Filter device

Abstract

Provided is a filter device with which it is possible to reduce loss without lowering electric power resistance.　A filter device according to the present invention comprises a plurality of elastic wave resonators each having a support member 8, a piezoelectric film that is provided on the support member 8 and includes a piezoelectric layer 5, and an IDT electrode 11 that is provided on the piezoelectric film and has a plurality of electrode fingers. The plurality of elastic wave resonators include at least one series arm resonator and at least one parallel arm resonator P1a. In each of the plurality of elastic wave resonators, an acoustic reflection part (cavity part 9a) is provided in the support member 8 at a position overlapping the IDT electrode 11 as seen in plan view, and d / p is 0.5 or greater, where d is the thickness of the piezoelectric film, and p is the distance between the centers of adjacent electrode fingers in the IDT electrode 11. The number of the plurality of electrode fingers is lowest in the parallel arm resonator, among the plurality of elastic wave resonators, and the number of the plurality of electrode fingers in the parallel arm resonator is lower than the number of the plurality of electrode fingers in any series arm resonator.

Description

Filter Device

[0001] The present invention relates to a filter device having a plurality of acoustic wave resonators.

[0002] Conventionally, filter devices having acoustic wave resonators have been widely used in mobile phones, etc. In recent years, a filter device having multiple resonators using a horizontal shear acoustic mode has been proposed, as described in Patent Document 1 listed below. In this filter device, the multiple resonators include series arm resonators and shunt resonators.

[0003] Each of the multiple resonators in Patent Document 1 is configured by providing an IDT (Interdigital Transducer) on a piezoelectric plate. The IDT has multiple fingers. The overlap dimension between adjacent fingers is represented by a distance AP called the "aperture." Adjacent fingers are connected to different potentials. By applying an AC voltage between adjacent fingers, bulk waves in a thickness-shear mode are excited as a horizontal shear acoustic mode.

[0004] Special Publication No. 2021-527344

[0005] The filter device described in Patent Document 1 may not be able to reduce loss and prevent a decrease in power durability at the same time. In conventional surface acoustic wave resonators, loss is improved by lowering the electrical resistance of the IDT. Specifically, as described in Patent Document 1, loss is improved by increasing the number of fingers and shortening the distance AP. In this case, the shorter distance AP improves heat dissipation and power durability.

[0006] However, the inventors have found that applying the above-described configuration to an elastic wave resonator that uses thickness-shear bulk waves as its main mode actually increases loss, making it difficult to achieve both loss reduction and suppression of a decrease in power durability for the entire filter device.

[0007] An object of the present invention is to provide a filter device that can reduce loss without reducing power durability.

[0008] A filter device according to the present invention includes a support member, a plurality of acoustic wave resonators each including a piezoelectric film provided on the support member and including a piezoelectric layer, and an IDT electrode provided on the piezoelectric film and having a plurality of electrode fingers, the plurality of acoustic wave resonators including at least one series arm resonator and at least one parallel arm resonator, each of the plurality of acoustic wave resonators having an acoustic reflecting portion provided on the support member at a position overlapping with the IDT electrode in a planar view, wherein d is a thickness of the piezoelectric film and p is a center-to-center distance between adjacent electrode fingers in the IDT electrode, and d / p is 0.5 or less, and the number of the plurality of electrode fingers in the parallel arm resonator is the smallest among the plurality of acoustic wave resonators, and the number of the plurality of electrode fingers in the parallel arm resonator is smaller than the number of the plurality of electrode fingers in any of the series arm resonators.

[0009] According to the filter device of the present invention, loss can be reduced without reducing power durability.

[0010] FIG. 1 is a circuit diagram of a filter device according to a first embodiment of the present invention. FIG. 2 is a schematic plan view of a filter device according to the first embodiment of the present invention. FIG. 3 is a schematic plan view of an acoustic wave resonator according to the first embodiment of the present invention. FIG. 4 is a diagram illustrating the relationship between the number of electrode fingers, the overlap width, and the impedance-frequency characteristics. FIG. 5 is a diagram illustrating the relationship between the number of electrode fingers, the overlap width, and the return loss. FIG. 6 is a diagram illustrating the vicinity of a gap between an electrode finger and a bus bar in the first reference example. FIG. 7 is a diagram illustrating the vicinity of a gap between an electrode finger and a bus bar in the second reference example. FIG. 8 is a diagram illustrating the impedance-frequency characteristics in the first and second reference examples. FIG. 9 is a diagram illustrating the return loss in the first and second reference examples. FIG. 10 is a schematic diagram illustrating the attenuation-frequency characteristics of each acoustic wave resonator in the filter device before and after a temperature increase. FIG. 11 is a schematic cross-sectional view taken along line II in FIG. 3. FIG. 12 is a schematic cross-sectional view taken along line II-II in FIG. 3. Fig. 13 is a schematic front cross-sectional view of an elastic wave resonator according to a second embodiment of the present invention. Fig. 14 is a diagram showing the relationship between d / p and the relative bandwidth of an elastic wave resonator. Fig. 15 is a diagram showing the relationship between the relative bandwidth of an elastic wave resonator and the magnitude of normalized spurious signals. Fig. 16 is a diagram showing the relationship between d / p, the metallization ratio MR, and the relative bandwidth. Fig. 17 is a diagram showing the relationship between d / p and the relative 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.

