Patent Publication Number: US-2022216850-A1

Title: Acoustic wave device, filter device, and multiplexer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-176873 filed on Sep. 27, 2019 and is a Continuation Application of PCT Application No. PCT/JP2020/035314 filed on Sep. 17, 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 device, a filter device, and a multiplexer. 
     2. Description of the Related Art 
     Acoustic wave devices have provided a wide range of applications such as filters of mobile phones. Japanese Unexamined Patent Application Publication No. 2019-106622 describes an example of a multiplexer including filters having acoustic wave resonators. The acoustic wave resonator is formed by disposing an interdigital transducer (IDT) electrode on a piezoelectric substrate. On the substrate, a pair of reflectors are disposed on both sides with respect to the IDT electrode in the propagation direction of acoustic wave. A plurality of filter devices are coupled to a common terminal by common connection. The common terminal is coupled to an antenna. 
     When such an acoustic wave resonator is used in filters of a multiplexer, concern arises about degradation of the receive sensitivity of the multiplexer in the case in which the pass band of filter devices included in the multiplexer is a particular band. 
     More specifically, when the multiplexer, which includes a transmit filter and a receive filter, outputs a transmit signal from the transmit filter to an antenna, another signal may be inputted from the antenna. At this time, if the communication band of the transmit filter and receive filter is, for example, Band 25, the inputted signal may act as an interference wave signal. As a result, intermodulation distortion (IMD) may occur in the receive band of Band 25, resulting in degradation of the receive sensitivity. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide acoustic wave devices, filter devices, and multiplexers that are each able to reduce IMD and reduce or prevent degradation of the receive sensitivity. 
     An acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric substrate including a piezoelectric layer, a first interdigital transducer (IDT) electrode on the piezoelectric substrate, a pair of reflectors on both sides of the first IDT electrode in a propagation direction of an acoustic wave on the piezoelectric substrate, and a second IDT electrode on the piezoelectric substrate and facing the first IDT electrode with one reflector of the pair of reflectors interposed therebetween. The first IDT electrode and the second IDT electrode each include a pair of busbars and a plurality of electrode fingers, some electrode fingers of the plurality of electrode fingers are coupled to one busbar of the pair of busbars in each of the first IDT electrode and the second IDT electrode, and other electrode fingers of the plurality of electrode fingers are coupled to another busbar of the pair of busbars in each of the first IDT electrode and the second IDT electrode. The first IDT electrode and the second IDT electrode each include an intersecting area in which the plurality of electrode fingers overlap in the propagation direction of acoustic wave, and at least a portion of the intersecting area of the first IDT electrode and at least a portion of the intersecting area of the second IDT electrode overlap in the propagation direction of acoustic wave. One busbar of the pair of busbars of the second IDT electrode is coupled to one busbar of the pair of busbars of the first IDT electrode, and another busbar of the pair of busbars of the second IDT electrode is coupled to a ground potential. A resonant frequency of the second IDT electrode is in a frequency band of an interference wave signal. 
     A filter device according to a preferred embodiment of the present invention is configured to be coupled to an antenna. The filter device includes a series arm resonator and a parallel arm resonator. At least one of the series arm resonator and the parallel arm resonator is an acoustic wave device according to a preferred embodiment of the present invention. 
     A multiplexer according to a preferred embodiment of the present invention includes an antenna terminal to be coupled to an antenna, and a plurality of filter devices coupled to the antenna terminal by a common connection. At least one of the filter devices is a filter device according to a preferred embodiment of the present invention. 
     The acoustic wave devices, the filter devices, and the multiplexers according to preferred embodiments of the present invention are each able to reduce intermodulation distortion (IMD) and reduce or prevent degradation of the receive sensitivity. 
     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 
         FIG. 1  is an elevational cross-sectional view of an acoustic wave device according to a first preferred embodiment of the present invention. 
         FIG. 2  is a schematic plan view illustrating an electrode structure of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIG. 3  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a first comparative example. 
         FIG. 4  illustrates the level of third-order intermodulation distortion (IMD3) with respect to the acoustic wave device according to the first preferred embodiment of the present invention and the acoustic wave device according to the first comparative example. 
         FIG. 5  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a second comparative example. 
         FIG. 6  illustrates the level of IMD3 with respect to the acoustic wave device according to the first preferred embodiment of the present invention, the acoustic wave device according to the first comparative example, and the acoustic wave device according to the second comparative example. 
         FIG. 7  illustrates the relationship between the number of pairs of electrode fingers of a second IDT electrode and the level of IMD3 with respect to the first preferred embodiment and the first comparative example. 
         FIG. 8  is an elevational cross-sectional view of an acoustic wave device according to a first modification of the first preferred embodiment of the present invention. 
         FIG. 9  is an elevational cross-sectional view of an acoustic wave device according to a second modification of the first preferred embodiment of the present invention. 
         FIG. 10  is an elevational cross-sectional view of an acoustic wave device according to a third modification of the first preferred embodiment of the present invention. 
         FIG. 11  is an elevational cross-sectional view of an acoustic wave device according to a fourth modification of the first preferred embodiment of the present invention. 
         FIG. 12  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention. 
