Elastic wave devices, filters, and multiplexers

Abstract

To provide a surface acoustic wave device capable of improving the temperature characteristics.SOLUTION: A surface acoustic wave device comprises: a first piezoelectric layer 14a; a second piezoelectric layer 14b that is laminated onto the first piezoelectric layer 14a, and includes a direction of intrinsic polarization opposite to a direction of intrinsic polarization of the first piezoelectric layer 14a; a first electrode that is provided at the opposite side to the second piezoelectric layer 14b in the first piezoelectric layer 14a; a second electrode that is provided at the opposite side to the first piezoelectric layer 14a in the second piezoelectric layer 14b, and sandwiches at least one part of the first piezoelectric layer 14a and at least one part of the second piezoelectric layer 14b between itself and the first electrode; and an insulation layer that is provided in at least one of spaces between the first piezoelectric layer 14a and the first electrode and between the second piezoelectric layer 14b and the second electrode, and includes a temperature coefficient of an elastic constant of a sign opposite to a sign of the temperature coefficient of the elastic constant of the first piezoelectric layer 14a and the second piezoelectric layer 14b.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to acoustic wave devices, filters, and multiplexers. [Background technology]

[0002] Bulk Acoustic Wave (BAW) resonators such as FBAR (Film Bulk Acoustic Resonator) and SMR (Solid Mounted Resonator) are used as filters and duplexers for high-frequency circuits in wireless terminals such as mobile phones. BAW resonators are also called piezoelectric thin-film resonators. A piezoelectric thin-film resonator has a structure in which a pair of electrodes are provided on either side of a piezoelectric layer, and the resonance region in which the pair of electrodes face each other with at least a portion of the piezoelectric layer on either side is the region in which elastic waves resonate. It is known that elastic waves of even-order modes such as the second harmonic can be excited by stacking two piezoelectric layers with opposite spontaneous polarization directions (for example, Patent Document 1). It is known that a temperature compensation film can be provided inside the piezoelectric layer or between the piezoelectric layer and the electrodes (for example, Patent Documents 2-4). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 64-71207 [Patent Document 2] Japanese Patent Application Publication No. 58-137317 [Patent Document 3] Japanese Patent Publication No. 2008-182512 [Patent Document 4] Japanese Patent Publication No. 2007-159123 [Overview of the project] [Problems that the invention aims to solve]

[0004] Patent documents 2 to 4 do not have two piezoelectric layers with different directions of spontaneous polarization, and are not elastic wave devices with the second harmonic as the main mode, as in Patent Document 1, but rather elastic wave devices with odd-order modes such as the fundamental wave as the main mode. In elastic wave devices with the fundamental wave as the main mode, the piezoelectric layer becomes thinner as the frequency handled increases. In elastic wave devices with the second harmonic as the main mode, the piezoelectric layer can be made thicker, which is advantageous for high-frequency applications. However, in elastic wave devices with even-order modes such as the second harmonic as the main mode, the preferred position for providing a temperature compensation film to improve temperature characteristics is unknown.

[0005] This invention has been made in view of the above problems, and aims to improve temperature characteristics. [Means for solving the problem]

[0006] The present invention is an elastic wave device comprising: a first piezoelectric layer; a second piezoelectric layer laminated on the first piezoelectric layer and having a spontaneous polarization direction opposite to the spontaneous polarization direction of the first piezoelectric layer; a first electrode provided on the first piezoelectric layer opposite to the second piezoelectric layer; a second electrode provided on the second piezoelectric layer opposite to the first piezoelectric layer and sandwiching at least a portion of the first piezoelectric layer and at least a portion of the second piezoelectric layer with the first electrode; and an insulating film provided between the first piezoelectric layer and the first electrode and between the second piezoelectric layer and the second electrode, at least one of which has a temperature coefficient of elasticity opposite to the sign of the temperature coefficient of elasticity of the first piezoelectric layer and the second piezoelectric layer.

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

[0008] In the above configuration, the thickness of the insulating film can be set to be 1 / 30 times or more of the total thickness of the first piezoelectric layer and the second piezoelectric layer.

[0009] In the above configuration, the thicknesses of the first piezoelectric layer and the second piezoelectric layer can be set to be the same.

[0010] In the above configuration, another insulating film having a temperature coefficient of elastic constant with a sign opposite to that of the temperature coefficient of elastic constant of the first piezoelectric layer and the second piezoelectric layer is provided between the first piezoelectric layer and the first electrode and between the second piezoelectric layer and the second electrode, and the configuration can be such that it is provided.

[0011] The present invention is an elastic wave device including a first piezoelectric layer, a second piezoelectric layer laminated on the first piezoelectric layer, having a thickness different from the thickness of the first piezoelectric layer, and having a direction of spontaneous polarization opposite to the direction of spontaneous polarization of the first piezoelectric layer, a first electrode provided on the opposite side of the first piezoelectric layer with respect to the second piezoelectric layer, a second electrode provided on the opposite side of the second piezoelectric layer with respect to the first piezoelectric layer, and sandwiching at least a part of the first piezoelectric layer and at least a part of the second piezoelectric layer together with the first electrode, and an insulating film provided between the first piezoelectric layer and the second piezoelectric layer and having a temperature coefficient of elastic constant with a sign opposite to that of the temperature coefficient of elastic constant of the first piezoelectric layer and the second piezoelectric layer.

[0012] In the above configuration, the first piezoelectric layer and the second piezoelectric layer can be a 158° or more and 168° or less rotated Y-cut lithium niobate substrate, or the first piezoelectric layer and the second piezoelectric layer can be a 158° or more and 168° or less rotated Y-cut lithium tantalate substrate.

[0013] In the above configuration, the ratio of the thickness of the second piezoelectric layer to the thickness of the first piezoelectric layer can be set to be 0.2 or more and 0.9 or less.

[0014] In the above configuration, the angle formed by the X-axis direction of the crystal orientation in the first piezoelectric layer and the X-axis direction of the crystal orientation in the second piezoelectric layer can be set to 5° or less.

[0015] In the above configuration, the insulating film can be configured to have silicon oxide as a main component.

[0016] The present invention is a filter including the above elastic wave device.

[0017] The present invention is a multiplexer including the above filter.

Advantages of the Invention

[0018] According to the present invention, the temperature characteristics can be improved.

