Method for fabricating devices utilizing higher-order mode surface acoustic waves - Patent Application 20070122967

Abstract

To provide a higher order mode surface acoustic wave device capable of obtaining a good characteristic even in a high frequency band of 3.8 GHz or more, and capable of holding a sufficient mechanical strength.SOLUTION: A higher order mode surface acoustic wave device (10) includes: a piezoelectric substrate (11) constructed by LiTaO3 crystal or LiNbO3 crystal; and an interdigital electrode (12) that is embedded onto a front surface of the piezoelectric substrate (11), and is formed so as to be projected from the front surface of the piezoelectric substrate (11), and uses a surface acoustic wave in a higher order mode. Also, it may have a thin film (13) laminated onto the piezoelectric substrate (11) or the substrate, or may have a support substrate (14) and / or a multilayer film (15) that are / is provided so as to be contacted to a surface opposite to the front surface of the piezoelectric substrate (11) where the interdigital electrode (12) is provided. The higher order mode surface acoustic wave device can obtain a good characteristic even in a high frequency band of 3.8 GHz or more, and can hold a sufficient mechanical strength.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to higher order mode surface acoustic wave (SAW) devices that provide for the utilization of higher order modes that are overtones of the fundamental mode. [Background technology]

[0002] In recent years, the frequency band between 700 MHz and 3 GHz, which is mainly used by smartphones, etc., has become extremely congested, with nearly 80 bands. To address this issue, the fifth-generation mobile communication system (5G), the next-generation wireless communication system, is planned to use the frequency band between 3.6 GHz and 4.9 GHz, and the next generation is also planned to use frequency bands above 6 GHz.

[0003] In contrast to these plans, surface acoustic wave devices, which are typical acoustic wave devices, have limitations in power handling and manufacturing technology, making it impossible to reduce the period (λ) of the interdigital transducer (IDT), limiting their ability to achieve higher frequencies. Figures 1(a) and (b) show plan and cross-sectional views of an example of a conventional SAW device, in which a 42° rotated Y-plate LiTaO3 crystal is used as the piezoelectric substrate and the X-direction propagating interdigital transducer 52 is formed from Al. The cross-sectional view in Figure 1(b) shows a cross section taken along line II in the plan view of Figure 1(a).

[0004] Figure 1(c) shows the frequency characteristics of the impedance obtained when the period of the interdigital transducer 52 is 1.2 μm. The resonant frequency is approximately 3.2 GHz, the relative bandwidth is 3.8%, and the impedance ratio is 65 dB. A small response, likely a higher-order mode, is observed at 17.2 GHz, but it is not at a usable level. Even if the period of the interdigital transducer 52 is reduced to 1 μm, the resonant frequency is approximately 3.8 GHz. As such, conventional SAW devices cannot cover the frequency band required for 5G, let alone beyond 5G.

[0005] Patent Document 1 discloses a surface acoustic wave device in which electrodes of Pt, Cu, Mo, Ni, Ta, W, or other materials heavier than Al with a metallization ratio of 0.45 or less are embedded in a LiNbO3 substrate with Euler angles (0°, 80-130°, 0°) to excite the fundamental mode of Love waves and achieve a wide bandwidth. Non-Patent Document 1 also discloses a surface acoustic wave device in which a Cu electrode of 0.1 wavelength or less is embedded in a 42° rotated Y-cut LiTaO3 substrate, on which an Al electrode is formed, to excite in the fundamental mode and achieve a high Q value. Meanwhile, bulk acoustic wave devices (FBARs; film bulk acoustic resonators) using AlN or ScAlN piezoelectric thin films have been studied as acoustic wave filters with a frequency band of 1.9 GHz (see, for example, Non-Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2014 / 054580 [Non-patent literature]

[0007] [Non-Patent Document 1] T. Kimura, M. Kadota, and Y. IDA, “High Q SAW resonator using upper-electrodes on Grooved-electrode in LiTaO3“, Proc. IEEE Microwave Symp. (IMS), p.1740, 2010. [Non-patent document 2] Keiichi Umeda et al., “PIEZOELECTRIC PROPERTIES OF ScAlN THIN FILMS FOR PIRZO-MEMS DEVICES”, MEMS 2013, Taipei, Taiwan January 20-24, 2013 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the technologies described in Patent Document 1 and Non-Patent Document 1 use heavy metals for the electrodes and have small metallization ratios, which prevent sufficient performance in the high-frequency bands above 3.6 GHz required for 5G and beyond. The bulk acoustic wave device described in Non-Patent Document 2 uses a polycrystalline piezoelectric thin film, resulting in an impedance ratio of only 55 dB at 1.9 GHz, and high attenuation at ultra-high frequencies makes it difficult to achieve good characteristics. Furthermore, the frequency of an FBAR is determined by the sound velocity in the thin film divided by (2 × the thickness of the thin film), and increasing the frequency requires an extremely thin thin film. Current FBARs have a self-supporting piezoelectric thin film, which makes it difficult to maintain mechanical strength in the ultra-high-frequency band where the film becomes extremely thin.

[0009] The present invention has been made in light of these problems, and aims to provide a higher-order mode surface acoustic wave device that can obtain good characteristics even in high frequency bands of 3.8 GHz or more, while maintaining sufficient mechanical strength. [Means for solving the problem]

[0010] To achieve the above object, a higher-order mode surface acoustic wave device according to the present invention includes a piezoelectric substrate including LiTaO3 crystal or LiNbO3 crystal, and an interdigital transducer embedded in the surface of the piezoelectric substrate, and utilizes higher-order mode surface acoustic waves.

[0011] Higher-order mode surface acoustic wave devices can excite higher-order SAW modes (such as first, second, and third modes) by embedding interdigital transducers on the surface of a piezoelectric substrate, resulting in higher-order modes with large impedance ratios. By utilizing these higher-order modes, higher-order mode surface acoustic wave devices can achieve higher frequencies and achieve good characteristics even in high-frequency bands above 3.8 GHz. Furthermore, by utilizing higher-order modes, there is no need to make the piezoelectric substrate ultra-thin or reduce the period of the interdigital transducers, even in high-frequency bands above 3.8 GHz, and sufficient mechanical strength can be maintained. Piezoelectric substrates also include piezoelectric thin films and piezoelectric thin plates.

[0012] In a higher-order mode surface acoustic wave device, the interdigital transducers may be formed to protrude from the surface of the piezoelectric substrate, which also provides a higher-order mode with a large impedance ratio.

[0013] The higher-order mode surface acoustic wave device may have a thin film or substrate provided in contact with the piezoelectric substrate. It may also have a support substrate and / or a multilayer film provided in contact with the surface of the piezoelectric substrate opposite to the surface on which the interdigital transducers are provided. When a support substrate is provided, the support substrate may be made of a material other than metal. The support substrate may also be made of at least one of silicon, crystal, sapphire, glass, quartz, germanium, and alumina. When a multilayer film is provided, the multilayer film may be made of an acoustic multilayer film formed by stacking multiple layers with different acoustic impedances. In these cases, a higher-order mode with a large impedance ratio can also be obtained.

[0014] In such a higher-order mode surface acoustic wave device, the metallization ratio of the interdigital transducer is preferably 0.45 or more and 0.9 or less, and more preferably 0.63 or more. In this case, a higher-order mode with a larger impedance ratio can be obtained, and the bandwidth can also be expanded.

