Patent ID: 12224733

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be clarified by describing preferred embodiments of the present invention with reference to the drawings.

The preferred embodiments described in the present specification are exemplary, and partial replacement or combination of components between different preferred embodiments is possible.

FIG.1is a plan view of an acoustic wave device according to a first preferred embodiment of the present invention.

An acoustic wave device1includes a piezoelectric substrate2. An IDT electrode3is provided on the piezoelectric substrate2. An acoustic wave is excited by applying an AC voltage to the IDT electrode3. A pair of reflectors8A and8B is provided on both sides of the IDT electrode3in an acoustic wave propagation direction on the piezoelectric substrate2. The acoustic wave device1of the present preferred embodiment is an acoustic wave resonator, for example. However, the acoustic wave device1according to the present invention is not limited to an acoustic wave resonator, and may be, for example, a filter device including a plurality of acoustic wave resonators, a multiplexer including the filter device, or the like.

The IDT electrode3includes a first busbar16and a second busbar17facing each other. The IDT electrode3includes a plurality of first electrode fingers18each including one end connected to the first busbar16. Further, the IDT electrode3includes a plurality of second electrode fingers19each including one end connected to the second busbar17. The plurality of first electrode fingers18and the plurality of second electrode fingers19are interdigitated with each other.

The IDT electrode3is made of, for example, a laminated metal film in which a Ti layer, an Al layer, and a Ti layer are laminated in this order from the piezoelectric substrate2side. The reflector8A and the reflector8B are made of the same material as that of the IDT electrode3. Note that the materials of the IDT electrode3, the reflector8A, and the reflector8B are not limited to those described above. Alternatively, the IDT electrode3, the reflector8A, and the reflector8B may be made of, for example, a single-layer metal film.

FIG.2is a front sectional view taken along a line I-I inFIG.1, illustrating the vicinity of a pair of electrode fingers in the IDT electrode3of the acoustic wave device according to the first preferred embodiment of the present invention.

The piezoelectric substrate2of the acoustic wave device1includes a support substrate4, a silicon nitride film5provided on the support substrate4, a silicon oxide film6provided on the silicon nitride film5, and a piezoelectric layer7provided on the silicon oxide film6. The IDT electrode3, the reflector8A, and the reflector8B are provided on the piezoelectric layer7.

The support substrate4of the present preferred embodiment is a silicon substrate, for example. The plane orientation of the support substrate4is, for example, Si (111). Here, the plane orientation of the support substrate4is the plane orientation of the support substrate4on the side of the piezoelectric layer7. Si (111) refers to a substrate obtained by cutting a crystal structure of silicon having a diamond structure along a (111) plane perpendicular or substantially perpendicular to a crystal axis represented by a Miller index [111]. Note that the (111) plane is the plane illustrated inFIG.3. However, other crystallographically equivalent planes are also included. In the present preferred embodiment, the Euler angles of the (111) plane of the support substrate4are (about −45°, about −54.7°, Ψ). Here, Ψ in the Euler angles of the support substrate4is a propagation angle of the support substrate4. The propagation angle of the support substrate4is an angle between the acoustic wave propagation direction and the crystal axis [1-10] of silicon in the (111) plane. From the symmetry of the crystal, Ψ=Ψ+about 120°.

In the acoustic wave device1, silicon nitride of the silicon nitride film5is, for example, SiN, and silicon oxide of the silicon oxide film6is, for example, SiO2. However, the rate of nitrogen in the silicon nitride film5and the rate of oxygen in the silicon oxide film6are not limited to the above.

The piezoelectric layer7is, for example, a lithium tantalate layer. To be more specific, LiTaO3of Y-cut X-SAW propagation is used for the piezoelectric layer7. Here, when a wavelength defined by an electrode finger pitch of the IDT electrode3is λ, a film thickness of the piezoelectric layer7is equal to or less than about 1λ. Note that the “film thickness” of a certain layer (film) refers to the size in a thickness direction of the layer, and refers to the size in a direction in which the support substrate4, the silicon nitride film5, the silicon oxide film6, and the piezoelectric layer7are laminated. The Euler angles of the piezoelectric layer7are (within a range of about 0°±5°, θ, within a range of about 0°±5°) or (within a range of about 0°±5°, θ, within a range of about 180°±5°). θ in the Euler angles of the piezoelectric layer7is about 95.5°≤θ<about 117.5° or about −84.5°≤θ<about −62.5°, which are equivalent to each other.

The present preferred embodiment preferably includes the following features. 1) The piezoelectric substrate2is a multilayer body in which the support substrate4that is a silicon substrate, the silicon nitride film5, the silicon oxide film6, and the piezoelectric layer7including Y-cut X-SAW propagation lithium tantalate are laminated in this order. 2) The film thickness of the piezoelectric layer7is equal to or less than about 1λ. 3) The relationship between θ in the Euler angles of the piezoelectric layer7and the film thickness of the silicon nitride film5is a combination shown in Table 5 or 6 described below. As a result, the higher-order mode can be reduced or prevented. This will be described hereinafter in detail. Note that after the case shown in Table 5 is described, the case shown in Table 6 is described.

