Elastic wave device

Abstract

Provided is an elastic wave device capable of suppressing spurious waves in the vicinity of a resonant frequency.　An elastic wave device according to the present invention comprises: a support member; a piezoelectric layer 6 which is provided on the support member and which is made of lithium niobate; an IDT electrode 7 which is provided on the piezoelectric layer 6 and which has a plurality of electrode fingers (a plurality of first and second electrode fingers 18, 19) made of a material that contains aluminum as a main component; and a dielectric film 8 which is provided on the piezoelectric layer 6 so as to cover the IDT electrode 7 and which is made of a material that contains silicon nitride as a main component. In plan view, an acoustic reflection part is formed at a position in the support member so as to overlap the IDT electrode 7. A region between adjacent electrode fingers that does not include a portion where an electrode finger is provided is an inter-electrode finger region B. A region where an electrode finger is provided is an electrode finger coverage region M. When the average thickness value of the piezoelectric layer 6 in the entirety of the inter-electrode finger region B and the electrode finger coverage region M is d, and the center-to-center distance between adjacent electrode fingers is p, d / p is not more than 0.5. When the thickness of the dielectric film 8 in the inter-electrode finger region B is tb, the thickness of the piezoelectric layer 6 in the inter-electrode finger region B is db, the thickness of the piezoelectric layer 6 in the electrode finger coverage region M is dm, the thickness of an electrode finger is hm, and tb / db is a thickness ratio rb, and hm / dm is a thickness ratio rm, one of the relationships indicated by expressions (1) and (2) holds true.　Expression (1): rm < -1.1 rb + 0.78 Expression (2): rm > -0.7 rb +1.00

Description

Elastic Wave Device

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

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

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

[0004] However, in an elastic wave device such as that described in Patent Document 1, unwanted waves may occur. Examples of unwanted waves include longitudinal wave modes. The frequency at which longitudinal wave modes occur is easily affected by processing variations. Longitudinal wave modes as unwanted waves may occur near the resonant frequency of the main mode due to processing variations and other factors. This may result in degradation of the electrical characteristics of the elastic wave device.

[0005] An object of the present invention is to provide an acoustic wave device that can suppress unwanted waves near the resonant frequency.

[0006] An elastic wave device according to the present invention includes a support member, a piezoelectric layer made of lithium niobate and provided on the support member, an IDT electrode made of a material mainly containing aluminum and provided on the piezoelectric layer, and a dielectric film made of a material mainly containing silicon nitride and provided on the piezoelectric layer so as to cover the IDT electrode, wherein an acoustic reflecting portion is formed on the support member at a position overlapping with the IDT electrode in a plan view, and a region between adjacent electrode fingers that does not include a portion where the electrode fingers are provided is an inter-electrode finger region. where d is an average thickness of the piezoelectric layer in the inter-electrode-finger region and the entire electrode finger covered region, p is a center-to-center distance between adjacent electrode fingers, d / p is 0.5 or less, tb is a thickness of the dielectric film in the inter-electrode-finger region, db is a thickness of the piezoelectric layer in the inter-electrode-finger region, dm is a thickness of the piezoelectric layer in the electrode finger covered region, hm is a thickness of the electrode fingers, tb / db is a thickness ratio rb, and hm / dm is a thickness ratio rm, one of the following relationships holds: rm<-1.1rb+0.78...Equation (1) rm>-0.7rb+1.00...Equation (2)

[0007] According to an acoustic wave device according to a preferred embodiment of the present invention, unwanted waves can be suppressed near the resonant frequency.

[0008] 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 showing the vicinity of a pair of electrode fingers according to the first preferred embodiment of the present invention. FIG. 4 is a graph showing the relationship between the thickness ratio rb and the thickness ratio rm and the frequency ratio S1 / A1. FIG. 5 is a graph showing the relationship between the thickness ratio rm and the frequency ratio S1 / A1 when the thickness ratio rb is 0.2. FIG. 6 is a graph showing admittance-frequency characteristics for three different thickness ratios rm. 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 graph showing the relationship between d / p and the fractional bandwidth of an elastic wave resonator. FIG. 9 is a graph showing the relationship between the fractional bandwidth of an elastic wave resonator and the magnitude of normalized spurious signals. FIG. 10 is a graph showing the relationship between d / p, the metallization ratio MR, and the fractional bandwidth. FIG. 11 is a graph showing the relationship between d / p and the fractional bandwidth 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.

