Patent Publication Number: US-2022239280-A1

Title: Acoustic wave filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-199253 filed on Oct. 31, 2019 and is a Continuation Application of PCT Application No. PCT/JP2020/040849 filed on Oct. 30, 2020. The entire contents of each 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 including a surface acoustic wave resonator and a bulk acoustic wave resonator. 
     2. Description of the Related Art 
     Low-loss and high-attenuation acoustic wave filters such as longitudinally coupled surface acoustic wave (SAW) filters and SAW ladder filters are used in communication devices, including mobile phones. In recent years, along with the development of multiband operation of communication devices, demand has increased for miniaturized acoustic wave filters usable in various frequency bands. 
     Japanese Unexamined Patent Application Publication No. 2007-037102 discloses an integrated filter constructed by forming a SAW resonator and a bulk acoustic wave (BAW) resonator on the same substrate. This can obtain an acoustic wave filter usable in various frequency ranges. 
     In the integrated filter disclosed in Japanese Unexamined Patent Application Publication No. 2007-037102, however, the SAW resonator and the BAW resonator are formed in different areas on the substrate, and thus, a problem arises in which the size of the filter is increased. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide miniaturized acoustic wave filters each including a SAW resonator and a BAW resonator. 
     An acoustic wave filter according to a preferred embodiment of the present invention includes a surface acoustic wave resonator and a bulk acoustic wave resonator. The surface acoustic wave resonator includes a substrate with piezoelectricity and an interdigital transducer (IDT) electrode on the substrate. The IDT electrode includes a pair of comb-shaped electrodes interdigitated with each other. Each of the pair of comb-shaped electrodes includes a plurality of electrode fingers extending parallel or substantially parallel in a direction crossing a surface acoustic wave propagation direction and a busbar electrode connecting the plurality of electrode fingers to each other at one end of each electrode finger of the plurality of electrode fingers. The bulk acoustic wave resonator includes a lower electrode including a portion of the busbar electrode, a piezoelectric film on the busbar electrode, and an upper electrode on the piezoelectric film. 
     Each of the acoustic wave filters according to preferred embodiments of the present invention is able to provide a miniaturized acoustic wave filter including a SAW resonator and a BAW resonator. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  provide plan views of an electrode and a sectional view of a structure that illustrate a basic structure of an acoustic wave filter according to a first preferred embodiment of the present invention. 
         FIGS. 2A and 2B  provide plan views of electrodes and a sectional view of a structure of an acoustic wave filter according to a first practical example of a preferred embodiment of the present invention. 
         FIGS. 3A and 3B  provide circuit configuration diagrams of the acoustic wave filter according to the first practical example and a graph illustrating a comparison between the acoustic wave filter according to the first practical example and an acoustic wave filter according to a first comparative example with respect to the bandpass characteristic. 
         FIGS. 4A and 4B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a first modification of a preferred embodiment of the present invention. 
         FIGS. 5A and 5B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a second modification of a preferred embodiment of the present invention. 
         FIGS. 6A and 6B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a third modification of a preferred embodiment of the present invention. 
         FIGS. 7A and 7B  provide plan views of electrodes and a sectional view of a structure of an acoustic wave filter according to a second practical example of a preferred embodiment of the present invention. 
         FIGS. 8A and 8B  provide circuit configuration diagrams of the acoustic wave filter according to the second practical example and a graph illustrating a comparison between the acoustic wave filter according to the second practical example and an acoustic wave filter according to a second comparative example with respect to the bandpass characteristic. 
         FIGS. 9A and 9B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a fourth modification of a preferred embodiment of the present invention. 
         FIGS. 10A and 10B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a fifth modification of a preferred embodiment of the present invention. 
         FIGS. 11A and 11B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter according to a sixth modification of a preferred embodiment of the present invention. 
         FIGS. 12A to 12C  provide graphs illustrating bandpass characteristics with respect to different film thicknesses of the piezoelectric film of the acoustic wave filter according to the first practical example. 
         FIGS. 13A to 13D  provide plan views of electrodes illustrating variations of the shape of a piezoelectric film of an acoustic wave filter according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The preferred embodiments described below are comprehensive or specific examples. Specific features including numerical values, shapes, materials, elements, arrangements of the elements, and connection configurations described in the following preferred embodiments are mere examples and are not intended to limit the present invention. Among the elements in the following preferred embodiments, elements not recited in any of the independent claims are described as arbitrary or optional elements. The size or size ratio of the elements illustrated in the drawings is not necessarily presented in an exact manner. 
     Preferred Embodiment 
     1. Basic Structure of Acoustic Wave Filter  1   
       FIGS. 1A and 1B  provide plan views of an electrode and a sectional view of a structure that illustrate a basic structure of an acoustic wave filter  1  according to a first preferred embodiment. In the drawings,  FIG. 1A  is a plan view of elements including an interdigital transducer (IDT) electrode when a substrate  10  is viewed in plan view, and  FIG. 1B  is a sectional view taken along line Ib-Ib in  FIG. 1A . As illustrated in the drawings, the acoustic wave filter  1  includes the substrate  10  having piezoelectricity, an IDT electrode  20 , a piezoelectric film  30   a , upper electrodes  40   a  and  40   b , and a protective layer  50 . 
     The substrate  10  is a substrate having piezoelectricity, such as, for example, a single-crystal piezoelectric substrate made of a piezoelectric material. Examples of the single-crystal piezoelectric substrate include piezoelectric single crystals, such as LiNbO 3  and LiTaO 3 , and quartz-crystal. 
