Acoustic wave filter

An acoustic wave filter includes first and second series-arm resonators, each including an IDT electrode including electrode fingers and a busbar electrode connecting first ends of the electrode fingers to each other. A direction in which second ends of the electrode fingers are aligned with each other crosses a propagation direction of an acoustic wave. The electrode fingers of the IDT electrodes of the first and second series-arm resonators each include an electrode-finger central portion and a wide portion located at the second end and being wider than the electrode-finger central portion. The length of the wide portion of each of the electrode fingers in the first series-arm resonator is greater than the length of the wide portion of each of the electrode fingers in the second series-arm resonator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2020-014780 filed on Jan. 31, 2020. The entire contents of this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave filter.

2. Description of the Related Art

A ladder acoustic wave filter including series-arm resonators and parallel-arm resonators is known. Japanese Unexamined Patent Application Publication No. 2000-114923 discloses an acoustic wave filter including series-arm resonators and parallel-arm resonators each of which includes an interdigital transducer (IDT) electrode and reflector electrodes.

According to Japanese Unexamined Patent Application Publication No. 2000-114923, each reflector electrode includes two busbars. One of the busbars is adjacent to a signal wiring electrode, and the other busbar is connected to one comb teeth-shaped electrode closer than another comb teeth-shaped electrode of the IDT electrode to the ground such that the resonator is provided with the resultant capacitance generated between the signal wiring electrode and the ground. Japanese Unexamined Patent Application Publication No. 2000-114923 also describes that providing a series-arm resonator with capacitance causes a shift of the anti-resonant frequency of the series-arm resonator toward a lower frequency side.

As described in Japanese Unexamined Patent Application Publication No. 2000-114923, the shift of the anti-resonant frequency of the series-arm resonator toward a lower frequency side enables the attenuation slope in a frequency range higher than the pass band of the acoustic wave filter to become steeper.

When the capacitance is provided parallel to a series-arm resonator, the Q-factor at the anti-resonant frequency is deteriorated due to the resistive component of the capacitance, and the return loss (insertion loss) in the pass band increases accordingly.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave filters that each reduce return loss in a pass band while enabling an attenuation slope in a frequency range higher than the pass band to become steeper.

According to a preferred embodiment of the present invention, an acoustic wave filter includes a first series-arm resonator and a second series-arm resonator that are disposed on a path connecting a first terminal and a second terminal. The first series-arm resonator has a lower anti-resonant frequency than any other series-arm resonator included in the acoustic wave filter. The first series-arm resonator and the second series-arm resonator each include an IDT electrode including a pair of comb teeth-shaped electrodes provided on a substrate including a piezoelectric layer. Electrodes of the pair of comb teeth-shaped electrodes of the first series-arm resonator and electrodes of the pair of comb teeth-shaped electrodes of the second series-arm resonator each include electrode fingers and a busbar electrode. The electrode fingers extend in a direction orthogonal or substantially orthogonal to a propagation direction of an acoustic wave. The busbar electrode connects first ends of the electrode fingers to each other. A direction in which second ends of the electrode fingers are aligned with each other crosses the propagation direction of the acoustic wave. The electrode fingers of the IDT electrode of the first series-arm resonator and the electrode fingers of the IDT electrode of the second series-arm resonator each include an electrode-finger central portion and a wide portion. The wide portion is located at the second end and is wider than the electrode-finger central portion. The length of the wide portion of each of the electrode fingers in the first series-arm resonator in the direction in which the electrode fingers extend is greater than the length of the wide portion of each of the electrode fingers in the second series-arm resonator in the direction in which the electrode fingers extend.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail with reference to preferred embodiments and the accompanying drawings. The following preferred embodiments are general or specific examples. Details, such as values, shapes, materials, components, and arrangements and connection configurations of the components in the following preferred embodiments, are provided merely as examples and should not be construed as limiting the present invention. Of the components in the following preferred embodiments, those not mentioned in an independent claim are described as optional components. The sizes and the relative proportions of the components illustrated in the drawings are not necessarily to scale. Redundant description of the same or corresponding components, which are denoted by the same reference signs in the drawings, will be omitted or described in brief. The expression “connected to” in the description of the following preferred embodiments includes not only direct connection but also electrical connection through another element or the like.

1-1 Basic Configuration of Acoustic Wave Filter

The following describes a basic configuration of an acoustic wave filter according to Preferred Embodiment 1 of the present invention with reference toFIG.1.

FIG.1is a circuit configuration diagram of an acoustic wave filter10according to Preferred Embodiment 1.

The acoustic wave filter10includes a series-arm resonator110s, series-arm resonators121sto124s, parallel-arm resonators121pto124p, and inductors121L and122L. The series-arm resonators110sand121sto124sare disposed on a path connecting a first terminal Port1and the second terminal Port2. The parallel-arm resonators121pto124pare disposed between the path and a reference terminal (i.e., ground).

The series-arm resonator110sis hereinafter also referred to as a first series-arm resonator110s. The series-arm resonators different from the first series-arm resonator110sare hereinafter referred to as second series-arm resonators121s,122s,123s, and124s.

The first series-arm resonator110sand the second series-arm resonators121sto124sare connected in series on the path (series arm) connecting the first terminal Port1and the second terminal Port2. The second series-arm resonator121s, the second series-arm resonator122s, the first series-arm resonator110s, the second series-arm resonator123s, and the second series-arm resonator124sare connected in series in the stated order in the direction from the first terminal Port1to the second terminal Port2.

The parallel-arm resonators121pto124pare connected in parallel and disposed on respective paths (parallel arms) each of which connects the reference terminal and the corresponding one of points at which the first series-arm resonator110sand the second series-arm resonators121sto124sare connected to each other. Specifically, one end of the parallel-arm resonator121pis connected to a node between the series-arm resonators121sand122s, and the other end of the parallel-arm resonator121pis connected directly to the reference terminal. One end of the parallel-arm resonator122pis connected to a node between the series-arm resonators122sand110s, and the other end of the parallel-arm resonator122pis connected to the reference terminal with the inductor121L therebetween. One end of the parallel-arm resonator123pis connected to a node between the series-arm resonators110sand123s, and the other end of the parallel-arm resonator123pis connected to the reference terminal with the inductor121L therebetween. One end of the parallel-arm resonator124pis connected to a node between the series-arm resonators123sand124s, and the other end of the parallel-arm resonator124pis connected to the reference terminal with the inductor122L therebetween.

