Acoustic wave device

An acoustic wave device includes: a piezoelectric substrate that is made of a single crystal piezoelectric material, and includes a first region including an upper surface, and a second region that is located under the first region and has a density less than a density of the first region; and an IDT located on the upper surface of 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. 2016-055379, filed on Mar. 18, 2016, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to an acoustic wave device.

BACKGROUND

In acoustic wave devices, an Interdigital Transducer (IDT) exciting an acoustic wave is formed on a piezoelectric substrate. The piezoelectric substrate is, for example, a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate. When the Li compositions in lithium tantalate and lithium niobate are stoichiometric, they are called a stoichiometry composition. When the Li composition is a little less than the stoichiometric composition, it is called a congruent composition. Most of the lithium tantalate substrates and the lithium niobate substrates have a congruent composition.

Japanese Patent Application Publication No. 2013-66032 (Patent Document 1) describes that lithium is diffused to the surface of a substrate with a congruent composition to form a region with a stoichiometry composition on the substrate surface. Japanese Patent Application Publication Nos. 2011-135245 and 2002-305426 (Patent Documents 2 and 3) describe that a piezoelectric substance with a stoichiometry composition is used for an acoustic wave device. International Publication No. 2013/031651 (Patent Document 4) describes that a dielectric film is provided under a piezoelectric film.

To reduce the loss of the acoustic wave device, it is required to reduce the leak of the acoustic wave excited by the IDT. However, there is no known preferable structure in the piezoelectric substrate for reducing the loss of the acoustic wave device.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided an acoustic wave device including: a piezoelectric substrate that is made of a single crystal piezoelectric material, and includes a first region including an upper surface, and a second region that is located under the first region and has a density less than a density of the first region; and an IDT located on the upper surface of the piezoelectric substrate.

DETAILED DESCRIPTION

A description will be given of embodiments of the present invention with reference to the accompanying drawings.

First Embodiment

An acoustic wave resonator will be described as an acoustic wave device.FIG. 1Ais a plan view of an acoustic wave resonator in accordance with a first embodiment, andFIG. 1Bis a cross-sectional view taken along line A-A inFIG. 1A. As illustrated inFIG. 1AandFIG. 1B, an IDT21and reflectors22are formed on a piezoelectric substrate10. The IDT21and the reflectors22are formed of a metal film12formed on the piezoelectric substrate10. The IDT21includes a pair of comb-shaped electrodes20facing each other. The comb-shaped electrode20includes a plurality of electrode fingers14and a bus bar18to which the electrode fingers14are connected. The pair of comb-shaped electrodes20are arranged so as to face each other so that the electrode fingers14of one of the comb-shaped electrodes20and the electrode fingers14of the other are arranged substantially in an alternate order. The acoustic wave excited by the IDT21mainly propagates in the alignment direction of the electrode fingers14. The pitch of the electrode fingers14is approximately equal to the wavelength λ of the acoustic wave. The piezoelectric substrate10is a lithium tantalate substrate or a lithium niobate substrate. The metal film12is, for example, an aluminum film, a copper film, a titanium film, or a chrome film, or a composite film of at least two of them. The metal film12has a film thickness of, for example, 100 to 400 nm.

As illustrated inFIG. 1B, the piezoelectric substrate10includes a first region10a, a second region10b, and a third region10c. The first region10aincludes the upper surface of the piezoelectric substrate10. The IDT21and the reflectors22are located on the upper surface of the piezoelectric substrate10. The second region10bis located under the first region10a. The third region10cis located between the first region10aand the second region10b. The first region10ais a region having a congruent composition. The second region10bhas a stoichiometry composition. The third region10cis a transition region from the congruent composition to the stoichiometry composition. In the stoichiometry composition, a composition ratio of lithium to lithium and tantalum (or niobium) (hereinafter, described as a lithium composition ratio) is 49.5% or greater and 50.5% or less. In the congruent composition, the lithium composition ratio is 49.5% or less. The lithium composition ratio is, for example, 48% or greater. In each of the first region10aand the second region10b, the lithium composition ratio is substantially constant. In the third region10c, the lithium composition ratio gradually changes. The third region10cmay not necessarily be located.

