Surface acoustic wave resonator and preparation method thereof

Abstract

The invention relates to the technical field of integrated circuit manufacturing, and particularly discloses a surface acoustic wave resonator and a preparation method thereof. The IDT electrode comprises a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer and a second electrode layer which are sequentially stacked on the piezoelectric substrate, and the acoustic impedance of the first electrode layer is smaller than that of the second electrode layer; and a protective film covering the IDT electrode. The first layer is made of the low-acoustic-impedance metal, the second layer is made of the high-acoustic-impedance metal, the size of the resonator can be smaller under the same thickness and frequency, the frequency-temperature characteristic is more excellent, and high-order stray is not prone to being generated.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuit manufacturing, and in particular to a surface acoustic wave resonator and a preparation method thereof. Background Art

[0002] With the miniaturization of electronic devices, the integration of multi-mode and multi-frequency functions, and the driving demands of cost and mass production, the size of filters needs to be further reduced. It is necessary to continuously promote technological innovation to form differentiated market competition, while breaking through the original technical barriers and forming new technical barriers.

[0003] Related literature and patents have proposed using a high-acoustic-impedance metal as the first layer of the IDT near the piezoelectric layer, which can significantly reduce the speed of sound v. According to the formula f = v / λ, as v decreases, the wavelength λ also decreases, which can reduce the filter area.

[0004] Based on this technical background, the present invention proposes a surface acoustic wave resonator and a method for preparing the same. Summary of the Invention

[0005] In response to the shortcomings of the existing technology, the present invention proposes a surface acoustic wave resonator and a preparation method thereof. The resonator uses a low-acoustic impedance metal as the first layer and a high-acoustic impedance metal as the second layer. Under the same thickness and frequency, the resonator can be smaller in size, have better frequency-temperature characteristics, and is less likely to generate high-order spurious signals.

[0006] In order to achieve the above object, the present invention provides a surface acoustic wave resonator in a first aspect, comprising:

[0007] Piezoelectric substrate;

[0008] The IDT electrode comprises a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer sequentially stacked on the piezoelectric substrate, wherein the acoustic impedance of the first electrode layer is smaller than the acoustic impedance of the second electrode layer;

[0009] A protective film covers the IDT electrode.

[0010] A second aspect of the present invention provides a method for preparing the above-mentioned resonator, comprising:

[0011] providing a piezoelectric substrate;

[0012] stacking a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer on the piezoelectric substrate in sequence, and setting the acoustic impedance of the first electrode layer to be smaller than the acoustic impedance of the second electrode layer;

[0013] The IDT electrode is covered with a protective film.

[0014] The technical effects of the present invention include:

[0015] (1) The surface acoustic wave resonator proposed in the present invention uses a low acoustic impedance metal as the first layer and a high acoustic impedance metal as the second layer. This can achieve a smaller resonator size, better frequency-temperature characteristics, and less prone to high-order spurious emissions at the same thickness and frequency.

[0016] (2) The surface acoustic wave resonator proposed in the present invention achieves a larger effective electromechanical coupling coefficient by using a low-angle substrate (0° to 30° or 160° to 180°). While suppressing high-order modal spurious conditions, it can accept a wider range of protective film thickness, thereby improving the resonator frequency temperature coefficient performance.

[0017] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0019] Figure 1 This is an enlarged schematic front cross-sectional view of the surface acoustic wave resonator proposed in the present invention.

[0020] Figure 2 (a) and (b) are respectively a front cross-section of the overall structure of the surface acoustic wave resonator proposed in the present invention and a top view of the IDT electrode.

[0021] Figure 3 This is a schematic diagram of the structure of the surface acoustic wave resonator proposed in the present invention applied to the radio frequency front-end filter.

[0022] Figure 4 Schematic diagram of the relationship between the effective electromechanical coupling coefficient (Keff2) and the cutting angle θ in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0023] Figure 5 This is a schematic diagram of the relationship between a typical cut angle and impedance in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0024] Figure 6-9 Schematic diagrams of the relationship between the SiO2 film thickness and (a) the impedance and (b) the phase with the product of frequency and wavelength in a specific embodiment of the surface acoustic wave resonator proposed in the present invention.

