JP7657515B2 - 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-shaped electrodes is known. 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 (e.g., 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 (e.g., 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 (e.g., Patent Documents 3 and 4). It is known that the thickness of the piezoelectric layer on the support substrate is made different between a transmitting filter and a receiving filter (e.g., Patent Document 5).

JP 2017-34363 A JP 2019-201345 A JP 2018-93487 A Special table 2018-506930 publication JP 2019-186655 A JP 2015-73331 A JP 2017-152868 A

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 caused by unwanted waves such as bulk waves. However, in a ladder filter, if a spurious response occurs within the passband, the pass characteristics will deteriorate.

The present invention was developed in consideration of the above problems, and aims to improve the transmission characteristics.

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

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

In the above configuration, the thickness of the piezoelectric layer in the one or more series resonators can be configured to be approximately equal.

In the above configuration, the piezoelectric layer in all of the one or more parallel resonators can be thinner than the piezoelectric layer in all of the one or more series resonators.

In the above configuration, the thickness of the piezoelectric layer in the one or more series resonators can be approximately equal, and the thickness of the piezoelectric layer in the one or more parallel resonators can be approximately equal.

In the above configuration, the average pitch of the electrode fingers of the pair of comb electrodes in the at least one parallel resonator is larger than the average pitch of the 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.

In the above configuration, the thickness of the piezoelectric layer in at least one of the parallel resonators can be approximately equal, and the thickness of the piezoelectric layer in the other parallel resonator and the one or more series resonators can be approximately equal.

In the above configuration, the thickness of the piezoelectric layer in the multiple elastic wave resonators can be configured to be 1.4 times or less than the average pitch.

In the above configuration, the multiple acoustic wave resonators can be configured to include an insulating layer provided between the piezoelectric layer and the support substrate.

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

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

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

The present invention can improve transmission characteristics.

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

The following describes an embodiment of the present invention with reference to the drawings.

The elastic wave resonator used in the ladder filter according to the first embodiment will be described. Fig. 1(a) and Fig. 1(b) are a plan view and a cross-sectional view of the elastic 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 stacking 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.

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 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 ( _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 a shear horizontal (SH) 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.

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 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, and is, 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.

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.

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.

[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 (SiO ₂ )
Piezoelectric layer 14: 42° Y-cut X-propagation lithium tantalate substrate Pitch 2×D(λ): 5.7 μm

Figure 2 shows the pass characteristic of the shunt-connected elastic 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 elastic 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 Figure 2, in the shunt-connected elastic wave resonator, the insertion loss is maximum at the resonance frequency fr and minimum at the anti-resonance frequency fa. A spurious response is observed at a frequency fs of around 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.

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. Figure 3 shows (fs-fr)/fr versus thickness T4 in simulation 1. Dots indicate simulation points, and straight lines connect the dots. As shown in Figure 3, as the thickness T4 of the piezoelectric layer 14 decreases, fs shifts to the higher 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 increases, fs shifts to the lower 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 between 0.2λ and 0.5λ.

The phenomenon in which the frequency fs at which spurious responses due to bulk waves occur depends on the thickness T4 of the piezoelectric layer 14 has not been known until now. 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.

The thickness T4 of the piezoelectric layer 14 and the thickness T2 of the temperature compensation film 12 were changed, and the electromechanical coupling coefficient ^k2 and the Q value Qa at the anti-resonance frequency fa were simulated. Figures 4(a) and 4(b) are diagrams showing ^k2 and Qa, respectively, for the thickness T4 in simulation 1. The thickness T2 of the temperature compensation film 12 is 0.2λ. As shown in Figures 4(a) and 4(b), when the thickness T4 of the piezoelectric layer 14 is 0.7λ or less, ^k2 increases and Qa decreases as the thickness T4 decreases. When the thickness T4 is 0.2λ, compared to when the thickness T4 is 0.6λ, ^k2 is about 1.3 times and Qa is 0.65 times.

The ladder-type filter according to the first embodiment will be described based on 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 the series resonators and parallel resonators. 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 the path between the input terminal Tin and the output terminal Tout, and the other end is connected to the ground terminal Gnd and grounded.

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 region 44, and a parallel resonator P is provided in region 46. The thicknesses of the piezoelectric layer 14 and the temperature compensation film 12 in region 44 are T4a and T2a, respectively, and the thicknesses of the piezoelectric layer 14 and the temperature compensation film 12 in region 46 are T4b and T2b, respectively. The thicknesses T2a and T2b are approximately equal to the manufacturing error. The thickness T4a is greater 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 filter, the anti-resonance frequency fa of the series resonator S is higher than the pass band, and the resonant frequency fr of the series resonator P is lower than the pass band. Therefore, the pitch Da is smaller than the pitch Db. Besides 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, changing the duty ratio and the thickness of the metal film 16 does not change the resonant frequency fr and the anti-resonant frequency fa as much as changing the pitch D. Therefore, the resonant frequency fr and the anti-resonant frequency fa are set to the desired frequency by mainly changing the pitch D.