For example, in an acoustic wave device using a leaky wave, the acoustic wave excited by the IDT21is mainly a leaky wave. The IDT21emits a bulk wave in addition to the surface acoustic wave. Since the bulk wave does not contribute to resonance, as the energy of the bulk wave increases, the loss of the resonator increases.

FIG. 2is a cross-sectional view of the piezoelectric substrate illustrating an image of a leaky wave and a bulk wave in the piezoelectric substrate. InFIG. 2, the x1 direction is the propagation direction of a leaky wave on the surface of the piezoelectric substrate10, the x2 direction is a direction perpendicular to the x1 direction on the surface of the piezoelectric substrate10, and the x3 direction is the depth direction of the piezoelectric substrate10. The main displacement component of a leaky wave is an SH wave. Thus, the leaky wave is displaced mainly in the x2 direction. On the other hand, the leaky wave propagates while emitting a bulk wave into the piezoelectric substrate10. The emission of the bulk wave causes the loss of the acoustic wave device.

FIG. 3is a graph of acoustic velocity versus depth in the piezoelectric substrate in the first embodiment. In the following description, the acoustic velocity of the bulk wave will be focused on, but the relationship between the acoustic velocity of the bulk wave and the lithium composition ratio is substantially the same as the relationship between the acoustic velocity of the surface acoustic wave and the lithium composition ratio. Therefore, a description will be simply given of the acoustic velocity. The acoustic velocity in the stoichiometry composition is greater than the acoustic velocity in the congruent composition. Thus, as illustrated inFIG. 3, the acoustic velocity in the first region10ais less than the acoustic velocity in the second region10b. In the third region10c, the acoustic velocity gradually changes. The energy of the acoustic wave concentrates in the region in which the acoustic velocity is low. For example, when a leaky wave is used, the boundary between the first region10aand the second region10bis structured to be located substantially between the leaky wave and the bulk wave inFIG. 2. This structure inhibits the emission of a bulk wave because the velocity of the bulk wave is fast in a deep region. Accordingly, the energy concentrates in the first region10a. Therefore, the insertion loss of the acoustic wave resonator can be improved.

The measured acoustic velocity of a Rayleigh wave in a 42° rotated Y-cut X-propagation lithium tantalate substrate by a linear focused beam acoustic microscope is approximately 3125 m/second in the congruent composition, and is approximately 3170 m/second in the stoichiometry composition. The acoustic velocity of the surface acoustic wave is proportional to the square root of (elastic modulus/density). The elastic modulus relates to a Young's modulus and a Poisson ratio. Between the stoichiometry composition and the congruent composition, the Young's moduluses and the Poisson ratios are approximately the same. In contrast, the density of the congruent composition is greater than the density of the stoichiometry composition. For example, in a lithium tantalate substrate, the density of the congruent composition is 7454 kg/m3, while the density of the stoichiometry composition is 7420 to 7440 kg/m3. Thus, the acoustic velocity in the stoichiometry composition is greater than the acoustic velocity in the congruent composition.

In Patent Document 4, located under a lithium niobate substrate is a dielectric film such as a silicon oxide film or a silicon nitride film. The silicon oxide film or the silicon nitride film has an acoustic velocity greater than that of lithium niobate. However, in this structure, a bulk wave is reflected by a boundary face between the lithium niobate substrate and the dielectric film. As a result, spurious due to the bulk wave occurs. On the other hand, the first embodiment provides the first region10ain which the acoustic velocity is high and the second region10bin which the acoustic velocity is low by making the densities different in a single crystal piezoelectric material. This structure can confine the acoustic wave in the second region10bwithout making the bulk wave reflected.

