Patent Publication Number: US-2021194457-A1

Title: Acoustic wave device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2018-176085 filed on Sep. 20, 2018 and is a Continuation Application of PCT Application No. PCT/JP2019/036624 filed on Sep. 18, 2019. 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 interdigital transducer (IDT) electrode. 
     2. Description of the Related Art 
     At a high-frequency front-end unit of a mobile communication device, a surface acoustic wave filter having features of a small size, low loss, and high selectivity is used. A surface acoustic wave element defining the surface acoustic wave filter has a structure in which an IDT electrode is formed on a surface of a piezoelectric layer, and interconversion is performed between an electrical signal and an acoustic wave signal. In the surface acoustic wave element, various excitation modes are used as surface acoustic waves. 
     In International Publication No. 2012/086639, a range of Euler angles of the piezoelectric layer in which excitation of an SH wave vibrating in a horizontal plane is more dominant than an SV wave vibrating in a vertical plane is defined, with respect to the surface of the piezoelectric layer. According to this, with the electromechanical coupling coefficient of the SH wave increased and the electromechanical coupling coefficient of the SV wave decreased, a wide-band acoustic wave filter with favorable attenuation characteristics can be configured, in which the SH wave is taken as a principal mode and SV-wave spurious is excluded. 
     As mobile communication systems advance from 3G and 4G to 5G, the frequency band that is used is made higher to a band equal to or higher than 3 GHz. To make the frequency of the acoustic wave element higher, possible solutions are (1) to decrease an IDT wavelength defined by the pitch of the IDT electrode and (2) to make the acoustic velocity faster. When the IDT wavelength is decreased, problems occur, such as limitation of processing accuracy of the IDT electrode and high resistance of the IDT electrode. Thus, to increase the frequency of the acoustic wave element, it is important to increase the acoustic velocity. 
     In the substrate including the piezoelectric layer, volume waves (bulk waves) of three types are present, that is, a “slow transversal wave”, a “fast transversal wave”, and a “longitudinal wave”. The phase velocity of the leaky surface acoustic wave (LSAW) where the components of the transversal wave are dominant is positioned between the “slow transversal wave” and the “fast transversal wave”. In International Publication No. 2012/086639, by trapping the leaky surface acoustic wave (LSAW) in the high acoustic velocity layer, a wide-band acoustic wave filter with favorable attenuation characteristics is achieved. In contrast, the phase velocity of the longitudinal leaky surface acoustic wave (LLSAW) is fast, positioned between the “fast transversal wave” and the “longitudinal wave”. Thus, to increase the frequency of the acoustic wave element, a possible solution is to use the longitudinal leaky surface acoustic wave (LLSAW). 
     However, the longitudinal leaky surface acoustic wave (LLSAW) is in a mode in which the surface acoustic waves propagate as being leaked to the substrate including the piezoelectric layer. Thus, a problem of Q-factor deterioration occurs. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide acoustic wave devices each having an excellent Q factor by using a longitudinal acoustic wave, that is, by efficiently trapping a longitudinal leaky surface acoustic wave (LLSAW) in the piezoelectric layer and the IDT electrode. 
     An acoustic wave device according to a preferred embodiment of the present invention uses a longitudinal acoustic wave, and the acoustic wave device includes a piezoelectric layer including a first principal surface and a second principal surface opposite to each other, an IDT electrode provided directly or indirectly on the first principal surface, and a high acoustic velocity member provided directly or indirectly on the second principal surface and including a 4H-type or 6H-type crystal polytype silicon carbide. 
     According to preferred embodiments of the present invention, acoustic wave devices using the longitudinal leaky surface acoustic wave (LLSAW) and having an excellent Q factor are able to 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 sectional view of an acoustic wave device according to a preferred embodiment of the present invention. 
         FIG. 2A  depicts graphs depicting reverse-velocity surfaces of Si, 3C—SiC, and 4H(6H)—SiC. 
         FIG. 2B  is a diagram describing crystal face orientations in a hexagonal crystal. 
         FIG. 3A  is a sectional view of an acoustic wave device according to Modified Example 1 of a preferred embodiment of the present invention. 
         FIG. 3B  is a sectional view of an acoustic wave device according to Modified Example 2 of a preferred embodiment of the present invention. 
         FIG. 4A  is a graph showing a relationship between the film thickness of an IDT electrode (Al) and acoustic velocity in a multilayer structure including an LN piezoelectric layer and a high acoustic velocity member. 
         FIG. 4B  is a graph showing a relationship between the film thickness of an IDT electrode (Cu) and acoustic velocity in a multilayer structure including an LN piezoelectric layer and a high acoustic velocity member. 
         FIG. 5A  is a graph showing a relationship between the propagation angle and acoustic velocity in a c-face 4H—SiC high acoustic velocity member. 
         FIG. 5B  is a graph showing a reverse-velocity surface in the c-face 4H—SiC high acoustic velocity member. 
         FIG. 6A  is a graph showing a relationship between the propagation angle and acoustic velocity in an a-face 4H—SiC high acoustic velocity member. 
         FIG. 6B  is a graph showing a reverse-velocity surface in the a-face 4H—SiC high acoustic velocity member. 
         FIG. 7A  is a graph showing a relationship between the propagation angle and acoustic velocity in an m-face 4H—SiC high acoustic velocity member. 
