ACOUSTIC WAVE DEVICE

An acoustic wave device includes a support, a piezoelectric layer including lithium niobate or lithium tantalate, and an interdigital transducer electrode including busbars and electrode fingers. An acoustic reflection portion overlaps a portion of the IDT electrode. d/p is about 0.5 or less. An intersecting region includes a central region and edge regions. Gap regions are located between the intersecting region and the busbars. At least one mass addition film is provided in at least one of the edge regions or the gap regions, where any two points, in an electrode finger facing direction, of a portion in which the mass addition film is located are first and second points, thicknesses of the mass addition film at at least a pair of the first point and the second point are different from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to acoustic wave devices.

2. Description of the Related Art

In the related art, an acoustic wave device has been widely used for a filter or the like of a mobile phone.

In recent years, as described in U.S. Pat. No. 10,491,192, an acoustic wave device using a bulk wave in a thickness shear mode has been proposed. In the acoustic wave device, a piezoelectric layer is provided on a support. A pair of electrodes are provided on the piezoelectric layer. The pair of electrodes face each other on the piezoelectric layer and are connected to different potentials. By applying an alternating-current (AC) voltage between the electrodes, the bulk wave in the thickness shear mode is excited.

In the acoustic wave device using the bulk wave in the thickness shear mode as described in U.S. Pat. No. 10,491,192, an unnecessary wave is generated at a frequency that is lower than a resonant frequency and is located near the resonant frequency. Therefore, there is a concern that electrical characteristics are deteriorated.

SUMMARY OF THE INVENTION

Example embodiments of the present invention provide acoustic wave devices that are each able to reduce or prevent an unnecessary wave at a frequency that is lower than a resonant frequency and is located near the resonant frequency.

An example embodiment of the present invention provides an acoustic wave device including a support including a support substrate, a piezoelectric layer on the support and including lithium niobate or lithium tantalate, and an interdigital transducer (IDT) electrode on the piezoelectric layer and including a pair of busbars and a plurality of electrode fingers. An acoustic reflection portion is provided at a position overlapping at least a portion of the IDT electrode in plan view viewed in a laminating direction of the support and the piezoelectric layer. Where d is a thickness of the piezoelectric layer and p is a center-to-center distance between the electrode fingers adjacent to each other, d/p is about 0.5 or less. Some electrode fingers among the plurality of electrode fingers are connected to one of the pair of busbars of the IDT electrode, remaining electrode fingers among the plurality of electrode fingers are connected to another of the pair of busbars, and the electrode fingers connected to the one of the pair of busbars and the electrode fingers connected to the another of the pair of busbars are interdigitated with each other. Where a direction in which the adjacent electrode fingers face each other is an electrode finger facing direction, a region in which the adjacent electrode fingers overlap each other when viewed in the electrode finger facing direction is an intersecting region. Where a direction in which the plurality of electrode fingers extend is an electrode finger extending direction, the intersecting region includes a central region and a pair of edge regions sandwiching the central region in the electrode finger extending direction, a pair of gap regions are located between the intersecting region and the pair of busbars. At least one mass addition film is provided in at least one of the pair of edge regions or the pair of gap regions, and where any two points, in the electrode finger facing direction, of a portion in which the mass addition film is located are a first point and a second point, thicknesses of the mass addition film at at least a pair of the first point and the second point are different from each other.

According to example embodiments of the present invention, it is possible to provide acoustic wave devices that are each able to reduce or prevent an unnecessary wave at a frequency that is lower than a resonant frequency and is located near the resonant frequency.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Hereinafter, the present invention will be clarified by describing example embodiments of the present invention with reference to the accompanying drawings.

Each of example embodiments described in the present specification is merely an example, and partial replacement or combination of the configurations can be made between different example embodiments.

FIG.1is a schematic plan view of an acoustic wave device according to a first example embodiment of the present invention.FIG.2is a schematic sectional view taken along line I-I inFIG.1.

As illustrated inFIG.1, the acoustic wave device10includes a piezoelectric substrate interdigital transducer (IDT) electrode11. As illustrated inFIG.2, the piezoelectric substrate includes a12support13and a piezoelectric layer14. In the present example embodiment, the support13includes a support substrate16and an insulating layer15. The insulating layer15is provided on the support substrate16. The piezoelectric layer14is provided on the insulating layer15. The support13may include only the support substrate16.

The piezoelectric layer14includes a first main surface14aand a second main surface14b. The first main surface14aand the second main surface14bface each other. Of the first main surface14aand the second main surface14b, the second main surface14bis located on the support13side.

As the material of the support substrate16, for example, a semiconductor such as silicon, a ceramic such as aluminum oxide, or the like can be used. As the material of the insulating layer15, for example, an appropriate dielectric such as silicon oxide or tantalum oxide can be used. The piezoelectric layer14is, for example, a lithium niobate layer such as a LiNbO3layer or a lithium tantalate layer such as a LiTaO3layer.

As illustrated inFIG.2, a recess portion is provided in the insulating layer15. The piezoelectric layer14is provided on the insulating layer15to close the recess portion. As a result, a hollow portion is provided. The hollow portion is a cavity portion10a. In the present example embodiment, the support13and the piezoelectric layer14are disposed such that a portion of the support13and a portion of the piezoelectric layer14face each other with the cavity portion10ainterposed therebetween. The recess portion in the support13may be provided over the insulating layer15and the support substrate16. Alternatively, the recess portion provided only in the support substrate16may be closed by the insulating layer15. The recess portion may be provided in the piezoelectric layer14. The cavity portion10amay be a through hole provided in the support13.

The IDT electrode11is provided on the first main surface14aof the piezoelectric layer14. The acoustic wave device10according to the present example embodiment is an acoustic wave resonator configured to use a bulk wave in a thickness shear mode. The acoustic wave device according to example embodiments of the present invention may be, for example, a filter device or a multiplexer having a plurality of acoustic wave resonators.

