FIG. 3B is a schematic sectional view illustrating a thickness-shear first mode bulk wave propagating in a piezoelectric layer of the first preferred embodiment. FIG. 4 is a schematic sectional view illustrating an amplitude direction of the thickness-shear first mode bulk wave propagating in the piezoelectric layer of the first preferred embodiment.

FIG. 3A illustrates the acoustic wave device as described in Japanese Unexamined Patent Application Publication No. 2012-257019, and a Lamb wave propagates in a piezoelectric layer. As illustrated in FIG. 3A, the wave propagates in a piezoelectric layer 201 as indicated by an arrow. Here, the piezoelectric layer 201 includes a first principal surface 201a and a second principal surface 201b, and a thickness direction connecting the first principal surface 201a to the second principal surface 201b is the Z direction. The X direction is a direction in which the electrodes 3 and 4 of the IDT electrode are aligned. As illustrated in FIG. 3A, the Lamb wave propagates in the X direction as illustrated in the drawing. It is a plate wave, and thus, the wave propagates in the X direction although the piezoelectric layer 201 vibrates as a whole. Therefore, resonance characteristics are obtained by reflectors being disposed on both sides. As a result, propagation loss of the wave occurs, and a Q factor decreases when attempting the size reduction (that is, when the number of pairs of the electrodes 3 and 4 is reduced).

On the other hand, in the acoustic wave device of the first preferred embodiment, as illustrated in FIG. 3B, vibration displacement is a thickness shear direction. Therefore, the wave propagates substantially in the direction connecting the first principal surface 2a to the second principal surface 2b of the piezoelectric layer 2 (that is, in the Z direction) and resonates. That is, an X-direction component of the wave is extremely smaller than a Z-direction component of the wave. Then, since the resonance characteristics are obtained by the wave propagation in the Z direction, reflectors are unnecessary. Thus, propagation loss caused during propagation to the reflectors does not occur. Therefore, even when the number of electrode pairs of the electrode 3 and the electrode 4 is reduced for size reduction, the Q factor is less likely to decrease.

Note that, as illustrated in FIG. 4, amplitude directions of the thickness-shear first mode bulk wave are opposite in a first range 451 included in the excitation range C (see FIG. 1B) of the piezoelectric layer 2 and a second range 452 included in the excitation range C. FIG. 4 schematically illustrates a bulk wave in a case where a voltage having a higher potential at the electrode 4 than the electrode 3 is applied between the electrode 3 and the electrode 4. The first range 451 is a range in the excitation range C, located between an imaginary plane VP1 orthogonal to the thickness direction of the piezoelectric layer 2 and dividing the piezoelectric layer 2 into two and the first principal surface 2a. The second range 452 is a range in the excitation range C, located between the imaginary plane VP1 and the second principal surface 2b.

In the acoustic wave device 1, although at least one pair of electrodes including the electrode 3 and the electrode 4 is provided, this configuration is not for propagating the wave in the X direction. Therefore, it is not always necessary that the pair of the electrode 3 and the electrode 4 includes a plurality of pairs of the electrode 3 and the electrode 4. That is, the number of pairs may be any number as long as at least one pair of electrodes is provided.

For example, the electrode 3 is an electrode connected to a hot potential, and the electrode 4 is an electrode connected to a ground potential. Note that the electrode 3 may be connected to the ground potential, and the electrode 4 may be connected to the hot potential. In the first preferred embodiment, each electrode in the at least one pair of electrodes is an electrode connected to the hot potential or an electrode connected to the ground potential as described above, and a floating electrode is not provided.

FIG. 5 is an explanatory diagram illustrating an example of resonance characteristics of the acoustic wave device of the first preferred embodiment. Note that design parameters of the acoustic wave device 1 which obtains the resonance characteristics illustrated in FIG. 5 are as follows.

- Piezoelectric layer 2: LiNbO₃having Euler angles (0°, 0°, 90°)
- Thickness of piezoelectric layer 2: about 400 nm
- Length of excitation range C (see FIG. 1B): about 40 μm
- Number of electrode pairs of electrode 3 and electrode 4: 21 pairs
- Center-to-center spacing distance (pitch) between electrode 3 and electrode 4: about 3 μm
- Width of electrode 3 and electrode 4: about 500 nm
- d/p: about 0.133
- Dielectric film 7: silicon oxide film having thickness of about 1 μm
- Support substrate 8: Si

Note that the excitation range C (see FIG. 1B) is an overlapping range of the electrode 3 and the electrode 4 when seen in the X direction orthogonal or substantially orthogonal to the longitudinal direction of the electrode 3 and the electrode 4. The length of the excitation range C is a dimension of the excitation range C in the longitudinal direction of the electrode 3 and the electrode 4.

