Acoustic wave device

An acoustic wave device includes a piezoelectric layer and first and second electrodes. The first and second electrodes face each other in a direction intersecting with a thickness direction of the piezoelectric layer. The acoustic wave device uses a bulk wave of a thickness-shear primary mode. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The piezoelectric layer is on a first main surface of the silicon substrate. The acoustic wave device further includes a trap region on a side of a second main surface of the piezoelectric layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave device, and more particularly to an acoustic wave device including a piezoelectric layer.

2. Description of the Related Art

Conventionally, a surface acoustic wave device including a support substrate, a low acoustic velocity film, a piezoelectric thin film, and an IDT electrode has been known (for example, see International Publication No. 2015/098678).

A material of the support substrate is, for example, silicon. A material of the low acoustic velocity film is, for example, silicon oxide. A material of the piezoelectric thin film is, for example, LiTaO3.

For a surface acoustic wave device disclosed in International Publication No. 2015/098678, it is difficult to cope with a further increase in frequency. Additionally, with the surface acoustic wave device disclosed in International Publication No. 2015/098678, linearity may be degraded due to harmonic distortion, intermodulation distortion (IMD), or the like.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave devices that are each able to handle higher frequencies and achieve improved linearity.

An acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric layer, and a first electrode and a second electrode. The first electrode and the second electrode face each other in a direction intersecting with a thickness direction of the piezoelectric layer. The acoustic wave device uses a bulk wave of a thickness-shear primary mode. The acoustic wave device further includes a silicon substrate. The silicon substrate includes a first main surface and a second main surface opposed to each other. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The piezoelectric layer is on the first main surface of the silicon substrate. The acoustic wave device further includes a trap region in the silicon substrate.

An acoustic wave device according to a preferred embodiment of the present invention includes a piezoelectric layer, and a first electrode and a second electrode. The first electrode and the second electrode face each other in a direction intersecting with a thickness direction of the piezoelectric layer. The first electrode and the second electrode are adjacent to each other. In the acoustic wave device, in any cross section along the thickness direction of the piezoelectric layer, d/p is equal to or less than about 0.5, when a distance between center lines of the first electrode and the second electrode is represented by p, and a thickness of the piezoelectric layer is represented by d. The acoustic wave device further includes a silicon substrate. The silicon substrate includes a first main surface and a second main surface opposed to each other. A material of the piezoelectric layer is lithium niobate or lithium tantalate. The piezoelectric layer is on the first main surface of the silicon substrate. The acoustic wave device further includes a trap region in the silicon substrate.

With each of the acoustic wave devices according to preferred embodiments of the present invention, it is possible to handle higher frequencies and improve linearity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS.1to8,13,14, and16to28Care schematic diagrams, and ratios of sizes and thicknesses of respective elements and portions in the diagrams do not necessarily reflect actual dimensional ratios.

Hereinafter, an acoustic wave device1according to Preferred Embodiment 1 of the present invention will be described with reference toFIGS.1to5.

(1.1) Overall Configuration of Acoustic Wave Device

As illustrated inFIG.1, the acoustic wave device1according to Preferred Embodiment 1 includes a piezoelectric layer4, and a first electrode51and a second electrode52. As illustrated inFIG.2, the first electrode51and the second electrode52face each other in a direction D2(hereinafter, also referred to as a second direction D2) intersecting with a thickness direction D1(hereinafter, also referred to as a first direction D1) of the piezoelectric layer4. The acoustic wave device1uses a bulk wave of a thickness-shear primary mode. The second direction D2is orthogonal to a polarization direction PZ1of the piezoelectric layer4. The bulk wave of the thickness-shear primary mode is a bulk wave whose propagation direction is the thickness direction D1of the piezoelectric layer4due to thickness-shear vibration of the piezoelectric layer4and whose number of nodes in the thickness direction D1of the piezoelectric layer4is one. The thickness-shear vibration is excited by the first electrode51and the second electrode52. The thickness-shear vibration is excited, in the piezoelectric layer4, in a specified region45between the first electrode51and the second electrode52in a plan view from the thickness direction D1. In the acoustic wave device1, when the second direction D2is orthogonal to the polarization direction PZ1of the piezoelectric layer4, an electromechanical coupling coefficient (hereinafter, also referred to as a coupling coefficient) of the bulk wave of the thickness-shear primary mode is large. Here, “orthogonal” is not limited to a case of being strictly orthogonal, and may be substantially orthogonal (an angle between the second direction D2and the polarization direction PZ1is, for example, about 90°±10°).

As illustrated inFIGS.1and2, the acoustic wave device1includes a plurality of first electrodes51and a plurality of second electrodes52. That is, when the first electrode51and the second electrode52define and function as an electrode set defining a pair of electrodes, the acoustic wave device1includes a plurality of electrode sets each of which includes a pair of the first electrode51and the second electrode52. In the acoustic wave device1, the plurality of first electrodes51and the plurality of second electrodes52are alternately provided one by one in the second direction D2. As illustrated inFIG.1, the acoustic wave device1further includes a first wiring portion61connected to the first electrode51and a second wiring portion62connected to the second electrode52. The first wiring portion61is connected to a first terminal T1. The second wiring portion62is connected to a second terminal T2different from the first terminal T1. The plurality of first electrodes51are commonly connected to the first wiring portion61. The plurality of second electrodes52are commonly connected to the second wiring portion62.

As illustrated inFIG.2, the acoustic wave device1includes, for example, a silicon substrate2, the piezoelectric layer4, the plurality of first electrodes51, and the plurality of second electrodes52. The piezoelectric layer4is provided on the silicon substrate2. As an example, the piezoelectric layer4is provided on the silicon substrate2with a silicon oxide film7interposed therebetween. The plurality of first electrodes51and the plurality of second electrodes52are provided on the piezoelectric layer4. The acoustic wave device1includes, as a resonator, an acoustic wave resonator5including the first electrode51and the second electrode52, and the piezoelectric layer4. The silicon substrate2includes at least a portion of a cavity26facing a portion of the piezoelectric layer4. The cavity26overlaps the plurality of first electrodes51and the plurality of second electrodes52in a plan view from the thickness direction D1of the piezoelectric layer4. Here, the cavity26overlaps the plurality of first electrodes51, the plurality of second electrodes52, and a plurality of specified regions45in a plan view from the thickness direction D1of the piezoelectric layer4. Each of the plurality of regions45is a portion between the first electrode51and the second electrode52that are adjacent to each other. The first electrode51and the second electrode52are “adjacent to each other” refers to a case where the first electrode51and the second electrode52face each other with an interval interposed therebetween.

The acoustic wave device1further includes a trap region10(seeFIGS.2to4) that reduces or prevents movement of charges.

(1.2) Elements of Acoustic Wave Device

Next, elements of the acoustic wave device1will be described with reference to the drawings.

As illustrated inFIG.2, the silicon substrate2supports the piezoelectric layer4. In the acoustic wave device1according to Preferred Embodiment 1, the silicon substrate2supports the piezoelectric layer4, the plurality of first electrodes51, and the plurality of second electrodes52with the silicon oxide film7interposed therebetween.

The silicon substrate2includes a first main surface21and a second main surface22that are opposed to each other. The first main surface21and the second main surface22are opposed to each other in a thickness direction of the silicon substrate2. The thickness direction of the silicon substrate2is a direction along the thickness direction D1of the piezoelectric layer4. In a plan view from the thickness direction D1of the piezoelectric layer4, an outer peripheral shape of the silicon substrate2is a rectangular or substantially rectangular shape, but is not limited thereto, and may be, for example, a square or substantially square shape.

The thickness of the silicon substrate2is, for example, equal to or more than about 100 μm and equal to or less than about 500 μm. The silicon substrate2is a single-crystal silicon substrate including a first main surface and a second main surface that are opposed to each other. As a plane orientation of the first main surface of the single-crystal silicon substrate, for example, a (100) plane, a (110) plane, or a (111) plane may be used. A propagation orientation of the bulk wave described above can be set without being restricted by the plane orientation of the single-crystal silicon substrate. A resistivity of the single-crystal silicon substrate is, for example, equal to or more than 1 kΩcm, preferably equal to or more than about 2 kΩcm, and more preferably equal to or more than about 4 kΩcm.

The first main surface21of the silicon substrate2includes a rough surface211. The rough surface211is formed by roughening the first main surface of the single-crystal silicon substrate. In the acoustic wave device1according to Preferred Embodiment 1, the entire or substantially the entire first main surface21of the silicon substrate2is the rough surface211. The rough surface211does not overlap the acoustic wave resonator in a plan view from the thickness direction D1of the piezoelectric layer4. The silicon substrate2includes a bulk region2B and a surface region2S. The bulk region2B is positioned on a side of the surface region2S opposite to the piezoelectric layer4side. The surface region2S is, for example, an amorphous silicon layer. The amorphous silicon layer is formed, for example, by degrading a lattice structure of the single-crystal silicon substrate when the first main surface of the single-crystal silicon substrate is roughened. The surface region2S includes the first main surface21of the silicon substrate2. The surface region2S has a thickness, for example, equal to or more than about 1 nm and equal to or less than about 700 nm. The bulk region2B is, for example, a single-crystal silicon layer. The single-crystal silicon layer is a remaining portion of the single-crystal silicon substrate when the surface region2S is provided in the single-crystal silicon substrate. The bulk region2B includes the second main surface22of the silicon substrate2. In the acoustic wave device1, the trap region10includes the surface region2S. The surface region2S is not limited to an amorphous silicon layer and may be, for example, a polycrystalline silicon layer. The surface region2S can be formed, for example, by grinding a portion of the single-crystal silicon substrate from the first main surface of the single-crystal silicon substrate, but the forming method is not limited thereto. The surface region2S may be, for example, an amorphous silicon layer or a polycrystalline silicon layer deposited on the single-crystal silicon substrate defining the bulk region2B. In the silicon substrate2, when the surface region2S is an amorphous silicon layer or a polycrystalline silicon layer deposited on the bulk region2B, the first main surface21of the silicon substrate2may include the rough surface211or does not need to include the rough surface211. Further, the surface region2S may be formed, for example, by implanting ions of at least one type of element selected from a group of argon, silicon, oxygen, and carbon into the single-crystal silicon substrate from the first main surface of the single-crystal silicon substrate. Further, the surface region2S may be formed, for example, by irradiating the single-crystal silicon substrate with radiation from the first main surface of the single-crystal silicon substrate. When the surface region2S is formed by ion implantation or radiation irradiation, the first main surface21of the silicon substrate2may include the rough surface211or does not need to include the rough surface211.