At least a portion of the IDT electrode11overlaps the cavity portion10aof the support13in plan view. In the present specification, “in plan view” means viewing in a laminating direction of the support13and the piezoelectric layer14, that is, in a direction from an upper side inFIG.2. InFIG.2, for example, the piezoelectric layer14side is an upper side of the support substrate16and the piezoelectric layer14.

As illustrated inFIG.1, the IDT electrode11includes a pair of busbars and a plurality of electrode fingers. Specifically, the pair of busbars include a first busbar26and a second busbar27. The first busbar26and the second busbar27face each other. The plurality of electrode fingers are, specifically, a plurality of first electrode fingers28and a plurality of second electrode fingers29. One end of each of the plurality of first electrode fingers28is connected to the first busbar26. One end of each of the plurality of second electrode fingers29is connected to the second busbar27. The plurality of first electrode fingers28and the plurality of second electrode fingers29are interdigitated with each other. The IDT electrode11may be made of a single metal film or a laminated metal film.

Hereinafter, the first electrode finger28and the second electrode finger29may be simply referred to as an electrode finger. The first busbar26and the second busbar27may be simply referred to as a busbar. In a case where a direction in which the plurality of electrode fingers extend is an electrode finger extending direction and a direction in which the electrode fingers adjacent to each other face each other is an electrode finger facing direction, the electrode finger extending direction and the electrode finger facing direction are orthogonal or substantially orthogonal to each other in the present example embodiment.

In the acoustic wave device10, in a case where d is a thickness of the piezoelectric layer14and p is a center-to-center distance between the adjacent electrode fingers, d/p is, for example, about 0.5 or less. As a result, the bulk wave in the thickness shear mode is suitably excited.

The cavity portion10aillustrated inFIG.2is an acoustic reflection portion. The acoustic reflection portion can effectively confine the energy of an acoustic wave to the piezoelectric layer14side. As the acoustic reflection portion, an acoustic reflection film such as, for example, an acoustic multilayer film described later may be provided.

Returning toFIG.1, the IDT electrode11includes an intersecting region F. The intersecting region F is a region in which the adjacent electrode fingers overlap each other when viewed in the electrode finger facing direction. The intersecting region F includes a central region H and a pair of edge regions. The pair of edge regions are, specifically, a first edge region E1and a second edge region E2. The first edge region E1and the second edge region E2face each other with the central region H interposed therebetween in the electrode finger extending direction. The first edge region E1is located on the first busbar26side. The second edge region E2is located on the second busbar27side.

The IDT electrode11includes a pair of gap regions. The pair of gap regions are located between the intersecting region F and the pair of busbars. The pair of gap regions are, specifically, a first gap region G1and a second gap region G2. The first gap region G1is located between the first busbar26and the first edge region E1. The second gap region G2is located between the second busbar27and the second edge region E2.

One mass addition film24is provided in each of the first edge region E1and the second edge region E2. Each mass addition film24has a strip shape. Each mass addition film24is provided on the first main surface14aof the piezoelectric layer14to cover the plurality of electrode fingers.

More specifically, as illustrated inFIG.2, each of the plurality of electrode fingers includes a first surface11a, a second surface11b, and a side surface11c. The first surface11aand the second surface11bface each other. The side surface11cis connected to the first surface11aand the second surface11b. Of the first surface11aand the second surface11b, the second surface11bis a surface on the piezoelectric layer14side. The mass addition film24is provided on the first surface11aof each electrode finger. The mass addition film24is continuously provided on the first surface11aand in a region between the electrode fingers on the piezoelectric layer. The mass addition film24also covers the side surface11cof each electrode finger.

The mass addition film24includes a third surface24aand a fourth surface24b. The third surface24aand the fourth surface24bface each other. Of the third surface24aand the fourth surface24b, the fourth surface24bis a surface on the piezoelectric layer14side. A thickness of a portion, of the mass addition film24, provided on the first surface11aof the electrode finger is a distance between the first surface11aof the electrode finger and the third surface24aof the mass addition film. A thickness of a portion, of the mass addition film24, provided in the region between the electrode fingers is a distance between the first main surface14aof the piezoelectric layer14and the third surface24aof the mass addition film24.

The thicknesses of the portions of the mass addition film24provided on the first surfaces11aof the electrode fingers are different from each other. Therefore, the thickness of the mass addition film24is not uniform. More specifically, in the present example embodiment, the thickness of the portion of the mass addition film24provided on the first surface11ais increased from one side to the other side in the electrode finger facing direction. The same applies to the mass addition film24provided in the second edge region E2. The thickness of the mass addition film24not being uniform is not limited to the above example.

In the present example embodiment, the mass addition film24is provided only in both the edge regions. The mass addition film24may be provided in the gap regions. At least one mass addition film24need only be provided in at least either the edge regions or the gap regions.

Hereinafter, in the electrode finger facing direction, any two points of a portion in which the mass addition film24is located are referred to as a first point O1and a second point O2. The first point O1and the second point O2illustrated inFIG.1or2are examples. A feature of the present example embodiment is that at least one mass addition film24is provided in at least either the edge regions or the gap regions, and the thicknesses of the mass addition films24at at least a set of the first point O1and the second point O2are different from each other. That is, in the present example embodiment, the thickness of the mass addition film24is not uniform in the electrode finger facing direction. As a result, it is possible to reduce or prevent an unnecessary wave at a frequency that is lower than a resonant frequency and is located near the resonant frequency. The details will be described later. In the following description, in a case where the unnecessary wave is simply described, unless otherwise specified, the unnecessary wave refers to the unnecessary wave that is generated at the frequency that is lower than the resonant frequency and is located near the resonant frequency.