TABLE 5FILM THICKNESS OF SiN FILMθLOWER LIMIT [λ]UPPER LIMIT [λ]95.5 ≤ θ < 96.50.00050.74696.5 ≤ θ < 97.50.00050.73297.5 ≤ θ < 98.50.00050.73698.5 ≤ θ < 99.50.00050.72699.5 ≤ θ < 100.50.00050.724100.5 ≤ θ < 101.50.00050.718101.5 ≤ θ < 102.50.00050.712102.5 ≤ θ < 103.50.00050.71103.5 ≤ θ < 104.50.00050.704104.5 ≤ θ < 105.50.00050.702105.5 ≤ θ < 106.50.00050.698106.5 ≤ θ < 107.50.00050.695107.5 ≤ θ < 108.50.00050.695108.5 ≤ θ < 109.50.00050.692109.5 ≤ θ < 110.50.00050.689110.5 ≤ θ < 111.50.00050.689111.5 ≤ θ < 112.50.00050.69112.5 ≤ θ < 113.50.00050.686113.5 ≤ θ < 114.50.00050.684114.5 ≤ θ < 115.50.00050.684115.5 ≤ θ < 116.50.00050.682116.5 ≤ θ < 117.50.00050.666

TABLE 6FILM THICKNESS OF SiN FILMθLOWER LIMIT [λ]UPPER LIMIT [λ]−84.5 ≤ θ < −83.50.00050.746−83.5 ≤ θ < −82.50.00050.732−82.5 ≤ θ < −81.50.00050.736−81.5 ≤ θ < −80.50.00050.726−80.5 ≤ θ < −79.50.00050.724−79.5 ≤ θ < −78.50.00050.718−78.5 ≤ θ < −77.50.00050.712−77.5 ≤ θ < −76.50.00050.71−76.5 ≤ θ < −75.50.00050.704−75.5 ≤ θ < −74.50.00050.702−74.5 ≤ θ < −73.50.00050.698−73.5 ≤ θ < −72.50.00050.695−72.5 ≤ θ < −71.50.00050.695−71.5 ≤ θ < −70.50.00050.692−70.5 ≤ θ < −69.50.00050.689−69.5 ≤ θ < −68.50.00050.689−68.5 ≤ θ < −67.50.00050.69−67.5 ≤ θ < −66.50.00050.686−66.5 ≤ θ < −65.50.00050.684−65.5 ≤ θ < −64.50.00050.684−64.5 ≤ θ < −63.50.00050.682−63.5 ≤ θ < −62.50.00050.666

In the acoustic wave device including the same laminated structure as that of the first preferred embodiment, the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film, and the phase of the higher-order mode was obtained. Table 5 shows θ and the range of the film thickness of the silicon nitride film in which the higher-order mode is equal to or less than about −70° and the higher-order mode can be effectively reduced or prevented. Here, conditions of the acoustic wave device are as follows.

Support substrate: material: silicon (Si), plane orientation: Si (111), Euler angles in (111) plane: (about −45°, about −54.7°, about 46°)

Silicon nitride film: film thickness: about 0.0005λ or more and about 1.5λ or less

Silicon oxide film: film thickness: about 0.15λ

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (about 0°, about 96°≤θ≤about 117°, about 0°)

IDT electrode: material: Ti/Al/Ti from the piezoelectric substrate side, film thickness of each layer: about 0.006λ/about 0.05λ/about 0.002λ from the piezoelectric substrate side

Wavelength λ of IDT electrode: about 2 μm

Note thatFIG.4toFIG.7show results of the case in which θ in the Euler angles of the piezoelectric layer was changed in a range of about 96°≤θ≤about 117°. It is known that, for example, when the effect of reducing or preventing a spurious component such as the higher-order mode is obtained in the case where θ is θ1, the same or substantially the same advantageous effect is obtained within the range of about θ1±0.5°. Therefore, Table 5 shows the case in the range of about 95.5°≤θ<about 96.5° to the case in the range of about 116.5°≤θ<about 117.5°. Similarly, in tables other than Table 5, a range within θ±about 0.5° may be used for description.

FIG.4is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 96°.FIG.5is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 97°≤θ≤about 103°.FIG.6is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 104°≤θ≤about 110°.FIG.7is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 111°≤θ≤about 117°.

As shown inFIG.4, when θ is about 96°, the phase of the higher-order mode is equal to or less than about −70° in the case where the film thickness of the silicon nitride film is in the range of equal to or more than about 0.0005λ and equal to or less than about 0.746λ. The results are shown in Table 5. Similarly, as shown inFIG.5toFIG.7, Table 5 shows the range of the film thickness of the silicon nitride film in which the phase of the higher-order mode is equal to or less than about −70° in the case where θ in the Euler angles of the piezoelectric layer is changed from about 97° to about 117° in increments of about 1°. As described above, in the present preferred embodiment in which the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film is the combination shown in Table 5, it is understood that the higher-order mode can be effectively reduced or prevented. The reason why the higher-order mode can be reduced or prevented is as follows.

