Patent Publication Number: US-2022224305-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-178096 filed on Sep. 27, 2019 and is a Continuation Application of PCT Application No. PCT/JP2020/036395 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 relates to an acoustic wave device, and more particularly, to an acoustic wave device including a piezoelectric layer. 
     2. Description of the Related Art 
     In the past, an acoustic wave device utilizing a plate wave propagating through a piezoelectric film made of LiNbO 3  or LiTaO 3  has been known. For example, Japanese Unexamined Patent Application Publication No. 2012-257019 discloses an acoustic wave device utilizing a Lamb wave as a 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 a plurality of first electrode fingers and a plurality of second electrode fingers of the IDT electrode. Accordingly, a Lamb wave is excited. A reflector is provided on each side of the IDT electrode. Accordingly, an acoustic wave resonator utilizing a plate wave is configured. 
     In the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2012-257019, it is conceivable to decrease the number of the first electrode fingers and the second electrode fingers in order to achieve size reduction. However, when the number of the first electrode fingers and the second electrode fingers is decreased, a Q-value is decreased. In addition, in the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2012-257019, when the number of the first electrode fingers and the second electrode fingers is decreased in order to achieve size reduction, it has been difficult to increase capacitance of the acoustic wave resonator. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide acoustic wave devices with each of which a Q-value and a capacitance are able to be increased, while achieving size reduction. 
     An acoustic wave device according to 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 a thickness direction of the piezoelectric layer. In the acoustic wave device, a bulk wave in a thickness-shear primary mode is utilized. A material of the piezoelectric layer is lithium niobate or lithium tantalate. At least a portion of each of the first electrode and the second electrode is embedded in the piezoelectric layer. 
     An acoustic wave device according to 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 a thickness direction of the piezoelectric layer. The first electrode and the second electrode are electrodes adjacent to each other. In the acoustic wave device, d/p is equal to or less than about 0.5, where in a section along the thickness direction of the piezoelectric layer, when an inter-centerline distance between the first electrode and the second electrode is denoted as p, and a thickness of the piezoelectric layer is denoted as d. A material of the piezoelectric layer is lithium niobate or lithium tantalate. At least a portion of each of the first electrode and the second electrode is embedded in the piezoelectric layer. 
     An acoustic wave device according to 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 a thickness direction of the piezoelectric layer. In the acoustic wave device, a bulk wave in a thickness-shear primary mode is utilized. The acoustic wave device further includes an acoustic reflection layer. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The piezoelectric layer is on the acoustic reflection layer. At least a portion of each of the first electrode and the second electrode is embedded in the acoustic reflection layer and is in contact with the piezoelectric layer. 
     An acoustic wave device according to 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 a thickness direction of the piezoelectric layer. In the acoustic wave device, d/p is equal to or less than about 0.5, where in a section along the thickness direction of the piezoelectric layer, an inter-centerline distance between the first electrode and the second electrode is denoted as p, and a thickness of the piezoelectric layer is denoted as d. The acoustic wave device further includes an acoustic reflection layer. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The piezoelectric layer is on the acoustic reflection layer. At least a portion of each of the first electrode and the second electrode is embedded in the acoustic reflection layer and is in contact with the piezoelectric layer. 
     In each of the acoustic wave devices according to preferred embodiments of the present invention, it is possible to increase a Q-value and a capacitance while achieving size reduction. 
     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 taken along a line A-A of  FIG. 1  and related to the acoustic wave device according to Preferred Embodiment 1 of the present invention. 
         FIG. 3  is a plan view of a main portion of the acoustic wave device according to Preferred Embodiment 1 of the present invention. 
         FIG. 4  is a sectional view of an acoustic wave device according to Reference Preferred Embodiment 1 of the present invention. 
         FIG. 5A  is an explanatory diagram of a Lamb wave.  FIG. 5B  is an explanatory diagram of a bulk wave in a thickness-shear primary mode. 
         FIG. 6  is an operation explanatory diagram of the acoustic wave device according to Preferred Embodiment 1 of the present invention. 
         FIG. 7A  is a graph showing a relationship between fractional bandwidth in a thickness-shear mode and [thicknesses of piezoelectric layer]/[inter-centerline distance between first electrode and second electrode], for a structural model of an acoustic wave device according to Reference Preferred Embodiment of the present invention.  FIG. 7B  is a graph showing a relationship between fractional bandwidth in the thickness-shear mode and [thicknesses of piezoelectric layer]/[inter-centerline distance between two electrodes forming a pair] and related to the above structural model and is a graph in which a range of about 0 to about 0.2 on a horizontal axis of  FIG. 7A  is enlarged. 
         FIG. 8  is a graph showing a relationship between fractional bandwidth in the thickness-shear mode and normalized spurious level and related to the above structural model of the acoustic wave device. 
         FIG. 9  is an impedance-frequency characteristic diagram of the above structural model of the acoustic wave device. 
         FIG. 10  is a diagram for explaining a distribution of fractional bandwidth in a combination of [thickness of piezoelectric layer]/[inter-centerline distance between first electrode and second electrode] and a structural parameter, related to the above structural model of the acoustic wave device. 
         FIG. 11  is an impedance-frequency characteristic diagram of each of an acoustic wave device according to an application example of Preferred Embodiment 1 of the present invention and an acoustic wave device according to an example of Reference Preferred Embodiment 1 of the present invention. 
         FIG. 12  is a plan view of an acoustic wave device according to Modified Example 1 of Preferred Embodiment 1 of the present invention. 
         FIG. 13  is an equivalent circuit diagram of the acoustic wave device according to Modified Example 1 of Preferred Embodiment 1 of the present invention. 
         FIG. 14  is a sectional view of an acoustic wave device according to Modified Example 2 of Preferred Embodiment 1 of the present invention. 
         FIG. 15  is a sectional view of an acoustic wave device according to Modified Example 3 of Preferred Embodiment 1 of the present invention. 
         FIG. 16  is a sectional view of an acoustic wave device according to Modified Example 4 of Preferred Embodiment 1 of the present invention. 
         FIG. 17  is a sectional view of an acoustic wave device according to Modified Example 5 of Preferred Embodiment 1 of the present invention. 
         FIG. 18  is a sectional view of an acoustic wave device according to Modified Example 6 of Preferred Embodiment 1 of the present invention. 
         FIG. 19  is a sectional view of an acoustic wave device according to Modified Example 7 of Preferred Embodiment 1 of the present invention. 
         FIG. 20  is a plan view of an acoustic wave device according to Modified Example 8 of Preferred Embodiment 1 of the present invention. 
         FIG. 21  is a sectional view of an acoustic wave device according to Preferred Embodiment 2 of the present invention. 
         FIG. 22  is a sectional view of an acoustic wave device according to a Modified Example of Preferred Embodiment 2 of the present invention. 
         FIGS. 23A to 23D  are sectional views each illustrating another shape of a pair of electrodes of the acoustic wave device according to the Modified Example of Preferred Embodiment 2 of the present invention. 
         FIGS. 24A to 24C  are sectional views each illustrating another configuration example of the acoustic wave device according to the Modified Example of Preferred Embodiment 2 of the present invention. 
         FIG. 25  is a plan view of an acoustic wave device according to Preferred Embodiment 3 of the present invention. 
         FIG. 26  is a sectional view taken along a line A-A of  FIG. 25  and related to the acoustic wave device according to Preferred Embodiment 3 of the present invention. 
         FIG. 27  is a plan view of an acoustic wave device according to a Modified Example of Preferred Embodiment 3 of the present invention. 
         FIG. 28  is a sectional view of an acoustic wave device according to another Modified Example of Preferred Embodiment 3 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below with reference to the drawings. 
       FIGS. 1 to 6, 12, and 14 to 28  referred to in the following preferred embodiments and the like are all schematic diagrams, and ratios of sizes and thicknesses of respective components in the diagrams do not necessarily reflect actual dimensional ratios. 
     Preferred Embodiment 1 
     Hereinafter, an acoustic wave device  1  according to Preferred Embodiment 1 of the present invention will be described with reference to  FIGS. 1 to 3 . 
     (1.1) Overall Configuration of Acoustic Wave Device 
     As illustrated in  FIG. 1 , the acoustic wave device  1  according to Preferred Embodiment 1includes a piezoelectric layer  4 , a first electrode  51 , and a second electrode  52 . As illustrated in  FIG. 2 , the first electrode  51  and the second electrode  52  face each other in a direction D 2  (hereinafter, also referred to as a second direction D 2 ) intersecting a thickness direction D 1  (hereinafter, also referred to as a first direction D 1 ) of the piezoelectric layer  4 . The acoustic wave device  1  utilizes a bulk wave in a thickness-shear primary mode. The second direction D 2  is orthogonal or substantially orthogonal to a polarization direction PZ 1  of the piezoelectric layer  4 . A bulk wave in the thickness-shear primary mode 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 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 . Thickness-shear vibration is excited in a defined region  45  in the piezoelectric layer  4  between the first electrode  51  and the second electrode  52  in plan view from the thickness direction D 1 . In the acoustic wave device  1 , when the second direction D 2  is orthogonal or substantially orthogonal to the polarization direction PZ 1  of the piezoelectric layer  4 , an electromechanical coupling coefficient (hereinafter, also referred to as a coupling coefficient) of a bulk wave in the thickness-shear primary mode is large. Here, “orthogonal” is not limited to a case of being strictly orthogonal and may also refer to substantially orthogonal (an angle formed by the second direction D 2  and the polarization direction PZ 1  is, for example, about 90°±10°). 
     As illustrated in  FIGS. 1 and 2 , the first electrode  51  and the second electrode  52  intersect each other when viewed from the second direction D 2 . “Intersect each other when viewed from the second direction D 2 ” means mutual overlapping when viewed from 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 . The first wiring portion  61  is connected to a first terminal T 1 . The second wiring portion  62  is connected to a second terminal T 2  different from the first terminal T 1 . The acoustic wave device  1  includes a plurality of first electrodes  51  and a plurality of second electrodes  52 . That is, when the first electrode  51  and the second electrode  52  define a pair of electrodes, the acoustic wave device  1  includes a plurality of pairs of the electrodes. In the acoustic wave device  1 , the first electrodes  51  and the second electrodes  52  are alternately provided in the second direction D 2  one by one. In the acoustic wave device  1 , as illustrated in  FIG. 1 , the plurality of first electrodes  51  are connected to the one first wiring portion  61 , and the plurality of second electrodes  52  are connected to the one second wiring portion  62 . 
     As illustrated in  FIG. 2 , the acoustic wave device  1  includes a support substrate  2 , an acoustic reflection layer  3 , the piezoelectric layer  4 , the first electrode  51 , and the second electrode  52 . The acoustic reflection layer  3  is provided on the support substrate  2 . The piezoelectric layer  4  is provided on the acoustic reflection layer  3 . The first electrode  51  and the second electrode  52  are in contact with the piezoelectric layer  4 . The acoustic reflection layer  3  includes at least one (for example, two) high acoustic impedance layer  32  and at least one (for example, three) low acoustic impedance layer  31 . The low acoustic impedance layer  31  has an acoustic impedance lower than that of the high acoustic impedance layer  32 . The acoustic wave device  1  includes, as a resonator, an acoustic wave resonator  5  including the first electrode  51 , the second electrode  52 , and the piezoelectric layer  4  described above. In the acoustic wave device  1 , the acoustic wave resonator  5  further includes the acoustic reflection layer  3  described above. 
     (1.2) Each Component of Acoustic Wave Device 
     Next, each component of the acoustic wave device  1  will be described with reference to the drawings. 
