Elastic wave device

Abstract

An IDT electrode (11) is disposed on a piezoelectric substrate (10). The IDT electrode has a first bus bar electrode (16) and a second bus bar electrode (18) that face each other, a first electrode finger (17) extending from the first bus bar electrode (16), and a second electrode finger (19) extending from the second bus bar electrode (18). The front ends of the first bus bar electrode (16) and the second electrode finger (19) face each other with a gap (15) therebetween, and the bottom surfaces (31) of the first bus bar electrode (16) and the second electrode finger (19) face each other with a first gap (100) therebetween. On the top surface (30) side, there is a portion where the first bus bar electrode (16) and the second electrode finger (19) face each other with a second gap (101) having a length shorter than that of the first gap (100). A cavity is provided in a second region (302) that is sandwiched between the piezoelectric substrate (10) and the first bus bar electrode (16) or the second electrode finger (19) in a first region (301) that is sandwiched between a first side surface (33) and a second side surface (34).

Description

Technical Field

[0001] The present disclosure relates to an elastic wave device. Background Art

[0002] There is known a surface acoustic wave element having a piezoelectric substrate and at least one IDT electrode disposed on the piezoelectric substrate, as described in Japanese Patent Application Laid-Open No. 2014-143657 (Patent Document 1). The IDT electrode includes a pair of first comb-shaped electrodes and a second comb-shaped electrode. Each comb-shaped electrode has a bus bar electrode and a plurality of electrode fingers extending from the bus bar electrode and having tips located at positions with a gap relative to the other comb-shaped electrode.

[0003] Conventionally, in a surface acoustic wave element, if an electric field is generated in the gap, a distortion current is generated due to the non-linear characteristics of the piezoelectric substrate, and sometimes the electrical characteristics deteriorate due to the distortion wave based on the distortion current.

[0004] Therefore, for example, Patent Document 1 describes a technique in which an insulating film having a dielectric constant smaller than that of the piezoelectric substrate is disposed between the bus bar electrode and the piezoelectric substrate. It is disclosed that, thereby, the electric field is concentrated in the insulating film, the electric field generated in the gap is weakened, and the distortion wave is suppressed.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-143657 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In the elastic wave device described in Patent Document 1, when it is desired to sufficiently reduce the electric field generated in the gap, it is necessary to increase the thickness of the insulating film to concentrate more electric fields. However, in this case, there is a problem that the electromechanical coupling coefficient decreases and the characteristics of the elastic wave device deteriorate.

[0010] In view of the above problems, an object of the present invention is to simultaneously achieve suppression of the distortion wave by reducing the electric field generated in the gap and suppression of deterioration of the characteristics.

[0011] Means for Solving the Problems

[0012] The elastic wave device related to the present disclosure is characterized in that it includes a piezoelectric substrate and an IDT electrode provided on the piezoelectric substrate. The IDT electrode has a pair of bus bar electrodes facing each other and a plurality of electrode fingers. One end of the plurality of electrode fingers is connected to any one of the pair of bus bar electrodes, and the other end is opposed to the other of the pair of bus bar electrodes with a gap therebetween. On at least one of the opposing surfaces of the bus bar electrodes and the opposing surfaces of the electrode fingers that oppose each other across the gap, a convex portion that protrudes from a portion away from the piezoelectric substrate toward the gap side is provided, such that the interval of the gap is narrower at the portion away from the piezoelectric substrate than at the portion in contact with the piezoelectric substrate, and there is a cavity between the convex portion and the piezoelectric substrate.

[0013] Advantages of the Invention

[0014] According to the present disclosure, while suppressing deterioration of characteristics, it is possible to reduce the intensity of the electric field generated in the gap in the piezoelectric substrate to suppress distortion waves. Description of the Drawings

[0015] Figure 1 is a top view showing an elastic wave device according to an embodiment of the present invention.

[0016] Figure 2 is a schematic cross-sectional view of an elastic wave device according to the first embodiment.

[0017] Figure 3 is for explaining Figure 1 the operation of the elastic wave device.

[0018] Figure 4 is a schematic cross-sectional view showing a modification of the elastic wave device according to the first embodiment.

[0019] Figure 5 is a diagram for explaining the manufacturing method of the elastic wave device according to the first embodiment.

[0020] Figure 6 is a diagram for explaining the manufacturing method of the elastic wave device according to the first embodiment.

[0021] Figure 7 is a schematic cross-sectional view of an elastic wave device according to the second embodiment.

[0022] Figure 8 is a schematic cross-sectional view of an elastic wave device according to the third embodiment. Detailed Embodiments

[0023] Hereinafter, an example of a mode in which the present invention is implemented will be described. However, the following embodiments are merely illustrative, and the present invention is not limited to the following embodiments at all.

[0024] In addition, in each of the drawings referred to in the embodiments and the like, components that are substantially the same in function are referred to by the same reference numerals. The drawings referred to in the embodiments and the like are schematically drawn, and there are cases where the ratio of the dimensions of the objects depicted in the drawings, etc., is different from the ratio of the dimensions of the actual objects. Moreover, there are also cases where the ratio of the dimensions of the objects is different between the drawings. The ratio of the dimensions of specific objects should be judged with reference to the following description.

[0025] In addition, in the description of this specification and the claims, regarding the elastic wave device, any direction can be regarded as the upper or lower direction. However, for convenience hereinafter, an orthogonal coordinate system xyz is defined, and it is assumed that the positive side in the z - direction ( Figure 2 the upper side of the paper surface) is regarded as the upper direction, and the negative side in the z - direction ( Figure 2 the lower side of the paper surface) is regarded as the lower direction, and terms such as upper, lower, above, and below are used.

[0026] Here, the terms "top surface" and "bottom surface" are referred to as terms for referring to any surface of the IDT electrode 11. Specifically, the surface of the IDT electrode 11 on the piezoelectric substrate 10 side is regarded as the bottom surface, and the surface of the IDT electrode 11 opposite to the bottom surface is regarded as the top surface, and terms such as bottom surface and top surface are used. If the above - mentioned definitions of upper and lower directions are used, it can be paraphrased as the top surface is above the bottom surface, and the bottom surface is below the top surface. Moreover, the term "side surface" refers to the surface of the IDT electrode 11 sandwiched between the top surface and the bottom surface. In other words, the surface of the IDT electrode 11 that does not belong to the top surface and the bottom surface is defined as the side surface.

