ACOUSTIC WAVE DEVICE

An acoustic wave device includes a piezoelectric film and an IDT electrode on the piezoelectric film. The IDT electrode includes first and second busbars, at least one first electrode finger, and at least one second electrode finger. When an overlap region is defined as a region in which the first and second electrode fingers overlap each other in an acoustic wave propagation direction, points A2, B2, C2, and D2, defined as follows, are all outside the cavity when, at the points A2, B2, C2, and D2, xa>about 25 μm, ya>about 25 μm, xb>about 25 μm, yb>about 25 μm, xc>about 25 μm, yc>about 25 μm, xd>about 25 μm, and yd>about 25 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave device having a structure in which a piezoelectric film is located over a cavity.

2. Description of the Related Art

Hitherto, acoustic wave devices having a structure in which a piezoelectric film is located over a cavity are known. An example of such an acoustic wave device is described in U.S. Pat. No. 10,491,192.

In an acoustic wave device having a structure in which a piezoelectric film is located over a cavity, cracks may occur in the piezoelectric film located over the cavity during a manufacturing process or in use.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave devices in each of which cracks are less likely to occur in a piezoelectric film located over a cavity.

An acoustic wave device according to a preferred embodiment of the present invention includes a support substrate including a recess, a piezoelectric film covering the recess of the support substrate and defining a cavity together with the recess, and a functional electrode on the piezoelectric film. The functional electrode includes first and second busbars, at least one first electrode finger including one end connected to the first busbar, and at least one second electrode finger including one end connected to the second busbar. When an overlap region is defined as a region in which the first and second electrode fingers overlap each other in an acoustic wave propagation direction, points A2, B2, C2, and D2are all outside the cavity when, at the points A2, B2, C2, and D2, xa>about 25 μm, ya>about 25 μm, xb>about 25 μm, yb>about 25 μm, xc>about 25 μm, yc>about 25 μm, xd>about 25 μm, and yd>about 25 μm. The points A2, B2, C2, and D2are defined as follows: point A2: a point shifted from a point A1as a starting point by xa in an x direction and by ya in a y direction toward the outside of the overlap region, point B2: a point shifted from a point B1as a starting point by xb in the x direction and by yb in the y direction toward the outside of the overlap region, point C2: a point shifted from a point C1as a starting point by xc in the x direction and by yc in the y direction toward the outside of the overlap region, point D2: a point shifted from a point D1as a starting point by xd in the x direction and by yd in the y direction toward the outside of the overlap region. The points A1, B1, C1, and D1are defined as follows: point A1: an intersection of an outer edge of an outermost electrode on one end side in the acoustic wave propagation direction and the first busbar, point B1: an intersection of an extension line of an outer edge of an outermost electrode on another end side in the acoustic wave propagation direction and the first busbar, point C1: an intersection of an extension line of the outer edge of the outermost electrode on the one end side in the acoustic wave propagation direction and the second busbar, point D1: an intersection of the outer edge of the outermost electrode on the other end side in the acoustic wave propagation direction and the second busbar.

An acoustic wave device according to a preferred embodiment of the present invention includes a support substrate including a recess, a piezoelectric film covering the recess of the support substrate and defining a cavity together with the recess, and a functional electrode on the piezoelectric film. The functional electrode includes first and second busbars, at least one first electrode finger including one end connected to the first busbar, and at least one second electrode finger including one end connected to the second busbar. When an overlap region is defined as a region in which the first and second electrode fingers overlap each other in an acoustic wave propagation direction, points A2, B2, C2, and D2are all inside the cavity when, at the points A2, B2, C2, and D2, xa<about 2 μm, ya<about 2 μm, xb<about 2 μm, yb<about 2 μm, xc<about 2 μm, yc<about 2 μm, xd<about 2 μm, and yd<about 2 μm. The points A2, B2, C2, and D2are defined as follows: point A2: a point shifted from a point A1as a starting point by xa in an x direction and by ya in a y direction toward the outside of the overlap region, point B2: a point shifted from a point B1as a starting point by xb in the x direction and by yb in the y direction toward the outside of the overlap region, point C2: a point shifted from a point C1as a starting point by xc in the x direction and by yc in the y direction toward the outside of the overlap region, point D2: a point shifted from a point D1as a starting point by xd in the x direction and by yd in the y direction toward the outside of the overlap region. The points A1, B1, C1, and D1are defined as follows: point A1: an intersection of an outer edge of an outermost electrode on one end side in the acoustic wave propagation direction and the first busbar, point B1: an intersection of an extension line of an outer edge of an outermost electrode on another end side in the acoustic wave propagation direction and the first busbar, point C1: an intersection of an extension line of the outer edge of the outermost electrode on the one end side in the acoustic wave propagation direction and the second busbar, point D1: an intersection of the outer edge of the outermost electrode on the other end side in the acoustic wave propagation direction and the second busbar.

