Trapezoidal filter and multiplexer

Abstract

The present application relates to ladder filters and multiplexers. A ladder filter includes a support substrate, a piezoelectric layer disposed on the support substrate, a parallel resonator including first electrode fingers disposed on the piezoelectric layer and having a first average pitch and a first average duty cycle, a maximum first average pitch being equal to or greater than twice a thickness of the piezoelectric layer, a first end of the parallel resonator coupled to a path between an input terminal and an output terminal, a second end of the parallel resonator coupled to a ground, and a series resonator connected in series between the input terminal and the output terminal, the series resonator including second electrode fingers disposed on the piezoelectric layer and having a second average pitch and a second average duty cycle, the second average duty cycle in at least one of the series resonators being less than a minimum first average duty cycle.

Description

Technical Field

[0001] One aspect of this embodiment relates to a ladder filter and a multiplexer. Background Technology

[0002] As acoustic resonators used in communication devices such as smartphones, it is known that acoustic resonators have a pair of comb-shaped electrodes formed on a piezoelectric layer. For example, as disclosed in Japanese Patent Application Publication No. 2017-34363 (Patent Document 1), it is known to bond the piezoelectric layer to a support substrate and adjust the thickness of the piezoelectric layer to be equal to or less than the wavelength of the sound wave. For example, as disclosed in Japanese Patent Application Publications Nos. 2019-201345 and 2015-73331 (Patent Documents 2 and 3), it is known to provide an insulating layer between the support substrate and the piezoelectric layer. For example, as disclosed in Japanese Patent Application Publication No. 2018-196028 (Patent Document 4), it is known to adjust the duty ratio of the parallel resonators to be less than the duty ratio of the series resonators in a trapezoidal filter using acoustic resonators in which a thick dielectric layer for temperature compensation is provided on the comb-shaped electrodes. Summary of the Invention

[0003] In a trapezoidal filter using an acoustic resonator with a piezoelectric layer and comb-shaped electrodes bonded to a support substrate, it is desirable to reduce the temperature coefficient of frequency (TCF).

[0004] According to one aspect of this disclosure, a trapezoidal filter is provided, comprising: a support substrate; a piezoelectric layer disposed on the support substrate; one or more parallel resonators, each parallel resonator including a plurality of first electrode fingers disposed on the piezoelectric layer, the first electrode fingers having a first average pitch and a first average duty cycle, the largest first average pitch being equal to or greater than twice the thickness of the piezoelectric layer, a first end of each of the one or more parallel resonators being connected to a path between an input terminal and an output terminal, and a second end of each of the one or more parallel resonators being connected to ground; and one or more series resonators connected in series between the input terminal and the output terminal, each of the one or more series resonators including a plurality of second electrode fingers disposed on the piezoelectric layer, the second electrode fingers having a second average pitch and a second average duty cycle, the second average duty cycle of at least one series resonator being less than the smallest first average duty cycle.

[0005] According to another aspect of this disclosure, a multiplexer including the trapezoidal filter described above is provided. Attached Figure Description

[0006] Figure 1AThis is a plan view of the acoustic resonator used in the trapezoidal filter according to the first embodiment, and Figure 1B and Figure 1C It is along Figure 1A A cross-sectional view taken from line AA in the diagram;

[0007] Figure 2A It is a graph showing the resonant frequency fr of the acoustic resonator in the first embodiment relative to the pitch D, and Figure 2B It is a curve of the resonant frequency fr relative to the duty cycle R;

[0008] Figure 3 This is a circuit diagram of a ladder filter according to the first embodiment;

[0009] Figure 4 This is a plan view of the trapezoidal filter according to the first embodiment;

[0010] Figure 5 This is a cross-sectional view of resonator B in Experiment 1;

[0011] Figure 6 This is a graph of the temperature coefficient (TCF) of the anti-resonant frequency fa and the resonant frequency fr relative to the duty cycle in Experiment 1.

[0012] Figure 7 This is a cross-sectional view of the resonator in Experiment 2;

[0013] Figure 8 This is a graph showing the temperature coefficient (TCF) of the anti-resonant frequency fa and the resonant frequency fr relative to the duty cycle in Experiment 2.

[0014] Figure 9A and Figure 9B The transmission characteristics of parallel and series resonators are schematically illustrated. Figure 9C The temperature dependence of the filter's transmission characteristics is illustrated schematically;

[0015] Figure 10A and Figure 10B The transmission characteristics of filters and parallel resonators are illustrated schematically;

[0016] Figure 11A and Figure 11B The transmission characteristics of filters and series resonators are illustrated schematically;

[0017] Figures 12A to 12C These are cross-sectional views of the acoustic resonators according to the second to fourth modifications of the first embodiment;

[0018] Figures 13A to 13C These are cross-sectional views of the acoustic resonators according to the fifth to seventh modifications of the first embodiment; and

[0019] Figure 14 This is a circuit diagram of a duplexer according to an implementation method. Detailed Implementation

[0020] Patent Document 4 describes an acoustic resonator in which a dielectric film thicker than the comb electrode is formed on the comb electrode disposed on a lithium niobate substrate, wherein the temperature coefficient of resonant frequency (TCF) and the temperature coefficient of anti-resonant frequency (TCF) depend on the duty cycle. In such an acoustic resonator, the Rayleigh wave is the dominant mode. Furthermore, it has been considered that when no thick dielectric film is disposed on the comb electrode, the temperature coefficients of the anti-resonant frequency and the temperature coefficients of the resonant frequency do not depend on the duty cycle. One such acoustic resonator uses a lithium tantalate substrate and has a shear level (SH) wave as the dominant mode. The inventors have discovered that even in an acoustic resonator where no thick dielectric film covering the comb electrode is disposed and the SH wave is the dominant mode, the temperature coefficients of the resonant frequency and the temperature coefficients of the anti-resonant frequency depend on the duty cycle when the piezoelectric layer is thinned. Embodiments based on the above findings will be described below.