The first series-arm resonator110s, the second series-arm resonators121sto124s, and the parallel-arm resonators121pto124pare connected as described above to define the acoustic wave filter10that is, for example, a ladder band-pass filter.

The resonant frequency of the first series-arm resonator110sand the resonant frequencies of the second series-arm resonators121sto124sare within the pass band of the acoustic wave filter10. The second series-arm resonators121sto124smay include respective number of electrode finger pairs, respective intersecting widths, and respective resonant frequencies. The anti-resonant frequency of the first series-arm resonator110sis lower than any of the anti-resonant frequencies of the second series-arm resonators121sto124s. That is, the first series-arm resonator110shas a lower anti-resonant frequency than any other series-arm resonator (i.e., the series-arm resonators121sto124s) included in the acoustic wave filter10.

The position of the first series-arm resonator110sis not limited to the point between the second series-arm resonators122sand123s. The first series-arm resonator110smay be disposed between the second series-arm resonators121sand122sor may be disposed between the second series-arm resonators123sand124s, for example. Alternatively, the first series-arm resonator110smay be disposed between the first terminal Port1and the second series-arm resonator121sor may be disposed between the second series-arm resonator124sand the second terminal Port2, for example.

Instead of including one first series-arm resonator (i.e., the first series-arm resonator110s), the acoustic wave filter10may include two or more first series-arm resonators. The acoustic wave filter10does not necessarily include four second series-arm resonators (i.e., the second series-arm resonators121sto124s) and four parallel-arm resonators (i.e., the parallel-arm resonators121pto124p. The acoustic wave filter10includes at least one second series-arm resonator and at least one parallel-arm resonator.

1-2 Basic Structures of Resonators

The following describes basic structures of the resonators (i.e., the series-arm resonators and the parallel-arm resonators) of the acoustic wave filter10. The resonators are preferably surface acoustic wave (SAW) resonators, for example.

FIG.2includes a schematic plan view and a schematic sectional view of a resonator of the acoustic wave filter10. The resonator illustrated inFIG.2represents a typical structure of the resonators described above. Details such as the number and the length of electrode fingers of each electrode may be changed.

When viewed in plan as inFIG.2, the resonator includes a pair of comb teeth-shaped electrodes and a pair of resonators. Electrodes of the pair of comb teeth-shaped electrodes face each other and are denoted by32aand32b, respectively. Reflectors of the pair of reflectors are denoted by32cand are adjacent to the comb teeth-shaped electrodes32aand32bin a propagation direction of an acoustic wave. The pair of comb teeth-shaped electrodes, or more specifically, the comb teeth-shaped electrodes32aand32bdefine an interdigital transducer (IDT) electrode. Either of the two reflectors32cmay be omitted when constraints arise from, for example, the mounting layout.

The comb teeth-shaped electrode32aincludes electrode fingers322a, offset electrode fingers323a, and a busbar electrode321a. The electrode fingers322aand the offset electrode fingers323aare disposed in parallel. The busbar electrode321aconnects first ends e1of the electrode finger322ato each other and also connects first ends e1of the offset electrode fingers323ato each other. The comb teeth-shaped electrode32bincludes electrode fingers322b, offset electrode fingers323b, and a busbar electrode321b. The electrode fingers322band the offset electrode fingers323bare parallel or substantially parallel to each other. The busbar electrode321bconnects first ends e1of the electrode fingers322bto each other and also connects first ends e1of the offset electrode fingers323bto each other.

The electrode fingers322a, the electrode fingers322b, the offset electrode fingers323a, and the offset electrode fingers323bextend in a direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave (i.e., in a direction orthogonal or substantially orthogonal to the X-axis). Each of the electrode fingers322afaces the corresponding one of the offset electrode fingers323bin the direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave, and each of the electrode fingers322bfaces the corresponding one of the offset electrode fingers323ain the direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave.

The direction in which second ends e2of the electrode fingers322a(i.e., end portions that are not connected to the busbar electrode321a) are aligned with each other is denoted by D and crosses the propagation direction of the acoustic wave at a predetermined angle. The direction in which second ends e2of the electrode fingers322b(i.e., end portions that are not connected to the busbar electrode321b) are aligned with each other is denoted by D and crosses the propagation direction of the acoustic wave at the predetermined angle. The direction in which second ends e2of the offset electrode fingers323a(i.e., end portions that are not connected to the busbar electrode321a) are aligned with each other is denoted by D and crosses the propagation direction of the acoustic wave at the predetermined angle. The direction in which second ends e2of the offset electrode fingers323b(i.e., end portions that are not connected to the busbar electrode321b) are aligned with each other is denoted by D and crosses the propagation direction of the acoustic wave at the predetermined angle.

That is, the first series-arm resonator110s, the second series-arm resonators121sto124s, and the parallel-arm resonators121pto124peach includes an inclined IDT electrode whose electrode fingers extend in a direction crossing the propagation direction of the acoustic wave.

Meanwhile, when a one-port SAW resonator including a piezoelectric layer is included in an acoustic wave filter, a transverse mode ripple may appear between the resonant frequency and the anti-resonant frequency of the resonator and can cause degradation of transmission characteristics in the pass band of the filter. To address this problem, the acoustic wave filter10according to the present preferred embodiment includes resonators whose IDT electrodes are inclined IDTs.

Another feature of the acoustic wave filter10according to the present preferred embodiment is that the second ends e2of the electrode fingers322a, the second ends e2of the electrode fingers322b, the second ends e2of the offset electrode fingers323a, and the second ends e2of the offset electrode fingers323beach have an atypical shape, or more specifically, the second ends e2are preferably T-shaped (seeFIGS.3A and3B), for example. The atypical shape will be described in detail later.

