Acoustic wave device and method of fabricating the same, filter, and multiplexer

An acoustic wave device includes: a support substrate; a single piezoelectric substrate that is located on the support substrate and is single-crystal; first electrodes located on a first surface of the piezoelectric substrate; second electrodes located on a second surface of the piezoelectric substrate; and an acoustic mirror that is bonded on the support substrate, is located between the support substrate and the first electrodes in resonance regions where the first electrodes and the second electrodes face each other across at least a part of the piezoelectric substrate, is not located between the support substrate and the first electrodes in at least a part of a region between the resonance regions, and reflects an acoustic wave propagating through the piezoelectric substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2017-220765, filed on Nov. 16, 2017, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to an acoustic wave device and a method of fabricating the same, a filter, and a multiplexer.

BACKGROUND

Acoustic wave devices including piezoelectric thin film resonators have been used as filters and multiplexers for wireless devices such as, for example, mobile phones. It has been known to use a single-crystal piezoelectric substrate as a piezoelectric layer of the piezoelectric thin film resonator as disclosed in, for example, Japanese Patent Application Publication Nos. 2012-16512, 2010-136317, 2013-223025, and H10-51262. It has been known to stack piezoelectric layers having opposite polarization directions as disclosed in, for example, Japanese Patent Application Publication Nos. 2012-165132, H10-51262, and 2007-36915.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an acoustic wave device including: a support substrate; a single piezoelectric substrate that is located on the support substrate and is single-crystal; first electrodes located on a first surface of the piezoelectric substrate; second electrodes located on a second surface of the piezoelectric substrate; and an acoustic mirror that is bonded on the support substrate, is located between the support substrate and the first electrodes in resonance regions where the first electrodes and the second electrodes face each other across at least a part of the piezoelectric substrate, is not located between the support substrate and the first electrodes in at least a part of a region between the resonance regions, and reflects an acoustic wave propagating through the piezoelectric substrate.

According to a second aspect of the present invention, there is provided a filter including the above acoustic wave device.

According to a third aspect of the present invention, there is provided a multiplexer including the above filter.

According to a fourth aspect of the present invention, there is provided a method of fabricating an acoustic wave device, the method including: forming first electrodes on a first surface of a single piezoelectric substrate that is single-crystal; forming second electrodes on a second surface of the piezoelectric substrate so that resonance regions where the first electrodes and the second electrodes face each other across at least a part of the piezoelectric substrate are formed; forming an acoustic mirror, which reflects an acoustic wave propagating through the piezoelectric substrate, on the first surface so that the acoustic mirror covers the first electrodes; leaving the acoustic mirror in regions to be the resonance regions, and removing the acoustic mirror in at least a part of a region between the regions to be the resonance regions; and bonding the acoustic mirror to a support substrate after the removing of the acoustic mirror.

DETAILED DESCRIPTION

The use of a single-crystal piezoelectric substrate as the piezoelectric layer of the piezoelectric thin film resonator improves the characteristics of the piezoelectric thin film resonator. It is difficult to form the single-crystal piezoelectric substrate made of lithium tantalate or lithium niobate on a support substrate. Thus, in Japanese Patent Application Publication No. 2012-165132, a single piezoelectric substrate having a plurality of piezoelectric thin film resonators is attached to the support substrate. However, a fabrication step of forming a hollow becomes complicating, and damage and/or distortion tends to be introduced into the piezoelectric substrate having the piezoelectric thin film resonators formed thereon.

First Embodiment

A ladder-type filter will be described as an example of the acoustic wave device.FIG. 1is a circuit diagram of a ladder-type filter in a first embodiment. As illustrated inFIG. 1, series resonators S1through S4are connected in series and the parallel resonators P1through P3are connected in parallel between an input terminal Tin and an output terminal Tout.