For example, the higher-order mode nearly twice the resonant frequency is a Rayleigh-based mode, and has a propagation direction component and a depth direction component. In a certain range of the cut-angles of the piezoelectric body of the piezoelectric layer, a coupling coefficient of the Rayleigh wave becomes small. Similarly, it is considered that the cut-angles of the piezoelectric body also affect the higher-order mode. In addition, it is considered that the plane orientation of silicon of the support substrate affects the higher-order mode. Therefore, it is considered that the higher-order mode can be reduced or prevented by setting the film thickness of the silicon nitride film to such a degree that the wave reaches the support substrate.

Also in the case where θ in the Euler angles of the piezoelectric layer7is about −84.5°≤θ<about −62.5°, which is equivalent to the case of about 95.5°≤θ<about 117.5°, the higher-order mode can be reduced or prevented in the same or substantially the same manner as described above. Therefore, even in the case where the relationship between θ and the film thickness of the silicon nitride film5is the combination shown in Table 6, the higher-order mode can be reduced or prevented.

In the above description, the case where the propagation angle Ψ in the (111) plane of the support substrate is about 46° has been described, the present invention is not limited thereto. By having the following configuration, in the case where θ in the Euler angles of the piezoelectric layer is about 95.5°≤θ<about 117.5° or about −84.5°≤θ<about −62.5°, the higher-order mode can be further reduced or prevented. 1) The film thickness of the silicon nitride film is equal to or less than about 0.5λ, and the plane orientation of the support substrate is Si (111).2a) When the Euler angles of the piezoelectric layer is (within a range of about 0°±5°, θ, within a range of about 0°±5°), n represents any integer (0, ±1, ±2), θ in the Euler angles of the piezoelectric layer is about 95.5°≤θ<about 117.5° or about −84.5°≤θ<about −62.5° and the propagation angle Ψ of the support substrate is within a range of about 60°±50°+120°×n.2b) When the Euler angles of the piezoelectric layer is (within a range of about 0°±5°, θ, within a range of about 180°±5°), n represents any integer (0, ±1, ±2), and θ in the Euler angles of the piezoelectric layer is about 95.5°≤θ<about 117.5° or θ is about −84.5°≤θ<about −62.5°, the propagation angle Ψ of the support substrate is within a range of about 0°±50°+120°×n. This will be described in detail. Note that after the above-described case of2a) is described, and the above-described case of2b) is described.

In the acoustic wave device having the same or substantially the same laminated structure as that of the first preferred embodiment, the relationship between θ in the Euler angles of the piezoelectric layer, the film thickness of the silicon nitride film, and the propagation angle Ψ of the support substrate, and the phase of the higher-order mode was obtained. Conditions of the acoustic wave device are as follows.

Support substrate: material: silicon (Si), plane orientation: Si (111), Euler angles in (111) plane: (about −45°, about −54.7°, about 0°≤Ψ<about 360°)

Silicon nitride film: film thickness: about 0.5λ or less

Silicon oxide film: film thickness: about 0.15λ

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: 0.2λ, Euler angles: (about 0°, about 96°≤θ≤about 117°, about 0°)

IDT electrode: material: Ti/Al/Ti from the piezoelectric substrate side, film thickness of each layer: about 0.006λ/about 0.05λ/about 0.002λ from the piezoelectric substrate side

Wavelength λ of IDT electrode: about 2 μm

FIG.8shows the relationship between the propagation angle Ψ of the support substrate and the phase of the higher-order mode.FIG.9is an enlarged view ofFIG.8. Here, since Ψ=Ψ+about 120°,FIG.8shows the case where Ψ is about 0° to about 120°.

As shown inFIG.8, the phase of the higher-order mode is less than about −77° regardless of the magnitude of the propagation angle Ψ, and the higher-order mode is sufficiently reduced or prevented. In particular, as shown inFIG.9, it can be seen that in the case where Ψ is equal to or more than about 10°, the value of the phase of the higher-order mode is further reduced. Similarly, in the case where Ψ is equal to or less than about 110°, the value of the phase of the higher-order mode is further reduced. It can be seen that the higher-order mode can be further reduced or prevented in the case where Ψ is in the range of about 60°±50°+120°×n.

Further, as shown inFIG.8, as the propagation angle Ψ of the support substrate increases to approach 26°, the value of the phase of the higher-order mode rapidly decreases, and in the case of about 26°≤Ψ≤about 60°, the phase of the higher-order mode is further reduced or prevented. Similarly, as Ψ decreases to approach 94°, the value of the phase of the higher-order mode rapidly decreases, and in the case of about 60°≤Ψ≤about 94°, the higher-order mode is further reduced or prevented. Therefore, when Ψ is within the range of about 60°±34°+120°×n, the higher-order mode can be further reduced or prevented. The same or substantially the same applies to the case of (about 0°, about −84°≤θ≤about −63°, about 0°).

