is disposed similarly to the first preferred embodiment. More specifically, the first layer  35   a  is provided between each of the first busbar  16  and the piezoelectric layer  14  and the second busbar  17  and the piezoelectric layer  14 . The first layer  35   a  is not provided, on the piezoelectric layer  14 , at a portion between the first busbar  16  and the second busbar  17 . 
     The second layer  35   b  is provided between the first busbar  16  and the piezoelectric layer  14  and between the second busbar  17  and the piezoelectric layer  14 . The second layer  35   b  is also provided, on the piezoelectric layer  14 , at a portion between the first busbar  16  and the second busbar  17 . More specifically, the second layer  35   b  is also provided between the plurality of electrode fingers of the IDT electrode  11  and the piezoelectric layer  14 , and at a portion between the electrode fingers on the piezoelectric layer  14 . 
     The thickness of a portion of the dielectric film  35  between the first busbar  16  and the second busbar  17  in a plan view is referred to as h 1 . The thickness of a portion of the dielectric film  35  between the first busbar  16  and the piezoelectric layer  14  is referred to as h 2 . The thickness of a portion of the dielectric film  35  between the second busbar  17  and the piezoelectric layer  14  is referred to as h 3 . The thickness h 1  is smaller than both the thickness h 2  and the thickness h 3 . The thickness h 1 , however, may be smaller than at least one of the thickness h 2  and the thickness h 3 . 
     In the present preferred embodiment, the dielectric film  35  is also provided between the first busbar  16  and the piezoelectric layer  14  and between the second busbar  17  and the piezoelectric layer  14 . Therefore, the electromechanical coupling coefficient varies compared with a case where the dielectric film  35  is not provided. Consequently, it is possible to reduce or prevent an influence of unwanted bulk waves on frequency characteristics and possible to reduce or prevent ripples in the frequency characteristics. 
       FIG.  9    is a sectional view illustrating a portion of an acoustic wave device according to a third preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
     The present preferred embodiment differs from the first preferred embodiment in that a plurality of dielectric films are provided. Specifically, a first dielectric film  45 A is provided between the first busbar  16  and the piezoelectric layer  14 . A second dielectric film  45 B is provided between the second busbar  17  and the piezoelectric layer  14 . The present preferred embodiment further differs from the first preferred embodiment in that the thickness of the first dielectric film  45 A and the thickness of the second dielectric film  45 B differ from each other. In terms of features other than the above-described features, the acoustic wave device in the present preferred embodiment has a configuration the same as or similar to that of the acoustic wave device  10  in the first preferred embodiment. The first dielectric film  45 A and the second dielectric film  45 B, however, may be integrally provided. 
     In the present preferred embodiment, the thickness of the second dielectric film  45 B is larger than the thickness of the first dielectric film  45 A. Consequently, it is possible to cause the form of propagation of unwanted bulk waves to be different between the first dielectric film  45 A and the second dielectric film  45 B. It is thus possible to cause the form of propagation of unwanted bulk waves to be different also between the first busbar  16  and the second busbar  17 . Therefore, it is possible to effectively reduce or prevent an influence of the unwanted bulk waves on the frequency characteristics and possible to effectively reduce or prevent ripples in the frequency characteristics. 
     It is sufficient that the thickness of the first dielectric film  45 A and the thickness of the second dielectric film  45 B differ from each other. In a modification of the third preferred embodiment illustrated in  FIG.  10   , the thickness of the first dielectric film  45 A is larger than the thickness of the second dielectric film  45 B. In this case, it is also possible to effectively reduce or prevent ripples in the frequency characteristics. 
       FIG.  11    is a sectional view illustrating a portion of an acoustic wave device according to a fourth preferred embodiment of the present invention, the portion corresponding to the section illustrated in  FIG.  3   . 
     The present preferred embodiment differs from the first preferred embodiment in that the thickness of a portion of a piezoelectric layer  54  overlapping the first busbar  16  in a plan view and the thickness of a portion of the piezoelectric layer  54  overlapping the second busbar  17  in the plan view differ from each other. The present preferred embodiment further differs from the first preferred embodiment in that the dielectric film  15  is not provided between the second busbar  17  and the piezoelectric layer  14 . In terms of features other than the above-described features, the acoustic wave device in the present preferred embodiment has a configuration the same as or similar to that of the acoustic wave device  10  in the first preferred embodiment. 
