Patent Publication Number: US-2022216845-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-178100 filed on Sep. 27, 2019 and is a Continuation Application of PCT Application No. PCT/JP2020/036397 filed on Sep. 25, 2020. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an acoustic wave device, and more particularly to an acoustic wave device including a piezoelectric layer. 
     2. Description of the Related Art 
     There has been known an acoustic wave device using a plate wave propagating through a piezoelectric film made of LiNbO 3  or LiTaO 3 . For example, Japanese Unexamined Patent Application Publication No. 2012-257019 discloses an acoustic wave device using a Lamb wave as the plate wave. Here, an IDT electrode is provided on an upper surface of a piezoelectric substrate made of LiNbO 3  or LiTaO 3 . A voltage is applied between multiple first electrode fingers and multiple second electrode fingers of the IDT electrode. Thus, a Lamb wave is excited. Each side of the IDT electrode is provided with one reflector. Thus, an acoustic wave resonator using a plate wave is formed. 
     SUMMARY OF THE INVENTION 
     With respect to the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2012-257019, it is conceivable to decrease the number of first electrode fingers and second electrode fingers in order to reduce the size thereof. However, when the number of first electrode fingers and second electrode fingers is decreased, a Q factor decreases. Further, since a support body  10  has a wall portion, it is hard to reduce in size. 
     Preferred embodiments of the present invention provide acoustic wave devices each capable of increasing a Q factor and maintaining strength even when the size thereof is further reduced. 
     An acoustic wave device according to an aspect of a preferred embodiment of the present invention includes a piezoelectric layer, a first electrode, and a second electrode. The first electrode and the second electrode face each other in a direction intersecting with a thickness direction of the piezoelectric layer. The acoustic wave device uses a thickness shear primary mode bulk wave. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The acoustic wave device includes multiple electrode portions including the first electrode and the second electrode. The acoustic wave device further includes a first divided resonator, a second divided resonator, and a support substrate. The first divided resonator and the second divided resonator are connected in series without another resonator being connected therebetween, or are connected in parallel with each other to the same connection node on a path connecting an input terminal and an output terminal. The support substrate includes a first main surface and a second main surface opposed to each other. The first divided resonator includes a first installation portion. The first installation portion includes a first electrode portion of the multiple electrode portions and a first region of the piezoelectric layer. The first region is provided with the first electrode portion. The second divided resonator includes a second installation portion. The second installation portion includes a second electrode portion of the multiple electrode portions and a second region of the piezoelectric layer. The second region is provided with the second electrode portion. The piezoelectric layer is directly or indirectly provided on the support substrate. The support substrate includes a first energy confinement layer and a second energy confinement layer. The first energy confinement layer at least partially overlaps with the first region of the piezoelectric layer in a plan view from the thickness direction of the piezoelectric layer. The second energy confinement layer at least partially overlaps with the second region of the piezoelectric layer in a plan view from the thickness direction of the piezoelectric layer. In the support substrate, the first energy confinement layer and the second energy confinement layer are integrally provided. 
     An acoustic wave device according to an aspect of a preferred embodiment of the present invention includes the piezoelectric layer, the first electrode, and the second electrode. The first electrode and the second electrode face each other in a direction intersecting with a thickness direction of the piezoelectric layer. The first electrode and the second electrode are adjacent to each other. In any section along the thickness direction of the piezoelectric layer, when a distance between center lines of the first electrode and the second electrode is denoted as p and a thickness of the piezoelectric layer is denoted as d, d/p is about 0.5 or less. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The acoustic wave device includes multiple electrode portions including the first electrode and the second electrode. The acoustic wave device further includes a first divided resonator, a second divided resonator, and a support substrate. The first divided resonator and the second divided resonator are connected in series without another resonator being connected therebetween, or are connected in parallel with each other to a same connection node on a path connecting an input terminal and an output terminal. The support substrate includes a first main surface and a second main surface opposed to each other. The first divided resonator includes a first installation portion. The first installation portion includes a first electrode portion of the multiple electrode portions and a first region of the piezoelectric layer. The first region is provided with the first electrode portion. The second divided resonator includes a second installation portion. The second installation portion includes a second electrode portion of the multiple electrode portions and a second region of the piezoelectric layer. The second region is provided with the second electrode portion. The piezoelectric layer is directly or indirectly provided on the support substrate. The support substrate includes a first energy confinement layer and a second energy confinement layer. The first energy confinement layer at least partially overlaps with the first region of the piezoelectric layer in a plan view from the thickness direction of the piezoelectric layer. The second energy confinement layer at least partially overlaps with the second region of the piezoelectric layer in a plan view from the thickness direction of the piezoelectric layer. In the support substrate, the first energy confinement layer and the second energy confinement layer are integrally provided. 
     With the use of the acoustic wave devices according to the above-described aspects of preferred embodiments of the present invention, the Q factor may be increased even when the size is further reduced, and it is possible to further reduce the size when the first divided resonator and the second divided resonator are formed. 
     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 Preferred Embodiment 1 of the present invention. 
         FIG. 2  is a sectional view of the acoustic wave device taken along a line A 1 -A 1  in  FIG. 1 . 
         FIG. 3  is an equivalent circuit diagram of the acoustic wave device. 
         FIG. 4  is a plan view of a resonator included in the acoustic wave device. 
         FIG. 5  is a plan view of a main portion of the resonator in the acoustic wave device. 
         FIG. 6A  is an explanatory diagram of a Lamb wave.  FIG. 6B  is an explanatory diagram of a thickness shear primary mode bulk wave. 
         FIG. 7  is a sectional view of the acoustic wave device taken along a line A 2 -A 2  in  FIG. 4 . 
         FIG. 8  is an explanatory diagram of the operation of the acoustic wave device. 
         FIG. 9  is an explanatory diagram of a structural model of an acoustic wave device according to a reference mode. 
         FIG. 10A  is a graph of a relationship between the fractional bandwidth of a thickness shear mode and [thickness of piezoelectric layer]/[distance between center lines of first electrode and second electrode], in the structural model of the acoustic wave device.  FIG. 10B  is a graph of a relationship between the fractional bandwidth of the thickness shear mode and [thickness of piezoelectric layer]/[distance between center lines of two electrodes defining a pair], in the structural model of the acoustic wave device. The range of 0 to about 0.2 on the horizontal axis of  FIG. 10A  is enlarged and illustrated in  FIG. 10B . 
         FIG. 11  is a graph of a relationship between the fractional bandwidth of the thickness shear mode and a normalized spurious level in the structural model of the acoustic wave device. 
         FIG. 12  is an impedance-frequency characteristics diagram of the structural model of the acoustic wave device. 
         FIG. 13  is a diagram to explain a distribution of the fractional bandwidth against the combination of [thickness of piezoelectric layer]/[distance between center lines of first electrode and second electrode] and a structural parameter, in the structural model of the acoustic wave device. 
         FIG. 14  is a plan view of an acoustic wave device according to Modification 1 of Preferred Embodiment 1 of the present invention. 
         FIG. 15  is a sectional view of an acoustic wave device according to Modification 2 of Preferred Embodiment 1 of the present invention. 
         FIG. 16  is a plan view of an acoustic wave device according to Modification 4 of Preferred Embodiment 1 of the present invention. 
         FIG. 17  is a plan view of an acoustic wave device according to Modification 5 of Preferred Embodiment 1 of the present invention. 
         FIG. 18  is a sectional view of an acoustic wave device according to Preferred Embodiment 2 of the present invention. 
         FIG. 19  is a plan view of the acoustic wave device according to Preferred Embodiment 2 of the present invention. 
         FIG. 20  is a plan view of an acoustic wave device according to Modification 2 of Preferred Embodiment 2 of the present invention. 
         FIGS. 21A to 21D  are sectional views illustrating other shapes of a pair of electrodes of the acoustic wave device. 
         FIGS. 22A to 22C  are sectional views illustrating other configuration examples of the acoustic wave device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     All of  FIG. 1 ,  FIG. 2 ,  FIG. 4  to  FIG. 9 ,  FIG. 14  to  FIG. 20 ,  FIGS. 21A to 21D , and  FIGS. 22A to 22C  referred to in the following preferred embodiments or the like are schematic diagrams, and ratios of the sizes or the thicknesses of elements or portions in the figures do not necessarily reflect actual dimensional ratios. 
     Preferred Embodiment 1 
     Hereinafter, an acoustic wave device  1  according to Preferred Embodiment 1 will be described with reference to  FIG. 1  to  FIG. 5 . 
     (1) Overall Configuration of Acoustic Wave Device 
     The acoustic wave device  1  according to Preferred Embodiment 1 includes a piezoelectric layer  4 , a first electrode  51 , and a second electrode  52  as illustrated in  FIG. 1 . The first electrode  51  and the second electrode  52  face each other in a direction D 2  (hereinafter, also referred to as second direction D 2 ) intersecting with a thickness direction (first direction) D 1  of the piezoelectric layer  4  as illustrated in  FIG. 2 . The acoustic wave device  1  uses a thickness shear primary mode bulk wave. The second direction D 2  is orthogonal to polarization directions PZ 1  and PZ 2  of the piezoelectric layer  4 . The thickness shear primary mode bulk wave is a bulk wave whose propagation direction is the thickness direction D 1  of the piezoelectric layer  4  due to a thickness shear vibration of the piezoelectric layer  4 , and in which the number of nodes in the thickness direction D 1  of the piezoelectric layer  4  is one. The thickness shear vibration is excited by the first electrode  51  and the second electrode  52 . The thickness shear vibration is excited in the defined region  45  between the first electrode  51  and the second electrode  52  in a plan view from the thickness direction D 1  in the piezoelectric layer  4 . In the acoustic wave device  1 , when the second direction D 2  is orthogonal to the polarization directions PZ 1  and PZ 2  of the piezoelectric layer  4 , the electromechanical coupling coefficient (hereinafter, also referred to as coupling coefficient) of the thickness shear primary mode bulk wave is large. Here, “orthogonal” is not limited only to a case of being strictly orthogonal, but may be a case of being substantially orthogonal (an angle defined by the second direction D 2  and the polarization directions PZ 1  and PZ 2  is about 90°±10°, for example). 
     The acoustic wave device  1  includes multiple pairs of the first electrode  51  and the second electrode  52  as illustrated in  FIG. 1  and  FIG. 2 . In the acoustic wave device  1 , the multiple first electrodes  51  and the multiple second electrodes  52  are alternately arranged one by one in the second direction D 2 . The acoustic wave device  1  further includes a first wiring portion  61  connected to the first electrode  51  and a second wiring portion  62  connected to the second electrode  52  as illustrated in  FIG. 1 . The multiple first electrodes  51  are commonly connected to the first wiring portion  61 . The multiple second electrodes  52  are commonly connected to the second wiring portion  62 . 