FIG. 2is a plan view of an acoustic wave device in accordance with the first embodiment.FIG. 3Ais a cross-sectional view taken along line A-A inFIG. 2,FIG. 3Bis a cross-sectional view taken along line B-B inFIG. 2, andFIG. 3Cis a cross-sectional view taken along line C-C inFIG. 2. As illustrated inFIG. 3A, an acoustic mirror30and an insulating film20are located on a support substrate10. Lower electrodes12are located on the acoustic mirror30and the insulating film20. A piezoelectric substrate14is located on the lower electrodes12. Upper electrodes16are located on the piezoelectric substrate14.

The support substrate10is an insulating substrate such as, for example, a silicon substrate, an alumina substrate, a quartz substrate, a spinel substrate, a glass substrate, or a crystal substrate. The upper surface of the support substrate10is, for example, flat. The acoustic mirror30includes first films32aand second films32balternately stacked. The acoustic impedance of the second film32bis higher than the acoustic impedance of the first film32a. The first film32aand the second film32bare, for example, insulating films or metal films. The first film32ais, for example, a silicon oxide film, and the second film32bis, for example, a tungsten (W) film, a ruthenium (Ru) film, or a molybdenum (Mo) film. When the wavelength of the acoustic wave in the primary mode propagating through the piezoelectric substrate14in the longitudinal direction is represented by λ, the film thickness of each of the first film32aand the second film32bis approximately ¼λ. The insulating film20is a film more flexible than the acoustic mirror30(i.e., a film having a less Young's modulus), and is, for example, a resin film such as a polyimide film.

The piezoelectric substrate14is single-crystal, and is shared by the piezoelectric thin film resonators. That is, the piezoelectric substrate14is a single substrate. A lower surface22and an upper surface24of the piezoelectric substrate14are flat. The piezoelectric substrate14is, for example, a single-crystal lithium tantalate substrate, a single-crystal lithium niobate substrate, a single-crystal aluminum nitride substrate, or a single-crystal crystal substrate. The lithium tantalate substrate or the lithium niobate substrate are, for example, an X-cut substrate (i.e., the normal direction of the upper surface of the piezoelectric substrate14corresponds to the X-axis orientation of the crystal orientation). The piezoelectric substrate14has a film thickness of approximately ½λ.

The lower electrodes12are located on the lower surface22of the piezoelectric substrate14, and the upper electrodes16are located on the upper surface24. The region where the lower electrode12and the upper electrode16face each other across at least a part of the piezoelectric substrate14is a resonance region50. The resonance region50is a region in which the acoustic wave resonates in the longitudinal direction. The planar shape of the resonance region50is, for example, an elliptical shape, a circular shape, or a polygonal shape. The lower electrode12and the upper electrode16are formed of a single-layer metal film made of, for example, Ru, chrome (Cr), aluminum (Al), titanium (Ti), copper (Cu), Mo, W, tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Jr), or a multilayered metal film of at least two of them.

The acoustic mirror30is located so as to include the resonance region50in plan view. The acoustic mirror30is not located in at least a part of a region54between the resonance regions50. The insulating film20is located in the region where no acoustic mirror30is located. The acoustic wave excited in the resonance region50of the piezoelectric substrate14is reflected by the acoustic mirror30. Accordingly, the acoustic wave is confined in the piezoelectric substrate14. Since the insulating film20is located between the resonance regions50, the interference between the resonance regions50is reduced.

As illustrated inFIG. 2, the piezoelectric thin film resonators include the series resonators S1through S4and the parallel resonators P1through P3. The lower electrode12and/or the upper electrode16connects between the resonance regions50of the piezoelectric thin film resonators. A metal film such as an Au film or a Cu film for reducing the resistance may be located under the lower electrode12and/or on the upper electrode16in regions other than the resonance regions50. The input terminal Tin, the output terminal Tout, and ground terminals Tgnd connecting to the parallel resonators P1through P3include the upper electrode16.

As illustrated inFIG. 2andFIG. 3B, in a region52other than the resonance region50and the region54, the acoustic mirror30and the lower electrode12may support the piezoelectric substrate14. The acoustic mirror30may not be necessarily located in all the regions other than the resonance regions50and the region54.

