JP7627174B2 - Ladder Filters and Multiplexers - Google Patents

Description

The present invention relates to ladder-type filters and multiplexers, for example ladder-type filters and multiplexers having piezoelectric thin-film resonators.

Ladder-type filters having piezoelectric thin-film resonators are known as filters used in communication devices such as mobile phones. A piezoelectric thin-film resonator has a structure in which a piezoelectric film is sandwiched between a lower electrode and an upper electrode. The region where the lower electrode and the upper electrode overlap, sandwiching at least a portion of the piezoelectric film, is the resonance region. It is known to insert an insertion film between the lower electrode and the upper electrode in the resonance region (e.g., Patent Documents 1 to 3). It is known to use piezoelectric thin-film resonators in ladder-type filters (e.g., Patent Documents 4 to 7).

JP 2015-95729 A JP 2016-29766 A JP 2017-34358 A JP 2003-22074 A JP 2004-173236 A JP 2005-223808 A JP 2007-324823 A

In ladder type filters, the electromechanical coupling coefficients of the piezoelectric thin film resonators are sometimes made different in order to increase the steepness of the skirt characteristics and broaden the bandwidth. To manufacture a ladder type filter having piezoelectric thin film resonators with different electromechanical coupling coefficients, the number of manufacturing steps increases, resulting in higher manufacturing costs.

The present invention was developed in consideration of the above problems, and aims to improve characteristics and prevent an increase in the manufacturing process.

The present invention relates to a piezoelectric thin-film resonator comprising a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other, at least some of the piezoelectric thin-film resonators having an insertion film provided between the first electrode and the second electrode in the resonance region, a first series resonator included in the plurality of piezoelectric thin-film resonators that is connected in series between a first terminal and a second terminal and is electrically closest to the second terminal, the first series resonator having the thinnest insertion film among the plurality of series resonators, and at least one second series resonator other than the first series resonator having a thicker insertion film than the first series resonator, and a series resonator having one end that is connected in a path between the first terminal and the second terminal. The ladder-type filter includes a first parallel resonator that is electrically closest to the second terminal and is connected to at least one first resonator among the plurality of piezoelectric thin-film resonators and has the thinnest insertion film or does not have an insertion film, and at least one second parallel resonator other than the first parallel resonator has a thicker insertion film than the first parallel resonator; an inductor having one end connected to at least one first resonator among the plurality of parallel resonators and the other end grounded; and a second resonator that is at least one of the plurality of parallel resonators and a parallel resonator to which the grounded inductor is not connected, and has an insertion film that is substantially the same thickness as the insertion film of the parallel resonator to which the inductor is connected.

In the above configuration, the first resonator may include the first parallel resonator, and the second resonator may include the first series resonator.

In the above configuration, among the plurality of series resonators and the plurality of parallel resonators, the first parallel resonator may have the thinnest insertion film, and the second series resonator may have the second thinnest insertion film.

In the above configuration, the third parallel resonator, which is electrically closest to the first terminal among the multiple parallel resonators, can be configured to have the thickest insertion film among the multiple series resonators and the multiple parallel resonators.

The present invention relates to a piezoelectric thin-film resonator comprising a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other, at least some of the piezoelectric thin-film resonators having an insertion film provided between the first electrode and the second electrode in the resonance region, a first series resonator included in the plurality of piezoelectric thin-film resonators that is electrically closest to the second terminal, has the thinnest insertion film among the plurality of series resonators, and at least one second series resonator other than the first series resonator has a thicker insertion film than the first series resonator, and one end of the series resonator is connected to a path between the first terminal and the second terminal. , the other end of which is grounded, and a first parallel resonator included in the plurality of piezoelectric thin-film resonators and electrically closest to the second terminal has the thinnest insertion film or does not have an insertion film among the plurality of parallel resonators, and at least one second parallel resonator other than the first parallel resonator has a thicker insertion film than the first parallel resonator; a capacitor connected in parallel to the plurality of series resonators and at least one first resonator among the plurality of parallel resonators; and a second resonator that is at least one of the plurality of series resonators and the plurality of parallel resonators, has no capacitor connected in parallel, and has an insertion film substantially the same thickness as the insertion film of the resonator to which the capacitor is connected in parallel.

In the above configuration, the first resonator may include the first series resonator, and the second resonator may include the first parallel resonator.

In the above configuration, among the plurality of series resonators and the plurality of parallel resonators, the first parallel resonator may have the thinnest insertion film, and the second series resonator may have the second thinnest insertion film.

In the above configuration, the third parallel resonator, which is electrically closest to the first terminal among the multiple parallel resonators, can be configured to have the thickest insertion film among the multiple series resonators and the multiple parallel resonators.

In the above configuration, the first resonator may include the third parallel resonator.

In the above configuration, the thickness of the piezoelectric film in the multiple piezoelectric thin film resonators can be substantially the same, the thickness of the first electrodes can be substantially the same, and the thickness of the second electrodes can be substantially the same.

The present invention relates to a plurality of piezoelectric thin-film resonators each including a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other; a first series resonator that is connected in series between a first terminal and a second terminal and is included in the plurality of piezoelectric thin-film resonators, the first series resonator being electrically closest to the second terminal and having the largest single electromechanical coupling coefficient among the plurality of series resonators, and at least one second series resonator other than the first series resonator having a smaller single electromechanical coupling coefficient than the first series resonator; and a plurality of series resonators each having one end connected to a path between the first terminal and the second terminal and the other end grounded and included in the plurality of piezoelectric thin-film resonators, The ladder-type filter includes a first parallel resonator electrically closest to the second terminal, which has the largest single electromechanical coupling coefficient among the multiple parallel resonators, and at least one second parallel resonator other than the first parallel resonator has a smaller single electromechanical coupling coefficient than the first parallel resonator, an inductor having one end connected to at least one first resonator among the multiple parallel resonators and the other end grounded, and a second resonator that is at least one of the multiple series resonators and a parallel resonator to which the grounded inductor is not connected, and has a single electromechanical coupling coefficient substantially the same as the single electromechanical coupling coefficient of the parallel resonator to which the inductor is connected.

The present invention relates to a plurality of piezoelectric thin-film resonators each including a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other; a first series resonator that is connected in series between a first terminal and a second terminal and is included in the plurality of piezoelectric thin-film resonators, and is electrically closest to the second terminal, has the largest single electromechanical coupling coefficient among the plurality of series resonators, and at least one second series resonator other than the first series resonator has a smaller single electromechanical coupling coefficient than the first series resonator; and a plurality of series resonators each having one end connected to a path between the first terminal and the second terminal and the other end grounded and included in the plurality of piezoelectric thin-film resonators, the second series resonator having the largest single electromechanical coupling coefficient among the plurality of series resonators. The ladder-type filter includes a plurality of parallel resonators, in which a first parallel resonator electrically closest to the terminal has the largest single electromechanical coupling coefficient among the plurality of parallel resonators, and at least one second parallel resonator other than the first parallel resonator has a single electromechanical coupling coefficient smaller than that of the first parallel resonator, a capacitor connected in parallel to the plurality of series resonators and at least one first resonator among the plurality of parallel resonators, and a second resonator that is at least one of the plurality of series resonators and the plurality of parallel resonators, in which a capacitor is not connected in parallel, and which has a single electromechanical coupling coefficient substantially the same as the single electromechanical coupling coefficient of the resonator to which the capacitor is connected in parallel.