     The acoustic wave device  1  includes a support substrate  2 , the piezoelectric layer  4 , the multiple first electrodes  51 , and the multiple second electrodes  52  as illustrated in  FIG. 2 . The piezoelectric layer  4  is provided on the support substrate  2 . As an example, the piezoelectric layer  4  is provided on the support substrate  2  via a silicon oxide film  7 . The multiple first electrodes  51  and the multiple second electrodes  52  are provided on the piezoelectric layer  4 . The acoustic wave device  1  includes an acoustic wave resonator  5  including the first electrode  51 , the second electrode  52 , and the piezoelectric layer  4 , as a resonator. The support substrate  2  includes at least a portion of the cavity  26  that partially exposes the piezoelectric layer  4 . The cavity  26  overlaps with the entire acoustic wave resonator  5  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . Here, the cavity  26  overlaps with the multiple first electrodes  51 , the multiple second electrodes  52 , and the multiple defined regions  45  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . Each of the multiple defined regions  45  is a portion between the first electrode  51  and the second electrode  52  defining a pair. 
     The acoustic wave device  1  according to Preferred Embodiment 1 is an acoustic wave filter (here, a ladder filter) as illustrated in  FIG. 1  and  FIG. 3 . The acoustic wave device  1  includes an input terminal  15 , an output terminal  16 , multiple (two) series-arm resonators RS 1 , and multiple (two) parallel-arm resonators RS 2 . The multiple (two) series-arm resonators RS 1  are provided on a first path  12  connecting the input terminal  15  and the output terminal  16 . The multiple (two) parallel-arm resonators RS 2  are respectively provided one by one on multiple (two) second paths  13  and  14  connecting multiple (two) nodes N 1  and N 2  on the first path  12  and ground (ground terminals  17  and  18 ). The ground terminals  17  and  18  may be made common as one ground. 
     Each of the multiple series-arm resonators RS 1  includes the first divided resonator RS 3  and the second divided resonator RS 4 . The first divided resonator RS 3  and the second divided resonator RS 4  are connected in series. The first divided resonator RS 3  and the second divided resonator RS 4  (multiple divided resonators) are resonators obtained by dividing the series-arm resonator RS 1 , and are connected in a line without any of the parallel-arm resonators RS 2  being connected therebetween. The number of divided resonators is not limited to two, and may be three or more. 
     Note that, in each of the multiple series-arm resonators RS 1 , the first divided resonator RS 3  and the second divided resonator RS 4  may be connected in parallel. In the case above, the first divided resonator RS 3  and the second divided resonator RS 4  are connected in parallel with each other to a connection node on the first path  12  connecting the input terminal  15  and the output terminal  16 . 
     Further, each of the multiple parallel-arm resonators RS 2  may include a first divided resonator and a second divided resonator. In each of the multiple parallel-arm resonators RS 2 , the first divided resonator and the second divided resonator are connected in series or in parallel. 
     In the acoustic wave device  1 , each of the first divided resonator RS 3 , the second divided resonator RS 4 , and the multiple parallel-arm resonators RS 2  is the acoustic wave resonator  5 . The first divided resonator RS 3  and the second divided resonator RS 4  are included in the multiple series-arm resonators RS 1 . Each of the multiple acoustic wave resonators  5  is a resonator including the multiple first electrodes  51  and the multiple second electrodes  52 , but is not limited thereto. It is sufficient that each of the multiple acoustic wave resonators  5  is a resonator including at least a pair of electrodes (first electrode  51  and second electrode  52 ). In the acoustic wave device  1 , the piezoelectric layer  4  is shared by the multiple acoustic wave resonators  5 . The resonant frequency of the parallel-arm resonator RS 2  is lower than that of the series-arm resonator RS 1 . Here, whereas the acoustic wave resonator  5  of the parallel-arm resonator RS 2  includes a silicon oxide film provided on a first main surface  41  of the piezoelectric layer  4 , for example, the acoustic wave resonator  5  of the series-arm resonator RS 1  does not include a silicon oxide film on the first main surface  41  of the piezoelectric layer  4 . The acoustic wave resonator  5  of the series-arm resonator RS 1  may include a silicon oxide film on the first main surface  41  of the piezoelectric layer  4 . In the case above, it is required that the thickness of the silicon oxide film of the acoustic wave resonator  5  of the series-arm resonator RS 1  is thinner than the thickness of the silicon oxide film of the acoustic wave resonator  5  of the parallel-arm resonator RS 2 . 
     (2) Elements of Acoustic Wave Device 
     Next, elements of the acoustic wave device  1  will be described with reference to the drawings. 
     (2.1) Support Substrate 
     The support substrate  2  supports the piezoelectric layer  4  as illustrated in  FIG. 2 . In the acoustic wave device  1  according to Preferred Embodiment 1, the support substrate  2  supports the piezoelectric layer  4 , the multiple first electrodes  51 , and the multiple second electrodes  52  via the silicon oxide film  7 . Note that, the silicon oxide film  7  is not an essential element. Further, another layer in addition to the silicon oxide film  7  may be laminated between the support substrate  2  and the piezoelectric layer  4 . 
     The support substrate  2  includes a first main surface  21  and a second main surface  22  opposed to each other. The first main surface  21  and the second main surface  22  are opposed to each other in the thickness direction of the support substrate  2 . The thickness direction of the support substrate  2  is a direction along the thickness direction D 1  of the piezoelectric layer  4 . The outer peripheral shape of the support substrate  2  is a rectangular shape in a plan view from the thickness direction D 1  of the piezoelectric layer  4 , but is not limited thereto, and may be a square shape, for example. 
     The support substrate  2  is a silicon substrate, for example. The thickness of the support substrate  2  is about 100 μm or more and about 500 μm or less, for example. The support substrate  2  is preferably a single-crystal silicon substrate including a first main surface and a second main surface opposed to each other. When the support substrate  2  is a single-crystal silicon substrate, the plane orientation of the first main surface  21  may be a ( 100 ) plane, a ( 110 ) plane, or a ( 111 ) plane, for example. The propagation orientation of the bulk wave described above may be determined without being restricted by the plane orientation of the single-crystal silicon substrate. The resistivity of the single-crystal silicon substrate is about 1 kΩ·cm or more, for example, but is preferably about 2 kΩ·cm or more, and more preferably about 4 kΩcm or more, for example. 
     The support substrate  2  is not limited to a silicon substrate, but may be a quartz substrate, a glass substrate, a sapphire substrate, a lithium tantalate substrate, a lithium niobate substrate, an alumina substrate, a spinel substrate, a gallium arsenide substrate, or a silicon carbide substrate, for example. 
     The support substrate  2  includes at least a portion of the cavity  26  that partially exposes the piezoelectric layer  4 . The cavity  26  overlaps with the acoustic wave resonator  5  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1  according to Preferred Embodiment 1, the cavity  26  is larger than the acoustic wave resonator  5  and overlaps with the entire acoustic wave resonator in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . Further, in the acoustic wave device  1  according to Preferred Embodiment 1, the cavity  26  also partially overlaps with the first wiring portion  61  and the second wiring portion  62  respectively in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . The opening shape of the cavity in a plan view from the thickness direction D 1  of the piezoelectric layer  4  is a rectangular shape, but is not limited thereto. 
     (2.2) Silicon Oxide Film 
     The silicon oxide film  7  is provided between the first main surface  21  of the support substrate  2  and the piezoelectric layer  4 . In the acoustic wave device  1  according to Preferred Embodiment 1, the silicon oxide film  7  overlaps with the entire first main surface  21  of the support substrate  2  in the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1  according to Preferred Embodiment 1, the support substrate  2  and the piezoelectric layer  4  are bonded to each other via the silicon oxide film  7 . The thickness of the silicon oxide film  7  is about 0.1 μm or more and about 10 μm or less, for example. 
     (2.3) Piezoelectric Layer 
     The piezoelectric layer  4  has the first main surface  41  and a second main surface  42  opposed to each other as illustrated in  FIG. 2 . The first main surface  41  and the second main surface  42  are opposed to each other in the thickness direction D 1  of the piezoelectric layer  4 . The piezoelectric layer  4  is provided on the first main surface  21  of the support substrate  2 . Here, the piezoelectric layer  4  overlaps with the first main surface  21  of the support substrate  2  and the cavity  26  in a plan view from the thickness direction D 1 . In the piezoelectric layer  4 , of the first main surface  41  and the second main surface  42 , the first main surface  41  is positioned on the first electrode  51  and the second electrode  52  side, and the second main surface  42  is positioned on the support substrate  2  side. The first main surface  41  of the piezoelectric layer  4  is the main surface of the piezoelectric layer  4  opposite to the support substrate  2  side. The second main surface  42  of the piezoelectric layer  4  is the main surface of the piezoelectric layer  4  on the support substrate  2  side. 
     In the acoustic wave device  1 , the distance between the first main surface  41  of the piezoelectric layer  4  and the support substrate  2  is longer than the distance between the second main surface  42  of the piezoelectric layer  4  and the support substrate  2 . The material of the piezoelectric layer  4  is lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). The piezoelectric layer  4  is Z-cut LiNbO 3  or Z-cut LiTaO 3 , for example. With respect to Euler angles (φ, θ, ψ) of the piezoelectric layer  4 , φ is 0°±10° and θ is 0°±10°. ψ is an angle of any value. From the viewpoint of increasing the coupling coefficient, the piezoelectric layer  4  is preferably Z-cut LiNbO 3  or Z-cut LiTaO 3 . The piezoelectric layer  4  may be rotated Y-cut LiNbO 3 , rotated Y-cut LiTaO 3 , X-cut LiNbO 3 , or X-cut LiTaO 3 . The propagation orientation may be a Y-axis direction, an X-axis direction, or a direction rotated within the range of ±90° from the X-axis, in a crystal axis (X, Y, Z) defined to the crystal structure of the piezoelectric layer  4 . The piezoelectric layer  4  is a single-crystal, but is not limited thereto, and may be a twin-crystal or ceramics, for example. 
     The thickness of the piezoelectric layer  4  is about 50 nm or more and about 1000 nm or less, for example, and an example thereof is about 400 nm. 
     The piezoelectric layer  4  has the defined region  45  (see  FIG. 5 ). In a plan view from the thickness direction D 1  of the piezoelectric layer  4 , the defined region  45  is the region of the piezoelectric layer  4  that overlaps with both of the first electrode  51  and the second electrode  52  in the direction D 2  in which the first electrode  51  and the second electrode  52  face each other, and is positioned between the first electrode  51  and the second electrode  52 . 
     (2.4) Electrode 
     The multiple first electrodes  51  and the multiple second electrodes  52  are provided on the first main surface  41  of the piezoelectric layer  4 . 
     In the acoustic wave device  1 , the first electrode  51  and the second electrode  52  defining a pair have electric potential different from each other. In the acoustic wave device  1 , of the first electrode  51  and the second electrode  52  defining a pair, the first electrode  51  is a hot electrode and the second electrode  52  is a ground electrode. 
     In the acoustic wave device  1 , the multiple first electrodes  51  and the multiple second electrodes  52  are alternately arranged one by one to be separated from each other. Accordingly, the first electrode  51  and the second electrode  52  adjacent to each other are separated from each other. The distance between the center lines of the first electrode  51  and the second electrode  52  adjacent to each other is about 1 μm or more and about 10 μm or less, for example, an example thereof is about 3 μm. Here, the first electrode  51  and the second electrode  52  being “adjacent to each other” refers to a case that the first electrode  51  and the second electrode  52  face each other with a gap therebetween. A group of electrodes including the multiple first electrodes  51  and the multiple second electrodes  52  may have a configuration as follows. The multiple first electrodes  51  and the multiple second electrodes  52  are arranged side by side to be separated from each other in the second direction D 2 , or the multiple first electrodes and the multiple second electrodes  52  are not alternately arranged side by side to be separated from each other. For example, there may be mixed a region in which the first electrode  51  and the second electrode  52  are arranged one by one to be separated from each other, and a region in which two first electrodes  51  or two second electrodes  52  are arranged side by side in the second direction D 2 . 
