Ladder Filters and Multiplexers

Abstract

To provide a ladder type filter which improves pass characteristics, and a multiplexer comprising the same.SOLUTION: A ladder type filter comprises: a support substrate 10; a plurality of acoustic wave resonators each including a piezoelectric layer 14 provided on the support substrate 10 and a pair of interdigital electrodes each provided on the piezoelectric layer 14 and formed from a metal film 16; one or more series resonators S1 to S4 connected in series between an input terminal Tin and an output terminal Tout and included in the plurality of acoustic wave resonators; and one or more parallel resonators P1 to P4 of which one end is connected to a path between the input terminal Tin and the output terminal Tout and the other end is ground and which are included in the plurality of acoustic wave resonators. A thickness T4b of the piezoelectric layer 14 in at least one parallel resonator P is equal to or less than an average pitch Db of electrode fingers 18 of the pair of interdigital electrodes in at least one parallel resonator and smaller than a thickness T4a of the piezoelectric layer 14 in at least one series resonator S in the one or more series resonators S1 to S4.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a ladder-type filter and a multiplexer, for example, a ladder-type filter and a multiplexer having a pair of comb-type electrodes. [Background technology]

[0002] An elastic wave resonator having a pair of comb-shaped electrodes is known as an elastic wave resonator used in communication devices such as smartphones. It is known that when a piezoelectric layer forming an elastic wave resonator is bonded to a support substrate, the thickness of the piezoelectric layer is set to be equal to or less than the wavelength of the elastic wave (for example, Patent Document 1). It is known that the sum of the thickness of a temperature compensation film provided between the piezoelectric layer and the support substrate and the thickness of the piezoelectric layer is set to be equal to or less than twice the wavelength of the elastic wave (for example, Patent Document 2). It is known that the piezoelectric layer of an elastic wave resonator having a large elastic wave wavelength is made thicker than the piezoelectric layer of an elastic wave resonator having a small elastic wave wavelength (for example, Patent Documents 3 and 4). It is known that the thicknesses of the piezoelectric layers on the support substrate are made different between a transmitting filter and a receiving filter (for example, Patent Document 5). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-34363 A [Patent Document 2] JP 2019-201345 A [Patent Document 3] JP 2018-93487 A [Patent Document 4] Special Publication No. 2018-506930 [Patent Document 5] JP 2019-186655 A [Patent Document 6] JP 2015-73331 A [Patent Document 7] JP 2017-152868 A Summary of the Invention [Problem to be solved by the invention]

[0004] By making the thickness of the piezoelectric layer equal to or less than the wavelength of the elastic wave, it is possible to suppress spurious responses due to unwanted waves such as bulk waves. However, in a ladder filter, if a spurious response occurs within the passband, the pass characteristic is deteriorated.

[0005] The present invention has been made in consideration of the above-mentioned problems, and has an object to improve the passing characteristics. [Means for solving the problem]

[0006] The present invention is a ladder-type filter including: a plurality of acoustic wave resonators each including a support substrate, a piezoelectric layer provided on the support substrate, and a pair of comb electrodes provided on the piezoelectric layer; one or more series resonators included in the plurality of acoustic wave resonators and connected in series between an input terminal and an output terminal; and one or more parallel resonators included in the plurality of acoustic wave resonators, each having one end connected to a path between the input terminal and the output terminal and the other end grounded, wherein a thickness of the piezoelectric layer in at least one parallel resonator is equal to or less than an average pitch of electrode fingers of the pair of comb electrodes in the at least one parallel resonator and is smaller than a thickness of the piezoelectric layer in at least one of the one or more series resonators.

[0007] In the above configuration, the piezoelectric layer in the at least one parallel resonator may be thinner than the piezoelectric layers in all of the one or more series resonators.

[0008] In the above configuration, the thicknesses of the piezoelectric layers in the one or more series resonators may be substantially equal.

[0009] In the above configuration, the piezoelectric layer in each of the one or more parallel resonators may be thinner than the piezoelectric layer in each of the one or more series resonators.

[0010] In the above configuration, the piezoelectric layer in the one or more series resonators may have a substantially equal thickness, and the piezoelectric layer in the one or more parallel resonators may have a substantially equal thickness.

[0011] In the above configuration, an average pitch of electrode fingers of the pair of comb electrodes in the at least one parallel resonator can be larger than an average pitch of electrode fingers of the pair of comb electrodes in other parallel resonators among the one or more parallel resonators, and the piezoelectric layer in the at least one parallel resonator can be thinner than the piezoelectric layer in the other parallel resonators.

[0012] In the above configuration, the piezoelectric layer in the at least one parallel resonator may have a substantially equal thickness, and the piezoelectric layer in the other parallel resonator and the one or more series resonators may have a substantially equal thickness.

[0013] In the above configuration, the thickness of the piezoelectric layer in each of the acoustic wave resonators may be 1.4 times or less the average pitch.

[0014] In the aforementioned configuration, the plurality of acoustic wave resonators may include an insulating layer provided between the piezoelectric layer and the support substrate.