According to preferred embodiments of the present invention, it is possible to provide acoustic wave devices in each of which cracks are less likely to occur in a piezoelectric film located over a cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings to clarify the present invention.

The preferred embodiments described herein are merely examples, and partial replacement or combination of elements of different preferred embodiments is possible.

An acoustic wave device according to a preferred embodiment of the present invention includes a support substrate including a recess, a piezoelectric film covering the recess of the support substrate and defining a cavity together with the recess, and a functional electrode provided on the piezoelectric film. The functional electrode includes first and second busbars, at least one first electrode finger including one end connected to the first busbar, and at least one second electrode finger including one end connected to the second busbar.

Preferred embodiments of the present invention provide acoustic wave devices having a structure in which a piezoelectric film is located over a cavity and cracks are less likely to occur in the piezoelectric film located over the cavity.

First, first to third structural examples as basic structures of acoustic wave devices according to preferred embodiments of the present invention will be described with reference toFIGS.1to8.

In the first structural example, a bulk wave in a thickness shear mode is used. In the second structural example, the first electrode finger and the second electrode finger are adjacent to each other, and when a thickness of the piezoelectric film is d and a distance between the centers of the first electrode finger and the second electrode finger is p, d/p is about 0.5 or less. With this configuration, in the first and second structural examples, a Q factor can be increased even when the size of the acoustic wave device is reduced.

In the third structural example, a Lamb wave is used as a plate wave. Thus, resonance characteristics due to the Lamb wave can be obtained.

Hereinafter, the first, second, and third specific structural examples will be described with reference to the drawings to clarify the present invention.

FIG.1Ais a schematic perspective view of an acoustic wave device according to a first preferred embodiment of the present invention,FIG.1Bis a plan view illustrating an electrode structure on a piezoelectric film, andFIG.2is a cross-sectional view taken along line A-A inFIG.1A. An acoustic wave device1illustrated inFIGS.1A and1BandFIG.2illustrates the first structural example to be described later, but the other structural examples have the same or substantially the same shape.

The acoustic wave device1includes a piezoelectric film2made of lithium niobate, for example. To be more specific, the piezoelectric film2is a LiNbO3layer, for example. However, the piezoelectric film2may be a lithium tantalate layer such as a LiTaO3layer, for example. The cut-angle of LiNbO3or LiTaO3is Z-cut in the present preferred embodiment, but may be rotated Y-cut or X-cut.

A thickness of the piezoelectric film2is not limited, but is preferably, for example, about 40 nm or more and about 1000 nm or less in order to effectively excite a thickness shear mode.

The piezoelectric film2includes first and second principal surfaces2aand2bfacing each other. Electrode fingers3and4are provided on the first principal surface2a. Here, the electrode fingers3are an example of “first electrode finger”, and the electrode fingers4are an example of “second electrode finger”. InFIGS.1A and1B, a plurality of electrode fingers3are connected to a first busbar5. A plurality of electrode fingers4are connected to a second busbar6. The plurality of electrode fingers3and the plurality of electrode fingers4are interdigitated with each other. The electrode fingers3, the electrode fingers4, the first busbar5, and the second busbar6define a functional electrode. In the present preferred embodiment, the functional electrode is, for example, an interdigital transducer (IDT) electrode including the plurality of electrode fingers3and the plurality of electrode fingers4. Although the plurality of electrode fingers3and the plurality of electrode fingers4are provided, it is sufficient that at least one electrode finger3and at least one electrode finger4are provided.

The electrode fingers3and the electrode fingers4have a rectangular or substantially rectangular shape and have a lengthwise direction. In a direction orthogonal to the lengthwise direction, the electrode fingers3and the adjacent electrode fingers4face each other. Both the lengthwise direction of the electrode fingers3and4and the direction orthogonal to the lengthwise direction of the electrode fingers3and4intersect a thickness-wise direction of the piezoelectric film2. Therefore, it can also be said that the electrode fingers3and the adjacent electrode fingers4face each other in a direction intersecting the thickness-wise direction of the piezoelectric film2.

