ACOUSTIC WAVE DEVICE

An acoustic wave device includes a support substrate, a piezoelectric layer, and a functional electrode. As seen in a first direction of the support substrate, the piezoelectric layer overlaps the support substrate. The functional electrode extends over a first major surface of the piezoelectric layer. A space is opposite to the first major surface of the piezoelectric layer and at or adjacent to a second major surface of the piezoelectric layer. In the first direction, the functional electrode extends over an overlap region that overlaps the space, and a non-overlap region that does not overlap the space. In the non-overlap region, at least one of an insulating film and a void is located between the functional electrode and the piezoelectric layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an acoustic wave device with a piezoelectric layer including lithium niobate or lithium tantalate.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication No. 2012-257019 describes an acoustic wave device.

SUMMARY OF THE INVENTION

For the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2012-257019, a demand exists to reduce ripples in frequency characteristics.

Preferred embodiments of the present invention provide acoustic wave devices that each allows for reduced ripples in frequency characteristics.

An acoustic wave device according to an aspect of an example embodiment of the present disclosure includes a support substrate, a piezoelectric layer overlapping the support substrate as seen in a first direction, and a functional electrode extending over at least a first major surface of the piezoelectric layer. A space is provided in a location opposite to the first major surface of the piezoelectric layer and at or adjacent to a second major surface of the piezoelectric layer. The space is at least partially covered by the piezoelectric layer. In the first direction, the functional electrode extends over an overlap region that overlaps the space, and a non-overlap region that does not overlap the space. In the overlap region, the functional electrode, and the first major surface of the piezoelectric layer are in contact with each other. In the non-overlap region of the functional electrode, at least one of an insulating film and a void is between the functional electrode and the piezoelectric layer.

An acoustic wave device according to an aspect of an example embodiment of the present disclosure includes a support substrate, a piezoelectric layer overlapping the support substrate as seen in a first direction, a first resonator extending over at least a first major surface of the piezoelectric layer, and a second resonator at a location of the support substrate different from a location of the first resonator. The first resonator includes a first space defined by a hollow in a portion of the support substrate, or a first space defined by an air gap between the support substrate and the piezoelectric layer, the piezoelectric layer that covers at least a portion of the first space, and a first electrode and a second electrode of the first resonator, the first electrode and the second electrode of the first resonator each extending over a first overlap region and a non-overlap region, the first overlap region overlapping the first space as seen in the first direction, the non-overlap region not overlapping the first space as seen in the first direction, the first electrode and the second electrode of the first resonator facing each other in a second direction. The second resonator includes a second space defined by a hollow in a portion of the support substrate, or a second space defined by an air gap between the support substrate and the piezoelectric layer, the piezoelectric layer that covers at least a portion of the second space, and a first electrode and a second electrode of the second resonator, the first electrode and the second electrode of the second resonator each extending over a second overlap region and the non-overlap region, the second overlap region overlapping the second space as seen in the first direction, the non-overlap region not overlapping the second space as seen in the first direction, the first electrode and the second electrode of the second resonator facing each other in the second direction. The second space is at a location different from a location of the first space. In at least a portion of the non-overlap region sandwiched between the first space and the second space, at least one of an insulating film and a void is in at least one of a location between the first electrode of the first resonator and the piezoelectric layer and a location between the second electrode of the second resonator and the piezoelectric layer. The first electrode of the first resonator, and the second electrode of the second resonator are in the non-overlap region sandwiched between the first space and the second space.

Preferred embodiments of the present disclosure reduce ripples in frequency characteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described below in detail with reference to the drawings. These preferred embodiments, however, are not intended to be limiting of the present disclosure. The disclosed preferred embodiments are intended to be illustrative only. Modifications that allow features to be partially combined or replaced with each other between different preferred embodiments, and matters described with reference to the second and subsequent preferred embodiments that are identical to those described with reference to the first preferred embodiment will not be described in further detail, and the following description will focus only on differences. In particular, the same or similar operational effects provided by the same or similar features will not be described for each individual preferred embodiment.

First Preferred Embodiment

FIG.1Ais a perspective view of an acoustic wave device according to a first preferred embodiment.FIG.1Bis a plan view of an arrangement of electrodes according to the first preferred embodiment.

An acoustic wave device1according to the first preferred embodiment includes a piezoelectric layer2made of LiNbO3. The piezoelectric layer2may be made of LiTaO3. The LiNbO3or LiTaO3used has a Z-cut angle according to the first preferred embodiment. The LiNbO3or LiTaO3used may have a rotated Y-cut angle or an X-cut angle. Preferred orientations of propagation are Y-propagation and X-propagation±30°.

Although the thickness of the piezoelectric layer2is not particularly limited, from the viewpoint of effectively exciting a first-order thickness shear mode, the piezoelectric layer2preferably has a thickness of greater than or equal to about 50 nm and less than or equal to about 1000 nm, for example.

The piezoelectric layer2has a first major surface2aand a second major surface2bthat are opposite to each other in a Z-direction. An electrode3and an electrode4extend over the first major surface2a.

The electrode3corresponds to an example of a “first electrode”, and the electrode4corresponds to an example of a “second electrode”. InFIGS.1A and1B, a plurality of electrodes3(hereinafter referred to in the singular as “electrode3” for convenience unless otherwise indicated) are connected with a first busbar5. A plurality of electrodes4(hereinafter referred to in the singular as “electrode4” for convenience unless otherwise indicated) are connected with a second busbar6. Each electrode3and each electrode4are interdigitated with each other.