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

[0010] It should be noted that the embodiments described in this specification are merely examples, and partial substitution or combination of configurations is possible between different embodiments.

[0011] 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. Note that a dielectric film, which will be described later, is omitted from Fig. 1.

[0012] As shown in Fig. 1, the acoustic wave device 1 has a piezoelectric substrate 2 and an IDT electrode 7. The piezoelectric substrate 2 is a substrate having piezoelectric properties. As shown in Fig. 2, the piezoelectric substrate 2 has a support member 3 and a piezoelectric layer 6. In this embodiment, the support member 3 includes a support substrate 4 and an insulating layer 5. The insulating layer 5 is provided on the support substrate 4. The piezoelectric layer 6 is provided on the insulating layer 5. However, the support member 3 may be composed of only the support substrate 4.

[0013] The support substrate 4 may be made of a semiconductor such as silicon or a ceramic such as aluminum oxide. The insulating layer 5 may be made of an appropriate dielectric such as silicon oxide or tantalum oxide. The piezoelectric layer 6 may be made of LiNbO ３ In this specification, a member made of a material includes a member containing a trace amount of impurities that does not significantly degrade the electrical characteristics of the acoustic wave device.

[0014] The piezoelectric layer 6 has a first principal surface 6a and a second principal surface 6b. The first principal surface 6a and the second principal surface 6b face each other. Of the first principal surface 6a and the second principal surface 6b, the second principal surface 6b is located on the support member 3 side. An IDT electrode 7 is provided on the first principal surface 6a of the piezoelectric layer 6.

[0015] As shown in FIG. 1 , the IDT electrode 7 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 ends of the plurality of first electrode fingers 18 are connected to the first bus bar 16, respectively. One ends of the plurality of second electrode fingers 19 are connected to the second bus bar 17, respectively. 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.

[0016] The first electrode fingers 18 and the second electrode fingers 19 are made of a material whose main component is aluminum. In this specification, the term "main component" refers to a component that occupies more than 50% by weight of the material.

[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, a dielectric film 8 is provided on the first main surface 6a of the piezoelectric layer 6 so as to cover the IDT electrodes 7. The dielectric film 8 is made of a material containing silicon nitride as a main component.

[0019] In the present invention, a plurality of regions are defined. Returning to 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.

[0020] FIG. 3 is a schematic front cross-sectional view showing the vicinity of a pair of electrode fingers in the first embodiment.

[0021] The region between adjacent electrode fingers that does not include the portion where the electrode fingers are provided is the inter-electrode finger region B. On the other hand, the region where the electrode fingers are provided is the electrode finger covered region M. The inter-electrode finger region B is a region that includes the piezoelectric layer 6 and the dielectric film 8. The electrode finger covered region M is a region that includes the piezoelectric layer 6, the IDT electrode 7, and the dielectric film 8. On the other hand, the intersection region F and the excitation region C shown in FIG. 1 are regions of the piezoelectric layer 6 that are defined based on the configuration of the IDT electrode 7.

[0022] 3 , the thickness of the dielectric film 8 in the inter-electrode-finger region B is denoted by tb, and the thickness of the dielectric film 8 in the electrode-finger-covered region M is denoted by tm. The thickness of the piezoelectric layer 6 in the inter-electrode-finger region B is denoted by db, and the thickness of the piezoelectric layer 6 in the electrode-finger-covered region M is denoted by dm. The thickness of the electrode finger is denoted by hm.

[0023] In this embodiment, db = dm. The thicknesses db and dm of the piezoelectric layer 6 may be different from each other. However, it is preferable that the thicknesses db and dm of the piezoelectric layer 6 are substantially the same. In this case, the elastic wave device 1 is easier to manufacture, and the productivity of the elastic wave device 1 can be increased. In this specification, "the thickness of one portion and the thickness of another portion of the piezoelectric layer 6 are substantially the same" means that the difference in thickness between the two portions is 5% or less relative to the thickness of either portion.

