Acoustic Wave Devices, Filters and Multiplexers

Abstract

To provide an acoustic wave device capable of suppressing unwanted waves.SOLUTION: The acoustic wave device includes a first piezoelectric layer 14b that is a rotary Y-cut lithium tantalate substrate and has a first thickness Tb, a second piezoelectric layer 14a that is a rotary Y-cut lithium niobate substrate, is stacked on the first piezoelectric layer 14b, has a second thickness Ta smaller than the first thickness Tb, and has a direction of spontaneous polarization that is roughly opposite to the direction of spontaneous polarization of the first piezoelectric layer 14b, a first electrode that is provided on a surface of the first piezoelectric layer 14b opposite to the second piezoelectric layer 14a, and a second electrode that is provided on a surface of the second piezoelectric layer 14a opposite to the first piezoelectric layer 14b and sandwiches at least a portion of the first piezoelectric layer 14b and at least a portion of the second piezoelectric layer 14a with the first electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an acoustic wave device, a filter, and a multiplexer, for example, an acoustic wave device, a filter, and a multiplexer having a resonator. [Background technology]

[0002] BAW (Bulk Acoustic Wave) resonators, such as FBAR (Film Bulk Acoustic Resonators) and SMR (Solid Mounted Resonators), are used as filters and duplexers for high-frequency circuits in wireless terminals such as mobile phones. BAW resonators are also known as thin-film piezoelectric resonators. A thin-film piezoelectric resonator has a structure in which a pair of electrodes are provided across a piezoelectric layer, and the resonance region where the pair of electrodes face each other across at least a portion of the piezoelectric layer is the region in which elastic waves resonate. It is known to stack two piezoelectric layers with spontaneous polarizations in opposite directions as the piezoelectric layer (e.g., Patent Documents 1 to 5). It is also known to use a single-crystal lithium niobate substrate and a single-crystal lithium tantalate substrate for the two stacked piezoelectric layers (e.g., Patent Document 5). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 64-71207 [Patent Document 2] Japanese Patent Application Publication No. 49-25883 [Patent Document 3] Japanese Patent Application Publication No. 3-123214 [Patent Document 4] Japanese Patent Application Publication No. 10-51262 [Patent Document 5] Japanese Patent Application Publication No. 7-254836 Summary of the Invention [Problem to be solved by the invention]

[0004] By stacking piezoelectric layers with spontaneous polarizations in opposite directions, it is possible to excite second-harmonic elastic waves. When second-harmonic elastic waves are used as the main mode, the fundamental wave becomes an unwanted wave. When one of the stacked piezoelectric layers is a rotated Y-cut lithium niobate substrate and the other is a rotated Y-cut lithium tantalate substrate, it is necessary to suppress the excitation of the fundamental wave, which is an unwanted wave.

[0005] The present invention has been made in view of the above-mentioned problems, and has an object to suppress unwanted waves. [Means for solving the problem]

[0006] The present invention is an acoustic wave device comprising: a first piezoelectric layer which is a rotated Y-cut lithium tantalate substrate and has a first thickness; a second piezoelectric layer which is a rotated Y-cut lithium niobate substrate and is laminated on the first piezoelectric layer, has a second thickness which is smaller than the first thickness, and has a spontaneous polarization direction which is approximately opposite to the spontaneous polarization direction of the first piezoelectric layer; a first electrode provided on the side of the first piezoelectric layer opposite the second piezoelectric layer; and a second electrode provided on the side of the second piezoelectric layer opposite the first piezoelectric layer, sandwiching at least a portion of the first piezoelectric layer and at least a portion of the second piezoelectric layer between the first electrode and the second electrode.

[0007] In the above configuration, the first piezoelectric layer may be a Y-cut lithium tantalate substrate rotated at an angle of 158° or more and 168° or less, the second piezoelectric layer may be a Y-cut lithium niobate substrate rotated at an angle of 158° or more and 168° or less, and the second thickness may be 0.5 times or more the first thickness.

[0008] In the above configuration, the second thickness may be 0.56 to 0.92 times the first thickness.

[0009] In the above configuration, the second thickness may be 0.68 to 0.78 times the first thickness.