[0011] The present invention will be clarified below by describing specific embodiments of the present invention with reference to the drawings.

[0012] 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.

[0013] FIG. 1 is a circuit diagram of a filter device according to a first embodiment of the present invention.

[0014] The filter device 1 is a ladder-type filter. The filter device 1 has a first signal terminal 2, a second signal terminal 3, and a plurality of acoustic wave resonators. In this embodiment, the second signal terminal 3 is an antenna terminal. The antenna terminal is connected to an antenna.

[0015] The plurality of acoustic wave resonators include a plurality of series arm resonators and a plurality of parallel arm resonators. In this embodiment, the plurality of series arm resonators specifically include a series arm resonator S1a, a series arm resonator S1b, a series arm resonator S2, a series arm resonator S3, and a series arm resonator S4. The plurality of parallel arm resonators specifically include a parallel arm resonator P1a, a parallel arm resonator P1b, a parallel arm resonator P2a, a parallel arm resonator P2b, a parallel arm resonator P3a, a parallel arm resonator P3b, a parallel arm resonator P4a, a parallel arm resonator P4b, a parallel arm resonator P4c, and a parallel arm resonator P4d.

[0016] In the circuit configuration, a series arm connects the first signal terminal 2 and the second signal terminal 3. Each series arm resonator is arranged in the series arm. On the other hand, multiple parallel arms connect the series arm and the ground potential. Each parallel arm resonator is arranged in the parallel arm.

[0017] Fig. 2 is a schematic plan view of the filter device according to the first embodiment, in which an IDT electrode (described later) is shown as a rectangle with two diagonal lines added.

[0018] The filter device 1 has a piezoelectric substrate 9. The piezoelectric substrate 9 is a substrate having piezoelectricity. Specifically, the piezoelectric substrate 9 is a laminated substrate including a piezoelectric film. The piezoelectric film includes at least a piezoelectric layer 5. The piezoelectric layer 5 is a layer made of a piezoelectric material. On the other hand, in this specification, a piezoelectric film is a film having piezoelectricity, and does not necessarily refer to a film made of a piezoelectric material. However, in this embodiment, the piezoelectric film is a single-layer piezoelectric layer 5, which is a film made of a piezoelectric material. It should be noted that in the present invention, the piezoelectric film may be a laminated film including the piezoelectric layer 5.

[0019] The piezoelectric layer 5 is made of, for example, LiNbO ３ Alternatively, the material may be made of lithium niobate such as LiTaO ３In this embodiment, the piezoelectric layer 5 is made of lithium niobate. In this specification, a certain member being made of a certain material includes a case where a small amount of impurity is contained to the extent that the electrical characteristics of the elastic wave resonator are not significantly deteriorated.

[0020] A plurality of IDT electrodes 11 are provided on the piezoelectric layer 5 serving as a piezoelectric film. This forms a plurality of acoustic wave resonators, each having the IDT electrode 11. In this embodiment, the acoustic wave resonators share the same piezoelectric substrate 9. However, each acoustic wave resonator may have its own individual piezoelectric substrate 9.

[0021] A first signal terminal 2, a second signal terminal 3, and a plurality of ground terminals 4 are provided on the piezoelectric substrate 9. The ground terminal 4 is a terminal connected to the ground potential. Each of the above terminals is configured as an electrode pad. However, each of the above terminals may also be configured as wiring.

[0022] Fig. 3 is a schematic plan view of an elastic wave resonator according to the first preferred embodiment. Note that other elastic wave resonators are omitted from Fig. 3. Fig. 3 shows a portion of the wiring, and the remaining portion of the wiring is omitted.

[0023] The acoustic wave resonator shown in FIG. 3 is a parallel arm resonator P1a. The IDT electrode 11 of the parallel arm resonator P1a 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, and one ends of the plurality of second electrode fingers 19 are 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 11 may be made of a single-layer metal film or a laminated metal film.