     The substrate  10  is not necessarily a single piezoelectric layer. The substrate  10  may include a high acoustic velocity support substrate, a low acoustic velocity film, and a piezoelectric film that are stacked in this order. The piezoelectric film in this case may be made of, for example, a piezoelectric single crystal or piezoelectric ceramic. The high acoustic velocity support substrate supports elements including the low acoustic velocity film, the piezoelectric film, and the IDT electrode  20 . The acoustic velocity of bulk waves in the high acoustic velocity support substrate is higher than the acoustic velocity of surface acoustic waves and boundary waves propagating along the piezoelectric film. The function of this structure is that the multilayer body of the piezoelectric film and the low acoustic velocity film confine surface acoustic waves so that the surface acoustic waves are prevented from leaking into the portion including the high acoustic velocity support substrate and portions lower than the high acoustic velocity support substrate. The high acoustic velocity support substrate may be, for example, a silicon substrate. In the low acoustic velocity film, bulk waves propagate at an acoustic velocity lower than bulk waves propagating in the piezoelectric film. The low acoustic velocity film is disposed between the piezoelectric film and the high acoustic velocity support substrate. This structure and a property of acoustic wave in which energy is naturally concentrated in low-acoustic-velocity media hinder leakage of surface acoustic wave energy outside the IDT electrode  20 . The low acoustic velocity film may be mainly made of, for example, silicon dioxide. By comparison to a single-layer piezoelectric substrate, the layered structure of the substrate  10  described above can increase the Q factor at the resonant frequency and anti-resonant frequency of a surface acoustic wave resonator. As such, an acoustic wave resonator with a relatively high Q factor can be provided, and as a result, a filter with low insertion loss can be provided by using the acoustic wave resonator. 
     The high acoustic velocity support substrate may have a structure provided by stacking a support substrate and a high acoustic velocity film in which bulk waves propagate at an acoustic velocity higher than acoustic waves such as surface acoustic waves and boundary waves propagating along the piezoelectric film. In this case, examples of a material used for the support substrate include piezoelectric materials such as sapphire, lithium tantalate, lithium niobate, and quartz-crystal, various ceramics such as alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite, dielectric materials such as glass, semiconductors such as silicon and gallium nitride, and resin substrates. The high acoustic velocity film may include various high-acoustic-velocity materials including, for example, aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, DLC films, or diamond, a medium mainly including the above material, and a medium mainly including a mixture of the above materials. 
     The substrate  10  may include a support substrate, an energy confining layer, and a piezoelectric film that are stacked in this order. The IDT electrode  20  may be provided on the piezoelectric film. For example, a piezoelectric single crystal or piezoelectric ceramic may be used for the piezoelectric film. The support substrate supports the piezoelectric film, the energy confining layer, and the IDT electrode  20 . The energy confining layer includes one or more layers. The velocity of bulk acoustic waves propagating in at least one layer of the layers is higher than the velocity of acoustic waves propagating close to the piezoelectric film. For example, the energy confining layer may have a layered structure including a low acoustic velocity layer and a high acoustic velocity layer. The acoustic velocity of bulk waves in the low acoustic velocity layer is lower than the acoustic velocity of acoustic waves propagating in the piezoelectric film. The acoustic velocity of bulk waves in the high acoustic velocity layer is higher than the acoustic velocity of acoustic waves propagating in the piezoelectric film. The support substrate may be defined by a high acoustic velocity layer. The energy confining layer may be an acoustic impedance layer provided by alternately layering a low acoustic impedance layer with a relatively low acoustic impedance and a high acoustic impedance layer with a relatively high acoustic impedance. 
     The IDT electrode  20  includes a pair of comb-shaped electrodes  20   a  and  20   b  that are provided on the substrate  10  and that are interdigitated with each other. The comb-shaped electrode  20   a  extends in a direction crossing a surface acoustic wave (SAW) propagation direction. The comb-shaped electrode  20   a  includes a plurality of electrode fingers  120   a  disposed parallel or substantially parallel to each other and a busbar electrode  110   a  connecting the electrode fingers  120   a  to each other at one end of each electrode finger  120   a . The comb-shaped electrode  20   b  extends in a direction crossing the SAW propagation direction. The comb-shaped electrode  20   b  includes a plurality of electrode fingers  120   b  disposed parallel or substantially parallel to each other and a busbar electrode  110   b  connecting the electrode fingers  120   b  to each other at one end of each electrode finger  120   b.    
     The IDT electrode  20  has a layered structure including, for example, a fixing layer and a main electrode layer. The fixing layer improves the fixing strength between the substrate  10  and the main electrode layer. An example of the material used for the fixing layer is Ti. An example of the material of the main electrode layer is Al including about 1% Cu. The material of the fixing layer and the material of the main electrode layer are not limited to the materials described above. The IDT electrode  20  is not necessarily defined by the layered structure described above. The IDT electrode  20  may be made of, for example, a metal, such as Ti, Al, Cu, Pt, Au, Ag, or Pd, or an alloy thereof, or may include a plurality of multilayer bodies made of the metal or alloy. 
     The protective layer  50  is provided on the IDT electrode  20  to cover the comb-shaped electrodes  20   a  and  20   b . The protective layer  50  is used for the following purposes: (1) to protect the main electrode layer of the IDT electrode  20  from the external environment, (2) to control the frequency temperature characteristic, and (3) to increase moisture resistance. The protective layer  50  is a dielectric film mainly made of, for example, silicon dioxide. 
     In the structure of the acoustic wave filter  1  described above, the substrate  10 , the IDT electrode  20 , and the protective layer  50  together define a SAW resonator. The SAW resonator exhibits resonance such that surface acoustic waves are excited along a surface of the piezoelectric substrate and in the comb-shaped electrodes by applying electrical energy between the busbar electrodes facing each other. The surface acoustic waves generated in the SAW resonator propagate between the electrode fingers  120   a  and  120   b  along the surface of the substrate  10 . 
