Elastic wave device

Abstract

Provided is an elastic wave device that makes it possible to reduce the size of a filter device, and to suppress degradation of filter characteristics.　An elastic wave device 10 according to the present invention comprises: a piezoelectric layer 14 composed of lithium niobate; a first comb-shaped electrode 17 which is provided on the piezoelectric layer 14 and includes a first bus bar 22 and a plurality of first electrode fingers 25 each having one end connected to the first bus bar 22, the first comb-shaped electrode 17 being connected to an input potential; a second comb-shaped electrode 18 which is provided on the piezoelectric layer 14 and includes a second bus bar 23 and a plurality of second electrode fingers 26 each having one end connected to the second bus bar 23, the plurality of second electrode fingers 26 being interdigitated with the plurality of first electrode fingers 25, and the second comb-shaped electrode 18 being connected to an output potential; and a reference potential electrode 19 which includes, in a direction in which the first electrode fingers 25 and the second electrode fingers 26 are arrayed, a plurality of third electrode fingers 27 that are provided on the piezoelectric layer 14 so as to be disposed side by side with the first electrode fingers 25 and the second electrode fingers 26. The reference potential electrode 19 includes a connection electrode (third bus bar 24) connecting adjacent ones of the third electrode fingers 27, and is connected to a reference potential. The order in which the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27 are arrayed is such that, when starting from a first electrode finger 25, the first electrode finger 25, a third electrode finger 27, a second electrode finger 26, and a third electrode finger 27 constitute one period. In the first comb-shaped electrode 17 and the second comb-shaped electrode 18, the center-to-center distance between adjacent ones of the first electrode fingers 25 and the second electrode fingers 26 is constant. In the reference potential electrode 19, the plurality of third electrode fingers 27 are disposed at equal intervals, and the center-to-center distance between adjacent ones of the first electrode fingers 25 and the third electrode fingers 27, and the center-to-center distance between adjacent ones of the second electrode fingers 26 and the third electrode fingers 27 are not constant.

Description

Elastic Wave Device

[0001] The present invention relates to an acoustic wave device.

[0002] Conventionally, acoustic wave devices have been widely used in filters for mobile phones and the like. Recently, an acoustic wave device using thickness-shear mode bulk waves has been proposed, as described in Patent Document 1 below. In this acoustic wave device, a piezoelectric layer is provided on a support. A pair of electrodes is provided on the piezoelectric layer. The pair of electrodes face each other on the piezoelectric layer and are connected to different potentials. By applying an AC voltage between the electrodes, thickness-shear mode bulk waves are excited.

[0003] U.S. Pat. No. 1,049,192

[0004] An acoustic wave device is, for example, an acoustic wave resonator used in, for example, a ladder-type filter. To obtain good characteristics in a ladder-type filter, it is necessary to increase the capacitance ratio between multiple acoustic wave resonators. In this case, it is necessary to increase the capacitance of some of the acoustic wave resonators in the ladder-type filter.

[0005] Increasing the capacitance of an elastic wave resonator requires, for example, increasing the size of the elastic wave resonator. Therefore, when such an elastic wave resonator is used in a ladder-type filter, the ladder-type filter tends to be large. In particular, ladder-type filters that include elastic wave resonators that utilize thickness-shear mode bulk waves, which have small capacitance, tend to be large.

[0006] The present inventors discovered that when an acoustic wave device is used in a filter device, a suitable filter waveform can be obtained without increasing the device size by configuring the acoustic wave device as follows: In this configuration, an electrode connected to a reference potential is disposed between an electrode connected to an input potential and an electrode connected to an output potential.

[0007] In addition, the inventors have found that simply adopting the above configuration may result in degradation of filter characteristics.

[0008] An object of the present invention is to provide an acoustic wave device that enables the miniaturization of a filter device and suppresses degradation of filter characteristics.

[0009] In one broad aspect of the present invention, an elastic wave device includes a piezoelectric layer made of lithium niobate; a first interdigital electrode provided on the piezoelectric layer, the first interdigital electrode having a first bus bar and a plurality of first electrode fingers, each connected at one end to the first bus bar, and connected to an input potential; a second interdigital electrode provided on the piezoelectric layer, the second interdigital electrode having a second bus bar and a plurality of second electrode fingers, each connected at one end to the second bus bar, and connected to an output potential; a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned; and a reference potential electrode connected to a reference potential, wherein the order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period, the center-to-center distance between adjacent first electrode fingers and second electrode fingers in the first comb-shaped electrode and the second comb-shaped electrode is constant, the plurality of third electrode fingers are arranged at equal intervals in the reference potential electrode, and the center-to-center distance between adjacent first electrode fingers and third electrode fingers and the center-to-center distance between adjacent second electrode fingers and third electrode fingers are not constant.

[0010] In another broad aspect of the elastic wave device according to the present invention, there is provided an elastic wave device including: a piezoelectric layer made of lithium niobate; a first interdigital electrode provided on the piezoelectric layer, the first interdigital electrode having a first bus bar and a plurality of first electrode fingers, one end of each of which is connected to the first bus bar, and connected to an input potential; a second interdigital electrode provided on the piezoelectric layer, the second interdigital electrode having a second bus bar and a plurality of second electrode fingers, one end of each of which is connected to the second bus bar, and interdigitated with the plurality of first electrode fingers, and connected to an output potential; and a plurality of second interdigital electrodes provided on the piezoelectric layer to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned. The piezoelectric element is provided with a reference potential electrode having three third electrode fingers and a connection electrode connecting adjacent third electrode fingers, the reference potential electrode being connected to a reference potential, and at least one fourth electrode finger being provided on the piezoelectric layer and adjacent to the first electrode finger, the second electrode finger, or the third electrode finger, and in an area where the fourth electrode finger is not provided, the order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is such that, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period, and the fourth electrode finger is not connected to an input potential, an output potential, or a reference potential.

[0011] In yet another broad aspect of the elastic wave device according to the present invention, there is provided an elastic wave device including: a piezoelectric layer made of lithium niobate; a first interdigital electrode provided on the piezoelectric layer, the first interdigital electrode having a first bus bar and a plurality of first electrode fingers, each connected at one end to the first bus bar, and connected to an input potential; a second interdigital electrode provided on the piezoelectric layer, the second interdigital electrode having a second bus bar and a plurality of second electrode fingers, each connected at one end to the second bus bar, and interdigitated with the plurality of first electrode fingers, and connected to an output potential; a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers, a connection electrode connecting adjacent third electrode fingers, and a reference potential electrode connected to a reference potential; the order in which the first electrode fingers, the second electrode fingers, and the third electrode fingers are arranged is such that, starting from the first electrode fingers, one period consists of the first electrode fingers, the third electrode fingers, the second electrode fingers, and the third electrode fingers; and when the width of the first electrode fingers is w1 and the width of the second electrode fingers is w2, w1≠w2.

[0012] In yet another broad aspect of the elastic wave device according to the present invention, there is provided an elastic wave device including: a piezoelectric layer made of lithium niobate; a first interdigital electrode provided on the piezoelectric layer, the first interdigital electrode having a first bus bar and a plurality of first electrode fingers, each connected at one end to the first bus bar, and connected to an input potential; a second interdigital electrode provided on the piezoelectric layer, the second interdigital electrode having a second bus bar and a plurality of second electrode fingers, each connected at one end to the second bus bar, and connected to an output potential; a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned; and a connection electrode connecting adjacent third electrode fingers to each other. and a reference potential electrode connected to a reference potential, wherein the order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period, and in each of the first comb electrode and the second comb electrode, the center-to-center distance between adjacent first electrode fingers and the center-to-center distance between adjacent second electrode fingers are constant, and in the reference potential electrode, the center-to-center distance between adjacent third electrode fingers is not constant, and the center-to-center distance between adjacent first electrode fingers and third electrode fingers is different from the center-to-center distance between adjacent second electrode fingers and third electrode fingers.

[0013] According to the present invention, it is possible to provide an acoustic wave device that can reduce the size of a filter device and suppress deterioration of filter characteristics.

[0014] FIG. 1 is a schematic front cross-sectional view of an elastic wave device according to a first preferred embodiment of the present invention. FIG. 2 is a schematic plan view of an elastic wave device according to the first preferred embodiment of the present invention. FIG. 3 is a schematic front cross-sectional view showing the vicinity of first to third electrode fingers according to the first preferred embodiment of the present invention. FIG. 4 is a schematic plan view of an elastic wave device according to a reference example. FIG. 5 is a diagram showing the pass characteristics of an elastic wave device according to a reference example. FIG. 6 is a diagram showing the pass characteristics of an elastic wave device according to the first preferred embodiment of the present invention. FIG. 7 is a diagram showing the pass characteristics of a LiNbO ３FIG. 8 is a diagram illustrating a map of the fractional bandwidth versus Euler angles (0°, θ, ψ). FIG. 8 is a schematic plan view of an elastic wave device according to a modified example of the first embodiment of the present invention. FIG. 9 is a schematic plan view of an elastic wave device according to a second embodiment of the present invention. FIG. 10 is a diagram illustrating the pass characteristics of elastic wave devices according to a second embodiment of the present invention and a reference example. FIG. 11 is a schematic plan view of an elastic wave device according to a first modified example of the second embodiment of the present invention. FIG. 12 is a schematic plan view of an elastic wave device according to a second modified example of the second embodiment of the present invention. FIG. 13 is a schematic plan view of an elastic wave device according to a third embodiment of the present invention. FIG. 14 is a diagram illustrating the pass characteristics of elastic wave devices according to a third embodiment of the present invention and a reference example. FIG. 15 is a schematic plan view of an elastic wave device according to a fourth embodiment of the present invention. FIG. 16 is a diagram illustrating the pass characteristics of elastic wave devices according to a fourth embodiment of the present invention where p1<p2 and a reference example where p1=p2. FIG. 17 is a diagram illustrating the pass characteristics of elastic wave devices according to the fourth embodiment of the present invention where p1>p2 and a reference example where p1=p2. FIG. 18( a ) is a schematic perspective view showing the appearance of an elastic wave device that uses bulk waves in thickness shear mode, and FIG. 18( b ) is a plan view showing the electrode structure on the piezoelectric layer. FIG. 19 is a cross-sectional view of a portion taken along line A-A in FIG. 18( a ). FIG. 20( a ) is a schematic front cross-sectional view illustrating Lamb waves propagating through a piezoelectric film of an elastic wave device, and FIG. 20( b ) is a schematic front cross-sectional view illustrating bulk waves in thickness shear mode propagating through a piezoelectric film in the elastic wave device. FIG. 21 is a diagram showing the amplitude direction of bulk waves in thickness shear mode. FIG. 22 is a diagram showing the resonance characteristics of an elastic wave device that uses bulk waves in thickness shear mode. FIG. 23 is a diagram showing the relationship between d / p and the fractional bandwidth as a resonator, where p is the center-to-center distance between adjacent electrodes and d is the thickness of the piezoelectric layer. FIG. 24 is a plan view of an elastic wave device that uses bulk waves in thickness shear mode. FIG. 25 is a diagram showing the resonance characteristics of a reference example elastic wave device in which spurious signals appear. Fig. 26 is a diagram showing the relationship between the fractional bandwidth and the amount of phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. Fig. 27 is a diagram showing the relationship between d / 2p and the metallization ratio MR. Fig. 28 is a diagram showing the relationship between d / p and the metallization ratio MR of LiNbO when d / p approaches 0. ３29 is a front cross-sectional view of an acoustic wave device having an acoustic multilayer film. FIG. 30 is a partially cutaway perspective view illustrating an acoustic wave device utilizing Lamb waves.

