Elastic wave device

Abstract

Provided is an elastic wave device in which peeling of an acoustic reflective film from a piezoelectric layer can be suppressed.　An elastic wave device 1 of the present invention comprises: an acoustic reflective film 4 that contains one or more high-acoustic impedance layers 6a, 6b which have relatively high acoustic impedance and one or more low-acoustic impedance layers 5a-5c which have relatively low acoustic impedance; a piezoelectric layer 7 that is provided on the acoustic reflective film 4 and that has a first main surface 7a and a second main surface 7b which are opposite from each other; and an IDT electrode 8 that is provided on the first main surface 7a of the piezoelectric layer 7 and that has a plurality of electrode fingers (a plurality of first and second electrode fingers 18 and 19). The low-acoustic impedance layers and the high-acoustic impedance layers are alternately layered. The one or more low-acoustic impedance layers 5a, 5b are low-density silicon oxide layers which have a density of less than 2 g / cm3. The low-density silicon oxide layers each have a third main surface 5x and a fourth main surface 5y which are opposite from each other. The acoustic reflective film 4 further contains at least one adhesion layer 11 which is provided between a high-acoustic impedance layer and at least one from among the third main surface 5x and the fourth main surface 5y of a low-density silicon oxide layer. The density of the adhesion layer 11 is higher than the density of a low-density silicon oxide layer which is in contact with the adhesion layer 11, and is lower than the density of a high-acoustic impedance layer which is in contact with the adhesion layer 11.

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. Patent Document 1 listed below discloses an example of a thin film resonator as an acoustic wave device. In this thin film resonator, a piezoelectric material is provided on a support structure. A pair of conductive electrode layers is provided on both main surfaces of the piezoelectric material. The support structure is an acoustic reflector. Patent Document 1 discloses an example of an acoustic reflector in such an acoustic element as a thin film resonator.

[0003] The acoustic reflector includes a substrate, a low-impedance material, and a high-impedance material. The low-impedance material and the high-impedance material are alternately laminated. A laminated film made of the low-impedance material and the high-impedance material is provided on the substrate. The piezoelectric material is provided on this laminated film. The low-impedance material is SiO ２ The density of this low impedance material is 1.0 g / cm ３ and ordinary SiO ２ lower than the density of

[0004] Japanese Patent Application Laid-Open No. 2002-041052

[0005] However, if the density of the low-impedance material is low, the film stress of the high-impedance material may damage the laminated film of the low-impedance material and the high-impedance material, which may cause the laminated film to peel off from the piezoelectric material.

[0006] An object of the present invention is to provide an acoustic wave device that can prevent peeling of an acoustic reflection film from a piezoelectric layer.

[0007] An elastic wave device according to the present invention includes an acoustic reflection film including at least one high acoustic impedance layer having a relatively high acoustic impedance and at least one low acoustic impedance layer having a relatively low acoustic impedance; a piezoelectric layer provided on the acoustic reflection film and having a first main surface and a second main surface facing each other; and an IDT electrode provided on the first main surface of the piezoelectric layer and having a plurality of electrode fingers, wherein the low acoustic impedance layers and the high acoustic impedance layers are alternately stacked, and at least one of the low acoustic impedance layers has a density of 2 g / cm ３ the low-density silicon oxide layer has a third main surface and a fourth main surface facing each other, the acoustic reflection film further includes at least one adhesion layer provided between at least one of the third main surface and the fourth main surface of the low-density silicon oxide layer and the high acoustic impedance layer, and the density of the adhesion layer is higher than the density of the low-density silicon oxide layer in contact with the adhesion layer and lower than the density of the high acoustic impedance layer in contact with the adhesion layer.

[0008] According to the acoustic wave device of the present invention, peeling of the acoustic reflection film from the piezoelectric layer can be suppressed.

[0009] FIG. 1 is a schematic plan view of an elastic wave device according to a first preferred embodiment of the present invention. FIG. 2 is a schematic cross-sectional view taken along line II in FIG. 1. FIG. 3 is a schematic front cross-sectional view of an elastic wave device according to a first modified example of the first preferred embodiment of the present invention. FIG. 4 is a schematic front cross-sectional view of an elastic wave device according to a second modified example of the first preferred embodiment of the present invention. FIG. 5 is a schematic plan view of an elastic wave device according to a third modified example of the first preferred embodiment of the present invention. FIG. 6(a) shows a cross-sectional view of an elastic wave device according to a first preferred embodiment of the present invention. １ 6(b) is a schematic diagram showing a Lamb wave in the S mode. ０FIG. 7 is a schematic diagram showing Lamb waves in a mode. FIG. 7 is a schematic front cross-sectional view of an elastic wave device according to a second preferred embodiment of the present invention. FIG. 8 is a diagram showing the relationship between d / p and the fractional bandwidth of an elastic wave resonator. FIG. 9 is a diagram showing the relationship between the fractional bandwidth of an elastic wave resonator and the magnitude of normalized spurious signals. FIG. 10 is a diagram showing the relationship between d / p, the metallization ratio MR, and the fractional bandwidth. FIG. 11 is a diagram showing the relationship between d / p and the metallization ratio MR of a LiNbO resonator 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 .lambda.

[0010] The present invention will be clarified below by describing specific embodiments of the present invention with reference to the drawings.

[0011] 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.

[0012] Fig. 1 is a schematic plan view of an elastic wave device according to a first preferred embodiment of the present invention, and Fig. 2 is a schematic cross-sectional view taken along line II in Fig. 1.