As described above, in the first example embodiment, the thicknesses of the mass addition film24are different between portions provided on the first surfaces11aof respective electrode fingers. Therefore, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce the intensity of the unnecessary wave. Therefore, it is possible to reduce or prevent the unnecessary wave. The details of this advantageous effect will be described later with reference to a reference example.

The reference example is different from the first example embodiment in that a pair of mass addition films are provided over the pair of edge regions and the pair of gap regions, and the thicknesses of the pair of mass addition films are constant or substantially constant. Here, a plurality of acoustic wave devices of a plurality of reference examples in which the thicknesses of the mass addition films are different from each other are prepared. The phase characteristics of each prepared acoustic wave device are measured.

FIG.3is a diagram illustrating a relationship between the thickness of the mass addition film in the edge regions and the gap regions, and the phase characteristics.FIG.4is an enlarged view ofFIG.3near 4000 MHZ.

As illustrated inFIGS.3and4, it can be seen that the frequencies at which ripples caused by the unnecessary waves occur are different in a case where the thicknesses of the mass addition films are different. The same applies to a case where the mass addition film is provided only in the edge regions as in the first example embodiment. Further, the same applies to a case where the mass addition film is provided only in the gap regions. As illustrated inFIG.1, in the first example embodiment, one mass addition film24includes two or more portions having different thicknesses from each other in the electrode finger facing direction. More specifically, in the first example embodiment, the thicknesses of the mass addition film24are different between respective portions provided on the first electrode fingers28or the second electrode fingers29provided on the first surface11a. As a result, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce the intensity of the unnecessary wave. Therefore, it is possible to reduce or prevent the unnecessary wave.

As illustrated inFIG.1, the intersecting region F in the IDT electrode11of the acoustic wave device10includes a plurality of excitation regions C. More specifically, the excitation region C is a region between the centers of the adjacent electrode fingers. By applying an alternating-current (AC) voltage to the IDT electrode11, the acoustic waves are excited in the plurality of excitation regions C. On the other hand, in the acoustic wave device using a surface acoustic wave, the intersecting region is one excitation region.

Unlike the acoustic wave device using the surface acoustic wave, the acoustic wave device10using the bulk wave in the thickness shear mode has the same or substantially the same configuration as a configuration in which a plurality of resonators each including the excitation region C are connected in parallel. Therefore, in the acoustic wave device10, even in a case where the thickness of the mass addition film24is not uniform in the electrode finger facing direction, the waveforms of the frequency characteristics such as the phase characteristics are less likely to be disturbed. Therefore, in the first example embodiment, it is possible to reduce or prevent the unnecessary wave without the electrical characteristics being deteriorated.

Further, by providing the mass addition film24only in the edge regions, an amount of change in the fractional bandwidth can be reduced. As a result, it is possible to stabilize the electrical characteristics of the acoustic wave device10.

In the first example embodiment, the mass addition film24is provided, and thus a low acoustic velocity region is provided in each edge region. The low acoustic velocity region is a region in which the acoustic velocity is lower than the acoustic velocity in the central region H. In the electrode finger extending direction, the central region H and the low acoustic velocity region are disposed in this order from an inner side portion to an outer side portion of the IDT electrode11. As a result, a piston mode is provided, and a transverse mode can be reduced or prevented.

The acoustic wave device according to the present example embodiment uses the bulk wave in the thickness shear mode instead of the surface acoustic wave. In this case, even in a case where the mass addition film24is provided in each gap region, the piston mode can be suitably provided.

It is preferable to use, as the material of the mass addition film24, at least one dielectric selected from, for example, silicon oxide, tungsten oxide, niobium oxide, tantalum oxide, and hafnium oxide. In this case, the piston mode can be more reliably provided, and the transverse mode can be more reliably reduced or prevented.

In the first example embodiment, the thicknesses of the respective portions of the mass addition film24located on the adjacent electrode fingers are different from each other. The present invention is not limited thereto. For example, in a first modified example of the first example embodiment illustrated inFIG.5, a mass addition film34A includes a plurality of flat portions34c. In each flat portion34c, a third surface34ais flat. Therefore, in each flat portion34c, a distance between the first main surface14aof the piezoelectric layer14and the third surface34aof the mass addition film34A is constant or substantially constant. In addition, in each flat portion34c, the thicknesses of the respective portions, of the mass addition film34A, provided on the first surfaces11aof the adjacent electrode fingers are the same or substantially the same. On the other hand, the thicknesses of the respective portions, of the mass addition film34A, provided on the first surfaces11aof the electrode fingers are different from each other between the plurality of flat portions34c. Also in this case, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

Returning toFIG.2, in the first example embodiment, in a portion in which the mass addition film24and the electrode finger are laminated, the electrode finger and the mass addition film24are laminated in this order from the piezoelectric layer14side. That is, in the portion in which the mass addition film24and the electrode finger are laminated, the mass addition film24is provided on the first surface11aof the electrode finger. The order of laminating the mass addition film24and the electrode finger is not limited to the above example. In a second modified example of the first example embodiment illustrated inFIG.6, in a portion in which a mass addition film34B and the electrode finger are laminated, the mass addition film34B and the electrode finger are laminated in this order from the piezoelectric layer14side. That is, the mass addition film34B is provided between the second surfaces11bof the plurality of electrode fingers and the piezoelectric layer14.

In the present modified example, a third surface34eof the mass addition film34B is inclined. More specifically, the thickness of the mass addition film34B is increased from one side to the other side in the electrode finger facing direction. As a result, also in the present modification, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

The configuration in which the mass addition film34A includes the flat portion34cin the first modified example illustrated inFIG.5can also be applied to a configuration other than the first modified example. Similarly, the configuration in which the mass addition film34B is provided between the second surfaces11bof the plurality of electrode fingers and the piezoelectric layer14in the second modified example illustrated inFIG.6can be applied to a configuration other than the second modified example.