[0021] First Implementation Method

[0022] The first embodiment is an exemplary trapezoidal filter. Figure 1A It is a plan view of the acoustic resonator used in the trapezoidal filter according to the first embodiment, and Figure 1B and Figure 1C It is along Figure 1A The cross-sectional view taken from line AA in the diagram. Figure 1B and Figure 1C These are cross-sectional views of the parallel resonator P and the series resonator S, respectively. The direction in which the electrode fingers are arranged (the arrangement direction of the electrode fingers) is defined as the X-direction, the direction in which the electrode fingers extend (the extension direction of the electrode fingers) is defined as the Y-direction, and the direction in which the supporting substrate and the piezoelectric layer are stacked (the stacking direction) is defined as the Z-direction. The X, Y, and Z directions do not necessarily correspond to the X-axis and Y-axis orientations of the piezoelectric layer's crystal orientation. In the case where the piezoelectric layer is a Y-cut X-propagation substrate, the X-direction is the X-axis orientation of the crystal orientation.

[0023] like Figures 1A to 1C As shown, a piezoelectric layer 14 is disposed above a support substrate 10. A temperature compensation film 12 is interposed between the support substrate 10 and the piezoelectric layer 14. An acoustic resonator 26 is disposed on the piezoelectric layer 14. The acoustic resonator 26 includes an interdigital transducer (IDT) 22 and a reflector 24. The reflector 24 is located on both sides of the IDT 22 in the X direction. The IDT 22 and the reflector 24 are formed from a metal film 16 on the piezoelectric layer 14.

[0024] The IDT 22 includes a pair of comb electrodes 20 facing each other. Each comb electrode 20 includes a plurality of electrode fingers 18 and a busbar 19 connecting the electrode fingers 18. Viewed from the X direction, the area where the electrode fingers 18 of one comb electrode in the pair overlap with the electrode fingers 18 of the other comb electrode in the pair is called an overlapping region 25. The length of the overlapping region 25 is the aperture length. The electrode fingers 18 of one comb electrode in the pair and the electrode fingers 18 of the other comb electrode in the pair are alternately arranged in at least a portion of the overlapping region 25. The acoustic waves excited primarily by the electrode fingers 18 in the overlapping region 25 propagate primarily in the X direction. The pitch of the electrode fingers 18 of one comb electrode in the pair is approximately equal to the wavelength λ of the comb electrode 20. When the pitch of the electrode fingers 18 (the pitch between the centers of the electrode fingers 18) is represented by D, the pitch of the electrode fingers 18 of one of the comb electrodes 20 is equal to twice the pitch D of the electrode fingers 18. The reflector 24 reflects the acoustic waves (surface acoustic waves) excited by the electrode fingers 18 of the IDT 22. As a result, the acoustic waves are confined within the overlapping region 25 of the IDT 22.

[0025] The piezoelectric layer 14 is, for example, a single-crystal lithium tantalate (LiTaO3) layer, and for example, a rotated Y-cut X-propagating lithium tantalate layer. In the case of a rotated Y-cut X-propagating lithium tantalate layer with a rotation of 36° or more and 48° or less, the SH wave is the dominant acoustic wave. The thickness of the piezoelectric layer 14 is equal to or less than the wavelength λ of the acoustic wave (i.e., equal to or less than twice the pitch D).

[0026] The support substrate 10 is, for example, a sapphire substrate, alumina substrate, silicon substrate, spinel substrate, crystal substrate, quartz substrate, or silicon carbide substrate. The sapphire substrate is a monocrystalline Al₂O₃ substrate. The alumina substrate is a polycrystalline or amorphous Al₂O₃ substrate. The silicon substrate is a monocrystalline or polycrystalline silicon substrate. The spinel substrate is a polycrystalline or amorphous MgAl₂O₄ substrate. The crystal substrate is a monocrystalline SiO₂ substrate. The quartz substrate is a polycrystalline or amorphous SiO₂ substrate. The silicon carbide substrate is a polycrystalline or monocrystalline SiC substrate. The linear expansion coefficient of the support substrate 10 in the X-direction is less than that of the piezoelectric layer 14 in the X-direction. This configuration can reduce the frequency-temperature dependence of the acoustic resonator.