[0013] As shown in Fig. 1, the acoustic wave device 1 includes a piezoelectric substrate 2 and an IDT electrode 8. The piezoelectric substrate 2 is a substrate having piezoelectric properties. As shown in Fig. 2, the piezoelectric substrate 2 includes a support substrate 3, an acoustic reflection film 4, and a piezoelectric layer 7. Specifically, the acoustic reflection film 4 is provided on the support substrate 3. The piezoelectric layer 7 is provided on the acoustic reflection film 4.

[0014] The support substrate 3 may be made of a semiconductor such as silicon or a ceramic such as aluminum oxide. The piezoelectric layer 7 may be made of LiNbO ３ Alternatively, the material may be made of lithium niobate such as LiTaO ３ In this embodiment, piezoelectric layer 7 is made of lithium niobate. In this specification, a certain component made of a certain material includes a component containing a trace amount of impurities that does not significantly degrade the electrical characteristics of the acoustic wave device.

[0015] The piezoelectric layer 7 has a first principal surface 7a and a second principal surface 7b. The first principal surface 7a and the second principal surface 7b face each other. In this embodiment, of the first principal surface 7a and the second principal surface 7b, the second principal surface 7b is located on the acoustic reflection film 4 side. An IDT electrode 8 is provided on the first principal surface 7a of the piezoelectric layer 7.

[0016] As shown in FIG. 1 , the IDT electrode 8 has a pair of bus bars and a plurality of electrode fingers. The pair of bus bars is specifically a first bus bar 16 and a second bus bar 17. The first bus bar 16 and the second bus bar 17 face each other. The plurality of electrode fingers is specifically a plurality of first electrode fingers 18 and a plurality of second electrode fingers 19. One end of each of the plurality of first electrode fingers 18 is connected to the first bus bar 16. One end of each of the plurality of second electrode fingers 19 is connected to the second bus bar 17. The plurality of first electrode fingers 18 and the plurality of second electrode fingers 19 are interdigitated with each other. The first electrode fingers 18 and the second electrode fingers 19 are connected to different potentials. The IDT electrode 8 may be made of a laminated metal film or a single-layer metal film.

[0017] Hereinafter, the first electrode fingers 18 and the second electrode fingers 19 may be collectively referred to simply as electrode fingers. The direction in which the electrode fingers 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.

[0018] 2, the acoustic reflection film 4 is a laminate including a plurality of acoustic impedance layers and at least one adhesive layer 11. Specifically, the acoustic reflection film 4 has a plurality of low acoustic impedance layers, a plurality of high acoustic impedance layers, and a plurality of adhesive layers 11. The low acoustic impedance layers are layers with a relatively low acoustic impedance. More specifically, the low acoustic impedance layers are layers with a lower acoustic impedance than adjacent layers in the acoustic reflection film 4. More specifically, the plurality of low acoustic impedance layers in the acoustic reflection film 4 are low acoustic impedance layers 5a, 5b, and 5c.

[0019] On the other hand, a high acoustic impedance layer is a layer with a relatively high acoustic impedance. More specifically, a high acoustic impedance layer is a layer with a higher acoustic impedance than an adjacent layer in the acoustic reflecting film 4. More specifically, the multiple high acoustic impedance layers in the acoustic reflecting film 4 are high acoustic impedance layers 6a and 6b. The low acoustic impedance layers and high acoustic impedance layers are stacked alternately. Note that the low acoustic impedance layer 5a is the acoustic impedance layer located closest to the piezoelectric layer 7 in the acoustic reflecting film 4.

[0020] In this embodiment, the plurality of low acoustic impedance layers includes a plurality of low-density silicon oxide layers. Specifically, the low-density silicon oxide layers are made of silicon oxide and have a density of 2 g / cm ３ The low-density silicon oxide layer is made of, for example, silicon oxide aerogel. Therefore, the density of the low-density silicon oxide layer is lower than that of a general SiO ２ The density is lower than that of general SiO ２ The density of ３ That's about it.

[0021] The low acoustic impedance layers 5a and 5b in the acoustic reflection film 4 are both low-density silicon oxide layers. On the other hand, the low acoustic impedance layer 5c is not a low-density silicon oxide layer. In the present invention, it is sufficient that at least one low acoustic impedance layer is a low-density silicon oxide layer.

[0022] The material of the low acoustic impedance layer other than the low-density silicon oxide layer is, for example, a silicon dioxide layer having a density of 2 g / cm ３ The high acoustic impedance layer can be made of a dielectric material such as silicon oxide, or a metal such as aluminum or titanium. The high acoustic impedance layer can be made of a dielectric material such as aluminum nitride, silicon nitride, ytterbium oxide, hafnium oxide, or tungsten oxide, or a metal such as platinum or tungsten.

[0023] It is preferable that at least the material of the acoustic impedance layer closest to the piezoelectric layer 7 in the acoustic reflection film 4 be a material with a low dielectric constant. Specifically, the material of the acoustic impedance layer is preferably a material with a dielectric constant of 30 or less. It is more preferable that at least the material of the acoustic impedance layer closest to the piezoelectric layer 7 in the acoustic reflection film 4 be a material with a dielectric constant of 15 or less. In this case, it is possible to further broaden the bandwidth of the elastic wave device 1. Here, broadening the bandwidth refers to increasing the absolute value of the difference between the resonant frequency and the antiresonant frequency.

[0024] 2, the low acoustic impedance layer 5a, which is a low-density silicon oxide layer, has a third principal surface 5x and a fourth principal surface 5y. The third principal surface 5x and the fourth principal surface 5y face each other. Of the third principal surface 5x and the fourth principal surface 5y, the third principal surface 5x is located on the piezoelectric layer 7 side. Similarly, the low acoustic impedance layer 5b, which is a low-density silicon oxide layer, also has a third principal surface 5x and a fourth principal surface 5y.