     The piezoelectric layer  54  includes a first region E 1 , a second region E 2 , and a third region E 3 . The first region E 1  is a region overlapping the first busbar  16  as the first electrode film in a plan view. The second region E 2  is a region overlapping the second busbar  17  as the second electrode film in the plan view. The third region E 3  is a region not overlapping the first electrode film and the second electrode film in the plan view. The thickness of the piezoelectric layer  54  in the first region E 1  is referred to as d 1 , the thickness of the piezoelectric layer  54  in the second region E 2  is referred to as d 2 , and the thickness of the piezoelectric layer  54  in the third region E 3  is referred to as d 3 . 
     In the present preferred embodiment, the thickness d 1  and the thickness d 2  differ from each other. More specifically, the thickness d 1  is larger than the thickness d 2 . Consequently, it is possible to cause the form of propagation of unwanted bulk waves to be different from each other between the first region E 1  and the second region E 2 . It is thus possible to cause the form of propagation of unwanted bulk waves to be different also between the first busbar  16  and the second busbar  17 . In addition, the dielectric film  15  is provided between the first busbar  16  and the piezoelectric layer  14 . Consequently, the electromechanical coupling coefficient varies compared with a case where the dielectric film  15  is not provided. Therefore, it is possible to effectively reduce or prevent an influence of the unwanted bulk waves on the frequency characteristics and possible to effectively reduce or prevent ripples in the frequency characteristics. 
     In the present preferred embodiment, the dielectric film  15  is provided only between the first busbar  16  and the piezoelectric layer  14 . It is sufficient that the dielectric film  15  is provided between one of the first busbar  16  and the piezoelectric layer  14  and the second busbar  17  and the piezoelectric layer  14 . 
     The thickness d 3  of the piezoelectric layer  54  in the third region E 3  is the same or substantially the same as the thickness d 2  of the piezoelectric layer  54  in the second region E 2 . The relationship of thicknesses of the piezoelectric layer  54  is, however, not limited to the above, and it is sufficient that the thickness d 1  in the first region E 1  and the thickness d 2  differ from each other. In a modification of the fourth preferred embodiment illustrated in  FIG.  12   , the thickness d 1  and the thickness d 3  are the same or substantially the same each other. Meanwhile, the thickness d 2  is smaller than the thickness d 1  and the thickness d 3 . The dielectric film  15  is provided between the second busbar  17  and a piezoelectric layer  54 A. Meanwhile, the dielectric film  15  is not provided between the first busbar  16  and the piezoelectric layer  54 A. In this case, it is also possible to effectively reduce or prevent ripples in the frequency characteristics. 
     In the present modification, the total of the thickness d 2  and the thickness of the dielectric film  15  provided between the second busbar  17  and the piezoelectric layer  54 A is the same or substantially the same as the thickness d 1  and the thickness d 3 . Consequently, the IDT electrode  11  is easily formed, which can improve productivity. The relationship between the thicknesses of the piezoelectric layer  54 A in the regions and the thickness of the dielectric film  15  is, however, not limited to the above. 
     In the first to fourth preferred embodiments, an example in which, in a case where a signal of unwanted bulk waves propagated from one of the one pair of busbars is extracted by the other one of the busbars, the influence of the signal can be reduced or prevented. Propagation and extraction of the signal of unwanted bulk waves may occur also between wiring electrode films in a filter device. In the following fifth preferred embodiment, an example in which the acoustic wave device is a filter device is presented. 
       FIG.  13    is a schematic plan view of an acoustic wave device according to a fifth preferred embodiment of the present invention. In  FIG.  13   , acoustic wave resonators are each schematically illustrated as a polygonal shape with two diagonal lines. 
