Patent Publication Number: US-2023143523-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-122999 filed on Jul. 17, 2020 and is a Continuation Application of PCT Application No. PCT/JP2021/023629 filed on Jun. 22, 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 first and second busbars, each of which includes a plurality of cavities inside. 
       2 . Description of the Related Art 
     In the related art, in order to suppress transverse mode ripples, an acoustic wave device using a piston mode has been known. For example, in Japanese Unexamined Patent Application Publication No. 2018-174595, in an intersection region of an interdigital transducer (IDT) electrode, a central region and a low acoustic velocity region on each side of the central region are provided. In addition, a busbar includes a plurality of cavities in a propagation direction. A portion in the busbar on an intersection region side of each cavity is an inner busbar portion, and an outer side portion opposite to the intersection region is an outer busbar portion. The inner busbar portion and the outer busbar portion are connected to each other by a plurality of fine connecting portions. As a result, a portion including the cavity is defined, together with the inner busbar portion, as a high acoustic velocity region. 
     SUMMARY OF THE INVENTION 
     In the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2018-174595, the inner busbar portion and the outer busbar portion are connected to each other by the plurality of fine connecting portions. Therefore, there has been a problem in which the Q characteristics degrade. 
     Preferred embodiments of the present invention provide acoustic wave devices that are each less likely to generate degradation of Q characteristics. 
     According to a preferred embodiment of the present invention, an acoustic wave device includes a piezoelectric substrate, and an IDT electrode on the piezoelectric substrate. The IDT substrate includes a first busbar and a second busbar facing each other, a plurality of first electrode fingers extending toward the second busbar from the first busbar, and a plurality of second electrode fingers extending toward the first busbar from the second busbar, an intersection region in which the first electrode fingers and the second electrode fingers overlap each other in an acoustic wave propagation direction includes a central region located at a center in an extending direction of the first electrode fingers and the second electrode fingers, and a first low acoustic velocity region and a second low acoustic velocity region outside the central region on respective sides in the extending direction of the first electrode fingers and the second electrode fingers, acoustic velocities in the first low acoustic velocity region and the second low acoustic velocity region are lower than an acoustic velocity in the central region, the first busbar and the second busbar each include a plurality of cavities in the acoustic wave propagation direction, a portion on an intersection region side of the plurality of cavities in the first busbar and a portion on an intersection region side of the plurality of cavities in the second busbar are a first inner busbar portion and a second inner busbar portion, respectively, a portion on a side opposite to the intersection region of the plurality of cavities in the first busbar and a portion on a side opposite to the intersection region of the plurality of cavities in the second busbar are a first outer busbar portion and a second outer busbar portion, respectively, the first inner busbar portion and the second inner busbar portion are connected to the first outer busbar portion and the second outer busbar portion by a plurality of connecting portions, and at least one of the plurality of connecting portions is a wide width connecting portion with a dimension in the acoustic wave propagation direction larger than a dimension of each of the remaining connecting portions. 
     According to preferred embodiments of the present invention, acoustic wave devices that are each less likely to generate degradation of the Q characteristics can be provided. 
     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 a schematic plan view for explaining an electrode structure of an acoustic wave device according to a first preferred embodiment of the present invention. 
         FIG.  2    is a schematic view illustrating the electrode structure of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIG.  3    is a front sectional view of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIG.  4    is a partial cutaway view illustrating a main portion of an IDT electrode of the acoustic wave device according to the first preferred embodiment of the present invention. 
         FIGS.  5 A and  5 B  are partial plan views illustrating portions indicated by arrows A and B in  FIG.  1   , in an enlarged view. 
         FIG.  6    is a graph showing Q characteristics of Example 1 and Comparative Example 1. 
         FIG.  7    is a graph showing a portion of the Q characteristics graph of  FIG.  6    in an enlarged view. 
         FIG.  8    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.  9    is a graph showing Q characteristics of acoustic wave devices of Examples 1 to 4. 
         FIG.  10    is a graph showing a portion of  FIG.  9    in an enlarged view. 
         FIG.  11    is a partial cutaway view illustrating an electrode structure of an acoustic wave device according to a third preferred embodiment of the present invention. 
         FIG.  12    is a front sectional view of an acoustic wave device according to a fourth preferred embodiment of the present invention. 