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

The present invention can improve properties while minimizing the increase in manufacturing steps.

FIG. 1A is a circuit diagram of a ladder-type filter, and FIG. 1B is a circuit diagram in which the ladder-type filter of FIG. 1A is divided into sections. FIG. 2A is a diagram showing the pass characteristic of a filter designed using the simultaneous Chebyshev approximation, and FIG. 2B is an equivalent circuit of a piezoelectric thin film resonator. 3A and 3B are circuit diagrams of the ladder filter in Simulation 1. FIG. FIG. 4A is a diagram showing the electromechanical coupling coefficients of the resonators in the filters A1 to A3, and FIG. 4B is a diagram showing the electromechanical coupling coefficients of the resonators in the filters A4 to A6. 5A and 5B are cross-sectional views of the piezoelectric thin film resonator in the first embodiment. FIG. 6 is a diagram showing the electromechanical coupling coefficient ^k2 versus the thickness T8 of the insertion film. FIG. 7 is a circuit diagram of the ladder-type filter according to the first embodiment. FIG. 8A is an equivalent circuit when an inductance Lg is connected in series to a resonator, and FIG. 8B is a diagram showing the electromechanical coupling coefficient versus the magnitude of the inductance Lg. 9A and 9B are diagrams showing the pass characteristics of the filters B1 to B3 in the simulation 2. FIG. 10A and 10B are circuit diagrams of ladder-type filters according to first and second modifications of the first embodiment, respectively. FIG. 11A is an equivalent circuit when a capacitance Cp is connected in parallel to a resonator, and FIG. 11B and FIG. 11C are diagrams showing the electromechanical coupling coefficient versus the magnitude of the capacitance Cp. 12A and 12B are diagrams showing the pass characteristics of the filters B1, C1 and C2 in Simulation 3. FIG. 13A and 13B are diagrams showing the pass characteristics of the filters B1, C3 and C4 in Simulation 3. FIG. 14A and 14B are cross-sectional views of film bulk acoustic resonators according to third and fourth modifications of the first embodiment, respectively. FIG. 15 is a circuit diagram of a duplexer in accordance with the second embodiment.

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

[Ladder type filters and design methods]
Before describing the first embodiment, a design method of a ladder filter will be described. FIG. 1A is a circuit diagram of a ladder filter. As shown in FIG. 1A, in the ladder filter, a plurality of series resonators S1 to S3 are connected in series between terminals T1 (first terminal) and T2 (second terminal). A plurality of parallel resonators P1 to P3 are connected in parallel between terminals T1 and T2. One end of the parallel resonators P1 to P3 is connected to a path between terminals T1 and T2, and the other end is connected (grounded) to a ground terminal Gnd. One of the terminals T1 and T2 is an input terminal to which a high-frequency signal is input, and the other of the terminals T1 and T2 is an output terminal to which a filtered high-frequency signal is output.

[Image impedance method]
FIG. 1(b) is a circuit diagram in which the ladder filter of FIG. 1(a) is divided into sections. As shown in FIG. 1(b), the series resonator S1 is divided into two series resonators S1a and S1b, and the series resonator S2 is divided into two series resonators S2a and S2b. The parallel resonator P2 is divided into two parallel resonators P2a and P2b, and the parallel resonator P3 is divided into two parallel resonators P3a and P3b. Sections 21 to 25 including one parallel resonator and one series resonator are set. In this way, the ladder filter of FIG. 1(a) is a five-stage ladder filter having five sections 21 to 25. In the image impedance method, the filter is designed using the sections 21 to 25. In the design of the constant k type, the impedance of the sections 21 to 25 is designed to be a desired value. In the design of the induced m type, in addition to the constant k type, an attenuation pole is provided to make the skirt characteristic steep. However, the attenuation away from the passband is less than that of the fixed-k type.

The image impedance method is widely used in the design of ladder filters because the design calculations are simple. In a ladder filter with piezoelectric thin-film resonators designed using the image impedance method, the electromechanical coupling coefficients of multiple series resonators are almost the same, and the electromechanical coupling coefficients of multiple parallel resonators are almost the same. However, ladder filters designed using the image impedance method have a large number of piezoelectric thin-film resonators, which leads to deterioration of characteristics. There are limitations to the design of ladder filters that have steep skirt characteristics and a wide bandwidth.

[Dynamic Parameter Method]
The dynamic parameter method is a method for directly calculating the parameters of a filter circuit that most closely resembles the desired characteristics. The filter can be constructed with a smaller number of resonators, which improves characteristics. As an example of the dynamic parameter method, we considered designing a ladder filter using the simultaneous Chebyshev approximation. In the simultaneous Chebyshev approximation, a steep skirt characteristic can be obtained by forming a ripple in the passband and an attenuation pole in the attenuation band. We considered using the simultaneous Chebyshev approximation to design a ladder filter using piezoelectric thin film resonators.

FIG. 2(a) is a diagram showing the pass characteristic of a filter designed using the simultaneous Chebyshev approximation. As shown in FIG. 2(a), the maximum loss in the passband Pass of the filter is PdB. The bandwidth at the loss PdB is BW1. The minimum attenuation in the attenuation band Att is AdB. The bandwidth at the attenuation AdB is BWn. The slope of the skirt between the passband Pass and the attenuation band Att is the filter drop rate θ. The reflection coefficient ρ [%] = (10 ^RdB/1 -1)/(10 ^RdB /10), and θ = arcsin(BW1/BWn). n is the order. In the simultaneous Chebyshev approximation, a reference LPF (Low Pass Filter) is determined by determining three values out of four: characteristic parameters ρ, θ, and AdB, and design parameter n. A BPF (Band Pass Filter) having desired characteristics can be designed by converting the circuit of the reference LPF.

The BVD (Butterworth-Van Dyke) model is used for the equivalent circuit of the piezoelectric thin film resonator. FIG. 2B shows the equivalent circuit of the piezoelectric thin film resonator. As shown in FIG. 2B, the motional inductance Lm and motional capacitance Cm are connected in series between the terminals T01 and T02. The motional inductance Lm and motional capacitance Cm are connected in parallel with the capacitance C0. A ladder filter is designed based on the simultaneous Chebyshev approximation using the BVD model as the equivalent circuit of the piezoelectric thin film resonator. The resonant frequency fr of the resonator is fr=1/(2π√(Lm×Cm)), the anti-resonant frequency fa is fa=fr√(1+Cm/C0), and the electromechanical coupling coefficient k ² is k ² =(π/2) ² (fr/fa)(fa-fr)/fa.