[0010] In the above configuration, the X-axis direction of the crystal orientation of the rotated Y-cut lithium tantalate substrate in the first piezoelectric layer and the 2 The piezoelectric layer may be configured such that the angle formed between the crystal orientation of the rotated Y-cut lithium niobate substrate and the X-axis direction is 5° or less.

[0011] In the above configuration, the first piezoelectric layer and the second piezoelectric layer may be directly bonded to each other.

[0012] The above-described configuration may include a support substrate, and the first piezoelectric layer may be provided between the support substrate and the second piezoelectric layer.

[0013] The above-described configuration may include a support substrate, and the second piezoelectric layer may be provided between the support substrate and the first piezoelectric layer.

[0014] The present invention also provides a filter including the above acoustic wave device.

[0015] The present invention is a multiplexer including the above filter. [Effects of the Invention]

[0016] According to the present invention, the electromechanical coupling coefficient can be adjusted. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1(a) is a plan view of a piezoelectric thin film resonator according to Example 1, and FIG. 1(b) is a cross-sectional view taken along the line AA of FIG. 1(a). [Figure 2] 2(a) and 2(b) are diagrams illustrating the operation of the piezoelectric thin film resonators in Comparative Example 1 and Example 1. FIG. [Figure 3] FIG. 3 is a diagram showing the crystal orientation of the piezoelectric layer in Example 1. As shown in FIG. [Figure 4] FIG. 4 is a diagram showing |Y| versus frequency in Comparative Example 2. In FIG. [Figure 5]FIG. 5 is a diagram illustrating |Y| versus frequency in the first embodiment. [Figure 6] FIG. 6(a) is a diagram showing k2 of the fundamental wave versus Ta / Tb, and FIG. 6(b) is a diagram showing Δk2 of the fundamental wave versus Ta / Tb. [Figure 7] FIG. 7 is a diagram showing k2 of the fundamental wave versus Ta / Tb. [Figure 8] FIG. 8 is a diagram showing k2 and k2 / k2 (Ta / Tb=1) versus Ta / Tb. [Figure 9] 9(a) and 9(b) are diagrams showing Ta / Tb versus θ. [Figure 10] 10(a) to 10(c) are cross-sectional views of film bulk acoustic resonators according to first to third modifications of the first embodiment. [Figure 11] FIG. 11 is a cross-sectional view of a film bulk acoustic resonator according to a fourth modification of the first embodiment. [Figure 12] FIG. 12(a) is a circuit diagram of a filter according to the second embodiment, and FIG. 12(b) is a circuit diagram of a duplexer according to a first modification of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [Example]

[0019] The acoustic wave device will be described using a film bulk acoustic resonator as an example. Fig. 1(a) is a plan view of the film bulk acoustic resonator according to Example 1, and Fig. 1(b) is a cross-sectional view taken along line AA in Fig. 1(a). The stacking direction of the piezoelectric layers 14a and 14b is the Z direction, the extension direction of the lower electrode 12 is the X direction, and the direction perpendicular to the X and Z directions is the Y direction.

[0020] As shown in FIGS. 1(a) and 1(b), a piezoelectric layer 14 is provided on a substrate 10. The piezoelectric layer 14 includes stacked piezoelectric layers 14a and 14b. The thicknesses of the piezoelectric layers 14a and 14b are Ta and Tb, respectively. The thickness of the piezoelectric layer 14 is T. The piezoelectric layer 14a is a rotated Y-cut lithium niobate (LiNbO) substrate, and the piezoelectric layer 14b is a rotated Y-cut lithium tantalate (LiTaO) substrate. The polarization direction 52a of the piezoelectric layer 14a is downward. The polarization direction 52b of the piezoelectric layer 14b is upward, and the piezoelectric layers 14a and 14b are directly bonded together by, for example, surface activation. When the piezoelectric layers 14a and 14b are bonded together by surface activation, an amorphous layer may be formed between the piezoelectric layers 14a and 14b. In this case, since the thickness of the amorphous layer is sufficiently smaller than the thickness of the piezoelectric layers 14a and 14b, the piezoelectric layers 14a and 14b can be considered to be directly bonded to each other.