In the first embodiment, the piezoelectric substrate10is made of a single crystal piezoelectric material, and includes the first region10aincluding the upper surface, and the second region10blocated under the first region10aand having a density less than that of the first region10a. The IDT21is located on the upper surface of the piezoelectric substrate10. This structure makes the energy of the bulk wave concentrate in the first region10a, improving the insertion loss of the acoustic wave device. The densities of the first and second regions10aand10bcan be estimated from the lithium composition ratio by X-ray diffractometry.

In addition, the velocity of the acoustic wave in the second region10bis greater than the velocity of the acoustic wave in the first region10a. This structure allows the energy of the bulk wave to concentrate in the first region10a.

Furthermore, when the piezoelectric substrate10is a lithium tantalate substrate or a lithium niobate substrate, the first region10ahas a congruent composition, and the second region10bhas a stoichiometry composition. This structure can make the velocity of the acoustic wave in the second region10bgreater than that in the first region10a.

Located between the first region10aand the second region10bis the third region10cof which the density changes from the first region10ato the second region10b. This structure can inhibit the reflection of the bulk wave due to the rapid change in density.

The thickness of the first region10ais preferably equal to or greater than the pitch λ of the electrode fingers14in the IDT21. The surface acoustic wave energy concentrates in a region from the upper surface of the piezoelectric substrate10to the depth of approximately λ. Thus, when the thickness of the first region10ais less than λ, the surface acoustic wave attenuates. Therefore, the thickness of the first region10ais preferably equal to or greater than the pitch λ of the electrode fingers14in the IDT21. The thickness of the first region10ais preferably 2λ or greater, more preferably 5λ or greater. To concentrate the energy of the bulk wave in the first region10a, the thickness of the first region10ais preferably 20λ or less, more preferably 10λ or less.

To concentrate the energy of the bulk wave in the first region10a, the thickness of the second region10bis preferably 10λ or greater, more preferably 20λ or greater. To inhibit the reflection of the bulk wave, the thickness of the third region10cis preferably 1λ or greater, more preferably 2λ or greater. To concentrate the energy of the bulk wave in the first region10a, the thickness of the third region10cis preferably 5λ or less, more preferably 10λ or less.

An exemplary case where the lithium composition (i.e., the density) is approximately constant in each of the first region10aand the second region10bhas been described, but the lithium composition (the density) may be inclined in the thickness direction in each of the first region10aand the second region10b. For example, it is only required that the average density of the first region10ais greater than the average density of the second region10b.

A description will next be given of a fabrication method of the first embodiment.FIG. 4AthroughFIG. 4Dare cross-sectional views illustrating a method of fabricating the acoustic wave resonator in accordance with the first embodiment. As illustrated inFIG. 4A, a piezoelectric substrate10dwith a congruent composition is prepared. A lithium tantalate substrate or a lithium niobate substrate is prepared as the piezoelectric substrate10d.

As illustrated inFIG. 4B, the second region10bwith a stoichiometry composition is formed by diffusing lithium to the upper and lower surfaces of the piezoelectric substrate10d. As a method of diffusing lithium, employed is, for example, the method disclosed in Patent Document 1. A region between the second regions10bbecomes the first region10awith a congruent composition. Between the first region10aand the second region10b, formed is the third region10cof which the lithium composition gradually changes. The above processes form a piezoelectric substrate10e. The first region10amay be formed only on the lower surface of the upper and lower surfaces by diffusing lithium only to the lower surface of the lower and upper surfaces of the piezoelectric substrate10d.

As illustrated inFIG. 4C, the upper surface of the piezoelectric substrate10is polished to expose the first region10a. This process forms the piezoelectric substrate10including the first region10a, the second region10b, and the third region10c. As illustrated inFIG. 4D, the metal film12is formed on the upper surface of the piezoelectric substrate10. The IDT21and the reflectors22are formed of the metal film12. The metal film12is formed by, for example, evaporation and liftoff. The metal layer12may be formed by sputtering and etching. Then, the separation into individual chips by dicing or the like is performed.