FIG.7is a plan view of an acoustic wave device according to a second example embodiment of the present invention.FIG.8is a sectional view taken along line II-II inFIG.7.

As illustrated inFIGS.7and8, the present example embodiment is different from the first example embodiment in that the mass addition film44is provided only in both the gap regions, and in the shape of the mass addition film44. Except for the above points, the acoustic wave device according to the present example embodiment has the same or substantially the same configurations as the acoustic wave device10according to the first example embodiment.

As illustrated inFIG.7, one of the pair of mass addition films44is provided in the first gap region G1. The other of the pair of mass addition films44is provided in the second gap region G2. The mass addition film44does not extend to an end portion on the busbar side in the gap region. The mass addition film44may be provided in the entirety or substantially the entirety of the gap region. The mass addition film44need only be provided in at least a portion of the gap region in the electrode finger extending direction.

As illustrated inFIG.8, the third surface44aof the mass addition film44is inclined. The thickness of the mass addition film44is increased from one side to the other side in the electrode finger facing direction. As described above, the thickness of the mass addition film44is not uniform in the electrode finger facing direction. As a result, as in the first example embodiment, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

The thickness of the mass addition film44is continuously changed to be increased from one side to the other side in the electrode finger facing direction. For example, the thickness of the mass addition film44may be changed to be thick and then changed to be thin from one side to the other side in the electrode finger facing direction.

FIG.9is a plan view of an acoustic wave device according to a third example embodiment of the present invention.

The present example embodiment is different from the first example embodiment in that the mass addition film54is provided over the edge regions and the gap regions, and in the shape of the mass addition film54. Except for the above points, the acoustic wave device according to the present example embodiment has the same or substantially the same configurations as the acoustic wave device10according to the first example embodiment.

One of the pair of mass addition films54is provided over the first edge region E1and the first gap region G1. The other of the pair of mass addition films54is provided over the second edge region E2and the second gap region G2. In the present example embodiment, the thickness of the mass addition film54is not uniform in the electrode finger facing direction. As a result, as in the first example embodiment, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

In the first to third example embodiments described above, the mass addition film has a strip shape. For example, in the first example embodiment illustrated inFIG.1, the mass addition film24is continuously provided on the plurality of electrode fingers and in the region between the electrode fingers. The mass addition film may be provided only on the electrode fingers, for example. This example is described with reference to a fourth example embodiment of the present invention.

FIG.10is a plan view of an acoustic wave device according to the fourth example embodiment.FIG.11is a sectional view taken along line I-I inFIG.10.

As illustratedFIG.10, the present example embodiment is different from the first example embodiment in that a protective film65is provided on the first main surface14aof the piezoelectric layer14to cover the IDT electrode11and that a plurality of mass addition films64are provided in each edge region. Except for the above points, the acoustic wave device according to the present example embodiment has the same or substantially the same configurations as the acoustic wave device10according to the first example embodiment. The protective film65does not have to be provided.

The plurality of mass addition films64are arranged in the electrode finger facing direction. In plan view, each mass addition film64and each electrode finger overlap each other. More specifically, in the first edge region E1, each mass addition film64is provided only on the first surface11aof one first electrode finger28or only on the first surface11aof one second electrode finger29. The same applies to the second edge region E2. As described above, the plurality of mass addition films64are provided only in the regions overlapping the electrode fingers in plan view.

As illustrated inFIG.11, in the present example embodiment, in a portion in which the electrode finger, the mass addition film64, and the protective film65are laminated, the electrode finger, the mass addition film64, and the protective film65are laminated in this order. The order of the lamination is not limited to the above example.

The thicknesses of the mass addition films64provided in the first edge regions E1are different from each other. Specifically, the thickness of the mass addition film64is increased for every other thickness from one side to the other side in the electrode finger facing direction. More specifically, among three mass addition films64provided side by side in the electrode finger facing direction, the lengths of the two adjacent mass addition films64are the same, and the lengths of the two mass addition films64and the length of the remaining one mass addition film64are different from each other. In this way, the thickness of the mass addition film64is periodically changed.

As described above, the thicknesses of the plurality of mass addition films64are not uniform in the electrode finger facing direction. The same applies to the second edge region E2. As a result, as in the first example embodiment, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

In each of the edge regions, the thicknesses of at least two mass addition films64of the plurality of mass addition films64need only be different from each other. In this case, the thicknesses of the mass addition films64at at least a set of the first point O1and the second point O2are different from each other. As a result, the unnecessary wave can be reduced or prevented.

Each mass addition film64is not in contact with both electrode fingers connected to the potentials different from each other. In this case, metal can be used as the material of the plurality of mass addition films64. A dielectric may be used as the material of the plurality of mass addition films64.

As in the first example embodiment, the plurality of mass addition films64are provided only in both of the edge regions. As a result, also in the present example embodiment, the amount of change in the fractional bandwidth can be reduced, and the electrical characteristics of the acoustic wave device can be stabilized. The plurality of mass addition films64may be provided over the first edge region E1and the first gap region G1illustrated inFIG.10. Similarly, the plurality of mass addition films64may be provided over the second edge region E2and the second gap region G2. Alternatively, the plurality of mass addition films64may be provided only in both the gap regions.

In the present example embodiment, the protective film65is provided. As a result, the IDT electrodes11are less likely to be damaged. As the material of the protective film65, for example, silicon oxide, silicon nitride, silicon oxynitride, or the like can be used. The configuration in which the protective film65is provided can also be used in a configuration other than the present example embodiment.

In a case where the materials of the protective film65and the mass addition film64are the same, the thickness of the protective film65is the thickness of the protective film65in the central region H illustrated inFIG.10. The thickness of the mass addition film64is obtained by subtracting the thickness of the protective film65from the total thickness of the protective film65and the mass addition film64.