[0021] An upper electrode 16 and a lower electrode 12 are provided above and below the piezoelectric layer 14, respectively. The region where the lower electrode 12 and the upper electrode 16 overlap in a planar view, sandwiching at least a portion of the piezoelectric layer 14a and at least a portion of the piezoelectric layer 14b, is a resonance region 50. A gap 34 is provided between the substrate 10 and the lower electrode 12. An elastic wave is reflected at the interface between the lower electrode 12 and the gap 34. In a planar view, the gap 34 overlaps the resonance region 50, and the gap 34 is the same size as or larger than the resonance region 50.

[0022] The substrate 10 is, for example, a silicon substrate, a sapphire substrate, an alumina substrate, a spinel substrate, a quartz substrate, a crystal substrate, a glass substrate, a ceramic substrate, a GaAs substrate, etc. The lower electrode 12 and the upper electrode 16 are, for example, a single layer film of ruthenium (Ru), chromium (Cr), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir), etc., or a laminated film in which these films are laminated.

[0023] 2(a) and 2(b) are diagrams illustrating the operation of the piezoelectric thin film resonators in Comparative Example 1 and Example 1. FIGS. 2(a) and 2(b) show the lower electrode 12, the piezoelectric layer 14, and the upper electrode 16. The diagram on the right shows the displacement of the acoustic wave with respect to the position in the Z direction. Note that in FIG. 2(b), for ease of explanation, the thicknesses Ta and Tb are shown as being the same.

[0024] As shown in FIG. 2(a), in Comparative Example 1, the piezoelectric layer 14 is a single piezoelectric layer with a polarization direction 52. In this case, as shown in the right diagram, when the displacement of the top surface of the piezoelectric layer 14 is 0, the displacement of the bottom surface of the piezoelectric layer 14 is E for the fundamental wave. As a result, the potentials of the bottom electrode 12 and the top electrode 16 are different for the fundamental wave, and when an AC signal of a predetermined frequency is applied between the bottom electrode 12 and the top electrode 16, the fundamental wave is excited in the piezoelectric layer 14. On the other hand, for the second harmonic, the potentials of the bottom electrode 12 and the top electrode 16 are approximately the same. Therefore, the second harmonic is hardly excited in the piezoelectric layer 14. The thickness T of the piezoelectric layer 14 is approximately half the wavelength of the fundamental wave. To fabricate a piezoelectric thin-film resonator with a high resonant frequency, the thickness T of the piezoelectric layer 14 must be thinned. However, due to the presence of a gap 34 between the piezoelectric layer 14 and the support substrate, thinning the piezoelectric layer 14 makes the piezoelectric layer 14 more susceptible to damage. Furthermore, due to manufacturing variations in the thickness T of the piezoelectric layer 14, characteristics such as the resonant frequency tend to vary.

[0025] As shown in FIG. 2(b), in Example 1, the polarization direction 52a of the piezoelectric layer 14a is downward, and the polarization direction 52b of the piezoelectric layer 14b is upward. As shown in the right diagram, when the displacement of the upper surface of the piezoelectric layer 14 is 0 in the second harmonic, the displacement of the lower surface of the piezoelectric layer 14 is also 0. The displacement near the center of the piezoelectric layer 14 in the Z direction is E. Because the polarization direction 52a of the piezoelectric layer 14a and the polarization direction 52b of the piezoelectric layer 14b are opposite directions, the potentials of the lower electrode 12 and the upper electrode 16 are different in the second harmonic. 。When an AC signal of a predetermined frequency is applied between the lower electrode 12 and the upper electrode 16, a second harmonic is excited. On the other hand, in the case of the fundamental wave, the potentials of the lower electrode 12 and the upper electrode 16 are approximately the same. Therefore, the fundamental wave is hardly excited in the piezoelectric layer 14. The thickness T is approximately the wavelength of the second harmonic. Therefore, when fabricating a piezoelectric thin film resonator having the same resonant frequency, the thickness T of the piezoelectric layer 14 in Example 1 can be approximately twice that of Comparative Example 1. This makes it possible to suppress damage to the piezoelectric layer 14. Furthermore, it is possible to suppress variations in characteristics such as the resonant frequency.