     (1.2.1) Support Substrate 
     As illustrated in  FIG. 2 , the support substrate  2  supports the piezoelectric layer  4 . In the acoustic wave device  1  according to Preferred Embodiment 1, the support substrate  2  also supports the acoustic reflection layer  3  and supports the piezoelectric layer  4  and the first electrode  51  and the second electrode  52  via the acoustic reflection layer  3 . 
     The support substrate  2  includes a first main surface  21  and a second main surface  22  facing each other. The first main surface  21  and the second main surface  22  face each other in a 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 . In plan view from the thickness direction D 1  of the piezoelectric layer  4 , an outer peripheral shape of the support substrate  2  is a rectangular or substantially rectangular shape but is not limited thereto and may be, for example, a square or substantially square shape. 
     The support substrate  2  is, for example, a silicon substrate. A thickness of the support substrate  2  is, for example, about 120 μm but is not limited thereto. The silicon substrate is, for example, a single crystal silicon substrate. When the support substrate  2  is a silicon substrate, as a plane orientation of the first main surface  21 , for example, a ( 100 ) plane, a ( 110 ) plane, or a ( 111 ) plane may be used. A propagation orientation of the bulk wave described above can be set without being restricted by the plane orientation of the silicon substrate. Resistivity of the silicon substrate is, for example, equal to or greater than about 1 kΩcm, preferably equal to or greater than about 2 kΩcm, and more preferably equal to or greater than about 4 kΩcm. 
     The support substrate  2  is not limited to a silicon substrate and may be, for example, 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. 
     (1.2.2) Acoustic Reflection Layer 
     As illustrated in  FIG. 2 , the acoustic reflection layer  3  is provided on the first main surface  21  of the support substrate  2 . The acoustic reflection layer  3  faces the first electrode  51  and the second electrode  52  in the thickness direction D 1  of the piezoelectric layer  4 . 
     The acoustic reflection layer  3  reduces or prevents leakage of a bulk wave (bulk wave in the above-described thickness-shear primary mode) excited by the first electrode  51  and the second electrode  52  to the support substrate  2 . Since the acoustic wave device  1  includes the acoustic reflection layer  3 , an effect of confining acoustic wave energy inside the piezoelectric layer  4  can be improved. Thus, the acoustic wave device  1  can reduce a loss and increase a Q-value, as compared with a case where the acoustic reflection layer  3  is not included. 
     The acoustic reflection layer  3  has a laminated structure including (for example, three) low acoustic impedance layers  31  and (for example, two) high acoustic impedance layers  32  are alternately arranged one by one in the thickness direction D 1  of the piezoelectric layer  4 . An acoustic impedance of the low acoustic impedance layer  31  is lower than an acoustic impedance 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 of closeness to 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 of closeness to 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 provided in this order from a side of the support substrate  2 . Thus, the acoustic reflection layer  3  can reflect a bulk wave (bulk wave in the thickness-shear primary mode) from the piezoelectric layer  4  at each of an interface between the third low acoustic impedance layer  313  and the second high acoustic impedance layer  322 , an interface between the second high acoustic impedance layer  322  and the second low acoustic impedance layer  312 , an interface between the second low acoustic impedance layer  312  and the first high acoustic impedance layer  321 , and an interface between the first high acoustic impedance layer  321  and the first low acoustic impedance layer  311 . 
     A material of the high acoustic impedance layers  32  is, for example, Pt (platinum). Further, a material of the low acoustic impedance layers  31  is, for example, silicon oxide. A thickness of each of the high acoustic impedance layers  32  is, for example, about 94 nm. Further, a thickness of each of the low acoustic impedance layers  31  is, for example, about 188 nm. Since each of the two high acoustic impedance layers  32  is made of platinum, the acoustic reflection layer  3  includes two conductive layers. 
     The material of the high acoustic impedance layers  32  is not limited to Pt and may be, for example, a metal such as W (tungsten) or Ta (tantalum). In addition, the material of the high acoustic impedance layers  32  is not limited to metal and may be, for example, an insulator. 
     Further, the high acoustic impedance layers  32  are not limited to being made of the same material and, for example, may be made of materials different from each other. Further, the low acoustic impedance layers  31  are not limited to being made of the same material and, for example, may be made of materials different from each other. 
     Further, in the acoustic reflection layer  3 , the number of the high acoustic impedance layers  32  is not limited to two and may be three or more, and the number of the low acoustic impedance layers  31  is not limited to three and may be four or more. In addition, the number of the high acoustic impedance layers  32  and the number of the low acoustic impedance layers  31  are not limited to being different and may be the same, or the number of the low acoustic impedance layers  31  may be one less than the number of the high acoustic impedance layers  32 . In addition, the thickness of each of the high acoustic impedance layer  32  and the low acoustic impedance layer  31  is appropriately set according to a desired frequency of the acoustic wave device  1  and a material applied to each of the high acoustic impedance layer  32  and the low acoustic impedance layer  31  so that favorable reflection is obtained in the acoustic reflection layer  3 . 
     (1.2.3) Piezoelectric Layer 
     As illustrated in  FIG. 2 , the piezoelectric layer  4  includes a first main surface  41  and a second main surface  42  that face each other. The first main surface  41  and the second main surface  42  face each other in the thickness direction D 1  of the piezoelectric layer  4 . In the piezoelectric layer  4 , of the first main surface  41  and the second main surface  42 , the first main surface  41  is located on a side of the first electrode  51  and the second electrode  52 , and the second main surface  42  is located on a side of the acoustic reflection layer  3 . Thus, in the acoustic wave device  1 , a distance between the first main surface  41  of the piezoelectric layer  4  and the acoustic reflection layer  3  is longer than a distance between the second main surface  42  of the piezoelectric layer  4  and the acoustic reflection layer  3 . A material of the piezoelectric layer  4  is, for example, lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). The piezoelectric layer  4  is, for example, a Z-cut LiNbO 3  or a Z-cut LiTaO 3 . With respect to Euler angles (φ, θ, ψ) of the piezoelectric layer  4 , φ is about 0°±10°, and θ is about 0°±10°. ψ is any angle. From a viewpoint of increasing a coupling coefficient, the piezoelectric layer  4  is preferably a Z-cut LiNbO 3  or a Z-cut LiTaO 3 , for example. The piezoelectric layer  4  may be a rotated Y-cut LiNbO 3 , a rotated Y-cut LiTaO 3 , an X-cut LiNbO 3 , or an X-cut LiTaO 3 . A propagation orientation may be a Y-axis direction, may be an X-axis direction, or may be a direction rotated within a range of about ±90° from an X-axis in crystal axes (X, Y, Z) defined for a crystal structure of the piezoelectric layer  4 . The piezoelectric layer  4  is a single crystal but is not limited thereto or may be, for example, a twin crystal or ceramics. 
     A thickness of the piezoelectric layer  4  is, for example, equal to or greater than about 50 nm and equal to or less than about 1000 nm and is, for example, about 400 nm. 
     The piezoelectric layer  4  includes the defined region  45 . The defined region  45  is a region in the piezoelectric layer that intersects both the first electrode  51  and the second electrode  52  in a direction in which the first electrode  51  and the second electrode  52  face each other and that is located between the first electrode  51  and the second electrode  52 , in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     (1.2.4) Electrode 
     In the acoustic wave device  1 , for example, of the first electrode  51  and the second electrode  52 , the first electrode is a hot electrode, and the second electrode is a ground electrode. In the acoustic wave device  1 , the first electrodes  51  and the second electrodes  52  are alternately provided one by one so as to be separated from each other. Thus, the first electrode  51  and the second electrode  52  adjacent to each other are separated from each other. An inter-centerline distance between the first electrode  51  and the second electrode  52  adjacent to each other is, for example, equal to or greater than about 1 μm and equal to or less than about 10 μm and is, for example, about 3 μm. Here, the case where the first electrode  51  and the second electrode  52  are “adjacent to each other” refers to a case where there is no electrode connected to a hot electrode or a ground electrode, including the other first electrode  51  and second electrode  52 , between the first electrode  51  and the second electrode  52 . A group of electrodes including the plurality of first electrodes  51  and the plurality of second electrodes  52  only needs to have a configuration in which the first electrodes  51  and the second electrodes  52  are separated from each other in the second direction D 2  and may have a configuration in which the first electrodes  51  and the second electrodes  52  are not alternately provided so as to be separated from each other. For example, a region in which the first electrodes  51  and the second electrodes  52  are provided one by one so as to be spaced apart from each other and a region in which two of the first electrodes  51  or the second electrodes  52  are provided in the second direction D 2  may be mixed. 
     The first electrodes  51  and the second electrodes  52  each have an elongated shape (linear shape) in plan view from the thickness direction D 1  of the piezoelectric layer  4 , with a third direction D 3  orthogonal or substantially 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. 1 . A length of each of the first electrodes  51  is, for example, about 20 μm but is not limited thereto. A width H 1  (first electrode width H 1 ) of each of the first electrodes  51  is, for example, in the range from about 50 nm to about 1000 nm and is, for example, about 500 nm. A length of each of the second electrodes  52  is, for example, about 20 μm but is not limited thereto. A width H 2  (second electrode width H 2 ) of each of the second electrodes  52  is, for example, in the range from about 50 nm to about 1000 nm and is, for example, about 500 nm. 
     The first electrode  51  includes a first electrode main portion  510 . The first electrode main portion  510  intersects the second electrode  52  in the direction in which the first electrode and the second electrode  52  face each other. Further, the second electrode  52  includes a second electrode main portion  520 . The second electrode main portion  520  intersects 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 electrodes  51  have the same or substantially the same first electrode width H 1  but are not limited thereto. In addition, in the acoustic wave device  1  according to Preferred Embodiment 1, the second electrodes  52  have the same or substantially the same second electrode width H 2  but are not limited thereto. 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 or substantially 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, although  FIG. 1  is shown with the number of each of the first electrodes  51  and the second electrodes  52  as five, for example, the number of each of first electrodes  51  and second electrodes  52  is not limited to five and may be, for example, one, two to four, six or more, or fifty or more. 
     The second direction D 2  in which the adjacent first electrode  51  and second electrode  52  face each other is preferably orthogonal or substantially orthogonal to the polarization direction PZ 1  (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 or substantially orthogonal to the third direction D 3  which is the longitudinal direction. Note that, both the first electrode  51  and the second electrode  52  are not rectangular or substantially rectangular, in some cases. In this case, the third direction D 3 , which is the longitudinal direction, may be a long side direction of a circumscribed polygon circumscribing the first electrode  51  and the second electrode  52  in plan view of the first electrode  51  and the second electrode  52 . Note that, when the first wiring portion  61  is connected to the first electrode  51  and the second wiring portion  62  is connected to the second electrode  52 , the “circumscribed polygon circumscribing the first electrode  51  and the second electrode  52 ” at least includes a polygon circumscribing a portion of the first electrode  51  excluding a portion connected to the first wiring portion  61  and a portion of the second electrode  52  excluding a portion connected to the second wiring portion  62 . 
     In the acoustic wave device  1 , as illustrated in  FIG. 2 , at least a portion of each of the first electrodes  51  is embedded in the piezoelectric layer  4 . In addition, in the acoustic wave device  1 , at least a portion of each of the second electrodes  52  is embedded in the piezoelectric layer  4 . 