         FIG. 13  illustrates the level of IMD3 with respect to the acoustic wave device according to the second preferred embodiment of the present invention, the acoustic wave device according to the first comparative example, and an acoustic wave device according to a third comparative example. 
         FIG. 14  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention. 
         FIG. 15  illustrates the relationship between the number of pairs of electrode fingers of a second IDT electrode and the level of IMD3 with respect to the first preferred embodiment of the present invention, the third preferred embodiment of the present invention, and the first comparative example. 
         FIG. 16  is a schematic circuit diagram of a filter device according to a fourth preferred embodiment of the present invention. 
         FIG. 17  is a schematic circuit diagram of a filter device according to a modification of the fourth preferred embodiment of the present invention. 
         FIG. 18  is a schematic diagram of a duplexer according to a fifth preferred embodiment of the present invention. 
         FIG. 19  is a schematic diagram of a multiplexer according to a sixth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, the present invention will be clarified by describing preferred embodiments of the present invention with reference to the drawings. 
     The preferred embodiments described in this specification are merely examples, and configurations of different preferred embodiments may be partially replaced or combined. 
       FIG. 1  is an elevational cross-sectional view of an acoustic wave device according to a first preferred embodiment of the present invention. 
     The acoustic wave device  1  includes a piezoelectric substrate  2 . A first interdigital transducer (IDT) electrode  3  is disposed on the piezoelectric substrate  2 . The piezoelectric substrate  2  and the first IDT electrode  3  define a first IDT  3 A. When alternating-current voltage is applied to an IDT electrode, acoustic waves are produced. A pair of reflectors  4 A and  4 B are disposed on both sides of the first IDT electrode  3  in the propagation direction of acoustic wave on the piezoelectric substrate  2 . Additionally, a second IDT electrode  5  is disposed on the piezoelectric substrate  2  and facing the first IDT electrode  3  with the reflector  4 A interposed therebetween. The piezoelectric substrate  2  and the second IDT electrode  5  define a second IDT  5 A. As such, the acoustic wave device  1  includes the first IDT  3 A, which is defined by the first IDT electrode  3 , and the second IDT  5 A, which is defined by the second IDT electrode  5 . The acoustic wave device  1  also includes the reflectors  4 A and  4 B. The acoustic wave device  1  according to the present preferred embodiment is an acoustic wave resonator. 
       FIG. 2  is a schematic plan view illustrating an electrode structure of the acoustic wave device according to the first preferred embodiment. 
     The first IDT electrode  3  includes a first busbar  12  and a second busbar  13  in a pair. The first busbar  12  and the second busbar  13  face each other. The first IDT electrode  3  includes a plurality of first electrode fingers  14  and a plurality of second electrode fingers  15 . The first electrode fingers  14  include some of the plurality of electrode fingers included in the first IDT electrode  3 , and the second electrode fingers  15  include others of the plurality of electrode fingers included in the first IDT electrode  3 . One end of each of the first electrode fingers  14  is connected to the first busbar  12 . One end of each of the second electrode fingers  15  is connected to the second busbar  13 . The first electrode fingers  14  and the second electrode fingers  15  are interdigitated with each other. The first IDT electrode  3  includes a first intersecting area A. In the first intersecting area A, the first electrode fingers  14  and the second electrode fingers  15  overlap in the propagation direction of acoustic wave. 
     At least one of the first busbar  12  and the second busbar  13  is coupled to a signal potential. More specifically, when the acoustic wave device  1  is used as, for example, a series arm resonator of a filter such as a ladder filter, the first busbar  12  and the second busbar  13  are both coupled to the signal potential. When the acoustic wave device  1  is used as, for example, a parallel arm resonator of a filter such as a ladder filter, one of the first busbar  12  and the second busbar  13  is coupled to the signal potential, while the other is coupled to the ground potential. 
     In the present preferred embodiment, the acoustic wave device  1  is coupled to an antenna. The first busbar  12  is positioned closer to the antenna than the second busbar  13 . In this specification, being positioned closer to an antenna than another busbar or element denotes being connected electrically closer to the antenna than the other busbar or element. 
     The first IDT electrode  3  is coupled to an antenna via an antenna terminal  11 A. More specifically, the first busbar  12  is coupled to the antenna terminal  11 A. The first busbar  12  may be coupled directly or indirectly via, for example, another element to the antenna terminal  11 A. The second busbar  13  is positioned closer to a signal terminal  11 B side different from the antenna terminal  11 A than the first busbar  12 . The signal terminal  11 B is coupled to the signal potential. The second busbar  13  may be coupled directly or indirectly via, for example, another element to the signal terminal  11 B. 
     The second IDT electrode  5  includes a third busbar  16  and a fourth busbar  17  in a pair. The third busbar  16  and the fourth busbar  17  face each other. The second IDT electrode  5  includes a plurality of third electrode fingers  18  and a plurality of fourth electrode fingers  19 . The third electrode fingers  18  include some of the plurality of electrode fingers included in the second IDT electrode  5 , and the fourth electrode fingers  19  include others of the plurality of electrode fingers included in the second IDT electrode  5 . One end of each of the third electrode fingers  18  is connected to the third busbar  16 . One end of each of the fourth electrode fingers  19  is connected to the fourth busbar  17 . The third electrode fingers  18  and the fourth electrode fingers  19  are interdigitated with each other. The second IDT electrode  5  includes a second intersecting area B. In the second intersecting area B, the third electrode fingers  18  and the fourth electrode fingers  19  overlap in the propagation direction of acoustic wave. 