[0026] When a lithium tantalate substrate with a rotational cut angle of 163° or a lithium niobate substrate with a rotational cut angle of 163° is used for the piezoelectric layers 14a and 14b, thickness-shear vibration is excited in the piezoelectric layer 14. The thickness-shear vibration direction is the Y direction. Furthermore, thickness-extensional vibration is hardly excited in the piezoelectric layer 14. For this reason, if thickness-shear vibration is the main mode, it is preferable to use a rotational cut angle of 163°.

[0027] If the piezoelectric layers 14a and 14b are made of the same material, the piezoelectric characteristics are determined by the material. Therefore, by using a lithium tantalate substrate for the piezoelectric layer 14a and a lithium niobate substrate for the piezoelectric layer 14b, the electromechanical coupling coefficient k 2 The piezoelectric characteristics can be between the piezoelectric characteristics when the piezoelectric layers 14a and 14b are formed on a lithium tantalate substrate and the piezoelectric characteristics when the piezoelectric layers 14a and 14b are formed on a lithium niobate substrate.

[0028] 3 is a diagram showing the crystal orientation of the piezoelectric layer in Example 1. The X-axis direction, Y-axis direction, and Z-axis direction are axial directions of the crystal orientation, and the X-axis direction, Y-axis direction, and Z-axis direction are the directions shown in FIGS. 1(a) and 1(b). A 163° rotated Y-cut lithium niobate substrate is used as the piezoelectric layer 14a, and a 163° rotated Y-cut lithium tantalate substrate is used as the piezoelectric layer 14b.

[0029] As shown in Figure 3, the +X-axis direction of the crystal orientation of piezoelectric layer 14a and the +X-axis direction of the crystal orientation of piezoelectric layer 14b are approximately the same, i.e., the X-direction. In piezoelectric layer 14a, the +Y-axis direction and the +Z-axis direction are respectively designated as the -Z-direction and +Y-direction, as indicated by the dashed arrows. The Y-axis and Z-axis are rotated θ=163° from the +Z-axis direction toward the +Y-axis direction around the X-axis. Lithium tantalate substrates and lithium niobate substrates have a crystalline structure similar to trigonal ilmenite, and the +Z-axis direction corresponds to the c-axis direction of the crystal orientation. The +Z-axis direction is the direction of spontaneous polarization. In this case, the top and bottom surfaces of piezoelectric layer 14a are referred to as the -plane and +plane, respectively, and the polarization direction 52a is illustrated as pointing downward.

[0030] In the piezoelectric layer 14b, the +Y-axis direction and the +Z-axis direction are the +Z-axis direction and the -Y-axis direction, respectively, as indicated by the dashed arrows. The Y-axis and Z-axis are rotated θ=163° from the +Z-axis direction toward the +Y-axis direction around the X-axis. The +Z-axis direction at this time corresponds to the c-axis direction of the crystal orientation. The +Z-axis direction is the direction of spontaneous polarization. In this case, the top and bottom surfaces of the piezoelectric layer 14b are referred to as the +plane and the -plane, respectively, and the polarization direction 52b is illustrated as pointing upward. The spontaneous polarization direction of the piezoelectric layers 14a and 14b can be confirmed by identifying the Z-axis direction using X-ray diffraction.

[0031] Spontaneous polarization is the state of polarization when no electric field is applied. In lithium tantalate, Li and Ta are cations and O is an anion. If the Li, Ta, and O atoms are positioned so that their charges cancel out, spontaneous polarization will not occur. However, because these atoms are not positioned so that they cancel out their charges, spontaneous polarization occurs. Spontaneous polarization also occurs in lithium niobate.

[0032] By making the +X-axis direction of piezoelectric layer 14a and the +X-axis direction of piezoelectric layer 14b approximately the same, the spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14a and the spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14b become approximately opposite directions. For example, if the +X-axis direction of piezoelectric layer 14a and the X-axis direction of piezoelectric layer 14b are different, the spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14a and the spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14b will not be opposite directions. The spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14a and the spontaneous polarization direction (+Z-axis direction) of piezoelectric layer 14b being approximately opposite directions is allowed to some extent when the angle between the +X-axis direction of piezoelectric layer 14a and the +X-axis direction of piezoelectric layer 14b is within a range of 5° or less.

