Elastic Wave Device

Abstract

To provide an acoustic wave device in which the Q value can be enhanced even when the size reduction is advanced and the position or size of a spurious can be controlled.SOLUTION: An acoustic wave device 1 includes a piezoelectric layer 2 including lithium niobate and lithium tantalate and at least a pair of electrodes 3 and 4 facing each other in a direction intersecting with a thickness direction of the piezoelectric layer 2. When the thickness of the piezoelectric layer 2 is d and the distance between centers of the adjacent electrodes of at least the pair of electrodes 3 and 4 is p, d / p is 0.5 or less. At least the pair of electrodes 3 and 4 have the length direction. At least the pair of electrodes include the first electrode 3 and the second electrode 4 with different cross-sectional shapes in any cross section in a direction orthogonal to the length direction of at least the pair of electrodes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an acoustic wave device having a piezoelectric layer made of LiNbO3 or LiTaO3. [Background technology]

[0002] Conventionally, acoustic wave devices using plate waves propagating through a piezoelectric film made of LiNbO3 or LiTaO3 have been known. For example, Patent Document 1 below discloses an acoustic wave device using Lamb waves as plate waves. In this device, an IDT electrode is provided on the upper surface of a piezoelectric film made of LiNbO3 or LiTaO3. A voltage is applied between multiple electrode fingers connected to one potential of the IDT electrode and multiple electrode fingers connected to the other potential. This excites Lamb waves. Reflectors are provided on both sides of this IDT electrode. This forms an acoustic wave resonator using plate waves. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-257019 Summary of the Invention [Problem to be solved by the invention]

[0004] In the acoustic wave device described in Patent Document 1, it is possible to reduce the number of electrode fingers in order to achieve miniaturization. However, reducing the number of electrode fingers reduces the Q value. Furthermore, spurious resonance characteristics may occur.

[0005] An object of the present invention is to provide an acoustic wave device that can increase the Q value and control the position and size of spurious components even when miniaturized. [Means for solving the problem]

[0006] The present invention provides an elastic wave device comprising a piezoelectric layer made of lithium niobate or lithium tantalate, and at least one pair of electrodes facing each other in a direction intersecting the thickness direction of the piezoelectric layer, wherein d / p is 0.5 or less, where d is the thickness of the piezoelectric layer and p is the center-to-center distance between adjacent electrodes of the at least one pair of electrodes, and the at least one pair of electrodes has a length direction, and the at least one pair of electrodes includes a first electrode and a second electrode whose cross-sectional shapes are different from each other in any cross section in a direction perpendicular to the length direction of the at least one pair of electrodes. [Effects of the Invention]

[0007] In an acoustic wave device according to a preferred embodiment of the present invention, even when miniaturized, the Q value can be increased and the size and position of spurious components can be controlled. [Brief explanation of the drawings]

[0008] [Figure 1] 1(a) and 1(b) are a schematic perspective view showing the appearance of an acoustic wave device according to a first preferred embodiment of the present invention and a plan view showing an electrode structure on a piezoelectric layer. [Figure 2] FIG. 2 is a cross-sectional view of a portion taken along line AA in FIG. 1(a). [Figure 3] Figure 3(a) is a schematic cross-sectional front view illustrating Lamb waves propagating through a piezoelectric film of a conventional elastic wave device, and Figure 3(b) is a schematic cross-sectional front view illustrating bulk waves in thickness-shear first-order mode propagating through a piezoelectric layer in an elastic wave device according to one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing the amplitude direction of a bulk wave in the first thickness-shear mode. [Figure 5] FIG. 5 is a diagram showing the phase characteristics of the resonators according to Examples 1 to 3. In FIG. [Figure 6A] FIG. 6A is a diagram showing the relationship between d / 2p and the fractional bandwidth of a resonator, where p is the center-to-center distance or the average center-to-center distance between adjacent electrodes and d is the thickness of the piezoelectric layer. [Figure 6B]FIG. 6B is a diagram showing the relationship between d / 2p and the fractional bandwidth of a resonator, where p is the center-to-center distance or the average center-to-center distance between adjacent electrodes and d is the thickness of the piezoelectric layer. [Figure 7] FIG. 7 is a diagram showing the resonance characteristics of the elastic wave device of the reference example in which spurious components appear. [Figure 8] FIG. 8 is a diagram showing the relationship between the fractional bandwidth and the magnitude of the normalized spurious. [Figure 9] FIG. 9 is a diagram showing the relationship between d / 2p, metallization ratio MR, and fractional bandwidth. [Figure 10] FIG. 10 is a diagram showing a map of the fractional bandwidth when d / p approaches 0 in LiNbO 3 with Euler angles (0°, θ, ψ). [Figure 11] FIG. 11 is a plan view illustrating the electrode structure of an elastic wave device according to a second preferred embodiment of the present invention. [Figure 12] FIG. 12 is a plan view illustrating the electrode structure of the elastic wave device according to the third preferred embodiment. [Figure 13] FIG. 13 is a plan view illustrating the electrode structure of the elastic wave device according to the fourth preferred embodiment. [Figure 14] FIG. 14 is a plan view illustrating the electrode structure of the elastic wave device according to the fifth preferred embodiment. [Figure 15] FIG. 15 is a plan view illustrating the electrode structure of an elastic wave device according to a sixth preferred embodiment of the present invention. [Figure 16] FIG. 16 is a front cross-sectional view showing the electrode structure of an elastic wave device according to a seventh preferred embodiment of the present invention. [Figure 17] FIG. 17 is a front cross-sectional view illustrating an elastic wave device according to an eighth preferred embodiment of the present invention. [Figure 18] FIG. 18 is a front cross-sectional view illustrating an elastic wave device according to a ninth preferred embodiment of the present invention. [Figure 19] FIG. 19(a) is a front cross-sectional view illustrating an elastic wave device according to a tenth preferred embodiment of the present invention, and FIG. 19(b) is a front cross-sectional view illustrating a modification thereof. [Figure 20]FIG. 20(a) is a front cross-sectional view illustrating an elastic wave device according to an eleventh preferred embodiment of the present invention, and FIG. 20(b) is a front cross-sectional view illustrating a modification thereof. [Figure 21] FIG. 21 is a front cross-sectional view of an elastic wave device according to a twelfth preferred embodiment of the present invention. [Figure 22] FIG. 22 is a front cross-sectional view showing a piezoelectric layer and a pair of electrodes in a thirteenth embodiment of the present invention. [Figure 23] FIG. 23(a) is a front cross-sectional view showing a piezoelectric layer and a pair of electrodes in a fourteenth embodiment of the present invention, and FIGS. 23(b) to 23(d) are front cross-sectional views for explaining modifications thereof. [Figure 24] 24(a) to 24(c) are front cross-sectional views illustrating still another modification of an elastic wave device according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be clarified below by describing specific embodiments of the present invention with reference to the drawings.

