Patent Publication Number: US-2023155569-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-126674 filed on Jul. 27, 2020 and is a Continuation Application of PCT Application No. PCT/JP2021/027694 filed on Jul. 27, 2021. 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 including an inclined IDT electrode. 
     2. Description of the Related Art 
     International Publication No. 2015/098756 discloses an acoustic wave device having an inclined IDT electrode, and further having a structure for suppressing a transverse mode. In this acoustic wave device, in order to provide a low acoustic velocity region in an intersection region, an edge portion of an electrode finger is formed as a large-width portion, and is made wider than a width of the electrode finger in a central region. 
     SUMMARY OF THE INVENTION 
     In the acoustic wave device described in International Publication No. 2015/098756, since the large-width portion is provided in the low acoustic velocity region, the transverse mode can be suppressed. However, due to such an electrode structure, another ripple could be generated. In particular, in a case of an acoustic wave resonator, a ripple could appear between a resonant frequency and an anti-resonant frequency. 
     Preferred embodiments of the present invention provide acoustic wave devices each capable of suppressing a ripple between a resonant frequency and an anti-resonant frequency. 
     An acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric substrate, and an IDT electrode provided on the piezoelectric substrate, wherein the IDT electrode includes a first busbar, a second busbar separated from the first busbar, a plurality of first electrode fingers including one end connected to the first busbar, a plurality of second electrode fingers including one end connected to the second busbar, a plurality of first dummy electrodes connected to the second busbar, tip ends of the first dummy electrodes facing tip ends of the first electrode fingers with a second gap interposed therebetween, and a plurality of second dummy electrodes connected to the first busbar, tip ends of the second dummy electrodes facing tip ends of the second electrode fingers with a first gap interposed therebetween, and when a first virtual line connecting the tip ends of the plurality of second electrode fingers is inclined with respect to an acoustic wave propagation direction that is a direction orthogonal to a direction in which the first and second electrode fingers extend, a distance between a tip end of a second electrode finger on one side, of a pair of second electrode fingers included in the second electrode fingers adjacent to any of the first electrode fingers, and a base end of the first electrode finger is shorter than a distance between a tip end of the second electrode finger on another side of the pair and the base end of the first electrode finger, and a direction extending in a direction in which the distance is longer in the first virtual line is an inclination direction, at least one of a projecting portion protruding toward the first electrode finger or the second electrode finger, in at least one of a side opposite to the inclination direction at a tip end of the second electrode finger, a side in the inclination direction at a tip end of the second dummy electrode, a side in the inclination direction of the first electrode finger on an extension in the inclination direction from the tip end of the second dummy electrode, and a side opposite to the inclination direction of the first electrode finger on an extension in the inclination direction from the tip end of the second electrode finger, and a recessed portion in at least one of a side in the inclination direction at the tip end of the second electrode finger, a side opposite to the inclination direction at the tip end of the second dummy electrode, a side opposite to the inclination direction of the first electrode finger positioned on the extension in the inclination direction from the tip end of the second dummy electrode, and a side in the inclination direction of the first electrode finger positioned on the extension in the inclination direction from the tip end of the second electrode finger, is provided. 