FIG.12is a plan view of an acoustic wave device according to a fifth example embodiment of the present invention.FIG.13is a sectional view taken along line I-I inFIG.12.

As illustrated inFIG.12, the present example embodiment is different from the third example embodiment in that the protective film65is provided and the mass addition film54is provided on the protective film65. Except for the above points, the acoustic wave device according to the present example embodiment has the same or substantially the same configurations as the acoustic wave device according to the third example embodiment.

As in the third example embodiment, one of the pair of mass addition films54is provided over the first edge region E1and the first gap region G1. The other of the pair of mass addition films54is provided over the second edge region E2and the second gap region G2. Each mass addition film54is continuously provided in a region overlapping the plurality of electrode fingers and overlapping the region between the electrode fingers in plan view.

As illustrated inFIG.13, the mass addition film54is indirectly provided on the plurality of electrode fingers with the protective film65interposed therebetween. The mass addition film54is continuously provided on the plurality of electrode fingers and in the region between the electrode fingers. In addition, the thickness of the mass addition film54is increased from one side to the other side in the electrode finger facing direction. As described above, the thickness of the mass addition film54is not uniform in the electrode finger facing direction. As a result, as in the third example embodiment, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

In the present example embodiment, the mass addition film54is not in contact with the electrode fingers. In this case, metal can be used as the material of the mass addition film54. Here, the mass addition film54faces the plurality of electrode fingers with the protective film65interposed therebetween. Therefore, in a case where metal is used as the material of the mass addition film54, the electrostatic capacity of the acoustic wave device can be increased. Therefore, an area of the IDT electrode11can be reduced to obtain a desired electrostatic capacity. Therefore, the size of the acoustic wave device can be reduced. A dielectric may be used as the material of the mass addition film54.

FIG.14is a plan view of an acoustic wave device according to a sixth example embodiment of the present invention.FIG.15is a sectional view taken along line I-I inFIG.14.

As illustrated inFIG.14, the present example embodiment is different from the fifth example embodiment in that the plurality of mass addition films64are provided. Except for the above points, the acoustic wave device according to the present example embodiment has the same substantially the same configurations as the acoustic wave device according to the fifth example embodiment.

The plurality of mass addition films64are provided on the protective film65. Among all of the mass addition films64, some mass addition films64are provided over the first edge region E1and the first gap region G1, and some other mass addition films64are provided over the second edge region E2and the second gap region G2.

As illustrated inFIG.15, the thickness of the mass addition film64is periodically increased from one side to the other side in the electrode finger facing direction. As described above, also in the present example embodiment, the thickness of the mass addition film64is not uniform in the electrode finger facing direction. As a result, as in the fifth example embodiment, it is possible to disperse the frequency at which the unnecessary wave is generated, and it is possible to reduce or prevent the unnecessary wave.

Hereinafter, the details of the thickness shear mode will be described. The “electrode” in the IDT electrode described later corresponds to an electrode finger. The support in the following example corresponds to a support substrate. Hereinafter, a case where a certain member is made of a certain material includes a case where a small amount of an impurity is included to such an extent that the electrical characteristics of the acoustic wave device are not significantly deteriorated.

FIG.16Ais a schematic perspective view illustrating an acoustic wave device using the bulk wave in the thickness shear mode, andFIG.16Bis a plan view illustrating the electrode structure on the piezoelectric layer, andFIG.17is a sectional view of a portion taken along line A-A inFIG.16A.

An acoustic wave device1includes a piezoelectric layer2made of, for example, LiNbO3. The piezoelectric layer2may be made of, for example, LiTaO3. A cut-angle of LiNbO3or LiTaO3is a Z cut, but may be a rotated Y cut or an X cut. The thickness of the piezoelectric layer2is not particularly limited, but is preferably, for example, about 40 nm or more and about 1000 nm or less, and more preferably, for example, about 50 nm or more and about 1000 nm or less in order to effectively excite the thickness shear mode. The piezoelectric layer2includes first and second main surfaces2aand2bfacing each other. Electrodes3and4are provided on the first main surface2a. Here, the electrode3is an example of a “first electrode” and the electrode4is an example of a “second electrode”. InFIGS.16A and16B, the plurality of electrodes3are a plurality of first electrode fingers connected to a first busbar5. The plurality of electrodes4are a plurality of second electrode fingers connected to a second busbar6. The plurality of electrodes3and the plurality of electrodes4are interdigitated with each other. Each of the electrodes3and4has a rectangular or substantially rectangular shape and a length direction. The electrode3and the electrode4adjacent thereto face each other in a direction orthogonal or substantially orthogonal to the length direction. Both of the length direction of the electrodes3and4and the direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4are directions intersecting a thickness direction of the piezoelectric layer2. Therefore, it can be said that the electrode3and the electrode4adjacent thereto face each other in the direction intersecting the thickness direction of the piezoelectric layer2. In addition, the length direction of the electrodes3and4may be interchanged with the direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4illustrated inFIGS.16A and16B. That is, inFIGS.16A and16B, the electrodes3and4may extend in the direction in which the first busbar5and the second busbar6extend. In this case, the first busbar5and the second busbar6extend in the direction in which the electrodes3and4extend inFIGS.16A and16B. A plurality of pairs each including a structure in which the electrode3connected to one potential and the electrode4connected to the other potential are adjacent to each other are provided in a direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4. Here, a case where the electrodes3and4are adjacent to each other does not mean a case where the electrodes3and4are disposed to be in direct contact with each other, but means a case where the electrodes3and4are disposed with a gap therebetween. In a case where the electrodes3and4are adjacent to each other, electrodes connected to a hot electrode or a ground electrode, including other electrodes3and4, are not disposed between the electrodes3and4adjacent to each other. The number of pairs does not have to be integer pairs, but may be 1.5 pairs, 2.5 pairs, or the like. The center-to-center distance, that is, the pitch between the electrodes3and4is preferably in a range of, for example, about 1 μm or more and about 10 μm or less. The widths of the electrodes3and4, that is, the dimensions of the electrodes3and4in the facing direction are preferably in a range of, for example, about 50 nm or more and about 1000 nm or less, and more preferably in a range of about 150 nm or more and about 1000 nm or less. The center-to-center distance between the electrodes3and4is a distance connecting the center of the dimension (width dimension) of the electrode3in the direction orthogonal to the length direction of the electrode3and the center of the dimension (width dimension) of the electrode4in the direction orthogonal to the length direction of the electrode4.