As illustrated inFIG. 2andFIG. 3C, when the lower electrode12and the upper electrode16are electrically connected, a through hole is formed in the piezoelectric substrate14. A through electrode18is located in the through hole. The through electrode18electrically connects the lower electrode12and the upper electrode16. The input terminal Tin, the output terminal Tout, and the ground terminals Tgnd are external connection terminals for electrically connecting to an external element. A bump or a bonding wire is in contact with the external connection terminal from above. Thus, the external connection terminal is formed on the upper surface of the piezoelectric substrate14. When the lower electrode12is coupled to the external connection terminal, the lower electrode12and the upper electrode16are electrically connected with use of the through electrode18. The through electrode18is formed of, for example, a metal layer such as an Au layer or a Cu layer.

Fabrication Method of the First Embodiment

FIG. 4AthroughFIG. 6Bare cross-sectional views illustrating a method of fabricating the acoustic wave device of the first embodiment. As illustrated inFIG. 4A, the single-crystal piezoelectric substrate14in a wafer state is prepared. The piezoelectric substrate14has a film thickness of, for example, hundreds of micrometers, and is fabricated by the Czochralski method or the like. InFIG. 4AthroughFIG. 4D, the lower surface22of the piezoelectric substrate14is at the upper side. As illustrated inFIG. 4B, the lower electrode12is formed on the lower surface22of the piezoelectric substrate14. The lower electrode12having a desired shape is formed by, for example, sputtering or vacuum evaporation, and etching or liftoff.

As illustrated inFIG. 4C, the acoustic mirror30is formed on the piezoelectric substrate14so as to cover the lower electrode12. The first films32aand the second films32bare alternately formed as the acoustic mirror30. The first film32aand the second film32bare formed by, for example, sputtering, vacuum evaporation, or Chemical Vapor Deposition (CVD). As illustrated inFIG. 4D, the acoustic mirror30is patterned into a desired shape by removing a part of the acoustic mirror30by, for example, etching.

As illustrated inFIG. 5A, the insulating film20is formed on the support substrate10. The insulating film20is formed by, for example, applying resin or attaching a sheet. The piezoelectric substrate14is arranged above the support substrate10so that an opposite surface34of the acoustic mirror30from the piezoelectric substrate14faces the support substrate10. At this time, the piezoelectric substrate14and the support substrate10are in a wafer state. As illustrated inFIG. 5B, the surface34of the acoustic mirror30is attached to the surface of the support substrate10. As an example, when a thermoset resin is used as the insulating film20, a resin layer before thermally cured is formed on the support substrate10. Then, the acoustic mirror30is attached to the support substrate10while deforming the resin layer. Thereafter, the resin layer is hardened by heating, and the insulating film20is thereby formed. The insulating film20may remain between the support substrate10and the acoustic mirror30to function as an adhesive agent that bonds the support substrate10and the acoustic mirror30. As another example, an aperture having the size substantially identical to the size of the acoustic mirror30is formed in the insulating film20. Then, the acoustic mirror30is embedded in the aperture. The acoustic mirror30and the support substrate10may be indirectly bonded through an adhesive agent, or may be directly bonded.

As illustrated inFIG. 6A, the upper surface of the piezoelectric substrate14is polished by, for example, Chemical Mechanical Polishing (CMP). This process makes the piezoelectric substrate14have a desired thickness (for example, from 1 μm to 10 μm). As illustrated inFIG. 6B, the upper electrode16is formed on the upper surface24of the piezoelectric substrate14. The upper electrode16having a desired shape is formed by, for example, sputtering or vacuum evaporation, and etching or liftoff. Thereafter, the support substrate10, the insulating film20, and the piezoelectric substrate14are cut by dicing or the like. The above-described processes complete the acoustic wave device of the first embodiment.