     An acoustic wave device  60  includes a plurality of the acoustic wave resonators. The acoustic wave device  60  is, for example, a filter device. The acoustic wave resonators of the acoustic wave device  60  each include an IDT electrode. Each IDT electrode includes one pair of busbars and a plurality of electrode fingers as functional electrodes. In the present preferred embodiment, a first electrode film  68  and a second electrode film  69  are wiring electrode films. 
     The first electrode film  68  and the second electrode film  69  are provided on the piezoelectric layer  14 , and the plurality of acoustic wave resonators are provided on the piezoelectric layer  14 . The wiring electrode film as the first electrode film  68  and the wiring electrode film as the second electrode film  69  are extracted from the acoustic wave resonators that differ from each other. In other words, the first electrode film  68  and the second electrode film  69  are each connected to the plurality of electrode fingers as the functional electrodes with a busbar interposed therebetween. The first electrode film  68  is connected to a hot potential, and the second electrode film  69  is connected to a ground potential. 
       FIG.  14    is a sectional view along line in  FIG.  13   . 
     The first electrode film  68  and the second electrode film  69  face each other. Similarly to the first preferred embodiment, the dielectric film  15  is provided between the first electrode film  68  and the piezoelectric layer  14  and between the second electrode film  69  and the piezoelectric layer  14 . Due to the dielectric film  15 , the electromechanical coupling coefficient varies compared with a case where the dielectric film  15  is not provided. Consequently, it is possible to reduce or prevent an influence of unwanted bulk waves on frequency characteristics and possible to reduce or prevent ripples in the frequency characteristics. Hereinafter, a circuit configuration in the present preferred embodiment will be described. 
       FIG.  15    is a circuit diagram of the acoustic wave device according to the third preferred embodiment. 
     The acoustic wave device  60  is, for example, a ladder filter. The plurality of acoustic wave resonators in the present preferred embodiment include a series arm resonator S 1 , a series arm resonator S 2 , a series arm resonator S 3 , a series arm resonator S 4 , a series arm resonator S 5 , a series arm resonator S 6 , a parallel arm resonator P 1 , a parallel arm resonator P 2 , and a parallel arm resonator P 3 . Further, the acoustic wave device  60  includes a first signal terminal  65  and a second signal terminal  66 . 
     The series arm resonator S 1 , the series arm resonator S 2 , the series arm resonator S 3 , the series arm resonator S 4 , the series arm resonator S 5 , and the series arm resonator S 6  are connected in series to each other in this order between the first signal terminal  65  and the second signal terminal  66 . The parallel arm resonator P 1  is connected between the ground potential and a connection point between the series arm resonator S 1  and the series arm resonator S 2 . The parallel arm resonator P 2  is connected between the ground potential and a connection point between the series arm resonator S 3  and the series arm resonator S 4 . The parallel arm resonator P 3  is connected between the ground potential and a connection point between the series arm resonator S 5  and the series arm resonator S 6 . As illustrated in  FIG.  13   , the acoustic wave device  60  includes a plurality of ground terminals  67 . The parallel arm resonators are each connected to the ground potential with any one of the ground terminals  67  interposed therebetween. The above-described circuit configuration is one example, and the circuit configuration of the acoustic wave device  60  is not particularly limited. 
     In the acoustic wave device  60 , the first electrode film  68  is a wiring electrode film connecting the series arm resonator S 1  and the parallel arm resonator P 1  to each other. The second electrode film  69  is a wiring electrode film connecting the parallel arm resonator P 2  and the ground terminals  67  to each other. A distance L between the first electrode film  68  and the second electrode film  69  is a length that differs from aperture lengths M of the acoustic wave resonators that are different from the acoustic wave resonators to which the first electrode film  68  and the second electrode film  69  are connected. The aperture length of an acoustic wave resonator refers to a distance between one pair of busbars of the acoustic wave resonator. As illustrated in  FIG.  13   , for example, the aperture length of the series arm resonator S 3  is M 3 , and the aperture length of the series arm resonator S 6  is M 6 . The distance L may be longer or shorter than, for example, the shortest aperture length among the aperture lengths M of the above-described different acoustic wave resonators. In a case where the aperture length M 6  is not constant as with the series arm resonator S 6 , the distance L may be longer or shorter than, for example, the shortest aperture length of the aperture length M 6 . 