         FIG. 7B  is a graph showing a reverse-velocity surface in the m-face 4H—SiC high acoustic velocity member. 
         FIG. 8A  is a graph showing a relationship between the propagation angle and acoustic velocity in an r-face 4H—SiC high acoustic velocity member. 
         FIG. 8B  is a graph showing a reverse-velocity surface in the r-face 4H—SiC high acoustic velocity member. 
         FIG. 9A  is a graph in which a comparison is made in impedance characteristics between a multilayer structure including an LT piezoelectric layer and a SiC member according to a preferred embodiment and one according to Comparative Example 1. 
         FIG. 9B  is a graph in which a comparison is made in conductance characteristics between the multilayer structure including the LT piezoelectric layer and the SiC member according to a preferred embodiment of the present invention and one according to Comparative Example 1. 
         FIG. 10  is a graph in which a comparison is made in impedance characteristics between a multilayer structure including an LN piezoelectric layer, an intermediate layer, and a high acoustic velocity member according to Modified Example 2 and one according to Comparative Example 2. 
         FIG. 11A  is a graph showing changes in impedance characteristics when the film thickness of a piezoelectric layer in a multilayer structure including an LN piezoelectric layer and a high acoustic velocity member is changed. 
         FIG. 11B  is a graph showing a relationship between the film thickness of the piezoelectric layer in the multilayer structure including the LN piezoelectric layer and the high acoustic velocity member, and spurious occurrence frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments of the present invention are described in detail with reference to preferred embodiments and the drawings. Note that any of the preferred embodiments described below represents a generic or specific example. Numerical values, shapes, materials, components, arrangement and connection configurations of the components, and the like which are described in the preferred embodiments below are merely examples, and are not intended to restrict the present invention. Of the components of the preferred embodiments below, those not described in an independent claim are described as arbitrary or optional components. Also, the size of the components shown in the drawings or a ratio in size thereof is presented not necessarily in a strict sense. 
     PREFERRED EMBODIMENT 
     1-1. Structure of Acoustic Wave Device 
       FIG. 1  is a sectional view of an acoustic wave device  1  according to a preferred embodiment of the present invention. As shown in  FIG. 1 , the acoustic wave device  1  includes a piezoelectric layer  10 , an interdigital transducer (IDT) electrode  110 , and a high acoustic velocity member  20 . 
     The piezoelectric layer  10  is a thin film or thin plate which includes a first principal surface and a second principal surface opposite to each other and is made of a piezoelectric material. As the material of the piezoelectric layer  10 , for example, any of LiNbO 3 , LiTaO 3 , ZnO, AlN, quartz, and so forth is preferably selected as appropriate in consideration of a resonance Q factor, an electromechanical coupling coefficient, and other characteristics of the acoustic wave device  1 . 
     The IDT electrode  110  is provided on the first principal surface of the piezoelectric layer  10 , and is preferably made of, for example, a metal selected from Al, Cu, Pt, Au, Ti, Ni, Cr, Ag, W, Mo, Ta, and so forth or an alloy or a multilayer body made of two or more metals thereof. The IDT electrode  110  includes a pair of comb-shaped electrodes opposite to each other when the piezoelectric layer  10  is viewed in plan. Each of the paired comb-shaped electrodes includes a plurality of electrode fingers parallel or substantially parallel to each other and a busbar electrode connecting the plurality of electrode fingers. The plurality of electrode fingers included in one comb-shaped electrode and the plurality of electrode fingers included in the other comb-shaped electrode are interdigitated with one another along a direction orthogonal or substantially orthogonal to a principal-mode acoustic wave propagation direction. Here, a pitch between a plurality of electrode fingers included in one comb-shaped electrode is defined as an IDT wavelength λ. In other words, a doubling of the pitch between adjacent electrode fingers of the plurality of electrode fingers of the paired comb-shaped electrodes is defined as an IDT wavelength. Note that in the piezoelectric layer  10  viewed in plan, a reflector may be provided adjacent to the IDT electrode  110  in the principal-mode acoustic wave propagation direction. 
     The high acoustic velocity member  20  is a support substrate provided on the second principal surface of the piezoelectric layer  10  and is preferably made of a 4H-type or 6H-type crystal polytype silicon carbide, for example. 
     The acoustic wave device  1  having the above-described structure is an acoustic wave device using a longitudinal acoustic wave, and is, for example, an acoustic wave device capable of exciting, among leaky surface acoustic waves (LSAW), a longitudinal leaky SAW (LLSAW) where longitudinal wave components are more dominant than transversal wave components. In other words, the acoustic wave device  1  is an acoustic wave device capable of using the longitudinal leaky SAW (LLSAW) as a principal-mode acoustic wave. 
     As mobile communication systems advance from 3G and 4G to 5G, the frequency band used is made higher to a band equal to or higher than 3 GHz. To make the frequency of the acoustic wave device higher, possible solutions are (1) to decrease an IDT wavelength defined by the pitch of the IDT electrode and (2) to make the acoustic velocity faster. When the IDT wavelength is decreased, problems occur such as limitation of processing accuracy of the IDT electrode and high resistance of the IDT electrode. Thus, to increase the frequency of the acoustic wave device, it is important to increase the acoustic velocity. 