[0024] Hereinafter, the first electrode fingers 18 and the second electrode fingers 19 may be collectively referred to simply as electrode fingers. The first bus bar 16 and the second bus bar 17 may be collectively referred to simply as bus bars. 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.

[0025] The first electrode fingers 18 and the second bus bars 17 face each other across a gap. The second electrode fingers 19 and the first bus bars 16 face each other across a gap. When the dimension of these gaps along the direction in which the electrode fingers extend is defined as an IB-gap, in this embodiment, the IB-gap in each gap is the same.

[0026] The parallel arm resonator P1a has an intersection region F. The intersection region F is a region where adjacent first electrode fingers 18 and second electrode fingers 19 overlap in the direction perpendicular to the electrode fingers. Hereinafter, the dimension of the intersection region F along the electrode finger extension direction will be referred to as the intersection width.

[0027] The intersection region F includes a plurality of excitation regions C. More specifically, the excitation region C is a region where adjacent first electrode fingers 18 and second electrode fingers 19 overlap in the direction perpendicular to the electrode fingers, and is a region between the centers of the adjacent first electrode fingers 18 and second electrode fingers 19. Note that only two of the plurality of excitation regions C are shown in FIG. 3 .

[0028] The parallel arm resonator P1a is an elastic wave resonator configured to utilize thickness-shear mode bulk waves as the main mode. By applying an AC voltage to the IDT electrode 11, thickness-shear mode bulk waves are excited in each excitation region C. More specifically, in the parallel arm resonator P1a, when the thickness of the piezoelectric film is d and the center-to-center distance between the adjacent first electrode finger 18 and second electrode finger 19 is p, d / p is 0.5 or less. This allows thickness-shear mode bulk waves to be suitably excited in each excitation region C. In this embodiment, the thickness d is the thickness of the piezoelectric layer 5.

[0029] 1 are also elastic wave resonators configured to utilize thickness-shear mode bulk waves. Each of the elastic wave resonators of the filter device 1 also has a crossover region and a plurality of excitation regions.

[0030] The multiple acoustic wave resonators of the filter device 1 have different design parameters depending on the desired characteristics. However, in each of the multiple acoustic wave resonators, when the thickness of the piezoelectric film is d and the center-to-center distance between adjacent electrode fingers in the IDT electrode 11 is p, d / p is 0.5 or less. This allows thickness-shear mode bulk waves to be suitably excited in each acoustic wave resonator.

[0031] This embodiment is characterized by the following configuration: 1) The multiple acoustic wave resonators are configured to use thickness-shear bulk waves as the main mode. 2) Among the multiple acoustic wave resonators, a parallel arm resonator has the smallest number of electrode fingers, and the number of electrode fingers in the parallel arm resonator is smaller than the number of electrode fingers in any of the series arm resonators. More specifically, for example, the number of electrode fingers in only one parallel arm resonator may be the smallest number among the multiple acoustic wave resonators. Alternatively, for example, two parallel arm resonators may have the same number of electrode fingers, and the number of electrode fingers in the parallel arm resonator may be the smallest number among the multiple acoustic wave resonators.

[0032] The filter device 1 having the above configuration can reduce loss without reducing the power durability of the entire filter device 1. This will be described below.

[0033] In an acoustic wave resonator using thickness-shear bulk waves as the main mode, the impedance frequency characteristics and return loss were measured for each number of electrode fingers. By varying the number of electrode fingers and the overlap width, the capacitance was kept almost constant.

[0034] The design parameters of the acoustic wave resonator in this study are as follows: Note that, among the parameters of the IDT electrode, the width of the electrode fingers is the dimension of the electrode fingers along the direction perpendicular to the electrode fingers.

[0035] Number of electrode fingers: 137, 69 or 35; Intersection width: 26.5 μm, 53 μm or 106.1 μm; Center-to-center distance between adjacent electrode fingers: 4.26 μm; Electrode finger width: 0.97 μm; Duty ratio: 0.228; IB-gap: 5 μm; Piezoelectric layer: Material... LiNbO ３ , thickness...370 nm

[0036] Fig. 4 is a diagram showing the relationship between the number of electrode fingers, the overlap width, and the impedance frequency characteristics, and Fig. 5 is a diagram showing the relationship between the number of electrode fingers, the overlap width, and the return loss.

[0037] As shown in Fig. 4, in each of the acoustic wave resonators studied, the resonant frequency is located near 4800 MHz, and the antiresonant frequency is located near 5250 MHz to 5350 MHz. As shown in Fig. 5, between the resonant frequency and the antiresonant frequency, the fewer the number of electrode fingers, the smaller the absolute value of the return loss. In addition, the wider the crossover width, the smaller the absolute value of the return loss.

