Elastic wave device and manufacturing method therefor

In an elastic wave device, a piezoelectric substrate is stacked on a support substrate and an IDT electrode is provided on the piezoelectric substrate. Wiring line portions are provided on the piezoelectric substrate. A first hollow portion is provided in the support substrate at least below at least one of the wiring line portions and or below a region between the wiring line portions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an elastic wave device such as an elastic wave resonator or an elastic wave filter, and to a manufacturing method therefor.

2. Description of the Related Art

Heretofore, various elastic wave devices have been used as resonators and band pass filters. Japanese Unexamined Patent Application Publication No. 2007-251910 discloses an elastic wave device in which Lamb waves are utilized as plate waves. In Japanese Unexamined Patent Application Publication No. 2007-251910, an IDT electrode is formed on a piezoelectric substrate. A reinforcement substrate having an opening therein that opens upward is bonded to a lower surface of the piezoelectric substrate. The opening is provided below a part of the piezoelectric substrate where the IDT electrode is provided. The opening is closed by the piezoelectric substrate and a hollow part is thus formed.

In elastic wave devices, in addition to an IDT electrode, a plurality of wiring lines are provided that are connected to the IDT electrode. Although there is no particular mention of such wiring lines in Japanese Unexamined Patent Application Publication No. 2007-251910, it is sometimes not possible to obtain good characteristics due to parasitic capacitances between wiring lines and so forth when an elastic wave device is actually manufactured.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide elastic wave devices that reduce capacitance between wiring lines and in which the characteristics thereof are unlikely to be degraded by parasitic capacitances and so forth.

An elastic wave device according to a preferred embodiment of the present invention includes a support substrate, a piezoelectric substrate that is stacked on the support substrate, an IDT electrode that is provided on the piezoelectric substrate, and a plurality of wiring line portions that are provided on the piezoelectric substrate and are electrically connected to the IDT electrode. A first hollow portion, which is covered by the piezoelectric substrate, is provided in the support substrate at least below at least one wiring line portion or below a region between wiring line portions among the plurality of wiring line portions.

In an elastic wave device according to a preferred embodiment of the present invention, a plate wave is utilized as an elastic wave, and an excitation-use second hollow portion, which is closed by the piezoelectric substrate, is provided in the support substrate below a region in which the IDT electrode is provided.

In an elastic wave device according to a preferred embodiment of the present invention, the first hollow portion and the second hollow portion are isolated from each other by a partition wall that is provided in the support substrate. In this case, the mechanical strength is increased by the partition wall.

In an elastic wave device according to a preferred embodiment of the present invention, the first hollow portion and the second hollow portion may be connected to each other.

In an elastic wave device according to a preferred embodiment of the present invention, a plate wave is utilized as an elastic wave, and the elastic wave device further includes an acoustic reflection film that is stacked on a lower surface of the piezoelectric substrate below the IDT electrode. A plate wave may be excited by providing an acoustic reflection film below the IDT electrode in this manner.

In an elastic wave device according to a preferred embodiment of the present invention, a leaky wave is utilized as an elastic wave.

An elastic wave device manufacturing method according to a preferred embodiment of the present invention includes a step of preparing the support substrate; a step of forming the first hollow portion, the first hollow portion being open at an upper surface of the support substrate; a step of stacking the piezoelectric substrate on the support substrate; and a step of forming the IDT electrode and the plurality of wiring line portions on the piezoelectric substrate.

In a method of manufacturing an elastic wave device according to a preferred embodiment of the present invention, the method further includes a step of forming the excitation-use second hollow portion in the support substrate.

In a method of manufacturing an elastic wave device according to a preferred embodiment of the present invention, the first hollow portion and the second hollow portion are simultaneously formed. In this case, an elastic wave device according to a preferred embodiment of the present invention can be provided while not increasing the number manufacturing steps.

With the elastic wave devices and the elastic wave device manufacturing methods according to various preferred embodiments of the present invention, the capacitance between wiring line portions is able to be reduced, and as a result, degradation of the characteristics of the elastic wave device caused by parasitic capacitances and so forth is effectively reduced or prevented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the present invention will be made clearer by describing specific preferred embodiments of the present invention while referring to the drawings.

