JP7656487B2 - Ladder Filters and Multiplexers - Google Patents

Description

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 electrodes.

As an elastic wave resonator used in communication devices such as smartphones, an elastic wave resonator having a pair of comb electrodes provided on a piezoelectric layer is known. It is known that the piezoelectric layer is bonded to a support substrate and the thickness of the piezoelectric layer is set to be equal to or less than the wavelength of the elastic wave (e.g., Patent Document 1). It is known that an insulating layer is provided between the support substrate and the piezoelectric layer (e.g., Patent Documents 2 and 3). It is known that in a ladder-type filter using an elastic wave resonator in which a thick dielectric layer for temperature compensation is provided on the comb electrodes, the duty ratio of the parallel resonator is made smaller than the duty ratio of the series resonator (e.g., Patent Document 4).

JP 2017-34363 A JP 2019-201345 A JP 2015-73331 A JP 2018-196028 A

In ladder-type filters using elastic wave resonators with comb-shaped electrodes and piezoelectric layers bonded to a support substrate, there is a demand for a small temperature coefficient of frequency (TCF).

The present invention was developed in consideration of the above problems, and aims to reduce the frequency temperature coefficient.

The present invention is a ladder type filter comprising: a support substrate; a piezoelectric layer provided on the support substrate; one or more parallel resonators each including a plurality of first electrode fingers provided on the piezoelectric layer and having a first average pitch and a first average duty ratio, where the largest first average pitch is more than twice the thickness of the piezoelectric layer, and each having one end connected to a path between an input terminal and an output terminal and the other end connected to ground; and one or more series resonators each including a plurality of second electrode fingers provided on the piezoelectric layer and having a second average pitch and a second average duty ratio, where the second average duty ratio of at least one series resonator is smaller than the smallest first average duty ratio, and connected in series between the input terminal and the output terminal.

In the above configuration, the piezoelectric layer can be configured as a Y-cut X-propagation lithium tantalate layer rotated at 36° or more and 48° or less.

In the above configuration, a dielectric film covering the first electrode fingers and having a thickness greater than the thickness of the first electrode fingers is not provided on the piezoelectric layer in the one or more parallel resonators, and a dielectric film covering the second electrode fingers and having a thickness greater than the thickness of the second electrode fingers is not provided on the piezoelectric layer in the one or more series resonators.

In the above configuration, the one or more series resonators may be multiple series resonators, and the at least one series resonator may include a first series resonator having the largest second average pitch.

In the above configuration, the second average duty ratio of a second series resonator among the plurality of series resonators, the second average pitch of which is smaller than the second average pitch of the first series resonator, can be configured to be larger than the second average duty ratio of the first series resonator.

In the above configuration, the largest second average duty ratio can be smaller than the smallest first average duty ratio.

In the above configuration, the largest second average pitch can be smaller than the smallest first average pitch.

In the above configuration, the difference between the largest second average duty ratio and the smallest second average duty ratio can be greater than the difference between the largest first average duty ratio and the smallest first average duty ratio.

The above configuration may include a temperature compensation film provided between the support substrate and the piezoelectric layer, the temperature coefficient of the elastic constant having a sign opposite to that of the temperature coefficient of the elastic constant of the piezoelectric layer.

The present invention is a multiplexer equipped with the above ladder-type filter.

The present invention makes it possible to reduce the frequency temperature coefficient.

FIG. 1A is a plan view of an acoustic wave resonator used in a ladder-type filter in accordance with a first embodiment, and FIG. 1B and FIG. 1C are cross-sectional views taken along line AA of FIG. FIG. 2A is a graph showing the resonance frequency fr versus the pitch D of the acoustic wave resonator in the first embodiment, and FIG. 2B is a graph showing the resonance frequency fr versus the duty ratio R. FIG. 3 is a circuit diagram of a ladder-type filter in accordance with the first embodiment. FIG. 4 is a plan view of the ladder-type filter in accordance with the first embodiment. FIG. 5 is a cross-sectional view of the resonator B in the experiment 1. As shown in FIG. FIG. 6 is a diagram showing the TCF of the resonant frequency fr and the anti-resonant frequency fa versus the duty ratio in Experiment 1. FIG. 7 is a cross-sectional view of the resonator in Experiment 2. FIG. 8 is a diagram showing the TCF of the resonant frequency fr and the anti-resonant frequency fa versus the duty ratio in Experiment 2. 9A and 9B are schematic diagrams showing the pass characteristics of a parallel resonator and a series resonator, and FIG. 9C is a schematic diagram showing the temperature change of the pass characteristics of a filter. 10A and 10B are schematic diagrams showing the pass characteristics of a filter and a parallel resonator. 11A and 11B are schematic diagrams showing the pass characteristics of a filter and a series resonator. 12A to 12C are cross-sectional views of acoustic wave resonators according to second to fourth modifications of the first embodiment, respectively. 13A to 13C are cross-sectional views of acoustic wave resonators according to fifth to seventh modifications of the first embodiment, respectively. FIG. 14 is a circuit diagram of a duplexer according to an embodiment.