In the acoustic wave device1, since the Z-cut piezoelectric layer is used, the direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4is a direction orthogonal to a polarization direction of the piezoelectric layer2. This is not a case when a piezoelectric material with a different cut-angle is used as the piezoelectric layer2. Here, “orthogonal” is not limited to being strictly orthogonal, but may be substantially orthogonal (angle between the direction orthogonal to the length direction of the electrodes3and4and the polarization direction is, for example, in a range of about 90°±10°).

A support8is laminated on the second main surface2bside of the piezoelectric layer2with an insulating layer7interposed therebetween. The insulating layer7and the support8have a frame shape and include through holes7aand8aas illustrated inFIG.17. As a result, a cavity portion9is provided. The cavity portion9is provided not to disturb the vibration of the excitation region C of the piezoelectric layer2. Therefore, the support8is laminated on the second main surface2bwith the insulating layer7interposed therebetween at a position not overlapping the portion in which at least a pair of electrodes3and4are provided. It should be noted that the insulating layer7does not have to be provided. Therefore, the support8can be directly or indirectly laminated on the second main surface2bof the piezoelectric layer2.

The insulating layer7is made of, for example, silicon oxide. In addition to silicon oxide, an appropriate insulating material such as, for example, silicon oxynitride or alumina can be used. The support8is made of, for example, Si. A plane orientation of the plane of Si on the piezoelectric layer2side may be (100), (110), or (111). Si included in the support8is preferably high resistance having a resistivity of, for example, about 4 kΩcm or more. The support8can also be made of an appropriate insulating material or semiconductor material.

Examples of the material of the support8include piezoelectric materials such as aluminum oxide, lithium tantalate, lithium niobate, and quartz crystal, various ceramics such as alumina, magnesia, sapphire, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite, dielectrics such as diamond and glass, and semiconductors such as gallium nitride.

The plurality of electrodes3and4and the first and second busbars5and6are made of appropriate metals or alloys, such as, for example, Al and AlCu alloys. In the present example embodiment, the electrodes3and4and the first and second busbars5and6include an Al film laminated on a Ti film. An adhesion layer other than the Ti film may be used.

The AC voltage for driving is applied between the plurality of electrodes3and the plurality of electrodes4. More specifically, the AC voltage is applied between the first busbar5and the second busbar6. As a result, it is possible to obtain the resonance characteristics using the bulk wave in the thickness shear mode excited in the piezoelectric layer2. In the acoustic wave device1, in a case where d is the thickness of the piezoelectric layer2and p is the center-to-center distance between any adjacent electrodes3and4among the plurality of pairs of electrodes3and4, d/p is, for example, about 0.5 or less. As a result, the bulk wave in the thickness shear mode is effectively excited, and good resonance characteristics can be obtained. More preferably, for example, d/p is about 0.24 or less, and in this case, further improved resonance characteristics can be obtained.

In the acoustic wave device1, since the above-described configuration is provided, even in a case where the number of pairs of the electrodes3and4is reduced in order to reduce the size, a Q value is less likely to be decreased. This is because the propagation loss is small even in a case where the number of electrode fingers in the reflectors on both sides is small. In addition, the number of electrode fingers can be reduced because the bulk wave in the thickness shear mode is used. A difference between the Lamb wave used in the acoustic wave device and the bulk wave in the thickness shear mode will be described with reference toFIGS.18A and18B.

FIG.18Ais a schematic elevational sectional view illustrating the Lamb wave that propagates through the piezoelectric film of the acoustic wave device as described in Japanese Unexamined Patent Application Publication No. 2012-257019. Here, the wave propagates in a piezoelectric film201as indicated by an arrow. Here, in the piezoelectric film201, a first main surface201aand a second main surface201bface each other, and a thickness direction connecting the first main surface201aand the second main surface201bis a Z direction. An X direction is a direction in which the electrode fingers of the IDT electrodes are arranged. As illustrated inFIG.18A, in the Lamb wave, the wave propagates in the X direction as illustrated in the figure. Since the wave is a plate wave, although the piezoelectric film201vibrates as a whole, the wave propagates in the X direction, and thus the reflectors are disposed on both sides to obtain the resonance characteristics. Therefore, the propagation loss of the wave occurs, and the Q value is decreased in a case where the size reduction is attempted, that is, in a case where the number of pairs of the electrode fingers is decreased.

On the other hand, as illustrated inFIG.18B, in the acoustic wave device1, since the vibration displacement is a thickness shear direction, the wave propagates and resonates in the direction connecting the first main surface2aand the second main surface2bof the piezoelectric layer2, that is, the Z direction. That is, an X-direction component of the wave is significantly smaller than a Z-direction component. In addition, since the resonance characteristics are obtained by the propagation of the wave in the Z direction, the propagation loss is less likely to occur even when the number of the electrode fingers of the reflector is reduced. Therefore, even in a case where the number of pairs each including the electrodes3and4is reduced when the size reduction is attempted, the Q value is less likely to be decreased.

