Elastic wave device

Abstract

This invention provides an elastic wave device capable of suppressing unwanted waves. The elastic wave device 1 of this invention comprises: a sound-reflecting membrane 4, which includes at least one high-impedance layer 6a, 6b with relatively high sound impedance, and a plurality of low-impedance layers with relatively low sound impedance; a piezoelectric layer 7 disposed on the sound-reflecting membrane 4, having a first main surface 7a and a second main surface 7b facing each other; and an IDT electrode 8 disposed on the first main surface 7a of the piezoelectric layer 7, having a plurality of electrode fingers (a plurality of first and second electrode fingers 18, 19). The low-impedance layers and the high-impedance layers are alternately stacked. The plurality of low-impedance layers include at least one first low-impedance layer 5a and at least one second low-impedance layer 15b, 15c. When the density of the first low acoustic impedance layer 5a is set to ρ1 and the Young's modulus is set to E1, and the density of the second low acoustic impedance layers 15b and 15c is set to ρ2 and the Young's modulus is set to E2, at least one of ρ1 > ρ2 and E1 > E2 is satisfied.

Description

[Technical Field]

[0001] This invention relates to an elastic wave device. [Previous Technology]

[0002] In the past, elastic wave devices have been widely used in filters for mobile phones, etc. Patent Document 1 below discloses an example of an elastic wave device. In this elastic wave device, a sound-reflecting layer is provided on a support substrate. A piezoelectric layer is provided on the sound-reflecting layer. An IDT (Interdigital Transducer) electrode is provided on the piezoelectric layer.

[0003] The sound-reflecting layer is formed by alternating layers of low acoustic impedance and high acoustic impedance. The acoustic impedance of the high acoustic impedance layer is higher than that of the low acoustic impedance layer. Patent Document 1 cites an example of a sound-reflecting layer composed of three low acoustic impedance layers made of SiO2 and three high acoustic impedance layers made of W. Other examples include sound-reflecting layers composed of five low acoustic impedance layers made of SiO2 and five high acoustic impedance layers made of AlN. By providing a sound-reflecting layer, the energy of elastic waves is contained within the piezoelectric layer. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2012 / 086441 [Summary of the Invention]

[0005] [Problem to be solved by the invention] However, in the elastic wave device such as Patent Document 1, there is a risk that unnecessary waves may not be sufficiently suppressed.

[0006] The object of this invention is to provide an elastic wave device capable of suppressing unwanted waves. [Means of the Problem]

[0007] The elastic wave device of the present invention comprises: a sound-reflecting membrane, which includes at least one high sound impedance layer with relatively high sound impedance and a plurality of low sound impedance layers with relatively low sound impedance; a piezoelectric layer disposed on the aforementioned sound-reflecting membrane, having a first main surface and a second main surface facing each other; and an IDT electrode disposed on the aforementioned first main surface of the aforementioned piezoelectric layer, having a plurality of electrode fingers; the aforementioned low sound impedance layers and the aforementioned high sound impedance layers are interleaved, the aforementioned plurality of low sound impedance layers including at least one first low sound impedance layer and at least one second low sound impedance layer, wherein when the density of the aforementioned first low sound impedance layer is set to ρ1 and the Young's modulus is set to E1, and the density of the aforementioned second low sound impedance layer is set to ρ2 and the Young's modulus is set to E2, at least one of ρ1 > ρ2 and E1 > E2 is satisfied. [Effects of the Invention]

[0008] The elastic wave device according to the present invention can suppress unnecessary waves.

Implementation Method

[0010] Hereinafter, the present invention will be explained by referring to the drawings and describing specific embodiments thereof.

[0011] In addition, the embodiments described in this specification are illustrative. It is hereby explained that the components may be replaced or combined in different embodiments.

[0012] FIG1 is a schematic top view of the elastic wave device according to the first embodiment of the present invention. FIG2 is a schematic cross-sectional view along line I-I in FIG1.

[0013] As shown in Figure 1, the elastic wave device 1 includes a piezoelectric substrate 2 and an IDT electrode 8. The piezoelectric substrate 2 is a piezoelectric substrate. As shown in Figure 2, the piezoelectric substrate 2 includes a support substrate 3, a sound-reflecting film 4, and a piezoelectric layer 7. Specifically, the sound-reflecting film 4 is disposed on the support substrate 3. The piezoelectric layer 7 is disposed on the sound-reflecting film 4.

[0014] As the material for the support substrate 3, for example, semiconductors such as silicon or ceramics such as alumina can be used. The piezoelectric layer 7 can be made of lithium niobate such as LiNbO3, or lithium tantalate such as LiTaO3. In this embodiment, the piezoelectric layer 7 is made of lithium niobate. The statement in this specification that a component is made of a certain material includes cases where there are trace amounts of impurities that do not significantly degrade the electrical characteristics of the elastic wave device.