[0027] The temperature compensation film 12 has a temperature coefficient of elasticity with the opposite sign to that of the piezoelectric layer 14. For example, the temperature coefficient of elasticity of the piezoelectric layer 14 has a negative value, while the temperature coefficient of elasticity of the temperature compensation film 12 has a positive value. The temperature compensation film 12 is an insulating film mainly composed of silicon oxide (SiO2), for example, an unadded silicon oxide (SiO2) film or a silicon oxide (SiO2) film with added elements such as fluorine, and is, for example, polycrystalline or amorphous. This configuration can reduce the frequency temperature coefficient of the acoustic resonator. When the temperature compensation film 12 is a silicon oxide film, the sound speed of the bulk wave propagating through the temperature compensation film 12 is less than the sound speed of the bulk wave propagating through the piezoelectric layer 14. The term "a layer mainly composed of a certain material" means that the layer may intentionally or unintentionally contain impurities, and the concentration of the certain material in the layer is, for example, 50 atomic percent or more, or 80 atomic percent or more. When the temperature compensation film 12 is mainly composed of silicon oxide, the sum of the oxygen concentration and silicon concentration in the temperature compensation film 12 is, for example, 50 atomic% or more, or 80 atomic% or more. The oxygen concentration and silicon concentration in the temperature compensation film 12 are each, for example, 10 atomic% or more, or 20 atomic% or more.

[0028] In order for the temperature compensation film 12 to have a temperature compensation function, the energy of the acoustic wave, which is the dominant mode, needs to be present in the temperature compensation film 12 to a certain extent. Although the energy concentration of the surface acoustic wave depends on the type of surface acoustic wave, the energy of the surface acoustic wave is generally concentrated in the portion from the upper surface of the piezoelectric layer 14 to a depth of 2λ (λ is the wavelength of the acoustic wave), especially in the portion from the upper surface of the piezoelectric layer 14 to a depth of λ. 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.

[0029] The metal film 16 is primarily composed of, for example, aluminum (Al), copper (Cu), or molybdenum (Mo). An adhesive film, such as a titanium (Ti) film or a chromium (Cr) film, can be disposed between the electrode finger 18 and the piezoelectric layer 14. The adhesive film is thinner than the electrode finger 18. An insulating film, thinner than the electrode finger 18, can be disposed to cover the electrode finger 18. The insulating film serves as a protective film.

[0030] When Figure 1B When the pitch of electrode fingers 18 in the parallel resonator P is represented by D1 and the width of electrode fingers 18 in the parallel resonator P is represented by E1, the duty cycle R1 = E1 / D1. When... Figure 1CWhen the pitch of the electrode finger 18 in the series resonator S is represented by D2 and the width of the electrode finger 18 in the series resonator S is represented by E2, the duty cycle R2 = E2 / D2. The pitch D2 of the series resonator S is smaller than the pitch D1 of the parallel resonator P, and the duty cycle R2 of the series resonator S is smaller than the duty cycle R1 of the parallel resonator P.

[0031] Figure 2A It is a graph showing the resonant frequency fr of the acoustic resonator in the first embodiment relative to the pitch D, and Figure 2B This is a graph of the resonant frequency fr relative to the duty cycle R. For example... Figure 2A As shown, the resonant frequency fr increases as the pitch D decreases. When the pitch D changes by 0.3 μm, the resonant frequency fr changes by approximately 500 MHz.

[0032] like Figure 2B As shown, the resonant frequency *fr* changes with the duty cycle *R*. Within a duty cycle *R* range of 60% or less, the resonant frequency *fr* increases as *R* decreases. Within a duty cycle *R* range of 70% or greater, the resonant frequency *fr* increases as *R* increases. When the duty cycle *R* changes by 40% from 60% to 20%, the resonant frequency *fr* changes by approximately 160 MHz. As can be seen from the above, the change in resonant frequency *fr* caused by the duty cycle *R* is relatively small. Therefore, the resonant frequency *fr* is primarily determined by the pitch *D*.

[0033] Figure 3 This is a circuit diagram of a ladder filter according to the first embodiment. For example... Figure 3 As shown, series resonators S1 to S5 are connected in series between the input terminal Tin and the output terminal Tout. The first ends of parallel resonators P1 to P4 are connected to the series path between the input terminal Tin and the output terminal Tout. The second ends of parallel resonators P1 and P2 are connected to the ground terminal Gnd through inductor L1, and the second ends of parallel resonators P3 and P4 are connected to the ground terminal Gnd through inductor L2.

[0034] Figure 4 This is a plan view of the trapezoidal filter according to the first embodiment. For example... Figure 4 As shown, acoustic resonators 26, wiring 30, and pads 32 are disposed on piezoelectric layer 14. Acoustic resonators 26 include an IDT 22 and a reflector 24. Acoustic resonators 26 include series resonators S1 to S5 and parallel resonators P1 to P4. Wiring 30 electrically connects the acoustic resonators 26 and also electrically connects the acoustic resonators 26 to the pads 32. Pads 32 are terminals for electrical connection to external devices and correspond to input terminal Tin, output terminal Tout, and ground terminal Gnd. Wiring 30 and pads 32 are formed of a metal layer including, for example, a gold layer, a copper layer, or an aluminum layer.

[0035] The number of series resonators S in a ladder filter can be one or more, and the number of parallel resonators P in a ladder filter can be one or more, and these numbers can be freely determined.