     In another broad aspect of an acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric substrate, and an IDT electrode on the piezoelectric substrate, wherein the IDT electrode includes a first busbar, a second busbar separated from the first busbar, a plurality of first electrode fingers including one end connected to the first busbar, a plurality of second electrode fingers including one end connected to the second busbar, a plurality of first dummy electrodes connected to the second busbar, tip ends of the first dummy electrodes facing tip ends of the first electrode fingers with a second gap interposed therebetween, and a plurality of second dummy electrodes connected to the first busbar, tip ends of the second dummy electrodes facing tip ends of the second electrode fingers with a first gap interposed therebetween, a first virtual line connecting the tip ends of the plurality of second electrode fingers is inclined with respect to an acoustic wave propagation direction that is a direction orthogonal to a direction in which the first and second electrode fingers extend, a distance between a tip end of a second electrode finger on one side, of a pair of second electrode fingers included in the second electrode fingers adjacent to any first electrode finger included in the first electrode fingers, and a base end of the first electrode finger is shorter than a distance between a tip end of the second electrode finger on an other side of the pair and the base end of the first electrode finger, a side closer to the second electrode finger for which the distance is shorter, of sides of the first electrode finger, and a side closer to the second electrode finger for which the distance is shorter, of sides of a first dummy electrode included in the first dummy electrodes facing the first electrode finger, are each a first side, and a side opposite to the first side is a second side, a distance between a tip end of a first electrode finger on one side, of a pair of first electrode fingers included in the first electrode fingers adjacent to any second electrode finger included in the second electrode fingers, and a base end of the second electrode finger is shorter than a distance between a tip end of the first electrode finger on another side of the pair and the base end of the second electrode finger, a side closer to the first electrode finger for which the distance is shorter, of sides of the second electrode finger, and a side closer to the first electrode finger for which the distance is shorter, of sides of a second dummy electrode included in the second dummy electrodes facing the second electrode finger, are each a second side, and a side opposite to the second side is a first side, a line connecting respective centers of first gaps, each being the first gap interposed between the tip ends, is a second virtual line, of a portion closer to the first gap of the second dummy electrode, a region closer to the first side is a first region and a region closer to the second side is a second region, of a portion closer to the first gap of the second electrode finger, a region closer to the first side is a fifth region and a region closer to the second side is a sixth region, and in the first electrode finger that is adjacent, in a portion closer to the first busbar with respect to the second virtual line, a region closer to the first side is a third region and a region closer to the second side is a fourth region, and in the first electrode finger, in a portion closer to the second busbar with respect to the second virtual line, a region closer to the first side is a seventh region and a region closer to the second side is an eighth region, and in the first region, the third region, the sixth region, and the eighth region, an angle defined by the first side or the second side in each region, and the first virtual line is an acute angle, and in the second region, the fourth region, the fifth region, and the seventh region, an angle defined by the first side or the second side positioned in each region, and the first virtual line is an obtuse angle, and at least one of a recessed portion provided in at least one region of the first region, the third region, the sixth region, and the eighth region, and a projecting portion provided in at least one region of the second region, the fourth region, the fifth region, and the seventh region is provided. 
     According to preferred embodiments of the present invention, it is possible to provide acoustic wave devices each capable of suppressing a ripple between a resonant frequency and an anti-resonant frequency. 
     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 A  is a schematic plan view for describing an electrode structure of an acoustic wave device according to a first preferred embodiment of the present invention and  FIG.  1 B  is an enlarged view of a main portion thereof. 
         FIG.  2    is a schematic plan view illustrating a main portion of an IDT electrode for describing first to eighth regions. 
         FIG.  3    is a partially cutaway enlarged plan view for describing a modified example of the first preferred embodiment of the present invention. 
         FIG.  4    is a front sectional view of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIG.  5    is a schematic plan view for describing a displacement distribution in an existing acoustic wave device. 
         FIG.  6    is an enlarged plan view for describing a relationship between the displacement distribution in the existing acoustic wave device and a shape of an electrode finger. 
         FIG.  7    is a schematic plan view for describing a main portion of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIG.  8    is a graph showing impedance-frequency characteristics of Example 1 and Comparative Example 1. 
         FIG.  9    is a graph showing return loss characteristics of Example 1 and Comparative Example 1. 
         FIG.  10    is a front sectional view for describing an acoustic wave device according to a second preferred embodiment of the present invention. 
         FIG.  11    is a front sectional view for describing an acoustic wave device according to a third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, specific preferred embodiments of the present invention will be described with reference to the accompanying drawings to clarify the present invention. 
     The preferred embodiments described in the present specification are merely examples, and partial replacement or combination of configurations is possible between different preferred embodiments. 
       FIG.  1 A  is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a first preferred embodiment of the present invention and  FIG.  1 B  is an enlarged view of a main portion thereof. Further,  FIG.  4    is a front sectional view of the acoustic wave device according to the first preferred embodiment. 
     As illustrated in  FIG.  4   , an acoustic wave device  1  includes a piezoelectric substrate  2 . An IDT electrode  7  and reflectors  8  and  9  are provided on the piezoelectric substrate  2 . As a result, a one-port-type acoustic wave resonator is provided. 
     The piezoelectric substrate  2  has a structure in which a support substrate  3 , a high acoustic velocity material layer  4 , a low acoustic velocity material layer  5 , and a piezoelectric film  6  are laminated in this order. The support substrate  3  is made of an appropriate semiconductor or dielectric material such as Si or alumina. 
