WO2022210809A1

WO2022210809A1 - Elastic wave device

Info

Publication number: WO2022210809A1
Application number: PCT/JP2022/015737
Authority: WO
Inventors: 優太石井; 勝己鈴木
Original assignee: 株式会社村田製作所
Priority date: 2021-03-31
Filing date: 2022-03-29
Publication date: 2022-10-06

Abstract

An elastic wave device comprising: a support substrate having a thickness in a first direction; a piezoelectric layer provided in the first direction of the support substrate and having a first main surface and a second main surface, the second main surface being on a side opposite the first main surface and on the support substrate side; and a functional electrode provided on the piezoelectric layer. A space portion is provided between the support substrate and the piezoelectric layer in a position where at least a part of the space portion overlaps the functional electrode when viewed in plan in the first direction. The piezoelectric layer has at least one through hole extending therethrough and in communication with the space portion. The opening area of the through hole in the first main surface of the piezoelectric layer is greater than the opening area of the through hole in the second main surface of the piezoelectric layer.

Description

Acoustic wave device

The present disclosure relates to elastic wave devices.

Patent Document 1 describes an elastic wave device.

JP 2012-257019 A

In Patent Document 1, a through-hole communicating with the cavity may be provided, and the sacrificial layer in the portion that will become the cavity may be etched through the through-hole. The longer the etchant is injected into the through-hole, the more likely the etchant will cause process damage to the piezoelectric layer.

An object of the present disclosure is to solve the above-described problems, and to suppress the influence of process damage on the piezoelectric layer.

An acoustic wave device according to one aspect includes a support substrate having a thickness in a first direction, a first main surface provided in the first direction of the support substrate, and a side opposite to the first main surface. a piezoelectric layer having a second principal surface on the side of the supporting substrate; and functional electrodes provided on the piezoelectric layer. Between the supporting substrate and the piezoelectric layer, the second A space is provided at a position that at least partially overlaps with the functional electrode when viewed in one direction, and at least one through hole penetrates the piezoelectric layer, and the through hole communicates with the space. The opening area of the through hole on the first main surface of the piezoelectric layer is larger than the opening area of the through hole on the second main surface of the piezoelectric layer.

According to the present disclosure, the influence of process damage on the piezoelectric layer is suppressed.

FIG. 1A is a perspective view showing the elastic wave device of this embodiment. FIG. 1B is a plan view showing the electrode structure of this embodiment. FIG. 2 is a cross-sectional view of a portion along line II-II of FIG. 1A. FIG. 3A is a schematic cross-sectional view for explaining Lamb waves propagating through the piezoelectric layer of the comparative example. FIG. 3B is a schematic cross-sectional view for explaining a thickness-shear primary mode bulk wave propagating through the piezoelectric layer of the present embodiment. FIG. 4 is a schematic cross-sectional view for explaining the amplitude direction of a thickness-shear primary mode bulk wave propagating through the piezoelectric layer of the present embodiment. FIG. 5 is an explanatory diagram showing an example of resonance characteristics of the elastic wave device of this embodiment. FIG. 6 shows that, in the elastic wave device of the present embodiment, d/2p and d/2p, where p is the center-to-center distance between adjacent electrodes or the average distance of the center-to-center distances, and d is the average thickness of the piezoelectric layer. FIG. 5 is an explanatory diagram showing a relationship with a fractional band; FIG. 7 is a plan view showing an example in which a pair of electrodes are provided in the acoustic wave device of this embodiment. FIG. 8 is a reference diagram showing an example of resonance characteristics of the elastic wave device of this embodiment. FIG. 9 shows the ratio of the bandwidth of the elastic wave device of the present embodiment when a large number of elastic wave resonators are configured, and the amount of phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. It is an explanatory view showing a relationship. FIG. 10 is an explanatory diagram showing the relationship between d/2p, metallization ratio MR, and fractional bandwidth. FIG. 11 is an explanatory diagram showing a map of the fractional band with respect to the Euler angles (0°, θ, ψ) of LiNbO ₃ when d/p is infinitely close to 0. FIG. FIG. 12 is a partially cutaway perspective view for explaining the elastic wave device according to this embodiment. FIG. 13 is a plan view of the elastic wave device according to the first embodiment; FIG. 14 is a diagram showing a cross section along line XIV-XIV in FIG. FIG. 15 is an explanatory diagram showing the relationship between the first angle formed by the second main surface of the piezoelectric layer and the side wall of the through hole and the maximum von Mises stress. FIG. 16 is a partially enlarged cross-sectional view around the through hole of the second embodiment. FIG. 17 is a partially enlarged cross-sectional view around the through-hole of the third embodiment. FIG. 18 is a plan view of an elastic wave device according to a fourth embodiment; FIG. 19 is a diagram showing another cross-sectional example of the functional electrode in the fourth embodiment.

Below, embodiments of the present disclosure will be described in detail based on the drawings. Note that the present disclosure is not limited by this embodiment. It should be noted that each embodiment described in the present disclosure is an example, and a modification that allows partial replacement or combination of configurations between different embodiments, and the first embodiment after the second embodiment A description of common matters with the form will be omitted, and only different points will be explained. In particular, similar actions and effects due to similar configurations will not be mentioned sequentially for each embodiment.

FIG. 1A is a perspective view showing the elastic wave device of this embodiment. FIG. 1B is a plan view showing the electrode structure of this embodiment.