Next, from the conditions of the acoustic wave device when the relationship ofFIG.8was obtained, by changing only the condition of Ψ in the Euler angles (φ, θ, ψ) of the piezoelectric layer, the relationship between ψ, the film thickness of the silicon nitride film, and the propagation angle Ψ of the support substrate, and the phase of the higher-order mode was obtained.

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (about 0°, about 96°≤θ≤about 117°, about 180°)

FIG.10is a diagram showing the relationship between the propagation angle Ψ of the support substrate and the phase of the higher-order mode.

As shown inFIG.10, the phase of the higher-order mode is less than about −77° regardless of the magnitude of the propagation angle Ψ, and the higher-order mode is sufficiently reduced or prevented. In particular, it can be seen that in the case where Ψ is equal to or more than about −50°, the value of the phase of the higher-order mode is further reduced. Similarly, in the case where Ψ is equal to or less than about 50°, the value of the higher-order mode is further reduced. It can be seen that in the case where Ψ is within the range of about 0°±50°+120°×n, the higher-order mode can be further reduced or prevented.

Further, as shown inFIG.10, as the propagation angle Ψ increases to approach −34°, the value of the phase of the higher-order mode rapidly decreases, and in the case of about −34° about 0°, the phase of the higher-order mode is further reduced or prevented. Similarly, as Ψ decreases to approach 34°, the value of the phase of the higher-order mode rapidly decreases, and in the case of about 0°≤Ψ≤about 34°, the higher-order mode is further reduced or prevented. Therefore, in the case where Ψ is within the range of about 0°±34°+120°×n, the higher-order mode can be further reduced or prevented.

FIG.11is a front sectional view illustrating the vicinity of a pair of electrode fingers in an IDT electrode of an acoustic wave device according to a second preferred embodiment of the present invention.

The present preferred embodiment is different from the first preferred embodiment in that a dielectric layer28is provided between the piezoelectric layer7and the IDT electrode3. The IDT electrode3is directly provided on the piezoelectric layer7in the first preferred embodiment, but the IDT electrode3may be indirectly provided on the piezoelectric layer7with the dielectric layer28interposed therebetween as in the present preferred embodiment.

An acoustic velocity of the bulk wave propagating through the dielectric layer28is higher than an acoustic velocity of the acoustic wave propagating through the piezoelectric layer7. In the present preferred embodiment, the dielectric layer28is, for example, a silicon nitride layer. Note that the dielectric layer28may have a relatively high acoustic velocity, and is not limited to a silicon nitride layer.

Hereinafter, phase characteristics of the acoustic wave device having the configuration of the second preferred embodiment and the acoustic wave device having the configuration of the first preferred embodiment will be described. The conditions of each of the acoustic wave devices are as follows.

Support substrate: material: silicon (Si), plane orientation: Si (111), Euler angles in (111) plane: (−45°, −54.7°, 47°)

Silicon nitride film: film thickness: about 0.15λ,

Silicon oxide film: film thickness: about 0.15λ,

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (about 0°, about 110°, about 0°)

IDT electrode: material: Ti/Al/Ti from the piezoelectric substrate side, film thickness of each layer: about 0.006λ/about 0.05λ/about 0.002λ from the piezoelectric substrate side

Wavelength λ of IDT electrode: about 2 μm

FIG.12is a diagram showing phase characteristics of the acoustic wave devices according to the first preferred embodiment and the second preferred embodiment.FIG.13is an enlarged view ofFIG.12. InFIG.12andFIG.13, the solid line indicates the result of the second preferred embodiment, and the broken line indicates the result of the first preferred embodiment.

As shown inFIG.12andFIG.13, in the first preferred embodiment and the second preferred embodiment, the higher-order mode can be effectively reduced or prevented. In particular, it can be seen that the higher-order mode can be further reduced or prevented in the second preferred embodiment. In the second preferred embodiment, since the high-acoustic-velocity dielectric layer is provided, the acoustic velocity of the higher-order mode increases. As a result, the higher-order mode is leaked in a bulk direction, so that the higher-order mode on the high-frequency side can be further reduced or prevented.

Here, for example, in the case where an SH wave or the like is used as a main mode, a Rayleigh wave becomes spurious. In the case where the dielectric layer28is a SiN layer, a Rayleigh wave as a spurious component can also be reduced or prevented by having the configuration described later. This will be described in detail.

The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the dielectric layer, and the phase of the Rayleigh wave in the acoustic wave device having the configuration of the second preferred embodiment was obtained. Conditions of the acoustic wave device are as follows.

Support substrate: material: silicon (Si), plane orientation: Si (111), Euler angles in (111) plane: (−45°, −54.7°, 47°)

Silicon nitride film: film thickness: about 0.15λ

Silicon oxide film: film thickness: about 0.15λ

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (about 0°, about 96°≤θ≤about 117°, about 0°)

IDT electrode: material: Ti/Al/Ti from the piezoelectric substrate side, film thickness of each layer: about 0.006λ/about 0.05λ/about 0.002λ from the piezoelectric substrate side

Wavelength λ of IDT electrode: about 2 μm

Dielectric layer: material: SiN, film thickness: about 0.0025λ or more and about 0.0975λ or less

FIG.14is a diagram showing the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride layer, and the phase of the Rayleigh wave in the case where the dielectric layer is a silicon nitride layer.