[0036] Experiment 1

[0037] For resonators A and B, measure the temperature coefficient of the resonant frequency and the temperature coefficient of the anti-resonant frequency. Resonator A has... Figures 1A to 1C The structure shown. Figure 5 This is a cross-sectional view of resonator B from Experiment 1. (Example) Figure 5 As shown, in resonator B, the temperature compensation film 12 is not provided, and the piezoelectric layer 14 is disposed on the supporting substrate 10. Other structures are similar to... Figures 1A to 1C The structure is the same as that of the other two, so its description is omitted.

[0038] The experimental conditions are as follows.

[0039] Support substrate 10: Sapphire substrate

[0040] Temperature compensation film 12: Silicon oxide film

[0041] Piezoelectric layer 14: 42° rotated Y-cut X-propagation lithium tantalate substrate

[0042] Metal film 16: Aluminum film

[0043] Resonator A

[0044] The wavelength λ of the sound wave is 5.0 μm.

[0045] The thickness T2 of the temperature compensation film 12 is 2.0 μm (0.4λ).

[0046] The thickness T4 of piezoelectric layer 14 is 2.0 μm (0.4λ).

[0047] Resonator B

[0048] Sound wavelength λ: 4.9μm

[0049] The thickness T4 of piezoelectric layer 14 is 20 μm (4.1λ).

[0050] For resonators A and B, the temperature coefficient of resonant frequency fr (TCF) and the temperature coefficient of anti-resonant frequency fa (TCF) are measured for different duty cycles R. Figure 6 This is a graph showing the temperature coefficients of the resonant frequency *fr* and the anti-resonant frequency *fa* in Experiment 1, relative to the duty cycle. Points represent measurement points, while straight lines are approximate lines fitted using the least squares method. (Example:) Figure 6As shown, in both resonators A and B, the difference between the temperature coefficient of the resonant frequency *fr* and the temperature coefficient of the anti-resonant frequency *fa* is approximately 15 ppm / K. In both resonators A and B, the temperature coefficient of the resonant frequency *fr* is close to 0, but the temperature coefficient of the anti-resonant frequency *fa* is negative and large (i.e., low). Furthermore, in resonator B, the TCF hardly changes even when the duty cycle R changes. On the other hand, in resonator A, as the duty cycle R decreases, the TCF shifts towards the positive side and becomes closer to 0.

[0051] Experiment 2

[0052] In Experiment 1, the difference between resonator A and resonator B lies in the presence of a temperature compensation film 12. Furthermore, the thickness T4 of the piezoelectric layer 14 in resonator B is 4.1λ, while in resonator A it is 0.6λ. Therefore, Experiment 2 was conducted to investigate whether the dependence of the TCF on the duty cycle R in resonator A is due to the thickness T4 of the piezoelectric layer 14 or due to the temperature compensation film 12.

[0053] Figure 7 This is a cross-sectional view of the resonator in Experiment 2. (For example...) Figure 7 As shown, the piezoelectric layer 14 is a single layer and has neither a supporting substrate nor an insulating layer. Other structures are similar to... Figures 1A to 1C The structures shown are identical, therefore their descriptions are omitted. The experimental conditions are as follows.

[0054] Piezoelectric layer 14: 42° rotated Y-cut X-propagation lithium tantalate substrate

[0055] Metal film 16: Aluminum film

[0056] Resonator C

[0057] Sound wave wavelength λ: 2.2μm

[0058] The thickness T4 of piezoelectric layer 14 is 1.32 μm (0.6λ).

[0059] Resonator D

[0060] Sound wave wavelength λ: 2.2μm

[0061] The thickness T4 of piezoelectric layer 14 is 8.8 μm (4.0λ).

[0062] For resonators C and D, the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa are measured for different duty cycles R. Figure 8 This is a graph showing the temperature coefficient of friction (TCF) of the resonant frequency *fr* and the temperature coefficient of friction (TCF) of the anti-resonant frequency *fa* relative to the duty cycle in Experiment 2. Points represent measurement points, while straight lines are approximate lines fitted using the least squares method. (Example:) Figure 8As shown, in resonator D, the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa depend slightly on the duty cycle R, while in resonator C, as the duty cycle R decreases, the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa become closer to 0.

[0063] As can be seen from the above, as the thickness T4 of the piezoelectric layer 14 decreases, the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa become dependent on the duty cycle R. Therefore, the reason why the TCF in resonator A in Experiment 1 depends on the duty cycle R is thought to be because the thickness T4 of the piezoelectric layer 14 is small.

[0064] It is unclear why the TCF depends on the duty cycle R when the thickness T4 of the piezoelectric layer 14 is small, but this may be related to the fact that surface acoustic waves (such as SH waves) are concentrated at a depth from the surface of the piezoelectric layer 14 to approximately the wavelength λ of the sound wave. Considering this fact, when the thickness T4 of the piezoelectric layer 14 is λ or less, the TCF depends on the duty cycle R. When the thickness of the piezoelectric layer 14 is adjusted to λ or less, the mechanical strength of the piezoelectric layer 14 decreases. Therefore, the piezoelectric layer 14 is bonded to the support substrate 10.

[0065] Problems in filter characteristics

[0066] A description of the problem will be given for the case where the trapezoidal filter is formed such that the duty cycle R1 in the parallel resonator P in resonator A or B is the same as the duty cycle R2 in the series resonator S.