[0015] In the above configuration, the multiple elastic wave resonators can be configured to have a temperature compensation film provided between the piezoelectric layer and the support substrate, the temperature compensation film having a sign of a temperature coefficient of an elastic constant that is opposite to the sign of the temperature coefficient of an elastic constant of the piezoelectric layer, and in at least one parallel resonator, the distance between a surface of the temperature compensation film facing the support substrate and a surface of the piezoelectric layer facing the pair of comb electrodes can be configured to be less than twice the average pitch.

[0016] In the above configuration, the piezoelectric layer may be a rotated Y-cut X-propagation lithium tantalate substrate having an angle of 36° or more and 48° or less.

[0017] The present invention is a multiplexer including the above filter. Effect of the Invention

[0018] According to the present invention, the transmission characteristics can be improved. [Brief description of the drawings]

[0019] [Figure 1] 1(a) and 1(b) are a plan view and a cross-sectional view of an acoustic wave resonator in accordance with a first embodiment. [Diagram 2] FIG. 2 is a diagram showing the transmission characteristics of a shunt-connected acoustic wave resonator in Simulation 1. In FIG. [Diagram 3] FIG. 3 is a diagram showing (fs-fr) / fr versus thickness T4 in simulation 1. [Figure 4] 4(a) and 4(b) are diagrams showing k2 and Qa, respectively, versus thickness T4 in simulation 1. [Diagram 5] FIG. 5(a) is a circuit diagram of the ladder-type filter in accordance with the first embodiment, and FIG. 5(b) is a cross-sectional view of a series resonator and a parallel resonator. [Figure 6] FIG. 6 is a diagram showing the pass characteristic of the ladder filter in the simulation 2. In FIG. [Figure 7] FIG. 7 is a cross-sectional view of a series resonator and a parallel resonator in the second embodiment. [Figure 8] FIG. 8 is a diagram showing the pass characteristic of the ladder filter in the simulation 3. In FIG. [Figure 9] FIG. 9 is a diagram showing the pass characteristic of the ladder filter in Simulation 3. In FIG. [Figure 10] 10(a) and 10(b) are cross-sectional views showing other examples of the series resonator and the parallel resonator in the first and second embodiments. [Figure 11] 11(a) and 11(b) are cross-sectional views showing other examples of the series resonator and the parallel resonator in the first and second embodiments. [Figure 12]FIG. 12 is a cross-sectional view showing another example of the series resonator and the parallel resonator in the first and second embodiments. [Figure 13] FIG. 13 is a circuit diagram of a duplexer in accordance with the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, embodiments of the present invention will be described with reference to the drawings. EXAMPLES

[0021] An acoustic wave resonator used in a ladder-type filter according to a first embodiment will be described. Fig. 1(a) and Fig. 1(b) are a plan view and a cross-sectional view of the acoustic wave resonator in the first embodiment. The arrangement direction of the electrode fingers is the X direction, the extension direction of the electrode fingers is the Y direction, and the lamination direction of the support substrate and the piezoelectric layer is the Z direction. The X direction, the Y direction, and the Z direction do not necessarily correspond to the X-axis direction and the Y-axis direction of the crystal orientation of the piezoelectric layer. When the piezoelectric layer is a rotated Y-cut X-propagation substrate, the X direction is the X-axis direction of the crystal orientation.

[0022] As shown in Figures 1(a) and 1(b), a piezoelectric layer 14 is provided on a support substrate 10. A temperature compensation film 12 is provided between the support substrate 10 and the piezoelectric layer 14. An acoustic wave resonator 26 is provided on the piezoelectric layer 14. The acoustic wave resonator 26 has an IDT 22 and a reflector 24. The reflectors 24 are provided on both sides of the IDT 22 in the X direction. The IDT 22 and the reflectors 24 are formed by a metal film 16 on the piezoelectric layer 14.

[0023] The IDT 22 includes a pair of opposing comb electrodes 20. The comb electrodes 20 include a plurality of electrode fingers 18 and a bus bar 19 to which the plurality of electrode fingers 18 are connected. When viewed from the X direction, a region where the electrode fingers 18 of the pair of comb electrodes 20 intersect is an intersection region 25. The length of the intersection region 25 is an aperture length. The pair of comb electrodes 20 have the electrode fingers 18 alternately provided in at least a part of the intersection region 25. An acoustic wave mainly excited by the plurality of electrode fingers 18 in the intersection region 25 propagates mainly in the X direction. The pitch of the electrode fingers 18 of one of the pair of comb electrodes 20 is approximately the wavelength λ of the acoustic wave. If the pitch of the plurality of electrode fingers 18 (the pitch between the centers of the electrode fingers 18) is D, the pitch of the electrode fingers 18 of one of the comb electrodes 20 is the pitch D of two electrode fingers 18. The reflector 24 reflects the acoustic wave (surface acoustic wave) excited by the electrode fingers 18 of the IDT 22. As a result, the acoustic wave is confined within the intersection region 25 of the IDT 22.