Each of the electrode3and the electrode4is rectangular or substantially rectangular in shape, and has a longitudinal direction. In a direction orthogonal to the longitudinal direction, the electrode3, and the electrode4adjacent to the electrode3face each other. The longitudinal direction of the electrodes3and4, and a direction orthogonal to the longitudinal direction of the electrodes3and4are each a direction that crosses the thickness direction of the piezoelectric layer2. It can thus be said that the electrode3, and the electrode4adjacent to the electrode3face each other in a direction that crosses the thickness direction of the piezoelectric layer2. In the following description of the first preferred embodiment, it will be sometimes assumed that the thickness direction of the piezoelectric layer2is a Z-direction (or a first direction), a direction orthogonal to the longitudinal direction of the electrodes3and4is an X-direction (or a second direction), and the longitudinal direction of the electrodes3and4is a Y-direction (or a third direction).

The longitudinal direction of the electrodes3and4may be interchanged with the direction orthogonal to the longitudinal direction of the electrodes3and4illustrated inFIGS.1A and1B. That is, the electrode3and the electrode4may extend in a direction in which the first busbar5and the second busbar6extend inFIGS.1A and1B. In that case, the first busbar5and the second busbar6extend in a direction in which the electrode3and the electrode4extend inFIGS.1A and1B. A plurality of pairs of mutually adjacent electrodes3and4, each pair including the electrode3connected with one potential and the electrode4connected with the other potential, are disposed in the direction orthogonal to the longitudinal direction of the electrodes3and4.

When it is stated herein that the electrode3and the electrode4are adjacent to each other, it is not meant that the electrode3and the electrode4are disposed in direct contact with each other but it is meant that the electrode3and the electrode4are disposed with a spacing therebetween. Further, if the electrode3and the electrode4are adjacent to each other, no electrode connected with a hot electrode or a ground electrode, such as another electrode3or4, is disposed between the adjacent electrodes3and4. The number of such electrode pairs does not necessary be an integer but may be 1.5, 2.5, or other non-integer.

The center-to-center distance, that is, the pitch between the electrodes3and4is preferably greater than or equal to about 1 μm and less than or equal to about 10 μm, for example. The center-to-center distance between the electrodes3and4refers to the distance between the center of the width dimension of the electrode3in a direction orthogonal to the longitudinal direction of the electrode3, and the center of the width dimension of the electrode4in a direction orthogonal to the longitudinal direction of the electrode4.

Further, if at least one of the number of electrodes3and the number of electrodes4is more than one (i.e., if, with the electrode3and the electrode4defined as one pair of electrodes, there are 1.5 or more pairs of electrodes), the center-to-center distance between the electrodes3and4refers to the mean of the center-to-center distances of mutually adjacent electrodes3and4among the 1.5 or more pairs of electrodes3and4.

The width of each of the electrodes3and4, that is, the dimension of each of the electrodes3and4in a direction in which the electrodes3and4face each other is preferably greater than or equal to about 150 nm and less than or equal to about 1000 nm, for example. The center-to-center distance between the electrodes3and4refers to the distance between the center of the dimension (width dimension) of the electrode3in the direction orthogonal to the longitudinal direction of the electrode3, and the center of the dimension (width dimension) of the electrode4in the direction orthogonal to the longitudinal direction of the electrode4.

Since the piezoelectric layer according to the first preferred embodiment is a Z-cut piezoelectric layer, the direction orthogonal to the longitudinal direction of the electrodes3and4is a direction orthogonal to the polarization direction of the piezoelectric layer2. This, however, does not hold if a piezoelectric with another cut-angle is used as the piezoelectric layer2. As used herein, the term “orthogonal” may encompass not only strictly orthogonal but also substantially orthogonal (i.e., when the direction orthogonal to the longitudinal direction of the electrodes3and4, and the polarization direction define an angle of, for example, about 90°±10°).

A support member8is stacked over the second major surface2bof the piezoelectric layer2with an intermediate layer7interposed therebetween. The intermediate layer7and the support member8have a frame shape, and respectively have a cavity7aand a cavity8aas illustrated inFIG.2. Due to the configuration mentioned above, a hollow (air gap)9is formed.

The hollow9is provided so that vibration of an excitation region C of the piezoelectric layer2is not prevented. Accordingly, the support member8extends over the second major surface2bwith the intermediate layer7interposed therebetween, at a location not overlapping an area where at least one pair of electrodes3and4is present. No intermediate layer7may be provided. Accordingly, the support member8can be stacked directly or indirectly over the second major surface2bof the piezoelectric layer2.

The intermediate layer7is an insulating layer, and made of silicon oxide. The intermediate layer7may, however, be made of any suitable insulating material other than silicon oxide, such as silicon oxynitride or alumina.

The support member8is also referred to as support substrate, and made of Si. The plane orientation of a face of Si near the piezoelectric layer2may be (100), or may be (110) or (111). Preferably, the Si used has a high resistivity greater than or equal to about 4 kΩ. It is to be noted, however, that the support member8may as well be made of any suitable insulating material or semiconductor material. Examples of suitable materials of the support member8may include piezoelectrics such as aluminum oxide, lithium tantalate, lithium niobate, and quartz, 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 electrodes3, the electrodes4, the first busbar5, and the second busbar6are each made of any suitable metal or alloy such as Al or AlCu alloy. According to the first preferred embodiment, each of the electrode3, the electrode4, the first busbar5, and the second busbar6is a stack of an Al film over a Ti film. It is to be noted, however, that an adhesion layer other than a Ti film may be used.