The silicon substrate2includes at least a portion of the cavity26facing the second main surface42of the piezoelectric layer4. The cavity26is positioned on a side opposite to the first electrode51and the second electrode52with the piezoelectric layer4interposed therebetween. The cavity26overlaps the acoustic wave resonator5in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1according to Preferred Embodiment 1, the cavity26is larger than the acoustic wave resonator5and overlaps the entire or substantially the entire acoustic wave resonator5in a plan view from the thickness direction D1of the piezoelectric layer4. Additionally, in the acoustic wave device1according to Preferred Embodiment 1, the cavity26also overlaps a portion of each of the first wiring portion61and the second wiring portion in a plan view from the thickness direction D1of the piezoelectric layer4. An opening shape of the cavity26in a plan view from the thickness direction D1of the piezoelectric layer4is, for example, a rectangular or substantially rectangular shape, but is not limited thereto.

(1.2.2) Silicon Oxide Film

The silicon oxide film7is provided between the first main surface21of the silicon substrate2and the piezoelectric layer4. In the acoustic wave device1according to Preferred Embodiment 1, the silicon oxide film7overlaps the entire or substantially the entire first main surface21of the silicon substrate2in the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1according to Preferred Embodiment 1, since the entire or substantially the entire first main surface21is the rough surface211, the silicon oxide film7overlaps the rough surface211of the silicon substrate2in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1according to Preferred Embodiment 1, the silicon substrate2and the piezoelectric layer4are bonded to each other with the silicon oxide film7interposed therebetween.

A thickness of the silicon oxide film7is, for example, equal to or more than about 0.1 μm and equal to or less than about 10 μm.

As illustrated inFIG.2, the piezoelectric layer4includes a first main surface41and a second main surface42that are opposed to each other. The first main surface41and the second main surface42are opposed to each other in the thickness direction D1of the piezoelectric layer4. The piezoelectric layer4is provided on the first main surface21of the silicon substrate2. Here, the piezoelectric layer4overlaps the first main surface21of the silicon substrate2and the cavity26in a plan view from the thickness direction D1. In the piezoelectric layer4, the second main surface42among the first main surface41and the second main surface42is positioned on the silicon substrate2side. The first main surface41of the piezoelectric layer4is a main surface of the piezoelectric layer4on a side opposite to the silicon substrate2side. The second main surface42of the piezoelectric layer4is a main surface of the piezoelectric layer4on the silicon substrate2side.

In the acoustic wave device1, a distance between the first main surface41of the piezoelectric layer4and the silicon substrate2is longer than a distance between the second main surface42of the piezoelectric layer4and the silicon substrate2. A material of the piezoelectric layer4is, for example, lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The piezoelectric layer4is, for example, Z-cut LiNbO3or Z-cut LiTaO3. With regard to Euler angles (φ, θ, ψ) of the piezoelectric layer4, φ is about 0°±10° and θ is about 0°±10°, for example. ψ is any angle. From the viewpoint of increasing the coupling coefficient, the piezoelectric layer4is preferably Z-cut LiNbO3or Z-cut LiTaO3. The piezoelectric layer4may be, for example, rotated Y-cut LiNbO3, rotated Y-cut LiTaO3, X-cut LiNbO3, or X-cut LiTaO3. The propagation orientation may be a Y-axis direction, or an X-axis direction of crystallographic axes (X, Y, Z) defined for a crystal structure of the piezoelectric layer4, or may be, for example, a direction rotated within a range of about ±90° from the X-axis. The piezoelectric layer4is single crystal, but is not limited thereto. For example, the piezoelectric layer4may be a twin crystal or may be made of ceramics.

The thickness of the piezoelectric layer4is, for example, equal to or more than about 50 nm and equal to or less than about 1000 nm and is about 400 nm as an example.

The piezoelectric layer4includes the specified region45. The specified region45is a region of the piezoelectric layer4that intersects with both the first electrode51and the second electrode52in a direction in which the first electrode51and the second electrode52face each other and that is positioned between the first electrode51and the second electrode52in a plan view from the thickness direction D1of the piezoelectric layer4.

The plurality of first electrodes51and the plurality of second electrodes52are provided on the first main surface41of the piezoelectric layer4.

In the acoustic wave device1, the first electrode51and the second electrode52define a pair with different potentials from each other. In the acoustic wave device1, one of the pair of the first electrode51and the second electrode52is an electrode having a potential at a non-ground (signal) side when an AC voltage is applied, and the other is an electrode having a ground potential.

In the acoustic wave device1, the plurality of first electrodes51and the plurality of second electrodes52are alternately provided one by one so as to be separated from each other. Thus, the first electrode51and the second electrode52that are adjacent to each other are separated from each other. A distance between center lines of a pair of the first electrode51and the second electrode52is, for example, equal to or more than about 1 μm and equal to or less than about 10 μm, and is about 3 μm as an example. A group of electrodes including the plurality of first electrodes51and the plurality of second electrodes52preferably has a configuration in which the plurality of first electrodes51and the plurality of second electrodes52are separated from each other in the second direction D2, and may have a configuration in which the plurality of first electrodes51and the plurality of second electrodes52are not alternately provided so as to be separated from each other. For example, a region in which the first electrodes51and the second electrodes52are provided one by one so as to be separated from each other and a region in which two first electrodes51or two second electrodes52are provided side by side in the second direction D2may be mixed.

The plurality of first electrodes51and the plurality of second electrodes52have an elongated shape (linear shape), as illustrated inFIG.1, in a third direction D3orthogonal or substantially orthogonal to the second direction D2defining and functioning as a longitudinal direction and to the second direction D2defining and functioning as a width direction in a plan view from the thickness direction D1of the piezoelectric layer4. A length of each of the plurality of first electrodes51is, for example, about 20 μm, but is not limited thereto. A width H1(first electrode width H1) of each of the plurality of first electrodes51is, for example, equal to or more than about 50 nm and equal to or less than about 1000 nm, and is about 500 nm as an example. A length of each of the plurality of second electrodes52is, for example, about 20 μm, but is not limited thereto. A width H2(second electrode width H2) of each of the plurality of second electrodes52is, for example, equal to or more than about 50 nm and equal to or less than about 1000 nm, and is about 500 nm as an example.

Each of the plurality of first electrodes51includes a first electrode main portion510. The first electrode main portion510is a portion of the first electrode51that intersects with the second electrode52in the direction in which the first electrode51and the second electrode52face each other. Additionally, each of the plurality of second electrodes52includes a second electrode main portion520. The second electrode main portion520is a portion of the second electrode52that intersects with the first electrode51in the direction in which the first electrode51and the second electrode52face each other.

In the acoustic wave device1according to Preferred Embodiment 1, the first electrode width H1of each of the plurality of first electrodes51is the same or substantially the same, but is not limited thereto, and may be different from each other. Also, in the acoustic wave device1according to Preferred Embodiment 1, the second electrode width H2of each of the plurality of second electrodes52is the same or substantially the same, but is not limited thereto, and may be different from each other. In the acoustic wave device1according to Preferred Embodiment 1, the first electrode width H1and the second electrode width H2are the same or substantially the same, but are not limited thereto, and the first electrode width H1and the second electrode width H2may be different from each other.

Regarding the acoustic wave device1according to Preferred Embodiment 1, although each of the number of the first electrodes51and the number of the second electrodes52is, for example, five inFIG.1, each of the number of the first electrodes51and the number of the second electrodes52is not limited to five, and may be one, may be from two to four, may be six or more, or may be50or more, for example.

The second direction D2in which the first electrode51and second electrode52that are adjacent to each other and face each other is preferably orthogonal or substantially orthogonal to the polarization direction PZ1(seeFIG.2) of the piezoelectric layer4, but is not limited to this configuration. For example, when the piezoelectric layer4is not a Z-cut piezoelectric body, the first electrode51and the second electrode52may face each other in a direction orthogonal or substantially orthogonal to the third direction D3defining and functioning as the longitudinal direction. The first electrode51and the second electrode52may not be rectangular or substantially rectangular. In this case, the third direction D3defining and functioning as the longitudinal direction may be a long side direction of a circumscribed polygon circumscribed to the first electrode51and the second electrode52in a plan view of the first electrode51and the second electrode52. When the first wiring portion61is connected to the first electrode51and the second wiring portion62is connected to the second electrode52, the “circumscribed polygon circumscribed to the first electrode51and the second electrode52” includes a polygon circumscribed to at least a portion of the first electrode51excluding a portion connected to the first wiring portion61and a portion of the second electrode52excluding a portion connected to the second wiring portion62.

As illustrated inFIG.2, each of the plurality of first electrodes51includes a first main surface511and a second main surface512that intersect with the thickness direction D1of the piezoelectric layer4, and two side surfaces513and513that intersect with the width direction of the first electrode51. In each of the plurality of first electrodes51, the second main surface512among the first main surface511and the second main surface512is positioned on the first main surface41side of the piezoelectric layer4and is in planar contact with the first main surface41of the piezoelectric layer4.

Each of the plurality of second electrodes52includes a first main surface521and a second main surface522that intersect with the thickness direction D1of the piezoelectric layer4, and two side surfaces523and523that intersect with the width direction of the second electrode52. In each of the plurality of second electrodes52, the second main surface522among the first main surface521and the second main surface522is positioned on the first main surface41side of the piezoelectric layer4and is in planar contact with the first main surface41of the piezoelectric layer4.

The plurality of first electrodes51and the plurality of second electrodes52have electrical conductivity. A material of each first electrode51and a material of each second electrode52are, for example, aluminum (Al), copper (Cu), platinum (Pt), gold (Au), silver (Ag), titanium (Ti), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), or an alloy including any of these metals as a main component. Further, each first electrode51and each second electrode52may have a structure in which a plurality of metal films made of these metals or the alloy are laminated. Each first electrode51and each second electrode52include, for example, a laminated film of an adhesion film made of a Ti film and a main electrode film made of an Al film or an AlCu film on the adhesion film. The adhesion film has a thickness of, for example, about 10 nm. Additionally, the main electrode film has a thickness of, for example, about 80 nm. In the AlCu film, a concentration of Cu is preferably, for example, equal to or more than about 1 wt % and equal to or less than about 20 wt %.