         FIG.  13    is a front sectional view of an acoustic wave device according to a fifth 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 drawings in order to clarify the present invention. 
     The various preferred embodiments described herein are illustrative. Partial substitutions and combinations of the features and elements are possible between different preferred embodiments. 
       FIG.  3    is a front sectional view of an acoustic wave device according to a first preferred embodiment of the present invention. An acoustic wave device  1  includes a piezoelectric substrate  2 . The piezoelectric substrate  2  includes a piezoelectric film  3 , a low acoustic velocity material layer  4 , a high acoustic velocity material layer  5 , and a supporting substrate  6 . An IDT electrode  7  and reflectors  8  and  9  are provided on the piezoelectric substrate  2 . As illustrated in a schematic view of  FIG.  2   , the electrode structure of the acoustic wave device  1  includes the IDT electrode  7 , and the reflectors  8  and  9  on respective sides of the IDT electrode  7  in an acoustic wave propagation direction. As a result, a one-port acoustic wave resonator is configured. 
     The acoustic wave device  1  is characterized by the specific electrode structure of the IDT electrode  7 . The electrode structure will be described in detail with reference to  FIGS.  1 ,  4 ,  5 A and  5 B . 
       FIG.  1    is a schematic plan view illustrating the electrode structure of the acoustic wave device  1 . The IDT electrode  7  includes a first busbar  11  and a second busbar  12  facing each other. The details of the IDT electrode  7  is illustrated in a partial cutaway view of  FIG.  4   . A plurality of first electrode fingers  13  are connected to the first busbar  11 . Each first electrode finger  13  extends toward the second busbar  12  from the first busbar  11 . A plurality of second electrode fingers  14  are connected to the second busbar  12 . Each second electrode finger  14  extends toward the first busbar  11  from the second busbar  12 . An intersection region K is a region in which the first electrode finger  13  and the second electrode finger  14  overlap each other when viewed in the acoustic wave propagation direction. The intersection region K includes a central region C, and a first low acoustic velocity region D1 and a second low acoustic velocity region D2 provided outside the central region C on respective sides in an extending direction of the first and second electrode fingers  13  and  14 . Note that other regions may exist outside the first and second low acoustic velocity regions D1 and D2. 
     The IDT electrode  7  is made of a suitable metal or alloy. In the first and second velocity regions D1 and D2, as illustrated by the hatched portions with oblique lines, mass adding films  13   a  and  14   a  are provided on the first and second electrode fingers  13  and  14 , respectively. The mass adding films  13   a  and  14   a  are made of metals or dielectrics. As illustrated on the right side of  FIG.  4   , since the mass adding films  13   a  and  14   a  are provided, compared to an acoustic velocity V1 in the central region C, acoustic velocities V2 in the first and second low acoustic velocity regions D1 and D2 are low. 
     Note that the mass adding films  13   a  and  14   a  may be laminated on the lower surface sides of the first and second electrode fingers  13  and  14 , respectively. 
     Note that in  FIG.  4   , the mass adding films  13   a  and  14   a  are laminated on the first and second electrode fingers  13  and  14 , respectively, but when the mass adding films  13   a  and  14   a  are made of dielectric films, a mass adding film may be structured to extend to a region between the first and second electrode fingers  13  and  14 . 
     The first busbar  11  includes a plurality of cavities  11   a  in the acoustic wave propagation direction. In the first busbar  11 , an inner busbar portion  11   b  is provided on an intersection region K side of the cavities  11   a . An outer busbar portion  11   c  is provided in an outer region of the cavities  11   a  opposite to an intersection region K. In the first busbar  11 , the inner busbar portion  11   b  and the outer busbar portion  11   c  are connected to each other by a plurality of connecting portions  11   d . Although not particularly limited, each connecting portion  11   d  extends in a direction orthogonal to the acoustic wave propagation direction and, in the present preferred embodiment, is provided at a position at which the first electrode finger  13  is extended. Note that the inner busbar portion  11   b  is a first inner busbar portion in the present invention, and the outer busbar portion  11   c  is a first outer busbar portion. 
     The second busbar  12  is configured in a similar manner. That is, the second busbar  12  also includes a plurality of cavities  12   a . In addition, an inner busbar portion  12   b  is provided on an intersection region K side of the cavities  12   a . An outer busbar portion  12   c  is provided in an outer region of the cavities  12   a  opposite to the intersection region K. The inner busbar portion  12   b  and the outer busbar portion  12   c  are connected to each other by a plurality of connecting portions  12   d . Note that the inner busbar portion  12   b  is a second inner busbar portion, and the outer busbar portion  12   c  is a second outer busbar portion. 
     Since the IDT electrode  7  has the above-described structure, acoustic velocities V10, V11, V12, and V13 illustrated in  FIG.  4    are acoustic velocities in regions outside the first and second low acoustic velocity regions D1 and D2. That is, high acoustic velocity regions indicated by the acoustic velocities V10, V11, and V12 are located outside the first and second low acoustic velocity regions D1 and D2 in the extending direction of the first and second electrode fingers  13  and  14 . Therefore, transverse mode ripples can be effectively confined. 