[0038] From these results, it can be seen that in an elastic wave resonator that uses thickness-shear bulk waves as the main mode, the fewer the electrode fingers, the lower the loss. Furthermore, it can be seen that the wider the crossover width, the lower the loss. This reduction in loss is due to the suppression of acoustic wave leakage in the electrode finger extension direction.

[0039] More specifically, the greater the gaps between the electrode fingers and the bus bars, the more likely the acoustic wave leakage in the electrode finger extension direction is. The fewer the number of electrode fingers, the smaller the gap. The wider the crossing width, the smaller the ratio of the IB-gap to the crossing width. These factors suppress the acoustic wave leakage in the electrode finger extension direction. Below, an example in which the acoustic wave leakage in the electrode finger extension direction is suppressed and the return loss is reduced is shown by comparing the first and second reference examples.

[0040] Fig. 6 is a diagram showing the vicinity of gaps between electrode fingers and bus bars in a first reference example. Fig. 7 is a diagram showing the vicinity of gaps between electrode fingers and bus bars in a second reference example. In Fig. 7, through holes, which will be described later, are indicated by cross-hatching.

[0041] 6, no through-hole is provided in the piezoelectric layer 105 at a portion located in the gap between the electrode fingers 109 and the bus bar 106. In the second reference example shown in FIG. 7, a through-hole 105c is provided in the piezoelectric layer 105 at a portion located in the gap between the electrode fingers 109 and the bus bar 106.

[0042] In the first and second reference examples, the impedance frequency characteristics and the return loss were measured.

[0043] Fig. 8 is a diagram showing impedance frequency characteristics in the first and second reference examples, and Fig. 9 is a diagram showing return loss in the first and second reference examples.

[0044] As shown in Fig. 8, in the first and second reference examples, the resonant frequency is located near 4240 MHz, and the anti-resonant frequency is located near 4580 MHz. As shown in Fig. 9, the absolute value of the return loss in the second reference example is smaller than the absolute value of the return loss in the first reference example between the resonant frequency and the anti-resonant frequency. This shows that the loss is reduced as the portion of the piezoelectric layer 105 located in the gap between the electrode fingers 109 and the bus bar 106 becomes smaller.

[0045] However, reducing the number of electrode fingers requires a wider crossover width to obtain the desired capacitance. For example, when power is applied to an IDT electrode 11 as shown in FIG. 3, acoustic waves are excited and heat is generated. At this time, the amount of heat generated is particularly large near the center of the excitation region C in the electrode finger extension direction. When the crossover width is widened, the distance from the center of the excitation region C in the electrode finger extension direction to the bus bar increases. This tends to reduce heat dissipation. As a result, when power is applied, excessive temperature rise occurs locally, which may make the electrode fingers and other components more susceptible to damage. This may result in a reduced power durability.

[0046] In contrast, in this embodiment, the parallel arm resonator has the fewest number of electrode fingers among the multiple acoustic wave resonators in the filter device 1. The parallel arm resonators are connected to the ground potential. Specifically, in this embodiment, as shown in FIG. 2 , each parallel arm resonator is connected to the ground terminal 4. Therefore, heat is propagated from the parallel arm resonator to the outside via the wiring and the ground terminal 4. Therefore, even if the crossover width of the parallel arm resonator is required to be wide, the heat dissipation performance can be improved. Therefore, in this embodiment, loss can be reduced without degrading the power handling capability.

[0047] Additionally, in general, the temperature coefficient of frequency (TCF) of an acoustic wave resonator is often a negative value. In this embodiment, the TCF of each of the acoustic wave resonators in the filter device 1 is a negative value. This makes it difficult for the power durability of the filter device 1 to decrease. This will be described in detail below.

[0048] 10 is a schematic diagram showing the attenuation-frequency characteristics of each acoustic wave resonator in a filter device before and after a temperature increase. In FIG. 10, arrow L indicates the lowest frequency in the pass band of the filter device. Arrow H indicates the highest frequency in the pass band of the filter device. The pass band here refers to the frequency band of signals that are intended to be passed by the filter device, and does not necessarily coincide with the band in which the insertion loss of the filter device is small.

[0049] When the TCF of an acoustic wave resonator is negative, when power is applied to the acoustic wave resonator and the temperature of the acoustic wave resonator rises, the resonant frequency and other parameters decrease. Therefore, when the temperature of each acoustic wave resonator in the filter device increases, the band with low insertion loss in the filter device shifts toward the lower frequency side. Even in this state, the frequency indicated by arrow L is located in the band with low insertion loss. On the other hand, the frequency indicated by arrow H is located in the band with high insertion loss. When power is applied in the band with high insertion loss, the power is easily converted into heat.