[0067] Figure 9A and Figure 9B The transmission characteristics of parallel and series resonators are schematically illustrated. Figure 9C The temperature dependence of the filter's transmission characteristics is illustrated. Figure 9B Enlarged Figure 9A The attenuation in the filter. The transmission characteristic of the filter is represented by F, the transmission characteristic of the parallel resonator P is represented by P, and the transmission characteristic of the series resonator S is represented by S. The transmission characteristic of the parallel resonator P is the transmission characteristic of the parallel resonator P in the case of shunt connection.

[0068] like Figure 9A and Figure 9BAs shown, the attenuation poles of filter F at frequencies below the passband Pass are formed by the resonant frequency frp of the parallel resonator P, while the attenuation poles at frequencies above the passband Pass are 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 passband Pass. From the above, it can be seen that the low-frequency side skirt of the passband Pass in the trapezoidal filter is mainly formed by the resonant frequency frp of the parallel resonator P, while the high-frequency side skirt of the passband Pass in the trapezoidal filter is formed by the anti-resonant frequency fas of the series resonator S.

[0069] like Figure 9C As shown, even when the temperature changes from -30℃ to 80℃, the frequency of the low-frequency side skirt of the passband Pass changes very little, but the frequency of the high-frequency side skirt of the passband Pass decreases as the temperature changes from -30℃ to 80℃. The reason is as follows: The temperature coefficient of the frequency of the low-frequency side skirt of the passband Pass, formed by the resonant frequency frp of the parallel resonator P, is small. This is because the temperature coefficient of the resonant frequency fr is close to 0 in both resonators A and B. Figure 6 As shown. However, since the temperature coefficient of the anti-resonant frequency fa is 15 ppm / K smaller than that of the resonant frequency fr, the temperature coefficient of the high-frequency side skirt formed by the anti-resonant frequency fas of the series resonator S in the passband Pass is negative and large.

[0070] In the first embodiment, the duty cycle R2 of the series resonator S is adjusted to be smaller than the duty cycle R1 of the parallel resonator P. This configuration allows the temperature coefficient of the anti-resonant frequency fas of the series resonator S to be closer to 0, such as... Figure 6 As shown. For example, the duty cycle R1 in the parallel resonator P is adjusted to 50%, and the duty cycle R2 in the series resonator S is adjusted to 30%. This configuration enables the temperature coefficient of the resonant frequency frp of the parallel resonator P to be approximately -6 ppm / K, and the temperature coefficient of the anti-resonant frequency fas of the series resonator S to be approximately -16 ppm / K. Compared to the case where the duty cycle R2 in the series resonator S is adjusted to 50%, the temperature coefficient of the anti-resonant frequency fas of the series resonator S is increased by approximately 6 ppm / K. By reducing the duty cycle R by 5%, the temperature coefficient of the anti-resonant frequency fa is increased by approximately 1 ppm / K. Therefore, the duty cycle R1 - duty cycle R2 (the difference between duty cycles R1 and R2) is preferably 5% or greater, more preferably 10% or greater, and even more preferably 20% or greater.

[0071] First variation of the first embodiment

[0072] A first variation of the first embodiment is an example in which the resonant frequencies frp of the parallel resonators P1 to P4 are different from each other, and the anti-resonant frequencies fas of the series resonators S1 to S5 are different from each other. Table 1 lists the pitch and duty cycle R1 of the parallel resonators P1 to P4, which is 2×D1.

[0073] Table 1

[0074] P1 P2 P3 P4 2×D1[μm] 1.675 1.652 1.652 1.666 R1[％] 50 50 50 50

[0075] Table 2 lists the pitch and duty cycle R2 of the series resonators S1 to S5, which are 2×D2.

[0076] Table 2

[0077] S1 S2 S3 S4 S5 2×D2[μm] 1.519 1.555 1.596 1.555 1.515 R1[％] 50 40 30 40 50

[0078] As shown in Table 1, the pitches D1 of the parallel resonators P1 to P4 are different from each other. The duty cycle R1 is 50% and is equal for all of them. As shown in Table 2, the pitches D2 of the series resonators S1 to S5 are different from each other. In series resonator S3, which has the largest pitch D2 among the series resonators S1 to S5, the duty cycle R1 is 30%. In series resonators S2 and S4, which have a smaller pitch D2 than series resonator S3, the duty cycle R2 is 40%. In series resonators S1 and S5, which have a smaller pitch D2 than series resonators S2 and S4, the duty cycle R2 is 50%.

[0079] Figure 10A and Figure 10B The transmission characteristics of filters and parallel resonators are illustrated schematically. Figure 10B Enlarged Figure 10A Attenuation in. For example... Figure 10A and Figure 10B As shown, the resonant frequencies frp and anti-resonant frequencies 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 from each other, the attenuation range below the passband Pass can be widened. The low-frequency side skirt of the passband Pass is formed by the parallel resonators P2 and P3, which have the highest resonant frequency frp. Figure 6 As shown, the temperature coefficient of the resonant frequency fr is close to 0. Therefore, the duty cycle R1 in the parallel resonators P1 to P4 is determined taking into account the resonant characteristics. For example, the duty cycle R1 is adjusted to 50%.