(1.2.5) First Wiring Portion and Second Wiring Portion

The first wiring portion61includes a first busbar611. The first busbar611is a conductor to cause the plurality of first electrodes51to have the same or substantially the same potential. The first busbar611has an elongated shape (linear shape) whose longitudinal direction is in the second direction D2. The first busbar611is connected to the plurality of first electrodes51. The plurality of first electrodes51connected to the first busbar611extend toward a second busbar621. In the acoustic wave device1, a first conductor including the plurality of first electrodes51and the first busbar611has a comb shape in a plan view from the thickness direction D1of the piezoelectric layer4. The first busbar611is integrally provided with the plurality of first electrodes51, but is not limited to this.

The second wiring portion62includes the second busbar621. The second busbar621is a conductor to cause the plurality of second electrodes52to have the same or substantially the same potential. The second busbar621has an elongated shape (linear shape) whose longitudinal direction is in the second direction D2. The second busbar621is connected to the plurality of second electrodes52. The plurality of second electrodes52connected to the second busbar621extend toward the first busbar611. In the acoustic wave device1, a second conductor including the plurality of second electrodes52and the second busbar621has a comb shape in a plan view from the thickness direction D1of the piezoelectric layer4. The second busbar621is integrally provided with the plurality of second electrodes52, but is not limited to this.

The first busbar611and the second busbar621face each other in the third direction D3.

The first wiring portion61and the second wiring portion62have electrical conductivity. A material of the first wiring portion61and a material of the second wiring portion62are, for example, Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, or an alloy including any of these metals as a main component. Further, the first wiring portion61and the second wiring portion62may have a structure including a plurality of metal films made of these metals or the alloy that are laminated. The first wiring portion and the second wiring portion62include, for example, a laminated film including an adhesion film made of a Ti film and a main wiring film made of an Al film or an AlCu film on the adhesion film. The adhesion film has a thickness of, for example, about 10 nm. In addition, the main wiring film has a thickness of, for example, about 80 nm. In the AlCu film, a concentration of Cu is preferably, for example, equal to or more than about 1 wt % and equal to or less than about 20 wt %.

In the acoustic wave device1, each of the first busbar611and the second busbar621may include a metal film on the main wiring film from the viewpoint of reducing the resistance of the first busbar611and the second busbar621. Further, the thickness of each of the first wiring portion61and the second wiring portion62may be larger than the thicknesses of the first electrode51and the second electrode52.

(1.2.6) Trap Region

The trap region10is provided on the second main surface side of the piezoelectric layer4. The trap region10is provided in the silicon substrate2. The trap region10reduces or prevents movement of charges along the first main surface21of the silicon substrate2. Here, in the acoustic wave device1according to Preferred Embodiment 1, when there is a potential difference between the first wiring portion61and the second wiring portion62, the trap region10reduces or prevents the charges near an interface between the first main surface21of the silicon substrate2and the silicon oxide film7to move between the first wiring portion61and the second wiring portion62along the first main surface21of the silicon substrate2.

In the acoustic wave device1according to Preferred Embodiment 1, a trap density of the surface region2S included in the trap region10is higher than a trap density of the bulk region2B. Here, the trap density is a density of traps that trap charges (free charge carriers). Additionally, in the trap region10, a carrier mobility in the surface region2S is lower than a carrier mobility in the bulk region2B. In the acoustic wave device1according to Preferred Embodiment 1, the cavity26included in the silicon substrate2overlaps a portion of each of the first wiring portion61and the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4, and is included in the trap region10. That is, in the acoustic wave device1according to Preferred Embodiment 1, the trap region10includes the surface region2S of the silicon substrate2and the cavity26of the silicon substrate2.

(1.3) Method of Manufacturing Acoustic Wave Device

In a non-limiting example of a method of manufacturing the acoustic wave device1, for example, after a single-crystal silicon substrate including a first main surface and a second main surface that are opposed to each other is prepared, first to sixth processes are performed. In the first process, the silicon substrate2including the surface region2S and the bulk region2B is formed by roughening the first main surface of the single-crystal silicon substrate. In the second process, the silicon oxide film7is formed on the first main surface21of the silicon substrate2. In the third process, a piezoelectric substrate of which the piezoelectric layer4is formed and the silicon substrate2are bonded to each other with the silicon oxide film7interposed therebetween. In the fourth process, the piezoelectric layer4made of a portion of the piezoelectric substrate is formed by thinning the piezoelectric substrate. In the fifth process, the plurality of first electrodes51, the plurality of second electrodes52, the first wiring portion61, the second wiring portion62, the first terminal T1, and the second terminal T2are formed on the first main surface41of the piezoelectric layer4. In the sixth process, the cavity26is formed from the second main surface22of the silicon substrate2. In the above-described fifth process, the plurality of first electrodes51, the plurality of second electrodes52, the first wiring portion61, the second wiring portion62, the first terminal T1, and the second terminal T2are formed by, for example, a photolithography technique, an etching technique, a thin film formation technique, or the like. Further, in the above-described sixth process, a region of the silicon substrate2in which the cavity26is to be formed is etched by, for example, a photolithography technique, an etching technique, or the like. In the sixth process, the silicon substrate2is etched by, for example, the silicon oxide film7as an etching stopper layer, and then, an unnecessary portion of the silicon oxide film7is removed by performing etching to expose a portion of the second main surface42of the piezoelectric layer4. Furthermore, in preparing the single-crystal silicon substrate, a single-crystal silicon wafer is prepared, and in the third process, a piezoelectric wafer is used as the piezoelectric substrate. In the method of manufacturing the acoustic wave device1, a wafer including a plurality of acoustic wave devices1is cut with, for example, a dicing machine to obtain the plurality of acoustic wave devices1(chips).

The method of manufacturing the acoustic wave device1described above is a non-limiting example and is not particularly limited thereto. For example, the piezoelectric layer4may be formed by a film formation technique. In this case, the method of manufacturing the acoustic wave device1includes a process of forming the piezoelectric layer4instead of the third process and the fourth process. The piezoelectric layer4formed as a film by the film formation technique may be, for example, a single crystal or a twin crystal. Examples of the film formation technique include a chemical vapor deposition (CVD) method, but the film formation technique is not limited thereto.

(1.4) Operation and Characteristics of Acoustic Wave Device

The acoustic wave device1according to Preferred Embodiment 1 uses a bulk wave of the thickness-shear primary mode. As described above, the bulk wave of the thickness-shear primary mode is a bulk wave whose propagation direction is the thickness direction D1of the piezoelectric layer4due to thickness-shear vibration of the piezoelectric layer4and whose number of nodes in the thickness direction D1of the piezoelectric layer4is one. The thickness-shear vibration is excited by the first electrode51and the second electrode52. The thickness-shear vibration is excited in the specified region45between the first electrode51and the second electrode52that are adjacent to each other in a plan view from the thickness direction D1in the piezoelectric layer4. The thickness-shear vibration can be checked by using, for example, a finite element method (FEM). More specifically, the thickness-shear vibration can be checked by, for example, analyzing a strain by analyzing a displacement distribution by FEM by using parameters of the piezoelectric layer4(the material, the Euler angles, the thickness, and the like), parameters of the first electrode51and the second electrode52(the materials, the thicknesses, the distance between the center lines of the first electrode51and the second electrode52, and the like), and the like. The Euler angles of the piezoelectric layer4can be obtained by analysis.

Here, a difference between a Lamb wave used in the conventional acoustic wave device and the bulk wave of the thickness-shear primary mode will be described with reference toFIGS.6A and6B.

FIG.6Ais a schematic front cross-sectional view for explaining a Lamb wave propagating in a piezoelectric thin film of a conventional acoustic wave device such as the surface acoustic wave device described in International Publication No. 2015/098678. In the conventional acoustic wave device, an acoustic wave propagates in a piezoelectric thin film400as indicated by an arrow. Here, the piezoelectric thin film400includes a first main surface401and a second main surface402that are opposed to each other. InFIG.6A, a Z direction and an X direction are illustrated separately from the piezoelectric thin film400. InFIG.6A, the Z direction is a thickness direction connecting the first main surface401and the second main surface402of the piezoelectric thin film400. The X direction is a direction in which a plurality of electrode fingers of an IDT electrode are disposed. The Lamb wave is a plate wave in which an acoustic wave propagates in the X direction as illustrated inFIG.6A. Thus, in the conventional acoustic wave device, since an acoustic wave propagates in the X direction, two reflectors are disposed one by one at both sides of the IDT electrode to obtain desired resonance characteristics. Thus, in the conventional acoustic wave device, since a propagation loss of an acoustic wave occurs, when the size is reduced, that is, when the number of pairs of electrode fingers is reduced, a Q value decreases.

On the other hand, in the acoustic wave device1according to Preferred Embodiment 1, since vibration displacement is generated in a thickness-shear direction, an acoustic wave substantially propagates in a direction connecting the first main surface41and the second main surface42of the piezoelectric layer4, that is, in the Z direction, and resonates, as illustrated inFIG.6B. That is, a component in the X direction of the acoustic wave is significantly smaller than a component in the Z direction. In the acoustic wave device1according to Preferred Embodiment 1, since resonance characteristics are obtained by propagation of the acoustic wave in the Z direction, a reflector is not necessarily required. Thus, in the acoustic wave device1according to Preferred Embodiment 1, no propagation loss occurs when the acoustic wave propagates into the reflector. Thus, in the acoustic wave device1according to Preferred Embodiment 1, even when the number of pairs of electrodes, each pair of which includes the first electrode51and the second electrode52, is reduced in order to reduce the size of the acoustic wave device1, a decrease in the Q value is unlikely to occur.

In the acoustic wave device1according to Preferred Embodiment 1, as illustrated inFIG.7, a bulk wave of the thickness-shear primary mode has opposite amplitude directions in a first region451included in the specified region45of the piezoelectric layer4and a second region452included in the specified region45. InFIG.7, a two-dot chain line schematically indicates a bulk wave when a voltage is applied between the first electrode51and the second electrode52such that the second electrode52has a higher potential than that of the first electrode51. The first region451is a region of the specified region45between the first main surface41and a virtual plane VP1that is orthogonal or substantially orthogonal to the thickness direction D1of the piezoelectric layer4and that divides the piezoelectric layer4into two. The second region452is a region of the specified region45between the virtual plane VP1and the second main surface42.

Characteristics for a structural model1r(seeFIG.8) of an acoustic wave device according to a reference preferred embodiment of the present invention that uses a bulk wave of the thickness-shear primary mode were simulated. Regarding the structural model1r, elements the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

The structural model1rdiffers from the acoustic wave device1according to Preferred Embodiment 1 in that the first wiring portion61, the second wiring portion62, and the trap region10are not included. In the simulation, the number of pairs of the first electrode51and the second electrode52was made infinite, and the piezoelectric layer4was made of 120° rotated Y-cut X-propagation LiNbO3.