[0024] In this embodiment, tb = tm. The thicknesses tb and tm of the dielectric film 8 may be different from each other. However, it is preferable that the thicknesses tb and tm of the dielectric film 8 are substantially the same. In this case, the elastic wave device 1 is easier to manufacture, and the productivity of the elastic wave device 1 can be increased. In this specification, "the thickness of one portion and the thickness of another portion of the dielectric film 8 are substantially the same" means that the difference in thickness between the two portions is 1% or less with respect to the thickness of either portion.

[0025] 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, when the average thickness of the piezoelectric layer 6 in the inter-electrode-finger region B and the entire electrode-finger covering region M is d and the center-to-center distance between adjacent first electrode fingers 18 and second electrode fingers 19 is p, d / p is 0.5 or less. This allows thickness-shear mode bulk waves to be suitably excited. Note that the elastic wave device according to the present invention may also be a filter device or a multiplexer having multiple elastic wave resonators.

[0026] Here, the ratio of the thickness tb of the dielectric film 8 to the thickness db of the piezoelectric layer 6 in the inter-electrode finger region B is defined as the thickness ratio rb, i.e., rb = tb / db. The ratio of the thickness hm of the electrode fingers to the thickness dm of the piezoelectric layer 6 in the electrode finger covered region M is defined as the thickness ratio rm, i.e., rm = hm / dm.

[0027] A feature of this embodiment is that it is configured to enable the use of thickness-shear bulk waves as the main mode, and one of the relationships shown in Equation (1) and Equation (2) below is established. This allows for the suppression of unwanted waves near the resonant frequency of the main mode. Note that in this specification, the resonant frequency refers to the resonant frequency of the main mode unless otherwise specified. The above effects are described in detail below.

[0028] rm<-1.1rb+0.78...Equation (1) rm>-0.7rb+1.00...Equation (2)

[0029] The thickness-shear bulk wave can be referred to as the shear wave mode. In an acoustic wave device that uses the shear wave mode as the main mode, the longitudinal wave mode is an unwanted wave. Conventionally, the frequency at which the longitudinal wave mode occurs has approached the frequency at which the shear wave mode occurs due to processing variations in the manufacturing process of an acoustic wave device. In response to this, the present inventors discovered that the ratio between the frequency at which the longitudinal wave mode occurs and the frequency at which the shear wave mode occurs depends on the relationship between the thickness ratio rb in the inter-electrode finger region B and the thickness ratio rm in the electrode finger covered region M.

[0030] The ratio of the frequency at which the longitudinal wave mode occurs to the frequency at which the transverse wave mode occurs is defined as the frequency ratio S1 / A1. The closer the value of S1 / A1 is to 1, the closer the frequency at which the longitudinal wave mode occurs to the frequency at which the transverse wave mode occurs. The closer the value of S1 / A1 is to 0 compared to 1, or the larger the value of S1 / A1 is compared to 1, the further apart the frequency at which the longitudinal wave mode occurs to the frequency at which the transverse wave mode occurs.

[0031] The relationship between the thickness ratio rb and the thickness ratio rm and the frequency ratio S1 / A1 was derived by simulation. The design parameters of the acoustic wave device studied were as follows. Note that, among the design parameters, the width of the electrode fingers is the dimension of the electrode fingers in the direction perpendicular to the electrode fingers.

[0032] Piezoelectric layer: Material... LiNbO ３, thickness db...390 nm, thickness dm...390 nm, average thickness d...390 nm IDT electrode: material...Al, center distance p...5.0 μm, electrode finger width...1.1 μm, thickness hm...varied in the range of more than 0 nm and not more than 585 nm. Dielectric film: material...SiN, thickness tb...varied in the range of more than 0 nm and not more than 195 nm. Ratio rb (=tb / db): varied in the range of more than 0 and not more than 0.5. Ratio rm (=hm / dm): varied in the range of more than 0 and not more than 1.5.

[0033] FIG. 4 is a diagram showing the relationship between the thickness ratio rb and the thickness ratio rm and the frequency ratio S1 / A1.

[0034] As shown in FIG. 4, it can be seen that the frequency ratio S1 / A1 depends on the thickness ratio rb and the thickness ratio rm. Here, when (S1 / A1) > 1.1 and when (S1 / A1) < 0.9, unwanted waves can be suppressed near the resonance frequency. Note that dashed line D1 in FIG. 4 indicates the boundary where (S1 / A1) ≥ 1.1 in all ranges. Dashed line D2 indicates the boundary where (S1 / A1) ≤ 0.9 in all ranges. In FIG. 4, when the region is below dashed line D1 and when the region is above dashed line D2, unwanted waves can be suppressed near the resonance frequency.