     As illustrated in  FIG. 2 , the first intersecting area A of the first IDT electrode  3  and the second intersecting area B of the second IDT electrode  5  overlap in the propagation direction of acoustic wave. More specifically, given that the direction perpendicular or substantially perpendicular to the propagation direction of acoustic wave is an overlap-width direction, with respect to the propagation direction of acoustic wave, a middle portion of the first intersecting area A in the overlap-width direction overlaps a middle portion of the second intersecting area B in the overlap-width direction. It is only necessary that at least a portion of the first intersecting area A and at least a portion of the second intersecting area B overlap in the propagation direction of acoustic wave. 
     The third busbar  16  is coupled to the signal potential. The third busbar  16  is connected to the first busbar  12  of the first IDT electrode  3 . More specifically, a hot wire  6 A connects the third busbar  16  to the first busbar  12 . The first busbar  12  and the third busbar  16  are coupled to the signal potential via the hot wire  6 A. 
     The fourth busbar  17  of the second IDT electrode  5  is coupled to the ground potential. More specifically, the fourth busbar  17  is coupled to the ground potential via a ground wire  6 B. It is only necessary that the wiring is configured such that one of the third busbar  16  and the fourth busbar  17  is coupled to the signal potential, and the other is coupled to the ground potential. 
     In the present preferred embodiment, the resonant frequency of the first IDT  3 A of the acoustic wave device  1  is, for example, in the range of about 1850 MHz to about 1915 MHz, which is the transmit band of Band 25. The resonant frequency of the second IDT  5 A of the acoustic wave device  1  is, for example, in a frequency band of interference wave signals. More specifically, the resonant frequency of the second IDT  5 A of the acoustic wave device  1  is, for example, in the range of about 1770 MHz to about 1835 MHz, which is the interference-wave frequency band of Band 25. In this specification, the interference wave signal indicates a signal of a frequency given by 2Tx−Rx, where Tx is a frequency of the transmit band, and Rx is a frequency of the receive band. 
     Of the acoustic wave device  1 , the resonant frequency of the first IDT  3 A and the resonant frequency of the second IDT  5 A are not limited to the examples described above. 
     Referring back to  FIG. 1 , the piezoelectric substrate  2  is a multilayer substrate including a high acoustic velocity support substrate  7  defining and functioning as a high acoustic velocity material layer, a low acoustic velocity film  8 , and a piezoelectric layer  9  stacked in this order. The piezoelectric substrate  2  may include only the piezoelectric layer  9 . 
     The piezoelectric layer  9  is, for example, a lithium tantalate layer. The material of the piezoelectric layer  9  is not limited to the above example. For example, lithium niobate, zinc oxide, aluminum nitride, quartz-crystal, or PZT may also be used as the material of the piezoelectric layer  9 . 
     The low acoustic velocity film  8  is, a film with a relatively low acoustic velocity. More specifically, the acoustic velocity of bulk waves propagating in the low acoustic velocity film  8  is lower than the acoustic velocity of bulk waves propagating in the piezoelectric layer  9 . The low acoustic velocity film  8  is, for example, a silicon oxide film. Silicon oxide can be indicated by SiO x . In the present preferred embodiment, the low acoustic velocity film  8  is, for example, a SiO 2  film. The material of the low acoustic velocity film  8  is not limited to the above example. For example, glass, silicon oxynitride, tantalum oxide, or a material containing as a principal component a compound formed by adding fluorine, carbon, or boron to silicon oxide may also be used as the material of the low acoustic velocity film  8 . 
     The high acoustic velocity material layer is a layer with a relatively high acoustic velocity. More specifically, the acoustic velocity of bulk waves propagating in the high acoustic velocity material layer is higher than the acoustic velocity of acoustic waves propagating in the piezoelectric layer  9 . The high acoustic velocity support substrate  7  defining and functioning as the high acoustic velocity material layer is, for example, a silicon substrate. The material of the high acoustic velocity support substrate  7  is not limited to the above example. For example, a medium containing as a principal component a material such as aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, sapphire, lithium tantalate, lithium niobate, quartz-crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, diamond-like carbon (DLC) film, or diamond may be used as the material of the high acoustic velocity support substrate  7 . 
     Because the piezoelectric substrate  2  has a layered structure including the high acoustic velocity support substrate  7 , the low acoustic velocity film  8 , and the piezoelectric layer  9 , acoustic waves can be effectively confined in the piezoelectric layer  9  side. 