The preferred embodiments described in the present specification are illustrative examples and it should be noted that elements and features of the configurations illustrated in different preferred embodiments can be substituted for one another or combined with one another.

FIG. 1is a schematic plan view of an elastic wave device1according to a first preferred embodiment of the present invention, andFIG. 2illustrates the circuit configuration of the elastic wave device1.

As illustrated inFIG. 2, the elastic wave device1preferably is a ladder filter, for example. A plurality of series-arm resonators S1to S4are provided in a series arm that connects an input terminal2and an output terminal3. A parallel arm resonator P1is provided in a parallel arm that connects a connection point between the series arm resonator S1and the series arm resonator S2, and the ground potential. A parallel arm resonator P2is provided in a parallel arm that connects a connection point between the series arm resonator S2and the series arm resonator S3, and the ground potential. A parallel arm resonator P3is provided in a parallel arm that connects a connection point between the series arm resonator S3and the series arm resonator S4, and the ground potential. A parallel arm resonator P4is provided in a parallel arm that connects a node between the series arm resonator S4and an output terminal and the ground potential. Ground-potential-side end portions of the parallel-arm resonators P1to P3are commonly connected to a connection point4.

The series arm resonators S1to S4and the parallel arm resonators P1to P4each include an elastic wave resonator. As illustrated inFIG. 1, the elastic wave device1includes a piezoelectric substrate5. The piezoelectric substrate5includes a piezoelectric single crystal such as LiTaO3or LiNbO3in this preferred embodiment.

The schematically illustrated electrode structure is provided on the piezoelectric substrate5. In more detail, the input terminal2, the output terminal3and ground terminals6and7are provided on the piezoelectric substrate5. Wiring line portions11to15are provided in order to define the series arm that connects the input terminal2and the output terminal3to each other. One end of the wiring line portion11is connected to the input terminal2and the other end of the wiring portion is connected to the series arm resonator S1. InFIG. 1, a portion where the series arm resonator S1is provided is schematically illustrated as a rectangular shape, for example. In reality, a one-port-type elastic wave resonator is provided preferably by forming an IDT electrode and reflectors, which are arranged on both sides of the IDT electrode in the elastic wave propagation direction. A portion in which a one-port-type elastic wave resonator is provided is illustrated as a rectangular frame shape, for example. In addition, a broken line X inside a rectangular frame shape indicates a portion where a second hollow portion, which is used for excitation, is provided below the piezoelectric substrate5, as will be described later. The outer edge of the second hollow portion is indicated by the broken line X in a plan view of the second hollow portion.

The other series-arm resonators S2to S4and the parallel arm resonators P1to P4are also schematically illustrated as similar rectangular frame shapes and regions in which the second hollow portions are provided are indicated by broken lines.

The wiring line portion12connects the series arm resonator S1and the series arm resonator S2to each other. The wiring line portion13connects the series arm resonator S2and the series arm resonator S3to each other. The wiring line portion14connects the series arm resonator S3and the series arm resonator S4to each other. The wiring line portion15connects the series arm resonator S4and the output terminal3to each other.

On the other hand, a wiring line portion16is connected to an end portion of the parallel arm resonator P1that is on the opposite side from the side that is connected to the series arm resonator S1. The wiring line portion16branches into first and second branch wiring line portions16aand16b. The first branch wiring line portion16ais connected to the ground terminal6. The second branch wiring line portion16bis connected to end portions of the parallel arm resonator P2and the parallel arm resonator P3that are on the ground potential side. Therefore, the connection point4is defined by the wiring line portion16.

The parallel arm resonator P4is connected to a wiring line portion17. The wiring line portion17is commonly connected to the wiring line portion15and is connected to the series arm resonator S4. In addition, a wiring line portion18is connected to an end portion of the parallel arm resonator P4that is on the ground potential side. The wiring line portion18is connected to the ground terminal7. The ladder circuit illustrated inFIG. 2is provided on the piezoelectric substrate5in the manner described above.