[0024] The piezoelectric layer 14 is, for example, a single crystal lithium tantalate (LiTaO3) layer or a single crystal lithium niobate (LiNbO3) layer, for example, a rotated Y-cut X-propagation lithium tantalate layer or a rotated Y-cut X-propagation lithium niobate layer. When the surface acoustic wave excited by the electrode fingers is an SH (Shear Horizontal) wave, the piezoelectric layer 14 is a rotated Y-cut X-propagation lithium tantalate substrate with an angle of 36° or more and 48° or less.

[0025] The support substrate 10 is, for example, a sapphire substrate, an alumina substrate, a silicon substrate, a spinel substrate, a quartz substrate, a quartz substrate, or a silicon carbide substrate. The sapphire substrate is a single crystal Al2O3 substrate, the alumina substrate is a polycrystalline or amorphous Al2O3 substrate, the silicon substrate is a single crystal or polycrystalline silicon substrate, the spinel substrate is a polycrystalline or amorphous MgAl2O4 substrate, the quartz substrate is a single crystal SiO2 substrate, the quartz substrate is a polycrystalline or amorphous SiO2 substrate, and the silicon carbide substrate is a polycrystalline or single crystal SiC substrate. The linear expansion coefficient of the support substrate 10 in the X direction is smaller than the linear expansion coefficient of the piezoelectric layer 14 in the X direction. This can reduce the frequency temperature dependency of the elastic wave resonator.

[0026] The temperature compensation film 12 has a temperature coefficient of elastic constant with the opposite sign to that of the piezoelectric layer 14. For example, the temperature coefficient of elastic constant of the piezoelectric layer 14 is negative, and the temperature coefficient of elastic constant of the temperature compensation film 12 is positive. The temperature compensation film 12 is an insulating film mainly composed of silicon oxide (SiO2), for example, a silicon oxide (SiO2) film with no additives or with additive elements such as fluorine, and is, for example, polycrystalline or amorphous. This makes it possible to reduce the frequency temperature coefficient of the elastic wave resonator. When the temperature compensation film 12 is a silicon oxide film, the sound velocity of the bulk wave propagating through the temperature compensation film 12 is slower than the sound velocity of the bulk propagating through the piezoelectric layer 14.

[0027] In order for the temperature compensation film 12 to have a temperature compensation function, it is required that the energy of the main mode elastic wave is present to a certain extent within the temperature compensation film 12. Although the range in which the energy of the surface acoustic wave is concentrated depends on the type of surface acoustic wave, typically, the energy of the surface acoustic wave is concentrated within a range of 2λ (λ is the wavelength of the elastic wave) from the upper surface of the piezoelectric layer 14, and is particularly concentrated within a range of λ from the upper surface of the piezoelectric layer 14. Therefore, the distance between the lower surface of the temperature compensation film 12 and the upper surface of the piezoelectric layer 14 is preferably 2λ or less, more preferably 1λ or less, and even more preferably 0.6λ or less.

[0028] The metal film 16 is a film whose main component is, for example, aluminum (Al), copper (Cu) or molybdenum (Mo). An adhesive film such as a titanium (Ti) film or a chromium (Cr) film may be provided between the electrode fingers 18 and the piezoelectric layer 14. The adhesive film is thinner than the electrode fingers 18. An insulating film may be provided so as to cover the electrode fingers 18. The insulating film functions as a protective film or a temperature compensation film.

[0029] [Simulation 1] The characteristics of the acoustic wave resonator were simulated by changing the thickness T4 of the piezoelectric layer 14 and the thickness T2 of the temperature compensation film 12. The simulation conditions were as follows. Support substrate 10: sapphire substrate Temperature compensation film 12: Silicon oxide (SiO2) Piezoelectric layer 14: 42° Y-cut X-propagation lithium tantalate substrate Pitch 2×D(λ): 5.7μm

[0030] FIG. 2 is a diagram showing the pass characteristic of the shunt-connected acoustic wave resonator in Simulation 1. The thickness T2 of the temperature compensation film 12 is 0.2λ, and T4 is 0.4λ and 0.2λ. The pass characteristic of the shunt-connected acoustic wave resonator corresponds to the pass characteristic of a parallel resonator in a ladder filter. A graph showing an enlarged attenuation is also shown. As shown in FIG. 2, in the shunt-connected acoustic wave resonator, the insertion loss is maximum at the resonance frequency fr and is minimum at the anti-resonance frequency fa. A spurious response is observed at a frequency fs of about 750 MHz. Ripples are observed due to the spurious response. When the thickness T4 of the piezoelectric layer 14 changes from 0.4λ to 0.2λ, the frequency fs increases.