     In the acoustic wave device  1  according to Preferred Embodiment 1, a thickness of each of the first electrodes  51  is less than the thickness of the piezoelectric layer  4 . Each of the first electrodes  51  includes a first main surface  511  and a second main surface  512  intersecting the thickness direction D 1  of the piezoelectric layer  4 , and two side surfaces  513  and  513  intersecting the width direction of the first electrode  51 . In each of the first electrodes  51 , of the first main surface  511  and the second main surface  512 , the second main surface  512  is located on a side of the acoustic reflection layer  3 . Thus, in the acoustic wave device  1 , a shortest distance between the first main surface  511  of the first electrode  51  and the acoustic reflection layer  3  is greater than a shortest distance between the second main surface  512  of the first electrode  51  and the acoustic reflection layer  3 . In each of the first electrodes  51 , the second main surface  512  and the two side surfaces  513  and  513  are in planar contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1  according to Preferred Embodiment 1, a thickness of each of the second electrodes  52  is less than the thickness of the piezoelectric layer  4 . Each of the second electrodes  52  includes a first main surface  521  and a second main surface  522  intersecting the thickness direction D 1  of the piezoelectric layer  4 , and two side surfaces  523  and  523  intersecting the width direction of the second electrode  52 . In each of the second electrodes  52 , of the first main surface  521  and the second main surface  522 , the second main surface  522  is located on the side of the acoustic reflection layer  3 . Thus, in the acoustic wave device  1 , a shortest distance between the first main surface  521  of the second electrode  52  and the acoustic reflection layer  3  is greater than a shortest distance between the second main surface  522  of the second electrode  52  and the acoustic reflection layer  3 . In each of the second electrodes  52 , the second main surface  522  and the two side surfaces  523  and  523  are in planar contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1 , a portion of the piezoelectric layer  4  is interposed between the side surface  513  of the first electrode  51  and the side surface  523  of the second electrode  52  that face each other, in the second direction D 2 . In each of the first electrodes  51 , the side surface  513  facing the second electrode  52  in the second direction D 2  is in contact with the piezoelectric layer  4 . In each of the second electrodes  52 , the side surface  523  facing the first electrode  51  in the second direction D 2  is in contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1  according to Preferred Embodiment 1, the first main surfaces  511  of the respective first electrodes  51  are flush with the first main surface  41  of the piezoelectric layer  4  but are not limited thereto. In the acoustic wave device  1  according to Preferred Embodiment 1, the first main surfaces  521  of the respective second electrodes  52  are flush with the first main surface  41  of the piezoelectric layer  4  but are not limited thereto. 
     The plurality of first electrodes  51  and the plurality of second electrodes  52  are electrically conductive. A material of the first electrode  51  and the second electrode  52  is, for example, Al (aluminum), Cu (copper), Pt (platinum), Au (gold), Ag (silver), Ti (titanium), Ni (nickel), Cr (chromium), Mo (molybdenum), W (tungsten), alloys including any of these metals as a main component, or the like. Further, the first electrode  51  and the second electrode  52  may each have a structure in which metal films made of these metals or alloys are laminated. Each of the first electrode  51  and the second electrode  52  includes, for example, 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. A thickness of the adhesion film is, for example, about 10 nm. Further, a thickness of the main electrode film is, for example, about 80 nm. In the AlCu film, Cu concentration is preferably from about 1 wt % to about 20 wt %, for example. 
     (1.2.5) First Wiring Portion and Second Wiring Portion 
     The first wiring portion  61  includes a first busbar  611 . The first busbar  611  is a conductor portion making the first electrodes  51  have the same potential. The first busbar  611  has an elongated shape (linear shape) with the second direction D 2  as a longitudinal direction. The first electrodes  51  connected to the first busbar  611  extend toward a second busbar  621 . In the acoustic wave device  1 , a first conductor portion including the plurality of first electrodes  51  and the first busbar  611  has a comb shape in plan view from the thickness direction D 1  of the piezoelectric layer  4 . The first busbar  611  is integrally provided with the plurality of first electrodes  51  but is not limited thereto. Similar to the plurality of first electrodes  51 , at least a portion of the first busbar  611  is embedded in the piezoelectric layer  4  but is not limited thereto. A location of the first busbar  611  in the thickness direction D 1  of the piezoelectric layer  4  is not particularly limited. 
     The second wiring portion  62  includes the second busbar  621 . The second busbar  621  is a conductor portion making the second electrodes  52  have the same potential. The second busbar  621  has an elongated shape (linear shape) with the second direction D 2  as a longitudinal direction. The second electrodes  52  connected to the second busbar  621  extend toward the first busbar  611 . In the acoustic wave device  1 , a second conductor portion including the plurality of second electrodes  52  and the second busbar  621  has a comb-like shape in plan view from the thickness direction D 1  of the piezoelectric layer  4 . The second busbar  621  is integrally provided with the plurality of second electrodes  52  but is not limited thereto. Similar to the plurality of second electrodes  52 , at least a portion of the second busbar  621  is embedded in the piezoelectric layer  4  but is not limited thereto. A location of the second busbar  621  in the thickness direction D 1  of the piezoelectric layer  4  is not particularly limited. 
     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  are electrically conductive. A material of the first wiring portion  61  and the second wiring portion  62  is, for example, Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, an alloy including any of these metals as a main component, or the like. Further, the first wiring portion  61  and the second wiring portion  62  may each have a structure in which metal films made of these metals or alloys are laminated. Each of the first wiring portion  61  and the second wiring portion  62  includes, for example, a laminated film including an adhesion film made of a Ti film and a main wiring film made of an Al film or an AlCu film provided on the adhesion film. A thickness of the adhesion film is, for example, about 10 nm. Further, a thickness of the main wiring film is, for example, about 80 nm. In the AlCu film, Cu concentration is preferably, for example, from about 1 wt % to about 20 wt %. 
     In the acoustic wave device  1 , from a viewpoint of reducing 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. 
     (1.3) Method of Manufacturing Acoustic Wave Device 
     In a non-limiting example of a method of manufacturing the acoustic wave device  1 , for example, a first step to a fourth step are performed after the support substrate  2  is prepared. In the first step, the acoustic reflection layer  3  is formed on the first main surface  21  of the support substrate  2 . In the second step, a piezoelectric substrate, from which the piezoelectric layer  4  is formed, and the support substrate  2  are bonded to each other with the acoustic reflection layer  3  interposed therebetween. In the third step, by thinning the piezoelectric substrate, the piezoelectric layer  4  formed from a portion of the piezoelectric substrate is formed. In the fourth step, the first electrode  51 , the second electrode  52 , the first wiring portion  61 , the second wiring portion  62 , the first terminal T 1 , and the second terminal T 2  are formed by utilizing, for example, a photolithography technique, an etching technique, a thin film forming technique, or the like. In addition, in the first step to the fourth step, a silicon wafer is used as the support substrate  2 , and a piezoelectric wafer is used as the piezoelectric substrate. In the above-described method of manufacturing the acoustic wave device  1 , a wafer including a plurality of acoustic wave devices  1  is diced to obtain a plurality of 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 by utilizing a film forming technique. In this case, the method of manufacturing the acoustic wave device  1  includes a step of forming the piezoelectric layer  4 , instead of the second step and the third step. The piezoelectric layer  4  formed by the film forming technique may be, for example, a single crystal or a twin crystal. Examples of the film forming technique include, but are not limited to, a CVD (Chemical Vapor Deposition) method, for example. 
     (1.4) Operation and Characteristics of Acoustic Wave Device 
     First, an acoustic wave device  1   r  according to Reference Preferred Embodiment 1 of the present invention utilizing a bulk wave in a thickness-shear primary mode will be described with reference to  FIGS. 4, 5B, and 6 . In the acoustic wave device  1   r  according to Reference Preferred Embodiment 1, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   r  according to Reference Preferred Embodiment 1 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that the acoustic wave device  1   r  is provided on the first main surface  41  of the piezoelectric layer  4 . That is, the acoustic wave device  1   r  according to Reference Preferred Embodiment 1 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that the plurality of first electrodes  51  and the plurality second electrodes  52  are not embedded in the piezoelectric layer  4 . 
     As with the acoustic wave device  1  according to Preferred Embodiment 1, the acoustic wave device  1   r  according to Reference Preferred Embodiment 1 is an acoustic wave device that utilizes a bulk wave in the thickness-shear primary mode. As described above, a bulk wave in the thickness-shear primary mode 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 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  in the piezoelectric layer  4  between the first electrode  51  and the second electrode  52  in plan view from the thickness direction D 1 . The thickness-shear vibration can be confirmed by, for example, an FEM (Finite Element Method). More specifically, the thickness-shear vibration can be confirmed by analyzing a displacement distribution by the FEM and analyzing deformation by using, for example, parameters of the piezoelectric layer  4  (the material, the Euler angles, the thickness, and the like), parameters of the first electrode  51  and the second electrode  52  (the material, the thickness, the inter-centerline distance between the first electrode  51  and the second electrode  52 , and the like), and parameters of the acoustic reflection layer  3  (the material, the thickness, and the like). The Euler angles of the piezoelectric layer  4  can be obtained by analysis. 
     A difference between a Lamb wave utilized in an existing acoustic wave device and a bulk wave in the thickness-shear primary mode will be described with reference to  FIGS. 5A and 5B . 
       FIG. 5A  is a schematic elevational sectional view for explaining a Lamb wave propagating through a piezoelectric substrate of an acoustic wave device as described in Japanese Unexamined Patent Application Publication No. 2012-257019. In this acoustic wave device, an acoustic wave propagates through a piezoelectric substrate  400  as indicated by an arrow. Here, the piezoelectric substrate  400  includes a first main surface  401  and a second main surface  402  facing each other. In  FIG. 5A , a Z direction and an X direction are illustrated separately from the piezoelectric substrate  400 . In  FIG. 5A , the Z direction is a thickness direction of the piezoelectric substrate  400  in which the first main surface  401  and the second main surface  402  are connected. The X direction is a direction in which a plurality of first electrode fingers and a plurality of second electrode fingers of an IDT electrode are provided. The Lamb wave is a plate wave in which an acoustic wave propagates in the X direction as illustrated in  FIG. 5A . Thus, in the existing acoustic wave device, since the acoustic wave propagates in the X direction, two reflectors are disposed on respective sides of the IDT electrode to obtain desired resonance characteristics. Thus, in the existing acoustic wave device, a propagation loss of the acoustic wave occurs, and thus, when size reduction is desired, that is, when the number of pairs of the first electrode finger and the second electrode finger is reduced, a Q-value is reduced. 
     On the other hand, in the acoustic wave device  1   r  according to Reference Preferred Embodiment 1, since a vibration is displaced in a thickness-shear direction, the acoustic wave propagates in or substantially in a direction in which the first main surface  41  and the second main surface  42  of the piezoelectric layer  4  are connected, that is, in or substantially in the Z direction and resonates, as illustrated in  FIG. 5B . That is, an X-direction component of the acoustic wave is significantly less than a Z-direction component. In the acoustic wave device  1   r  according to Reference Preferred Embodiment 1, since resonance characteristics are obtained by propagation of the acoustic wave in the Z direction, a reflector is not necessary. Thus, in the acoustic wave device  1   r  according to Reference Preferred Embodiment 1, no propagation loss occurs when an acoustic wave propagates to the reflector. Thus, in the acoustic wave device  1   r  according to Reference Preferred Embodiment 1, even when the number of electrode pairs each including the first electrode  51  and the second electrode  52  is reduced to achieve size reduction, a decrease in the Q-value is less likely to occur. 