[0010] 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.

[0011] The first and second inventions of the present application comprise a piezoelectric layer made of lithium niobate or lithium tantalate and at least one pair of electrodes facing each other in a direction intersecting the thickness direction of the piezoelectric layer. The at least one pair of electrodes has a length direction, and the at least one pair of electrodes includes a first electrode and a second electrode that have different cross-sectional shapes in a cross section in a direction perpendicular to the length direction of the at least one pair of electrodes. This makes it possible to control the size and position of spurious emissions.

[0012] The first invention utilizes bulk waves in the thickness-shear first-order mode. In the second invention, d / p is set to 0.5 or less, where d is the thickness of the piezoelectric layer and p is the center-to-center distance between adjacent electrodes in at least one pair of electrodes. This allows the first and second inventions to increase the Q value even when miniaturization is promoted.

[0013] FIG. 1(a) is a schematic perspective view showing the appearance of an elastic wave device according to a first embodiment of the first and second inventions, FIG. 1(b) is a plan view showing the electrode structure on a piezoelectric layer, and FIG. 2 is a cross-sectional view of a portion along line AA in FIG. 1(a).

[0014] The acoustic wave device 1 has a piezoelectric layer 2 made of LiNbO3. The piezoelectric layer 2 may also be made of LiTaO3. To utilize the first-order thickness shear mode, the thickness of the piezoelectric layer 2 is preferably 50 nm or more and 1000 nm or less. In this embodiment, the cut angle of the piezoelectric layer 2 is Z-cut, but it may also be rotated Y-cut or X-cut. Furthermore, the propagation direction is preferably Y-propagation or X-propagation ±30°.

[0015] 1(b), the electrodes 3 and 4 are rectangular and have a length direction. The direction perpendicular to the length direction of the electrodes 3 and 4 is the width direction of the electrodes 3 and 4. The width of the electrodes 3 and 4 refers to the dimension along the width direction of the electrodes 3 and 4.

[0016] The electrodes 3 and 4 have a first electrode and a second electrode that have different cross-sectional shapes in a cross section perpendicular to the longitudinal direction of the electrodes 3 and 4. More specifically, in this preferred embodiment, the width of the electrode 4 is greater than the width of the electrode 3. Therefore, for example, in a cross section taken along line P-P in FIG. 1(b), i.e., in a cross section perpendicular to the longitudinal direction of the electrodes 3 and 4, the cross-sectional shapes of the first electrode (electrode 3) and the second electrode (electrode 4) are different. In the elastic wave device 1 of this preferred embodiment, the electrodes 3 and 4 are configured as described above, which allows the size and position of spurious components to be controlled. This will be described in more detail later based on specific examples.

[0017] The piezoelectric layer 2 has first and second principal surfaces 2a and 2b facing each other. At least one pair of electrodes 3 and 4 is provided on the first principal surface 2a. In FIGS. 1(a) and 1(b), the plurality of electrodes 3 are connected to a first bus bar 5. The plurality of electrodes 4 are connected to a second bus bar 6. The plurality of electrodes 3 and 4 are interdigitated with each other. The electrodes 3 and 4 face each other in a direction perpendicular to the thickness direction of the piezoelectric layer 2. The electrodes 3 and 4 are rectangular and have a length direction. The electrode 3 faces the electrode 4 adjacent to the electrode 3 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 faces the adjacent electrode 4 in a direction perpendicular to the thickness direction of the piezoelectric layer 2. Furthermore, the length direction of the electrodes 3, 4 may be reversed to the direction perpendicular to the length direction of the electrodes 3, 4 shown in FIGS. 1(a) and 1(b). That is, the electrodes 3, 4 may extend in the direction in which the first bus bar 5 and the second bus bar 6 extend in FIGS. 1(a) and 1(b). In this case, the first bus bar 5 and the second bus bar 6 extend in the direction in which the electrodes 3, 4 extend in FIGS. 1(a) and 1(b). A plurality of pairs of adjacent electrodes, 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, 4. The number of pairs does not need to be an integer and may be, for example, 1.5 pairs or 2.5 pairs.