In the first preferred embodiment, a distance between electrodes in each electrode pair of the electrode 3 and the electrode 4 is equal in all of the plurality of electrode pairs. That is, the electrode 3 and the electrode 4 are positioned with an equal pitch.

As is apparent in FIG. 5, favorable resonance characteristics having a fractional bandwidth of about 12.5% is obtained regardless of not having a reflector.

Meanwhile, assuming that the thickness of the piezoelectric layer 2 is d and the center-to-center spacing distance between the electrode 3 and the electrode 4 is p, in the first preferred embodiment, d/p is about 0.5 or smaller, and more preferably, about 0.24 or smaller, for example. This is described with reference to FIG. 6.

Similarly to the acoustic wave device which obtains the resonance characteristics illustrated in FIG. 5, a plurality of acoustic wave devices are obtained while changing d/2p. FIG. 6 is an explanatory diagram illustrating relation between d/2p and a fractional bandwidth as a resonator in the acoustic wave device of the first preferred embodiment, where a center-to-center spacing distance or an average distance of the center-to-center spacing distances between electrodes adjacent to each other is p and an average thickness of the piezoelectric layer 2 is d.

As illustrated in FIG. 6, when d/2p exceeds about 0.25 (that is, d/p>about 0.5), the fractional bandwidth is smaller than about 5% regardless of adjustment of d/p. On the other hand, in a case of d/2p≤about 0.25 (that is, d/p≤about 0.5), the fractional bandwidth can be brought to about 5% or higher when d/p is changed within the range, for example. That is, a resonator having a high coupling coefficient can be configured. Moreover, when d/2p is about 0.12 or smaller (that is, d/p is about 0.24 or smaller), the fractional bandwidth can be increased to about 7% or higher, for example. In addition to this, by d/p being adjusted within the range, a resonator with a further wider fractional bandwidth can be obtained, and a resonator having a further higher coupling coefficient can be realized. Therefore, like the second invention of this application, by d/p being about 0.5 or smaller, a resonator having a high coupling coefficient utilizing the thickness-shear first mode bulk wave can be configured.

Note that the at least one pair of electrodes may be one pair, and in this case, the p is the center-to-center spacing distance between the electrode 3 and the electrode 4 adjacent to each other. Moreover, in a case of 1.5 or more pairs of electrodes being provided, an average distance of the center-to-center spacing distances between the electrode 3 and the electrode 4 adjacent to each other may be considered as the p.

Moreover, also in terms of the thickness d of the piezoelectric layer 2, when the piezoelectric layer 2 has variations in thickness, a value of an averaged thickness may be adopted.

FIG. 7 is a plan view illustrating an example where one pair of electrodes are provided in the acoustic wave device of the first preferred embodiment. In an acoustic wave device 101, one pair of electrodes including the electrode 3 and the electrode 4 are provided on the first principal surface 2a of the piezoelectric layer 2. Note that K in FIG. 7 is an intersecting width. As described above, in the acoustic wave device of the present disclosure, the number of pairs of electrodes may be one. Also in this case, a thickness-shear first mode bulk wave can effectively be excited when d/p is about 0.5 or smaller, for example.

In the acoustic wave device 1, preferably, a metallization ratio MR of the electrode 3 and the electrode 4 adjacent to each other among the plurality of electrodes 3 and electrodes 4 with respect to the excitation range C which is a range where the adjacent electrodes 3 and 4 overlap each other in the opposing direction desirably satisfies MR≤about 1.75(d/p)+0.075. In this case, a spurious can effectively be reduced. This is described with reference to FIGS. 8 and 9.

FIG. 8 is a reference diagram illustrating one example of resonance characteristics of the acoustic wave device of the first preferred embodiment. A spurious indicated by an arrow B appears between a resonant frequency and an anti-resonant frequency. Note that it is assumed that d/p=about 0.08 and the Euler angles of the LiNbO₃are (0°, 0°, 90°), for example. Moreover, the metallization ratio MR=about 0.35 is satisfied.