[0035] The broken line D1 can be expressed as an equation: rm = -1.1rb + 0.78. The broken line D2 can be expressed as an equation: rm = -0.7rb + 1.00. Therefore, when one of the relationships in equation (1) and equation (2) is established, it is possible to suppress unwanted waves at the resonant frequency. A specific example of this is shown below.

[0036] rm<-1.1rb+0.78...Equation (1) rm>-0.7rb+1.00...Equation (2)

[0037] 5 is a diagram showing the relationship between the thickness ratio rm and the frequency ratio S1 / A1 when the thickness ratio rb is 0.2. The range surrounded by a frame E in FIG. 5 is the range where 0.9≦(S1 / A1)≦1.1.

[0038] Under the condition indicated by the dashed-dotted line G1, rm = 0.125 and rb = 0.2. Under this condition, (S1 / A1) > 1.1. Therefore, the condition indicated by the dashed-dotted line G1 is within the range of formula (1), which is below the dashed line D1 in FIG. 4.

[0039] Under the condition indicated by the dashed-dotted line G2 in Fig. 5, rm = 0.625 and rb = 0.2. Under this condition, 0.9 ≤ (S1 / A1) ≤ 1.1. Therefore, the condition indicated by the dashed-dotted line G2 is a condition in the region between the dashed lines D1 and D2 in Fig. 4. In other words, this condition is outside the range of formulas (1) and (2).

[0040] Under the condition indicated by the dashed-dotted line G3 in Fig. 5, rm = 1.25 and rb = 0.2. Under this condition, (S1 / A1) < 0.9. Therefore, the condition indicated by the dashed-dotted line G3 is within the range of formula (2), which is above the dashed line D2 in Fig. 4.

[0041] Under the conditions indicated by the dashed dotted line G1, dashed dotted line G2, and dashed dotted line G3, the admittance frequency characteristics were derived by FEM (Finite Element Method) simulation.

[0042] 6 is a diagram showing the admittance frequency characteristics for three thickness ratios rm. Note that the symbols G1 to G3 in FIG. 6 correspond to the symbols G1 to G3 indicated by the dashed dotted lines in FIG. 5.

[0043] As described above, rm = 0.625 and rb = 0.2, and the condition indicated by the dashed-dotted line G2 in Fig. 5 is outside the range of formulas (1) and (2). In this case, as shown by the area surrounded by the two-dot chain line in Fig. 6, a strong unwanted wave is generated near the resonance frequency. This unwanted wave is a longitudinal wave mode.

[0044] On the other hand, the condition indicated by the dashed-dotted line G1 in Fig. 5 where rm = 0.125 and rb = 0.2 is within the range of formula (1). In this case, as shown in Fig. 6, no strong unwanted waves are generated near the resonance frequency.

[0045] rm=1.25, rb=0.2, and the condition indicated by the dashed dotted line G3 in Fig. 5 is within the range of formula (2). In this case, as shown in Fig. 6, no strong unwanted waves are generated near the resonance frequency.

[0046] As described above, when either Equation (1) or Equation (2) is satisfied, it is possible to suppress unwanted waves near the resonant frequency. Note that when the coefficient of the thickness ratio rb in Equation (1) is expressed to two decimal places, it is -1.10. When the coefficient of the thickness ratio rb in Equation (2) is expressed to two decimal places, it is -0.70.

[0047] Returning to FIG. 4 , when (S1 / A1) > 1.2 and when (S1 / A1) < 0.8, unwanted waves can be further suppressed near the resonant frequency. Note that dashed line D3 in FIG. 4 indicates the boundary where (S1 / A1) > 1.2 in all ranges. Dashed line D4 indicates the boundary where (S1 / A1) < 0.8 in all ranges. In the region below dashed line D3 or above dashed line D4 in FIG. 4 , unwanted waves can be further suppressed near the resonant frequency.

[0048] The broken line D3 can be expressed as rm = -0.95rb + 0.60. The broken line D4 can be expressed as rm = -0.77rb + 1.25. It is preferable that one of the following equations (3) and (4) holds. This makes it possible to further suppress unwanted waves near the resonant frequency.