Brief Description of the Drawings

[0019] [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 line A-A of FIG. 1(a). [Figure 2] FIG. 2 is a diagram showing the crystal orientation of the piezoelectric layer in Example 1. [Figure 3] FIGS. 3(a) to 3(c) are schematic diagrams of the piezoelectric thin film resonators in resonators A1 to A3. [Figure 4] FIGS. 4(a) to 4(c) are schematic diagrams of the piezoelectric thin film resonators in resonators B1 to B3. [Figure 5] FIGS. 5(a) to 5(c) are diagrams showing the normalized elastic strain energy with respect to the position Z in the thickness direction in resonators A1 to A3, respectively. [Figure 6] FIGS. 6(a) to 6(c) are diagrams showing the normalized elastic strain energy with respect to the position Z in the thickness direction in resonators B1 to B3, respectively. [Figure 7] FIGS. 7(a) and 7(b) are diagrams showing the temperature coefficient of velocity TCV and the electromechanical coupling coefficient k2 with respect to the thickness T18 of resonators A2 and B2 in Simulation 2. [Figure 8] Figures 8(a) and 8(b) show the temperature rate coefficient TCV and electromechanical coupling coefficient k2 for the thicknesses T18a and T18b of resonators A3 and B3 in Simulation 2. [Figure 9] Figure 9(a) is a plan view of the piezoelectric thin-film resonator according to Example 2, and Figure 9(b) is a cross-sectional view AA of Figure 9(a). [Figure 10] Figure 10(a) is a schematic diagram of the piezoelectric thin-film resonator in resonator C2, and Figure 10(b) is a diagram showing the normalized elastic strain energy with respect to position Z in the thickness direction in resonator C2. [Figure 11] Figures 11(a) and 11(b) show the temperature rate coefficient TCV and electromechanical coupling coefficient k2 for the ratio of the thicknesses T14b / T14a of the resonator C2 in Simulation 3. [Figure 12] Figure 12(a) is a cross-sectional view of a piezoelectric thin-film resonator according to Modification 1 of Examples 1 and 2, and Figure 12(b) is a cross-sectional view of a piezoelectric thin-film resonator according to Modification 2 of Examples 1 and 2. [Figure 13] Figure 13(a) is a circuit diagram of the filter according to Example 3, and Figure 13(b) is a circuit diagram of the duplexer according to Modification 1 of Example 3. [Modes for carrying out the invention]

[0020] The embodiments of the present invention will be described below with reference to the drawings. [Examples]

[0021] A piezoelectric thin-film resonator will be used as an example of an elastic wave device. Figure 1(a) is a plan view of the piezoelectric thin-film resonator according to Example 1, and Figure 1(b) is a cross-sectional view AA of Figure 1(a). The stacking direction of the piezoelectric layers 14a and 14b is the Z direction, the extraction direction of the lower electrode 12 is the X direction, and the direction perpendicular to the X and Z directions is the Y direction.

[0022] As shown in Figures 1(a) and 1(b), a piezoelectric layer 14 is provided on the substrate 10. The piezoelectric layer 14 comprises laminated piezoelectric layers 14a and 14b. The polarization direction 52a of piezoelectric layer 14a is downward. The polarization direction 52b of piezoelectric layer 14b is upward, and piezoelectric layers 14a and 14b are directly joined, for example, by a surface activation method. When piezoelectric layers 14a and 14b are joined by a surface activation method, an amorphous layer may be formed between piezoelectric layers 14a and 14b. In this case, since the thickness of the amorphous layer is sufficiently smaller than the thickness of piezoelectric layers 14a and 14b, piezoelectric layers 14a and 14b can be considered to be directly joined.

[0023] An upper electrode 16 and a lower electrode 12 are provided above and below the piezoelectric layer 14, respectively, so as to sandwich the piezoelectric layer 14. A temperature compensation film 18a is provided between the lower electrode 12 and the piezoelectric layer 14a, and a temperature compensation film 18b is provided between the piezoelectric layer 14b and the upper electrode 16. The region where the lower electrode 12 and the upper electrode 16 overlap in a plan view, with at least a portion of the piezoelectric layer 14a and at least a portion of the piezoelectric layer 14b sandwiched between them, is the resonance region 50. In a plan view, the temperature compensation films 18a and 18b are provided over the entire surface of the resonance region 50. It is sufficient that the temperature compensation films 18a and 18b are provided in at least a portion of the resonance region 50. An air gap 34 is provided between the substrate 10 and the lower electrode 12. Elastic waves are reflected at the interface between the lower electrode 12 and the air gap 34. In a plan view, the air gap 34 overlaps the resonance region 50, and the air gap 34 is the same size as or larger than the resonance region 50.

[0024] The substrate 10 is, for example, a silicon substrate, sapphire substrate, alumina substrate, spinel substrate, quartz substrate, crystal substrate, glass substrate, ceramic substrate, or GaAs substrate. The lower electrode 12 and upper electrode 16 are, for example, monolayer films of ruthenium (Ru), chromium (Cr), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Ir), or multilayer films of these films.

[0025] The piezoelectric layers 14a and 14b are, for example, a lithium niobate single crystal substrate, a lithium tantalate single crystal substrate, or a quartz single crystal substrate. The piezoelectric layers 14a and 14b may also be polycrystalline, such as an aluminum nitride layer. The temperature compensation films 18a and 18b have a temperature coefficient of elasticity with the opposite sign to that of the temperature coefficient of elasticity of the piezoelectric layers 14a and 14b. The sign of the temperature coefficient of elasticity of a piezoelectric material is generally negative. Therefore, the temperature coefficient of elasticity of the temperature compensation films 18a and 18b is positive. Such materials include silicon oxide (SiO2) or silicon oxide containing fluorine (SiOF). Therefore, the temperature compensation films 18a and 18b are, for example, a silicon oxide film or a silicon oxide film containing fluorine.

[0026] Figure 2 shows the crystal orientation of the piezoelectric layer in Example 1. The X, Y, and Z axes are the axes of the crystal orientation, and the X, Y, and Z directions are the directions shown in Figures 1(a) and 1(b). The figures show the case where a 163° rotated Y-cut lithium niobate substrate is used as the piezoelectric layer 14a and 14b, or the case where a 163° rotated Y-cut lithium tantalate substrate is used as the piezoelectric layer 14a and 14b.

[0027] As shown in Figure 2, 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 direction, and are in the X direction. In piezoelectric layer 14a, the +Y axis direction and the +Z axis direction are designated as the -Z direction and +Y direction, respectively, as shown by the dashed arrows. The Y axis and Z axis are rotated by θ = 163° around the X axis from the +Z axis direction to the +Y axis direction. The lithium tantalate substrate and the lithium niobate substrate have a crystal 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 upper and lower surfaces of piezoelectric layer 14a are referred to as the - plane and the + plane, respectively, and the polarization direction 52a is shown pointing downwards.

[0028] In piezoelectric layer 14b, the +Y axis direction and the +Z axis direction are defined as the +Z direction and -Y direction, respectively, as shown by the dashed arrows. The Y and Z axes are rotated by θ = 163° around the X axis from the +Z axis direction to the +Y axis direction. At this time, the +Z axis direction corresponds to the c axis direction of the crystal orientation. The +Z axis direction is the direction of spontaneous polarization. At this time, the upper and lower surfaces of piezoelectric layer 14b are referred to as the + plane and - plane, respectively, and the polarization direction 52b is shown pointing upward. The direction of spontaneous polarization of piezoelectric layers 14a and 14b can be confirmed by identifying the Z axis direction using X-ray diffraction.