[Simulation 1]
Six filters A1 to A6 were designed based on the simultaneous Chebyshev approximation by changing the characteristic parameters and design parameters. The parameters of each filter A1 to A6 are as follows.
Filter A1: θ=60°, AdB=59 dB, ρ=20%, number of stages: 5
Filter A2: θ=70°, AdB=44 dB, ρ=20%, number of stages: 5
Filter A3: θ=80°, AdB=29 dB, ρ=20%, number of stages: 5
Filter A4: θ=51°, AdB=48 dB, ρ=20%, number of stages: 3
Filter A5: θ=51°, AdB=73 dB, ρ=20%, number of stages: 5
Filter A6: θ=51°, AdB=98 dB, ρ=20%, number of stages: 7

Figure 1(a) shows a circuit diagram of a ladder filter with five stages. Figures 3(a) and 3(b) show circuit diagrams of ladder filters in Simulation 1. As shown in Figure 3(a), in a ladder filter with three stages, two series resonators S1 and S2 are connected in series between terminals T1 and T2, and two parallel resonators P1 and P2 are connected in parallel. As shown in Figure 3(b), in a ladder filter with seven stages, four series resonators S1 to S4 are connected in series between terminals T1 and T2, and four parallel resonators P1 to P4 are connected in parallel. The rest of the configuration is the same as in Figure 1(a), so a description is omitted.

Tables 1 to 6 show the capacitance C0, dynamic inductance Lm, dynamic capacitance Cm, resonance frequency fr, anti-resonance frequency fa, and electromechanical coupling coefficient ^k2 in the filters A1 to A6.

FIG. 4(a) is a diagram showing the electromechanical coupling coefficients of the resonators in the filters A1 to A3, and FIG. 4(b) is a diagram showing the electromechanical coupling coefficients of the resonators in the filters A4 to A6. As shown in FIG. 4(a) and FIG. 4(b), the electromechanical coupling coefficients k ² of the resonators are different. In the filters A1 to A6, the resonator with the largest electromechanical coupling coefficient k ² is the parallel resonator closest to the terminal T2 among the parallel resonators. The resonator with the second largest electromechanical coupling coefficient k ² is the series resonator closest to the terminal T2 among the series resonators. In the case of three and five stages, the resonator with the smallest electromechanical coupling coefficient k ² is the parallel resonator closest to the terminal T1 among the parallel resonators. The largest electromechanical coupling coefficient k ² is nearly twice the smallest electromechanical coupling coefficient k ^2. In this way, in a ladder filter designed using the simultaneous Chebyshev approximation, the electromechanical coupling coefficient k ² is made to differ greatly for each resonator. Furthermore, the electromechanical coupling coefficient ^k2 of the series resonator and parallel resonator close to the terminal T2 is increased regardless of the characteristic parameters and design parameters. The terminal T2 may be an input terminal or an output terminal.

Examples of piezoelectric thin film resonators with different electromechanical coupling coefficients ^k2 will be described. Figures 5(a) and 5(b) are cross-sectional views of the piezoelectric thin film resonator in Example 1. As shown in Figure 5(a), resonators 11a and 11b are provided on a substrate 10. A cavity 30 consisting of a recess is provided on the upper surface of the substrate 10. A lower electrode 12 (first electrode) is provided on the substrate 10 and the cavity 30, a piezoelectric film 14 is provided on the lower electrode 12, and an upper electrode 16 (second electrode) is provided on the piezoelectric film 14. The region where the lower electrode 12 and the upper electrode 16 overlap (face each other) through at least a part of the piezoelectric film 14 is a resonance region 50. The planar shape of the resonance region 50 can be set to any shape, such as an ellipse, a polygon (rectangle or pentagon), or the like. The resonator 11a is not provided with an insertion film 18. In the resonator 11b, the piezoelectric film 14 includes a lower piezoelectric film 14a provided on the lower electrode 12 and an upper piezoelectric film 14b provided on the lower piezoelectric film 14a. An insertion film 18 is provided between the lower piezoelectric film 14a and the upper piezoelectric film 14b.

In the resonator 11a, the thicknesses of the lower electrode 12, the piezoelectric film 14, and the upper electrode 16 are T2, T4, and T6, respectively. In the resonator 11b, the thicknesses of the lower piezoelectric film 14a, the upper piezoelectric film 14b, and the insertion film 18 are T4a, T4b, and T8, respectively. T4=T4a+T4b. In the resonator 11b in which the insertion film 18 is not provided, the thickness T8 of the insertion film 18 is 0. In the resonators 11a and 11b, the lower electrode 12, the piezoelectric film 14, and the upper electrode 16 are formed simultaneously. Therefore, the thicknesses T2 of the lower electrodes 12 are substantially the same within the manufacturing error range in the resonators 11a and 11b, the thicknesses T4 of the piezoelectric film 14 are substantially the same within the manufacturing error range, and the thicknesses T6 of the upper electrodes 16 are substantially the same within the manufacturing error range.

As shown in FIG. 5(b), in the resonator 11a, the piezoelectric film 14 comprises a lower piezoelectric film 14a provided on the lower electrode 12 and an upper piezoelectric film 14b provided on the lower piezoelectric film 14a. An insertion film 18 is provided between the lower piezoelectric film 14a and the upper piezoelectric film 14b. The thickness T8 of the insertion film 18 of the resonator 11a is thinner than the thickness T8 of the insertion film 18 of the resonator 11b. The other configuration is the same as in FIG. 4(a), and a description thereof will be omitted.

The substrate 10 is an insulating substrate or a semiconductor substrate, such as a silicon substrate, a sapphire substrate, a spinel substrate, an alumina substrate, a quartz substrate, a glass substrate, a crystal substrate, a ceramic substrate, or a GaAs substrate. The lower electrode 12 and the upper electrode 16 are a single layer film of a metal, such as ruthenium (Ru), chromium (Cr), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Ir), or a laminate film of these metals.