     In the substrate including the piezoelectric layer, volume waves (bulk waves) of three types are present, that is, a “slow transversal wave”, a “fast transversal wave”, and a “longitudinal wave”. The phase velocity of the leaky surface acoustic wave (LSAW) where the components of the transversal wave are dominant is positioned between the “slow transversal wave” and the “fast transversal wave”. In contrast, the phase velocity of the longitudinal leaky SAW (LLSAW) is fast, positioned between the “fast transversal wave” and the “longitudinal wave”. Thus, to increase the frequency of the acoustic wave device, a possible solution is to use the longitudinal leaky SAW (LLSAW). 
     On the other hand, in an acoustic resonator having a multilayer structure including a piezoelectric layer and a support substrate, when the bulk wave acoustic velocity of the support substrate is higher than that of the piezoelectric layer, a mode excited by the IDT electrode is not coupled to the bulk wave of the support substrate and acoustic energy in that mode is distributed to be concentrated onto the piezoelectric layer and the IDT electrode. Here, the longitudinal leaky SAW (LLSAW) is no longer leaked, and becomes a guided longitudinal acoustic wave. 
     As described above, the longitudinal leaky SAW (LLSAW) has a very high acoustic velocity compared with the leaky surface acoustic wave (LSAW), and is easily coupled to the transversal wave (for example, SV bulk wave and SH bulk wave) of the piezoelectric layer. To efficiently trap the longitudinal leaky SAW (LLSAW) in the piezoelectric layer and the IDT electrode, the high acoustic velocity member preferably has a transversal bulk wave acoustic velocity faster than the acoustic velocity of the longitudinal leaky SAW (LLSAW). 
     However, generally, the longitudinal leaky SAW (LLSAW) has an acoustic velocity (for example, about 6000 m/s or higher) higher than acoustic velocity of the transversal bulk wave of the high acoustic velocity member, such as, for example, silicon for use in a multilayer structure including a thin piezoelectric layer and a high acoustic velocity support substrate. Thus, it is difficult to trap the longitudinal leaky SAW (LLSAW) in the piezoelectric layer and to ensure a sufficient Q factor. 
     By contrast, according to the above-described structure of the present preferred embodiment, the high acoustic velocity member  20  defining a support substrate is preferably made of a 4H-type or 6H-type crystal polytype silicon carbide, for example. Thus, the transversal bulk wave acoustic velocity of the high acoustic velocity member  20  can be made faster than the acoustic velocity of the longitudinal leaky SAW (LLSAW) propagating through the piezoelectric layer  10 . Therefore, the longitudinal leaky SAW (LLSAW) can be efficiently trapped in the piezoelectric layer  10  and the IDT electrode  110 , thus allowing the Q factor of the resonance characteristics of the acoustic wave device  1  to be improved. Accordingly, by using the longitudinal acoustic wave, the acoustic wave device  1  with an excellent Q factor can be provided. 
     Silicon carbide is one of materials with the smallest viscosity losses and also has large thermal conductivity. Therefore, favorable electrical characteristics and power durability characteristics can be provided. Some of silicon carbide are crystal polytype, called a 3C-type, 4H-type, and 6H-type. Of these, the 3C-type has a slow bulk wave acoustic velocity, and when used as the high acoustic velocity member in contact with the piezoelectric layer, is not sufficient to trap the longitudinal leaky SAW (LLSAW). On the other hand, in the 4H-type and the 6H-type, the slowest transversal bulk wave acoustic velocity is approximately 6000 m/s to 7000 m/s, for example. 
       FIG. 2A  shows graphs depicting reverse-velocity surfaces of a c face (Euler angle notation (0, 0, ψ)) of Si, 3C—SiC, and 4H(6H)—SiC. As shown in  FIG. 2A , when applied to the high acoustic velocity member with the c face as a principal surface, Si and 3C-type silicon carbide has an acoustic velocity equal to or lower than approximately 6000 m/s. By contrast, the 4H-type or 6H-type crystal polytype silicon carbide has an acoustic velocity of approximately 6000 m/s to 7000 m/s, for example. With this, it has been discovered that when the 4H-type or 6H-type crystal polytype silicon carbide is used as the high acoustic velocity member  20 , it is advantageous to efficiently trap the longitudinal leaky SAW (LLSAW) in the piezoelectric layer  10  and the IDT electrode  110 . 
     Below are preferable crystal orientations when a 4H-type/6H-type crystal polytype silicon carbide is used as the high acoustic velocity member  20 . Note in the following that Euler angle notation is used to describe crystal orientations. 
       FIG. 2B  is a diagram describing crystal face orientations in a hexagonal crystal. As shown in  FIG. 2B , in the present preferred embodiment, a c-face orientation is denoted as Euler angles (0, 0, ψ), an a-face orientation is denoted as Euler angles (90, 90, ψ), an m-face orientation is denoted as Euler angles (0, 90, ψ), and an r-face orientation is denoted as Euler angles (0, 122.23, ψ). Here, ψ is defined as follows. As for the c face, ψ denotes an angle defined by an electrode-finger orthogonal direction when viewed from above on a principal surface side where the IDT electrode is provided and the Miller indices of silicon carbide, crystal orientation [1000]. Furthermore, as for the a face, ψ denotes an angle defined by the electrode-finger orthogonal direction when viewed from above on the principal surface side where the IDT electrode is provided and the Miller indices of silicon carbide, crystal orientation [0001]. Still further, as for the r surface, ψ denotes an angle defined by the electrode-finger orthogonal direction when viewed from above on the principal surface side where the IDT electrode is defined and the Miller indices of silicon carbide, crystal orientation [1-10-1]. 