[0029] Spontaneous polarization is a state of polarization that occurs without the application of an electric field. In lithium tantalate, Li and Ta become cations, and O becomes an anion. If the Li, Ta, and O atoms were positioned to cancel each other's charges, spontaneous polarization would not occur. Spontaneous polarization occurs because these atoms are misaligned from their charge-canceling positions. Spontaneous polarization also occurs in lithium niobate in a similar manner.

[0030] By using a 163° rotated lithium niobate substrate or a lithium tantalate substrate as the piezoelectric layer 14, the main mode of elastic waves excited in the piezoelectric layer 14 becomes thickness-slip vibration, and thickness-longitudinal vibration elastic waves are hardly excited within the piezoelectric layer 14. This suppresses spurious emissions caused by thickness-longitudinal vibration.

[0031] By aligning the +X axis direction of the piezoelectric layer 14a with the +X axis direction of the piezoelectric layer 14b, the direction of spontaneous polarization of the piezoelectric layer 14a (+Z axis direction) and the direction of spontaneous polarization of the piezoelectric layer 14b (+Z axis direction) are approximately opposite. For example, if the +X axis direction of the piezoelectric layer 14a and the X axis direction of the piezoelectric layer 14b are different, the direction of spontaneous polarization of the piezoelectric layer 14a (+Z axis direction) and the direction of spontaneous polarization of the piezoelectric layer 14b (+Z axis direction) will not be opposite. The fact that the direction of spontaneous polarization of the piezoelectric layer 14a (+Z axis direction) and the direction of spontaneous polarization of the piezoelectric layer 14b (+Z axis direction) are opposite (approximately opposite) means that the angle between the +X axis direction of the piezoelectric layer 14a and the +X axis direction of the piezoelectric layer 14b is allowed to some extent within a range of 5° or less.

[0032] [Simulation 1] First, simulations were performed using the two-dimensional finite element method with an anisotropy-considered model for resonators A1-A3, which use the fundamental wave as the main mode, and resonators B1-B3, which use the second harmonic as the main mode. Resonators A1-A3, B1, and B2 correspond to comparative examples, while resonator B3 corresponds to an example.

[0033] Figures 3(a) to 3(c) are schematic diagrams of piezoelectric thin-film resonators in resonators A1 to A3. The right-hand figure in Figure 3(a) shows the displacement of thickness-slip vibration in the thickness direction (Z direction) of the piezoelectric layer 14. The lower surface of the lower electrode 12 is set to Z=0.

[0034] As shown in Figure 3(a), in resonator A1, the piezoelectric layer 14 is a single piezoelectric layer with a polarization direction 52 in one direction. No temperature compensation film is provided. The thicknesses of the lower electrode 12, piezoelectric layer 14, and upper electrode 16 are T12, T14, and T16, respectively. As shown in the figure on the right, in the case of the fundamental wave, when the displacement of the lower surface of the lower electrode 12 is 0, the displacement of the upper surface of the upper electrode 16 is E. As a result, the potentials of the lower electrode 12 and the upper electrode 16 are different, and when an AC signal of a predetermined frequency is applied between the lower electrode 12 and the upper electrode 16, the piezoelectric layer 14 is excited by the fundamental wave. On the other hand, in the case of the second harmonic, the potentials of the lower electrode 12 and the upper electrode 16 are almost the same. Therefore, the piezoelectric layer 14 is hardly excited by the second harmonic. The sum of the thicknesses of the lower electrode 12, piezoelectric layer 14, and upper electrode 16, T12 + T14 + T16, is approximately half the wavelength of the fundamental wave.

[0035] As shown in Figure 3(b), in the resonator A2, the piezoelectric layer 14 comprises piezoelectric layers 14a and 14b, and the polarization direction 52a of piezoelectric layer 14a and the polarization direction 52b of piezoelectric layer 14b are in the same direction. The temperature compensation film 18 is provided between the piezoelectric layers 14a and 14b. The thicknesses of the lower electrode 12, piezoelectric layers 14a and 14b, temperature compensation film 18, and upper electrode 16 are T12, T14a, T14b, T18, and T16, respectively.

[0036] As shown in Figure 3(c), in resonator A3, the piezoelectric layer 14 is a single piezoelectric layer with a polarization direction 52 in one direction. The temperature compensation film 18a is provided between the lower electrode 12 and the piezoelectric layer 14, and the temperature compensation film 18b is provided between the piezoelectric layer 14 and the upper electrode 16. The thicknesses of the lower electrode 12, piezoelectric layer 14, temperature compensation films 18a and 18b, and upper electrode 16 are T12, T14, T18a, T18b, and T16, respectively.

[0037] Figures 4(a) to 4(c) are schematic diagrams of piezoelectric thin-film resonators in resonators B1 to B3. The right-hand figure in Figure 4(a) shows the displacement E of the thickness-slip vibration in the thickness direction (Z direction) of the piezoelectric layer 14.

[0038] As shown in Figure 4(a), in the resonator B1, the piezoelectric layer 14 comprises stacked piezoelectric layers 14a and 14b. The polarization direction 52a of piezoelectric layer 14a is downward, and the polarization direction 52b of piezoelectric layer 14b is upward. As shown in the right figure of Figure 4(a), at the second harmonic, when the displacement of the lower surface of the lower electrode 12 is zero, the displacement of the upper surface of the upper electrode 16 is also zero. The displacement at the interface between piezoelectric layers 14a and 14b is E. The polarization direction 52a of piezoelectric layer 14a and the polarization direction 52b of piezoelectric layer 14b are in opposite directions. Therefore, the direction of displacement with respect to the polarity (positive / negative) of the potential applied to the lower electrode 12 is opposite to the direction of displacement with respect to the polarity (positive / negative) of the potential applied to the lower electrode 16. For example, when the upper electrode 16 is at a positive potential and the lower electrode 12 is at a negative potential, the piezoelectric layer 14a has a displacement of 0 on the negative potential side, and the piezoelectric layer 14b has a displacement of 0 on the positive potential side. In other words, when an AC signal of a predetermined frequency is applied between the lower electrode 12 and the upper electrode 16, the second harmonic is excited. On the other hand, in order to excite the fundamental wave in the structure shown in the left diagram of Figure 4(a), the displacements of the part of the piezoelectric layer 14b on the upper electrode 16 side and the part of the piezoelectric layer 14a on the lower electrode 12 side must be reversed (for example, as shown in the right diagram of Figure 3(a), the displacement is set to E on the upper electrode 16 side and the displacement is set to 0 on the lower electrode 12 side of the piezoelectric layer 14a). To achieve such a displacement, the polarity of the potential of the upper electrode 16 and the polarity of the potential of the lower electrode 12 must be made the same. In this way, when the upper electrode 16 and the lower electrode 12 have the same potential polarity, an electric field is not generated in the piezoelectric layer 14, and the fundamental wave is hardly excited. The total thickness T12 + T14a + T14b + T16 of the lower electrode 12, piezoelectric layers 14a and 14b, and upper electrode 16 is approximately equal to the wavelength of the second harmonic.