Patent Document 4 describes that in an elastic wave resonator in which a dielectric film thicker than the comb electrodes is formed on a comb electrode provided on a lithium niobate substrate, the temperature coefficient of frequency (TCF) of the resonant frequency and anti-resonant frequency depends on the duty ratio. Such an elastic wave resonator has a Rayleigh wave as its main mode. On the other hand, if no thick dielectric film is provided on the comb electrodes, it was thought that the TCF of the resonant frequency and anti-resonant frequency does not depend on the duty ratio. One such elastic wave resonator is an elastic wave resonator that uses a lithium tantalate substrate and has SH (Shear Horizontal) waves as its main mode. The inventors have discovered that even in an elastic wave resonator that does not have a thick dielectric film covering the comb electrodes and has SH waves as its main mode, the TCF of the resonant frequency and anti-resonant frequency depends on the duty ratio by making the piezoelectric layer thinner. Below, an embodiment based on the above knowledge will be described.

Example 1 is an example of a ladder-type filter. FIG. 1(a) is a plan view of an acoustic wave resonator used in the ladder-type filter according to Example 1, and FIG. 1(b) and FIG. 1(c) are cross-sectional views taken along the line A-A of FIG. 1(a). FIG. 1(b) and FIG. 1(c) are cross-sectional views of a parallel resonator P and a series resonator S, respectively. 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, Y direction, and Z direction do not necessarily correspond to the X-axis direction and 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.

As shown in Figs. 1(a) to 1(c), 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 elastic wave resonator 26 is provided on the piezoelectric layer 14. The elastic 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 reflector 24 are formed by a metal film 16 on the piezoelectric layer 14.

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. The region where the electrode fingers 18 of the pair of comb electrodes 20 intersect when viewed from the X direction is the intersection region 25. The length of the intersection region 25 is the aperture length. The pair of comb electrodes 20 have the electrode fingers 18 alternately provided in at least a portion of the intersection region 25. The elastic waves excited mainly by the plurality of electrode fingers 18 in the intersection region 25 propagate 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 elastic 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. This confines the acoustic wave within the intersection region 25 of the IDT 22.

The piezoelectric layer 14 is, for example, a single crystal lithium tantalate (LiTaO ₃ ) layer, for example, a rotated Y-cut X-propagation lithium tantalate layer. When the rotated Y-cut X-propagation lithium tantalate layer is 36° or more and 48° or less, the SH wave is the main mode of the elastic wave. The thickness of the piezoelectric layer 14 is equal to or less than the wavelength λ of the elastic wave (i.e., twice the pitch D).

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 makes it possible to reduce the frequency temperature dependency of the elastic wave resonator.

The temperature compensation film 12 has a temperature coefficient of elastic constant with a sign opposite to that of the temperature coefficient of elastic constant 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 (SiO ₂ ), for example, a silicon oxide (SiO ₂ ) film with no additives or with additive elements such as fluorine, for example, polycrystalline or amorphous. This can 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. Note that, when a certain layer is composed mainly of a certain material, impurities that are intentionally or unintentionally contained are allowed, and the concentration of the certain material in a certain layer is, for example, 50 atomic % or more, 80 atomic % or more. When the temperature compensation film 12 is mainly composed of silicon oxide, the total of the oxygen concentration and the silicon concentration in the temperature compensation film 12 is, for example, 50 atomic % or more and 80 atomic % or more. Also, the oxygen concentration and the silicon concentration in the temperature compensation film 12 are, for example, 10 atomic % or more and 20 atomic % or more, respectively.

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 from the lower surface of the temperature compensation film 12 to the upper surface of the piezoelectric layer 14 is preferably 2λ or less, more preferably λ or less, and even more preferably 0.6λ or less.

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 thinner than the electrode fingers 18 may be provided so as to cover the electrode fingers 18. The insulating film functions as a protective film.