[0018] A support member 8 is provided on the second principal surface 2b 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 have openings 7a and 8a, as shown in FIG. 2 . This forms an air gap 9. The air gap 9 is provided so as not to interfere with the vibration of the excitation region of the piezoelectric layer 2. That is, the air gap 9 is formed on the side opposite to the side on which the at least one pair of electrodes 3 and 4 is provided, in a region that overlaps with at least a portion of the portion on which the at least one pair of electrodes 3 and 4 is provided, in a planar view. 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 on which the at least one pair of electrodes 3 and 4 is provided. Note that the insulating layer 7 does not necessarily 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. Furthermore, the support member 8 may be provided not only at a position that does not overlap with the portion on which the at least one pair of electrodes 3 and 4 is provided, but also at a position that overlaps with the portion on which the at least one pair of electrodes 3 and 4 is provided, in a planar view. In this case, an air gap 9 is provided between the piezoelectric layer 2 and the support member 8 at a position that overlaps with the portion where at least one pair of electrodes 3, 4 is provided in a plan view.

[0019] The insulating layer 7 is made of silicon oxide. However, in addition to silicon oxide, other suitable insulating materials such as silicon oxynitride and alumina can be used. The support member 8 is made of Si. The plane orientation of the Si on the piezoelectric layer 2 side may be (100), (110), or (111). Preferably, Si has a high resistivity of 4 kΩ or more. However, the support member 8 can also be made of other insulating or semiconductor materials.

[0020] 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 this embodiment, the electrodes 3 and 4 have a structure in which an Al film is laminated on a Ti film. The Ti film is an adhesive layer, and the adhesive layer may be made of a material other than Ti.

[0021] 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 using bulk waves in a thickness-shear mode excited in the piezoelectric layer 2. In addition, 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 bulk waves in the thickness-shear mode, resulting in good resonance characteristics. More preferably, d / p is 0.24 or less, which provides even better resonance characteristics.

[0022] Note that "the electrodes 3, 4 are adjacent" does not mean that the electrodes 3, 4 are in direct contact, but rather that the electrodes 3, 4 are arranged adjacent to each other with a gap between them. Furthermore, when the electrodes 3, 4 are adjacent to each other, there are no other electrodes connected to the hot electrodes or ground electrodes, including the electrodes 3, 4, between the electrodes 3, 4. Furthermore, when there are multiple electrodes 3, 4 (when the electrodes 3, 4 are considered as a pair of electrodes, there are 1.5 or more pairs of electrodes), the center-to-center distance of the electrodes 3, 4 refers to the average value of the center-to-center distances of adjacent electrodes 3, 4 among the 1.5 or more pairs of electrodes 3, 4.

[0023] The center-to-center distance between electrodes 3 and 4 is the distance between the center of the dimension (width dimension) of electrode 3 in a direction perpendicular to the length direction of electrode 3 and the center of the dimension (width dimension) of electrode 4 in a direction perpendicular to the length direction of electrode 4.

[0024] Furthermore, in this embodiment, since a Z-cut piezoelectric layer is used, 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 as the piezoelectric layer 2. Here, "perpendicular" is not limited to 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 PZ1 is 90°±10°).

[0025] Because the elastic wave device 1 of this preferred embodiment has the above configuration, even if the number of pairs of electrodes 3 and 4 is reduced in an attempt to reduce the device's size, the Q value is unlikely to decrease. This is because waves can be confined even with a small number of reflecting electrode fingers in the reflector, resulting in low propagation loss. Another advantage is that bulk waves in the first-order thickness-shear mode are used. The difference between the Lamb waves used in conventional elastic wave devices and the bulk waves in the first-order thickness-shear mode is described below with reference to FIGS. 3(a) and 3(b).

[0026] FIG. 3(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 Patent Document 1. Here, waves propagate through a piezoelectric film 201 as indicated by arrows. The piezoelectric film 201 has a first principal surface 201a and a second principal surface 201b that face each other. 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. 3(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. Therefore, when miniaturization is attempted, i.e., when the number of pairs of reflecting electrode fingers in the reflectors is reduced, wave propagation loss occurs and the Q value decreases.

[0027] In contrast, as shown in FIG. 3(b), in the acoustic wave device of this embodiment, vibration displacement is in the thickness shear direction, so the waves propagate and resonate almost entirely in the direction connecting the first principal surface 2a and the second principal surface 2b of the piezoelectric layer 2, i.e., the Z direction. That is, the X direction component of the waves is significantly smaller than the Z direction component. Furthermore, because resonance characteristics are obtained through wave propagation in this Z direction, a reflector is not necessarily required. Therefore, even if the number of electrode pairs consisting of electrodes 3 and 4 is reduced in an effort to reduce the device size, the Q value is unlikely to decrease.

[0028] 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, the number of electrode pairs, consisting of electrodes 3 and 4, does not necessarily need to be multiple. In other words, it is sufficient that at least one pair of electrodes is provided.

[0029] As shown in Figure 4, the amplitude direction of the bulk wave in the first thickness-shear mode is opposite between a first region 451 included in the excitation region of the piezoelectric layer 2 and a second region 452 included in the excitation region. Figure 4 schematically shows the bulk wave when a voltage is applied between electrodes 3 and 4 such that electrode 4 has a higher potential than electrode 3. The first region 451 is the region of the excitation region 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 the region of the excitation region between the imaginary plane VP1 and the second principal surface 2b.

[0030] 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 this embodiment, 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.

[0031] Next, the ability to control the magnitude and position of spurious components in acoustic wave device 1 according to this preferred embodiment will be demonstrated by comparing Examples 1 to 3 below. In all of Examples 1 to 3, a LiNbO3 film with Euler angles of (0°, 0°, 90°) was used as piezoelectric layer 2. The thickness was 400 nm.

[0032] The pitch, which is the center-to-center distance between electrodes 3 and 4, was 3 μm. The material for electrodes 3 and 4 was a structure in which a 100 nm thick Al film was laminated on a 10 nm thick Ti film. The width dimensions of electrode 3 and electrode 4 were as shown in Table 1 below. Therefore, d / p was 0.133.