     The IDT electrodes of the series arm resonator S 1  and the parallel arm resonator P 1  are each connected to the first electrode film  68 . Meanwhile, the IDT electrode of the parallel arm resonator P 2  is connected to the second electrode film  69 . An IDT electrode connected to both the first electrode film  68  and the second electrode film  69  may be provided. 
       FIG.  16    is a plan view of the acoustic wave device according to the fourth preferred embodiment.  FIG.  17    is a sectional view along line IV-IV in  FIG.  16   . 
     As illustrated in  FIG.  16    and  FIG.  17   , a functional electrode includes an upper electrode  71 A and a lower electrode  71 B in the present preferred embodiment. The upper electrode  71 A is provided on the first main surface  14   a  of the piezoelectric layer  14 . The lower electrode  71 B is provided on the second main surface  14   b  of the piezoelectric layer  14 . The upper electrode  71 A and the lower electrode  71 B face each other with the piezoelectric layer  14  interposed therebetween. The upper electrode  71 A and the lower electrode  71 B are connected to electric potentials that differ from each other. A region in which the upper electrode  71 A and the lower electrode  71 B face each other is an excitation region. 
     As illustrated in  FIG.  16   , the first main surface  14   a  of the piezoelectric layer  14  is provided with the first electrode film  68  and the second electrode film  69 . In the present preferred embodiment, the first electrode film  68  and the second electrode film  69  are wiring electrode films. The first electrode film  68  is connected to the upper electrode  71 A. Meanwhile, the second main surface  14   b  of the piezoelectric layer  14  is provided with a connection electrode  72 . The connection electrode  72  is connected to the lower electrode  71 B. The piezoelectric layer  14  includes a through hole. The connection electrode  72  passes through the through hole and is connected to the second electrode film  69 . Therefore, the second electrode film  69  is connected to the lower electrode  71 B with the connection electrode  72  interposed therebetween. 
     The first electrode film  68  and the second electrode film  69  face each other. As in the configuration in the third preferred embodiment illustrated in  FIG.  14   , the dielectric film  15  is provided between the first electrode film  68  and the piezoelectric layer  14  and between the second electrode film  69  and the piezoelectric layer  14  in the present preferred embodiment. Due to the dielectric film  15 , the electromechanical coupling coefficient varies compared with a case where the dielectric film  15  is not provided. Consequently, it is possible to reduce or prevent an influence of unwanted bulk waves on frequency characteristics and possible to reduce or prevent ripples in the frequency characteristics. 
     Bulk acoustic waves (BAW) such as those in the acoustic wave device in the present preferred embodiment may be applied to a filter device such as that illustrated in  FIG.  13   . In this case, as in the third preferred embodiment, the first electrode film and the second electrode film may be wiring electrode films that are connected to different acoustic wave resonators. 
     Hereinafter, details of the thickness shear mode and plate waves will be described. The support member in the following example corresponds to the support substrate. 
       FIG.  18 A  is a schematic perspective view illustrating an appearance of an acoustic wave device that uses bulk waves of the thickness shear mode, and  FIG.  18 B  is a plan view illustrating an electrode structure on a piezoelectric layer.  FIG.  19    is a sectional view of a portion along line A-A in  FIG.  18 A . 