[0027] (First Embodiment)

[0028] Figure 1 is a top view of the elastic wave device 1 according to an embodiment of the present invention. In addition, the elastic wave device 1 according to an embodiment of the present invention is, for example, a surface acoustic wave device. As Figure 1 shown, the elastic wave device 1 has a piezoelectric substrate 10. The piezoelectric substrate 10 contains lithium tantalate. In addition, the piezoelectric substrate 10 may also contain a piezoelectric single crystal other than lithium tantalate, such as lithium niobate, and an appropriate piezoelectric ceramic.

[0029] As Figure 1As shown, the piezoelectric substrate 10 has a main surface 10a, and an IDT electrode 11 is provided on the main surface 10a. By applying an alternating voltage to the IDT electrode 11, elastic waves are excited. Reflectors 14 including a reflector 14a and a reflector 14b are arranged on both sides of the IDT electrode 11 in the elastic wave propagation direction (x direction). The reflector 14a and the reflector 14b include a plurality of reflecting electrode fingers extending in a direction (y direction) orthogonal to the elastic wave propagation direction (x direction) and parallel to each other.

[0030] The IDT electrode 11 has a first bus bar electrode 16 and a second bus bar electrode 18 that face each other. Moreover, the IDT electrode 11 has a plurality of first electrode fingers 17 with one end connected to the first bus bar electrode 16 and extending toward the second bus bar electrode 18, and a plurality of second electrode fingers 19 with one end connected to the second bus bar electrode 18 and extending toward the first bus bar electrode 16. The first bus bar electrode 16 and the first electrode fingers 17 form a first comb-shaped electrode 12, and the second bus bar electrode 18 and the second electrode fingers 19 form a second comb-shaped electrode 13. In addition, hereinafter, there are also cases where the first comb-shaped electrode 12 and the second comb-shaped electrode 13 are only referred to as comb-shaped electrodes, and sometimes they are not distinguished.

[0031] The plurality of first electrode fingers 17 and the plurality of second electrode fingers 19 are interlaced with each other. That is, in a pair of comb-shaped electrodes, the plurality of first electrode fingers 17 and the plurality of second electrode fingers 19 are arranged to be interlaced with each other. However, it is not necessary for all of the plurality of first electrode fingers 17 and the plurality of second electrode fingers 19 to be interlaced, as long as a part of them is in an interlaced form.

[0032] Moreover, the front end portions of the first bus bar electrode 16 and the plurality of second electrode fingers 19 or the front end portions of the second bus bar electrode 18 and the plurality of first electrode fingers 17 face each other with a gap 15 therebetween in a direction ( Figure 1 in the x direction) orthogonal to the elastic wave propagation direction ( Figure 1 in the y direction).

[0033] Here, the first bus bar electrode 16 and the second bus bar electrode 18 may also have dummy electrode fingers extending toward the opposing bus bar electrode. In this case, it is hereinafter assumed that the dummy electrode fingers are included in the bus bar electrode for explanation. Even if the dummy electrode fingers and the bus bar electrode are treated as different things, since the effects achieved by the present invention are the same, separate explanations and interpretations regarding the dummy electrode fingers are omitted.

[0034] The IDT electrode 11 includes a laminated metal film in which a plurality of metal layers are laminated. As the metal, for example, Al, W, Mo, Ta, Hf, Cu, Pt, Ti, Au, Ag, Ni, Zn, Cr, or an alloy mainly composed of them can be cited. In addition, the IDT electrode may also include a single metal layer.

[0035] The distance p between the centers of adjacent first electrode fingers 17 in the x direction is defined as the IDT electrode pitch p. Here, the thickness of the piezoelectric substrate 10 is preferably 2p or less. In this case, the piezoelectric substrate 10 may be laminated on the support substrate. In this way, when the thickness of the piezoelectric substrate 10 is 2p or less, the effects of improving the Q value and coupling coefficient of the surface acoustic wave device are achieved.

[0036] In addition, the elastic wave device 1 may also have, in addition to the above, an adhesive layer interposed between the IDT electrode 11 and the reflector 14 and the piezoelectric substrate 10, or a protective layer covering the IDT electrode 11 and the reflector 14 on the main surface 10a of the piezoelectric substrate 10, etc. In Figure 1 the wiring and pads for inputting and outputting signals to and from the IDT electrode 11 are omitted from the illustration.

[0037] Figure 2 is a schematic cross-sectional view of the elastic wave device according to the first embodiment. Specifically, it is a simplified cross-sectional view when the IDT electrode 11 shown in Figure 1 is cut along the line A-A. Here, Figure 2 (a), (b), and (c) in

[0038] refer to elastic wave devices having the same structure, but the structure and regions of the elastic wave device according to the first embodiment described later are described separately in each figure.

[0039] As Figure 2As shown in (a), the first busbar electrode 16 of the first comb-shaped electrode 12 and the plurality of second electrode fingers 19 of the second comb-shaped electrode 13 on the piezoelectric substrate 10 face each other with a gap 15 therebetween. In addition, the gap 15 in this specification means the void where the material of the IDT electrode 11 does not exist between the first busbar electrode 16 and the second electrode finger 19. Further, in the first busbar electrode 16 and the second electrode finger 19, the surface on the piezoelectric substrate 10 side is defined as the bottom surface 31, and the surface opposite to the bottom surface 31 is defined as the top surface 30. Moreover, the surface sandwiched between the top surface 30 and the bottom surface 31 is defined as the side surface. Otherwise stated, the surface of the IDT electrode 11 that does not belong to the top surface and the bottom surface is defined as the side surface.

[0040] Here, the side surface of the first busbar electrode 16 on the second electrode finger 19 side is defined as the first side surface 33, and the side surface of the second electrode finger 19 on the first busbar electrode 16 side is defined as the second side surface 34. In this case, the first side surface 33 and the second side surface 34 face each other with the gap 15 therebetween.