Second Embodiment

FIG. 5is a cross-sectional view of an acoustic wave resonator in accordance with a second embodiment. As illustrated inFIG. 5, the upper surface of a support substrate11and the lower surface of the piezoelectric substrate10are bonded together. The bonded surface of the piezoelectric substrate10and the support substrate11is a plane surface and flat. The support substrate11is, for example, an insulating substrate such as a sapphire substrate, an alumina substrate, or a spinel substrate, or a semiconductor substrate such as a silicon substrate.

FIG. 6is a graph of acoustic velocity versus depth in the piezoelectric substrate and the support substrate in the second embodiment. As illustrated inFIG. 6, the acoustic velocity of the support substrate11is greater than that in the second region10b. Thus, the energy of the bulk wave concentrates in the first region10amore than that in the first embodiment. Therefore, the insertion loss of the acoustic wave device can be further improved.

As described above, in the second embodiment, the support substrate11is bonded under the second region10b, and has an acoustic velocity greater than that in the second region10b. This structure can further improve the insertion loss of the acoustic wave device. In addition, by making the linear thermal expansion coefficient of the support substrate11less than that of the piezoelectric substrate10, the frequency temperature dependence of the acoustic wave device can be reduced.

FIG. 7AthroughFIG. 7Care cross-sectional views illustrating a method of fabricating the acoustic wave resonator in accordance with the second embodiment. As illustrated inFIG. 7A, the piezoelectric substrate10einFIG. 4Bof the first embodiment is bonded onto the support substrate11.

The example of the bonding of the piezoelectric substrate10eand the support substrate11will be described. The upper surface of the support substrate11and the lower surface of the piezoelectric substrate10eare irradiated with the ion beam, the neutral beam, or plasma of an inert gas. This process forms an amorphous layer with a thickness of a several nanometers on the upper surface of the support substrate11and the lower surface of the piezoelectric substrate10e. Dangling bonds are formed on the surface of the amorphous layer. The presence of the dangling bonds puts the upper surface of the support substrate11and the lower surface of the piezoelectric substrate10ein an active state. The dangling bond on the upper surface of the support substrate11bonds to the dangling bond on the lower surface of the piezoelectric substrate10e. Thus, the support substrate11and the piezoelectric substrate10eare bonded together at normal temperature. The amorphous layer is integrally formed between the bonded support substrate11and the bonded piezoelectric substrate10e. The amorphous layer has a thickness of, for example, 1 to 8 nm.

As illustrated inFIG. 7B, the upper surface of the piezoelectric substrate10eis polished so that the first region10ais exposed. As illustrated inFIG. 7C, as inFIG. 4D, the IDT21and the reflectors22formed of the metal film12are formed. Then, the lower surface of the support substrate11may be polished. Thereafter, performed is the separation into individual chips by dicing or the like.

Third Embodiment

A third embodiment uses the acoustic wave resonator of any one of the first and second embodiments for a filter or a duplexer.FIG. 8Ais a circuit diagram of a ladder-type filter in accordance with the third embodiment. As illustrated inFIG. 8A, series resonators S1through S4are connected in series between an input terminal In and an output terminal Out. Parallel resonators P1through P3are connected in parallel between the input terminal In and the output terminal Out. At least one of the series resonators S1through S4and the parallel resonators P1through P3may be the acoustic wave resonator of the first or second embodiment. The number of and the connection of the series resonators and the parallel resonators may be appropriately designed. The acoustic wave resonator of the first or second embodiment may be used for a multimode filter.

FIG. 8Bis a block diagram of a multiplexer in accordance with a variation of the third embodiment. As illustrated inFIG. 8B, a transmit filter80is connected between a common terminal Ant and a transmit terminal Tx. A receive filter82is connected between the common terminal Ant and a receive terminal Rx. The transmit filter80transmits signals in the transmit band to the common terminal Ant among signals input from the transmit terminal Tx, and suppresses signals in other bands. The receive filter82allows signals in the receive band among signals input from the common terminal Ant to pass therethrough, and suppresses signals in other bands. At least one of the transmit filter80or the receive filter82may be the filter of the third embodiment. A duplexer has been described as a multiplexer, but at least one filter in a triplexer or a quadplexer may be the filter of the third embodiment.