     The piezoelectric film  6  is made of a piezoelectric single crystal such as LiTaO 3 . The high acoustic velocity material layer  4  is made of a high acoustic velocity material by which acoustic velocity of a bulk wave propagating therethrough is higher than acoustic velocity of an acoustic wave propagating through the piezoelectric film  6 . As such a high acoustic velocity material, various materials can be used such as aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, quartz, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, a diamond-like carbon (DLC) film or diamond, a medium containing the above-described material as a main component, and a medium containing a mixture of the above-described materials as a main component. 
     The low acoustic velocity material layer  5  is made of a low acoustic velocity material by which acoustic velocity of a bulk wave propagating therethrough is lower than acoustic velocity of a bulk wave propagating through the piezoelectric film  6 . As such a low acoustic velocity material, various materials can be used such as silicon oxide, glass, silicon oxynitride, tantalum oxide, a compound obtained by adding fluorine, carbon, boron, hydrogen, or a silanol group to silicon oxide, a medium containing the above-described material as a main component, and the like. 
     Since the piezoelectric substrate  2  has the structure described above, an acoustic wave excited in the piezoelectric film  6  can be effectively confined in the piezoelectric film  6 . Note that, the support substrate  3  may be a high acoustic velocity support substrate formed of a material similar to that of the high acoustic velocity material layer  4 . In this case, the piezoelectric substrate  2  need not have the high acoustic velocity material layer  4 . That is, the layer configuration of the piezoelectric substrate  2  may be a configuration in which a high acoustic velocity support substrate, a low acoustic velocity material layer, and a piezoelectric film are laminated in this order. 
     The IDT electrode  7  and the reflectors  8  and  9  are made of an appropriate metal or alloy. A 1 ternatively, the IDT electrode  7  and the reflectors  8  and  9  may include a multilayer body including a plurality of metal films. 
     As illustrated in  FIGS.  1 A and  1 B , the IDT electrode  7  has a so-called inclined structure. The IDT electrode  7  has a first busbar  11  and a second busbar  12 . In  FIGS.  1 A and  1 B , the first and second busbars  11  and  12  are inclined downward relative to a horizontal direction, while heading from a left side to a right side of the figure. The first busbar  11  and the second busbar  12  are parallel or substantially parallel to each other. 
     A plurality of first electrode fingers  13  is connected to the first busbar  11 . A plurality of second electrode fingers  14  is connected to the second busbar  12 . The plurality of first electrode fingers  13  and the plurality of second electrode fingers  14  are interdigitated with each other. On the other hand, a plurality of second dummy electrodes  16  is connected to the first busbar  11 . A plurality of first dummy electrodes  15  is connected to the second busbar  12 . Respective tip ends of the second dummy electrode  16  and the second electrode finger  14  face each other with a first gap G 1  interposed therebetween. Similarly, respective tip ends of the first electrode finger  13  and the first dummy electrode  15  face each other with a second gap G 2  interposed therebetween. 
     As illustrated in  FIG.  2   , an acoustic wave propagation direction D is a direction orthogonal to a direction in which the first and second electrode fingers  13  and  14  extend. A first virtual line A is inclined with respect to the acoustic wave propagation direction D. The first virtual line A is a virtual straight line connecting tip ends of the plurality of second electrode fingers  14 . Note that, a virtual straight line connecting centers of the plurality of first gaps G 1  is a second virtual line B. Further, on a side of the second gap G 2 , a virtual line connecting tip ends of the plurality of first electrode fingers  13  is a third virtual line A 1 . A virtual line connecting centers of the plurality of second gaps G 2  is a fourth virtual line B 1 . 
     The first virtual line A and the third virtual line A 1  described above are inclined with respect to the acoustic wave propagation direction D. 
     As illustrated in  FIG.  1 A , when viewed along the acoustic wave propagation direction D, a region in which the first electrode finger  13  and the second electrode finger  14  overlap each other is an intersection region K. The intersection region K includes a central region C and first and second low acoustic velocity regions L 1  and L 2  provided on respective outer side portions of the central region C in a direction in which the first and second electrode fingers  13  and  14  extend. Here, a projecting portion  17 , which will be described later, is provided in each of the first and second low acoustic velocity regions L 1  and L 2 , thus achieving low acoustic velocity. 