In driving, an alternating-current voltage is applied between the electrodes3and the electrodes4. More specifically, an alternating-current voltage is applied between the first busbar5and the second busbar6. This makes it possible to provide resonance characteristics employing bulk waves in first-order thickness shear mode excited in the piezoelectric layer2.

The acoustic wave device1is designed such that d/p is less than or equal to about 0.5, for example, where d is the thickness of the piezoelectric layer2, and p is the center-to-center distance between any mutually adjacent electrodes3and4among a plurality of pairs of electrodes3and4. This makes it possible to effectively excite the bulk waves in first-order thickness shear mode mentioned above, and consequently provide resonance characteristics. More preferably, d/p is less than or equal to about 0.24, for example, in which case further improved resonance characteristics can be provided.

It is to be noted that if at least one of the number of electrodes3and the number of electrodes4is more than one as with the first preferred embodiment, that is, if, with the electrode3and the electrode4defined as one pair of electrodes, there are 1.5 or more pairs of electrodes3and4, the center-to-center distance p between mutually adjacent electrodes3and4refers to the mean of the center-to-center distances of the respective pairs of mutually adjacent electrodes3and4.

The above-mentioned configuration of the acoustic wave device1according to the first preferred embodiment helps to reduce a decrease in Q-factor, even if the number of pairs of electrodes3and4is reduced to achieve miniaturization. This is because the resulting resonator does not require a reflector on each side, and thus has no insertion loss. The reason why no reflector is required as mentioned above is because bulk waves in first-order thickness shear mode are used.

FIG.3Ais a schematic cross-sectional illustration for explaining Lamb waves propagating in a piezoelectric layer according to Comparative Example.FIG.3Bis a schematic cross-sectional illustration for explaining bulk waves in first-order thickness shear mode that propagate in the piezoelectric layer according to the first preferred embodiment.FIG.4is a schematic cross-sectional illustration for explaining the amplitude directions of bulk waves in first-order thickness shear mode that propagate in the piezoelectric layer according to the first preferred embodiment.

FIG.3Aillustrates an acoustic wave device like the one described in Japanese Unexamined Patent Application Publication No. 2012-257019, with Lamb waves propagating in the piezoelectric layer. As illustrated inFIG.3A, the waves propagate within a piezoelectric layer201as indicated by arrows. The piezoelectric layer201has a first major surface201a,and a second major surface201b.The thickness direction connecting the first major surface201aand the second major surface201bis defined as the Z-direction. The X-direction refers to a direction in which the fingers of an interdigital transducer (IDT) electrode are arranged. As illustrated inFIG.3A, Lamb waves propagate in the X-direction. Although the piezoelectric layer201vibrates as a whole due to the Lamb waves being plate waves, since the waves propagate in the X-direction, a reflector is disposed on each side to provide resonance characteristics. This results in wave propagation loss. Therefore, an attempt for miniaturization, that is, a reduction in the number of pairs of electrode fingers results in a decrease in Q-factor.

By contrast, with the acoustic wave device according to the first preferred embodiment, vibration displacement occurs in the thickness shear direction as illustrated inFIG.3B. This results in the waves propagating substantially in the direction connecting the first major surface2aand the second major surface2bof the piezoelectric layer2, that is, in the Z-direction, to achieve resonance. That is, the waves have an extremely small X-direction component relative to their Z-direction component. Since the wave propagation in the Z-direction provides the resonance characteristics, no reflector is required. This means that no propagation loss due to wave propagation through the reflector occurs. This helps to reduce a decrease in Q-factor, even if the number of pairs of electrodes3and4is reduced in an attempt to achieve further miniaturization.

As illustrated inFIG.4, the amplitude direction of bulk waves in first-order thickness shear mode is opposite between a first region451and a second region452, which are included in the excitation region C of the piezoelectric layer2(seeFIG.1B).FIG.4schematically illustrates bulk waves generated upon application of a voltage between the electrode3and the electrode4such that the electrode4is at a higher potential than the electrode3. The first region451is a portion of the excitation region C located between a virtual plane VP1and the first major surface2a,the virtual plane VP1being orthogonal to the thickness direction of the piezoelectric layer2and dividing the piezoelectric layer2into two regions. The second region452is a portion of the excitation region C located between the virtual plane VP1and the second major surface2b.

As described above, the acoustic wave device1includes at least one pair of electrodes including the electrode3and the electrode4. Since the acoustic wave device1is not designed for wave propagation in the X-direction, the acoustic wave device1does not necessarily need to include a plurality of such electrode pairs each including the electrode3and the electrode4. That is, the acoustic wave device1may simply include at least one pair of electrodes.

For example, the electrode3is an electrode to be connected with a hot potential, and the electrode4is an electrode to be connected with a ground potential. Alternatively, however, the electrode3may be connected with a ground potential, and the electrode4may be connected with a hot potential. According to the first preferred embodiment, at least one pair of electrodes includes an electrode to be connected with a hot potential or an electrode to be connected with a ground potential as described above, and no floating electrode is provided.

FIG.5illustrates an example of the resonance characteristics of the acoustic wave device according to the first preferred embodiment. The acoustic wave device1with the resonance characteristics illustrated inFIG.5has design parameters described below.