[0015] Furthermore, the higher-order mode surface acoustic wave device may have the following configuration to achieve a higher-order mode with a larger impedance ratio. Specifically, the piezoelectric substrate may be made of LiTaO3 crystal, and the interdigital transducer may be made of at least one of a Ti, Al, and Mg alloy. In this case, the interdigital transducer is preferably buried from the surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.075 to 0.3 (0.15 to 0.6 when the wavelength / metallization ratio is 0.5), and more preferably to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.115 to 0.3 (0.23 to 0.6 when the wavelength / metallization ratio is 0.5). Here, if the cross section of the buried electrode is not perpendicular to the substrate surface, the metallization ratio and electrode width refer to the effective metallization ratio and electrode width. The same applies hereinafter.

[0016] Alternatively, the piezoelectric substrate may be made of LiTaO3 crystal, and the interdigital transducers may be made of at least one of Ag, Mo, Cu, and Ni. In this case, the interdigital transducers are preferably buried from the surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.08 to 0.3 (0.16 to 0.6 when the wavelength / metallization ratio is 0.5), and more preferably to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.09 to 0.3 (0.18 to 0.6 when the wavelength / metallization ratio is 0.5).

[0017] Alternatively, the piezoelectric substrate may be made of LiTaO3 crystal, and the interdigital transducers may be made of at least one of Pt, Au, W, Ta, and Hf. In this case, the interdigital transducers are preferably buried from the surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.08 to 0.3 (0.16 to 0.6 when the wavelength / metallization ratio is 0.5), and more preferably to a depth where the wavelength / metallization ratio of the surface acoustic wave falls within a range of 0.125 to 0.3 (0.25 to 0.6 when the wavelength / metallization ratio is 0.5).

[0018] Alternatively, the piezoelectric substrate may be made of LiNbO3 crystal, and the interdigital transducer may be made of at least one of a Ti, Al, and Mg alloy. In this case, the interdigital transducer is preferably buried to a depth from the surface of the piezoelectric substrate where the wavelength / metallization ratio of the surface acoustic wave is 0.07 to 0.3 (0.14 to 0.6 when the wavelength / metallization ratio is 0.5), and more preferably where the wavelength / metallization ratio of the surface acoustic wave is 0.105 to 0.3 (0.21 to 0.6 when the wavelength / metallization ratio is 0.5).

[0019] Alternatively, the piezoelectric substrate may be made of LiNbO3 crystal, and the interdigital transducer may be made of at least one of Ag, Mo, Cu, and Ni. In this case, the interdigital transducer is preferably buried to a depth from the surface of the piezoelectric substrate where the wavelength / metallization ratio of the surface acoustic wave is 0.065 to 0.3 (0.13 to 0.6 wavelengths when the wavelength / metallization ratio is 0.5), and more preferably where the wavelength / metallization ratio of the surface acoustic wave is 0.09 to 0.3 (0.18 to 0.6 wavelengths when the wavelength / metallization ratio is 0.5).

[0020] Alternatively, the piezoelectric substrate may be made of LiNbO3 crystal, and the interdigital transducer may be made of at least one of Pt, Au, W, Ta, and Hf. In this case, the interdigital transducer is preferably buried to a depth from the surface of the piezoelectric substrate where the wavelength / metallization ratio of the surface acoustic wave is 0.075 to 0.3 (0.15 to 0.6 when the wavelength / metallization ratio is 0.5), and more preferably where the wavelength / metallization ratio of the surface acoustic wave is 0.115 to 0.3 (0.23 to 0.6 wavelengths when the wavelength / metallization ratio is 0.5).

[0021] Furthermore, it is preferable that the piezoelectric substrate is made of LiTaO3 crystal and that the Euler angles are in the range of (0°±20°, 112° to 140°, 0°±5°) or are crystallographically equivalent to these, and it is even more preferable that the Euler angles are in the range of (0°±10°, 120° to 132°, 0°±5°) or are crystallographically equivalent to these.

[0022] Furthermore, it is preferable that the piezoelectric substrate is made of LiNbO3 crystal and that the Euler angles are in the range of (0°±25°, 78° to 153°, 0°±5°) or are crystallographically equivalent to these, and it is even more preferable that the Euler angles are in the range of (0°±20°, 87° to 143°, 0°±5°) or are crystallographically equivalent to these.

[0023] Here, the Euler angles (φ, θ, ψ) are in a right-handed system and represent the cross section of the piezoelectric substrate and the propagation direction of the surface acoustic wave. Specifically, the X' axis is obtained by rotating the X axis counterclockwise by φ around the Z axis, relative to the X, Y, and Z crystal axes of the LiTaO3 or LiNbO3 crystals that make up the piezoelectric substrate. Next, the Z axis is rotated counterclockwise by θ around the X' axis to obtain the Z' axis. The Z' axis is then considered the normal, and the plane containing the X' axis is the cross section of the piezoelectric substrate. The direction of counterclockwise rotation of the X' axis by ψ around the Z' axis is defined as the propagation direction of the surface acoustic wave. The axis perpendicular to the X' and Z' axes, obtained by moving the Y axis through these rotations, is defined as the Y' axis.

[0024] By defining the Euler angles in this way, for example, propagation in the X direction of a 40° rotated Y-plate is expressed as Euler angles (0°, 130°, 0°), and propagation in the 90° X direction of a 40° rotated Y-plate is expressed as Euler angles (0°, 130°, 90°). When cutting a piezoelectric substrate to the desired Euler angles, an error of up to ±0.5° may occur in each component of the Euler angles. Regarding the shape of the interdigital transducer, an error of approximately ±3° may occur with respect to the propagation direction ψ. Regarding the characteristics of elastic waves, a deviation of approximately ±5° between ψ and ψ, among the Euler angles (φ, θ, ψ), makes almost no difference in the characteristics.

[0025] The piezoelectric substrate may include at least one of a support substrate, a thin film, and a multilayer film, which is provided in contact with the surface of the piezoelectric substrate opposite to the surface on which the interdigital transducer is provided, and the shear wave acoustic velocity or equivalent shear wave acoustic velocity of the support substrate may be in the range of 2000 to 3000 m / s or 6000 to 8000 m / s, and the thickness of the piezoelectric substrate may be in the range of 0.2 to 20 wavelengths.

[0026] The piezoelectric substrate may include at least one of a support substrate, a thin film, and a multilayer film, which is provided in contact with the surface of the piezoelectric substrate opposite to the surface on which the interdigital transducer is provided, and the shear wave acoustic velocity or equivalent shear wave acoustic velocity of the support substrate may be in the range of 3000 to 6000 m / s, and the thickness of the piezoelectric substrate may be in the range of 2 to 20 wavelengths.

[0027] The piezoelectric substrate includes a support substrate provided in contact with the surface opposite to the surface on which the interdigital transducer is provided, and at least one of a thin film and a multilayer film, and the linear expansion coefficient of the support substrate is 10.4×10 -6 / ° C. or less, and the thickness ratio TR of the support substrate / piezoelectric substrate may be equal to or greater than the value of TR defined by the following formula (1), where α is a linear expansion coefficient.