[0049] rm<-0.95rb+0.60...Equation (3) rm>-0.77rb+1.25...Equation (4)

[0050] The configuration of this embodiment will be described in more detail below.

[0051] As shown in FIG. 2 , a recess is provided in the insulating layer 5. A piezoelectric layer 6 is provided on the insulating layer 5 so as to close the recess. This forms a hollow portion. This hollow portion is the cavity 2a. In the first embodiment, the support member 3 and the piezoelectric layer 6 are arranged so that a portion of the support member 3 and a portion of the piezoelectric layer 6 face each other with the cavity 2a in between. However, the recess in the support member 3 may be provided across the insulating layer 5 and the support substrate 4. Alternatively, a recess provided only in the support substrate 4 may be closed by the insulating layer 5. The recess may be provided in the piezoelectric layer 6, for example. The cavity 2a may be a through-hole provided in the support member 3.

[0052] In plan view, at least a portion of the IDT electrode 7 overlaps with the cavity 2a. The cavity 2a is the acoustic reflecting portion of the present invention. The acoustic reflecting portion can reflect the elastic wave toward the piezoelectric layer 6. This allows the energy of the elastic wave to be effectively confined to the piezoelectric layer 6. It is preferable that the multiple excitation regions C overlap with the acoustic reflecting portion in plan view. This allows the energy of the elastic wave to be more reliably and effectively confined to the piezoelectric layer 6.

[0053] The acoustic reflecting portion is not limited to the cavity 2a. Another example of an acoustic reflecting portion in an acoustic wave device will be described in a second embodiment.

[0054] FIG. 7 is a schematic front cross-sectional view of an elastic wave device according to a second preferred embodiment of the present invention.

[0055] This embodiment differs from the first embodiment in that the acoustic reflection portion is an acoustic reflection film 25. This embodiment also differs from the first embodiment in that the support member 23 is formed only from a support substrate. Except for the above points, the elastic wave device of this embodiment has the same configuration as the elastic wave device 1 of the first embodiment.

[0056] An acoustic reflection film 25 is provided on the surface of the support member 23. A piezoelectric layer 6 is provided on the acoustic reflection film 25. The support member 23 and the piezoelectric layer 6 may be arranged such that at least a portion of the support member 23 and at least a portion of the piezoelectric layer 6 face each other with the acoustic reflection film 25 sandwiched therebetween.

[0057] The acoustic reflecting film 25 is a laminate of multiple acoustic impedance layers. Specifically, the acoustic reflecting film 25 has multiple low acoustic impedance layers and multiple high acoustic impedance layers. 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 the adjacent layers in the acoustic reflecting film 25. More specifically, the multiple low acoustic impedance layers in the acoustic reflecting film 25 are low acoustic impedance layer 26a, low acoustic impedance layer 26b, and low acoustic impedance layer 26c.

[0058] 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 the adjacent layers in the acoustic reflecting film 25. More specifically, the multiple high acoustic impedance layers in the acoustic reflecting film 25 are high acoustic impedance layers 27a and 27b. The low acoustic impedance layers and high acoustic impedance layers are alternately stacked. Note that the low acoustic impedance layer 26a is the acoustic impedance layer located closest to the piezoelectric layer 6 in the acoustic reflecting film 25.

[0059] The acoustic reflection film 25 has three low acoustic impedance layers and two high acoustic impedance layers, although it is sufficient that the acoustic reflection film 25 has at least one low acoustic impedance layer and one high acoustic impedance layer.

[0060] The low acoustic impedance layer may be made of a dielectric material such as silicon oxide or a metal such as aluminum or titanium, while the high acoustic impedance layer may be made of a dielectric material such as aluminum nitride, silicon nitride, or hafnium oxide or a metal such as platinum or tungsten.

[0061] A preferred configuration of the present invention will be described below with reference to Figures 1 and 3. However, the preferred configuration described below can also be applied to configurations of the present invention other than the first embodiment.

[0062] In the first embodiment, when the average thickness of the piezoelectric layer 6 in the inter-electrode finger region B and the entire electrode finger covering region M shown in FIG. 3 is d and the center-to-center distance between adjacent electrode fingers is p, 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 shown in FIG. 1, and enables the value of the fractional bandwidth of the elastic wave resonator to be sufficiently large. The fractional bandwidth is expressed by (|fa-fr| / fr)×100[%], where fr is the resonant frequency and fa is the antiresonant frequency.