[0025] Figure 10 (a) is a schematic diagram of the relationship between the SiO2 film thickness and the maximum phase of the high-order mode in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0026] Figure 10 (b) is a schematic diagram showing the relationship between SiO2 film thickness and relative bandwidth in a specific embodiment of the surface acoustic wave resonator proposed in the present invention.

[0027] Figure 11-15 Schematic diagrams of the relationship between the cut angle θ and (a) the impedance and (b) the phase versus the product of frequency and wavelength in a specific embodiment of the surface acoustic wave resonator proposed in the present invention.

[0028] Figure 16 This is a schematic diagram of the relationship between the cut angle θ and the maximum phase of the high-order mode in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0029] Figure 17-18 They are schematic diagrams of the relationship between the relative bandwidth of the SH wave and the cutting angle θ when the thickness of the second electrode layer changes in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0030] Figure 19-21 Schematic diagrams of the relationship between the IDT metallization ratio and (a) the impedance and (b) the phase with the product of frequency and wavelength in a specific embodiment of the surface acoustic wave resonator proposed in the present invention.

[0031] Figure 22 This is a schematic diagram of the relationship between the IDT metallization ratio and the maximum phase of the high-order mode in a specific embodiment of the surface acoustic wave resonator proposed by the present invention.

[0032] Description of reference numerals:

[0033] Rayleigh-wave Coupling of coefficient-Rayleigh wave coupling coefficient, SH-wave couplingof coefficient-horizontal shear wave coupling coefficient, impedance-impedence, acoustic velocity-acoustic velocity, Phase-phase, Rayleigh BW-Rayleigh wave relative bandwidth, thickness of SiO2-SiO2 thickness, SH-waveBW-horizontal shear wave relative bandwidth, ratio of metalization-metallization ratio. DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0035] In the present invention, unless otherwise specified, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of a device in normal use, and "inner" and "outer" refer to positions relative to the device's outline. Furthermore, the terms "first, second, and third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the number of the technical features indicated. Therefore, features defined as "first, second, and third" may explicitly or implicitly include one or more of such features. In the description of the present invention, "plurality" means two or more, unless otherwise explicitly and specifically defined.

[0036] The present invention provides a surface acoustic wave resonator, such as Figure 1 As shown, including:

[0037] Piezoelectric substrate;

[0038] The IDT electrode comprises a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer sequentially stacked on the piezoelectric substrate, wherein the acoustic impedance of the first electrode layer is smaller than the acoustic impedance of the second electrode layer;

[0039] Protective film, covering the IDT electrode.

[0040] In the present invention, low acoustic impedance metal is used as the first layer and high acoustic impedance metal is used as the second layer, so that at the same thickness and frequency, the resonator can be smaller in size, have better frequency-temperature characteristics, and is less likely to generate high-order spurious.

[0041] According to the present invention, the material of the piezoelectric substrate is LiNbO 3 , and the material of the protective film is SiO 2 .

[0042] According to the present invention, the Euler angle of LiNbO3 is (0°±10°, θ, 0°±10°), wherein the cutting angle θ ranges from 0° to 30° or from 85° to 130° or from 160° to 180°.

[0043] Preferably, the cutting angle θ is in the range of 0° to 30° or 160° to 180°.

[0044] In the present invention, a larger effective electromechanical coupling coefficient is achieved by using a low-cut angle substrate (0°~30° or 160°~180°). While suppressing high-order modal spurious conditions, a wider range of protective film thickness can be accepted, thereby improving the resonator frequency temperature coefficient performance.

[0045] According to the present invention, normalized by the acoustic wavelength λ, the thickness of the first electrode layer is less than or equal to 0.1λ, the total thickness of the first electrode layer and the second electrode layer is not greater than 0.3λ, and the thickness of the protective film is greater than or equal to 0.12λ and less than or equal to 0.45λ.