Further, the lengthwise direction of the electrode fingers3and4may be interchanged with the direction orthogonal to the lengthwise direction of the electrode fingers3and4illustrated inFIGS.1A and1B. That is, inFIGS.1A and1B, the electrode fingers3and4may 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 electrode fingers3and4extend inFIGS.1A and1B.

A plurality of pairs of an electrode finger3connected to one potential and an electrode finger4connected to another potential that are adjacent to each other are provided in the direction orthogonal to the lengthwise direction of the electrode fingers3and4. Here, “an electrode finger3and an electrode finger4are adjacent to each other” does not mean that the electrode finger3and the electrode finger4are disposed so as to be in direct contact with each other, but means that the electrode finger3and the electrode finger4are disposed with a gap interposed therebetween. When an electrode finger3and an electrode finger4are adjacent to each other, there are no electrodes connected to a hot electrode or a ground electrode between the electrode finger3and the electrode finger4, including the other electrode fingers3and4. The number of pairs need not be an integer, but may be, for example, 1.5 pairs, 2.5 pairs, or the like.

A center-to-center distance between the electrode fingers3and4, that is, a pitch, is preferably in a range of about 1 μm or more and about 10 μm or less, for example. The center-to-center distance between the electrode fingers3and4is a distance between the centers of width dimensions of the electrode fingers3in the direction orthogonal to the lengthwise direction of the electrode fingers3and the centers of width dimensions of the electrode fingers4in the direction orthogonal to the lengthwise direction of the electrode fingers4. The widths of the electrode fingers3and4, that is, the dimensions of the electrode fingers3and4in the direction in which the electrode fingers3and4face each other, are preferably in a range of about 50 nm or more and about 1000 nm or less, for example.

Further, in the present preferred embodiment, since the Z-cut piezoelectric film is used, the direction orthogonal to the lengthwise direction of the electrode fingers3and4is a direction orthogonal to a polarization direction of the piezoelectric film2. This is not the case when a piezoelectric material having a different cut-angle is used as the piezoelectric film2. Here, “orthogonal” is not limited to strictly orthogonal, but may also be substantially orthogonal (an angle between the direction orthogonal to the lengthwise direction of the electrode fingers3and4and the polarization direction is within a range of, for example, about 90°±10°).

A support member11is stacked on the second principal surface2bside of the piezoelectric film2. The support member11includes an insulating layer7and a support substrate8stacked on the insulating layer7. The insulating layer7and the support substrate8are frame-shaped and include cavities7aand8a, as illustrated inFIG.2. The cavities7aand8aare covered (blocked) by the piezoelectric film2. Thus, a hollow portion9(cavity) is provided. The hollow portion9is provided so that the piezoelectric film2can vibrate in an excitation region. Therefore, the support substrate8is stacked on the second principal surface2bwith the insulating layer7interposed therebetween at a position not overlapping a portion where at least one pair of electrode fingers3and4are provided. The insulating layer7need not be provided. Thus, the support substrate8can be directly or indirectly stacked on the second principal surface2bof the piezoelectric film2.

The hollow portion9may be defined by a recess provided in the support substrate8instead of the cavity extending through the support substrate8as illustrated inFIG.2. In other words, the hollow portion9may be surrounded by the recess of the support substrate8, the cavity7aof the insulating layer7, and the piezoelectric film2. The cavity8ais not necessarily provided in the support substrate8, and the cavity7amay be provided only in the insulating layer7. In addition, the insulating layer7may be provided with a recess instead of the cavity extending through the insulating layer7as illustrated inFIG.2. Further, the cavity7ais not necessarily provided in the insulating layer7, and the cavity8amay be provided only in the support substrate8. That is, it is sufficient that the hollow portion9is provided in the support member11including the support substrate8(and the insulating layer7in some cases).

The hollow portion9is not necessarily provided in the support member11, but may be provided in the piezoelectric film. For example, the hollow portion9may be configured to include a recess provided on the principal surface of the piezoelectric film on the support member11side.

The insulating layer7is made of silicon oxide, for example. However, instead of silicon oxide, a suitable insulating material such as, for example, silicon oxynitride or alumina may be used. The support substrate8is made of Si, for example. The plane orientation of a surface of Si on the piezoelectric film2side may be (100), (110), or (111). High-resistivity Si having a resistivity of about 4 kΩ or more is preferable, for example. However, the support substrate8can also be made of an appropriate insulating material or semiconductor material. For example, piezoelectric materials such as aluminum oxide, lithium tantalate, lithium niobate, and quartz; various ceramics such as alumina, sapphire, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite; dielectric materials such as diamond and glass; semiconductors such as gallium nitride; and resins can also be used.