[0015] The present invention will be clarified below by describing specific embodiments of the present invention with reference to the drawings.

[0016] It should be noted that the embodiments described in this specification are merely examples, and partial substitution or combination of configurations is possible between different embodiments.

[0017] Fig. 1 is a schematic front cross-sectional view of an elastic wave device according to a first preferred embodiment of the present invention. Fig. 2 is a schematic plan view of an elastic wave device according to the first preferred embodiment of the present invention. Note that Fig. 1 is a schematic cross-sectional view taken along line II in Fig. 2. In Fig. 2, each electrode is indicated by hatching. Electrodes may also be similarly indicated by hatching in other schematic plan views besides Fig. 2.

[0018] 1 is configured to be able to utilize a thickness shear mode. The acoustic wave device 10 is an acoustically coupled filter. The configuration of the acoustic wave device 10 will be described below.

[0019] The acoustic wave device 10 includes a piezoelectric substrate 12 and a functional electrode 11. The piezoelectric substrate 12 is a substrate having piezoelectric properties. Specifically, the piezoelectric substrate 12 includes a support member 13 and a piezoelectric layer 14. In this embodiment, the support member 13 includes a support substrate 16 and an insulating layer 15. The insulating layer 15 is provided on the support substrate 16. The piezoelectric layer 14 is provided on the insulating layer 15. However, the support member 13 may be composed of only the support substrate 16. Note that the support member 13 is not necessarily provided.

[0020] The piezoelectric layer 14 has a first main surface 14 a and a second main surface 14 b. The first main surface 14 a and the second main surface 14 b face each other. Of the first main surface 14 a and the second main surface 14 b, the second main surface 14 b is located on the support member 13 side.

[0021] The piezoelectric layer 14 is made of lithium niobate. More specifically, in this embodiment, the lithium niobate used in the piezoelectric layer 14 is LiNbO ３ This LiNbO ３ The Euler angles (φ, θ, ψ) of the piezoelectric layer 14 are (0°, 0°, 90°). However, the Euler angles (φ, θ, ψ) of the piezoelectric layer 14 are not limited to the above. In this specification, a certain member being made of a certain material also includes a case where the member contains a trace amount of impurities that do not significantly degrade the electrical characteristics of the elastic wave device.

[0022] A functional electrode 11 is provided on the first main surface 14a of the piezoelectric layer 14. As shown in Fig. 2, the functional electrode 11 has a pair of comb electrodes and a reference potential electrode 19. The reference potential electrode 19 is connected to a reference potential. The pair of comb electrodes is specifically a first comb electrode 17 and a second comb electrode 18. The first comb electrode 17 is connected to an input potential. The second comb electrode 18 is connected to an output potential.

[0023] The first comb electrode 17 and the second comb electrode 18 are provided on the first main surface 14a of the piezoelectric layer 14. The first comb electrode 17 has a first bus bar 22 and a plurality of first electrode fingers 25. One ends of the plurality of first electrode fingers 25 are connected to the first bus bar 22. The second comb electrode 18 has a second bus bar 23 and a plurality of second electrode fingers 26. One ends of the plurality of second electrode fingers 26 are connected to the second bus bar 23.

[0024] The first bus bar 22 and the second bus bar 23 face each other. The first electrode fingers 25 and the second electrode fingers 26 are interdigitated with each other. The first electrode fingers 25 and the second electrode fingers 26 are alternately arranged in a direction perpendicular to the direction in which the first electrode fingers 25 and the second electrode fingers 26 extend.

[0025] The reference potential electrode 19 has a third bus bar 24 as a connection electrode and a plurality of third electrode fingers 27. The plurality of third electrode fingers 27 are provided on the first main surface 14a of the piezoelectric layer 14. The plurality of third electrode fingers 27 extend parallel to the plurality of first electrode fingers 25 and the plurality of second electrode fingers. Hereinafter, the direction in which the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27 extend is referred to as the electrode finger extension direction, and the direction perpendicular to the electrode finger extension direction is referred to as the electrode finger perpendicular direction. In this specification, the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27 may be collectively referred to simply as electrode fingers.

[0026] The third electrode fingers 27 are arranged so as to be aligned with the first electrode fingers 25 and the second electrode fingers 26 in the direction in which the first electrode fingers 25 and the second electrode fingers 26 are arranged. Thus, the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27 are aligned in one direction. When the direction in which the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27 are aligned is defined as the electrode finger arrangement direction, the electrode finger arrangement direction is parallel to the electrode finger orthogonal direction. The two third electrode fingers 27 are located at both ends in the electrode finger orthogonal direction in the region where multiple electrode fingers are provided. The multiple third electrode fingers 27 other than the two third electrode fingers 27 are arranged between the first electrode fingers 25 and the second electrode fingers 26.

[0027] FIG. 3 is a schematic front cross-sectional view showing the vicinity of the first to third electrode fingers in the first embodiment.

[0028] The order in which the multiple electrode fingers are arranged is such that, starting from the first electrode finger 25, one period consists of the first electrode finger 25, the third electrode finger 27, the second electrode finger 26, and the third electrode finger 27. Therefore, the order in which the multiple electrode fingers are arranged is the first electrode finger 25, the third electrode finger 27, the second electrode finger 26, the third electrode finger 27, the first electrode finger 25, the third electrode finger 27, the second electrode finger 26, and so on. If the input potential is represented by IN, the output potential is represented by OUT, and the reference potential is represented by GND, the order of the multiple electrode fingers is IN, GND, OUT, GND, IN, GND, OUT, and so on.

[0029] In this embodiment, in the region where a plurality of electrode fingers are provided, the electrode fingers located at both ends in the direction perpendicular to the electrode fingers are the third electrode fingers 27. The electrode fingers located at the ends in the direction perpendicular to the electrode fingers may be any of the first electrode fingers 25, the second electrode fingers 26, and the third electrode fingers 27.

[0030] As shown in Fig. 3 , the center-to-center distance between adjacent electrode fingers is not constant. Note that, as shown in Fig. 2 , the center-to-center distance between adjacent first electrode fingers 25 and second electrode fingers 26 is constant in the first comb electrode 17 and the second comb electrode 18. In the reference potential electrode 19, the plurality of third electrode fingers 27 are arranged at equal intervals. Note that, in this specification, "equally spaced" electrode fingers is synonymous with "the electrode fingers are arranged so that the center-to-center distance between the electrode fingers is constant." Meanwhile, in the reference potential electrode 19, the first electrode fingers 25 and the second electrode fingers 26 are each located at a position shifted from the center of the region between adjacent third electrode fingers 27.

[0031] Each electrode finger of the functional electrode 11 is made of a laminated metal film. Specifically, in each electrode finger, a Ti layer, an AlCu layer, and a Ti layer are laminated in this order from the piezoelectric layer 14 side. Note that the material of each electrode finger is not limited to the above. Alternatively, each electrode finger may be made of a single-layer metal film.

[0032] 2 , a third bus bar 24 serving as a connection electrode for the reference potential electrode 19 electrically connects the plurality of third electrode fingers together. Specifically, the third bus bar 24 is located in a region between the first bus bar 22 and the tips of the plurality of second electrode fingers 26. The plurality of first electrode fingers 25 are also located in this region. However, the third bus bar 24 and the plurality of first electrode fingers 25 are electrically insulated from each other by an insulating film 28.

[0033] More specifically, the third bus bar 24 includes a plurality of first connection electrodes 24A and one second connection electrode 24B. Each first connection electrode 24A connects the tips of two adjacent third electrode fingers 27. The first connection electrode 24A and the two third electrode fingers 27 form a U-shaped electrode. The plurality of first connection electrodes 24A are connected to each other by a second connection electrode 24B. An insulating film 28 is provided between this second connection electrode 24B and the plurality of first electrode fingers 25.

[0034] More specifically, an insulating film 28 is provided on the first main surface 14a of the piezoelectric layer 14 so as to cover a portion of the plurality of first electrode fingers 25. The insulating film 28 is provided in a region between the first bus bar 22 and the tips of the plurality of second electrode fingers 26. The insulating film 28 has a strip shape.