[0031] The thickness T4 of the piezoelectric layer 14 and the thickness T2 of the temperature compensation film 12 were changed to simulate the change in frequency fs. FIG. 3 is a diagram showing (fs-fr) / fr versus thickness T4 in simulation 1. Dots indicate simulation points, and straight lines connect the dots. As shown in FIG. 3, when the thickness T4 of the piezoelectric layer 14 becomes smaller, fs shifts to the high frequency side. When the thickness T4 is 0.5λ or more, fs hardly changes even if T4 changes. When the thickness T4 is 0.5λ or less, fs changes significantly with respect to the thickness T4. When the thickness T2 of the temperature compensation film 12 becomes thicker, fs shifts to the low frequency side. The difference between (fs-fr) / fr when the thickness T4 is 0.5λ and (fs-fr) / fr when the thickness T4 is 0.2λ is almost the same regardless of the thickness T2 being 0.2λ to 0.5λ.

[0032] Thus, it has not been known until now that the frequency fs at which spurious responses due to bulk waves occur depends on the thickness T4 of the piezoelectric layer 14. This phenomenon is considered to be unique to the thickness T4 of the piezoelectric layer 14 being equal to or less than the wavelength λ of the elastic wave, and is prominent when the thickness T4 is 0.5λ or less.

[0033] The thickness T4 of the piezoelectric layer 14 and the thickness T2 of the temperature compensation film 12 are changed, and the electromechanical coupling coefficient k 2The Q value Qa at the anti-resonant frequency fa was simulated. Figures 4(a) and 4(b) show the relationship between the thickness T4 and k in Simulation 1. 2 4(a) and 4(b), when the thickness T4 of the piezoelectric layer 14 is 0.7λ or less, the k 2 becomes larger and Qa becomes lower. When the thickness T4 is 0.6λ, compared with 0.2λ, k 2 becomes approximately 1.3 times and Qa becomes 0.65 times.

[0034] A ladder-type filter according to a first embodiment will be described based on a simulation 1. Fig. 5(a) is a circuit diagram of the ladder-type filter according to the first embodiment, and Fig. 5(b) is a cross-sectional view of a series resonator and a parallel resonator. As shown in Fig. 5(a), the series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. The parallel resonators P1 to P4 are connected in parallel between the input terminal Tin and the output terminal Tout. One end of the parallel resonators P1 to P4 is connected to a path between the input terminal Tin and the output terminal Tout, and the other end is connected to the ground terminal Gnd and grounded.

[0035] As shown in FIG. 5(b), a temperature compensation film 12 is provided on a support substrate 10, and a piezoelectric layer 14 is provided on the temperature compensation film 12. A series resonator S is provided in an area 44, and a parallel resonator P is provided in an area 46. The thicknesses of the piezoelectric layer 14 and the temperature compensation film 12 in the area 44 are T4a and T2a, respectively, and the thicknesses of the piezoelectric layer 14 and the temperature compensation film 12 in the area 46 are T4b and T2b, respectively. The thicknesses T2a and T2b are approximately equal to the manufacturing error. The thickness T4a is larger than the thickness T4b. The pitch of the electrode fingers 18 in the series resonator S is Da, and the pitch in the parallel resonator P is Db. In the ladder type filter, the anti-resonant frequency fa of the series resonator S is set higher than the pass band, and the resonant frequency fr of the series resonator P is set lower than the pass band. For this reason, the pitch Da is smaller than the pitch Db. In addition to increasing the pitch, the resonant frequency fr and the anti-resonant frequency fa can be increased by decreasing the duty ratio or thinning the metal film 16. However, even if the duty ratio and the thickness of the metal film 16 are changed, the resonance frequency fr and the anti-resonance frequency fa do not change as much as changing the pitch D. Therefore, the resonance frequency fr and the anti-resonance frequency fa are set to desired frequencies mainly by changing the pitch D.

[0036] [Simulation 2] The pass characteristics of the ladder type filters in Example 1 and Comparative Example 1 were simulated. The simulation conditions were as follows. Piezoelectric layer 14: 42° rotated Y-cut X-propagation lithium tantalate substrate Temperature compensation film 12: Silicon oxide film with a thickness of 1.1 μm Metal film 16: Aluminum film with a thickness of 550 nm (0.1 λb) Duty ratio: 50% Average Pitch Series resonator S1~S4: 2×Da(λa)=5.1μm Parallel resonator P1~P4: 2×Db(λb)=5.5μm average resonant frequency fr Series resonator S1~S4: 730MHz Parallel resonator P1~P4: 690MHz average antiresonance frequency fa Series resonator S1~S4: 760MHz Parallel resonator P1~P4:710MHz Thickness of piezoelectric layer 14 in Comparative Example 1 Series resonators S1~S4 and parallel resonators P1~P4: 2.2μm (0.4λb) Thickness of the piezoelectric layer 14 in Example 1 Series resonator S1~S4: 2.2μm (0.4λb) Parallel resonator P1~P4: 1.1μm (0.2λb)