[0028] TR = α × 0.55 × 10 6 + 2.18 (1) [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a higher-order mode surface acoustic wave device that can obtain good characteristics even in a high frequency band of 3.8 GHz or more and maintain sufficient mechanical strength. [Brief explanation of the drawings]

[0030] [Figure 1] Figure 1(a) is a plan view of a conventional surface acoustic wave device [Al interdigital transducer / 42° rotated Y-plate X-propagation LiTaO3 crystal], Figure 1(b) is a cross-sectional view, and Figure 1(c) is a graph showing the frequency characteristics of the impedance of the surface acoustic wave devices of Figures 1(a) and 1(b). [Figure 2] FIG. 2(a) shows a higher-order mode surface acoustic wave device according to the present embodiment, FIG. 2(b) shows a modified example of FIG. 2(a) having a thin film, FIG. 2(c) shows a modified example of FIG. 2(a) having protruding interdigital electrodes, FIG. 2(d) shows a modified example of FIG. 2(a) having a supporting substrate, FIG. 2(e) shows a modified example of FIG. 2(d) having protruding interdigital electrodes, and FIG. 2(f) is a cross-sectional view of a modified example of FIG. 2(d) having a multilayer film between a piezoelectric substrate and a supporting substrate. [Figure 3] FIG. 3 is a cross-sectional view of the higher-order mode surface acoustic wave device shown in FIGS. 2(a) to 2(f) when the side surface of the embedded electrode is not perpendicular to the substrate surface. [Figure 4] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode / (0°, 126.5°, 0°) LiTaO3 crystal substrate], Figure 4(a) is a graph showing the frequency characteristics of impedance when the metallization ratio of the interdigital transducer is 0.5, Figure 4(b) is a graph showing an enlarged view of the first-mode resonant frequency of Figure 4(a), Figure 4(c) is a graph showing the displacement distribution at the first-mode resonant frequency, and Figure 4(d) is a graph showing the frequency characteristics of the first-mode impedance when the metallization ratio of the interdigital transducer is 0.7. [Figure 5] 2(c) is a graph showing the frequency characteristics of impedance for the higher-order mode surface acoustic wave device [Al electrode (metallization ratio 0.5) / (0°, 126.5°, 0°) LiTaO3 crystal substrate]. [Figure 6] For the higher-order mode surface acoustic wave device shown in Figure 2(d) [Al electrode (metallization ratio 0.5) / (0°, 126.5°, 0°) LiTaO3 crystal substrate / support substrate], Figure 6(a) is a graph showing the frequency characteristics of impedance when the support substrate is made of a Si substrate, and Figure 6(b) is a graph showing the frequency characteristics of impedance when the support substrate is made of a quartz substrate. [Figure 7] 2(f) is a graph showing the frequency characteristics of the impedance of the first mode for the higher-order mode surface acoustic wave device [Al electrode (metallization ratio 0.5) / (0°, 126.5°, 0°) LiTaO3 crystal substrate]. [Figure 8] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode (metallization ratio 0.5) / (0°, 116°, 0°) LiNbO3 crystal substrate], Figure 8(a) is a graph showing the frequency characteristics of impedance, and Figure 8(b) is a graph enlarging the vicinity of the first-order mode resonance frequency in Figure 8(a). [Figure 9] 2(a) is a graph showing the frequency characteristics of impedance for the higher-order mode surface acoustic wave device [Cu electrode (metallization ratio 0.5) / (0°, 116°, 0°) LiNbO3 crystal substrate] shown in FIG. [Figure 10] Figure 10(a) is a graph showing the relationship between the thickness of each electrode and the relative bandwidth of the first mode, and Figure 10(b) is a graph showing the relationship between the thickness of each electrode and the impedance ratio of the first mode for the higher-order mode surface acoustic wave device shown in Figure 2(a) [interdigital electrode (metallization ratio 0.5) / (0°, 126.5°, 0°) LiTaO3 crystal substrate], where the interdigital electrodes are Al electrodes, Cu electrodes, and Au electrodes. [Figure 11] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode (metallization ratio 0.5) / (0°, θ, 0°) LiTaO3 crystal substrate], Figure 11(a) is a graph showing the relationship between θ and the fractional bandwidth of the first mode, and Figure 11(b) is a graph showing the relationship between θ and the impedance ratio of the first mode. [Figure 12]For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode (metallization ratio 0.5) / (0°, θ, 0°) LiTaO3 crystal substrate], Figure 12 is a graph showing the relationship between φ and the impedance ratio of the first mode in the (φ, 126.5°, 0°) LiTaO3 crystal substrate. [Figure 13] Figure 13(a) is a graph showing the relationship between the thickness of each electrode and the relative bandwidth of the first mode, and Figure 13(b) is a graph showing the relationship between the thickness of each electrode and the impedance ratio of the first mode for the higher-order mode surface acoustic wave device shown in Figure 2(a) [interdigital electrode (metallization ratio 0.5) / (0°, 116°, 0°) LiNbO3 crystal substrate], where the interdigital electrodes are Al electrodes, Cu electrodes, and Au electrodes. [Figure 14] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode (metallization ratio 0.5) / (0°, θ, 0°) LiNbO3 crystal substrate], Figure 14(a) is a graph showing the relationship between θ and the fractional bandwidth of the first mode, and Figure 14(b) is a graph showing the relationship between θ and the impedance ratio of the first mode. [Figure 15] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode (metallization ratio 0.5) / (φ, θ, 0°) LiNbO3 crystal substrate], Figure 15 is a graph showing the relationship between φ and the impedance ratio of the first mode in the (φ, 116°, 0°) LiNbO3 crystal substrate. [Figure 16] For the higher-order mode surface acoustic wave device shown in Figure 2(a) [Al electrode / (0°, 126.5°, 0°) LiTaO3 crystal substrate], Figure 16(a) is a graph showing the relationship between the metallization ratio of the Al electrode and the phase velocity of the first mode, and Figure 16(b) is a graph showing the relationship between the metallization ratio of the Al electrode and the impedance ratio of the first mode. [Figure 17] 2(a) is a graph showing the frequency characteristics of impedance of the higher-order mode surface acoustic wave device [Al electrode (metallization ratio 0.85) / (0°, 126.5°, 0°) LiTaO3 crystal substrate] shown in FIG. [Figure 18]For the higher-order mode surface acoustic wave device shown in FIG. 17, FIG. 18(a) is a graph showing the relationship between the thickness of the interdigital transducer and the phase velocities of the zeroth to third modes, and FIG. 18(b) is a graph showing the relationship between the thickness of the interdigital transducer and the impedance ratios of the zeroth to third modes. [Figure 19] 2(d) is a graph showing the dependency of the impedance ratio of the first mode on the thickness of the LiTaO3 crystal substrate for the higher-order mode surface acoustic wave device [Cu electrode (metallization ratio 0.5) with a groove depth of 0.2λ / (0°, 126.5°, 0°) LiTaO3 crystal substrate / support substrate] when the support substrate is made of c-sapphire, Si, quartz crystal, Pyrex (registered trademark) glass, or lead glass. [Figure 20] This graph shows the dependence of the impedance ratio of the first mode on the substrate thickness of the LiNbO3 crystal for the higher-order mode surface acoustic wave device shown in Figure 2(d) [Cu electrode (metallization ratio 0.5) with a groove depth of 0.23λ / (0°, 116°, 0°) LiNbO3 crystal substrate / support substrate] when the support substrate is made of sapphire, Si, quartz crystal, Pyrex glass, or lead glass. [Figure 21] This graph shows the dependence of the frequency temperature coefficient of the higher-order mode surface acoustic wave device shown in Figure 2(d) [Al electrode (metallization ratio 0.5) with a groove depth of 0.3λ / (0°, 126.5°, 0°) LiTaO3 crystal substrate and (0°, 116°, 0°) LiNbO3 crystal substrate / support substrate] on the thickness of the LiTaO3 crystal substrate and LiNbO3 crystal for each linear expansion coefficient of the support substrate. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figures 2 to 21 relate to higher-order mode surface acoustic wave devices according to embodiments of the present invention. As shown in Figure 2(a), a higher-order mode surface acoustic wave device 10 utilizes a higher-order mode surface acoustic wave (SAW) and includes a piezoelectric substrate 11 and an interdigital transducer (IDT) 12.