The pair of reflectors (i.e., the reflectors32c) are adjacent to the pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) in the direction D. Specifically, the reflectors32care disposed with the comb teeth-shaped electrodes32aand32btherebetween in the direction D. The reflectors32ceach include reflector electrode fingers parallel or substantially parallel to each other and reflector busbar electrodes connecting the reflector electrode fingers to each other. The reflector busbar electrodes of each reflector32cextend in the direction D.

When viewed in a cross-section as inFIG.2, the IDT electrode including the electrode fingers322a, the electrode fingers322b, the offset electrode fingers323a, the offset electrode fingers323b, and the busbar electrodes321aand321bhas a multilayer structure including an adhesive layer324and a main electrode layer325. The structure of each reflector32cviewed in a cross-section is the same as or similar to the structure of the IDT electrode viewed in cross-section and will not be further described here.

The adhesive layer324improves the adhesion between a piezoelectric layer327and the main electrode layer325and is preferably made of, for example, Ti.

The main electrode layer325is preferably made mainly of Al and has a Cu content of about 1%, for example.

The IDT electrode is covered with a protective layer326. The protective layer326is provided, for example, to protect the main electrode layer325from the external environment, to adjust the frequency-temperature characteristics, and to improve the moisture resistance. The protective layer326is preferably made mainly of, for example, silicon dioxide.

The materials of the adhesive layer324, the main electrode layer325, and the protective layer326are not limited to the materials described above. It is not required that the IDT electrode have the multilayer structure. The IDT electrode may be made of a metal such as Ti, Al, Cu, Pt, Au, Ag, or Pd or may be made of an alloy, for example. The IDT electrode may include multilayer bodies made of these metals or alloys. The protective layer326is optional.

The IDT electrode and the reflectors32care disposed on a main surface of a substrate320, which will be described below. The following describes a multilayer structure of the substrate320.

As illustrated in the lower section ofFIG.2, the substrate320includes a high-acoustic-velocity support substrate329, a low-acoustic-velocity film328, and the piezoelectric layer327. The high-acoustic-velocity support substrate329, the low-acoustic-velocity film328, and the piezoelectric layer327are stacked on top of each other in the stated order.

The piezoelectric layer327is, for example, a piezoelectric film. The IDT electrode and the reflectors32care disposed on a main surface of the piezoelectric layer327. The piezoelectric layer327is preferably made of, for example, a θ°-rotated Y cut X SAW propagation LiTaO3piezoelectric single crystal or θ°-rotated Y cut X SAW propagation LiTaO3piezoelectric ceramics obtained by cutting a lithium tantalate single crystal or ceramics along a plane whose normal line is an axis rotated from a Y-axis by θ° with an X-axis as the central axis. The surface acoustic wave propagates in the X-axis direction through a single crystal or ceramics. The piezoelectric layer327preferably has a thickness of, for example, about 3.5λ or less, where λ denotes the wavelength of the acoustic wave and is determined by the electrode-to-electrode pitch of the IDT electrode. For example, the piezoelectric layer327preferably has a thickness of about 600 nm.

The high-acoustic-velocity support substrate329supports the low-acoustic-velocity film328, the piezoelectric layer327, and the IDT electrode. The acoustic velocity of a bulk wave propagating through the high-acoustic-velocity support substrate329is higher than the acoustic velocity of an acoustic wave such as a surface acoustic wave or a boundary wave propagating though the piezoelectric layer327. The high-acoustic-velocity support substrate329confines the surface acoustic wave in the portion where the piezoelectric layer327is stacked on the low-acoustic-velocity film328, and the surface acoustic wave is thus reduced or prevented from leaking to underneath the high-acoustic-velocity support substrate329. The high-acoustic-velocity support substrate329is preferably, for example, a silicon substrate having a thickness of about 125 μm. Examples of the material of the high-acoustic-velocity support substrate329include: (1) piezoelectric materials such as aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon, sapphire, lithium tantalate, lithium niobate, and quartz; (2) various ceramics such as alumina, zirconia, cordierite, mullite, steatite, and forsterite; (3) magnesia diamond; (4) materials containing any of the above materials as a principal component; and (5) materials containing a mixture of the above materials as a principal component.

The acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film328is lower than the velocity of an acoustic wave propagating through the piezoelectric layer327. The low-acoustic-velocity film328is disposed between the piezoelectric layer327and the high-acoustic-velocity support substrate329. Energy of an acoustic wave inherently concentrates in a low-acoustic-velocity medium. Together with this property, the above structure helps eliminate or reduce the possibility that energy of the surface acoustic wave will leak out of the IDT electrode. The low-acoustic-velocity film328preferably includes, for example, silicon dioxide as a principal component. The low-acoustic-velocity film328preferably has a thickness of, for example, about 2λ or less, where λ denotes the wavelength of the acoustic wave and is determined by the electrode-to-electrode pitch of the IDT electrode. For example, the low-acoustic-velocity film328preferably has a thickness of about 670 nm.

The Q-factor at the resonant frequency and the Q-factor at the anti-resonant frequency of the resonator on the substrate320, that is, on the multilayer structure described above may be much higher than the corresponding Q-factors of a resonator on a known structure including a single piezoelectric substrate. That is, the multilayer structure may be used to obtain a SAW resonator with a high Q-factor, and the SAW resonator may be used to obtain an acoustic wave filter having a small insertion loss.

The high-acoustic-velocity support substrate329may be a multilayer structure including a support substrate and a high-acoustic-velocity film stacked on the support substrate, where the acoustic velocity of a bulk wave propagating through the high-acoustic-velocity film is higher than the acoustic velocity of an acoustic wave such as a surface acoustic wave or a boundary wave propagating through the piezoelectric layer327. In this case, examples of the material of the support substrate include: piezoelectric materials such as sapphire, lithium tantalate, lithium niobate, and quartz; various ceramics such as alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite; a dielectric material such as glass; a semiconductor such as silicon and gallium nitride; and resin. Examples of the material of the high-acoustic-velocity film include: aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, a diamond-like carbon (DLC) film, and diamond; mediums containing the above materials as a principal component; mediums containing a mixture of the above materials as a principal component; and other various high-acoustic-velocity materials.