[0037] FIG. 6 is a diagram showing the pass characteristic of the ladder filter in Simulation 2. A graph showing an enlarged attenuation is also shown. As shown in FIG. 6, the pass band Pass is 703 MHz to 748 MHz, which corresponds to the transmission band of the LTE (Long Term Evolution) band 28. In the range of large attenuation, the pass characteristic is almost the same between Comparative Example 1 and Example 1. When the attenuation in the pass band Pass is enlarged, the insertion loss at the high frequency end of the pass band Pass in Comparative Example 1 is reduced. This corresponds to the spurious response near the frequency fs of the thickness T4=0.4λ in FIG. 2 in the parallel resonators P1 to P4. In Example 1, the insertion loss at the high frequency end of the pass band Pass is smaller than that in Comparative Example 1. This is because, as shown in FIG. 2, the thickness T4 of the piezoelectric layer 14 of the parallel resonators P1 to P4 is set to 0.2λ, and the frequency fs of the spurious response moves to the high frequency side, and the frequency fs moves outside the pass band Pass. If the thickness T4 of the piezoelectric layer 14 in the series resonators S1 to S4 is 0.2λ, then the electromechanical coupling coefficient k 2 4(b), the Q value Qa becomes lower, and the insertion loss in the passband becomes large. In the first embodiment, the thickness T4 of the piezoelectric layer 14 is set to 0.4λ, thereby increasing the steepness at the high frequency end and reducing the insertion loss in the passband. EXAMPLES

[0038] In the first embodiment, the thickness T4b of the piezoelectric layer 14 in the parallel resonators P1 to P4 is set smaller than the thickness T4a of the piezoelectric layer 14 in the series resonators S1 to S4. In this case, the electromechanical coupling coefficient k 2 Since the Q value of the parallel resonators P1 to P4 is low, the steepness of the low-frequency end of the passband Pass is low. Also, since the Q value of the parallel resonators P1 to P4 is low, the insertion loss in the passband Pass is large. Therefore, in the second embodiment, it was considered to thin the piezoelectric layer 14 in part of the parallel resonators P1 to P4.

[0039] Fig. 7 is a cross-sectional view of a series resonator and a parallel resonator in Example 2. As shown in Fig. 7, a part of the parallel resonators P among the parallel resonators P1 to P4, that is, a parallel resonator P, is provided in an area 46 where the piezoelectric layer 14 has a thickness T4b, and the remaining parallel resonator P' among the parallel resonators P1 to P4 is provided in an area 44 where the piezoelectric layer 14 has a thickness T4a. The pitch Db of the electrode fingers 18 in the parallel resonator P is larger than the pitch Dc of the electrode fingers 18 in the parallel resonator P'.

[0040] [Simulation 3] A simulation was performed on the pass characteristics of the ladder type filters in Example 2 and Comparative Example 2. The simulation conditions were as follows. Pitch 2×D(λ) P1: 5.56 μm P2: 5.66 μm P3: 5.51 μm P4: 5.60μm resonant frequency fr P1: 690MHz P2: 680MHz P3: 696MHz P4: 686MHz anti-resonance frequency fa P1: 715MHz P2: 705MHz P3: 721MHz P4: 711MHz

[0041] The thickness of the piezoelectric layer 14 of each of the parallel resonators P1 to P4 is set as follows. Comparative Example 1 P1~P4: 0.4λb Example 1 P1~P4: 0.2λb Example 2a P1, P2, P4: 0.2λb P3:0.4λb Example 2b P2, P4: 0.2λb P1, P3: 0.4λb Example 2c P2:0.2λb P1, P3, P4: 0.4λb Other simulation conditions were the same as in Simulation 2.

[0042] Fig. 8 is a diagram showing the pass characteristic of the ladder filter in Simulation 3. As shown in Fig. 8, in the range of large attenuation, the pass characteristic of Example 2b is almost the same as that of Comparative Example 1 and Example 1. When the attenuation in the pass band Pass is enlarged, in Example 2b, the decrease in insertion loss at the high frequency end of the pass band Pass caused by the spurious response as in Comparative Example 1 is small. Furthermore, the insertion loss in the pass band Pass is larger than that of Example 1.

[0043] Fig. 9 is a diagram showing the pass characteristic of the ladder filter in Simulation 3. In Fig. 9, the attenuation in the pass band Pass is enlarged compared to Fig. 8. As shown in Fig. 9, the insertion loss in the pass band Pass decreases in the order of Example 1, Examples 2a, 2b, and 2c. The insertion loss at the low frequency end (703 MHz) and high frequency end (748 MHz) of the pass band Pass is shown below. Comparative example 1: 0.745dB, 1.088dB Example 1: 0.847 dB, 0.760 dB Example 2a: 0.826 dB, 0.750 dB Example 2b: 0.778 dB, 0.741 dB Example 2c: 0.764 dB, 0.746 dB