[0041] Next, as Figure 2 shown in (b), the portion where the first side surface 33 is in contact with the piezoelectric substrate 10 is defined as the first point 41, and the portion where the second side surface 34 is in contact with the piezoelectric substrate 10 is defined as the second point 42. When the portion between the first point 41 and the second point 42 is defined as the first gap 100, the first portion 43 and the second portion 44 face each other with a second gap 101 that is shorter in length than the first gap 100. In addition, the third portion 45 and the fourth portion 46 face each other with the first gap 100. Here, the third portion 45 and the fourth portion 46 are an example of the "first facing portion 47", and the first portion 43 and the second portion 44 are an example of the "second facing portion 48". In addition, the length of the gap in the present invention means the length along the direction (y direction) orthogonal to the elastic wave propagation direction (x direction).

[0042] Moreover, the first portion 43 is located on the top surface 30 side relative to the first point 41, and the second portion 44 is located on the top surface 30 side relative to the second point 42. Here, the first portion 43 and the third portion 45 are part of the first side surface 33, and the second portion 44 and the fourth portion 46 are part of the second side surface 34. In a simplified cross-sectional view, the corresponding portions can be either points or lines. In this Figure 2 figure, the portions corresponding to the first portion 43, the second portion 44, the third portion 45, and the fourth portion 46 are shown by lines.

[0043] In addition, the region sandwiched between the first side surface 33 and the second side surface 34 is referred to as the first region 301. More precisely regarding the first region 301, when assuming a first virtual line 200 connecting from the top surface 30 of the first bus bar electrode 16 to the top surface 30 of the second electrode finger 19, the region sandwiched by the first virtual line 200, the piezoelectric substrate 10, the first bus bar electrode 16, and the second electrode finger 19 is taken as the first region 301.

[0044] Here, if the first virtual line 200 is accurately described, the straight line connecting the part of the top surface of the first bus bar electrode 16 closest to the opposing second electrode finger 19 to the part of the top surface of the second electrode finger 19 closest to the opposing first bus bar electrode 16 is assumed to be the first virtual line 200. Since the top surface 30 of the IDT electrode 11 is mainly flat, the first virtual line 200 of the present application is substantially parallel to the piezoelectric substrate 10.

[0045] In addition, the cavity in the present invention refers to the part not filled with solid and liquid materials including insulators, etc. More specifically, for example, it is a state mainly configured with gases such as air.

[0046] Next, as shown in (c) of Figure 2 In the above-mentioned first region 301, a cavity is provided in the second region 302 sandwiched by the piezoelectric substrate 10 and the first bus bar electrode 16 or the second electrode finger 19. Specifically, the second region 302 is the region sandwiched by the first part 43 of the second opposing portion 48 and the piezoelectric substrate 10, and the region sandwiched by the second part 44 of the second opposing portion 48 and the piezoelectric substrate 10.

[0047] In addition, when a cavity is provided in a certain region, it is not necessary to provide a cavity in the entire region. It should be noted that the case where a cavity is provided only in a part of the region can also be included in the case of "providing a cavity".

[0048] Next, the part of the first bus bar electrode 16 protruding more toward the first gap 100 side than the first point 41, and the part of the second electrode finger 19 protruding more toward the first gap 100 side than the second point 42 are referred to as the convex portions 70. That is, the first part 43 and the second part 44 of the second opposing portion 48 are convex portions with respect to the third part 45 and the fourth part 46 of the first opposing portion 47. More precisely, draw a perpendicular line B to the piezoelectric substrate 10 passing through the first point 41. Similarly, draw a perpendicular line C to the piezoelectric substrate 10 passing through the second point 42. The parts of the first bus bar electrode 16 and the second electrode finger 19 existing in the region sandwiched by these two perpendicular lines B and perpendicular line C are referred to as the convex portions 70.

[0049] That is, voids are provided in at least one of the regions sandwiched between the convex portion 70 of the first bus bar electrode 16 and the piezoelectric substrate 10 and the regions sandwiched between the convex portion 70 of the second electrode finger 19 and the piezoelectric substrate 10.

[0050] That is, on the first side surface 33 of the first bus bar electrode 16 and the second side surface 34 of the second electrode finger 19 that face each other with a gap 15 therebetween, convex portions 70 that protrude from the portion away from the piezoelectric substrate 10 toward the gap 15 side are provided such that the distance from the portion away from the piezoelectric substrate 10 to the gap 15 is narrower than the distances from the portion away from the piezoelectric substrate 10 to the first point 41 and the second point 42 that are in contact with the piezoelectric substrate 10. Further, voids exist between the convex portions 70 and the piezoelectric substrate 10.

[0051] Here, the height from the main surface 10a of the piezoelectric substrate 10 to the piezoelectric substrate side surface of the convex portion 70 is defined as the first height 20. Generally, the first height 20 from the main surface 10a of the piezoelectric substrate 10 to the piezoelectric substrate side surfaces of the convex portion 70 from the first bus bar electrode 16 and the convex portion 70 from the second electrode finger 19 is constituted by substantially equal heights from the viewpoint of the manufacturing method. However, in the case where the first heights 20 are different, the shorter one is adopted as the first height 20.

[0052] Hereinafter, by using Figure 1 and Figure 3 , the mechanism by which the electrical distortion wave caused by the nonlinear characteristics of the surface acoustic wave element is generated and the principle of suppressing the distortion wave by the present invention are shown.

[0053] In Figure 1 , it is assumed that the first comb-shaped electrode 12 is in a state where the potential is higher than that of the second comb-shaped electrode 13. At this time, an electric field E is generated in the region between the first comb-shaped electrode 12 and the second comb-shaped electrode 13 of the piezoelectric substrate 10. That is, an electric field Ex along the elastic wave propagation direction (x direction) and an electric field Ey in the gap region along the direction orthogonal to the elastic wave propagation direction (y direction) are generated. Moreover, when such an electric field E is generated, a distortion current is generated due to the nonlinear characteristics of the piezoelectric substrate 10 including the piezoelectric body.

[0054] Specifically, when an electric field is applied to the piezoelectric substrate, a distortion current corresponding to the electric field flows due to the second-order nonlinear characteristics of the dielectric constant of the piezoelectric substrate 10. Based on this mechanism, in an actual surface acoustic wave element, an electric field is excited inside the piezoelectric crystal by the IDT electrode formed on the surface of the piezoelectric crystal, and the electric field has a component in the direction parallel to the main surface 10a of the piezoelectric substrate 10 and a component in the perpendicular direction (depth direction). For this electric field, distortion currents caused by the nonlinear characteristics of the anisotropic dielectric constant are generated respectively.