     Note that in the acoustic wave device  1  according to the present preferred embodiment, the piezoelectric layer  10  may preferably have, for example, a film thickness equal to or smaller than about three times of an IDT wavelength defined by a doubling of a pitch between the plurality of electrode fingers configuring the IDT electrode  110 . This allows the longitudinal leaky SAW (LLSAW) to be efficiently trapped in the piezoelectric layer  10  and the IDT electrode  110 . Thus, the Q factor of the resonance characteristics of the acoustic wave device  1  can be further improved. 
     Furthermore, in the acoustic wave device  1  according to the present preferred embodiment, the film thickness of the piezoelectric layer  10  is preferably, for example, equal to or larger than about 0.05 times of the IDT wavelength and equal to or smaller than about 0.5 times thereof. This can improve both of the electromechanical coupling coefficient and the Q factor of the resonance characteristics of the acoustic wave device  1 . 
     Also, the piezoelectric layer  10  may preferably be made of lithium tantalate, for example. This can provide the acoustic wave device  1  with excellent temperature stability. 
     Also, the piezoelectric layer  10  may preferably be made of lithium niobate, for example. This can provide the acoustic wave device  1  with an excellent electromechanical coupling coefficient. 
     Also, when Euler angles of the piezoelectric layer  10  are denoted as (φ, θ, ψ), −5°≤φ≤5° and 0°≤θ≤90° and 85°≤ψ≤95° may preferably be maintained. For example, the piezoelectric layer  10  is preferably made of YZ—LiNbO 3  (0°, 90°, 90°). This allows the use of a longitudinal leaky SAW (LLSAW) mode with high acoustic velocity and less spurious responses. 
     Furthermore, when Euler angles of the piezoelectric layer  10  are denoted as (φ, θ, ψ), 15°≤φ≤95° and 85°≤θ≤95° and 0°≤ψ≤60° may preferably be maintained. For example, the piezoelectric layer  10  is preferably made of LiNbO 3  (90°, 90°, 36°) or LiTaO 3  (90°, 90°, 31°). This allows the use of a longitudinal leaky SAW (LLSAW) mode with high acoustic velocity and in a wide band. 
     Still further, when Euler angles of the piezoelectric layer  10  are denoted as (φ, θ, ψ), −5°≤φ≤5° and 120°≤θ≤160° and −5°≤ψ≤5° may preferably be maintained. For example, the piezoelectric layer  10  is preferably made of 41°-LiNbO 3  (0°, −49°, 0°) or 36° YX—LiTaO 3  (0°, −54°, 0°). This allows the use of a longitudinal leaky SAW (LLSAW) mode with a high Q factor. 
       FIG. 3A  is a sectional view of an acoustic wave device  1 A according to Modified Example 1 of a preferred embodiment of the present invention. As shown in  FIG. 3A , the acoustic wave device  1 A includes the piezoelectric layer  10 , the IDT electrode  110 , and a high acoustic velocity member  20 . The high acoustic velocity member  20  includes a high acoustic velocity layer  21  and a support substrate  22 . The acoustic wave device  1 A according to the present modified example is different in the structure of the high acoustic velocity member  20  compared with the acoustic wave device  1  according to the preferred embodiment. In the following, as for the acoustic wave device  1 A according to the present modified example, description of structures the same or substantially the same those of the acoustic wave device  1  according to the present preferred embodiment is omitted, and different structures are mainly described. 
     The support substrate  22  provided on a second principal surface side of the piezoelectric layer  10  across the high acoustic velocity layer  21  and supporting the piezoelectric layer  10  and the high acoustic velocity layer  21 . The support substrate  22  is preferably made of, for example, any of silicon, sapphire, and silicon carbide. This can improve crystallinity of the high acoustic velocity layer  21  made of a crystal polytype silicon carbide and provided on the support substrate  22  and can reduce acoustic losses. 
     The high acoustic velocity layer  21  is a thin film or thin plate between the support substrate  22  and the piezoelectric layer  10  and is preferably made of a 4H-type or 6H-type crystal polytype silicon carbide, for example. 
     According to the above-described structure of the present modified example, since the high acoustic velocity layer  21  is made of a 4H-type or 6H-type crystal polytype silicon carbide, the acoustic velocity of the transversal bulk wave of the high acoustic velocity layer  21  can be made faster than the acoustic velocity of the transversal bulk wave coupling to the longitudinal leaky SAW (LLSAW) propagating through the piezoelectric layer  10 . Thus, the longitudinal leaky SAW (LLSAW) can be efficiently trapped in the piezoelectric layer  10  and the IDT electrode  110 , thus allowing an improvement of the Q factor of the resonance characteristics of the acoustic wave device  1 A. Thus, by using the longitudinal acoustic wave, the acoustic wave device  1 A with an excellent Q factor can be provided. Also, it is sufficient to use a 4H-type or 6H-type crystal polytype silicon carbide, for example, not for the support substrate  22  being a thick plate, but to the high acoustic velocity layer  21  being a thin film or having a thin-plate shape, thus simplifying the process of manufacture. 
       FIG. 3B  is a sectional view of an acoustic wave device  1 B according to Modified Example 2 of a preferred embodiment of the present invention. As shown in  FIG. 3B , the acoustic wave device  1 B includes the piezoelectric layer  10 , the IDT electrode  110 , the high acoustic velocity member  20 , a functional layer  31 , and an intermediate layer  32 . The high acoustic velocity member  20  includes the high acoustic velocity layer  21  and the support substrate  22 . The acoustic wave device  1 B according to the present modified example is different in that the functional layer  31  and the intermediate layer  32  are added, compared with the acoustic wave device  1 A according to Modified Example 1. In the following, as for the acoustic wave device  1 B according to the present modified example, description of structures the same or substantially the same those of the acoustic wave device  1 A according to Modified Example 1 is omitted, and different structures are mainly described. 