[0025] The piezoelectric layer 7 is located on the third principal surface 5x side of the low acoustic impedance layer 5a. The high acoustic impedance layer 6a is located on the fourth principal surface 5y side of the low acoustic impedance layer 5a. The high acoustic impedance layer 6a is located on the third principal surface 5x side of the low acoustic impedance layer 5b. The high acoustic impedance layer 6b is located on the fourth principal surface 5y side of the low acoustic impedance layer 5b.

[0026] An adhesive layer 11 is provided between the fourth main surface 5y of the low acoustic impedance layer 5a and the high acoustic impedance layer 6a. Similarly, an adhesive layer 11 is provided between the fourth main surface 5y of the low acoustic impedance layer 5b and the high acoustic impedance layer 6b. On the other hand, no adhesive layer 11 is provided between the third main surface 5x of the low acoustic impedance layer 5b and the high acoustic impedance layer 6a.

[0027] The acoustic reflection film 4 has three low acoustic impedance layers, two high acoustic impedance layers, and two adhesive layers 11. It is sufficient that the acoustic reflection film 4 has at least one low acoustic impedance layer, at least one high acoustic impedance layer, and at least one adhesive layer 11.

[0028] The present embodiment is characterized by the following configuration: 1) The density of the low-density silicon oxide layer is 2 g / cm ３ 2) At least one adhesion layer 11 is provided between the high acoustic impedance layer and at least one of the third main surface 5x and the fourth main surface 5y of the low-density silicon oxide layer. 3) The density of the adhesion layer 11 is higher than the density of the low-density silicon oxide layer in contact with the adhesion layer 11 and lower than the density of the high acoustic impedance layer in contact with the adhesion layer 11. This makes it possible to suppress peeling of the acoustic reflection film 4 from the piezoelectric layer 7. This will be explained below.

[0029] The density of the low acoustic impedance layer, which is a low-density silicon oxide layer, is 2 g / cm ３ In particular, when the density of the low acoustic impedance layer is low like this, there is a risk that the low acoustic impedance layer will be damaged when a large membrane stress is applied from the high acoustic impedance layer.

[0030] In contrast, an adhesive layer 11 is provided between the low-density silicon oxide layer and the high acoustic impedance layer. As a result, the film stress of the high acoustic impedance layer is indirectly applied to the low-density silicon oxide layer via the adhesive layer 11. This makes it possible to reduce the film stress applied from the high acoustic impedance layer to the low-density silicon oxide layer. In addition, the density of the adhesive layer 11 is lower than the density of the high acoustic impedance layer. Therefore, the film stress applied from the adhesive layer 11 to the low-density silicon oxide layer is also small. Therefore, the low-density silicon oxide layer is less likely to be damaged.

[0031] Furthermore, the density of the adhesion layer 11 is higher than the density of the low-density silicon oxide layer. Therefore, even if the film stress of the high acoustic impedance layer is directly applied to the adhesion layer 11, the adhesion layer 11 is unlikely to be damaged. Therefore, the acoustic reflection film 4 is unlikely to be damaged. Therefore, peeling of the acoustic reflection film 4 from the piezoelectric layer 7 can be suppressed.

[0032] In addition, the density of the low-density silicon oxide layer is 2 g / cm ３ Since the acoustic impedance of the low-density silicon oxide layer is lower than that of the high-density silicon oxide layer, the acoustic impedance of the low-density silicon oxide layer is particularly low. This allows for a large acoustic impedance ratio. In this specification, the acoustic impedance ratio is the value obtained by dividing the acoustic impedance of the high acoustic impedance layer by the acoustic impedance of the low acoustic impedance layer in adjacent high and low acoustic impedance layers. A large acoustic impedance ratio allows the acoustic reflecting film 4 to effectively reflect acoustic waves toward the piezoelectric layer 7. As a result, loss in the acoustic wave device 1 can be reduced.

[0033] Furthermore, the low-density silicon oxide layer included in the acoustic reflection film 4 is made of, for example, silicon oxide aerogel. This prevents a circuit configuration in which conductance is arranged in parallel with the acoustic wave resonator when the acoustic wave device 1 is used as an acoustic wave resonator in a filter device.

[0034] The material of the adhesive layer 11 is not particularly limited as long as it satisfies the above density relationship. For example, the material of the adhesive layer 11 may be a silicon dioxide layer having a density of 2 g / cm, similar to low acoustic impedance layers other than the low-density silicon oxide layer. ３ The above-mentioned dielectric materials such as silicon oxide, and metals such as aluminum and titanium can be used. When the material of the adhesion layer 11 is the same as the material of the low acoustic impedance layer other than the low-density silicon oxide layer, even if the adhesion layer 11 is provided between the low-density silicon oxide layer and the high acoustic impedance layer, the effect of reflecting the elastic wave toward the piezoelectric layer 7 is unlikely to be reduced.

[0035] The configuration of the first embodiment will be described in more detail below.

[0036] The elastic wave device 1 is an elastic wave resonator configured to utilize thickness-shear mode bulk waves as the main mode. More specifically, in the elastic wave device 1, where d is the thickness of the piezoelectric layer 7 and p is the center-to-center distance between adjacent first electrode fingers 18 and second electrode fingers 19, d / p is 0.5 or less. This allows the thickness-shear mode bulk waves to be suitably excited.