The piezoelectric film 14 is a film mainly composed of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO ₃ ), single crystal lithium tantalate (LiTaO ₃ ), single crystal lithium niobate (LiNbO ₃ ), or the like. For example, the piezoelectric film 14 may be mainly composed of aluminum nitride and contain other elements to improve resonance characteristics or piezoelectricity. For example, the piezoelectricity of the piezoelectric film 14 is improved by using scandium (Sc), two elements of a group 2 element and a group 4 element, or two elements of a group 2 element and a group 5 element as an additive element. Therefore, the effective electromechanical coupling coefficient of the piezoelectric thin film resonator can be improved. The group 2 element is, for example, calcium (Ca), magnesium (Mg), strontium (Sr), or zinc (Zn). The group 4 element is, for example, titanium, zirconium (Zr), or hafnium (Hf). The Group 5 element is, for example, tantalum, niobium (Nb) or vanadium (V). Furthermore, the piezoelectric film 14 may be mainly composed of aluminum nitride and may also contain boron (B).

The acoustic impedance of the insertion film 18 is preferably smaller than the acoustic impedance of the piezoelectric film 14. When the piezoelectric film 14 is mainly composed of aluminum nitride, the insertion film 18 is, for example, a silicon oxide film, an aluminum film, a gold film, a copper film, a titanium film, a platinum film, or a tantalum film.

FIG. 6 is a diagram showing the electromechanical coupling coefficient ^k2 with respect to the thickness T8 of the insertion film. The lower electrode 12 is a ruthenium film with a thickness T2 of 130 nm, the piezoelectric film 14 is an aluminum nitride film with a thickness T4 of 800 nm, the upper electrode 16 is a ruthenium film with a thickness T6 of 145 nm, and the insertion film 18 is a silicon oxide film. As shown in FIG. 6, the electromechanical coupling coefficient ^k2 decreases as the thickness T8 of the insertion film 18 increases. When the thickness T8 of the insertion film 18 is set to about 100 nm, the electromechanical coupling coefficient ^k2 is about half that of a resonator without the insertion film 18. In this way, by changing the thickness T8 of the insertion film 18, the electromechanical coupling coefficient ^k2 of the resonator can be greatly changed.

Figure 7 is a circuit diagram of the ladder filter in Example 1. As shown in Figure 7, inductors L1 and L2 are provided. One end of inductor L1 is connected to parallel resonator P2, and the other end is connected to ground terminal Gnd. One end of inductor L2 is connected to parallel resonator P3, and the other end is connected to ground terminal Gnd. The other circuit configuration is the same as in Figure 1 (a), and the description will be omitted.

Figure 8(a) shows an equivalent circuit when an inductance Lg is connected in series to a resonator, and Figure 8(b) shows the electromechanical coupling coefficient versus the magnitude of inductance Lg. As shown in Figure 8(a), inductance Lg is connected between node N01, to which motional capacitance Cm and capacitance C0 are connected, and terminal T01. The resonant frequency fr' of a resonator to which inductance Lg is connected is fr' = 1/(2π√(Lm x Cm + Lg x Cm)). The anti-resonant frequency fa is the same as in Figure 2(b) where inductance Lg is not provided, and is fa = fr√(1 + Cm/C0).

FIG. 8B is a diagram showing the calculation result of the change amount Δk ² of the electromechanical coupling coefficient with respect to the inductance Lg of the inductors L1 and L2 connected in series to the parallel resonators P2 and P3. The change amount Δk ² is the change amount of the substantial electromechanical coupling coefficient k ² when the inductors L1 and L2 are connected in series with respect to the electromechanical coupling coefficient k ² of the single unit in the resonator without the inductance Lg connected in series. That is, Δk ² = (substantial k ² -single unit k ² )/(substantial k ² )×100[%]. The electromechanical coupling coefficient k ² of the single unit of the parallel resonators P2 and P3 is 3.3% and 5.6%, respectively. As shown in FIG. 8B, the electromechanical coupling coefficient k ² increases when the inductance Lg is increased. In the parallel resonator P3 with a large electromechanical coupling coefficient k ² of the single unit of the resonator, the change amount Δk ² is larger with respect to the same inductance Lg than the parallel resonator P2.

In the piezoelectric thin film resonator, the number of steps is increased in order to make the thickness T8 of the insertion film 18 different. Therefore, if a ladder filter designed based on the simultaneous Chebyshev approximation is to be realized using a piezoelectric thin film resonator, the number of manufacturing steps increases and the manufacturing cost increases. Therefore, in the series resonators S1 to S3 and the parallel resonators P1 to P3 having similar electromechanical coupling coefficients ^k2 , the thickness T8 of the insertion film 18 is made substantially the same, and the electromechanical coupling coefficients ^k2 of the piezoelectric thin film resonators are made substantially the same. In the piezoelectric thin film resonators having substantially the same electromechanical coupling coefficients ^k2 , the inductance Lg is connected to the parallel resonators whose substantial electromechanical coupling coefficients ^k2 should be increased. This allows the level of the thickness T8 of the insertion film 18 to be reduced, and the number of manufacturing steps to be reduced.

[Simulation 2]
The pass characteristic of a ladder filter was simulated using a transmission filter for LTE (Long Term Evolution) band 7 as an example. In LTE band 7, the transmission band is 2500 MHz to 2570 MHz, and the reception band is 2620 MHz to 2690 MHz. The width of the guard band between the transmission band and the reception band is 50 MHz. On the other hand, the width of the transmission band and the reception band is 70 MHz, respectively, and the relative bandwidth, which is the bandwidth relative to the frequency, is 2.7%. Thus, a filter used in a band with a small guard band width is required to have a steep skirt characteristic. In addition, it is required to increase the passband width to 70 MHz and suppress ripples in the passband.

The pass characteristics were simulated for filters B1 to B3. Filters B1 and B2 are comparative examples, and filter B3 corresponds to Example 1.

1A, the filter B1 does not include the inductors L1 and L2, and the electromechanical coupling coefficient ^k2 of all the resonators is set to a value designed based on the simultaneous Chebyshev approximation. The filter B1 uses piezoelectric thin film resonators having six levels of electromechanical coupling coefficient ^k2 (i.e., thickness T8 of the insertion film 18).

1A, the filter B2 does not include the inductors L1 and L2, and uses piezoelectric thin film resonators with the same electromechanical coupling coefficient ^k2, which is close to the electromechanical coupling coefficient ^k2 designed based on the simultaneous Chebyshev approximation. The electromechanical coupling coefficients ^k2 of the parallel resonators P1 and P2 are set to be the same, and the electromechanical coupling coefficients ^k2 of the parallel resonators P3 and the series resonators S3 are set to be the same. Filter B2 uses piezoelectric thin film resonators with four levels of electromechanical coupling coefficient ^k2 (i.e., thickness T8 of the insertion film 18).

Filter B3 includes inductors L1 and L2 as shown in FIG. 7. The electromechanical coupling coefficient ^k2 of each resonator is the same as that of filter B2. There are four levels of the electromechanical coupling coefficient ^k2 of the single unit (i.e., the thickness T8 of the insertion film 18). By connecting inductors L1 and L2 in series to parallel resonators P2 and P3, the effective electromechanical coupling coefficient ^k2 is made larger than the electromechanical coupling coefficient ^k2 of the single unit in parallel resonators P2 and P3. As a result, the effective electromechanical coupling coefficient ^k2 of all resonators is set to a value designed based on the simultaneous Chebyshev approximation.