     The intermediate layer  32  is a low acoustic velocity layer between the piezoelectric layer  10  and the high acoustic velocity layer  21 . The acoustic velocity of the slowest transversal bulk wave of the transversal bulk waves propagating through the intermediate layer  32  is slower than the acoustic velocity in the principal mode propagating through the piezoelectric layer  10 . With this structure and the property of acoustic waves intrinsically concentrating energy onto a low acoustic velocity medium, leakage of surface acoustic wave energy to the high acoustic velocity member  20  is reduced. Thus, a desired mode can be more efficiently excited. The intermediate layer  32  is a thin film or thin plate made of a dielectric film and preferably including, for example, silicon dioxide as a main component. Furthermore, the intermediate layer  32  may include a dielectric, such as SiN, AlN, Al 2 O 3 , Ta 2 O 3 , HfO 2 , or DLC, and may be a semiconductor such as amorphous silicon Ge, GaN, InP, or a compound thereof, for example. 
     The functional layer  31  is on the first principal surface of the piezoelectric layer  10  and is in contact with at least a portion of the IDT electrode  110 . The functional layer  31  may be an adjusting layer between the IDT electrode  110  and the piezoelectric layer  10 , or may be a protective layer covering the IDT electrode  110 . When the functional layer  31  is an adjusting layer, the electromechanical coupling coefficient can be adjusted in accordance with the film thickness of the adjusting layer. Also, when the functional layer  31  is a protective layer, it is possible to protect the IDT electrode  110  from the external environment, adjust frequency temperature characteristics, and improve moisture resistance. The functional layer  31  is preferably, for example, a dielectric film having silicon dioxide as a main component. 
     Furthermore, when the functional layer  31  as an adjusting layer includes silicon dioxide as a main component and includes at least one of a carbon atom and a hafnium atom, for example, the electromechanical coupling coefficient (fractional band width) can be adjusted by permittivity modulation. Also, when the functional layer  31  as a protective layer includes silicon dioxide as a main component and includes at least one of a fluorine atom, a boron atom, and a nitrogen atom, for example, the temperature characteristics can be more improved. 
     Also, with the intermediate layer  32  and the functional layer  31  each defined by a dielectric film, it is possible to provide an acoustic wave device with less influence, such as parasitic capacitance from a metal material or semiconductor material. 
     Note that in the acoustic wave device  1 B according to the present modified example, either of the functional layer  31  and the intermediate layer  32  may be omitted. 
     1-2. Suitable Conditions of High Acoustic Velocity Member 
     In the following, description is provided to explain suitable conditions under which the acoustic velocity of the transversal bulk wave of the high acoustic velocity member  20  made of a 4H-type or 6H-type crystal polytype silicon carbide can be made faster than the acoustic velocity of the transversal bulk wave coupling to the longitudinal leaky SAW (LLSAW) propagating through the piezoelectric layer  10 . In a multilayer structure including the IDT electrode  110  made of Al, the piezoelectric layer  10  made of LiTaO 3 , and the high acoustic velocity member  20  made of a 4H-type crystal polytype silicon carbide, a relationship between the crystal orientation (propagation angle ψ in Euler angle notation) of the 4H-type crystal polytype silicon carbide and acoustic velocity has been discovered by a simulation using a finite element solution. Parameters used in the present simulation are depicted in Table 1. 
     Note that the 4H-type crystal polytype silicon carbide and the 6H-type crystal polytype silicon carbide have the same acoustic characteristics. Therefore, the following suitable conditions discovered in the high acoustic velocity member  20  made of the 4H-type crystal polytype silicon carbide are also applied to the high acoustic velocity member  20  made of the 6H-type crystal polytype silicon carbide. 
       FIG. 4A  is a graph showing a relationship between the film thickness of an IDT electrode (Al) and acoustic velocity in a multilayer structure including an LN piezoelectric layer and a high acoustic velocity member. Also,  FIG. 4B  is a graph showing a relationship between the film thickness of an IDT electrode (Cu) and acoustic velocity in a multilayer structure including an LN piezoelectric layer and a high acoustic velocity member. In  FIGS. 4A and 4B , acoustic velocity and fractional band width (value obtained by dividing a frequency difference between an anti-resonant frequency and a resonant frequency by the resonant frequency) of an acoustic wave propagating through the IDT electrode and the piezoelectric layer are shown when the film thickness of the IDT electrode is changed. 
     As shown in  FIGS. 4A and 4B , as the film thickness of the IDT electrode is reduced, the acoustic velocity becomes faster and the fractional band width increases (the electromechanical coupling coefficient increases). To support a higher frequency of the acoustic wave device and achieve a wider band, about 10% or more is preferably ensured as a fractional band width of a resonator, and about 6000 m/s to about 7000 m/s is preferably ensured as the acoustic velocity. Also from this, as the high acoustic velocity member  20 , the acoustic velocity of the transversal bulk wave on the order of about 7500 m/s or higher is preferably ensured to trap an acoustic wave with an acoustic velocity of about 6000 m/s to about 7000 m/s in the IDT electrode and the piezoelectric layer. 
       FIG. 5A  is a graph showing a relationship between the propagation angle ψ and acoustic velocity in a c-face 4H—SiC (silicon carbide) high acoustic velocity member. Also,  FIG. 5B  is a graph showing a reverse-velocity surface in the c-face 4H—SiC high acoustic velocity member. In  FIG. 5A , the relationship between the propagation angle ψ and acoustic velocity of a “longitudinal bulk wave”, a “fast transversal bulk wave”, and a “slow transversal bulk wave” of the high acoustic velocity member  20  having the c face (0, 0, ψ) as a principal surface is shown. 