     In a plan view from the thickness direction D 1  of the piezoelectric layer  4 , the multiple first electrodes  51  and the multiple second electrodes  52  have an elongated shape (linear shape) with a third direction D 3  orthogonal to the second direction D 2  as a longitudinal direction and the second direction D 2  as a width direction, as illustrated in  FIG. 4 . The length of each of the multiple first electrodes  51  is about 20 μm, for example, but is not limited thereto. A width H 1  (first electrode width H 1 ) of each of the multiple first electrodes  51  is in the range from about 50 nm to about 1000 nm, for example, and an example thereof is about 500 nm. The length of each of the multiple second electrodes  52  is about 20 μm, for example, but is not limited thereto. A width H 2  (second electrode width H 2 ) of each of the multiple second electrodes  52  is in the range from about 50 nm to about 1000 nm, for example, and an example thereof is about 500 nm. 
     Each of the multiple first electrodes  51  includes a first electrode main portion  510 . The first electrode main portion  510  is a portion of the first electrode  51  that overlaps with the second electrode  52  in the direction in which the first electrode  51  and the second electrode  52  face each other. Further, each of the multiple second electrodes  52  includes a second electrode main portion  520 . The second electrode main portion  520  is a portion of the second electrode  52  that overlaps with the first electrode  51  in the direction in which the first electrode  51  and the second electrode  52  face each other. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the first electrode widths H 1  of the multiple first electrodes  51  are the same, but are not limited thereto, and may be different from each other. Further, in the acoustic wave device  1  according to Preferred Embodiment 1, the second electrode widths H 2  of the multiple second electrodes  52  are the same, but are not limited thereto, and may be different from each other. In the acoustic wave device  1  according to Preferred Embodiment 1, the first electrode width H 1  and the second electrode width H 2  are the same, but are not limited thereto, and the first electrode width H 1  and the second electrode width H 2  may be different from each other. 
     With respect to the acoustic wave device  1  according to Preferred Embodiment 1, each of the number of first electrodes  51  and the number of second electrodes  52  is illustrated as five in  FIG. 4 . However, the number of each of the first electrodes  51  and the second electrodes  52  is not limited to five, and may be one, two to four, six or more, or fifty or more. 
     The second direction D 2  in which the first electrode  51  and the second electrode  52  face each other is preferably orthogonal to the polarization directions PZ 1  and PZ 2  (see  FIG. 2 ) of the piezoelectric layer  4 , but is not limited thereto. For example, when the piezoelectric layer  4  is not a Z-cut piezoelectric body, the first electrode  51  and the second electrode  52  may face each other in a direction orthogonal to the third direction D 3  being the longitudinal direction. Note that, there is a case that the first electrode  51  and the second electrode  52  do not have a rectangular shape. In the case above, in a plan view of the first electrode  51  and the second electrode  52 , the third direction D 3  being the longitudinal direction may be a direction of a long side of a circumscribed polygon circumscribing the first electrode  51  and the second electrode  52 . Note that, when the first wiring portion  61  and the second wiring portion  62  are connected to the first electrode  51  and the second electrode  52 , the “circumscribed polygon circumscribing the first electrode and the second electrode  52 ” at least includes a polygon circumscribing the portion of the first electrode  51  and the second electrode  52  excluding the portion connected to the first wiring portion  61  or the second wiring portion  62 . 
     Each of the multiple first electrodes  51  includes a first main surface  511  and a second main surface  512  intersecting with the thickness direction D 1  of the piezoelectric layer  4 , as illustrated in  FIG. 7 . In each of the multiple first electrodes  51 , of the first main surface  511  and the second main surface  512 , the second main surface  512  is positioned on the first main surface  41  side of the piezoelectric layer  4 , and is in planar contact with the first main surface  41  of the piezoelectric layer  4 . 
     Each of the multiple second electrodes  52  includes a first main surface  521  and a second main surface  522  intersecting with the thickness direction D 1  of the piezoelectric layer  4 . In each of the multiple second electrodes  52 , of the first main surface  521  and the second main surface  522 , the second main surface  522  is positioned on the first main surface  41  side of the piezoelectric layer  4 , and is in planar contact with the first main surface  41  of the piezoelectric layer  4 . 
     The multiple first electrodes  51  and the multiple second electrodes  52  have electrical conductivity. The material of each first electrode  51  and each second electrode  52  is Al (aluminum), Cu (copper), Pt (platinum), Au (gold), Ag (silver), Ti (titanium), Ni (nickel), Cr (chromium), Mo (molybdenum), W (tungsten), an alloy containing any of these metals as a main component, or the like, for example. Further, each first electrode  51  and each second electrode  52  may have a structure in which multiple metal films made of these metals or alloys are laminated. Each first electrode  51  and each second electrode  52  include a laminated film including an adhesion film made of a Ti film, and a main electrode film made of an Al film or an AlCu film formed on the adhesion film, for example. The thickness of the adhesion film is about 10 nm, for example. Further, the thickness of the main electrode film is about 80 nm, for example. In the AlCu film, the concentration of Cu is preferably about 1 wt % or more and about 20 wt % or less. 
     (2.5) First Wiring Portion and Second Wiring Portion 
     The first wiring portion  61  includes a first busbar  611 , as illustrated in  FIG. 4 . The first busbar  611  is a conductor portion for making the multiple first electrodes  51  have the same electric potential. The first busbar  611  has an elongated shape (linear shape) whose longitudinal direction is the second direction D 2 . The first busbar  611  is connected to the multiple first electrodes  51 . The multiple first electrodes  51  connected to the first busbar  611  extend toward a second busbar  621 . In the acoustic wave device  1 , the first conductor portion including the multiple first electrodes  51  and the first busbar  611  has a comb-like shape in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . The first busbar  611  is integrally provided with the multiple first electrodes  51 , but is not limited thereto. 
     The second wiring portion  62  includes the second busbar  621 . The second busbar  621  is a conductor portion for making the multiple second electrodes  52  have the same electric potential. The second busbar  621  has an elongated shape (linear shape) whose longitudinal direction is the second direction D 2 . The second busbar  621  is connected to the multiple second electrodes  52 . The multiple second electrodes  52  connected to the second busbar  621  extend toward the first busbar  611 . In the acoustic wave device  1 , the second conductor portion including the multiple second electrodes  52  and the second busbar  621  has a comb-like shape in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . The second busbar  621  is integrally provided with the multiple second electrodes  52 , but is not limited thereto. 
     The first busbar  611  and the second busbar  621  face each other in the third direction D 3 . 
     The first wiring portion  61  and the second wiring portion  62  have electrical conductivity. The material of the first wiring portion  61  and the second wiring portion  62  is Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, an alloy including any of these metals as a main component, or the like, for example. Further, the first wiring portion  61  and the second wiring portion  62  may have a structure in which multiple metal films made of these metals or alloys are laminated. The first wiring portion  61  and the second wiring portion  62  include a laminated film of an adhesion film made of a Ti film, and a main wiring film made of an Al film or an AlCu film formed on the adhesion film, for example. The thickness of the adhesion film is about 10 nm, for example. Further, the thickness of the main wiring film is about 80 nm, for example. In the AlCu film, the concentration of Cu is preferably about 1 wt % or more and about 20 wt % or less. 
     In the acoustic wave device  1 , from the viewpoint of reducing the resistance of the first busbar  611  and the second busbar  621 , each of the first busbar  611  and the second busbar  621  may include a metal film on the main wiring film. Further, the thickness of each of the first wiring portion  61  and the second wiring portion  62  may be thicker than the thickness of each of the first electrode  51  and the second electrode  52 . 
     (2.6) First Divided Resonator and Second Divided Resonator 
     The first divided resonator RS 3  includes a first installation portion  401 . The first installation portion  401  includes a first electrode portion  501  of multiple electrode portions  50  and a first region  451  of the piezoelectric layer  4 . The first region  451  is provided with the first electrode portion  501 . 
     The second divided resonator RS 4  includes a second installation portion  402 . The second installation portion  402  includes a second electrode portion  502  of the multiple electrode portions  50  and a second region  452  of the piezoelectric layer  4 . The second region  452  is provided with the second electrode portion  502 . 
     The support substrate  2  includes multiple cavities  26 . The multiple cavities  26  include a first cavity  26   a  and a second cavity  26   b.  Here, the first cavity  26   a  is an example of a “first energy confinement layer”, and the second cavity  26   b  is an example of a “second energy confinement layer”. The first cavity  26   a  at least partially exposes the first region  451  of the piezoelectric layer  4 . The second cavity  26   b  at least partially exposes the second region  452  of the piezoelectric layer  4 . In the support substrate  2 , the first cavity  26   a  and the second cavity  26   b  are integrally provided. That is, a member such as a wall portion is not present between the first cavity  26   a  and the second cavity  26   b.    
     In a plan view from the thickness direction D 1 , the first cavity  26   a  overlaps with the first electrode  51  and the second electrode  52  of the first electrode portion  501 , and a portion of the piezoelectric layer  4  between the first electrode  51  and the second electrode  52  of the first electrode portion  501 . In a plan view from the thickness direction D 1 , the second cavity  26   b  overlaps with the first electrode  51  and the second electrode  52  of the second electrode portion  502 , and a portion of the piezoelectric layer  4  between the first electrode  51  and the second electrode  52  of the second electrode portion  502 . In other words, in a plan view from the thickness direction D 1  of the piezoelectric layer  4 , the first cavity  26   a  overlaps with the first region  451  on which the first electrode portion  501  is provided, and the second cavity  26   b  overlaps with the second region  452  on which the second electrode portion  502  is provided. In  FIG. 2 , the first cavity  26   a  overlaps with the entire first region  451  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 , and the second cavity  26   b  overlaps with the entire second region  452  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . However, it is sufficient that the first cavity  26   a  at least partially overlaps with the first region  451 , and the second cavity  26   b  at least partially overlaps with the second region  452 . Further, it is not necessary that the cavity  26  partially overlaps with each of the first wiring portion  61  and the second wiring portion  62  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     Furthermore, it is not required that the first region  451  is at least partially exposed and the second region  452  is at least partially exposed, as long as the first cavity  26   a  overlaps with the first region  451  and the second cavity  26   b  overlaps with the second region  452  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . Specifically, it is allowed that a dielectric film laminated on the first region  451  of the piezoelectric layer  4  is present between the first region  451  and the first cavity  26   a,  and a dielectric film laminated on the second region  452  of the piezoelectric layer  4  is present between the second region  452  and the second cavity  26   b.    