Tables 7 to 9 show the capacitance C0, dynamic inductance Lm, dynamic capacitance Cm, resonance frequency fr, anti-resonance frequency fa, and electromechanical coupling coefficient ^k2 in filters B1 to B3. Table 9 shows the inductance Lg and unit ^k2 in filter B3. In Table 9, ^k2 is the substantial ^k2 of the resonator to which inductors L1 and L2 are connected, and unit ^k2 is the ^k2 of the resonator alone.

As shown in Tables 7 and 9, the parameters of the resonators in filters B1 and B3 are almost the same. As shown in Tables 7 to 9, the parameters of the parallel resonators P2 and P3 in filter B2 are different from those in filters B1 and B3.

Figures 9(a) and 9(b) show the pass characteristics of filters B1 to B3 in simulation 2. Figure 9(b) is an enlarged view of the pass band area of Figure 9(a). As shown in Figures 9(a) and 9(b), filters B1 and B3 have similar pass band widths, ripples in the pass band, and steepness of the skirt on the low-frequency side. Filter B2 has a narrower pass band width than filters B1 and B3, and the steepness of the skirt on the low-frequency side is reduced.

Thus, in filter B1, the passband width is wide and the skirt steepness is high, but the manufacturing process increases and the manufacturing cost increases because a piezoelectric thin film resonator having six levels of electromechanical coupling coefficient ^k2 is prepared. In filter B2, the electromechanical coupling coefficient ^k2 is set to four levels, so the manufacturing process can be reduced and the manufacturing cost can be suppressed. However, the filter characteristics are deteriorated. In filter B3, by providing inductors L1 and L2, the electromechanical coupling coefficient ^k2 of the resonator alone is set to four levels, so the manufacturing process can be reduced and the manufacturing cost can be suppressed. By connecting inductors L1 and L2, the substantial electromechanical coupling coefficient ^k2 can be made to be approximately the same as that of filter B1. This allows the filter characteristics to be approximately the same as that of filter B1.

[Modification 1 of Example 1]
Fig. 10A is a circuit diagram of a ladder filter according to Modification 1 of the first embodiment. As shown in Fig. 10A, capacitors C1 and C2 are provided. The capacitor C1 is connected in parallel to the series resonator S2, and the capacitor C2 is connected in parallel to the series resonator S3. The other circuit configurations are the same as those in Fig. 1A, and therefore description thereof will be omitted.

[Modification 2 of Example 1]
Fig. 10B is a circuit diagram of a ladder filter according to the second modification of the first embodiment. As shown in Fig. 10B, capacitors C3 and C4 are provided. The capacitor C3 is connected in parallel to the parallel resonator P1, and the capacitor C4 is connected in parallel to the parallel resonator P4. The other circuit configurations are the same as those in Fig. 1A, and therefore description thereof will be omitted.

Figure 11(a) shows an equivalent circuit when a capacitance Cp is connected in parallel to a resonator, and Figures 11(b) and 11(c) show the electromechanical coupling coefficient versus the magnitude of capacitance Cp. As shown in Figure 11(a), capacitance Cp is connected in parallel to capacitance C0. The anti-resonant frequency fa' of a resonator to which capacitance Cp is connected is fa' = fr√(1 + Cm/(C0 + Cp)). The resonant frequency fr is the same as in Figure 2(b) where capacitance Cp is not provided, and is fr = 1/(2π√(Lm × Cm)).

11B is a diagram showing the calculation results of the change ^Δk2 in the electromechanical coupling coefficient with respect to the capacitance Cp of the capacitors C1 and C2 connected in parallel to the series resonators S2 and ^S3 . The change ^Δk2 is the substantial change in the electromechanical coupling coefficient k2 when the capacitors C1 and C2 are connected in parallel with respect to the electromechanical coupling coefficient ^k2 of a single resonator without the capacitance Cp connected in parallel. The electromechanical coupling coefficients ^k2 of the single series resonators S2 and S3 are 3.8% and 6.7%, respectively.

11C is a diagram showing the calculation results of the change ^Δk2 in the electromechanical coupling coefficient with respect to the capacitance Cp of the capacitors C3 and C4 connected in parallel to the parallel resonators P1 and P2. The electromechanical coupling coefficients ^k2 of the parallel resonators P1 and P2 alone are 3.4% and 4.5%, respectively.

11B and 11C, increasing the capacitance Cp reduces the electromechanical coupling coefficient ^k2 . In a resonator with a large electromechanical coupling coefficient ^k2 of the resonator alone, the change amount ^Δk2 for the same capacitance Cp is large. In this way, by providing the capacitance Cp in parallel, the effective electromechanical coupling coefficient ^k2 can be made smaller than the electromechanical coupling coefficient ^k2 of the single resonator.

[Simulation 3]
The pass characteristics of filters C1 to C4 were simulated using as an example a transmission filter for LTE band 7. Filters C1 and C3 are comparative examples, filter C2 corresponds to modified example 1 of embodiment 1, and filter C3 corresponds to modified example 2 of embodiment 1.

The filters C1 and C3 do not include the capacitors C1 to C4 as shown in FIG. 1A, and use piezoelectric thin-film resonators in which the electromechanical coupling coefficient ^k2 of all the resonators is set to the same electromechanical coupling coefficient ^k2 that is close to the electromechanical coupling coefficient ^k2 designed based on the simultaneous Chebyshev approximation.

In the filter C1, the electromechanical coupling coefficient ^k2 of the parallel resonator P2 is the same as that of the series resonator S2, and the electromechanical coupling coefficient ^k2 of the parallel resonator P3 is the same as that of the series resonator S3. There are four levels of the electromechanical coupling coefficient ^k2 (i.e., the thickness T8 of the insertion film 18).

In the filter C3, the electromechanical coupling coefficient ^k2 of the parallel resonator P1 is the same as that of the series resonator S2, and the electromechanical coupling coefficient ^k2 of the series resonator S1 is the same as that of the parallel resonator P2. There are four levels of the electromechanical coupling coefficient ^k2 (i.e., the thickness T8 of the insertion film 18).

In filter C2, the electromechanical coupling coefficient ^k2 of the single piezoelectric thin film resonator is the same as that of filter C1, and the level of the single electromechanical coupling coefficient ^k2 (i.e., the thickness T8 of the insertion film 18) is four. As shown in Fig. 10(a), by connecting capacitors C1 and C2 in parallel to the series resonators S2 and S3, the substantial electromechanical coupling coefficient ^k2 in the series resonators S2 and S3 is made smaller than the single electromechanical coupling coefficient in the series resonators S2 and S3. As a result, the electromechanical coupling coefficient ^k2 of all resonators is set to a value designed based on the simultaneous Chebyshev approximation.