[0015] The piezoelectric layer 7 has a first main surface 7a and a second main surface 7b. The first main surface 7a and the second main surface 7b face each other. In this embodiment, the second main surface 7b is located on the side of the sound-reflecting membrane 4. An IDT electrode 8 is provided on the first main surface 7a of the piezoelectric layer 7.

[0016] As shown in Figure 1, the IDT electrode 8 has a pair of buses and a plurality of electrode fingers. Specifically, the pair of buses is a first bus 16 and a second bus 17. The first bus 16 and the second bus 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 16. One end of each of the plurality of second electrode fingers 19 is connected to the second bus 17. The plurality of first electrode fingers 18 and the plurality of second electrode fingers 19 are arranged alternately. The first electrode fingers 18 and the second electrode fingers 19 are connected to different potentials. The IDT electrode 8 may be constructed of a multilayer metal film, or it may be constructed of a single-layer metal film.

[0017] Hereinafter, the first electrode finger 18 and the second electrode finger 19 will sometimes be referred to as electrode fingers together. The direction in which the plurality of electrode fingers extend is defined as the electrode finger extension direction, and the direction orthogonal to the electrode finger extension direction is defined as the electrode finger orthogonal direction.

[0018] As shown in Figure 2, the sound reflective film 4 is a laminate comprising a plurality of sound impedance layers. Specifically, the sound reflective film 4 has a plurality of low sound impedance layers and a plurality of high sound impedance layers. The low sound impedance layers and high sound impedance layers are alternately laminated.

[0019] Furthermore, the material of the acoustic impedance layer closest to the piezoelectric layer 7 in the acoustic reflective film 4 is preferably 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. The material of the acoustic impedance layer closest to the piezoelectric layer 7 in the acoustic reflective film 4 is more preferably a material with a dielectric constant of 15 or less. In this case, the elastic wave device 1 can be made more broadband. Broadbanding here refers to increasing the absolute value of the difference between the resonant frequency and the anti-resonant frequency.

[0020] The high acoustic impedance layer is a layer with relatively high acoustic impedance. More specifically, the high acoustic impedance layer is a layer with higher acoustic impedance than the adjacent layers in the sound reflective film 4. The plurality of high acoustic impedance layers in the sound reflective film 4 are, more specifically, high acoustic impedance layer 6a and high acoustic impedance layer 6b.

[0021] On the other hand, the low acoustic impedance layer is a layer with relatively low acoustic impedance. More specifically, the low acoustic impedance layer is a layer with lower acoustic impedance compared to the layers adjacent to it in the sound reflective film 4. The plurality of low acoustic impedance layers in the sound reflective film 4 specifically includes at least one first low acoustic impedance layer and at least one second low acoustic impedance layer. More specifically, the plurality of low acoustic impedance layers in this embodiment are a first low acoustic impedance layer 5a, a second low acoustic impedance layer 15b, and a second low acoustic impedance layer 15c.

[0022] The characteristic of this embodiment is that when the density of the first low acoustic impedance layer is set to ρ1 and the Young's modulus is set to E1, and the density of the second low acoustic impedance layer is set to ρ2 and the Young's modulus is set to E2, at least one of ρ1 > ρ2 and E1 > E2 is satisfied. Therefore, the acoustic impedance of the first low acoustic impedance layer and the acoustic impedance of the second low acoustic impedance layer are different from each other. That is, the sound reflector 4 is a composite component comprising low acoustic impedance layers with different acoustic impedances. When unwanted waves are transmitted to this sound reflector 4, the unwanted waves are easily scattered. Therefore, unwanted waves are difficult to reflect towards the piezoelectric layer 7. Accordingly, unwanted waves can be suppressed.

[0023] On the other hand, the dominant mode is most easily reflected at the interface between the low acoustic impedance layer and the high acoustic impedance layer, which is closest to the piezoelectric layer 7. Accordingly, the dominant mode can be appropriately reflected towards the piezoelectric layer 7 by means of the sound reflection film 4. Therefore, loss can be suppressed.

[0024] In this invention, the sound reflective film only needs to include at least one first low sound impedance layer, at least one second low sound impedance layer, and at least one high sound impedance layer.

[0025] However, in this embodiment, it is preferable that the sound reflective film 4 includes a plurality of high acoustic impedance layers. Accordingly, the interfaces between the low acoustic impedance layers and the high acoustic impedance layers increase. Thereby, even when the main mode passes through the interface closest to the piezoelectric layer 7 in the sound reflective film 4, the main mode can be reflected more reliably toward the piezoelectric layer 7.

[0026] On the other hand, unwanted waves pass through the interfaces in the sound-reflecting membrane 4 more easily than the dominant mode. Furthermore, the sound-reflecting membrane 4 includes a first low acoustic impedance layer and a second low acoustic impedance layer. Since the density or Young's modulus differs between the first and second low acoustic impedance layers, the speed of sound transmitted by the unwanted waves also differs. Therefore, even if a portion of the unwanted wave is reflected towards the piezoelectric layer 7 at multiple interfaces, the phases of the unwanted waves reflected at each interface are different from each other. Accordingly, the response caused by unwanted waves can be reduced more reliably.