[0032] Piezoelectric substrate 11 is made of LiTaO3 crystal or LiNbO3 crystal. Interdigital transducer 12 is embedded in the surface of piezoelectric substrate 11. The upper surface of interdigital transducer 12 may be flush with the surface of piezoelectric substrate 11, or may be below that plane and protrude from the surface of piezoelectric substrate 11. Hereinafter, the electrode thickness refers to the thickness of the electrode embedded in the groove.

[0033] As shown in FIG. 2(b), the higher-order mode surface acoustic wave device 10 may have a thin film 13 provided to cover the surface of the piezoelectric substrate 11 in the gaps between the interdigital transducers 12. The thin film 13 is, for example, a SiO2 thin film. The upper surface of the interdigital transducer 12 is on the same plane as the surface of the thin film 13. In the higher-order mode surface acoustic wave device 10, the interdigital transducer 12 may be flush with the surface of the piezoelectric substrate 11 or may be below it, as shown in FIG. 2(a). As shown in FIG. 2(c), the interdigital transducer 12 may be provided to protrude from the surface of the piezoelectric substrate 11.

[0034] As shown in FIG. 2(d), the higher-order mode surface acoustic wave device 10 may have a support substrate 14, in which the piezoelectric substrate 11 is made of a thin plate with a small thickness, and the support substrate 14 may be provided in contact with the surface of the piezoelectric substrate 11 opposite to the surface on which the interdigital transducers 12 are provided. The support substrate 14 may be, for example, a semiconductor or insulating substrate such as a silicon substrate, a quartz substrate, a sapphire substrate, a glass substrate, a quartz substrate, a germanium substrate, or an alumina substrate. In addition to the configuration shown in FIG. 2(d), a thin film 13 may be formed on the surface of the piezoelectric substrate 11, as shown in FIG. 2(b). In addition to the configuration shown in FIG. 2(d), the higher-order mode surface acoustic wave device 10 may have the interdigital transducers 12 protruding from the surface of the piezoelectric substrate 11, as shown in FIG. 2(c).

[0035] 2(d), the higher-order mode surface acoustic wave device 10 may further include a multilayer film 15 disposed between the piezoelectric substrate 11 and the support substrate 14, as shown in FIG. 2(f). The multilayer film 15 is, for example, an acoustic multilayer film formed by stacking multiple layers with different acoustic impedances. In addition to the configuration shown in FIG. 2(f), a thin film 13 may be formed on the surface of the piezoelectric substrate 11, as shown in FIG. 2(b), or an interdigital transducer 12 may protrude from the surface of the piezoelectric substrate 11, as shown in FIG. 2(c).

[0036] By embedding interdigital transducers 12 on the surface of piezoelectric substrate 11, higher-order mode surface acoustic wave device 10 can excite higher-order SAW modes (such as first, second, and third modes) and achieve higher-order modes with large impedance ratios. Higher-order modes are also called overtones, which excite frequencies approximately two, three, or four times higher. By utilizing these higher-order modes, higher-order mode surface acoustic wave device 10 can achieve higher frequencies and achieve good characteristics even in high-frequency bands of 3.8 GHz and above. Furthermore, by utilizing the higher-order modes, it is not necessary to make piezoelectric substrate 11 ultra-thin or reduce the period of the interdigital transducers, even in high-frequency bands of 3.8 GHz and above, and sufficient mechanical strength can be maintained.

[0037] The higher-order mode surface acoustic wave device 10 can be manufactured, for example, as follows. First, electrode grooves for embedding the interdigital transducers 12 are formed on the surface of the piezoelectric substrate 11. That is, a resist or the like is applied to the portions of the surface of the piezoelectric substrate 11 where the electrode grooves are not to be formed, and dry etching is performed using ions such as Ar to form the electrode grooves on the surface of the piezoelectric substrate 11. In this case, instead of the resist, a material other than the resist whose etching speed is slower than the etching speed of the piezoelectric substrate 11 may be used. Furthermore, instead of dry etching, a wet etching method may also be used.

[0038] Next, a metal film for the electrodes is formed over the entire surface of piezoelectric substrate 11 to a thickness sufficient to fill the electrode grooves down to the surface of piezoelectric substrate 11. The resist is then removed by wet etching or washing, etc., thereby forming interdigital electrodes 12 embedded in the electrode grooves. If the thickness of interdigital electrode 12 is not the desired thickness, a further step of adjusting the thickness of interdigital electrode 12 by etching or the like may be performed.

[0039] The impedance ratio, fractional bandwidth, and other characteristics were determined for the higher-order mode surface acoustic wave device 10 of each configuration shown in FIG. 2. Referring to FIG. 1(c), the impedance ratio is given by 20×log(Za / Zr), which is the ratio of the resonant impedance Zr at the lowest resonant frequency fr to the antiresonant impedance Za at the highest antiresonant frequency fa among the impedances of the resonant characteristics. The fractional bandwidth is given by (fa-fr) / fr. Referring to FIG. 1(a), the metallization ratio of the interdigital transducer 52 is given by the ratio of the width F of the electrode fingers of the interdigital transducer 52 along the propagation direction of the surface acoustic wave, divided by half the period (λ) of the electrode fingers (the sum of the width F of the electrode fingers and the gap G between the electrode fingers), i.e., F / (F+G)=2×F / λ.

[0040] As shown in Figure 3, the electrodes of the interdigital transducer 12 may be embedded in the substrate at an angle rather than perpendicular to the substrate surface. In such cases, the metallization ratio and electrode width are the effective metallization ratio and electrode width. That is, when the angle γ between the side of the electrode groove and the surface of the piezoelectric substrate 11 is less than 90 degrees, the surface width of each electrode is a, the bottom width is b, and the embedded depth is d, then the effective width c of the surface electrode is given by (a + b) / 2 and the metallization ratio is given by (c / (c + e)). The embedded electrode depth remains d.

[0041] Here, the period (λ) of interdigital transducer 12 is 1 μm, and the metallization ratio is 0.5. In other words, the width of the electrode fingers is 0.25 μm, and the gap between the electrode fingers is 0.25 μm. In the following, the Euler angles (φ, θ, ψ) are simply represented as (φ, θ, ψ). Furthermore, the thicknesses of piezoelectric substrate 11 and interdigital transducer 12 are represented as a ratio of the wavelength λ (interdigital transducer period) of the surface acoustic wave device used.

[0042] Figure 4 shows the impedance-frequency characteristics of a higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(a). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.36λ and is embedded 0.36λ from the surface of the piezoelectric substrate 11. Figures 4(a) and 4(b) show the impedance-frequency characteristics when the interdigital transducer 12 has a metallization ratio of 0.5, and Figure 4(c) shows the displacement distribution at the first-order mode resonant frequency. Figure 4(b) is an enlarged view of the first-order mode resonant frequency of Figure 4(a). Figure 4(d) shows the impedance-frequency characteristics when the interdigital transducer 12 has a metallization ratio of 0.7.