     The SAW resonator described above does not necessarily include the protective layer  50 . A reflector may be provided adjacent to the IDT electrode  20  in the SAW propagation direction. 
     The piezoelectric film  30   a  is provided on the busbar electrode  110   a . The upper electrode  40   a  is provided on the piezoelectric film  30   a.    
     The piezoelectric film  30   a  is mainly made of, for example, at least one of zinc oxide (ZnO), aluminum nitride (AlN), lead zirconate titanate (PZT), potassium niobate (KN), lithium niobate (LN), lithium tantalate (LT), quartz-crystal, and lithium borate (LiBO). When the piezoelectric film  30   a  is made of zinc oxide (ZnO) or aluminum nitride (AlN), it is preferable that the piezoelectric film  30   a  is a c-axis oriented film. 
     The upper electrode  40   a  is made of, for example, the same material as the main electrode layer of the IDT electrode  20 , which may be Al including about 1% Cu. 
     In the structure of the acoustic wave filter  1  described above, the busbar electrode  110   a , the piezoelectric film  30   a , and the upper electrode  40   a  together define a bulk acoustic wave (BAW) resonator. The BAW resonator exhibits resonance such that bulk acoustic waves are excited in the piezoelectric film  30   a  by applying electrical energy between the busbar electrode  110   a  and the upper electrode  40   a . The busbar electrode  110   a  defines and functions as a lower electrode of the BAW resonator. The bulk acoustic waves generated in the BAW resonator propagate between the busbar electrode  110   a  and the upper electrode  40   a  in the vertical direction relative to the surface of the substrate  10 . To define the BAW resonator, the protective layer  50  is disposed between the busbar electrode  110   b  and the upper electrode  40   a  so that the busbar electrode  110   b  is not in direct contact with the upper electrode  40   a.    
     On the busbar electrode  110   b , the upper electrode  40   b  is provided in a cavity above the busbar electrode  110   b . As such, the busbar electrode  110   b  and the upper electrode  40   b  define two-layer wiring. The two-layer wiring can reduce interconnection resistance, and as a result, it is possible to reduce transmission loss of radio-frequency signal. The upper electrode  40   b  is made of, for example, the same material as the upper electrode  40   a . The upper electrodes  40   a  and  40   b  are formed by the same deposition process. 
     With the structure described above, the acoustic wave filter  1  includes the SAW resonator and the BAW resonator on the same substrate. Additionally, the busbar electrode  110   a  of the SAW resonator also defines and functions as the lower electrode of the BAW resonator. This means that the SAW resonator and the BAW resonator are provided in a shared area on the substrate  10 . Consequently, by comparison to the structure in which a SAW resonator and a BAW resonator are separately provided in discrete areas on a substrate, this structure can reduce the size of the acoustic wave filter  1 . Furthermore, the busbar electrode  110   a  of the SAW resonator and the lower electrode of the BAW resonator can be formed at the same time, and additionally, the upper electrode  40   b  of the two-layer wiring of the SAW resonator and the upper electrode  40   a  of the BAW resonator can be formed by the same manufacturing process. Accordingly, it is possible to simplify the manufacturing process and also reduce costs. 
     2. Structure of Acoustic Wave Filter According to First Practical Example 
       FIGS. 2A and 2B  provide plan views of electrodes and a sectional view of a structure of an acoustic wave filter  2 A according to a first practical example of a preferred embodiment of the present invention.  FIG. 2A  provides a plan view of elements including IDT electrodes and reflectors when the substrate  10  is viewed in plan view, and an enlarged plan view of busbar electrodes of adjacent IDT electrodes, and  FIG. 2B  is a sectional view taken along line IIb-IIb in the enlarged plan view of  FIG. 2A . 
     As illustrated in  FIGS. 2A and 2B , the acoustic wave filter  2 A includes the substrate  10  with piezoelectricity, IDT electrodes  20 A,  20 B,  20 C,  20 D, and  20 E, reflectors  90 A and  90 B, the piezoelectric film  30   a , the upper electrode  40   a , and the protective layer  50 . 
     As illustrated in  FIG. 2A , the IDT electrodes  20 A to  20 E each include a pair of comb-shaped electrodes. Each comb-shaped electrode includes a plurality of electrode fingers extending in a direction crossing the SAW propagation direction and a busbar electrode connecting the electrode fingers to each other at one end of each electrode finger. The pair of comb-shaped electrodes face each other such that the electrode fingers are interdigitated with each other. The IDT electrodes  20 A to  20 E are disposed between the reflectors  90 A and  90 B in the SAW propagation direction. 
     The IDT electrodes  20 A to  20 E and the reflectors  90 A and  90 B are arranged in the SAW propagation direction in the following order: the reflector  90 A, the IDT electrodes  20 A,  20 B,  20 C,  20 D, and  20 E, and the reflector  90 B. Of each of the IDT electrodes  20 A,  20 C, and  20 E, the busbar electrode on one side of the pair of comb-shaped electrodes is coupled to an input-side wire  70   a . Of each of the IDT electrodes  20 B and  20 D, the busbar electrode on the other side of the pair of comb-shaped electrodes is coupled to an input-side ground wire  70   b . Of each of the IDT electrodes  20 A,  20 C, and  20 E, the busbar electrode on the other side of the pair of comb-shaped electrodes is coupled to an output-side ground wire  70   d . Of each of the IDT electrodes  20 B and  20 D, the busbar electrode on the one side of the pair of comb-shaped electrodes is coupled to an output-side wire  70   c . With the structure described above, in the acoustic wave filter  2 A according to the present practical example, the substrate  10 , the IDT electrodes  20 A to  20 E, and the reflectors  90 A and  90 B include a longitudinally coupled resonator including five SAW resonators. 