[0027] In this embodiment, as described above, ρ1 > ρ2 or E1 > E2. Therefore, the acoustic impedance in the first low acoustic impedance layer 5a shown in FIG2 is higher than the acoustic impedance of the second low acoustic impedance layer 15b and the second low acoustic impedance layer 15c. Consequently, the Q value of the first low acoustic impedance layer 5a is high. Accordingly, mechanical damping is effectively suppressed in the first low acoustic impedance layer 5a.

[0028] Here, the concentration of elastic wave energy is higher closer to the piezoelectric layer 7. Furthermore, the first low acoustic impedance layer 5a, which has a high Q value, is the layer closest to the piezoelectric layer 7 in the sound-reflecting membrane 4. Accordingly, the loss of elastic waves within the layer where the elastic wave energy is particularly concentrated can be effectively reduced. Therefore, loss can be effectively suppressed in the elastic wave device 1.

[0029] The composition of the first embodiment will be explained in more detail below.

[0030] The elastic wave device 1 is configured as an elastic wave resonator that can utilize the bulk wave of the thickness-slip mode as the main mode. More specifically, in the elastic wave device 1, when the thickness of the piezoelectric layer 7 is set as d and the center-to-center distance between adjacent first electrode fingers 18 and second electrode fingers 19 is set as p, d / p is 0.5 or less. Accordingly, the bulk wave of the thickness-slip mode can be appropriately excited.

[0031] As shown in Figure 1, when viewed from the orthogonal direction of the electrode fingers, the area where adjacent first electrode fingers 18 and second electrode fingers 19 overlap is the intersection region F. The intersection region F contains a plurality of excitation regions C. More specifically, the region where adjacent first electrode fingers 18 and second electrode fingers 19 overlap in the orthogonal direction of the electrode fingers, and the region between the centers of adjacent first electrode fingers 18 and second electrode fingers 19, is the excitation region C. In addition, Figure 1 shows only two of the plurality of excitation regions C.

[0032] By applying an alternating voltage to the IDT electrode 8, bulk waves of the thickness-slip mode are excited in a plurality of excitation regions C. The propagation direction of the bulk waves of the thickness-slip mode is parallel to the thickness direction of the piezoelectric layer 7. Therefore, in the elastic wave device 1 that can utilize the bulk waves of the thickness-slip mode as the main mode, the configuration of reflecting elastic waves by means of the sound-reflecting membrane 4 shown in FIG2 is particularly suitable.

[0033] The material used as the low acoustic impedance layer in the sound reflector 4 can be, for example, a dielectric such as silicon oxide, or a metal such as aluminum or titanium.

[0034] For example, silicon oxide may also be used as the material for both the first low acoustic impedance layer and the second low acoustic impedance layer. In this case, it is sufficient to satisfy at least one of the following two relationships: The first relationship is that the density of silicon oxide used in the second low acoustic impedance layer is less than the density of silicon oxide used in the first low acoustic impedance layer. The second relationship is that the Young's modulus of silicon oxide used in the second low acoustic impedance layer is less than the Young's modulus of silicon oxide used in the first low acoustic impedance layer.

[0035] When silicon oxide is used as the material for the first low acoustic impedance layer and the second low acoustic impedance layer, these layers can be formed, for example, by sputtering or vacuum evaporation. In this case, it is sufficient to satisfy at least one of the above two relationships by making the conditions for forming the first low acoustic impedance layer different from the conditions for forming the second low acoustic impedance layer.

[0036] Of course, the first low acoustic impedance layer and the second low acoustic impedance layer may also be made of materials containing different elements.

[0037] The material used as the high acoustic impedance layer in the sound reflector 4 can be, for example, a dielectric such as aluminum nitride, silicon nitride, ytterbium oxide, hafnium oxide or tungsten oxide, or a metal such as platinum or tungsten.

[0038] All the low acoustic impedance layers and all the high acoustic impedance layers in the sound-reflecting film 4 may also be made of dielectric material. Alternatively, all the low acoustic impedance layers and all the high acoustic impedance layers in the sound-reflecting film 4, except for the low-density silicon oxide layer, may also be made of metal. Furthermore, either the low acoustic impedance layer or the high acoustic impedance layer may be made of dielectric material, and the other may be made of metal. Here, "layer" refers to an acoustic impedance layer.

[0039] The plurality of low acoustic impedance layers preferably include a low acoustic impedance layer made of silicon oxide. In this case, when the elastic wave device 1 is used as an elastic wave resonator in a filter device, the circuit configuration can be prevented from being configured to have conductivity in parallel with the elastic wave resonator. Of course, for example, the plurality of low acoustic impedance layers may also include a low acoustic impedance layer made of aluminum.