The metallization ratio MR is described with reference to FIG. 1B. In the electrode structure illustrated in FIG. 1B, assume that only one pair of the electrode 3 and the electrode 4 is provided when focusing on the one pair of the electrode 3 and the electrode 4. In this case, a portion surrounded by a one-dot chain line is the excitation range C. The excitation range C is, when the electrode 3 and the electrode 4 are seen in the direction orthogonal or substantially orthogonal to the longitudinal direction of the electrode 3 and the electrode 4 (that is, in the opposing direction thereof), a range of the electrode 3 overlapping the electrode 4, a range of the electrode 4 overlapping the electrode 3, and a range between the electrode 3 and the electrode 4 where the electrode 3 and the electrode 4 overlap each other. An area of the electrode 3 and the electrode 4 within the excitation range C with respect to an area of the excitation range C is the metallization ratio MR. That is, the metallization ratio MR is a ratio of the area of the metallization portion to the area of the excitation range C.

Note that when a plurality of pairs of the electrode 3 and the electrode 4 are provided, a ratio of a metallization portion included in the entire excitation range C with respect to the total area of the excitation range C may be considered as the MR.

FIG. 9 is an explanatory diagram illustrating relation of a fractional bandwidth and an amount of phase rotation of impedance of a spurious standardized at 180 degrees as spurious magnitude when a plurality of acoustic wave resonators are configured in the acoustic wave device of the first preferred embodiment. Note that the fractional bandwidth is adjusted while the film thickness of the piezoelectric layer 2 and the dimensions of the electrode 3 and the electrode 4 are variously changed. Moreover, although FIG. 9 illustrates results when a Z-cut LiNbO₃piezoelectric layer 2 is used, similar tendency can be obtained when the piezoelectric layer 2 having other cut-angles is used.

In a range surrounded by an oval J in FIG. 9, a spurious is large at about 1.0, for example. As is apparent in FIG. 9, when the fractional bandwidth exceeds about 0.17 (that is, exceeds about 17%), for example, a large spurious at 1 or larger spurious level appears in a pass band even when the parameter that constitutes the fractional bandwidth is changed. That is, like the resonance characteristics illustrated in FIG. 8, the large spurious indicated by the arrow B appears in the band. Therefore, the fractional bandwidth is preferably about 17% or lower, for example. In this case, the spurious can be made smaller by adjusting the film thickness of the piezoelectric layer 2, the dimensions of the electrode 3 and the electrode 4, and other factors.

FIG. 10 is an explanatory diagram illustrating relation between d/2p, the metallization ratio MR, and the fractional bandwidth. In the acoustic wave device 1 of the first preferred embodiment, various acoustic wave devices 1 with different d/2p and MR are configured, and the fractional bandwidths thereof are measured. A hatched portion on the right side of a broken line D in FIG. 10 is a range where the fractional bandwidth is about 17% or lower, for example. A boundary between the hatched range and a range without hatching is represented by MR=about 3.5(d/2p)+0.075, for example. That is, MR=about 1.75(d/p)+0.075 is satisfied, and thus, preferably, MR≤about 1.75(d/p)+0.075 is satisfied, for example. In this case, the fractional bandwidth can easily be brought to about 17% or lower, for example. A range on the right side of a one-dot chain line D1 (MR=about 3.5(d/2p)+0.05), for example, in FIG. 10 is more preferable. That is, the fractional bandwidth can certainly be about 17% or lower in a case of MR≤about 1.75(d/p)+0.05, for example.

FIG. 11 is an explanatory diagram illustrating a map of the fractional bandwidth with respect to Euler angles (0°, θ, ψ) of LiNbO₃when d/p is brought infinitely closer to zero. A hatched portion in FIG. 11 is a range where a fractional bandwidth of at least about 5% or higher can be obtained. A range expressed by the following Formulas (1), (2), and (3) is obtained when the ranges are approximated.

(0°±10°, 0° to 20°, any ψ) (1)

(0°±10°, 20° to 80°, 0° to 60° (1−(θ−50)²/900)^1/2) or (0°±10°, 20° to 80°, [180°−60° (1−(θ−50)²/900)^1/2] to 180°) (2)

(0°±10°, [180°−30° (1−(ψ−90)²/8100)^1/2] to 180°, any ψ) (3)

Therefore, in the case of the Euler angle range in the Formula (1), (2), or (3), the fractional bandwidth can sufficiently be widened, which is preferable.