[0035] The insulating film 28 does not reach the first connection electrode 24A of the reference potential electrode 19. A second connection electrode 24B is provided on the insulating film 28 and over the multiple first connection electrodes 24A. Specifically, the second connection electrode 24B has a bar portion 24a and multiple protrusions 24b. Each protrusion 24b extends from the bar portion 24a toward each first connection electrode 24A. Each protrusion 24b is connected to each first connection electrode 24A. As a result, the multiple third electrode fingers 27 are electrically connected to each other by the first connection electrode 24A and the second connection electrode 24B.

[0036] In this embodiment, the third bus bar 24 is located in a region between the first bus bar 22 and the tips of the second electrode fingers 26. Therefore, the tips of the second electrode fingers 26 face the third bus bar 24 across a gap in the electrode finger extension direction. On the other hand, the tips of the first electrode fingers 25 face the second bus bar 23 across a gap in the electrode finger extension direction.

[0037] The third bus bar 24 may be located in a region between the second bus bar 23 and the tips of the first electrode fingers 25. In this case, the tips of the first electrode fingers 25 face the third bus bar 24 across a gap. On the other hand, the tips of the second electrode fingers 26 face the first bus bar 22 across a gap.

[0038] The elastic wave device 10 is an elastic wave resonator configured to utilize bulk waves in thickness-shear mode. As shown in FIG. 2 , the elastic wave device 10 has multiple excitation regions C. In the multiple excitation regions C, bulk waves in thickness-shear mode and elastic waves in other modes are excited. Note that FIG. 2 shows only two of the multiple excitation regions C.

[0039] A part of all the excitation regions C, a plurality of excitation regions C, are regions where the adjacent first electrode fingers 25 and third electrode fingers 27 overlap when viewed from the electrode finger orthogonal direction, and are regions between the centers of the adjacent first electrode fingers 25 and third electrode fingers 27. The remaining plurality of excitation regions C are regions where the adjacent second electrode fingers 26 and third electrode fingers 27 overlap when viewed from the electrode finger orthogonal direction, and are regions between the centers of the adjacent second electrode fingers 26 and third electrode fingers 27. These excitation regions C are lined up in the electrode finger orthogonal direction.

[0040] The configuration of the functional electrode 11, excluding the reference potential electrode 19, is the same as that of an IDT (Interdigital Transducer) electrode. When viewed from the electrode finger orthogonal direction, the region where adjacent first electrode fingers 25 and second electrode fingers 26 overlap is the intersection region E. The intersection region E includes multiple excitation regions C. The intersection region E and the excitation regions C are regions of the piezoelectric layer 14 that are defined based on the configuration of the functional electrode 11.

[0041] This embodiment is characterized by the following configuration: 1) In the first interdigital electrode 17 and the second interdigital electrode 18, the center-to-center distance between adjacent first electrode fingers 25 and adjacent second electrode fingers 26 is constant. 2) In the reference potential electrode 19, a plurality of third electrode fingers 27 are equally spaced. 3) The center-to-center distance between adjacent first electrode fingers 25 and adjacent third electrode fingers 27, and the center-to-center distance between adjacent second electrode fingers 26 and adjacent third electrode fingers 27, are not constant. This enables the filter device to be miniaturized and the degradation of filter characteristics to be suppressed when the acoustic wave device 10 is used in a filter device. This is explained below by comparing this embodiment with a reference example.

[0042] 4 , the reference example differs from the first embodiment in that first electrode fingers 25, second electrode fingers 26, and third electrode fingers 27 are arranged at equal intervals. Like elastic wave device 10 of the first embodiment, elastic wave device 100 of the reference example is an acoustically coupled filter. The configurations of first interdigital electrode 17, second interdigital electrode 18, and reference potential electrode 19 of the reference example are the same as those of the first embodiment. However, as described above, the reference example differs from the first embodiment in the positional relationship between the multiple electrode fingers.

[0043] The passband characteristics of the first embodiment and the reference example were compared. The design parameters of the acoustic wave device 10 having the configuration of the first embodiment were as follows. Note that the design parameters of the reference example were the same as those of the first embodiment, except for the center-to-center distance between adjacent electrode fingers.

[0044] Piezoelectric layer: Material...LiNbO ３ , Euler angles (φ, θ, ψ)...(0°, 0°, 90°), thickness...400 nm First to third electrode fingers: Layer structure...Ti layer / AlCu layer / Ti layer from the piezoelectric layer side, thickness of each layer...10 nm / 390 nm / 4 nm from the piezoelectric layer side Order of the first to third electrode fingers expressed by the potential to be connected: IN, GND, OUT, GND is repeated. Center-to-center distance between the first electrode finger and the second electrode finger: 2.8 μm Duty ratio of the functional electrode: 0.3

[0045] 5 and 6 are graphs illustrating the passband characteristics of an elastic wave device according to a reference example and a first preferred embodiment of the present invention, respectively. The passband characteristics are represented by S parameters.

[0046] 5 and 6, it can be seen that a suitable filter waveform can be obtained even with a single acoustic wave device. This is because the acoustic wave devices of the first preferred embodiment and the reference example are acoustically coupled filters.

[0047] 2 , the elastic wave device 10 of the first embodiment has an excitation region C located between the centers of adjacent first electrode fingers 25 and third electrode fingers 27, and an excitation region C located between the centers of adjacent second electrode fingers 26 and third electrode fingers 27. Elastic waves of multiple modes, including thickness-shear bulk waves, are excited in these excitation regions C. By coupling these modes, a filter waveform can be suitably obtained even in a single elastic wave device 10.

[0048] When the elastic wave device 10 is used as an elastic wave resonator in a filter device, a suitable filter waveform can be obtained even if the filter device includes only one or a small number of elastic wave resonators, thereby contributing to the miniaturization of the filter device.

[0049] However, in the reference example shown in Fig. 5, the steepness is low on the low-frequency side and the high-frequency side of the pass band. In contrast, in the first embodiment shown in Fig. 6, the steepness is high on the low-frequency side of the pass band. In this specification, high steepness means that the amount of change in frequency is small for a certain amount of attenuation or change in S-parameter near the end of the pass band.

[0050] As shown in FIG. 2 , in the first embodiment, the center-to-center distance between adjacent first electrode fingers 25 and third electrode fingers 27, and the center-to-center distance between adjacent second electrode fingers 26 and third electrode fingers 27 are not constant. This allows the mode frequency to be changed. This allows an attenuation pole to be provided at low frequencies. This therefore increases the steepness at the low-frequency side of the passband. In this way, the filter characteristics can be improved.

[0051] On the other hand, the center-to-center distance between adjacent first electrode fingers 25 and second electrode fingers 26 is constant in the first comb electrode 17 and the second comb electrode 18. In the reference potential electrode 19, a plurality of third electrode fingers 27 are arranged at equal intervals. This makes it possible to more reliably suppress deterioration of the filter characteristics.

[0052] The configuration of the first embodiment will be described in more detail below.

[0053] 1, the support member 13 is made up of a support substrate 16 and an insulating layer 15. The piezoelectric substrate 12 is a laminate of the support substrate 16, the insulating layer 15, and the piezoelectric layer 14. That is, the piezoelectric layer 14 and the support member 13 overlap when viewed from a direction in which the first main surface 14a and the second main surface 14b of the piezoelectric layer 14 face each other.

[0054] The support substrate 16 may be made of a semiconductor such as silicon or a ceramic such as aluminum oxide. The insulating layer 15 may be made of a suitable dielectric such as silicon oxide or tantalum oxide. The piezoelectric layer 14 may be made of, for example, LiNbO ３ The layer is a lithium niobate layer, such as a layer.

[0055] A recess is provided in the insulating layer 15. A piezoelectric layer 14 is provided on the insulating layer 15 so as to close the recess. This forms a hollow portion. This hollow portion is the cavity 10a. In the first embodiment, the support member 13 and the piezoelectric layer 14 are arranged so that a portion of the support member 13 and a portion of the piezoelectric layer 14 face each other with the cavity 10a in between. However, the recess in the support member 13 may be provided across the insulating layer 15 and the support substrate 16. Alternatively, a recess provided only in the support substrate 16 may be closed by the insulating layer 15. The recess may also be provided in the piezoelectric layer 14. The cavity 10a may be a through-hole provided in the support member 13.

[0056] The cavity 10a is the acoustic reflector of the present invention. The acoustic reflector can effectively confine the energy of the elastic wave to the piezoelectric layer 14 side. The acoustic reflector may be provided at a position on the support member 13 that overlaps with at least a portion of the functional electrode 11 in a planar view. More specifically, at least a portion of each of the first electrode finger 25, the second electrode finger 26, and the third electrode finger 27 may overlap with the acoustic reflector in a planar view. It is preferable that a plurality of excitation regions C overlap with the acoustic reflector in a planar view.

[0057] In this specification, a plan view refers to a view along the stacking direction of the support member 13 and the piezoelectric layer 14 from a direction corresponding to the top in FIG. 1. In FIG. 1, for example, the piezoelectric layer 14 side is the top, out of the support substrate 16 side and the piezoelectric layer 14 side. Furthermore, in this specification, a plan view is synonymous with a view from the principal surface opposing direction. The principal surface opposing direction is the direction in which the first principal surface 14a and the second principal surface 14b of the piezoelectric layer 14 face each other. More specifically, the principal surface opposing direction is, for example, the normal direction of the first principal surface 14a.

[0058] The acoustic reflecting portion may be an acoustic reflecting film such as an acoustic multilayer film, which will be described later. For example, an acoustic reflecting film may be provided on the surface of the support member.

[0059] In the first embodiment, the center-to-center distance between adjacent first electrode fingers 25 and third electrode fingers 27 and the center-to-center distance between adjacent second electrode fingers 26 and third electrode fingers 27 are not constant. Hereinafter, the longest distance among the center-to-center distance between adjacent first electrode fingers 25 and third electrode fingers 27 and the center-to-center distance between adjacent second electrode fingers 26 and third electrode fingers 27 is defined as p. In this case, when the thickness of the piezoelectric layer 14 is d, d / p is preferably 0.5 or less, and more preferably 0.24 or less. This allows thickness-shear mode bulk waves to be suitably excited.