     In the acoustic wave device  1   r  according to the reference preferred embodiment, as illustrated in  FIG. 6 , an amplitude direction of a bulk wave in the thickness-shear primary mode is inverted between a first region  451  included in the defined region  45  of the piezoelectric layer  4 , and a second region  452  included in the defined region  45 . In  FIG. 6 , a bulk wave is schematically illustrated by a two-dot chain line when a voltage is applied between the first electrode  51  and the second electrode  52  such that the second electrode  52  is higher than the first electrode  51  in potential. The first region  451  is a portion, of the defined region  45 , between a virtual plane VP 1  and the first main surface  41 , the virtual plane VP 1  being orthogonal or substantially orthogonal to the thickness direction D 1  of the piezoelectric layer  4  and dividing the piezoelectric layer  4  into two. The second region  452  is a portion of the defined region  45  between the virtual plane VP 1  and the second main surface  42 . 
     Hereinafter, a result of a characteristic simulation performed on a structural model of an acoustic wave device of Reference Preferred Embodiment 2 utilizing a bulk wave in a thickness-shear primary mode will be described. With respect to the structural model, the same or similar components to those of the acoustic wave device  1   r  according to Reference Preferred Embodiment 1 will be denoted by the same reference numerals, and described. The structural model differs from that of the acoustic wave device  1   r  according to Reference Preferred Embodiment 1 in that the acoustic reflection layer  3  is not provided. In the simulation, the number of pairs of the first electrode  51  and the second electrode  52  was assumed to be infinite, and the piezoelectric layer  4  was a 120° rotated Y-cut X-propagation LiNbO 3 . In the structural model, the piezoelectric layer  4  is a membrane, and the second main surface  42  of the piezoelectric layer  4  is in contact with air. The structural model will be described in which, in a section ( FIG. 6 ) along the thickness direction D 1  of the piezoelectric layer  4 , an inter-centerline distance between the first electrode  51  and the second electrode  52  is denoted as p, and a thickness of the piezoelectric layer  4  is denoted as d. In addition, in plan view from the thickness direction D 1  of the piezoelectric layer  4 , an area of the first electrode main portion  510  is denoted as S1, an area of the second electrode main portion  520  is denotes as S2, an area of the defined region  45  is denoted as S0, and a structural parameter defined by (S1+S2)/(S1+S2+S0) is denoted as MR. Note that, when at least one of the first electrode  51  and the second electrode  52  is plurally provided on the piezoelectric layer  4  (in other words, when the first electrodes  51  and the second electrodes  52  define a pair of electrode sets, and, for example, 1.5 or more pairs of the electrode sets are provided on the piezoelectric layer  4 ), the above inter-centerline distance p is each inter-centerline distance between the adjacent first electrodes  51  and second electrodes  52 . 
       FIGS. 7A and 7B  are graphs each showing a relationship between fractional bandwidth and d/p of the structural model of the acoustic wave device according to Reference Preferred Embodiment 2 utilizing a thickness-shear mode. In  FIGS. 7A and 7B , a horizontal axis indicates d/p and a vertical axis indicates the fractional bandwidth.  FIGS. 7A and 7B  each shows a case where the piezoelectric layer  4  is a 120° rotated Y-cut X-propagation LiNbO 3 , but the same or similar tendency appears in cases of other cut angles. In addition, in the structural model of the acoustic wave device of the Reference Preferred Embodiment 2, even when the material of the piezoelectric layer  4  is, for example, LiTaO 3 , a relationship between fractional bandwidth and d/p has the same or similar tendency to that in  FIGS. 7A and 7B . In addition, in the structural model, regardless of the number of pairs of the first electrode  51  and the second electrode  52 , a relationship between fractional bandwidth and d/p has the same or similar tendency to that in  FIGS. 7A and 7B . Further, in the structural model, not only when the second main surface  42  of the piezoelectric layer  4  is in contact with air, but also when the second main surface  42  is in contact with the acoustic reflection layer  3 , the relationship between the fractional bandwidth and d/p has the same or similar tendency to that in  FIGS. 7A and 7B . 
     From  FIG. 7A , it can be seen that a value of the fractional bandwidth changes drastically with d/p=about 0.5 as an inflection point in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2. In the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, when d/p≥about 0.5, a coupling coefficient is low and the fractional bandwidth is less than about 5%, regardless of how much d/p is changed within a range of about 0.5&lt;d/p&lt;about 1.6. On the other hand, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, when d/p≤about 0.5, by changing d/p within a range of about 0&lt;d/p≤about 0.5, the coupling coefficient can be increased and the fractional bandwidth can be set to be equal to or greater than about 5%. 
     In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, when d/p≤about 0.24, by changing d/p within a range of about 0&lt;d/p≤about 0.24, the coupling coefficient can be further increased and the fractional bandwidth can be further increased. Even in the acoustic wave device  1  according to Preferred Embodiment 1, as illustrated in  FIG. 2 , in a section along the thickness direction D 1  of the piezoelectric layer  4 , when the inter-centerline distance between 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, a relationship between fractional bandwidth thereof and d/p exhibits the same or similar tendency to the relationship between fractional bandwidth and d/p of the structural model of the acoustic wave device according to Reference Preferred Embodiment 2. 
     In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, when d/p≤about 0.10, by changing d/p within a range of about 0&lt;d/p≤about 0.10, the coupling coefficient can be further increased and the fractional bandwidth can be further increased. 
       FIG. 7B  is a graph in which a portion of  FIG. 7A  is enlarged. As illustrated in  FIG. 7B , in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, the fractional bandwidth changes with d/p=about 0.096 as an inflection point, thus, when d/p≤about 0.096, by changing d/p within a range of d/p≤about 0.096, the coupling coefficient can be further increased and the fractional bandwidth can be further increased as compared with a case of about 0.096&lt;d/p. Further, as shown in  FIG. 7B , the fractional bandwidth changes with d/p=about 0.072 and about 0.048 as respective inflection points, and by setting about 0.048≤d/p≤about 0.072, it is possible to reduce or prevent a change in the coupling coefficient due to a change in d/p and to set the fractional bandwidth to a substantially constant value. 
       FIG. 8  is a graph obtained by plotting a spurious level in a frequency band between a resonant frequency and an anti-resonant frequency, when the thickness d of the piezoelectric layer  4 , the inter-centerline distance p between the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2  are changed in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2 utilizing the thickness-shear mode. In  FIG. 8 , a horizontal axis indicates fractional bandwidth and a vertical axis indicates normalized spurious level. The normalized spurious level is a value obtained by normalizing a spurious level when a spurious level is defined as about 1 in a fractional bandwidth (for example, about 22%) in which a spurious level has the same or substantially the same value even when the thickness d of the piezoelectric layer  4 , the inter-centerline distance p between the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2  are changed.  FIG. 8  shows a case where a Z-cut LiNbO 3  with which the thickness-shear mode can be more suitably excited is provided for the piezoelectric layer  4 , but the same or similar tendency appears in cases of other cut angles. In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, even when the material of the piezoelectric layer  4  is, for example, LiTaO 3 , a relationship between a normalized spurious level and fractional bandwidth has the same or similar tendency to that in  FIG. 8 . In addition, in the structural model, regardless of the number of pairs of the first electrode  51  and the second electrode  52 , a relationship between a normalized spurious level and fractional bandwidth has the same or similar tendency to that in  FIG. 8 . In addition, in the structural model, not only when the second main surface  42  of the piezoelectric layer  4  is in contact with air but also when the second main surface  42  is in contact with the acoustic reflection layer  3 , a relationship between a normalized spurious level and fractional bandwidth has the same or similar tendency to that in  FIG. 8 . 
     It can be seen from  FIG. 8  that when the fractional bandwidth exceeds about 17%, the normalized spurious level is aggregated to about 1. This indicates that, when the fractional bandwidth is equal to or greater than about 17%, some sort of sub-resonance exists in a band between a resonant frequency and an anti-resonant frequency, as in frequency characteristics of impedance illustrated in  FIG. 9 .  FIG. 9  shows frequency characteristics of impedance when a Z-cut LiNbO 3  having Euler angles of about (0°, 0°,90°) is provided as the piezoelectric layer 4, and d/p=about 0.08, and MR=about 0.35 are set. In  FIG. 9 , a portion indicating 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 inter-centerline distance p between the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode width H 2  are changed, a large spurious level is included in the bandwidth between the resonant frequency and the anti-resonant frequency. Such a spurious level is generated by an overtone in a planar direction, mainly in the direction in which the first electrode  51  and the second electrode face each other. Thus, from a viewpoint of reducing or preventing a spurious level in the band, the fractional bandwidth is preferably equal to or less than about 17%, for example. Since the acoustic wave device  1  according to Preferred Embodiment 1 exhibits the same or similar tendency to that in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2 even in a relationship between a normalized spurious level and fractional bandwidth, the fractional bandwidth is preferably equal to or less than about 17%, for example. 
       FIG. 10  shows, with respect to the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, a first distribution region DA 1  in which a fractional bandwidth exceeds about 17% and a second distribution region DA 2  in which a fractional bandwidth is equal to or less than about 17% are illustrated, when d/p and MR are used as parameters, in a case where a Z-cut LiNbO 3  is provided as the piezoelectric layer  4 , and the thickness d of the piezoelectric layer  4 , the inter-centerline distance p between the first electrode  51  and the second electrode  52 , the first electrode width H 1 , and the second electrode widths H 2  are changed. In  FIG. 10 , the first distribution region DA 1  and the second distribution region DA 2  are different in dot density, and the dot density in the first distribution region DA 1  is higher than the dot density in the second distribution region DA 2 . In addition, in  FIG. 10 , an approximate straight line DL 1  of 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 an equation of MR=1.75×(d/p)+0.075. Thus, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, the fractional bandwidth can be equal to or less than about 17%, by satisfying a condition of MR≤1.75×(d/p)+0.075. FIG. 10  shows a case where a Z-cut LiNbO 3  with which the thickness-shear mode can be more suitably excited is provided for the piezoelectric layer  4 , but the same or similar tendency appears in cases of other cut angles. In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, the approximate straight line DL 1  is the same or substantially the same, even when the material of the piezoelectric layer  4  is, for example, LiTaO 3 . In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, the approximate straight line DL 1  is the same or substantially the same, regardless of the number of pairs of the first electrode  51  and the second electrode  52 . In addition, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, the approximate straight line DL 1  is the same or substantially the same, not only when the second main surface  42  of the piezoelectric layer  4  is in contact with air, but also when the second main surface  42  is in contact with an acoustic reflection layer. As in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2, in the acoustic wave device  1  according to Preferred Embodiment 1, the fractional bandwidth is easily set to be equal to or less than about 17%, by satisfying the condition of MR≤1.75×(d/p)+0.075. An approximate straight line DL 2  (hereinafter, also referred to as a second approximate straight line DL 2 ) indicated in  FIG. 10  by an alternate long and short dash line separately from the approximate straight line DL 1  (hereinafter, also referred to as a first approximate straight line DL 1 ) is a line indicating a boundary for reliably setting the fractional bandwidth to equal to or less than 17%. The second approximate straight line DL 2  is expressed by an equation of MR=1.75×(d/p)+0.05. Thus, in the structural model of the acoustic wave device according to Reference Preferred Embodiment 2 and the acoustic wave device  1  according to Preferred Embodiment 1, the fractional bandwidth can be reliably set to be equal to or less than about 17%, by satisfying a condition of MR≤1.75×(d/p)+0.05. 