A polycrystalline aluminum nitride (AlN) film or a polycrystalline zinc oxide (ZnO) film is used as the piezoelectric film of the piezoelectric thin film resonator. When the aluminum nitride film is used in the piezoelectric thin film resonator, the electromechanical coupling coefficient is from 6% to 7%, and the temperature coefficient of frequency (TCF) such as the temperature coefficient of the resonant frequency is −30 ppm/° C. When zinc oxide is used in the piezoelectric thin film resonator, the electromechanical coupling coefficient is 8.5%, but the TCF is −60 ppm/T.

In the first embodiment, the piezoelectric substrate14that is single-crystal is used in the piezoelectric thin film resonator. For example, when an X-cut single-crystal lithium tantalate substrate or an X-cut single-crystal lithium niobate substrate is used, the electromechanical coupling coefficient is 18%, and the TCF of the resonant frequency is several ppm/° C. As described above, the use of the single-crystal piezoelectric substrate14improves the characteristics such as the electromechanical coupling coefficient and the TCF. However, when the piezoelectric layer is formed on the support substrate10, the piezoelectric layer is not formed as a single-crystal layer. Thus, as illustrated inFIG. 5B, the single-crystal piezoelectric substrate14is located on the support substrate10by bonding the piezoelectric substrate14in a wafer state on the support substrate10in a wafer state.

Hereinafter, first and second comparative examples referring to the second embodiment of Patent Application Publication No. 2012-165132 using a method of bonding the piezoelectric substrate14to the support substrate10will be described.

Fabrication Method of the First Comparative Example

FIG. 7AthroughFIG. 8Care cross-sectional views illustrating a method of fabricating an acoustic wave device in accordance with the first comparative example. As illustrated inFIG. 7A, the lower electrode12having a desired shape is formed on the lower surface22of the piezoelectric substrate14that is single-crystal. The lower surface22of the piezoelectric substrate14is bonded to the upper surface of a support layer21. As illustrated inFIG. 7B, the upper surface of the piezoelectric substrate14is polished to thin the piezoelectric substrate14. As illustrated inFIG. 7C, the upper electrode16is formed on the upper surface24of the piezoelectric substrate14.

As illustrated inFIG. 8A, the lower surface of the support layer21is polished to thin the support layer21. As illustrated inFIG. 8B, holes31apenetrating through the support layer21are formed. As illustrated inFIG. 8C, the upper surface of the support substrate10is bonded to the lower surface of the support layer21. This process forms an air gap31from the hole31aso that the air gap31includes the resonance region50in plan view.

In the first comparative example, when the hole31ais formed in the support layer21inFIG. 8B, damage or the like tends to be introduced into the lower electrode12and/or the piezoelectric substrate14in the resonance region50. Additionally, inFIG. 8C, when the support layer21is bonded to the support substrate10, the air gap31is located between the piezoelectric substrate and the support substrate10in the resonance region50. Thus, due to shock or stress when the support layer21is bonded to the support substrate10, distortion tends to occur in the piezoelectric substrate14in the resonance region50. In addition, during the steps fromFIG. 8AthroughFIG. 8C, the wafer is handled with the piezoelectric substrate14and the support layer21being thin. Thus, distortion tends to be introduced into the piezoelectric substrate14and/or the piezoelectric substrate14tends to be damaged.

Fabrication Method of the Second Comparative Example

FIG. 9AthroughFIG. 10Care cross-sectional views illustrating a method of fabricating an acoustic wave device in accordance with the second comparative example. As illustrated inFIG. 9A, a sacrifice layer38is embedded in a recessed portion on the upper surface of the support layer21. The piezoelectric substrate14having the lower electrode12formed thereon is bonded to the upper surface of the support layer21. As illustrated inFIG. 9B, the piezoelectric substrate14is thinned. As illustrated inFIG. 9C, the upper electrode16is formed on the upper surface24of the piezoelectric substrate14.