[0040] A plurality of high acoustic impedance layers are preferably high acoustic impedance layers comprising at least one of ytterbium oxide, hafnium oxide, and tungsten oxide as materials. In this case, when the elastic wave device 1 is used as an elastic wave resonator in a filter device, the circuit configuration can be prevented from being configured to have conductivity arranged in parallel with the elastic wave resonator.

[0041] Of course, for example, a plurality of high acoustic impedance layers may also include high acoustic impedance layers made of metals such as platinum or tungsten. In this case, the acoustic impedance ratio can be easily increased. The acoustic impedance ratio referred to in this specification 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 acoustic impedance layers and low acoustic impedance layers. Because the acoustic impedance ratio is large, the elastic wave can be effectively reflected towards the piezoelectric layer 7 by the sound reflecting film 4. As a result, the loss in the elastic wave device 1 can be reduced.

[0042] Preferably, in at least one group of adjacent high acoustic impedance layers and low acoustic impedance layers in the sound reflector 4, the acoustic impedance ratio is 6 or more. More preferably, in all adjacent high acoustic impedance layers and low acoustic impedance layers in the sound reflector 4, the acoustic impedance ratio is 6 or more. Accordingly, the degradation of the electrical characteristics of the main mode can be suppressed more reliably. For example, losses can be reduced in the elastic wave device 1.

[0043] As described above, in the first embodiment, the first low acoustic impedance layer is the layer closest to the piezoelectric layer 7 in the sound reflective film 4. This refers to the acoustic impedance layer. Furthermore, the arrangement of the first and second low acoustic impedance layers is not limited to the above.

[0044] For example, in the first modification of the first embodiment shown in FIG3, the second low acoustic impedance layer 15a is the layer closest to the piezoelectric layer 7 in the sound reflective film 4A. In this modification, the low acoustic impedance layers other than the second low acoustic impedance layer 15a are the first low acoustic impedance layers 5b and 5c. Similarly, in this case, unwanted waves can be scattered in the sound reflective film 4A. Accordingly, unwanted waves can be suppressed.

[0045] In the elastic wave device 1 of the first embodiment shown in FIG. 2, no dielectric film is provided on the first main surface 7a and the second main surface 7b of the piezoelectric layer 7. Alternatively, a dielectric film may be provided on at least one of the first main surface 7a and the second main surface 7b of the piezoelectric layer 7. In the second variation of the first embodiment shown in FIG. 4, a first dielectric film 9A is provided on the first main surface 7a of the piezoelectric layer 7 to cover the IDT electrode 8. In this case, the IDT electrode 8 is protected by the first dielectric film 9A. Therefore, the IDT electrode 8 is less likely to be damaged.

[0046] 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 sound-reflecting film 4. In this modified example, the piezoelectric substrate 2B includes a support substrate 3, a sound-reflecting film 4, a second dielectric film 9B, and a piezoelectric layer 7.

[0047] Materials for the first dielectric film 9A and the second dielectric film 9B may include, for example, silicon oxide, silicon nitride, or silicon oxynitride. Furthermore, the materials for 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 also be the same as the material of the low acoustic impedance layer or the high acoustic impedance layer. The material of the second dielectric film 9B is also the same.

[0048] In this modified example, similar to the first embodiment, unwanted waves can be scattered in the sound-reflecting membrane 4. Accordingly, unwanted waves can be suppressed. Furthermore, the configuration of having at least one of the first dielectric film 9A and the second dielectric film 9B in this modified example can also be applied to configurations of the present invention other than this modified example.

[0049] In this invention, the elastic wave device can also be configured to utilize plate waves as the main mode. In the third modification of the first embodiment shown in FIG5, the elastic wave device 1C is configured to utilize plate waves as the main mode. In this modification, a pair of reflectors 12A and 12B are provided on the first main surface 7a of the piezoelectric layer 7. More specifically, reflectors 12A and 12B face each other across the IDT electrode 8 in the orthogonal direction of the electrodes.

[0050] Reflector 12A has a pair of reflector buses 13a and 13b, and a plurality of reflector electrode fingers 14. Reflector buses 13a and 13b face each other. One end and the other end of each of the plurality of reflector electrode fingers 14 are short-circuited together by means of reflector buses 13a and 13b. Reflector 12B is configured the same as reflector 12A.

[0051] When the thickness of the piezoelectric layer 7 is set as d, and the center-to-center distance between adjacent first electrode fingers 18 and second electrode fingers 19 is set as p, it is preferable that d ≤ 2p. In this case, the plate wave is appropriately excited. In addition, by providing reflectors 12A and 12B, the resonance characteristics can be improved. Furthermore, when the plate wave is used as the main mode, the cross region F is the excitation region.

[0052] When the piezoelectric layer 7 is composed 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°. Accordingly, the Lamb wave of the A1 mode shown in FIG. 6(a), one type of plate wave, can be appropriately excited. The same applies when the piezoelectric layer 7 is composed of lithium tantalate.