     As illustrated in  FIG. 2 , in the present preferred embodiment, the acoustic wave device  1  has the following structure: 1) the acoustic wave device  1  includes the second IDT electrode  5 , and the second IDT electrode  5  and the first IDT electrode  3  face each other with the reflector  4 A interposed therebetween; 2) at least a portion of the first intersecting area A and at least a portion of the second intersecting area B overlap in the propagation direction of acoustic wave; 3) the third busbar  16  of the second IDT electrode  5  is connected to the first busbar  12  of the first IDT electrode  3 , and the fourth busbar  17  of the second IDT electrode  5  is coupled to the ground potential. With this structure, waves at frequencies close to interference-wave frequencies are caused at the second IDT electrode  5 , and as a result, it is possible to reduce or prevent the occurrence of intermodulation distortion (IMD) due to inputs of interference wave signals. Accordingly, when the acoustic wave device  1  is used in a filter device of, for example, a multiplexer, it is possible to reduce or prevent degradation of the receive sensitivity of another filter device coupled to a signal potential by common connection with the filter device. This will be further described in detail below by comparing the present preferred embodiment to a first comparative example and a second comparative example. 
     The acoustic wave device  1  having the structure of the first preferred embodiment and an acoustic wave device  101  of the first comparative example illustrated in  FIG. 3  were prepared. The first comparative example differs from the first preferred embodiment in that it does not include the second IDT electrode  5 . The design parameters of the acoustic wave device  1  are described below. The design parameters of the prepared acoustic wave device  101  are the same or substantially the same as the design parameters of the acoustic wave device  1 , except that the acoustic wave device  101  does not include the second IDT electrode  5 . 
     The first IDT electrode  3 : number of pairs of electrode fingers 99 pairs, wavelength about 2.104 μm, duty ratio about 0.5 
     The second IDT electrode  5 : number of pairs of electrode fingers 10 pairs, wavelength about 2.104 μm, duty ratio about 0.5 
     The reflectors  4 A and  4 B: number of electrode fingers 21 fingers, wavelength about 2.104 μm 
     The resonant frequency of the first IDT  3 A of the acoustic wave device  1  and the resonant frequency of the acoustic wave device  101  are in the transmit band of Band 25. When a signal in the transmit band of Band 25 and an interference wave signal are inputted at the same time to such acoustic wave resonators, the third-order IMD occurs in the range of about 1930 MHz to about 1995 MHz, which is the receive band of Band 25. The measurement of the level of third-order intermodulation distortion (IMD3) was conducted in the condition in which a signal in the transmit band and an interference wave signal were inputted at the same time to the acoustic wave device  1  and the acoustic wave device  101 . 
       FIG. 4  illustrates the level of IMD3 with respect to the acoustic wave device according to the first preferred embodiment and the acoustic wave device according to the first comparative example. 
     As illustrated in  FIG. 4 , the level of IMD3 in the acoustic wave device  1  having the structure of the first preferred embodiment is lower than the level of IMD3 in the acoustic wave device  101  of the first comparative example. As for the first preferred embodiment, the level of IMD3 is particularly low at frequencies close to 1936 MHz. At the frequencies close to 1936 MHz, the level of IMD3 of the first preferred embodiment is better by approximately 24 dB than the level of IMD3 of the first comparative example. 
     When a signal in the transmit band and an interference wave signal are inputted at the same to the acoustic wave device  101  of the first comparative example, a wave of a frequency close to the interference-wave frequency is caused at the first IDT electrode  3  in addition to a wave based on the signal in the transmit band. This results in IMD. 
     In contrast, the acoustic wave device  1  according to the first preferred embodiment includes the second IDT electrode  5  in addition to the first IDT electrode  3 . Further, the fourth busbar  17  of the second IDT electrode  5  is coupled to the ground potential. As a result, when a signal in the transmit band and an interference wave signal are inputted at the same time to the acoustic wave device  1 , a wave of a frequency close to the interference-wave frequency is caused in not only the first IDT electrode  3  but also the second IDT electrode  5 . Furthermore, the second IDT electrode  5  is disposed facing the first IDT electrode  3  with the reflector  4 A interposed therebetween, and the first intersecting area A and the second intersecting area B overlap in the propagation direction of acoustic wave. As a result, the IMD signal caused in the first IDT electrode  3  and the IMD signal caused in the second IDT electrode  5  interfere with each other to cancel each other out. As such, IMD can be reduced. Accordingly, when the acoustic wave device  1  is used in a filter device of, for example, a multiplexer, it is possible to reduce or prevent degradation of the receive sensitivity of another filter device coupled to a signal potential by common connection with the filter device. 
     An acoustic wave device  111  according to the second comparative example illustrated in  FIG. 5  was prepared. In  FIG. 5 , the terminals are not illustrated. The second comparative example differs from the first preferred embodiment in that the first IDT electrode  3  and the second IDT electrode  5  are spaced apart from each other in the overlap-width direction, and the entire first intersecting area A and the second intersecting area B do not overlap in the propagation direction of acoustic wave. The design parameters of the prepared acoustic wave device  111  are the same or substantially the same as the design parameters of the acoustic wave device  1 . 
       FIG. 6  illustrates the level of IMD3 with respect to the acoustic wave device according to the first preferred embodiment, the acoustic wave device according to the first comparative example, and the acoustic wave device according to the second comparative example. 
     As illustrated in  FIG. 6 , the level of IMD3 in the acoustic wave device  1  having the structure of the first preferred embodiment is lower than the level of IMD3 in the acoustic wave device  111  of the second comparative example. It can be understood that the first preferred embodiment can reduce IMD because the first intersecting area A overlaps the second intersecting area B in the propagation direction of acoustic wave. 