[0039] As shown in Figure 4(b), in resonator B2, the temperature compensation film 18 is provided between the piezoelectric layers 14a and 14b. As shown in Figure 4(c), in resonator B3, the temperature compensation film 18a is provided between the lower electrode 12 and the piezoelectric layer 14a, and the temperature compensation film 18b is provided between the piezoelectric layer 14b and the upper electrode 16.

[0040] The conditions for Simulation 1 for resonators A1-A3 and B1-B3 are as follows: Piezoelectric layers 14, 14a, 14b: 163° rotated Y-cut lithium niobate substrate Lower electrode 12, upper electrode 16: Aluminum film Temperature compensation films 18, 18a, and 18b: Silicon oxide (SiO2) films

[0041] Table 1 shows the thickness of each layer in resonators A1-A3 and B3-B3, the resonant frequency fr, the anti-resonant frequency fa, and the electromechanical coupling coefficient k. 2 The temperature coefficient of velocity (TCV) at the resonant frequency fr is also shown. The resonant frequencies fr and anti-resonant frequencies fa of resonators A1 to A3 are the fundamental resonant frequencies and anti-resonant frequencies, respectively, while the resonant frequencies fr and anti-resonant frequencies fa of resonators B1 to B3 are the second harmonic resonant frequencies and anti-resonant frequencies, respectively. [Table 1]

[0042] The temperature coefficient of frequency (TCF) of a piezoelectric thin-film resonator is determined by the temperature dependence of the sound velocity of elastic waves due to the temperature rate coefficient (TCV), and the change in the thickness of the piezoelectric layer 14 due to temperature. The temperature coefficient of frequency (TCF) is not determined solely by the temperature rate coefficient (TCV), but by bringing the temperature rate coefficient (TCV) closer to zero, the temperature coefficient of frequency (TCF) can be brought closer to zero.

[0043] As shown in Table 1, T12, T16, T14, T14a, and T14b in resonators B1 to B3 are doubles of T12, T16, T14, T14a, and T14b in resonators A1 to A3, respectively. The resonant frequency fr is approximately the same for resonators B1 to B3 and their corresponding resonators A1 to A3, and the anti-resonant frequency fa is also approximately the same.

[0044] In resonators A1 to A3, to fabricate piezoelectric thin-film resonators with high resonant frequencies fr and anti-resonant frequencies fa, the thickness T14 of the piezoelectric layer 14 must be reduced. However, because an air gap 34 exists between the piezoelectric layer 14 and the support substrate, reducing the thickness of the piezoelectric layer 14 makes it more susceptible to damage. Furthermore, variations in the manufacturing thickness of the piezoelectric layer 14 can easily lead to variations in characteristics such as the resonant frequency fr and anti-resonant frequency fa. In resonators B1 to B3, when fabricating piezoelectric thin-film resonators with the same resonant frequencies fr and anti-resonant frequencies fa as resonators A1 to A3, the thickness of the piezoelectric layer 14 can be approximately twice that of resonators A1 to A3. This can suppress damage to the piezoelectric layer 14. It can also suppress variations in characteristics such as the resonant frequency fr and anti-resonant frequency fa.

[0045] Figures 5(a) to 5(c) show the normalized elastic strain energy with respect to position Z in the thickness direction for resonators A1 to A3, respectively. Figures 6(a) to 6(c) show the normalized elastic strain energy with respect to position Z in the thickness direction for resonators B1 to B3, respectively. For resonators A1 to A3, the normalized elastic strain energy at the fundamental wave resonance frequency is shown, and for resonators B1 to B3, the normalized elastic strain energy at the second harmonic resonance frequency is shown. Normalized elastic strain energy is elastic strain energy normalized so that the maximum elastic strain energy is 1. The normalized elastic strain energy changes in a step-like manner with respect to Z. This is due to the mesh of the finite element method, and it is thought that the actual normalized elastic strain energy changes smoothly with respect to Z.

[0046] As shown in Figure 5(a), in resonator A1, the elastic strain energy at the lower electrode 12 and the upper electrode 16 is small, and the elastic strain energy is maximum at the center of the piezoelectric layer 14 in the Z direction. As shown in Table 1, the electromechanical coupling coefficient k in resonator A1 is small. 2 Although the coefficient is large, the absolute value of the temperature rate coefficient (TCV) is also large.

[0047] As shown in Figure 5(b), in resonator A2, the temperature compensation film 18 is placed in a location with high elastic strain energy, resulting in high elastic strain energy in the temperature compensation film 18. As shown in Table 1, the absolute value of the temperature rate coefficient TCV in resonator A2 is smaller than that in resonator A1. Electromechanical coupling coefficient k 2 It is smaller than resonator A1.

[0048] As shown in Figure 5(c), in resonator A3, the temperature compensation films 18a and 18b are placed in areas with low elastic strain energy, and the elastic strain energy in the temperature compensation films 18a and 18b is not as large as in resonator A2. As shown in Table 1, the absolute value of the temperature rate coefficient TCV in resonator A3 is smaller than that of resonator A1, but larger than that of resonator A2. Electromechanical coupling coefficient k 2 It is comparable to that of resonator A2.

[0049] Thus, in resonators A1 to A3, the absolute value of the temperature rate coefficient can be reduced by providing a temperature compensation film 18 between the piezoelectric layers 14a and 14b, as in resonator A2. This is thought to be because, in resonator A2, the temperature compensation film 18 is provided in a location where the elastic strain energy is large.

[0050] As shown in Figure 6(a), in resonator B1, the elastic strain energy at the lower electrode 12 and the upper electrode 16 is small, and the elastic strain energy between the piezoelectric layers 14a and 14b is also small. The elastic strain energy is maximum at the center of the piezoelectric layer 14a in the Z direction and at the center of the piezoelectric layer 14b in the Z direction. As shown in Table 1, the electromechanical coupling coefficient k in resonator B1 is small. 2 Although the coefficient is large, the absolute value of the temperature rate coefficient (TCV) is also large.

[0051] As shown in Figure 6(b), in resonator B2, the temperature compensation film 18 is placed in a region with low elastic strain energy, resulting in low elastic strain energy in the temperature compensation film 18. As shown in Table 1, the absolute value of the temperature rate coefficient TCV in resonator B2 is smaller than that of resonator B1, but not as small as that of resonator A2. Electromechanical coupling coefficient k 2 It is smaller than resonator B1.

[0052] As shown in Fig. 6(c), in resonator B3, the temperature compensation films 18a and 18b are provided at positions with larger elastic strain energy than the temperature compensation film 18 of resonator B2, and the elastic strain energy in the temperature compensation films 18a and 18b is larger than that of resonator B2. As shown in Table 1, in resonator B3, the absolute value of the temperature coefficient of velocity TCV becomes smaller than that of resonator B2. The electromechanical coupling coefficient k 2 is slightly smaller than that of resonator B2.