The plurality of electrode fingers3and4and the first and second busbars5and6are made of a suitable metal or alloy such as, for example, Al or an Al—Cu alloy. In the present preferred embodiment, the electrode fingers3and4and the first and second busbars5and6include an Al film stacked on a Ti film defining and functioning as an adhesion layer. An adhesion layer other than the Ti film may be used.

For driving, an AC voltage is applied between the plurality of electrode fingers3and the plurality of electrode fingers4. More specifically, an AC voltage is applied between the first busbar5and the second busbar6. This makes it possible to obtain resonance characteristics using a bulk wave in a thickness shear mode such as a first-order thickness shear mode excited in the piezoelectric film2. In the acoustic wave device1, when the thickness of the piezoelectric film2is d and the center-to-center distance between any adjacent electrode fingers3and4among the plurality of pairs of electrode fingers3and4is p, d/p is about 0.5 or less, for example. That is, the acoustic wave device1also corresponds to the second structural example described above. Therefore, the bulk wave in the thickness shear mode is effectively excited, and good resonance characteristics can be obtained. More preferably, d/p is about 0.24 or less, for example, in which case better resonance characteristics can be obtained.

When there are a plurality of electrode fingers3or a plurality of electrode fingers4, or both, as in the present preferred embodiment, that is, when there are 1.5 or more pairs of electrode fingers3and4, with each pair of electrodes including one electrode finger3and one electrode finger4, the center-to-center distance p between the adjacent electrode fingers3and4means the center-to-center distance between the electrode fingers3and4adjacent to each other.

Since the acoustic wave device1according to the present preferred embodiment has the above configuration, even when the number of pairs of electrode fingers3and4is reduced in order to reduce the size of the acoustic wave device1, a Q factor is less likely to decrease. This is because the acoustic wave device1is a resonator that does not require a reflector on either side and has low propagation loss. In addition, reflectors as mentioned above are not necessarily required because the bulk wave in the thickness shear mode is used. A difference between a Lamb wave used in a conventional acoustic wave device and a bulk wave in the thickness shear mode will be described with reference toFIGS.3A and3B.

FIG.3Ais a schematic elevational cross-sectional view illustrating a Lamb wave propagating through a piezoelectric film of an acoustic wave device as described in U.S. Pat. No. 10,491,192. Here, a wave propagates through a piezoelectric film201as indicated by arrows. Here, in the piezoelectric film201, a first principal surface201aand a second principal surface201bface each other, and a thickness-wise direction connecting the first principal surface201aand the second principal surface201bis the Z direction. The X direction is a direction in which electrode fingers of an IDT electrode are arranged. As illustrated inFIG.3A, the Lamb wave propagates in the X direction as illustrated. Although the piezoelectric film201vibrates as a whole, the wave is a plate wave and thus propagates in the X direction; therefore, reflectors are arranged on both sides to obtain resonance characteristics. This results in wave propagation loss, and when the size of the acoustic wave device is reduced, that is, when the number of pairs of electrode fingers is reduced, the Q factor decreases.

On the other hand, as illustrated inFIG.3B, in the acoustic wave device1of the present preferred embodiment, since the vibration displacement is in the thickness shear direction, the wave substantially propagates in the direction connecting the first principal surface2aand the second principal surface2bof the piezoelectric film2, that is, in the Z direction, and resonates. That is, the X-direction component of the wave is significantly smaller than the Z-direction component of the wave. Since resonance characteristics are obtained by propagation of the wave in the Z direction, reflectors are not necessarily required. Therefore, there is no propagation loss that occurs when the wave propagates to the reflectors. Thus, even when the number of pairs of electrodes of the electrode fingers3and4is reduced in order to reduce the size of the acoustic wave device1, the Q factor is less likely to decrease.

As illustrated inFIG.4, in the piezoelectric film2, the amplitude direction of the bulk wave in the thickness shear mode is opposite between a first region451included in the excitation region and a second region452included in the excitation region.FIG.4schematically illustrates the bulk wave when a voltage is applied between the electrode fingers3and the electrode fingers4so that the electrode fingers4have a higher potential than the electrode fingers3. The first region451is a region in the excitation region between a virtual plane VP1that is orthogonal to the thickness-wise direction of the piezoelectric film2and divides the piezoelectric film2into two parts, and the first principal surface2a. The second region452is a region in the excitation region between the virtual plane VP1and the second principal surface2b.