In the structural model1r, the piezoelectric layer4is a membrane, and the second main surface42of the piezoelectric layer4is in contact with air. In the structural model1r, in any cross section (FIG.8) along the thickness direction D1of the piezoelectric layer4, a distance between center lines of the first electrode51and the second electrode52that are adjacent to each other is represented by p, and a thickness of the piezoelectric layer4is represented by d. Further, in the structural model1r, in a plan view from the thickness direction D1of the piezoelectric layer4, an area of the first electrode main portion510is represented by S1, an area of the second electrode main portion520is represented by S2, an area of the specified region45is represented by S0, and a structural parameter defined by (S1+S2)/(S1+S2+S0) is represented by MR. When a plurality of at least either the first electrodes51or the second electrodes52are provided on the piezoelectric layer4, the above-described distance p between the center lines indicates each distance between the center lines of any pair of the first electrode51and the second electrode52that are adjacent to each other, among the plurality of at least either the first electrodes51or the second electrodes52.

FIGS.9A and9Bare graphs showing a relationship between a fractional bandwidth and d/p when different potentials are applied to the first electrode51and the second electrode52for the structural model1r. InFIGS.9A and9B, the horizontal axis represents d/p and the vertical axis represents the fractional bandwidth.FIGS.9A and9Bindicate a case where the piezoelectric layer4is made of 120° rotated Y-cut X-propagation LiNbO3, but similar tendencies are seen in the cases of other cut angles. Further, with the structural model1rof the acoustic wave device, even when the material of the piezoelectric layer4is LiTaO3, the relationship between the fractional bandwidth and d/p has tendencies the same as or similar to those inFIGS.9A and9B. Furthermore, with the structural model1rof the acoustic wave device, the relationship between the fractional bandwidth and d/p has tendencies the same as or similar to those in theFIGS.9A and9Bregardless of the number of pairs of the first electrode51and the second electrode52. In addition, with the structural model1rof the acoustic wave device, not only when the second main surface42of the piezoelectric layer4is in contact with air but also when the second main surface42is in contact with an acoustic reflection layer, the relationship between the fractional bandwidth and d/p has tendencies the same as or similar to those inFIGS.9A and9B.

It can be seen fromFIG.9Athat, with the structural model1rof the acoustic wave device, a value of the fractional bandwidth drastically changes with d/p=about 0.5 being as an inflection point. With the structural model1rof the acoustic wave device, when a relationship of d/p> about 0.5 is satisfied, a coupling coefficient is low and the fractional bandwidth is less than about 5% regardless of how much d/p is changed within the range of about 0.5<d/p< about 1.6. On the other hand, with the structural model1rof the acoustic wave device, when a relationship of d/p about 0.5 is satisfied, it is possible to increase the coupling coefficient and set the fractional bandwidth to be equal to or more than about 5% by changing d/p within the range of about 0<d/p≤ about 0.5.

In addition, with the structural model1rof the acoustic wave device, when a relationship of d/p≤ about 0.24 is satisfied, by changing d/p within the range of about 0<d/p≤ about 0.24, the coupling coefficient can be further increased and the fractional bandwidth can be further increased. Also with the acoustic wave device1according to Preferred Embodiment 1, as illustrated inFIG.2, in any cross section along the thickness direction D1of the piezoelectric layer4, when the distance between the center lines of the first electrode51and the second electrode52is represented by p and the thickness of the piezoelectric layer4is represented by d, the relationship between the fractional bandwidth and d/p has a tendency the same as or similar to that of the relationship between the fractional bandwidth and d/p of the structural model1rof the acoustic wave device.

Furthermore, as is clear fromFIG.9A, when a relationship of d/p≤ about 0.10 is satisfied, by changing d/p within the range of about 0<d/p≤ about 0.10, it is possible to further increase the coupling coefficient and further increase the fractional bandwidth.

FIG.9Bis a graph obtained by enlarging a part ofFIG.9A. As shown inFIG.9B, since the fractional bandwidth changes with d/p=0.096 being as an inflection point, when a relation of d/p≤0.096 is satisfied, by changing d/p within the range of 0<d/p≤0.096, it is possible to further increase the coupling coefficient and further increase the fractional bandwidth as compared with the case where a relation of 0.096<d/p is satisfied. Further, as shown inFIG.9B, the fractional bandwidth changes with d/p=about 0.072 and d/p=about 0.048 being as inflection points, and when a relationship of about 0.048≤d/p≤ about 0.072 is satisfied, it is possible to reduce or prevent a change in the coupling coefficient due to a change in d/p and to set the fractional bandwidth to a constant or substantially constant value.

FIG.10is a graph plotting a spurious level in a frequency band between a resonant frequency and an anti-resonant frequency when the thickness d of the piezoelectric layer4, the distance p between the center lines of the first electrode51and the second electrode52, the first electrode width H1, and the second electrode width H2are changed in the structural model1rof the acoustic wave device according to the reference preferred embodiment using the thickness-shear mode. InFIG.10, the horizontal axis represents the fractional bandwidth and the vertical axis represents a normalized spurious level. The normalized spurious level is a value obtained by normalizing a spurious level with a spurious level of a fractional bandwidth (for example, about 22%) in which the spurious level has the same or substantially the same value as1even when the thickness d of the piezoelectric layer4, the distance p between the center lines of the first electrode51and the second electrode52, the first electrode width H1, and the second electrode width H2are changed.FIG.10shows a case where Z-cut LiNbO3capable of more suitably exciting the thickness-shear mode is used for the piezoelectric layer4, but the same or similar tendencies are observed in the cases of other cut angles. Additionally, with the structural model1rof the acoustic wave device, when the material of the piezoelectric layer4is made of LiTaO3, the relationship between the normalized spurious level and the fractional bandwidth has a tendency the same as or similar to that shown inFIG.10. Further, with the structural model1rof the acoustic wave device, the relationship between the normalized spurious level and the fractional bandwidth has a tendency the same as or similar to that inFIG.10regardless of the number of pairs of the first electrode51and the second electrode52. Furthermore, with the structural model1rof the acoustic wave device, the relationship between the normalized spurious level and the fractional bandwidth has a tendency the same as or similar to that inFIG.10not only when the second main surface42of the piezoelectric layer4is in contact with air but also when the second main surface42is in contact with the acoustic reflection layer.

It can be seen fromFIG.10that when the fractional bandwidth exceeds about 17%, the normalized spurious levels are aggregated to 1. This means that, when the fractional bandwidth is equal to or more than about 17%, some kind of sub-resonance exists in the band between the resonant frequency and the anti-resonant frequency as in frequency characteristics of impedance shown inFIG.11as an example.FIG.11shows the frequency characteristics of impedance when Z-cut LiNbO3having Euler angles (0°, 0°, 90°) is used as the piezoelectric layer4, d/p=about 0.08, and MR=about 0.35. InFIG.11, a portion indicating the sub-resonance is surrounded by a broken line.

As described above, when the fractional bandwidth exceeds about 17%, even when the thickness d of the piezoelectric layer4, the first electrode width H1, and the second electrode width H2are changed, a large spurious component is included in the band between the resonant frequency and the anti-resonant frequency. Such a spurious component is generated due to an overtone in a planar direction, mainly in a direction in which the first electrode51and the second electrode52face each other. Thus, from the viewpoint of reducing or preventing the spurious component in the band, the fractional bandwidth is preferably equal to or less than about 17%, for example. Since the acoustic wave device1according to Preferred Embodiment 1 also exhibits a tendency the same as or similar to that of the structural model1rof the acoustic wave device with regard to the relationship between the normalized spurious level and the fractional bandwidth, the fractional bandwidth is preferably equal to or less than about 17%, for example.

FIG.12shows, for the structural model1rof the acoustic wave device, a first distribution region DA1having a fractional bandwidth exceeding about 17% and a second distribution region DA2having a fractional bandwidth being equal to or less than about 17% by using d/p and MR as parameters when Z-cut LiNbO3is used for the piezoelectric layer4and the thickness d of the piezoelectric layer4, the distance p between the center lines of the first electrode51and the second electrode52, the first electrode width H1, and the second electrode width H2are changed. InFIG.12, the first distribution region DA1and the second distribution region DA2have different dot densities, and the dot density of the first distribution region DA1is higher than the dot density of the second distribution region DA2. Additionally, inFIG.12, an approximately straight line DL1of a boundary between the first distribution region DA1and the second distribution region DA2is indicated by a broken line. An approximately straight line DL1is expressed by an equation of MR=1.75×(d/p)+0.075. Thus, in the structural model1rof the acoustic wave device, the fractional bandwidth is likely to be equal to or less than about 17% by satisfying the condition of MR≤1.75×(d/p)+0.075.FIG.12shows a case where Z-cut LiNbO3capable of more suitably exciting the thickness-shear mode is used for the piezoelectric layer4, but the same or similar tendencies are observed in the cases of other cut angles. Additionally, with the structural model1rof the acoustic wave device, the approximately straight line DL1is the same or substantially the same even when the material of the piezoelectric layer4is LiTaO3. Further, with the structural model1rof the acoustic wave device, the approximately straight line DL1is the same or substantially the same regardless of the number of pairs of the first electrode51and the second electrode52. Furthermore, with the structural model1rof the acoustic wave device, the approximately straight line DL1is the same or substantially the same not only when the second main surface42of the piezoelectric layer4is in contact with air but also when the second main surface42is in contact with the acoustic reflection layer. The acoustic wave device1according to Preferred Embodiment 1 satisfies the condition of MR≤1.75×(d/p)+0.075, as with the structural model1rof the acoustic wave device, so that the fractional bandwidth is likely to be equal to or less than about 17%. InFIG.12, an approximately straight line DL2(hereinafter, also referred to as a second approximately straight line DL2) indicated by a dashed-dotted line separately from the approximately straight line DL1(hereinafter, also referred to as the first approximately straight line DL1) is a line indicating a boundary for reliably setting the fractional bandwidth to being equal to or less than about 17%. The second approximately straight line DL2is expressed by an equation of MR=1.75×(d/p)+0.05. Thus, with the acoustic wave device1according to Preferred Embodiment 1 and the structural model1rof the acoustic wave device, the fractional bandwidth can be reliably set to be equal to or less than about 17% by satisfying the condition of MR≤1.75×(d/p)+0.05.