The acoustic wave device 1 of this embodiment has a piezoelectric layer 2 made of LiNbO ₃ . The piezoelectric layer 2 may consist of LiTaO ₃ . The cut angle of LiNbO ₃ and LiTaO ₃ is Z-cut in this embodiment. The cut angles of LiNbO ₃ and LiTaO ₃ may be rotated Y-cut or X-cut. Preferably, the Y-propagation and X-propagation ±30° propagation orientations are preferred.

Although the thickness of the piezoelectric layer 2 is not particularly limited, it is preferably 50 nm or more and 1000 nm or less in order to effectively excite the thickness shear primary mode.

The piezoelectric layer 2 has a first main surface 2a and a second main surface 2b facing each other in the Z direction.

Electrode fingers

3 and 4 are provided on the first main surface 2a.

Here, the electrode finger 3 is an example of the "first electrode finger" and the electrode finger 4 is an example of the "second electrode finger". In FIGS. 1A and 1B , the multiple electrode fingers 3 are multiple “first electrode fingers” connected to the first busbar electrodes 5 . The multiple electrode fingers 4 are multiple “second electrode fingers” connected to the second busbar electrodes 6 . The plurality of electrode fingers 3 and the plurality of electrode fingers 4 are interdigitated with each other. Thereby, the functional electrode 30 including the electrode finger 3, the electrode finger 4, the first busbar electrode 5, and the second busbar electrode 6 is configured. Such a functional electrode 30 is also called an IDT (Interdigital Transducer) electrode.

The

electrode fingers

3 and 4 have a rectangular shape and a length direction. The electrode finger 3 and the electrode finger 4 adjacent to the electrode finger 3 face each other in a direction perpendicular to the length direction. Both the length direction of the

electrode fingers

3 and 4 and the direction orthogonal to the length direction of the

electrode fingers

3 and 4 are directions that intersect the thickness direction of the piezoelectric layer 2 . Therefore, it can be said that the electrode finger 3 and the electrode finger 4 adjacent to the electrode finger 3 face each other in the direction intersecting the thickness direction of the piezoelectric layer 2 . In the following description, the thickness direction of the piezoelectric layer 2 is defined as the Z direction (or first direction), the length direction of the

electrode fingers

3 and 4 is defined as the Y direction (or second direction), and the

electrode fingers

3 and 4 4 may be described as the X direction (or the third direction).

Further, the length direction of the

electrode fingers

3 and 4 may be interchanged with the direction orthogonal to the length direction of the

electrode fingers

3 and 4 shown in FIGS. 1A and 1B. That is, in FIGS. 1A and 1B, the

electrode fingers

3 and 4 may extend in the direction in which the first busbar electrodes 5 and the second busbar electrodes 6 extend. In that case, the first busbar electrode 5 and the second busbar electrode 6 extend in the direction in which the

electrode fingers

3 and 4 extend in FIGS. 1A and 1B. A pair of structures in which the electrode fingers 3 connected to one potential and the electrode fingers 4 connected to the other potential are adjacent to each other are arranged in a direction perpendicular to the length direction of the

electrode fingers

3 and 4. Multiple pairs are provided.

Here, the electrode finger 3 and the electrode finger 4 are adjacent to each other, not when the electrode finger 3 and the electrode finger 4 are arranged so as to be in direct contact, but when the electrode finger 3 and the electrode finger 4 are arranged with a gap therebetween. It refers to the case where the When the electrode finger 3 and the electrode finger 4 are adjacent to each other, there are electrodes connected to the hot electrode and the ground electrode, including

other electrode fingers

3 and 4, between the electrode finger 3 and the electrode finger 4. is not placed. The logarithms need not be integer pairs, but may be 1.5 pairs, 2.5 pairs, or the like.

The center-to-center distance, that is, the pitch, between the

electrode fingers

3 and 4 is preferably in the range of 1 μm or more and 10 μm or less. Further, the center-to-center distance between the

electrode fingers

3 and 4 means the center of the width dimension of the electrode fingers 3 in the direction orthogonal to the length direction of the electrode fingers 3 and the distance orthogonal to the length direction of the electrode fingers 4 . It is the distance connecting the center of the width dimension of the electrode finger 4 in the direction of

Furthermore, when at least one of the

electrode fingers

3 and 4 is plural (when there are 1.5 or more pairs of electrodes when the

electrode fingers

3 and 4 are paired as a pair of electrode pairs), the electrode fingers 3. The center-to-center distance of the electrode fingers 4 refers to the average value of the center-to-center distances of adjacent electrode fingers 3 and electrode fingers 4 among 1.5 or more pairs of electrode fingers 3 and electrode fingers 4 .

Also, the width of the

electrode fingers

3 and 4, that is, the dimension in the facing direction of the

electrode fingers

3 and 4 is preferably in the range of 150 nm or more and 1000 nm or less. Note that the center-to-center distance between the

electrode fingers

3 and 4 is the distance between the center of the dimension (width dimension) of the electrode finger 3 in the direction perpendicular to the length direction of the electrode finger 3 and the length of the electrode finger 4. It is the distance connecting the center of the dimension (width dimension) of the electrode finger 4 in the direction orthogonal to the direction.

In addition, since the Z-cut piezoelectric layer is used in this embodiment, the direction perpendicular to the length direction of the

electrode fingers

3 and 4 is the direction perpendicular to the polarization direction of the piezoelectric layer 2 . This is not the case when a piezoelectric material with a different cut angle is used as the piezoelectric layer 2 . Here, "perpendicular" is not limited to being strictly perpendicular, but substantially perpendicular (the angle formed by the direction perpendicular to the length direction of the electrode fingers 3 and electrode fingers 4 and the polarization direction is, for example, 90° ± 10°).