[0033] [simulation] The frequency characteristics of the piezoelectric thin film resonator were simulated using the finite element method under the following simulation conditions: Thickness T of the piezoelectric layer 14: 300 nm Bottom electrode 12: 30 nm thick aluminum film Upper electrode 16: 30 nm thick aluminum film The thickness Ta of the piezoelectric layer 14a relative to the thickness Tb of the piezoelectric layer 14b is defined as Ta / Tb. When Ta / Tb=1, the thicknesses Ta and Tb of the piezoelectric layers 14a and 14b are both 150 nm.

[0034] As Comparative Example 2, we simulated the frequency characteristics when both the piezoelectric layers 14a and 14b were formed on 163° rotated Y-cut lithium niobate substrates. The piezoelectric layers 14a and 14b were bonded so that their polarization directions were opposite to each other. FIG. 4 shows |Y| versus frequency in Comparative Example 2. |Y| is the absolute value of admittance. As shown in FIG. 4, when Ta / Tb=1, the resonant frequency fr2 and the anti-resonant frequency fa2 are observed as responses to the second harmonic elastic wave. No fundamental wave response is observed. When Ta / Tb=0.9048, the resonant frequency fr2 and the anti-resonant frequency fa2 are observed as responses to the second harmonic elastic wave, and the resonant frequency fr1 and the anti-resonant frequency fa1 are observed as responses to the fundamental elastic wave.

[0035] When the piezoelectric layers 14a and 14b are made of the same material as in Comparative Example 2, the fundamental wave, which is an unwanted wave, is hardly excited by setting Ta / Tb=1. On the other hand, when Ta / Tb is changed from 1, the fundamental wave, which is an unwanted wave, is excited.

[0036] In Example 1, we simulated the frequency characteristics when a 163° rotated Y-cut lithium niobate substrate was used for the piezoelectric layer 14a and a 163° rotated Y-cut lithium tantalate substrate was used for the piezoelectric layer 14b. The piezoelectric layers 14a and 14b were bonded so that their polarization directions were opposite to each other. FIG. 5 shows |Y| versus frequency in Example 1. |Y| is the absolute value of admittance. As shown in FIG. 5, when Ta / Tb=1, the resonant frequency fr2 and anti-resonant frequency fa2 are observed as responses to the second harmonic elastic wave, and the resonant frequency fr1 and anti-resonant frequency fa1 are observed as responses to the fundamental elastic wave. When Ta / Tb=0.7178, the resonant frequency fr2 and anti-resonant frequency fa2 are observed as responses to the second harmonic elastic wave, and no fundamental response is observed.

[0037] In this way, when a rotated Y-cut lithium niobate substrate is used for the piezoelectric layer 14a and a rotated Y-cut lithium tantalate substrate is used for the piezoelectric layer 14b, the fundamental wave can be suppressed by making the thickness Ta smaller than the thickness Tb. Therefore, by changing Ta / Tb, the electromechanical coupling coefficient k 2 was simulated.

[0038] Figure 6(a) shows the k of the fundamental wave versus Ta / Tb. 2 Fig. 6(b) shows the relationship between Ta / Tb and Δk 2 In Fig. 6(a), the black circles represent the simulated points, and the curves are the lines connecting the black circles. In Fig. 6(b), Δk 2 is the k for Ta / Tb 2 This is the rate of change of the temperature, which corresponds to the slope between adjacent black circles in Figure 6(a).

[0039] As shown in Figure 6(a) and Figure 6(b), the electromechanical coupling coefficient k 2 When Ta / Tb is 0.70 to 0.75, k is about 0.06. When Ta / Tb is 0.7 or less, k 2 When Ta / Tb is 0.75 or more, k 2 becomes larger.

[0040] By setting θ to 158°, 163° and 168°, and changing Ta / Tb, the k 2 Figure 7 shows the k of the fundamental wave for Ta / Tb. 2 As shown in FIG. 7, when θ is 158°, 163°, or 168°, the k 2 The ranges R1 to R3 are the minimum when θ is 158°, 163°, and 168°, respectively. 2 k when Ta / Tb=1 2 If Ta / Tb is within the range R1 to R3, the k 2 is k when Ta / Tb=1 2 (Ta / Tb=1) or less.