[0033] The length of the overlapping region of the electrodes 3 and 4 when viewed in a direction perpendicular to the longitudinal direction of the electrodes 3 and 4, that is, the excitation region, was set to 20 μm. The number of pairs of the electrodes 3 and 4 was 100.

[0034] [Table 1]

[0035] Fig. 5 is a diagram showing the phase characteristics of the resonators of Examples 1 to 3. As is clear from Fig. 5, by varying the widths of electrode 3 and electrode 4 as shown in Examples 1 to 3, the magnitude and position of the spurious responses appearing near 5040 MHz to 5070 MHz and the magnitude and position of the spurious responses appearing near 5280 MHz to 5400 MHz change. Therefore, in elastic wave device 1 of this preferred embodiment, the pair of electrodes 3 and 4 includes an electrode whose cross-sectional shape differs from that of the other electrodes in any cross section perpendicular to the longitudinal direction thereof, as described above. By adjusting the manner in which this difference is made, the spurious responses can be reduced or shifted away from the band.

[0036] In the first embodiment, the width of electrode 3 and the width of electrode 4 are different, but when multiple pairs of electrodes 3, 4 are provided, it is not necessary to make the widths of all electrodes 3 equal. Also, it is not necessary to make the widths of all electrodes 4 equal. When multiple pairs of electrodes 3, 4 are provided, it is sufficient that there are electrodes with different cross-sectional shapes in any cross section in a direction perpendicular to the longitudinal direction of the electrodes 3, 4.

[0037] As described above, in this embodiment, d / p is 0.5 or less, and more preferably 0.24 or less, where d is the thickness of the piezoelectric layer 2 and p is the center-to-center distance between the electrodes 3 and 4. This will be explained with reference to FIGS. 6A and 6B.

[0038] 6A and 6B are graphs showing the relationship between d / 2p and the fractional bandwidth of the acoustic wave device as a resonator.

[0039] As can be seen from FIG. 6A, when d / 2p exceeds 0.25, i.e., when d / p > 0.5, adjusting d / p results in a fractional bandwidth of less than 5%. In contrast, when d / 2p ≦ 0.25, i.e., 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 / 2p is 0.12 or less, i.e., when d / p is 0.24 or less, the fractional bandwidth can be increased to 7% or more. More preferably, when d / 2p is 0.05 or less, i.e., when d / p is 0.025 or less, the coupling coefficient can be further increased. Furthermore, adjusting d / p within this range can result in a resonator with a wider fractional bandwidth and a higher coupling coefficient. Therefore, by setting d / p to 0.5 or less, as in the second invention of the present application, a resonator with a high coupling coefficient can be constructed using the bulk wave in the thickness-shear first-order mode. Furthermore, as is clear from Figure 6B, d / 2p is more preferably 0.048 or less. In this case, the coupling coefficient can be further increased. Even more preferably, d / 2p is in the range of 0.024 to 0.036. In this case, even if the pitch p fluctuates, the change in the coupling coefficient is small.

[0040] As mentioned above, the number of electrodes may be one pair, and in the case of one pair of electrodes, the above p is the center-to-center distance between adjacent electrodes 3 and 4. In the case of 1.5 or more pairs of electrodes, the average distance between the centers of adjacent electrodes 3 and 4 may be used as p.

[0041] In the acoustic wave device 1, it is preferable that the metallization ratio MR of at least one pair of electrodes 3, 4 with respect to the excitation region, which is the region where at least one pair of electrodes 3, 4 overlap when viewed in the opposing direction, 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. 7 and 8. FIG. 7 is a reference diagram showing an example of the resonance characteristics of the acoustic wave device 1. A spurious signal indicated by arrow B appears between the resonance frequency and the antiresonance frequency. Note that d / p=0.08 and the Euler angles of LiNbO3 are (0°, 0°, 90°). The metallization ratio MR is 0.35.

[0042] The metallization ratio MR will be explained with reference to FIG. 1(b). In the electrode structure of FIG. 1(b), focusing on a pair of electrodes 3 and 4, it is assumed that only this pair of electrodes 3 and 4 is provided. In this case, the area surrounded by the dashed-dotted line C is the excitation region. When electrodes 3 and 4 are viewed in a direction perpendicular to the longitudinal direction of electrodes 3 and 4, i.e., in the opposing direction, this excitation region includes the area of electrode 3 that overlaps with electrode 4, the area of electrode 4 that overlaps with electrode 3, and the area between electrodes 3 and 4 where electrodes 3 and 4 overlap. The area of electrodes 3 and 4 within excitation region C relative to the area of this excitation region is the metallization ratio MR. In other words, the metallization ratio MR is the ratio of the area of the metallization portion to the area of the excitation region.

[0043] 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.

[0044] FIG. 8 shows the relationship between the bandwidth ratio when multiple acoustic wave resonators are configured according to this embodiment and the amount of phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. The bandwidth ratio was adjusted by changing the film thickness of the piezoelectric layer and the dimensions of the electrodes. While FIG. 8 shows the results when a piezoelectric layer made of Z-cut LiNbO3 is used, similar trends are observed when a piezoelectric layer with a different cut angle is used.

[0045] In the region surrounded by ellipse J in Figure 8, the spurious is as large as 1.0. As is clear from Figure 8, 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. In other words, as in the resonance characteristics shown in Figure 7, large spurious signals indicated by arrow B appear within the band. 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.