[0080] Figure 11A and Figure 11B The transmission characteristics of filters and series resonators are illustrated schematically. Figure 11B Enlarged Figure 11A Attenuation in. For example... Figure 11A and Figure 11BAs shown, the resonant frequencies frs and anti-resonant frequencies fas of the series resonators S1 to S5 depend on the pitch D2. Figure 2B As shown, the resonant frequency frs also depends on the duty cycle R2, but primarily on the pitch D2 rather than the duty cycle R2. Therefore, even when the duty cycles R2 of the series resonators S1 to S5 are different from each other, the magnitude relationship between the anti-resonant frequencies fas essentially depends on the magnitude relationship between the pitches D2. By making the anti-resonant frequencies fas of the series resonators S1 to S5 different from each other, the attenuation range above the passband Pass can be widened. The high-frequency side skirt of the passband Pass is formed by the parallel resonator S3, which has the lowest anti-resonant frequency fas.

[0081] In a first variation of the first embodiment, the duty cycle R2 in the series resonator S3, which has the lowest anti-resonant frequency fas (i.e., the maximum pitch D2), is minimized to bring the temperature coefficient of the anti-resonant frequency fas closer to 0. This configuration allows the TCF of the high-frequency side skirt of the passband Pass to be closer to 0. The series resonators that secondarily affect the high-frequency side skirt of the passband Pass are the series resonators S2 and S4, which have the second lowest anti-resonant frequency fas. Therefore, the duty cycle R2 in the series resonators S2 and S4 is the second lowest to bring the temperature coefficient of the anti-resonant frequency fas closer to 0. The series resonators S1 and S5, which have high anti-resonant frequencies fas, have little effect on the low-frequency side skirt of the passband Pass. Therefore, considering the resonance characteristics, the duty cycle R2 in the series resonators S2 and S4 is adjusted to be the same as the duty cycle R1 in the parallel resonators P1 to P4.

[0082] As described above, the duty cycle R2 in the series resonator S3, which affects the high-frequency side of the passband, is adjusted to be smaller than the duty cycle R1 in the parallel resonators P1 to P4, so that the high-frequency side of the passband is smaller. The duty cycle R2 in the series resonators S1 and S5, which have little effect on the low-frequency side of the passband, is adjusted to be basically the same as the duty cycle R1 in the parallel resonators P1 to P4. This configuration can improve the resonance characteristics.

[0083] Second variation of the first embodiment

[0084] Figure 12A This is a cross-sectional view of an acoustic resonator according to a second modification of the first embodiment. (See diagram below.) Figure 12AAs shown, in a second variation of the first embodiment, a boundary layer 11, serving as an insulating layer, is inserted between the temperature compensation film 12 and the support substrate 10. The sound velocity of bulk waves propagating through the boundary layer 11 is greater than that of bulk waves propagating through the temperature compensation film 12. The boundary layer 11 is a polycrystalline or amorphous film, for example, composed of aluminum oxide, aluminum nitride, silicon, silicon nitride, or silicon carbide. The provision of the boundary layer 11 allows the dominant acoustic wave to be confined within the piezoelectric layer 14 and the temperature compensation film 12, and unwanted waves, such as bulk waves, are attenuated in the boundary layer 11. Therefore, stray emissions caused by unwanted waves can be reduced. The thickness of the boundary layer 11 is, for example, 1λ to 5λ. Other structures are the same as in the first embodiment, and therefore their description is omitted.

[0085] Third variation of the first embodiment

[0086] Figure 12B This is a cross-sectional view of the acoustic resonator according to the third modification of the first embodiment. For example... Figure 12B As shown, in a third variation 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 to the temperature compensation film 12. A bonding layer 13 can be provided when it is difficult to directly bond the piezoelectric layer 14 to the temperature compensation film 12. The bonding layer 13 is, for example, an alumina film, a silicon film, an aluminum nitride film, a silicon nitride film, or a silicon carbide film. The thickness of the bonding layer 13 is preferably 20 nm or less, more preferably 10 nm or less, so as not to impair the function of the piezoelectric layer 14 and the temperature compensation film 12. To avoid impairing the function of the bonding layer 13, the thickness of the bonding layer 13 is preferably 1 nm or more, more preferably 2 nm or more. To confine the sound wave, which is the dominant mode, within the piezoelectric layer 14, the sound velocity of the bulk wave propagating through the bonding layer 13 is preferably greater than the sound velocity of the bulk wave propagating through the temperature compensation film 12. Other structures are the same as those in the first embodiment, and therefore their description is omitted.

[0087] Fourth variation of the first embodiment

[0088] Figure 12C This is a cross-sectional view of the acoustic resonator according to the fourth modification of the first embodiment. For example... Figure 12C As shown, in the fourth variation of the first embodiment, no temperature compensation film is provided between the piezoelectric layer 14 and the support substrate 10. The piezoelectric layer 14 is directly bonded to the support substrate 10, for example, by surface activation. The other structures are the same as those in the first embodiment, and therefore their description is omitted.

[0089] Fifth variation of the first embodiment

[0090] Figure 13A This is a cross-sectional view of the acoustic resonator according to the fifth modification of the first embodiment. For example... Figure 13AAs shown, in the fifth variation of the first embodiment, protrusions and / or recesses arranged at regular intervals or irregularly arranged are provided on the boundary surface between the support substrate 10 and the boundary layer 11. The protrusions and / or recesses scatter unnecessary waves, thereby reducing stray emission. The boundary surface between the boundary layer 11 and the temperature compensation film 12 is substantially flat. Other structures are the same as those in the third variation of the first embodiment, and therefore their description is omitted.

