Acoustic wave device

An acoustic wave device in which a cavity defining an acoustic reflector is formed on a first main surface side of a substrate, an excitation portion is structured above the cavity in a manner that a first electrode, a piezoelectric thin film, and a second electrode are laminated, and a periodic pattern is provided in a normal direction of a side of the excitation portion on at least one of a first extraction electrode and a second extraction electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave device including an excitation portion on an acoustic reflector.

2. Description of the Related Art

Acoustic wave devices including an acoustic reflector have been generally known. In the acoustic wave device described in Japanese Patent No. 4877966 below, for example, a concave portion is formed on an upper surface of a supporting substrate. A multilayer body is provided on the supporting substrate. The multilayer body includes a lower electrode, a piezoelectric thin film, and an upper electrode. The lower electrode and the upper electrode are mutually overlapped with the piezoelectric thin film interposed therebetween, above the concave portion. In this configuration, a cavity which is formed by the concave portion defines an acoustic reflector.

In acoustic wave devices as the one described in Japanese Patent No. 4877966, a portion in which a lower electrode and an upper electrode are opposed to each other above a cavity serves as an excitation portion. The lower electrode and the upper electrode are joined to an extraction electrode used for electrical connection with an outside. Here, a lower electrode and an upper electrode sometimes extend to an outside of a cavity over a region above the cavity due to a manufacturing error or the like. In this case, the lower electrode and the upper electrode are mutually opposed with a piezoelectric thin film interposed therebetween in an outside region of the cavity. Therefore, a voltage is applied to the piezoelectric thin film also in this opposing portion, generating vibration. Accordingly, the vibration generated in the opposing portion is sometimes leaked to the extraction electrode joined to the lower electrode or the upper electrode.

If a leakage mode is thus propagated to the extraction electrode, spurious components may be generated in the acoustic wave device.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave devices that are each able to reduce or prevent leakage mode propagation.

An acoustic wave device according to a preferred embodiment of the present invention includes a substrate that includes a first main surface, a first electrode on the first main surface of the substrate, a piezoelectric thin film on the first main surface of the substrate and covering at least a portion of the first electrode, a second electrode on the piezoelectric thin film and including a portion which is opposed to the first electrode with the piezoelectric thin film interposed therebetween, and an acoustic reflector on the substrate. The first electrode is laminated on an upper surface of the acoustic reflector. A portion in which the first electrode and the second electrode are opposed to each other with the piezoelectric thin film interposed therebetween above the acoustic reflector is an excitation portion. The acoustic wave device further includes a first extraction electrode and a second extraction electrode. The first extraction electrode is joined to the first electrode and extends from the excitation portion to an exterior of the excitation portion. The second extraction electrode is joined to the second electrode and extends from the excitation portion to the exterior of the excitation portion. A periodic pattern is provided along a direction separating from the excitation portion on at least one of the first extraction electrode and the second extraction electrode.

Acoustic wave devices according to the preferred embodiments of the present invention are each able to reduce or prevent leakage mode propagation toward at least one of the first extraction electrode and second extraction electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained below by describing specific preferred embodiments of the present invention with reference to the accompanying drawings.

Each of the preferred embodiments described in this specification is provided as an example, and partial substitutions or combinations of features may be implemented between different preferred embodiments.

FIG.1is a plan view of an acoustic wave device according to a first preferred embodiment of the present invention, andFIG.2is a front sectional view showing portions of the acoustic wave device.

An acoustic wave device1includes a substrate2. The substrate2is made of insulation ceramics, for example, alumina or silicon. The material of the substrate2is not particularly limited. The substrate2includes a first main surface2aand a second main surface2b. A concave portion2cis provided on the first main surface2a. The concave portion2cincludes a cavity X that defines an acoustic reflector.

A first electrode3is provided on the first main surface2a. The first electrode3extends above the concave portion2c. An end portion3aof the first electrode3extends to an outside region of the concave portion2cover the concave portion2c. A piezoelectric thin film4is laminated to cover at least a portion of the first electrode3. A second electrode5is provided on the piezoelectric thin film4. The second electrode5includes a portion opposed to the first electrode3with the piezoelectric thin film4interposed therebetween.

The laminate of the first electrode3and the piezoelectric thin film4closes the concave portion2c, to provide the cavity X. The cavity X defines and functions as an acoustic reflector.