In the filter C4, the electromechanical coupling coefficient ^k2 of the single piezoelectric thin film resonator is the same as that of the filter C3, and the level of the electromechanical coupling coefficient ^k2 (i.e., the thickness T8 of the insertion film 18) is four. As shown in Fig. 10B, by connecting the capacitors C3 and C4 in parallel to the parallel resonators P1 and P2, the substantial electromechanical coupling coefficient ^k2 in the parallel resonators P1 and P2 is made smaller than the electromechanical coupling coefficient ^k2 of the single resonator in the series resonators S1 and S2. As a result, the electromechanical coupling coefficient ^k2 of all the resonators is set to a value designed based on the simultaneous Chebyshev approximation.

Tables 10 to 13 show the capacitance C0, dynamic inductance Lm, dynamic capacitance Cm, resonance frequency fr, anti-resonance frequency fa, and electromechanical coupling coefficient ^k2 in filters C1 to C4. Tables 11 and 13 show the capacitance Cp and unit ^k2 in filters C2 and C4. In Tables 11 and 13, ^k2 is the substantial ^k2 of the resonator in which capacitors C1 to C4 are connected in parallel, and unit ^k2 is the ^k2 of the resonator alone.

As shown in Tables 11 and 13, the parameters of the resonators in filters C2 and C4 are almost the same as those of filter B1 in Table 7. As shown in Tables 10 and C12, the parameters of the series resonators S2 and S3 in filter C1 are different from those of filters B1 and C2. The parameters of the parallel resonators P1 and P2 in filter C3 are different from those of filters B1 and C4.

Figures 12(a) and 12(b) are diagrams showing the pass characteristics of filters B1, C1, and C2 in simulation 3. Figure 12(b) is an enlarged view of the pass band of Figure 12(a). As shown in Figures 12(a) and 12(b), the pass characteristics of filters B1 and C2 almost overlap. Filter C1 has a wider pass band than filters B1 and C2, but ripples are formed within the pass band and the steepness of the skirt on the high-frequency side is reduced.

Figures 13(a) and 13(b) are diagrams showing the pass characteristics of filters B1, C3, and C4 in simulation 3. Figure 13(b) is an enlarged view of the pass band of Figure 13(a). As shown in Figures 13(a) and 13(b), the pass characteristics of filters B1 and C4 almost overlap. In filter C3, a ripple is formed on the shoulder on the high frequency side of the pass band compared to filters B1 and C4. This narrows the width of the pass band.

In this way, in filters C2 and C4, by providing capacitors C1 to C4, the electromechanical coupling coefficient ^k2 of the resonator alone can be set to four levels, thereby reducing the manufacturing process and suppressing the manufacturing cost. By connecting capacitors C1 to C4, the substantial electromechanical coupling coefficient ^k2 can be made to be approximately the same as that of filter B1. This allows the filter characteristics to be approximately the same as that of filter B1.

According to the first embodiment and its modified example, when a ladder filter is designed based on the simultaneous Chebyshev approximation, as in the filters A1 to A6 in the simulation 1, the series resonator S3 (first series resonator) electrically closest to the terminal T2 has the largest electromechanical coupling coefficient ^k2 among the multiple series resonators S1 to S3. That is, the series resonator S3 has the thinnest insertion film 18 among the multiple series resonators S1 to S3. At least one of the series resonators S1 and S2 (second series resonators) other than the series resonator S3 has a smaller electromechanical coupling coefficient ^k2 than the series resonator S3. That is, the series resonators S1 and S2 have a thicker insertion film 18 than the series resonator S3. Note that the resonator provided with the insertion film 18 is expressed as having a thicker insertion film 18 than the resonator not provided with the insertion film 18. Note that the resonator not provided with the insertion film 18 is expressed as having a thinner insertion film 18 than the resonator provided with the insertion film 18.

Similarly, the parallel resonator P3 (first parallel resonator) that is electrically closest to the terminal T2 has the largest electromechanical coupling coefficient ^k2 among the multiple parallel resonators P1 to P3. That is, the parallel resonator P3 has the thinnest insertion film 18 among the multiple parallel resonators P1 to P3 or does not have an insertion film 18. At least one of the parallel resonators P1 and P2 (second parallel resonators) other than the parallel resonator P3 has a smaller electromechanical coupling coefficient ^k2 than the parallel resonator P3. That is, the parallel resonators P1 and P2 have a thicker insertion film 18 than the parallel resonator P3.

In the first embodiment, as shown in FIG. 7, in the filter B3, one end of the inductors L1 and L2 is connected to at least one of the parallel resonators P2 and P3 (first resonator) among the multiple parallel resonators P1 to P3, and the other end is grounded. This allows the substantial electromechanical coupling coefficient ^k2 in the parallel resonators P2 and P2 to be greater than the electromechanical coupling coefficient ^k2 of the resonator alone. The electromechanical coupling coefficient ^k2 of at least one resonator (second resonator) among the multiple series resonators S1 to S3 and the parallel resonator P1 to which the grounded inductor is not connected is made substantially the same as the electromechanical coupling coefficient k2 of the parallel resonators ^P2 and P3 alone. That is, the parallel resonator P3 and the series resonator S3 have substantially the same electromechanical coupling coefficient ^k2 (5.6% in the filter B3) (that is, the thickness of the insertion film 18 is substantially the same). The parallel resonators P2 and P1 have substantially the same electromechanical coupling coefficient ^k2 (3.3% in the filter B3) (i.e., the thickness of the insertion film 18 is substantially the same). This makes it possible to reduce the number of levels of the electromechanical coupling coefficient ^k2 of the piezoelectric thin film resonators in the ladder filter without increasing the number of manufacturing steps. By connecting the inductors L1 and L2 in series to the parallel resonators P2 and P3, the substantial electromechanical coupling coefficient ^k2 of the parallel resonators P2 and P3 can be made larger than the electromechanical coupling coefficient ^k2 of the parallel resonators P2 and P3. This makes it possible to improve the filter characteristics. Therefore, it is possible to improve the characteristics and suppress an increase in the manufacturing steps.

When a ladder filter is designed based on the simultaneous Chebyshev approximation, the electromechanical coupling coefficient ^k2 of the series resonator S3 becomes larger than that of the parallel resonator P3, as in the filters A1 to A6 of Simulation 1. Therefore, the parallel resonator (first resonator) to which the inductor L2 is connected is designated as the parallel resonator P3 (first parallel resonator), and the series resonator S3 (first series resonator) has a single electromechanical coupling coefficient ^k2 substantially the same as that of the parallel resonator P3. That is, the thicknesses T8 of the insertion films 18 of the series resonator S3 (first series resonator) and the parallel resonator P3 (first parallel resonator) are made substantially the same. This makes it possible to improve the characteristics and suppress an increase in the number of manufacturing steps.