[0037] As shown in Fig. 1 , when viewed from the electrode finger orthogonal direction, the region where adjacent first electrode fingers 18 and second electrode fingers 19 overlap is the intersection region F. The intersection region F includes a plurality of excitation regions C. More specifically, the excitation region C is the region where adjacent first electrode fingers 18 and second electrode fingers 19 overlap in the electrode finger orthogonal direction and is the region between the centers of the adjacent first electrode fingers 18 and second electrode fingers 19. Note that Fig. 1 shows only two of the plurality of excitation regions C.

[0038] By applying an AC voltage to the IDT electrode 8, thickness-shear mode bulk waves are excited in multiple excitation regions C. The propagation direction of the thickness-shear mode bulk waves is parallel to the thickness direction of the piezoelectric layer 7. Therefore, in an elastic wave device 1 that can utilize thickness-shear mode bulk waves as the main mode, a configuration in which elastic waves are reflected by an acoustic reflection film 4 as shown in FIG.

[0039] Incidentally, all of the low acoustic impedance layers and all of the high acoustic impedance layers in the acoustic reflection film 4 may be made of a dielectric. Alternatively, all of the low acoustic impedance layers and all of the high acoustic impedance layers in the acoustic reflection film 4 other than the low-density silicon oxide layer may be made of a metal. Note that any one of all of the low acoustic impedance layers and all of the high acoustic impedance layers may be made of a dielectric, and any one of the other layers may be made of a metal. The layer referred to here is an acoustic impedance layer.

[0040] The plurality of low acoustic impedance layers are made of a material having a density of 2 g / cm ３It is preferable that the acoustic wave device 1 includes a low acoustic impedance layer made of silicon oxide. In this case, when the acoustic wave device 1 is used as an acoustic wave resonator in a filter device, a circuit configuration in which conductance is arranged in parallel with the acoustic wave resonator can be prevented. However, for example, the plurality of low acoustic impedance layers may include a low acoustic impedance layer made of aluminum.

[0041] Preferably, the plurality of high acoustic impedance layers include a high acoustic impedance layer made of at least one of ytterbium oxide, hafnium oxide, and tungsten oxide. In this case, when the acoustic wave device 1 is used as an acoustic wave resonator in a filter device, a circuit configuration in which conductance is arranged in parallel with the acoustic wave resonator can be prevented.

[0042] However, for example, the plurality of high acoustic impedance layers may include a high acoustic impedance layer made of a metal such as platinum or tungsten, which makes it easy to increase the acoustic impedance ratio.

[0043] Preferably, the acoustic impedance ratio of at least one pair of adjacent high acoustic impedance layers and low acoustic impedance layers in the acoustic reflecting film 4 is 6 or greater. More preferably, the acoustic impedance ratio of all adjacent high acoustic impedance layers and low acoustic impedance layers in the acoustic reflecting film 4 is 6 or greater. This more reliably suppresses deterioration of the electrical characteristics in the main mode. For example, loss can be reduced in the acoustic wave device 1.

[0044] Note that the adjacent high acoustic impedance layer and low acoustic impedance layer referred to here include both cases where an adhesive layer 11 is provided between the high acoustic impedance layer and the low acoustic impedance layer, and cases where an adhesive layer 11 is not provided between the high acoustic impedance layer and the low acoustic impedance layer. For example, the high acoustic impedance layer 6a and the low acoustic impedance layer 5a shown in Fig. 2 are adjacent high acoustic impedance layer and low acoustic impedance layer. The high acoustic impedance layer 6b and the low acoustic impedance layer 5c are also adjacent high acoustic impedance layer and low acoustic impedance layer.

[0045] The layer of the acoustic reflection film 4 that is closest to the piezoelectric layer 7 has a particularly large effect on the electrical characteristics of the acoustic wave device 1. In this embodiment, the low acoustic impedance layer 5a, which is a low-density silicon oxide layer, is the layer of the acoustic reflection film 4 that is closest to the piezoelectric layer 7. This makes it possible to particularly reduce the acoustic impedance of the layer of the acoustic reflection film 4 that is closest to the piezoelectric layer 7. This makes it possible to more reliably suppress deterioration of the electrical characteristics of the acoustic wave device 1.

[0046] The arrangement of the low-density silicon oxide layer is not limited to the above. For example, a low acoustic impedance layer other than the low-density silicon oxide layer may be the layer in the acoustic reflection film 4 that is located closest to the piezoelectric layer 7. In this case, the bonding strength between the acoustic reflection film 4 and the piezoelectric layer 7 can be more reliably increased. A low acoustic impedance layer other than the low-density silicon oxide layer may be the layer in the acoustic reflection film 4 that is located closest to the support substrate 3. In this case, the bonding strength between the acoustic reflection film 4 and the support substrate 3 can be more reliably increased.

[0047] Alternatively, all of the low acoustic impedance layers may be low-density silicon oxide layers, which can effectively reduce loss in the acoustic wave device 1.

[0048] In the acoustic reflection film 4, the thickness of the adhesive layer 11 is thinner than the thickness of the low-density silicon oxide layer in contact with the adhesive layer 11. The thickness of the adhesive layer 11 is preferably 30% or less of the thickness of the low-density silicon oxide layer in contact with the adhesive layer 11. This can increase the contribution of the low-density silicon oxide layer to the reflection of elastic waves in the acoustic reflection film 4.

[0049] On the other hand, the thickness of the adhesion layer 11 is preferably 1.25% or more of the thickness of the low-density silicon oxide layer in contact with the adhesion layer 11. In this case, it is possible to more reliably reduce the film stress applied to the low-density silicon oxide layer from the high acoustic impedance layer, thereby more reliably suppressing damage to the low-density silicon oxide layer and peeling of the acoustic reflection film 4 from the piezoelectric layer 7.