Thickness of piezoelectric layer2: 400 nm

Length of excitation region C (seeFIG.1B): 40 μm

Number of electrode pairs each including the electrode3and the electrode4: 21

Width of electrodes3and4: 500 nm

Intermediate layer7: silicon oxide film with thickness of 1 μm

Support member8: Si

The excitation region C (seeFIG.1B) refers to a region where the electrodes3and4overlap each other when viewed in the X-direction, which is a direction orthogonal to the longitudinal direction of the electrodes3and4. The length of the excitation region C refers to a dimension of the excitation region C in the longitudinal direction of the electrodes3and4.

According to the first preferred embodiment, the center-to-center distance is set equal or substantially equal between all pairs of electrodes3and4. That is, the electrodes3and4are disposed at equal or substantially equal pitches.

As can be appreciated fromFIG.5, improved resonance characteristics with a fractional band width of about 12.5% are obtained, even though no reflector is provided.

According to the first preferred embodiment, d/p is less than or equal to about 0.5, more preferably less than or equal to about 0.24, for example, where d is the thickness of the piezoelectric layer2, and p is the center-to-center distance between the electrode3and the electrode4. This is explained below with reference toFIG.6.

A plurality of acoustic wave devices are obtained in the same manner as with the acoustic wave device having the resonant characteristics illustrated inFIG.5, but with varying values of d/2p.FIG.6illustrates, for the acoustic wave device according to the first preferred embodiment, the relationship between d/2p, and the fractional band width of the acoustic wave device serving as a resonator, where p is the center-to-center distance between mutually adjacent electrodes or the mean center-to-center distance, and d is the mean thickness of the piezoelectric layer.

As illustrated inFIG.6, when d/2p exceeds about 0.25, that is, when d/p>about 0.5, the fractional band width remains below about 5% even as d/p is adjusted. By contrast, when d/2p≤about 0.25, that is, when d/p≤about 0.5, varying d/p within this range makes it possible to provide a fractional band width of greater than or equal to about 5%, that is, a resonator with a high coupling coefficient. When d/2p is less than or equal to about 0.12, that is, when d/p is less than or equal to about 0.24, the fractional band width can be increased to be greater than or equal to about 7%. In addition, adjusting d/p within this range makes it possible to provide a resonator with an even greater fractional band width, and consequently with an even higher coupling coefficient. It can therefore be appreciated that setting d/p less than or equal to about 0.5, for example, makes it possible to provide a resonator with a high coupling coefficient that employs the bulk waves in first-order thickness shear mode mentioned above.

It is to be noted that the at least one pair of electrodes mentioned above may be one pair of electrodes, in which case the value of p mentioned above is the center-to-center distance between mutually adjacent electrodes3and4. If there are 1.5 or more pairs of electrodes, the mean of the center-to-center distances of mutually adjacent electrodes3and4may be defined as p.

For example, if the piezoelectric layer2has thickness variations, its averaged thickness may be used.

FIG.7is a plan view of an example of the acoustic wave device according to the first preferred embodiment that includes one pair of electrodes. The acoustic wave device1includes one pair of electrodes3and4extending over the first major surface2aof the piezoelectric layer2. InFIG.7, K represents intersecting width. As previously mentioned, the acoustic wave device according to the present disclosure may include one pair of electrodes. In this case as well, bulk waves in first-order thickness shear mode can be effectively excited if the value of d/p mentioned above is less than or equal to about 0.5, for example.

FIG.8is a partially cut-away perspective view of an acoustic wave device according to a modification of the first preferred embodiment. An acoustic wave device81includes a support substrate82. The support substrate82has a recess that opens at the top. A piezoelectric layer83is stacked over the support substrate82. Due to the configuration mentioned above, the hollow9in the support substrate82is formed. An IDT electrode84is disposed above the hollow9and over the piezoelectric layer83. Reflectors85and86are disposed beside opposite sides of the IDT electrode84in the direction of acoustic wave propagation. InFIG.8, the peripheral edges of the hollow9are represented by broken lines. In this case, the IDT electrode84includes a first busbar84a,a second busbar84b,a plurality of electrodes84cserving as first electrode fingers, and a plurality of electrodes84dserving as second electrode fingers. The electrodes84care connected with the first busbar84a.The electrodes84dare connected with the second busbar84b.Each electrode84cand each electrode84dare interdigitated with each other.

In the acoustic wave device81, Lamb waves, which are plate waves, are excited through application of an alternating-current electric field to the IDT electrode84extending over the hollow9. The presence of the reflectors85and86beside opposite sides of the IDT electrode84makes it possible to provide resonance characteristics due to the Lamb waves.

FIG.9is a plan view of an acoustic wave device according to Comparative Example.FIG.10is a cross-sectional view taken along a line X-X inFIG.9. As illustrated inFIGS.9and10, in the acoustic wave device according to Comparative Example, a single support member8A supports a first resonator RS1and a second resonator RS2.

The acoustic wave device illustrated inFIGS.9and10includes the support member8A, and a piezoelectric layer with the first major surface2aand the second major surface2b.Electrodes extend over the first major surface2a,and hollows9A and9B are located near the second major surface2b.Over the piezoelectric layer2, the first electrode3of the first resonator RS1, and the second electrode4of the second resonator RS2, which are electrodes at mutually different potentials, are disposed with a non-overlap region NSA interposed therebetween, the non-overlap region NSA not overlapping the hollows9A and9B as seen in plan view in the Z-direction.