[0046] FIG. 9 illustrates the relationship between d / 2p, metallization ratio MR, and bandwidth fraction. Various acoustic wave devices with different d / 2p and MR were fabricated and their bandwidth fractions were measured. The hatched area to the right of dashed line D in FIG. 9 represents the bandwidth fraction of 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, it is preferable that MR ≦ 1.75(d / p) + 0.075. In this case, it is easy to achieve a bandwidth fraction of 17% or less. More preferably, it is the area to the right of MR = 3.5(d / 2p) + 0.05, indicated by dashed line D1 in FIG. 9. That is, if MR ≦ 1.75(d / p) + 0.05, the bandwidth fraction can be reliably maintained at 17% or less.

[0047] Fig. 10 is a diagram showing a map of the fractional bandwidth versus the Euler angles (0°, θ, ψ) of LiNbO3 when d / p approaches 0. The hatched areas in Fig. 9 are regions E, F, G, and H where a fractional bandwidth of at least 5% or more can be obtained. The ranges of regions E, F, G, and H can be approximated as the ranges expressed by the following equations (1), (2), and (3).

[0048] (0°±10°, 0°~20°, any ψ) ...Equation (1) ...Area E (0°±10°,20°~80°,0°~60°(1-(θ-50) 2 / 900) 1 / 2 ) or (0°±10°, 20°~80°, [180°-60°(θ-50) 2 / 900) 1 / 2 ]~180°) ...Equation (2) ...Area F or G (0°±10°,[180°-30°(1-(ψ-90) 2 / 8100) 1 / 2 ]~180°, any ψ) ...Equation (3) ...Area H

[0049] Therefore, in the case of the Euler angle range of the above formula (1), formula (2) or formula (3), the relative bandwidth can be made sufficiently wide, which is preferable.

[0050] As described above, in the acoustic wave devices according to the first and second aspects of the present invention, good resonance characteristics can be obtained even with a reduced number of electrode fingers in the reflector, and therefore a high Q value can be achieved even with increased miniaturization. Furthermore, by differentiating the cross-sectional shapes of electrodes 3 and 4 in either the longitudinal direction orthogonal to the longitudinal direction of electrodes 3 and 4, the magnitude and position of spurious emissions can be controlled.

[0051] 11 is a plan view showing the electrode structure of an elastic wave device according to a second preferred embodiment of the present invention. In the second preferred embodiment, a pair of electrodes 3 and 4 is provided on a piezoelectric layer 2. As such, the number of pairs in the electrode structure made up of the electrodes 3 and 4 may be one. Again, the width of electrode 4 is wider than the width of electrode 3. Other preferred embodiments and modifications of the present invention will be described below.

[0052] In the elastic wave device 1 of the first preferred embodiment, at least one pair of electrodes 3 and 4 is configured so that the width of electrode 3 is different from the width of electrode 4. In contrast, the present invention is not limited to such a configuration in which the widths are different, and any one of the pair of electrodes 3 and 4 may have a cross-sectional shape different from that of the other electrode in any cross section in a direction perpendicular to the longitudinal direction, and the shape may be modified in various ways.

[0053] 12 to 16 , the electrode structures of elastic wave devices according to third to seventh embodiments will be described. In the elastic wave device according to the third embodiment shown in FIG. 12 , electrodes 3 and 4 have tapered shapes that become thinner from the base end to the tip end. That is, they have an isosceles trapezoidal shape. Here, electrodes 3 and 4 have different cross-sectional shapes when viewed in a cross section perpendicular to the longitudinal direction of electrodes 3 and 4. That is, in the elastic wave device according to the third embodiment, electrode 3 is an example of a “first electrode,” and electrode 4 is an example of a “second electrode.” For example, in a cross section taken along line P1-P1 in FIG. 12 , the cross-sectional shapes of electrodes 3 and 4 are different. In this way, by forming electrodes 3 and 4 into a shape other than a rectangle, such as an isosceles trapezoid, the cross-sectional shapes of electrodes 3 and 4 can be made different in a cross section perpendicular to the longitudinal direction of electrodes 3 and 4.

[0054] In the third embodiment, it is not always necessary for all of the electrodes 3 to have the same shape.

[0055] Although the electrodes 3 and 4 have an isosceles trapezoidal shape, they may have other shapes such as a non-isosceles trapezoid or a parallelogram.

[0056] FIG. 13 is a plan view illustrating the electrode structure of an elastic wave device according to a fourth preferred embodiment of the present invention. In the fourth preferred embodiment, a rectangular electrode 3 is connected to a first bus bar 5. A plurality of electrodes 4 and an electrode 4A are connected to a second bus bar 6. The electrode 4 has a rectangular shape, the same shape as the electrode 3. On the other hand, the electrode 4A has a plurality of recesses 4a on one side extending in the length direction. In other words, the electrode 4A has a shape in which the recesses 4a are provided. In this case, the length direction is the length direction of the rectangle R circumscribing the electrode 4A. Although the fourth preferred embodiment includes recesses 4a, it is sufficient that the electrode 4 has at least one recess 4a.

[0057] In the fourth preferred embodiment, the cross-sectional shape of electrode 4A taken along line P2-P2, which passes through the position where recess 4a is provided, is different from that of electrodes 3 and 4. That is, in the elastic wave device of the fourth preferred embodiment, electrode 4A is an example of a "first electrode," and electrode 3 or electrode 4 is an example of a "second electrode." Therefore, in this case as well, the size and position of spurious components can be controlled by adjusting the cross-sectional shapes of electrodes 3, 4, and 4A.

[0058] FIG. 14 is a plan view illustrating the electrode structure of an elastic wave device according to a fifth preferred embodiment. In the elastic wave device according to the fifth preferred embodiment, one ends of a plurality of electrodes 3 and 3B are connected to a first bus bar 5. A plurality of electrodes 4B and a plurality of electrodes 4 are connected to a second bus bar 6. Here, the electrodes 3 and 4 have a rectangular shape. In contrast, the electrodes 3B and 4B have a curved shape as shown in the figure. That is, the electrode 3B has a pair of curved side edges 3d and 3e. Similarly, the electrode 4B has a pair of curved side edges 4d and 4e. For the curved electrodes 3B and 4B, the length direction of the circumscribing rectangular shape corresponds to the length direction of the electrodes 3B and 4B. Note that the term "rectangular shape circumscribing electrodes 3B and 4B" includes at least a rectangular shape circumscribing electrodes 3B and 4B excluding the portions connected to the first bus bar 5 or the second bus bar 6.