     When a filter device includes an element especially for hindering the input of interference wave, the design flexibility of the filter device may be degraded under the influence of the element on impedance. Moreover, the insertion loss may be increased. In this respect, when the acoustic wave device  1  is used in a filter device, it is possible to reduce IMD with the structure of the acoustic wave device  1  defining and functioning as an acoustic wave resonator. There is, thus, no need for an element especially for hindering the input of interference wave. Accordingly, it is possible to reduce IMD without degradation of the design flexibility and increases in the insertion loss. 
     The measurement of the level of IMD3 was conducted in the condition in which the second IDT electrode  5  of the acoustic wave device  1  according to the first preferred embodiment was varied with respect to the number of pairs of electrode fingers. The number of pairs of electrode fingers varied among five, ten, and twenty pairs. The level of IMD3 of the first comparative example, in which the second IDT electrode  5  is not included, and the number of pairs of electrode fingers of the second IDT electrode  5  is thus considered as zero, is also illustrated. 
       FIG. 7  illustrates the relationship between the number of pairs of electrode fingers of the second IDT electrode and the level of IMD3 with respect to the first preferred embodiment and the first comparative example. 
     As illustrated in  FIG. 7 , regardless of the variations in the number of pairs of electrode fingers of the second IDT electrode  5  of the first preferred embodiment, IMD in the first preferred embodiment is lower than the first comparative example. In the first preferred embodiment, when the number of pairs of electrode fingers of the second IDT electrode  5  is ten or less, as the number of pairs of electrode fingers increases, IMD decreases. It is preferable that the number of pairs of electrode fingers of the second IDT electrode  5  is in the range of five to twenty, for example. As such, IMD can be further reduced. 
     As described above, in the piezoelectric substrate  2  of the first preferred embodiment, the piezoelectric layer  9  is disposed over the high acoustic velocity support substrate  7  with the low acoustic velocity film  8  interposed therebetween, without direct contact with the high acoustic velocity support substrate  7 . The structure of the piezoelectric substrate  2  is, however, not limited to the example described above. The following describes first to third modifications of the first preferred embodiment. The first to third modifications differ from the first preferred embodiment only in the structure of the piezoelectric substrate. The first to third modifications can also reduce IMD similarly to the first preferred embodiment. Additionally, the first to third modifications can effectively confine the energy of acoustic wave in the piezoelectric layer  9  side. 
     In the first modification illustrated in  FIG. 8 , the high acoustic velocity material layer is a high acoustic velocity film  27 . A piezoelectric substrate  22 A includes a support substrate  26 , the high acoustic velocity film  27 , the low acoustic velocity film  8 , and the piezoelectric layer  9 . The high acoustic velocity film  27  is disposed on the support substrate  26 . The low acoustic velocity film  8  is disposed on the high acoustic velocity film  27 . The piezoelectric layer  9  is disposed on the low acoustic velocity film  8 . When the piezoelectric substrate  22 A includes the high acoustic velocity film  27 , it is unnecessary to use a relatively high acoustic velocity material for the support substrate  26 . 
     Examples of the material of the support substrate  26  include, for example, piezoelectric materials such as aluminum oxide, lithium tantalate, lithium niobate, and quartz-crystal, ceramics such as alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite, dielectric materials such as sapphire, diamond, and glass, and semiconductors or resins such as silicon and gallium nitride. 
     As the material of the high acoustic velocity film  27 , for example, a medium including as a principal component a material such as aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, quartz-crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, DLC film, or diamond may be used. 
     In the second modification illustrated in  FIG. 9 , a piezoelectric substrate  22 B includes the support substrate  26 , the high acoustic velocity film  27 , and the piezoelectric layer  9 . In this modification, the piezoelectric layer  9  is disposed on the high acoustic velocity film  27  defining and functioning as a high acoustic velocity material layer with direct contact with the high acoustic velocity film  27 . 
     In the third modification illustrated in  FIG. 10 , a piezoelectric substrate  22 C includes the high acoustic velocity support substrate  7  and the piezoelectric layer  9 . In this modification, the piezoelectric layer  9  is disposed on the high acoustic velocity support substrate  7  defining and functioning as a high acoustic velocity material layer with direct contact with the high acoustic velocity support substrate  7 . 
     In contrast, in a fourth modification of the first preferred embodiment illustrated in  FIG. 11 , a piezoelectric substrate  22 D includes only a piezoelectric layer. Also in this case, it is possible to reduce IMD similarly to the first preferred embodiment. 
       FIG. 12  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention. 
     The present preferred embodiment differs from the first preferred embodiment in that the fourth busbar  17  of the second IDT electrode  5  is connected to the second busbar  13  of the first IDT electrode  3 , and the third busbar  16  of the second IDT electrode  5  is coupled to the ground potential. More specifically, the hot wire  6 A connects the fourth busbar  17  of the second IDT electrode  5  to the second busbar  13  of the first IDT electrode  3 . The third busbar  16  is coupled to the ground potential via the ground wire  6 B. Apart from the points described above, an acoustic wave device  31  according to the present preferred embodiment is the same or substantially the same as the acoustic wave device  1  according to the first preferred embodiment. 