A support substrate 8 is laminated on the second main surface 2b side of the piezoelectric layer 2 with an intermediate layer 7 interposed therebetween. The intermediate layer 7 and the support substrate 8 have a frame shape and, as shown in FIG. 2,

openings

7a and 8a. A cavity (air gap) 9 is thereby formed.

The cavity 9 is provided so as not to disturb the vibration of the excitation region C of the piezoelectric layer 2 . Therefore, the support substrate 8 is laminated on the second main surface 2b with the intermediate layer 7 interposed therebetween at a position not overlapping the portion where at least one pair of

electrode fingers

3 and 4 are provided. Note that the intermediate layer 7 may not be provided. Therefore, the support substrate 8 can be directly or indirectly laminated to the second main surface 2b of the piezoelectric layer 2 .

The intermediate layer 7 is made of silicon oxide. However, the intermediate layer 7 can be formed of an appropriate insulating material other than silicon oxide, such as silicon nitride and alumina. Here, the intermediate layer 7 is an example of the "intermediate layer".

The support substrate 8 is made of Si. The plane orientation of the surface of Si on the piezoelectric layer 2 side may be (100), (110), or (111). Preferably, high-resistance Si having a resistivity of 4 kΩ or more is desirable. However, the support substrate 8 can also be constructed using an appropriate insulating material or semiconductor material. Materials for the support substrate 8 include, for example, aluminum oxide, lithium tantalate, lithium niobate, piezoelectric materials such as crystal, alumina, magnesia, sapphire, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, and steer. Various ceramics such as tight and forsterite, dielectrics such as diamond and glass, and semiconductors such as gallium nitride can be used.

The plurality of electrode fingers 3, electrode fingers 4, first busbar electrodes 5, and second busbar electrodes 6 are made of appropriate metals or alloys such as Al and AlCu alloys. In this embodiment, the electrode fingers 3, the electrode fingers 4, the first busbar electrodes 5, and the second busbar electrodes 6 have a structure in which an Al film is laminated on a Ti film. Note that materials other than the Ti film may be used for the adhesion layer.

When driving, an AC voltage is applied between the multiple electrode fingers 3 and the multiple electrode fingers 4 . More specifically, an AC voltage is applied between the first busbar electrode 5 and the second busbar electrode 6 . As a result, it is possible to obtain resonance characteristics using a thickness-shear primary mode bulk wave excited in the piezoelectric layer 2 .

Further, in the elastic wave device 1, when the thickness of the piezoelectric layer 2 is d, and the center-to-center distance between any one of the plurality of pairs of

electrode fingers

3 and 4 adjacent to each other is p, d/p is set to 0.5 or less. As a result, the thickness-shear primary mode bulk wave is effectively excited, and good resonance characteristics can be obtained. More preferably, d/p is 0.24 or less, in which case even better resonance characteristics can be obtained.

When at least one of the

electrode fingers

3 and 4 is plural as in the present embodiment, that is, when the

electrode fingers

3 and 4 form a pair of electrode sets, the

electrode fingers

3 and 4 , the center-to-center distance p between the

adjacent electrode fingers

3 and 4 is the average distance between the center-to-center distances between the

adjacent electrode fingers

3 and 4 .

Since the acoustic wave device 1 of the present embodiment has the above configuration, even if the logarithm of the

electrode fingers

3 and 4 is reduced in an attempt to reduce the size, the Q value is unlikely to decrease. This is because the resonator does not require reflectors on both sides, and the propagation loss is small. The reason why the above reflector is not required is that the bulk wave of the thickness-shlip primary mode is used.

FIG. 3A is a schematic cross-sectional view for explaining Lamb waves propagating through the piezoelectric layer of the comparative example. FIG. 3B is a schematic cross-sectional view for explaining a thickness-shear primary mode bulk wave propagating through the piezoelectric layer of the present embodiment. FIG. 4 is a schematic cross-sectional view for explaining the amplitude direction of a thickness-shear primary mode bulk wave propagating through the piezoelectric layer of the present embodiment.

FIG. 3A shows an acoustic wave device as described in Patent Document 1, in which Lamb waves propagate through the piezoelectric layer. As shown in FIG. 3A, waves propagate through the piezoelectric layer 201 as indicated by arrows. Here, the piezoelectric layer 201 has a first principal surface 201a and a second principal surface 201b, and the thickness direction connecting the first principal surface 201a and the second principal surface 201b is the Z direction. . The X direction is the direction in which the

electrode fingers

3 and 4 of the functional electrode 30 are aligned. As shown in FIG. 3A, in the Lamb wave, the wave propagates in the X direction as shown. Since it is a plate wave, although the piezoelectric layer 201 vibrates as a whole, the wave propagates in the X direction, so reflectors are arranged on both sides to obtain resonance characteristics. Therefore, wave propagation loss occurs, and when miniaturization is attempted, that is, when the number of logarithms of the

electrode fingers

3 and 4 is reduced, the Q value is lowered.

On the other hand, as shown in FIG. 3B, in the acoustic wave device of this embodiment, since the vibration displacement is in the thickness sliding direction, the wave is generated on the first main surface 2a and the second main surface of the piezoelectric layer 2. 2b, ie, the Z direction, and resonate. That is, the X-direction component of the wave is significantly smaller than the Z-direction component. Further, since resonance characteristics are obtained by propagating waves in the Z direction, no reflector is required. Therefore, no propagation loss occurs when propagating to the reflector. Therefore, even if the number of electrode pairs consisting of the

electrode fingers

3 and 4 is reduced in an attempt to promote miniaturization, the Q value is unlikely to decrease.