[0046] According to the present invention, the materials of the first electrode adhesion layer and the second electrode adhesion layer are independently selected from at least one of Ti, TiN, AlN, Cr, Ni and Ta;

[0047] The material of the first electrode layer is selected from at least one of Ag, Al and Cu;

[0048] The material of the second electrode layer is selected from at least one of Pt, W, Mo, Au, Ru and Cu.

[0049] According to the present invention, the material of the first electrode layer is an alloy with Al or Cu as a main component.

[0050] The present invention also provides a method for preparing the above-mentioned resonator, comprising:

[0051] providing a piezoelectric substrate;

[0052] stacking a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer on the piezoelectric substrate in sequence, and setting the acoustic impedance of the first electrode layer to be smaller than the acoustic impedance of the second electrode layer;

[0053] The IDT electrode is covered with a protective film.

[0054] According to the present invention, it also includes:

[0055] The material of the piezoelectric substrate is selected as LiNbO3, and the material of the protective film is selected as SiO2;

[0056] The Euler angle of LiNbO3 is controlled to be (0°±10°, θ, 0°±10°), wherein the range of the cutting angle θ is 0° to 30° or 85° to 130° or 160° to 180°.

[0057] According to the present invention, it also includes:

[0058] Normalized by the acoustic wavelength λ, the thickness of the first electrode layer is controlled to be less than or equal to 0.1λ, the total thickness of the first electrode layer and the second electrode layer is not greater than 0.3λ, and the thickness of the protective film is greater than or equal to 0.12λ and less than or equal to 0.45λ.

[0059] The present invention will be described in more detail below through specific examples.

[0060] Example 1

[0061] like Figure 1 As shown, this embodiment provides a surface acoustic wave resonator, including:

[0062] LiNbO3 piezoelectric substrate;

[0063] an IDT electrode disposed on the piezoelectric layer; and a silicon dioxide film disposed on the piezoelectric layer to cover the IDT electrode; the IDT electrode comprising a first electrode adhesion layer, a first electrode layer stacked on the first electrode layer, a second electrode adhesion layer stacked on the first electrode layer, and a second electrode layer stacked on the second electrode adhesion layer, wherein the acoustic impedance of the first electrode layer is less than the acoustic impedance of the second electrode layer;

[0064] SiO2 protective film covering the IDT electrode;

[0065] The main cross-section of the overall structure of the surface acoustic wave resonator and the top view of the IDT electrode in this embodiment are shown in FIG. Figure 2 As shown in (a) and (b);

[0066] In this embodiment, the first electrode adhesion layer is required to ensure high-reliability adhesion between the first electrode layer and the substrate, and the optional material includes at least one of Ti, TiN, AlN, Cr, Ni, and Ta. The second electrode adhesion layer is optional, and the material includes at least one of Ti, TiN, AlN, Cr, Ni, and Ta. The first electrode layer material includes at least one of Ag, Al, and Cu, or an alloy with Al or Cu as a main component. The second electrode layer can be composed of at least one of Pt, W, Mo, Au, Ru, and Cu, or an alloy with these metals as a main component.

[0067] In this embodiment, the first electrode layer is close to the piezoelectric layer, and the second electrode layer is above the first electrode layer; the thickness of the first electrode layer is less than 0.1λ normalized by the acoustic wavelength λ; the total thickness h of the first electrode layer and the second electrode layer satisfies h≤0.3λ; the thickness of SiO2 is greater than 0.12λ and less than 0.45λ;

[0068] In the ZXZ Euler angles of the LiNbO3 piezoelectric substrate (0°±10°, θ, 0°±10°), the cutting angle θ is in the range of 0°~30°, 85°~130° and 160°~180°, among which 0°~30° and 160°~180° can achieve a larger electromechanical coupling coefficient.

[0069] It can be seen from Table 1 that, compared with the existing technical solutions, the technical solution of the present application can effectively reduce the size of the IDT.

[0070] The surface acoustic wave resonator of the present invention is applied to the schematic diagram of the radio frequency front-end filter structure, as shown in FIG. Figure 3 shown.