Above the cavity X, a portion in which the first electrode3and the second electrode5are opposed to each other with the piezoelectric thin film4interposed therebetween is an excitation portion Y. When an AC electric field is applied to the first electrode3and the second electrode5, the piezoelectric thin film4expands and contracts and an acoustic wave is excited. In the first preferred embodiment, an acoustic wave in a thickness slip mode or thickness extension mode, for example, is excited as an acoustic wave. That is, the acoustic wave device1is a BAW device utilizing one of these modes.

The materials of the first electrode3and the second electrode5are not particularly limited. Examples of the materials may include metals, for example, Al, Cu, Mo, W, and Ru or alloys including these metals.

The piezoelectric thin film4is made of a piezoelectric material. Examples of the piezoelectric material may include a piezoelectric thin film material, for example, aluminum nitride, zinc oxide, and piezoelectric zirconate titanate (PZT), and a single crystal of lithium niobate or lithium tantalate. Aluminum nitride may be doped with other elements. As an element to dope aluminum nitride, at least one rare earth element selected from the group of scandium, yttrium, lanthanum, and erbium may be included.

The first electrode3and the second electrode5are electrically connected to an exterior of the acoustic wave device1to apply a voltage to a portion between the first electrode3and the second electrode5. To provide an electrical connection, a first extraction electrode6and a second extraction electrode7are respectively joined to the first electrode3and the second electrode5.

The first and second extraction electrodes6and7are made of the same or substantially the same material as the first electrode3and second electrode5. Preferably, the first and second extraction electrodes6and7are integrally formed with the first and second electrodes3and5, respectively, with the same material.

InFIG.2, a portion between a dashed line Z1and a dashed line Z2is the excitation portion Y described above.

The excitation portion Y preferably has a larger area on the cavity X in the acoustic wave device1, for example, and the first electrode3extends to the end portion of the cavity X. To make the first electrode3securely extend to the end portion of the cavity X, the end portion3amay be extended over the cavity X to the second extraction electrode7side. With the end portion3aextending over the cavity X to the second extraction electrode side, and with the piezoelectric thin film4interposed therebetween, a portion of the first electrode3from an outer peripheral edge of the cavity X to the end portion3ais opposed to the second electrode5with the piezoelectric thin film4interposed therebetween. That is, an AC electric field is applied to the piezoelectric thin film4in the region between the dashed line Z2and the dashed line Z3inFIG.2. Accordingly, vibration is also generated in the portion between the dashed line Z2and the dashed line Z3and the vibration may be propagated to the second extraction electrode7side as a leakage mode.

If a leakage mode is propagated to the second extraction electrode7side, the leakage mode appears as a spurious mode on resonance characteristics when a piezoelectric resonator is configured, for example.

The acoustic wave device1according to the present preferred embodiment is provided with a periodic pattern11, thus reducing or preventing a leakage mode.

As illustrated inFIG.1, the excitation portion Y has a pentagonal or substantially pentagonal shape in plan view, and includes five sides12ato12e, in the first preferred embodiment. In a normal direction B1of one side12aamong the five sides12ato12e, a plurality of protrusion portions11aand11bare provided at intervals. More specifically, though not always limited, the second extraction electrode7extends in the normal direction B1of the side12ain the first preferred embodiment. Further, the second extraction electrode7includes a pair of side edges7aand7b. The side edges7aand7bextend in the normal direction B1of the side12a. The plurality of protrusion portions11aare provided from the side edge7ain the direction orthogonal or substantially orthogonal to the normal direction B1. Also on the side edge7b, the plurality of protrusion portions11bare provided from the side edge7bin the direction orthogonal or substantially orthogonal to the normal direction B1. The plurality of protrusion portions11aare located at regular intervals in the normal direction B1. The protrusion portions11bare also located at regular intervals in the normal direction B1. Thus, the pattern11including the plurality of protrusion portions11aand11bis a periodic pattern along the normal direction B1. Here, the “periodic” state includes not only a state in which a pattern includes a plurality of protrusion portions whose intervals are all equal to each other, but also a state in which a pattern includes a plurality of protrusion portions whose intervals are different from each other within about ±20% when any one interval between protrusion portions is set as a reference.

Further, one protrusion portion11aand one protrusion portion11bare opposed to each other in the direction parallel or substantially parallel to the side12a, with the second extraction electrode7interposed therebetween, in the present preferred embodiment. Thus, a plurality of pairs of protrusion portions, each of which includes one protrusion portion11aand one protrusion portion11b, are provided.