As illustrated inFIG. 10A, the support layer21is thinned. As illustrated inFIG. 10B, the sacrifice layer38is removed. This process forms the hole31apenetrating through the support layer21. As illustrated inFIG. 10C, the upper surface of the support substrate10is bonded to the lower surface of the support layer21.

In the second comparative example, damage tends to be introduced into the lower electrode12and/or the piezoelectric substrate14in the resonance region50when the sacrifice layer38is removed as illustrated inFIG. 10B. In addition, as in the first comparative example, when the support layer21is bonded to the support substrate10inFIG. 10C, distortion tends to occur in the piezoelectric substrate14in the resonance region50. Furthermore, during the steps fromFIG. 10AthroughFIG. 10C, since the piezoelectric substrate14and the support layer21are thin, distortion tends to be introduced into the piezoelectric substrate14and/or the piezoelectric substrate14tends to be damaged.

Fabrication Method of the Third Comparative Example

FIG. 11AthroughFIG. 11Care cross-sectional views illustrating a method of fabricating an acoustic wave device in accordance with a third comparative example. As illustrated inFIG. 11A, before the piezoelectric substrate14is thinned and the upper electrode16is formed, the support layer21is thinned and the air gap31is formed. Then, the support layer21is bonded to the support substrate10. As illustrated inFIG. 11B, the piezoelectric substrate14is thinned. As illustrated inFIG. 11C, the upper electrode16is formed on the upper surface of the piezoelectric substrate14.

In the third comparative example, the support layer21is bonded to the support substrate10with the piezoelectric substrate14being thick. Thus, unlike the first and second comparative examples, the wafer is not handled while the piezoelectric substrate14and the support layer21are thin. Thus, distortion and/or breakage of the piezoelectric substrate14is inhibited. However, since the piezoelectric substrate14is thinned while the air gap31exists as illustrated inFIG. 11B, distortion may be introduced into the piezoelectric substrate14in the resonance region50and/or the piezoelectric substrate14may be damaged.

In addition, as in the first and second comparative examples, since the air gap31is formed in the support layer21in the resonance region, damage tends to be introduced into the lower electrode12and/or the piezoelectric substrate14in the resonance region50. Furthermore, the support layer21is bonded to the support substrate10while the air gap31is located in the support layer21in the resonance region50. Thus, distortion tends to be introduced into the piezoelectric substrate14in the resonance region50due to the shock at the time of bonding.

Advantage of the First Embodiment

In the acoustic wave device of the first embodiment, as illustrated inFIG. 2throughFIG. 3C, the lower electrodes12(first electrodes) are located on the lower surface22(a first surface) of the single piezoelectric substrate14that is single-crystal, and the upper electrodes16(second electrodes) are located on the upper surface24(a second surface) of the piezoelectric substrate14. The acoustic mirror30is bonded on the support substrate10, is located between the support substrate10and the lower electrodes12in the resonance regions50, and is not located between the support substrate10and the lower electrodes12in at least a part of the region54between the resonance regions50.

As a method of fabricating the acoustic wave device, as illustrated inFIG. 4C, the acoustic mirror30is formed on the lower surface22so as to cover the lower electrodes12. As illustrated inFIG. 4D, the acoustic mirror30is left in the regions to be the resonance regions50, and the acoustic mirror30in at least a part of the region between the regions to be the resonance regions is removed. As illustrated inFIG. 5B, thereafter, the acoustic mirror30is bonded to the support substrate10.

Compared to the first through third comparative examples, the fabrication process does not become complicating because the air gap31is not formed in the resonance region50. Thus, damage is inhibited from being introduced into the lower electrode12and/or the piezoelectric substrate14in the resonance region50. As illustrated inFIG. 5B, the acoustic mirror30is bonded to the support substrate10with the acoustic mirror30being formed in the resonance region50. Thus, distortion is inhibited from being introduced into the piezoelectric substrate14in the resonance region50because of the shock at the time of bonding.