     The second busbar  13  of the first IDT electrode  3  is positioned closer to the signal terminal  11 B side different from the antenna terminal  11 A than the first busbar  12 . The present preferred embodiment can also reduce IMD similarly to the first preferred embodiment. This will be further described in detail below by comparing the present preferred embodiment to the first comparative example and a third comparative example. 
     The first comparative example does not include the second IDT electrode similarly to the comparative example compared to the first preferred embodiment. The third comparative example differs from the second preferred embodiment in that the first IDT electrode and the second IDT electrode are spaced apart from each other in the overlap-width direction, and the first intersecting area and the second intersecting area do not overlap in the propagation direction of acoustic wave. 
     The acoustic wave device  31  having the structure of the second preferred embodiment, the acoustic wave device according to the first comparative example, and the acoustic wave device according to the third comparative example were prepared. The design parameters of the prepared acoustic wave device  31  having the structure of the second preferred embodiment and the design parameters of the acoustic wave device according to the third comparative example are described below. It should be noted that the design parameters of the prepared acoustic wave device  101  of the first comparative example are the same as the design parameters of the acoustic wave device  31  except that the acoustic wave device  101  does not include the second IDT electrode  5 . 
     The first IDT electrode  3 : number of pairs of electrode fingers 99 pairs, wavelength about 2.104 μm, duty ratio about 0.5 
     The second IDT electrode  5 : number of pairs of electrode fingers 10 pairs, wavelength about 2.104 μm, duty ratio about 0.5 
     The reflectors  4 A and  4 B: number of electrode fingers 21 fingers, wavelength about 2.104 μm 
     The measurement of the level of IMD3 was conducted in the condition in which a signal in the transmit band of Band 25 and an interference wave signal were inputted at the same time to the acoustic wave devices presented above. 
       FIG. 13  illustrates the level of IMD3 with respect to the acoustic wave device according to the second preferred embodiment, the acoustic wave device according to the first comparative example, and the acoustic wave device according to the third comparative example. 
     As illustrated in  FIG. 13 , the level of IMD3 in the acoustic wave device  31  having the structure of the second preferred embodiment is lower than the level of IMD3 in the acoustic wave device  101  according to the first comparative example and the level of IMD3 in the acoustic wave device according to the third comparative example. As for the second preferred embodiment, the level of IMD3 is particularly low at frequencies close to 1942 MHz. At the frequencies close to 1942 MHz, the level of IMD3 of the second preferred embodiment is better by approximately 22 dB than the level of IMD3 of the first comparative example. As such, the second preferred embodiment can reduce IMD. 
       FIG. 14  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention. 
     The present preferred embodiment differs from the first preferred embodiment in that the present preferred embodiment includes, in addition to the second IDT electrode  5 , another second IDT electrode  45  other than the second IDT electrode  5 . An acoustic wave device  41  includes the second IDT electrode  5  and the second IDT electrode  45  in a pair. Apart from the points described above, the acoustic wave device  41  according to the present preferred embodiment is the same or substantially the same as the acoustic wave device  1  according to the first preferred embodiment. 
     The second IDT electrode  45  is disposed on the piezoelectric substrate  2 . The second IDT electrode  45  faces the first IDT electrode  3  across the reflector  4 B. The piezoelectric substrate  2  and the second IDT electrode  45  define a second IDT  45 A. 
     The second IDT electrode  45  is the same or substantially the same as the second IDT electrode  5 . More specifically, the second IDT electrode  45  includes a third busbar  46  and a fourth busbar  47  in a pair, and a plurality of third electrode fingers  48  and a plurality of fourth electrode fingers  49 . The second IDT electrode  45  includes a second intersecting area C. In the second intersecting area C, the third electrode fingers  48  and the fourth electrode fingers  49  overlap in the propagation direction of acoustic wave. The resonant frequency of the second IDT  45 A is in the interference-wave frequency band similarly to the second IDT  5 A. The design parameters of the second IDT electrode  45  are not necessarily the same as the design parameters of the second IDT electrode  5 . 
     Both the third busbar  16  of the second IDT electrode  5  and the third busbar  46  of the second IDT electrode  45  are connected by the hot wire  6 A to the first busbar  12  of the first IDT electrode  3 . The fourth busbar  17  of the second IDT electrode  5  and the fourth busbar  47  of the second IDT electrode  45  are coupled to the ground potential via the ground wire  6 B. The first intersecting area A of the first IDT electrode  3 , the second intersecting area B of the second IDT electrode  5 , and the second intersecting area C of the second IDT electrode  45  overlap in the propagation direction of acoustic wave. 