However, the plurality of protrusion portions11aand11bdo not always have to make pairs. Also, the plurality of protrusion portions11aand11bextend in the direction orthogonal or substantially orthogonal to the side edges7aand7b, that is, the direction orthogonal or substantially orthogonal to the normal direction B1, but the protrusion portions11aand11bmay extend in an intersecting direction other than the orthogonal or substantially orthogonal direction.

In the acoustic wave device1, when an AC electric field is applied between the first electrode3and the second electrode5, an acoustic wave in the thickness slip mode or thickness extension mode described above is excited in the excitation portion Y. Thus, the acoustic wave device1is able to provide resonance characteristics.

The AC electric field is also applied to the portion between the dashed line Z2and the dashed line Z3, and a leakage mode is generated. The leakage mode is propagated in the normal direction B1of the side12afrom the excitation portion Y toward the second extraction electrode7. However, the propagated leakage mode is Bragg-reflected by the periodic pattern11, and thus the propagation of the leakage mode toward the second extraction electrode7is able to be reduced or prevented. Accordingly, a resonator having favorable resonance characteristics is able to be provided.

The periodic pattern11is provided along the normal direction B1in the first preferred embodiment. However, the pattern11may instead only be provided in a periodic manner on the first extraction electrode6and the second extraction electrode7in a direction separating from the excitation portion Y.

A material of the periodic pattern11is preferably the same metal as that of the second extraction electrode7. However, a metal different from that of the second extraction electrode7may be included. Also, the second extraction electrode7is not limited to a metal, and a dielectric, for example, silicon oxide, may be included.

Leakage mode propagation is able to be reduced or prevented without increasing a manufacturing cost by only including the periodic pattern11.

The plurality of protrusion portions11amay be made of the same or substantially the same material as the second extraction electrode7. Accordingly, the protrusion portions11aare able to be joined to the side edge7aas illustrated inFIG.1. Similar features apply to the plurality of protrusion portions11b. However, the plurality of protrusion portions11aand11bmay be made of a material different from that of the second extraction electrode7. Further, the thickness of the plurality of protrusion portions11aand11bis not particularly limited, and the thickness of the protrusion portions11aand11bmay be equivalent to or smaller than the sum of the thickness of the piezoelectric thin film4and the thickness of the second extraction electrode7. The protrusion portions11aand11bmay even have the thickness larger than the sum of the thickness of the piezoelectric thin film4and the thickness of the second extraction electrode7.

Preferably, for example, the plurality of protrusion portions11aand11bare integrally provided with the second extraction electrode7with the same material of the second extraction electrode7. Accordingly, a manufacturing process is able to be simplified.

In a second preferred embodiment illustrated inFIG.3, the protrusion portion11aand the protrusion portion11bare coupled with each other by a coupling portion11cand are integrated with each other. The protrusion portion11aand the protrusion portion11bmay be thus integrated with each other through the coupling portion11c. Here, the coupling portion11cis provided on the second extraction electrode7inFIG.3, but the coupling portion11cmay be provided on a lower surface of the second extraction electrode7. Accordingly, the second extraction electrode7may be formed after forming the pattern11.

FIG.4is a plan view of an acoustic wave device according to a third preferred embodiment of the present invention. In an acoustic wave device31, a plurality of protrusion portions11aand a plurality of protrusion portions11binclude an outer peripheral edge not defining a rectangle but defining a curve. In particular, the protrusion portions11aand the protrusion portions lib have a shape obtained by cutting out a portion of an ellipse. The protrusion portions11aand11bdefining the pattern11may have a curved outer peripheral edge.

FIG.5is a plan view of an acoustic wave device according to a fourth preferred embodiment of the present invention. In an acoustic wave device41, a plurality of convex portions11dare provided in the second extraction electrode7in plan view, as illustrated inFIG.5. The periodic pattern11is thus provided.FIG.6is a sectional view illustrating a portion taken along the B-B line ofFIG.5. As illustrated inFIG.6, the plurality of convex portions11dare formed to protrude upward from the second extraction electrode7. The plurality of convex portions11dare located at regular intervals in the normal direction of the side12a. The periodic pattern11is thus formed in the normal direction.

The periodic pattern11may be provided by forming the convex portions11dillustrated inFIG.6, that is, by mass addition.