FIG. 12 is a modification of the first preferred embodiment and is a partial sectional view taken along the line II-II in FIG. 1A. In an acoustic wave device 41, an acoustic multilayer film 42 is laminated to the second principal surface 2b of the piezoelectric layer 2. The acoustic multilayer film 42 has a multilayer structure including low acoustic impedance layers 42a, 42c, and 42e having a relatively low acoustic impedance and high acoustic impedance layers 42b and 42d having a relatively high acoustic impedance. When the acoustic multilayer film 42 is used, a thickness-shear first mode bulk wave can be confined in the piezoelectric layer 2 without use of the hollow portion 9 of the acoustic wave device 1. Also in the acoustic wave device 41, by d/p being about 0.5 or smaller, for example, the resonance characteristics based on the thickness-shear first mode bulk wave can be obtained. Note that, in the acoustic multilayer film 42, the number of laminated layers of the low acoustic impedance layers 42a, 42c, and 42e and the high acoustic impedance layers 42b and 42d is not particularly limited, as long as at least one high acoustic impedance layer 42b, 42d is disposed farther than the low acoustic impedance layer 42a, 42c, 42e, from the piezoelectric layer 2.

The low acoustic impedance layers 42a, 42c, and 42e and the high acoustic impedance layers 42b and 42d can be configured by suitable material, as long as it satisfies the acoustic impedance relation described above. For example, as the material of the low acoustic impedance layers 42a, 42c, and 42e, silicon oxide, silicon oxynitride, or the like may be used. Moreover, as the material of the high acoustic impedance layers 42b and 42d, alumina, silicon nitride, metal such as platinum, or the like may be used.

FIG. 13 is a partially cutaway perspective view illustrating the acoustic wave device according to the present preferred embodiment of the present disclosure. In FIG. 13, a peripheral edge of the hollow portion 9 is indicated by a broken line. The acoustic wave device of the present disclosure may utilize a plate wave. In this case, an acoustic wave device 301 includes reflectors 310 and 311 as illustrated in FIG. 13. The reflectors 310 and 311 are provided to both sides of the electrodes 3 and 4 of the piezoelectric layer 2 in the acoustic wave propagation direction. In the acoustic wave device 301, by an AC electric field being applied to the electrodes 3 and 4 above the hollow portion 9, a Lamb wave as a plate wave is excited. At this time, since the reflectors 310 and 311 are provided on both sides, resonance characteristics by the Lamb wave as the plate wave can be obtained.

As described above, in the acoustic wave devices 1 and 101, a thickness-shear first mode bulk wave is utilized. Moreover, in the acoustic wave devices 1 and 101, when the first electrode 3 and the second electrode 4 are electrodes adjacent to each other and it is assumed that the thickness of the piezoelectric layer 2 is d and the center-to-center spacing distance between the first electrode 3 and the second electrode 4 is p, d/p is about 0.5 or smaller, for example. Therefore, a Q factor can be increased even when the size of the acoustic wave device is reduced.

In the acoustic wave devices 1 and 101, the piezoelectric layer 2 is formed by lithium niobate or lithium tantalate. The first electrode 3 and the second electrode 4 opposed to each other in the direction intersecting with the thickness direction of the piezoelectric layer 2 are provided to the first principal surface 2a or the second principal surface 2b of the piezoelectric layer 2, and a protection film desirably covers over the first electrode 3 and the second electrode 4.

In the acoustic wave devices 1 and 41, an energy confining layer is provided between the piezoelectric layer 2 and the support substrate 8. The energy confining layer is a layer which confines an acoustic wave such as a thickness-shear first mode bulk wave in the piezoelectric layer 2. As the energy confining layer, for example, the hollow portion 9, an acoustic reflection film 11, an acoustic reflection layer, or a multilayer body of a low acoustic velocity layer and a high acoustic velocity layer, which will be described later, may be used. The acoustic reflection layer is a layer including the low acoustic impedance layer and the high acoustic impedance layer laminated to the low acoustic impedance layer. Here, the acoustic multilayer film 42 is one example of the acoustic reflection layer and the multilayer body of the low acoustic velocity layer and the high acoustic velocity layer.

FIG. 14 is a plan view illustrating one example of the first preferred embodiment. As illustrated in FIG. 14, an acoustic wave device 1A according to the example of the first preferred embodiment includes a first resonator 20 which utilizes the thickness-shear first mode and a second resonator 20A which does not utilize the thickness-shear first mode, the first resonator 20 and the second resonator 20A being provided to the support substrate 8.