As shown in FIG. 4, the amplitude direction of the bulk wave of the primary thickness-shear mode is the first region 451 included in the excitation region C (see FIG. 1B) of the piezoelectric layer 2 and the first region 451 included in the excitation region C (see FIG. 1B). 2 area 452 is reversed. FIG. 4 schematically shows bulk waves when a voltage is applied between the electrode fingers 3 so that the electrode fingers 4 have a higher potential than the electrode fingers 3 . The first region 451 is a region of the excitation region C between the first main surface 2a and a virtual plane VP1 that is perpendicular to the thickness direction of the piezoelectric layer 2 and bisects the piezoelectric layer 2 . The second region 452 is a region of the excitation region C between the virtual plane VP1 and the second main surface 2b.

In the elastic wave device 1, at least one pair of electrodes consisting of the

electrode fingers

3 and 4 is arranged. It is not always necessary to have a plurality of pairs of electrode pairs. That is, it is sufficient that at least one pair of electrodes is provided.

For example, the electrode finger 3 is an electrode connected to a hot potential, and the electrode finger 4 is an electrode connected to a ground potential. However, the electrode finger 3 may be connected to the ground potential and the electrode finger 4 to the hot potential. In this embodiment, at least one pair of electrodes is an electrode connected to a hot potential or an electrode connected to a ground potential, as described above, and no floating electrodes are provided.

FIG. 5 is an explanatory diagram showing an example of resonance characteristics of the elastic wave device of this embodiment. The design parameters of the acoustic wave device 1 that obtained the resonance characteristics shown in FIG. 5 are as follows.

Piezoelectric layer ₂ : LiNbO3 with Euler angles (0°, 0°, 90°)
Thickness of piezoelectric layer 2: 400 nm

Length of excitation region C (see FIG. 1B): 40 μm
Number of electrode pairs consisting of electrode fingers 3 and 4: 21 pairs Center-to-center distance (pitch) between electrode fingers 3 and 4: 3 μm
Width of electrode fingers 3 and 4: 500 nm
d/p: 0.133

　Middle layer 7: Silicon oxide film with a thickness of 1 μm

Support substrate 8: Si

The excitation region C (see FIG. 1B) is a region where the

electrode fingers

3 and 4 overlap when viewed in the X direction perpendicular to the length direction of the

electrode fingers

3 and 4. . The length of the excitation region C is the dimension along the length direction of the

electrode fingers

3 and 4 of the excitation region C. As shown in FIG. Here, the excitation region C is an example of the "intersection region".

In the present embodiment, the inter-electrode distances of the electrode pairs consisting of the

electrode fingers

3 and 4 are all made equal in the plurality of pairs. That is, the electrode fingers 3 and the electrode fingers 4 are arranged at equal pitches.

As is clear from FIG. 5, good resonance characteristics with a specific bandwidth of 12.5% are obtained in spite of having no reflector.

By the way, when the thickness of the piezoelectric layer 2 is d, and the center-to-center distance between the

electrode fingers

3 and 4 is p, in the present embodiment, d/p is 0.5 or less, more preferably 0.5. 24 or less. This will be explained with reference to FIG.

A plurality of elastic wave devices were obtained by changing d/2p in the same manner as the elastic wave device that obtained the resonance characteristics shown in FIG. FIG. 6 shows that, in the elastic wave device of the present embodiment, when p is the center-to-center distance between adjacent electrodes or the average distance of the center-to-center distances, and d is the average thickness of the piezoelectric layer 2, d/2p is used as a resonator. 2 is an explanatory diagram showing the relationship between , and the fractional band. FIG.

As shown in FIG. 6, when d/2p exceeds 0.25, that is, when d/p>0.5, even if d/p is adjusted, the fractional bandwidth is less than 5%. On the other hand, when d/2p≦0.25, that is, when d/p≦0.5, the specific bandwidth can be increased to 5% or more by changing d/p within that range. , that is, a resonator having a high coupling coefficient can be constructed. Further, when d/2p is 0.12 or less, that is, when d/p is 0.24 or less, the specific bandwidth can be increased to 7% or more. In addition, by adjusting d/p within this range, a resonator with a wider specific band can be obtained, and a resonator with a higher coupling coefficient can be realized. Therefore, by setting d/p to 0.5 or less, it is possible to construct a resonator having a high coupling coefficient using the thickness-shear primary mode bulk wave.

Note that at least one pair of electrodes may be one pair, and the above p is the center-to-center distance between

adjacent electrode fingers

3 and 4 in the case of one pair of electrodes. In the case of 1.5 pairs or more of electrodes, the average distance between the centers of

adjacent electrode fingers

3 and 4 should be p.

Also, for the thickness d of the piezoelectric layer 2, if the piezoelectric layer 2 has variations in thickness, a value obtained by averaging the thickness may be adopted.

FIG. 7 is a plan view showing an example in which a pair of electrodes are provided in the elastic wave device of this embodiment. In elastic wave device 101 , a pair of electrodes having

electrode fingers

3 and 4 are provided on first main surface 2 a of piezoelectric layer 2 . Note that K in FIG. 7 is the intersection width. As described above, in the elastic wave device of the present disclosure, the number of pairs of electrodes may be one. Even in this case, if the above d/p is 0.5 or less, it is possible to effectively excite the bulk wave in the primary mode of thickness shear.