Although the θ°-rotated Y cut X SAW propagation LiTaO3single crystal is used as the piezoelectric layer327in the present preferred embodiment, the cut-angle of the single crystal material is not limited to the specified angle. The multilayer structure, the material, and the thickness of the substrate may be changed as appropriate in accordance with, for example, the bandpass characteristics required of the acoustic wave filter device concerned. A SAW filter including a LiTaO3piezoelectric substrate or a LiNbO3piezoelectric substrate having a cut-angle different from the specified angle may produce the same or substantially the same advantageous effects described above.

The following describes electrode parameters of the IDT electrode included in the SAW resonator.

The wavelength of the SAW resonator is determined by the wavelength λ, which the repetition period of the electrode fingers322aor the electrode fingers322bincluded in the IDT electrode (see the middle section ofFIG.2). The electrode-to-electrode pitch is half the wavelength λ and is expressed as (W+S), where W denotes the line width of each of the electrode fingers322aincluded in the comb teeth-shaped electrode32aor each of the electrode fingers322bincluded in the comb teeth-shaped electrode32b, and S denotes the space width, or more specifically, the distance between the electrode finger322aand the electrode finger322badjacent to each other. The intersecting width of the pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) is denoted by L and is the length of an overlap between each electrode finger322aand each electrode finger322bviewed in the direction D (see the upper section ofFIG.2). The electrode duty ratio of each resonator refers to the proportion of the line width of the electrode fingers322aand322b, or more specifically, the ratio of the line width to the value obtained by adding the line width to the space width of the electrode fingers322aand322band is expressed as W/(W+S). Each electrode parameter will be specifically described later.

The following describes the structure of the IDT electrode of the first series-arm resonator110sand the IDT electrode of each of the second series-arm resonators121sto124swith reference toFIGS.3A and3B, respectively.

FIG.3Aillustrates the IDT electrode of the first series-arm resonator110sincluded in the acoustic wave filter10.FIG.3Billustrates the IDT electrode of the second series-arm resonator121sincluded in the acoustic wave filter10. The second series-arm resonator121sinFIG.3Bwill be described below as an example of the second series-arm resonators121sto124s.

Referring toFIG.3A, the electrode fingers322band the offset electrode fingers323ain the first series-arm resonator110seach have an atypical shape, or more specifically, are preferably T-shaped, for example. Referring toFIG.3B, the electrode fingers322band the offset electrode fingers323ain the second series-arm resonator121seach have an atypical shape, or more specifically, are preferably T-shaped, for example.

The structure of the second ends e2of the electrode fingers322aand the second ends e2of the offset electrode fingers323b(not illustrated in the enlarged views inFIGS.3A and3B) is the same or substantially the same as the structure of the second ends e2of the electrode fingers322band the second ends e2of the offset electrode fingers323a. That is, the electrode fingers322aand the offset electrode fingers323bin the first series-arm resonator110sand the electrode fingers322aand the offset electrode fingers323bin the second series-arm resonator121seach have an atypical shape, or more specifically, are preferably T-shaped, for example.

Each electrode finger has an atypical shape. That is, the second end e2that is not connected to the busbar electrode is wider than the central portion of the electrode finger. Specifically, the electrode fingers322aand322beach include an electrode-finger central portion cp and a wide portion wp located at the second end e2and being wider than the electrode-finger central portion cp. The wide portion wp is preferably rectangular or substantially rectangular, for example. Alternatively, the wide portion wp may be substantially octagonal, cross-shaped, or convex, for example. The electrode-finger central portion cp refers to a portion of each electrode finger except for end portions opposite each other in the direction in which the electrode finger extends.

Each electrode finger322aincludes the wide portion wp such that the gap in the X direction between the wide portion wp of the electrode finger322aand the electrode finger322badjacent to the wide portion wp is smaller than the gap between the electrode-finger central portions cp of the electrode fingers322aand322badjacent to each other. Each electrode finger322bincludes the wide portion wp such that the gap between the wide portion wp of the electrode finger322band the electrode finger322aadjacent to the wide portion wp is smaller than the gap between the electrode-finger central portions cp of the electrode fingers322band322aadjacent to each other.

For example, when the electrode duty ratio is about 0.5, the gap between the electrode fingers322aand322badjacent to each other is preferably about 0.25λ, and the gap between the wide portion wp of the electrode finger322aand the electrode finger322badjacent to the wide portion wp is preferably not less than about 0.1λ and not more than about 0.2λ.

In the present preferred embodiment, L1is greater than L2(L2<L1), where L1denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the first series-arm resonator110sin the direction in which the electrode fingers extend, and L2denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the second series-arm resonators121sto124sin the direction in which the electrode fingers extend. The same holds true for the offset electrode fingers. That is, L1is greater than L2, where L1denotes the length of the wide portion wp of each of the offset electrode fingers323aand323bin the first series-arm resonator110sin the direction in which the offset electrode fingers extend, and L2denotes the length of the wide portion wp of each of the offset electrode fingers323aand323bin the second series-arm resonators121sto124sin the direction in which the offset electrode fingers extend. That is, the wide portion wp of each electrode finger in the first series-arm resonator110sis longer than the wide portion wp of each electrode in any of the other series-arm resonators (i.e., the series-arm resonators121sto124s).

The length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sis preferably, for example, not less than about 0.1λ and not more than about 0.4λ, where λ denotes the wavelength of the acoustic wave filter10. The intersecting width L of the electrode fingers322aand322bis preferably not greater than about 20λ, for example.