The range other than the hatched range shown inFIG.14is a range in which the phase of the Rayleigh wave is equal to or less than about −70°, for example. Here, the relationship shown inFIG.14is expressed by Equation 1 described below. Note that in Equation 1, the film thickness of the silicon nitride layer is referred to as SiN film thickness.
Phase of Rayleigh wave=(−81.6949045454545)+(−0.67490613636364)×(θ−110)+(−189.247997265892)×((“SiN film thickness [λ]”)−0.05)+0.111730638111889×((θ−110)×(θ−110)−40)+36.1595358851675×((θ−110)×((“SiN film thickness [λ]”)−0.05))+5258.22469396632×(((“SiN film thickness [λ]”)−0.05)×((“SiN film thickness [λ]”)−0.05)−0.00083125)  Equation 1

θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride layer are preferably an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 1 is equal to or less than about −70°, for example. In this case, the Rayleigh wave can be effectively reduced or prevented in addition to the reduction or prevention of the higher-order mode.

As described above, the dielectric layer is not limited to a silicon nitride layer. For example, even in the case where the dielectric layer is an aluminum oxide layer or an aluminum nitride layer, the Rayleigh wave can be effectively reduced or prevented in addition to the higher-order mode. This will be shown hereinafter.

The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum oxide layer, and the phase of the Rayleigh wave was obtained. On the other hand, the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum nitride layer, and the phase of the Rayleigh wave was obtained. Note that the conditions of the acoustic wave device are the same or substantially the same as the conditions when obtaining the relationship shown inFIG.14except that the material of the dielectric layer is Al2O3or AlN.

FIG.15is a diagram showing the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum oxide layer, and the phase of the Rayleigh wave in the case where the dielectric layer is the aluminum oxide layer.

The range other than the hatched range shown inFIG.15is a range in which the phase of the Rayleigh wave is equal to or less than about −70°, for example. Here, the relationship shown inFIG.15is expressed by Equation 2 described below. Note that in Equation 2, the film thickness of the aluminum oxide layer is referred to as Al2O3film thickness.
Phase of Rayleigh wave=(−80.1333863636364)+(−0.554522499999998)×(θ−110)+(−173.463554340396)×((“Al2O3 film thickness [λ]”)−0.05)+0.149698033216783×((θ−110)×(θ−110)−40)+41.1703301435407×((θ−110)*((“Al2O3 film thickness [λ]”)−0.05))+4990.83763126825×(((“Al2O3 film thickness [λ]”)−0.05)×((“Al2O3 film thickness [λ]”)−0.05)−0.00083125)  Equation 2

θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum oxide layer are preferably an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 2 is equal to or less than about −70°, for example. In this case, the Rayleigh wave can be effectively reduced or prevented in addition to the reduction or prevention of the higher-order mode.

FIG.16is a diagram showing the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum nitride layer, and the phase of the Rayleigh wave in the case where the dielectric layer is the aluminum nitride layer.

The range other than the hatched range shown inFIG.16is a range in which the phase of the Rayleigh wave is equal to or less than about −70°, for example. Here, the relationship shown inFIG.16is expressed by Equation 3 described below. Note that in Equation 3, the film thickness of the aluminum nitride layer is referred to as AlN film thickness or AlN β.
Phase of Rayleigh wave=(−87.3504136363636)+(−0.270137500000004)×(θ−110)+(−97.7367464114832)×((“AlN film thickness [λ]”)−0.05)+0.0257423222610727×((θ−110)×(θ−110)−40)+14.0575563909775×((θ−110)×((“AlN β [λ]”)−0.05))+3335.40856272914×(((“AlN film thickness [λ]”)−0.05)×((“AlN film thickness [λ]”)−0.05)−0.00083125)  Equation 3

θ in the Euler angles of the piezoelectric layer and the film thickness of the aluminum nitride layer are preferably an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 3 is equal to or less than about −70°, for example. In this case, the Rayleigh wave can be effectively reduced or prevented in addition to the reduction or prevention of the higher-order mode.

In the first preferred embodiment and the second preferred embodiment, the case where θ in the Euler angles of the piezoelectric layer satisfies about 95.5 0<117.5° is described. Hereinafter, the configuration capable of reducing or preventing the higher-order mode even in the case of about 117.5°≤θ<129.5° or about 85.5°≤θ<95.5° will be described. Details of a third preferred embodiment of the present invention having the same or substantially the same configuration as the first preferred embodiment except that about 117.5°≤θ<129.5° is satisfied will be described with reference toFIG.17toFIG.21. Details of a fourth preferred embodiment of the present invention having the same or substantially the same configuration as the first preferred embodiment except that about 85.5°≤θ<95.5° is satisfied will be described with reference toFIG.22toFIG.24.