[0055] Here, if we focus on the distortion current Ix in the x-direction caused by the electric field Ex, regarding the distortion current Ix flowing between the first electrode fingers 17, since the distortion current Ixin flowing into the first electrode finger 17 and the distortion current Ixout flowing out from the first electrode finger 17 are in opposite directions in the y-direction, they cancel each other out. Therefore, the distortion wave based on the distortion current Ix is basically not output to the outside of the surface acoustic wave device 1. Similarly, even in the second electrode fingers 19, the distortion current Ix cancels each other out. Therefore, the distortion current Ix in the x-direction of the IDT electrode 11 cancels each other out and is not output to the outside of the surface acoustic wave device 1 as a distortion wave.

[0056] On the other hand, if we focus on the electric field Ey in the y-direction, the distortion currents Iy caused by the electric field Ey in the gap 15 are in the same direction, so these distortion currents Iy do not cancel each other out.

[0057] In this way, it can be considered that the distortion current Iy that does not cancel each other out and remains becomes one of the causes of the generation of the distortion wave in the gap 15.

[0058] On the other hand, use Figure 3 to illustrate the principle of suppressing the distortion wave by the present invention. Figure 3 is Figure 1 A cross-sectional view when the IDT electrode 11 shown in

[0059] is cut along the A-A line, (a) is a cross-sectional view of a conventional surface acoustic wave device, and (b) is a cross-sectional view of the surface acoustic wave device according to the present invention. Figure 3 As shown in (a) of

[0060] in a general surface acoustic wave device, the length of the gap is generally kept the same from the bottom surface to the top surface. Therefore, an electric field Ey with an equal penetration depth (length in the z-direction) is generated between the gaps 15. In addition, the penetration depth refers to the length from the minimum value to the maximum value of the z-axis component in an arbitrary electric field Ey. In the piezoelectric substrate 10, the more on the upper surface side of the piezoelectric substrate 10 closer to the electrode fingers, the more electric field can pass through, so the electric field Ey is larger on the upper surface side of the piezoelectric substrate 10. Figure 3 On the other hand, as shown in (b) of Figure 3is shallow in (a), and the electric field 5 generated between the second gaps 101 can hardly reach the piezoelectric substrate 10.

[0061] As a result, although there is an influence of the distortion wave caused by the electric field generated between the first gaps 100, it is possible to make it less susceptible to the influence of the distortion wave caused by the electric field generated between the second gaps 101, and the total amount of the electric field that can reach the piezoelectric substrate is reduced. Accordingly, a reduction in the influence of the distortion wave can be achieved.

[0062] Based on these characteristics, compared with the elastic wave device of (a) of Figure 3 in the elastic wave device of (b) of Figure 3 as one mode of the embodiment of the present invention, an effect of reducing the electric field Ey generated in the piezoelectric substrate 10 can be exerted.

[0063] Moreover, even if it is assumed that the electric field generated on the side surface of the first bus bar electrode 16 and the side surface of the second electrode finger 19 facing each other across the second gap 101 can reach the piezoelectric substrate 10, by passing through the cavity filled with air having a relatively low dielectric constant disposed in the second region, a reduction effect of the electric field intensity can also be obtained. As described above, it can be considered that the electric field Ey generated in the gap is one of the causes of the generation of the distortion wave. Therefore, by weakening the intensity of the electric field Ey, the distortion wave is also suppressed.

[0064] Here, the electric field generated in the gap means, in detail, that an electric field is generated in the region directly below the gap in the piezoelectric substrate.

[0065] Furthermore, based on the relationship between the length defined by the second gap 101 and the length defined by the first height 20, a further reduction in the electric field applied to the piezoelectric substrate can be achieved. Specifically, by making the value of the distance of the first height 20 greater than the value of the length of the second gap 101, a further reduction in the electric field applied to the piezoelectric substrate can be achieved. As described above, the penetration depth of the electric field between the gaps is approximately equal to the length of the gap. Therefore, by making the value of the distance of the first height 20 greater than the value of the length of the second gap 101, an effect is achieved that the electric field generated in the convex portion 70 hardly reaches the piezoelectric substrate.

[0066] (Modification of the First Embodiment)

[0067] Hereinafter, a modification of the first embodiment will be described.

[0068] Regarding the elastic wave device according to this modification, the description of the same structure as that of the elastic wave device 1 according to the first embodiment will be omitted, and the description will be centered on the different parts.

[0069] The modification 1 of the first embodiment is shown in Figure 4(a). As shown in the figure, the sides of the first bus bar electrode 16 and the second electrode finger 19 facing each other across the second gap 101 are not necessarily parallel and may be inclined. Even in this case, the same effects as those of the first embodiment can be obtained.

[0070] In this modification 1, the length of the gap does not continuously decrease from the piezoelectric substrate 10 toward the top surface 30 (the first virtual line 200) of the IDT electrode 11, but decreases stepwise. Additionally, the stepwise decrease in the length of the gap means that the portions of the first bus bar electrode 16 and the second electrode finger 19 protruding into the gap portion have a stepped shape, and the length changes unevenly.

[0071] Furthermore, in the case where there are multiple gaps shorter than the first gap 100, the gap with the shortest length is defined as the second gap 101. Moreover, in the first side surface and the second side surface, the first portion 43 and the second portion 44 facing each other across the second gap 101 correspond to the second facing portion 48.

[0072] Next, as shown in Figure 4 (b) of the second modification of the first embodiment, it is not necessary for the entire top surface 30 of the IDT electrode 11 including the first bus bar electrode 16 and the second electrode finger 19 to be parallel to the piezoelectric substrate 10, and it may be partially inclined. Viewed from another perspective, in the IDT electrode 11, the shape is not limited to a fixed distance from the main surface 10a of the piezoelectric substrate 10 to the top surface 30 of the IDT electrode 11. Here too, the gap decreases stepwise, and the second facing portion 48 is equal to the first portion 43 and the second portion 44, which are the side surfaces on the top surface side among the two parts divided by the first side surface and the second side surface. Even in this case, the same effects as those of the first embodiment can be obtained.