     Incidentally, a thickness d 1  of the first divided resonator RS 3  and a thickness d 2  of the second divided resonator RS 4  are different from each other. The thickness d 1  of the first divided resonator RS 3  is the thickness of the first installation portion  401 , and is the thickness of the piezoelectric layer  4  in the first region  451 , here. Note that, the thickness of the first installation portion  401  may be the total thickness being the sum of the thickness of the first electrode portion  501  included in the first installation portion  401  and the thickness of the piezoelectric layer  4  in the first region  451 . Also, the thickness of the first installation portion  401  may be the thickness of only the first electrode portion  501  included in the first installation portion  401  or the thickness of only the piezoelectric layer  4  in the first region  451 . In a case that an insulation layer or the like is provided on the first main surface  41  of the piezoelectric layer  4  or the second main surface  42  of the piezoelectric layer  4  in the first region  451 , the thickness of the first installation portion  401  may be the thickness of only the insulation layer, or the total thickness being the sum of the thickness of the first electrode portion  501 , the thickness of the piezoelectric layer  4  in the first region  451 , and the thickness of the insulation layer in the first region  451 . The thickness of the second divided resonator RS 4  is the thickness of the second installation portion  402 , and is the thickness of the piezoelectric layer  4  in the second region  452 , here. Note that, the thickness of the second installation portion  402  may be the total thickness being the sum of the thickness of the second electrode portion  502  included in the second installation portion  402  and the thickness of the piezoelectric layer  4  in the second region  452 . Also, the thickness of the second installation portion  402  may be the thickness of only the second electrode portion  502  included in the second installation portion  402  or the thickness of only the piezoelectric layer  4  in the second region  452 . Further, in a case that such as an insulation layer is provided on the first main surface  41  of the piezoelectric layer  4  or the second main surface  42  of the piezoelectric layer  4  in the second region  452 , the thickness of the second installation portion  402  may be the thickness of only the insulation layer, or the total thickness being the sum of the thickness of the second electrode portion  502 , the thickness of the piezoelectric layer  4  in the second region  452 , and the thickness of the insulation layer in the second region  452 . That is, the case that the thickness d 1  of the first divided resonator RS 3  and the thickness d 2  of the second divided resonator RS 4  are different from each other includes not only a case that the total thickness of the first installation portion  401  and the total thickness of the second installation portion  402  are different from each other, but also a case that the total thickness of the first installation portion  401  and the total thickness of the second installation portion  402  are the same, and the thickness of the element included in the first installation portion  401  and the thickness of the element included in the second installation portion  402  are different from each other. With this, the resonant frequency of the ripple generated in the first divided resonator RS 3  and the resonant frequency of the ripple generated in the second divided resonator RS 4  may be made different from each other. In other words, it is possible to disperse the unnecessary wave ripples by making the resonant frequency of the unnecessary wave of the first electrode portion  501  and the resonant frequency of the unnecessary wave of the second electrode portion  502  different from each other. 
     Further, the polarity of the first divided resonator RS 3  and the polarity of the second divided resonator RS 4  are different from each other. In Preferred Embodiment 1, the polarization direction PZ 1  of the piezoelectric layer  4  in the first divided resonator RS 3  and the polarization direction PZ 2  of the piezoelectric layer  4  in the second divided resonator RS 4  are different from each other. With this, linearity may be improved. 
     (3) Method of Manufacturing Acoustic Wave Device 
     In the method of manufacturing the acoustic wave device  1 , the first to fifth processes are performed after the support substrate  2  is prepared, for example. In the first process, the silicon oxide film  7  is provided on the first main surface  21  of the support substrate  2 . In the second process, the piezoelectric substrate is defined by the piezoelectric layer  4  and the support substrate  2  being bonded to each other via the silicon oxide film  7 . In the third process, the piezoelectric layer  4  made of part of the piezoelectric substrate is formed by thinning the piezoelectric substrate. In the fourth process, the multiple first electrodes  51 , the multiple second electrodes  52 , the first wiring portion  61 , and the second wiring portion  62  are formed on the first main surface  41  of the piezoelectric layer  4 . In the fifth process, the cavity  26  is formed from the second main surface  22  of the support substrate  2 . In the fourth process described above, the first electrode  51 , the second electrode  52 , the first wiring portion  61 , and the second wiring portion  62  are formed by using such as a photolithography technique, an etching technique, or a thin film forming technique. Further, in the fifth process described above, a region of the support substrate  2  where the cavity  26  is to be formed is etched using such as a photolithography technique or an etching technique. In the fifth process, the support substrate  2  is etched using the silicon oxide film  7  as an etching stopper layer, and then the unnecessary portion of the silicon oxide film  7  is removed by etching to partially expose the second main surface  42  of the piezoelectric layer  4 . When preparing the single-crystal silicon substrate, a single-crystal silicon wafer is prepared, and in the second process, a piezoelectric wafer is used as the piezoelectric substrate. In the method of manufacturing the acoustic wave device  1 , a wafer including the multiple acoustic wave devices  1  is cut with a dicing machine to obtain the multiple acoustic wave devices  1  (chips). 
     The method of manufacturing the acoustic wave device  1  is an example and is not particularly limited. For example, the piezoelectric layer  4  may be formed using a film formation technique. In the case above, the method of manufacturing the acoustic wave device  1  includes a process of forming the piezoelectric layer  4  instead of the third process and the fourth process. The piezoelectric layer  4  formed by the film formation technique may be a single-crystal or a twin-crystal, for example. The film formation technique is a CVD (Chemical Vapor Deposition) method, for example, but is not limited thereto. Further, the piezoelectric layer  4  is subjected to poling treatment. 
     (4) Operation and Characteristics of Acoustic Wave Device 
     The acoustic wave device  1  according to Preferred Embodiment 1 is an acoustic wave device using the thickness shear primary mode bulk wave. As described above, the thickness shear primary mode bulk wave is a bulk wave whose propagation direction is the thickness direction D 1  of the piezoelectric layer  4  due to the thickness shear vibration of the piezoelectric layer  4 , and is a bulk wave in which the number of nodes in the thickness direction D 1  of the piezoelectric layer  4  is one. The thickness shear vibration is excited by the first electrode  51  and the second electrode  52 . The thickness shear vibration is excited in the defined region  45  between the first electrode  51  and the second electrode  52  in a plan view from the thickness direction D 1  in the piezoelectric layer  4 . The thickness shear vibration can be confirmed using FEM (Finite Element Method), for example. More particularly, the thickness shear vibration can be confirmed by the analysis of strain based on the analysis of a displacement distribution using FEM, for example. The analysis uses such as parameters of the piezoelectric layer  4  (such as material, Euler angles, thickness), and parameters of the first electrode  51  and the second electrode  52  (such as material, thickness, distance between center lines of the first electrode  51  and the second electrode  52 ). The Euler angles of the piezoelectric layer  4  can be obtained by analysis. 
     Here, the difference between the Lamb wave used in the acoustic wave device in the past and the thickness shear primary mode bulk wave will be described with reference to  FIG. 6A  and  FIG. 6B . 
       FIG. 6A  is a schematic elevational cross-sectional view for explaining a Lamb wave propagating through the piezoelectric substrate  460  of the acoustic wave device described in Patent Document 1. In the acoustic wave device above, a wave propagates through the piezoelectric substrate  460  as indicated by an arrow. Here, the piezoelectric substrate  460  includes a first main surface  461  and a second main surface  462  opposed to each other. In  FIG. 6A , the Z-direction and the X-direction are illustrated separately from the piezoelectric substrate  460 . In  FIG. 6A , the Z-direction is a thickness direction connecting the first main surface  461  and the second main surface  462  of the piezoelectric substrate  460 . The X-direction is a direction in which multiple first electrode fingers and multiple second electrode fingers of an IDT electrode are arranged side by side. In the Lamb wave, the acoustic wave is a plate wave propagating in the X-direction as illustrated in  FIG. 6A . Since an acoustic wave propagates in the X-direction, two reflectors are disposed one by one on both sides of an IDT electrode to obtain desired resonant characteristics in a conventional acoustic wave device. Therefore, in a conventional acoustic wave device, since propagation loss of an acoustic wave occurs, when the size is reduced, that is, when the number of pairs of the first electrode finger and the second electrode finger is reduced, the Q factor decreases. 
     Meanwhile, in the acoustic wave device according to a reference mode, since the vibration displacement is in the thickness shear direction, the acoustic wave substantially propagates in the direction connecting the first main surface  41  and the second main surface  42  of the piezoelectric layer  4 , that is, in the Z-direction, and resonates, as illustrated in  FIG. 6B . That is, the X-direction component of the acoustic wave is remarkably smaller than the Z-direction component. In the acoustic wave device according to the reference mode, since resonant characteristics are obtained by the propagation of a wave in the Z-direction, a reflector is not necessarily required. Therefore, in the acoustic wave device according to the reference mode, no propagation loss occurs when a wave propagates to the reflector. With this, in the acoustic wave device according to the reference mode, even when the number of electrode pairs includes the first electrode  51  and the second electrode  52  is decreased in order to further reduce the size, a decrease in Q factor is unlikely to occur. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the amplitude directions of the thickness shear primary mode bulk wave are opposite to each other in the first region  451  and the second region  452  that are included in the defined region  45  of the piezoelectric layer  4 , as illustrated in  FIG. 8 . In  FIG. 8 , a dashed-and-double-dotted line schematically indicates a bulk wave when a voltage, in which electric potential of the second electrode  52  is higher than that of the first electrode  51 , is applied between the first electrode  51  and the second electrode  52 . The first region  451  is a region of the defined region  45  between the first main surface  41  and a virtual plane VP 1  that is orthogonal to the thickness direction D 1  of the piezoelectric layer  4  and divides the piezoelectric layer  4  into two. The second region  452  is a region of the defined region  45  between the second main surface  42  and the virtual plane VP 1 . 
     A structural model  1   r  (see  FIG. 9 ) of the acoustic wave device according to the reference mode using a thickness shear primary mode bulk wave was simulated to obtain characteristics. With respect to the structural model  1   r,  the same elements as those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference signs, and a description thereof will be omitted. 
     The structural model  1   r  differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that the first wiring portion  61  and the second wiring portion  62  are not included. In the simulation, the number of pairs of the first electrode  51  and the second electrode  52  was infinite, and the piezoelectric layer  4  was 120° rotated Y-cut X-propagation LiNbO 3 . 
     In the structural model  1   r,  the piezoelectric layer  4  is a membrane, and the second main surface  42  of the piezoelectric layer  4  is in contact with air. In the structural model  1   r,  in any section ( FIG. 9 ) along the thickness direction D 1  of the piezoelectric layer  4 , the distance between the center lines of the first electrode  51  and the second electrode  52  is denoted as p, and the thickness of the piezoelectric layer  4  is denoted as d. Further, in the structural model  1   r,  in a plan view from the thickness direction D 1  of the piezoelectric layer  4 , the area of the first electrode main portion  510  is denoted as S 1 , the area of the second electrode main portion  520  is denoted as S 2 , the area of the defined region  45  is denoted as S 0 , and a structural parameter defined by (S 1 +S 2 )/(S 1 +S 2 +S 0 ) is denoted as MR. Note that, when at least one of the first electrode  51  and the second electrode  52  is provided in multiple in the piezoelectric layer  4  (in other words, when the first electrode  51  and the second electrode  52  make an electrode pair, 1.5 or more electrode pairs are provided in the piezoelectric layer  4 ), distances p between the center lines are each the distance between the center lines of the adjacent first electrode  51  and the second electrode  52 . 