       FIG.  5 A  is a partial plan view illustrating a main portion of the IDT electrode  7  illustrated in  FIG.  4   , that is, the portion indicated by an arrow A in  FIG.  1   , in an enlarged view.  FIG.  5 B  is a partial plan view illustrating the portion indicated by an arrow B in  FIG.  1   , in an enlarged view. 
     The first busbar  11  includes a wide width connecting portion  11   d   1 . The wide width connecting portion  11   d   1  has a dimension extending in the acoustic wave propagation direction, that is, a width larger than the remaining connecting portions  11   d  ( FIG.  5 A ) . In  FIG.  5 B , a boundary between the outer busbar portion  11   c  and the wide width connecting portion  11   d   1  is indicated by a broken line X1. In addition, a boundary between the wide width connecting portion  11   d   1  and the inner busbar portion  11   b  is indicated by a broken line X2. 
     Since the width of the wide width connecting portion  11   d   1  is wider than the width of each of the remaining connecting portions  11   d , in the first busbar  11 , electric resistance between the outer busbar portion  11   c  and the inner busbar portion  11   b  can be reduced. Therefore, degradation of the Q characteristics can be suppressed. 
     In particular, as illustrated in  FIG.  1   , in the present preferred embodiment, a pair of the wide width connecting portions  11   d   1  and  11   d   1  is provided so as to be separated in the acoustic wave propagation direction of the first busbar  11 . In the second busbar  12 , wide width connecting portions  12   d   1  and  12   d   1  are also provided so as to be separated in the acoustic wave propagation direction. Since a plurality of wide width connecting portions  11   d   1  and  11   d   1  or a plurality of wide width connecting portions  12   d   1  and  12   d   1  are provided, electric resistance between the above outer busbar portions  11   c  and  12   c  and the inner busbar portions  11   b  and  12   b  can be further reduced. Therefore, degradation of the Q characteristics can be further effectively suppressed. 
     Note that in a preferred embodiment of the present invention, the wide width connecting portion may be provided in at least one place in at least one of the first busbar  11  and the second busbar  12 . Therefore, only one wide width connecting portion  11   d   1  may be provided in the first busbar  11 . 
     It is preferable that the wide width connecting portions are provided in both of the first busbar  11  and the second busbar  12 . In addition, as in the present preferred embodiment, it is more preferable that a plurality of wide width connecting portions  11   d   1  and  11   d   1  and a plurality of wide width connecting portions  12   d   1  and  12   d   1  are provided in the first busbar  11  and the second busbar  12 , respectively. 
     As illustrated in  FIG.  1   , a pair of the wide width connecting portions  11   d   1  and  11   d   1  is provided at respective ends in the acoustic wave propagation direction of the first busbar  11 . In the second busbar  12  as well, a pair of the wide width connecting portions  12   d   1  and  12   d   1  is provided at respective ends in the acoustic wave propagation direction. Therefore, leakage of acoustic wave energy is reduced, and, in the present preferred embodiment, reduction of a Q value is even more unlikely to occur. Therefore, more preferably, it is desirable that, in at least one of the first busbar  11  and the second busbar  12 , the wide width connecting portions  11   d   1  and  11   d   1  or the wide width connecting portions  12   d   1  and  12   d   1  are provided at respective ends in the acoustic wave propagation direction. 
     The width of each of the above wide width connecting portions  11   d   1  and  12   d   1  is sufficient as long as the width is larger than the width of each of the remaining connecting portions  11   d  and  12   d , but is preferably equal to or more than about 10 µm and equal to or less than about 30 µm, for example. In addition, when λ is a wavelength determined by electrode finger pitches of the first and second electrode fingers  13  and  14 , the width of each of the above wide width connecting portions  11   d   1  and  12   d   1  is preferably equal to or more than about 1 λ and equal to or less than about 10 λ, for example. When the width is equal to or more than 1 λ, a decrease in the above Q value can be more effectively suppressed. In addition, when the width is equal to or less than 10 λ, transverse mode ripples can be more effectively suppressed. 
     In the reflectors  8  and  9 , both ends of a plurality of electrode fingers are short circuited by a pair of busbars. That is, the reflectors  8  and  9  are grating reflectors, and the structures thereof are not particularly limited. 
     As illustrated in  FIG.  3   , the piezoelectric substrate  2  includes a structure in which the high acoustic velocity material layer  5 , the low acoustic velocity material layer  4 , and the piezoelectric film  3  are laminated on the supporting substrate  6  in this order. The supporting substrate  6  is made of a suitable semiconductor or dielectric material such as Si or alumina. 
     As the piezoelectric film  3 , a piezoelectric monocrystal such as LiTao 3  can be used. The low acoustic velocity material layer  4  is made of a low acoustic velocity material. The low acoustic velocity material refers to a material in which an acoustic velocity of a bulk wave that propagates through the low acoustic velocity material is lower than an acoustic velocity of a bulk wave that propagates through the piezoelectric film  3 . Examples of such a low acoustic velocity material include glass, silicon oxide, silicon oxynitride, lithium oxide, tantalum pentoxide, or a material containing compounds obtained by adding fluorine, carbon, or boron to silicon oxide as principal components, or the like. 