[0053] Thus, in resonators B1 to B3, like resonator B3, by providing the temperature compensation films 18a and 18b between the lower electrode 12 and the piezoelectric layer 14a and between the piezoelectric layer 14b and the upper electrode 16, the absolute value of the temperature coefficient of velocity can be reduced. This is considered to be because in resonator B3, the temperature compensation films 18a and 18b are provided at positions with larger elastic strain energy compared to resonator B2.

[0054] [Simulation 2] Regarding resonators A2, A3, B2, and B3, the thicknesses T18, T18a, and T18b of the temperature compensation films 18, 18a, and 18b were changed, and the temperature coefficient of velocity TCV and the electromechanical coupling coefficient k 2 were simulated. In Simulation 2, as the piezoelectric layers 14, 14a, and 14b, a 165° rotated Y-cut lithium niobate substrate and a 165° rotated Y-cut lithium tantalate substrate were used. Other simulation conditions were the same as those in Simulation 1. For resonators A2 and A3, the temperature coefficient of velocity TCV and the electromechanical coupling coefficient k 2 at the fundamental resonance frequency were simulated, and for resonators B2 and B3, the temperature coefficient of velocity TCV and the electromechanical coupling coefficient k 2 at the second harmonic resonance frequency were simulated.

[0055] Figs. 7(a) and 7(b) show the temperature coefficient of velocity TCV and the electromechanical coupling coefficient k 2These figures illustrate the respective values. Figures 8(a) and 8(b) show the temperature rate coefficient TCV and electromechanical coupling coefficient k for the thicknesses T18a and T18b of resonators A3 and B3 in Simulation 2. 2 These are diagrams illustrating the respective conditions. In Figures 7(a) and 7(b), when the thickness T18 is 0, resonators A2 and B2 correspond to resonators A1 and B1, respectively. In Figures 8(a) and 8(b), when the thicknesses T18a and T18b are 0, resonators A3 and B3 correspond to resonators A1 and B1, respectively. LN indicates that the piezoelectric layers 14, 14a and 14b are 165° rotated Y-cut lithium niobate substrates, and LT indicates that the piezoelectric layers 14, 14a and 14b are 165° rotated Y-cut lithium tantalate substrates.

[0056] As shown in Figure 7(a), in resonator A2, increasing the thickness T18 of both LN and LT increases the temperature rate coefficient TCV, changing from negative to positive, and the temperature rate coefficient TCV becomes almost 0 when the thickness T18 is around 10 nm. In resonator B2, increasing the thickness T18 of LT decreases the temperature rate coefficient TCV, while increasing the thickness T18 of LN increases the temperature rate coefficient TCV. In both LT and LT, increasing the thickness T18 does not make the temperature rate coefficient 0.

[0057] As shown in Figure 7(b), in both resonators A2 and B2, the electromechanical coupling coefficient k increases as the thickness T18 increases. 2 The electromechanical coupling coefficient k becomes smaller. When the thickness T18 is the same, the electromechanical coupling coefficient k of resonator B2 is smaller than that of resonator A2. 2 Although the coefficient is large, the thickness of the piezoelectric layer in resonator B2 is approximately twice that of the piezoelectric layer in resonator A2, and the electromechanical coupling coefficient k between resonators A2 and B2 is large. 2 They cannot be simply compared.

[0058] As shown in Figures 7(a) and 7(b), in a resonator where the fundamental wave is the main mode, the absolute value of the temperature rate coefficient TCV can be reduced by providing a temperature compensation film 18 between the piezoelectric layers 14a and 14b, as in resonator A2. In a resonator where the second harmonic is the main mode, even if a temperature compensation film 18 is provided between the piezoelectric layers 14a and 14b, as in resonator B2, the absolute value of the temperature rate coefficient TCV cannot be brought close to zero.

[0059] This is thought to be because, as shown in Figure 5(b), in resonator A2, the temperature compensation film 18 is placed in a location with high elastic strain energy, whereas, as shown in Figure 6(b), in resonator B2, the temperature compensation film 18 is placed in a location with low elastic strain energy. Thus, in resonator B2, which has the second harmonic as the main mode, the elastic strain energy is small in the area between the piezoelectric layers 14a and 14b, and the temperature compensation effect is small. This finding was first obtained by the inventors through simulations.

[0060] As shown in Figure 8(a), in both resonators A3 and B3, increasing the thicknesses T18a and T18b in the LN region increases the temperature rate coefficient TCV, changing from negative to positive. In the LT region, increasing the thicknesses T18a and T18b decreases the temperature rate coefficient TCV, but then it increases again, changing from negative to positive. In resonator A3, the temperature rate coefficient TCV is almost zero for both LN and LT when the thicknesses T18a and T18b are between 30 nm and 40 nm. In resonator B3, the temperature rate coefficient TCV is almost zero for both LN and LT when the thicknesses T18a and T18b are 20 nm.

[0061] As shown in Figure 8(b), increasing the thicknesses T18a and T18b of both resonators A3 and B3 increases the electromechanical coupling coefficient k 2 When thicknesses T18a and T18b are the same, the electromechanical coupling coefficient k of resonator B3 becomes smaller. 2 The electromechanical coupling coefficient k of resonator A3 is 2It becomes larger. In resonator A3, the electromechanical coupling coefficient k is at thicknesses T18a and T18b of 30 nm to 40 nm (see Figure 8(a)), where the temperature rate coefficient TCV is 0. 2 It is 5% or less. In resonator B3, the electromechanical coupling coefficient k is at a thickness of 20 nm (see Figure 8(a)) where the temperature rate coefficient TCV is 0, T18a and T18b. 2 This is 10% or more. Thus, in resonator B3, compared to resonator A3, the electromechanical coupling coefficient k at thicknesses T18a and T18b where the temperature rate coefficient TCV is 0 is 2 It's big.

[0062] As shown in Figures 8(a) and 8(b), in a resonator where the fundamental wave is the main mode, if we try to reduce the absolute value of the temperature rate coefficient TCV by providing temperature compensation films 18a and 18b, as in resonator A3, the electromechanical coupling coefficient k 2 The coefficient becomes smaller. On the other hand, in a resonator with the second harmonic as the main mode, as in resonator B3, by providing a temperature compensation film 18a between the lower electrode 12 and the piezoelectric layer 14a, and a temperature compensation film 18b between the piezoelectric layer 14b and the upper electrode 16, the electromechanical coupling coefficient k 2 This can suppress the decrease and reduce the absolute value of the temperature rate coefficient TCV.

[0063] As shown in Figures 6(a) to 6(c), in resonators B1 to B3, where the second harmonic is the dominant mode, the elastic strain energy is smallest between the piezoelectric layers 14a and 14b. Although the elastic strain energy is smallest on the lower surface of the lower electrode 12 and the upper surface of the upper electrode 16, the elastic strain energy is large between the lower electrode 12 and the piezoelectric layer 14a, and between the piezoelectric layer 14b and the upper electrode 16, corresponding to the thickness T12 of the lower electrode 12 and the thickness T16 of the upper electrode 16. For this reason, it is considered that in resonator B3, by providing temperature compensation films 18a and 18b, the absolute value of the temperature rate coefficient TCV can be reduced compared to resonator B2. Thus, in a resonator with the second harmonic as the dominant mode, the elastic strain energy is greater between the lower electrode 12 and the piezoelectric layer 14a, and between the piezoelectric layer 14b and the upper electrode 16, than between the piezoelectric layers 14a and 14b. The fact that the temperature compensation effect is enhanced by providing temperature compensation films 18a and 18b is a finding obtained for the first time through simulations conducted by the inventors.