       FIGS. 10A and 10B  are graphs illustrating the relationship between the fractional bandwidth and d/p when the first electrode  51  and the second electrode  52  are applied electric potential different from each other in the structural model  1   r . In  FIGS. 10A and 10B , the horizontal axis represents d/p and the vertical axis represents the fractional bandwidth.  FIGS. 10A and 10B  illustrate a case that the piezoelectric layer  4  is 120° rotated Y-cut X-propagation LiNbO 3 , but the same tendency is observed in the cases of other cut-angles. Further, in the structural model  1   r  of the acoustic wave device, even when the material of the piezoelectric layer  4  is LiTaO 3 , the relationship between the fractional bandwidth and d/p has the same tendency as that in  FIGS. 10A and 10B . Furthermore, in the structural model  1   r  of the acoustic wave device, regardless of the number of pairs of the first electrode  51  and the second electrode  52 , the relationship between the fractional bandwidth and d/p has the same tendency as that in  FIGS. 10A and 10B . Still further, in the structural model  1   r  of the acoustic wave device, when the second main surface  42  of the piezoelectric layer  4  is in contact with not only air but also an acoustic reflection layer, the relationship between the fractional bandwidth and d/p has the same tendency as that in  FIGS. 10A and 10B . 
     It can be seen in  FIG. 10A  that, in the structural model  1   r  of the acoustic wave device, the value of the fractional bandwidth drastically changes with d/p=about 0.5 as an inflection point. In the structural model  1   r  of the acoustic wave device, when d/p&gt;about 0.5, the coupling coefficient is low and the fractional bandwidth is less than about 5%, no matter how much d/p is changed within the range of about 0.5&lt;d/p&lt;about 1.6. Meanwhile, in the structural model  1   r  of the acoustic wave device, when d/p ≤about 0.5, it is possible to increase the coupling coefficient and make the fractional bandwidth 5% or more, by changing d/p within the range of 0&lt;d/p≤about 0.5. 
     Further, in the structural model  1   r  of the acoustic wave device, when d/p≤about 0.24, it is possible to further increase the coupling coefficient and make the fractional bandwidth still larger, by changing d/p within the range of 0&lt;d/p≤about 0.24. Also in the acoustic wave device  1  according to Preferred Embodiment 1, the relationship between the fractional bandwidth and d/p thereof has the same tendency as the relationship between the fractional bandwidth and d/p in the structural model  1   r  of the acoustic wave device, when a distance between center lines of the first electrode  51  and the second electrode  52  defining a pair is denoted as p, and the thickness of the piezoelectric layer  4  is denoted as d in any section along the thickness direction D 1  of the piezoelectric layer  4 , as illustrated in  FIG. 7 . 
     Furthermore, as is clear in  FIG. 10A , when d/p≤about 0.10, it is possible to further increase the coupling coefficient and make the fractional bandwidth still larger, by changing d/p within the range of 0&lt;d/p≤about 0.10. 
       FIG. 10B  is a graph obtained by enlarging a portion of  FIG. 10A . As seen in  FIG. 10B , since the fractional bandwidth changes with d/p=about 0.096 as an inflection point, when d/p≤about 0.096, it is possible to further increase the coupling coefficient and make the fractional bandwidth still larger in comparison with the case of about 0.096&lt;d/p, by changing d/p within the range of 0&lt;d/p≤about 0.096. Further, as seen in  FIG. 10B , since the fractional bandwidth changes with d/p=about 0.072 and about 0.048 as inflection points, when about 0.048≤d/p about 0.072, it is possible to suppress the change of the coupling coefficient due to the change of d/p and to make the fractional bandwidth a substantially constant value. 
       FIG. 11  is a graph plotting spurious levels in a frequency band between the resonant frequency and the anti-resonant frequency in the structural model r 1  of the acoustic wave device of the reference mode using the thickness shear mode. The spurious levels are plotted changing the thickness d of the piezoelectric layer  4 , the distance p between the center lines of the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2 . In  FIG. 11 , the horizontal axis represents the fractional bandwidth and the vertical axis represents the normalized spurious level. The normalized spurious level is a value in which the spurious level is normalized with the spurious level at a fractional bandwidth (about 22%, for example) as 1. In the fractional bandwidth, the spurious level has the same value even when the thickness d of the piezoelectric layer  4 , the first electrode width H 1 , and the second electrode width H 2  are changed.  FIG. 11  illustrates a case that Z-cut LiNbO 3  capable of more suitably exciting the thickness shear mode is preferably used as the piezoelectric layer  4 , but the same tendency is observed in the cases of other cut-angles. Further, in the structural model  1   r  of the acoustic wave device, when the material of the piezoelectric layer  4  is LiTaO 3 , the relationship between the normalized spurious level and the fractional bandwidth has the same tendency as that in  FIG. 11 . Furthermore, in the structural model  1   r  of the acoustic wave device, regardless of the number of pairs of the first electrode  51  and the second electrode  52 , the relationship between the normalized spurious level and the fractional bandwidth has the same tendency as that in  FIG. 11 . Still further, in the structural model  1   r  of the acoustic wave device, when the second main surface  42  of the piezoelectric layer  4  is in contact with not only air but also an acoustic reflection layer, the relationship between the normalized spurious level and the fractional bandwidth has the same tendency as that in  FIG. 11 . 
     It can be seen in  FIG. 11  that when the fractional bandwidth exceeds about 17%, the normalized spurious level is aggregated to 1. This indicates that, when the fractional bandwidth is about 17% or more, some kind of sub-resonance is present in the band between the resonant frequency and the anti-resonant frequency as in the frequency characteristics of impedance exemplified in  FIG. 12 .  FIG. 12  illustrates the frequency characteristics of impedance when Z-cut LiNbO 3  having Euler angles (0°, 0°, 90°) is preferably used as the piezoelectric layer  4 , d/p=about 0.08, and MR=about 0.35. In  FIG. 12 , a portion of the sub-resonance is surrounded by a broken line. 
     As described above, when the fractional bandwidth exceeds about 17%, even when the thickness d of the piezoelectric layer  4 , the distance p between the center lines of the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2  are changed, large spurious components are included in the band between the resonant frequency and the anti-resonant frequency. The spurious components above are generated by an overtone in a planar direction, mainly in the direction in which the first electrode  51  and the second electrode  52  face each other. Therefore, from the viewpoint of suppressing the spurious components in the band, the fractional bandwidth is preferably about 17% or less. Since the acoustic wave device  1  according to Preferred Embodiment 1 exhibits the same tendency as that of the structural model  1   r  of the acoustic wave device with respect to the relationship between the normalized spurious level and the fractional bandwidth, the fractional bandwidth is preferably about 17% or less. 
       FIG. 13  illustrates a first distribution region DA 1  having the fractional bandwidth of more than about 17% and a second distribution region DA 2  having the fractional bandwidth of about 17% or less in the structural model  1   r  of the acoustic wave device with d/p and MR used as parameters. In  FIG. 13 , Z-cut LiNbO 3  is used as the piezoelectric layer  4 , and changed are the thickness d of the piezoelectric layer  4 , the distance p between the center lines of the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2 . In  FIG. 13 , the first distribution region DA 1  and the second distribution region DA 2  have different dot densities, and the dot density of the first distribution region DA 1  is higher than the dot density of the second distribution region DA 2 . Further, in  FIG. 13 , an approximate straight line DL 1  as a boundary line between the first distribution region DA 1  and the second distribution region DA 2  is indicated by a broken line. The approximate straight line DL 1  is expressed by the equation of MR =about 1.75×(d/p)+0.075. Therefore, in the structural model  1   r  of the acoustic wave device, the fractional bandwidth is likely to be about 17% or less by satisfying the condition of MR about 1.75×(d/p)+0.075.  FIG. 13  illustrates a case that Z-cut LiNbO 3  capable of more suitably exciting the thickness shear mode is preferably used as the piezoelectric layer  4 , but the same tendency is observed in the cases of other cut-angles. Further, in the structural model  1   r  of the acoustic wave device, even when the material of the piezoelectric layer  4  is LiTaO 3 , the approximate straight line DL 1  is the same. Furthermore, in the structural model  1   r  of the acoustic wave device, regardless of the number of pairs of the first electrode  51  and the second electrode  52 , the approximate straight line DL 1  is the same. Still further, in the structural model  1   r  of the acoustic wave device, when the second main surface  42  of the piezoelectric layer  4  is in contact with not only air but also an acoustic reflection layer, the approximate straight line DL 1  is the same. In the acoustic wave device  1  according to Preferred Embodiment 1, similarly to the structural model  1   r  of the acoustic wave device, the fractional bandwidth is likely to be about 17% or less by satisfying the condition of MR about 1.75×(d/p)+0.075. Note that, in  FIG. 13 , an approximate straight line DL 2  (hereinafter, also referred to as second approximate straight line DL 2 ) indicated by a dashed-and-dotted line separately from the approximate straight line DL 1  (hereinafter, also referred to as first approximate straight line DL 1 ) is a line indicating a boundary for surely making the fractional bandwidth about 17% or less. The second approximate straight line DL 2  is expressed by the equation of MR=about 1.75×(d/p)+0.05. Therefore, in the structural model  1   r  of the acoustic wave device according to the reference mode and the acoustic wave device  1  according to Preferred Embodiment 1, the fractional bandwidth may surely be made 17% or less by satisfying the condition of MR about 1.75×(d/p)+0.05. 
     (5) Effect 
     The acoustic wave device  1  according to Preferred Embodiment 1 includes the piezoelectric layer  4 , the first electrode  51 , and the second electrode  52 , and uses a thickness shear primary mode bulk wave. Further, the acoustic wave device  1  includes the first divided resonator RS 3  and the second divided resonator RS 4 . Furthermore, the first cavity  26   a  at least partially overlapping with the first region  451  of the piezoelectric layer  4  and the second cavity  26   b  at least partially overlapping with the second region  452  of the piezoelectric layer  4  are provided in the support substrate  2 , when viewed from the thickness direction of the piezoelectric layer  4 . Further, in the support substrate  2 , the first cavity  26   a  and the second cavity  26   b  are integrally provided. With this, the Q factor may be increased even when the size is further reduced, and it is possible to further reduce the size when the first divided resonator RS 3  and the second divided resonator RS 4  are provided. 
     The acoustic wave device  1  according to Preferred Embodiment 1 includes the piezoelectric layer  4 , the first electrode  51 , and the second electrode  52 . With respect to the distance p between the center lines of the first electrode  51  and the second electrode  52 , and the thickness d of the piezoelectric layer  4 , d/p is about 0.5 or less, for example. Further, the acoustic wave device  1  includes the first divided resonator RS 3  and the second divided resonator RS 4 . Furthermore, the first cavity  26   a  at least partially overlapping with the first region  451  of the piezoelectric layer  4  and the second cavity  26   b  at least partially overlapping with the second region  452  of the piezoelectric layer  4  are provided in the support substrate  2 , when viewed from the thickness direction D 1  of the piezoelectric layer  4 . Further, in the support substrate  2 , the first cavity  26   a  and the second cavity  26   b  are integrally provided. With this, the Q factor may be increased even when the size is further reduced, and it is possible to further reduce the size when the first divided resonator RS 3  and the second divided resonator RS 4  are provided. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the thickness d 1  of the first divided resonator RS 3  and the thickness d 2  of the second divided resonator RS 4  are different from each other. With this, the ripples may be dispersed. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the polarity of the first divided resonator RS 3  and the polarity of the second divided resonator RS 4  are different from each other. With this, the linearity may further be improved. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, d/p is about 0.24 or less. With this, it is possible to make the fractional bandwidth still larger. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, in a plan view from the thickness direction D 1  of the piezoelectric layer  4 , when the area of the first electrode main portion is denoted as S 1 , the area of the second electrode main portion is denoted as S 2 , the area of the defined region is denoted as S 0 , and a structural parameter defined by (S 1 +S 2 )/(S 1  +S 2 +S 0 ) is denoted as MR, where MR satisfies MR 1.75×(d/p) +0.075. With this, it is possible to suppress the spurious components in the band. 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the electric potential of the first electrode  51  and the electric potential of the second electrode  52  are different from each other. With this, it is possible to prevent the first electrode  51  and the second electrode  52  defining a pair from taking floating electric potential. 