     The high acoustic velocity material layer  5  is made of a high acoustic velocity material in which an acoustic velocity of a bulk wave that propagates through the high acoustic velocity material is higher than an acoustic velocity of an acoustic wave that propagates through the piezoelectric film  3 . Examples of such a high acoustic velocity material include silicon, aluminum oxide, silicon carbide, silicon oxynitride, sapphire, lithium tantalate, lithium niobate, crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, or media containing the above materials as principal components such as a diamond-like carbon (DLC) film, diamond, or the like. 
     Note that the low acoustic velocity material layer  4  may be omitted. 
     Since the piezoelectric substrate  2  has the laminated structure described above, the acoustic wave energy can be effectively confined in the piezoelectric film  3 . 
     Next, the Q characteristics of Example 1 of the first preferred embodiment and Comparative Example 1 will be shown to clarify the effects of the present preferred embodiment. 
     Design parameters of Example 1 are as follows. 
     Material of the IDT electrode  7  and the reflectors  8  and  9 : Al. Wavelength λ determined by electrode finger pitches = 2.12 µm. Electrode film thickness = 0.047 λ. Dimension of the intersection region K = 23.58 λ. Length of the central region C = 21.77 λ. Length of the first and second low acoustic velocity regions D1 and D2 = 0.90 λ. Width of the inner busbar portions  11   b  and  12   b  (dimension in the extending direction of the first and second electrode fingers  13  and  14 ) = 0.20 λ. Dimension of the cavities  11   a  and  12   a  extending in the acoustic wave propagation direction = 2.00 λ. Dimension in the extending direction of the first and second electrode fingers  13  and  14  = 0.43 λ. Width of the connecting portions  11   d  and  12   d  = 1.13 λ. Width of the wide width connecting portions  11   d   1  and  12   d   1  = 100.46 λ. Width of the outer busbar portions  11   c  and  12   c  (dimension in the extending direction of the first and second electrode fingers  13  and  14 ) = 5.66 λ. Number of pairs of electrode fingers = 100. The piezoelectric film  3  = LiTaO 3  having a thickness of 0.19 λ. The low acoustic velocity material layer  4 : SiN having a thickness of 0.14 λ. The high acoustic velocity material layer  5 : SiO 2  having a thickness of 0.14 λ. The supporting substrate  6 : Si. 
     In Comparative Example 1, the above wide width connecting portions  11   d   1  and  11   d   1 , and  12   d   1  and  12   d   1  are not provided, and the portions including the wide width connecting portions  11   d   1  and  11   d   1 , and  12   d   1  and  12   d   1  in Example 1 are configured in the same manner as the portions including the remaining connecting portions  11   d  and  12   d . Other configurations in Comparative Example 1 are the same as those in Example 1. 
       FIG.  6    is a graph showing the Q characteristics of Example 1 as an example of the acoustic wave device  1  of the first preferred embodiment and Comparative Example 1. The solid line indicates the results of Example 1, and the broken line indicates the results of Comparative Example 1.  FIG.  7    is a graph showing a portion of  FIG.  6    in an enlarged view. As it is evident from  FIGS.  6  and  7   , around 1900 MHz where the Q values are the highest, according to Example 1, the Q value can be effectively increased compared to Comparative Example 1. That is, compared to Comparative Example 1, degradation of the Q characteristics is less likely to be generated. 
       FIG.  8    is a schematic plan view illustrating an electrode structure of an acoustic wave device according to a second preferred embodiment of the present invention. In an acoustic wave device  21 , in the IDT electrode  7 , the positions of the wide width connecting portions  11   d   1  and  12   d   1  extending in the acoustic wave propagation direction are changed. Other configurations of the acoustic wave device  21  are the same as those of the acoustic wave device  1 . 
     As illustrated in  FIG.  8   , the wide width connecting portions  11   d   1  and  11   d   1  are located inside the respective ends in the acoustic wave propagation direction of the first busbar  11  in the acoustic wave propagation direction. More precisely, when L represents a distance between one end in the acoustic wave propagation direction of the first busbar  11  and the wide width connecting portion  11   d   1 , L &gt;  0  is satisfied. The wide width connecting portion  12   d   1  in the second busbar  12  is also disposed in the same manner. 
     In the acoustic wave device  21 , as described above, the wide width connecting portions  11   d   1  and  12   d   1  are located inside the end portions of the first and second busbars  11  and  12 , respectively, in the acoustic wave propagation direction. In this manner, the wide width connecting portions  11   d   1  and  12   d   1  may be located inside the end portions of the first and second busbars  11  and  12  in the acoustic wave propagation direction. In this case as well, by reducing the electric resistance, degradation of the Q characteristics can be suppressed. 
     Acoustic wave devices of the following Examples 2 to 4 are configured using the same design parameters as the acoustic wave device of the above Example 1 except for the distance L.  
     