[0064] As shown in Figures 6(a) to 6(c), the elastic strain energy is maximum at the center of the piezoelectric layer 14a in the Z direction and at the center of the piezoelectric layer 14b in the Z direction. Therefore, it is thought that the temperature characteristics of the piezoelectric thin-film resonator can be improved by providing a temperature compensation film at the center of the piezoelectric layer 14a in the Z direction and at the center of the piezoelectric layer 14b in the Z direction. However, in this case, four layers of piezoelectric layers 14a and 14b would be stacked, which would increase the manufacturing process and costs.

[0065] According to Example 1, the piezoelectric layer 14a (first piezoelectric layer) and the piezoelectric layer 14b (second piezoelectric layer) are laminated. The piezoelectric layer 14b has a spontaneous polarization direction that is approximately opposite to the spontaneous polarization direction of the piezoelectric layer 14a. The lower electrode 12 (first electrode) is provided on the side of the piezoelectric layer 14a opposite to the piezoelectric layer 14b. The upper electrode 16 (second electrode) is provided on the side of the piezoelectric layer 14b opposite to the piezoelectric layer 14a, and sandwiches at least a portion of the piezoelectric layer 14a and at least a portion of the piezoelectric layer 14b between the lower electrode 12 and the upper electrode 16. The temperature compensation films 18a and 18b (insulating films) are provided between the piezoelectric layer 14a and the lower electrode 12 and between the piezoelectric layer 14b and the upper electrode 16. As a result, the temperature rate coefficient TCV can be reduced, as in the resonator B3, and the temperature characteristics of the piezoelectric thin-film resonator can be improved. Furthermore, since only two piezoelectric layers 14a and 14b need to be laminated, it is possible to reduce the manufacturing process and costs compared to laminating four piezoelectric layers.

[0066] At least one of the temperature compensation films 18a and 18b is required. By providing both temperature compensation films 18a and 18b, a greater temperature compensation effect can be obtained.

[0067] The piezoelectric layers 14a and 14b are either a lithium niobate substrate rotated at 158° or more and 168° or less using the Y-cut method, or a lithium tantalate substrate rotated at 158° or more and 168° or less using the Y-cut method. This suppresses spurious emissions caused by the fundamental wave mode. Furthermore, the results of simulations 1 and 2 can be applied.

[0068] In order to make the direction of spontaneous polarization of the piezoelectric layer 14a and the direction of spontaneous polarization of the piezoelectric layer 14b opposite (approximately opposite), the angle between the X-axis direction of the crystal orientation in the piezoelectric layer 14a and the X-axis direction of the crystal orientation in the piezoelectric layer 14b is preferably 5° or less, and more preferably 2° or less.

[0069] The temperature compensation films 18a and 18b are silicon oxide films or silicon oxide films containing fluorine. That is, the temperature compensation films 18a and 18b are mainly composed of silicon oxide. The statement that the temperature compensation films 18a and 18b are mainly composed of silicon oxide means that they may contain impurities (e.g., fluorine) that are intentionally or unintentionally added. The combined oxygen concentration and silicon concentration in the temperature compensation films 18a and 18b are, for example, 50 atomic% or more, 80 atomic% or more, or 90 atomic% or more, and the oxygen concentration and silicon concentration are, for example, 10 atomic% or more, or 20 atomic% or more, respectively.

[0070] As shown in Figure 8(a), when the thicknesses T18a and T18b are 10 nm or more, the temperature rate coefficient TCV largely depends on the thicknesses T18a and T18b. The total thickness T14a + T14b of the piezoelectric layers 14a and 14b is 300 nm. Therefore, the thickness T18a or T18b of the temperature compensation film 18a or 18b is preferably 1 / 30 or more, more preferably 1 / 20 or more, and even more preferably 1 / 15 or more, of the total thickness T14a + T14b of the piezoelectric layers 14a and 14b. The thickness T18a or T18b is preferably 1 / 5 or less of the total thickness T14a + T14b. As a result, as shown in Figure 8(b), the electromechanical coupling coefficient k 2 This can suppress the decrease in temperature. When both temperature compensation films 18a and 18b are provided, the total thickness T18a + T18b of the temperature compensation films 18a and 18b is preferably 1 / 15 or more of the total thickness T14a + T14b, and more preferably 1 / 10 or more. The thickness T18a or T18b is preferably 1 / 2.5 or less of the total thickness T14a + T14b.

[0071] If the thickness T14a of the piezoelectric layer 14a and the thickness T14b of the piezoelectric layer 14b are different, spurious emissions will occur due to the elastic waves of the fundamental wave mode. Also, the electromechanical coupling coefficient k 2The effect decreases. Therefore, it is preferable that the thicknesses T14a and T14b are the same. Thicknesses T14a and T14b being the same (approximately the same) means that a difference in thickness between T14a and T14b is allowed to the extent that spurious emissions caused by elastic waves of the fundamental wave mode do not become a problem. For example, it is sufficient if 2 × |T14a - T14b| / (T14a + T14b) is 0.1 or less.

[0072] The piezoelectric layers 14a and 14b are directly joined, 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). This allows the piezoelectric layer 14 to be excited by a second harmonic. [Examples]

[0073] Figure 9(a) is a plan view of the piezoelectric thin-film resonator according to Example 2, and Figure 9(b) is a cross-sectional view AA of Figure 9(a). As shown in Figures 9(a) and 9(b), the thickness T14a of the piezoelectric layer 14a and the thickness T14b of the piezoelectric layer 14b are different. A temperature compensation film 18 is provided between the piezoelectric layers 14a and 14b. No temperature compensation film is provided between the lower electrode 12 and the piezoelectric layer 14a, and between the piezoelectric layer 14b and the upper electrode 16. The other configurations are the same as in Example 1 and will not be described.

[0074] [Simulation 3] For resonators C1 and C2, which have the second harmonic as the dominant mode, simulations were performed using the two-dimensional finite element method with an anisotropic model. Resonator C1 corresponds to a comparative example, and resonator C2 corresponds to an example.

[0075] The schematic diagram of the piezoelectric thin-film resonator in resonator C1 is the same as that of resonator B2 in Figure 4(b). Figure 10(a) is a schematic diagram of the piezoelectric thin-film resonator in resonator C2. As shown in Figure 10(a), in resonator C2, the thickness T14a of the piezoelectric layer 14a is greater than the thickness T14b of the piezoelectric layer 14b. The other configurations are the same as those of resonator B2.