     Here, as illustrated in the enlarged plan view in  FIG. 2A , the piezoelectric film  30   a  is provided on the busbar electrode  110   a  of the IDT electrode  20 D of the adjacent IDT electrodes  20 C and  20 D. The upper electrode  40   a  is provided on the piezoelectric film  30   a . The busbar electrode  110   a  is connected to the input-side ground wire  70   b . The upper electrode  40   a  is connected to the input-side wire  70   a . Although  FIG. 2A  does not illustrate the protective layer  50 , the protective layer  50  is disposed between the busbar electrode  110   a  and the upper electrode  40   a.    
     In the structure of the acoustic wave filter  2 A, as illustrated in  FIG. 2B , the busbar electrode  110   a , the piezoelectric film  30   a , and the upper electrode  40   a  together define a BAW resonator  60 D. The busbar electrode  110   a  defines and functions as a lower electrode of the BAW resonator  60 D. The bulk acoustic waves generated in the BAW resonator  60 D propagate between the busbar electrode  110   a  and the upper electrode  40   a  in the vertical direction relative to the surface of the substrate  10 . 
     The upper electrode  40   a  is directly connected to a busbar electrode  111   b  of the IDT electrode  20 C. This means that the busbar electrode  111   b  and the upper electrode  40   a  define a two-layer wiring on the busbar electrode  111   b . As such, the busbar electrode  111   b  is coupled to the input-side wire  70   a.    
     As described above, in the acoustic wave filter  2 A according to the present practical example, the piezoelectric film  30   a  is provided on the busbar electrode  110   a  connected to the input-side ground wire  70   b , and the upper electrode  40   a  is provided on the piezoelectric film  30   a  and on the adjacent busbar electrode  111   b  coupled to the input-side wire  70   a.    
       FIGS. 3A and 3B  provide circuit configuration diagrams of the acoustic wave filter  2 A according to the first practical example and a graph illustrating a comparison between the acoustic wave filter according to the first practical example and an acoustic wave filter according to a first comparative example with respect to the bandpass characteristic. The acoustic wave filter according to the first comparative example has a structure provided by removing the BAW resonator  60 D from the acoustic wave filter  2 A according to the first practical example. 
     As illustrated in  FIG. 3A , the acoustic wave filter  2 A according to the present practical example includes the BAW resonator  60 D between the busbar electrode  110   a  at a ground potential of the IDT electrode  20 D and the input-side busbar electrode  111   b  at a HOT potential of the IDT electrode  20 C, in addition to the longitudinally coupled resonator including the SAW resonators that include the IDT electrodes  20 A to  20 E. 
     As illustrated in  FIG. 3B , there is almost no difference between the acoustic wave filter  2 A according to the first practical example and the acoustic wave filter according to the first comparative example with respect to the insertion loss in a pass band. This is because the longitudinally coupled resonator effects characteristics in the pass band. In contrast, in an attenuation band higher than the pass band, the acoustic wave filter  2 A according to the first practical example is greater with respect to attenuation. It is concluded that attenuation outside the band is improved because oscillations caused by vibrations of bulk acoustic waves generated in the BAW resonator  60 D occur at frequencies higher than the pass band. 
     The number of IDT electrodes of the acoustic wave filter  2 A is not limited to five. The number of IDT electrodes constituting the acoustic wave filter  2 A only needs to be, for example, three or more. Additionally, two or more BAW resonators may be provided to define the acoustic wave filter  2 A. 
     In the structure of the acoustic wave filter  2 A according to the first practical example, the SAW resonator defining the longitudinally coupled resonator and the BAW resonator  60 D are provided at the same substrate  10 . Additionally, the busbar electrode  110   a  of the SAW resonator also defines and functions as the lower electrode of the BAW resonator  60 D. This means that the SAW resonator and the BAW resonator  60 D are provided in a shared area on the substrate  10 . Consequently, in comparison to the structure in which the SAW resonators and the BAW resonator  60 D are separately provided in discrete areas on a substrate, this structure can improve the attenuation characteristic of the acoustic wave filter  2 A and also make the acoustic wave filter  2 A smaller. Furthermore, the busbar electrode  110   a  of the SAW resonator and the lower electrode of the BAW resonator  60 D can be formed at the same time, and additionally, the upper electrode of the busbar electrode  111   b  of the SAW resonator and the upper electrode of the BAW resonator  60 D can be defined by the same upper electrode  40   a . Accordingly, it is possible to simplify the manufacturing process and also reduce costs. 
       FIGS. 4A and 4B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  2 B according to a first modification of a preferred embodiment of the present invention. The acoustic wave filter  2 B illustrated in  FIGS. 4A and 4B  differs from the acoustic wave filter  2 A according to the first practical example in that it includes two BAW resonators  60 B and  60 D. The following description of the acoustic wave filter  2 B according to the present modification omits the same or corresponding configurations as the acoustic wave filter  2 A according to the first practical example and mainly focuses on configurations different from the acoustic wave filter  2 A. 
     As illustrated in  FIG. 4A , the acoustic wave filter  2 B according to the present modification includes the BAW resonators  60 B and  60 D in addition to the longitudinally coupled resonator defined by the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  60 B is provided between the busbar electrode (lower electrode) at the ground potential of the IDT electrode  20 B and an upper electrode coupled to the busbar electrode at the HOT potential of the IDT electrode  20 C. The BAW resonator  60 D is provided between the busbar electrode (lower electrode) at the ground potential of the IDT electrode  20 D and the upper electrode coupled to the busbar electrode at the HOT potential of the IDT electrode  20 C. 