     To make the acoustic velocity of the transversal bulk wave of the high acoustic velocity member  20  faster than the acoustic velocity of the transversal bulk wave coupling to the longitudinal leaky SAW (LLSAW) propagating through the piezoelectric layer  10 , the acoustic velocity of the “slowest transversal bulk wave” of the high acoustic velocity member  20  is preferably faster than the acoustic velocity of the longitudinal leaky SAW (LLSAW). 
     Here, since the acoustic velocity of the longitudinal leaky SAW (LLSAW) is about 6000 m/s to about 7500 m/s, the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is preferably equal to or higher than about 7500 m/s, for example. However, as shown in  FIG. 5A , the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  does not become equal to or higher than about 7500 m/s in the entire angle range of the propagation angle ψ. 
     Thus, when Euler angles of the 4H-type or 6H-type crystal polytype silicon carbide included in the high acoustic velocity member  20  are denoted as (0, 0, ψ), no suitable condition for ψ is provided to widen the band while ensuring more favorable Q characteristics. 
       FIG. 6A  is a graph showing a simulation result of a relationship between the propagation angle ψ and acoustic velocity in an a-face 4H—SiC high acoustic velocity member. Also,  FIG. 6B  is a graph showing a reverse-velocity surface in the a-face 4H—SiC high acoustic velocity member. In  FIG. 6A , the relationship between the propagation angle ψ and acoustic velocity of a “longitudinal bulk wave”, a “fast transversal bulk wave”, and a “slow transversal bulk wave” of the high acoustic velocity member having the a face (90, 90, ψ) as a principal surface is shown. 
     Here, since the acoustic velocity of the longitudinal leaky SAW (LLSAW) is about 6000 m/s to about 7500 m/s, the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is desirably set equal to or higher than about 7500 m/s, for example. With this, from  FIG. 6A , a suitable range in which the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is equal to or higher than about 7500 m/s is any of 20°≤ψ≤40°, 140°≤ψ≤160°, 200°≤ψ≤220°, and 320°≤ψ≤340°, for example. 
     With this, the acoustic velocity of the transversal bulk wave of the high acoustic velocity member  20  can be made equal to or higher than approximately 7500 m/s, thus allowing trapping of the longitudinal leaky SAW with higher velocity (to about 7500 m/s). Thus, a wider-band acoustic wave device can be provided with high accuracy. 
       FIG. 7A  is a graph depicting a simulation result of a relation between the propagation angle ψ and acoustic velocity in a multilayer structure including an LT piezoelectric layer and an m-face 4H—SiC high acoustic velocity member. Also,  FIG. 7B  is a graph showing a simulation result of a reverse-velocity surface in the multilayer structure including the LT piezoelectric layer and the m-face 4H—SiC high acoustic velocity member. In  FIG. 7A , the relationship between the propagation angle ψ and acoustic velocity of a “longitudinal bulk wave”, a “fast transversal bulk wave”, and a “slow transversal bulk wave” of the high acoustic velocity member  20  having the m face (0, 90, ψ) as a principal surface is shown, the relationship being determined by the above-described finite element solution simulation. 
     Here, since the acoustic velocity of the longitudinal leaky SAW (LLSAW) is about 6000 m/s to about 7500 m/s, the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is preferably set equal or higher than about 7500 m/s, for example. With this, from  FIG. 7A , a suitable range in which the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is equal to or higher than about 7500 m/s is any of 20°≤ψ≤40°, 140°≤ψ≤160°, 200°≤ψ≤220°, and 320°≤ψ≤340°, for example. 
     With this, the acoustic velocity of the transversal bulk wave of the high acoustic velocity member  20  can be made equal to or higher than approximately 7500 m/s, thus allowing trapping of the longitudinal leaky SAW with higher velocity (to about 7500 m/s), for example. Thus, a wider-band acoustic wave device can be provided with high accuracy. 
       FIG. 8A  is a graph showing a simulation result of a relationship between the propagation angle ψ and acoustic velocity in a multilayer structure including an LT piezoelectric layer and an r-face 4H—SiC high acoustic velocity member. Also,  FIG. 8B  is a graph showing a simulation result of a reverse-velocity surface in the multilayer structure including the LT piezoelectric layer and the r-face 4H—SiC high acoustic velocity member. In  FIG. 8A , the relationship between the propagation angle ψ and acoustic velocity of a “longitudinal bulk wave”, a “fast transversal bulk wave” is shown, and a “slow transversal bulk wave” of the high acoustic velocity member  20  having the r face (0, about 122.23, ψ) as a principal surface, the relationship being determined by the above-described finite element solution simulation. 
     Here, since the acoustic velocity of the longitudinal leaky SAW (LLSAW) is about 6000 m/s to about 7500 m/s, the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is preferably set equal or higher than about 7500 m/s, for example. With this, from  FIG. 8A , a suitable range in which the acoustic velocity of the “slow transversal bulk wave” of the high acoustic velocity member  20  is equal to or higher than about 7500 m/s is any of 20°≤ψ≤50°, 130°≤ψ≤160°, 200°≤ψ≤230°, and 310°≤ψ≤340°, for example. 
     With this, the acoustic velocity of the transversal bulk wave of the high acoustic velocity member  20  can be made equal to or higher than approximately 7500 m/s, thus allowing trapping of the longitudinal leaky SAW with higher velocity (to about 7500 m/s). Thus, a wider-band acoustic wave device can be provided with high accuracy. 
     1-3. Resonance Characteristics of Acoustic Wave Device 
       