[0044] At the low frequency end (703 MHz) of the pass band Pass, the insertion loss is smallest in Comparative Example 1, followed by Examples 2c, 2b, 2a, and 1 in that order. This corresponds to the fact that the insertion loss decreases as the number of resonators with high Q values ​​(i.e., thicker piezoelectric layers 14) increases among the parallel resonators P1 to P4 and the series resonators S1 to S4. At the high frequency end (748 MHz) of the pass band Pass, the insertion loss is largest in Comparative Example 1, followed by Examples 1, 2a, 2c, and 2b in that order. This is believed to be because, in Comparative Example 1, the spurious response of the parallel resonator P2 with the lowest resonant frequency fr is located within the pass band Pass, and when the piezoelectric layer 14 of the parallel resonator P2 is made thinner, the spurious response moves to the higher frequency side than the pass band Pass. The spurious responses of the parallel resonators P1 and P3 with high resonant frequencies fr are not located within the pass band Pass, so the insertion loss at the high frequency end does not increase even if the piezoelectric layers 14 of the parallel resonators P1 and P3 are made thicker. Rather, the insertion loss is reduced because the Q value is increased. The spurious response of the parallel resonator P4, which has the second lowest resonance frequency, slightly overlaps with the passband Pass. Therefore, the insertion loss at the low frequency end of the embodiment 2b is smaller than that of the embodiment 2c.

[0045] 10(a) to 12 are cross-sectional views showing other examples of the series resonator and the parallel resonator in Examples 1 and 2. As shown in Fig. 10(a), a boundary layer 11 is provided between a support substrate 10 and a temperature compensation film 12. An insulating layer 13 includes the temperature compensation film 12 and the boundary layer 11. The thicknesses of the boundary layer 11 in regions 44 and 46 are T1a and T1b, respectively. The other configurations are the same as those in Fig. 5(b).

[0046] The acoustic velocity of the bulk wave propagating through the boundary layer 11 is faster than that of the bulk wave propagating through the temperature compensation film 12. This confines the main mode elastic wave within the piezoelectric layer 14 and the temperature compensation film 12. This reduces loss. Furthermore, the acoustic velocity of the bulk wave propagating through the boundary layer 11 is slower than that of the bulk wave propagating through the support substrate 10. The boundary layer 11 is, for example, polycrystalline or amorphous, and is an insulating layer such as an aluminum oxide film, a silicon nitride film, an aluminum nitride film, or a silicon film. The boundary layer 11 may be a laminate of multiple layers made of different materials.

[0047] By providing the boundary layer 11, the high-frequency unnecessary waves reaching the boundary layer 11 from the temperature compensation film 12 are attenuated while passing through the boundary layer 11. Therefore, the unnecessary waves reflected at the interface with the support substrate 10 and reaching the IDT can be suppressed. When the boundary layer 11 is thin, the main mode elastic waves are less likely to be confined in the piezoelectric layer 14 and the temperature compensation film 12. From this viewpoint, the thicknesses T1a and T1b of the boundary layer 11 are preferably 2.2 times (1.1λ) or more of the average pitch D of the electrode fingers 18, and more preferably 3.0 times (1.5λ) or more. If the boundary layer 11 is made thicker, the manufacturing process will increase in number and the difficulty of the manufacturing process will increase. From this viewpoint, the thicknesses T1a and T1b of the boundary layer 11 are preferably 10 times (5λ) or less of the average pitch D of the electrode fingers 18, and more preferably 8 times (4λ) or less.

[0048] If the thicknesses T2a and T2b of the temperature compensation film 12 are different, the temperature characteristics of the series resonator S and the parallel resonator P will differ. Furthermore, if a step is formed on the upper surface of the piezoelectric layer 14, it will be difficult to form wiring that connects the series resonator S and the parallel resonator P. By making the thickness T1b of the boundary layer 11 larger than T1a, the thicknesses T2a and T2b of the temperature compensation film 12 can be made substantially equal, and the formation of steps on the upper surface of the support substrate 10 and the upper surface of the piezoelectric layer 14 can be suppressed.

[0049] As shown in Fig. 10(b), the piezoelectric layer 14 may be directly connected to the support substrate 10. By forming a step on the upper surface of the support substrate 10 between the regions 44 and 46, it is possible to prevent a step from being formed on the upper surface of the piezoelectric layer 14. The other configurations are the same as those in Fig. 5(b).

[0050] As shown in Fig. 11(a), thicknesses T1a and T1b of the boundary layer 11 are approximately equal, and thicknesses T2a and T2b of the temperature compensation film 12 are approximately equal. A step is formed on the upper surface of the piezoelectric layer 14 between regions 44 and 46. The other configurations are the same as those in Fig. 10(a), and therefore description thereof will be omitted.

[0051] As shown in Fig. 11(b), thicknesses T2a and T2b of the temperature compensation film 12 are substantially equal. A step is formed on the upper surface of the piezoelectric layer 14 between regions 44 and 46. The other configurations are the same as those in Fig. 5(b), and therefore description thereof will be omitted.

[0052] As shown in Fig. 12, no step is formed on the upper surface of the support substrate 10 between the regions 44 and 46, but a step is formed on the upper surface of the piezoelectric layer 14. The other configurations are the same as those in Fig. 10(b), and therefore description thereof will be omitted.

[0053] As shown in FIGS. 11(a) and 11(b), a step may be formed on the upper surface of the piezoelectric layer 14 between the regions 44 and 46, as long as wiring can be formed to connect the series resonator S and the parallel resonator P.