[0041] Figure 8 shows the k versus Ta / Tb ratio. 2 and k 2 / k 2 (Ta / Tb=1). When θ is 158°, 163°, and 168°, k 2 / k 2 Ta / Tb where (Ta / Tb=1) is 1.0, 0.8, 0.6, 0.5, 0.4 and 0.2, and k 2 The figure shows Ta / Tb such that the Ta / Tb ratio is 0.1%.

[0042] 9(a) and 9(b) are diagrams showing Ta / Tb versus θ. 2 / k 2 The range of Ta / Tb where (Ta / Tb=1) is smaller than 0.5 is shown in Fig. 9(b). 29(a) and 9(b), the black circles represent the simulation points, the straight lines are approximate lines of the black circles, and the hatched areas represent the range between the straight lines.

[0043] As shown in Figure 9(a) and Figure 9(b), k 2 / k 2 The range of Ta / Tb where (Ta / Tb=1) is less than 0.5, and the k 2 The range of Ta / Tb where is 0.1% or less does not depend much on θ.

[0044] According to the first embodiment, as shown in FIGS. 1(a) and 1(b), a piezoelectric layer 14b (first piezoelectric layer) and a piezoelectric layer 14a (second piezoelectric layer) are laminated. The piezoelectric layer 14b is a rotated Y-cut lithium tantalate substrate and has a thickness Tb (first thickness). The piezoelectric layer 14a is a rotated Y-cut lithium niobate substrate and has a thickness Ta (second thickness), and has a spontaneous polarization direction that is approximately opposite to the spontaneous polarization direction of the piezoelectric layer 14b. The lower electrode 12 (first electrode) is provided on the surface of the piezoelectric layer 14b opposite to the piezoelectric layer 14a. The upper electrode 16 (second electrode) is provided on the surface of the piezoelectric layer 14a opposite to the piezoelectric layer 14b, and sandwiches at least a portion of the piezoelectric layer 14b and at least a portion of the piezoelectric layer 14a between the lower electrode 12 and the upper electrode 16. In this structure, the thickness Ta is made smaller than the thickness Tb. This reduces the k of the fundamental wave. 2 This can reduce the magnitude of the spontaneous polarization, thereby suppressing unwanted waves. Note that the directions of the spontaneous polarization of the piezoelectric layers 14a and 14b being substantially opposite to each other only need to be opposite enough to suppress the fundamental wave. The angle formed by the directions of the spontaneous polarization of the piezoelectric layers 14a and 14b is preferably 175° or more and 185° or less, more preferably 177° or more and 183° or less, and even more preferably 178° or more and 182° or less.

[0045] 8 to 9(b), the magnitude of the fundamental wave does not depend much on θ. From this, it is considered that if the piezoelectric layer 14b is a Y-cut lithium tantalate substrate rotated at an angle of 158° or more and 168° or less, and the piezoelectric layer 14a is a Y-cut lithium niobate substrate rotated at an angle of 158° or more and 168° or less, results similar to those in FIGS. 8 to 9(b) can be obtained. From FIG. 8, it can be seen that if the thickness Ta is 0.5 times or more the thickness Tb, k 2 / k 2 (Ta / Tb=1) can be made smaller than 1. In order to excite thickness-shear vibration elastic waves and not thickness-extensional vibration elastic waves, the θ of piezoelectric layers 14b and 14a is preferably 160° or more and 166° or less, and more preferably 161° or more and 165° or less.

[0046] From Figure 8, when Ta / Tb is 0.52 or more and 0.97 or less, k 2 / k 2 (Ta / Tb=1) can be made 0.8 or less. When Ta / Tb is 0.55 or more and 0.94 or less, k 2 / k 2 (Ta / Tb=1) can be made 0.6 or less. When Ta / Tb is 0.56 or more and 0.92 or less, k 2 / k 2 (Ta / Tb=1) can be made 0.5 or less. When Ta / Tb is 0.58 or more and 0.90 or less, k 2 / k 2 (Ta / Tb=1) can be made 0.4 or less. When Ta / Tb is 0.62 or more and 0.85 or less, k 2 / k 2 (Ta / Tb=1) can be made 0.2 or less. When Ta / Tb is 0.68 or more and 0.78 or less, k 2 can be reduced to less than 0.1%.