For example, a leaky wave LW due to leakage of a wave excited by the electrode3of the first resonator RS1may potentially be reflected by the support member8A and propagate to the electrode4of the second resonator RS2.FIG.11Ais an illustration for explaining frequency characteristics according to Comparative Example.FIG.11Bis an illustration for explaining part of the frequency characteristics illustrated inFIG.11A. InFIGS.11A and11B, the vertical axis represents bandpass characteristics [dB], and the horizontal axis represents frequency. A band of frequencies between a resonant frequency Fr and an anti-resonant frequency Fa, which are illustrated inFIG.11A, is herein referred to as pass band.FIG.11Bis an enlarged illustration of a pass band QQ illustrated inFIG.11A. Within the pass band inFIGS.11A and11B, insertion loss is depicted, and outside the pass band inFIG.11A, attenuation is depicted.

As illustrated inFIG.11B, the acoustic wave device according to Comparative Example has a large number of ripples in the filter pass band QQ due to the leaky wave LW. This can lead to degradation of filter characteristics.

FIG.12is a plan view of the acoustic wave device according to the first preferred embodiment.FIG.13is a cross-sectional view taken along a line XIII-XIII inFIG.12. As illustrated inFIGS.12and13, in the acoustic wave device according to the first preferred embodiment, a single support member8A supports the first resonator RS1and the second resonator RS2. The second resonator RS2is at a location different from that of the first resonator RS1.

The acoustic wave device illustrated inFIGS.12and13includes the support member8A, and the piezoelectric layer2with the first major surface2aand the second major surface2b.The first electrode3and the second electrode4extend over the first major surface2a,and the hollows9A and9B are located near the second major surface2b.The hollow9B is disposed in the Y-direction relative to the hollow9A. One of the first electrode3of the first resonator RS1, and the second electrode4of the second resonator RS2is a hot electrode, and the other is a ground electrode. The first electrode3of the first resonator RS1, and the second electrode4of the second resonator RS2have mutually different potentials. According to the first preferred embodiment, the first electrode3of the first resonator RS1is a ground electrode, and the second electrode4of the second resonator RS2is a hot electrode.

Over the first major surface2aof the piezoelectric layer2, the first electrode3of the first resonator RS1, and the second electrode4of the second resonator RS2are disposed with the non-overlap region NSA interposed therebetween, the non-overlap region NSA not overlapping the hollows9A and9B as seen in plan view in the Z-direction. As seen in the Z-direction, the first electrode3of the first resonator RS1, and the second electrode4of the second resonator RS2each extend over an overlap region SA, which overlaps the hollow9A or9B, and the non-overlap region NSA, which does not overlap the hollow9A or9B.

As illustrated inFIG.12, the first electrode3includes a plurality of electrode fingers33, a first busbar5, and a wiring layer22. The electrode fingers33extend in the Y-direction. The first busbar5is connected with the electrode fingers33. The wiring layer22is connected with an electrode terminal25. The second electrode4includes a plurality of electrode fingers34, a second busbar6, and a wiring layer24. The electrode fingers34extend in the Y-direction. The second busbar6is connected with the electrode fingers34. The wiring layer24is connected with an electrode terminal26. Each of the first electrode3and the second electrode4defines a functional electrode, and is also called an IDT electrode.

In the first electrode3of the first resonator RS1, the electrode fingers33extending over the first major surface2aof the piezoelectric layer2, and the first busbar5correspond to a first portion21. The wiring layer22corresponds to a second portion that is disposed at least partially over the first portion21, and that connects with the first portion21. A connection portion CP1, which defines at least a portion of the first electrode3and connects the first portion21and the wiring layer22, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer22excluding the connection portion CP overlaps the non-overlap region NSA.

In the second electrode4of the second resonator RS2, the electrode fingers34extending over the first major surface2aof the piezoelectric layer2, and the second busbar6correspond to a first portion23. The wiring layer24corresponds to a second portion that is disposed at least partially over the first portion23, and that connects with the first portion23. A connection portion CP2, which defines at least a portion of the second electrode4and connects the first portion23and the wiring layer24, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer24excluding the connection portion CP2overlaps the non-overlap region NSA.

As illustrated inFIGS.12and13, in the non-overlap region NSA, an insulating film31is interposed between the piezoelectric layer2, and each of the wiring layer22of the first resonator RS1and the wiring layer24of the second resonator RS2. The insulating film31is one of silicon oxide, silicon nitride, and resin. The resin to be used is not particularly limited as long as the resin is insulating. Examples of such resin may include polyimide resin and epoxy resin.

Although not particularly limited, the thickness of the insulating film31is preferably greater than or equal to about 500 nm and less than or equal to about 3000 nm, for example, from the viewpoint of the deposition accuracy when the electrode3or the electrode4is positioned adjacent to the insulating film31. If the insulating film31is present in the overlap region SA, this would cause a change in electromechanical coupling coefficient, and consequently affect the filter characteristics within the pass band. For this reason, preferably, the insulating film31is not provided in the overlap region SA. In the overlap region SA, the first electrode3and the second electrode4, which are functional electrodes, are in contact with the first major surface of the piezoelectric layer2.

As described above, the acoustic wave device according to the first preferred embodiment includes the support member8A corresponding to a support substrate, the piezoelectric layer2that, as seen in the Z-direction of the support member8A, overlaps the support member8A, and the first electrode3and the second electrode4. The first electrode3and the second electrode4extend over the first major surface2aof the piezoelectric layer2, face each other in the X-direction that crosses the Z-direction, and have mutually different potentials. The hollow9A and the hollow9B, which are disposed in the support member8A, are spaces located near the second major surface2bof the piezoelectric layer2. The hollow9A and the hollow9B are covered by the piezoelectric layer2. As seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA that does not overlap the hollow9B. In the non-overlap region NSA of the first electrode3, the insulating film31is disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA of the second electrode4, the insulating film31is disposed between the second electrode4and the piezoelectric layer2.