FIG. 15A is one example of a partial sectional view taken along a line A-A′ in FIG. 14. The first resonator 20 is a resonator laminated to the piezoelectric layer 2 in the Z direction. The first resonator 20 is a resonator including a first IDT electrode 30 and a multilayer body of which at least a portion overlaps the first IDT electrode 30 in plan view in the Z direction. In the example in FIG. 15A, the multilayer body of the first resonator 20 includes the first IDT electrode 30, a portion of the piezoelectric layer 2, and the hollow portion 9.

The first IDT electrode 30 includes the first electrode 3, the second electrode 4, the first busbar electrode 5, and the second busbar electrode 6. The first IDT electrode 30 is configured such that at least one pair of the first electrode 3 and the second electrode 4 can resonate, and d/p is preferably about 0.5 or smaller, for example. The first resonator 20 is configured to utilize a thickness-shear first mode bulk wave.

FIG. 15B is one example of a partial sectional view taken along a line B-B′ in FIG. 14. The second resonator 20A is a resonator laminated to the piezoelectric layer 2 in the Z direction. The second resonator 20A is a resonator including a second IDT electrode 30A and a multilayer body of which at least a portion overlaps the second IDT electrode 30A in plan view in the Z direction. Moreover, in the example in FIG. 15B, the multilayer body of the second resonator 20A includes the second IDT electrode 30A, a portion of the piezoelectric layer 2 and a hollow portion 9A. The second IDT electrode 30A is configured such that at least one pair of a third electrode 3A and a fourth electrode 4A can resonate, and d/p is preferably larger than 0.5. The second resonator 20A is configured to utilize a wave other than a thickness-shear first mode bulk wave, and in the first preferred embodiment, is configured to utilize an S0 mode plate wave.

The second IDT electrode 30A includes the first electrode 3A, the second electrode 4A, a third busbar electrode 5A, and a fourth busbar electrode 6A. Here, an angle between a direction orthogonal or substantially orthogonal to a longitudinal direction of the first electrode 3A or the second electrode 4A and the direction orthogonal or substantially orthogonal to the longitudinal direction of the first electrode 3 or the second electrode 4 is preferably within a range of about 90°±10°, for example.

FIG. 16 is an explanatory diagram illustrating frequency characteristics of the acoustic wave device according to the example of the first preferred embodiment. The piezoelectric layer 2 of the acoustic wave device according to FIG. 16 is LiNbO₃having Euler angles (0°, 37.5°, 0°), and has a thickness of about 0.45 μm and a propagation angle of about 90°, for example. Moreover, the distance between electrodes of the second IDT electrode of the acoustic wave device according to FIG. 16 is about 1.7 μm in Example 1, about 1.6 μm in Example 2, and about 1.5 μm in Example 3.

Values fr, fa, and (fa−fr)/fr obtained based on the frequency characteristics of the acoustic wave device 1A according to FIG. 16 are shown in Table 1. Here, the fr is a resonant frequency of the first resonator 20 of the acoustic wave device 1A. The fa is an anti-resonant frequency of the first resonator 20. Moreover, the (fa−fr)/fr is a fractional bandwidth of the first resonator 20.

TABLE 1

Resonant
Anti-resonant
Fractional

frequency
frequency
bandwidth

fr (GHz)
fa (GHz)
(fa − fr)/fr (%)

Example 1
3730
3970
6.4

Example 2
3950
4200
6.3

Example 3
4200
4460
6.2

As is apparent from FIG. 16 and Table 1, by the spacing between the electrodes of the second resonator 20A being adjusted, in terms of the first resonator 20, the resonant frequency can be adjusted while influence on the fractional bandwidth is suppressed.

Although the example of the acoustic wave device according to the first preferred embodiment is described above, the configuration of the acoustic wave device 1A is not limited to the configuration illustrated in FIGS. 15A and 15B. Other modifications are described below with reference to the drawings.

FIG. 17A is a first modification of the partial sectional view taken along the line A-A′ in FIG. 14. FIG. 17B is the first modification of the partial sectional view taken along the line B-B′ in FIG. 14. As illustrated in FIGS. 17A and 17B, the support substrate 8 and the piezoelectric layer 2 may be laminated to each other with the acoustic multilayer film 42 interposed therebetween. That is, a first resonator 21 and a second resonator 21A may include the acoustic multilayer film 42 as the energy confining layer. Also in this case, the resonant frequency of the first resonator 21 can be adjusted. Note that although, in the first modification, the low acoustic impedance layers 42a, 42c, and