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 LLSAW mode 
               
               
                   
                   
               
             
            
               
                   
                 IDT electrode 110 
                 Al 
               
               
                   
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                   
                 about 0.03 
               
               
                   
                 Piezoelectric layer 10 
                 LiTaO 3  (90°, 90°, 31°) 
               
               
                   
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                   
                 about 0.2 
               
               
                   
                 High acoustic 
                 4H-type crystal polytype silicon carbide 
               
               
                   
                 velocity member 20 
                 (c face, a face, m face, r face) 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 9A  is a graph in which a comparison is made in impedance characteristics between a multilayer structure including an LT piezoelectric layer, and a SiC high acoustic velocity member according to the present preferred embodiment and one according to Comparative Example 1.  FIG. 9B  is a graph in which a comparison is made in conductance characteristics between the multilayer structure including the LT piezoelectric layer, and the SiC high acoustic velocity member according to the present preferred embodiment and one according to Comparative Example 1. In  FIGS. 9A and 9B , results of impedance characteristics and conductance characteristics of the acoustic wave devices according to the present preferred embodiment and Comparative Example 1 determined by a finite element solution simulation are shown. 
     Here, the acoustic wave device according to the present preferred embodiment has a multilayer structure and parameters depicted in Table 1. As the high acoustic velocity member  20 , a 4H-type crystal polytype silicon carbide is used. On the other hand, in the acoustic wave device according to Comparative Example 1, the IDT electrode and the piezoelectric layer each have a structure similar to that of the acoustic wave device according to the present preferred embodiment, while a 3C-type silicon carbide is used as the high acoustic velocity member. 
     In the acoustic wave device according to the present preferred embodiment, it is discovered that trapping of the longitudinal leaky SAW (LLSAW) in the piezoelectric layer  10  and the IDT electrode  110  is maintained and, as depicted in  FIG. 9A , impedance characteristics with a resonance point and an anti-resonance point each with a large Q factor have been obtained. Also, as shown in  FIG. 9B , in the acoustic wave device according to the present preferred embodiment, it is discovered that favorable conductance characteristics have been obtained. 
     On the other hand, in the acoustic wave device according to Comparative Example 1, as shown in  FIGS. 9A and 9B , no steep peak appears in the impedance characteristics and the conductance characteristics and trapping of the longitudinal leaky SAW (LLSAW) in the piezoelectric layer and the IDT electrode is insufficient. Thus, it is determined that the longitudinal leaky SAW (LLSAW) is leaked to the high acoustic velocity member  20 . 
     That is, as can be seen from the comparison among the reverse-velocity surfaces in  FIG. 2A , the 4H-type and 6H-type crystal polytype silicon carbides having a transversal bulk wave acoustic velocity equal to or higher than about 7000 m/s can sufficiently trap the longitudinal leaky SAW (LLSAW). By contrast, the 3C-type has a transversal bulk wave acoustic velocity of about 4000 m/s, and is not sufficient to trap the mode. That is, it can be determined that, even among the same silicon carbides, one that can obtain a favorable Q factor in the longitudinal leaky SAW (LLSAW) mode is the 4H-type or 6H-type crystal polytype silicon carbide when used as the high acoustic velocity member  20 . 
       FIG. 10  is a graph in which a comparison is made in impedance characteristics between a multilayer structure including a piezoelectric layer, an intermediate layer, and a high acoustic velocity member according to Modified Example 2 and one according to Comparative Example 2. In  FIG. 10 , results of impedance characteristics of the acoustic wave devices according to Modified Example 2 and Comparative Example 2 are shown found by a finite element solution simulation. 
     Here, the acoustic wave device  1 B according to Modified Example 2 has a multilayer structure shown in  FIG. 3B . However, the functional layer  31  is not included. On the other hand, in the acoustic wave device according to Comparative Example 2, as for the IDT electrode, the piezoelectric layer, and the intermediate layer, the structure is similar to that of the acoustic wave device  1 B according to Modified Example 2. However, the material structure of the high acoustic velocity layer is different. Parameters of the acoustic wave devices according to Modified Example 2 and Comparative Example 2 used in this simulation are shown in Table 2. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 LLSAW mode 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Modified Example 2 
                   
               
               
                 IDT electrode 110 
                 Al 
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.04 
               
               
                 Piezoelectric layer 10 
                 LiNbO 3  (90°, 90°, 40°) 
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.2 
               
               
                 Intermediate layer 32 
                 SiO 2   
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.05 
               
               
                 High acoustic 
                 4H-type crystal polytype silicon carbide 
               
               
                 velocity member 20 
                 (c face) 
               
               
                 Comparative Example 2 
               
               
                 IDT electrode 
                 Al 
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.04 
               
               
                 Piezoelectric layer 
                 LiNbO3 (90°, 90°, 40°) 
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.2 
               
               
                 Intermediate layer 
                 SiO 2   
               
               
                   
                 Film thickness: IDT wavelength λ × 
               
               
                   
                 about 0.05 
               
               
                 High acoustic 
                 Diamond (0°, 0°, 0°) 
               