[0043] As shown in Figure 4(a), by embedding the interdigital transducer 12 in the piezoelectric substrate 11, it was confirmed that a zeroth-order mode with a resonant frequency of 4.5 GHz was obtained, which is 1.36 times the resonant frequency of 3.3 GHz of the zeroth-order mode of the conventional SAW device shown in Figure 1. Furthermore, as shown in Figures 4(a) and 4(b), it was confirmed that the first-order mode with a resonant frequency of 9.6 GHz, approximately twice the resonant frequency of the zeroth-order mode with a resonant frequency of 4.5 GHz, was excited to a greater extent. The fractional bandwidth of the first-order mode was 3%, and the impedance ratio was 67 dB, confirming that a larger impedance ratio was obtained than in the conventional SAW device shown in Figure 1. The resonant frequency of the first-order mode was approximately 2.9 times that of the conventional SAW device.

[0044] Furthermore, as shown in Figure 4(c), the first-order mode with a resonant frequency of 9.5 GHz is composed only of SH (shear horizontal) components. Since the resonant frequency of conventional SAW devices is also composed of SH components, it is clear that this is a higher-order mode (first-order mode) of the fundamental mode (zeroth order). Note that "L" in Figure 4(c) represents the longitudinal wave component, and "SV" represents the shear vertical component. Furthermore, as shown in Figure 4(d), by increasing the metallization ratio to 0.7, the resonant frequency of the first-order mode increased to 11.2 GHz, 1.2 times that of a metallization ratio of 0.5; the relative bandwidth increased to 3.4%, 13% wider; and the impedance ratio increased to 70 dB, 3 dB higher.

[0045] Figure 5 shows the impedance-frequency characteristics of a higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(c). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal. The interdigital transducer 12 is an Al electrode with a thickness of 0.38λ, embedded 0.36λ below the surface of the piezoelectric substrate 11, and protruding 0.02λ above the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5.

[0046] As shown in Figure 5, it was confirmed that the resonance frequency of the higher-order mode (first-order mode) was slightly higher than in Figures 4(a) and 4(b). Also, although the impedance ratio was reduced to 50 dB, the relative bandwidth was narrowed to 1%, which is suitable for narrowbanding. It was also confirmed that the excitation of the fundamental mode (zeroth order), which causes spurious noise, was small.

[0047] Figure 6 shows the impedance-frequency characteristics of the higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(d). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal with a thickness of 0.5λ. The interdigital transducer 12 is an Al electrode with a thickness of 0.36λ, and is embedded 0.36λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5. The support substrate 14 is made of silicon or quartz, both of which are 350 μm thick, and is bonded to the piezoelectric substrate 11 by adhesive or direct bonding. Figure 6(a) shows the impedance-frequency characteristics when the support substrate 14 is a silicon substrate, and Figure 6(b) shows the impedance-frequency characteristics when the support substrate 14 is a quartz substrate.

[0048] As shown in FIG. 6(a), when a Si support substrate is used, the resonant frequency of the first mode is 9 GHz, the fractional bandwidth is 2.8%, and the impedance ratio is 71 dB. Furthermore, as shown in FIG. 6(b), when a quartz substrate is used, the resonant frequency of the first mode is 9 GHz, the fractional bandwidth is 3.5%, and the impedance ratio is 68 dB. Comparing FIGS. 6(a) and 6(b) with FIG. 4(b), it is confirmed that the provision of the support substrate 14 increases the impedance ratio. To achieve a larger impedance ratio, the piezoelectric substrate 11 is preferably thinner than the support substrate 14, and is more preferably 20 wavelengths or less, and even more preferably 10 wavelengths or less.

[0049] Figure 7 shows the impedance-frequency characteristics of a higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(f). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal with a thickness of 0.5λ. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.36λ, and is embedded to a depth of 0.36λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5. The multilayer film 15 is composed of an acoustic multilayer film in which six SiO2 layers (thickness 0.25 μm) and Ta layers (thickness 0.25 μm) with different acoustic impedances are alternately stacked. The support substrate 14 is composed of a Si substrate with a thickness of 350 μm. Note that the number of layers in this acoustic film may be other than six.

[0050] As shown in Fig. 7, it was confirmed that the resonant frequency of the first mode was 9.5 GHz, the relative bandwidth was 2.6%, and the impedance ratio was 69 dB. Comparing Fig. 7 with Fig. 6(a), it was confirmed that the provision of the multilayer film 15 slightly narrowed the bandwidth and slightly reduced the impedance ratio.

[0051] Figure 8 shows the impedance-frequency characteristics of the higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(a). Figure 8(a) shows the impedance-frequency characteristics, and Figure 8(b) is an enlarged view of the first-order mode resonant frequency of Figure 8(a). The piezoelectric substrate 11 is a (0°, 116°, 0°) LiNbO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.35λ and is buried to a depth of 0.35λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5.

[0052] As shown in Figures 8(a) and 8(b), when the piezoelectric substrate 11 is made of LiNbO3 crystal, it was confirmed that a high-order mode (first mode) at 10.4 GHz was excited strongly, as in the case of LiTaO3 crystal (see Figure 4). The fractional bandwidth of the first mode was 6.4%, and the impedance ratio was 68 dB, which confirmed that the bandwidth was wider and the impedance ratio was larger than the first mode of LiTaO3 crystal shown in Figure 4(b).

[0053] Figure 9 shows the impedance-frequency characteristics of a higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(a). The piezoelectric substrate 11 is a (0°, 116°, 0°) LiNbO3 crystal. The interdigital transducer 12 is composed of a Cu electrode with a thickness of 0.24λ, and is buried to a depth of 0.24λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5.

[0054] As shown in FIG. 9, when the interdigital transducer 12 is made of Cu electrodes, the resonant frequency of the first mode is 9.5 GHz, which is slightly lower than that of the interdigital transducer 12 made of Al electrodes (see FIG. 8(a)). However, it was confirmed that even when the interdigital transducer 12 is made thinner (shallower) than the Al electrodes, an impedance ratio of 68 dB, which is similar to that of the Al electrodes, can be obtained.

[0055] FIG. 10 shows the relationship between the thickness of the interdigital transducer 12 and the relative bandwidth and impedance ratio of the first mode for a higher-order mode surface acoustic wave device 10 having the structure shown in FIG. 2(a). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal. The interdigital transducer 12 is composed of Al electrodes, Cu electrodes, or Au electrodes. The metallization ratio of the interdigital transducer 12 is 0.5. The relationship between the thickness of each electrode and the relative bandwidth and the relationship between the thickness of each electrode and the impedance ratio of the first mode when the thickness (depth) of each electrode is changed from 0.02λ to 0.6λ are shown in FIG. 10(a) and FIG. 10(b), respectively.

[0056] As shown in Figure 10(a), for the same thickness (depth), the Al electrode had the widest bandwidth, followed by the Cu electrode and the Au electrode, which had narrower bandwidths. It was also confirmed that the bandwidth increased with increasing thickness (depth) for each electrode. As shown in Figure 10(b), the impedance ratio was confirmed to be 50 dB or greater when the Al electrode was 0.15λ to 0.6λ, and when the Cu and Au electrodes were 0.16λ to 0.6λ. The impedance ratio was confirmed to be 60 dB or greater when the Al electrode was 0.23λ to 0.6λ, the Cu electrode was 0.18λ to 0.6λ, and the Au electrode was 0.25λ to 0.6λ. Furthermore, the impedance ratio was confirmed to be 65 dB or greater when the Al electrode was 0.3λ to 0.6λ, the Cu electrode was 0.29λ to 0.6λ, and the Au electrode was 0.55λ to 0.6λ.