     An acoustic wave device  1  includes a piezoelectric layer  2  made of, for example, LiNbO 3 . The piezoelectric layer  2  may be made of, for example, LiTaO 3 . The cut-angle of LiNbO 3  or LiTaO 3  is, for example, a Z-cut and, however, may be a rotated Y-cut or an X-cut. The thickness of the piezoelectric layer  2  is not particularly limited. To effectively excite the thickness shear mode, the thickness of the piezoelectric layer  2  is, for example, preferably about 40 nm or more and about 1000 nm or less and more preferably about 50 nm or more and about 600 nm or less. The piezoelectric layer  2  includes a first main surface  2   a  and a second main surface  2   b  facing each other. An electrode  3  and an electrode  4  are provided on the first main surface  2   a . Here, the electrode  3  is one example of the “first electrode”, and the electrode  4  is one example of the “second electrode”. In  FIGS.  18 A and  18 B , a plurality of the electrodes  3  are connected to a first busbar  5 . A plurality of the electrodes  4  are connected to a second busbar  6 . The plurality of electrodes  3  and the plurality of electrodes  4  are interdigitated with each other. The electrodes  3  and the electrodes  4  each have a rectangular or substantially rectangular shape and have a length direction. In a direction orthogonal or substantially orthogonal to the length direction, the electrodes  3  and the electrodes  4  adjacent to the electrodes  3  face each other. The length direction of the electrodes  3  and  4  and the direction orthogonal or substantially orthogonal to the length direction of the electrodes  3  and  4  are each a direction intersecting with the thickness direction of the piezoelectric layer  2 . Therefore, the electrodes  3  and the electrodes  4  adjacent to the electrodes  3  are considered to face each other in the direction intersecting with the thickness direction of the piezoelectric layer  2 . The length direction of the electrodes  3  and  4  may be replaced with the direction orthogonal or substantially orthogonal to the length direction of the electrodes  3  and  4  illustrated in  FIGS.  18 A and  18 B . In other words, in  FIGS.  18 A and  18 B , the electrodes  3  and  4  may be extended in a direction in which the first busbar  5  and the second busbar  6  extend. In such a case, the first busbar  5  and the second busbar  6  extend in the direction in which the electrodes  3  and  4  extend in  FIGS.  18 A and  18 B . A plurality of pairs of structures in each of which one electrode  3  connected to one electric potential and one electrode  4  connected to the other electric potential are adjacent to each other are provided in the direction orthogonal or substantially orthogonal to the length direction of the electrodes  3  and  4 . Here, “one electrode  3  and one electrodes  4  are adjacent to each other” denotes a case where the electrode  3  and the electrode  4  are disposed with a gap therebetween, not a case where the electrode  3  and the electrode  4  are disposed in direct contact with each other. In the case where one electrode  3  and one electrode  4  are adjacent to each other, electrodes, including the other electrodes  3  and  4 , connected to the hot electrode and the ground electrode are not disposed between the one electrode  3  and the one electrode  4 . The number of pairs of one electrode  3  and one electrode  4  is not necessarily an integer and may be, for example, 1.5, 2.5, or the like. The distance, in other words, the pitch between the centers of the electrode  3  and the electrode  4  is preferably, for example, in a range of about 1 μm or more and about 10 μm or less. The width of the electrodes  3  and  4 , in other words, the dimension of the electrodes  3  and  4  in the facing direction is, for example, preferably in a range of about 50 nm or more and about 1000 nm or less and more preferably in a range of about 150 nm or more and about 1000 nm or less. The distance between the centers of the electrodes  3  and  4  is a distance that connects the center of the dimension (width dimension) of the electrode  3  in the direction orthogonal or substantially orthogonal to the length direction of the electrode  3  and the center of the dimension (width dimension) of the electrode  4  in the direction orthogonal or substantially orthogonal to the length direction of the electrode  4 . 
     In the acoustic wave device  1 , a piezoelectric layer of a Z-cut is used. The direction orthogonal or substantially orthogonal to the length direction of the electrodes  3  and  4  is thus a direction orthogonal or substantially orthogonal to the polarization direction of the piezoelectric layer  2 . The above shall not apply to a case where a piezoelectric body of another cut angle is used as the piezoelectric layer  2 . Here, “orthogonal” is not limited only to strictly orthogonal and may be substantially orthogonal (an angle formed by the direction orthogonal to the length direction of the electrodes  3  and  4  and the polarization direction may be, for example, in a range of about 90°±10°). 