[0071] Table 1 Comparison of IDT dimensions between existing solutions and the technical solution of this application

[0072]

[0073] Test Example 1

[0074] In this test example, the relationship between the effective electromechanical coupling coefficient (Keff2) and the cutting angle θ and the impedance of the typical cutting angle are tested.

[0075] In this embodiment, the structural parameters of the surface acoustic wave resonator are:

[0076] The piezoelectric substrate is LiNbO3, and the Euler angle is (0°, θ, 0°);

[0077] The first electrode adhesion layer is a Ti film with a thickness of 30 nm;

[0078] The first electrode layer is an Al film with a thickness of 0.033λ;

[0079] No second electrode layer adhesion layer is used;

[0080] The second electrode layer is a W film with a thickness of 0.11λ;

[0081] IDT electrode: duty cycle 0.45;

[0082] The protective film is SiO2.

[0083] When h / λ=0.15,h_SiO2=0.36λ, the relationship between the effective electromechanical coupling coefficient (Keff2) and the cutting angle θ is shown in the figure below: Figure 4 As shown, when the cutting angle is 10°, 120°, and 170°, the corresponding impedance relationship diagrams are as follows: Figure 5 (a), (b) and (c), where h is the thickness of the IDT electrode.

[0084] Test Example 2

[0085] In this test example, the relationship between SiO2 film thickness and impedance, phase and frequency multiplied by wavelength, maximum phase of high-order modes, and relative bandwidth BW are tested.

[0086] In this embodiment, the structural parameters of the surface acoustic wave resonator are:

[0087] The piezoelectric substrate is LiNbO3, and the Euler angle is (0°, 120, 0°);

[0088] The first electrode adhesion layer is a Ti film with a thickness of 30 nm;

[0089] The first electrode layer is an Al film with a thickness of 0.033λ;

[0090] No second electrode layer adhesion layer is used;

[0091] The second electrode layer is a W film with a thickness of 0.11λ;

[0092] IDT electrode: duty cycle 0.45;

[0093] The protective film is SiO2.

[0094] The relative bandwidth BW (%) is calculated by the formula: relative bandwidth (%) = anti-resonance frequency fp - resonance frequency fs) / resonance frequency fs * 100.

[0095] Figures 6 to 9 , showing the characteristic diagrams of (a) impedance size and (b) phase as a function of the product of frequency and wavelength under different SiO2 film thicknesses, where the values ​​of the SiO2 film thickness normalized by wavelength are 0.22λ, 0.30λ, 0.4λ, and 0.47λ, respectively; it can be seen from the figure that as the SiO2 film thickness increases, the spurious signal of the high-order mode near the sound velocity of 4700m / s becomes larger; in order to suppress the degradation of the overall characteristics of the device caused by the influence of the high-order mode, it is necessary to limit the maximum phase of the high-order mode to below -25°.

[0096] Figure 10 (a) shows the relationship between the thickness of the SiO2 film and the maximum phase of the high-order mode. It can be seen from the figure that if the SiO2 film thickness is set to more than 0.45λ, the high-order mode will become larger and the filter out-of-band characteristics will deteriorate.

[0097] Figure 10 (b) shows the relationship between SiO2 thickness and relative bandwidth BW; it can be seen from the figure that as the SiO2 thickness increases, the relative bandwidth decreases, and the thickness of SiO2 is preferably below 0.45λ.

[0098] Test Example 3

[0099] In this test example, the impedance magnitude and phase at different cutting angles θ are tested as a function of the product of frequency and wavelength, as well as the relationship between the cutting angle θ and the maximum phase of the high-order mode.

[0100] In this embodiment, the structural parameters of the surface acoustic wave resonator are:

[0101] The piezoelectric substrate is LiNbO3, and the Euler angle is (0°, θ, 0°);

[0102] The first electrode adhesion layer is a Ti film with a thickness of 30 nm;

[0103] The first electrode layer is an Al film with a thickness of 0.033λ;

[0104] No second electrode layer adhesion layer is used;

[0105] The second electrode layer is a W film with a thickness of 0.11λ;

[0106] IDT electrode: duty cycle 0.45;

[0107] The protective film is SiO2 with a film thickness of 0.4λ.