[0057] The product of electrode thickness and metallization ratio is constant, so for example, if the metallization ratio is 0.5 and the electrode thickness is 0.15λ, and the metallization ratio is 0.75, the electrode thickness is 0.5 × 0.15λ / 0.75 = 0.10λ. Therefore, for example, if the metallization ratio is 0.5 and the Al electrode thickness is 0.15λ, then if the metallization ratio is 0.75, the Al electrode thickness must be 0.10λ or greater.

[0058] It is considered that the relationship between the impedance ratio and the thickness of each electrode is the same not only for the structure shown in Fig. 2(a) but also for the structures shown in Fig. 2(b) to Fig. 2(f). Also, the relationship between the impedance ratio and the thickness of each electrode is the same for a density of 1500 to 6000 kg / m 3 The electrode material (e.g., Ti, Mg alloy) is Al electrode and has a density of 6000 to 12000 kg / m 3 The electrode materials (e.g., Ag, Mo, Ni) are used with Cu electrodes and have a density of 12000-23000 kg / m 3Electrode materials (e.g., Pt, W, Ta, Hf) show the same tendency as Au electrodes. Also, when the electrode material used is an alloy or a laminate of different metals, the tendency of the relationship between the impedance ratio and the electrode thickness is determined by the average density calculated from the thickness and density of each material.

[0059] FIG. 11 shows the relationship between the Euler angles of the piezoelectric substrate 11 and the first-mode fractional bandwidth and impedance ratio for a higher-order mode surface acoustic wave device 10 having the structure shown in FIG. 2(a). The piezoelectric substrate 11 is a (0°, θ, 0°) LiTaO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.36λ and is embedded to a depth of 0.36λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5. The relationship between θ and the fractional bandwidth and the relationship between θ and the impedance ratio when the Euler angles, θ, are varied from 0° to 180° are shown in FIG. 11(a) and FIG. 11(b), respectively.

[0060] As shown in Figures 11(a) and 11(b), when θ = 112° to 140°, the relative bandwidth was 2.5 to 3.2%, and the impedance ratio was confirmed to be 50 dB or more. Furthermore, when θ = 120° to 132°, the relative bandwidth was 2.6 to 2.7%, and the impedance ratio was confirmed to be 60 dB or more. Furthermore, as shown in Figure 12, when φ = -20° to 20°, the impedance ratio was confirmed to be 50 dB or more, and when φ = -10° to 10°, the impedance ratio was confirmed to be 60 dB or more.

[0061] FIG. 13 shows the relationship between the thickness of the interdigital transducer 12 and the first-mode fractional bandwidth and impedance ratio for the higher-order mode surface acoustic wave device 10 having the structure shown in FIG. 2(a). The piezoelectric substrate 11 is a (0°, 116°, 0°) LiNbO3 crystal. The interdigital transducer 12 is composed of Al electrodes, Cu electrodes, or Au electrodes. The metallization ratio of the interdigital transducer 12 is 0.5. The relationship between the thickness of each electrode and the fractional bandwidth and the relationship between the thickness of each electrode and the impedance ratio when the thickness (depth) of each electrode is changed from 0.02λ to 0.6λ are shown in FIG. 13(a) and FIG. 13(b), respectively.

[0062] As shown in Figure 13(a), when the thickness (depth) is 0.1λ or greater, the Al electrode has the widest bandwidth, followed by the Cu electrode and the Au electrode, which have narrower bandwidths. Furthermore, for each electrode, when the thickness (depth) is 0.4λ or less, the bandwidth increases with increasing thickness (depth). As shown in Figure 13(b), the impedance ratio was confirmed to be 50 dB or greater when the Al electrode was 0.14λ to 0.6λ, the Cu electrode was 0.13λ to 0.6λ, and the Au electrode was 0.15λ to 0.6λ. Furthermore, the impedance ratio was confirmed to be 60 dB or greater when the Al electrode was 0.21λ to 0.6λ, the Cu electrode was 0.18λ to 0.6λ, and the Au electrode was 0.23λ to 0.6λ. As mentioned above, the electrode thickness x metallization ratio was constant.

[0063] It is considered that the relationship between the impedance ratio and the thickness of each electrode is the same not only for the structure shown in Fig. 2(a) but also for the structures shown in Fig. 2(b) to Fig. 2(f). Also, the relationship between the impedance ratio and the thickness of each electrode is the same for a density of 1500 to 6000 kg / m 3 The electrode material (e.g., Ti, Mg alloy) is Al electrode and has a density of 6000 to 12000 kg / m 3 The electrode materials (e.g., Ag, Mo, Ni) are used with Cu electrodes and have a density of 12000-23000 kg / m 3Electrode materials (e.g., Pt, W, Ta, Hf) show the same tendency as Au electrodes. Also, when the electrode material used is an alloy or a laminate of different metals, the tendency of the relationship between the impedance ratio and the electrode thickness is determined by the average density calculated from the thickness and density of each material.

[0064] FIG. 14 shows the relationship between the Euler angles of the piezoelectric substrate 11 and the first-mode fractional bandwidth and impedance ratio for a higher-order mode surface acoustic wave device 10 having the structure shown in FIG. 2(a). The piezoelectric substrate 11 is a (0°, θ, 0°) LiNbO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.3λ and is embedded to a depth of 0.3λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.5. The relationship between θ and the fractional bandwidth and the relationship between θ and the impedance ratio when the Euler angles, θ, are varied from 50° to 180° are shown in FIG. 14(a) and FIG. 14(b), respectively.

[0065] As shown in Figures 14(a) and 14(b), when θ = 78° to 153°, the relative bandwidth was 4.4 to 6.5%, and the impedance ratio was confirmed to be 50 dB or more. Furthermore, when θ = 87° to 143°, the relative bandwidth was 5.2 to 6.5%, and the impedance ratio was confirmed to be 60 dB or more. Furthermore, when θ = 94° to 135°, the relative bandwidth was 5.7 to 6.5%, and the impedance ratio was confirmed to be 65 dB or more. Furthermore, as shown in Figure 15, the impedance ratio was confirmed to be 50 dB or more when φ = -25° to 25°, 60 dB or more when φ = -20° to 20°, and 70 dB or more when φ = -10° to 10°.

[0066] FIG. 16 shows the relationship between the metallization ratio of the interdigital transducer 12 and the phase velocity and impedance ratio of the first mode in a higher-order mode surface acoustic wave device 10 having the structure shown in FIG. 2(a). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.36λ, and is buried to a depth of 0.36λ from the surface of the piezoelectric substrate 11. The relationship between the metallization ratio of the Al electrode and the phase velocity and the relationship between the metallization ratio and the impedance ratio are shown in FIG. 16(a) and FIG. 16(b), respectively, when the metallization ratio of the Al electrode is changed from 0.3 to 0.9.

[0067] As shown in Figure 16(a), the phase velocity was approximately 10,000 to 11,500 m / s, and it was confirmed that the phase velocity generally increased as the metallization ratio increased. Furthermore, as shown in Figure 16(b), it was confirmed that when the metallization ratio was 0.4 or greater, the impedance ratio was 50 dB or greater, when the metallization ratio was 4.5 or greater, the impedance ratio was 60 dB or greater, when the metallization ratio was 0.52 or greater, the impedance ratio was 65 dB or greater, and when the metallization ratio was 0.63 or greater, the impedance ratio was 70 dB or greater.

[0068] Figure 17 shows the impedance-frequency characteristics of a higher-order mode surface acoustic wave device 10 having the structure shown in Figure 2(a). The piezoelectric substrate 11 is a (0°, 126.5°, 0°) LiTaO3 crystal. The interdigital transducer 12 is composed of an Al electrode with a thickness of 0.2λ, and is embedded to a depth of 0.2λ from the surface of the piezoelectric substrate 11. The metallization ratio of the interdigital transducer 12 is 0.85.