     Modification  1  of Preferred Embodiment 1 
     Hereinafter, an acoustic wave device la according to Modification  1  will be described with reference to  FIG. 14 . With respect to the acoustic wave device la according to Modification  1 , the same elements as those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference signs, and a description thereof will be omitted. 
     The acoustic wave device la according to Modification  1  differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that each of the multiple acoustic wave resonators  5  further includes two reflectors  8 . 
     Each of the two reflectors  8  is a short-circuited grating. Each reflector  8  does not reflect a bulk wave in the primary shear mode but reflects an unnecessary surface acoustic wave propagating along the first main surface  41  of the piezoelectric layer  4 . One reflector  8  of the two reflectors  8  is positioned on the side of an end first electrode  51  opposite to the second electrode  52  side. The end first electrode  51  is positioned at the end of the multiple first electrodes  51  in the direction along the propagation direction of the unnecessary surface acoustic wave of the acoustic wave device la. The remaining one reflector  8  of the two reflectors  8  is positioned on the side of an end second electrode opposite to the first electrode  51  side. The end second electrode  52  is positioned at the end of the multiple second electrodes  52  in the direction along the propagation direction of the unnecessary surface acoustic wave of the acoustic wave device  1   a.    
     Each reflector  8  has multiple (four, for example) electrode fingers  81 . One ends of the multiple electrode fingers  81  are short-circuited to each other, and the other ends are short-circuited to each other. In each reflector  8 , the number of electrode fingers  81  is not particularly limited. 
     Each reflector  8  has electrical conductivity. The material of each reflector  8  is Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, an alloy including any one of these metals as a main component, or the like, for example. Further, each reflector  8  may have a structure in which a multiple metal films made of these metals or alloys are laminated. Each reflector  8  includes a laminated film including an adhesion film made of a Ti film on the piezoelectric layer  4 , and a main electrode film made of an Al film on the adhesion film, for example. The thickness of the adhesion film is about 10 nm, for example. Further, the thickness of the main electrode film is about 80 nm, for example. 
     In the acoustic wave device la according to Modification  1 , each reflector  8  is a short-circuited grating, but is not limited thereto. Each reflector  8  may be an open grating, a positive-negative reflection grating, or a grating in which a short-circuited grating and an open grating are combined, for example. Further, in the acoustic wave device la, each acoustic wave resonator  5  includes the two reflectors  8 , but may include only one of the two reflectors  8 . 
     Modification 2 of Preferred Embodiment 1 
     In Modification 2 of Preferred Embodiment 1, as illustrated in  FIG. 15 , the first electrode  51  and the second electrode  52  in the second divided resonator RS 4  may be provided on the side opposite to the first electrode  51  and the second electrode  52  in the first divided resonator RS 3 . In the example of  FIG. 15 , the first electrode  51  and the second electrode  52  in the first divided resonator RS 3  are provided on the first main surface  41  of the piezoelectric layer  4 , and the first electrode  51  and the second electrode  52  in the second divided resonator RS 4  are provided on the second main surface  42  of the piezoelectric layer  4 . In Modification 2, the polarization direction PZ 1  of the piezoelectric layer  4  in the first divided resonator RS 3  is the same as the polarization direction PZ 2  of the piezoelectric layer  4  in the second divided resonator RS 4 . With this, the linearity may further be improved. 
     Modification 3 of Preferred Embodiment 1 
     In Modification 3 of Preferred Embodiment 1, the distance p between the center lines in the first divided resonator RS 3  and the distance p between the center lines in the second divided resonator RS 4  may be different from each other. With this, the ripples may be dispersed without affecting the main resonance. 
     Modification 4 of Preferred Embodiment 1 
       FIG. 1  illustrates a configuration in which the series-arm resonators (first divided resonator RS 3  and second divided resonator RS 4 ) overlap with the same cavity  26  in a plan view. However, as Modification 4 of Preferred Embodiment 1, the series-arm resonator RS 1  and the parallel-arm resonator RS 2  may overlap with the same cavity  26  in a plan view, as illustrated in  FIG. 16 . 
     Modification 5 of Preferred Embodiment 1 
     In Modification 5 of Preferred Embodiment 1, as illustrated in  FIG. 17 , the first divided resonator RS 3  and the second divided resonator RS 4  may have a configuration as follows. The second busbar  621  of the pair of busbars (first busbar  611  and second busbar  621 ) included in the first divided resonator RS 3 , and the first busbar  611  of the pair of busbars (first busbar  611  and second busbar  621 ) included in the second divided resonator RS 4  may be a shared inter-stage busbar  63 . 
     Preferred Embodiment 2 
     Hereinafter, an acoustic wave device  1   b  according to Preferred Embodiment 2 will be described with reference to  FIG. 18  and  FIG. 19 . With respect to the acoustic wave device  1   b  according to Preferred Embodiment 2, the same elements as those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference signs, and the description thereof will be omitted. 
     (1) Configuration of Acoustic Wave Device 
     The acoustic wave device  1   b  according to Preferred Embodiment 2 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that an acoustic reflection layer  3  is provided between the support substrate  2  and the piezoelectric layer  4 , as illustrated in  FIG. 18 . 
     In the acoustic wave device  1   b  according to Preferred Embodiment 2, the acoustic reflection layer  3  is provided on the first main surface  21  of the support substrate  2 , and the piezoelectric layer  4  is provided on the acoustic reflection layer  3 . In the acoustic wave device  1   b , the acoustic wave resonator  5  includes at least a pair of electrodes (first electrode  51  and second electrode  52 ) and the piezoelectric layer  4 . In the acoustic wave device  1   b , the acoustic wave resonator  5  further has the acoustic reflection layer  3  described above. 
     The acoustic reflection layer  3  includes a first acoustic reflection layer  3   a  overlapping with the first region  451  and a second acoustic reflection layer  3   b  overlapping with the second region  452  in a plan view in the thickness direction of the piezoelectric layer  4 . The first acoustic reflection layer  3   a  is an example of the “first energy confinement layer”, and the second acoustic reflection layer  3   b  is an example of the “second energy confinement layer”. The first acoustic reflection layer  3   a  and the second acoustic reflection layer  3   b  are integrally provided. 
     The acoustic wave device  1   b  according to Preferred Embodiment 2 is an acoustic wave filter (here, a ladder filter), similarly to the acoustic wave device  1  according to Preferred Embodiment 1, as illustrated in  FIG. 19 . The acoustic wave device  1   b  includes the input terminal  15 , the output terminal  16 , the multiple (two) series-arm resonators RS 1 , and the multiple (two) parallel-arm resonators RS 2 . The multiple (two) series-arm resonators RS 1  are provided on the first path  12  (see  FIG. 3 ) connecting the input terminal  15  and the output terminal  16 . The multiple (two) parallel-arm resonators RS 2  are respectively provided one by one on the multiple (two) second paths  13  and  14  (see  FIG. 3 ) connecting the multiple (two) nodes N 1  and N 2  (see 
       FIG. 3 ) on the first path  12  and the ground (ground terminals  17  and  18 ). The ground terminals  17  and  18  may be made common as one ground. 
     Each of the multiple series-arm resonators RS 1  includes the first divided resonator RS 3  and the second divided resonator RS 4 . The first divided resonator RS 3  and the second divided resonator RS 4  are connected in series. Note that, in each of the multiple series-arm resonators RS 1 , the first divided resonator RS 3  and the second divided resonator RS 4  may be connected in parallel. Further, each of the multiple parallel-arm resonators RS 2  may include a first divided resonator and a second divided resonator. In each of the multiple parallel-arm resonators RS 2 , the first divided resonator and the second divided resonator are connected in series or in parallel. 
     In the acoustic wave device  1   b , each of the first divided resonator RS 3 , the second divided resonator RS 4 , and the multiple parallel-arm resonators RS 2  is the acoustic wave resonator  5 . The first divided resonator RS 3  and the second divided resonator RS 4  are included in the multiple series-arm resonators RS 1 . Each of the multiple acoustic wave resonators  5  is a resonator including at least a pair of electrodes (first electrode  51  and second electrode  52 ). In the acoustic wave device  1   b , the piezoelectric layer  4  is shared by the multiple acoustic wave resonators  5 . Further, in the acoustic wave device  1   b , the acoustic reflection layer  3  is divided by the multiple acoustic wave resonators  5 . The resonant frequency of the parallel-arm resonator RS 2  is lower than that of the series-arm resonator RS 1 . 
     (2) Acoustic Reflection Layer 
     The acoustic reflection layer  3  faces the multiple first electrodes  51  and the multiple second electrodes  52  in the thickness direction D 1  of the piezoelectric layer  4 , as illustrated in  FIG. 18 . 
     The acoustic reflection layer  3  has a function of suppressing the leakage of a bulk wave (thickness shear primary mode bulk wave described above), excited by the first electrode  51  and the second electrode  52  defining a pair, to the support substrate  2 . By having the acoustic reflection layer  3 , the acoustic wave device  1   b  may increase the effect of confining acoustic wave energy in the piezoelectric layer  4 . Therefore, the acoustic wave device  1   b  may reduce the loss and increase the Q factor in comparison with a case that the acoustic reflection layer  3  is not provided. 
     The acoustic reflection layer  3  has a laminated structure in which at least one (three) low acoustic impedance layer  31  and at least one (two) high acoustic impedance layer  32  are alternately arranged in layer by layer in the thickness direction D 1  of the piezoelectric layer  4 . The acoustic impedance of the low acoustic impedance layer  31  is lower than that of the high acoustic impedance layer  32 . 
     Hereinafter, for convenience of description, in the acoustic reflection layer  3 , the two high acoustic impedance layers  32  may be referred to as a first high acoustic impedance layer  321  and a second high acoustic impedance layer  322  in order from the first main surface  21  of the support substrate  2 . Further, the three low acoustic impedance layers  31  may be referred to as a first low acoustic impedance layer  311 , a second low acoustic impedance layer  312 , and a third low acoustic impedance layer  313  in order from the first main surface  21  of the support substrate  2 . 
     In the acoustic reflection layer  3 , the first low acoustic impedance layer  311 , the first high acoustic impedance layer  321 , the second low acoustic impedance layer  312 , the second high acoustic impedance layer  322 , and the third low acoustic impedance layer  313  are arranged in this order from the support substrate  2  side. Accordingly, the acoustic reflection layer  3  may reflect a bulk wave (thickness shear primary mode bulk wave) from the piezoelectric layer  4  at each of the interface between the third low acoustic impedance layer  313  and the second high acoustic impedance layer  322 , the interface between the second high acoustic impedance layer  322  and the second low acoustic impedance layer  312 , the interface between the second low acoustic impedance layer  312  and the first high acoustic impedance layer  321 , and the interface between the first high acoustic impedance layer  321  and the first low acoustic impedance layer  311 . 