[0076] The conditions for Simulation 1 of resonators C1 and C2 are as follows: Piezoelectric layers 14a, 14b: 165° rotated Y-cut lithium niobate substrate Lower electrode 12, upper electrode 16: Aluminum film Temperature compensation film 18: Silicon oxide (SiO2) film

[0077] Table 2 shows the thickness T12 of the lower electrode 12 in resonators C1 and C2, the thickness T16 of the upper electrode 16, the total thickness T14a + T14b of the piezoelectric layers 14a and 14b, the ratio T14b / T14a of thickness T14a, the thickness T18 of the temperature compensation film 18, the resonant frequency fr, the anti-resonant frequency fa, and the electromechanical coupling coefficient k. 2 The temperature rate coefficient TCV at the resonant frequency fr is also shown. The resonant frequency fr and anti-resonant frequency fa are the resonant frequency and anti-resonant frequency at the second harmonic. [Table 2]

[0078] As shown in Table 2, the thickness T14b + T14a of resonator C2 is the same as the thickness T14a + T14b of resonator C1, making the ratio of the thicknesses of resonator C2 T14b / T14a smaller than the ratio T14b / T14a of resonator C1.

[0079] Figure 10(b) shows the normalized elastic strain energy with respect to position Z in the thickness direction in resonator C2. As shown in Figure 10(b), in resonator C2, the temperature compensation film 18 is provided at locations with large elastic strain energy, and the elastic strain energy in the temperature compensation film 18 is greater than that of resonator B2 in Figure 6(b). As shown in Table 2, the absolute value of the temperature rate coefficient TCV in resonator C2 is smaller than that of resonator C1. Electromechanical coupling coefficient k 2 It is slightly smaller than the resonator C1.

[0080] For the resonator C2, the ratio of thicknesses T14b / T14a is changed, and the electromechanical coupling coefficient k 2 The temperature rate coefficient (TCV) was also simulated. Piezoelectric layers 14a and 14b are 165° rotated Y-cut lithium niobate substrates.

[0081] Figures 11(a) and 11(b) show the temperature rate coefficient TCV and electromechanical coupling coefficient k for the ratio of the thicknesses T14b / T14a of the resonator C2 in Simulation 3. 2 This figure shows the result. When the thickness ratio T14b / T14a is 1, it corresponds to the resonator C1.

[0082] As shown in Figure 11(a), the absolute value of the temperature rate coefficient TCV decreases as the thickness ratio T14b / T14a decreases. When the thickness ratio T14b / T14a is less than 0.3, the temperature rate coefficient TCV becomes almost constant. This is thought to be because, when the thickness ratio T14b / T14a is 1 / 3, the temperature compensation film 18 is provided at the location where the elastic strain energy is maximum.

[0083] As shown in Figure 11(b), reducing the thickness ratio T14b / T14a reduces the electromechanical coupling coefficient k 2 The spurious emissions decrease. This is thought to be due to the presence of the temperature compensation film 18, as well as the difference in thickness between T14a and T14b. Furthermore, as the thickness ratio T14b / T14a decreases, the spurious emissions caused by the fundamental wave mode increase.

[0084] Thus, by making the thickness ratio T14b / T14a less than 1, the elastic strain energy increases in the region between the piezoelectric layers 14a and 14b. Therefore, by providing a temperature compensation film 18 between the piezoelectric layers 14a and 14b, the temperature compensation effect is increased. This is a finding that was first obtained by the inventors through simulations.

[0085] According to Example 2, the thickness T14b of the piezoelectric layer 14b is different from the thickness T14a of the piezoelectric layer 14a, and the temperature compensation film 18 (insulating film) is provided between the piezoelectric layers 14a and 14b. This makes it possible to reduce the temperature rate coefficient TCV, as in the resonator C2, and improve the temperature characteristics of the piezoelectric thin-film resonator. Furthermore, since only two layers of piezoelectric layers 14a and 14b need to be stacked, it is possible to reduce the manufacturing process and costs compared to stacking four piezoelectric layers.

[0086] In the example of resonator C2, we described a case where the thickness ratio T14b / T14a is less than 1, but the thickness ratio T14a / T14b may also be less than 1.

[0087] The piezoelectric layers 14a and 14b are either a lithium niobate substrate rotated at 158° or more and 168° or less using the Y-cut method, or a lithium tantalate substrate rotated at 158° or more and 168° or less using the Y-cut method. This suppresses spurious emissions caused by the fundamental wave mode. Furthermore, the results of Simulation 3 can be applied.

[0088] In order to make the direction of spontaneous polarization of the piezoelectric layer 14a and the direction of spontaneous polarization of the piezoelectric layer 14b opposite (approximately opposite), the angle between the X-axis direction of the crystal orientation in the piezoelectric layer 14a and the X-axis direction of the crystal orientation in the piezoelectric layer 14b is preferably 5° or less, and more preferably 2° or less.

[0089] The temperature compensation film 18 is a silicon oxide film or a silicon oxide film containing fluorine. That is, the temperature compensation film 18 is mainly composed of silicon oxide. The sum of the oxygen concentration and silicon concentration in the temperature compensation film 18 is, for example, 50 atomic% or more, 80 atomic% or more, or 90 atomic% or more, and the oxygen concentration and silicon concentration are, for example, 10 atomic% or more, or 20 atomic% or more, respectively.

[0090] When the thickness ratio T14b / T14a is 0.9 or less, the absolute value of the temperature rate coefficient TCV can be reduced, improving the temperature characteristics. As shown in Figure 11(a), when the thickness ratio T14b / T14a is 0.8 or more, the absolute value of the temperature rate coefficient TCV does not change much even when the thickness ratio T14b / T14a changes, but when it is 0.8 or less, the absolute value of the temperature rate coefficient TCV decreases rapidly as the thickness ratio T14b / T14a increases. Thus, by setting the thickness ratio T14b / T14a to 0.8 or less, the temperature characteristics can be further improved. By setting the thickness ratio T14b / T14a to 0.7 or less, the temperature characteristics can be further improved.

[0091] When the thickness ratio T14b / T14a is around 0.2, the absolute value of the temperature rate coefficient TCV does not change much even if the thickness ratio T14b / T14a is reduced, whereas the electromechanical coupling coefficient k 2 The coefficient of electromechanical coupling coefficient k becomes smaller. Therefore, the thickness ratio T14b / T14a is preferably 0.2 or more, and more preferably 0.3 or more. By setting the thickness ratio T14b / T14a to 0.5 or more, the electromechanical coupling coefficient k 2 This can further suppress the decline.

[0092] Figure 12(a) is a cross-sectional view of a piezoelectric thin-film resonator according to Modification 1 of Examples 1 and 2. As shown in Figure 12(a), 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 in Example 1 and will not be described. As in Modification 1 of Examples 1 and 2, the polarization direction 52a of the piezoelectric layer 14a and the polarization direction 52b of the piezoelectric layer 14b may be in approximately opposite directions. In Example 2, the polarization direction 52a of the piezoelectric layer 14a may be upward, and the polarization direction 52b of the piezoelectric layer 14b may be downward.