[0047] In order to make the direction of spontaneous polarization of piezoelectric layer 14a and the direction of spontaneous polarization of piezoelectric layer 14b approximately opposite to each other, the angle formed between the X-axis direction of the crystal orientation of the lithium tantalate substrate in piezoelectric layer 14a and the X-axis direction of the crystal orientation of the lithium niobate substrate in piezoelectric layer 14b is preferably 5° or less, and more preferably 2° or less.

[0048] The piezoelectric layers 14a and 14b are directly bonded to each other, and the interface between the piezoelectric layers 14a and 14b is a flat surface (for example, the arithmetic surface roughness Ra is 10 nm or less), which allows the piezoelectric layer 14 to generate second harmonic vibrations.

[0049] [Modification 1 of Example 1] 10(a) to 10(c) are cross-sectional views of film bulk acoustic resonators according to first to third modifications of the first embodiment. As shown in Fig. 10(a), in the first modification of the first embodiment, the polarization direction 52a of the piezoelectric layer 14a is upward, and the polarization direction 52b of the piezoelectric layer 14b is downward. The other configurations are the same as those of the first embodiment, and therefore the description thereof will be omitted.

[0050] [Modification 2 of Example 1] 10(b), in Modification 2 of Example 1, the upper surface of piezoelectric layer 14a is bonded to the lower surface of piezoelectric layer 14b, a lower electrode 12 is provided on the lower surface of piezoelectric layer 14a, and an upper electrode 16 is provided on the upper surface of piezoelectric layer 14b. The polarization direction 52a of piezoelectric layer 14a is upward, and the polarization direction 52b of piezoelectric layer 14b is downward. The other configurations are the same as in Example 1, and therefore a description thereof will be omitted.

[0051] [Modification 3 of Example 1] 10(c), in Modification 3 of Example 1, the polarization direction 52a of the piezoelectric layer 14a is downward, and the polarization direction 52b of the piezoelectric layer 14b is upward. The other configurations are the same as those of Modification 2 of Example 1, and therefore, description thereof will be omitted.

[0052] As in Example 1 and its Modification 1, the piezoelectric layer 14b may be provided between the substrate 10 (support substrate) and the piezoelectric layer 14a. As in Modifications 2 and 3 of Example 1, the piezoelectric layer 14a may be provided between the substrate 10 and the piezoelectric layer 14b. The Ta / Tb ratio that suppresses the fundamental harmonic is determined by the amount of charge generated by spontaneous polarization when the fundamental harmonic is excited. Therefore, the preferable range of Ta / Tb obtained by the above simulation can also be applied to Modifications 1 to 3 of Example 1. Furthermore, the preferable range of Ta / Tb can be applied regardless of the frequency, such as the resonant frequency, of the piezoelectric thin film resonator, the materials of the lower electrode 12 and the upper electrode 16, and the piezoelectric layers 14a, 14b, the lower electrode 12, and the upper electrode 16.

[0053] [Modification 4 of Example 1] FIG. 11 is a cross-sectional view of a piezoelectric thin film resonator according to a fourth modification of the first embodiment. As shown in FIG. 11, in the fourth modification of the first embodiment, an acoustic reflection film 30 is provided between the substrate 10 and the lower electrode 12, instead of the gap 34. The acoustic reflection film 30 is formed by alternating low-acoustic-impedance films 31 and high-acoustic-impedance films 32. The thicknesses of the films 31 and 32 are, for example, approximately λ / 4 (λ is the wavelength of the acoustic wave). This allows the acoustic reflection film 30 to reflect acoustic waves. The number of layers of the films 31 and 32 can be set arbitrarily. The acoustic reflection film 30 overlaps the resonance region 50, and the acoustic reflection film 30 is the same size as or larger than the resonance region 50. The film 31 of the acoustic reflection film 30 is made of, for example, silicon oxide, silicon nitride, or the like. The film 32 is made of, for example, tungsten, tantalum, molybdenum, ruthenium, or the like.