       FIG. 11  shows frequency characteristics of impedance of the acoustic wave device  1  according to Preferred Embodiment 1 and the acoustic wave device  1   r  according to Reference Preferred Embodiment 1. In  FIG. 11 , a horizontal axis indicates frequency, and a vertical axis indicates impedance Z [dB] of the acoustic wave device  1 . Z [dB] is a value obtained by Z=20×log 10 |Z0|, when impedance of the acoustic wave device  1  is Z0. In  FIG. 11 , an example of the frequency characteristics of impedance of the acoustic wave device  1  according to Preferred Embodiment 1 is indicated by a solid line, and an example of the frequency characteristics of impedance of the acoustic wave device  1   r  according to Reference Preferred Embodiment 1 is indicated by a broken line. It can be seen from  FIG. 11  that, in the acoustic wave device  1  according to Preferred Embodiment 1, capacitance can be made larger, as compared with the acoustic wave device  1   r  according to Reference Preferred Embodiment 1. 
     (1.5) Advantageous Effects 
     The acoustic wave device  1  according to Preferred Embodiment 1 includes 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 D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . The acoustic wave device  1  utilizes a bulk wave in a thickness-shear primary mode. A material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. Each of the first electrode  51  and the second electrode  52  is embedded in the piezoelectric layer  4 . 
     In the acoustic wave device  1  according to Preferred Embodiment 1, a Q-value can be increased and a capacitance can be increased, while reducing in size the acoustic wave device  1  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . Here, in the acoustic wave device  1  according to Preferred Embodiment 1, a resonant frequency is not restricted by an inter-centerline distance between the first electrode  51  and the second electrode  52 , and the resonant frequency can be increased by reducing a thickness of the piezoelectric layer  4 , and thus, high frequency can be supported while reducing in size the acoustic wave device  1  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . In addition, in the acoustic wave device  1  according to Preferred Embodiment 1, each of the first electrode  51  and the second electrode  52  is embedded in the piezoelectric layer  4 , thus, even when a capacitance generated between the first electrode  51  and the second electrode is decreased by reducing the thickness of the piezoelectric layer  4 , the capacitance between the first electrode  51  and the second electrode  52  can be increased without increasing a planar size of each of the first electrode  51  and the second electrode  52 . 
     Further, the acoustic wave device  1  according to Preferred Embodiment 1 includes 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 D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1 , in a section along the thickness direction D 1  of the piezoelectric layer  4 , d/p is equal to or less than about 0.5, where p is the inter-centerline distance between the first electrode  51  and the second electrode  52 , and d is the thickness of the piezoelectric layer  4 . The material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. Each of the first electrode  51  and the second electrode  52  is embedded in the piezoelectric layer  4 . 
     In the acoustic wave device  1  according to Preferred Embodiment 1, a Q-value can be increased and a capacitance can be increased, while reducing in size the acoustic wave device  1  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     In addition, in the acoustic wave device  1  according to Preferred Embodiment 1, the second main surface  42  of the piezoelectric layer  4  is constrained by the acoustic reflection layer  3 , and thus unnecessary waves can be reduced or prevented. In addition, in the acoustic wave device  1  according to Preferred Embodiment 1, the material of the piezoelectric layer  4  is, for example. LiNbO 3  or LiTaO 3 , and a material of the low acoustic impedance layer  31  is silicon oxide. Here, frequency-temperature characteristics of each of LiNbO 3  and LiTaO 3  have a negative inclination, and frequency-temperature characteristics of silicon oxide have a positive inclination. Thus, in the acoustic wave device  1  according to the preferred embodiment, an absolute value of a TCF (Temperature Coefficient of Frequency) can be reduced, and the frequency-temperature characteristics can be improved. 
     (1.6) Modified Example 
     Preferred Embodiment 1 described above is merely one of various preferred embodiments of the present invention. Preferred Embodiment 1 described above can be modified in various ways according to design and the like, as long as at least one of the advantageous effects of various preferred embodiments of the present invention can be achieved. 
     Modified Example 1 
     Hereinafter, an acoustic wave device  1   a  according to Modified Example 1 of Preferred Embodiment 1 will be described with reference to  FIGS. 12 and 13 . Note that, with respect to the acoustic wave device  1   a  according to Modified Example 1, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   a  according to Modified Example 1 is an acoustic wave filter (here, a ladder filter). The acoustic wave device  1   a  includes an input terminal  15 , an output terminal  16 , (for example, two) series-arm resonators RS 1  provided on a first path  12  connecting the input terminal  15  and the output terminal  16 , and (for example, two) parallel-arm resonators RS 2  provided on respective (for example, two) second paths  13  and  14  connecting (for example, two) nodes N 1  and N 2  on the first path  12  and grounds (ground terminals  17  and  18 ), respectively. The ground terminals  17  and  18  may be one common ground. 
     In the acoustic wave device la, each of the series-arm resonators RS 1  and the parallel-arm resonators RS 2  is the acoustic wave resonator  5 . Each of the acoustic wave resonators  5  is a resonator including at least one first electrode  51  and one second electrode  52 . In the acoustic wave device  1   a , the piezoelectric layer  4  is shared by the plurality of acoustic wave resonators  5 . In addition, in the acoustic wave device  1   a , the acoustic reflection layer  3  is shared by the plurality of acoustic wave resonators  5 . A resonant frequency of the parallel-arm resonator RS 2  is lower than a resonant frequency of the series-arm resonator RS 1 . Here, the acoustic wave resonator  5  defining the parallel-arm resonator RS 2  includes, for example, a silicon oxide film provided on the first main surface  41  of the piezoelectric layer  4 , whereas the acoustic wave resonator  5  defining 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  defining the series-arm resonator RS 1  may include a silicon oxide film on the first main surface  41  of the piezoelectric layer  4 , and in this case, a thickness of a silicon oxide film of the acoustic wave resonator  5  defining the series-arm resonator RS 1  only needs to be less than a thickness of a silicon oxide film of the acoustic wave resonator  5  defining the parallel-arm resonator RS 2 . 
     In the acoustic wave device  1   a , the support substrate  2  and the acoustic reflection layer  3  are shared by the plurality of acoustic wave resonators  5 . However, the high acoustic impedance layer  32  (second high acoustic impedance layer  322 ) closest to the piezoelectric layer  4  among the high acoustic impedance layers  32  may be isolated for each acoustic wave resonator  5 . 
     Modified Example 2 
     Hereinafter, an acoustic wave device  1   b  according to Modified Example 2 of Preferred Embodiment 1 will be described with reference to  FIG. 14 . With respect to the acoustic wave device  1   b  according to Modified Example 2, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   b  according to Modified Example 2 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that first electrode  51  and the second electrode  52  penetrate the piezoelectric layer  4 . 
     In the acoustic wave device  1   b  according to Modified Example 2, the second main surface  512  of each of the first electrodes  51  and the second main surface  42  of the piezoelectric layer  4  are flush with each other, and the first main surface  511  of each of the first electrodes  51  and the first main surface  41  of the piezoelectric layer  4  are flush with each other. The second main surface  512  of each of the first electrodes  51  is in contact with the acoustic reflection layer  3 . 
     In addition, in the acoustic wave device  1   b , the second main surface  522  of each of the second electrodes  52  and the second main surface  42  of the piezoelectric layer  4  are flush with each other, and the first main surface  521  of each of the second electrodes  52  and the first main surface  41  of the piezoelectric layer  4  are flush with each other. The second main surface  522  of each of the second electrodes  52  is in contact with the acoustic reflection layer  3 . 
     In the acoustic wave device  1   b  according to Modified Example 2, an area of the side surface  513  of the first electrode  51  that faces the second electrode  52  and is in contact with the piezoelectric layer  4 , and an area of the side surface  523  of the second electrode  52  that faces the first electrode  51  and is in contact with the piezoelectric layer  4 , can be increased, as compared with the acoustic wave device  1  according to Preferred Embodiment 1. Accordingly, a capacitance of the acoustic wave device  1   b  according to Modified Example 2 can be increased without changing a size of the piezoelectric layer  4  in plan view from the thickness direction D 1 , as compared with the acoustic wave device  1  according to Preferred Embodiment 1. 
     Modified Example 3 
     Hereinafter, an acoustic wave device  1   c  according to Modified Example 3 of Preferred Embodiment 1 will be described with reference to  FIG. 15 . With respect to the acoustic wave device  1   c  according to Modified Example 3, the same or similar components to those of the acoustic wave device  1   b  according to Modified Example 2 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   c  according to Modified Example 3 differs from the acoustic wave device  1   b  according to Modified Example 2 in that the first main surface  511  of each of the first electrodes  51  is recessed from the first main surface  41  of the piezoelectric layer  4 , and the first main surface  521  of each of the second electrodes  52  is recessed from the first main surface  41  of the piezoelectric layer  4 . 
     Modified Example 4 
     Hereinafter, an acoustic wave device  1   d  according to Modified Example 4 of Preferred Embodiment 1 will be described with reference to  FIG. 16 . With respect to the acoustic wave device  1   d  according to Modified Example 4, the same or similar components to those of the acoustic wave device  1   b  according to Modified Example 2 are denoted by the same reference numerals, and a description thereof is omitted. 
     In the acoustic wave device  1   d  according to Modified Example 4, the first electrode  51  and the second electrode  52  each include a portion protruding from the piezoelectric layer  4 . Here, the first electrode  51  includes a penetrating portion  51 A penetrating the piezoelectric layer  4 , a first protruding portion  51 B connected to the penetrating portion  51 A and protruding from the first main surface  41  of the piezoelectric layer  4 , and a second protruding portion  51 C connected to the penetrating portion  51 A and protruding from the second main surface  42  of the piezoelectric layer  4 , and the second electrode  52  includes a penetrating portion  52 A penetrating the piezoelectric layer  4 , a first protruding portion  52 B connected to the penetrating portion  52 A and protruding from the first main surface  41  of the piezoelectric layer  4 , and a second protruding portion  52 C connected to the penetrating portion  52 A and protruding from the second main surface  42  of the piezoelectric layer  4 . Thus, in the acoustic wave device  1   d  according to Modified Example 4, a capacitance between the first electrode  51  and the second electrode  52  can be increased, as compared with the acoustic wave device  1   b  according to Modified Example 2. 
     In the acoustic wave device  1   d  according to Modified Example 4, in the first electrode  51 , a protruding dimension HB 1  of the first protruding portion  51 B is greater than a protrusion dimension HC 1  of the second protrusion portion  51 C, and in the second electrode  52 , a protruding dimension HB 2  of the first protruding portion  52 B is greater than a protrusion dimension HC 2  of the second protrusion portion  52 C. Thus, in the acoustic wave device  1   d  according to Modified Example 4, it is possible to improve heat dissipation as compared with a case where the protruding dimension HB 1  of the first protruding portion  51 B is the same or substantially the same as the protruding dimension HC 1  of the second protruding portion  51 C, and the protruding dimension HB 2  of the first protruding portion  52 B is the same or substantially the same as the protruding dimensions HC 2  of the second protruding portion  52 C. 
     In the acoustic wave device  1   d  according to Modified Example 4, the first electrode  51  includes both the first protruding portion  51 B and the second protruding portion  51 C, and the second electrode  52  includes both the first protruding portion  52 B and the second protruding portion  52 C. However, the present invention is not limited thereto, and a configuration may be provided in which only one of the first protruding portions  51 B and  52 B, and only one of the second protruding portions  51 C and  52 C are provided. 