[0050] When power is applied near the frequency indicated by the arrow H, heat is likely to be generated in each elastic wave resonator. Furthermore, the frequency indicated by the arrow H is often located near an intermediate frequency between the resonant frequency and the antiresonant frequency of the series arm resonator. Elastic wave resonators are particularly likely to generate heat when power is applied at a frequency intermediate between the resonant frequency and the antiresonant frequency.

[0051] On the other hand, the parallel arm resonator constitutes the low-frequency side of the band in which the insertion loss of the filter device is small. The frequency indicated by arrow H is higher than the resonant frequency of the parallel arm resonator. Therefore, even if power is applied near the frequency indicated by arrow H, the amount of heat generated in the parallel arm resonator is smaller than the amount of heat generated in the series arm resonator. Even if power is applied near the frequency indicated by arrow L, the power is unlikely to be converted into heat.

[0052] Therefore, even if the crossover width needs to be widened in the parallel arm resonator having the fewest number of electrode fingers in this embodiment, the power handling capability of the parallel arm resonator is unlikely to be reduced, and therefore, the loss of the filter device 1 as a whole can be reduced without a more reliable reduction in power handling capability.

[0053] The number of electrode fingers in each parallel arm resonator is preferably smaller than the number of electrode fingers in each series arm resonator. In other words, the largest number of electrode fingers in all parallel arm resonators is preferably smaller than the smallest number of electrode fingers in all series arm resonators. In this case, a decrease in power durability can be more reliably suppressed.

[0054] The configuration of this embodiment will be described in more detail below.

[0055] 1, the series arm resonators S1a, S2, S3, and S4 are connected in series between the first signal terminal 2 and the second signal terminal 3. Specifically, in terms of the circuit configuration, the series arm resonators S1a, S2, S3, and S4 are arranged in this order from the first signal terminal 2 side. The series arm resonators S1a and S1b are connected in parallel between the first signal terminal 2 and the series arm resonator S2.

[0056] The parallel arm resonators P1a and P1b are connected in parallel with each other between the connection point between the series arm resonators S1a and S2 and the ground potential. The parallel arm resonators P2a and P2b are connected in parallel with each other between the connection point between the series arm resonators S2 and S3 and the ground potential. The parallel arm resonators P3a and P3b are connected in parallel with each other between the connection point between the series arm resonators S3 and S4 and the ground potential. The parallel arm resonators P4a, P4b, P4c, and P4d are connected in parallel with each other between the second signal terminal 3 and the ground potential.

[0057] The circuit configuration of the filter device 1 is not limited to the above. The plurality of acoustic wave resonators in the filter device 1 may include at least one series arm resonator and at least one parallel arm resonator.

[0058] 1 , the filter device 1 of the first embodiment is a ladder-type filter. However, for example, the filter device of the present invention may also include a longitudinally coupled resonator filter. In this case, it is sufficient that the filter device includes at least one series arm resonator and at least one parallel arm resonator as the multiple acoustic wave resonators.

[0059] Fig. 11 is a schematic cross-sectional view taken along line II in Fig. 3. Fig. 12 is a schematic cross-sectional view taken along line II-II in Fig. 3. Fig. 12 shows a portion of the wiring, with the remaining portion of the wiring omitted.

[0060] 11 and 12 , the piezoelectric substrate 9 includes a support member 8 and a piezoelectric layer 5 as a piezoelectric film. In this embodiment, the support member 8 includes a support substrate 7 and an insulating layer 6. The insulating layer 6 is provided on the support substrate 7. The piezoelectric layer 5 is provided on the insulating layer 6. However, the support member 8 may be composed of only the support substrate 7.

[0061] The material of the support substrate 7 can be, for example, a semiconductor such as silicon, or a ceramic such as aluminum oxide. The material of the insulating layer 6 can be an appropriate dielectric such as silicon oxide or tantalum oxide.

[0062] The piezoelectric layer 5 has a first main surface 5a and a second main surface 5b. The first main surface 5a and the second main surface 5b face each other. Of the first main surface 5a and the second main surface 5b, the second main surface 5b is located on the support member 8 side. The first main surface 5a and the second main surface 5b of the piezoelectric layer 5 are the first main surface and the second main surface of the piezoelectric film in this embodiment.

[0063] A recess is provided in the insulating layer 6. A piezoelectric layer 5 serving as a piezoelectric film is provided on the insulating layer 6 so as to close the recess. This forms a hollow portion. This hollow portion is the cavity 9a. In this embodiment, the support member 8 and the piezoelectric film are arranged so that a portion of the support member 8 and a portion of the piezoelectric film face each other with the cavity 9a in between. However, the recess in the support member 8 may be provided across the insulating layer 6 and the support substrate 7. Alternatively, a recess provided only in the support substrate 7 may be closed by the insulating layer 6. The recess may be provided in the piezoelectric layer 5, for example. The cavity 9a may be a through-hole provided in the support member 8.