FIG.7is a sectional view showing a periodic pattern in an acoustic wave device according to a modification of the fourth preferred embodiment of the present invention.FIG.7is also a sectional view of the portion corresponding to the B-B line ofFIG.5. In the fourth preferred embodiment, a plurality of concave portions11eare formed instead of the convex portions11d. That is, the plurality of concave portions11eare located at regular intervals in the normal direction of the side12ainFIG.5. The periodic pattern11is thus formed. The periodic pattern may be formed by providing the concave portions11einstead of the convex portions11d.

A portion of the second extraction electrode7remains on the bottom portions of the concave portions11einFIG.7. On the other hand, the bottom portions of the concave portions11emay be positioned on the piezoelectric thin film4. In particular, the concave portions11emay be provided as electrode-free portions in which there are no electrode materials.

The material of the pattern11is preferably the same metal as that of the second extraction electrode7, for example.

However, a metal different from that of the second extraction electrode7may be provided for the material of the pattern11. Also, the material of the pattern11is not limited to a metal, and a dielectric, for example, silicon oxide, may be included.

The features described above are able to reduce or prevent leakage mode propagation without increasing a manufacturing cost because the structure only requires that the periodic pattern11is provided.

In the structures provided with the convex portions11dand the concave portions11e, respectively illustrated inFIG.6andFIG.7, periodic mismatching is provided in the second extraction electrode7. Accordingly, leakage mode propagation is able to be more effectively reduced or prevented.

FIG.8is a plan view of an acoustic wave device according to a fifth preferred embodiment of the present invention. In an acoustic wave device51according to the fifth preferred embodiment, the excitation portion Y has a pentagonal or substantially pentagonal shape in plan view, and includes five sides12ato12e, similar to the first preferred embodiment. Periodic patterns11are respectively provided in the normal direction B1of the side12aand the normal direction B2of the side12b, in the fifth preferred embodiment. That is, the second extraction electrode7is extracted from both of the side12aand the side12baway from the excitation portion Y. A plurality of protrusion portions11aand a plurality of protrusion portions11bare provided along the normal directions B1and B2of the sides12aand12b, respectively. Thus, the second extraction electrode7may straddle the plurality of sides12aand12bof the excitation portion Y having a polygonal planar shape. The periodic pattern11is preferably provided, for example, along both of the normal directions B1and B2of the side12aand the side12b, as the fifth preferred embodiment. An interval between the plurality of protrusion portions11ain the normal direction B1of the side12amay be equal to or different from an interval between the plurality of protrusion portions11bprovided in the normal direction B2of the side12b. The interval or a pitch between the plurality of protrusion portions11aand a pitch between the plurality of protrusion portions11b, for example, may be determined with respect to acoustic characteristics of an acoustic stack and the like.

Further, the plurality of protrusion portions11aand11bare provided to extend to an interior of the second extraction electrode7in the acoustic wave device51.

The protrusion portion11aand the protrusion portion11bmay be coupled with each other by the coupling portion11cas an acoustic wave device61, according to a sixth preferred embodiment of the present invention illustrated inFIG.9. In the sixth preferred embodiment, the structure including the protrusion portions11a, the coupling portions11c, and the protrusion portions11bis radially expanded from the excitation portion Y.

FIG.10is a plan view of an acoustic wave device according to a seventh preferred embodiment of the present invention. In an acoustic wave device71according to the seventh preferred embodiment, the excitation portion Y has an elliptical or substantially elliptical shape in plan view. Thus, the planar shape of the excitation portion Y is not limited to a polygonal shape including a plurality of sides but may be a shape which has a curve in at least a portion thereof. The excitation portion Y has the elliptical or substantially elliptical shape, and this shape may be a shape provided by significantly increasing the number of sides of the polygonal of the excitation portion Y in the above-described preferred embodiments. Accordingly, when the second extraction electrode7is extended from the outer peripheral edge of the excitation portion Y to the outside, the normal direction B1may be defined as a direction orthogonal or substantially orthogonal to a virtual line which is obtained by connecting two connection points on which the side edges7aand7bof the second extraction electrode7are connected to the excitation portion Y, as illustrated inFIG.10. Furthermore, in the normal direction B1, the plurality of protrusion portions11aand11bare located at regular intervals in the normal direction B1. The periodic pattern11is thus provided. The structure described above also includes the periodic pattern11and is able to reduce or prevent leakage mode propagation as is the case with the first preferred embodiment.