An acoustic wave device according to the third preferred embodiment can reduce or prevent the higher-order mode with the following configuration. 1) The piezoelectric substrate is a multilayer body including a support substrate that is a silicon substrate, a silicon nitride film, a silicon oxide film, and a piezoelectric layer using lithium tantalate that are laminated in this order. 2) The film thickness of the piezoelectric layer is equal to or less than about 1λ. 3) The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film is a combination shown in Table 7 described below.

TABLE 7FILM THICKNESS OF SiN FILMθLOWER LIMIT [λ]UPPER LIMIT [λ]117.5 ≤ θ < 118.50.00050.092117.5 ≤ θ < 118.50.1660.597118.5 ≤ θ < 119.50.00050.07118.5 ≤ θ < 119.50.1680.554119.5 ≤ θ < 120.50.0050.063119.5 ≤ θ < 120.50.1520.522120.5 ≤ θ < 121.50.0040.06120.5 ≤ θ < 121.50.1320.492121.5 ≤ θ < 122.50.0220.052121.5 ≤ θ < 122.50.120.466122.5 ≤ θ < 123.50.220.43123.5 ≤ θ < 124.50.240.4124.5 ≤ θ < 125.50.250.38125.5 ≤ θ < 126.50.250.38126.5 ≤ θ < 127.50.250.36127.5 ≤ θ < 128.50.250.31128.5 ≤ θ < 129.50.250.28

From the conditions of the acoustic wave device when the relationship ofFIG.4toFIG.7was obtained, by changing only the condition of θ in the Euler angles of the piezoelectric layer, the relationship between θ and the film thickness of the silicon nitride film, and the phase of the higher-order mode was obtained.

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (0°, 118°≤θ≤129°, 0°)

FIG.17is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 118°.FIG.18is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 119°≤θ≤122°.FIG.19is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 123°≤θ≤126°.FIG.20is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 127°≤θ≤129°.FIG.21is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 130°.

As shown inFIG.17, in the case where θ is about 118°, the phase of the higher-order mode is equal to or less than about −70° in the range of the film thickness of the silicon nitride film of equal to or more than about 0.0005λ and equal to or less than about 0.092λ or in the range of equal to or more than about 0.166λ and equal to or less than about 0.597λ. The results are shown in Table 7. Similarly, as shown inFIG.18toFIG.20, in the case where θ in the Euler angles of the piezoelectric layer is changed from about 119° to about 129° in increments of 1°, the range of the film thickness of the silicon nitride film in which the phase of the higher-order mode is equal to or less than about −70° is shown in Table 7.

On the other hand, as shown inFIG.21, in the case where θ is about 130°, the phase of the higher-order mode exceeds about −70° in the range in which the film thickness of the silicon nitride film is equal to or less than about 1.5λ, and it is difficult to sufficiently suppress the phase of the higher-order mode. As described above, it can be seen that in the case where the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film is the combination shown in Table 7, the higher-order mode can be effectively reduced or prevented.

An acoustic wave device according to the fourth preferred embodiment can suppress the higher-order mode by having the following configuration. 1) The piezoelectric substrate is a multilayer body including a support substrate that is a silicon substrate, a silicon nitride film, a silicon oxide film, and a piezoelectric layer using lithium tantalate that are laminated in this order. 2) The film thickness of the piezoelectric layer is equal to or less than about 1λ. 3) The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film is a combination shown in Table 8 described below.

TABLE 8FILM THICKNESS OF SiN FILMθLOWER LIMIT [λ]UPPER LIMIT [λ]85.5 ≤ θ < 86.50.00050.0385.5 ≤ θ < 86.50.430.4686.5 ≤ θ < 87.50.00050.03886.5 ≤ θ < 87.50.420.4787.5 ≤ θ < 88.50.00050.04887.5 ≤ θ < 88.50.4120.48588.5 ≤ θ < 89.50.00050.05888.5 ≤ θ < 89.50.4040.50689.5 ≤ θ < 90.50.00050.0789.5 ≤ θ < 90.50.3880.5290.5 ≤ θ < 91.50.00050.08590.5 ≤ θ < 91.50.370.53891.5 ≤ θ < 92.50.00050.10291.5 ≤ θ < 92.50.3460.5692.5 ≤ θ < 93.50.00050.12892.5 ≤ θ < 93.50.3150.5993.5 ≤ θ < 94.50.00050.16693.5 ≤ θ < 94.50.2950.61694.5 ≤ θ < 95.50.00050.22694.5 ≤ θ < 95.50.2840.66

From the conditions of the acoustic wave device when the relationship ofFIG.4toFIG.7was obtained, by changing only the condition of θ in the Euler angles of the piezoelectric layer, the relationship between θ and the film thickness of the silicon nitride film, and the phase of the higher-order mode was obtained.

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (0°, 86° 95°, 0°)

FIG.22is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 75°≤θ≤85°.FIG.23is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angles of the piezoelectric layer is about 86°≤θ≤88°.FIG.24is a diagram showing the relationship between the film thickness of the silicon nitride film and the phase of the higher-order mode in the case where θ in the Euler angle of the piezoelectric layer is about 89°≤θ≤95°.