     In the present preferred embodiment, when a signal in the transmit band and an interference wave signal are inputted at the same time to the acoustic wave device  41 , a wave of a frequency close to the interference-wave frequency is caused in not only the first IDT electrode  3  but also the second IDT electrode  5  and the second IDT electrode  45  in a pair. Furthermore, the second IDT electrode  5  and the second IDT electrode  45  in a pair are disposed individually facing the first IDT electrode  3  with the pair of reflectors  4 A and  4 B interposed therebetween. The first intersecting area A, the second intersecting area B, and the second intersecting area C overlap in the propagation direction of acoustic wave. As a result, the IMD signal caused in the first IDT electrode  3  and the IMD signals caused in the second IDT electrode  5  and the second IDT electrode  45  in a pair interfere with each other to cancel each other out. Accordingly, it is possible to reduce IMD similarly to the first preferred embodiment. Accordingly, when the acoustic wave device  41  is used in a filter device of, for example, a multiplexer, it is possible to reduce or prevent degradation of the receive sensitivity of another filter device coupled to a signal potential by common connection with the filter device. 
     The measurement of the level of IMD3 was conducted in the condition in which the second IDT electrode  5  and the second IDT electrode  45  in a pair of the acoustic wave device  41  according to the third preferred embodiment were varied with respect to the number of pairs of electrode fingers. The number of pairs of electrode fingers varied among two, five, and ten pairs. The result of the first preferred embodiment and the result of the first comparative example are also illustrated. 
       FIG. 15  illustrates the relationship between the number of pairs of electrode fingers of the second IDT electrode and the level of IMD3 with respect to the first preferred embodiment, the third preferred embodiment, and the first comparative example. 
     As illustrated in  FIG. 15 , regardless of the variations in the number of pairs of electrode fingers of the second IDT electrode  5  and the second IDT electrode  45  in a pair of the third preferred embodiment, IMD in the third preferred embodiment is lower than the first comparative example. In the third preferred embodiment, when the number of pairs of electrode fingers of the second IDT electrode  5  and the second IDT electrode  45  in a pair is ten or less, for example, as the number of pairs of electrode fingers increases, IMD decreases. 
       FIG. 16  is a schematic circuit diagram of a filter device according to a fourth preferred embodiment of the present invention. In  FIG. 16 , the second IDT electrode  5  is represented by a simplified symbol of a rectangle with two diagonal lines. The first IDT electrode  3 , and the reflectors  4 A and  4 B are represented by resonator symbols. In the present preferred embodiment, the acoustic wave device  1  serving as an acoustic wave resonator includes the first IDT electrode  3 , the second IDT electrode  5 , and the reflectors  4 A and  4 B. 
     The filter device  52  is a ladder filter including a plurality of series arm resonators and a plurality of parallel arm resonators. The filter device  52  is a band pass filter. The filter device  52  includes a first signal end  51 A and a second signal end  51 B. In the present preferred embodiment, the first signal end  51 A is an antenna end coupled to an antenna. The first signal end  51 A and the second signal end  51 B may be provided by electrode pads or wires. In the present preferred embodiment, the first signal end  51 A and the second signal end  51 B are provided by electrode pads. 
     Between the first signal end  51 A and the second signal end  51 B, series arm resonators S 51 , S 52 , S 53 , and S 54  and the acoustic wave device  1  are coupled in series with each other. In the present preferred embodiment, the acoustic wave device  1  is a series arm resonator. 
     A parallel arm resonator P 51  is coupled between a node between the series arm resonators S 51  and S 52 , and the ground potential. A parallel arm resonator P 52  is coupled between a node between the series arm resonators S 52  and S 53 , and the ground potential. A parallel arm resonator P 53  is coupled between a node between the series arm resonators S 53  and S 54 , and the ground potential. A parallel arm resonator P 54  is coupled between a node between the series arm resonator S 54  and the acoustic wave device  1 , and the ground potential. 
     In the present preferred embodiment, each series arm resonator and each parallel arm resonator are acoustic wave resonators. In the filter device  52 , a closest acoustic wave resonator of the plurality of series arm resonators and the plurality of parallel arm resonators to the first signal end  51 A is the acoustic wave device  1 . The first IDT electrode  3  and the second IDT electrode  5  illustrated in  FIG. 2  are connected by the hot wire  6 A to the first signal end  51 A provided as an electrode pad. The hot wire  6 A may be provided together with the first signal end  51 A. The circuit configuration of the filter device  52  is not limited to the above example, and the filter device  52  only needs to include the acoustic wave device according to the present invention. For example, the acoustic wave device  1  may be used as a parallel arm resonator in a ladder filter. 
     Because the filter device  52  includes the acoustic wave device  1  according to the first preferred embodiment, the filter device  52  can reduce IMD. Accordingly, when the filter device  52  is used in, for example, a multiplexer, it is possible to reduce or prevent degradation of the receive sensitivity of another filter device coupled to the signal potential on the first signal end  51 A side by common connection with the filter device  52 . 
     It is preferable that the acoustic wave device  1  is a closest resonator to the first signal end  51 A. As a result, it is possible to effectively reduce or prevent degradation of the receive sensitivity of another filter device coupled to the signal potential by common connection. It should be noted that the acoustic wave device  1  is not necessarily a closest resonator to the first signal end  51 A. 
       FIG. 17  is a schematic circuit diagram of a filter device according to a modification of the fourth preferred embodiment. 