[0064] In a plan view, at least a portion of the IDT electrode 11 overlaps the hollow portion 9a of the support member 8. In this specification, a plan view refers to a view from a direction corresponding to the top in FIG. 11 along the stacking direction of the support member 8 and the piezoelectric film. In FIG. 11, for example, of the support substrate 7 side and the piezoelectric layer 5 side, the piezoelectric layer 5 side is the top. Furthermore, in this specification, a plan view is synonymous with a view from the principal surface opposing direction. The principal surface opposing direction is the direction in which the first principal surface 5a and the second principal surface 5b of the piezoelectric layer 5 face each other. More specifically, the principal surface opposing direction is, for example, the normal direction of the first principal surface 5a.

[0065] The cavity 9a shown in Figures 11 and 12 is the acoustic reflecting portion of the present invention. The acoustic reflecting portion can reflect the elastic wave toward the piezoelectric layer 5. This allows the energy of the elastic wave to be effectively confined to the piezoelectric layer 5. It is preferable that the multiple excitation regions C overlap with the acoustic reflecting portion in a plan view. This allows the energy of the elastic wave to be more reliably and effectively confined to the piezoelectric layer 5.

[0066] In this embodiment, in each of the plurality of acoustic wave resonators, an acoustic reflecting portion is provided on the support member 8 at a position that overlaps with the IDT electrode 11 in a plan view.

[0067] As shown in Fig. 12, a wiring electrode 15 is provided on a first bus bar 16. Similarly, a wiring electrode 15 is provided on a second bus bar 17. The acoustic wave resonators are connected to each other via the wiring electrodes 15. However, only a portion of each wiring electrode 15 is shown in Fig. 3 above. The thickness of the wiring electrode 15 is greater than the thickness of the bus bar and the thickness of the electrode fingers. This allows the electrical resistance of the wiring electrode 15 to be suitably low.

[0068] In the first embodiment, the acoustic reflecting portion is a cavity 9 a. However, the acoustic reflecting portion is not limited to the cavity 9 a. Another example of an acoustic reflecting portion in an acoustic wave resonator of a filter device is shown in a second embodiment.

[0069] FIG. 13 is a schematic front cross-sectional view of an elastic wave resonator according to the second preferred embodiment.

[0070] The circuit configuration of the filter device according to this embodiment is the same as that of the first embodiment. The acoustic wave resonator shown in FIG. 13 is a parallel arm resonator P21a. This parallel arm resonator P21a corresponds to the parallel arm resonator P1a in the circuit configuration shown with reference to FIG. 1. The present embodiment shown in FIG. 13 differs from the first embodiment in that the acoustic reflection portion is an acoustic reflection film 24. The present embodiment also differs from the first embodiment in that the support member 28 is formed only by a support substrate. Apart from the above, the filter device according to this embodiment has the same configuration as the filter device 1 according to the first embodiment.

[0071] An acoustic reflection film 24 is provided on the surface of a support member 28. A piezoelectric layer 5 serving as a piezoelectric film is provided on the acoustic reflection film 24. The support member 28 and the piezoelectric film may be arranged such that at least a portion of the support member 28 and at least a portion of the piezoelectric film face each other with the acoustic reflection film 24 in between.

[0072] The acoustic reflecting film 24 is a laminate of multiple acoustic impedance layers. Specifically, the acoustic reflecting film 24 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 24. More specifically, the multiple low acoustic impedance layers in the acoustic reflecting film 24 are low acoustic impedance layer 26a, low acoustic impedance layer 26b, and low acoustic impedance layer 26c.

[0073] 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 24. More specifically, the multiple high acoustic impedance layers in the acoustic reflecting film 24 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 5 in the acoustic reflecting film 24.

[0074] The acoustic reflection film 24 has three low acoustic impedance layers and two high acoustic impedance layers, although it is sufficient that the acoustic reflection film 24 has at least one low acoustic impedance layer and one high acoustic impedance layer.

[0075] 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.

[0076] Similar to the parallel arm resonator P21a, the parallel arm resonators and the series arm resonators other than the parallel arm resonator P21a also have acoustic reflection films 24. The parallel arm resonators and the series arm resonators may share the same acoustic reflection film 24. Alternatively, the parallel arm resonators and the series arm resonators may each have their own individual acoustic reflection films 24.