Additionally, as illustrated inFIG. 3B, the insulating film20made of a material different from that of the acoustic mirror30is located between the support substrate10and the piezoelectric substrate14in at least a part of the region54between the resonance regions50. This structure protects the piezoelectric substrate14. Thus, introduction of distortion into the piezoelectric substrate14and/or damage to the piezoelectric substrate14is further inhibited.

The insulating film20preferably has less acoustic impedance than the first film32aand the second film32bof the acoustic mirror30. This configuration attenuates the acoustic wave in the insulating film20. Thus, the interference of the acoustic wave between the resonance regions50is inhibited. In addition, the insulating film20preferably has a less relative permittivity than the first film32aand the second film32bof the acoustic mirror30. This configuration reduces the electric connection between the resonance regions50. The use of a resin layer for the insulating film20enables to fabricate the acoustic wave device inexpensively.

Furthermore, the piezoelectric substrate14is a lithium tantalate substrate or a lithium niobate substrate. This configuration improves the electromechanical coupling coefficient and the TCF.

In the first embodiment, the acoustic mirror30is located between the piezoelectric substrate14and the support substrate10. Thus, as illustrated inFIG. 6A, even when the piezoelectric substrate14is thinned after the acoustic mirror30is bonded to the support substrate10, introduction of distortion into the piezoelectric substrate14and breakage of the piezoelectric substrate14described in the third comparative example is inhibited. As illustrated inFIG. 6B, after the piezoelectric substrate14is thinned, the upper electrode16is formed on the upper surface24of the piezoelectric substrate14. Accordingly, unlike the first and second comparative examples, it is not necessary to execute a process while the piezoelectric substrate14and the support layer21are thin. Thus, distortion and/or breakage of the piezoelectric substrate14is inhibited.

The lower electrode12and/or the upper electrode16electrically connects between the piezoelectric thin film resonators corresponding to the resonance regions50. This structure enables to connect between the piezoelectric thin film resonators. The piezoelectric thin film resonators include one or more series resonators S1through S4and one or more parallel resonators P1through P3. Accordingly, the ladder-type filter can be formed.

First Variation of the First Embodiment

FIG. 12AthroughFIG. 12Care cross-sectional views illustrating a method of fabricating an acoustic wave device in accordance with a first variation of the first embodiment. As illustrated inFIG. 12A, the same steps as the steps fromFIG. 4AtoFIG. 4Dof the first embodiment are executed. No insulating film is located on the upper surface of the support substrate10. As illustrated inFIG. 12B, the acoustic mirror30is bonded to the support substrate10. As illustrated inFIG. 12C, the steps ofFIG. 6AandFIG. 6Bof the first embodiment are executed. This process forms an air gap28between the support substrate10and the piezoelectric substrate14in at least a part of the region54between the resonance regions50. Thus, interference of the acoustic wave between the resonance regions50and/or the electrical connection between the resonance regions50is inhibited.

Second Variation of the First Embodiment

FIG. 13AthroughFIG. 14Bare cross-sectional views illustrating a method of fabricating an acoustic wave device in accordance with a second variation of the first embodiment. As illustrated inFIG. 13A, piezoelectric substrates14aand14bare bonded. The piezoelectric substrates14aand14bare bonded by, for example, surface activation under normal temperature. A polarization direction56aof the piezoelectric substrate14ais opposite to a polarization direction56bof the piezoelectric substrate14b. For example, when the piezoelectric substrates14aand14bare X-cut lithium tantalate substrates or X-cut lithium niobate substrates, the X-axis orientations of the piezoelectric substrates14aand14bare made to be opposite to each other. InFIG. 13A, the polarization directions56aand56bare directions facing each other, but the polarization directions56aand56bmay be directions away from each other. The polarization direction56amay correspond to the planar direction of the piezoelectric substrate14a, and the polarization direction56bmay correspond to the planar direction of the piezoelectric substrate14band be opposite to the polarization direction56a.