               
                 velocity member 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, in the acoustic wave device  1 B according to Modified Example 2, the material of the IDT electrode  110  is Al, the material of the piezoelectric layer  10  is LiNbO 3  (Euler angle notation (90°, 90°, 40°)), the material of the high acoustic velocity member  20  is a 4H-type crystal polytype silicon carbide (Euler angle notation (0°, 0°, 0°)), and the material of the intermediate layer  32  is SiO 2 . 
     On the other hand, in the acoustic wave device according to the comparative example, the materials of the IDT electrode, the piezoelectric layer, and the intermediate layer are similar to those in Modified Example 2, and the material of the high acoustic velocity member is diamond (Euler angle notation (0°, 0°, 0°)). 
     As evident from the impedance characteristics in  FIG. 10 , in the acoustic wave device according to Comparative Example 2 using diamond with extremely fast acoustic velocity (transversal bulk wave acoustic velocity: about 10000 m/s or higher) as the high acoustic velocity member, a minimum point (resonance point) and a maximum point (anti-resonance point) of impedance can be seen near 4 GHz, and it can be discovered that the LLSAW mode is trapped in the IDT electrode, the piezoelectric layer, and the intermediate layer. However, at the same time, the spurious mode having an acoustic velocity higher than the longitudinal bulk wave acoustic velocity of the longitudinal leaky SAW (LLSAW) is also trapped in the IDT electrode, the piezoelectric layer, and the intermediate layer, and thus many spurious modes occur in a band on a high frequency side from about 5 GHz. 
     By contrast, in the acoustic wave device  1 B according to Modified Example 2, the transversal bulk wave acoustic velocity of the 4H-type crystal polytype silicon carbide is relatively near the longitudinal leaky SAW (LLSAW). Thus, the spurious mode is leaked to the high acoustic velocity member  20  and a large spurious mode does not remain. That is, in the acoustic wave device  1 B according to Modified Example 2, it is possible to reduce spurious modes by the high acoustic velocity member  20  made of the 4H-type crystal polytype silicon carbide. 
     The transversal bulk wave acoustic velocity of diamond is extremely fast (equal to or higher than about 10000 m/s). This bulk wave with high acoustic velocity is preferable in view of viscosity losses but, on the other hand, induces the occurrence of spurious modes outside the band. This is because a high-order plate wave mode, which has an acoustic velocity higher than that of the longitudinal leaky SAW (LLSAW), is simultaneously trapped by diamond which is the high acoustic velocity member. In contrast, the transversal bulk wave acoustic velocity of the 4H-type or 6H-type crystal polytype silicon carbide is faster by approximately 10% than that of the longitudinal leaky SAW (LLSAW), and is slower than a spurious mode as described above. Thus, it is possible to leak the spurious mode to the high acoustic velocity member and reduce the occurrence of spurious modes. 
       FIG. 11A  is a graph showing changes in impedance characteristics when the film thickness of a piezoelectric layer in a multilayer structure including an LN piezoelectric layer, and a high acoustic velocity member is changed. In  FIG. 11A , impedance (resonance) characteristics when the film thickness of a piezoelectric layer made of LiNbO 3  is changed are shown. As shown in  FIG. 11A , it can be discovered that as the film thickness of the piezoelectric layer is increased, the spurious mode becomes closer to a basic mode. 
       FIG. 11B  is a graph showing a relationship between the film thickness of the piezoelectric layer in the multilayer structure including the LN piezoelectric layer, and the high acoustic velocity member, and spurious mode occurrence frequency. As shown in  FIG. 11B , as the film thickness of the piezoelectric layer is increased, spurious mode (resonant frequency)/basic mode (anti-resonant frequency) becomes smaller. In other words, as the film thickness of the piezoelectric layer is increased, a gap between the occurrence frequency of the spurious mode and the occurrence frequency of the basic mode is decreased. When the film thickness of the piezoelectric layer is increased, in the long run, a spurious mode enters the pass band of the acoustic wave device, thus deteriorating bandpass characteristics. 
     With this, as spurious mode (resonant frequency)/basic mode (anti-resonant frequency) to prevent spurious modes from entering the pass band of the acoustic wave device, approximately 1.2 or more is preferably ensured. When the acoustic velocity of the principal mode of the longitudinal leaky SAW (LLSAW) is set at approximately 6500 m/s to 6700 m/s, a spurious mode having an acoustic velocity equal to or higher than approximately 8000 m/s (acoustic velocity of the principal mode of LLSAW×1.2), for example, is to be leaked. Thus, the transversal bulk wave acoustic velocity of the high acoustic velocity member  20  as a support substrate is preferably equal to or lower than about 8000 m/s, for example. 
     With this, for example, a high-order plate wave mode having a velocity higher than about 8000 m/s and causing an unwanted spurious mode to occur is leaked. Thus, it is possible to selectively trap a specific longitudinal leaky SAW. Therefore, an acoustic wave device with excellent Q characteristics and with reduced spurious modes can be provided. This means that the acoustic wave devices  1 ,  1 A, and  1 B according to the present preferred embodiment can have features of a small size, low loss, and high selectivity in configuring a high-frequency filter, multiplexer, and RF front-end circuit even in a high-frequency band equal to or higher than about 3 GHz. 
     OTHER PREFERRED EMBODIMENTS 
     While the acoustic wave devices according to preferred embodiments of the present invention have been described in the foregoing, the present invention is not limited to the above-described preferred embodiments. The present invention also includes other preferred embodiments achieved by combining any of the components or features of the above-described preferred embodiments, a modified example obtained by making various modifications conceived by a person skilled in the art to the above-described preferred embodiments in a range not deviating from the gist of the present invention, and various devices having incorporated therein an acoustic wave device according to a preferred embodiment of the present invention. 
     Note that while an X cut is exemplarily described as the cut angle of the piezoelectric layer emphatically exciting the longitudinal leaky SAW (LLSAW) mode in the above-described preferred embodiment, this is not restrictive. Also, it is evident that the same is true for the leaky surface acoustic wave (LSAW) mode. 
     Preferred embodiments of the present invention are widely used for various electronic devices and communication devices. Examples of electronic devices include high-frequency filters, multiplexers, and RF frontend components. 
     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.