[0050] In this embodiment, the adhesion layer 11 is located on the fourth principal surface 5y side of the low acoustic impedance layer 5b, which is a low-density silicon oxide layer. Meanwhile, the high acoustic impedance layer 6b is located on the third principal surface 5x side. In this manner, the adhesion layer 11 may be located on one side of the third principal surface 5x or the fourth principal surface 5y, and the high acoustic impedance layer 6b may be located on the other side.

[0051] In this case, the membrane stress applied from one main surface of the low-density silicon oxide layer can be reduced. The acoustic impedance ratio can be more reliably increased on the other main surface of the low-density silicon oxide layer. This makes it possible to more reliably and effectively reduce loss in the acoustic wave device 1 and to prevent peeling of the acoustic reflection film 4 from the piezoelectric layer 7.

[0052] The arrangement of the adhesion layer 11 is not limited to the above. For example, in a first modification of the first embodiment shown in Fig. 3, the adhesion layer 11 is located on both the third principal surface 5x side and the fourth principal surface 5y side of the low acoustic impedance layer 5b, which is a low-density silicon oxide layer. In this case, the film stress applied from both principal surface sides of the low-density silicon oxide layer can be reduced. Therefore, peeling of the acoustic reflection film 4A from the piezoelectric layer 7 side can be effectively suppressed.

[0053] In the elastic wave device 1 of the first preferred embodiment shown in FIG. 2 , no dielectric film is provided on the first principal surface 7 a or the second principal surface 7 b of the piezoelectric layer 7. However, a dielectric film may be provided on at least one of the first principal surface 7 a or the second principal surface 7 b of the piezoelectric layer 7. In a second modified example of the first preferred embodiment shown in FIG. 4 , a first dielectric film 9A is provided on the first principal surface 7 a of the piezoelectric layer 7 so as to cover the IDT electrode 8. In this case, the IDT electrode 8 is protected by the first dielectric film 9A. This makes the IDT electrode 8 less susceptible to damage.

[0054] A second dielectric film 9B is provided on the second main surface 7b of the piezoelectric layer 7. More specifically, the second dielectric film 9B is provided between the piezoelectric layer 7 and the acoustic reflection film 4. In this modification, the piezoelectric substrate 2B includes a support substrate 3, an acoustic reflection film 4, the second dielectric film 9B, and the piezoelectric layer 7.

[0055] The first dielectric film 9A and the second dielectric film 9B may be made of, for example, silicon oxide, silicon nitride, or silicon oxynitride. The materials of the first dielectric film 9A and the second dielectric film 9B may be the same or different. The material of the first dielectric film 9A may be the same as the material of the low acoustic impedance layer or the high acoustic impedance layer. The same applies to the material of the second dielectric film 9B.

[0056] In this modification, as in the first embodiment, the low-density silicon oxide layer is less likely to be damaged in the acoustic reflection film 4. This makes it possible to prevent the acoustic reflection film 4, together with the second dielectric film 9B, from peeling off from the piezoelectric layer 7. Note that the configuration in this modification that includes at least one of the first dielectric film 9A and the second dielectric film 9B can also be applied to configurations of the present invention other than this modification.

[0057] According to the present invention, the elastic wave device may be configured to use plate waves as the main mode. In a third modification of the first embodiment shown in FIG. 5 , an elastic wave device 1C is configured to use plate waves as the main mode. In this modification, a pair of reflectors 13A and 13B are provided on the first main surface 7a of the piezoelectric layer 7. More specifically, the reflectors 13A and 13B face each other with the IDT electrode 8 sandwiched between them in the direction perpendicular to the electrode fingers.

[0058] The reflector 13A has a pair of reflector bus bars 14a and 14b and a plurality of reflector electrode fingers 15. The reflector bus bars 14a and 14b face each other. One end and the other end of each of the plurality of reflector electrode fingers 15 are short-circuited by the reflector bus bars 14a and 14b. The reflector 13B has a similar configuration to the reflector 13A.

[0059] When the thickness of the piezoelectric layer 7 is d and the center-to-center distance between the adjacent first electrode fingers 18 and second electrode fingers 19 is p, the thickness of the piezoelectric layer 7 is preferably d≦2p. In this case, plate waves are suitably excited. In addition, the provision of the reflectors 13A and 13B can improve resonance characteristics. When plate waves are used as the main mode, the crossing region F is the excitation region.

[0060] When the piezoelectric layer 7 is made of lithium niobate, the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 are preferably within the range of 0°±5°, 30°±30°, or 0°±5°. Alternatively, the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 are preferably within the range of 0°±5°, 0°±30°, or 90°±5°. As a result, the A shown in FIG. 6( a), which is a type of plate wave, can be obtained. １ The same applies when the piezoelectric layer 7 is made of lithium tantalate.

[0061] When the piezoelectric layer 7 is made of lithium niobate, it is preferable that the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 are within the range of 90°±5°, 90°±5°, or 40°±20°. As a result, the S wave shown in FIG. 6(b), which is a type of plate wave, can be generated. ０ The same applies when the piezoelectric layer 7 is made of lithium tantalate.

[0062] In this modification, the low-density silicon oxide layer is also less likely to be damaged in the acoustic reflection film 4 shown in FIG. 2. Therefore, as in the first embodiment, peeling of the acoustic reflection film 4 from the piezoelectric layer 7 can be suppressed.

[0063] In the first embodiment and its modifications, the second main surface of the piezoelectric layer is located on the acoustic reflection film side. However, this is not limiting. An example in which the first main surface of the piezoelectric layer is located on the acoustic reflection film side is shown in the second embodiment.