     Modified Example 5 
     Hereinafter, an acoustic wave device  1   e  according to Modified Example 5 of Preferred Embodiment 1 will be described with reference to  FIG. 17 . With respect to the acoustic wave device  1   e  according to Modified Example 5, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   e  according to Modified Example 5 differs from the acoustic wave device  1  according to 
     Preferred Embodiment 1 in that the first electrode  51  and the second electrode  52  are embedded in the piezoelectric layer  4 , and the first main surface  511  of the first electrode  51  and the first main surface  521  of the second electrode  52  are also in contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1   e  according to Modified Example 5, a portion of the piezoelectric layer  4  is located between the first main surface  511  of the first electrode  51  and the first main surface  41  of the piezoelectric layer  4 . In addition, in the acoustic wave device  1   e  according to Modified Example 5, a portion of the piezoelectric layer  4  is located between the first main surface  521  of the second electrode  52  and the first main surface  41  of the piezoelectric layer  4 . In the acoustic wave device  1   e  according to Modified Example 5, at the time of manufacturing thereof, the piezoelectric layer  4  only needs to be formed by, for example, a film forming technique. 
     The acoustic wave device  1   e  according to Modified Example 5 has an advantage, as compared with the acoustic wave device  1  according to Preferred Embodiment 1, in that variations in a capacitance between the first electrode  51  and the second electrode  52  can be easily reduced or prevented. 
     Modified Example 6 
     Hereinafter, an acoustic wave device  1   f  according to Modified Example 6 of Preferred Embodiment 1 will be described with reference to  FIG. 18 . With respect to the acoustic wave device  1   f  according to Modified Example 6, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   f  according to Modified Example 6 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that, in addition to portions  514  and  524  embedded in the piezoelectric layer  4 , the first electrode  51  and the second electrode  52  respectively include portions  515  and  525  embedded in the low acoustic impedance layer  31  of the acoustic reflection layer  3 . 
     In the acoustic wave device  1   f  according to Modified Example 6, a capacitance between the first electrode  51  and the second electrode  52  can be increased, as compared with the acoustic wave device  1  according to Preferred Embodiment 1. Note that, a material of the high acoustic impedance layer  32  is not limited to a conductor such as, for example, Pt and may be an insulator (for example, silicon nitride, aluminum nitride, alumina, tantalum oxide, or the like). When the material of the high acoustic impedance layer  32  is an insulator, the first electrode  51  and the second electrode  52  may be embedded in the low acoustic impedance layer  31  and the high acoustic impedance layer  32 . In addition, in the acoustic reflection layer  3 , a layer in contact with the second main surface  42  of the piezoelectric layer  4  may be the high acoustic impedance layer  32 , and in this case, the first electrode  51  and the second electrode  52  may be embedded in the high acoustic impedance layer  32 , or may be embedded in the high acoustic impedance layer  32  and the low acoustic impedance layer  31 . 
     Modified Example 7 
     Hereinafter, an acoustic wave device  1   g  according to Modified Example 7 of Preferred Embodiment 1 will be described with reference to  FIG. 19 . With respect to the acoustic wave device  1   g  according to Modified Example 7, similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     In the acoustic wave device  1   g  according to Modified Example 7, in the first electrode  51  and the second electrode  52 , the first electrode  51  includes (for example, three) conductor portions  516 ,  517 , and  518  arranged in the thickness direction D 1  of the piezoelectric layer  4 , and the second electrode  52  includes (for example, three) conductor portions  526 ,  527 , and  528  arranged in the thickness direction D 1  of the piezoelectric layer  4 . 
     In the first electrode  51 , the conductor portion  516 , the conductor portion  517 , and the conductor portion  518  are provided in this order from a side of the acoustic reflection layer  3 . The conductor portion  516  is embedded in the low acoustic impedance layer  31  (third low acoustic impedance layer  313 ) of the acoustic reflection layer  3 , and the piezoelectric layer  4 . The conductor portion  517  is embedded in the piezoelectric layer  4 . In addition, in the first electrode  51 , a part of the conductor portion  518  is embedded in the piezoelectric layer  4 . 
     In addition, in the second electrode  52 , the conductor portion  526 , the conductor portion  527 , and the conductor portion  528  are provided in this order from the side of the acoustic reflection layer  3 . The conductor portion  526  is embedded in the low acoustic impedance layer  31  (third low acoustic impedance layer  313 ) of the acoustic reflection layer  3 , and the piezoelectric layer  4 . The conductor portion  527  is embedded in the piezoelectric layer  4 . In addition, in the second electrode  52 , a part of the conductor portion  528  is embedded in the piezoelectric layer  4 . 
     In the acoustic wave device  1   g  according to Modified Example 7, a capacitance between the first electrode  51  and the second electrode  52  can be increased, as compared with the acoustic wave device  1  according to Preferred Embodiment 1. 
     In the acoustic wave device  1   g  according to Modified Example 7, the three conductor portions  516 ,  517 , and  518  are included in the first electrode  51 . However, the number of the conductor portions included in the first electrode  51  is not limited to three and may be two, or four or more. In addition, in the acoustic wave device  1   g  according to Modified Example 7, the three conductor portions  526 ,  527 , and  528  are included in the second electrode  52 . However, the number of the conductor portions included in the second electrode  52  is not limited to three and may be two, or four or more. In the acoustic wave device  1   g  according to Modified Example 7, at the time of manufacturing thereof, for example, it is sufficient that the piezoelectric layer  4  is formed in a plurality of steps by, for example, a film forming technique, and the first electrode  51  and the second electrode  52  are formed in a plurality of steps. An air layer may be formed between two of the conductor portions,  516  and  517  adjacent to each other in the thickness direction D 1  of the piezoelectric layer  4 , or an air layer may be formed between two of the conductor portions,  517  and  518 . Such a structure can be formed by utilizing, for example, a micromachining technique using a sacrificial layer. 
     Modified Example 8 
     Hereinafter, an acoustic wave device  1   h  according to Modified Example 8 of Preferred Embodiment 1 will be described with reference to  FIG. 20 . With respect to the acoustic wave device  1   h  according to Modified Example 8, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   h  according to Modified Example 8 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that two reflectors  8  are further provided. 
     Each of the two reflectors  8  is a short-circuited grating. Each reflector  8  does not reflect a bulk wave in a 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 located on a side opposite to a side of the second electrode  52  of the first electrode  51  of a plurality of first electrodes  51  located at an end in a direction along a propagation direction of an unnecessary surface acoustic wave of the acoustic wave device  1   h.  The other one reflector  8  of the two reflectors  8  is located on a side opposite to a side of the first electrode  51  of the second electrode  52  of a plurality of second electrodes  52  located at an end in the direction along the propagation direction of the unnecessary surface acoustic wave of the acoustic wave device  1   h.    
     Each reflector  8  includes a plurality of (for example, three) electrode fingers  81 , and one end of each of the respective electrode fingers  81  is short-circuited to each other, and another end is short-circuited to each other. In each reflector  8 , the number of the electrode fingers  81  is not particularly limited. 
     Each reflector  8  is electrically conductive. A material of each reflector  8  is, for example, Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, an alloy mainly containing any one of these metals, or the like. Further, each reflector  8  may have a structure in which metal films made of these metals or alloys are laminated. Each reflector  8  includes, for example, 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. A thickness of the adhesion film is, for example, about 10 nm. Further, a thickness of the main electrode film is, for example, about 80 nm. 
     In addition, in the acoustic wave device  1   h  according to Modified Example 8, each reflector  8  is a short-circuited grating but is not limited thereto and may be, for example, an open grating, a positive-negative reflection grating, a grating in which a short-circuited grating and an open grating are combined, or the like. In addition, in the acoustic wave device  1   h,  two reflectors  8  are provided, however, only one of the two reflectors  8  may be provided. 
     Preferred Embodiment 2 
     Hereinafter, an acoustic wave device  1   i  according to Preferred Embodiment 2 of the present invention will be described with reference to  FIG. 21 . With respect to the acoustic wave device  1   i  according to Preferred Embodiment 2, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   i  according to Preferred Embodiment 2 differs from the acoustic wave device  1  according to Preferred Embodiment 1 in that the first electrode  51  and the second electrode  52  are embedded in the acoustic reflection layer  3  and are in contact with the piezoelectric layer  4 . 
     The first electrode  51  and the second electrode  52  are in planar contact with the second main surface  42  of the piezoelectric layer  4 . Further, a portion of the low acoustic impedance layer  31  (third low acoustic impedance layer  313 ) of the acoustic reflection layer  3  is interposed between the first electrode  51  and the second electrode  52  in the second direction D 2 . In the acoustic reflection layer  3 , a material of the low acoustic impedance layer  31  is, for example, silicon oxide, and a material of the high acoustic impedance layer  32  is, for example, Pt. The portion of the low acoustic impedance layer  31  interposed between the first electrode  51  and the second electrode  52  also has a function as a dielectric portion interposed between the first electrode  51  and the second electrode  52 . A thickness of each of the first electrode  51  and the second electrode  52  is less than a thickness of the low acoustic impedance layer  31 . Thus, the first electrode  51  and the second electrode  52  are not in contact with the high acoustic impedance layer  32 . 
     The acoustic wave device  1   i  according to Preferred Embodiment 2 includes 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 D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . The acoustic wave device  1  utilizes a bulk wave in a thickness-shear primary mode. A material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. The piezoelectric layer  4  is provided on the acoustic reflection layer  3 . At least a portion of each of the first electrode  51  and the second electrode  52  is embedded in the acoustic reflection layer  3  and is in contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1   i  according to Preferred Embodiment 2, a Q-value can be increased and a capacitance can be increased, while achieving size reduction of the acoustic wave device  1   i  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     Further, the acoustic wave device  1   i  according to Preferred Embodiment 2 includes 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 D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1 , in a section along the thickness direction D 1  of the piezoelectric layer  4 , d/p is equal to or less than about 0.5, where p is an inter-centerline distance between the first electrode  51  and the second electrode  52 , and d is a thickness of the piezoelectric layer  4 . The material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. The piezoelectric layer  4  is provided on the acoustic reflection layer  3 . At least a portion of each of the first electrode  51  and the second electrode  52  is embedded in the acoustic reflection layer  3  and is in contact with the piezoelectric layer  4 . 
     In the acoustic wave device  1   i  according to Preferred Embodiment 2, a Q-value can be increased and capacitance can be increased, while achieving size reduction of the acoustic wave device  1   i  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1   i  according to Preferred Embodiment 2, since a portion of the low acoustic impedance layer  31  made of, for example, silicon oxide, which is a type of dielectric, is interposed between the first electrode  51  and the second electrode  52  facing each other, the capacitance between the first electrode  51  and the second electrode  52  can be increased. In addition, in the acoustic wave device  1   i  according to Preferred Embodiment 2, since the first electrode  51  and the second electrode  52  face the high acoustic impedance layer  32  made of, for example, Pt, which is a type of conductor, with the low acoustic impedance layer  31  interposed therebetween, the capacitance between the first electrode  51  and the second electrode  52  can be further increased. 
     When the material of the high acoustic impedance layer (second high acoustic impedance layer  322 ) is not metal but an insulator, the first electrode  51  and the second electrode  52  may be in contact with the high acoustic impedance layer  32  or may be embedded in the high acoustic impedance layer  32 . 
     Preferred Embodiment 2 described above is merely one of various preferred embodiments of the present invention. Preferred Embodiment 2 described above can be modified in various ways according to design and the like, as long as at least one of the advantageous effects of various preferred embodiments of the present invention can be achieved. 
     Modified Example 1 of Preferred Embodiment 2 
     Hereinafter, an acoustic wave device  1   j  according to Modified Example 1 of Preferred Embodiment 2 will be described with reference to  FIG. 22 . With respect to the acoustic wave device  1   j  according to Modified Example 1, the same or similar components to those of the acoustic wave device  1   i  according to Preferred Embodiment 2 are denoted by the same reference numerals, and a description thereof is omitted. 