Furthermore, when a ladder filter is designed based on the simultaneous Chebyshev approximation, among the multiple series resonators S1 to S3, the parallel resonator P3 closest to the terminal T2 has the largest electromechanical coupling coefficient ^k2 , and the series resonator S3 closest to the terminal T2 has the second largest electromechanical coupling coefficient ^k2 . That is, the parallel resonator P3 has the thinnest insertion film 18, and the series resonator S3 has the second thinnest insertion film. The electromechanical coupling coefficient ^k2 of the single unit of the series resonator S3 is 1.2 times or more or 1.4 times or more the smallest electromechanical coupling coefficient ^k2 of the single unit of the multiple series resonators S1 to S3. Furthermore, the electromechanical coupling coefficient k2 of the single unit of the parallel resonator P3 closest to the terminal ^T2 is 1.2 times or more or 1.4 times or more the smallest electromechanical coupling coefficient ^k2 of the single unit of the multiple parallel resonators P1 to P3. To achieve this, the difference in thickness between the insertion film 18 of the series resonator S3 and the thickest insertion film 18 of the multiple series resonators S1 to S3 is 5 nm or more and 10 nm or more. The difference in thickness between the insertion film 18 of the parallel resonator P3 and the thickest insertion film 18 of the multiple parallel resonators P1 to P3 is 5 nm or more and 10 nm or more.

Furthermore, when a ladder filter is designed based on the simultaneous Chebyshev approximation, as in the filters A1 to A6 of Simulation 1, in which the number of stages is five or less, the parallel resonator P1 (third parallel resonator) that is electrically closest to the terminal T1 has the smallest electromechanical coupling coefficient ^k2 of the series resonators S1 to S3 and the parallel resonators P1 to P3 (i.e., the insertion film 18 has the largest).

In the first and second modifications of the first embodiment, as shown in the filters C2 and C4 and in FIG. 10(a) and FIG. 10(b), the capacitors C1 to C4 are connected in parallel to at least one resonator (first resonator) among the series resonators S1 to S3 and the parallel resonators P1 to P3. A resonator (second resonator) is provided that has the same single electromechanical coupling coefficient k ² as the single electromechanical coupling coefficient k ² of the resonator to which the capacitor C1 is connected in parallel (i.e., the thickness T8 of the insertion film 18 is substantially the same). For example, in the filter C2, the series resonator S2 to which the capacitor C1 is connected in parallel and the parallel resonator P2 to which the capacitor is not connected in parallel have substantially the same single electromechanical coupling coefficient k ² (3.8%). The series resonator S3 to which the capacitor C2 is connected in parallel and the parallel resonator P3 to which the capacitor is not connected in parallel have substantially the same single electromechanical coupling coefficient k ² (6.7%). In the filter C4, the parallel resonator P1 to which the capacitor C3 is connected in parallel and the series resonator S2 to which no capacitor is connected have substantially the same single electromechanical coupling coefficient k ² (3.4%). The parallel resonator P2 to which the capacitor C4 is connected in parallel and the series resonator S1 to which no capacitor is connected have substantially the same single electromechanical coupling coefficient k ² (4.5%). This makes it possible to reduce the number of levels of the single electromechanical coupling coefficient k ² of the piezoelectric thin film resonator in the ladder filter without increasing the number of manufacturing steps. By connecting the capacitors C1 to C4 in parallel to the resonators, the effective electromechanical coupling coefficient k ² of the resonators can be made smaller than the single electromechanical coupling coefficient k ^2. This makes it possible to improve the filter characteristics. Therefore, it is possible to improve the characteristics and suppress an increase in the manufacturing process.

When a ladder filter is designed based on the simultaneous Chebyshev approximation, the electromechanical coupling coefficient ^k2 of the series resonator S3 becomes larger than the electromechanical coupling coefficient ^k2 of the parallel resonator P3, as in the filters A1 to A6 of Simulation 1. Therefore, in the filter C2, the resonator (first resonator) to which the capacitor C2 is connected in parallel is set as the series resonator S3 (first series resonator), and the parallel resonator P3 (first parallel resonator) has a single electromechanical coupling coefficient ^k2 substantially the same as that of the series resonator S3. That is, the thicknesses of the insertion films 18 of the parallel resonator P3 (first parallel resonator) and the series resonator S3 (first series resonator) are substantially the same. This makes it possible to improve the characteristics and suppress an increase in the number of manufacturing steps.

Furthermore, when a ladder filter is designed based on the simultaneous Chebyshev approximation, as in filters A1 to A6 in Simulation 1, when the number of stages is five or less, the parallel resonator P1 (third parallel resonator) electrically closest to the terminal T1 has the smallest electromechanical coupling coefficient ^k2 of the series resonators S1 to S3 and the parallel resonators P1 to P3 (i.e., the insertion film 18 has the largest electromechanical coupling coefficient k2). Therefore, as in filter C4, the electromechanical coupling coefficient ^k2 of the parallel resonator P1 is made substantially the same as that of the other resonators (series resonator S2 in filter ^C4 ), and a capacitor C3 is connected in parallel to the parallel resonator P1. This makes it possible to improve the characteristics and suppress an increase in the number of manufacturing steps.

The electromechanical coupling coefficients ^k2 of the plurality of resonators being substantially the same means that, when the maximum electromechanical coupling coefficient of the plurality of resonators is kmax and the minimum electromechanical coupling coefficient of the plurality of resonators is kmax, (kmax-kmin)/(kmax+kmin) is, for example, 0.1 or less or 0.05 or less. The thickness of a certain layer of the plurality of resonators being substantially the same means that, when the maximum thickness of a certain layer of the plurality of resonators is Tmax and the minimum thickness is Tmin, (Tmax-Tmin)/(Tmax+Tmin) is, for example, 0.1 or less or 0.05 or less.

As a method for varying the electromechanical coupling coefficient ^k2 in the piezoelectric thin film resonator, in addition to the method of varying the thickness of the insertion film, a method of adding impurities to the piezoelectric film 14 to vary the piezoelectricity, a method of changing the ratio of the planar area of the insertion film 18 in the resonance region 50 to the planar area of the resonance region 50, etc. may be used. When a ladder type filter is designed based on the simultaneous Chebyshev approximation like the filters A1 to A6, the largest electromechanical coupling coefficient ^k2 in the series resonator and parallel resonator is 1.5 times or more, and in some cases, 2 times or more, of the smallest electromechanical coupling coefficient ^k2 . Thus, as a method for greatly varying the electromechanical coupling coefficient ^k2 , a method of varying the thickness T2 of the insertion film 18 is preferable.