[0073] In addition, in Modification 2, both the portion 45 of the second facing portion 48 in the first side surface 33 that is closest to the piezoelectric substrate 10 and the portion 46 of the second facing portion 48 in the second side surface 34 that is closest to the piezoelectric substrate 10 are exposed in the cavity. The intensity of the electric field generated between the gaps becomes stronger in inverse proportion to the length between the gaps. Therefore, the intensity of the electric field from the second facing portion 48 becomes higher among the intensities of the electric fields generated between the gaps. Moreover, the electric field generated between the portions closer to the piezoelectric substrate 10 is more likely to reach the piezoelectric substrate 10 than the electric field generated between the portions farther from the piezoelectric substrate 10. Therefore, by exposing the portion of the second facing portion 48 that is closest to the piezoelectric substrate 10 to the cavity filled with air having a relatively low dielectric constant, the effect of further enhancing the reduction effect of the electric field intensity is achieved. However, it is not necessary for both the portions of the second facing portion 48 in the first side surface 33 and the second side surface 34 that are closest to the piezoelectric substrate 10 to be exposed in the cavity, and it is sufficient if at least one of them is exposed.

[0074] Regarding the cavity in which both the portions of the second facing portion 48 in the first side surface 33 and the second side surface 34 that are closest to the piezoelectric substrate 10 are exposed, it may be the case like Modification 2 where the cavity coincides with the cavity disposed in the second region 302, or it may be the case where the cavity is different from the cavity disposed in the second region 302.

[0075] The modification 3 of the first embodiment is shown in Figure 4 (c). As shown in the figure, the shape may also be such that the length of the gap continuously becomes shorter from the piezoelectric substrate 10 toward the top surface 30 (the first virtual line 200) instead of being stepwise shorter. More specifically, the shape may also be such that the side surfaces of the first bus bar electrode 16 and the second electrode finger 19 facing each other across the gap are inclined. In addition, in Modification 3, the first side surface 33 except for the first point 41 becomes the first portion 43, and the second side surface 34 except for the second point 42 becomes the second portion 44. Even with such a configuration, similar to the first embodiment, since the penetration depth of the electric field between the gaps becomes shallower, the effect that the electric field between the gaps becomes difficult to reach the piezoelectric substrate is achieved.

[0076] Here, in Modification 3, both the portion where the first side surface 33 is in contact with the top surface 30 of the first bus bar electrode 16 and the portion where the second side surface 34 is in contact with the top surface 30 of the second electrode finger 19 are exposed in the cavity. As described above, by allowing an electric field that easily reaches the piezoelectric substrate 10 and has a strong electric field strength to pass through air with a low dielectric constant, the following effect is achieved, that is, the effect of further reducing the electric field strength is improved. However, it is not necessary for both the portion where the first side surface 33 is in contact with the top surface 30 of the first bus bar electrode 16 and the portion where the second side surface 34 is in contact with the top surface 30 of the second electrode finger 19 to be exposed in the cavity, and it is sufficient if at least one of them is exposed. In addition, this cavity does not necessarily have to coincide with the cavity disposed in the second region 302.

[0077] As Modification 4 of the first embodiment, instead of the shape in which the length of the gap continuously becomes shorter at all portions from the piezoelectric substrate 10 toward the top surface 30, it may be a shape in which the length continuously becomes shorter at a part of the portions from the piezoelectric substrate 10 toward the top surface 30 as shown in Figure 4 (d). Specifically, the length of the gap is kept fixed up to a point in the middle from the piezoelectric substrate 10 toward the top surface 30, and it has an arc shape such that the distance between the gaps continuously becomes shorter compared to the first gap 100 from this point toward the top surface 30. However, an example of the shape in which the length of the gap continuously becomes shorter is shown here, and it is not necessarily limited to the arc shape as shown in the figure.

[0078] Here, the first portion 43 in Modification 4 is the portion of the first side surface 33 shown by the curve, and in the second portion 44, it is also the portion of the second side surface 34 shown by the curve, and they are opposed with a gap having a length shorter than the first gap 100.

[0079] As in Figure 4 (d), even if it is a shape in which the length continuously becomes shorter at a part of the portions from the piezoelectric substrate 10 toward the top surface 30, the effect of the first embodiment that the electric field between the gaps becomes difficult to reach the piezoelectric substrate 10 can be achieved as well.

[0080] In the embodiments described above, among the first bus bar electrode 16 and the second electrode finger 19 disposed on the piezoelectric substrate 10, both have characteristic shapes, but it is not limited thereto.

[0081] For example, Modification 5 of the first embodiment is shown in Figure 4 (e). It may be as Figure 4Like the surface acoustic wave device shown in (e), it has the following shape, that is, among the opposed comb electrodes, only the first bus bar electrode 16 on one side has the convex portion 70, and the second electrode finger 19 on the other side does not have the convex portion 70. Even in this case, since the second gap 101 shorter than the first gap 100 is provided on the top surface 30 side, the effect that the electric field between the gaps becomes difficult to reach the piezoelectric substrate 10 can be obtained. Here, the mode in which the convex portion 70 is provided only on the first bus bar electrode 16 is illustrated, but the case where only the second electrode finger 19 has the convex portion 70 may also be used.

[0082] Modification 5 has a shape in which the length of the gap gradually becomes shorter from the piezoelectric substrate 10 toward the top surface 30 of the IDT electrode 11. Here, the second opposed portion 48 in Modification 5 will be described. First, in the first side surface 33 of the first bus bar electrode 16, the first portion 43 which is the side surface on the top surface side among the side surfaces divided into two parts becomes the second opposed portion 48. On the other hand, in the second side surface 34 of the second electrode finger 19, the second portion 44 which is opposed to the second opposed portion 48 of the first bus bar electrode 16 with the second gap 101 therebetween becomes the second opposed portion 48.

[0083] In other words, in the present invention, the electrodes opposed across the gap, for example, the first bus bar electrode 16 and the second electrode finger 19, may be line-symmetric or non-line-symmetric with respect to the perpendicular line passing through the midpoint of the first gap 100, and as long as the shape has the second gap 101 shorter than the first gap 100 on the top surface 30 side of the first bus bar electrode 16 or the second electrode finger 19, the desired effect can be obtained.