As shown inFIG.22, in the case where θ is about 75° 85°, the phase of the higher-order mode exceeds about −70° in the range in which the film thickness of the silicon nitride film is equal to or less than about 1.5λ, and it is difficult to sufficiently reduce or prevent the phase of the higher-order mode.

On the other hand, as shown inFIG.23, in the case where θ is about 86°, the phase of the higher-order mode is equal to or less than about −70° in the range in which the film thickness of the silicon nitride film is equal to or more than 0.0005λ and equal to or less than about 0.03λ or in the range of in which the film thickness is equal to or more than about 0.43λ and equal to or less than about 0.46λ. The results are shown in Table 8. Similarly, as shown inFIG.23andFIG.24, in the case where θ in the Euler angles of the piezoelectric layer is changed from about 87° to about 95° in increments of 1°, the range of the film thickness of the silicon nitride film in which the phase of the higher-order mode is equal to or less than about −70° is shown in Table 8. As described above, it can be seen that in the case where the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the silicon nitride film is the combination shown in Table 8, the higher-order mode can be effectively reduced or prevented.

FIG.25is a front sectional view illustrating the vicinity of a pair of electrode fingers in an IDT electrode of an acoustic wave device according to a fifth preferred embodiment of the present invention.

The present preferred embodiment is different from the first preferred embodiment in that a protective film39is provided on the piezoelectric layer7so as to cover the IDT electrode3. A material of the protective film39of the present preferred embodiment is, for example, silicon nitride. More specifically, the protective film39is, for example, a SiN protective film made of SiN. Note that the rate of nitrogen in the silicon nitride of the protective film39is not limited to the above. Alternatively, the material of the protective film39is not limited to silicon nitride, and may be, for example, aluminum nitride or aluminum oxide.

The acoustic wave device of the present preferred embodiment can reduce or prevent not only the higher-order mode, but also a Rayleigh wave as a spurious component by having the configuration described later. This will be described in detail.

The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the protective film and the phase of the Rayleigh wave in the acoustic wave device having the configuration of the fifth preferred embodiment was obtained. Conditions of the acoustic wave device are as follows.

Support substrate: material: silicon (Si), plane orientation: Si (111), Euler angles in (111) plane: (−45°, −54.7°, 46°)

Silicon nitride film: film thickness: about 0.15λ,

Silicon oxide film: film thickness: about 0.15λ,

Piezoelectric layer: material: LiTaO3of Y-cut X-SAW propagation, film thickness: about 0.2λ, Euler angles: (0°, 96°≤θ≤117°, 0°)

IDT electrode: material: Ti/Al/Ti from the piezoelectric substrate side, film thickness of each layer: about 0.006λ/about 0.05λ/about 0.002λ from the piezoelectric substrate side

Wavelength λ of IDT electrode: about 2 μm

Protective film: material: SiN, film thickness: about 0.005λ or more and about 0.05λ or less

FIG.26is a diagram showing the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the SiN protective film, and the phase of the Rayleigh wave and the higher-order mode.

The range other than the hatched range shown inFIG.26is a range in which the phase of the higher-order mode is equal to or less than about −70° and the phase of the Rayleigh wave is equal to or less than about −70°. Here, the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the SiN protective film, and the phase of the Rayleigh wave shown inFIG.26is expressed by Equation 4 described below. Similarly, the relationship between θ and the film thickness of the SiN protective film, and the phase of the higher-order mode is expressed by Equation 5 described below. Note that in Equation 4 and Equation 5, the film thickness of the SiN protective film is simply referred to as SiN protective film.
Rayleigh wave=(−51.52545)+1.78434436363636×(θ−127.5)+551.57103030303×((“SiN protective film [λ]”)−0.0275)+0.102490363636364×((θ−127.5)×(θ−127.5)−206.25)+60.3989730027549×((θ−127.5)×((“SiN protective film [λ]”)−0.0275))  Equation 4
Higher-order mode=(−26.58444)+2.61766496969697×(θ−127.5)+1545.84533333333×((“SiN protective film [λ]”)−0.0275)+(−0.0379121515151515)×((θ−127.5)×(θ−127.5)−206.25)+56.9468179981635×((θ−127.5)×((“SiN protective film [λ]”)−0.0275))  Equation 5

In the present preferred embodiment, θ in the Euler angles of the piezoelectric layer and the film thickness of the SiN protective film are an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 4 is equal to or less than about −70° and the phase of the higher-order mode derived by Equation 5 is equal to or less than about −70°. This makes it possible to effectively reduce or prevent not only the higher-order mode but also the Rayleigh wave.

The lower limit of the phase of the Rayleigh wave derived by Equation 4 is preferably about −90°, for example. This makes it possible to more reliably and effectively reduce or prevent the Rayleigh wave. The lower limit of the phase of the higher-order mode derived by Equation 5 is preferably about −90°, for example. This makes it possible to more reliably and effectively reduce or prevent the higher-order mode. The same applies to Equation 6 and Equation 8, and Equation 7 and Equation 9 described later.