[0069] As shown in Figure 17, it was confirmed that the first, second, and third modes, which are higher-order modes of the zeroth mode, were excited. As shown in Figures 4 and 8, when the metallization ratio was 0.5, the second and third modes were barely detectable, which suggests that higher-order modes such as the second and third modes are excited by increasing the metallization ratio.

[0070] Figures 18(a) and 18(b) show the relationship between the thickness of interdigital transducer 12 and the phase velocities of the zeroth- to third-order modes, and the relationship between the thickness of interdigital transducer 12 and the impedance ratios of the zeroth- to third-order modes, when the thickness of interdigital transducer 12 is varied from 0.05λ to 0.55λ with the same structure as in Figure 17 . As shown in Figure 18(a), when the thickness of interdigital transducer 12 is 0.3λ, for example, it was confirmed that higher-order modes, such as the first-order mode, whose phase velocity is approximately 2.7 times that of the zeroth-order mode, the second-order mode, whose phase velocity is 4.7 times that of the zeroth-order mode, and the third-order mode, whose phase velocity is approximately 6.9 times that of the zeroth-order mode, are excited. Furthermore, as shown in Figure 18(b), the impedance ratios were 47 dB for the zeroth-order mode, 57 dB for the first-order mode, 40 dB for the second-order mode, and 45 dB for the third-order mode, confirming that these were sufficiently high levels for use.

[0071] Table 1 shows the density, longitudinal wave velocity, and shear wave velocity of the support substrate for the higher-order mode surface acoustic wave device [groove electrode / LiTaO3 crystal or LiNbO3 crystal substrate / support substrate] shown in Figure 2(d). The longitudinal wave velocity is ((c 33 / density)), and the shear wave speed is ((c 44 / density) square root), where C ij is the elastic stiffness constant. They are grouped into five groups, A, B, C, D, and E, according to the shear wave velocity.

[0072] [Table 1]

[0073] Figure 19 shows the dependence of the impedance ratio on the LiTaO3 crystal substrate thickness for the higher-order mode surface acoustic wave device shown in Figure 2(d) [Cu electrodes (metallization ratio 0.5) with a groove depth of 0.2λ / (0°, 126.5°, 0°) LiTaO3 crystal substrate / support substrate] when the support substrate is made of sapphire, silicon, quartz, Pyrex glass, or lead glass. In the figure, open symbols indicate frequency characteristics without ripples within the band, while solid symbols indicate characteristics with ripples within the band. For both support substrates, the impedance ratio of 62 dB for a LiTaO3 crystal substrate alone without a support substrate is consistent for LiTaO3 crystal thicknesses of 20 wavelengths or more. However, the impedance ratio is significantly higher for LiTaO3 crystal thicknesses of 20 wavelengths or less.

[0074] In the case of lead glass (Group A, 2414 m / s, shear wave velocity 2000-3000 m / s, Table 1), which has a shear wave velocity of 2414 m / s, much slower than the 3604 m / s of the LiTaO3 crystal shown in Table 1, an impedance ratio of 62 dB can be obtained without in-band ripple for LiTaO3 crystal thicknesses of 0.2 λ or more and less than 20 λ, and an impedance ratio of 63 dB or more can be obtained for 10 wavelengths or less. In the case of sapphire (Group E, 6073 m / s, shear wave velocity 6001-8000 m / s, Table 1), which has a shear wave velocity much faster than LiTaO3 crystal, an impedance ratio of 62 dB or more can be obtained without in-band ripple for LiTaO3 crystal thicknesses of 0.2 λ or more and less than 20 λ, and an impedance ratio of 63 dB or more can be obtained for LiTaO3 crystal thicknesses of 0.2 λ or more and 10 λ.

[0075] However, in the case of support substrates such as Pyrex glass in Group B, which has a shear wave sound velocity of 3000 to 4220 m / s, quartz crystal in Group C, which has a shear wave sound velocity of 4220 to 5000 m / s, and a Si substrate in Group D, which has a shear wave sound velocity of 4220 to 5000 m / s, which is close to the shear wave sound velocity of LiTaO3 crystal, ripples occur within the band when the LiTaO3 crystal thickness is 0.2λ or more and less than 2λ, an impedance ratio of 62 dB or more is obtained when the LiTaO3 crystal thickness is 2λ to less than 20λ, and an impedance ratio of 64.5 dB or more is obtained when the LiTaO3 crystal thickness is 2λ to 10λ.

[0076] Figure 20 shows the dependence of the impedance ratio on the LiTaO3 crystal substrate thickness for the higher-order mode surface acoustic wave device shown in Figure 2(d) [Cu electrodes (metallization ratio 0.5) with a groove depth of 0.23λ / (0°, 112°, 0°) LiNbO3 crystal substrate / support substrate] when the support substrate is composed of sapphire, Si, quartz, Pyrex glass, or lead glass. In the figure, open symbols indicate frequency characteristics without ripple within the band, while solid symbols indicate characteristics with ripple within the band. For both support substrates, the impedance ratio of 68 dB for a LiNbO3 crystal substrate alone without a support substrate is consistent for LiNbO3 crystal thicknesses of 20 wavelengths or more, but the impedance ratio is significantly higher for LiNbO3 crystal thicknesses of less than 20 wavelengths.

[0077] In the case of lead glass (Group A, Table 1) with a shear wave velocity of 2414 m / s, which is much slower than the 3604 m / s shear wave velocity of the LiTaO3 crystal shown in Table 1, an impedance ratio of 68 to 80 dB or more can be obtained without in-band ripples for LiTaO3 crystal thicknesses of 0.2 λ or more and less than 20 λ, and an impedance ratio of 71.5 dB or more can be obtained for 10 wavelengths or less. In the case of sapphire (Group C, Table 1) with a shear wave velocity of 6073 m / s, which is much faster than LiTaO3 crystal, an impedance ratio of 68 to 71 dB can be obtained without in-band ripples for LiTaO3 crystal thicknesses of 0.2 λ or more and less than 20 λ, and an impedance ratio of 70 dB or more can be obtained for LiTaO3 crystal thicknesses of 10 wavelengths or less.

[0078] However, in the case of Pyrex, quartz, or Si support substrates with shear wave acoustic velocities of 3000 to 6000 m / s, which are groups B, C, and D in Table 1 and are close to the shear wave acoustic velocity of LiTaO3 crystal, ripples occur within the band when the LiTaO3 crystal thickness is 0.2λ or more and less than 2λ, and an impedance ratio of 68 to 77 dB is obtained when the LiTaO3 crystal thickness is from 2λ to less than 20λ, and an impedance ratio of 71.5 to 77 dB is obtained when the LiTaO3 crystal thickness is from 2λ to less than 10λ.

[0079] Furthermore, if there is a thin film such as an SiO2 film, SiO film, or SiOF or other SiO compound film between the LiTaO3 crystal or LiNbO3 crystal piezoelectric plate and the support substrate, this is considered to be the average shear wave sound velocity between that film and the support substrate underneath. Even if an SiO2 film, SiO compound film, or acoustic multilayer film is interposed between the piezoelectric substrate and the support substrate, the apparent average sound velocity within two wavelengths of these films will be determined by the sound velocity that belongs to Groups A, B, or C in Table 1, and the optimum film thickness for the piezoelectric substrate will be determined. In this case, the weight of the material of the first layer in contact with the piezoelectric substrate will be 70%, and all subsequent layers will be 30%. For example, if the first layer of SiO2 film (shear wave sound velocity 3572 m / s) is 0.5 wavelengths thick and the sapphire (shear wave sound velocity 6073 m / s) support substrate is 1.5 wavelengths thick, then the thickness will be (3572 x 0.5 x 0.7 + 6073 x 1.5 x 0.3) = 3983 m / s, and a LiTaO3 crystal or LiNbO3 crystal substrate with the optimal substrate thickness for Group E should be used.