     In comparison to an acoustic wave filter including a single BAW resonator, the acoustic wave filter  2 B according to the first modification can increase attenuation in the attenuation band because the acoustic wave filter  2 B includes the two BAW resonators  60 B and  60 D. When the piezoelectric film of the BAW resonator  60 B is provided differently from the piezoelectric film of the BAW resonator  60 D with respect to thickness, attenuation can be achieved in two kinds of different attenuation bands. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  2 B and also make the acoustic wave filter  2 B smaller. 
       FIGS. 5A and 5B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  2 C according to a second modification of a preferred embodiment of the present invention. The acoustic wave filter  2 C illustrated in  FIGS. 5A and 5B  differs from the acoustic wave filter  2 A according to the first practical example in that a BAW resonator  60 C is coupled on the output-side instead of the input-side with respect to the longitudinally coupled resonator. The following description of the acoustic wave filter  2 C according to the present modification omits the same or corresponding configurations as the acoustic wave filter  2 A according to the first practical example and mainly focuses on configurations different from the acoustic wave filter  2 A. 
     As illustrated in  FIG. 5A , the acoustic wave filter  2 C according to the present modification includes the BAW resonator  60 C in addition to the longitudinally coupled resonator composed of the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  60 C is provided between the busbar electrode (lower electrode) at the ground potential of the IDT electrode  20 C and an upper electrode coupled to the output-side busbar electrode at a HOT potential of the IDT electrode  20 B. 
     The acoustic wave filter  2 C according to the second modification can increase attenuation in the attenuation band because the BAW resonator  60 C is disposed on the output-side with respect to the longitudinally coupled resonator. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  2 C and also make the acoustic wave filter  2 C smaller. 
       FIGS. 6A and 6B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  2 D according to a third modification of a preferred embodiment of the present invention. The acoustic wave filter  2 D illustrated in  FIGS. 6A and 6B  differs from the acoustic wave filter  2 A according to the first practical example in that a BAW resonator  61 B is coupled between an IDT electrode and a reflector instead of between IDT electrodes of the longitudinally coupled resonator. The following description of the acoustic wave filter  2 D according to the present modification omits the same or corresponding configurations as the acoustic wave filter  2 A according to the first practical example and mainly focuses on configurations different from the acoustic wave filter  2 A. 
     As illustrated in  FIG. 6A , the acoustic wave filter  2 D according to the present modification includes the BAW resonator  61 B in addition to the longitudinally coupled resonator composed of the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  61 B is provided between a busbar electrode of the reflector  90 B and an upper electrode coupled to an input-side busbar electrode at the HOT potential of the IDT electrode  20 E. 
     The acoustic wave filter  2 D according to the third modification can increase attenuation in the attenuation band because the BAW resonator  61 B is disposed between an IDT electrode and a reflector on the input-side with respect to the longitudinally coupled resonator. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  2 D and also make the acoustic wave filter  2 D smaller. 
     3. Structure of Acoustic Wave Filter According to Second Practical Example 
       FIGS. 7A and 7B  provide plan views of electrodes and a sectional view of a structure of an acoustic wave filter  3 A according to a second practical example of a preferred embodiment of the present invention.  FIG. 7A  provides a plan view of elements including IDT electrodes and reflectors when the substrate  10  is viewed in plan view, and an enlarged plan view of busbar electrodes of adjacent IDT electrodes, and  FIG. 7B  is a sectional view taken along line VIIb-VIIb in the enlarged plan view of  FIG. 7A . 
     As illustrated in  FIGS. 7A and 7B , the acoustic wave filter  3 A includes the substrate  10  with piezoelectricity, the IDT electrodes  20 A,  20 B,  20 C,  20 D, and  20 E, the reflectors  90 A and  90 B, the piezoelectric film  30   a , the upper electrode  40   a , and the protective layer  50 . The acoustic wave filter  3 A according to the present practical example differs from the acoustic wave filter  2 A according to the first practical example in that a lower electrode of a BAW resonator is coupled to a HOT wire while an upper electrode is coupled to a ground wire. The following description of the acoustic wave filter  3 A according to the present practical example omits the same or corresponding configurations as the acoustic wave filter  2 A according to the first practical example and mainly focuses on configurations different from the acoustic wave filter  2 A. 
     As illustrated in the enlarged plan view in  FIG. 7A , the piezoelectric film  30   a  is provided on a busbar electrode  111   a  of the IDT electrode  20 B of the adjacent IDT electrodes  20 B and  20 C. The upper electrode  40   a  is provided on the piezoelectric film  30   a . The busbar electrode  111   a  is connected to the output-side wire  70   c . The upper electrode  40   a  is connected to the output-side ground wire  70   d . Although  FIG. 7A  does not illustrate the protective layer  50 , the protective layer  50  is disposed between the busbar electrode  111   a  and the upper electrode  40   a.    
     In the structure of the acoustic wave filter  3 A, as illustrated in  FIG. 7B , the busbar electrode  111   a , the piezoelectric film  30   a , and the upper electrode  40   a  together define a BAW resonator  62 B. The busbar electrode  111   a  defines and functions as a lower electrode of the BAW resonator  62 B. The bulk acoustic waves generated in the BAW resonator  62 B propagate between the busbar electrode  111   a  and the upper electrode  40   a  in the vertical direction relative to the surface of the substrate  10 . 
     The upper electrode  40   a  is directly connected to the busbar electrode  110   a  of the IDT electrode  20 C. This means that the busbar electrode  110   a  and the upper electrode  40   a  define a two-layer wiring on the busbar electrode  110   a . As such, the busbar electrode  110   a  is coupled to the output-side wire  70   c.    