[0077] In the second embodiment, the plurality of acoustic wave resonators are also configured to use thickness-shear bulk waves as the main mode. Among the plurality of acoustic wave resonators, the parallel arm resonator has the fewest number of electrode fingers, which is smaller than the number of electrode fingers in any of the series arm resonators. This allows for reduced loss in the entire filter device without reducing power handling capability, as in the first embodiment.

[0078] In the present invention, the acoustic reflecting portion of at least one elastic wave resonator may be the acoustic reflecting film 24. Similarly, the acoustic reflecting portion of at least one elastic wave resonator may be the cavity 9a shown in FIG. 11. For example, the acoustic reflecting portions of a plurality of elastic wave resonators may be the acoustic reflecting film 24, and the acoustic reflecting portions of a plurality of other elastic wave resonators may be the cavity 9a shown in FIG. 11. Alternatively, the acoustic reflecting portions of all the elastic wave resonators may be the acoustic reflecting film 24. On the other hand, the acoustic reflecting portions of all the elastic wave resonators may be the cavity 9a shown in FIG. 11.

[0079] A preferred configuration of an elastic wave resonator according to the present invention will be described below with reference to Fig. 3. However, the following preferred configuration can also be applied to configurations of the present invention other than the first embodiment.

[0080] In each elastic wave resonator according to the first embodiment, where d is the thickness of the piezoelectric layer 5 and p is the center-to-center distance between adjacent electrode fingers, d / p is 0.5 or less. Preferably, d / p is 0.24 or less. This allows thickness-shear mode bulk waves to be more effectively excited and enables the elastic wave resonator to have a sufficiently large fractional bandwidth. The fractional bandwidth is expressed as (|fa-fr| / fr)×100[%], where fr is the resonant frequency and fa is the antiresonant frequency.

[0081] FIG. 14 is a graph showing the relationship between d / p and the fractional bandwidth of an elastic wave resonator.

[0082] As is clear from FIG. 14 , when d / p > 0.5, the fractional bandwidth is less than 5%. In contrast, when d / p ≦ 0.5, the fractional bandwidth can be 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 7% or more. This effectively increases the electromechanical coupling coefficient of the thickness-shear mode bulk wave. For these reasons, it is preferable that d / p ≦ 0.24 be satisfied for at least one acoustic wave resonator in the filter device of the present invention. It is more preferable that d / p ≦ 0.24 be satisfied for all acoustic wave resonators.

[0083] 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.

[0084] In this specification, the metallization ratio MR of the electrode fingers to the excitation region C is the ratio of the portion of the piezoelectric layer 5 covered with the metal constituting the electrode fingers to the excitation region C 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 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. The width of the electrode fingers is the dimension of the electrode fingers in the direction perpendicular to the electrode fingers.

[0085] Fig. 15 is a diagram showing the relationship between the bandwidth ratio and the normalized magnitude of spurious in an elastic wave resonator. Fig. 15 shows the results of measuring the amount of phase rotation of spurious every time the bandwidth ratio is changed by changing the thickness of the piezoelectric layer and the dimensions of the electrode fingers. The normalized magnitude of spurious in Fig. 15 is specifically the value obtained by normalizing the amount of phase rotation of the spurious impedance by 180°. The results shown in Fig. 15 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.

[0086] In the region surrounded by ellipse D in Fig. 15, 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.

[0087] 16 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.

[0088] In Figure 16, 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 E. Dashed line E 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.

[0089] On the other hand, the dashed-dotted line E1 in FIG. 16 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 E, and where the fractional bandwidth is 17% or less across the entire range. The dashed-dotted line E1 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 to 17% or less. For these reasons, in at least one acoustic wave resonator in the filter device of the present invention, MR ≦ 1.75(d / p) + 0.075 is preferable, and MR ≦ 1.75(d / p) + 0.05 is even more preferable. In all acoustic wave resonators, MR ≦ 1.75(d / p) + 0.075 is even more preferable, and MR ≦ 1.75(d / p) + 0.05 is even more preferable.

[0090] FIG. 17 shows the results of LiNbO when d / p approaches 0. ３ 17 is a diagram showing a map of fractional bandwidths with respect to Euler angles (0°, θ, ψ) of the frequency band of the optical fiber 10. The hatched area in FIG. 17 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 to the ranges expressed by the following formulas (1), (2), and (3).

[0091] (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)

[0092] It is preferable that the Euler angles (φ, θ, ψ) of the lithium niobate constituting the piezoelectric layer 5 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 5 is made of lithium tantalate.

[0093] Examples of the configuration of the filter device according to the present invention will be summarized below.