[0064] FIG. 7 is a schematic front cross-sectional view of an elastic wave device according to a second preferred embodiment of the present invention.

[0065] This embodiment differs from the first embodiment in that, of the first principal surface 7a and the second principal surface 7b of the piezoelectric layer 7, the first principal surface 7a is located on the acoustic reflection film 4 side. The IDT electrode 8 is embedded in the acoustic reflection film 4. Specifically, the IDT electrode 8 is embedded in the low acoustic impedance layer 5a of the acoustic reflection film 4. Except for the above points, the elastic wave device 21 of this embodiment has a similar configuration to the elastic wave device 1 of the first embodiment.

[0066] In this embodiment, as in the first embodiment, the low-density silicon oxide layer is less likely to be damaged in the acoustic reflection film 4. Therefore, peeling of the acoustic reflection film 4 from the piezoelectric layer 7 can be suppressed.

[0067] 4 may be provided even in a configuration in which the first main surface 7a of the piezoelectric layer 7 is located on the acoustic reflection film 4 side. For example, even in a case in which the first dielectric film 9A is provided on the first main surface 7a of the piezoelectric layer 7, the IDT electrode 8 may be embedded in the acoustic reflection film 4. In this specification, the IDT electrode being embedded in the acoustic reflection film means that part of the acoustic reflection film is located between the electrode fingers of the IDT electrode.

[0068] A preferred configuration of the present invention will be described below with reference to Fig. 1. However, the preferred configuration described below can also be applied to configurations of the present invention other than the first embodiment that utilize thickness-shear bulk waves as the main mode.

[0069] In the first embodiment, where d is the thickness of the piezoelectric layer 7 and p is the center-to-center distance between adjacent electrode fingers, d / p is 0.5 or less. It is preferable that d / p is 0.24 or less. This allows thickness-shear mode bulk waves to be more effectively excited in each excitation region C, and enables the value of the fractional bandwidth of the elastic wave resonator to be sufficiently large. The fractional bandwidth is expressed as (|fa-fr| / fr)×100[%], where fr is the resonant frequency and fa is the antiresonant frequency.

[0070] FIG. 8 is a graph showing the relationship between d / p and the bandwidth ratio of an elastic wave resonator.

[0071] As is clear from Figure 8, when d / p > 0.5, the fractional bandwidth is less than 5%. In contrast, when d / p ≤ 0.5, the fractional bandwidth can be increased to 5% or more. This increases the electromechanical coupling coefficient of the thickness-shear mode bulk wave. When d / p ≤ 0.24, the fractional bandwidth can be increased to 7% or more. This effectively increases the electromechanical coupling coefficient of the thickness-shear mode bulk wave.

[0072] When the metallization ratio of the electrode fingers to the excitation region C is MR, it is preferable to satisfy MR≦1.75(d / p)+0.075. In this case, the value of the fractional bandwidth of the acoustic wave resonator does not become too large, and spurious emissions between the resonant frequency and the antiresonant frequency can be suppressed. Details of this are described below.

[0073] In this specification, the metallization ratio MR of the electrode fingers to the excitation region C is the proportion of the portion of the piezoelectric layer 7 that is covered with the metal that constitutes the electrode fingers in the excitation region C when viewed in a plan view. Specifically, the metallization ratio MR is the ratio of the area of ​​the first electrode fingers 18 and the second electrode fingers 19 in the excitation region C to the area of ​​the excitation region C when viewed in a plan view. When the width of the electrode fingers located in the excitation region C is constant, the metallization ratio MR can also be calculated by dividing the sum of the widths of the electrode fingers located in the excitation region C by the dimension of the excitation region C in the direction perpendicular to the electrode fingers.

[0074] Fig. 9 is a diagram showing the relationship between the relative bandwidth of an elastic wave resonator and the magnitude of normalized spurious. Fig. 9 shows the results of measuring the amount of phase rotation of spurious every time the relative bandwidth is changed by changing the thickness of the piezoelectric layer and the dimensions of the electrode fingers. The normalized magnitude of spurious in Fig. 9 is specifically a value obtained by normalizing the amount of phase rotation of the spurious impedance by 180°. The results shown in Fig. 9 are for a Z-cut LiNbO ３ Although this is the result when a piezoelectric layer made of this material was used, the same tendency is observed when a piezoelectric layer having another cut angle is used.

[0075] In the region surrounded by ellipse A in Figure 9, the normalized magnitude of the spurious response between the resonant frequency and the anti-resonant frequency is 1.0. If the bandwidth fraction of the elastic wave resonator exceeds 17%, the normalized magnitude of the spurious response may be 1.0 or more. For this reason, it is preferable that the bandwidth fraction be 17% or less. This makes it possible to suppress the spurious response between the resonant frequency and the anti-resonant frequency.

[0076] 10 is a diagram showing the relationship between d / p, metallization ratio MR, and bandwidth fraction, in which the bandwidth fraction is calculated for each of different d / p and metallization ratio MR.

[0077] In Figure 10, the hatched area is the area where the fractional bandwidth is 17% or less. The boundary between this hatched area and the non-hatched area is roughly represented by dashed line B. Dashed line B is represented by MR = 1.75(d / p) + 0.075. It is preferable that MR ≤ 1.75(d / p) + 0.075. In this case, it is easy to keep the fractional bandwidth at 17% or less.

[0078] On the other hand, the dashed-dotted line B1 in Figure 10 indicates the boundary where the slope of the change in metallization ratio MR with respect to the change in d / p is the same as that of the dashed line B, and where the fractional bandwidth is 17% or less over the entire range. The dashed-dotted line B1 is represented by MR = 1.75(d / p) + 0.05. It is more preferable that MR ≤ 1.75(d / p) + 0.05. In this case, the fractional bandwidth can be more reliably kept at 17% or less.