[0060] Note that the elastic wave device according to the present invention does not necessarily have to be configured to utilize the thickness shear mode. The elastic wave device according to the present invention may be configured to utilize plate waves. In this case, the excitation region is the intersection region E shown in FIG. 2 .

[0061] In the first embodiment, the piezoelectric layer 14 is made of LiNbO ３ In this case, the bandwidth ratio of acoustic wave device 10 depends on the Euler angles (φ, θ, ψ) of the lithium niobate used in piezoelectric layer 14. The bandwidth ratio is expressed as (|fa-fr| / fr)×100[%], where fr is the resonant frequency and fa is the antiresonant frequency.

[0062] The relationship between the bandwidth fraction of acoustic wave device 10 and the Euler angles (φ, θ, ψ) of piezoelectric layer 14 was derived when d / p was set as close to 0 as possible. Note that φ in the Euler angles was set to 0°.

[0063] FIG. 7 shows the results of LiNbO when d / p approaches 0. ３ FIG. 10 is a diagram showing a map of fractional bandwidths versus Euler angles (0°, θ, ψ) of the .

[0064] The hatched region R in Fig. 7 is a region where a fractional bandwidth of at least 2% or more can be obtained. The range of region R can be approximated as the ranges expressed by the following formulas (1), (2), and (3). Note that when φ in the Euler angles (φ, θ, ψ) is within a range of 0°±10°, the relationship between θ and ψ and the fractional bandwidth is the same as the relationship shown in Fig. 7. When the piezoelectric layer 14 is a lithium tantalate layer, the relationship between θ and ψ in the Euler angles (θ, ψ within a range of 0°±10°) and the fractional bandwidth is also the same as the relationship shown in Fig. 7.

[0065] (within the range of 0°±10°, 0° to 25°, any ψ) ... Equation (1) (within the range of 0°±10°, 25° to 100°, 0° to 75° [(1-(θ-50) ２ / 2500) １／２ or 180°-75°[(1-(θ-50) ２ / 2500) １／２ ~180°) ...Equation (2) (within the range of 0°±10°, 180°-40°[(1-(ψ-90)２ / 8100)] １／２ ~180°, any ψ) ...Equation (3)

[0066] The Euler angles are preferably within the range of the above formula (1), formula (2), or formula (3). This allows the fractional bandwidth to be sufficiently wide, which makes the acoustic wave device 10 suitable for use as a filter device.

[0067] As shown in FIG. 2 , in the first embodiment, the reference potential electrode 19 has a third bus bar 24 as a connection electrode and a plurality of third electrode fingers 27. The reference potential electrode 19 is a comb-shaped electrode. However, the reference potential electrode 19 does not have to be a comb-shaped electrode. For example, in a modification of the first embodiment shown in FIG. 8 , the reference potential electrode 39 has a meandering shape. In this modification, the insulating film 28 is not provided on the piezoelectric layer 14. The connection electrode 35 includes only portions corresponding to the plurality of first connection electrodes 24A in the first embodiment. The connection electrode 35 in this modification is not a third bus bar.

[0068] More specifically, the reference potential electrode 39 has a plurality of connection electrodes 35 located on the first bus bar 22 side and a plurality of connection electrodes 35 located on the second bus bar 23 side. The tips of two adjacent third electrode fingers 27 on the first bus bar 22 side or the tips of two adjacent third electrode fingers 27 on the second bus bar 23 side are connected by a connection electrode 35. For example, of the multiple third electrode fingers 27, the third electrode fingers 27 other than those at both ends in the direction perpendicular to the electrode fingers have one connection electrode 35 connected to each of their tips on the first bus bar 22 side and the second bus bar 23 side. Each third electrode finger 27 is connected to the adjacent third electrode fingers 27 on both sides by each connection electrode 35. By repeating this structure, the reference potential electrode 39 has a meandering shape.

[0069] In this modification, as in the first embodiment, the filter device can be made smaller. In addition, the plurality of electrode fingers are arranged in the same manner as in the first embodiment. Specifically, in the first interdigital electrode 17 and the second interdigital electrode 18, the center-to-center distance between adjacent first electrode fingers 25 and second electrode fingers 26 is constant. In the reference potential electrode 39, the plurality of third electrode fingers 27 are arranged at equal intervals. The center-to-center distance between adjacent first electrode fingers 25 and third electrode fingers 27, and the center-to-center distance between adjacent second electrode fingers 26 and third electrode fingers 27, are not constant. This suppresses degradation of the filter characteristics. Specifically, it suppresses degradation of the steepness on the lower side of the pass band.

[0070] FIG. 9 is a schematic plan view of an elastic wave device according to a second preferred embodiment of the present invention.

[0071] This embodiment differs from the first embodiment in that the plurality of electrode fingers includes a plurality of fourth electrode fingers 48, and the plurality of electrode fingers are arranged at equal intervals. The fourth electrode fingers 48 are floating electrodes. A floating electrode is an electrode that is not connected to any of the input potential, output potential, and reference potential. Except for the above points, the elastic wave device 40 of this embodiment has a similar configuration to the elastic wave device 10 of the first embodiment.

[0072] The functional electrode 41 of the acoustic wave device 40 has a configuration in which at least one of the plurality of third electrode fingers 27 in the reference example shown in Fig. 4 is replaced with a fourth electrode finger 48. More specifically, as shown in Fig. 9 , the third electrode finger 27 or the fourth electrode finger 48 is positioned between the first electrode finger 25 and the second electrode finger 26.

[0073] The configuration of the elastic wave device of the present invention may be such that at least one of the plurality of first electrode fingers 25 or the plurality of second electrode fingers 26 in the reference example is replaced with a fourth electrode finger 48.

[0074] In the region where the fourth electrode finger 48 is not provided, the order in which the multiple electrode fingers are arranged is the same as in the first embodiment and the reference example. That is, starting from the first electrode finger 25, the order is such that one period consists of the first electrode finger 25, the third electrode finger 27, the second electrode finger 26, and the third electrode finger 27. However, in the region corresponding to the configuration in which the third electrode finger 27 is replaced with the fourth electrode finger 48, the order in which the electrode fingers are arranged is not the above order.

[0075] In this embodiment, the first electrode fingers 25, the second electrode fingers 26, the third electrode fingers 27, and the fourth electrode fingers 48 are arranged at equal intervals. Therefore, the center-to-center distance between adjacent first electrode fingers 25 and third electrode fingers 27, and the center-to-center distance between adjacent second electrode fingers 26 and third electrode fingers 27 are constant.

[0076] This embodiment is characterized by the following configuration: 1) The functional electrode 41 is provided on the piezoelectric layer 14 and has at least one fourth electrode finger 48 adjacent to the first electrode finger 25, the second electrode finger 26, or the third electrode finger 27. 2) The fourth electrode finger 48 is a floating electrode. This allows for the miniaturization of the filter device and suppresses degradation of the filter characteristics. Specifically, the passband frequency can be adjusted without significantly changing the bandwidth from the desired bandwidth. This is specifically demonstrated by comparing the second embodiment with the reference example.

[0077] The design parameters of the acoustic wave device 40 having the configuration of the second preferred embodiment were as follows: Note that the design parameters of the reference example were the same as those of the second preferred embodiment except for the fourth electrode finger 48 .

[0078] Piezoelectric layer: Material...LiNbO ３, Euler angles (φ, θ, ψ)...(0°, 0°, 90°), thickness...400 nm First to third electrode fingers: Layer structure...Ti layer / AlCu layer / Ti layer from the piezoelectric layer side, thickness of each layer...10 nm / 390 nm / 4 nm from the piezoelectric layer side Order of the first to third electrode fingers expressed by the potential to be connected: IN, GND, OUT, GND is repeated. Center-to-center distance between the first electrode finger and the second electrode finger: 2.8 μm Duty ratio of the functional electrode: 0.3

[0079] FIG. 10 is a diagram illustrating the pass characteristics of the elastic wave devices according to the second preferred embodiment and the reference example.

[0080] 10, it can be seen that the passband frequencies are different between the second embodiment and the reference example. Furthermore, it can be seen that the bandwidth in the second embodiment is not significantly different from the bandwidth in the reference example.

[0081] When the elastic wave device 40 according to the second embodiment is used as an elastic wave resonator in a filter device, a suitable filter waveform can be obtained even if the filter device includes only one or a small number of elastic wave resonators. As described above, in the second embodiment, the filter device can be made smaller, and the frequency can be adjusted without significantly changing the bandwidth.

[0082] The fourth electrode finger 48 may be adjacent to the first electrode finger 25, the second electrode finger 26, or the third electrode finger 27. Below, first and second modified examples of the second embodiment are shown, which differ from the second embodiment only in the arrangement of the fourth electrode finger 48. In the first and second modified examples, as in the second embodiment, it is possible to further reduce the size of the filter device and suppress deterioration of the filter characteristics.

[0083] The configuration of the first modified example shown in Fig. 11 is a configuration in which at least one of the plurality of second electrode fingers 26 in the reference example shown in Fig. 4 is replaced with a fourth electrode finger 48. In a region where the fourth electrode finger 48 is not provided, the order in which the plurality of electrode fingers are arranged is the same as in the first embodiment and the reference example. That is, starting from the first electrode finger 25, the order is such that one period includes the first electrode finger 25, the third electrode finger 27, the second electrode finger 26, and the third electrode finger 27. However, in a region corresponding to the configuration in which the second electrode finger 26 is replaced with the fourth electrode finger 48, the order in which the electrode fingers are arranged is different from the above order.

[0084] The fourth electrode finger 48 is located between two third electrode fingers 27. Therefore, the fourth electrode finger 48 is adjacent to the third electrode finger 27. In this modification as well, the multiple electrode fingers are arranged at equal intervals.