[0093] Figure 12(b) is a cross-sectional view of a piezoelectric thin-film resonator according to a modification 2 of Examples 1 and 2. As shown in Figure 12(b), instead of the air gap 34, an acoustic reflective film 30 is provided between the substrate 10 and the lower electrode 12. The acoustic reflective film 30 is made up of alternating layers of film 31 with low acoustic impedance and film 32 with high acoustic impedance. The thickness of films 31 and 32 is, for example, approximately λ / 4 (where λ is the wavelength of the elastic wave). As a result, the acoustic reflective film 30 reflects elastic waves. The number of layers of films 31 and 32 can be arbitrarily set. The acoustic reflective film 30 overlaps the resonance region 50, and the acoustic reflective film 30 is the same size as or larger than the resonance region 50. Film 31 of the acoustic reflective film 30 is, for example, silicon oxide, silicon nitride, etc. Film 32 is, for example, tungsten, tantalum, molybdenum, ruthenium, etc.

[0094] As in Examples 1 and 2 and their Modification 1, the piezoelectric thin-film resonator may be an FBAR in which the air gap 34 reflects elastic waves. As in Modification 2 of Examples 1 and 2, the piezoelectric thin-film resonator may be an SMR in which the acoustic reflective film 30 reflects elastic waves. In Examples 1 and 2 and their Modifications, examples in which the second harmonic is the dominant mode were described, but any even-order mode may be used as the dominant mode. Although an example in which the planar shape of the resonance region 50 is rectangular was described, the resonance region 50 may also be an elliptical or polygonal shape such as a pentagon. [Examples]

[0095] Example 3 is an example of a filter and duplexer using piezoelectric thin-film resonators of Examples 1 and 2 and their variations. Figure 13(a) is a circuit diagram of the filter according to Example 3. As shown in Figure 13(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. Piezoelectric thin-film resonators of Examples 1 and 2 and their variations can be used for at least one of the series resonators S1 to S4 and the parallel resonators P1 to P4. The number of resonators in the ladder-type filter can be set as appropriate.

[0096] Figure 13(b) is a circuit diagram of a duplexer according to Modification 1 of Example 3. As shown in Figure 13(b), a transmit filter 40 is connected between the common terminal Ant and the transmit terminal Tx. A receive filter 42 is connected between the common terminal Ant and the receive terminal Rx. The transmit filter 40 allows signals in the transmit band from the signal input from the transmit terminal Tx to pass to the common terminal Ant as the transmit signal, and suppresses signals of other frequencies. The receive filter 42 allows signals in the receive band from the signal input from the common terminal Ant to pass to the receive terminal Rx as the received signal, and suppresses signals of other frequencies. At least one of the transmit filter 40 and the receive filter 42 can be the filter of Example 3.

[0097] I used Duplexa as an example of a multiplexer, but Triplexa or Quadplexa would also work.

[0098] Although embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention as described in the claims. [Explanation of Symbols]

[0099] 10 circuit boards 12 Lower electrode 14, 14a, 14b Piezoelectric layer 16 Upper electrode 18, 18a, 18b Temperature compensation membrane 30 Acoustic reflective film 34 void 40 Transmission Filters 42 Receiving Filter 50 resonance area 52a, 52b Polarization direction

Claims

1. First piezoelectric layer and A second piezoelectric layer is laminated on the first piezoelectric layer and has a spontaneous polarization direction opposite to the spontaneous polarization direction of the first piezoelectric layer, A first electrode provided on the side of the first piezoelectric layer opposite to the second piezoelectric layer, A second electrode is provided on the side of the second piezoelectric layer opposite to the first piezoelectric layer, and sandwiches at least a portion of the first piezoelectric layer and at least a portion of the second piezoelectric layer with the first electrode, An insulating film is provided between the first piezoelectric layer and the first electrode and between the second piezoelectric layer and the second electrode, and the insulating film has an elastic constant temperature coefficient with a sign opposite to the sign of the elastic constant temperature coefficient of the first piezoelectric layer and the second piezoelectric layer, Equipped with, An elastic wave device in which, in a resonant region where the first electrode and the second electrode face each other across the first piezoelectric layer and the second piezoelectric layer, there is no metal film connected to ground between the first piezoelectric layer and the second piezoelectric layer.

2. The first piezoelectric layer and the second piezoelectric layer are Y-cut lithium niobate substrates rotated at 158° or more and 168° or less, or The elastic wave device according to claim 1, wherein the first piezoelectric layer and the second piezoelectric layer are lithium tantalate substrates rotated at 158° or more and 168° or less (Y-cut).

3. The elastic wave device according to claim 2, wherein the thickness of the insulating film is 1 / 30 times or more the combined thickness of the first piezoelectric layer and the second piezoelectric layer.

4. The elastic wave device according to any one of claims 1 to 3, wherein the thickness of the first piezoelectric layer and the second piezoelectric layer are the same.

5. The elastic wave device according to any one of claims 1 to 3, wherein the insulating film is provided both between the first piezoelectric layer and the first electrode and between the second piezoelectric layer and the second electrode.

6. First piezoelectric layer and A second piezoelectric layer is laminated on the first piezoelectric layer, has a thickness different from that of the first piezoelectric layer, and has a spontaneous polarization direction opposite to that of the first piezoelectric layer. A first electrode provided on the side of the first piezoelectric layer opposite to the second piezoelectric layer, A second electrode is provided on the side of the second piezoelectric layer opposite to the first piezoelectric layer, and sandwiches at least a portion of the first piezoelectric layer and at least a portion of the second piezoelectric layer with the first electrode, An insulating film is provided between the first piezoelectric layer and the second piezoelectric layer, having an elastic constant temperature coefficient with a sign opposite to that of the elastic constant temperature coefficients of the first piezoelectric layer and the second piezoelectric layer, Equipped with, An elastic wave device in which, in a resonant region where the first electrode and the second electrode face each other across the first piezoelectric layer and the second piezoelectric layer, there is no metal film connected to ground between the first piezoelectric layer and the second piezoelectric layer.

7. The first piezoelectric layer and the second piezoelectric layer are Y-cut lithium niobate substrates rotated at 158° or more and 168° or less, or The elastic wave device according to claim 6, wherein the first piezoelectric layer and the second piezoelectric layer are lithium tantalate substrates rotated at 158° or more and 168° or less (Y-cut).

8. The elastic wave device according to claim 7, wherein the ratio of the thickness of the second piezoelectric layer to the thickness of the first piezoelectric layer is 0.2 or more and 0.9 or less.

9. The elastic wave device according to any one of claims 2, 3, 7, and 8, wherein the angle between the X-axis direction of the crystal orientation in the first piezoelectric layer and the X-axis direction of the crystal orientation in the second piezoelectric layer is 5° or less.

10. The elastic wave device according to any one of claims 2, 3, 7, and 8, wherein the insulating film is mainly composed of silicon oxide.

11. A filter comprising an elastic wave device according to any one of claims 1 to 3 and 6 to 8.

12. A multiplexer comprising the filter described in claim 11.

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