     Note that, in the intersection region K, other regions may be further provided on respective outer side portions of the first and second low acoustic velocity regions L 1  and L 2  in the direction in which the first and second electrode fingers  13  and  14  extend. 
     In the acoustic wave device  1 , high acoustic velocity regions are further provided on the respective outer side portions of the first and second low acoustic velocity regions L 1  and L 2 , thus suppressing a ripple caused by a transverse mode. Such a structure for suppressing a transverse mode is similar to that in a case of the acoustic wave device described in International Publication No. 2015/098756. 
     As illustrated in  FIG.  1 A , the reflectors  8  and  9  each have a structure in which both ends of a plurality of electrode fingers are short-circuited by respective busbars. In each of the reflectors  8  and  9  as well, the respective busbars on both sides are inclined similarly to the first and second busbars  11  and  12 . 
     The inclined IDT electrode described above is also disclosed in International Publication No. 2015/098756. In addition, in the acoustic wave device described in International Publication No. 2015/098756, a large-width portion is provided at a tip end of each of first and second electrode fingers in order to suppress a transverse mode. However, the inventor of the present application has discovered that a ripple appears between a resonant frequency and an anti-resonant frequency due to the provision of such a large-width portion. 
     In the acoustic wave device  1 , it is possible to suppress the ripple between the resonant frequency and the anti-resonant frequency. This is possible because the projecting portion  17  is provided at each of the first electrode finger  13 , the second electrode finger  14 , the first dummy electrode  15 , and the second dummy electrode  16 . This will be described in more detail. 
     As illustrated in  FIG.  1 B , the first electrode finger  13  includes a first side  13   a  and a second side  13   b,  and the second electrode finger  14  includes a first side  14   a  and a second side  14   b.  A 1 so, the first dummy electrode  15  includes a first side  15   a  and a second side  15   b,  and the second dummy electrode  16  includes a first side  16   a  and a second side  16   b.    
     A direction in which the first busbar  11  is inclined downward relative to the horizontal direction in  FIG.  2    is an inclination direction. As described above, the IDT electrode  7  has the inclined structure. Accordingly, a distance between a tip end of the second electrode finger  14  on one side, of the second electrode fingers  14  adjacent to any first electrode finger  13 , and a base end of the first electrode finger  13  is shorter than a distance between a tip end of the second electrode finger  14  on the other side and the base end. In the present preferred embodiment, a side closer to the second electrode finger  14  for which the distance is shorter, of sides of the first electrode finger  13 , is the first side  13   a.  A side closer to the second electrode finger  14  for which the distance is shorter, of sides of the first dummy electrode  15  facing the first electrode finger  13 , is the first side  15   a.  A side opposite to the first side  13   a  is the second sides  13   b  and a side opposite to the first side  15   a  is the second side  15   b.  Similarly, a distance between a tip end of the first electrode finger  13  on one side, of the first electrode fingers  13  adjacent to any second electrode finger  14 , and a base end of the second electrode finger  14  is shorter than a distance between a tip end of the first electrode finger  13  on the other side and the base end. In the present preferred embodiment, a side closer to the first electrode finger  13  for which the distance is shorter, of sides of the second electrode finger  14 , is the second side  14   b.  A side closer to the first electrode finger  13  for which the distance is shorter, of sides of the second dummy electrode  16  facing the second electrode finger  14 , is the second side  16   b.  A side opposite to the second side  14   b  is the first side  14   a,  and a side opposite to the second side  16   b  is the first side  16   a.    
     As illustrated in  FIG.  2   , of a portion closer to the first gap G 1  of the second dummy electrodes  16 , a region closer to the first side  16   a  is a first region R 1  and a region closer to the second side  16   b  is a second region R 2 . Of the second electrode finger  14  facing the first gap G 1 , a region closer to the first side  14   a  is a fifth region R 5  and a region closer to the second side  14   b  is a sixth region R 6 . In the first electrode finger  13  adjacent in the inclination direction to the second dummy electrode  16 , in a portion on the side closer to the first busbar  11  with respect to the second virtual line B, a region closer to the first side  13   a  is a third region R 3  and a region closer to second side  13   b  is a fourth region R 4 , in the first electrode finger  13 , in a portion on the side closer to the second busbar  12  with respect to the second virtual line B, a region closer to the first side  13   a  is a seventh region R 7  and a region closer to the second side  13   b  is an eighth region R 8 . 