     As described above, in the acoustic wave filter  3 A according to the present practical example, the piezoelectric film  30   a  is provided on the busbar electrode  111   a  coupled to the output-side wire  70   c , and the upper electrode  40   a  is provided on the piezoelectric film  30   a  and on the busbar electrode  110   a  coupled to the output-side ground wire  70   d.    
       FIGS. 8A and 8B  provide circuit configuration diagrams of the acoustic wave filter  3 A according to the second practical example and a graph illustrating the comparison between the acoustic wave filter according to the second practical example and an acoustic wave filter according to a second comparative example with respect to the bandpass characteristic. The acoustic wave filter according to the second comparative example has a structure provided by removing the BAW resonator  62 B from the acoustic wave filter  3 A according to the second practical example. 
     As illustrated in  FIG. 8A , the acoustic wave filter  3 A according to the present practical example includes the BAW resonator  62 B provided between the busbar electrode  111   a  at the HOT potential of the IDT electrode  20 B and the output-side busbar electrode  110   a  at the ground potential of the IDT electrode  20 C, in addition to the longitudinally coupled resonator defined by the SAW resonators including the IDT electrodes  20 A to  20 E. 
     As illustrated in  FIG. 8B , there is almost no difference between the acoustic wave filter  3 A according to the second practical example and the acoustic wave filter according to the second comparative example with respect to the insertion loss in a pass band. This is because the longitudinally coupled resonator effects characteristics in the pass band. In contrast, in an attenuation band higher than the pass band, the acoustic wave filter  3 A according to the second practical example is greater with respect to attenuation. It is concluded that attenuation outside the band is improved because oscillations caused by vibrations of bulk acoustic waves generated in the BAW resonator  62 B occur at frequencies higher than the pass band. 
     The number of IDT electrodes of the acoustic wave filter  3 A is not limited to five. The number of IDT electrodes constituting the acoustic wave filter  3 A only needs to be, for example, three or more. Additionally, for example, two or more BAW resonators may be provided to define the acoustic wave filter  3 A. 
     In the structure of the acoustic wave filter  3 A according to the second practical example, the SAW resonator defining the longitudinally coupled resonator and the BAW resonator  62 B are provided at the same substrate  10 . Additionally, the busbar electrode  111   a  of the SAW resonator also defines and functions as the lower electrode of the BAW resonator  62 B. This means that the SAW resonator and the BAW resonator  62 B are provided in a shared area on the substrate  10 . Consequently, in comparison to the structure in which the SAW resonators and the BAW resonator  62 B are separately provided in discrete areas on a substrate, this structure can improve the attenuation characteristic of the acoustic wave filter  3 A and also make the acoustic wave filter  3 A smaller. Furthermore, the busbar electrode  111   a  of the SAW resonator and the lower electrode of the BAW resonator  62 B can be formed at the same time, and additionally, the upper electrode of the busbar electrode  110   a  of the SAW resonator and the upper electrode of the BAW resonator  62 B can be provided by the same upper electrode  40   a . Accordingly, it is possible to simplify the manufacturing process and also reduce costs. 
       FIGS. 9A and 9B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  3 B according to a fourth modification of a preferred embodiment of the present invention. The acoustic wave filter  3 B illustrated in the drawing differs from the acoustic wave filter  3 A according to the second practical example in that it includes two BAW resonators  62 B and  62 D. The following description of the acoustic wave filter  3 B according to the present modification omits the same or corresponding configurations as the acoustic wave filter  3 A according to the second practical example and mainly focuses on configurations different from the acoustic wave filter  3 A. 
     As illustrated in  FIG. 9A , the acoustic wave filter  3 B according to the present modification includes the BAW resonators  62 B and  62 D in addition to the longitudinally coupled resonator composed of the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  62 B is provided between the busbar electrode (lower electrode) at the HOT potential of the IDT electrode  20 B and an upper electrode coupled to the busbar electrode at the ground potential of the IDT electrode  20 C. The BAW resonator  62 D is provided between the busbar electrode (lower electrode) at the HOT potential of the IDT electrode  20 D and the upper electrode coupled to the busbar electrode at the ground potential of the IDT electrode  20 C. 
     In comparison to an acoustic wave filter including a single BAW resonator, the acoustic wave filter  3 B according to the fourth modification can increase attenuation in the attenuation band because the acoustic wave filter  3 B includes the two BAW resonators  62 B and  62 D. When the piezoelectric film of the BAW resonator  62 B is formed differently from the piezoelectric film of the BAW resonator  62 D with respect to thickness, attenuation can be achieved in two kinds of attenuation bands of different frequencies. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  3 B and also make the acoustic wave filter  3 B smaller. 
       FIGS. 10A and 10B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  3 C according to a fifth modification of a preferred embodiment of the present invention. The acoustic wave filter  3 C illustrated in the drawing differs from the acoustic wave filter  3 A according to the second practical example in that a BAW resonator  62 C is coupled on the input-side instead of the output-side with respect to the longitudinally coupled resonator. The following description of the acoustic wave filter  3 C according to the present modification omits the same or corresponding configurations as the acoustic wave filter  3 A according to the second practical example and mainly focuses on configurations different from the acoustic wave filter  3 A. 
     As illustrated in  FIG. 10A , the acoustic wave filter  3 C according to the present modification includes the BAW resonator  62 C in addition to the longitudinally coupled resonator composed of the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  62 C is provided between the busbar electrode (lower electrode) at the HOT potential of the IDT electrode  20 C and an upper electrode coupled to the input-side busbar electrode at the ground potential of the IDT electrode  20 D. 
     The acoustic wave filter  3 C according to the fourth modification can increase attenuation in the attenuation band because the BAW resonator  62 C is disposed on the input-side with respect to the longitudinally coupled resonator. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  3 C and also make the acoustic wave filter  3 C smaller. 