[0108] Figures 11-15 The characteristic diagrams of (a) impedance magnitude and (b) phase versus frequency times wavelength product are shown for the Euler angles (0°, θ, 0°) of the piezoelectric substrate with cut angles θ of 114°, 118°, 122°, 126°, and 128°, respectively. As can be seen from the figure, as θ decreases, the spurious emission of higher-order modes decreases.

[0109] Figure 16 The relationship between θ and the maximum phase of the high-order mode is shown. It can be seen from the figure that when the SiO2 thickness is 0.45λ, the maximum phase of the high-order mode is below -25° only when θ is above 118° and below 124°; when the SiO2 thickness is 0.4λ and the SiO2 thickness is 0.32λ, the θ range can be extended to 85°-130°.

[0110] In addition, from Figures 11-15 Large spurious emissions can also be observed near the main resonance (around a sound velocity of 2700 m / s). This is due to the excitation of unwanted SH waves in addition to the main Rayleigh wave mode. This spurious emission can be suppressed by reducing the electromechanical coupling coefficient of the SH wave.

[0111] Test Example 4

[0112] In this test example, the relationship between the relative bandwidth of the SH wave and the cutting angle θ when the thickness of the second electrode layer changes is tested.

[0113] In this embodiment, the structural parameters of the surface acoustic wave resonator are:

[0114] The piezoelectric substrate is LiNbO3, and the Euler angle is (0°, θ, 0°);

[0115] The first electrode adhesion layer is a Ti film with a thickness of 30 nm;

[0116] The first electrode layer is an Al film with a thickness of 0.033λ;

[0117] No second electrode layer adhesion layer is used;

[0118] The second electrode layer is a W film with a thickness of 0.11λ;

[0119] IDT electrode: duty cycle 0.45;

[0120] The protective film is SiO2 with a film thickness of 0.35λ.

[0121] The relative bandwidth BW (%) is calculated by the formula: relative bandwidth (%) = anti-resonance frequency fp - resonance frequency fs) / resonance frequency fs * 100.

[0122] Figure 17-18The relationship between θ and the relative bandwidth of the SH wave in the Euler angle (0°, θ, 0°) of the piezoelectric substrate when the W film thickness changes is shown, where the W film thickness is 0.075λ, 0.085λ, 0.095λ, 0.105λ, 0.115λ, and 0.125λ respectively.

[0123] according to Figure 17 From (a) to (c), it can be seen that when the thickness of the W film is in the range of 0.075λ to 0.095λ, as the thickness of the W film increases, the electromechanical coupling coefficient of the SH wave becomes the minimum value θ and becomes larger. On the other hand, according to Figure 13 (a) It can be seen that when the thickness of the W film is 0.075λ, the θ at which the electromechanical coupling coefficient of the SH wave reaches its minimum value becomes as small as 118°; Figure 13 (b) It can be seen that when the thickness of the W film is 0.085λ, θ is 119°; according to Figure 13 (c) It can be seen that when the film thickness of the W film is 0.095λ, θ is 120°.

[0124] Depend on Figure 18 From (a) to (c), it can be seen that when the W film thickness is between 0.105λ and 0.125λ, θ is 120°.

[0125] Test Example 5

[0126] In this test example, the relationship between the IDT metallization ratio and impedance size, the phase change with the product of frequency and wavelength, and the maximum phase of high-order modes are tested.

[0127] In this embodiment, the structural parameters of the surface acoustic wave resonator are:

[0128] The piezoelectric substrate is LiNbO3, and the Euler angles are (0°, 120°, 0°);

[0129] The first electrode adhesion layer is a Ti film with a thickness of 30 nm;

[0130] The first electrode layer is an Al film with a thickness of 0.033λ;

[0131] No second electrode layer adhesion layer is used;

[0132] The second electrode layer is a W film with a thickness of 0.11λ;

[0133] IDT electrode: metallization ratio;

[0134] The protective film is SiO2 with a film thickness of 0.32λ.