     In the first region R 1  and the third region R 3 , an angle F 2  defined by each of the first sides  16   a  and  13   a  positioned in the respective regions, and the first virtual line A is an acute angle. Similarly, in the sixth region R 6  and the eighth region R 8 , the angle F 2  defined by each of the second sides  14   b  and  13   b  positioned in the respective regions, and the first virtual line A is an acute angle. 
     On the other hand, in the second region R 2  and the fourth region R 4 , an angle F 1  defined by each of the second sides  16   b  and  13   b  and the first virtual line A is an obtuse angle. Similarly, in the fifth region R 5  and the seventh region R 7 , the angle F 1  defined by each of the first sides  14   a  and  13   a  and the first virtual line A is an obtuse angle. 
     Here, the angle defined by the first side or the second side and the first virtual line A in each of the first to eighth regions R 1  to R 8  refers to an intersection angle in a portion positioned in each of the first to eighth regions R 1  to R 8 . Further, the angle in each region refers to an intersection angle closer to the region, between a portion of the first side or the second side positioned in each region and the first virtual line A. 
     In a preferred embodiment of the present invention, at least one of, at least one recessed portion provided in at least one region of the first region R 1 , the third region R 3 , the sixth region R 6 , and the eighth region R 8 , and at least one projecting portion provided in at least one region of the second region R 2 , the fourth region R 4 , the fifth region R 5  and the seventh region R 7 , is provided. In other words, at least one of, a projecting portion protruding toward a first electrode finger or a second electrode finger, in at least one of a side opposite to an inclination direction at a tip end of the second electrode finger, a side in the inclination direction at a tip end of a second dummy electrode, a side in the inclination direction of the first electrode finger positioned on an extension in the inclination direction from the tip end of the second dummy electrode, and a side opposite to the inclination direction of the first electrode finger positioned on an extension in the inclination direction from the tip end of the second electrode finger, and a recessed portion provided in at least one of a side in the inclination direction at the tip end of the second electrode finger, a side opposite to the inclination direction at the tip end of the second dummy electrode, a side opposite to the inclination direction of the first electrode finger positioned on the extension in the inclination direction from the tip end of the second dummy electrode, and a side in the inclination direction of the first electrode finger positioned on the extension in the inclination direction from the tip end of the second electrode finger, is provided. As a result, it is possible to suppress ripples between the resonant frequency and the anti-resonant frequency. 
     In the present preferred embodiment, as illustrated in  FIGS.  1 A and  1 B , the projecting portion  17  is provided as the recessed portion or projecting portion. More specifically, in the second side  16   b  of the second dummy electrode  16 , the projecting portion  17  protruding toward the first electrode finger  13  is provided. That is, the projecting portion  17  is provided in the second region R 2 . Similarly, the projecting portion  17  is provided also in the fifth region R 5 . 
     In the existing acoustic wave device, a low acoustic velocity region was defined by providing the large-width portion at the tip end of the electrode finger. A displacement distribution in this case will be described with reference to  FIG.  5   .  FIG.  5    is a schematic plan view illustrating an enlarged portion of an electrode structure of an existing acoustic wave device  100 . Here, a large-width portion  102   a  is provided at a tip end of a second electrode finger  102 . Further, a tip end of a second dummy electrode  104  is also provided with a large-width portion  104   a.    
     The large-width portion  102   a  and the large-width portion  104   a  face each other with the first gap G 1  interposed therebetween. In this case, when a first electrode finger  101  connected to a first busbar becomes a hot side, since an IDT electrode has an inclined structure, a region where displacement on a positive potential side is large is a region H 1  illustrated by hatching. On the other hand, a region where displacement on a negative potential side is large is a region H 2  indicated by hatching. 
     As is apparent from  FIG.  5   , since the IDT electrode has the inclined structure, the portion having the large displacement is inclined with respect to a direction in which the first and second electrode fingers  101  and  102  extend. That is, as further enlarged and illustrated in  FIG.  6   , the region H 2  schematically illustrated is positioned in a region inclined with respect to a direction in which the second electrode finger  102  and the second dummy electrode  104  extend. Thus, it is considered that, when the large-width portion  102   a  or  104   a  is provided symmetrically about a central axis passing in a length direction of the second electrode finger  102  or the second dummy electrode  104 , the above-described ripple appears due to shifting from the inclination angles of the respective regions H 1  and H 2 . 