[0054] As in the first embodiment and its first to third modifications, the piezoelectric thin film resonator may be an FBAR in which the air gap 34 reflects elastic waves. As in the fourth modification of the first embodiment, the piezoelectric thin film resonator may be an SMR in which the acoustic reflection film 30 reflects elastic waves. Although the above description has been given of an example in which the planar shape of the resonance region 50 is rectangular, the resonance region 50 may be an elliptical shape or a polygonal shape such as a pentagon. [Example]

[0055] Example 2 is an example of a filter and a duplexer using the piezoelectric thin film resonator of Example 1. Fig. 12(a) is a circuit diagram of the filter according to Example 2. As shown in Fig. 12(a), one or more series resonators S1 to S4 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 to P4 are connected in parallel between the input terminal Tin and the output terminal Tout. The piezoelectric thin film resonator of Example 1 can be used for at least one of the one or more series resonators S1 to S4 and the one or more parallel resonators P1 to P4. The number of resonators in the ladder-type filter can be set as appropriate.

[0056] FIG. 12(b) is a circuit diagram of a duplexer according to a first modification of the second embodiment. As shown in FIG. 12(b), a transmit filter 40 is connected between a common terminal Ant and a transmit terminal Tx. A receive filter 42 is connected between the common terminal Ant and a receive terminal Rx. The transmit filter 40 passes signals in the transmit band, among the signals input from the transmit terminal Tx, to the common terminal Ant as transmit signals, and suppresses signals of other frequencies. The receive filter 42 passes signals in the receive band, among the signals input from the common terminal Ant, to the receive terminal Rx as receive signals, and suppresses signals of other frequencies. At least one of the transmit filter 40 and the receive filter 42 can be the filter of the second embodiment.

[0057] Although a duplexer has been described as an example of a multiplexer, a triplexer or a quadplexer may also be used.

[0058] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as defined in the claims. [Explanation of symbols]

[0059] 10 Substrate 12 Lower electrode 14, 14a, 14b Piezoelectric layers 16 Upper electrode 30 Acoustic reflective film 34 void 40 Transmission Filter 42 Receive Filter 50 resonance area 52a, 52b Polarization direction

Claims

1. a first piezoelectric layer, the first piezoelectric layer being a rotated Y-cut lithium tantalate substrate and having a first thickness; a second piezoelectric layer, the second piezoelectric layer being a rotated Y-cut lithium niobate substrate, laminated to the first piezoelectric layer, the second piezoelectric layer having a second thickness less than the first thickness and a direction of spontaneous polarization substantially opposite to the direction of spontaneous polarization of the first piezoelectric layer; a first electrode provided on a surface of the first piezoelectric layer opposite to the second piezoelectric layer; a second electrode provided on a surface of the second piezoelectric layer opposite to the first piezoelectric layer, the second electrode sandwiching at least a portion of the first piezoelectric layer and at least a portion of the second piezoelectric layer between the second electrode and the first electrode; 1. An acoustic wave device comprising:

2. the first piezoelectric layer is a Y-cut lithium tantalate substrate rotated at an angle of 158° or more and 168° or less; the second piezoelectric layer is a Y-cut lithium niobate substrate rotated 158° or more and 168° or less; The acoustic wave device according to claim 1 , wherein the second thickness is at least 0.5 times the first thickness.

3. The acoustic wave device according to claim 2 , wherein the second thickness is not less than 0.56 times and not more than 0.92 times the first thickness.

4. The acoustic wave device according to claim 2 , wherein the second thickness is not less than 0.68 times and not more than 0.78 times the first thickness.

5. 5. The acoustic wave device according to claim 2, wherein an angle between an X-axis direction of a crystal orientation of the rotated Y-cut lithium tantalate substrate in the first piezoelectric layer and an X-axis direction of a crystal orientation of the rotated Y-cut lithium niobate substrate in the second piezoelectric layer is 5° or less.

6. The acoustic wave device according to claim 1 , wherein the first piezoelectric layer and the second piezoelectric layer are directly bonded to each other.

7. A support substrate is provided, The acoustic wave device according to claim 1 , wherein the first piezoelectric layer is provided between the support substrate and the second piezoelectric layer.

8. A support substrate is provided, The acoustic wave device according to claim 1 , wherein the second piezoelectric layer is provided between the support substrate and the first piezoelectric layer.

9. A filter comprising the acoustic wave device according to any one of claims 1 to 8.

10. A multiplexer including the filter of claim 9.

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