       FIGS. 11A and 11B  provide circuit configuration diagrams and a plan view of electrodes of an acoustic wave filter  3 D according to a sixth modification of a preferred embodiment of the present invention. The acoustic wave filter  3 D illustrated in the drawing differs from the acoustic wave filter  3 A according to the second practical example in that a BAW resonator  63 A is coupled between an IDT electrode and a reflector instead of between IDT electrodes of the longitudinally coupled resonator. The following description of the acoustic wave filter  3 D according to the present modification omits the same or corresponding configurations as the acoustic wave filter  3 A according to the second practical example and mainly focuses on configurations different from the acoustic wave filter  3 A. 
     As illustrated in  FIG. 11A , the acoustic wave filter  3 D according to the present modification includes the BAW resonator  63 A in addition to the longitudinally coupled resonator defined by the SAW resonators including the IDT electrodes  20 A to  20 E. The BAW resonator  63 A is provided between a busbar electrode on the output-side of the reflector  90 A and an upper electrode coupled to an output-side busbar electrode at the ground potential of the IDT electrode  20 A. 
     The acoustic wave filter  3 D according to the sixth modification can increase attenuation in the attenuation band because the BAW resonator  63 A is disposed between an IDT electrode and a reflector on the output-side with respect to the longitudinally coupled resonator. As a result, by comparison to the structure in which the SAW resonators and the BAW resonators are separately formed in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter  3 D and also make the acoustic wave filter  3 D smaller. 
     4. Relationship Between Piezoelectric Film Thickness and Filter Bandpass Characteristic 
     Concerning a BAW resonator, the thickness of a piezoelectric film correlates with its resonant frequency. Specifically, the thicker the piezoelectric film is, the lower the resonant frequency of bulk wave vibration is. 
       FIGS. 12A to 12C  provide graphs illustrating the bandpass characteristic with respect to different film thicknesses of the piezoelectric film of the acoustic wave filter  2 A according to the first practical example. As illustrated  FIGS. 12A to 12C , as the thickness of the piezoelectric film  30   a  of the acoustic wave filter  2 A increases from a relatively thin film to a relatively thick film, the attenuation pole (arrow in  FIGS. 12A to 12C ) moves from the higher-frequency side to the lower-frequency side. 
     Accordingly, by controlling the film thickness of the piezoelectric film  30   a , attenuation can be achieved in a desired attenuation band. Moreover, when a plurality of BAW resonators are included as in the acoustic wave filter  2 B according to the first modification and the acoustic wave filter  3 B according to the fourth modification, two or more attenuation bands can be provided by configuring one BAW resonator of the plurality of BAW resonators differently from other BAW resonators. 
     5. Shape of Piezoelectric Film 
       FIGS. 13A to 13D  provide plan views of electrodes illustrating variations of the shape of the piezoelectric film  30   a  of the acoustic wave filter according to the above-described preferred embodiment. In the acoustic wave filter according to the above-described preferred embodiment, the piezoelectric film  30   a  may include curved corners when the substrate  10  is viewed in plan view, as in an acoustic wave filter  4 A illustrated in  FIG. 13A . 
     In the acoustic wave filter according to the above-described preferred embodiment, the piezoelectric film  30   a  may have a polygonal shape when the substrate  10  is viewed in plan view, as in an acoustic wave filter  4 B illustrated in  FIG. 13B . 
     In the acoustic wave filter according to the above-described preferred embodiment, the piezoelectric film  30   a  may have an oval or a circular shape when the substrate  10  is viewed in plan view, as in an acoustic wave filter  4 C illustrated in  FIG. 13C . 
     In the acoustic wave filter according to the above-described preferred embodiment, the piezoelectric film  30   a  may include a plurality of island-shaped areas when the substrate  10  is viewed in plan view, as in an acoustic wave filter  4 D illustrated in  FIG. 13D . 
     With the structure of the acoustic wave filters  4 A to  4 C, it is possible to reduce transverse-mode spurious responses as spurious waves unnecessary for vibrations of bulk acoustic wave in the BAW resonator. When the piezoelectric film is relatively small as in the acoustic wave filters  4 A to  4 C to reduce transverse-mode spurious responses, a plurality of piezoelectric films can be provided as illustrated in the acoustic wave filter  4 D so that sufficient capacitance is provided between the upper electrode and the lower electrode. 
     As described above, the acoustic wave filter  1  according to the above-described preferred embodiment includes a SAW resonator and a BAW resonator. The SAW resonator includes the substrate  10  with piezoelectricity and the IDT electrode  20  on the substrate  10 . The IDT electrode  20  includes the pair of comb-shaped electrodes  20   a  and  20   b  interdigitated with each other. Each comb-shaped electrode includes a plurality of electrode fingers extending parallel or substantially parallel in a direction crossing the SAW propagation direction and a busbar electrode connecting the plurality of electrode fingers to each other at one end of each electrode finger of the plurality of electrode fingers. The BAW resonator includes a lower electrode defined by a portion of the busbar electrode  110   a , the piezoelectric film  30   a  provided on the busbar electrode  110   a , and the upper electrode  40   a  provided on the piezoelectric film  30   a.    
     With this structure, the acoustic wave filter  1  includes the SAW resonator and the BAW resonator on the same substrate. Additionally, the busbar electrode  110   a  of the SAW resonator also defines and functions as the lower electrode of the BAW resonator. This means that the SAW resonator and the BAW resonator are provided in a shared area on the substrate  10 . Consequently, in comparison to the structure in which a SAW resonator and a BAW resonator are separately provided in discrete areas on a substrate, this structure can make the acoustic wave filter  1  smaller. Furthermore, because the busbar electrode  110   a  of the SAW resonator and the lower electrode of the BAW resonator can be formed at the same time, it is possible to simplify the manufacturing process and also reduce costs. 