     A support member  8  is laminated on the side of the second main surface  2   b  of the piezoelectric layer  2  with an electrically insulating layer  7  interposed therebetween. The electrically insulating layer  7  and the support member  8  each have a frame shape and, as illustrated in  FIG.  19   , include a through hole  7   a  and a through hole  8   a , respectively. A cavity portion  9  is thus provided. The cavity portion  9  is provided so that the vibration of the excitation regions C of the piezoelectric layer  2  is not hindered. Therefore, the support member  8  is laminated, at a position not overlapping a portion in which at least one pair of the electrodes  3  and  4  is provided, on the second main surface  2   b  with the electrically insulating layer  7  interposed therebetween. The electrically insulating layer  7  is not necessarily provided. Accordingly, the support member  8  can be laminated on the second main surface  2   b  of the piezoelectric layer  2  directly or indirectly. 
     The electrically insulating layer  7  is made of, for example, silicon oxide. However, an appropriate electrically insulative material other than silicon oxide, such as, for example, silicon nitride or alumina, may be used. The support member  8  is made of, for example, Si. The orientation of Si at a surface on the side of the piezoelectric layer  2  may be ( 100 ), may be ( 110 ), and may be ( 111 ). The Si of the support member  8  preferably has a high resistance with resistivity of about 2 kΩ, or more and more preferably has a high resistance with resistivity of about 4 kΩ, or more, for example. The support member  8 , however, also may be made of, for example, an appropriate electrically insulative material or a semiconductor material. 
     The plurality of electrodes  3  and  4 , and the first busbar  5 , and the second busbar  6  described above are made of an appropriate metal or alloy, such as, for example, Al or a AlCu alloy. In the present preferred embodiment, the electrodes  3  and  4 , the first busbar  5 , and the second busbar  6  each have a structure in which, for example, a Al film is laminated on a Ti film. A close contact layer other than the Ti film may be used. 
     An alternating-current voltage is applied for driving between the plurality of electrodes  3  and the plurality of electrodes  4 . More specifically, the alternating-current voltage is applied between the first busbar  5  and the second busbar  6 . It is thus possible to obtain resonance characteristics using bulk waves of the thickness shear mode excited at the piezoelectric layer  2 . In the acoustic wave device  1 , d/p is, for example, about 0.5 or less, where d is the thickness of the piezoelectric layer  2  and p is a distance between the centers of any mutually adjacent electrodes  3  and  4  of the plurality of pairs of the electrodes  3  and  4 . Therefore, the bulk waves of the thickness shear mode are effectively excited, and favorable resonance characteristics can be obtained. More preferably, d/p is, for example, about 0.24 or less. In this case, more favorable resonance characteristics can be obtained. 
     Since the acoustic wave device  1  has the above-described configuration, the Q-value does not easily decrease even when the number of pairs of the electrodes  3  and  4  is reduced for size reduction of the acoustic wave device  1 . This is because propagation loss is small even when the number of the electrode fingers of the reflectors on both sides is reduced. Reducing the number of the electrode fingers is enabled by using the bulk waves of the thickness shear mode. A difference between Lamb waves and the bulk waves of the thickness shear mode used in an acoustic wave device will be described with reference to  FIGS.  20 A and  20 B . 
       FIG.  20 A  is a schematic elevational cross-sectional view for describing Lamb waves propagating through a piezoelectric film of an acoustic wave device such as that described in Japanese Unexamined Patent Application Publication No. 2012-257019. In  FIG.  20 A , waves propagate as indicated by the arrows through a piezoelectric film  201 . Here, in the piezoelectric film  201 , a first main surface  201   a  and a second main surface  201   b  face each other, and a thickness direction connecting the first main surface  201   a  and the second main surface  201   b  to each other is the Z-direction. The X-direction is a direction in which electrode fingers of an IDT electrode are disposed side by side. As illustrated in  FIG.  20 A , waves of the Lamb waves propagate in the X-direction as illustrated. Since the waves are plate waves, the piezoelectric film  201  vibrates as a whole while the waves propagate in the X-direction. Therefore, resonance characteristics are obtained by disposing reflectors on both sides. Consequently, propagation loss of the waves occurs, and the Q-value decreases when a size reduction is performed, in other words, when the number of pairs of the electrode fingers is reduced. 