[0084] (Manufacturing method)

[0085] Next, an example of the manufacturing method of the surface acoustic wave device will be described using Figure 5 and Figure 6 A cross-sectional view of the manufacturing process of the surface acoustic wave device is shown in (a) to Figure 5 of Figure 5 and (d) of Figure 6 and (e) to Figure 6 and (g) of Figure 1 which are arranged in the order of the manufacturing process of the surface acoustic wave element and correspond to the cross-section at the A-A line of Figure 5 The manufacturing process proceeds from Figure 5 and (d) of Figure 6 and (e) to Figure 6 and (g) of

[0086] As shown in Figure 5 and (a), the piezoelectric substrate 10 is prepared.

[0087] Next, as shown in Figure 5 (b) of FIG. Figure 5 , on the main surface 10a of the piezoelectric substrate 10, a first electrode film 91 that is part of an electrode pattern including the IDT electrode 11, the reflector 14, etc. is formed using photolithography and etching techniques.

[0088] In addition, the method for forming the IDT electrode 11 and the reflector 14 is not limited to the photolithography process, and an etching process such as film formation, resist patterning, etching, and resist stripping can also be sequentially adopted. At this time, in order to suppress the corrosion of the IDT electrode 11, a thin passivation film (such as SiO2, SiN, etc.) can also be provided.

[0089] Next, as shown in Figure 5 (c) of FIG. Figure 5 , a sacrificial layer 92 is provided in the gap 15 of the first electrode film 91 generated up to Figure 5 (b) of FIG. Figure 5 . In addition, as the raw material of the sacrificial layer 92, for example, PI or ZnO is used. However, it is not limited to this material.

[0090] Then, as shown in Figure 5 (d) of FIG. Figure 5 , a resist pattern 93 is formed so that the length in the direction (y direction) orthogonal to the elastic wave propagation direction becomes shorter than that of the sacrificial layer 92.

[0091] Then, as shown in Figure 6 (e) of FIG. Figure 6 , a second electrode film 94 containing an electrode material (Al) is formed on the resist pattern 93 and the first electrode film 91 by methods such as evaporation and sputtering.

[0092] Next, as shown in Figure 6 (f) of FIG. Figure 6 , the resist pattern 93 is dissolved with a solvent, and the unnecessary electrode film attached thereto is removed.

[0093] Similarly, by also removing the sacrificial layer 92, it becomes a form as shown in Figure 6 (g) of FIG. Figure 6 in which the second electrode film 94 containing the electrode material (Al) is laminated on the first electrode film 91.

[0094] (Second Embodiment)

[0095] Figure 7 FIG. Figure 7 is a schematic cross-sectional view of the surface acoustic wave device 1 according to the second embodiment.

[0096] In the following description, components having substantially the same functions as those in the above-described first embodiment are referred to by the same reference numerals, and the description thereof is omitted.

[0097] Figure 7The (a), (b), and (c) refer to the same second embodiment. By separately illustrating the structure of the elastic wave device according to the second embodiment in each figure, the invention becomes clear.

[0098] First, in Figure 7 (a), on the piezoelectric substrate 10, the first bus bar electrode 16 of the first comb-shaped electrode 12 and a plurality of second electrode fingers 19 of the second comb-shaped electrode 13 face each other with a gap 15 therebetween at least in part. Moreover, in the first bus bar electrode 16 and the second electrode fingers 19, the surface on the piezoelectric substrate 10 side is defined as the bottom surface 31, and the surface opposite to the bottom surface 31 is defined as the top surface 30. In addition, when the surface connecting the top surface and the bottom surface is defined as the side surface, the side surface of the first bus bar electrode 16 on the second electrode finger 19 side is defined as the first side surface 33, and the side surface of the second electrode finger 19 on the first bus bar electrode 16 side is defined as the second side surface 34. At this time, the first side surface 33 and the second side surface 34 face each other with a gap therebetween.

[0099] The portion (second facing portion 48) where the first bus bar electrode 16 and the second electrode fingers 19 face each other with a second gap 101 therebetween is provided on the side closer to the top surface 30 than the first gap 100, and the length of the second gap 101 is shorter than the length of the first gap 100 which is the gap between the first point 41 and the second point 42.

[0100] In Figure 7 (b), there are a plurality of portions where the first bus bar electrode 16 and the second electrode fingers 19 face each other with a gap shorter than the length of the first gap 100, and these portions are provided on the side closer to the top surface 30 than the first gap 100. Therefore, there are a plurality of gaps 15 with lengths shorter than the first gap 100. In this case, the gap with the shortest length among the gaps where the first bus bar electrode 16 and the second electrode fingers 19 face each other is defined as the second gap 101. In addition, in a manner where the length of the gap gradually becomes shorter from the piezoelectric substrate 10 toward the first virtual line 200 as in this embodiment, the first portion 43 included in the side surface of the first bus bar electrode 16 and the second portion 44 included in the side surface of the second electrode fingers 19 that face each other with the second gap 101 therebetween are defined as the second facing portion 48. Referring to Figure 7 (b), in the first side surface 33 and the second side surface 34, the central side surface in the thickness direction of the piezoelectric substrate 10 among the side surfaces divided into three parts corresponds to the second facing portion 48 (43, 44). In addition, there is a first region 301 in the region sandwiched by the first side surface 33 and the second side surface 34.

[0101] Next, in Figure 7 (c), in the above-described first region 301, a cavity is provided in a second region 302 sandwiched by the piezoelectric substrate 10 and the first bus bar electrode 16 or the second electrode fingers 19.

[0102] Even in the second embodiment, a second gap 101 is provided closer to the top surface 30 than the first gap 100, where the length of the gap of the second gap 101 is shorter than that of the first gap 100 where the closest parts of the first bus bar electrode 16 and the second electrode finger 19 to the bottom surface 31 face each other. Therefore, as described above, by making the penetration depth of the electric field generated between the gaps shallower, the electric field that can reach the piezoelectric substrate can be reduced. In addition, even if the electric field generated on the side surfaces of the first bus bar electrode 16 and the second electrode finger 19 can reach the piezoelectric substrate 10, by passing through the voids filled with air having a relatively low dielectric constant disposed in the second region 302, the same effect as that of the first embodiment can be obtained, that is, the reduction effect of the electric field intensity can also be obtained.