In the present preferred embodiment, the offset electrode fingers323aand323beach include an electrode-finger central portion cp and a wide portion wp located at the second end e2and being wider than the electrode-finger central portion cp. The length L1of the wide portion wp of each of the offset electrode fingers323aand323bis equal to the length L1of the wide portion wp of each of the electrode fingers322aand322b.

1-4 Example 1 According to Preferred Embodiment 1

The following describes characteristics of the first series-arm resonator110saccording to Example 1 of Preferred Embodiment 1 with reference to Table 1 andFIGS.4to6B.

Table 1 shows the fractional bandwidth (%) and the return loss (dB) of the first series-arm resonator110swith variations in the length L1of the wide portion wp of each of the electrode fingers (i.e., the electrode fingers322a, the electrode fingers322b, the offset electrode fingers323a, and the offset electrode fingers323b) in the first series-arm resonator110s. The values of the fractional bandwidth are given byFIG.5, and the values of the return loss are given byFIGS.6A and6B.FIGS.5to6Bwill be described later.

Conditions in Example 1 were as follows: the wavelength λ of the acoustic wave as determined by the electrode-to-electrode pitch of the IDT electrode of the first series-arm resonator110swas about 2.1 μm; the intersecting width L was about 12λ; the number of electrode finger pairs was 200; and the electrode duty ratio was about 0.5. Referring to Table 1, 0λ provided as the length L1of the wide portion wp indicates that none of the electrode fingers included the wide portion wp and that the width of the electrode-finger central portion cp of each electrode finger was equal or substantially equal to the width of the second end e2of each electrode finger. Dividing the difference between the anti-resonant frequency and the resonant frequency by the resonant frequency and by multiplying the quotient by 100 gives the fractional bandwidth.

FIG.4is a graphical representation of the impedance characteristics of the first series-arm resonator110saccording to Example 1.FIG.4illustrates the impedance characteristics of the series-arm resonator110swith variations in the length L1of the wide portion wp within a range of 0 to about 0.7λ.

FIG.4indicates that as the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sincreased gradually within a range of 0 to about 0.7λ, the anti-resonant frequency of the first series-arm resonator110swas shifted toward the lower frequency side.

FIG.5is a graphical representation of the fractional bandwidth of the first series-arm resonator110saccording to Example 1.FIG.5illustrates the fractional bandwidth of the series-arm resonator110swith variations in the length L1of the wide portion wp within a range of 0 to about 0.7λ.

FIG.5indicates that when the length L1of the wide portion wp was within a range of about 0.1 to about 0.4λ, the fractional bandwidth decreased constantly as the length L1of the wide portion wp increased.FIG.5also indicates that when the length L1of the wide portion wp was not less than about 0.5λ, the fractional bandwidth decreased gradually at a rate lower than the rate of change in fractional bandwidth with the length L1within a range of 0.1 to about 0.4λ.

As can be seen fromFIGS.4and5, increasing the length L1of the wide portion wp provides a reduction in the fractional bandwidth and causes a shift of the anti-resonant frequency toward the low frequency side. The attenuation slope in a frequency range higher than the pass band of the acoustic wave filter10becomes steeper accordingly. When the length L1of the wide portion wp is unduly large, spurious waves can be generated in the pass band of the acoustic wave filter as will be described below.

FIG.6Ais a graphical representation of the return loss of the first series-arm resonator according to Example 1, where each electrode finger in the first series-arm resonator included a wide portion having a length of 0λ, about 0.5λ, about 0.6λ, or about 0.7λ.FIG.6Bis a graphical representation of the return loss of the first series-arm resonator according to Example 1, where each electrode finger in the first series-arm resonator included a wide portion having a length of about 0.1λ, about 0.2λ, about 0.3λ, or about 0.4λ. The pass band of the acoustic wave filter10including the first series-arm resonator110sinFIGS.6A and6Bwas in a range of about 1,730 MHz to about 1,850 MHz.

FIG.6Aindicates that when the length L1of the wide portion wp was 0λ (i.e., when none of the electrode fingers included the wide portion wp) or when the length L1of the wide portion wp was not less than about 0.5λ, spurious waves were generated in the pass band of the acoustic wave filter10, resulting in an increase in return loss. Meanwhile,FIG.6Bindicates that when the length L1of the wide portion wp was not less than about 0.1λ and not more than about 0.4λ, spurious waves were reduced or prevented, and the return loss of the first series-arm resonator110swas reduced accordingly. More specifically, when the length L1of the wide portion wp was not less than about 0.1λ and not more than about 0.4λ, the return loss of the first series-arm resonator110swas not more than about 0.8 dB, that is, did not exceed the level that would adversely affect the bandpass characteristics of the acoustic wave filter10. This indicates that setting the length L1of the wide portion wp to any value from about 0.1λ to about 0.4λ enables a reduction in the return loss in the pass band.

Referring toFIGS.6A and6B, the return loss observed at or around a frequency of about 1,940 MHz outside the pass band was an excitation mode in a waveguide on the wide portion wp and presumably had no direct impact on the pass band of the acoustic wave filter10.

The following describes the bandpass characteristics of the acoustic wave filter10according to Example 1 of Preferred Embodiment 1 with reference toFIG.7.

FIG.7is a graphical representation of the bandpass characteristics of the acoustic wave filter10according to Example 1.FIG.7shows the insertion loss of the acoustic wave filter according to Example 1 and the insertion loss of an acoustic wave filter according to Comparative Example 1.

The acoustic wave filter10according to Example 1 differed from the acoustic wave filter according to Comparative Example 1 in that the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110swas greater than the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124s. More specifically, the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110swas about 0.4λ, and the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124swas about 0.2λ.

Further, electrode fingers in any of the series-arm resonators110sand121sto124sof the acoustic wave filter according to Comparative Example 1 all included wide portions wp that were of the same length. More specifically, the wide portion wp of each electrode finger in the series-arm resonators110sand121sto124shad a length of about 0.2λ.