[0063] FIG. 8 is a graph showing the relationship between d / p and the bandwidth ratio of an elastic wave resonator.

[0064] 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.

[0065] 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.

[0066] 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 6 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.

[0067] 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.

[0068] In the region surrounded by ellipse H in Fig. 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.

[0069] 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.

[0070] 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 J. Dashed line J 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 achieve a fractional bandwidth of 17% or less.

[0071] On the other hand, the dashed-dotted line J1 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 J, and where the fractional bandwidth is 17% or less over the entire range. The dashed-dotted line J1 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.

[0072] 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 frequency band 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 (5), (6), and (7).

[0073] (0°±10°, 0° to 20°, any ψ) ... Equation (5) (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°) ...Equation (6) (0°±10°, [180°-30°(1-(ψ-90) ２ / 8100) １／２ ] to 180°, any ψ) ...Equation (7)

[0074] It is preferable that the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer are within the range of the above formula (5), formula (6), or formula (7), thereby making it possible to sufficiently widen the fractional bandwidth of the elastic wave resonator.

[0075] In the following, examples of the configuration of the acoustic wave device according to the present invention will be described together.

[0076] <1> A piezoelectric element comprising: a support member; a piezoelectric layer made of lithium niobate and provided on the support member; an IDT electrode having a plurality of electrode fingers made of a material mainly containing aluminum and provided on the piezoelectric layer; and a dielectric film made of a material mainly containing silicon nitride and provided on the piezoelectric layer so as to cover the IDT electrode, wherein, in a plan view, an acoustic reflection portion is formed in the support member at a position overlapping with the IDT electrode, and an inter-electrode finger region is a region between adjacent electrode fingers that does not include a portion where the electrode fingers are provided, an elastic wave device in which a region where electrode fingers are provided is an electrode finger covered region, and where d is an average thickness of the piezoelectric layer in the inter-electrode finger region and the entire electrode finger covered region, and p is a center-to-center distance between adjacent electrode fingers, d / p is 0.5 or less, and where tb is a thickness of the dielectric film in the inter-electrode finger region, db is a thickness of the piezoelectric layer in the inter-electrode finger region, dm is a thickness of the piezoelectric layer in the electrode finger covered region, and hm is a thickness ratio rb, tb / db is a thickness ratio rm, and hm / dm is a thickness ratio rm, one of the relationships in the following formula (1) and formula (2) is established: rm<-1.1rb+0.78...formula (1) rm>-0.7rb+1.00...formula (2)

[0077] <2> The elastic wave device according to <1>, wherein one of the following relationships, formula (3) and formula (4), is satisfied: rm<-0.95rb+0.60 (formula (3)) rm>-0.77rb+1.25 (formula (4))

[0078] <3> The acoustic wave device according to <1> or <2>, wherein a thickness db of the piezoelectric layer in the inter-electrode-finger region and a thickness dm of the piezoelectric layer in the electrode-finger-covered region are substantially the same.

[0079] <4> An elastic wave device described in any one of <1> to <3>, wherein when the thickness of the dielectric film in the electrode finger covering region is tm, the thickness tb of the dielectric film in the inter-electrode finger region and the thickness tm of the dielectric film in the electrode finger covering region are approximately the same.

[0080] <5> The acoustic wave device according to any one of <1> to <4>, wherein d / p is 0.24 or less.

[0081] <6> An elastic wave device described in any one of <1> to <5>, wherein the acoustic reflection portion is a hollow portion, and the support member and the piezoelectric layer are arranged so that a portion of the support member and a portion of the piezoelectric layer face each other across the hollow portion.

[0082] <7> An elastic wave device according to any one of <1> to <5>, wherein the acoustic reflection portion is an acoustic reflection film including a high acoustic impedance layer having a relatively high acoustic impedance and a low acoustic impedance layer having a relatively low acoustic impedance, and the support member and the piezoelectric layer are arranged so that at least a portion of the support member and at least a portion of the piezoelectric layer face each other with the acoustic reflection film in between.

[0083] <8> An elastic wave device according to any one of <1> to <7>, 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.