     In the acoustic wave device  1   j  according to Modified Example 1 of Preferred Embodiment 2, the first electrode  51  includes a bottom electrode  51 D, and a top electrode  51 E, and the second electrode  52  includes a bottom electrode  52 D, and a top electrode  52 E. The bottom electrodes  51 D and  52 D are embedded in the acoustic reflection layer  3  and are in planar contact with the second main surface  42  of the piezoelectric layer  4 . The top electrodes  51 E and  52 E are provided on the first main surface  41  of the piezoelectric layer  4  and are in planar contact with the first main surface  41 . 
     A capacitance of the acoustic wave device  1   j  according to Modified Example 1 of the Preferred Embodiment 2 can be made larger, as compared with the acoustic wave device  1   i  according to Preferred Embodiment 2. 
     Other Modified Examples of Preferred Embodiment 2 
     In the acoustic wave device  1   j,  a sectional shape of each of the top electrodes  51 E and  52 E is a rectangular or substantially rectangular shape but is not limited thereto. The top electrodes  51 E and  52 E may each have a shape in which a width of a lower end is greater than a width of an upper end, for example, as in any one of  FIGS. 23A to 23D . This makes it possible to increase a capacitance between the first electrode  51  and the second electrode  52 , without increasing a width of an upper surface of each of the top electrodes  51 E and  52 E. 
     Each of the top electrodes  51 E and  52 E illustrated in  FIG. 23A  includes a portion with a constant or substantially constant width on an upper end side, and a portion with a gradually increasing width on a lower end side. Further, the top electrodes  51 E and  52 E illustrated in  FIG. 23B  each have a trapezoidal or substantially trapezoidal shape in cross section. In addition, each of the top electrodes  51 E and  52 E illustrated in  FIG. 23C  has a shape spreading toward an end, and both side surfaces in a width direction are curved surfaces. In addition, the top electrodes  51 E and  52 E illustrated in  FIG. 23D  each include a portion with a trapezoidal or substantially trapezoidal shape in section on an upper end side, and a portion on a lower end side with a trapezoidal or substantially trapezoidal shape in section wider than the portion having the trapezoidal or substantially trapezoidal shape in section on the upper end side. 
     Additionally, as illustrated in any one of  FIGS. 24A to 24C , the acoustic wave device  1   j  may include a dielectric film  9  covering the first main surface  41  of the piezoelectric layer  4 , and the top electrodes  51 E and  52 E on the first main surface  41 . In  FIG. 24A , a thickness of the dielectric film  9  is less than a thickness of each of the top electrodes  51 E and  52 E, and a surface of the dielectric film  9  has a concavo-convex shape along a shape of a base. In  FIG. 24B , surface of the dielectric film  9  is flattened to be planar. In  FIG. 24C , a thickness of the dielectric film  9  is greater than a thickness of each of the top electrodes  51 E and  52 E, and a surface of the dielectric film  9  has a concavo-convex shape along a shape of a base. 
     These Modified Examples can be applied to the acoustic wave device  1  according to Preferred Embodiment 1 and an acoustic wave device  1   k  according to Preferred Embodiment 3 described later. 
     Preferred Embodiment 3 
     Hereinafter, an acoustic wave device  1   k  according to Preferred Embodiment 3 of the present invention will be described with reference to  FIGS. 25 and 26 . With respect to the acoustic wave device  1   k  according to Preferred Embodiment 3, the same or similar components to those of the acoustic wave device  1  according to Preferred Embodiment 1 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   k  according to Preferred Embodiment 3 does not include the acoustic reflection layer  3  of the acoustic wave device  1  according to Preferred Embodiment 1. In the acoustic wave device  1   k  according to Preferred Embodiment 3, the piezoelectric layer  4  is provided on the support substrate  2 . Here, the support substrate  2  is, for example, a silicon substrate. The piezoelectric layer  4  is bonded to the support substrate  2  with, for example, a silicon oxide film  7  interposed therebetween. The acoustic wave device  1   k  further includes a cavity  26 . The cavity  26  overlaps the acoustic wave resonator  5  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . The cavity  26  is located immediately below the acoustic wave resonator  5 . The acoustic wave resonator  5  includes the first electrode  51  and the second electrode  52  in plan view from the thickness direction D 1  of the piezoelectric layer  4 , and a portion (defined region  45 ) between the first electrode  51  and the second electrode  52  in the piezoelectric layer  4  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1   k  according to Preferred Embodiment 3, the cavity  26  extends through the support substrate  2  and the silicon oxide film  7 , and exposes a portion of the piezoelectric layer  4  (a portion of the second main surface  42 ). In the acoustic wave device  1   k  according to Preferred Embodiment 3, the acoustic wave resonator  5  does not include the acoustic reflection layer  3  of the acoustic wave device  1  according to Preferred Embodiment 1. The cavity  26  overlaps a portion of each of the first wiring portion  61  and the second wiring portion  62  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . However, the cavity  26  need not overlap a portion of each of the first wiring portion  61  and the second wiring portion  62  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     A thickness of the support substrate 2 is, for example, equal to or greater than about 100 μm and equal to or less than about 500 μm. A thickness of the silicon oxide film  7  is, for example, equal to or greater than about 0.1 μm and equal to or less than about 10 μm. A thickness of the piezoelectric layer  4  is the same or substantially the same as the thickness of the piezoelectric layer  4  of the acoustic wave device  1  according to Preferred Embodiment 1. 
     In a non-limiting example of a method of manufacturing the acoustic wave device  1   k,  for example, from a first step to a fifth step are performed after the support substrate  2  is prepared. In the first step, a silicon oxide film is formed on the first main surface  21  of the support substrate  2 . In the second step, a piezoelectric substrate from which the piezoelectric layer  4  is formed and the support substrate  2  are bonded to each other with a silicon oxide film interposed therebetween. In the third step, by thinning the piezoelectric substrate, the piezoelectric layer  4  formed from a portion of the piezoelectric substrate is formed. In the fourth step, the first electrode  51 , the second electrode  52 , the first wiring portion  61 , the second wiring portion  62 , the first terminal T 1 , and the second terminal T 2  are formed on the piezoelectric layer  4 . In the fifth step, the cavity  26  is formed. In the fourth step, the first electrode  51 , the second electrode  52 , the first wiring portion  61 , the second wiring portion  62 , the first terminal T 1 , and the second terminal T 2  are formed by utilizing, for example, a photolithography technique, an etching technique, a thin film forming technique, or the like. In the fifth step, a region of the support substrate  2  and the silicon oxide film where the cavity  26  is to be formed is etched by, for example, utilizing a photolithography technique, an etching technique, or the like. In the fifth step, etching is performed using, for example, the silicon oxide film as an etching stopper layer, and then an unnecessary portion of the silicon oxide film is removed by etching to expose a portion of the second main surface  42  of the piezoelectric layer  4 . In addition, in the first step to the fifth step, for example, a silicon wafer is used as the support substrate  2 , and a piezoelectric wafer is used as the piezoelectric substrate. In the method of manufacturing the acoustic wave device  1   k,  a wafer including a plurality of acoustic wave devices  1   k  is diced to obtain a plurality of acoustic wave devices  1   k  (chips). 
     The method of manufacturing the acoustic wave device  1   k  is an example and is not particularly limited. For example, the piezoelectric layer  4  may be formed by utilizing a film forming technique. In this case, the method of manufacturing the acoustic wave device  1   k  includes a step of forming the piezoelectric layer  4 , instead of the second step and the third step. The piezoelectric layer  4  formed by the film forming technique may be, for example, a single crystal or a twin crystal. Examples of the film forming technique include, but are not limited to, a CVD method. 
     The acoustic wave device  1   k  according to Preferred Embodiment 3 includes the piezoelectric layer  4 , the first electrode  51 , and the second electrode  52 , the same as or similar to the acoustic wave device  1  according to Preferred Embodiment 1. The first electrode  51  and the second electrode  52  face each other in the direction D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . The acoustic wave device  1  utilizes a bulk wave in a thickness-shear primary mode. A material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. Each of the first electrode  51  and the second electrode is embedded in the piezoelectric layer  4 . With the above configuration, in the acoustic wave device  1   k  according to Preferred Embodiment 3, a Q-value can be increased and a capacitance can be increased, while achieving reducing in size the acoustic wave device  1   k  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     Further, the acoustic wave device  1   k  according to Preferred Embodiment 3 includes the piezoelectric layer  4 , the first electrode  51 , and the second electrode  52 , the same as or similar to the acoustic wave device  1  according to Preferred Embodiment 1. The first electrode  51  and the second electrode  52  face each other in the direction D 2  intersecting the thickness direction D 1  of the piezoelectric layer  4 . In the acoustic wave device  1 , in a section along the thickness direction D 1  of the piezoelectric layer  4 , d/p is, for example, equal to or less than about 0.5, where p is an inter-centerline distance between the first electrode  51  and the second electrode  52 , and d is a thickness of the piezoelectric layer  4 . The material of the piezoelectric layer  4  is, for example, lithium niobate or lithium tantalate. Each of the first electrode  51  and the second electrode is embedded in the piezoelectric layer  4 . With the above configuration, in the acoustic wave device  1   k  according to Preferred Embodiment 3, a Q-value can be increased and a capacitance can be increased, while achieving reducing in size the acoustic wave device  1   k  in plan view from the thickness direction D 1  of the piezoelectric layer  4 . 
     Further, the acoustic wave device  1   k  according to Preferred Embodiment 3 includes the cavity  26 . Thus energy of a bulk wave is confined inside the piezoelectric layer  4 , and a favorable Q-value can be obtained. 
     In the acoustic wave device  1   k  according to Preferred Embodiment 3, the piezoelectric layer  4  is bonded to the support substrate  2  with the silicon oxide film  7  interposed therebetween. However, the silicon oxide film  7  is not necessary. Further, in addition to the silicon oxide film  7 , another layer may be laminated between the support substrate  2  and the piezoelectric layer  4 . In addition, in the acoustic wave device  1   k  according to Preferred Embodiment 3, the cavity  26  penetrates the support substrate  2  in a thickness direction thereof but is not limited thereto and may be defined by an internal space of a recessed portion in the first main surface  21  of the support substrate  2 , without penetrating the support substrate  2 . Further, the acoustic wave resonator  5  may include another film (for example, a dielectric film such as the silicon oxide film  7 ) laminated on the second main surface  42  of the piezoelectric layer  4 . Further, another substrate that overlaps the cavity  26  in plan view may be laminated on a side of the support substrate  2  opposite to a side of the piezoelectric layer  4 . A material of the other support substrate is, for example, Si. 
     Preferred Embodiment 3 described above is merely one of various preferred embodiments of the present invention. Preferred Embodiment 3 described above can be modified in various ways according to design and the like, as long as at least one of the advantageous effects of various preferred embodiments of the present invention can be achieved. 
     Modified Example of Preferred Embodiment 3 
     Hereinafter, an acoustic wave device  1   m  according to a Modified Example of Preferred Embodiment 3 will be described with reference to  FIG. 27 . Note that, with respect to the acoustic wave device  1   m  according to Preferred Embodiment 3, the same or similar components to those of the acoustic wave device  1   k  according to Preferred Embodiment 3 are denoted by the same reference numerals, and a description thereof is omitted. 