The presence of the insulating film31helps to reduce a leaky wave LW that emanates from the non-overlap region NSA of the first electrode3and the non-overlap region NSA of the second electrode4. The presence of the insulating film31also helps to reduce the possibility of the leaky wave LW reaching the non-overlap region NSA of the first electrode3or the non-overlap region NSA of the second electrode4. This leads to reduced ripples in the filter pass band QQ, and consequently reduced degradation of filter characteristics.

FIG.14is an illustration for explaining a filter including the acoustic wave device according to the first preferred embodiment. The filter illustrated inFIG.14is a ladder filter circuit including a plurality of resonators S1, S2, S3, P1, P2, and P3that each have unique resonant and anti-resonant frequencies, and that are connected in a ladder configuration so as to pass signals in a predetermined frequency band and attenuate signals in other frequency bands.

The first resonator RS1mentioned above is one of the resonators S1, S2, and S3in the series arm, and the second resonator RS2is one of the resonators P1, P2, and P3in the parallel arm. Alternatively, each of the first resonator RS1and the second resonator RS2may be one of the resonators S1, S2, S3, P1, P2, and P3.

FIG.15is a cross-sectional view of an acoustic wave device according to a first modification of the first preferred embodiment. According to the first modification of the first preferred embodiment, as seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and a non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and a non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the insulating film31is disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA2of the first electrode3, the insulating film31is not disposed between the second electrode4and the piezoelectric layer2. The connection portion CP1, which defines at least a portion of the first electrode3and connects the first portion21and the wiring layer22, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer22excluding the connection portion CP1overlaps the non-overlap region NSA1.

A first area, which is the area of the first electrode3in the non-overlap region NSA1of the first resonator RS1illustrated inFIG.15, is larger than a second area, which is the area of the second electrode4in the non-overlap region NSA2of the second resonator RS2. The insulating film31is present in the non-overlap region NSA1, which is the non-overlap region of the electrode with the relatively larger one of the first area and the second area, whereas the insulating film31is not present in the non-overlap region NSA2, which is the non-overlap region of the electrode with the relatively smaller one of the first area and the second area. The electrode with the relatively larger one of the first area and second area is also the one susceptible to greater vibration due to excitation. In the acoustic wave device according to the first modification of the first preferred embodiment, the insulating film31is present in the non-overlap area NSA1of the electrode with the relatively larger one of the first area and the second area as described above. This helps to reduce vibration of the electrode, and consequently reduce degradation of filter characteristics.

FIG.16is a cross-sectional view of an acoustic wave device according to a second modification of the first preferred embodiment. According to the second modification of the first preferred embodiment, as seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the insulating film31is not disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA2of the second electrode4, the insulating film31is disposed between the second electrode4and the piezoelectric layer2. The connection portion CP2, which defines at least a portion of the second electrode4and connects the first portion23and the wiring layer24, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer24excluding the connection portion CP2overlaps the non-overlap region NSA2.

A first area, which is the area of the first electrode3in the non-overlap region NSA1of the first resonator RS1illustrated inFIG.16, is larger than a second area, which is the area of the second electrode4in the non-overlap region NSA2of the second resonator RS2. The insulating film31is not present in the non-overlap region NSA1, which is the non-overlap region of the electrode with the relatively larger one of the first area and the second area, whereas the insulating film31is present in the non-overlap region NSA2, which is the non-overlap region of the electrode with the relatively smaller one of the first area and the second area. A leaky wave LW is more likely to propagate to the electrode with the relatively smaller one of the first area and the second area. In the acoustic wave device according to the second modification of the first preferred embodiment, the insulating film31is present in the non-overlap area NSA2of the electrode with the relatively smaller one of the first area and the second area as described above. This helps to reduce the possibility of the leaky wave LW reaching the electrode, and consequently reduce degradation of filter characteristics.

Second Preferred Embodiment

FIG.17is a cross-sectional view of an acoustic wave device according to a second preferred embodiment.FIG.17illustrates another cross-section taken along the line XIII-XIII inFIG.12. In the acoustic wave device according to the second preferred embodiment, a single support member8A supports the first resonator RS1and the second resonator RS2. The second resonator RS2is at a location different from that of the first resonator RS1. Features according to the second preferred embodiment that are identical to those according to the first preferred embodiment will be designated by the same reference signs and will not be described in further detail.

As seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the insulating film31and a void32are disposed between the first electrode3and the piezoelectric layer2. The void32in the non-overlap region NSA1is surrounded by the first portion21, the insulating film31, the piezoelectric layer2, and the wiring layer22, which corresponds to a second portion. In the non-overlap region NSA2of the second electrode4, the insulating film31and the void32are disposed between the second electrode4and the piezoelectric layer2. The void32in the non-overlap region NSA2is surrounded by the first portion23, the insulating film31, the piezoelectric layer2, and the wiring layer24, which corresponds to a second portion.

The connection portion CP1, which defines at least a portion of the first electrode3and connects the first portion21and the wiring layer22, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer22excluding the connection portion CP1overlaps the non-overlap region NSA1. The connection portion CP2, which defines at least a portion of the second electrode4and connects the first portion23and the wiring layer24, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer24excluding the connection portion CP2overlaps the non-overlap region NSA2.