As described above, in the acoustic wave device1, at least one pair of electrodes of the electrode fingers3and4are disposed. Since a wave does not propagate in the X direction, a plurality of pairs of electrodes composed of the electrode fingers3and4need not necessarily be provided. That is, it is sufficient that at least one pair of electrodes be provided.

For example, the electrode fingers3are electrodes connected to a hot potential, and the electrode fingers4are electrodes connected to a ground potential. However, the electrode fingers3may be connected to the ground potential and the electrode fingers4may be connected to the hot potential. In the present preferred embodiment, as described above, each of the at least one pair of electrodes is an electrode connected to the hot potential or an electrode connected to the ground potential, and no floating electrodes are provided.

FIG.5is a diagram illustrating resonance characteristics of the acoustic wave device1according to the first preferred embodiment of the present invention. The design parameters of the acoustic wave device1having these resonance characteristics are as follows.

The piezoelectric film2: made of LiNbO3with Euler angles (about 0°, about 0°, about 90°), and with a thickness of about 400 nm.

When viewed in a direction orthogonal to the lengthwise direction of the electrode fingers3and4, the length of the region in which the electrode fingers3and4overlap each other, that is, the length of the excitation region C, is about 40 μm, the number of pairs of electrodes composed of the electrode fingers3and4is 21, the distance between the centers of the electrode fingers3and4is about 3 μm, the widths of the electrode fingers3and4are about 500 nm, and d/p is about 0.133.

The insulating layer7: made of a silicon oxide film with a thickness of about 1 μm.

The support substrate8: made of Si.

The length of the excitation region C is a dimension along the lengthwise direction of the electrode fingers3and4in the excitation region C.

In the present preferred embodiment, the distances between the electrode fingers of the plurality of pairs of electrodes of the electrode fingers3and4are all set to be equal or substantially equal. That is, the electrode fingers3and the electrode fingers4are arranged at equal or substantially equal pitches.

As is apparent fromFIG.5, good resonance characteristics with a fractional bandwidth of about 12.5% are obtained in spite of including no reflectors.

When the thickness of the piezoelectric film2is d and the distance between the centers of the electrode fingers3and4is p, d/p is preferably about 0.5 or less, more preferably about 0.24 or less, in the present preferred embodiment as described above. This will be described with reference toFIG.6.

A plurality of acoustic wave devices were obtained in the same manner as the acoustic wave device having the resonance characteristics illustrated inFIG.5but with varying values of d/2p.FIG.6is a diagram illustrating a relationship between d/2p and the fractional bandwidth of the acoustic wave device as a resonator.

As is apparent fromFIG.6, when d/2p exceeds about 0.25, that is, when d/p>about 0.5, the fractional bandwidth is less than about 5% even when d/p is adjusted. On the other hand, when d/2p≤about 0.25, that is, d/p≤about 0.5, the fractional bandwidth can be increased to about 5% or more by changing d/p within this range; that is, a resonator having a high coupling coefficient can be obtained. When d/2p is about 0.12 or less, that is, when d/p is about 0.24 or less, the fractional bandwidth can be increased to about 7% or more. In addition, when d/p is adjusted within this range, a resonator having a wider fractional bandwidth can be obtained, and a resonator having a higher coupling coefficient can be achieved. Therefore, it can be seen that by setting d/p to about 0.5 or less as in the second structural example of the present application, a resonator having a high coupling coefficient using the bulk wave in the thickness shear mode can be generated.

As described above, the at least one pair of electrodes may be one pair, and in the case of one pair of electrodes, p is the center-to-center distance between the adjacent electrode fingers3and4.

When the piezoelectric film2has thickness variations, its average thickness may be used as the thickness d of the piezoelectric film.

FIG.7is a plan view illustrating a first modification of the acoustic wave device according to the first preferred embodiment of the present invention. In an acoustic wave device31, a pair of electrodes including the electrode fingers3and4are provided on the first principal surface2aof the piezoelectric film2. K inFIG.7is an overlap region. The dimension in the direction in which the electrode fingers3and4extend in the overlap region is the overlap width. As described above, in the acoustic wave device of the present invention, the number of pairs of electrodes may be one. Also in this case, when d/p is about 0.5 or less, the bulk wave in the thickness shear mode can be effectively excited.