     In contrast, as illustrated in  FIG.  20 B , vibration displacement in the acoustic wave device  1  is a thickness shear direction. Waves thus propagate substantially in a direction connecting the first main surface  2   a  and the second main surface  2   b  of the piezoelectric layer  2  to each other, in other words, in the Z-direction, and resonance occurs. That is, the X-direction component of the waves is much smaller than the Z-direction component of the waves. Since resonance characteristics are obtained due to the propagation of the waves in the Z-direction, propagation loss does not easily occur even when the number of the electrode fingers of reflectors is reduced. Further, even when the number of pairs of electrodes including the electrodes  3  and  4  is reduced to address a size reduction, the Q-value does not easily decrease. 
     As illustrated in  FIG.  21   , the amplitude direction of the bulk waves of the thickness shear mode in a first region  451  included in the excitation regions C of the piezoelectric layer  2  and the amplitude direction thereof in a second region  452  included in the excitation regions C are opposite each other. In  FIG.  21   , bulk waves when a voltage that causes the electrode  4  to have a higher potential than the electrode  3  is applied between the electrode  3  and the electrode  4  are schematically illustrated. The first region  451  is a region that is included in the excitation regions C and that is between the first main surface  2   a  and an imaginary plane VP 1  orthogonal or substantially orthogonal to the thickness direction of the piezoelectric layer  2  and dividing the piezoelectric layer  2  into two. The second region  452  is a region that is included in the excitation regions C and that is between the imaginary plane VP 1  and the second main surface  2   b.    
     As described above, although at least one pair of electrodes including one electrode  3  and one electrode  4  is disposed at the acoustic wave device  1 , the pair of electrodes is not for causing waves to propagate in the X-direction. The number of pairs of the electrodes including the electrodes  3  and  4  is thus not necessarily a plural number. In other words, it is sufficient that at least one pair of the electrodes is provided. 
     For example, the electrode  3  is an electrode connected to the hot potential, and the electrode  4  is an electrode connected to the ground potential. The electrode  3 , however, may be connected to the ground potential while the electrode  4  is connected to the hot potential. In the present preferred embodiment, the at least one pair of electrodes is, as described above, electrodes connected to the hot potential or the ground potential, and no floating electrode is provided. 
       FIG.  22    shows resonance characteristics of the acoustic wave device illustrated in  FIG.  19   . Design parameters of the acoustic wave device  1  with which the resonance characteristics are obtained are as follows. 
     Piezoelectric layer  2 : LiNbO 3  having Euler angles (0°, 0°, 90°); thickness=about 400 nm 
     The length of a region in which the electrode  3  and the electrode  4  overlap each other when viewed in a direction orthogonal to the length direction of the electrode  3  and the electrode  4 , that is, the length of the excitation region C=about 40 μm; the number of pairs of electrodes consisting of the electrode  3  and the electrode  4 = 21  pairs; the distance between the centers of the electrodes=about 3 μm; the width of the electrodes  3  and  4 =about 500 nm; d/p=about 0.133 
     Electrically insulating layer  7 : A silicon oxide film having a thickness of about 1 μm 
     Support member  8 : Si 
     The length of the excitation region C is a dimension extending in the length direction of the electrodes  3  and  4  in the excitation region C. 
     In the present preferred embodiment, the distance between electrodes of one pair of electrodes including one electrode  3  and one electrode  4  is equal or substantially equal among all of a plurality of the pairs. In other words, the electrodes  3  and the electrodes  4  are disposed at equal or substantially equal pitches. 
     As clearly shown in  FIG.  22   , favorable resonance characteristics in which the fractional bandwidth is about 12.5% are obtained despite the absence of reflectors. 
     Meanwhile, as described above, d/p is about 0.5 or less and more preferably about 0.24 or less, where d is the thickness of the piezoelectric layer  2  and p is the distance between the centers of the electrode  3  and the electrode  4 , in the present preferred embodiment. This will be described with reference to  FIG.  23   . 
     A plurality of acoustic wave devices are obtained similarly, while d/2p is varied, to the acoustic wave device with which the resonance characteristics shown in  FIG.  22    are obtained.  FIG.  23    shows a relationship between d/2p thereof and the fractional bandwidths as resonators of the acoustic wave devices. 