The input terminal2, the output terminal3, the ground terminals6and7and the wiring line portions11to18are composed of a metal. As examples of the metal, a suitable metal or alloy such as Cu, Al, an Al—Cu alloy, Ag, or an Ag—Pd alloy can be used. In addition, a single metal film may be used, or a multilayer metal film obtained by stacking a plurality of metal films may be used.

The IDT electrodes and reflectors of the series-arm resonators S1to S4and the parallel arm resonators P1to P4can be formed of the same metals as described above.

In the elastic wave device1, the second hollow portions indicated by the broken lines X described above are provided in order that the vibrations of the elastic wave resonators provided on the piezoelectric substrate5are not obstructed. This point will be explained while referring to the sectional view ofFIG. 3.

FIG. 3is a sectional view taken along a broken line A-A inFIG. 1. An IDT electrode21and reflectors22and23, which define the parallel arm resonator P2, are provided on the piezoelectric substrate5. The branch wiring line portion16band the wiring line portion13are located outside the reflectors22and23in a surface acoustic wave propagation direction.

In the elastic wave device1of this preferred embodiment, plate waves are preferably utilized as the elastic waves. Therefore, the entirety of the portion of the piezoelectric substrate5where the IDT electrode21is provided vibrates. The thickness of the piezoelectric substrate5is small and therefore a support substrate24is stacked on a lower surface of the piezoelectric substrate5. In other words, the piezoelectric substrate5is held by the support substrate24.

An excitation-use second hollow portion32is provided below a region where the IDT electrode21and the reflectors22and23are provided in order to allow an elastic wave to be excited in the IDT electrode21. In addition, first hollow portions31, which will be described next, are connected to both sides of the excitation-use second hollow portion32.

The first hollow portions31and the second hollow portion32are each defined by covering an opening, which opens at the upper surface of the support substrate24, with the piezoelectric substrate5.

The support substrate24can be made of a suitable insulating material, semiconductor material or piezoelectric material.

One of the unique features of the elastic wave device1of the present preferred embodiment is that the first hollow portions31are provided.

InFIG. 3, the first hollow portions31preferably are connected to and integrated with the second hollow portion32, which is to the inside of the first hollow portions31, with one-dot chain lines Y therebetween.

The first hollow portions31are respectively located below portions where the second branch wiring line portion16band the wiring line portion13are provided.

In this preferred embodiment, since the first hollow portions31are respectively provided below the second branch wiring line portion16band the wiring line portion13, the parasitic capacitance between the second branch wiring line portion16band the wiring line portion13is able to be reduced. As a result, degradation of characteristics is significantly reduced or prevented.

In the elastic wave device1, as described above, the thin piezoelectric substrate5is used, and in addition to the second hollow portions32, similar first hollow portions are provided in portions surrounded by one-dot chain lines Y inFIG. 1.

FIG. 4is a sectional taken along line B-B inFIG. 1. Here, the first hollow portions31are respectively provided below the wiring line portion11and below the wiring line portion15. Therefore, a parasitic capacitance between the wiring line portion11, which is connected to the input terminal2illustrated inFIG. 1, and the wiring line portion15, which is connected to the output terminal3, is able to be reduced. Thus, the first hollow portions31may be provided in order to reduce the parasitic capacitance between the wiring line portions11and15, which do not have an IDT electrode provided therebetween.

As described above, it is clear that the parasitic capacitance between the wiring line portions is able to be effectively reduced by providing the first hollow portions in parts indicated by the one-dot chain lines Y inFIG. 1. InFIG. 4, the first hollow portions31are provided below the wiring line portions11and15in order to reduce the parasitic capacitance between the wiring line portion11and wiring line portion15, which is considerably spaced apart from the wiring line portion11.

In contrast, first hollow portions31may be respectively provided below wiring line portions33and34, which are close to or adjacent to each other on the piezoelectric substrate5, as in a modification illustrated in the sectional view ofFIG. 5. In this modification, the first hollow portions31are adjacent to each other, and a partition wall24ais provided therebetween. Therefore, deformation of the piezoelectric substrate5toward the first hollow portions31is able to be reduced. Therefore, the mechanical strength is able to be increased compared with a configuration in which the first hollow portions31are connected to each other.