FIG.8is a partially cutaway perspective view for explaining the third structural example of the acoustic wave device according to the present invention.

An acoustic wave device81includes a support substrate82. The support substrate82includes a recess that is open on the upper surface. A piezoelectric film83is stacked on the support substrate82. Thus, the hollow portion9is provided. An IDT electrode84is provided on the piezoelectric film83over the hollow portion9. Reflectors85and86are provided on both sides of the IDT electrode84in the acoustic wave propagation direction. InFIG.8, a perimeter of the hollow portion9is indicated by a broken line. Here, the IDT electrode84includes first and second busbars84aand84b, a plurality of first electrode fingers84c, and a plurality of second electrode fingers84d. The plurality of first electrode fingers84care connected to the first busbar84a. The plurality of second electrode fingers84dare connected to the second busbar84b. The plurality of first electrode fingers84cand the plurality of second electrode fingers84dare interdigitated with each other.

In the acoustic wave device81, a Lamb wave as a plate wave is excited by applying an AC electric field to the IDT electrode84over the hollow portion9. Since the reflectors85and86are provided on both sides, resonance characteristics due to the Lamb wave can be obtained. Thus, the acoustic wave device according to a preferred embodiment of the present invention may use plate waves.

FIG.9is a schematic plan view for explaining a relationship between a position of the cavity and an electrode structure in the acoustic wave device according to the first preferred embodiment of the present invention illustrated inFIGS.1A,1B, and2.

As illustrated inFIG.9, an IDT electrode10is located over the hollow portion9located on the lower side. The IDT electrode10includes the first busbar5and the second busbar6described above. One end of each of the plurality of first electrode fingers3is connected to the first busbar5. One end of each of the plurality of second electrode fingers4is connected to the second busbar6. The plurality of first electrode fingers3and the plurality of second electrode fingers4are interdigitated with each other. Since the hollow portion9is located on the lower side, vibration is less likely to be disturbed when a voltage is applied between the electrode fingers3and the electrode fingers4. In addition, as described above, in the acoustic wave device1of the first preferred embodiment, good resonance characteristics using the thickness shear mode are obtained, and the Q factor can be increased because d/p is about 0.5 or less.

The inventors of preferred embodiments of the present application have discovered that in a structure in which the piezoelectric film2is located over the hollow portion9, cracks may occur in the piezoelectric film2located over the hollow portion9.

In the acoustic wave device1according to the first preferred embodiment, an overlap region is defined as a region in which the first and second electrode fingers3and4overlap each other in the acoustic wave propagation direction. In this case, at points A2, B2, C2, and D2,

The points A2, B2, C2, and D2are defined as follows:

point A2: a point shifted from a point A1as a starting point by xa in an x direction and by ya in a y direction toward the outside of the overlap region,

point B2: a point shifted from a point B1as a starting point by xb in the x direction and by yb in the y direction toward the outside of the overlap region,

point C2: a point shifted from a point C1as a starting point by xc in the x direction and by yc in the y direction toward the outside of the overlap region, and

point D2: a point shifted from a point D1as a starting point by xd in the x direction and by yd in the y direction toward the outside of the overlap region.

The points A1, B1, C1, and D1are defined as follows:

point A1: an intersection of an outer edge of an outermost electrode on one end side in the acoustic wave propagation direction and the first busbar,

point B1: an intersection of an extension line of an outer edge of an outermost electrode on another end side in the acoustic wave propagation direction and the first busbar,

point C1: an intersection of an extension line of the outer edge of the outermost electrode on the one end side in the acoustic wave propagation direction and the second busbar, and

point D1: an intersection of the outer edge of the outermost electrode on the other end side in the acoustic wave propagation direction and the second busbar.

Therefore, the acoustic wave device1in which cracks are less likely to occur in the piezoelectric film can be provided. The points A1, B1, C1, and D1and the distances xa to xd and ya to yd will be described with reference toFIGS.9and10.

First, as illustrated inFIG.9, each of the points A1, B1, C1, and D1is an intersection of a straight line passing through an outer edge of an outermost electrode finger in the acoustic wave propagation direction and the first busbar5or the second busbar6. For example, the point A1is an intersection of an outer edge of the outermost first electrode finger3and an inner edge5aof the first busbar5. The point C1is an intersection of a straight line passing through the outer edge of the first electrode finger3and an inner edge6aof the second busbar6.