     As clearly shown in  FIG.  23   , when d/2p exceeds about 0.25, that is, d/p&gt;about 0.5, the fractional bandwidth is less than about 5% even when d/p is adjusted. In contrast, when d/2p≤about 0.25, that is, d/p≤about 0.5, it is possible to cause the fractional bandwidth to be about 5% or more by changing d/p in the range, in other words, it is possible to configure a resonator having a high coupling coefficient. When d/2p is about 0.12 or less, that is, d/p is about 0.24 or less, it is possible to increase the fractional bandwidth to about 7% or more. In addition, by adjusting d/p in this range, it is possible to obtain a resonator in which the fractional bandwidth is wider, and possible to achieve a resonator having a higher coupling coefficient. Accordingly, it was discovered that, by setting d/p to about 0.5 or less, as in the fourth preferred embodiment of the present application, it is possible to configure a resonator that uses bulk waves of the thickness shear mode and that has a high coupling coefficient. 
     As described above, the at least one pair of the electrodes may be one pair. 
     When, for example, the piezoelectric layer  2  has a variation in thickness, a value obtained by averaging the thicknesses may be used. 
       FIG.  24    is a plan view of an acoustic wave device that uses bulk waves of the thickness shear mode. In an acoustic wave device  80 , one pair of electrodes including one electrode  3  and one electrode  4  is provided on the first main surface  2   a  of the piezoelectric layer  2 . In  FIG.  24   , K is an intersection width. As described above, the number of pairs of the electrodes may be one in the acoustic wave device according to the present invention. In this case, it is also possible when d/p is about 0.5 or less to effectively excite the bulk waves of the thickness shear mode. 
       FIG.  25    is a graph showing a map of the fractional bandwidth with respect to Euler angles (0°, θ, ψ) of LiNbO 3  when d/p is approximated to zero as closely as possible. Hatched portions in  FIG.  11    are regions in which the fractional bandwidth of at least 5% or more is obtained. When the range of the regions is approximated, the range is represented by the following expressions (1), (2), and (3). 
       (0°±10°,0° to 20°, optional ψ)  expression (1)
 
       (0°±10°,20° to 80°,0° to 60(1−(0−50) 2 /900) 1/2 ) or (0°±10°,20° to 80°,[180°−60(1−(0−50) 2 /900) 1/2 ] to 180°)   expression (2)
 
       (0±10,[180°−30(1−(ψ−90) 2 /8100) 1/2 ] to 180°, optional ψ)  expression (3)
 
     Accordingly, in a case of the Euler angle range of the expressions (1), (2), or (3), the fractional bandwidth can be sufficiently widened, which is preferable. 
       FIG.  26    is a partially omitted perspective view for describing an acoustic wave device that uses Lamb waves. The dashed line in  FIG.  26    indicates the position of the cavity portion  9  as viewed from the side of a piezoelectric layer  83 . 
     An acoustic wave device  81  includes a support substrate  82 . The support substrate  82  includes a recessed portion that opens on the upper surface thereof. The piezoelectric layer  83  is laminated on the support substrate  82 . The cavity portion  9  is thus provided. Above the cavity portion  9 , an IDT electrode  84  is provided on the piezoelectric layer  83 . Reflectors  85  and  86  are provided on both sides of the IDT electrode  84  in an acoustic wave propagation direction. In  FIG.  26   , the outer peripheral edge of the cavity portion  9  is indicated by a dashed line. Here, the IDT electrode  84  includes first and second busbars  84   a  and  84   b , electrodes  84   c  as a plurality of first electrode fingers, and electrodes  84   d  as a plurality of second electrode fingers. The plurality of electrodes  84   c  are connected to the first busbar  84   a . The plurality of electrodes  84   d  are connected to the second busbar  84   b . The plurality of electrodes  84   c  and the plurality of electrodes  84   d  are interdigitated with each other. 
     In the acoustic wave device  81 , an alternating-current electric field is applied to the IDT electrode  84  above the cavity portion  9 , thereby exciting Lamb waves as plate waves. Since the reflectors  85  and  86  are provided on both sides, it is possible to obtain the resonance characteristics due to the Lamb waves. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.