[0059] In this embodiment, too, the cross-sectional shape of electrode 3B and the cross-sectional shape of electrode 4B are different, for example, in a cross section along line P3-P3. Furthermore, the width of electrode 4B is greater than the widths of electrodes 3 and 4. That is, electrode 3B is an example of a "first electrode," and electrodes 3, 4, and 4B are examples of "second electrodes." Alternatively, electrode 4B is an example of a "first electrode," and electrodes 3, 3B, and 4 are examples of "second electrodes." Therefore, by adjusting the way in which the cross-sectional shapes are made different, the size and position of spurious components can be adjusted, as in the first embodiment.

[0060] FIG. 15 is a plan view illustrating the electrode structure of an elastic wave device according to a sixth preferred embodiment of the present invention. In this example, one end of electrodes 3C and 3D is connected to a first bus bar 5. One end of electrodes 4C and 4D is connected to a second bus bar 6. That is, two pairs of electrodes 3C, 4C, 3D, and 4D are provided. Note that electrode 3C has an isosceles trapezoidal shape. Electrode 4C has a trapezoidal shape that is thinner than electrode 3C. Furthermore, electrode 4D has a shape with a narrow portion 4d1 in the longitudinal center. Electrode 3D has a shape that widens from the base end to the tip. Electrodes 3C, 4C, 3D, and 4D with various shapes other than rectangular may also be used. Even in this case, electrodes 3C, 4C, 3D, and 4D have different electrode shapes in any cross section perpendicular to the longitudinal direction. That is, any one of electrodes 3C, 4C, 3D, and 4D is an example of a "first electrode," and the remaining electrodes are an example of a "second electrode." Therefore, by adjusting the way in which the electrode shapes are changed, the size and position of the spurious can be controlled.

[0061] FIG. 16 is a front cross-sectional view showing the electrode structure of an elastic wave device according to a seventh preferred embodiment of the present invention. In this preferred embodiment, electrodes 3 and 4 have wide portions 3f and 4f disposed on the piezoelectric layer 2 and rectangular cross-section portions 3g and 4g provided on the wide portions 3f and 4f. The side surfaces of the wide portions 3f and 4f are tapered from the first principal surface 2a toward the rectangular cross-section portions 3g and 4g. The taper of the wide portion 4f is different from the taper of the wide portion 3f, so that the side surface of the wide portion 4f is steeper than the side surface of the wide portion 3f. Therefore, the cross-sectional shapes of the electrodes 3 and 4 differ from each other in the cross-sections shown in the figure, i.e., in a cross section perpendicular to the longitudinal direction of the electrodes 3 and 4. In other words, electrode 3 is an example of a "first electrode," and electrode 4 is an example of a "second electrode." Therefore, by adjusting this difference, the magnitude and position of spurious components can be adjusted.

[0062] In the seventh embodiment, the inclination angles of the side surfaces of the wide portions 3f, 4f of the electrodes 3, 4 are different. However, the inclination angles of the entire side surfaces of the electrodes 3 and 4 may be different. That is, the inclination angle of at least a part of the side surface of the electrode 3 relative to the piezoelectric layer 2 may be different from the inclination angle of at least a part of the side surface of the electrode 4.

[0063] FIG. 17 is a front cross-sectional view of an elastic wave device according to an eighth preferred embodiment of the present invention. In an elastic wave device 81, the thickness of electrode 4 is greater than the thickness of electrode 3. Therefore, in this preferred embodiment, the cross-sectional shapes of electrodes 3 and 4 are different in a cross section perpendicular to the longitudinal direction of electrodes 3 and 4. Therefore, by adjusting the difference, the size and position of spurious components can be adjusted. Furthermore, a protective film 22 is laminated on the first principal surface 2 a of piezoelectric layer 2 so as to cover at least one pair of electrodes 3 and 4. An insulating material such as silicon oxide or silicon oxynitride is preferably used as protective film 22. Protective film 22 also covers the gap region between electrodes 3 and 4, but may also cover only a portion of the gap region.

[0064] 18, 19(a), 19(b), 20(a), 20(b), 21, and 22, the structures of electrodes 3 and 4 are the same as those in the first embodiment. That is, the width of electrode 4 is greater than the width of electrode 3. Therefore, by adjusting the difference in the width of the cross-sectional shapes of electrode 4 and electrode 3, the size and position of spurious components can be adjusted.

[0065] FIG. 18 is a front cross-sectional view of an elastic wave device according to a ninth preferred embodiment. In an elastic wave device 91, electrodes 3 and 4 with different widths are provided on a first principal surface 2a of a piezoelectric layer 2, as in the first preferred embodiment. An acoustic multilayer film 42 is laminated on a second principal surface 2b of the piezoelectric layer 2. The acoustic multilayer film 42 has a laminated structure of low acoustic impedance layers 42a, 42c, and 42e, each having a relatively low acoustic impedance, and high acoustic impedance layers 42b and 42d, each having a relatively high acoustic impedance. The use of the acoustic multilayer film 42 allows bulk waves in thickness-shear mode to be confined within the piezoelectric layer 2 without the air gap 9 used in the elastic wave device 1. In the elastic wave device 91, resonance characteristics based on bulk waves in thickness-shear mode can be achieved by setting the d / p ratio to 0.5 or less. The number of low acoustic impedance layers and high acoustic impedance layers in the acoustic multilayer film 42 is not particularly limited. It is sufficient that at least one high acoustic impedance layer be disposed farther from the piezoelectric layer 2 than the low acoustic impedance layer.