1(b) and 1(c), if the pitch of the electrode fingers 18 in the parallel resonator P is D1 and the width of the electrode fingers 18 is E1, then the duty ratio R1=E1/D1. If the pitch of the electrode fingers 18 in the series resonator S is D2 and the width of the electrode fingers 18 is E2, then the duty ratio R2=E2/D2. The pitch D2 in the series resonator S is smaller than the pitch D1 in the parallel resonator P, and the duty ratio R2 in the series resonator S is smaller than the duty ratio R1 in the parallel resonator P.

Figure 2(a) shows the resonant frequency fr versus pitch D of the elastic wave resonator in Example 1, and Figure 2(b) shows the resonant frequency fr versus duty ratio R. As shown in Figure 2(a), the resonant frequency fr increases as the pitch D decreases. A change in pitch D of 0.3 μm changes the resonant frequency fr by approximately 500 MHz.

As shown in Fig. 2(b), when the duty ratio R changes, the resonant frequency fr changes. When the duty ratio R is 60% or less, the resonant frequency fr increases as the duty ratio R decreases. When the duty ratio R is 70% or more, the resonant frequency fr increases as the duty ratio R increases. When the duty ratio R changes by 40%, from 60% to 20%, the resonant frequency fr changes by about 160 MHz. In this way, the change in the resonant frequency fr due to the duty ratio R is small. Therefore, the resonant frequency fr is mainly determined by the pitch D.

Figure 3 is a circuit diagram of the ladder filter according to the first embodiment. As shown in Figure 3, the series resonators S1 to S5 are connected in series between the input terminal Tin and the output terminal Tout. One end of the parallel resonators P1 to P4 is connected to a series path between the input terminal Tin and the output terminal Tout, and the other end is connected to the ground terminal Gnd via the inductors L1 and L2.

Figure 4 is a plan view of the ladder-type filter according to the first embodiment. As shown in Figure 4, an elastic wave resonator 26, wiring 30, and pads 32 are provided on a piezoelectric layer 14. The elastic wave resonator 26 includes an IDT 22 and a reflector 24. The multiple elastic wave resonators 26 include series resonators S1 to S5 and parallel resonators P1 to P4. The wiring 30 electrically connects the elastic wave resonators 26 together and electrically connects the elastic wave resonators 26 to the pads 32. The pads 32 are terminals for electrically connecting to the outside, and correspond to the input terminal Tin, the output terminal Tout, and the ground terminal Gnd. The wiring 30 and the pads 32 are metal layers including, for example, a gold layer, a copper layer, or an aluminum layer.

The number of series resonators S in a ladder filter may be one or more, and the number of parallel resonators P may be one or more, and these numbers can be set appropriately.

[Experiment 1]
The TCFs of the resonant frequency and anti-resonant frequency were measured for resonators A and B. Resonator A has the structure shown in Fig. 1(a) to Fig. 1(c). Fig. 5 is a cross-sectional view of resonator B in experiment 1. As shown in Fig. 5, resonator B does not have a temperature compensation film 12, and a piezoelectric layer 14 is provided on a support substrate 10. The other configurations are the same as those shown in Fig. 1(a) to Fig. 1(c), and therefore description thereof will be omitted.

The experimental conditions are as follows.
Support substrate 10: sapphire substrate Temperature compensation film 12: silicon oxide film Piezoelectric layer 14: 42° rotated Y-cut X-propagation lithium tantalate substrate Metal film 16: aluminum film resonator A
Elastic wave wavelength λ: 5.0 μm
Thickness T2 of the temperature compensation film 12: 2.0 μm (0.4 λ)
Thickness T4 of the piezoelectric layer 14: 2.0 μm (0.4 λ)
Resonator B
Elastic wave wavelength λ: 4.9 μm
Thickness T4 of the piezoelectric layer 14: 20 μm (4.1 λ)

The duty ratio R was changed for resonators A and B, and the TCFs of the resonant frequency fr and the anti-resonant frequency fa were measured. Figure 6 shows the TCFs of the resonant frequency fr and the anti-resonant frequency fa versus the duty ratio in experiment 1. The dots are measurement points, and the straight lines are approximate straight lines fitted using the least squares method. As shown in Figure 6, the difference between the TCF of the resonant frequency fr and the anti-resonant frequency fa is about 15 ppm/K for both resonators A and B. The TCF of the resonant frequency fr is close to 0 for both resonators A and B, but the TCF of the anti-resonant frequency fa is negatively large (i.e., low). Furthermore, in resonator B, the TCF hardly changes even when the duty ratio R is changed. On the other hand, in resonator A, when the duty ratio R is small, the TCF moves toward the positive side and approaches 0.