[0085] The configuration of the second modified example shown in Fig. 12 is a configuration in which at least one of the plurality of first electrode fingers 25 in the reference example shown in Fig. 4 is replaced with a fourth electrode finger 48. In a region where the fourth electrode finger 48 is not provided, the order in which the plurality of electrode fingers are arranged is the same as in the first embodiment and the reference example. However, in a region corresponding to the configuration in which the first electrode finger 25 is replaced with the fourth electrode finger 48, the order in which the electrode fingers are arranged is different from the order in the first embodiment and the reference example.

[0086] The fourth electrode finger 48 is located between two third electrode fingers 27. Therefore, the fourth electrode finger 48 is adjacent to the third electrode finger 27. In this modification as well, the multiple electrode fingers are arranged at equal intervals.

[0087] FIG. 13 is a schematic plan view of an elastic wave device according to a third preferred embodiment of the present invention.

[0088] The third embodiment differs from the first embodiment in that, when the width of first electrode finger 25 is w1 and the width of second electrode finger 26 is w2, w1≠w2. The third embodiment also differs from the first embodiment in that the plurality of electrode fingers are arranged at equal intervals. Except for the above points, the elastic wave device 50 of the third embodiment has a similar configuration to the elastic wave device 10 of the first embodiment.

[0089] 4 , functional electrode 51 of elastic wave device 50 is configured such that width w1 of the plurality of first electrode fingers 25 is wider than width w2 of the plurality of second electrode fingers 26. However, width w2 of the plurality of second electrode fingers 26 may be wider than width w1 of the plurality of first electrode fingers 25.

[0090] A feature of this embodiment is that w1≠w2. This allows for the miniaturization of the filter device and suppresses degradation of the filter characteristics. Specifically, it is possible to reduce ripples in the filter characteristics caused by unwanted waves. This will be explained below by comparing the third embodiment with the reference example.

[0091] The design parameters of acoustic wave device 50 having the configuration of the third preferred embodiment were as follows: Note that the design parameters of the reference example were the same as those of the third preferred embodiment except for the width of the electrode fingers.

[0092] Piezoelectric layer: Material...LiNbO ３ , Euler angles (φ, θ, ψ)...(0°, 0°, 90°), thickness...400 nm First to third electrode fingers: Layer structure...Ti layer / AlCu layer / Ti layer from the piezoelectric layer side, thickness of each layer...10 nm / 390 nm / 4 nm from the piezoelectric layer side Order of the first to third electrode fingers expressed by the potential to be connected: IN, GND, OUT, GND is repeated. Center-to-center distance between the first electrode finger and the second electrode finger: 2.8 μm Duty ratio of the functional electrode: 0.3

[0093] FIG. 14 is a diagram illustrating the pass characteristics of the elastic wave devices according to the third preferred embodiment and the reference example.

[0094] As indicated by the arrow F in Fig. 14, the third embodiment can reduce the ripple caused by the unwanted waves compared to the reference example. In the third embodiment, w1 ≠ w2, so that the frequencies at which the unwanted waves occur can be dispersed. This makes it possible to suppress the unwanted waves.

[0095] When the elastic wave device 50 according to the third embodiment is used as an elastic wave resonator in a filter device, a suitable filter waveform can be obtained even if the filter device includes only one or a small number of elastic wave resonators. As described above, the third embodiment allows for the miniaturization of the filter device and suppression of unwanted waves.

[0096] FIG. 15 is a schematic plan view of an elastic wave device according to a fourth preferred embodiment of the present invention.

[0097] This embodiment differs from the first embodiment in that the spacing between the multiple third electrode fingers 27 is not constant in the reference potential electrode 69 of the functional electrode 61. Except for the above difference, the elastic wave device 60 of this embodiment has a similar configuration to the elastic wave device 10 of the first embodiment.

[0098] More specifically, among the adjacent third electrode fingers 27, the center-to-center distance between the third electrode fingers 27 connected by the first connection electrode 24A is different from the center-to-center distance between the third electrode fingers 27 not connected by the first connection electrode 24A. Note that, among the adjacent third electrode fingers 27, the center-to-center distance between the third electrode fingers 27 connected by the first connection electrode 24A is constant. Similarly, among the adjacent third electrode fingers 27, the center-to-center distance between the third electrode fingers 27 not connected by the first connection electrode 24A is constant.

[0099] As shown in FIG. 15, in the first comb-shaped electrode 17 and the second comb-shaped electrode 18, the center-to-center distance between the adjacent first electrode fingers 25 and the adjacent second electrode fingers 26 is constant.

[0100] In this embodiment, when the center-to-center distance between the adjacent first electrode finger 25 and the adjacent third electrode finger 27 is p1 and the center-to-center distance between the adjacent second electrode finger 26 and the adjacent third electrode finger 27 is p2, p1≠p2 holds. Specifically, p1<p2 holds. However, p1<p2 may hold.

[0101] Note that p1 is constant in each portion where the first electrode finger 25 and the third electrode finger 27 are adjacent to each other. Similarly, p2 is constant in each portion where the second electrode finger 26 and the third electrode finger 27 are adjacent to each other.

[0102] This embodiment is characterized by the following configuration: 1) In each of the first comb electrode 17 and the second comb electrode 18, the center-to-center distance between adjacent first electrode fingers 25 and the center-to-center distance between adjacent second electrode fingers 26 are constant. 2) In the reference potential electrode 69, the center-to-center distance between adjacent third electrode fingers 27 is not constant. 3) p1 ≠ p2. This enables the filter device to be made more compact and allows the frequency to be changed without significant degradation of the filter characteristics. This will be explained below by comparing the fourth embodiment with the reference example shown in FIG. 4.

[0103] The design parameters of the acoustic wave device 60 having the configuration of the fourth preferred embodiment were as follows: The design parameters of the reference example were the same as those of the fourth preferred embodiment, except for the center-to-center distance between adjacent electrode fingers.

[0104] Piezoelectric layer: Material...LiNbO ３ , Euler angles (φ, θ, ψ)...(0°, 0°, 90°), thickness...400 nm First to third electrode fingers: Layer structure...Ti layer / AlCu layer / Ti layer from the piezoelectric layer side, thickness of each layer...10 nm / 390 nm / 4 nm from the piezoelectric layer side Order of the first to third electrode fingers expressed by the potential to be connected: IN, GND, OUT, GND is repeated. Center-to-center distance between the first electrode finger and the second electrode finger: 2.8 μm Duty ratio of the functional electrode: 0.3

[0105] The comparison was performed in both cases where the relationship between the center distance p1 and the center distance p2 in the fourth embodiment is p1<p2 and p1>p2.

[0106] 16 and 17 are graphs illustrating the passband characteristics of the elastic wave device according to the fourth embodiment where p1<p2 and the elastic wave device according to the reference example where p1=p2.

[0107] As shown in FIG. 16, the passband of the fourth embodiment is located slightly higher than that of the reference example. In addition, the value of the fractional bandwidth is larger. As described above, the fractional bandwidth is expressed by (|fa-fr| / fr) x 100 [%]. In a filter device using a normal surface acoustic wave resonator, if only the center-to-center distance p2 is increased compared to when p1 = p2, the frequency of the passband becomes lower. On the other hand, in an acoustically coupled filter such as the fourth embodiment, if p1 < p2, the frequency of the passband becomes higher.

[0108] As shown in Figure 17, the passband of the fourth embodiment is located slightly lower than the passband of the reference example. In addition, the value of the relative bandwidth is smaller. Compared to when p1 = p2, shortening only the center-to-center distance p2 increases the frequency of the passband in a filter device using a normal surface acoustic wave resonator. On the other hand, in an acoustically coupled filter such as the fourth embodiment, when p1 > p2, the frequency of the passband decreases. In the fourth embodiment, the frequency and relative bandwidth of the passband can be adjusted by adjusting the center-to-center distance p1 and the center-to-center distance p2.

[0109] When the elastic wave device 60 according to the fourth embodiment is used as an elastic wave resonator in a filter device, a suitable filter waveform can be obtained even if the filter device includes only one or a small number of elastic wave resonators. As described above, in the fourth embodiment, the filter device can be made smaller and the frequency can be changed without significantly degrading the filter characteristics.

[0110] The thickness-shear mode will be described in detail below using an example in which the functional electrode is an IDT electrode. Note that the IDT electrode does not have a third electrode finger. The "electrode" in the IDT electrode described below corresponds to the electrode finger. The support member in the following example corresponds to the support substrate in the present invention. Below, the reference potential may also be referred to as the ground potential.

[0111] Figure 18(a) is a schematic perspective view showing the appearance of an elastic wave device that utilizes thickness-shear mode bulk waves, Figure 18(b) is a plan view showing the electrode structure on the piezoelectric layer, and Figure 19 is a cross-sectional view of the portion along line A-A in Figure 18(a).