     The acoustic wave device  1   m  according to the Modified Example of Preferred Embodiment 3 differs from the acoustic wave device  1   k  according to Preferred Embodiment 3 in that the two reflectors  8  are further provided as in the acoustic wave device  1   h  (see  FIG. 20 ) according to Modified Example 8 of Preferred Embodiment 1. A configuration of each reflector  8  is the same as or similar to that of each reflector  8  of the acoustic wave device  1   h.    
     Other Modified Examples 
     For example, as in an acoustic wave device  1   n  illustrated in  FIG. 28 , respective sectional shapes of the first electrode  51  and the second electrode  52  may be different from each other. Here, the sectional shape is, for example, a shape in cross section orthogonal or substantially orthogonal to the thickness direction D 1  and the second direction D 2  of the piezoelectric layer  4 . In  FIG. 28 , the sectional shape of the first electrode  51  is the same or substantially the same as a sectional shape of the acoustic wave device  1   b  (see  FIG. 14 ), and the cross-sectional shape of the second electrode  52  is the same or substantially the same as a cross-sectional shape of the acoustic wave device  1   c  (see  FIG. 15 ), but the present invention is not limited to these combinations. 
     In addition, when an acoustic wave filter is configured as in the acoustic wave device  1   a  according to Modified Example 1 of the Preferred Embodiment 1, the first electrode  51  and the second electrode  52  may have different shapes, respectively, for each acoustic wave resonator  5 . Further, the respective shapes of the first electrode  51  and the second electrode  52  may be different between the acoustic wave resonator  5  defining the series-arm resonator RS 1  and the acoustic wave resonator  5  defining the parallel-arm resonator RS 2 . 
     Further, instead of the acoustic wave resonator  5  in the acoustic wave device  1   a  according to Modified Example 1 of Preferred Embodiment 1, the acoustic wave resonator  5  according any one of Modified Examples 2 to 8 of Preferred Embodiment 1, Preferred Embodiment 2, the Modified Examples of Preferred Embodiment 2, Preferred Embodiment 3, the Modified Example of Preferred Embodiment 3, other Modified Examples, and the like, may be provided. 
     Further, each of the first electrode  51  and the second electrode  52  is not limited to being linear in plan view from the thickness direction D 1  of the piezoelectric layer  4 . For example, each of 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. 
     In addition, in the acoustic wave device  1   k  according to Preferred Embodiment 3 and the acoustic wave device  1   m  according to the Modified Example of the Preferred Embodiment 3, for the first electrode  51  and the second electrode  52 , the structure the same as or similar to the first electrode  51  and the second electrode  52  according to any one of Modified Examples 3 to 8 of Preferred Embodiment 1, Preferred Embodiment 2, and the Modified Example of Preferred Embodiment 2 may be provided. For the configuration embedded in the acoustic reflection layer  3  and in contact with the piezoelectric layer  4 , for example, a configuration in contact with the second main surface  42  of the piezoelectric layer  4  and exposed by the cavity  26  may be provided. 
     The following aspects of the present invention are disclosed in the present specification from the above-described preferred embodiments and the like. 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ) includes 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 (D 2 ) intersecting a thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The acoustic wave device ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ) utilizes a bulk wave in a thickness-shear primary mode. A material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. At least a portion of each of the first electrode ( 51 ) and the second electrode ( 52 ) is embedded in the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), a Q-value can be increased and a capacitance can be increased while achieving size reduction. 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ) includes 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 (D 2 ) intersecting a thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The first electrode ( 51 ) and the second electrode ( 52 ) are electrodes adjacent to each other. In the acoustic wave device ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), in a section along the thickness direction (D 1 ) of the piezoelectric layer ( 4 ), d/p is equal to or less than about 0.5, where p is an inter-centerline distance between the first electrode ( 51 ) and the second electrode ( 52 ), and d is a thickness of the piezoelectric layer ( 4 ). A material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. At least a portion of each of the first electrode ( 51 ) and the second electrode ( 52 ) is embedded in the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), a Q-value can be increased and a capacitance can be increased while achieving size reduction. 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ) further includes a support substrate ( 2 ) and an acoustic reflection layer ( 3 ). The support substrate ( 2 ) includes a first main surface ( 21 ) and a second main surface ( 22 ) facing each other. The acoustic reflection layer ( 3 ) is on the first main surface ( 21 ) of the support substrate ( 2 ). The piezoelectric layer ( 4 ) is on the acoustic reflection layer ( 3 ). The acoustic reflection layer ( 3 ) includes at least one high acoustic impedance layer ( 32 ), and at least one low acoustic impedance layer ( 31 ). The at least one low acoustic impedance layer ( 31 ) has an acoustic impedance lower than that of the at least one high acoustic impedance layer ( 32 ). 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1   k;    1   m ) further includes a support substrate ( 2 ). The support substrate ( 2 ) includes a first main surface ( 21 ) and a second main surface ( 22 ) facing each other. The piezoelectric layer ( 4 ) is on the support substrate ( 2 ). The acoustic wave device ( 1   k;    1   m ) further includes a cavity ( 26 ). The cavity ( 26 ) overlaps an acoustic wave resonator ( 5 ) including the first electrode ( 51 ), the second electrode ( 52 ), and a portion (defined region  45 ) between the first electrode ( 51 ) and the second electrode ( 52 ) in the piezoelectric layer ( 4 ) in plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The cavity ( 26 ) is on a side of the support substrate ( 2 ) with respect to the piezoelectric layer ( 4 ). 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1   i ;  1   j ) includes 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 (D 2 ) intersecting a thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The acoustic wave device ( 1 ;  1   a ) utilizes a bulk wave in a thickness-shear primary mode. The acoustic wave device ( 1   i ;  1   j ) further includes an acoustic reflection layer ( 3 ). A material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. The piezoelectric layer ( 4 ) is on the acoustic reflection layer ( 3 ). The first electrode ( 51 ) and the second electrode ( 52 ) are embedded in the acoustic reflection layer ( 3 ) and are in contact with the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   i ;  1   j ), a Q-value can be increased, and a capacitance can be increased, while achieving size reduction. 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1   i ;  1   j ) includes 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 (D 2 ) intersecting a thickness direction (D 1 ) of the piezoelectric layer ( 4 ). In the acoustic wave device ( 1   i ;  1   j ), in a section along the thickness direction (D 1 ) of the piezoelectric layer ( 4 ), d/p is equal to or less than about 0.5, where p is an inter-centerline distance between the first electrode ( 51 ) and the second electrode ( 52 ), and d is a thickness of the piezoelectric layer ( 4 ). The acoustic wave device ( 1   i ;  1   j ) further includes an acoustic reflection layer ( 3 ). A material of the piezoelectric layer ( 4 ) is lithium niobate or lithium tantalate. The piezoelectric layer ( 4 ) is on the acoustic reflection layer ( 3 ). The first electrode ( 51 ) and the second electrode ( 52 ) are embedded in the acoustic reflection layer ( 3 ) and are in contact with the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   i ;  1   j ), a Q-value can be increased, and a capacitance can be increased, while achieving size reduction. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), d/p is equal to or less than about 0.24. 
     In the acoustic wave device according to the above-described preferred embodiment (( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), it is possible to increase a fractional bandwidth. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), the first electrode ( 51 ) and the second electrode ( 52 ) are adjacent to each other. The first electrode ( 51 ) includes a first electrode main portion ( 510 ), and the second electrode ( 52 ) includes a second electrode main portion ( 520 ). The first electrode main portion ( 510 ) intersects the second electrode ( 52 ) in the direction in which the first electrode ( 51 ) and the second electrode ( 52 ) face each other. The second electrode ( 52 ) intersects the first electrode ( 51 ) in the direction in which the first electrode ( 51 ) and the second electrode ( 52 ) face each other. The piezoelectric layer ( 4 ) includes a defined region ( 45 ). The defined region ( 45 ) intersects both 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 in the piezoelectric layer ( 4 ), and is located between the first electrode ( 51 ) and the second electrode ( 52 ) in plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). The acoustic wave device ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ) satisfies the following condition. The condition is MR≤1.75×(d/p)+0.075. Here, S1 is an area of the first electrode main portion ( 510 ) in plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). S2 is an area of the second electrode main portion ( 520 ) in plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). S0 is an area of the defined region ( 45 ) in plan view from the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). MR is a structural parameter defined by (S1+S2)/(S1+S2+S0). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   k ;  1   m ;  1   n ), it is possible to reduce or prevent a spurious level in a band. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   b ;  1   d ), the first electrode ( 51 ) penetrates the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   b ;  1   d ), the capacitance can be increased as compared with a case where the first electrode ( 51 ) does not penetrate the piezoelectric layer ( 4 ). 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   d;    1   g;    1   j ), the first electrode ( 51 ) includes a portion protruding from the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   d;    1   g;    1   j ), heat dissipation can be improved. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   d ), the piezoelectric layer ( 4 ) includes a first main surface ( 41 ) and a second main surface ( 42 ) facing each other in the thickness direction (D 1 ). The first electrode ( 51 ) includes a penetrating portion ( 51 A), a first protruding portion ( 51 B), and a second protruding portion ( 51 C). The penetrating portion ( 51 A) penetrates the piezoelectric layer ( 4 ). The first protruding portion ( 51 B) is connected to the penetrating portion ( 51 A), and protrudes from the first main surface ( 41 ) of the piezoelectric layer ( 4 ). The second protruding portion ( 51 C) is connected to the penetrating portion ( 51 A) and protrudes from the second main surface ( 42 ) of the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   d ), the capacitance can be further increased. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   d ), a protruding dimension (HB 1 ) of the first protruding portion ( 51 B) is greater than a protruding dimension (HC 1 ) of the second protruding portion ( 51 C). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   d ), heat dissipation can be improved. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   g ), the first electrode ( 51 ) includes a plurality of conductor portions ( 516 ,  517 ,  518 ) in the thickness direction (D 1 ) of the piezoelectric layer ( 4 ). 
     In the acoustic wave device according to the above-described preferred embodiment ( 1   g ), the capacitance can be further increased. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1   n ), the first electrode ( 51 ) and the second electrode ( 52 ) have respective sectional shapes different from each other. 
     In an acoustic wave device according to a preferred embodiment of the present invention ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   i ;  1   j;    1   k ;  1   m ;  1   n ), the first electrode ( 51 ) includes a plurality of first electrodes ( 51 ) and the second electrode ( 52 ) includes a plurality of second electrodes ( 52 ). The first electrodes ( 51 ) and the second electrodes ( 52 ) are alternately provided one by one. The acoustic wave device ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ; 1   g;    1   h ;  1   i ;  1   j;    1   k ;  1   m ;  1   n ), further includes a first wiring portion ( 61 ) and a second wiring portion ( 62 ). The plurality of first electrodes ( 51 ) is connected to the first wiring portion ( 61 ). The plurality of second electrodes ( 52 ) is connected to the second wiring portion ( 62 ). 
     In the acoustic wave device according to the above-described ( 1 ;  1   a ;  1   b ;  1   c ;  1   d ;  1   e ;  1   f ;  1   g;    1   h ;  1   i ;  1   j;    1   k ;  1   m ;  1   n ), the capacitance can be further increased. 
     An acoustic wave device according to a preferred embodiment of the present invention ( 1   a ), is an acoustic wave filter including a plurality of acoustic wave resonators ( 5 ). Each of the acoustic wave resonators ( 5 ) includes the first electrode ( 51 ) and the second electrode ( 52 ). The piezoelectric layer ( 4 ) is shared by the acoustic wave resonators ( 5 ). 
     The acoustic wave device according to the above-described preferred embodiment ( 1   a ) can support high frequency. 
     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.