FIG.11is a plan view of an acoustic wave device according to an eighth preferred embodiment of the present invention. In an acoustic wave device81according to the eighth preferred embodiment, the normal directions B1and B2are directions which are orthogonal or substantially orthogonal to tangents with respect to the outer peripheral edge of the excitation portion Y. The tangents respectively pass through contact points between the side edges7a,7band the outer peripheral edge of the excitation portion Y. Further, a plurality of protrusion portions11aand11b, protruding from the side edges7aand7bof the second extraction electrode7, are provided. The protrusion portions11aand11breach virtual lines extending from the above-mentioned contact points in the normal directions B1and B2, thus providing the radial pattern11. The protrusion portions11aand11borthogonally or substantially orthogonally intersect with the normal directions B1and B2respectively in the eighth preferred embodiment.

FIG.12is a plan view of an acoustic wave device according to a ninth preferred embodiment of the present invention. In an acoustic wave device91according to the ninth preferred embodiment, the second extraction electrode7is divided into segment extraction portions7A to7C. Thus, the second extraction electrode7may be divided into a plurality of lines of segment extraction portions7A to7C. A plurality of protrusion portions11aand11bare provided from side edges of the segment extraction portions7A to7C in the direction orthogonal or substantially orthogonal, to the normal direction B1of the side12a. Here, a protrusion portion11abetween adjacent segment extraction portions7B and7C and a protrusion portion11bbetween adjacent segment extraction portions7A and7B are provided as a single component.

In the ninth preferred embodiment, the plurality of protrusion portions11aand11bare located at intervals in the normal direction B1, and are able to reduce or prevent leakage mode propagation. The segment extraction portions7A to7C are preferably parallel or substantially parallel to the normal direction B1and are parallel or substantially parallel to each other. The above features are able to reduce a parasitic resistance. Accordingly, an energy confinement effect of the acoustic wave device91is able to be improved.

Further, the segment extraction portions7A to7C are preferably located at regular intervals in the direction in which the side12aextends. The structure and location of the segment extraction portions7A to7C provides a Bragg reflection effect not only in the normal direction B1but also in the direction in which the side12aextends. Accordingly, leakage mode propagation is able to be more effectively reduced or prevented. Here, the segment extraction portions7A to7C do not always have to be located at regular intervals. The Bragg reflection effect is able to be provided as long as the segment extraction portions7A to7C are located at intervals. Therefore, at least three segment extraction portions7A to7C are preferably provided, for example.

FIG.13is a front sectional view showing portions of an acoustic wave device according to a tenth preferred embodiment of the present invention. In an acoustic wave device101, the second extraction electrode7extends to the first extraction electrode6side over the cavity X. Therefore, vibration is also generated in a portion between dashed lines Z4and Z1on the first extraction electrode6side of the excitation portion Y, generating a leakage mode. In this case, the periodic pattern11may be provided on the first extraction electrode6. In particular, in the acoustic wave device101according to the tenth preferred embodiment, the periodic pattern11is provided to both of the first extraction electrode6and the second extraction electrode7and thus, leakage mode propagation is able to be reduced or prevented on both sides.

InFIG.13, when the end portion3aof the first electrode3is positioned in the cavity X, the periodic pattern11does not have to be provided on the second extraction electrode7side.

The periodic pattern11is provided on at least one of the first extraction electrode6and the second extraction electrode7, as described above.

FIG.14is a front sectional view showing portions of an acoustic wave device according to an eleventh preferred embodiment of the present invention. In an acoustic wave device111, an acoustic Bragg reflector112is provided instead of the cavity X. The acoustic Bragg reflector112defines and functions as an acoustic reflector. The acoustic Bragg reflector112has a structure provided by alternately laminating low acoustic impedance material layers112a,112c, and112eand high acoustic impedance material layers112b,112d, and112f. The low acoustic impedance material layers112a,112c, and112ehave a relatively low acoustic impedance, and the high acoustic impedance material layers112b,112d, and112fhave a relatively high acoustic impedance. Thus, the present invention may employ an acoustic Bragg reflector obtained by laminating low acoustic impedance material layers and high acoustic impedance material layers as an acoustic reflector. The above features do not particularly limit the numbers of laminated layers of low acoustic impedance material layers and high acoustic impedance material layers.