[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 resonators S1 to S4: 2 x Da (λa) = 5.1 μm
Parallel resonators P1 to P4: 2 x Db (λb) = 5.5 μm
Average resonance frequency fr
Series resonator S1 to S4: 730MHz
Parallel resonator P1 to P4: 690MHz
average antiresonance frequency fa
Series resonator S1 to S4: 760MHz
Parallel resonator P1 to P4: 710MHz
Thickness of the piezoelectric layer 14 in Comparative Example 1: Series resonators S1 to S4 and parallel resonators P1 to P4: 2.2 μm (0.4 λb)
Thickness of the piezoelectric layer 14 in the first embodiment: Series resonators S1 to S4: 2.2 μm (0.4 λb)
Parallel resonator P1 to P4: 1.1μm (0.2λb)

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. When the thickness T4 of the piezoelectric layer 14 in the series resonators S1 to S4 is 0.2λ, the electromechanical coupling coefficient ^k2 becomes high, as shown in Fig. 4(a), and the steepness of the high frequency end of the pass band (Pass) becomes low. Also, as shown in Fig. 4(b), the Q value Qa becomes low, and the insertion loss in the pass band (Pass) becomes large. In the first embodiment, the thickness T4 of the piezoelectric layer 14 is set to 0.4λ, which makes it possible to increase the steepness at the high frequency end and reduce the insertion loss in the pass band (Pass).

In the first embodiment, the thickness T4b of the piezoelectric layer 14 in the parallel resonators P1 to P4 is made smaller than the thickness T4a of the piezoelectric layer 14 in the series resonators S1 to S4. In this case, the electromechanical coupling coefficient ^k2 of the parallel resonators P1 to P4 becomes high, so the steepness of the low-frequency end of the pass band Pass becomes low. In addition, the Q value of the parallel resonators P1 to P4 becomes low, so the insertion loss in the pass band Pass becomes large. Therefore, in the second embodiment, it was considered to make the piezoelectric layer 14 in a part of the parallel resonators P1 to P4 thinner.

Figure 7 is a cross-sectional view of a series resonator and a parallel resonator in Example 2. As shown in Figure 7, some of the parallel resonators P1 to P4 are provided in an area 46 where the thickness of the piezoelectric layer 14 is T4b, and the remaining parallel resonators P' are provided in an area 44 where the thickness of the piezoelectric layer 14 is 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'.

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

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

Figure 8 is a diagram showing the pass characteristic of the ladder filter in Simulation 3. As shown in Figure 8, in the large attenuation range, 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.

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 losses at the low frequency end (703 MHz) and high frequency end (748 MHz) of the pass band (Pass) are 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

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 thought to be because in Comparative Example 1, the spurious response of the parallel resonator P2, which has 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, which have a high resonant frequency fr, are not located within the passband Pass, so even if the piezoelectric layer 14 of the parallel resonators P1 and P3 is made thicker, the insertion loss at the high frequency end does not increase. Rather, the insertion loss decreases because the Q value increases. The spurious response of the parallel resonator P4, which has the second lowest resonant frequency, overlaps slightly with the passband Pass. For this reason, the insertion loss at the low frequency end of Example 2b is smaller than that of Example 2c.

Figures 10(a) to 12 are cross-sectional views showing other examples of the series resonator and parallel resonator in Examples 1 and 2. As shown in Figure 10(a), a boundary layer 11 is provided between a support substrate 10 and a temperature compensation film 12. An insulating layer 13 includes a temperature compensation film 12 and a 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 Figure 5(b).

The sound velocity of the bulk wave propagating through the boundary layer 11 is faster than the sound velocity 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 sound velocity of the bulk wave propagating through the boundary layer 11 is slower than the sound velocity 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.

By providing the boundary layer 11, the high-frequency unwanted waves that reach the boundary layer 11 from the temperature compensation film 12 are attenuated while passing through the boundary layer 11. Therefore, the unwanted waves that are reflected at the interface with the support substrate 10 and reach 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 perspective, 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 perspective, 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.

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 approximately 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.

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 regions 44 and 46, the formation of a step on the upper surface of the piezoelectric layer 14 can be suppressed. The other configurations are the same as those in FIG. 5(b).