The acoustic wave device1according to Preferred Embodiment 1 includes the piezoelectric layer4, and the first electrode51and the second electrode52. The first electrode51and the second electrode52face each other in the direction D2intersecting with the thickness direction D1of the piezoelectric layer4. The acoustic wave device1uses a bulk wave of the thickness-shear primary mode. The acoustic wave device1further includes the silicon substrate2. The silicon substrate2includes the first main surface21and the second main surface22that are opposed to each other. The material of the piezoelectric layer4is lithium niobate or lithium tantalate. The piezoelectric layer4is provided on the first main surface21of the silicon substrate2. The acoustic wave device1further includes the trap region10provided in the silicon substrate2.

With the acoustic wave device1according to Preferred Embodiment 1 as described above, it is possible to handle higher frequencies and to improve linearity.

With the acoustic wave device1according to Preferred Embodiment 1, the resonant frequency is not limited by the distance between the center lines of the first electrode51and the second electrode52that are adjacent to each other, and the resonant frequency can be increased by reducing the thickness of the piezoelectric layer4. Thus, the acoustic wave device1can handle higher frequencies without increasing the planar size of the acoustic wave device. Further, with the surface acoustic wave device described in International Publication No. 2015/098678, there is a case where a sufficient Q value cannot be obtained when the number of electrode fingers of the IDT electrode is reduced. On the other hand, with the acoustic wave device1according to Preferred Embodiment 1, since a sufficient Q value can be obtained even when the number of pairs of the first electrode51and the second electrode52is reduced, it is possible to obtain a sufficient Q value while achieving miniaturization. Further, in the acoustic wave device1according to Preferred Embodiment 1, providing the trap region10can improve linearity.

With the acoustic wave device1according to Preferred Embodiment 1, the trap region10reduces or prevents the movement of charges along the first main surface21of the silicon substrate as compared with an acoustic wave device according to a comparative example including an interface between a single-crystal silicon substrate and a silicon oxide film without including the trap region10, and thus, the linearity can be improved.

Further, the acoustic wave device1according to Preferred Embodiment 1 includes the piezoelectric layer4, and the first electrode51and the second electrode52. The first electrode51and the second electrode52face each other in the direction D2intersecting with the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1, in any cross section along the thickness direction D1of the piezoelectric layer4, d/p is equal to or less than about 0.5, when p represents the distance between the center lines of the first electrode51and the second electrode52that are adjacent to each other, and d represents the thickness of the piezoelectric layer4. The acoustic wave device1further includes the silicon substrate2. The silicon substrate2includes the first main surface21and the second main surface22that are opposed to each other. The material of the piezoelectric layer4is lithium niobate or lithium tantalate. The piezoelectric layer4is provided on the first main surface21of the silicon substrate2. The acoustic wave device1further includes the trap region10provided in the silicon substrate2.

With the acoustic wave device1according to Preferred Embodiment 1 as described above, it is possible to handle higher frequencies and to improve linearity.

Further, in the acoustic wave device1according to Preferred Embodiment 1, the silicon substrate2includes a portion of the cavity26that exposes a portion of the second main surface42of the piezoelectric layer4. Note that the case where “the silicon substrate2includes a portion of the cavity26” refers to a case where a portion of the cavity26is surrounded by the silicon substrate2. “A case where a portion of the cavity26is surrounded by the silicon substrate2” is not limited to, for example, a case where the cavity26is covered by the substrate20on the second main surface22side of the silicon substrate2as illustrated inFIG.13to be described later, but also includes a case where the cavity26is not covered by the substrate20on the second main surface side of the silicon substrate2. Here, a portion of the cavity26also defines and functions as a gap27that overlaps both a portion of the first wiring portion61and a portion of the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. Additionally, in the acoustic wave device1according to Preferred Embodiment 1, the trap region10includes the surface region2S and the gap27. Thus, the acoustic wave device1according to Preferred Embodiment 1 can improve linearity as compared with a case where the trap region10does not include the gap27.

Another Example of Acoustic Wave Device According to Preferred Embodiment 1

In another example of the acoustic wave device1, for example, as illustrated inFIG.13, another substrate20may be laminated on the side of the silicon substrate2opposite to the piezoelectric layer4, that is, on the second main surface22of the silicon substrate2, so as to overlap the piezoelectric layer in a plan view from the thickness direction D1of the piezoelectric layer4. A material of the other substrate20describe above may be silicon, for example. In short, in the acoustic wave device1, a second silicon substrate including the other substrate20described above may be bonded to the second main surface22of the first silicon substrate2, which is the silicon substrate2. Note that the silicon substrate2and the other substrate20are not limited to being laminated, and may be integrally provided with one substrate.

Modification 1 of Preferred Embodiment 1

Hereinafter, an acoustic wave device1aaccording to Modification 1 of Preferred Embodiment 1 will be described with reference toFIGS.14and15. For the acoustic wave device1aaccording to Modification 1, elements that are the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1aaccording to Modification 1 is an acoustic wave filter (for example, a ladder filter). The acoustic wave device1aincludes an input terminal15, an output terminal16, a plurality of (for example, two) series arm resonators RS1provided on a first path12connecting the input terminal15and the output terminal16, and a plurality of (for example, two) parallel arm resonators RS2provided one by one on a plurality of (two) second paths13and14connecting a plurality of (two) nodes N1and N2on the first path12and the ground (ground terminals17and18). The ground terminals17and18may be commonly used as one ground.

In the acoustic wave device1a, each of the series arm resonators RS1and the parallel arm resonators RS2is, for example, the acoustic wave resonator5. Each of a plurality of acoustic wave resonators5is a resonator including a plurality of first electrodes51and a plurality of second electrodes52, but is not limited thereto, and is preferably a resonator including at least one first electrode51and one second electrode52. In the acoustic wave device1a, the piezoelectric layer4is shared by the plurality of acoustic wave resonators5. A resonant frequency of the parallel arm resonator RS2is lower than a resonant frequency of the series arm resonator RS1. The acoustic wave resonator5defining the parallel arm resonator RS2includes, for example, a silicon oxide film provided on the first main surface41of the piezoelectric layer4, while the acoustic wave resonator5defining the series arm resonator RS1does not include a silicon oxide film on the first main surface41of the piezoelectric layer4. The acoustic wave resonator5defining the series arm resonator RS1may include, for example, a silicon oxide film on the first main surface41of the piezoelectric layer4. In this case, the silicon oxide film of the acoustic wave resonator5defining the series arm resonator RS1may be thinner than the silicon oxide film of the acoustic wave resonator5defining the parallel arm resonator RS2.

In the acoustic wave device1a, the silicon substrate2includes a portion of the cavity26overlapping the plurality of acoustic wave resonators5in a plan view from the thickness direction D1of the piezoelectric layer4, but is not limited thereto, and may include, for example, a portion of each of a plurality of cavities26overlapping the plurality of acoustic wave resonators5in a one-by-one manner.

Modification 2 of Preferred Embodiment 1

Hereinafter, an acoustic wave device1jaccording to Modification 2 of Preferred Embodiment 1 will be described with reference toFIG.16. For the acoustic wave device1jaccording to Modification 2, elements the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

In the acoustic wave device1jaccording to Modification 2, the gap27of the acoustic wave device1according to Preferred Embodiment 1 is not provided, and the silicon substrate2and the silicon oxide film7overlap both a portion of the first wiring portion61and a portion of the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. Here, in the acoustic wave device1jaccording to Modification 2, the surface region2S overlapping the entire or substantially the entire silicon oxide film7overlaps both a portion of the first wiring portion61and a portion of the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. Thus, in the acoustic wave device1jaccording to Modification 2, the surface region2S included in the trap region10is provided so as to overlap a plurality of external connection terminals (the input terminal15, the output terminal16, and the ground terminals17and18) in a plan view.

Modification 3 of Preferred Embodiment 1

Hereinafter, an acoustic wave device1baccording to Modification 3 of Preferred Embodiment 1 will be described with reference toFIG.17. For the acoustic wave device1baccording to Modification 3, elements the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1baccording to Modification 3 differs from the acoustic wave device1according to Preferred Embodiment 1 in that two reflectors8are further provided.

Each of the two reflectors8is a short-circuited grating. Each reflector8does not reflect a bulk wave of a primary shear mode but reflects an unnecessary surface acoustic wave propagating along the first main surface41of the piezoelectric layer4. One reflector8of the two reflectors8is positioned on the side opposite to the second electrode52side of the first electrode51positioned at an end among the plurality of first electrodes51in a direction along a propagation direction of the unnecessary surface acoustic wave of the acoustic wave device1b. The remaining other reflector8of the two reflectors8is positioned on the side opposite to the first electrode51side of the second electrode52positioned at an end among the plurality of second electrodes52in the direction along the propagation direction of the unnecessary surface acoustic wave of the acoustic wave device1b.

Each reflector8includes a plurality of (for example, four) electrode fingers81, and one end of each of the plurality of electrode fingers81is short-circuited to each other, and the other end is short-circuited to each other. In each reflector8, the number of electrode fingers81is not particularly limited.

Each reflector8has electrical conductivity. A material of each reflector8is, for example, Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, or W, an alloy mainly including any one of these metals, or the like. Further, each reflector8may have a structure in which a plurality of metal films made of these metals or the alloy are laminated. Each reflector8includes, for example, a laminated film of an adhesion film made of a Ti film provided on the piezoelectric layer4and a main electrode film made of an Al film provided on the adhesion film. The adhesion film has a thickness of, for example, about 10 nm. Additionally, the main electrode film has a thickness of, for example, about 80 nm.

Further, in the acoustic wave device1baccording to Modification 2, each reflector8is the short-circuited grating, but is not limited thereto, and may be, for example, an open-circuited grating, a positive/negative reflection grating, a grating in which a short-circuited grating and an open-circuited grating are combined, or the like. In addition, the acoustic wave device1bincludes the two reflectors8, but may include only one of the two reflectors8.

The two reflectors8in the acoustic wave device1baccording to Modification 2 are also applicable to the acoustic wave device1aaccording to Modification 1. For example, the two reflectors8may be provided for each acoustic wave resonator5of the acoustic wave device1aaccording to Modification 1.

Hereinafter, an acoustic wave device1caccording to Preferred Embodiment 2 of the present invention will be described with reference toFIG.18. For the acoustic wave device1caccording to Preferred Embodiment 2, elements the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1caccording to Preferred Embodiment 2 includes, for example, a silicon nitride film11as an insulating film directly provided on the first main surface21of the silicon substrate2. Thus, a weight ratio of oxygen in the insulating film is smaller than that in silicon oxide. The silicon nitride film11is a sputtered thin film formed by sputtering.