Here, the protective film is not limited to the SiN protective film. For example, even in the case where the material of the protective film is aluminum oxide or aluminum nitride, the Rayleigh wave can be effectively reduced or prevented in addition to the higher-order mode. These will be described hereinafter as a first modification and a second modification of the fifth preferred embodiment. In the first modification, the protective film is, for example, an Al2O3protective film made of Al2O3. In the second modification, the protective film is, for example, an AlN protective film made of AlN. Note that the rate of oxygen or nitrogen in aluminum oxide or aluminum nitride of the protective film is not limited to the above.

The relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the Al2O3protective film, and the phase of the Rayleigh wave and the higher-order mode were obtained. Note that the conditions of the acoustic wave device are the same or substantially the same as the conditions for obtaining the relationship inFIG.26, except that the material of the protective film is Al2O3.

Protective film: material: Al2O3, film thickness: about 0.005λ, or more and about 0.05λ, or less

FIG.27is a diagram shown the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the Al2O3protective film, and the phase of the Rayleigh wave and the higher-order mode.

The range other than the hatched range shown inFIG.27is a range in which the phase of the higher-order mode is equal to or less than about −70° and the phase of the Rayleigh wave is equal to or less than about −70°. Here, the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the Al2O3protective film, and the phase of the Rayleigh wave shown inFIG.27is expressed Equation 6 described below. Similarly, the relationship between θ and the film thickness of the Al2O3protective film, and the phase of the higher-order mode is expressed by Equation 7 described below. Note that in Equation 6 and Equation 7, the film thickness of the Al2O3protective film is simply referred to as Al2O3protective film.
Rayleigh wave=(−40.9578496399338)+2.25601530917004×(θ−127.272727272727)+964.821146439353×((“Al2O3 protective film [λ]”)−0.0274747474747475)+0.100000607678288×((θ−127.272727272727)×(θ−127.272727272727)−203.168044077135)+74.7099873794682×((θ−127.272727272727)×((“Al2O3protective film [λ]”)−0.0274747474747475))  Equation 6
Higher-order mode=(−21.0375184962657)+2.73177575694271×(θ−127.272727272727)+1701.65880601573×((“Al2O3protective film [λ]”)−0.0274747474747475)+(−0.0499273041403373)×((θ−127.272727272727)×(θ−127.272727272727)−203.168044077135)+53.1637721066764×((θ−127.272727272727)×((“Al2O3 protective film [λ]”)−0.0274747474747475))+(−34811.3628963409)×(((“Al2O3 protective film [λ]”)−0.0274747474747475)×((“Al2O3 protective film [λ]”)−0.0274747474747475)−0.00020826956433017)  Equation 7

In the first modification, θ in the Euler angles of the piezoelectric layer and the film thickness of the Al2O3protective film are an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 6 is equal to or less than about −70° and the phase of the higher-order mode derived by Equation 7 is equal to or less than about −70°. This makes it possible to effectively reduce or prevent not only the higher-order mode but also the Rayleigh wave.

On the other hand, the relationship between θ in the Euler angle of the piezoelectric layer and the film thickness of the AlN protective film, and the phase of the Rayleigh wave and the higher-order mode was obtained. Note that the conditions of the acoustic wave device are the same as the conditions for obtaining the relationship inFIG.26, except that the material of the protective film is AlN.

Protective film: material: AlN, film thickness: about 0.005λ or more and about 0.05λ or less

FIG.28is a diagram showing the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the AlN protective film, and the phase of the Rayleigh wave and the higher-order mode.

The range other than the hatched range shown inFIG.28is a range in which the phase of the Rayleigh wave is equal to or less than about −70°. Here, the relationship between θ in the Euler angles of the piezoelectric layer and the film thickness of the AlN protective film, and the phase of the Rayleigh wave shown inFIG.28are expressed by Equation 8 described below. Similarly, the relationship between θ and the film thickness of the AlN protective film, and the phase of the higher-order mode are expressed by Equation 9 described below. Note that in Equation 8 and Equation 9, the film thickness of the AlN protective film is simply referred to as AlN protective film.
Rayleigh wave=(−46.4003800000001)+1.94268775757576×(θ−127.5)+755.618424242424×((“AlN protective film [λ]”)−0.0275)+0.109821151515152×((θ−127.5)×(θ−127.5)−206.25)+69.3031610651975×((θ−127.5)×((“AlN protective film [λ]”)−0.0275))  Equation 8
Higher-order mode=(−24.10841)+2.65936036363636×(θ−127.5)+1560.34145454545×((“AlN protective film [λ]”)−0.0275)+(−0.0415194696969697)×((θ−127.5)×(θ−127.5)−206.25)+54.3834578512397×((θ−127.5)×((“AlN protective film [λ]”)−0.0275))  Equation 9

In the second modification, θ in the Euler angles of the piezoelectric layer and the film thickness of the AlN protective film are an angle and a film thickness within a range in which the phase of the Rayleigh wave derived by Equation 8 is equal to or less than about −70° and the phase of the higher-order mode derived by Equation 9 is equal to or less than about −70°. This makes it possible to effectively reduce or prevent not only the higher-order mode but also the Rayleigh wave.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.