In the elastic wave device 1, preferably, the excitation region is an overlapping region of the plurality of

electrode fingers

3 and 4 when viewed in the direction in which any

adjacent electrode fingers

3 and 4 are facing each other. It is desirable that the metallization ratio MR of the

adjacent electrode fingers

3 and 4 with respect to the region C satisfies MR≦1.75(d/p)+0.075. In that case, spurious can be effectively reduced. This will be described with reference to FIGS. 8 and 9. FIG.

FIG. 8 is a reference diagram showing an example of resonance characteristics of the elastic wave device of this embodiment. A spurious signal indicated by an arrow B appears between the resonance frequency and the anti-resonance frequency. Note that d/p=0.08 and the Euler angles of LiNbO ₃ (0°, 0°, 90°). Also, the metallization ratio MR was set to 0.35.

The metallization ratio MR will be explained with reference to FIG. 1B. In the electrode structure of FIG. 1B, when focusing on the pair of

electrode fingers

3 and 4, it is assumed that only the pair of

electrode fingers

3 and 4 are provided. In this case, the excitation region C is the portion surrounded by the dashed-dotted line. The excitation region C is a region where the

electrode fingers

3 and 4 overlap with the electrode fingers 4 when viewed in a direction perpendicular to the length direction of the

electrode fingers

3 and 4, that is, in a facing direction. a region where the electrode fingers 3 overlap each other; and a region between the

electrode fingers

3 and 4 where the

electrode fingers

3 and 4 overlap each other. The area of the

electrode fingers

3 and 4 in the excitation region C with respect to the area of the excitation region C is the metallization ratio MR. That is, the metallization ratio MR is the ratio of the area of the metallization portion to the area of the excitation region C.

When a plurality of pairs of

electrode fingers

3 and 4 are provided, the ratio of the metallization portion included in the entire excitation region C to the total area of the excitation region C should be MR.

FIG. 9 shows the ratio of the bandwidth of the elastic wave device of the present embodiment when a large number of elastic wave resonators are configured, and the amount of phase rotation of the spurious impedance normalized by 180 degrees as the magnitude of the spurious. It is an explanatory view showing a relationship. The ratio band was adjusted by changing the film thickness of the piezoelectric layer 2 and the dimensions of the

electrode fingers

3 and 4 . FIG. 9 shows the results when the piezoelectric layer 2 made of Z-cut LiNbO ₃ is used, but the same tendency is obtained when the piezoelectric layer 2 with other cut angles is used.

In the area surrounded by ellipse J in FIG. 9, the spurious is as large as 1.0. As is clear from FIG. 9, when the fractional band exceeds 0.17, that is, exceeds 17%, a large spurious with a spurious level of 1 or more changes the parameters constituting the fractional band, even if the passband appear within. That is, as in the resonance characteristics shown in FIG. 8, a large spurious component indicated by arrow B appears within the band. Therefore, the specific bandwidth is preferably 17% or less. In this case, by adjusting the film thickness of the piezoelectric layer 2 and the dimensions of the

electrode fingers

3 and 4, the spurious response can be reduced.

FIG. 10 is an explanatory diagram showing the relationship between d/2p, metallization ratio MR, and fractional bandwidth. In the elastic wave device 1 of the present embodiment, various elastic wave devices 1 with different d/2p and MR were configured and the fractional bandwidth was measured. The hatched portion on the right side of the dashed line D in FIG. 10 is the area where the fractional bandwidth is 17% or less. The boundary between the hatched area and the non-hatched area is expressed by MR=3.5(d/2p)+0.075. That is, MR=1.75(d/p)+0.075. Therefore, preferably MR≤1.75(d/p)+0.075. In that case, it is easy to set the fractional bandwidth to 17% or less. More preferably, it is the area on the right side of MR=3.5(d/2p)+0.05 indicated by the dashed-dotted line D1 in FIG. That is, if MR≤1.75(d/p)+0.05, the fractional bandwidth can be reliably reduced to 17% or less.

FIG. 11 is an explanatory diagram showing a map of the fractional band with respect to the Euler angles (0°, θ, ψ) of LiNbO ₃ when d/p is infinitely close to 0. FIG. A hatched portion in FIG. 11 is a region where a fractional bandwidth of at least 5% or more is obtained. When the range of the area is approximated, it becomes the range represented by the following formulas (1), (2) and (3).

(0°±10°, 0° to 20°, arbitrary ψ) Equation (1)
(0°±10°, 20° to 80°, 0° to 60° (1-(θ-50) ² /900) ^1/2 ) or (0°±10°, 20° to 80°, [180 °-60° (1-(θ-50) ² /900) ^1/2 ] ~ 180°) Equation (2)
(0°±10°, [180°-30°(1-(ψ-90) ² /8100) ^1/2 ]~180°, arbitrary ψ) Equation (3)

Therefore, in the case of the Euler angle range of formula (1), formula (2), or formula (3), the fractional band can be sufficiently widened, which is preferable.

FIG. 12 is a partially cutaway perspective view for explaining the elastic wave device according to this embodiment. In FIG. 12, the outer periphery of the hollow portion 9 is indicated by broken lines. The elastic wave device of the present disclosure may utilize plate waves. In this case, the elastic wave device 301 has

reflectors

310 and 311 as shown in FIG.