[0112] The acoustic wave device 1 is made of LiNbO ３ The piezoelectric layer 2 is made of LiTaO ３ It may be made of LiNbO ３ and LiTaO ３The cut angle of the piezoelectric layer 2 is a Z-cut, but may also be a rotated Y-cut or X-cut. The thickness of the piezoelectric layer 2 is not particularly limited, but to effectively excite the thickness shear mode, it is preferably 40 nm or more and 1000 nm or less, and more preferably 50 nm or more and 1000 nm or less. The piezoelectric layer 2 has first and second principal surfaces 2a and 2b facing each other. An electrode 3 and an electrode 4 are provided on the first principal surface 2a. Here, the electrode 3 is an example of a "first electrode," and the electrode 4 is an example of a "second electrode." In Figures 18(a) and 18(b), multiple electrodes 3 are connected to a first bus bar 5. Multiple electrodes 4 are connected to a second bus bar 6. The multiple electrodes 3 and multiple electrodes 4 are interdigitated with each other. The electrodes 3 and 4 have a rectangular shape and a length direction. An electrode 3 faces an adjacent electrode 4 in a direction perpendicular to the length direction. The length direction of the electrodes 3 and 4 and the direction perpendicular to the length direction of the electrodes 3 and 4 both intersect the thickness direction of the piezoelectric layer 2. Therefore, it can be said that the electrode 3 and the adjacent electrode 4 face each other in a direction perpendicular to the thickness direction of the piezoelectric layer 2. The length directions of the electrodes 3 and 4 may be reversed with the direction perpendicular to the length direction of the electrodes 3 and 4 shown in FIGS. 18( a) and 18(b). That is, the electrodes 3 and 4 may extend in the direction in which the first bus bar 5 and the second bus bar 6 extend in FIGS. 18( a) and 18(b). In that case, the first bus bar 5 and the second bus bar 6 extend in the direction in which the electrodes 3 and 4 extend in FIGS. 18( a) and 18(b). Furthermore, a plurality of pairs of adjacent structures, each of which has an electrode 3 connected to one potential and an electrode 4 connected to the other potential, are provided in a direction perpendicular to the length direction of the electrodes 3 and 4. Here, "electrodes 3 and 4 are adjacent" does not mean that electrodes 3 and 4 are arranged so as to be in direct contact with each other, but rather that electrodes 3 and 4 are arranged with a gap therebetween. Furthermore, when electrodes 3 and 4 are adjacent to each other, no electrodes connected to hot electrodes or ground electrodes, including other electrodes 3 and 4, are arranged between electrodes 3 and 4. The number of pairs does not need to be an integer number, and may be 1.5 pairs, 2.5 pairs, or the like. The center-to-center distance between electrodes 3 and 4, i.e., the pitch, is preferably in the range of 1 μm or more and 10 μm or less.The width of the electrodes 3 and 4, i.e., the dimension in the opposing direction of the electrodes 3 and 4, is preferably in the range of 50 nm to 1000 nm, and more preferably in the range of 150 nm to 1000 nm. The center-to-center distance between the electrodes 3 and 4 is the distance connecting the center of the dimension (width dimension) of electrode 3 in the direction perpendicular to the longitudinal direction of electrode 3 and the center of the dimension (width dimension) of electrode 4 in the direction perpendicular to the longitudinal direction of electrode 4.

[0113] Furthermore, because the elastic wave device 1 uses a Z-cut piezoelectric layer, the direction perpendicular to the length direction of the electrodes 3 and 4 is perpendicular to the polarization direction of the piezoelectric layer 2. This does not apply if a piezoelectric material with a different cut angle is used for the piezoelectric layer 2. Here, "perpendicular" is not limited to being strictly perpendicular, but may also be approximately perpendicular (for example, the angle between the direction perpendicular to the length direction of the electrodes 3 and 4 and the polarization direction is within a range of 90°±10°).

[0114] A support member 8 is laminated on the second principal surface 2b side of the piezoelectric layer 2 via an insulating layer 7. The insulating layer 7 and the support member 8 have a frame-like shape and, as shown in FIG. 19 , have through holes 7a, 8a, thereby forming a cavity 9. The cavity 9 is provided so as not to interfere with vibration in the excitation region C of the piezoelectric layer 2. Therefore, the support member 8 is laminated on the second principal surface 2b via the insulating layer 7 at a position that does not overlap with the portion where at least one pair of electrodes 3, 4 is provided. Note that the insulating layer 7 does not have to be provided. Therefore, the support member 8 can be laminated directly or indirectly on the second principal surface 2b of the piezoelectric layer 2.

[0115] The insulating layer 7 is made of silicon oxide. However, in addition to silicon oxide, appropriate insulating materials such as silicon oxynitride and alumina can be used. The support member 8 is made of Si. The surface of the Si facing the piezoelectric layer 2 may have a (100), (110), or (111) plane orientation. The Si constituting the support member 8 desirably has a high resistivity of 4 kΩcm or more. However, the support member 8 can also be made of an appropriate insulating material or semiconductor material.

[0116] Examples of materials that can be used for the support member 8 include piezoelectric materials such as aluminum oxide, lithium tantalate, lithium niobate, and quartz crystal; various ceramics such as alumina, magnesia, sapphire, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite; dielectric materials such as diamond and glass; and semiconductors such as gallium nitride.

[0117] The electrodes 3 and 4 and the first and second bus bars 5 and 6 are made of an appropriate metal or alloy, such as Al or an AlCu alloy. In the elastic wave device 1, the electrodes 3 and 4 and the first and second bus bars 5 and 6 have a structure in which an Al film is laminated on a Ti film. Note that an adhesive layer other than a Ti film may also be used.

[0118] During operation, an AC voltage is applied between the plurality of electrodes 3 and the plurality of electrodes 4. More specifically, an AC voltage is applied between the first bus bar 5 and the second bus bar 6. This makes it possible to obtain resonance characteristics utilizing thickness-shear mode bulk waves excited in the piezoelectric layer 2. Furthermore, in the elastic wave device 1, where d is the thickness of the piezoelectric layer 2 and p is the center-to-center distance between any two adjacent electrodes 3 and 4 among the multiple pairs of electrodes 3 and 4, d / p is set to 0.5 or less. This effectively excites the thickness-shear mode bulk waves, resulting in good resonance characteristics. More preferably, d / p is 0.24 or less, which provides even better resonance characteristics.

[0119] Because the elastic wave device 1 has the above configuration, even if the number of pairs of electrodes 3 and 4 is reduced in an attempt to reduce the device size, the Q value is unlikely to decrease. This is because there is little propagation loss even when the number of electrode fingers in the reflectors on both sides is reduced. Furthermore, the use of thickness-shear bulk waves allows the number of electrode fingers to be reduced. The difference between the Lamb waves used in the elastic wave device and the thickness-shear bulk waves is described below with reference to Figures 20(a) and 20(b).

[0120] FIG. 20( a) is a schematic front cross-sectional view illustrating Lamb waves propagating through a piezoelectric film of an acoustic wave device such as that described in Japanese Patent Publication No. 2012-257019. Here, waves propagate through a piezoelectric film 201 as indicated by arrows. Here, the piezoelectric film 201 has a first principal surface 201a and a second principal surface 201b that face each other, and the thickness direction connecting the first principal surface 201a and the second principal surface 201b is the Z direction. The X direction is the direction in which the electrode fingers of the IDT electrode are arranged. As shown in FIG. 20( a), Lamb waves propagate in the X direction as shown. Because they are plate waves, the piezoelectric film 201 vibrates as a whole, but the waves propagate in the X direction. Therefore, reflectors are placed on both sides to achieve resonance characteristics. As a result, wave propagation loss occurs, and when miniaturization is attempted, i.e., when the number of pairs of electrode fingers is reduced, the Q value decreases.

[0121] In contrast, as shown in Figure 20(b), in elastic wave device 1, vibration displacement is in the thickness shear direction, so waves propagate and resonate almost entirely in the direction connecting first principal surface 2a and second principal surface 2b of piezoelectric layer 2, i.e., the Z direction. That is, the X direction component of the wave is significantly smaller than the Z direction component. Furthermore, because resonance characteristics are achieved through wave propagation in this Z direction, propagation loss is unlikely to occur even if the number of electrode fingers of the reflector is reduced. Furthermore, even if the number of electrode pairs consisting of electrodes 3 and 4 is reduced in an effort to achieve miniaturization, a decrease in the Q value is unlikely to occur.

[0122] As shown in Figure 21, the amplitude direction of the thickness-shear mode bulk wave is opposite between a first region 451 included in the excitation region C of the piezoelectric layer 2 and a second region 452 included in the excitation region C. Figure 21 schematically shows the bulk wave when a voltage is applied between electrode 3 and electrode 4 such that electrode 4 has a higher potential than electrode 3. The first region 451 is a region in the excitation region C between the first principal surface 2a and an imaginary plane VP1 that is perpendicular to the thickness direction of the piezoelectric layer 2 and divides the piezoelectric layer 2 in half. The second region 452 is a region in the excitation region C between the imaginary plane VP1 and the second principal surface 2b.

[0123] As described above, elastic wave device 1 includes at least one pair of electrodes, consisting of electrode 3 and electrode 4. However, because waves are not propagated in the X direction, there is no need for multiple pairs of electrodes, consisting of electrodes 3 and 4. In other words, it is sufficient that at least one pair of electrodes is provided.

[0124] For example, electrode 3 is an electrode connected to a hot potential, and electrode 4 is an electrode connected to a ground potential. However, electrode 3 may be connected to the ground potential, and electrode 4 may be connected to the hot potential. In elastic wave device 1, at least one pair of electrodes is an electrode connected to a hot potential or an electrode connected to a ground potential, as described above, and no floating electrodes are provided.

[0125] Fig. 22 is a diagram showing the resonance characteristics of the elastic wave device shown in Fig. 19. The design parameters of elastic wave device 1 that achieved these resonance characteristics are as follows:

[0126] Piezoelectric layer 2: LiNbO with Euler angles (0°, 0°, 90°) ３ , thickness = 400 nm. When viewed in a direction perpendicular to the longitudinal direction of electrodes 3 and 4, the region where electrodes 3 and 4 overlap, i.e., excitation region C, has a length of 40 μm, the number of electrode pairs consisting of electrodes 3 and 4 = 21 pairs, the center distance between electrodes = 3 μm, the width of electrodes 3 and 4 = 500 nm, and d / p = 0.133. Insulating layer 7: silicon oxide film with a thickness of 1 μm. Support member 8: Si.

[0127] The length of the excitation region C is the dimension of the excitation region C along the length direction of the electrodes 3 and 4.

[0128] In the elastic wave device 1, the inter-electrode distance between the electrodes 3 and 4 is the same for all pairs of electrodes. That is, the electrodes 3 and 4 are arranged at equal intervals.

[0129] As is clear from FIG. 22, good resonance characteristics with a relative bandwidth of 12.5% ​​are obtained despite the absence of a reflector.