       
         
           
             L 
               
               
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             . 
           
         
       
     
     
       
         
           
             L 
               
               
             = 
               
               
             20 
               
               
               
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             9 
             .4 
             λ 
               
             . 
           
         
       
     
     
       
         
           
             L 
               
               
             = 
               
               
             30 
               
               
               
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             14. 
             1 
             λ 
               
             . 
           
         
       
     
     The Q characteristics of the acoustic wave devices of Example 1 and Examples 2 to 4 are shown in  FIG.  9   . In addition, a portion of  FIG.  9    is shown in  FIG.  10    in an enlarged view. In  FIG.  10   , the solid line indicates the results of Example 1, the broken line indicates the results of Example 2, the dashed line indicates the results of Example 3, and the two-dot line indicates the results of Example 4. As it is evident from  FIG.  10   , compared to Examples 2 to 4, Example 1 is less likely to generate degradation of the Q characteristics and is more preferable. In addition, Examples 2 to 4 show that degradation of the Q characteristics can be suppressed as the above L decreases. Therefore, more preferably, L = less than 20 µm, and further more preferably, L = less than 9.4 λ. 
       FIG.  11    is a partial cutaway view for explaining an electrode structure according to a third preferred embodiment of the present invention. In the third preferred embodiment, each first electrode finger  13  and each second electrode finger  14  include a wide width portion  13   b  and a wide width portion  14   b , respectively. Regions including the wide width portions  13   b  and  14   b  are low acoustic velocity regions. Other configurations of an acoustic wave device  31  in the third preferred embodiment are the same as those of the acoustic wave device  1  of the first preferred embodiment. In this manner, in the present invention, the first and second low acoustic velocity regions may be configured by including not mass adding films, but the wide width portions  13   b  and the wide width portion  14   b  as a portion of the first electrode finger  13  and a portion of the second electrode finger  14 , respectively. 
       FIG.  12    is a front sectional view of an acoustic wave device according to a fourth preferred embodiment of the present invention. In an acoustic wave device  41 , the piezoelectric substrate  2  includes a high acoustic velocity supporting substrate 5A. The high acoustic velocity supporting substrate 5A is made of the high acoustic velocity material described earlier. In this case, the high acoustic velocity supporting substrate 5A is used in place of the supporting substrate  6  illustrated in  FIG.  3   . Therefore, the high acoustic velocity material layer  5  illustrated in  FIG.  3    can be omitted. 
       FIG.  13    is a front sectional view of an acoustic wave device according to a fifth preferred embodiment of the present invention. In an acoustic wave device  51 , as a piezoelectric substrate, a piezoelectric substrate  52  is used. The piezoelectric substrate  52  is made of a monocrystal such as LiNbO 3  or LiTaO 3 . In a preferred embodiment of the present invention, the piezoelectric substrate may be formed of a single piezoelectric monocrystal substrate as described above. 
     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.