[0066] The low acoustic impedance layers 42a, 42c, and 42e and the high acoustic impedance layers 42b and 42d can be made of any suitable material as long as the acoustic impedance relationship is satisfied. For example, the low acoustic impedance layers 42a, 42c, and 42e can be made of silicon oxide or silicon oxynitride. The high acoustic impedance layers 42b and 42d can be made of alumina, silicon nitride, or metal.

[0067] FIG. 19(a) is a front cross-sectional view illustrating an elastic wave device according to a tenth preferred embodiment, and FIG. 19(b) is a front cross-sectional view illustrating a modification thereof. FIG. 19(a) shows only a portion of an elastic wave device 101 according to the tenth preferred embodiment, i.e., a portion including a piezoelectric layer 2 and at least one pair of electrodes 3 and 4. In the elastic wave device 101 according to the tenth preferred embodiment, at least one pair of electrodes 3 and 4 is provided on the first principal surface 2a of the piezoelectric layer 2. At least one pair of electrodes 3 and 4 is also provided on the second principal surface 2b. In this manner, at least one pair of electrodes may also be provided on the second principal surface 2b. The electrodes 3 and 4 on the second principal surface 2b are preferably arranged to overlap the electrodes 3 and 4 on the first principal surface 2a with the piezoelectric layer 2 interposed therebetween.

[0068] 19(b), the electrodes 3, 4 on the first principal surface 2a may partially overlap the electrodes 3, 4 on the second principal surface 2b. That is, it is sufficient that the electrodes 3, 4 on the first principal surface 2a and the electrodes 3, 4 on the second principal surface 2b at least partially overlap each other.

[0069] 19(a), acoustic wave device 101 has the same configuration as acoustic wave device 1, except for piezoelectric layer 2 and electrodes 3 and 4. Therefore, similar to acoustic wave device 1, it is possible to obtain good resonance characteristics due to bulk waves in the thickness-shear first-order mode, and it is possible to increase the Q value even when the device is miniaturized.

[0070] FIG. 20(a) is a front cross-sectional view illustrating an elastic wave device according to an eleventh embodiment, and FIG. 20(b) is a front cross-sectional view illustrating a modified example thereof. Similar to FIG. 19(a), FIGS. 20(a) and 20(b) only show a portion of elastic wave device 111 where piezoelectric layer 2 and at least one pair of electrodes 3 and 4 are provided. In elastic wave device 111, first main surface 2a of piezoelectric layer 2 is roughened. In this case, frequency adjustment can be performed by adjusting the degree of roughness. The remaining configuration of elastic wave device 111 is similar to that of elastic wave device 1 according to the first embodiment.

[0071] 20(b), a modified elastic wave device 111A has a recess 2c formed by removing the gap between electrode 3 and electrode 4. The frequency can also be adjusted by adjusting the size and depth of recess 2c.

[0072] 21 is a front cross-sectional view of an elastic wave device according to a twelfth preferred embodiment of the present invention. In an elastic wave device 121, a mass-adding film 72 is provided on a first principal surface 2a of a piezoelectric layer 2. A mass-adding film 73 is also provided on a second principal surface 2b. The mass-adding films 72 and 73 are provided outside the excitation region, i.e., outside the region where the plurality of electrodes 3 and 4 are arranged. Although the mass-adding films 72 and 73 can be provided at any position outside the excitation region, in FIG. 21 , the mass-adding films 72 and 73 are provided at positions overlapping the insulating layer 7.

[0073] It is also possible to provide only one of the mass adding films 72 and 73. The mass adding films 72 and 73 can be made of an insulator such as silicon oxide, silicon oxynitride, or alumina, or a metal or alloy such as Al.

[0074] 22 is a front cross-sectional view illustrating a piezoelectric layer and a pair of electrodes of an elastic wave device according to a thirteenth preferred embodiment of the present invention. In elastic wave device 131, first principal surface 2a and second principal surface 2b of piezoelectric layer 2 are curved. As such, piezoelectric layer 2 does not need to be a flat piezoelectric film, and may be at least partially curved.

[0075] FIG. 23(a) is a front cross-sectional view illustrating a piezoelectric layer and a pair of electrodes in an elastic wave device according to a fourteenth preferred embodiment of the present invention. In an elastic wave device 141, at least one pair of electrodes 3, 4 has a cross-sectional shape that is irregular and not rectangular. That is, the electrodes 3, 4 each have a wide portion 3f, 4f located on the first principal surface 2a and a rectangular cross-sectional portion 3g, 4g provided on the wide portion 3f, 4f. The side surfaces of the wide portions 3f, 4f are tapered so that they become thinner from the first principal surface 2a toward the rectangular cross-sectional portions 3g, 4g. Providing the wide portions 3f, 4f reduces the distance between the electrodes 3 and 4. This increases the capacitance between the electrodes. This increases the capacitance without significantly changing the resonance characteristics.

[0076] In this way, the cross-sectional shape of at least one pair of electrodes 3, 4 may be a shape other than a rectangle, i.e., an irregular shape. Also, a part of the electrodes 3, 4 may have a portion extending toward the other electrode 4, 3.

[0077] The electrodes 3 and 4 may have any of the shapes shown in Figures 23(b) to 23(d). The electrodes 3 and 4 shown in Figure 23(b) have a trapezoidal cross section. The electrodes 3 and 4 shown in Figure 23(c) have a shape that flares outward, with both side surfaces in the width direction being curved. The electrodes 3 and 4 shown in Figure 23(d) have a trapezoidal cross section at the upper end and a trapezoidal cross section at the lower end that is wider than the trapezoidal cross section at the upper end.