     On the other hand, as illustrated in  FIG.  7   , in the present preferred embodiment, for example, the projecting portion  17  is provided in each of the second region R 2  and the fifth region R 5  to adapt to the inclination of the region H 2 . Thus, the above-described ripple due to the transverse mode is suppressed. This will be described based on a specific experimental example. 
     An acoustic wave device of Comparative Example 1 was configured based on the existing acoustic wave device described above, and an acoustic wave device of Example 1 was manufactured that was configured similarly to Comparative Example 1 except that the projecting portion  17  was provided instead of the large-width portion. Design parameters of the acoustic wave device of Example 1 were as follows. 
     A layer configuration of the piezoelectric substrate  2 , materials of each layer, thicknesses of each layer: piezoelectric film/low acoustic velocity material layer/high acoustic velocity support substrate, LiTaO 3 /SiO 2 /Si, 0.350 μm/0.450 μm/250 μm. 
     A material of the IDT electrode  7  and the reflectors  8  and  9 : Al. A thickness=60 nm. 
     A wavelength λ determined by an electrode finger pitch of the IDT electrode  7 =0.7 μm. 
     The number of pairs of electrode fingers: a one pair model was configured to have an infinite period by a boundary condition. 
     An angle defined by the first virtual line A and the acoustic wave propagation direction D=5°. 
     A dimension of each of the first and second gaps G 1  and G 2  in a direction in which an electrode finger extends=0.28 μm. 
     An amount of protrusion of the projecting portion  17  from a first side or a second side=0.07 μm. 
     A dimension of the projecting portion  17  in the direction in which the electrode finger extends=0.2 μm. 
       FIG.  8    shows impedance-frequency characteristics of the acoustic wave devices of Comparative Example 1 and Example 1 described above, and  FIG.  9    shows return loss characteristics. Note that, in each of  FIG.  8    and  FIG.  9   , a solid line shows a result of Example 1 and a broken line shows a result of Comparative Example 1. 
     As is clear from  FIG.  8    and  FIG.  9   , in Comparative Example 1, a plurality of large ripples appears between a resonant frequency and an anti-resonant frequency. On the other hand, according to Example 1, such ripples are effectively suppressed. Thus, according to Example 1, since the configuration was adopted in which the projecting portion  17  was provided such that a large-width portion of a tip end of each of the second electrode finger  14  and the second dummy electrode  16  was not symmetric, ripples between the resonant frequency and the anti-resonant frequency could be effectively suppressed. 
     Note that, as is clear from the regions H 2  and H 1  illustrated in  FIG.  7   , instead of providing the projecting portion  17 , conversely, it is desirable to provide a recessed portion in the first region R 1 , the sixth region R 6 , the third region R 3 , and the eighth region R 8 . Thus, as in a modified example illustrated in  FIG.  3   , it is preferable to further provide a recessed portion  17 A closer to the second side  14   b  on a tip end side of the second electrode finger  14 , and the recessed portion  17 A closer to the first side  16   a  in the second dummy electrode  16  as well. 
     However, in a preferred embodiment of the present invention, it is not necessary to provide the projecting portion  17  or the recessed portion  17 A in all of the first to eighth regions R 1  to R 8 . As described above, a projecting portion may be provided in at least one region of regions where a projecting portion is desirably provided, and a recessed portion may be provided in at least one region of regions where the recessed portion  17 A is desirably provided. In addition, it is sufficient that the projecting portion  17  or the recessed portion  17 A is provided in at least one region of the first to eighth regions R 1  to R 8 . 