[0053] When the piezoelectric layer 7 is composed of lithium niobate, the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 are preferably within the range of 90°±5°, 90°±5°, and 40°±20°. Accordingly, the Lamb wave of the S0 mode shown in FIG. 6(b), one type of plate wave, can be appropriately excited. The same applies when the piezoelectric layer 7 is composed of lithium tantalate.

[0054] In this modified example, the sound-reflecting membrane 4, as shown in FIG2, can scatter unwanted waves. Accordingly, similar to the first embodiment, unwanted waves can be suppressed.

[0055] In the first embodiment and its various modifications, the second main surface of the piezoelectric layer is located on the sound-reflecting membrane side. Of course, it is not limited to this. The second embodiment shows an example in which the first main surface of the piezoelectric layer is located on the sound-reflecting membrane side.

[0056] Figure 7 is a schematic front cross-sectional view of the elastic wave device of the second embodiment.

[0057] The difference between this embodiment and the first embodiment is that the first main surface 7a of the piezoelectric layer 7 and the second main surface 7b is located on the side of the sound reflecting membrane 4. The IDT electrode 8 is embedded in the sound reflecting membrane 4. Specifically, the IDT electrode 8 is embedded in the first low acoustic impedance layer 5a of the sound reflecting membrane 4. Apart from the above points, the elastic wave device 21 of this embodiment has the same configuration as the elastic wave device 1 of the first embodiment.

[0058] In this embodiment, similar to the first embodiment, unwanted waves can be scattered in the sound-reflecting membrane 4. Accordingly, unwanted waves can be suppressed.

[0059] In a configuration where the first main surface 7a of the piezoelectric layer 7 is located on the side of the sound-reflecting membrane 4, at least one of the first dielectric film 9A and the second dielectric film 9B shown in FIG. 4 may also be provided. For example, in the case where the first dielectric film 9A is provided on the first main surface 7a of the piezoelectric layer 7, the IDT electrode 8 may also be embedded in the sound-reflecting membrane 4. In addition, the term "IDT electrode embedded in sound-reflecting membrane" in this specification refers to a portion of the sound-reflecting membrane located between the electrode fingers of the IDT electrode.

[0060] Hereinafter, a preferred configuration of the present invention will be described using FIG1. ​​Of course, the preferred configuration shown below can also be applied to configurations of the present invention other than the first embodiment which utilizes the bulk wave of the thickness slip mode as the main mode.

[0061] In the first embodiment, when the thickness of the piezoelectric layer 7 is set as d and the distance between the centers of adjacent electrodes is set as p, d / p is 0.5 or less. Furthermore, it is preferable that d / p is 0.24 or less. Accordingly, in each excitation region C, the bulk wave of the thickness slip mode is more appropriately excited, and the fractional bandwidth of the elastic wave resonator is sufficiently large. When the resonant frequency is set as fr and the anti-resonant frequency is set as fa, the fractional bandwidth is expressed as (|fa-fr| / fr)×100 [%].

[0062] Figure 8 is a graph showing the relationship between d / p and part of the bandwidth of the elastic wave resonator.

[0063] As can be seen from Figure 8, when d / p > 0.5, the bandwidth of some frequencies is less than 5%. In contrast, when d / p ≦ 0.5, the bandwidth of some frequencies can reach more than 5%. Accordingly, the electromechanical coupling coefficient of the bulk wave in the thickness-slip mode can be increased. When d / p ≦ 0.24, the bandwidth of some frequencies can reach more than 7%. Accordingly, the electromechanical coupling coefficient of the bulk wave in the thickness-slip mode can be effectively increased.

[0064] When the metallization ratio of the electrode fingers relative to the excitation region C is set to MR, it is preferable to satisfy MR ≤ 1.75 (d / p) + 0.075. In this case, the value of a portion of the bandwidth of the elastic wave resonator is not too large, and spurious noise between the resonant frequency and the anti-resonant frequency can be suppressed. This detail is shown below.

[0065] Furthermore, in this specification, the metallization ratio MR of the electrode fingers relative to the excitation region C is the ratio of the portion of the piezoelectric layer 7 covered by the metal constituting the electrode fingers in the excitation region C when viewed from above. Specifically, the metallization ratio MR is the ratio of the area of ​​the first electrode finger 18 and the second electrode finger 19 within the excitation region C to the area of ​​the excitation region C when viewed from above. In addition, when the width of the electrode fingers located in the excitation region C is fixed, 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 along the orthogonal direction of the electrode fingers.

[0066] Figure 9 is a graph showing the relationship between a portion of the bandwidth in the elastic wave resonator and the magnitude of the normalized clutter. Figure 9 shows the results of measuring the phase rotation of the clutter by varying the thickness of the piezoelectric layer or the size of the electrode fingers, with each change in the portion of the bandwidth. Specifically, the magnitude of the normalized clutter in Figure 9 is the value of the phase rotation of the clutter impedance normalized by 180°. Although the results shown in Figure 9 are for a piezoelectric layer composed of Z-cut LiNbO3, the same trend is observed when using piezoelectric layers with other cut angles.