Amplitude directions of the bulk waves of the thickness shear mode are opposite to each other between a first region451included in the excitation region C of the piezoelectric layer2and a second region452included in the excitation region C, as illustrated inFIG.19.FIG.19schematically illustrates the bulk waves when the voltage is applied between the electrodes3and4such that the potential of the electrode4is higher than the potential of the electrode3. The first region451is a region of the excitation region C between a virtual plane VP1, which is orthogonal or substantially orthogonal to the thickness direction of the piezoelectric layer2and bisects the piezoelectric layer2, and the first main surface2a. The second region452is a region of the excitation region C between the virtual plane VP1and the second main surface2b.

As described above, in the acoustic wave device1, although at least a pair of electrodes including the electrodes3and4are disposed, the waves are not intended to propagate in the X direction, and thus the number of pairs of the electrode pair including the electrodes3and4does not have to be two or more. That is, at least one pair of electrodes need only be provided.

For example, the electrode3is an electrode connected to a hot potential and the electrode4is an electrode connected to a ground potential. The electrode3may be connected to the ground potential and the electrode4may be connected to the hot potential. In the present example embodiment, at least a pair of electrodes include the electrodes connected to the hot potential or the electrodes connected to the ground potential, as described above, and no floating electrodes are provided.

FIG.20is a diagram illustrating the resonance characteristics of the acoustic wave device illustrated inFIG.17. The design parameters of the acoustic wave device1with the resonance characteristics are as follows.

When viewed in the direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4, the length of the region in which the electrodes3and4overlap each other, that is, the length of the excitation region C=about 40 μm, the number of pairs of the electrodes including the electrodes3and4=21 pairs, the center-to-center distance between the electrodes=about 3 μm, the width of the electrodes3and4=about 500 nm, and d/p=about 0.133.

Insulating layer7: silicon oxide film having a thickness of about 1 μm.

The length of the excitation region C is the dimension of the excitation region C in the length direction of the electrodes3and4.

In the present example embodiment, the electrode-to-electrode distances in the electrode pairs each including the electrodes3and4are all equal or substantially equal in the plurality of pairs. That is, the electrodes3and4are disposed at equal or substantially equal pitches.

As is clear fromFIG.20, good resonance characteristics with the fractional bandwidth of about 12.5% are obtained regardless of excluding reflectors.

In a case where d is the thickness of the piezoelectric layer2and p is the center-to-center distance between the electrodes3and4, in the present example embodiment, as described above, d/p is, for example, about 0.5 or less, more preferably about 0.24 or less. This will be described with reference toFIG.21.

A plurality of acoustic wave devices are obtained similarly, but with different d/p, to the acoustic wave device that obtains the resonance characteristics illustrated inFIG.20.FIG.21is a diagram illustrating a relationship between d/p and the fractional bandwidth as the resonator of the acoustic wave device.

As is clear fromFIG.21, when d/p>about 0.5, the fractional bandwidth is less than about 5% even in a case where d/p is adjusted. On the other hand, in a case where d/p≤ about 0.5, when d/p is changed within this range, the fractional bandwidth of about 5% or more can be obtained, that is, the resonator having a high coupling coefficient can be provided. In addition, in a case where d/p is about 0.24 or less, the fractional bandwidth can be increased to about 7% or more. In addition, by adjusting d/p within this range, a resonator with a wider fractional bandwidth can be obtained, and a resonator with a higher coupling coefficient can be realized. Therefore, it is discovered that, by adjusting d/p to about 0.5 or less, it is possible to configure a resonator having a high coupling coefficient using the bulk wave in the thickness shear mode.

FIG.22is a plan view of the acoustic wave device using the bulk wave in the thickness shear mode. In an acoustic wave device80, the pair of electrodes including the electrode3and electrode4are provided on the first main surface2aof the piezoelectric layer2. K inFIG.22is an intersecting width. As described above, in the acoustic wave device according to the present invention, the number of pairs of the electrodes may be one pair. Even in this case, when d/p is about 0.5 or less, it is possible to effectively excite the bulk wave in the thickness shear mode.

In the acoustic wave device1, preferably, the metallization ratio MR of any adjacent electrodes3and4among the plurality of electrodes3and4to the excitation region C, which is the region in which the adjacent electrodes3and4overlap each other when viewed in the facing direction, satisfies, for example, MR≤about 1.75 (d/p)+0.075. In this case, the spurious response can be effectively reduced. This will be described with reference toFIGS.23and24.FIG.23is a reference diagram illustrating an example of the resonance characteristics of the acoustic wave device1. The spurious response indicated by an arrow B appears between the resonant frequency and the anti-resonant frequency. It should be noted that d/p=about 0.08 and the Euler angles of LiNbO3are (0°, 0°, 90°). Also, the metallization ratio MR is about 0.35.

The metallization ratio MR will be described with reference toFIG.16B. In the electrode structure ofFIG.16B, it is assumed that, when focusing on the pair of electrodes3and4, only the pair of electrodes3and4are provided. In this case, a portion surrounded by a one-dot chain line is the excitation region C. The excitation region C is, when the electrode3and the electrode4are viewed in the direction orthogonal or substantially orthogonal to the length direction of the electrodes3and4, that is, in the facing direction, a region of the electrode3that overlaps the electrode4, a region of the electrode4that overlaps the electrode3, and a region between the electrode3and the electrode4in which the electrode3and the electrode4overlap each other. An area of the electrodes3and4in the excitation region C with respect to an area of the excitation region C is the metallization ratio MR. That is, the metallization ratio MR is a ratio of an area of the metallization portion to the area of the excitation region C.

In a case where the plurality of pairs of electrodes are provided, a ratio of the metallization portion included in the entire excitation region to a total area of the excitation region need only be MR.