Of course, the first hollow portion31below the wiring line portion33and the first hollow portion31below the wiring line portion34may be integrated with each other as in a second modification illustrated inFIG. 6.

It is preferable that the parasitic capacitances between hot-side wiring lines such as the wiring line portion11and the wiring line portion15be made small. For example, if the parasitic capacitance between the wiring line portion11and the wiring line portion15is made small, attenuation of the ladder filter is able to be made sufficiently large. Therefore, it is preferable that first hollow portions31be provided in this manner below wiring line portions and/or below a region between wiring line portions in the case of such hot-side wiring line portions.

FIG. 7is a front sectional view of an elastic wave device according to a second preferred embodiment of the present invention and corresponds toFIG. 3, which illustrates the first preferred embodiment. An elastic wave device41of the second preferred embodiment is preferably the same as that of the first preferred embodiment except that first hollow portions31and a second hollow portion32are separated from each other by partition walls24a.

As is clear from comparing and contrastingFIG. 3andFIG. 7, in the second preferred embodiment, a first hollow portion31that is below the branch wiring line portion16band a second hollow portion32that is below the IDT electrode21are separated from each other by a partition wall24a. Similarly, a first hollow portion31that is below the wiring line portion is also separated from the second hollow portion32by a partition wall24a. Therefore, compared with the structure illustrated inFIG. 3, the mechanical strength is able to be more effectively increased with the structure of the second preferred embodiment illustrated inFIG. 7.

In various preferred embodiments of the present invention, the first hollow portions are provided in order to reduce the parasitic capacitances between wiring line portions. Therefore, although it is sufficient for the first hollow portions to be provided below wiring line portions, the first hollow portions may be provided between wiring line portions. In other words, it is sufficient for a first hollow portion31to be provided at least below at least one wiring line portion or below a region between wiring line portions.

FIG. 8is a front sectional view of an elastic wave device51according to a third preferred embodiment of the present invention. The elastic wave device51is preferably the same as that of the first preferred embodiment illustrated inFIG. 3except that the second hollow portion32is not provided and an acoustic reflection film52is provided in place of the second hollow portion32. Therefore, only this different portion will be described.

The acoustic reflection film52has a structure obtained by alternately stacking acoustic reflection films52a,52c, and52ehaving a relatively low acoustic impedance and acoustic reflection films52band52dhaving a relatively high acoustic impedance. Therefore, an elastic wave is reflected by the acoustic reflection film52and is confined to the piezoelectric substrate5. Thus, even though the second hollow portion32is not provided, an elastic wave is able to be effectively excited using a thin piezoelectric substrate5. The acoustic reflection film is not limited to above-described configuration, and it is sufficient that a layer having a relatively low acoustic impedance and a layer have a relatively high acoustic impedance be alternately stacked on top of one another.

Next, a non-limiting example of a method of manufacturing the elastic wave device1of the first preferred embodiment will be described.

Next, openings are formed in the support substrate24by performing etching or the like in portions of the support substrate24where the first hollow portions31and the second hollow portions32are to be formed. These openings, that is, openings that open at the upper surface of the support substrate24, are filled with a sacrificial layer. As a result, the upper surface of the support substrate24is made flat.

The sacrificial layer is preferably made of a material that can be removed through a solvent treatment in a later step.

After forming the sacrificial layer, the piezoelectric substrate5is stacked on the upper surface of the support substrate24. After that, the IDT electrodes21, the reflectors22and23, the plurality of wiring line portions11to18, and the input terminal2, the output terminal3and the ground terminals6and7are formed on the piezoelectric substrate5using a photolithography method. Next, through holes that are connected to the sacrificial layer are formed in the piezoelectric substrate5by performing etching. Finally, the sacrificial layer is removed by injecting a solvent from the through holes.