[0067] In the region surrounded by ellipse A in Figure 9, the normalized clutter magnitude between the resonant frequency and the anti-resonant frequency is 1.0. If a portion of the bandwidth of the elastic wave resonator exceeds 17%, the normalized clutter magnitude will be greater than 1.0. Therefore, the portion of the bandwidth is preferably less than 17%. Accordingly, clutter between the resonant frequency and the anti-resonant frequency can be suppressed.

[0068] Figure 10 is a graph showing the relationship between d / p, metallization ratio MR, and partial bandwidth. Figure 10 shows the results of calculating partial bandwidth for each different d / p and metallization ratio MR.

[0069] In Figure 10, the area indicated by the shaded line represents a region with a bandwidth of 17% or less. The boundary between the shaded region and the unshaded region can be roughly represented by the dashed line B. The dashed line B can be represented by MR = 1.75 (d / p) + 0.075. Moreover, it is preferable that MR ≤ 1.75 (d / p) + 0.075. In this case, it is easy to set the bandwidth to 17% or less.

[0070] On the other hand, the link line B1 in Figure 10 indicates that the slope of the change in the metallization ratio MR relative to the change in d / p is the same as that of the dashed line B, and that a portion of the bandwidth becomes the boundary of 17% or less across the entire range. The link line B1 can be represented by MR = 1.75 (d / p) + 0.05. Moreover, it is more preferably MR ≤ 1.75 (d / p) + 0.05. In this case, the portion of the bandwidth can be more reliably set to 17% or less.

[0071] Figure 11 is a diagram showing the distribution of a portion of the bandwidth relative to the Euler angles (0°, θ, ψ) of LiNbO3 when d / p approaches 0 infinitely. The portion indicated by the shaded lines in Figure 11 is the region where at least 5% of the bandwidth can be obtained. If the range of this region is approximated, it becomes the range that can be represented by the following equations (1), (2) and (3).

[0072] (0°±10°, 0°~20°, any ψ) …Equation (1) (0°±10°, 20°~80°, 0°~60°(1-(θ-50)2 / 900)1 / 2) or (0°±10°, 20°~80°, [180°-60°(1-(θ-50)2 / 900)1 / 2]~180°) …Equation (2) (0°±10°, [180°-30°(1​-(ψ-90)2 / 8100)1 / 2]~180°, any ψ) …Equation (3)

[0073] The Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 7 are preferably within the range of Equation (1), Equation (2), or Equation (3) above. Accordingly, a portion of the bandwidth of the elastic wave resonator can be sufficiently expanded. The same applies when the piezoelectric layer 7 is composed of lithium tantalate.

[0074] Hereinafter, examples of the form of the elastic wave device of the present invention will be summarized.

[0075] <1> An elastic wave device comprising: a sound-reflecting membrane comprising at least one high sound impedance layer having relatively high sound impedance and a plurality of low sound impedance layers having relatively low sound impedance; a piezoelectric layer disposed on the aforementioned sound-reflecting membrane, having a first main surface and a second main surface facing each other; and an IDT electrode disposed on the aforementioned first main surface of the aforementioned piezoelectric layer, having a plurality of electrode fingers; the aforementioned low sound impedance layers and the aforementioned high sound impedance layers are interleaved, the aforementioned plurality of low sound impedance layers comprising at least one first low sound impedance layer and at least one second low sound impedance layer, wherein when the density of the aforementioned first low sound impedance layer is set to ρ1 and the Young's modulus is set to E1, and the density of the aforementioned second low sound impedance layer is set to ρ2 and the Young's modulus is set to E2, at least one of ρ1>ρ2 and E1>E2 is satisfied.

[0076] <2> As described in <1>, the first low acoustic impedance layer is the layer closest to the piezoelectric layer in the aforementioned acoustic reflective film.

[0077] <3> As described in <1> or <2>, the aforementioned piezoelectric layer, specifically the first main surface and the second main surface, is located on the side of the aforementioned sound-reflecting membrane.

[0078] <4> As described in <1> or <2>, the first main surface of the aforementioned piezoelectric layer and the aforementioned second main surface are located on the side of the aforementioned sound reflection membrane, and the aforementioned IDT electrode is embedded in the aforementioned sound reflection membrane.

[0079] <5> The elastic wave device described in any of <1>~<4> further comprises: a dielectric film disposed on at least one of the aforementioned first main surface and the aforementioned second main surface of the piezoelectric layer.

[0080] <6> The elastic wave device described in any of <1> to <5> further includes a support substrate; the aforementioned sound-reflecting film is provided on the aforementioned support substrate.

[0081] <7> The elastic wave device described in any of <1> to <6>, wherein all of the aforementioned low acoustic impedance layer and all of the aforementioned high acoustic impedance layer are made of dielectric.

[0082] <8> The elastic wave device described in any of <1> to <6>, wherein either of the aforementioned low acoustic impedance layers and the aforementioned high acoustic impedance layers is a layer made of dielectric material, and the other is a layer made of metal.