[0091] The sixth variation of the first embodiment

[0092] Figure 13B This is a cross-sectional view of the acoustic resonator according to the sixth modification of the first embodiment. For example... Figure 13B As shown, in the sixth variation of the first embodiment, in addition to the boundary surface between the support substrate 10 and the boundary layer 11, protrusions and / or recesses arranged at regular intervals or irregularly arranged are also provided on the boundary surface between the boundary layer 11 and the temperature compensation film 12. The protrusions and / or recesses formed on the two layers scatter unnecessary waves, thereby reducing stray emissions. The other structures are the same as those in the fifth variation of the first embodiment, so their description is omitted.

[0093] The seventh variation of the first embodiment

[0094] Figure 13C This is a cross-sectional view of the acoustic resonator according to the seventh modification of the first embodiment. For example... Figure 13C As shown, in the seventh variation of the first embodiment, protrusions and / or recesses arranged at regular intervals or irregularly arranged are provided on the boundary surface between the support substrate 10 and the temperature compensation film 12. Other structures are the same as those in the first embodiment, and therefore their description is omitted.

[0095] Furthermore, in the second to seventh modifications of the first embodiment, by adjusting the thickness T4 of the piezoelectric layer 14 to be equal to or less than the wavelength λ, the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa depend on the duty cycle R. Therefore, by adjusting the duty cycle R2 in the series resonator S to be less than the duty cycle R1 in the parallel resonator P, the TCF of the skirt of the passband Pass can be made closer to 0. As in the first embodiment and its first to third and fifth to seventh modifications, an insulating layer can be provided between the support substrate 10 and the piezoelectric layer 14. The insulating layer can be a single layer composed of a uniform material, or it can be a layer formed by stacking multiple layers.

[0096] In the first embodiment and its variations, the parallel resonator P includes electrode fingers 18 (first electrode fingers) having an average pitch D1 (first average pitch) and an average duty cycle R1 (first average duty cycle). The series resonator S includes electrode fingers 18 (second electrode fingers) having an average pitch D2 (second average pitch) and an average duty cycle R2 (second average duty cycle). In this structure, the maximum average pitch D1 among the average pitches of the parallel resonators P1 to P4 is adjusted to be greater than twice the thickness T4 of the piezoelectric layer 14. That is, the thickness T4 of the piezoelectric layer 14 is adjusted to be equal to or less than the wavelength λ. This configuration makes the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa dependent on the duty cycle R, such as Figure 6 As shown, the average duty cycle R2 of at least one series resonator is adjusted to be less than the minimum average duty cycle R1 among the average duty cycles R1 of the parallel resonators. This configuration can reduce the TCF at the high-frequency end of the passband.

[0097] The maximum average pitch D1 among the average pitches of the parallel resonators P1 to P4 is preferably equal to or greater than 1.6 times the thickness T4 of the piezoelectric layer 14, more preferably equal to or greater than 1.2 times the thickness T4 of the piezoelectric layer 14. The difference between the average duty cycle R2 [%] and the minimum average duty cycle R1 [%] in at least one series resonator is preferably 5% or greater, more preferably 10% or greater. The average pitch D is calculated by dividing the width of the IDT 22 in the X direction by the number of electrode fingers 18. The average duty cycle R is calculated by dividing the width of the IDT 22 in the X direction by the sum of the widths of the electrode fingers 18.

[0098] When the piezoelectric layer is a Y-cutting X-propagating lithium tantalate layer rotating between 36° and 48°, the dominant acoustic mode is the SH wave. In the parallel resonator P and the series resonator S, no covering electrode finger 18 is provided on the piezoelectric layer 14, and a dielectric film with a greater thickness than the electrode finger 18 is formed. This structure causes the temperature coefficients of the resonant frequency fr and the anti-resonant frequency fa to depend on the duty cycle R, such as... Figure 6 As shown.

[0099] At least one series resonator includes a series resonator with the maximum average pitch D2 (Table 2 and Figure 11A S3 (the first series resonator). As in the first variation of the first embodiment, this configuration can reduce the temperature coefficient of the anti-resonance frequency fa of the series resonator that mainly forms the high-frequency side skirt of the passband, and thus can reduce the TCF of the high-frequency side skirt of the passband.

[0100] Series resonators with an average pitch D2 smaller than that in series resonator S3 (Table 2 and Figure 11AThe duty cycle R2 of the series resonators S1, S2, S4, and S5 (the second series resonator) is greater than the duty cycle R2 of the first series resonator S3. This configuration improves the resonance characteristics of the second series resonators S1, S2, S4, and S5. The difference between the duty cycle R2[%] of the first series resonator and the duty cycle R2[%] of the second series resonator is preferably 5% or greater, more preferably 10% or greater.

[0101] In the first embodiment, the maximum average duty cycle R2 is adjusted to be less than the minimum average duty cycle R1. That is, the average duty cycle R2 of each of the series resonators S1 to S5 is less than the average duty cycle R1 of all the parallel resonators P1 to P4. This configuration can reduce the TCF of the high-frequency side skirt in the passband. The difference between the maximum average duty cycle R2 and the minimum average duty cycle R1 is preferably 5% or greater, more preferably 10% or greater.