[0130] As described above, when the thickness of piezoelectric layer 2 is d and the center-to-center distance between electrodes 3 and 4 is p, in elastic wave device 1, d / p is 0.5 or less, and more preferably 0.24 or less. This will be explained with reference to FIG. 23 .

[0131] A number of elastic wave devices were obtained by varying the d / p ratio, similar to the elastic wave device having the resonance characteristics shown in Fig. 22. Fig. 23 shows the relationship between the d / p ratio and the fractional bandwidth of the elastic wave device as a resonator.

[0132] As is clear from Figure 23, when d / p > 0.5, adjusting d / p results in a fractional bandwidth of less than 5%. In contrast, when d / p ≤ 0.5, varying d / p within this range can increase the fractional bandwidth to 5% or more, thereby enabling the construction of a resonator with a high coupling coefficient. Furthermore, when d / p is 0.24 or less, the fractional bandwidth can be increased to 7% or more. Furthermore, adjusting d / p within this range can provide a resonator with an even wider fractional bandwidth and a higher coupling coefficient. Therefore, by setting d / p to 0.5 or less, it is possible to construct a resonator with a high coupling coefficient utilizing the thickness-shear mode bulk wave.

[0133] Figure 24 is a plan view of an elastic wave device utilizing thickness-shear mode bulk waves. In elastic wave device 80, a pair of electrodes, including electrode 3 and electrode 4, is provided on first main surface 2a of piezoelectric layer 2. Note that K in Figure 24 represents the crossover width. As mentioned above, in an elastic wave device of the present invention, the number of electrode pairs may be one. Even in this case, thickness-shear mode bulk waves can be effectively excited as long as the above d / p is 0.5 or less.

[0134] In the elastic wave device 1, it is preferable that the metallization ratio MR of adjacent electrodes 3, 4 relative to excitation region C, which is the region where adjacent electrodes 3, 4 overlap when viewed in the direction in which they face each other, satisfies MR≦1.75(d / p)+0.075. In this case, spurious signals can be effectively reduced. This will be explained with reference to FIGS. 25 and 26. FIG. 25 is a reference diagram showing an example of the resonance characteristics of the elastic wave device 1. A spurious signal indicated by arrow B appears between the resonance frequency and the antiresonance frequency. Note that, assuming d / p=0.08 and LiNbO ３ The Euler angles were (0°, 0°, 90°), and the metallization ratio was set to MR=0.35.

[0135] The metallization ratio MR will be explained with reference to FIG. 18( b). Focusing on a pair of electrodes 3 and 4 in the electrode structure of FIG. 18( b), let us assume that only this pair of electrodes 3 and 4 is provided. In this case, the area surrounded by the dashed line is the excitation region C. This excitation region C refers to the region of electrode 3 that overlaps with electrode 4, the region of electrode 4 that overlaps with electrode 3, and the region between electrodes 3 and 4 where electrodes 3 and 4 overlap when viewed in a direction perpendicular to the longitudinal direction of electrodes 3 and 4, i.e., the opposing direction. The metallization ratio MR is the ratio of the area of ​​the electrodes 3 and 4 within the excitation region C to the area of ​​the excitation region C. In other words, the metallization ratio MR is the ratio of the area of ​​the metallization portion to the area of ​​the excitation region C.

[0136] When multiple pairs of electrodes are provided, the ratio of the metallization portion included in all excitation regions to the total area of ​​the excitation regions may be defined as MR.

[0137] 26 is a graph showing the relationship between the relative bandwidth when a number of elastic wave resonators are configured according to the configuration of elastic wave device 1 and the amount of phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. The relative bandwidth was adjusted by changing the film thickness of the piezoelectric layer and the dimensions of the electrodes. Also, FIG. 26 shows the relationship between the relative bandwidth and the phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. ３Although this is the result when a piezoelectric layer having a cut angle of 100° is used, the same tendency is observed when a piezoelectric layer having a different cut angle is used.

[0138] In the region surrounded by ellipse J in Figure 26, the spurious is as large as 1.0. As is clear from Figure 26, when the fractional bandwidth exceeds 0.17, i.e., exceeds 17%, large spurious signals with a spurious level of 1 or more appear within the passband, even if the parameters constituting the fractional bandwidth are changed. That is, as in the resonance characteristics shown in Figure 25, large spurious signals indicated by arrow B appear within the passband. Therefore, it is preferable that the fractional bandwidth be 17% or less. In this case, the spurious signals can be reduced by adjusting the film thickness of piezoelectric layer 2 and the dimensions of electrodes 3 and 4, etc.

[0139] FIG. 27 illustrates the relationship between d / 2p, metallization ratio MR, and bandwidth fraction. Various acoustic wave devices with different d / 2p and MR were constructed, and the bandwidth fractions were measured. The hatched area to the right of dashed line D in FIG. 27 represents the region where the bandwidth fraction is 17% or less. The boundary between this hatched area and the unhatched area is represented by MR = 3.5(d / 2p) + 0.075. That is, MR = 1.75(d / p) + 0.075. Therefore, preferably, MR ≦ 1.75(d / p) + 0.075. In this case, the bandwidth fraction is easily maintained at 17% or less. More preferably, the region to the right of MR = 3.5(d / 2p) + 0.05, indicated by dashed line D1 in FIG. 27, is the region where MR ≦ 1.75(d / p) + 0.05. That is, if MR ≦ 1.75(d / p) + 0.05, the bandwidth fraction can be reliably maintained at 17% or less.

[0140] FIG. 28 shows the LiNbO ３ 28 is a diagram showing a map of the fractional bandwidth with respect to the Euler angles (0°, θ, ψ) of the piezoelectric layer. Each of the hatched regions R shown in FIG. 28 is a region where a fractional bandwidth of 2% or more is obtained. When φ in the Euler angles (φ, θ, ψ) is within the range of 0°±5°, the relationship between θ and ψ and the fractional bandwidth is the same as that shown in FIG. 28. When the piezoelectric layer is made of lithium tantalate (LiTaO ３), the relationship between θ and ψ in the Euler angles (θ, ψ in the range of 0°±5°) and BW is the same as the relationship shown in FIG.

[0141] Therefore, it is preferable that φ in the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer is within the range of 0°±5°, and θ and φ are within the range of any of the multiple regions R shown in Figure 28, as this can sufficiently widen the relative bandwidth.

[0142] FIG. 29 is a front cross-sectional view of an acoustic wave device having an acoustic multilayer film.

[0143] In the acoustic wave device 81, an acoustic multilayer film 82 is laminated on the second principal surface 2b of the piezoelectric layer 2. The acoustic multilayer film 82 has a laminated structure of low acoustic impedance layers 82a, 82c, and 82e, each having a relatively low acoustic impedance, and high acoustic impedance layers 82b and 82d, each having a relatively high acoustic impedance. Using the acoustic multilayer film 82 allows thickness-shear mode bulk waves to be confined within the piezoelectric layer 2 without using the cavity 9 of the acoustic wave device 1. In the acoustic wave device 81, by setting the d / p ratio to 0.5 or less, resonance characteristics based on thickness-shear mode bulk waves can also be obtained. Note that the number of layers of the low acoustic impedance layers 82a, 82c, and 82e and the high acoustic impedance layers 82b and 82d in the acoustic multilayer film 82 is not particularly limited. It is sufficient that at least one of the high acoustic impedance layers 82b and 82d be located farther from the piezoelectric layer 2 than the low acoustic impedance layers 82a, 82c, and 82e.

[0144] The low acoustic impedance layers 82a, 82c, and 82e and the high acoustic impedance layers 82b and 82d can be made of any suitable material as long as the acoustic impedance relationship is satisfied. For example, the low acoustic impedance layers 82a, 82c, and 82e can be made of silicon oxide or silicon oxynitride. The high acoustic impedance layers 82b and 82d can be made of alumina, silicon nitride, or metal.

[0145] FIG. 30 is a partially cutaway perspective view illustrating an elastic wave device that utilizes Lamb waves.

[0146] The acoustic wave device 91 includes a support substrate 92. The support substrate 92 has a recessed portion formed on its upper surface. A piezoelectric layer 93 is laminated on the support substrate 92, thereby forming a cavity 9. An IDT electrode 94 is provided on the piezoelectric layer 93 above the cavity 9. Reflectors 95 and 96 are provided on both sides of the IDT electrode 94 in the acoustic wave propagation direction. In FIG. 30 , the outer periphery of the cavity 9 is indicated by a dashed line. Here, the IDT electrode 94 includes first and second bus bars 94a and 94b, a plurality of first electrode fingers 94c, and a plurality of second electrode fingers 94d. The first electrode fingers 94c are connected to the first bus bar 94a. The second electrode fingers 94d are connected to the second bus bar 94b. The first electrode fingers 94c and the second electrode fingers 94d are interdigitated with each other.

[0147] In the acoustic wave device 91, Lamb waves as plate waves are excited by applying an AC electric field to the IDT electrode 94 above the cavity 9. Furthermore, since reflectors 95 and 96 are provided on both sides, resonance characteristics due to the Lamb waves can be obtained.

[0148] As described above, an elastic wave device of the present invention may utilize plate waves. In the example shown in FIG. 30 , an IDT electrode 94, a reflector 95, and a reflector 96 are provided on a principal surface corresponding to the first principal surface 14 a of the piezoelectric layer 14 shown in FIG. 1 , etc. Meanwhile, in an elastic wave device of the present invention, a pair of comb electrodes and a plurality of third electrode fingers are provided on the first principal surface 14 a. When an elastic wave device of the present invention utilizes plate waves, it is sufficient that a pair of comb electrodes, a plurality of third electrode fingers, and the reflectors 95 and 96 are provided on the first principal surface 14 a of the piezoelectric layer 14 in the first to fourth embodiments and their respective modifications. In this case, it is sufficient that the pair of comb electrodes and the plurality of third electrode fingers are sandwiched between the reflectors 95 and 96 in the direction perpendicular to the electrode fingers.