[Experiment 2]
In experiment 1, resonator A is provided with a temperature compensation film 12, unlike resonator B. Furthermore, the thickness T4 of the piezoelectric layer 14 is 4.1λ in resonator B and 0.6λ in resonator Ad. Therefore, experiment 2 was performed to investigate whether the dependency of the TCF on the duty ratio R in resonator A is due to the thickness T4 of the piezoelectric layer 14 or the temperature compensation film 12.

Fig. 7 is a cross-sectional view of the resonator in Experiment 2. As shown in Fig. 7, the piezoelectric layer 14 is a single layer, and a support substrate and an insulating layer are not provided. The other configurations are the same as those in Figs. 1(a) to 1(c), and therefore the description will be omitted.
The experimental conditions are as follows.
Piezoelectric layer 14: 42° rotated Y-cut X-propagation lithium tantalate substrate Metal film 16: Aluminum film resonator C
Elastic wave wavelength λ: 2.2 μm
Thickness T4 of the piezoelectric layer 14: 1.32 μm (0.6 λ)
Resonator D
Elastic wave wavelength λ: 2.2 μm
Thickness T4 of the piezoelectric layer 14: 8.8 μm (4.0 λ)

The TCF of the resonant frequency fr and the anti-resonant frequency fa was measured for resonators C and D by changing the duty ratio R. Figure 8 shows the TCF of the resonant frequency fr and the anti-resonant frequency fa versus the duty ratio in experiment 2. The dots are measurement points, and the straight lines are approximate straight lines fitted using the least squares method. As shown in Figure 8, the TCF of the resonant frequency fr and the anti-resonant frequency fa in resonator D is almost independent of the duty ratio R, but in resonator C, as the duty ratio R decreases, the TCF approaches the TCF of the resonant frequency fr and the anti-resonant frequency fa.

In this way, when the thickness T4 of the piezoelectric layer 14 becomes smaller, the TCF of the resonant frequency fr and the antiresonant frequency fa becomes dependent on the duty ratio R. Therefore, in Experiment 1, it is believed that the reason why the TCF of resonator A depends on the duty ratio R is because the thickness T4 of the piezoelectric layer 14 is thin.

The reason why the TCF depends on the duty ratio R as the thickness T4 of the piezoelectric layer 14 decreases is unclear, but it may be related to the fact that surface acoustic waves such as SH waves are concentrated from the surface of the piezoelectric layer 14 to about the wavelength λ of the elastic wave. Considering this, it is thought that the TCF depends on the duty ratio R when the thickness T4 of the piezoelectric layer 14 is λ or less. When the piezoelectric layer 14 is λ or less, the mechanical strength of the piezoelectric layer 14 is small, so the piezoelectric layer 14 is bonded to the support substrate 10.

[Issues with filter characteristics]
A problem that arises when a ladder filter is formed by setting the duty ratio R1 of the parallel resonator P and the duty ratio R2 of the series resonator S in the resonator A or B to be the same will be described.

Figures 9(a) and 9(b) are schematic diagrams showing the pass characteristics of a parallel resonator and a series resonator, and Figure 9(c) is a schematic diagram showing the temperature change in the pass characteristics of a filter. Figure 9(b) is an enlarged view of the attenuation in Figure 9(a). The pass characteristics of the filter are indicated by F, the pass characteristics of the parallel resonator P by P, and the pass characteristics of the series resonator S by S. The pass characteristics of the parallel resonator P are the pass characteristics when connected in shunt.

As shown in Figures 9(a) and 9(b), the attenuation pole on the low frequency side of the pass band Pass of the filter F is formed by the resonant frequency frp of the parallel resonator P, and the attenuation pole on the high frequency side of the pass band Pass is formed by the anti-resonant frequency fas of the series resonator S. The anti-resonant frequency fap of the parallel resonator P and the resonant frequency frs of the series resonator S are located within the pass band Pass. In this way, the shoulder on the low frequency side of the pass band Pass in the ladder filter is formed mainly by the resonant frequency frp of the parallel resonator P, and the shoulder on the high frequency side of the pass band Pass is formed mainly by the anti-resonant frequency fas of the series resonator S.

As shown in FIG. 9(c), the frequency of the low-frequency shoulder of the passband Pass remains almost unchanged when the temperature changes from -30° to 80°C, but the frequency of the high-frequency shoulder of the passband Pass decreases when the temperature changes from -30° to 80°C. This is because, as shown in FIG. 6, the TCF of the resonance frequency fr of both resonators A and B is close to 0, so the temperature coefficient of the frequency of the low-frequency shoulder of the passband Pass formed by the resonance frequency Frp of the parallel resonator P becomes small. However, the TCF of the anti-resonance frequency fa is 15 ppm/K lower than the TCF of the resonance frequency fr, so the temperature coefficient of the high-frequency shoulder of the passband Pass formed by the anti-resonance frequency Fas of the series resonator S becomes negative.