FIG.24is a diagram illustrating a relationship between a fractional bandwidth and a phase rotation amount of an impedance of the spurious standardized at about 180 degrees as a magnitude of the spurious response in a case where a large number of acoustic wave resonators are configured according to the present example embodiment. The fractional bandwidth is adjusted by changing the film thickness of the piezoelectric layer and the dimensions of the electrodes. Moreover,FIG.24illustrates the results in a case where the piezoelectric layer made of the Z-cut LiNbO3is used, but the same or substantially the same tendency is obtained in a case where piezoelectric layers with other cut-angles are used.

In a region surrounded by an ellipse J inFIG.24, the spurious response is as large as about 1.0. As is clear fromFIG.24, in a case where the fractional bandwidth exceeds about 0.17, that is, exceeds about 17%, a large spurious response with a spurious level of about 1 or more appears in a pass band even when the parameters constituting the fractional bandwidth are changed. That is, as in the resonance characteristics illustrated inFIG.23, a large spurious response indicated by an arrow B appears within the band. Therefore, the fractional bandwidth is preferably, for example, about 17% or less. In this case, by adjusting the film thickness of the piezoelectric layer2and the dimensions of the electrodes3and4, the spurious response can be reduced.

FIG.25is a diagram illustrating a relationship between d/2p, the metallization ratio MR, and the fractional bandwidth. Regarding the acoustic wave device described above, various acoustic wave devices having different d/2p and MR are configured, and the fractional bandwidth is measured. A hatched portion on a right side of a broken line D inFIG.25is a region in which the fractional bandwidth is about 17% or less. A boundary between the hatched region and a non-hatched region is expressed by MR=about 3.5(d/2p)+0.075. That is, MR=about 1.75 (d/p)+0.075. Therefore, preferably, for example, MR≤ about 1.75 (d/p)+0.075. In this case, it is easy to set the fractional bandwidth to about 17% or less. It is more preferable, for example, to have a region on a right side of MR=about 3.5 (d/2p)+0.05 indicated by a one-dot chain line D1inFIG.25. That is, in a case where MR≤ about 1.75 (d/p)+0.05, the fractional bandwidth can be reliably set to about 17% or less.

FIG.26is a diagram illustrating a map of the fractional bandwidth with respect to Euler angles (0°, θ, ψ) of LiNbO3in a case where d/p is infinitely close to 0. A hatched portion inFIG.26is a region in which the fractional bandwidth of at least about 5% or more is obtained, and in a case where a range of the region is approximated, the range is represented by Expressions (1), (2), and (3) below.

Therefore, in a case of the Euler angle range of Expression (1), Expression (2), or Expression (3) described above, the fractional bandwidth can be sufficiently widened, which is preferable. The same applies to a case where the piezoelectric layer2is the lithium tantalate layer.

FIG.27is an elevational sectional view of the acoustic wave device including the acoustic multilayer film.

In an acoustic wave device81, an acoustic multilayer film82is laminated on the second main surface2bof the piezoelectric layer2. The acoustic multilayer film82has a laminated structure including low acoustic impedance layers82a,82c, and82ehaving a relatively low acoustic impedance and high acoustic impedance layers82band82dhaving a relatively high acoustic impedance. In a case where the acoustic multilayer film82is used, the bulk wave in the thickness shear mode can be confined in the piezoelectric layer2without using the cavity portion9of the acoustic wave device1. Also in the acoustic wave device81, the resonance characteristics based on the bulk wave in the thickness shear mode can be obtained by setting d/p to about 0.5 or less, for example. In the acoustic multilayer film82, the number of laminated layers of the low acoustic impedance layers82a,82c, and82eand the high acoustic impedance layers82band82dis not particularly limited. At least one layer of the high acoustic impedance layers82band82dshould be disposed farther from the piezoelectric layer2than the low acoustic impedance layers82a,82c, and82e.

The low acoustic impedance layers82a,82c, and82eand the high acoustic impedance layers82band82dcan be made of an appropriate material as long as the above-described relationship of the acoustic impedance is satisfied. Examples of the material of the low acoustic impedance layers82a,82c, and82einclude silicon oxide and silicon oxynitride. In addition, examples of the material of the high acoustic impedance layers82band82dinclude alumina, silicon nitride, and metal.

In the acoustic wave devices according to the first to sixth example embodiments and each of the modified examples, for example, the acoustic multilayer film82illustrated inFIG.27may be provided as the acoustic reflection film between the support and the piezoelectric layer. Specifically, the support and the piezoelectric layer may be disposed such that at least a portion of the support and at least a portion of the piezoelectric layer face each other with the acoustic multilayer film82interposed therebetween. In this case, in the acoustic multilayer film82, the low acoustic impedance layer and the high acoustic impedance layer need only be alternately laminated. The acoustic multilayer film82may be the acoustic reflection portion in the acoustic wave device.

In the acoustic wave devices according to the first to sixth example embodiments and each of the modified examples that use the bulk wave in the thickness shear mode, as described above, d/p is preferably, for example, about 0.5 or less, and more preferably about 0.24 or less. As a result, better resonance characteristics can be obtained. Further, in the intersecting regions in the acoustic wave devices according to the first to sixth example embodiments and each of the modified examples that use the bulk wave in the thickness shear mode, as described above, preferably, for example, MR≤ about 1.75 (d/p)+0.075 is satisfied. In this case, it is possible to more reliably reduce or prevent the spurious response.

It is preferable that the piezoelectric layers in the acoustic wave devices according to the first to sixth example embodiments and each of the modified examples that use the bulk wave in the thickness shear mode is, for example, a lithium niobate layer or a lithium tantalate layer. In addition, it is preferable that the Euler angles (φ, θ, ψ) of lithium niobate or lithium tantalate of the piezoelectric layer are in the range of Expression (1), Expression (2), or Expression (3) described above. In this case, the fractional bandwidth can be sufficiently widened.