Referring toFIG.7, in a frequency range higher than the pass band, the attenuation slope of the acoustic wave filter10according to Example 1 is steeper than the attenuation slope of the acoustic wave filter according to Comparative Example 1. More specifically, the spacing (difference) between the frequency with an insertion loss of about 3 dB and the frequency with an insertion loss of about 55 dB in Comparative Example 1 was about 13.72 MHz, and the corresponding spacing (difference) in Example 1 was about 11.17 MHz. That is, a reduction of about 2.55 MHz was observed in Example 1. As to the acoustic wave filter10according to Example 1, no return loss causing potential problems was observed in the pass band of the acoustic wave filter, or more specifically, in a frequency range of about 1,710 MHz to about 1,785 MHz.

The above design, in which the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sis greater than the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124s, enables a reduction in the return loss in the pass band while enabling the attenuation slope in a frequency range higher than the pass band to become steeper.

1-5 Example 2 According to Preferred Embodiment 1

The following describes characteristics of the first series-arm resonator110saccording to Example 2 of Preferred Embodiment 1 with reference toFIG.8.

FIG.8is a graphical representation of the relationship between the fractional bandwidth and the intersecting width L of the IDT electrode of the first series-arm resonator110saccording to Example 2.FIG.8shows how the fractional bandwidth (%) changed when the intersecting width L was varied with the capacitance of the IDT electrode being fixed, that is, with the area determined by multiplying the intersecting width by the number of electrode finger pairs being fixed.

FIG.8indicates that when the intersecting width L was not less than about 7.5λ and not more than about 20λ, the fractional bandwidth (%) decreased as the intersecting width L was reduced.FIG.8also indicates that the fractional bandwidth did not change much when the intersecting width L was more than about 20λ. This indicates that setting the intersecting width L to about 20λ or less with no or substantially no change in the area of the IDT electrode enables a reduction in the fractional bandwidth. The attenuation slope in a frequency range outside the pass band of the acoustic wave filter10becomes steeper accordingly.

The following describes the bandpass characteristics of the acoustic wave filter10according to Example 2 of Preferred Embodiment 1 with reference toFIG.9.

FIG.9is a graphical representation of the bandpass characteristics of the acoustic wave filter10according to Example 2.FIG.9shows the insertion loss of the acoustic wave filter according to Example 2 and the insertion loss of an acoustic wave filter according to Comparative Example 2.

The acoustic wave filter10according to Example 2 differed from the acoustic wave filter according to Comparative Example 2 in that the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110swas about 0.4λ, and the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124swas about 0.2λ. The intersecting width L of the IDT electrode of the first series-arm resonator110swas about 12λ, and the intersecting width L of each of the IDT electrodes of the second series-arm resonators121sto124swas also about 12λ.

Further, none of the electrode fingers in any of the series-arm resonators110sand121sto124sof the acoustic wave filter according to Comparative Example 2 included the wide portion wp, and the electrode-finger central portions cp and the second ends e2of the electrode fingers had the same or substantially the same width. The intersecting width L of each of the IDT electrodes of the series-arm resonators110sand121sto124swas about 30λ.

Referring toFIG.9, in a frequency range higher than the pass band, the attenuation slope of the acoustic wave filter10according to Example 2 is steeper than the attenuation slope of the acoustic wave filter according to Comparative Example 2. More specifically, the spacing (difference) between the frequency with an insertion loss of about 3 dB and the frequency with an insertion loss of about 55 dB in Comparative Example 2 was about 15.09 MHz, and the corresponding spacing (difference) in Example 2 was about 11.17 MHz. That is, a reduction of about 3.92 MHz was observed in Example 2. As to the acoustic wave filter10according to Example 2, no return loss causing potential problems was observed in the pass band of the acoustic wave filter, or more specifically, in a frequency range of about 1,710 MHz to about 1,785 MHz.

Example 2 had an advantage over Comparative Example 2. That is, the above design, in which the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sis greater than the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124sand the intersecting width L of each IDT electrode is not more than about 20λ, prevents the return loss in the pass band from reaching a level causing potential problems while enabling the attenuation slope in a frequency range higher than the pass band to become steeper.

The following describes a basic configuration of an acoustic wave filter according to Preferred Embodiment 2 of the present invention with reference toFIGS.10and11. The acoustic wave filter according to Preferred Embodiment 2 includes a third series-arm resonator130sin place of the second series-arm resonator123sin Preferred Embodiment 1.

FIG.10is a circuit configuration diagram of an acoustic wave filter10A according to Preferred Embodiment 2.

The acoustic wave filter10A includes series-arm resonators121s,122s,110s,130s, and124s, parallel-arm resonators121pto124p, and inductors121L and122L. The series-arm resonators121s,122s,110s,130s, and124sare disposed on a path connecting a first terminal Port1and the second terminal Port2. The parallel-arm resonators121pto124pare disposed between the path and a reference terminal (i.e., ground).

The series-arm resonator121s,122s,110s,130s, and124sare connected in series on the path (series arm) connecting the first terminal Port1and the second terminal Port2. The second series-arm resonator121s, the second series-arm resonator122s, the first series-arm resonator110s, the third series-arm resonator130s, and the second series-arm resonator124sare connected in series in the stated order in the direction from the first terminal Port1to the second terminal Port2.

The parallel-arm resonators121pto124pare connected in parallel and disposed on respective paths (parallel arms) each of which connects the reference terminal and a corresponding one of points at which the series-arm resonators121s,122s,110s,130s, and124sare connected to each other.

The series-arm resonators121s,122s,110s,130s, and124sand the parallel-arm resonators121pto124pare connected as described above to define the acoustic wave filter10A that is, for example, a ladder band-pass filter.

The resonant frequency of the first series-arm resonator110s, the resonant frequencies of the second series-arm resonators121s,122s, and124s, and the resonant frequency of the third series-arm resonator130sare within the pass band of the acoustic wave filter10A. The anti-resonant frequency of the first series-arm resonator110sis lower than any of the anti-resonant frequencies of the second series-arm resonators121s,122s, and124s. The anti-resonant frequency of the third series-arm resonator130sis lower than any of the anti-resonant frequencies of the second series-arm resonators121s,122s, and124sand is higher than the anti-resonant frequency of the first series-arm resonator110s.