In the acoustic wave device1caccording to Preferred Embodiment 2, the silicon oxide film7is interposed between the silicon nitride film11and the piezoelectric layer4, and is in contact with the silicon nitride film11and the piezoelectric layer4. Thus, in the acoustic wave device1caccording to Preferred Embodiment 2, the silicon nitride film11and the surface region2S are provided between the silicon oxide film7and the bulk region2B of the silicon substrate2.

In the acoustic wave device1caccording to Preferred Embodiment 2, the trap region10includes the surface region2S of the silicon substrate2.

Since the acoustic wave device1caccording to Preferred Embodiment 2 includes the trap region10as in the acoustic wave device1according to Preferred Embodiment 1, linearity can be improved.

In the acoustic wave device1caccording to Preferred Embodiment 2, the silicon oxide film7is not necessarily included, and the silicon nitride film11and the piezoelectric layer4may be in contact with each other.

Also, in the acoustic wave device1caccording to Preferred Embodiment 2, the silicon oxide film7may be formed by, for example, sputtering without the silicon nitride film11.

Hereinafter, an acoustic wave device1daccording to Preferred Embodiment 3 of the present invention will be described with reference toFIGS.19to21. For the acoustic wave device1daccording to Preferred Embodiment 3, elements the same as or similar to those of the acoustic wave device1according to Preferred Embodiment 1 are denoted by the same reference characters, and description thereof will be omitted.

As illustrated inFIG.20, the acoustic wave device1daccording to Preferred Embodiment 3 differs from the acoustic wave device1according to Preferred Embodiment 1 in that the acoustic wave device1dincludes an acoustic reflection layer3interposed between the silicon substrate2and the piezoelectric layer4, and does not include the cavity26in the acoustic wave device1according to Preferred Embodiment 1. Additionally, the acoustic wave device1daccording to Preferred Embodiment 3 is different from the acoustic wave device1according to Preferred Embodiment 1 in that the silicon substrate2includes a portion of each of two gaps27(seeFIGS.19and21) individually overlapping a portion of the first wiring portion61and a portion of the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. The acoustic wave device1daccording to Preferred Embodiment 3 includes two trap regions10. Each of the two trap regions10includes the gap27. Each trap region10is provided in the silicon substrate2on the second main surface42side of the piezoelectric layer4, and reduces or prevents the movement of charges along the first main surface21of the silicon substrate2.

In the acoustic wave device1daccording to Preferred Embodiment 3, the acoustic reflection layer3is provided on the first main surface21of the silicon substrate2, and the piezoelectric layer4is provided on the acoustic reflection layer3. In the acoustic wave device1d, the acoustic wave resonator5includes the first electrode51and the second electrode52, and the piezoelectric layer4. In the acoustic wave device1d, the acoustic wave resonator5further includes the acoustic reflection layer3described above.

The acoustic reflection layer3is opposed to a plurality of first electrodes51and a plurality of second electrodes52in the thickness direction D1of the piezoelectric layer4.

The acoustic reflection layer3reduces or prevents leakage of a bulk wave excited by the first electrode51and the second electrode52(a bulk wave of the thickness-shear primary mode described above) to the silicon substrate2. By including the acoustic reflection layer3, the acoustic wave device1dcan improve the effect of confining acoustic wave energy in the piezoelectric layer4. Thus, the acoustic wave device1dcan reduce the loss and can increase the Q value as compared with a case where the acoustic reflection layer3is not provided.

The acoustic reflection layer3has a laminated structure including a plurality of (for example, three) low acoustic impedance layers31and a plurality of (for example, two) high acoustic impedance layers32that are alternately provided one by one in the thickness direction D1of the piezoelectric layer4. An acoustic impedance of the low acoustic impedance layer31is lower than an acoustic impedance of the high acoustic impedance layer32.

Hereinafter, for convenience of explanation, in the acoustic reflection layer3, the two high acoustic impedance layers32may be referred to as a first high acoustic impedance layer321and a second high acoustic impedance layer322in the order of proximity to the first main surface21of the silicon substrate2. Further, the three low acoustic impedance layers31may be referred to as a first low acoustic impedance layer311, a second low acoustic impedance layer312, and a third low acoustic impedance layer313in the order of proximity to the first main surface21of the silicon substrate2.

In the acoustic reflection layer3, the first low acoustic impedance layer311, the first high acoustic impedance layer321, the second low acoustic impedance layer312, the second high acoustic impedance layer322, and the third low acoustic impedance layer313are provided in this order from the silicon substrate2side. Thus, the acoustic reflection layer3can reflect a bulk wave (a bulk wave of the thickness-shear primary mode) from the piezoelectric layer4at each of an interface between the third low acoustic impedance layer313and the second high acoustic impedance layer322, an interface between the second high acoustic impedance layer322and the second low acoustic impedance layer312, an interface between the second low acoustic impedance layer312and the first high acoustic impedance layer321, and an interface between the first high acoustic impedance layer321and the first low acoustic impedance layer311.

A material of the plurality of high acoustic impedance layers32is, for example, platinum (Pt). Additionally, a material of the plurality of low acoustic impedance layers31is, for example, silicon oxide. A thickness of each of the plurality of high acoustic impedance layers32is, for example, about 94 nm. Further, a thickness of each of the plurality of low acoustic impedance layers31is, for example, about 188 nm. The acoustic reflection layer3includes two electrically conductive layers because each of the two high acoustic impedance layers32is made of platinum.

The material of the plurality of high acoustic impedance layers32is not limited to Pt, and may be metal such as W (tungsten) or Ta (tantalum), for example. In addition, the material of the plurality of high acoustic impedance layers32is not limited to metal, and may be, for example, an insulator.

Further, the plurality of high acoustic impedance layers32are not limited to being made of the same material, and may be made of different materials, for example. Furthermore, the plurality of low acoustic impedance layers31are not limited to being made of the same material, and may be made of different materials, for example.

Further, the number of the high acoustic impedance layers32and the number of the low acoustic impedance layers31in the acoustic reflection layer3are not limited to two and three, respectively, and may be, for example, one, three or more, or four or more. In addition, the number of high acoustic impedance layers32and the number of low acoustic impedance layers31are not limited to being different, and may be the same, or the number of low acoustic impedance layers31may be one less than the number of high acoustic impedance layers32. In addition, the thickness of each of the high acoustic impedance layer32and the low acoustic impedance layer31is appropriately set according to a designed frequency of the acoustic wave device1and the material applied to each of the high acoustic impedance layer32and the low acoustic impedance layer31so that favorable reflection can be obtained in the acoustic reflection layer3.

In the acoustic wave device1daccording to the Preferred Embodiment 3, the gap27is provided over the silicon substrate2and the acoustic reflection layer3, and exposes a portion of the second main surface42of the piezoelectric layer4.

In a non-limiting example of a method of manufacturing the acoustic wave device1d, for example, the silicon substrate2including the first main surface21and the second main surface22that are opposed to each other is prepared, and then, the first process to the fifth process are performed. In the first process, the acoustic reflection layer3is formed on the first main surface of the silicon substrate2. In the second process, a piezoelectric substrate from which the piezoelectric layer4is formed and the silicon substrate2are bonded to each other with the acoustic reflection layer3interposed therebetween. In the third process, the piezoelectric layer4made of a portion of the piezoelectric substrate is formed by thinning the piezoelectric substrate. In the fourth process, the plurality of first electrodes51, the plurality of second electrodes52, the first wiring portion61, the second wiring portion62, the first terminal T1, and the second terminal T2are formed on the piezoelectric layer4. In the fifth process, a portion of each of the silicon substrate2and the acoustic reflection layer3is etched from the second main surface22of the silicon substrate2to form the gap27. In the fifth process, a portion of the silicon substrate2may be etched from the first main surface21of the silicon substrate2. In the first to fifth processes, a silicon wafer is used as the silicon substrate2. Additionally, in the second process, a piezoelectric wafer is used as the piezoelectric substrate. In the method of manufacturing the acoustic wave device1d, a wafer including a plurality of acoustic wave devices1dis cut with a dicing machine to obtain the plurality of acoustic wave devices1d(chips).

The method of manufacturing the acoustic wave device1dis an example and is not particularly limited thereto. For example, the piezoelectric layer4may be formed by a film formation technique. In this case, the method of manufacturing the acoustic wave device1dincludes a process of forming the piezoelectric layer4instead of the second process and the third process. The piezoelectric layer4formed as a film by the film formation technique may be, for example, single crystal or twin crystal. Examples of the film formation technique include, but are not limited to, a CVD method.

As with the acoustic wave device1according to Preferred Embodiment 1, the acoustic wave device1daccording to Preferred Embodiment 3 uses a bulk wave of the thickness-shear primary mode. As a result, in the acoustic wave device1daccording to Preferred Embodiment 3, the resonant frequency is not limited by the distance between the center lines of the first electrode51and the second electrode52that are adjacent to each other, and the resonant frequency can be increased by reducing the thickness of the piezoelectric layer4. Thus, it is possible to handle higher frequencies without increasing the planar size of the acoustic wave device1d. In addition, the acoustic wave device1daccording to Preferred Embodiment 3 includes the trap region10as in the acoustic wave device1according to Preferred Embodiment 1, and thus, can improve linearity.

The acoustic wave device1dincludes the trap region10overlapping at least a portion of the first wiring portion61in the thickness direction D1of the piezoelectric layer4and the trap region10overlapping at least a portion of the second wiring portion62in the thickness direction D1of the piezoelectric layer4, but as long as the acoustic wave device1dincludes at least one of the trap regions10, linearity can be improved.

Further, in the acoustic wave device1daccording to Preferred Embodiment 3, an unnecessary wave can be reduced or prevented by the acoustic reflection layer3in the acoustic wave resonator5. Further, in the acoustic wave device1daccording to Preferred Embodiment 3, a material of the piezoelectric layer4is, for example, LiNbO3or LiTaO3, and a material of the low acoustic impedance layer31is, for example, silicon oxide. Here, frequency-temperature characteristics of each of LiNbO3and LiTaO3have a negative slope, and frequency-temperature characteristics of silicon oxide have a positive slope. Thus, with the acoustic wave device1daccording to Preferred Embodiment 3, an absolute value of a temperature coefficient of frequency (TCF) can be reduced, and the frequency-temperature characteristics can be improved.