As shown in FIG. 11(a), the thicknesses T1a and T1b of the boundary layer 11 are approximately equal, and the 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 rest of the configuration is the same as in FIG. 10(a), and a description thereof will be omitted.

As shown in FIG. 11(b), the 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 rest of the configuration is the same as in FIG. 5(b), and a description thereof will be omitted.

As shown in FIG. 12, no step is formed on the top surface of the support substrate 10 between regions 44 and 46, but a step is formed on the top surface of the piezoelectric layer 14. The rest of the configuration is the same as in FIG. 10(b), so a description is omitted.

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

In Figs. 10(a) to 12, an example is described in which a series resonator S is provided in region 44 and a parallel resonator P is provided in region 46, as in Example 1. However, as in Example 2, a series resonator S and a parallel resonator P' may be provided in region 44, and a parallel resonator P may be provided in region 46.

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 less than the average pitch Db (0.5 λb) of the electrode fingers 18 in 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 P1 to S4. This allows the spurious response of the parallel resonator P to be shifted to a higher frequency than the passband Pass.

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 (0.4 λb) or less, and more preferably 0.6 times (0.3 λb) or less, of the average pitch Db of the electrode fingers 18 in the parallel resonator P. In addition, the thickness T4b is preferably 0.1 Db (0.05 λb) or more smaller than the thickness T4a, and more preferably 0.2 Db (0.1 λb) 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 0.1 Db (0.05 λb) or more, and more preferably 0.2 Db (0.1 λb) or more. To increase the Q value of the series resonator S, the thicknesses T4a and T4b of the piezoelectric layers 14 of all resonators are preferably 1.4 Db (0.7 λb) or less, more preferably 1.2 Db (0.6 λb) or less, and even more preferably 1.0 Db (0.5 λb) or less.

In a typical ladder-type filter, the resonant frequency of the series resonators S1 to S4 is higher than the resonant frequency of the parallel resonators P1 to P4. Therefore, the spurious responses of the series resonators S1 to S4 rarely overlap the passband Pass. Therefore, the piezoelectric layer 14 in the parallel resonator P is made thinner than the piezoelectric layer 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 allows the number of manufacturing steps to be reduced. Note that the thickness being approximately equal to the manufacturing error allows an error of, for example, about 10%. 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.

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 prevent 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.

As in the second embodiment, the average pitch Db of the electrode fingers 18 of the comb-shaped electrodes 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 layer 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 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 thickness T4a of the piezoelectric layer 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.

As shown in Fig. 10(a) and Fig. 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 Fig. 5(b), Fig. 7, Fig. 10(a), Fig. 10(b), Fig. 11(a) and Fig. 11(b), the series resonators S1 to S4 and the parallel resonators P1 to P4 may include a temperature compensation film 12. It is preferable that 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 equal to or less than twice the pitch Db (λb). This allows the energy of the main mode elastic wave to exist in the piezoelectric layer 14 and the temperature compensation film 12. A bonding layer may be provided between the temperature compensation film 12 and the piezoelectric layer 14 to bond the temperature compensation film 12 and the piezoelectric layer 14 by a surface activation method. The bonding layer is, for example, an aluminum oxide layer or an aluminum oxynitride film having a thickness of 1 nm to 20 nm.

In the first and second embodiments, an example was described in which the number of series resonators was four and the number of parallel resonators was four, but the number of series resonators may be one or more, and the number of parallel resonators may be one or more.

Figure 13 is a circuit diagram of a duplexer according to the third embodiment. As shown in Figure 13, a transmission filter 40 is connected between the common terminal Ant and the transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and the 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 the filter of the first or second embodiment.

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 Insulating 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 44, 46 Area

Claims (12)

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: The ladder filter according to claim 1, wherein the piezoelectric layer in at least one parallel resonator is thinner than the piezoelectric layer in all of the one or more series resonators. The ladder filter of claim 2, wherein the thicknesses of the piezoelectric layers in the one or more series resonators are approximately equal. The ladder filter according to claim 1, wherein the piezoelectric layer in all of the one or more parallel resonators is thinner than the piezoelectric layer in all of the one or more series resonators. 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. 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. 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. The ladder filter according to any one of claims 1 to 7, wherein the thickness of the piezoelectric layer in the plurality of elastic wave resonators is 1.4 times or less than the average pitch. The ladder filter according to any one of claims 1 to 8, wherein the plurality of acoustic wave resonators include an insulating layer provided between the piezoelectric layer and the support substrate. 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. The ladder filter according to any one of claims 1 to 10, wherein the piezoelectric layer is a Y-cut X-propagation lithium tantalate substrate rotated 36° or more and 48° or less. A multiplexer comprising a filter according to any one of claims 1 to 11.

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