The third series-arm resonator130smay be disposed between the second series-arm resonator122sand the first series-arm resonator110s. That is, the third series-arm resonator130sis connected directly to the first series-arm resonator110s.

FIG.11illustrates an IDT electrode of the third series-arm resonator130sincluded in the acoustic wave filter10A according to Preferred Embodiment 2.

The IDT electrode of the third series-arm resonator130sis the same or substantially the same as the IDT electrode of the first series-arm resonator110s. That is, L3is greater than L2, where L3denotes the length of the wide portion wp of each of the electrode fingers in the third series-arm resonator130s, and L2denotes the length of the wide portion wp of each of the electrodes of the second series-arm resonators121s,122s, and124s. The length L3is not more than the length L1of the wide portion wp of each of the electrode fingers in the first series-arm resonator110s(L2<L3L1).

The acoustic wave filter10A according to Preferred Embodiment 2 includes the third series-arm resonator130sdisposed on the path. The first series-arm resonator110sand the third series-arm resonator130sare connected in series. The third series-arm resonator130sincludes an IDT electrode including a pair of comb teeth-shaped electrodes (i.e., a comb teeth-shaped electrode32aand a comb teeth-shaped electrode32b) provided on a substrate including a piezoelectric layer. The comb teeth-shaped electrode32aof the third series-arm resonator130sincludes electrode fingers322aand a busbar electrode321a, and the comb teeth-shaped electrode32bof the third series-arm resonator130sincludes electrode fingers322band a busbar electrode321b. The electrode fingers322aand322bextend in a direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave. The busbar electrode321aconnects first ends e1of the electrode fingers322ato each other, and the busbar electrode321bconnects first ends e1of the electrode fingers322bto each other.

The direction in which second ends e2of the electrode fingers322aare aligned with each other and second ends e2of the electrode fingers322bare aligned with each other is denoted by D and crosses the propagation direction of the acoustic wave. The electrode fingers of the IDT electrode of the third series-arm resonator130seach include an electrode-finger central portion cp and a wide portion wp located at the second end e2and being wider than the electrode-finger central portion cp. L3is greater than L2, where L3denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the third series-arm resonator130sin the direction in which the electrode fingers extend, and L2denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the second series-arm resonators121s,122s, and124sin the direction in which the electrode fingers extend.

As in Preferred Embodiment 1, the acoustic wave filter10A according to Preferred Embodiment 3 enables a reduction in the return loss in the pass band of the acoustic wave filter10A while enabling the attenuation slope in a frequency range higher than the pass band to become steeper.

Preferred Embodiment 1 describes that the acoustic wave filter10includes only a ladder filter structure. In some preferred embodiments of the present invention, the filter includes a longitudinally coupled filter structure in addition to the ladder filter structure. A filter according to Preferred Embodiment 3 of the present invention includes these structures as will be described below.

FIG.12is a circuit configuration diagram of an acoustic wave filter10B according to Preferred Embodiment 3.

As illustrated inFIG.12, the acoustic wave filter10B includes a second series-arm resonator121s, a first series-arm resonator110s, parallel-arm resonators121pand124p, and a longitudinally coupled resonator150. That is, the acoustic wave filter10B includes the longitudinally coupled resonator150in addition to the ladder filter structure.

The longitudinally coupled resonator150has a longitudinally coupled filter structure disposed between a first terminal Port1and a second terminal Port2. The longitudinally coupled resonator150in the present preferred embodiment is preferably closer than the first series-arm resonator110sto the second terminal Port2and includes, for example, two reflectors and nine IDTs disposed between the reflectors. In some preferred embodiments of the present invention, the longitudinally coupled resonator150may be disposed between the second series-arm resonator121sand the first series-arm resonator110s. The longitudinally coupled resonator150does not necessarily include nine IDTs and may include three or more IDTs, for example.

As in the above preferred embodiments, the acoustic wave filter10B enables a reduction in the return loss in the pass band of the acoustic wave filter10B while enabling the attenuation slope in a frequency range higher than the pass band to become steeper.

As described above, the acoustic wave filter10according to Preferred Embodiment 1 includes the first series-arm resonator110sand the second series-arm resonators121sto124s. The first series-arm resonator110sand the second series-arm resonators121sto124sare disposed on the path connecting the first terminal Port1and the second terminal Port2. The first series-arm resonator110shas a lower anti-resonant frequency than the any other series-arm resonator included in the acoustic wave filter10. The first series-arm resonator110sand the second series-arm resonators121sto124seach include an IDT electrode including a pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) provided on the substrate320including the piezoelectric layer327. Electrodes of the pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) of the first series-arm resonator110sand electrodes of the pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) of each of the second series-arm resonator121sto124seach include the electrode fingers322a, the electrode fingers322b, and the busbar electrode321a, and the busbar electrode321b. The electrode fingers322aand322bextend in the direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave. The busbar electrode321aconnects the first ends e1of the electrode fingers322ato each other, and the busbar electrode321bconnects the first ends e1of the electrode fingers322bto each other. The direction D in which the second ends e2of the electrode fingers322aare aligned with each other and the second ends e2of the electrode fingers322bare aligned with each other crosses the propagation direction of the acoustic wave. The electrode fingers of the IDT electrode of the first series-arm resonator110sand the electrode fingers of the IDT electrodes of the second series-arm resonators121sto124seach include the electrode-finger central portion cp and the wide portion wp located at the second end e2and being wider than the electrode-finger central portion cp. L1is greater than L2, where L1denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the first series-arm resonator110sin the direction in which the electrode fingers extend, and L2denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the second series-arm resonators121sto124sin the direction in which the electrode fingers extend.

The above design, in which the electrode fingers322aand322binclude the respective wide portions wp and the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sis greater than the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124s, enables a reduction in the return loss in the pass band of the acoustic wave filter10while enabling the attenuation slope in a frequency range higher than the pass band to become steeper.