According to the example manufacturing method of this preferred embodiment, the first hollow portions31and the second hollow portions32are simultaneously formed. Therefore, it is possible to avoid a complicated manufacturing process. The second hollow portions32do not necessarily have to be provided in a preferred embodiment of the present invention. In other words, according to various preferred embodiments of the present invention, a parasitic capacitance between wiring line portions is able to be reduced so long as at least one first hollow portion is provided below a wiring line portion or between wiring line portions. Thus, the attenuation outside the pass band of a ladder filter is able to be increased and the characteristics of the ladder filter are able to be improved.

In the above-described preferred embodiments, plate waves preferably are used and therefore the thickness of the piezoelectric substrate5is small. Therefore, an etching hole is able to be easily formed in the piezoelectric substrate5. Consequently, the sacrificial layer is able to be easily removed by performing etching.

FIG. 9is a schematic front sectional view for describing a plate wave device according to a fourth preferred embodiment of the present invention. In an elastic wave device of the fourth preferred embodiment, the first and second hollow portions31and32, which are provided in the support substrate24, open at the lower surface of the support substrate24. The first and second hollow portions31and32are connected to each other as in the case illustrated inFIG. 3. The piezoelectric substrate5is stacked on the support substrate24so as to cover the upper openings of the first and second hollow portions31and32. The IDT electrode21is provided on the piezoelectric substrate5. Thus, the first and hollow portions31and32, which are provided in the support substrate24, may open downwardly.

In addition, a structure may be adopted in which the first hollow portions31that are each provided below a wiring line portion or between wiring line portions also open downwardly as in the case of the second hollow portion32of the fourth preferred embodiment.

A non-limiting example of a manufacturing method of the fourth preferred embodiment will be described. The piezoelectric substrate is stacked on the surface of the support substrate24, and then the thickness of the piezoelectric substrate is reduced. A piezoelectric substrate5having a prescribed thickness is formed in this way. Next, the IDT electrodes21are formed at prescribed positions on the piezoelectric substrate5. After that, the first and second hollow portions31and32are formed in the support substrate24.

In the above-described preferred embodiments, plate waves are preferably used, for example, but elastic waves other than a plate wave may be used. Possible examples of such elastic waves include surface acoustic waves, leaky waves and bulk waves.

FIG. 10is a front sectional view of an elastic wave device according to a fifth preferred embodiment of the present invention. An elastic wave device71does not include the second hollow portion or an acoustic reflection film. The piezoelectric substrate5of this preferred embodiment is preferably made of LiTaO3(LT). The elastic wave device71utilizes leaky waves. Of course, the piezoelectric substrate5may be composed of a piezoelectric single crystal other than LiTaO3. In addition, elastic waves other than leaky waves may be used.

The first hollow portions31are provided in this preferred embodiment as well, and therefore, the parasitic capacitances between wiring line portions are able to be reduced.

An acoustic reflection film may be provided between the piezoelectric substrate and the support substrate. An elastic wave is able to be confined to the piezoelectric substrate in this way. Therefore, energy efficiency is able to be effectively increased.

Alternatively, a high acoustic velocity film may be stacked on the support substrate, a low acoustic velocity film may be stacked on the high acoustic velocity film, and a piezoelectric substrate may be stacked on the low acoustic velocity film. Here, “high acoustic velocity film” refers to a film in which the velocity of a bulk wave propagating therethrough is higher than that of an elastic wave propagating through the piezoelectric substrate. “Low acoustic velocity film” refers to a film in which the velocity of a bulk wave propagating therethrough is lower than the velocity of a bulk wave propagating through the piezoelectric substrate. An elastic wave is able to be confined to the piezoelectric substrate in this case as well.

In addition, a high acoustic velocity support substrate may be used in which a high acoustic velocity film and a support substrate are integrated with each other. The material of the high acoustic velocity support substrate may be silicon (Si), for example.

Furthermore, a ladder filter is described in the first preferred embodiment as a non-limiting example, but an elastic wave device of a preferred embodiment of the present invention may have the structure of another type of filter such as a longitudinally coupled resonator-type elastic wave filter, for example. In addition, preferred embodiments of the present invention are not limited to being applied to a filter, and preferred embodiments of the present invention can be applied to various elastic wave devices in which there is a demand to reduce the parasitic capacitances between wiring line portions.