[0083] <9> The elastic wave device described in any of <1> to <6>, wherein all of the aforementioned low acoustic impedance layer and all of the aforementioned high acoustic impedance layer are made of metal.

[0084] <10> The elastic wave device described in any of <1> to <8>, wherein the aforementioned plurality of low acoustic impedance layers comprises the aforementioned low acoustic impedance layers using silicon oxide as a material.

[0085] <11> The elastic wave device described in any of <1> to <8> or <10>, wherein the aforementioned sound reflecting film comprises the aforementioned high sound impedance layer using at least one of ytterbium oxide, hafnium oxide and tungsten oxide as a material.

[0086] <12> The elastic wave device described in any of <1> to <6>, <8> or <9>, wherein the aforementioned plurality of low acoustic impedance layers comprises the aforementioned low acoustic impedance layer using aluminum as the material.

[0087] <13> In any of the elastic wave devices described in <1> to <12>, when the sound impedance ratio is defined as the value obtained by dividing the sound impedance of the high sound impedance layer by the sound impedance of the low sound impedance layer in adjacent high sound impedance layers and low sound impedance layers, the sound impedance ratio is 6 or more in at least one group of adjacent high sound impedance layers and low sound impedance layers in the sound reflection film.

[0088] <14> The elastic wave device described in any of <1>~<13>, wherein when the thickness of the aforementioned piezoelectric layer is set as d and the distance between the centers of adjacent previously described electrodes is set as p, d / p is 0.5 or less.

[0089] <15> As described in <14>, the elastic wave device, wherein d / p is 0.24 or less.

[0090] <16> As described in <14> or <15>, when the direction orthogonal to the direction of extension of the aforementioned plurality of electrode fingers is defined as the electrode finger orthogonal direction, the region where adjacent previously described electrode fingers overlap each other in the aforementioned electrode finger orthogonal direction and the region between the centers of adjacent previously described electrode fingers is the excitation region, and when the metallization ratio of the aforementioned electrode fingers relative to the aforementioned excitation region is defined as MR, MR≦1.75(d / p)+0.075 is satisfied.

[0091] <17> As described in any of <1> to <16>, the aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are within the range of the following formulas (1), (2), or (3): (0°±10°, 0°~20°, any ψ) … formula (1); (0°±10°, 20°~80°, 0°~60°(1-(θ-50)2 / 900)1 / 2) or (0°±10°, 20°~80°, [180°-60°(1-(θ-50)2 / 900)1 / 2]~180°) … formula (2); (0°±10°, [180°-30°(1​-(ψ-90)2 / 8100)1 / 2]~180°, any ψ) …equation (3).

[0092] <18> As described in any of <1>~<13>, the aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are (within the range of 0°±5°, 30°±30°, and 0°±5°).

[0093] <19> As described in any of <1>~<13>, the aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are (within the range of 0°±5°, 0°±30°, and 90°±5°).

[0094] <20> The elastic wave device described in any of <1> to <13>, wherein the aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angle (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer is (within the range of 90°±5°, 90°±5°, and 40°±20°). [Simplified Explanation of the Diagram]

[0009] [Figure 1] Figure 1 is a schematic top view of the elastic wave device according to the first embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view along line I-I in Figure 1. [Figure 3] Figure 3 is a schematic front cross-sectional view of the elastic wave device according to the first modification of the first embodiment of the present invention. [Figure 4] Figure 4 is a schematic front cross-sectional view of the elastic wave device according to the second modification of the first embodiment of the present invention. [Figure 5] Figure 5 is a schematic top view of the elastic wave device according to the third modification of the first embodiment of the present invention. [Figure 6] Figure 6(a) is a schematic diagram showing the Lamb wave of the A1 mode. Figure 6(b) is a schematic diagram showing the Lamb wave of the S0 mode. [Figure 7] Figure 7 is a schematic front cross-sectional view of the elastic wave device according to the second embodiment of the present invention. [Figure 8] Figure 8 is a graph showing the relationship between d / p and a portion of the bandwidth of the elastic wave resonator. [Figure 9] Figure 9 shows the relationship between a portion of the bandwidth in the elastic wave resonator and the magnitude of the normalized clutter. [Figure 10] Figure 10 shows the relationship between d / p, metallization ratio (MR), and a portion of the bandwidth. [Figure 11] Figure 11 shows the distribution of a portion of the bandwidth relative to the Euler angles (0°, θ, ψ) of LiNbO3 when d / p approaches 0.

Claims

1. An elastic wave device comprising: a sound-reflecting membrane comprising at least one high sound impedance layer having relatively high sound impedance and a plurality of low sound impedance layers having relatively low sound impedance; a piezoelectric layer disposed on the sound-reflecting membrane, having a first main surface and a second main surface facing each other; and an IDT electrode disposed on the first main surface of the piezoelectric layer, having a plurality of electrode fingers; the aforementioned low sound impedance layers and the aforementioned high sound impedance layers being interlayered, the aforementioned plurality of low sound impedance layers comprising at least one first low sound impedance layer and at least one second low sound impedance layer, wherein when the density of the aforementioned first low sound impedance layer is set to ρ1 and the Young's modulus is set to E1, and the density of the aforementioned second low sound impedance layer is set to ρ2 and the Young's modulus is set to E2, at least one of ρ1 > ρ2 and E1 > E2 is satisfied.