     The material of the multiple high acoustic impedance layers  32  is Pt (platinum), for example. Further, the material of the multiple low acoustic impedance layers  31  is silicon oxide, for example. The thickness of each of the multiple high acoustic impedance layers  32  is about 94 nm, for example. Further, the thickness of each of the multiple low acoustic impedance layers  31  is about  188  nm, for example. The acoustic reflection layer  3  has two electrically conductive layers because each of the two high acoustic impedance layers  32  is formed of platinum. 
     The material of the multiple high acoustic impedance layers  32  is not limited to Pt, but may be a metal such as W (tungsten) or Ta (tantalum), for example. Further, the material of the multiple high acoustic impedance layers  32  is not limited to a metal, but may be an insulator, for example. 
     Furthermore, the multiple high acoustic impedance layers  32  are not limited to be made of the same material, but may be made of different materials, for example. Still further, the multiple low acoustic impedance layers  31  are not limited to be made of the same material, but may be made of different materials, for example. 
     The number of each of the high acoustic impedance layers and the low acoustic impedance layers  31  in the acoustic reflection layer  3  is not limited to two and three, and may be one, three or more, or four or more. Further, the number of high acoustic impedance layers  32  and the number of low acoustic impedance layers  31  are not limited to be different from each other, and may be the same, or the number of low acoustic impedance layers  31  may be less than the number of high acoustic impedance layers  32  by one. In order to obtain the preferable reflection in the acoustic reflection layer  3 , the thickness of each of the high acoustic impedance layer  32  and the low acoustic impedance layer  31  is appropriately set in accordance with: a desired frequency of the acoustic wave device  1 , and a material used in each of the high acoustic impedance layer  32  and the low acoustic impedance layer  31 . 
     (3) Method of Manufacturing Acoustic Wave Device 
     In the method of manufacturing the acoustic wave device  1   b , the first to fourth processes are performed after the support substrate  2  is prepared, for example. In the first process, the acoustic reflection layer  3  is formed on the first main surface  21  of the support substrate  2 . In the second process, the piezoelectric substrate is defined by the piezoelectric layer  4  and the support substrate  2  being bonded to each other via the acoustic reflection layer  3 . In the third process, the piezoelectric layer  4  defined by a portion of the piezoelectric substrate is formed by thinning the piezoelectric substrate. In the fourth process, the multiple first electrodes  51 , the multiple second electrodes  52 , the first wiring portion  61 , and the second wiring portion  62  are formed on the piezoelectric layer  4 . In the first process to the fourth process, a silicon wafer is used as the support substrate  2 . Further, in the second process, a piezoelectric wafer is used as the piezoelectric substrate. In the method of manufacturing the acoustic wave device  1   b , a wafer including the multiple acoustic wave devices  1   b  is cut with a dicing machine to obtain the multiple acoustic wave devices  1   b  (chips). 
     The method of manufacturing the acoustic wave device  1   b  is an example and is not particularly limited. For example, the piezoelectric layer  4  may be formed using a film formation technique. In the case above, the method of manufacturing the acoustic wave device  1   b  includes a process of forming the piezoelectric layer  4  instead of the second process and the third process. The piezoelectric layer  4  formed by the film formation technique may be a single-crystal or a twin-crystal, for example. The film formation technique is a CVD method, for example, but is not limited thereto. Further, the piezoelectric layer  4  is subjected to poling treatment. 
     (4) Effect 
     The acoustic wave device  1   b  according to Preferred Embodiment 2 uses a thickness shear primary mode bulk wave, the same as the acoustic wave device  1  according to Preferred Embodiment 1. With this, in the acoustic wave device  1   b  according to Preferred Embodiment 2, the resonant frequency is not restricted by the distance between the center lines of the first electrode  51  and the second electrode  52  defining a pair, but the resonant frequency may be made higher by reducing the thickness of the piezoelectric layer  4 . This makes it possible to handle a higher frequency without increasing the planar size of the acoustic wave device  1   b.    
     In the acoustic wave device  1   b  according to Preferred Embodiment 2, since the second main surface  42  of the piezoelectric layer  4  is constrained by the acoustic reflection layer  3  in the acoustic wave resonator  5 , it is possible to suppress an unnecessary wave. Further, in the acoustic wave device lb according to Preferred Embodiment 2, the material of the piezoelectric layer  4  is LiNbO 3  or LiTaO 3 , and the material of the low acoustic impedance layer  31  is silicon oxide. Here, the frequency-temperature characteristics of each of LiNbO 3  and LiTaO 3  have a negative slope, and the frequency-temperature characteristics of silicon oxide have a positive slope. Accordingly, in the acoustic wave device  1   b  according to Preferred Embodiment 2, the absolute value of TCF (Temperature Coefficient of Frequency) may be made smaller, and frequency-temperature characteristics may be improved. 
     The acoustic wave device  1   b  according to Preferred Embodiment 2 includes the acoustic reflection layer  3  provided between the support substrate  2  and the piezoelectric layer  4 . 
     Modification 1 of Preferred Embodiment 2 
     In the acoustic wave device  1   b  according to Preferred Embodiment 2, the acoustic reflection layer  3  is shared by the multiple acoustic wave resonators  5 . However, of the multiple high acoustic impedance layers  32 , the high acoustic impedance layer  32  (second high acoustic impedance layer  322 ) closest to the piezoelectric layer  4  may be separated for each acoustic wave resonator  5 . 
     Modification 2 of Preferred Embodiment 2 
     Hereinafter, an acoustic wave device  1   c  according to Modification 2 of Preferred Embodiment 2 will be described with reference to  FIG. 20 . With respect to the acoustic wave device  1   c  according to Modification 2, the same elements as those of the acoustic wave device  1   b  according to Preferred Embodiment 2 are denoted by the same reference signs, and a description thereof will be omitted. 
     The acoustic wave device  1   c  according to Modification 2 of Preferred Embodiment 2 differs from the acoustic wave device  1   b  according to Preferred Embodiment 2 in that each of the multiple acoustic wave resonators  5  further includes two reflectors  8 . The configuration of each reflector  8  is the same as that of each reflector  8  of the acoustic wave device la according to Modification 1 of Preferred Embodiment 1. 
     As another modification of Preferred Embodiment 2, the series-arm resonator RS 1  and the parallel-arm resonator RS 2  may overlap with the same cavity  26  in a plan view, the same as in Modification 4 of Preferred Embodiment 1. Otherwise, as in Modification 5 of Preferred Embodiment 1, the first divided resonator RS 3  and the second divided resonator RS 4  may have a configuration as follows. The second busbar  621  of the pair of busbars (first busbar  611  and second busbar  621 ) included in the first divided resonator RS 3 , and the first busbar  611  of the pair of busbars (first busbar  611  and second busbar  621 ) included in the second divided resonator RS 4  may be the shared inter-stage busbar  63 . 
     The above-described Preferred Embodiment 1, Preferred Embodiment 2, or the like is merely one of various preferred embodiments of the present invention. The above-described Preferred Embodiment 1, Preferred Embodiment 2, or the like may be modified in various ways depending on such as design as long as one or more of the advantages of various preferred embodiments of the present invention is achieved. 
     For example, in the acoustic wave device  1  according to Preferred Embodiment 1, the piezoelectric layer  4  is bonded to the support substrate  2  via the silicon oxide film  7 , but the silicon oxide film  7  is not an essential element. 
     Further, in the acoustic wave device  1  according to Preferred Embodiment 1, the cavity  26  is formed to penetrate through the support substrate  2  in the thickness direction thereof, but is not limited thereto. The cavity  26  may be include an internal space in a recessed portion on the first main surface  21  of the support substrate  2  without penetrating through the support substrate  2 . 
     In the acoustic wave devices  1  to  1   c , the sectional shape of the first electrode  51  and the second electrode  52  is a rectangular shape, but is not limited thereto. Here, the sectional shape is the shape of a section orthogonal to the thickness direction D 1  and the second direction D 2  of the piezoelectric layer  4 , for example. The first electrode  51  and the second electrode  52  may have a shape in which the width of the lower end is wider than the width of the upper end as in any of  FIGS. 21A to 21D , for example. With this, it is possible to increase capacitance between the first electrode  51  and second electrode  52  defining a pair without increasing the widths of the first main surface  511  (see  FIG. 7 ) of the first electrode  51  and the first main surface  521  (see  FIG. 7 ) of the second electrode  52 . 
     The first electrode  51  and the second electrode  52  in  FIG. 21A  have a portion with a substantially constant width on the upper end side and a portion with a gradually increasing width on the lower end side. Further, the first electrode  51  and the second electrode  52  in  FIG. 21B  have a trapezoidal shape in section. Furthermore, the first electrode  51  and the second electrode  52  in  FIG. 21C  have a shape spreading toward the lower end, and both side surfaces in the width direction are curved surfaces. Still further, the first electrode  51  and the second electrode  52  in  FIG. 21D  have a portion having a trapezoidal section on the upper end side and a portion having another trapezoidal section on the lower end side. The portion on the lower end side is wider than the portion on the upper end side. 
     The acoustic wave devices  1  to  1   c  may include a dielectric film  9  covering the first main surface  41  of the piezoelectric layer  4  and the first electrode  51  and the second electrode  52  on the first main surface  41 , as in any of  FIGS. 22A to 22C . In the acoustic wave devices  1  to  1   c , by including the dielectric film  9 , the capacitance between the first electrode  51  and the second electrode  52  defining a pair may be increased. In  FIG. 22A , the thickness of the dielectric film  9  is thinner than the thickness of the first electrode  51  and the second electrode  52 , and the surface of the dielectric film  9  has an uneven shape following the shape of the base. In  FIG. 22B , the surface of the dielectric film  9  is flattened to be a planar shape. In  FIG. 22C , the thickness of the dielectric film  9  is thicker than the thickness of the first electrode  51  and the second electrode  52 , and the surface of the dielectric film  9  has an uneven shape following the shape of the base. 
     In the acoustic wave devices  1  to  1   c , the sectional shape of the first electrode  51  and the sectional shape of the second electrode  52  may be different from each other. Here, the sectional shape is the shape of a section orthogonal to the thickness direction D 1  and the second direction D 2  of the piezoelectric layer  4 , for example. 
     In the acoustic wave devices  1  to  1   c , the acoustic wave resonator  5  includes the multiple first electrodes  51  and the multiple second electrodes  52 , but not limited thereto. It is sufficient that the acoustic wave resonator  5  includes at least a pair of electrodes (first electrode  51  and second electrode  52 ). 
     In the acoustic wave devices  1  to  1   c , the shape of the first electrode  51  and the second electrode  52  of one acoustic wave resonator  5  may be different from the shape of the first electrode  51  and the second electrode  52  of another acoustic wave resonator  5 . Further, the shape of the first electrode  51  and the second electrode  52  of the acoustic wave resonator  5  of the series-arm resonator RS 1  and the shape of the first electrode  51  and the second electrode  52  of the acoustic wave resonator  5  of the parallel-arm resonator RS 2  may be different from each other. 