As described above, the acoustic wave device according to the second preferred embodiment includes the support member8A corresponding to a support substrate, the piezoelectric layer2that, as seen in the Z-direction of the support member8A, overlaps the support member8A, and the first electrode3and the second electrode4. The first electrode3and the second electrode4extend over the first major surface2aof the piezoelectric layer2, face each other in the X-direction that crosses the Z-direction, and have mutually different potentials. The hollow9A and the hollow9B are spaces located near the second major surface2bof the piezoelectric layer2. The hollow9A and the hollow9B are covered by the piezoelectric layer2. As seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the void32is disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA2of the second electrode4, the void32is disposed between the second electrode4and the piezoelectric layer2.

The presence of the void32helps to reduce a leaky wave LW that emanates from the non-overlap region NSA1of the first electrode3and the non-overlap region NSA2of the second electrode4. The presence of the insulating film31also helps to reduce the possibility of the leaky wave LW reaching the non-overlap region NSA1of the first electrode3or the non-overlap region NSA2of the second electrode4. This leads to reduced ripples in the filter pass band QQ, and consequently reduced degradation of filter characteristics.

FIG.18is a cross-sectional view of an acoustic wave device according to a first modification of the second preferred embodiment. According to the first modification of the second preferred embodiment, as seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the insulating film31and the void32are disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA2of the second electrode4, the insulating film31and the void32are not disposed between the second electrode4and the piezoelectric layer2. The connection portion CP1, which defines at least a portion of the first electrode3and connects the first portion21and the wiring layer22, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer22excluding the connection portion CP1overlaps the non-overlap region NSA1.

A first area, which is the area of the first electrode3in the non-overlap region NSA1of the first resonator RS1illustrated inFIG.18, is larger than a second area, which is the area of the second electrode4in the non-overlap region NSA2of the second resonator RS2. The insulating film31and the void32are present in the non-overlap region NSA1, which is the non-overlap region of the electrode with the relatively larger one of the first area and the second area, whereas the insulating film31and the void32are not present in the non-overlap region NSA2, which is the non-overlap region of the electrode with the relatively smaller one of the first area and the second area. The electrode with the relatively larger one of the first area and second area is also the one susceptible to greater vibration due to excitation. In the acoustic wave device according to Modification 1 of the second preferred embodiment, the insulating film31or the void32is present in the non-overlap area NSA1of the electrode with the relatively larger one of the first area and the second area as described above. This helps to reduce vibration of the electrode, and consequently reduce degradation of filter characteristics.

FIG.19is a cross-sectional view of an acoustic wave device according to a second modification of the second preferred embodiment. According to the second modification of the second preferred embodiment, as seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA1that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA2that does not overlap the hollow9B. In the non-overlap region NSA1of the first electrode3, the insulating film31and the void32are not disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA2of the second electrode4, the insulating film31and the void32are disposed between the second electrode4and the piezoelectric layer2. The connection portion CP2, which defines at least a portion of the second electrode4and connects the first portion23and the wiring layer24, is disposed in the overlap region SA as seen in the Z-direction. As seen in the Z-direction, a portion of the wiring layer24excluding the connection portion CP2overlaps the non-overlap region NSA2.

A first area, which is the area of the first electrode3in the non-overlap region NSA1of the first resonator RS1illustrated inFIG.19, is larger than a second area, which is the area of the second electrode4in the non-overlap region NSA2of the second resonator RS2. The insulating film31and the void32are not present in the non-overlap region NSA1, which is the non-overlap region of the electrode with the relatively larger one of the first area and the second area, whereas the insulating film31and the void32are present in the non-overlap region NSA2, which is the non-overlap region of the electrode with the relatively smaller one of the first area and the second area. A leaky wave LW is more likely to propagate to the electrode with the relatively smaller one of the first area and the second area. In the acoustic wave device according to Modification 2 of the second preferred embodiment, the insulating film31or the void32is present in the non-overlap area of the electrode with the relatively smaller one of the first area and the second area as described above. This helps to reduce the possibility of the leaky wave LW reaching the electrode, and consequently reduce degradation of filter characteristics.

Third Preferred Embodiment

FIG.20is a cross-sectional view of an acoustic wave device according to a third preferred embodiment. Unlike in the acoustic wave device according to the first preferred embodiment, in the acoustic wave device according to the third preferred embodiment, the hollow9A and the hollow9B are disposed in the intermediate layer7. Features according to the third preferred embodiment that are identical to those according to the first preferred embodiment will be designated by the same reference signs and will not be described in further detail.

According to the third preferred embodiment, the presence of the hollow9A and the hollow9B in the intermediate layer7makes it possible to increase the accuracy of a membrane region of the piezoelectric layer2that overlaps the hollow9A and the hollow9B. According to the third preferred embodiment, the piezoelectric layer2may, in some cases, be provided with a hole for forming each of the hollow9A and the hollow9B. The piezoelectric layer2covers the hollow9A and the hollow9B except at the location of this hole. As described above, at least a portion of the hollow9A, and at least a portion of the hollow9B are covered by the piezoelectric layer2.