As illustrated inFIG. 13B, the piezoelectric substrate14ais thinned. As illustrated inFIG. 13C, the same steps as the steps fromFIG. 4BtoFIG. 5Bof the first embodiment are executed. As illustrated inFIG. 14A, the piezoelectric substrate14bis thinned. This process forms the piezoelectric substrate14from the piezoelectric substrates14aand14b. At this time, the film thicknesses of the piezoelectric substrates14aand14bare made to be approximately equal to each other to the extent of production errors. As illustrated inFIG. 14B, the upper electrode16is formed on the piezoelectric substrate14.

FIG. 15AandFIG. 15Billustrate the piezoelectric substrates of the first embodiment and the second variation of the first embodiment, respectively. As illustrated inFIG. 15A, in the first embodiment, a polarization direction56of the piezoelectric substrate14is the direction from the upper electrode16to the lower electrode12. The film thickness of the piezoelectric substrate14is represented by h. When the piezoelectric thin film resonator operates in a mode of a fundamental wave, the electric field distribution of the acoustic wave in the piezoelectric substrate14becomes negative (−) at the lower electrode12when being positive (+) at the upper electrode16. The displacement distribution of the acoustic wave in the piezoelectric substrate14becomes negative (−) at the lower electrode12when being positive (+) at the upper electrode16, for example. Accordingly, the piezoelectric substrate14elongates (or contracts) as indicated by an arrow57a. Thus, the wavelength λ of the acoustic wave becomes 2 h. The operation frequency becomes the acoustic velocity/the film thickness h.

As illustrated inFIG. 15B, in the second variation of the first embodiment, provided as the piezoelectric substrate14are the piezoelectric substrate14a(a first substrate) and the piezoelectric substrate14b(a second substrate) that have the same film thickness, are made of the same material, have opposite polarization directions, and are stacked. The piezoelectric substrate14aand the piezoelectric substrate14bhave the same film thickness, and the piezoelectric substrate14has a film thickness of h′. When it is assumed that h′=h, the electric field distribution of the acoustic wave in the piezoelectric substrate14becomes negative (−) at the lower electrode12when being positive (+) at the upper electrode16as inFIG. 15A. Since the polarization directions56aand56bof the piezoelectric substrates14aand14bare opposite to each other, the displacement distribution of the acoustic wave in the piezoelectric substrate14abecomes negative (−) at the upper electrode16side and positive (+) at the lower electrode12, for example, and the displacement distribution of the acoustic wave in the piezoelectric substrate14bbecomes positive (+) at the upper electrode16and negative (−) at the lower electrode12side, for example. Accordingly, the piezoelectric substrate14acontracts (or elongates) as indicated by an arrow57b, and the piezoelectric substrate14belongates (or contracts) as indicated by the arrow57a. The operation frequency becomes the acoustic velocity/the film thickness h′/2. Thus, the second embodiment operates in a mode of second harmonic of which the operation frequency is twice that of the fundamental waves of the first embodiment and the first variation thereof.

In the second embodiment, when it is assumed that h′=2 h, the operation frequency is equal to that of the first embodiment and the first variation thereof. In both the cases of h′=h and h′=2 h, since the second-order linear distortions are canceled out each other, second harmonic distortion is reduced.

Second Embodiment

FIG. 16is a circuit diagram of a ladder-type filter in a second embodiment. As illustrated inFIG. 16, the series resonator S1is divided into resonators S1aand S1bin series. The series resonator S4is divided into resonators S4aand S4bin series. The parallel resonator P1is divided into resonators P1aand P1bin series. The parallel resonator P3is divided into resonators P3aand P3bin series. Other structures are the same as those of the first embodiment illustrated inFIG. 1, and the description thereof is thus omitted.

FIG. 17is a plan view of an acoustic wave device in accordance with the second embodiment. As illustrated inFIG. 17, the resonators S1aand S1b, the resonators S4aand S4b, the resonators P1aand P1b, and the resonators P3aand P3bare connected by the lower electrodes12. The through electrode18penetrating through the piezoelectric substrate14is not provided. Other structures are the same as those of the first embodiment illustrated inFIG. 2, and the description thereof is thus omitted.