[0054] 10(a) to 12, an example has been described in which the series resonator S is provided in the region 44 and the parallel resonator P is provided in the region 46, as in the first embodiment. However, the series resonator S and the parallel resonator P' may be provided in the region 44, and the parallel resonator P may be provided in the region 46, as in the second embodiment.

[0055] According to the first and second embodiments, the thickness T4b of the piezoelectric layer 14 in at least one of the parallel resonators P1 to P4 is equal to or smaller than the average pitch Db (0.5 λb) of the electrode fingers 18 in the at least one of the parallel resonators P, and is thinner than the thickness T4a of the piezoelectric layer 14 in at least one of the series resonators P among the series resonators S1 to S4. This makes it possible to shift the spurious response of the parallel resonator P to the higher frequency side than the passband Pass.

[0056] In order to shift the spurious response of the parallel resonator P to the higher frequency side, the thickness T4b of the piezoelectric layer 14 in the parallel resonator P is preferably 0.8 times (0.4λb) or less, and more preferably 0.6 times (0.3λb) or less, of the average pitch Db of the electrode fingers 18 in the parallel resonator P. Moreover, the thickness T4b is preferably 0.1Db (0.05λb) or more smaller than the thickness T4a, and more preferably 0.2Db (0.1λb) or more smaller. As shown in FIG. 4(b), if the thickness T4b is too thin, the Q value decreases. From this viewpoint, the thickness T4b is preferably 0.1Db (0.05λb) or more, and more preferably 0.2Db (0.1λb) or more. To increase the Q value of the series resonator S, the thicknesses T4a and T4b of the piezoelectric layers 14 of all the resonators are preferably 1.4Db (0.7λb) or less, more preferably 1.2Db (0.6λb) or less, and even more preferably 1.0Db (0.5λb) or less.

[0057] In a typical ladder-type filter, the resonant frequency of the series resonators S1 to S4 is higher than that of the parallel resonators P1 to P4. Therefore, the spurious responses of the series resonators S1 to S4 rarely overlap with the passband Pass. Therefore, the piezoelectric layer 14 in the parallel resonator P is made thinner than the piezoelectric layers 14 in all of the series resonators S1 to S4. This makes it possible to suppress an increase in insertion loss caused by the thinning of the piezoelectric layer 14. The thickness T4 of the piezoelectric layer 14 in the series resonators S1 to S4 is approximately equal to the manufacturing error. This makes it possible to reduce the manufacturing process. Note that the thickness being approximately equal to the manufacturing error means that an error of, for example, about 10% is allowed. For example, if the thickest and thinnest thicknesses are Tmax and Tmin, respectively, 2×(Tmax-Tmin) / (Tmax+Tmin) is 0.1 or less and 0.05 or less.

[0058] When all the spurious responses of the parallel resonators P1 to P4 affect the passband Pass, the piezoelectric layers 14 in all the parallel resonators P1 to P4 are made thinner than the piezoelectric layers 14 in all the series resonators S1 to S4. This makes it possible to suppress the spurious responses of the parallel resonators P1 to P4 from affecting the passband Pass. The thickness T4a of the piezoelectric layers 14 in the series resonators S1 to S4 is approximately equal to the manufacturing error, and the thickness T4b of the piezoelectric layers 14 in the parallel resonators P1 to P4 is approximately equal to the manufacturing error. This makes it possible to reduce the number of manufacturing processes.

[0059] As in the second embodiment, the average pitch Db of the electrode fingers 18 of the comb-shaped electrode in the parallel resonator P is larger than the average pitch Dc of the electrode fingers 18 in the other parallel resonators P' among the parallel resonators P1 to P4. As a result, the parallel resonator P has a lower resonance frequency than the parallel resonator P'. Therefore, the spurious response of the parallel resonator P is more likely to affect the passband Pass than the spurious response of the parallel resonator P'. The piezoelectric layer 14 in the parallel resonator P is made thinner than the piezoelectric layers 14 in the other parallel resonators P'. This makes it possible to suppress the spurious response of the parallel resonator P from affecting the passband Pass. The Q value of the parallel resonator P', in which the spurious response is less likely to affect the passband Pass, is improved, and therefore the insertion loss can be suppressed. The thickness T4b of the piezoelectric layer 14 in the parallel resonator P is approximately equal to the manufacturing error, and the thicknesses T4a of the piezoelectric layers 14 in the parallel resonator P' and the series resonators S1 to S4 are approximately equal. This makes it possible to reduce the manufacturing process.