In the first embodiment, the duty ratio R2 of the series resonator S is set smaller than the duty ratio R1 of the parallel resonator P. This allows the TCF of the anti-resonant frequency fas of the series resonator S to approach 0 as shown in FIG. 6. For example, the duty ratio R1 of the parallel resonator P is set to 50%, and the duty ratio R2 of the series resonator S is set to 30%. This allows the TCF of the resonant frequency frp of the parallel resonator P to be approximately -6 ppm/K, and the TCF of the anti-resonant frequency fas of the series resonator S to be approximately -16 ppm/K. Compared to the case where the duty ratio R2 of the series resonator S is set to 50%, the TCF of the anti-resonant frequency fas of the series resonator S can be improved by approximately 6 ppm/K. The TCF of the anti-resonant frequency fa is improved by approximately 1 ppm/K by reducing the duty ratio R by 5%. Therefore, the duty ratio R1-R2 is preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more.

[Modification 1 of Example 1]
In the first modification of the first embodiment, the parallel resonators P1 to P4 have different resonant frequencies frp, and the series resonators S1 to S5 have different anti-resonant frequencies fas. Table 1 shows the pitch 2×D1 and duty ratio R1 in the parallel resonators P1 to P4.

Table 2 shows the pitch 2×D2 and the duty ratio R2 in the series resonators S1 to S5.

As shown in Table 1, the pitches D1 of the parallel resonators P1 to P4 are different from each other. The duty ratio R1 is 50% and is the same for each. As shown in Table 2, the pitches D2 of the series resonators S1 to S5 are different from each other. The duty ratio R1 of the series resonator S3, which has the largest pitch D2 among the series resonators S1 to S5, is 30%. The duty ratio R2 of the series resonators S2 and S4, which have a smaller pitch D2 than the series resonators S3, is 40%. The duty ratio R2 of the series resonators S1 and S5, which have a smaller pitch D2 than the series resonators S2 and S4, is 50%.

Figures 10(a) and 10(b) are schematic diagrams showing the pass characteristics of a filter and a parallel resonator. Figure 10(b) is an enlarged view of the attenuation of Figure 10(a). As shown in Figures 10(a) and 10(b), the resonant frequency frp and the anti-resonant frequency fap of the parallel resonators P1 to P4 depend on the pitch D1. By making the resonant frequencies frp of the parallel resonators P1 to P4 different, the attenuation range on the low-frequency side of the pass band Pass can be widened. The shoulder on the low-frequency side of the pass band Pass is formed by the parallel resonators P2 and P3 with the highest resonant frequency frp. As shown in Figure 6, since the TCF of the resonant frequency fr is close to 0, the duty ratio R1 of the parallel resonators P1 to P4 is determined taking into account the resonant characteristics. For example, the duty ratio R1 is set to 50%.

Figures 11(a) and 11(b) are schematic diagrams showing the pass characteristics of a filter and a series resonator. Figure 11(b) is an enlarged view of the attenuation of Figure 11(a). As shown in Figures 11(a) and 11(b), the resonance frequency frs and the anti-resonance frequency fas of the series resonators S1 to S5 depend on the pitch D2. As shown in Figure 2(b), the resonance frequency frs also depends on the duty ratio R2, but since it depends mainly on the pitch D2 rather than the duty ratio R2, the magnitude relationship of the anti-resonance frequency fas depends almost entirely on the magnitude relationship of the pitch D2, even if the duty ratio R2 is different in the series resonators S1 to S5. By making the anti-resonance frequencies fas of the series resonators S1 to S5 different, the attenuation range on the high-frequency side of the pass band Pass can be widened. The high-frequency shoulder of the pass band Pass is formed by the parallel resonator S3, which has the lowest anti-resonance frequency fas.