Modification 1 of Preferred Embodiment 3

Hereinafter, an acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 will be described with reference toFIG.22. For the acoustic wave device1eaccording to Modification 1, elements the same as or similar to those of the acoustic wave device1daccording to Preferred Embodiment 3 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 is an acoustic wave filter (here, a ladder filter) similar to the acoustic wave device1aaccording to Modification 1 of Preferred Embodiment 1. The acoustic wave device1eincludes the input terminal15, the output terminal16, a plurality of (for example, two) series arm resonators RS1provided on the first path12connecting the input terminal15and the output terminal16, and a plurality of (for example, two) parallel arm resonators RS2provided one by one on a plurality of (for example, two) second paths13and14connecting a plurality of (two) nodes N1and N2on the first path12and the ground (ground terminals17and18). The ground terminals17and18may be commonly used as one ground.

In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, each of the plurality of series arm resonators RS1and the plurality of parallel arm resonators RS2is the acoustic wave resonator5. Each of the acoustic wave resonators5is a resonator including the first electrode51and the second electrode52. In the acoustic wave device1e, the piezoelectric layer4is shared by the plurality of acoustic wave resonators5. Further, in the acoustic wave device1e, the acoustic reflection layer3is shared by the plurality of acoustic wave resonators5. A resonant frequency of the parallel arm resonator RS2is lower than a resonant frequency of the series arm resonator RS1. Here, the acoustic wave resonator5defining the parallel arm resonator RS2includes, for example, a silicon oxide film provided on the first main surface41of the piezoelectric layer4, whereas the acoustic wave resonator5defining the series arm resonator RS1does not include a silicon oxide film on the first main surface41of the piezoelectric layer4. The acoustic wave resonator5defining the series arm resonator RS1may include, for example, a silicon oxide film on the first main surface41of the piezoelectric layer4. In this case, the silicon oxide film of the acoustic wave resonator5defining the series arm resonator RS1is preferably thinner in thickness than the silicon oxide film of the acoustic wave resonator5defining the parallel arm resonator RS2.

In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, the acoustic reflection layer3is shared by the plurality of acoustic wave resonators5. However, the high acoustic impedance layer32(the second high acoustic impedance layer322) closest to the piezoelectric layer4among the plurality of high acoustic impedance layers32may be separated for each acoustic wave resonator5. Additionally, in the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, the first high acoustic impedance layer321is more preferably separated for each acoustic wave resonator5.

In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, the gap27overlaps a portion of each of the first wiring portion61and the second wiring portion62that are connected to each of the plurality of acoustic wave resonators5in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, the gap27does not overlap any of a plurality of external connection terminals (the input terminal15, the output terminal16, and the ground terminals17and18) in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment, most of the first path12overlaps the gap27and most of each of the second paths13and14overlaps the gap27in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, the gap27overlapping the second wiring portion62connected to the series arm resonator RS1and the gap27overlapping the second wiring portion62connected to the parallel arm resonator RS2are connected to each other. As a result, in the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3, movement of charges along the first main surface21of the silicon substrate2can be further suppressed. Since the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 includes the trap regions10, linearity can be improved.

Modification 2 of Preferred Embodiment 3

Hereinafter, an acoustic wave device1faccording to Modification 2 of the Preferred Embodiment 3 will be described with reference toFIG.23. For the acoustic wave device1faccording to Modification 2 of Preferred Embodiment 3, elements the same as or similar to those of the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1faccording to Modification 2 of Preferred Embodiment 3 is different from the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 in that the acoustic wave device1fhas a region of each of the second paths13and14overlapping the gap27is smaller than that of the acoustic wave device1eaccording to Modification of Preferred Embodiment 3 in a plan view from the thickness direction D1of the piezoelectric layer4. In the acoustic wave device1faccording to Modification 2 of Preferred Embodiment 3, the gap27overlapping the second wiring portion62connected to the series arm resonator RS1and the gap27overlapping the second wiring portion62connected to the parallel arm resonator RS2are separated from each other. The acoustic wave device1faccording to Modification 2 of Preferred Embodiment 3 can have a higher mechanical strength than that of the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3.

Modification 3 of Preferred Embodiment 3

Hereinafter, an acoustic wave device1gaccording to Modification 3 of Preferred Embodiment 3 will be described with reference toFIG.24. For the acoustic wave device1gaccording to Modification 3 of Preferred Embodiment 3, elements the same as or similar to those of the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1gaccording to Modification 3 of Preferred Embodiment 3 includes a gap28in a region different from the gap27of the acoustic wave device1eaccording to Modification 1 of Preferred Embodiment 3. Similarly to the gap27, the gap28is provided over the silicon substrate2and the acoustic reflection layer3. At least a portion of the gap28is positioned within a predetermined distance L11, L12from a region overlapping a portion of at least one of the first wiring portion and the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. The predetermined distance L11, L12is a distance between either of the first electrode51or the second electrode52and the silicon substrate2. The trap region10includes the gap28.

Since the acoustic wave device1gaccording to Modification 3 of Preferred Embodiment 3 includes the trap regions10, linearity can be improved.

Modification 4 of Preferred Embodiment 3

Hereinafter, an acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3 will be described with reference toFIG.25. For the acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3, elements the same as or similar to those of the acoustic wave device1daccording to Preferred Embodiment 3 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3 differs from the acoustic wave device1daccording to Preferred Embodiment 3 in that a material of each low acoustic impedance layer31and a material of each high acoustic impedance layer32in the acoustic reflection layer3are different dielectrics from each other. The material of each low acoustic impedance layer31is, for example, silicon oxide. The material of each high acoustic impedance layer32is, for example, any of silicon nitride, aluminum nitride, alumina, and tantalum oxide.

The acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3 does not include the gap27of the acoustic wave device1daccording to Preferred Embodiment 3. In the acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3, the silicon substrate2includes the surface region2S that overlaps a portion of at least one of the first wiring portion61and the second wiring portion62in a plan view from the thickness direction D1of the piezoelectric layer4. The surface region2S preferably overlaps a portion of at least one of the first wiring portion61and the second wiring portion62.

In the acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3, the silicon substrate2includes the bulk region2B and the surface region2S, similarly to the silicon substrate2of the acoustic wave device1according to Preferred Embodiment 1. The surface region2S is, for example, an amorphous silicon layer. The surface region2S includes a portion of the first main surface21of the silicon substrate2. The first main surface21of the silicon substrate2includes the rough surface211in the surface region2S. The surface region2S is not provided in a region overlapping the acoustic wave resonator5in a plan view from the thickness direction D1of the piezoelectric layer4.

In the acoustic wave device1haccording to Modification of Preferred Embodiment 3, the trap region10includes the surface region2S. Since the acoustic wave device1haccording to Modification 4 of Preferred Embodiment 3 includes the trap region10, linearity can be improved.

Modification 5 of Preferred Embodiment 3

Hereinafter, an acoustic wave device1iaccording to Modification 5 of Preferred Embodiment 3 will be described with reference toFIG.26. For the acoustic wave device1iaccording to Modification 5 of Preferred Embodiment 3, elements the same as or similar to those of the acoustic wave device1daccording to Preferred Embodiment 3 are denoted by the same reference characters, and description thereof will be omitted.

The acoustic wave device1iaccording to Modification 5 of Preferred Embodiment 3 is different from the acoustic wave device1daccording to Preferred Embodiment 3 in that the acoustic wave device1ifurther includes two reflectors8, similar to the acoustic wave device1b(seeFIG.17) according to Modification 2 of Preferred Embodiment 1. The configuration of each reflector8is the same as or similar to that of each reflector8of the acoustic wave device1b.

The above-described first to third preferred embodiments and the like are merely examples of various preferred embodiments of the present invention. The above-described first to third preferred embodiments and the like can be variously modified according to design and the like as long as the advantageous effects of preferred embodiments of the present invention can be achieved.

For example, in the acoustic wave device1according to Preferred Embodiment 1, the piezoelectric layer4is bonded to the silicon substrate2with the silicon oxide film7interposed therebetween. However, the silicon oxide film7is not a necessary element.

In addition, in the acoustic wave device1according to Preferred Embodiment 1, the cavity26penetrates the silicon substrate2in the thickness direction thereof, but is not limited thereto. The cavity26may not penetrate the silicon substrate2and an internal space of a recess may not be provided in the first main surface21of the silicon substrate2.

Further, in the acoustic wave devices1to1j, each of the first electrode51and the second electrode52has a rectangular or substantially rectangular shape in cross section, but is not limited to having this shape. Here, the cross-sectional shape is, for example, a shape of a cross section orthogonal or substantially orthogonal to the thickness direction D1of the piezoelectric layer4and the second direction D2. For example, the first electrode51and the second electrode52may have a shape in which a width of a lower end is wider than a width of an upper end as in any ofFIGS.27A to27D. This makes it possible to increase a capacitance between the first electrode51and the second electrode52that are adjacent to each other without increasing the widths of the first main surface511of the first electrode51(seeFIG.2) and the first main surface521of the second electrode52(seeFIG.2).

The first electrode51and the second electrode52illustrated inFIG.27Ainclude a portion with a constant or substantially constant width on the upper end side and a portion with a gradually increasing width on the lower end side. Further, the first electrode51and the second electrode52illustrated inFIG.27Bhave a trapezoidal or substantially trapezoidal shape in cross section. In addition, the first electrode51and the second electrode52illustrated inFIG.27Chave a shape spreading toward the end, and both side surfaces in the width direction are curved surfaces. Further, each of the first electrode51and the second electrode52illustrated inFIG.27Dincludes a portion with a trapezoidal or substantially trapezoidal shape in cross section on the upper end side and includes, on the lower end side, a portion having the trapezoidal or substantially trapezoidal shape in cross section wider in width than the portion with the trapezoidal or substantially trapezoidal shape in cross section on the upper end side.

In addition, the acoustic wave devices1to1jmay include a dielectric film9that covers the first main surface41of the piezoelectric layer4and the first electrode51and the second electrode52on the first main surface41as illustrated in any ofFIGS.28A to28C. By including the dielectric film9, the acoustic wave devices1to1jcan increase the capacitance between the first electrode51and the second electrode52that are adjacent to each other. InFIG.28A, the dielectric film9is thinner than the first electrode51and the second electrode52, and a surface of the dielectric film9has an uneven shape along a shape of a base. InFIG.28B, the surface of the dielectric film9is flattened to have a planar shape. InFIG.28C, the dielectric film9is thicker than the first electrode51and the second electrode52, and the surface of the dielectric film9has an uneven shape along the shape of the base.