The point B1is an intersection of a straight line passing through an outer edge of the outermost second electrode finger4and the inner edge5aof the first busbar5. The point D1is an intersection of the outer edge of the second electrode finger4and the inner edge6aof the second busbar6.

The points A2, B2, C2, and D2illustrated inFIG.10are defined based on the points A1to D1, respectively, defined in this way.

That is, when the acoustic wave propagation direction is X and a widthwise direction orthogonal or substantially orthogonal to the acoustic wave propagation direction X is Y, as described above, the point A2is a point shifted from the point A1by xa in the X direction and by ya in the Y direction. Similarly, the point B2is a point shifted from the point B1by xb in the X direction and by yb in the Y direction toward the outside of the overlap region K. The point C2is a point shifted from the point C1as a starting point by xc in the X direction and yc in the Y direction toward the outside of the overlap region K. The point D2is a point shifted from the point D1as a starting point by xd in the X direction and yd in the Y direction toward the outside of the overlap region K.

A large number of acoustic wave devices having a piezoelectric film with a thickness of about 500 nm were prepared with varying values of the parameters xa to xd and ya to yd and were inspected for the presence or absence of cracks in the piezoelectric film. The results are shown in Table 1 below. The unit of the parameters xa to xd and ya to yd is nm.

Hereinafter, the parameters xa to xd are collectively referred to as “parameter x”, and the parameters ya to yd are collectively referred to as “parameter y”.

The results of the experiment are illustrated in a graph inFIG.11.FIG.11is a diagram illustrating a relationship between the parameter x or y and the Q factor. A region E1surrounded by a broken line inFIG.11indicates a region in which no cracks occurred in Table 1. A region E2surrounded by a one-dot chain line indicates a region in which the Q factor, that is, the maximum value of Q, is about 470 or more, which is good. A region E3surrounded by a two-dot chain line indicates a region where no cracks occurred and the Q factor is good.

From the results shown in Table 1 andFIG.11above, when condition [1] shown in Table 2 below is satisfied, an acoustic wave device in which cracks are less likely to occur in a piezoelectric film can be provided, and the yield can be increased.

More preferably, when the following condition [2] is satisfied, an acoustic wave device having a higher Q factor and capable of effectively using acoustic wave energy can be provided.

The inventors of preferred embodiments of the present application have confirmed that, when the thickness of the piezoelectric film is within a range of about 500 nm±300 nm, cracks are less likely to occur when the above condition [1] is satisfied. That is, the inventors of preferred embodiments of the present application have confirmed that the above advantageous effect can be obtained when the piezoelectric film is made of one of lithium niobate and lithium tantalate.

FIG.12is a partially cutaway cross-sectional view for explaining an angle α between an inclined side surface of an electrode finger and a piezoelectric film in an acoustic wave device according to a second preferred embodiment of the present invention. The acoustic wave device according to the second preferred embodiment is the same or substantially the same as the acoustic wave device1according to the first preferred embodiment except that inclined side surfaces described below are provided. Therefore, the descriptions provided for the first preferred embodiment will be quoted for the portions other than the inclined side surfaces.

The cross section illustrated inFIG.12illustrates a portion obtained by cutting a first electrode finger3in a direction orthogonal to a direction in which the electrode finger extends, that is, in the Y direction. Preferably, the angle α between the first and second electrode fingers3and4and a principal surface of the piezoelectric film2is set to α>about 90°. More specifically, the first electrode finger3includes a first surface3alocated on the piezoelectric film2side, a second surface3bfacing the first surface3a, and side surfaces3cand3dconnected to the first surface3aand the second surface3b. The side surfaces3cand3dare inclined as illustrated inFIG.12. Therefore, the first electrode finger3is a side-surface-inclined electrode finger. The angle between the side surfaces3cand3dand the principal surface2aof the piezoelectric film2is α. In the first electrode finger3, α>about 90° in a portion where the side surface3cis connected to the first surface3a. Here, the angle α is an angle between the principal surface2aof the piezoelectric film2and the side surface3cor the side surface3dof the first electrode finger3, as illustrated inFIG.12. The first electrode finger3is preferably a side-surface-inclined electrode finger as illustrated inFIG.12, and has a reverse-tapered cross section.

In the present preferred embodiment, the angle α between the side surfaces3cand3dof the first electrode finger3, as well as a pair of side surfaces of the second electrode finger, and the principal surface2aof the piezoelectric film2is set to α>about 90°.