2. The elastic wave device as claimed in claim 1, wherein, The aforementioned first low acoustic impedance layer is the layer closest to the aforementioned piezoelectric layer in the aforementioned sound reflective film.

3. The elastic wave device as claimed in claim 1, wherein, The aforementioned piezoelectric layer has its first main surface and its second main surface located on the side of the aforementioned sound-reflecting membrane.

4. The elastic wave device as claimed in claim 1, wherein, In the aforementioned piezoelectric layer, the first main surface is located on the side of the aforementioned sound-reflecting membrane, and the aforementioned IDT electrode is embedded in the aforementioned sound-reflecting membrane.

5. The elastic wave device of claim 1, further comprising: a dielectric film disposed on at least one of the aforementioned first main surface and the aforementioned second main surface of the piezoelectric layer.

6. The elastic wave device of claim 1, further comprising a support substrate; wherein the aforementioned sound-reflecting film is disposed on the aforementioned support substrate.

7. The elastic wave device as claimed in claim 1, wherein, All of the aforementioned low acoustic impedance layers and all of the aforementioned high acoustic impedance layers are made of dielectrics.

8. The elastic wave device as claimed in claim 1, wherein, One of the aforementioned low acoustic impedance layers and one of the aforementioned high acoustic impedance layers is a layer made of dielectric material, and the other is a layer made of metal.

9. The elastic wave device as claimed in claim 1, wherein, All of the aforementioned low acoustic impedance layers and all of the aforementioned high acoustic impedance layers are made of metal.

10. The elastic wave device as claimed in claim 1, wherein, The aforementioned plurality of low acoustic impedance layers include the aforementioned low acoustic impedance layers using silicon oxide as a material.

11. The elastic wave device as claimed in claim 1, wherein, The aforementioned sound-reflecting film includes the aforementioned high sound impedance layer using at least one of ytterbium oxide, hafnium oxide, and tungsten oxide as a material.

12. The elastic wave device as claimed in claim 1, wherein, The aforementioned plurality of low acoustic impedance layers include the aforementioned low acoustic impedance layers using aluminum as the material.

13. The elastic wave device as claimed in claim 1, wherein, When the sound impedance ratio is defined as the sound impedance obtained by dividing the sound impedance of the aforementioned high sound impedance layer by the sound impedance of the aforementioned low sound impedance layer in the adjacent high sound impedance layer and the aforementioned low sound impedance layer, the sound impedance ratio is 6 or more in at least one group of adjacent high sound impedance layers and the aforementioned low sound impedance layers in the aforementioned sound reflective film.

14. The elastic wave device as claimed in claim 1, wherein, When the thickness of the aforementioned piezoelectric layer is set as d, and the distance between the centers of adjacent previously mentioned electrodes is set as p, d / p is 0.5 or less.

15. The elastic wave device as claimed in claim 14, wherein, d / p is below 0.

24.

16. The elastic wave device as claimed in claim 14, wherein, When the direction orthogonal to the direction of extension of the aforementioned plurality of electrode fingers is defined as the electrode finger orthogonal direction, the region where adjacent previously mentioned electrode fingers overlap each other in the aforementioned electrode finger orthogonal direction and the region between the centers of adjacent previously mentioned electrode fingers is the excitation region. When the metallization ratio of the aforementioned electrode fingers relative to the aforementioned excitation region is defined as MR, MR≦1.75(d / p)+0.075 is satisfied.

17. The elastic wave device as claimed in claim 1, wherein, The aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate. The Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are within the range of the following equations (1), (2), or (3): (0°±10°, 0°~20°, any ψ) … Equation (1); (0°±10°, 20°~80°, 0°~60°(1-(θ-50)2 / 900)1 / 2) or (0°±10°, 20°~80°, [180°-60°(1-(θ-50)2 / 900)1 / 2]~180°) … Equation (2); (0°±10°, [180°-30°(1-(ψ-90)2 / 8100)1 / 2]~180°, any ψ) …Formula (3).

18. The elastic wave device as claimed in claim 1, wherein, The aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are within the range of 0°±5°, 30°±30°, and 0°±5°.

19. The elastic wave device as claimed in claim 1, wherein, The aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are within the range of 0°±5°, 0°±30°, and 90°±5°.

20. The elastic wave device as claimed in claim 1, wherein, The aforementioned piezoelectric layer is composed of lithium niobate or lithium tantalate, and the Euler angles (φ, θ, ψ) of the lithium niobate or lithium tantalate constituting the aforementioned piezoelectric layer are within the range of 90°±5°, 90°±5°, and 40°±20°.

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