In the first modification of the first embodiment, the high-frequency shoulder of the passband Pass is mainly reduced by reducing the duty ratio R2 of the series resonator S3, which has the lowest anti-resonance frequency fas (i.e., the largest pitch D2), to bring the TCF of the anti-resonance frequency fas closer to 0. This allows the TCF of the high-frequency shoulder of the passband Pass to approach 0. The next series resonators that affect the high-frequency shoulder of the passband Pass are the series resonators S2 and S4, which have the next lowest anti-resonance frequency fas. Therefore, the duty ratio R2 of the series resonators S2 and S4 is next reduced to bring the TCF of the anti-resonance frequency fas closer to 0. The series resonators S1 and S5, which have high anti-resonance frequencies fas, do not have much effect on the low-frequency shoulder of the passband Pass. Therefore, the duty ratio R2 of the series resonators S2 and S4 is set to be approximately the same as the duty ratio R1 of the parallel resonators P1 to P4, taking into account the resonance characteristics.

In this way, the duty ratio R2 of the series resonator S3, which affects the shoulder on the high frequency side of the passband Pass, is made smaller than the duty ratio R1 of the parallel resonators P1 to P4, and the TCF of the shoulder on the high frequency side of the passband Pass is reduced. The duty ratio R2 of the series resonators S1 and S5, which does not affect the shoulder on the low frequency side of the passband Pass very much, is made approximately the same as the duty ratio R1 of the parallel resonators P1 to P4. This improves the resonance characteristics.

[Modification 2 of Example 1]
FIG. 12(a) is a cross-sectional view of an elastic wave resonator in the second modification of the first embodiment. As shown in FIG. 12(a), in the second modification of the first embodiment, a boundary layer 11 is provided as an insulating layer between the temperature compensation film 12 and the support substrate 10. The sound velocity of the bulk wave in the boundary layer 11 is faster than the sound velocity of the bulk wave in the temperature compensation film 12. The boundary layer 11 is, for example, a polycrystalline film or an amorphous film such as aluminum oxide, aluminum nitride, silicon, silicon nitride, or silicon carbide. By providing the boundary layer 11, the elastic wave of the main mode can be confined in the piezoelectric layer 14 and the temperature compensation film 12, and unwanted waves such as bulk waves can be attenuated in the boundary layer 11. Therefore, spurious due to unwanted waves can be suppressed. The thickness of the boundary layer 11 is, for example, 1λ to 5λ. The other configurations are the same as those in the first embodiment, and description thereof will be omitted.

[Modification 3 of Example 1]
FIG. 12B is a cross-sectional view of an elastic wave resonator in the third modification of the first embodiment. As shown in FIG. 12B, in the third modification of the first embodiment, a bonding layer 13 is provided between the piezoelectric layer 14 and the temperature compensation film 12. The bonding layer 13 bonds the piezoelectric layer 14 and the temperature compensation film 12. When it is difficult to directly bond the piezoelectric layer 14 and the temperature compensation film 12, the bonding layer 13 may be provided. The bonding layer 13 is, for example, an aluminum oxide film, a silicon film, an aluminum nitride film, a silicon nitride film, or a silicon carbide film. From the viewpoint of not impairing the functions of the piezoelectric layer 14 and the temperature compensation film 12, the thickness of the bonding layer 13 is preferably 20 nm or less, more preferably 10 nm or less. From the viewpoint of not impairing the function as the bonding layer 13, the thickness of the bonding layer 13 is preferably 1 nm or more, more preferably 2 nm or more. From the viewpoint of confining the main mode elastic wave in the piezoelectric layer 14, it is preferable that the acoustic velocity of the bulk wave propagating through the bonding layer 13 is faster than the acoustic velocity of the bulk wave propagating through the temperature compensation film 12. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[Fourth Modification of the First Embodiment]
Fig. 12(c) is a cross-sectional view of an acoustic wave resonator according to Modification 4 of Example 1. As shown in Fig. 12(c), in Modification 4 of Example 1, a temperature compensation film is not provided between the piezoelectric layer 14 and the support substrate 10. The piezoelectric layer 14 is bonded in series onto the support substrate 10 by, for example, a surface activation method. The other configurations are the same as those of Example 1, and therefore a description thereof will be omitted.

[Fifth Modification of the First Embodiment]
Fig. 13(a) is a cross-sectional view of an elastic wave resonator according to the fifth modification of the first embodiment. As shown in Fig. 13(a), in the fifth modification of the first embodiment, periodic or irregular irregularities are provided at the interface between the support substrate 10 and the boundary layer 11. The irregularities scatter unwanted waves, suppressing spurious emissions. The interface between the boundary layer 11 and the temperature compensation film 12 is a substantially flat surface. The other configurations are the same as those of the third modification of the first embodiment, and therefore will not be described.