[0078] As shown in any of FIGS. 24(a) to 24(c), the acoustic wave device 1 may also include a dielectric film 10 that covers the first principal surface 2a of the piezoelectric layer 2 and the electrodes 3 and 4 on the first principal surface 2a. In FIG. 24(a), the thickness of the dielectric film 10 is thinner than the thickness of the electrodes 3 and 4, and the surface of the dielectric film 10 has an uneven shape that conforms to the shape of the base. In FIG. 24(b), the surface of the dielectric film 10 is flattened and has a planar shape. In FIG. 24(c), the thickness of the dielectric film 10 is thicker than the thickness of the electrodes 3 and 4, and the surface of the dielectric film 10 has an uneven shape that conforms to the shape of the base. [Explanation of symbols]

[0079] 1...Elastic wave device 2...Piezoelectric layer 2a...first principal surface 2b...Second main surface 2c...recess 3,3B,3C,3D,4,4A,4B,4C,4D…electrode 3d,3e,4d,4e…side 3rd floor, 4th floor...wide section 3g,4g…Rectangular cross section 4a...recess 4d1...narrow part 5,6...1st and 2nd bus bars 7...Insulating layer 8...Support member 7a,8a…opening 9...Air gap 10...Dielectric film 22...Protective film 42…Acoustic multilayer film 42a, 42c, 42e...Low acoustic impedance layers 42b, 42d...High acoustic impedance layer 72... Mass-added membrane 73... Mass-added membrane 81, 91, 101, 101A, 111, 111A, 121, 131, 141...Elastic wave device 201...Piezoelectric film 201a...First principal surface 201b...Second main surface 451…First area 452…Second area

Claims

1. a piezoelectric layer made of lithium niobate or lithium tantalate; at least one pair of electrodes facing each other in a direction intersecting a thickness direction of the piezoelectric layer, where d is a thickness of the piezoelectric layer and p is a center-to-center distance between adjacent electrodes of the at least one pair of electrodes, d / p is 0.24 or less; the at least one pair of electrodes has a length; the at least one pair of electrodes includes a first electrode and a second electrode having cross-sectional shapes different from each other in a cross section in a direction perpendicular to a longitudinal direction of the at least one pair of electrodes; An elastic wave device in which, in any cross section in a direction perpendicular to the longitudinal direction of the at least one pair of electrodes, the inclination angle of at least a portion of the first electrode side surface relative to the piezoelectric layer is different from the inclination angle of at least a portion of the electrode side surface of the second electrode.

2. a piezoelectric layer made of lithium niobate or lithium tantalate; at least one pair of electrodes facing each other in a direction intersecting a thickness direction of the piezoelectric layer, where d is a thickness of the piezoelectric layer and p is a center-to-center distance between adjacent electrodes of the at least one pair of electrodes, d / p is 0.5 or less, the at least one pair of electrodes has a length; the at least one pair of electrodes includes a first electrode and a second electrode having cross-sectional shapes different from each other in a cross section in a direction perpendicular to a longitudinal direction of the at least one pair of electrodes; An elastic wave device in which, in any cross section in a direction perpendicular to the longitudinal direction of the at least one pair of electrodes, the inclination angle of at least a portion of the first electrode side surface relative to the piezoelectric layer is different from the inclination angle of at least a portion of the electrode side surface of the second electrode.

3. The acoustic wave device according to claim 1 , wherein a thickness of the first electrode is different from a thickness of the second electrode in any cross section taken along a direction perpendicular to a longitudinal direction of the at least one pair of electrodes.

4. 4. The acoustic wave device according to claim 1, wherein the width of the first electrode is different from the width of the second electrode.

5. The elastic wave device according to any one of claims 1 to 4, wherein a metallization ratio MR, which is a ratio of an area of the at least one pair of electrodes in the excitation region to an area where the at least one pair of electrodes overlaps each other when viewed in a direction in which the at least one pair of electrodes face each other, satisfies MR≦1.75(d / p)+0.

075.

6. a first bus bar and a second bus bar connected to the at least one pair of electrodes; The acoustic wave device according to claim 1 , wherein the at least one pair of electrodes includes an electrode connected to the first bus bar and an electrode connected to the second bus bar.

7. The elastic wave device according to any one of claims 1 to 6, wherein the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate are within the range of the following formula (1), formula (2), or formula (3): (0°±10°, 0° to 20°, any ψ) ...Equation (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 ] ~ 180°) …Formula (2) (0°±10°, [180°-30°(1-(ψ-90) ２ / 8100) １／２ ] to 180°, any ψ) ...Equation (3)

8. The acoustic wave device according to claim 1 , further comprising a support member provided on the piezoelectric layer on a side opposite to a side on which the at least one pair of electrodes is provided.

9. 9. The elastic wave device according to claim 8, wherein, in a plan view, an air gap is provided on the piezoelectric layer on a side opposite to the side on which the at least one pair of electrodes is provided, in a region that overlaps with at least a portion of the region on which the at least one pair of electrodes is provided.

10. The elastic wave device according to any one of claims 1 to 7, further comprising an acoustic multilayer film laminated on the piezoelectric layer on the side opposite to the side on which the at least one pair of electrodes is provided, the acoustic multilayer film having a laminated structure of a low acoustic impedance layer having a relatively low acoustic impedance and a high acoustic impedance layer having a relatively high acoustic impedance.

11. The acoustic wave device according to claim 1, wherein the at least one pair of electrodes faces each other on the same surface of the piezoelectric layer.

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