Reflectors

310 and 311 are provided on both sides of the

electrode fingers

3 and 4 of the piezoelectric layer 2 in the acoustic wave propagation direction. In the acoustic wave device 301, a Lamb wave as a plate wave is excited by applying an alternating electric field to the

electrode fingers

3 and 4 on the cavity 9. FIG. At this time, since the

reflectors

310 and 311 are provided on both sides, it is possible to obtain resonance characteristics due to Lamb waves as Lamb waves.

As described above, the elastic wave devices 1 and 101 use bulk waves in the primary mode of thickness shear. In the elastic wave devices 1 and 101, the

electrode fingers

3 and 4 are adjacent electrodes, and when the thickness of the piezoelectric layer 2 is d and the distance between the centers of the

electrode fingers

3 and 4 is p, d/p is 0.5 or less. As a result, the Q value can be increased even if the elastic wave device is miniaturized.

In elastic wave devices 1 and 101, piezoelectric layer 2 is made of lithium niobate or lithium tantalate. The first main surface 2a or the second main surface 2b of the piezoelectric layer 2 has

electrode fingers

3 and 4 facing each other in a direction intersecting the thickness direction of the piezoelectric layer 2. should be covered with a protective film.

(First embodiment)
FIG. 13 is a plan view of the elastic wave device according to the first embodiment; FIG. 14 is a diagram showing a cross section along line XIV-XIV in FIG. A portion Q in FIG. 14 is a partially enlarged sectional view around the through hole 2H. In the example shown in FIG. 13, the first busbar electrode 5 and the second busbar electrode 6 are connected to the wiring 12 provided on the first main surface 2a of the piezoelectric layer 2, but this configuration is Just an example.

As shown in FIGS. 13 and 14, in an elastic wave device 1A according to the first embodiment, a hollow portion 9 is provided on the surface of the support substrate 8 on the side of the piezoelectric layer 2 in the Z direction. The hollow portion 9 is provided so as to at least partially overlap the functional electrode 30 when viewed in the Z direction. As shown in FIG. 14 , the cavity 9 is provided in a part of the intermediate layer 7 and is a space surrounded by the piezoelectric layer 2 and the intermediate layer 7 . The cavity 9 may be a space surrounded by the piezoelectric layer 2 , the support substrate 8 and the intermediate layer 7 . The support substrate 8 is, for example, a silicon substrate. The intermediate layer 7 is, for example, silicon oxide. The piezoelectric layer also includes, for example, lithium niobate or lithium tantalate. The piezoelectric layer 2 may contain lithium niobate or lithium tantalate and inevitable impurities. The functional electrode 30 includes the first busbar electrode 5 and the second busbar electrode 6 facing each other, the electrode fingers 3 connected to the first busbar electrode 5, and the electrodes connected to the second busbar electrode 6. finger 4 and an IDT electrode. The functional electrode 30 is provided on the first main surface 2a of the piezoelectric layer 2, but may be provided on the second main surface of the piezoelectric layer 2 opposite to the first main surface 2a.

As shown in FIG. 13, in the acoustic wave device 1A according to the first embodiment, the piezoelectric layer 2 has a through hole 2H penetrating through the piezoelectric layer 2 at a position overlapping with the hollow portion 9 when viewed in plan in the Z direction. is provided.

The through hole 2H is an etching hole for etching the sacrificial layer embedded in the portion that will become the hollow portion 9 . The etchant is injected from the first main surface 2 a of the piezoelectric layer 2 . When the opening area of the through hole 2H on the first main surface 2a of the piezoelectric layer 2 is larger than the opening area of the through hole 2H on the second main surface 2b of the piezoelectric layer 2, the cross section of the side wall 2Ha of the through hole 2H is It has a tapered shape. The first angle θ formed by the second main surface 2b of the piezoelectric layer 2 and the side wall 2Ha of the through hole 2H becomes an acute angle, and the etchant is smoothly injected. As a result, process damage to the piezoelectric layer 2 is suppressed.

FIG. 15 is an explanatory diagram showing the relationship between the first angle formed by the second main surface of the piezoelectric layer and the side wall of the through hole and the maximum von Mises stress. As shown in FIG. 15, when the first angle θ becomes smaller than 40 degrees, the maximum von Mises stress increases. When the first angle θ is 40 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H. When the first angle θ is 60 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H.

As described above, the elastic wave device 1A according to the first embodiment includes the support substrate 8 having a thickness in the first direction, the piezoelectric layer 2 provided in the first direction on the support substrate 8, and the piezoelectric layer 2. and a functional electrode 30 provided in the first direction. The functional electrode 30 faces any one of the plurality of electrode fingers 3 extending in a second direction orthogonal to the first direction and a third direction orthogonal to the first direction and the second direction. and a plurality of electrode fingers 4 extending in a direction. A hollow portion (space portion) 9 is provided between the support substrate 8 and the piezoelectric layer 2 at a position at least partially overlapping the functional electrode 30 when viewed in the first direction. There is at least one through hole 2H that penetrates the piezoelectric layer 2 and the through hole 2H communicates with the cavity 9 . In at least one through hole 2H, the opening area of the through hole 2H on the first main surface 2a of the piezoelectric layer 2 is larger than the opening area of the through hole 2H on the second main surface 2b of the piezoelectric layer 2.

As a result, the etchant is smoothly injected into the through holes 2H. As a result, process damage to the piezoelectric layer 2 is suppressed.