[Modification 3 of Example 1]
Modifications 3 and 4 of Example 1 are examples in which the configuration of the gap is changed. Figures 14(a) and 14(b) are cross-sectional views of piezoelectric thin film resonators according to Modifications 3 and 4 of Example 1, respectively. As shown in Figure 14(a) , in Modification 3 of Example 1, the upper surface of the substrate 10 is flat, and a dome-shaped gap 30 is provided between the substrate 10 and the lower electrode 12. The other configurations are the same as those in Figure 5(a) of Example 1, and therefore description thereof will be omitted.

[Fourth Modification of the First Embodiment]
As shown in FIG. 14B, in the fourth modification of the first embodiment, an acoustic reflection film 31 is formed under the lower electrode 12 of the resonance region 50. The acoustic reflection film 31 is formed by alternating low acoustic impedance films 31a and high acoustic impedance films 31b. The thicknesses of the films 31a and 31b are, for example, approximately λ/4 (λ is the wavelength of the elastic wave). The number of layers of the films 31a and 31b can be set arbitrarily. The acoustic reflection film 31 may be formed by stacking at least two types of layers with different acoustic characteristics with a gap therebetween. The substrate 10 may be one of the at least two types of layers with different acoustic characteristics of the acoustic reflection film 31. For example, the acoustic reflection film 31 may be configured such that one film with different acoustic impedance is provided in the substrate 10. The other configurations are the same as those in FIG. 5A of the first embodiment, and will not be described.

As in Example 1 and its Modifications 1 to 3, the piezoelectric thin film resonator may be an FBAR (Film Bulk Acoustic Resonator) in which a gap 30 is formed between the substrate 10 and the lower electrode 12 in the resonance region 50. As in Modification 4 of Example 1, the piezoelectric thin film resonator may be an SMR (Solidly Mounted Resonator) that includes an acoustic reflection film 31 that reflects elastic waves propagating through the piezoelectric film 14 under the lower electrode 12 in the resonance region 50. The acoustic reflection layer that includes the resonance region 50 may include the gap 30 or the acoustic reflection film 31.

Figure 15 is a circuit diagram of a duplexer according to the second embodiment. As shown in Figure 15, 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 signals in the transmission band among the signals input from the transmission terminal Tx to the common terminal Ant as transmission signals, and suppresses signals of other frequencies. The reception filter 42 passes signals in the reception band among the signals input from the common terminal Ant to the reception terminal Rx as reception signals, 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 embodiment and its modified examples.

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

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

REFERENCE SIGNS LIST 10: Substrate 12: Lower electrode 14: Piezoelectric film 16: Upper electrode 18: Insertion film 40: Transmitting filter 42: Receiving filter 50: Resonance region

Claims (11)

a plurality of piezoelectric thin film resonators comprising a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other, at least some of the piezoelectric thin film resonators comprising an insertion film provided between the first electrode and the second electrode within the resonance region;
a plurality of series resonators connected in series between a first terminal and a second terminal, the plurality of series resonators including a first series resonator electrically closest to the second terminal, the first series resonator having the thinnest insertion film among the plurality of series resonators, and at least one second series resonator other than the first series resonator having a thicker insertion film than the first series resonator;
a first parallel resonator, one end of which is connected to a path between the first terminal and the second terminal and the other end of which is grounded and which is included in the plurality of piezoelectric thin film resonators and is electrically closest to the second terminal, has the thinnest insertion film or does not have an insertion film among the plurality of parallel resonators, and at least one second parallel resonator other than the first parallel resonator has a thicker insertion film than the first parallel resonator;
an inductor having one end connected to at least one first resonator among the plurality of parallel resonators and the other end grounded;
a second resonator which is at least one of the plurality of series resonators and a parallel resonator to which no grounded inductor is connected and has an insertion film having substantially the same thickness as an insertion film of the parallel resonator to which the inductor is connected;
A ladder type filter comprising: the first resonator includes the first parallel resonator,
2. The ladder filter according to claim 1, wherein the second resonator includes the first series resonator. The ladder filter according to claim 1 or 2, wherein the first parallel resonator has the thinnest insertion film among the plurality of series resonators and the plurality of parallel resonators, and the second series resonator has the second thinnest insertion film. The ladder filter according to any one of claims 1 to 3, wherein a third parallel resonator, which is electrically closest to the first terminal among the plurality of parallel resonators, has the thickest insertion film among the plurality of series resonators and the plurality of parallel resonators. a plurality of piezoelectric thin film resonators comprising a substrate, a piezoelectric film provided on the substrate, and a first electrode and a second electrode that sandwich at least a portion of the piezoelectric film and form a resonance region that overlaps with each other, at least some of the piezoelectric thin film resonators comprising an insertion film provided between the first electrode and the second electrode within the resonance region;
a plurality of series resonators connected in series between a first terminal and a second terminal, the plurality of series resonators including a first series resonator electrically closest to the second terminal, the first series resonator having the thinnest insertion film among the plurality of series resonators, and at least one second series resonator other than the first series resonator having a thicker insertion film than the first series resonator;
a first parallel resonator, one end of which is connected to a path between the first terminal and the second terminal and the other end of which is grounded and which is included in the plurality of piezoelectric thin film resonators and is electrically closest to the second terminal, has the thinnest insertion film or does not have an insertion film among the plurality of parallel resonators, and at least one second parallel resonator other than the first parallel resonator has a thicker insertion film than the first parallel resonator;
a capacitor connected in parallel to at least one first resonator among the plurality of series resonators and the plurality of parallel resonators;
a second resonator, which is at least one of the plurality of series resonators and the plurality of parallel resonators, in which a capacitor is not connected in parallel and which has an insertion film having substantially the same thickness as an insertion film of a resonator in which the capacitor is connected in parallel;
A ladder type filter comprising: the first resonator includes the first series resonator,
6. The ladder filter according to claim 5, wherein the second resonator includes the first parallel resonator. The ladder filter according to claim 5 or 6, wherein the first parallel resonator has the thinnest insertion film and the second series resonator has the second thinnest insertion film among the plurality of series resonators and the plurality of parallel resonators. The ladder filter according to any one of claims 5 to 7, wherein a third parallel resonator, which is electrically closest to the first terminal among the plurality of parallel resonators, has the thickest insertion film among the plurality of series resonators and the plurality of parallel resonators. The ladder filter according to claim 8, wherein the first resonator includes the third parallel resonator. The ladder filter according to any one of claims 1 to 9, wherein in the plurality of piezoelectric thin film resonators, the thickness of the piezoelectric film is substantially the same, the thickness of the first electrodes is substantially the same, and the thickness of the second electrodes is substantially the same. A multiplexer comprising the ladder filter according to any one of claims 1 to 10 .

Images