[0079] FIG. 11 shows the results of LiNbO when d / p approaches 0. ３ 11 is a map of the fractional bandwidth with respect to Euler angles (0°, θ, ψ) of the optical fiber 10. The hatched area in FIG. 11 is a region where a fractional bandwidth of at least 5% or more can be obtained, and the range of this region can be approximated as the ranges expressed by the following formulas (1), (2), and (3).

[0080] (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)

[0081] It is preferable that the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 be within the range of the above formula (1), formula (2), or formula (3). This allows the relative bandwidth of the elastic wave resonator to be sufficiently wide. The same applies when the piezoelectric layer 7 is made of lithium tantalate.

[0082] Examples of embodiments of the acoustic wave device according to the present invention will be described below.

[0083] <1> An acoustic reflection film including at least one high acoustic impedance layer having a relatively high acoustic impedance and at least one low acoustic impedance layer having a relatively low acoustic impedance; a piezoelectric layer provided on the acoustic reflection film and having a first main surface and a second main surface facing each other; and an IDT electrode provided on the first main surface of the piezoelectric layer and having a plurality of electrode fingers, wherein the low acoustic impedance layers and the high acoustic impedance layers are alternately laminated, and at least one of the low acoustic impedance layers has a density of 2 g / cm ３ the low-density silicon oxide layer has a third main surface and a fourth main surface facing each other, the acoustic reflection film further includes at least one adhesion layer provided between the high acoustic impedance layer and at least one of the third main surface and the fourth main surface of the low-density silicon oxide layer, and the density of the adhesion layer is higher than the density of the low-density silicon oxide layer in contact with the adhesion layer and lower than the density of the high acoustic impedance layer in contact with the adhesion layer.

[0084] <2> The acoustic wave device according to <1>, wherein the low-density silicon oxide layer is a layer of the acoustic reflection film that is closest to the piezoelectric layer.

[0085] <3> The acoustic wave device according to <1> or <2>, wherein the second principal surface of the first principal surface and the second principal surface of the piezoelectric layer is located on the acoustic reflection film side.

[0086] <4> The elastic wave device according to <1> or <2>, wherein the first principal surface of the piezoelectric layer is located on the acoustic reflection film side, and the IDT electrode is embedded in the acoustic reflection film.

[0087] <5> The elastic wave device according to any one of <1> to <4>, further comprising a dielectric film provided on at least one of the first principal surface and the second principal surface of the piezoelectric layer.

[0088] <6> The acoustic wave device according to any one of <1> to <5>, further comprising a support substrate, wherein the acoustic reflection film is provided on the support substrate.

[0089] <7> The acoustic wave device according to any one of <1> to <6>, wherein all of the low acoustic impedance layers and all of the high acoustic impedance layers are made of a dielectric material.

[0090] <8> The elastic wave device according to any one of <1> to <6>, wherein any one of all the low acoustic impedance layers and all the high acoustic impedance layers is a layer made of a dielectric, and any one of the other is a layer made of a metal.

[0091] <9> The acoustic wave device according to any one of <1> to <6>, wherein the acoustic reflection film includes a plurality of the low acoustic impedance layers, and all of the low acoustic impedance layers other than the low-density silicon oxide layer and all of the high acoustic impedance layers are made of metal.

[0092] <10> The acoustic reflection film includes a plurality of the low acoustic impedance layers, and the plurality of low acoustic impedance layers are made of a material having a density of 2 g / cm ３ The acoustic wave device according to any one of <1> to <8>, including the low acoustic impedance layer made of the above silicon oxide.

[0093] <11> The acoustic wave device according to any one of <1> to <8> or <10>, wherein the acoustic reflection film includes the high acoustic impedance layer made of at least one of ytterbium oxide, hafnium oxide, and tungsten oxide.

[0094] <12> The acoustic wave device according to any one of <1> to <6>, <8>, or <9>, wherein the acoustic reflection film includes a plurality of the low acoustic impedance layers, and the plurality of low acoustic impedance layers include the low acoustic impedance layer made of aluminum.

[0095] <13> The elastic wave device according to any one of <1> to <12>, wherein, when the acoustic impedance ratio is the value obtained by dividing the acoustic impedance of the high acoustic impedance layer by the acoustic impedance of the low acoustic impedance layer, the acoustic impedance ratio is 6 or more for at least one pair of adjacent high acoustic impedance layer and low acoustic impedance layer in the acoustic reflection film.

[0096] <14> The elastic wave device according to any one of <1> to <13>, 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 electrode fingers.

[0097] <15> The acoustic wave device according to <14>, wherein d / p is 0.24 or less.

[0098] <16> An elastic wave device according to <14> or <15>, wherein, when a direction perpendicular to the direction in which the plurality of electrode fingers extend is defined as an electrode finger perpendicular direction, adjacent electrode fingers overlap in the electrode finger perpendicular direction, and the region between the centers of adjacent electrode fingers is an excitation region, and when the metallization ratio of the electrode fingers to the excitation region is defined as MR, MR≦1.75(d / p)+0.075 is satisfied.

[0099] <17> The acoustic wave device according to any one of <1> to <16>, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within the range of the following formula (1), formula (2), or formula (3): (0°±10°, 0° to 20°, any ψ) ... formula (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)

[0100] <18> The elastic wave device according to any one of <1> to <13>, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are (within a range of 0°±5°, 30°±30°, or 0°±5°).