     Furthermore, the first electrode  51  and the second electrode  52  are not limited to being linear in a plan view from the thickness direction D 1  of the piezoelectric layer  4 . For example, the first electrode  51  and the second electrode  52  may have a curved shape or a shape including a linear portion and a curved portion. 
     Aspects 
     The present description discloses the following aspects. 
     Acoustic wave devices ( 1  to  1   c ) according to a first aspect include a piezoelectric layer ( 4 ), a first electrode ( 51 ), and a second electrode ( 52 ). The first electrode ( 51 ) and the second electrode ( 52 ) face each other in a direction (longitudinal direction D 2 ) intersecting with a thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The acoustic wave devices ( 1  to  1   c ) use a thickness shear primary mode bulk wave. The material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. The acoustic wave devices ( 1  to  1   c ) include the multiple electrode portions ( 50 ), each including the first electrode ( 51 ) and the second electrode ( 52 ). The acoustic wave devices ( 1  to  1   c ) further include a first divided resonator (RS 3 ), a second divided resonator (RS 4 ), and a support substrate ( 2 ). The first divided resonator (RS 3 ) and the second divided resonator (RS 4 ) are connected in series without another resonator being connected therebetween, or are connected in parallel with each other to the same connection node on a path (first path  12 ) connecting an input terminal ( 15 ) and an output terminal ( 16 ). The support substrate ( 2 ) includes a first main surface ( 21 ) and a second main surface ( 22 ) opposed to each other. The first divided resonator (RS 3 ) includes a first installation portion ( 401 ). The first installation portion ( 401 ) includes a first electrode portion ( 501 ) of multiple electrode portions ( 50 ) and a first region ( 451 ) of the piezoelectric layer ( 4 ). The first region ( 451 ) is provided with the first electrode portion ( 501 ). The second divided resonator (RS 4 ) includes a second installation portion ( 402 ). The second installation portion ( 402 ) includes a second electrode portion ( 502 ) of the multiple electrode portions ( 50 ) and a second region ( 452 ) of the piezoelectric layer ( 4 ). The second region ( 452 ) is provided with the second electrode portion ( 502 ). The piezoelectric layer ( 4 ) is directly or indirectly provided on the support substrate ( 2 ). The support substrate ( 2 ) includes a first energy confinement layer (a first cavity  26   a  and a first acoustic reflection layer  3   a ) and a second energy confinement layer (a second cavity  26   b  and a second acoustic reflection layer  3   b ). The first energy confinement layer at least partially overlaps with the first region ( 451 ) of the piezoelectric layer ( 4 ). The second energy confinement layer at least partially overlaps with the second region ( 452 ) of the piezoelectric layer ( 4 ). The first energy confinement layer and the second energy confinement layer are integrally provided in the support substrate ( 2 ). 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the first aspect, the Q factor may be increased even when the size is further reduced, and it is possible to further reduce the size when the first divided resonator (RS 3 ) and the second divided resonator (RS 4 ) are formed. 
     The acoustic wave devices ( 1  to  1   c ) according to a second aspect include the piezoelectric layer ( 4 ), the first electrode ( 51 ), and the second electrode ( 52 ). The first electrode ( 51 ) and the second electrode ( 52 ) face each other in the direction (second direction D 2 ) intersecting with the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The first electrode ( 51 ) and the second electrode ( 52 ) are adjacent to each other. In any section along the thickness direction (D 1 ) of the piezoelectric layer ( 4 ), when the distance between the center lines of the first electrode ( 51 ) and the second electrode ( 52 ) is denoted as p, and the thickness of the piezoelectric layer ( 4 ) is denoted as d, d/p is about 0.5 or less. The material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. The acoustic wave devices ( 1  to  1   c ) include the multiple electrode portions ( 50 ), each including the first electrode ( 51 ) and the second electrode ( 52 ). The acoustic wave devices ( 1  to  1   c ) further include the first divided resonator (RS 3 ), the second divided resonator (RS 4 ), and the support substrate ( 2 ). The first divided resonator (RS 3 ) and the second divided resonator (RS 4 ) are connected in series without another resonator being connected therebetween, or are connected in parallel with each other to the same connection node on the path (first path  12 ) connecting the input terminal ( 15 ) and the output terminal ( 16 ). The support substrate ( 2 ) has the first main surface ( 21 ) and the second main surface ( 22 ) opposed to each other. The first divided resonator (RS 3 ) includes the first installation portion ( 401 ). The first installation portion ( 401 ) includes the first electrode portion ( 501 ) of the multiple electrode portions ( 50 ) and the first region ( 451 ) of the piezoelectric layer ( 4 ). The first region ( 451 ) is provided with the first electrode portion ( 501 ). The second divided resonator (RS 4 ) includes the second installation portion ( 402 ). The second installation portion ( 402 ) includes the second electrode portion ( 502 ) of the multiple electrode portions ( 50 ) and the second region ( 452 ) of the piezoelectric layer ( 4 ). The second region ( 452 ) is provided with the second electrode portion ( 502 ). The piezoelectric layer ( 4 ) is directly or indirectly provided on the support substrate ( 2 ). The support substrate ( 2 ) includes the first energy confinement layer (a first cavity  26   a,  and a first acoustic reflection layer  3   a ) and the second energy confinement layer (a second cavity  26   b,  and a second acoustic reflection layer  3   b ). The first energy confinement layer at least partially exposes the first region ( 451 ) of the piezoelectric layer ( 4 ). The second energy confinement layer at least partially exposes the second region ( 452 ) of the piezoelectric layer ( 4 ). The first energy confinement layer and the second energy confinement layer are integrally provided in the support substrate ( 2 ). 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the second aspect, the Q factor may be increased even when the size is further reduced, and it is possible to further reduce the size when the first divided resonator (RS 3 ) and the second divided resonator (RS 4 ) are formed. 
     In the acoustic wave devices ( 1  to  1   c ) according to a third aspect, a thickness (d 1 ) of the first divided resonator (RS 3 ) is the thickness of the first installation portion ( 401 ), in the first or second aspect. The thickness of the second divided resonator (RS 4 ) is the thickness of the second installation portion ( 402 ). The thickness (d 1 ) of the first divided resonator (RS 3 ) and a thickness (d 2 ) of the second divided resonator (RS 4 ) are different from each other. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the third aspect, ripples may be dispersed. 
     In the acoustic wave devices ( 1  to  1   c ) according to a fourth aspect, the polarity of the first divided resonator (RS 3 ) and the polarity of the second divided resonator (RS 4 ) are different from each other, in any one of the first to third aspects. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the fourth aspect, linearity may further be improved. 
     In the acoustic wave devices ( 1   b ;  1   c ) according to a fifth aspect, at least one of the first energy confinement layer and the second energy confinement layer is an acoustic reflection layer ( 3 ), in any one of the first to fourth aspects. The acoustic reflection layer ( 3 ) has a high acoustic impedance layer ( 32 ) and a low acoustic impedance layer ( 31 ). The acoustic impedance of the low acoustic impedance layer ( 31 ) is lower than that of the high acoustic impedance layer ( 32 ). 
     With the use of the acoustic wave devices ( 1   b ;  1   c ) according to the fifth aspect, the linearity may further be improved. 
     In the acoustic wave devices ( 1 ; 1   a ) according to a sixth aspect, the first energy confinement layer and the second energy confinement layer are a cavity ( 26 ) , in any one of the first to fourth aspects. 
     In the acoustic wave devices ( 1  to  1   c ) according to a seventh aspect, a distance (p) between the center lines in the first divided resonator (RS 3 ) and a distance (p) between the center lines in the second divided resonator (RS 4 ) are different from each other, in the second aspect. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the seventh aspect, the ripples may be dispersed without affecting the main resonance. 
     In the acoustic wave devices ( 1  to  1   c ) according to an eighth aspect, d/p is about 0.24 or less, in the second aspect. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the eighth aspect, it is possible to make a fractional bandwidth still larger. 
     In the acoustic wave devices ( 1  to  1   c ) according to a ninth aspect, the first electrode ( 51 ) and the second electrode ( 52 ) are adjacent to each other, in the eighth aspect. The first electrode ( 51 ) has a first electrode main portion ( 510 ). The first electrode main portion ( 510 ) overlaps with the second electrode ( 52 ) in the direction (longitudinal direction D 2 ) in which the first electrode ( 51 ) and the second electrode ( 52 ) face each other. The second electrode ( 52 ) has a second electrode main portion ( 520 ). The second electrode main portion ( 520 ) overlaps with the first electrode ( 51 ) in the direction (longitudinal direction D 2 ) in which the first electrode ( 51 ) and the second electrode ( 52 ) face each other. The piezoelectric layer ( 4 ) has a defined region ( 45 ). In a plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ), the defined region ( 45 ) overlaps with both of the first electrode ( 51 ) and the second electrode ( 52 ) in the direction (longitudinal direction D 2 ) in which the first electrode ( 51 ) and the second electrode ( 52 ) face each other in the piezoelectric layer ( 4 ), and is positioned between the first electrode ( 51 ) and the second electrode ( 52 ). In a plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ), when the area of the first electrode main portion ( 510 ) is denoted as S 1 , the area of the second electrode main portion ( 520 ) is denoted as S 2 , and the area of the defined region ( 45 ) is denoted as S 0 , and a structural parameter defined by (S 1 +S 2 )/(S 1 +S 2 +S 0 ) is denoted as MR, the acoustic wave devices ( 1  to  1   c ) satisfy the condition MR about 1.75×(d/p)+0.075. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the ninth aspect, it is possible to suppress spurious components in a band. 
     In the acoustic wave devices ( 1  to  1   c ) according to a tenth aspect, each of the first electrode ( 51 ) and the second electrode ( 52 ) is the electrode to have hot electric potential or ground electric potential, in any one of the first to ninth aspects. 
     With the use of the acoustic wave devices ( 1  to  1   c ) according to the tenth aspect, it is possible to prevent the first electrode ( 51 ) and the second electrode ( 52 ) defining a pair from taking floating electric potential. 
     In the acoustic wave devices ( 1  to  1   c ) according to an eleventh aspect, in a plan view from the thickness direction (D 1 ), the first energy confinement layer (first cavity  26   a;  first acoustic reflection layer  3   a ) overlaps with the first electrode ( 51 ) and the second electrode ( 52 ) of the first electrode portion ( 501 ), and a portion of the piezoelectric layer ( 4 ) between the first electrode ( 51 ) and the second electrode ( 52 ) of the first electrode portion ( 501 ), in any one of the first to tenth aspects. In a plan view from the thickness direction (D 1 ) , the second energy confinement layer (a second cavity  26   b,  and a second acoustic reflection layer  3   b ) overlaps with the first electrode ( 51 ) and the second electrode ( 52 ) of the second electrode portion ( 502 ), and a portion of the piezoelectric layer ( 4 ) between the first electrode ( 51 ) and the second electrode ( 52 ) of the second electrode portion ( 502 ). 
     In the acoustic wave devices ( 1  to  1   c ) according to a twelfth aspect, the first electrode ( 51 ) and the second electrode ( 52 ) face each other on the same main surface (a first main surface  41 , and a second main surface  42 ) of the piezoelectric layer ( 4 ), in any one of the first to eleventh aspects. 
     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.