     The piezoelectric film  30   a  may be mainly made of, for example, at least one of zinc oxide (ZnO), aluminum nitride (AlN), PZT, potassium niobate (KN), LN, LT, quartz-crystal, and lithium borate (LiBO). 
     The piezoelectric film  30   a  may be, for example, a c-axis oriented film made of zinc oxide (ZnO) or aluminum nitride (AlN). 
     When the substrate  10  is viewed in plan view, the piezoelectric film  30   a  may have a polygonal, a circular, or an oval shape, for example. 
     This structure can reduce transverse-mode spurious responses as spurious waves unnecessary for vibrations of bulk acoustic wave in the BAW resonator. 
     In the acoustic wave filter  2 A according to the first practical example, the busbar electrode  110   a  and the lower electrode of the BAW resonator  60 D may be coupled to the input-side ground wire  70   b , and the upper electrode  40   a  of the BAW resonator  60 D may be coupled to the radio-frequency-signal input-side wire  70   a.    
     In the acoustic wave filter  3 A according to the second practical example, the busbar electrode  111   a  and the lower electrode of the BAW resonator  62 B may be coupled to the radio-frequency-signal output-side wire  70   c , and the upper electrode  40   a  of the BAW resonator  62 B may be coupled to the output-side ground wire  70   d.    
     The acoustic wave filter  2 A according to the first practical example and the acoustic wave filter  3 A according to the second practical example may include a plurality of SAW resonators and a BAW resonator, the plurality of SAW resonators may determine the pass band of the acoustic wave filter, and the BAW resonator may determine an attenuation pole. 
     The acoustic wave filter  2 B according to the first modification and the acoustic wave filter  3 B according to the fourth modification may include a plurality of SAW resonators and a plurality of BAW resonators. The plurality of SAW resonators may include a plurality of IDT electrodes corresponding to the plurality of SAW resonators. The plurality of BAW resonators may include a first BAW resonator and a second BAW resonator. The first BAW resonator may include a first lower electrode defined by a portion of the busbar electrode of a first IDT electrode of the plurality of IDT electrodes, a first piezoelectric film provided on the busbar electrode, and a first upper electrode provided on the first piezoelectric film. The second BAW resonator may include a second lower electrode defined by a portion of the busbar electrode of a second IDT electrode of the plurality of IDT electrodes, a second piezoelectric film provided on the busbar electrode, and an upper electrode provided on the second piezoelectric film. The first piezoelectric film may be thinner than the second piezoelectric film, and the frequency at an attenuation pole determined by the first BAW resonator is higher than the frequency at an attenuation pole determined by the second BAW resonator. 
     In contrast to an acoustic wave filter including a single BAW resonator, this structure can achieve attenuation in two kinds of attenuation bands of different frequencies. As a result, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, this structure can further improve the attenuation characteristic of the acoustic wave filter and also make the acoustic wave filter smaller. 
     In the acoustic wave filter according to the first or second practical example or any of the first to sixth modifications, the plurality of SAW resonators may define a longitudinally coupled resonator. The longitudinally coupled resonator may include the IDT electrodes  20 A to  20 E corresponding to the plurality of SAW resonators. The IDT electrodes  20 A to  20 E may be adjacent to each other in the SAW propagation direction. The BAW resonator may include a lower electrode defined by a portion of the busbar electrode of a first IDT electrode of the IDT electrodes  20 A to  20 E, a piezoelectric film provided on the busbar electrode, and an upper electrode provided on the piezoelectric film. The upper electrode may be coupled to the busbar electrode of a second IDT electrode adjacent to the first IDT electrode. 
     With this structure of the acoustic wave filter including a longitudinally coupled resonator, in comparison to the structure in which the SAW resonators and the BAW resonators are separately provided in discrete areas on a substrate, it is possible to improve the attenuation characteristic of the acoustic wave filter and also make the acoustic wave filter smaller. 
     Other Preferred Embodiments 
     The acoustic wave filter according to the present invention has been described with reference to a preferred embodiment of the present invention, practical examples, and modifications, but the present invention is not limited to the above-described preferred embodiment, practical examples, and modifications described above. The present invention also embraces other preferred embodiments including any combination of the elements of the above-described preferred embodiment, practical examples, and modifications, other modified examples obtained by making various modifications to the above-described preferred embodiment that occur to those skilled in the art without departing from the scope of the present invention, and various hardware devices including the acoustic wave filter of the present invention. 
     The first and second practical examples and the first to sixth modifications describe as an example the acoustic wave filter including a longitudinally coupled resonator, but the acoustic wave filter according to the present invention only needs to include one or more SAW resonators and one or more BAW resonators. For example, an acoustic wave filter according to a preferred embodiment of the present invention may be an acoustic wave ladder filter including a series arm SAW resonator and a parallel arm SAW resonator, and also include a BAW resonator including a lower electrode formed by a busbar electrode of the series arm SAW resonator or the parallel arm SAW resonator. 
     Further, at least either a series arm resonator or a parallel arm resonator may be coupled on the input-side or output-side with respect to the longitudinally coupled resonator included in the acoustic wave filter according to the first or second practical example or any of the first to sixth modifications. 
     In an acoustic wave filter according to a preferred embodiment of the present invention, an inductor and a capacitor may be coupled among the IDT electrodes and the wires. The inductor may include a wire inductor defined by a wire defining and functioning as an interconnection between elements. 
     Preferred embodiments of the present invention can be used as a small filter for a wide variety of wireless communication terminals required to achieve low loss performance in the pass band and high attenuation performance outside the pass band. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.