As a preferred embodiment, the thickness of the piezoelectric layer 2 is 2p or less, where p is the center-to-center distance between

adjacent electrode fingers

3 and 4 among the plurality of

electrode fingers

3 and 4. be. Thereby, the acoustic wave device 1 can be miniaturized and the Q value can be increased.

As a more desirable embodiment, the piezoelectric layer 2 contains lithium niobate or lithium tantalate. As a result, it is possible to provide an elastic wave device capable of obtaining good resonance characteristics.

In a more desirable mode, the Euler angles (φ, θ, ψ) of lithium niobate or lithium tantalate constituting the piezoelectric layer 2 are within the range of the following formula (1), formula (2), or formula (3). It is in. In this case, the fractional bandwidth can be widened sufficiently.

As a desirable aspect, the elastic wave device 1 is configured to be able to use bulk waves in the thickness shear mode. As a result, it is possible to provide an elastic wave device with a high coupling coefficient and good resonance characteristics.

As a more desirable aspect, d/p≦0.5, where d is the thickness of the piezoelectric layer 2 and p is the center-to-center distance between the

adjacent electrode fingers

3 and 4 . Thereby, the acoustic wave device 1 can be miniaturized and the Q value can be increased.

A more desirable aspect is that d/p is 0.24 or less. Thereby, the acoustic wave device 1 can be miniaturized and the Q value can be increased.

As a preferred embodiment, the region where the

adjacent electrode fingers

3 and 4 overlap in the facing direction is the excitation region C, and the metallization of the plurality of electrode fingers 3 and the plurality of electrode fingers 4 to the excitation region C. When the ratio is MR, MR≦1.75(d/p)+0.075 is satisfied. In this case, the fractional bandwidth can be reliably set to 17% or less.

As a desirable aspect, the elastic wave device 301 may be configured to be able to use plate waves. As a result, it is possible to provide an elastic wave device capable of obtaining good resonance characteristics.

(Second embodiment)
FIG. 16 is a partially enlarged cross-sectional view around the through hole of the second embodiment. In the second embodiment, compared with the first embodiment, the cross section of the side wall of the through hole 2H is tapered in two stages.

The first angle θ1 between the second main surface 2b of the piezoelectric layer 2 and the first side wall 2Ha1 of the through hole 2H is the same as the first angle θ in the first embodiment. When the first angle θ1 is 40 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H. When the first angle θ1 is 60 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H.

A second angle θ2 between the second side wall 2Ha2 of the through hole 2H intersecting the first main surface 2a of the piezoelectric layer 2 and the parallel surface L2b with the second main surface 2b of the piezoelectric layer 2 is the first angle θ1. It has a different angle. In the second embodiment, the first angle θ1 is smaller than the second angle θ2. Since the stress that causes cracks tends to occur on the second main surface 2b side of the piezoelectric layer 2, cracks can be suppressed by making the first angle θ1 smaller than the second angle θ2.

(Third Embodiment)
FIG. 17 is a partially enlarged cross-sectional view around the through-hole of the third embodiment. In the third embodiment, compared with the first embodiment, the cross section of the side wall of the through hole 2H is tapered in two stages.

A second angle θ2 between the second side wall 2Ha2 of the through hole 2H intersecting the first main surface 2a of the piezoelectric layer 2 and the parallel surface L2b with the second main surface 2b of the piezoelectric layer 2 is the first angle θ1. It has a different angle. In the third embodiment, the second angle θ2 is an acute angle. As a result, the opening area of the through holes 2H on the first main surface 2a of the piezoelectric layer 2 becomes larger than the opening area of the through holes 2H on the second main surface 2b of the piezoelectric layer 2 .

(Fourth embodiment)
FIG. 18 is a plan view of an elastic wave device according to a fourth embodiment; The through-hole 2H of the first embodiment is arranged at a position that is shifted in the X direction and does not overlap with the plurality of electrode fingers 3 and the plurality of electrode fingers 4, whereas the through-hole 2Hb of the fourth embodiment are arranged at positions that are displaced in the Y direction and do not overlap with the plurality of electrode fingers 3 and the plurality of electrode fingers 4 .

As shown in FIG. 18, the through hole 2Hb is a through hole provided in the piezoelectric layer 2. As shown in FIG. The through hole 2Hb is provided between the end 3a of at least one electrode finger 3 and the second busbar electrode 6 when viewed in plan in the Z direction. Alternatively, the through hole 2Hb is provided between the end portion 4a of at least one electrode finger 4 and the first busbar electrode 5 in a plan view in the Z direction. The through hole 2Hb is shifted outward in the Y direction from the imaginary line connecting the ends 3a of the electrode fingers 3 or the ends 4a of the electrode fingers 4 .

The through-hole 2Hb penetrates the piezoelectric layer 2 in the Z direction and communicates with the hollow portion 9 . A portion Q of the through hole 2Hb has a cross-sectional shape shown in FIG. 14, FIG. 16 or FIG. As a result, the opening area of the through holes 2Hb in the first main surface 2a of the piezoelectric layer 2 becomes larger than the opening area of the through holes 2Hb in the second main surface 2b of the piezoelectric layer 2 .

By providing the through hole 2Hb, it is possible to suppress the energy of the elastic wave from leaking in the Y direction, thereby suppressing the energy loss of the elastic wave. In addition, by providing the through hole 2Hb, spurious emissions can be selectively leaked and the occurrence of spurious emissions can be suppressed. Thereby, the Q value can be improved.