[0135] Figure 19-21The characteristic diagrams of (a) impedance magnitude and (b) phase versus frequency times wavelength product for different metallization ratios are shown, where the metallization ratios are 0.4, 0.5, and 0.6, respectively. As can be seen from the figure, there is no significant difference in the spurious suppression of high-order modes as the metallization ratio increases.

[0136] Figure 22 The relationship between the IDT electrode metallization ratio and the maximum phase of the high-order mode is shown in the figure. It can be seen from the figure that when the metallization ratio of the IDT is further increased, the maximum phase of the high-order mode is below -25°; in addition, if the metallization ratio becomes larger, the gap between adjacent electrode fingers becomes smaller, and the small size requires thicker electrodes, so the metallization ratio is best below 0.7.

[0137] The surface acoustic wave resonator proposed in the embodiment of the present invention uses low acoustic impedance metal as the first layer and high acoustic impedance metal as the second layer. Under the same thickness and frequency, the resonator can be smaller in size, have better frequency-temperature characteristics, and is less likely to generate high-order spurious.

[0138] While various embodiments of the present invention have been described above, the above description is intended to be illustrative, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A surface acoustic wave resonator, characterized in that: include: Piezoelectric substrate; The IDT electrode comprises a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer sequentially stacked on the piezoelectric substrate, wherein the acoustic impedance of the first electrode layer is smaller than the acoustic impedance of the second electrode layer; A protective film covers the IDT electrode.

2. The resonator according to claim 1, characterized in that The material of the piezoelectric substrate is LiNbO3, and the material of the protective film is SiO2.

3. The resonator according to claim 2, characterized in that The Euler angle of the LiNbO 3 is (0°±10°, θ, 0°±10°), wherein the range of the cutting angle θ is 0° to 30° or 85° to 130° or 160° to 180°.

4. The resonator according to claim 3, characterized in that The cutting angle θ ranges from 0° to 30° or from 160° to 180°.

5. The resonator according to claim 1, characterized in that Normalized by the acoustic wavelength λ, the thickness of the first electrode layer is less than or equal to 0.1λ, the total thickness of the first electrode layer and the second electrode layer is not greater than 0.3λ, and the thickness of the protective film is greater than or equal to 0.12λ and less than or equal to 0.45λ.

6. The resonator according to claim 1, characterized in that The materials of the first electrode adhesion layer and the second electrode adhesion layer are independently selected from at least one of Ti, TiN, AlN, Cr, Ni and Ta; The material of the first electrode layer is selected from at least one of Ag, Al and Cu; The material of the second electrode layer is selected from at least one of Pt, W, Mo, Au, Ru and Cu.

7. The resonator according to claim 1, characterized in that The material of the first electrode layer is an alloy with Al or Cu as a main component.

8. A method for preparing a surface acoustic wave resonator, characterized in that: include: providing a piezoelectric substrate; stacking a first electrode adhesion layer, a first electrode layer, an optional second electrode adhesion layer, and a second electrode layer on the piezoelectric substrate in sequence, and setting the acoustic impedance of the first electrode layer to be smaller than the acoustic impedance of the second electrode layer; The IDT electrode is covered with a protective film.

9. The preparation method according to claim 8, characterized in that Also includes: The material of the piezoelectric substrate is selected as LiNbO3, and the material of the protective film is selected as SiO2; The Euler angle of the LiNbO 3 is controlled to be (0°±10°, θ, 0°±10°), wherein the range of the cutting angle θ is 0° to 30° or 85° to 130° or 160° to 180°.

10. The preparation method according to claim 8, characterized in that Also includes: Normalized by the acoustic wavelength λ, the thickness of the first electrode layer is controlled to be less than or equal to 0.1λ, the total thickness of the first electrode layer and the second electrode layer is not greater than 0.3λ, and the thickness of the protective film is greater than or equal to 0.12λ and less than or equal to 0.45λ.

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