[0103] As Figure 7 shown, on the piezoelectric substrate 10, an intermediate layer 50 and a semiconductor layer 60 equivalent to a support substrate are laminated on the main surface opposite to the main surface 10a provided with the IDT electrode 11. In addition, the semiconductor layer contains silicon, for example.

[0104] Moreover, it may be a mode in which the intermediate layer 50 is provided between the piezoelectric substrate 10 and the semiconductor layer 60. By providing the intermediate layer 50, the following effects can also be obtained, that is, the electromechanical coupling coefficient can be improved and the deterioration of characteristics can be suppressed.

[0105] In the convex portion 70 defined in the same manner as in the above-described first embodiment, the height from the interface between the intermediate layer 50 and the support substrate including the semiconductor layer 60 to the surface of the piezoelectric substrate 10 on the side of the convex portion 70 is defined as the second height 21. However, when the second height 21 is different from the convex portions from the first bus bar electrode 16 and the convex portions from the second electrode finger 19 from the interface between the intermediate layer 50 and the semiconductor layer 60, it is preferable to use the shorter one as the second height 21. In this case, when the value of the second height 21 is greater than the value of the length of the second gap 101, the following effect is achieved, that is, the reduction effect of the distortion wave from the semiconductor layer becomes further larger. This effect is due to the fact that the electric field that can reach the semiconductor layer is reduced by utilizing the property that the penetration depth of the electric field between the gaps is approximately equal to the length of the gaps.

[0106] Here, an example of a manufacturing method of the surface acoustic wave device according to the second embodiment will be described. The basic manufacturing process is based on the manufacturing method of the surface acoustic wave device according to the above-described first embodiment. First, the manufacturing process is carried out in sequence according to (a) of Figure 5 to (d) of Figure 5 and (e) of Figure 6 to (f) of Figure 6 in order.

[0107] Next, ifFigure 6 Upon completion of (f), again in the same manner as Figure 5 (d), a resist pattern 93 is formed on the second electrode film 94 and the sacrificial layer 92, and then a third electrode film is formed so as to straddle the second electrode film 94.

[0108] Next, in the same manner as the processes in Figure 6 (f) and Figure 6 (g), the resist pattern 93 and the sacrificial layer 92 are sequentially removed, resulting in a configuration where the third electrode film is stacked on the first electrode film 91 and the second electrode film 94 Figure 7 in such a form.

[0109] (Third Embodiment)

[0110] Figure 8 FIG. is a schematic cross-sectional view of the surface acoustic wave device 1 according to the third embodiment.

[0111] For example, as in the surface acoustic wave device Figure 8 shown, in addition to the same structure as in the first embodiment, an additional film 80 is provided in a region 300 sandwiched by the first virtual line 200, the piezoelectric substrate 10, the first bus bar electrode 16, and the second electrode finger 19. For example, the additional film 80 is configured on the main surface 10a of the piezoelectric substrate 10.

[0112] The additional film 80 includes a metal layer that is not electrically connected to the pair of comb-shaped electrodes. In this case, a part of the electric field generated between the gaps passes through the metal layer, so that the electric field that can reach the piezoelectric substrate 10 can be reduced compared to the state before the additional film is provided, thereby suppressing the distortion wave.

[0113] In addition, it may be the case where the additional film 80 is a dielectric having a dielectric constant lower than that of the piezoelectric substrate 10 and including at least one of, for example, tantalum oxide, hafnium oxide, and zirconium oxide. In this case, the electric field generated between the gaps tends to concentrate on the additional film, and the electric field generated between the gaps is less likely to reach the piezoelectric substrate 10 compared to the case where the additional film is not mounted. As a result, the electric field generated between the gaps can be reduced to suppress the distortion wave. Moreover, compared to the case where the additional film is a metal layer, since the parasitic capacitance between the gaps is smaller, the parasitic effect of the additional film on the surface acoustic wave device is less.

[0114] Furthermore, Patent Document 1 discloses a configuration in which an additional film 80 is disposed between the IDT electrode 11 and the piezoelectric substrate 10. However, in the third embodiment, the additional film 80 is not disposed between the IDT electrode 11 and the piezoelectric substrate 10. In other words, in this embodiment, the IDT electrode 11 is directly provided on the piezoelectric substrate 10. Generally, the region between the IDT electrode 11 and the piezoelectric substrate 10 affects the electrical characteristics. In Patent Document 1, an additional film is provided in this dominant region, while in this embodiment, no additional film is provided in this dominant region. Therefore, compared with Patent Document 1, the effect of being able to improve the electromechanical coupling coefficient and suppress the deterioration of electrical characteristics is achieved.

[0115] Here, an example of a method for manufacturing an elastic wave device according to the third embodiment will be described. The basic manufacturing process is based on the manufacturing method of the elastic wave device according to the first embodiment described above. First, the manufacturing process is carried out in accordance with Figure 5 (a) of Figure 5 (b).

[0116] Next, an additional film 80 is provided in the gap 15 of the first electrode film 91 generated up to Figure 5 (b). In addition, as the raw material of the additional film 80, for example, tantalum oxide, hafnium oxide, or zirconium oxide is used.

[0117] Next, as shown in Figure 5 (c), a sacrificial layer 92 is provided between the gaps 15 of the first electrode film 91 so as to cover the additional film 80, and the process is sequentially carried out up to Figure 5 (d) of Figure 6 (e) to Figure 6 (g), thereby forming a structure having the additional film 80 in the region 300 as Figure 8 such.

[0118] (Other modification examples, etc.)

[0119] The elastic wave device according to the present invention may also have a structure in which a plurality of elastic wave devices are disposed on a single piezoelectric substrate.

[0120] In addition, in the above-described embodiments and modification examples, an elastic wave device having a surface acoustic wave device is exemplified. However, the elastic wave in the above-described embodiments and modification examples refers to various types of elastic waves excited by the IDT electrode, including elastic waves propagating on the surface of the composite substrate or at the interface of multiple materials. The elastic waves also include, for example, Love waves, leaky waves, Rayleigh waves, pseudo SAWs, and plate waves.