(Fifth embodiment)
FIG. 23 is a diagram showing another cross-sectional example of the functional electrode in the fifth embodiment. A functional electrode 30 of the fifth embodiment has an upper electrode 31 and a lower electrode 32 . The upper electrode 31 and the lower electrode 32 sandwich the piezoelectric layer 2 in the thickness direction. The elastic wave device of the fifth embodiment is sometimes called a BAW element (Bulk Acoustic Wave element).

When the first angle θ shown in FIG. 19 is 40 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H. When the first angle θ is 60 degrees or more, it becomes easier to suppress the occurrence of cracks in the piezoelectric layer 2 caused by the through holes 2H.

It should be noted that the above-described embodiments are intended to facilitate understanding of the present disclosure, and are not intended to limit and interpret the present disclosure. This disclosure may be modified/modified without departing from its spirit, and this disclosure also includes equivalents thereof.

1, 1A, 101, 301 elastic wave device 2 piezoelectric layer 2a first main surface 2b second main surface 2H, 2Hb through hole 2Ha side wall 2Ha1 first side wall 2Ha2 second side wall 3 electrode finger (first electrode finger)
4 electrode finger (second electrode finger)
5 First busbar electrode 6 Second busbar electrode 7 Intermediate layer 8 Support substrate 9 Cavity 12 Wiring 30 Functional electrode 201 Piezoelectric layer

Claims

a support substrate having a thickness in a first direction;
a piezoelectric layer provided in the first direction of the support substrate and having a first main surface and a second main surface opposite to the first main surface and on the side of the support substrate; ,
a functional electrode provided on the piezoelectric layer;
with
A space is provided between the support substrate and the piezoelectric layer at a position where at least a portion of the functional electrode overlaps with the functional electrode when viewed in plan in the first direction,
there is at least one through-hole penetrating the piezoelectric layer, the through-hole communicating with the space,
The elastic wave device, wherein the opening area of the through hole on the first main surface of the piezoelectric layer is larger than the opening area of the through hole on the second main surface of the piezoelectric layer.
The elastic wave device according to claim 1, wherein a first angle formed by said second main surface of said piezoelectric layer and said first side wall of said through hole in a cross section of said piezoelectric layer is an acute angle.
The elastic wave device according to claim 2, wherein the first angle is 40° or more.
The elastic wave device according to claim 2, wherein the first angle is 60° or more.
In the cross section of the piezoelectric layer, a second angle formed between a second side wall of the through hole intersecting the first main surface of the piezoelectric layer and a parallel plane parallel to the second main surface of the piezoelectric layer. is an acute angle.
In the cross section of the piezoelectric layer, the first angle formed by the second main surface of the piezoelectric layer and the first side wall of the through hole is defined by the parallel plane and the second side wall of the through hole. The elastic wave device according to claim 5, wherein the angle is smaller than the second angle.
The elastic wave device according to claim 1, further comprising an intermediate layer provided on said support substrate, wherein said intermediate layer has said space in a part thereof.
The functional electrode has one or more first electrode fingers extending in a second direction intersecting the first direction and faces the one or more first electrode fingers in a third direction perpendicular to the second direction. , and one or more second electrode fingers extending in the second direction.
The thickness of the piezoelectric layer is such that, of the one or more first electrode fingers and the one or more second electrode fingers, p is the center-to-center distance between adjacent first electrode fingers and second electrode fingers. The elastic wave device according to claim 8, wherein the elastic wave device is 2p or less when
The elastic wave device according to any one of claims 1 to 7, wherein the functional electrode has an upper electrode and a lower electrode facing each other with the piezoelectric layer interposed therebetween in the first direction.
The elastic wave device according to claim 1, wherein the piezoelectric layer contains lithium niobate or lithium tantalate.
The elastic wave device according to claim 8, configured to be able to use bulk waves in thickness shear mode.
Let d be the thickness of the piezoelectric layer, and let p be the center-to-center distance between the one or more first electrode fingers and the one or more second electrode fingers that are adjacent to each other. 9. The elastic wave device according to claim 8, wherein d/p≤0.5.
The elastic wave device according to claim 13, wherein d/p is 0.24 or less.
The functional electrode faces a plurality of first electrode fingers extending in a second direction that intersects with the first direction, and faces any one of the plurality of first electrode fingers in a third direction that intersects with the second direction. and a plurality of second electrode fingers extending in a second direction, and an excitation region is a region where the adjacent first electrode fingers and the second electrode fingers overlap when viewed in the facing direction. wherein MR≦1.75(d/p)+0.075 is satisfied, where MR is a metallization ratio of the plurality of first electrode fingers and the plurality of second electrode fingers to the excitation region. Item 8. The elastic wave device according to any one of Items 1 to 7.
The elastic wave device according to claim 1, configured to be able to use plate waves.
the piezoelectric layer comprises lithium niobate or lithium tantalate;
The elastic wave according to claim 1, wherein Euler angles (φ, θ, ψ) of the lithium niobate or the lithium tantalate are in the range of the following formula (1), formula (2), or formula (3) Device.
(0°±10°, 0° to 20°, arbitrary ψ) Equation (1)
(0°±10°, 20° to 80°, 0° to 60° (1-(θ-50) 2 /900) 1/2 ) or (0°±10°, 20° to 80°, [180 °-60° (1-(θ-50) 2 /900) 1/2 ] ~ 180°) Equation (2)
(0°±10°, [180°-30°(1-(ψ-90) 2 /8100) 1/2 ]~180°, arbitrary ψ) Equation (3)