[0084] <9> The acoustic wave device according to any one of <1> to <8>, wherein the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer are within the range of the following formula (5), formula (6), or formula (7): (0°±10°, 0° to 20°, any ψ) ... formula (5) (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°) ...Equation (6) (0°±10°, [180°-30°(1-(ψ-90) ２ / 8100) １／２ ] to 180°, any ψ) ...Equation (7)

[0085] REFERENCE SIGNS LIST 1...Acoustic wave device 2...Piezoelectric substrate 2a...Cavity 3...Support member 4...Support substrate 5...Insulating layer 6...Piezoelectric layer 6a, 6b...First and second principal surfaces 7...IDT electrode 8...Dielectric film 16, 17...First and second bus bars 18, 19...First and second electrode fingers 23...Support member 25...Acoustic reflection film 26a to 26c...Low acoustic impedance layers 27a, 27b...High acoustic impedance layers B...Inter-electrode finger region C...Excitation region F...Crossing region M...Electrode finger covering region

Claims

1. A piezoelectric element comprising: a support member; a piezoelectric layer formed on the support member and made of lithium niobate; an IDT electrode formed on the piezoelectric layer and having a plurality of electrode fingers made of a material primarily composed of aluminum; and a dielectric film formed on the piezoelectric layer so as to cover the IDT electrode and made of a material primarily composed of silicon nitride; wherein an acoustic reflecting portion is formed on the support member at a position overlapping with the IDT electrode in a planar view; a region between adjacent electrode fingers that does not include the portion where the electrode fingers are provided is an inter-electrode finger region, and a region where the electrode fingers are provided is an electrode finger covered region; where d is the average thickness of the piezoelectric layer in the inter-electrode finger region and the entire electrode finger covered region, and p is the center-to-center distance between adjacent electrode fingers, d / p is 0.5 or less; an elastic wave device in which one of the following relationships between formula (1) and formula (2) holds, where tb is the thickness of the dielectric film in the inter-electrode finger region, db is the thickness of the piezoelectric layer in the inter-electrode finger region, dm is the thickness of the piezoelectric layer in the electrode finger covered region, hm is the thickness of the electrode fingers, tb / db is the thickness ratio rb, and hm / dm is the thickness ratio rm: rm<-1.1rb+0.78 (1) rm>-0.7rb+1.00 (2) 2. The acoustic wave device according to claim 1, wherein one of the following relationships, formula (3) and formula (4), is satisfied: rm<-0.95rb+0.60 (formula (3)) rm>-0.77rb+1.25 (formula (4)) 3. The acoustic wave device according to claim 1 or 2, wherein the thickness db of the piezoelectric layer in the inter-electrode-finger region and the thickness dm of the piezoelectric layer in the electrode-finger-covered region are substantially the same.

4. An elastic wave device according to any one of claims 1 to 3, wherein when the thickness of the dielectric film in the electrode finger covering region is tm, the thickness tb of the dielectric film in the inter-electrode finger region and the thickness tm of the dielectric film in the electrode finger covering region are approximately the same.

5. The elastic wave device according to any one of claims 1 to 4, wherein d / p is 0.24 or less.

6. An elastic wave device according to any one of claims 1 to 5, wherein the acoustic reflection portion is a hollow portion, and the support member and the piezoelectric layer are arranged so that a portion of the support member and a portion of the piezoelectric layer face each other across the hollow portion.

7. An elastic wave device according to any one of claims 1 to 5, wherein the acoustic reflection portion is an acoustic reflection film including a high acoustic impedance layer with a relatively high acoustic impedance and a low acoustic impedance layer with a relatively low acoustic impedance, and the support member and the piezoelectric layer are arranged so that at least a portion of the support member and at least a portion of the piezoelectric layer face each other with the acoustic reflection film in between.

8. An elastic wave device according to any one of claims 1 to 7, wherein, when a direction perpendicular to the direction in which the plurality of electrode fingers extend is defined as an electrode finger perpendicular direction, the region where 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 MR, MR≦1.75(d / p)+0.075 is satisfied.

9. The acoustic wave device according to any one of claims 1 to 8, wherein the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer are within the range of the following formula (5), formula (6), or formula (7): (0°±10°, 0° to 20°, any ψ) ... formula (5) (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°) ...Equation (6) (0°±10°, [180°-30°(1-(ψ-90) ２ / 8100) １／２ ] to 180°, any ψ) ...Equation (7)

Images