[0149] In the elastic wave devices according to the first to fourth embodiments and their respective modifications, for example, an acoustic multilayer film 82 shown in FIG. 29 may be provided between the support member and the piezoelectric layer as an acoustic reflecting film. Specifically, the support member and the piezoelectric layer may be arranged so that at least a portion of the support member and at least a portion of the piezoelectric layer face each other with the acoustic multilayer film 82 in between. In this case, it is sufficient that low acoustic impedance layers and high acoustic impedance layers are alternately stacked in the acoustic multilayer film 82. The acoustic multilayer film 82 may be an acoustic reflecting portion in the elastic wave device.

[0150] In the elastic wave devices according to the first to fourth embodiments and their modifications that utilize thickness-shear mode bulk waves, as described above, d / p is preferably 0.5 or less, and more preferably 0.24 or less, thereby achieving even better resonance characteristics.

[0151] Furthermore, in the excitation region of the elastic wave devices according to the first to fourth embodiments and their modifications that utilize thickness-shear mode bulk waves, it is preferable to satisfy MR≦1.75(d / p)+0.075, as described above. More specifically, when the metallization ratio of the first electrode fingers and the third electrode fingers, and the second electrode fingers and the third electrode fingers to the excitation region is MR, it is preferable to satisfy MR≦1.75(d / p)+0.075. In this case, spurious signals can be more reliably suppressed.

[0152] REFERENCE SIGNS LIST 1...acoustic wave device 2...piezoelectric layer 2a, 2b...first and second principal surfaces 3, 4...electrodes 5, 6...first and second bus bars 7...insulating layer 7a...through hole 8...support member 8a...through hole 9...cavity 10...acoustic wave device 10a...cavity 11...functional electrode 12...piezoelectric substrate 13...support member 14...piezoelectric layer 14a, 14b...first and second principal surfaces 15...insulating layer 16...support substrate 17, 18...first and second interdigital electrodes 19...reference potential electrode 22-24...first to third bus bars 24A, 24B...first and second connecting electrodes 24a...bar portion 24b...projection portion 25-27...first to third electrode fingers 28...insulating film 29...reference potential electrode 35...connecting electrode 39...Reference potential electrode 40...Acoustic wave device 48...Fourth electrode finger 50...Acoustic wave device 51...Functional electrode 60...Acoustic wave device 61...Functional electrode 69...Reference potential electrode 80, 81...Acoustic wave device 82...Acoustic multilayer film 82a, 82c, 82e...Low acoustic impedance layer 82b, 82d...High acoustic impedance layer 91...Acoustic wave device 92...Support substrate 93...Piezoelectric layer 94...IDT electrode 94a, 94b...First and second bus bars 94c, 94d...First and second electrode fingers 95, 96...Reflector 100...Acoustic wave device 201...Piezoelectric film 201a, 201b...First and second principal surfaces 451, 452...First and second regions C...Excitation region E...Intersection region R...Region VP1...Virtual plane

Claims

1. a piezoelectric layer made of lithium niobate; a first comb electrode provided on the piezoelectric layer, the first comb electrode having a first bus bar and a plurality of first electrode fingers each having one end connected to the first bus bar, and connected to an input potential; a second comb electrode provided on the piezoelectric layer, the second comb electrode having a second bus bar and a plurality of second electrode fingers, one end of each of which is connected to the second bus bar and interdigitated with the plurality of first electrode fingers, the second comb electrode being connected to an output potential; a reference potential electrode including a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned, and a connection electrode connecting adjacent third electrode fingers to each other, the reference potential electrode being connected to a reference potential; Equipped with an order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period; An elastic wave device, wherein in the first comb electrode and the second comb electrode, the center-to-center distance between adjacent first electrode fingers and second electrode fingers is constant, and in the reference potential electrode, the multiple third electrode fingers are arranged at equal intervals, and the center-to-center distance between adjacent first electrode fingers and third electrode fingers, and the center-to-center distance between adjacent second electrode fingers and third electrode fingers are not constant.

2. a piezoelectric layer made of lithium niobate; a first comb electrode provided on the piezoelectric layer, the first comb electrode having a first bus bar and a plurality of first electrode fingers each having one end connected to the first bus bar, and connected to an input potential; a second comb electrode provided on the piezoelectric layer, the second comb electrode having a second bus bar and a plurality of second electrode fingers, one end of each of which is connected to the second bus bar and interdigitated with the plurality of first electrode fingers, the second comb electrode being connected to an output potential; a reference potential electrode including a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned, and a connection electrode connecting adjacent third electrode fingers to each other, the reference potential electrode being connected to a reference potential; at least one fourth electrode finger provided on the piezoelectric layer and adjacent to the first electrode finger, the second electrode finger, or the third electrode finger; Equipped with an order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged in a region in which the fourth electrode finger is not provided is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period; The fourth electrode finger is not connected to an input potential, an output potential, or a reference potential.

3. The acoustic wave device according to claim 2 , wherein the fourth electrode finger is located between the first electrode finger and the second electrode finger.

4. The acoustic wave device according to claim 2 , wherein the fourth electrode finger is adjacent to the third electrode finger.

5. a piezoelectric layer made of lithium niobate; a first comb electrode provided on the piezoelectric layer, the first comb electrode having a first bus bar and a plurality of first electrode fingers each having one end connected to the first bus bar, and connected to an input potential; a second comb electrode provided on the piezoelectric layer, the second comb electrode having a second bus bar and a plurality of second electrode fingers, one end of each of which is connected to the second bus bar and interdigitated with the plurality of first electrode fingers, the second comb electrode being connected to an output potential; a reference potential electrode including a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned, and a connection electrode connecting adjacent third electrode fingers to each other, the reference potential electrode being connected to a reference potential; Equipped with an order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period; The acoustic wave device, wherein when the width of the first electrode finger is w1 and the width of the second electrode finger is w2, w1≠w2 is satisfied.

6. a piezoelectric layer made of lithium niobate; a first comb electrode provided on the piezoelectric layer, the first comb electrode having a first bus bar and a plurality of first electrode fingers each having one end connected to the first bus bar, and connected to an input potential; a second comb electrode provided on the piezoelectric layer, the second comb electrode having a second bus bar and a plurality of second electrode fingers, one end of each of which is connected to the second bus bar and interdigitated with the plurality of first electrode fingers, the second comb electrode being connected to an output potential; a reference potential electrode including a plurality of third electrode fingers provided on the piezoelectric layer so as to be aligned with the first electrode fingers and the second electrode fingers in a direction in which the first electrode fingers and the second electrode fingers are aligned, and a connection electrode connecting adjacent third electrode fingers to each other, the reference potential electrode being connected to a reference potential; Equipped with an order in which the first electrode finger, the second electrode finger, and the third electrode finger are arranged is an order in which, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger form one period; An elastic wave device, wherein in each of the first comb electrode and the second comb electrode, the center-to-center distance between adjacent first electrode fingers and the center-to-center distance between adjacent second electrode fingers are constant, and in the reference potential electrode, the center-to-center distance between adjacent third electrode fingers is not constant, and the center-to-center distance between adjacent first electrode fingers and third electrode fingers is different from the center-to-center distance between adjacent second electrode fingers and third electrode fingers.

7. The elastic wave device according to claim 1 , wherein the elastic wave device is configured to be able to utilize plate waves.

8. The elastic wave device according to claim 1 , wherein the elastic wave device is configured to utilize bulk waves in a thickness shear mode.

9. A support member is laminated on the piezoelectric layer, an acoustic reflecting portion is formed at a position on the support member that overlaps with the plurality of first electrode fingers, the plurality of second electrode fingers, and the plurality of third electrode fingers in a plan view along a stacking direction of the support member and the piezoelectric layer, An elastic wave device as described in any one of claims 1, 2, 5 or 6, wherein, when the longest distance among the center-to-center distance between adjacent first electrode fingers and third electrode fingers and the center-to-center distance between adjacent second electrode fingers and third electrode fingers is p, and the thickness of the piezoelectric layer is d, d / p is 0.5 or less.

10. The acoustic wave device according to claim 9 , wherein d / p is equal to or less than 0.

24.

11. 10. The elastic wave device according to claim 9, wherein the acoustic reflecting portion is a cavity, and the support member and the piezoelectric layer are arranged such that a portion of the support member and a portion of the piezoelectric layer face each other with the cavity therebetween.

12. 10. The elastic wave device of claim 9, wherein the acoustic reflection portion is an acoustic reflection film including a high acoustic impedance layer having a relatively high acoustic impedance and a low acoustic impedance layer having a relatively low acoustic impedance, and the support member and the piezoelectric layer are arranged so that at least a portion of the support member and at least a portion of the piezoelectric layer face each other across the acoustic reflection film.

13. when a direction perpendicular to a direction in which the first electrode finger, the second electrode finger, and the third electrode finger extend is defined as an electrode finger orthogonal direction, a region in which adjacent first electrode fingers and the third electrode fingers overlap in the electrode finger orthogonal direction and between the centers of adjacent first electrode fingers and third electrode fingers, and a region in which adjacent second electrode fingers and third electrode fingers overlap in the electrode finger orthogonal direction and between the centers of adjacent second electrode fingers and third electrode fingers are excitation regions; The elastic wave device according to claim 9, wherein when a metallization ratio of the first electrode finger and the third electrode finger, and the second electrode finger and the third electrode finger to the excitation region is MR, MR≦1.75(d / p)+0.075 is satisfied.

14. 7. The elastic wave device according to claim 1, wherein the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer are within the range of the following formula (1), formula (2), or formula (3). (within the range of 0°±10°, 0° to 25°, any ψ) ...Equation (1) (Within the range of 0°±10°, 25° to 100°, 0° to 75° [(1-(θ-50) ２ / 2500)] １／２ or 180°-75°[(1-(θ-50) ２ / 2500)] １／２ ~180°) …Formula (2) (Within the range of 0°±10°, 180°-40°[(1-(ψ-90) ２ / 8100)] １／２ ~180°, any ψ) ...Equation (3)