Further, in the acoustic wave devices1to1j, the cross-sectional shape of the first electrode51may be different from the cross-sectional shape of the second electrode52. Here, the cross-sectional shape is, for example, a shape of a cross section orthogonal or substantially orthogonal to the thickness direction D1of the piezoelectric layer4and the second direction D2.

Furthermore, in the acoustic wave devices1to1j, the acoustic wave resonator5includes the plurality of first electrodes51and the plurality of second electrodes52. However, the acoustic wave resonator5is not limited thereto and may include at least one first electrode51and one second electrode52.

Additionally, in a case where an acoustic wave filter is configured as in the acoustic wave device1aaccording to Modification 1 of Preferred Embodiment 1, the first electrode51and the second electrode52may have different shapes for each acoustic wave resonator5. Further, the first electrodes51and the second electrodes52may have different shapes between the acoustic wave resonator5of the series arm resonator RS1and the acoustic wave resonator5of the parallel arm resonator RS2.

Further, the first electrode51and the second electrode52are not limited to being linear in a plan view from the thickness direction D1of the piezoelectric layer4. For example, the first electrode51and the second electrode52may have a curved shape or a shape including a linear portion and a curved portion.

The following features are disclosed in this specification from the above-described preferred embodiments and the like.

An acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention includes a piezoelectric layer (4), and a first electrode (51) and a second electrode (52). The first electrode (51) and the second electrode (52) face each other in a direction (D2) intersecting with a thickness direction (D1) of the piezoelectric layer (4). The acoustic wave device (1;1a) uses a bulk wave of a thickness-shear primary mode. The acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) further includes a silicon substrate (2). The silicon substrate (2) includes a first main surface (21) and a second main surface (22) opposed to each other. A material of the piezoelectric layer (4) is lithium niobate or lithium tantalate. The piezoelectric layer (4) is on the first main surface (21) of the silicon substrate (2). The acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) further includes a trap region (10) in the silicon substrate (2).

With the acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to the above-described preferred embodiment, it is possible to handle higher frequencies and improve linearity.

An acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention includes a piezoelectric layer (4), and a first electrode (51) and a second electrode (52). The first electrode (51) and the second electrode (52) face each other in a direction (D2) intersecting with a thickness direction (D1) of the piezoelectric layer (4). The first electrode (51) and the second electrode (52) are adjacent to each other. In the acoustic wave device (1;1a), in any cross section along the thickness direction (D1) of the piezoelectric layer (4), d/p is equal to or less than about 0.5, when a distance between center lines of the first electrode (51) and the second electrode (52) is represented by p and a thickness of the piezoelectric layer (4) is represented by d. The acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) further includes a silicon substrate (2). The silicon substrate (2) includes a first main surface (21) and a second main surface (22) opposed to each other. A material of the piezoelectric layer (4) is lithium niobate or lithium tantalate. The piezoelectric layer (4) is on the first main surface (21) of the silicon substrate (2). The acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) further includes a trap region (10) in the silicon substrate (2).

With the acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to the above-described preferred embodiment, it is possible to handle higher frequencies and improve linearity.

In an acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention, d/p is equal to or less than about 0.24.

With the acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to the above-described preferred embodiment, a fractional bandwidth can be further increased.

In an acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention, the first electrode (51) includes a first electrode main portion (510), and the second electrode (52) includes a second electrode main portion (520). The first electrode main portion (510) intersects with the second electrode (52) in a direction (D2) in which the first electrode (51) and the second electrode (52) face each other. The second electrode main portion (520) intersects with the first electrode (51) in the direction (D2) in which the first electrode (51) and the second electrode (52) face each other. The piezoelectric layer (4) includes a specified region (45) that intersects with both the first electrode (51) and the second electrode (52) in the direction (D2) in which the first electrode (51) and the second electrode (52) face each other and is between the first electrode (51) and the second electrode (52) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) satisfies the condition of MR≤1.75×d/p)+0.075. S1 represents an area of the first electrode main portion (510) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). S2 represents an area of the second electrode main portion (520) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). S0 represents an area of the specified region (45) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). MR is a structural parameter defined by (S1+S2)/(S1+S2+S0).

With the acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to the above-described preferred embodiment, it is possible to reduce or prevent a spurious component in a band.

In an acoustic wave device (1;1a;1h;1i;1j) according to a preferred embodiment of the present invention, at least a portion of the first main surface (21) of the silicon substrate (2) is a rough surface (211). The silicon substrate (2) includes a bulk region (2B) and a surface region (2S) including the rough surface (211). The trap region (10) includes the surface region (2S).

In an acoustic wave device (1;1a;1h;1i;1j) according to a preferred embodiment of the present invention, the silicon substrate (2) includes a bulk region (2B) and a surface region (2S) including the first main surface (21) of the silicon substrate (2). The surface region (2S) is an amorphous silicon layer. The trap region (10) includes the surface region (2S).

In an acoustic wave device (1;1a;1h;1i;1j) according to a preferred embodiment of the present invention, the silicon substrate (2) includes a bulk region (2B) and a surface region (2S) including the first main surface (21) of the silicon substrate (2). The surface region (2S) is a polycrystalline silicon layer. The trap region (10) includes the surface region (2S).

In an acoustic wave device (1;1a;1h;1i) according to a preferred embodiment of the present invention, the rough surface (211) does not overlap an acoustic wave resonator (5) including the first electrode (51) and the second electrode (52), and a portion of the piezoelectric layer (4) in a plan view from the thickness direction (D1) of the piezoelectric layer (4).

In an acoustic wave device (1c) according to a preferred embodiment of the present invention, the trap region (10) includes an insulating film (silicon nitride film11) directly on the first main surface (21) of the silicon substrate (2). A weight ratio of oxygen of the insulating film (silicon nitride film11) is smaller than a weight ratio of oxygen of silicon oxide.

In an acoustic wave device (1c) according to a preferred embodiment of the present invention, the trap region (10) includes an insulating film (silicon nitride film11) directly on the first main surface (21) of the silicon substrate (2). The insulating film is a silicon nitride film (11).

In an acoustic wave device (1c) according to a preferred embodiment of the present invention, the insulating film (silicon nitride film11) does not overlap an acoustic wave resonator (5) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The acoustic wave resonator (5) includes the first electrode (51) and the second electrode (52), and a portion of the piezoelectric layer (4).

With the acoustic wave device (1c) according to the above-described preferred embodiment, it is possible to improve a Q value of resonance characteristics of the acoustic wave resonator (5), compared to a case where the insulating film (silicon nitride film11) overlaps the acoustic wave resonator (5).

In an acoustic wave device (1;1a;1b;1c) according to a preferred embodiment of the present invention, the silicon substrate (2) includes at least a portion of a cavity (26) on a side opposite to the first electrode (51) and the second electrode (52) with the piezoelectric layer (4) interposed therebetween. The cavity (26) overlaps an entire or substantially an entire region of an acoustic wave resonator (5) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The acoustic wave resonator (5) includes the first electrode (51) and the second electrode (52), and a portion of the piezoelectric layer (4).

An acoustic wave device (1;1a;1b;1c;1d;1e;1f;1i) according to a preferred embodiment of the present invention includes a first wiring portion (61) and a second wiring portion (62). The first wiring portion (61) is connected to the first electrode (51). The second wiring portion (62) is connected to the second electrode (52). The silicon substrate (2) includes at least a portion of a gap (27). The gap (27) overlaps a portion of at least one of the first wiring portion (61) and the second wiring portion (62) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The trap region (10) includes the gap (27).

An acoustic wave device (1d;1e;1f;1i) according to a preferred embodiment of the present invention includes a first wiring portion (61), a second wiring portion (62), and an acoustic reflection layer (3). The first wiring portion (61) is connected to the first electrode (51). The second wiring portion (62) is connected to the second electrode (52). The acoustic reflection layer (3) is provided between the first main surface (21) of the silicon substrate (2) and the piezoelectric layer (4). The acoustic reflection layer (3) includes at least one high acoustic impedance layer (32) and at least one low acoustic impedance layer (31). The at least one low acoustic impedance layer (31) has an acoustic impedance lower than an acoustic impedance of the at least one high acoustic impedance layer (32). The silicon substrate (2) includes at least a portion of a gap (27). The gap (27) overlaps a portion of at least one of the first wiring portion (61) and the second wiring portion (62) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The trap region (10) includes the gap (27).

An acoustic wave device (1g) according to a preferred embodiment of the present invention includes a first wiring portion (61) and a second wiring portion (62). The first wiring portion (61) is connected to the first electrode (51). The second wiring portion (62) is connected to the second electrode (52). The silicon substrate (2) includes at least a portion of a gap (28). The gap (28) includes a portion positioned within a predetermined distance (L11, L12) from a region overlapping a portion of at least one of the first wiring portion (61) and the second wiring portion (62) in a plan view from the thickness direction (D1) of the piezoelectric layer (4). The predetermined distance (L11, L12) is a distance between either of the first electrode (51) or the second electrode (52) and the silicon substrate (2). The trap region (10) includes the gap (28).

An acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention includes a plurality of the first electrodes (51) and a plurality of the second electrodes (52). The plurality of the first electrodes (51) and the plurality of the second electrodes (52) are alternately provided one by one. The plurality of the first electrodes (51) are commonly connected to the first wiring portion (61). The plurality of the second electrodes (52) are commonly connected to the second wiring portion (62).

With an acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to the above-described preferred embodiment, the Q value can be further increased.

An acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention includes a silicon oxide film (7). The silicon oxide film (7) is between the silicon substrate (2) and the piezoelectric layer (4).

An acoustic wave device (1a;1e;1f;1g;1j) according to a preferred embodiment of the present invention is an acoustic wave filter including a plurality of acoustic wave resonators (5). Each of the plurality of acoustic wave resonators (5) is a resonator including the first electrode (51) and the second electrode (52). The piezoelectric layer (4) is shared by the plurality of acoustic wave resonators (5).

In an acoustic wave device (1;1a;1b;1c;1d;1e;1f;1g;1h;1i;1j) according to a preferred embodiment of the present invention, the piezoelectric layer (4) includes a first main surface (41) and a second main surface (42) opposed to each other. The first main surface (41) of the piezoelectric layer (4) is on a side opposite to a side of the silicon substrate (2). The second main surface (42) of the piezoelectric layer (4) is on the side of the silicon substrate (2). The first electrode (51) and the second electrode (52) face each other on the first main surface (41) of the piezoelectric layer (4).