The length L1of the wide portion wp of each electrode finger in the first series-arm resonator110smay preferably be, for example, not less than about 0.1λ and not more than about 0.4λ, where λ denotes the wavelength of the acoustic wave filter10.

The return loss in the pass band of the acoustic wave filter10may thus be prevented from reaching a level that causes potential problems.

The intersecting width L of the IDT electrode of the first series-arm resonator110smay preferably be not more than about 20λ, for example.

The fractional bandwidth of the first series-arm resonator110smay thus be reduced while the area determined by multiplying the intersecting width of the IDT electrode by the number of electrode finger pairs is fixed. This prevents the return loss in the pass band from reaching a level causing potential problems while enabling the attenuation slope in a frequency range higher than the pass band of the acoustic wave filter10to become steeper with the area being fixed.

The substrate320may include the piezoelectric layer327, the high-acoustic-velocity support substrate329, and the low-acoustic-velocity film328disposed between the high-acoustic-velocity support substrate329and the piezoelectric layer327. The piezoelectric layer327includes two main surfaces, and the IDT electrode may be disposed on one of the two main surfaces of the piezoelectric layer327. The acoustic velocity of the bulk wave propagating through the high-acoustic-velocity support substrate329is higher than the acoustic velocity of the acoustic wave propagating through the piezoelectric layer327. The acoustic velocity of the bulk wave propagating through the low-acoustic-velocity film328is lower than the acoustic velocity of the acoustic wave propagating through the piezoelectric layer327.

The Q-factor at the resonant frequency and the Q-factor at the anti-resonant frequency of a resonator having the structure mentioned above may be much higher than the corresponding Q-factors of a resonator having a known structure including a single piezoelectric substrate. That is, the multilayer structure may be used to obtain a SAW resonator with a high Q-factor, and the SAW resonator may be used to obtain an acoustic wave filter having a small insertion loss.

The acoustic wave filter10A according to Preferred Embodiment 2 also includes the third series-arm resonator130sdisposed on the path. The first series-arm resonator110sand the third series-arm resonator130sare connected in series. The anti-resonant frequency of the third series-arm resonator130sis lower than any of the anti-resonant frequencies of the second series-arm resonators121s,122s, and124s. The third series-arm resonator130sincludes an IDT electrode including a pair of comb teeth-shaped electrodes (i.e., the comb teeth-shaped electrodes32aand32b) on the substrate320including the piezoelectric layer327. The comb teeth-shaped electrode32aof the third series-arm resonator130sincludes the electrode fingers322aand the busbar electrode321a, and the comb teeth-shaped electrode32bof the third series-arm resonator130sincludes the electrode fingers322band the busbar electrode321b. The electrode fingers322aand322bextend in the direction orthogonal or substantially orthogonal to the propagation direction of the acoustic wave. The busbar electrode321aconnects the first ends e1of the electrode fingers322ato each other, and the busbar electrode321bconnects the first ends e1of the electrode fingers322bto each other. The direction D in which the second ends e2of the electrode fingers322aare aligned with each other and the second ends e2of the electrode fingers322bare aligned with each other crosses the propagation direction of the acoustic wave. The electrode fingers of the IDT electrode of the third series-arm resonator130seach include the electrode-finger central portion cp and the wide portion wp located at the second end e2and being wider than the electrode-finger central portion cp. L3is greater than L2, where L3denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the third series-arm resonator130sin the direction in which the electrode fingers extend, and L2denotes the length of the wide portion wp of each of the electrode fingers322aand322bin the second series-arm resonators121s,122s, and124sin the direction in which the electrode fingers extend.

The above design, in which L3is greater than L2and the third series-arm resonator130sis connected directly to the first series-arm resonator110s, enables a reduction in the return loss in the pass band of the acoustic wave filter10A while enabling the attenuation slope in a frequency range higher than the pass band to become much steeper.

The acoustic wave filters according to Preferred Embodiments 1, 2, and 3 of the present invention have been described above. Although the present invention has been described with reference to preferred embodiments, the present invention also includes other preferred embodiments provided by varying combinations of components of the aforementioned preferred embodiments, other modifications achieved through various alterations to the preferred embodiments that may be conceived by those skilled in the art within a range not departing from the spirit of the present invention, and various types of apparatuses including the acoustic wave filters according to preferred embodiments of the present invention.

The preferred embodiments described above each include the series-arm resonators including offset electrode fingers. In some preferred embodiments, however, none of the series-arm resonators includes the offset electrode fingers.

In the preferred embodiments described above, the length L1of the wide portion wp of each electrode finger in the first series-arm resonator110sis greater than the length L2of the wide portion wp of each electrode finger in the second series-arm resonators121sto124s. In some preferred embodiments, however, the length L1of the wide portions wp of, for example, about 50% or more of the electrode fingers in the first series-arm resonator110sis greater than the length L2.

The acoustic wave filter10may be used as a transmitting filter or a receiving filter. The acoustic wave filter10may be used as a transmitting filter in the following manner: a transmission wave generated by a transmitting circuit, such as a radio-frequency integrated circuit (RFIC), for example, and input to the acoustic wave filter10through the second terminal Port2is filtered in a predetermined transmission pass band, and the resultant wave is output to the first terminal Port1. The acoustic wave filter10may be used as a receiving filter in the following manner: a reception wave input to the acoustic wave filter10through the first terminal Port1is filtered in a predetermined reception pass band, and the resultant wave is output to the second terminal Port2.

The first terminal Port1may be an input terminal or an output terminal. Similarly, the second terminal Port2may be an input terminal or an output terminal. When the first terminal Port1is an input terminal, the second terminal Port2may be an output terminal. When the second terminal Port2is an input terminal, the first terminal Port1may be an output terminal.

Preferred embodiments of the present invention may be included, for example, in multiplexers including acoustic wave filters, front-end circuits, and communication devices and thus have wide applicability to communication apparatuses, such as mobile phones, for example.