[0101] <19> The elastic wave device according to any one of <1> to <13>, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within a range of 0°±5°, 0°±30°, or 90°±5°.

[0102] <20> The elastic wave device according to any one of <1> to <13>, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within a range of 90°±5°, 90°±5°, or 40°±20°.

[0103] REFERENCE SIGNS LIST 1, 1C... acoustic wave device 2, 2B... piezoelectric substrate 3... support substrate 4, 4A... acoustic reflection film 5a to 5c... low acoustic impedance layer 5x, 5y... third and fourth principal surfaces 6a, 6b... high acoustic impedance layer 7... piezoelectric layer 7a, 7b... first and second principal surfaces 8... IDT electrode 9A, 9B... first and second dielectric films 11... adhesion layer 13A, 13B... reflector 14a, 14b... reflector bus bar 15... reflector electrode finger 16, 17... first and second bus bar 18, 19... first and second electrode finger 21... acoustic wave device C... excitation region F... intersection region

Claims

1. An acoustic reflection film including at least one high acoustic impedance layer having a relatively high acoustic impedance and at least one low acoustic impedance layer having a relatively low acoustic impedance; a piezoelectric layer provided on the acoustic reflection film and having a first main surface and a second main surface facing each other; and an IDT electrode provided on the first main surface of the piezoelectric layer and having a plurality of electrode fingers, wherein the low acoustic impedance layers and the high acoustic impedance layers are alternately stacked, and at least one of the low acoustic impedance layers has a density of 2 g / cm ３ the acoustic reflection film further includes at least one adhesive layer provided between the high acoustic impedance layer and at least one of the third and fourth main surfaces of the low-density silicon oxide layer, and the adhesive layer has a density higher than that of the low-density silicon oxide layer in contact with the adhesive layer and lower than that of the high acoustic impedance layer in contact with the adhesive layer.

2. The acoustic wave device according to claim 1, wherein the low-density silicon oxide layer is the layer of the acoustic reflection film that is closest to the piezoelectric layer.

3. The acoustic wave device according to claim 1 or 2, wherein the second principal surface of the piezoelectric layer is located on the acoustic reflection film side.

4. An elastic wave device according to claim 1 or 2, wherein of the first and second principal surfaces of the piezoelectric layer, the first principal surface is located on the acoustic reflection film side, and the IDT electrode is embedded in the acoustic reflection film.

5. The elastic wave device according to any one of claims 1 to 4, further comprising a dielectric film provided on at least one of the first main surface and the second main surface of the piezoelectric layer.

6. The acoustic wave device according to any one of claims 1 to 5, further comprising a support substrate, wherein the acoustic reflection film is provided on the support substrate.

7. The acoustic wave device according to any one of claims 1 to 6, wherein all of the low acoustic impedance layers and all of the high acoustic impedance layers are made of a dielectric material.

8. The elastic wave device according to any one of claims 1 to 6, wherein either all of the low acoustic impedance layers or all of the high acoustic impedance layers are layers made of a dielectric material, and the other is a layer made of a metal.

9. The acoustic wave device according to any one of claims 1 to 6, wherein the acoustic reflection film includes a plurality of low acoustic impedance layers, and all of the low acoustic impedance layers other than the low-density silicon oxide layer and all of the high acoustic impedance layers are made of metal.

10. The acoustic reflection film includes a plurality of the low acoustic impedance layers, and the plurality of low acoustic impedance layers are made of a material having a density of 2 g / cm ３ The acoustic wave device according to claim 1, further comprising the low acoustic impedance layer made of the above silicon oxide.

11. The acoustic wave device according to claim 1, wherein the acoustic reflection film includes the high acoustic impedance layer made of at least one of ytterbium oxide, hafnium oxide, and tungsten oxide.

12. The elastic wave device according to any one of claims 1 to 6, 8, or 9, wherein the acoustic reflection film includes a plurality of low acoustic impedance layers, and the plurality of low acoustic impedance layers include a low acoustic impedance layer made of aluminum.

13. The elastic wave device according to any one of claims 1 to 12, wherein, when the acoustic impedance ratio is the value obtained by dividing the acoustic impedance of the high acoustic impedance layer by the acoustic impedance of the low acoustic impedance layer, the acoustic impedance ratio is 6 or greater for at least one pair of adjacent high acoustic impedance layer and low acoustic impedance layer in the acoustic reflection film.

14. The elastic wave device according to any one of claims 1 to 13, 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 electrode fingers.

15. The acoustic wave device according to claim 14, wherein d / p is 0.24 or less.

16. The elastic wave device according to claim 14 or 15, wherein, when a direction perpendicular to the direction in which the plurality of electrode fingers extend is defined as an electrode finger perpendicular direction, a region where adjacent electrode fingers overlap in the electrode finger perpendicular direction and a region between the centers of adjacent electrode fingers is an excitation region, and when a metallization ratio of the electrode fingers to the excitation region is defined as MR, MR≦1.75(d / p)+0.075 is satisfied.

17. The acoustic wave device according to any one of claims 1 to 16, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within the range of the following formula (1), formula (2), or formula (3): (0°±10°, 0° to 20°, any ψ) ... formula (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) 18. The elastic wave device according to any one of claims 1 to 13, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within a range of 0°±5°, 30°±30°, or 0°±5°.

19. The elastic wave device according to any one of claims 1 to 13, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within a range of 0°±5°, 0°±30°, or 90°±5°.

20. The elastic wave device according to any one of claims 1 to 13, wherein the piezoelectric layer is made of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the piezoelectric layer are within the range of 90°±5°, 90°±5°, or 40°±20°.

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