[0060] As shown in Figs. 10(a) and 11(a), the series resonators S1 to S4 and the parallel resonators P1 to P4 may include an insulating layer 13 provided between the piezoelectric layer 14 and the support substrate 10. The insulating layer 13 may be a single layer or a laminate of multiple layers. As shown in Figs. 5(b), 7, 10(a), 10(b), 11(a) and 11(b), the series resonators S1 to S4 and the parallel resonators P1 to P4 may include a temperature compensation film 12. The distance between the surface of the temperature compensation film 12 on the support substrate 10 side and the surface of the piezoelectric layer 14 on the pair of comb electrodes 20 side is preferably equal to or less than twice the pitch Db (λb). This allows the energy of the main mode acoustic wave to exist in the piezoelectric layer 14 and the temperature compensation film 12. A bonding layer for bonding the temperature compensation film 12 and the piezoelectric layer 14 by a surface activation method may be provided between the temperature compensation film 12 and the piezoelectric layer 14. The bonding layer is, for example, an aluminum oxide layer or an aluminum nitride oxide film having a thickness of 1 nm to 20 nm.

[0061] In the first and second embodiments, the number of series resonators is four and the number of parallel resonators is four. However, the number of series resonators may be one or more, and the number of parallel resonators may be one or more. EXAMPLES

[0062] FIG. 13 is a circuit diagram of a duplexer according to a third embodiment. As shown in FIG. 13, a transmission filter 40 is connected between a common terminal Ant and a transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and a reception terminal Rx. The transmission filter 40 passes a signal in a transmission band among high-frequency signals input from the transmission terminal Tx to the common terminal Ant as a transmission signal, and suppresses signals of other frequencies. The reception filter 42 passes a signal in a reception band among high-frequency signals input from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be the filter of the first or second embodiment.

[0063] Although a duplexer has been described as an example of a multiplexer, a triplexer or a quadplexer may also be used.

[0064] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0065] 10 Support substrate 11 Boundary layer 12 Temperature compensation membrane 13 Insulating layer 14 Piezoelectric layer 16 Metal Film 18 electrode fingers 20 comb electrode 22 IDT 25 Intersection Area 26 Elastic Wave Resonator 40 Transmission Filter 42 Receiving Filter 44, 46 area

Claims

1. A plurality of acoustic wave resonators each including a support substrate, a piezoelectric layer provided on the support substrate, and a pair of comb electrodes provided on the piezoelectric layer; one or more series resonators included in the plurality of acoustic wave resonators, the series resonators being connected in series between an input terminal and an output terminal; one or more parallel resonators, one end of which is connected to a path between the input terminal and the output terminal and the other end of which is grounded, which are included in the plurality of elastic wave resonators, the thickness of the piezoelectric layer in at least one parallel resonator being equal to or smaller than an average pitch of electrode fingers of the pair of comb electrodes in the at least one parallel resonator and smaller than the thickness of the piezoelectric layer in at least one of the one or more series resonators; A ladder type filter comprising:

2. 2. The ladder filter according to claim 1, wherein the piezoelectric layer in the at least one parallel resonator is thinner than the piezoelectric layer in all of the one or more series resonators.

3. 3. The ladder filter according to claim 2, wherein the thicknesses of the piezoelectric layers in the one or more series resonators are substantially equal.

4. 2. The ladder filter according to claim 1, wherein the piezoelectric layer in each of the one or more parallel resonators is thinner than the piezoelectric layer in each of the one or more series resonators.

5. the thickness of the piezoelectric layer in the one or more series resonators is substantially equal; 5. The ladder filter according to claim 4, wherein the thicknesses of the piezoelectric layers in the one or more parallel resonators are substantially equal to each other.

6. an average pitch of electrode fingers of the pair of comb electrodes in the at least one parallel resonator is larger than an average pitch of electrode fingers of the pair of comb electrodes in other parallel resonators among the one or more parallel resonators; 2. The ladder filter according to claim 1, wherein the piezoelectric layer in the at least one parallel resonator is thinner than the piezoelectric layers in the other parallel resonators.

7. the thicknesses of the piezoelectric layers in the at least one parallel resonator are substantially equal; 7. The ladder filter according to claim 6, wherein the thicknesses of the piezoelectric layers in the other parallel resonator and the one or more series resonators are substantially equal to each other.

8. 8. The ladder filter according to claim 1, wherein the thickness of the piezoelectric layer in each of the elastic wave resonators is equal to or smaller than 1.4 times the average pitch.

9. The ladder-type filter according to claim 1 , wherein the elastic wave resonators each include an insulating layer provided between the piezoelectric layer and the support substrate.

10. The plurality of elastic wave resonators each include a temperature compensation film provided between the piezoelectric layer and the support substrate, the temperature compensation film having a temperature coefficient of elastic constant whose sign is opposite to that of a temperature coefficient of elastic constant of the piezoelectric layer; 10. The ladder filter according to claim 1, wherein in the at least one parallel resonator, a distance between a surface of the temperature compensation film facing the supporting substrate and a surface of the piezoelectric layer facing the pair of comb electrodes is equal to or less than twice the average pitch.

11. 11. The ladder filter according to claim 1, wherein the piezoelectric layer is a rotated Y-cut X-propagation lithium tantalate substrate having an angle of 36 degrees or more and 48 degrees or less.

12. A multiplexer comprising a filter according to any one of claims 1 to 11.

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