As in the second embodiment, at least one (e.g., the series resonator S1) of the series resonators S1through S4and the parallel resonators P1through P3is divided into a first resonator (e.g., the resonator S1a) and a second resonator (e.g., the resonator S1b) in series between two nodes. As described above, in the case where the resonator is divided in series and the resonators S1aand S1bare connected by the lower electrode12or the upper electrode16, when the resonator S1aand the resonator S1bare viewed from one (e.g., input terminal Tin) of the two nodes, the polarization direction56aof the piezoelectric substrate14of the resonator S1ais opposite to the polarization direction56bof the piezoelectric substrate14of the resonator S1b. Accordingly, second harmonic distortion is reduced.

It is difficult to form a through hole in the single-crystal piezoelectric substrate14such as a lithium tantalate substrate, a lithium niobate substrate, or a crystal substrate. In the first variation of the second embodiment, by appropriately selecting a resonator to be divided in series, even when the through electrode18penetrating through the piezoelectric substrate14and connecting the lower electrode12and the upper electrode16is not provided, all the input terminal Tin, the output terminal Tout, and the ground terminals Tgnd connected to one or more parallel resonators can be formed on the upper surface24of the piezoelectric substrate14.

First Variation of the Second Embodiment

FIG. 18is a circuit diagram of a ladder-type filter in a first variation of the second embodiment. As illustrated inFIG. 18, the series resonator S1is divided into the resonators S1aand S1bin parallel. Other structures are the same as those of the first embodiment illustrated inFIG. 1, and the description thereof is thus omitted.

FIG. 19is a plan view of an acoustic wave device in accordance with the first variation of the second embodiment. As illustrated inFIG. 19, the through electrode18connects the lower electrode12and the upper electrode16between the resonators S1aand S1b. Other structures are the same as those of the first embodiment illustrated inFIG. 2, and the description thereof is thus omitted.

In the first variation of the second embodiment, at least one (e.g., the series resonator S1) of the series resonators S1through S4and the parallel resonators P1through P3is divided into the resonators S1aand S1bin parallel. When the resonators S1aand S1bare viewed from one (e.g., the input terminal Tin) of the two nodes, the polarization direction56aof the piezoelectric substrate14of the resonator S1ais opposite to the polarization direction56bof the piezoelectric substrate14of the resonator S1b. Accordingly, second harmonic distortion is reduced.

In the second embodiment and the first variation thereof, to further reduce the second harmonic distortion, the areas of the resonance regions50of the divided resonators (for example, S1aand S1b) are preferably approximately equal to each other to the extent of production errors. Additionally, the thicknesses of the piezoelectric substrates14of the divided resonators (for example, S1aand S1b) are preferably approximately equal to each other to the extent of production errors.

In the first and second embodiments and the variations thereof, the number of series resonators and the number of parallel resonators can be freely selected. The ladder-type filter has been described as an example of the filter, but the filter may be a multimode filter.

Third Embodiment

FIG. 20is a circuit diagram of a duplexer in accordance with a third embodiment. As illustrated inFIG. 20, a transmit filter40is connected between a common terminal Ant and a transmit terminal Tx. A receive filter42is connected between the common terminal Ant and a receive terminal Rx. The transmit filter40transmits signals in the transmit band to the common terminal Ant as transmission signals among high-frequency signals input from the transmit terminal Tx, and suppresses signals with other frequencies. The receive filter42transmits signals in the receive band to the receive terminal Rx as reception signals among high-frequency signals input from the common terminal Ant, and suppresses signals with other frequencies. At least one of the transmit filter40and the receive filter42may be the filter according to any one of the first and second embodiments and the variations thereof.

The duplexer has been described as an example of the multiplexer, but the multiplexer may be a triplexer or a quadplexer.