Thus, since the angle α is greater than about 90° and the first electrode finger3has a reverse-tapered shape, unnecessary modes can be effectively reduced in the acoustic wave device1. When the first electrode finger3has a forward-tapered cross section, unnecessary modes are likely to occur.

FIGS.13A to13Care partially cutaway elevational cross-sectional views, illustrating a non-limiting example of a process of manufacturing the acoustic wave device, for explaining a process of forming the first electrode finger3having the angle α. First, a layer of an electrode material3A is deposited on the principal surface2aof the piezoelectric film2. Subsequently, a resist layer is formed on the electrode material3A. Subsequently, the resist layer is patterned to form a resist pattern12illustrated inFIG.13B. This protects a portion where the electrode finger is to be formed. Thereafter, the electrode material3A outside the region where the electrode finger is to be formed is removed by dry etching. In addition, the resist pattern12is removed. In this way, as illustrated inFIG.13C, the reverse-tapered first electrode finger3can be formed. In this case, the angle α can be controlled by controlling the dry etching conditions.

As illustrated inFIG.12, the side-surface-inclined electrode finger may be formed such that the entire surfaces3cand3dform inclined surfaces, but the side-surface-inclined electrode finger is not limited to any particular structure as long as the side surfaces include inclined surface portions. Such modifications will be described below.

FIG.14is a partially cutaway elevational cross-sectional view for explaining a first modification of the acoustic wave device according to the second preferred embodiment.

The first electrode finger3illustrated inFIG.14is a side-surface-inclined electrode finger and includes the side surface3cand the side surface3d. However, the side surface3cincludes a first portion3cand a second portion3e. Similarly, the side surface3dincludes a first portion3d1and a second portion3f. The first portion3cis connected to the first surface3aand is located closer to the first surface3aside than is the second portion3e. The second portion3eis connected to the second surface3band is located closer to the second surface3bside than is the first portion3c1. There is also the same or similar relationship between the first portion3d1and the second portion3f.

The portions extending from the first surface3ato an intermediate height position form the first portions3cand3d1, which are reverse-tapered. The second portions3eand3fare forward-tapered. On the other hand, in the first portions3cand3d1, angle α>about 90°. Thus, also in this case, unnecessary modes can be effectively reduced. In this way, the side surfaces3cand3ddo not need to satisfy α>about 90° in their entirety. When the side surfaces of each electrode finger3are inclined side surfaces, the upper portions of the inclined side surfaces may be side surfaces forming forward tapers, that is, portions with α<about 90°.

FIG.15is a partially cutaway elevational cross-sectional view for explaining a second modification of the acoustic wave device according to the second preferred embodiment. As illustrated inFIG.15, in the first portions3cand3d1connected to the first surface3a, angle α>about 90°. In third portions3gand3hof the side surfaces3cand3d, angle α=about 90°. That is, the third portions3gand3hextend in a direction connecting the first surface3aand the second surface3b. Also in this case, since α>about 90° in the first portions3cand3d1, unnecessary modes can be suppressed. Furthermore, since α=about 90° in the third portions3gand3h, unnecessary modes can be reduced or prevented more effectively than in the first modification illustrated inFIG.14.

FIGS.16A to16Cinclude illustrations for explaining a non-limiting example of a process of forming the first electrode finger illustrated inFIG.14.FIGS.16A to16Care partially cutaway elevational cross-sectional views for explaining the process of forming the first electrode finger3illustrated inFIG.14.

First, as illustrated inFIG.16A, a lower electrode13is formed on the principal surface2aof the piezoelectric film2. Here, a material of the lower electrode13is Ti, for example. The material of the lower electrode may be, for example, Al, Mo, a composite material of, for example, another metal and carbon, or the like instead of Ti.

Subsequently, an upper electrode14illustrated inFIG.16Bis formed by, for example, vapor deposition and lift-off. The material of the upper electrode14may be, for example, Al, Cu, Pt, or an alloy including any of these elements. A material including Al or Cu, for example, is preferable. Thus, the electrical resistance can be lowered. In the present preferred embodiment, the upper electrode14is made of an Al—Cu alloy, for example.

Subsequently, the lower electrode13is removed except for a portion located below the upper electrode14by dry etching using, for example, CF4gas. In this case, the angle α of the side surfaces3cand3dof the lower electrode13can be controlled by controlling the dry etching conditions.