[Sixth Modification of the First Embodiment]
Fig. 13(b) is a cross-sectional view of an elastic wave resonator according to the sixth modification of the first embodiment. As shown in Fig. 13(b), in the sixth modification of the first embodiment, in addition to the interface between the support substrate 10 and the boundary layer 11, periodic or irregular irregularities are provided at the interface between the boundary layer 11 and the temperature compensation film 12. Unwanted waves are scattered by the two layers of irregularities, and spurious emissions can be suppressed. The other configurations are the same as those of the fifth modification of the first embodiment, and therefore description thereof will be omitted.

[Seventh Modification of the First Embodiment]
Fig. 13C is a cross-sectional view of an acoustic wave resonator according to the seventh modification of the embodiment 1. As shown in Fig. 13C, in the seventh modification of the embodiment 1, periodic or irregular unevenness is provided at the interface between the support substrate 10 and the temperature compensation film 12. The other configurations are the same as those of the embodiment 1, and therefore the description thereof will be omitted.

In the second to seventh variations of the first embodiment, the thickness T4 of the piezoelectric layer 14 is set to be equal to or less than the wavelength λ, so that the TCF of the resonant frequency fr and the anti-resonant frequency fa depends on the duty ratio R. Therefore, by making the duty ratio R2 of the series resonator S smaller than the duty ratio R1 of the parallel resonator P, the TCF of the shoulder of the passband Pass can be brought closer to 0. As in the first embodiment and its first to third and fifth to seventh variations, an insulating layer may be provided between the support substrate 10 and the piezoelectric layer 14. The insulating layer may be a single layer made of a uniform material, or a layer in which multiple layers are laminated.

According to the first embodiment and its modified example, the parallel resonator P has a plurality of electrode fingers 18 (first electrode fingers) having an average pitch D1 (first average pitch) and an average duty ratio R1 (first average duty ratio). The series resonator S has a plurality of electrode fingers 18 (second electrode fingers) having an average pitch D2 (second average pitch) and an average duty ratio R2 (second average duty ratio). At this time, the largest average pitch D1 among the parallel resonators P1 to P4 is set to be twice or more the thickness T4 of the piezoelectric layer 14. In other words, the thickness T4 of the piezoelectric layer 14 is set to be equal to or less than the wavelength λ. As a result, as shown in FIG. 6, the TCF of the resonance frequency fr and the anti-resonance frequency fa depends on the duty ratio R. The average duty ratio R2 in at least one series resonator is set to be smaller than the smallest average duty ratio R1 among the average duty ratios R1 of the parallel resonators. This makes it possible to reduce the TCF at the high frequency end of the passband.

The largest average pitch D1 among the parallel resonators P1 to P4 is preferably 1.6 times or more the thickness T4 of the piezoelectric layer 14, and more preferably 1.2 times or more. The difference between the average duty ratio R2 [%] and the smallest average duty ratio R1 [%] in at least one series resonator is preferably 5% or more, and more preferably 10% or more. The average pitch D can be calculated by dividing the width of the IDT 22 in the X direction by the number of electrode fingers 18. The average duty ratio R can be calculated by dividing the width of the IDT 22 in the X direction by the total width of the electrode fingers 18.

When the piezoelectric layer is a 36° or more and 48° or less rotated Y-cut X-propagation lithium tantalate layer, the main mode elastic wave is an SH wave. In addition, in the parallel resonator P and the series resonator S, no dielectric film that covers the electrode fingers 18 and is thicker than the electrode fingers 18 is provided on the piezoelectric layer 14. As a result, the TCF of the resonant frequency fr and the antiresonant frequency fa depends on the duty ratio R, as shown in Figure 6.

At least one of the series resonators includes a series resonator having the largest average pitch D2 (series resonator S3 in Table 2, FIG. 11(a): first series resonator). This allows the TCF of the anti-resonant frequency fa of the series resonator that mainly forms the shoulder on the high frequency side of the pass band to be small, as in the first modification of the first embodiment, and therefore allows the TCF of the shoulder on the high frequency side of the pass band to be small.

The duty ratio R2 in the series resonators (series resonators S1, S2, S4 and S5 in Table 2 and FIG. 11(a): second series resonators) having a smaller average pitch D2 than that in the series resonator S3 is greater than the duty ratio R2 in the first series resonator S3. This improves the resonance characteristics of the second series resonators S1, S2, S4 and S5. The difference in duty ratio R2 [%] between the first and second series resonators is preferably 5% or more, and more preferably 10% or more.

As in the first embodiment, the largest average duty ratio R2 is set smaller than the smallest average duty ratio R1. In other words, all average duty ratios R2 of the series resonators S1 to S5 are smaller than all average duty ratios R1 of the parallel resonators P1 to P4. This makes it possible to reduce the TCF of the shoulder on the high frequency side of the passband. The difference between the largest average duty ratio R2 and the smallest average duty ratio R1 is preferably 5% or more, and more preferably 10% or more.