[0102] The maximum average pitch D2 is less than the minimum average pitch D1. That is, each of the resonant frequencies frs of the series resonators S1 to S5 is higher than all the resonant frequencies frp of the parallel resonators P1 to P4. Therefore, a trapezoidal filter can be formed.

[0103] In a first variation of the first embodiment, since the temperature coefficient of the resonant frequency frp of the parallel resonators P1 to P4 is small, the average duty cycle R1 of the parallel resonators P1 to P4 is adjusted to be substantially equal to each other to improve the resonance characteristics. On the other hand, the duty cycle R2 of the series resonator S3, which mainly forms the high-frequency side skirt of the passband, is adjusted to be small, and the duty cycle R2 of the series resonators S1 and S5, which contribute almost nothing to forming the high-frequency side skirt of the passband, is adjusted to be substantially the same as the duty cycle R1 of the parallel resonators P1 to P4. This configuration improves the resonance characteristics of the series resonators S1 and S5. That is, the difference between the maximum average duty cycle R2 and the minimum average duty cycle R2 is preferably greater than the difference between the maximum average duty cycle R1 and the minimum average duty cycle R1.

[0104] Second Implementation Method

[0105] Figure 14 This is a circuit diagram of a duplexer according to an implementation method. For example... Figure 14As shown, the transmitting filter 40 is connected between the common terminal Ant and the transmitting terminal Tx. The receiving filter 42 is connected between the common terminal Ant and the receiving terminal Rx. The transmitting filter 40 transmits the signal in the transmitting frequency band of the high-frequency signal input from the transmitting terminal Tx to the common terminal Ant as a transmitting signal, and suppresses signals with other frequencies. The receiving filter 42 transmits the signal in the receiving frequency band of the high-frequency signal input from the common terminal Ant to the receiving terminal Rx as a receiving signal, and suppresses signals with other frequencies. At least one of the transmitting filter 40 or the receiving filter 42 can be a filter according to any one of the first embodiment and its variations.

[0106] As a multiplexer, a duplexer has been described, but a multiplexer can also be a tripartite or quadrupler.

[0107] Although embodiments of the invention have been described in detail, the invention is not limited to such specific embodiments, and it should be understood that various changes, substitutions and modifications can be made thereto without departing from the spirit and scope of the invention.

Claims

1. A trapezoidal filter, the trapezoidal filter comprising: Support substrate; A piezoelectric layer is disposed on the supporting substrate; One or more parallel resonators, each parallel resonator including a plurality of first electrode fingers disposed on the piezoelectric layer, the first electrode fingers having a first average pitch and a first average duty cycle, the largest first average pitch being equal to or greater than twice the thickness of the piezoelectric layer, a first end of each of the one or more parallel resonators being connected to a path between an input terminal and an output terminal, and a second end of each of the one or more parallel resonators being connected to ground; as well as One or more series resonators are connected in series between the input terminal and the output terminal. Each of the series resonators includes a plurality of second electrode fingers disposed on the piezoelectric layer. The second electrode fingers have a second average pitch and a second average duty cycle. The second average duty cycle of at least one series resonator is less than a minimum first average duty cycle. Wherein, the first average pitch is the average of all pitches of the plurality of first electrode fingers, and the first average duty cycle is the average of all duty cycles of the plurality of first electrode fingers; and Wherein, the second average pitch is the average of all pitches of the plurality of second electrode fingers, and the second average duty cycle is the average of all duty cycles of the plurality of second electrode fingers.

2. The trapezoidal filter according to claim 1, wherein, The piezoelectric layer is a Y-cut X-propagation lithium tantalate layer with a rotation of 36° or more and 48° or less.

3. The trapezoidal filter according to claim 2, in, In each of the one or more parallel resonators, no dielectric film covering the first electrode finger and having a thickness greater than that of the first electrode finger is disposed on the piezoelectric layer. In each of the one or more series resonators, no dielectric film covering the second electrode finger and having a thickness greater than that of the second electrode finger is disposed on the piezoelectric layer.

4. The trapezoidal 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 with the largest second average pitch.

5. The trapezoidal filter according to claim 4, wherein, In the series resonators, the second series resonator with a second average pitch smaller than that of the first series resonator has a greater second average duty cycle than the first series resonator.

6. The trapezoidal filter according to any one of claims 1 to 5, wherein, The second highest average duty cycle is less than the first lowest average duty cycle.

7. The trapezoidal filter according to any one of claims 1 to 6, wherein, The largest second average pitch is smaller than the smallest first average pitch.

8. The trapezoidal filter according to any one of claims 1 to 7, wherein, The difference between the largest and smallest second average duty cycles is greater than the difference between the largest and smallest first average duty cycles.

9. The trapezoidal filter according to any one of claims 1 to 8, the trapezoidal filter further comprising a temperature compensation film disposed between the support substrate and the piezoelectric layer, the temperature compensation film having a temperature coefficient of elastic constant with the opposite sign to that of the elastic constant of the piezoelectric layer.

10. A multiplexer, the multiplexer comprising: The trapezoidal filter according to any one of claims 1 to 9.

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