[0121] The embodiments disclosed herein should be considered illustrative in all respects and not restrictive. The scope of the present disclosure is shown not by the description of the above embodiments but by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0122] Description of Reference Numerals

[0123] 1 Elastic wave device, 2 Electrode fingers, 3 Bus bar electrode, 5 Electric field, 10 Piezoelectric substrate, 10a Main surface of piezoelectric substrate, 11 IDT electrode, 12 First comb-shaped electrode, 13 Second comb-shaped electrode, 14a, 14b Reflectors, 15 Gap, 16 First bus bar electrode, 17 First electrode finger, 18 Second bus bar electrode, 19 Second electrode finger, 20 First height, 21 Second height, 30 Top surface, 31 Bottom surface, 33 First side surface, 34 Second side surface, 41 First point, 42 Second point, 43 First part, 44 Second part, 45 Third part, 46 Fourth part, 47 First opposed portion, 48 Second opposed portion, 50 Intermediate layer, 60 Semiconductor layer, 70 Protrusion, 80 Additional film, 91 First electrode film, 92 Sacrificial layer, 93 Resist pattern, 94 Second electrode film, 100 First gap, 101 Second gap, 200 First virtual line, 301 First region, 302 Second region.

Claims

1. An elastic wave device, comprising: a piezoelectric substrate; and an IDT electrode disposed on the piezoelectric substrate, wherein the IDT electrode has: a pair of bus bar electrodes facing each other; and a plurality of electrode fingers, one end of the plurality of electrode fingers is connected to any one of the pair of bus bar electrodes, and the other end is opposed to the other of the pair of bus bar electrodes with a gap therebetween, on at least one of the opposing surfaces of the bus bar electrodes facing each other across the gap and the opposing surfaces of the electrode fingers, a convex portion protruding from a portion away from the piezoelectric substrate toward the gap side is provided so that the gap is narrower at a portion away from the piezoelectric substrate than at a portion in contact with the piezoelectric substrate, a cavity exists between the convex portion and the piezoelectric substrate.

2. An elastic wave device, comprising: a piezoelectric substrate; and an IDT electrode disposed on the piezoelectric substrate, wherein the IDT electrode has: a first bus bar electrode and a second bus bar electrode facing each other; a first electrode finger extending from the first bus bar electrode toward the second bus bar electrode; and a second electrode finger extending from the second bus bar electrode toward the first bus bar electrode, when the surface on the piezoelectric substrate side is taken as the bottom surface, the surface opposed to the bottom surface is taken as the top surface, and the surface connecting the top surface and the bottom surface is taken as the side surface in the first bus bar electrode and the second electrode finger, the first side surface, which is the side surface of the first bus bar electrode on the second electrode finger side, and the second side surface, which is the side surface of the second electrode finger on the first bus bar electrode side, face each other with a gap therebetween, when the gap from the first point where the first side surface contacts the piezoelectric substrate to the second point where the second side surface contacts the piezoelectric substrate is taken as the first gap, the gap from the first portion of the first side surface closer to the top surface than the first point to the second portion of the second side surface closer to the top surface than the second point is a second gap having a length shorter than the length of the first gap, a cavity is provided in a second region sandwiched between the piezoelectric substrate and the first bus bar electrode or the second electrode finger in a first region sandwiched between the first side surface and the second side surface.

3. The elastic wave device according to claim 2, wherein when a first virtual line connecting the top surface of the first bus bar electrode to the top surface of the second electrode finger is assumed, the length of the gap gradually becomes shorter from the piezoelectric substrate toward the first virtual line, when the surfaces facing each other across the first gap are taken as the first opposing portions and the surfaces facing each other across the second gap are taken as the second opposing portions in the first side surface and the second side surface, at least one of the portion of the second opposing portion of the first side surface closest to the piezoelectric substrate and the portion of the second opposing portion of the second side surface closest to the piezoelectric substrate is exposed in the cavity.

4. The elastic wave device according to claim 2, wherein When assuming a first virtual line connecting the top surface of the first bus bar electrode to the top surface of the second electrode finger, the length of the gap continuously shortens from the piezoelectric substrate toward the first virtual line.

5. The elastic wave device according to claim 4, wherein at least one of a portion where the first side surface and the top surface are joined and a portion where the second side surface and the top surface are joined exposes a cavity.

6. The elastic wave device according to claim 3, wherein the second opposing portion becomes a convex portion with respect to the first opposing portion, when setting the height from the main surface on the IDT electrode side of the piezoelectric substrate to the surface on the piezoelectric substrate side of the convex portion as a first height, the value of the first height is greater than the value of the length of the second gap.

7. The elastic wave device according to claim 3 or 6, wherein the second opposing portion becomes a convex portion with respect to the first opposing portion, a support substrate containing a semiconductor is laminated on a surface layer of the piezoelectric substrate that faces the surface on which the IDT electrode is provided, when setting the height from the main surface on the piezoelectric substrate side of the support substrate to the surface on the piezoelectric substrate side of the convex portion as a second height, the value of the second height is greater than the value of the length of the second gap.

8. The elastic wave device according to any one of claims 2 to 6, wherein a support substrate containing a semiconductor is laminated on a surface layer of the piezoelectric substrate that faces the surface on which the IDT electrode is provided.

9. The elastic wave device according to claim 8, wherein the semiconductor contains a material mainly composed of silicon.

10. The elastic wave device according to any one of claims 2 to 6, wherein an additional film is provided in the first region on the piezoelectric substrate.

11. The elastic wave device according to claim 10, wherein the additional film contains a dielectric.

12. The elastic wave device according to claim 11, wherein the material of the additional film contains at least one of tantalum oxide, hafnium oxide, and zirconium oxide.

13. The elastic wave device according to claim 12, wherein the additional film contains a metal layer not electrically connected to the IDT electrode.

14. The elastic wave device according to any one of claims 2 to 6, wherein the first bus bar electrode has dummy electrode fingers extending toward the second bus bar electrode, any one of the dummy electrode fingers faces the second electrode finger across the gap.

15. The elastic wave device according to any one of claims 2 to 6, wherein among the adjacent first electrode fingers among the plurality of first electrode fingers, when setting the center-to-center distance in the elastic wave propagation direction as the IDT electrode pitch p, the piezoelectric substrate has a thickness of 2p or less.

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