The largest average pitch D2 is smaller than the smallest average pitch D1. In other words, the resonant frequencies frs of all the series resonators S1 to S5 are higher than the resonant frequencies frp of all the parallel resonators P1 to P4. This allows a ladder-type filter to be formed.

As in the first modification of the first embodiment, since the TCF of the resonant frequency frp of the parallel resonators P1 to P4 is small, the average duty ratio R1 of the parallel resonators P1 to P4 is set to be approximately the same duty ratio R1 so as to improve the resonant characteristics. On the other hand, the duty ratio R2 of the series resonator S3, which mainly forms the shoulder on the high frequency side of the pass band, is made small, and the duty ratios R2 of the series resonators S1 and S5, which do not contribute much to forming the shoulder on the high frequency side of the pass band, are made approximately the same as the duty ratio R1 of the parallel resonators P1 to P4. This makes it possible to improve the resonant characteristics of the series resonators S1 and S5. In other words, it is preferable that the difference between the largest average duty ratio R2 and the smallest average duty ratio R2 is made larger than the difference between the largest average duty ratio R1 and the smallest average duty ratio R1.

Figure 14 is a circuit diagram of a duplexer according to an embodiment. As shown in Figure 14, 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 the transmission band among the 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 the reception band among the 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 a filter of the first embodiment and its modified examples.

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

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

REFERENCE SIGNS LIST 10 Support substrate 11 Boundary layer 12 Temperature compensation film 13 Bonding layer 14 Piezoelectric layer 16 Metal film 18 Electrode finger 20 Comb-shaped electrode 22 IDT
25 Intersection area 26 Acoustic wave resonator 40 Transmitting filter 42 Receiving filter

Claims (10)

A support substrate;
a piezoelectric layer provided on the support substrate;
one or more parallel resonators each including a plurality of first electrode fingers provided on the piezoelectric layer, each having a first average pitch and a first average duty ratio, the largest first average pitch being equal to or greater than twice the thickness of the piezoelectric layer, one end of each parallel resonator being connected to a path between an input terminal and an output terminal and the other end of each parallel resonator being connected to ground;
one or more series resonators each including a plurality of second electrode fingers provided on the piezoelectric layer and having a second average pitch and a second average duty ratio, the second average duty ratio of at least one series resonator being smaller than the smallest first average duty ratio, and connected in series between the input terminal and the output terminal;
Equipped with
the first average pitch and the first average duty ratio are a pitch and a duty ratio that are average values of the entirety of the plurality of first electrode fingers,
a ladder-type filter in which the second average pitch and the second average duty ratio are respectively average values of the pitch and duty ratio of the entire plurality of second electrode fingers ; The ladder filter of claim 1, wherein the piezoelectric layer is a Y-cut X-propagation lithium tantalate layer rotated 36° or more and 48° or less. a dielectric film covering the first electrode fingers and having a thickness greater than a thickness of the first electrode fingers is not provided on the piezoelectric layer in the one or more parallel resonators;
3. The ladder-type filter according to claim 2, wherein no dielectric film covering the second electrode fingers and having a thickness greater than that of the second electrode fingers is provided on the piezoelectric layer in the one or more series resonators. The ladder filter according to any one of claims 1 to 3, wherein the one or more series resonators are multiple series resonators, and the at least one series resonator includes a first series resonator having the largest second average pitch. The ladder filter according to claim 4, wherein the second average duty ratio of a second series resonator having a smaller second average pitch than the second average pitch of the first series resonator among the plurality of series resonators is larger than the second average duty ratio of the first series resonator. A ladder filter according to any one of claims 1 to 5, wherein the largest second average duty ratio is smaller than the smallest first average duty ratio. A ladder filter according to any one of claims 1 to 6, wherein the largest second average pitch is smaller than the smallest first average pitch. A ladder filter according to any one of claims 1 to 7, wherein the difference between the largest second average duty ratio and the smallest second average duty ratio is greater than the difference between the largest first average duty ratio and the smallest first average duty ratio. The ladder filter according to any one of claims 1 to 8, further comprising a temperature compensation film provided between the support substrate and the piezoelectric layer, the temperature coefficient of elastic constant having a sign opposite to that of the temperature coefficient of elastic constant of the piezoelectric layer. A multiplexer comprising the ladder filter according to any one of claims 1 to 9.

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