The acoustic wave device according to the third preferred embodiment includes the support member8A corresponding to a support substrate, the piezoelectric layer2that, as seen in the Z-direction of the support member8A, overlaps the support member8A, and the first electrode3and the second electrode4. The first electrode3and the second electrode4extend over the first major surface2aof the piezoelectric layer2, face each other in the X-direction that crosses the Z-direction, and have mutually different potentials. The hollow9A and the hollow9B, which are disposed in the intermediate layer7, are spaces located near the second major surface2bof the piezoelectric layer2. The hollow9A and the hollow9B are covered by the piezoelectric layer2. As seen in the Z-direction, the first electrode3extends over the overlap region SA that overlaps the hollow9A, and the non-overlap region NSA that does not overlap the hollow9A. As seen in the Z-direction, the second electrode4extends over the overlap region SA that overlaps the hollow9B, and the non-overlap region NSA that does not overlap the hollow9B. In the non-overlap region NSA of the first electrode3, the insulating film31is disposed between the first electrode3and the piezoelectric layer2. In the non-overlap region NSA of the second electrode4, the insulating film31is disposed between the second electrode4and the piezoelectric layer2.

The presence of the insulating film31helps to reduce a leaky wave LW that emanates from the non-overlap region NSA of the first electrode3and the non-overlap region NSA of the second electrode4. The presence of the insulating film31also helps to reduce the possibility of the leaky wave LW reaching the non-overlap region NSA of the first electrode3or the non-overlap region NSA of the second electrode4. This leads to reduced ripples in the filter pass band QQ, and consequently reduced degradation of filter characteristics.

Fourth Preferred Embodiment

FIG.21is a cross-sectional view of an acoustic wave device according to a fourth preferred embodiment. The acoustic wave device according to the fourth preferred embodiment includes an upper electrode91, which corresponds to a first electrode, and a lower electrode92, which corresponds to a second electrode, and piezoelectric layers2A and2B. The upper electrode91and the lower electrode92of the first resonator RS1sandwich the piezoelectric layer2A in the Z-direction. The upper electrode91and the lower electrode92of the second resonator RS2sandwich the piezoelectric layer2B in the Z-direction. In the acoustic wave device according to the fourth preferred embodiment, each of the upper electrode91and the lower electrode92defines a functional electrode. The acoustic wave device is also sometimes called a bulk acoustic wave (BAW) device.

In the acoustic wave device according to the fourth preferred embodiment, a single support member8B supports the first resonator RS1and the second resonator RS2. The second resonator RS2is at a location different from that of the first resonator RS1, The hollow9A and the hollow9B, which are disposed in the support member8B, are respectively covered by the piezoelectric layer2A and the piezoelectric layer2B. As seen in the Z-direction, the upper electrode91extends over an overlap region SX that overlaps the hollow9A, and the non-overlap region NSA that does not overlap the hollow9A. As seen in the Z-direction, the lower electrode92extends over the overlap region SX that overlaps the hollow9B, and the non-overlap region NSA that does not overlap the hollow9B. In the non-overlap regions NSA, an insulating film35is disposed between the upper electrode91and the piezoelectric layer2A. In the non-overlap region NSA of the lower electrode92, an insulating film36is disposed between the lower electrode92and the support member8B.

The insulating film35and the insulating film36are made of the same material as that of the insulating film31mentioned above. The presence of the insulating film35and the insulating film36helps to reduce a leaky wave LW that emanates from the non-overlap region NSA of the upper electrode91and the non-overlap region NSA of the lower electrode92. The presence of the insulating film35and the insulating film36also helps to reduce the possibility of the leaky wave LW reaching the overlap region SX. This leads to reduced ripples in the filter pass band QQ, and consequently reduced degradation of filter characteristics.

Fifth Preferred Embodiment

FIG.22is an illustration for explaining, for an acoustic wave device according to a fifth preferred embodiment, the relationship between d/2p, metallization ratio MR, and fractional band width. Features according to the fifth preferred embodiment that are identical to those according to the first preferred embodiment will be designated by the same reference signs and will not be described in further detail. As the acoustic wave device1according to the fifth preferred embodiment, acoustic wave devices1with different values of d/2p and MR are formed, and their fractional band widths are measured. The hatched region on the right-hand side of a broken line D inFIG.22represents a region with a fractional bandwidth of less than or equal to about 17%, for example. The boundary between the hatched region and a non-hatched region is represented as MR=about 3.5(d/2p)+0.075. That is, MR=about 1.75(d/p)+0.075. Accordingly, it is preferable that MR≤about 1.75(d/p)+0.075. In that case, a fractional band width of less than or equal to about 17% can be easily obtained. A more preferable example of the above-mentioned region is the region on the right-hand side of an alternate long and short dashed line D1inFIG.22that represents MR=about 3.5(d/2p)+0.05. In other words, if MR≤about 1.75(d/p) +0.05, this allows a fractional band width of less than or equal to about 17% to be obtained with reliability.

Sixth Preferred Embodiment

FIG.23illustrates, for an acoustic wave device according to a sixth preferred embodiment, a map of fractional band width with respect to the Euler angles (0°, θ, ψ) of LiNbO3with d/p set as close to zero as possible. Features according to the sixth preferred embodiment that are identical to those according to the first preferred embodiment will be designated by the same reference signs and will not be described in further detail. Hatched regions inFIG.23represent regions where a fractional band width of at least greater than or equal to about 5% is obtained. The ranges of individual regions are approximated by Expressions (1), (2), and (3) below.

Therefore, Euler angles within the range represented by Expression (1), (2), or (3) are preferred from the viewpoint of achieving a sufficiently large fractional band width.