[0094] <1> A filter device comprising: a support member; a plurality of elastic wave resonators each including: a piezoelectric film provided on the support member and including a piezoelectric layer; and an IDT electrode provided on the piezoelectric film and having a plurality of electrode fingers, wherein the plurality of elastic wave resonators include at least one series arm resonator and at least one parallel arm resonator; each of the plurality of elastic wave resonators has an acoustic reflecting portion provided on the support member at a position overlapping with the IDT electrode in a planar view; wherein, when a thickness of the piezoelectric film is d and a center-to-center distance between adjacent electrode fingers in the IDT electrode is p, d / p is 0.5 or less; and among the plurality of elastic wave resonators, the parallel arm resonator has the fewest number of the plurality of electrode fingers, and the number of the plurality of electrode fingers in the parallel arm resonator is smaller than the number of the plurality of electrode fingers in any of the series arm resonators.

[0095] <2> The filter device according to <1>, wherein the number of the plurality of electrode fingers in each of the parallel arm resonators is smaller than the number of the plurality of electrode fingers in each of the series arm resonators.

[0096] <3> The filter device according to <1> or <2>, wherein d / p is 0.24 or less in at least one acoustic wave resonator.

[0097] <4> A filter device according to any one of <1> to <3>, 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 in which adjacent electrode fingers overlap each other in the electrode finger perpendicular direction and a region between the centers of adjacent electrode fingers is defined as an excitation region, and when a metallization ratio of the electrode fingers to the excitation region is defined as MR, at least one of the elastic wave resonators satisfies MR≦1.75(d / p)+0.075.

[0098] <5> A filter device described in any one of <1> to <4>, wherein in at least one of the elastic wave resonators, the acoustic reflection portion is a cavity portion, and the support member and the piezoelectric film are arranged so that a part of the support member and a part of the piezoelectric film face each other across the cavity portion.

[0099] <6> A filter device according to any one of <1> to <4>, wherein in at least one of the elastic wave resonators, 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 film are arranged so that at least a portion of the support member and at least a portion of the piezoelectric film face each other with the acoustic reflection film in between.

[0100] <7> The filter device according to any one of <1> to <6>, 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)

[0101] REFERENCE SIGNS LIST 1...Filter device 2, 3...First and second signal terminals 4...Ground terminal 5...Piezoelectric layer 5a, 5b...First and second main surfaces 6...Insulating layer 7...Support substrate 8...Support member 9...Piezoelectric substrate 9a...Cavity 11...IDT electrode 15...Wiring electrode 16, 17...First and second bus bars 18, 19...First and second electrode fingers 24...Acoustic reflection film 26a to 26c...Low acoustic impedance layers 27a, 27b...High acoustic impedance layers 28...Support member 105...Piezoelectric layer 105c...Through hole 106...Bus bar 109...Electrode fingers C...Excitation region F...Crossing region P1a, P1b, P2a, P2b, P3a, P3b, P4a to P4d, P21a...Parallel arm resonator S1a, S1b, S2 to S4...series arm resonators

Claims

1. A filter device comprising: a support member; a plurality of elastic wave resonators each having a piezoelectric film provided on the support member and including a piezoelectric layer; and an IDT electrode provided on the piezoelectric film and having a plurality of electrode fingers; the plurality of elastic wave resonators including at least one series arm resonator and at least one parallel arm resonator; each of the plurality of elastic wave resonators having an acoustic reflector provided on the support member at a position overlapping with the IDT electrode in a planar view; where d is the thickness of the piezoelectric film and p is the center-to-center distance between adjacent electrode fingers in the IDT electrode, d / p is 0.5 or less; and among the plurality of elastic wave resonators, the parallel arm resonator has the fewest number of the plurality of electrode fingers, and the number of the plurality of electrode fingers in the parallel arm resonator is smaller than the number of the plurality of electrode fingers in any of the series arm resonators.

2. The filter device according to claim 1, wherein the number of the plurality of electrode fingers in each of the parallel arm resonators is smaller than the number of the plurality of electrode fingers in each of the series arm resonators.

3. The filter device according to claim 1 or 2, wherein d / p is 0.24 or less in at least one elastic wave resonator.

4. A filter device according to any one of claims 1 to 3, 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 defined as MR, at least one of the elastic wave resonators satisfies MR≦1.75(d / p)+0.

075.

5. A filter device according to any one of claims 1 to 4, wherein in at least one of the elastic wave resonators, the acoustic reflection portion is a cavity, and the support member and the piezoelectric film are arranged so that a portion of the support member and a portion of the piezoelectric film face each other across the cavity.

6. A filter device according to any one of claims 1 to 4, wherein in at least one of the elastic wave resonators, 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 film are arranged so that at least a portion of the support member and at least a portion of the piezoelectric film face each other with the acoustic reflection film in between.

7. A filter device according to any one of claims 1 to 6, 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)

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