     This modification differs from the fourth preferred embodiment in that the acoustic wave device  31  of the second preferred embodiment is used as a closest resonator to the first signal end  51 A. In this modification, the first IDT electrode  3  and the second IDT electrode  5  illustrated in  FIG. 12  are connected by the hot wire  6 A to the series arm resonator S 54 . The second IDT electrode  5  is also coupled to the IDT electrode of the parallel arm resonator P 54 . More specifically, the fourth busbar  17  of the second IDT electrode  5  is connected by a hot wire to one busbar of the pair of busbars of the IDT electrode of the parallel arm resonator P 54 . The first IDT electrode  3  and the second IDT electrode  5  are indirectly coupled to the signal potential on the second signal end  51 B side via the series arm resonators S 54 , S 53 , S 52 , and S 51 . This modification can also reduce IMD similarly to the fourth preferred embodiment. 
     The fourth preferred embodiment and its modification describe an example including the acoustic wave device according to the first preferred embodiment and the acoustic wave device according to the second preferred embodiment. Also with an acoustic wave device according to another preferred embodiment of the present invention such as the third preferred embodiment, it is possible to reduce IMD similarly to the fourth preferred embodiment. The above description describes an example in which the acoustic wave device according to a preferred embodiment of the present invention is used as a series arm resonator, but the acoustic wave device according to a preferred embodiment of the present invention may be used as a parallel arm resonator. 
       FIG. 18  is a schematic diagram of a duplexer according to a fifth preferred embodiment of the present invention. 
     A duplexer  60  includes a first filter device  62 A and a second filter device  62 B. The first filter device  62 A is a transmit filter and is also the filter device according to the fourth preferred embodiment. The second filter device  62 B is a receive filter. In the duplexer  60 , it is only necessary that at least the transmit filter is the filter device according to the fourth preferred embodiment. 
     The duplexer  60  includes a signal terminal  61 A. The signal terminal  61 A is an antenna terminal coupled to an antenna. The signal terminal  61 A may be provided by an electrode pad or wire. 
     The first filter device  62 A and the second filter device  62 B are coupled to the signal terminal  61 A by common connection. The communication band of the duplexer  60  is Band 25, for example. The pass band of the first filter device  62 A is about 1850 MHz to about 1915 MHz, for example. The pass band of the second filter device  62 B is about 1930 MHz to about 1995 MHz, for example. The communication band of the duplexer  60  is not limited to the example described above. 
     In the duplexer  60 , the first filter device  62 A can reduce IMD. As a result, it is possible to reduce or prevent degradation of the receive sensitivity of the second filter device  62 B coupled to the signal terminal  61 A by common connection with the first filter device  62 A. 
       FIG. 19  is a schematic diagram of a multiplexer according to a sixth preferred embodiment of the present invention. 
     A multiplexer  70  includes a first filter device  72 A, a second filter device  72 B, a third filter device  72 C, and a fourth filter device  72 D. The first filter device  72 A and the third filter device  72 C are each the filter device according to the fourth preferred embodiment. The multiplexer  70  only needs to include at least one filter device implemented by the filter device according to the present invention. For example, the first filter device  72 A, the second filter device  72 B, the third filter device  72 C, and the fourth filter device  72 D may each be the filter device according to the fourth preferred embodiment. 
     The multiplexer  70  also includes a plurality of filter devices other than the first filter device  72 A, the second filter device  72 B, the third filter device  72 C, and the fourth filter device  72 D. The number of filter devices included in the multiplexer  70  is not limited to a particular number. 
     The multiplexer  70  includes the signal terminal  61 A. The signal terminal  61 A is an antenna terminal coupled to an antenna. The first filter device  72 A, the second filter device  72 B, the third filter device  72 C, the fourth filter device  72 D, and the other filter devices are coupled to the signal terminal  61 A by common connection. 
     In the present preferred embodiment, the communication band of the multiplexer  70  includes Band  25  and Band  66 , for example. The first filter device  72 A is a transmit filter of Band  25 , for example. The second filter device  72 B is a receive filter of Band  25 , for example. The third filter device  72 C is a transmit filter of Band  66 , for example. The fourth filter device  72 D is a receive filter of Band  66 , for example. The pass band of the first filter device  72 A is about 1850 MHz to 1915 MHz, for example. The pass band of the second filter device  72 B is about 1930 MHz to about 1995 MHz, for example. The pass band of the third filter device  72 C is about 1710 MHz to about 1780 MHz, for example. The pass band of the fourth filter device  72 D is about 2110 MHz to about 2200 MHz, for example. The communication band of the multiplexer  70  is not limited to the example described above. 
     As described above, the first filter device  72 A and the third filter device  72 C that are transmit filters of the multiplexer  70  are each implemented by the filter device according to the fourth preferred embodiment. The filter device according to the present invention may be a transmit or receive filter. The multiplexer  70  only needs to include at least one receive filter. 
     In known technologies, when a signal in the transmit band of Band 25 and Band 66 are being transmitted, if the antenna receives an interference wave signal, the third-order IMD is caused. As a result, the receive sensitivity in the receive band of Band 25 and Band 66 can be degraded. 
     In contrast, in the multiplexer  70 , the first filter device  72 A and the third filter device  72 C can reduce IMD similarly to the fourth preferred embodiment. Consequently, it is possible to reduce or prevent degradation of the receive sensitivity of the second filter device  72 B and the fourth filter device  72 D. 
     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.