     Further, the first to eighth regions R 1  to R 8  are illustrated for the side of the first gap G 1 , but similarly for a side of the second gap G 2 , it is sufficient that the first to eighth regions R 1  to R 8  are defined, and the projecting portion  17  or the recessed portion  17 A is provided. That is, as illustrated in  FIG.  2   , the first to eighth regions R 1  to R 8  are defined based on the fourth virtual line B 1  connecting the centers of the second gaps G 2  and the third virtual line A 1  connecting the tip ends of the plurality of first electrode fingers  13 . At least one of the projecting portion  17  and the recessed portion  17 A described above is preferably provided in the first to eighth regions R 1  to R 8 . In other words, preferably, at least one of, a projecting portion protruding toward a first electrode finger or a second electrode finger in at least one of a side in an inclination direction at a tip end of the first electrode finger, a side opposite to the inclination direction at a tip end of a first dummy electrode, a side opposite to the inclination direction of the second electrode finger positioned on an extension opposite to the inclination direction from the tip end of the first dummy electrode, and a side in the inclination direction of the second electrode finger positioned on an extension opposite to the inclination direction from the tip end of the first electrode finger, and a recessed portion provided in at least one of a side opposite to the inclination direction at the tip end of the first electrode finger, a side in the inclination direction at the tip end of the first dummy electrode, a side in the inclination direction of the second electrode finger positioned on the extension opposite to the inclination direction from the tip end of the first dummy electrode, and a side opposite to the inclination direction of the second electrode finger positioned on the extension opposite to the inclination direction from the tip end of the first electrode finger, is provided. 
     Note that, on the second gap G 2  side, in the first region R 1 , the third region R 3 , the sixth region R 6 , and the eighth region R 8 , an angle defined by the first side  13   a  or  14   a,  or the second side  14   b  or  15   b,  and the third virtual line A 1  is an acute angle, and in the second region R 2 , the fourth region R 4 , the fifth region R 5 , and the seventh region R 7 , an angle defined by the second side  13   b  or  14   b,  or the first side  14   a  or  15   a,  and the third virtual line A 1  is an obtuse angle. Thus, it is sufficient that, a recessed portion is provided in at least one region of the first region R 1 , the third region R 3 , the sixth region R 6 , and the eighth region R 8 , and a projecting portion is provided in at least one region of the second region R 2 , the fourth region R 4 , the fifth region R 5 , and the seventh region R 7 . 
     Furthermore, preferably, as in the modified example illustrated in  FIG.  3   , in the second region R 2  and the third region R 3  facing each other in an acoustic wave propagation direction, when the projecting portion  17  is provided in the second region R 2 , the recessed portion  17 A is preferably provided in the third region R 3 . As a result, a distance between the second dummy electrode  16  and the first electrode finger  13  along the acoustic wave propagation direction can be increased. Accordingly, surge resistance can be enhanced. Thus, a projecting portion and a recessed portion are preferably provided in at least one of a portion where the second region R 2  and the third region R 3  face each other and a portion where the sixth region R 6  and the seventh region R 7  face each other. To be more specific, the IDT electrode preferably has at least one of a configuration in which a projecting portion is provided in the second region R 2  and a recessed portion is provided in the third region R 3 , and a configuration in which a recessed portion is provided in the sixth region R 6  and a projecting portion is provided in the seventh region R 7 . 
     Note that, in the modified example illustrated in  FIG.  3   , a tip end portion of an electrode finger including the projecting portion  17  and the recessed portion  17 A has a parallelogram shape. As described above, the shape of the tip end portion of the electrode finger including the projecting portion  17  and the recessed portion  17 A in preferred embodiments of the present invention is not limited to a rectangle but may be a parallelogram. 
       FIG.  10    is a front sectional view for describing an acoustic wave device according to a second preferred embodiment of the present invention. In an acoustic wave device  31 , a high acoustic velocity material layer  4   a  also defines and functions as a support substrate. That is, the high acoustic velocity material layer  4   a  is a high acoustic velocity support substrate made of a high acoustic velocity material. In this case, the support substrate  3  illustrated in  FIG.  4    can be omitted. Such a piezoelectric substrate  2   a  may be used. 
     Furthermore, the low acoustic velocity material layer  5  may be omitted in  FIG.  4    and  FIG.  10   . 
     Further,  FIG.  11    is a front sectional view for describing an acoustic wave device according to a third preferred embodiment of the present invention. In an acoustic wave device  41 , the piezoelectric substrate  2  is a single-plate piezoelectricity substrate made of a piezoelectric single crystal such as LiNbO 3 . In a preferred embodiment of the present invention, the piezoelectric substrate  2  may be formed by using such a single-plate piezoelectricity substrate. 
     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.