[0080] Table 2 shows the linear expansion coefficients of LiTaO3 crystals and LiNbO3 crystals, as well as the linear expansion coefficients of representative substrates that are smaller than LiTaO3 crystals and LiNbO3 crystals. Table 2 also shows the linear expansion coefficients of various support substrates used in the higher-order mode surface acoustic wave device [groove electrode / piezoelectric substrate / support substrate] structure shown in Figure 2(d).

[0081] [Table 2]

[0082] Figure 21 shows the dependence of the frequency temperature coefficient on the linear expansion coefficient of the support substrate for the higher-order mode surface acoustic wave device shown in Figure 2(d) [Al electrode (metallization ratio 0.5) with a groove depth of 0.3λ / (0°, 126.5°, 0°) LiTaO3 crystal substrate / support substrate] and Al electrode (metallization ratio 0.5) with a groove depth of 0.3λ / (0°, 112°, 0°) LiNbO3 crystal substrate / support substrate]. The frequency temperature coefficient on the vertical axis is expressed as the frequency change rate per 1°C of temperature when using a LiTaO3 crystal or LiNbO3 crystal / support substrate, i.e., (maximum frequency change between -20 and 80°C / (maximum temperature change between 100 and 20°C (80 in this case))). The left and right vertical axes are the frequency temperature coefficients when using a LiTaO3 crystal substrate and a LiNbO3 crystal substrate, respectively. The horizontal axis represents the ratio of the thickness of the support substrate to the thickness of the piezoelectric substrate, that is, (thickness of the support substrate / thickness of the LiTaO3 crystal substrate or LiNbO3 crystal substrate).

[0083] When an Al groove electrode is provided only on the support substrate, the frequency temperature coefficients are -45 and -100 ppm / °C, respectively, but the linear expansion coefficient is 0.5 × 10 -6 When a support substrate with a linear expansion coefficient of 3.35×10 is used, a frequency temperature coefficient better than -25 ppm / °C can be obtained with LiTaO3 crystal and -35 ppm / °C can be obtained with LiNbO3 crystal when the thickness ratio of the piezoelectric substrate to the support substrate is 2.5 or more. -6 / ℃, the thickness ratio of the piezoelectric substrate to the support substrate is 4 or more, and the linear expansion coefficient is 8.4 × 10 -6 / °C, the thickness ratio of the piezoelectric substrate to the support substrate is 6.7 or more, and the linear expansion coefficient is 10.4 × 10 -6 When a support substrate with a thickness ratio of 1 / °C is used, a frequency temperature coefficient better than -25 ppm / °C can be obtained for LiTaO3 crystal and -35 ppm / °C for LiNbO3 crystal when the thickness ratio of the piezoelectric substrate / support substrate is 8 or more. The relationship between this linear expansion coefficient α and the thickness ratio TR of the piezoelectric substrate / support substrate is expressed by the following equation (2): TR = α × 0.55 × 10 6 + 2.18 (2)

[0084] Therefore, it is sufficient to use a piezoelectric substrate and a support substrate with a thickness ratio of piezoelectric substrate / support substrate larger than the TR obtained by equation (2). Even when an SiO2 film, SiO compound film, or acoustic multilayer film is interposed between the piezoelectric substrate and the support substrate, the TR can be calculated from the average linear expansion coefficient according to the thickness and the total thickness. For the piezoelectric substrate and the support substrate, the linear expansion coefficient is smaller than that of LiTaO3 crystal and LiNbO3 crystal shown in Table 2, 10.4 × 10 -6 It is desirable to use a support substrate with a linear expansion coefficient of 1 / °C or less, and even better still, one with a linear expansion coefficient of 1 / °C or less. [Explanation of symbols]

[0085] 10 Higher-order mode surface acoustic wave devices 11 Piezoelectric substrate 12 Interdigital transducer (IDT) 13 Thin Film 14 Support substrate 15 Multilayer film

Claims

1. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiTaO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of a Ti, Al, and Mg alloy and is embedded into the one surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.075 to 0.

3.

2. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiTaO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Ag, Mo, Cu, Ni, Pt, Au, W, Ta, and Hf, and is embedded into the one surface of the piezoelectric substrate to a depth at which the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.08 to 0.

3.

3. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiTaO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of a Ti, Al, and Mg alloy and is embedded into the piezoelectric substrate from the one surface to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.115 to 0.

3.

4. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiTaO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Ag, Mo, Cu, and Ni and is embedded into the one surface of the piezoelectric substrate to a depth at which the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.09 to 0.

3.

5. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiTaO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Pt, Au, W, Ta, and Hf and is embedded into the one surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.125 to 0.

3.

6. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of a Ti, Al, and Mg alloy and is embedded into the one surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.07 to 0.

3.

7. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Ag, Mo, Cu, and Ni and is embedded into the one surface of the piezoelectric substrate to a depth at which the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.065 to 0.

3.

8. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Pt, Au, W, Ta, and Hf and is embedded into the one surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.075 to 0.

3.

9. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of a Ti, Al, and Mg alloy and is embedded into the piezoelectric substrate from the one surface to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.105 to 0.

3.

10. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; an interdigital transducer embedded in the surface of the piezoelectric substrate; Including, the interdigital transducer comprises at least one of Ag, Mo, Cu, and Ni and is embedded into the one surface of the piezoelectric substrate to a depth at which the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.09 to 0.

3.

11. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: LiNbO ３ forming an electrode groove on one surface of a piezoelectric substrate including a crystal; forming an interdigital electrode embedded in the electrode groove; Including, the interdigital transducer comprises at least one of Pt, Au, W, Ta, and Hf and is embedded into the one surface of the piezoelectric substrate to a depth where the wavelength / metallization ratio of the surface acoustic wave is in the range of 0.115 to 0.

3.

12. 6. The method of claim 1, wherein the piezoelectric substrate has Euler angles in the ranges (0°±20°, 112°-140°, 0°±5°), or crystallographically equivalent Euler angles thereto.

13. The method according to any one of claims 1 to 5, wherein the piezoelectric substrate has Euler angles in the ranges (0°±10°, 120°-132°, 0°±5°) or crystallographically equivalent Euler angles thereto.

14. 12. The method of claim 6, wherein the piezoelectric substrate has Euler angles in the ranges (0°±25°, 78°-153°, 0°±5°), or crystallographically equivalent Euler angles thereto.

15. 12. The method of claim 6, wherein the piezoelectric substrate has Euler angles in the ranges (0°±20°, 87°-143°, 0°±5°), or crystallographically equivalent Euler angles thereto.

16. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: forming a support substrate; On the support substrate, LiTaO ３ Crystal or LiNbO ３ forming a piezoelectric substrate including a crystal; forming an electrode groove on one surface of the piezoelectric substrate; forming an interdigital electrode embedded in the electrode groove; Including, the support substrate is provided in contact with a surface of the piezoelectric substrate opposite to the surface on which the interdigital transducer is provided, The method, wherein the shear wave acoustic velocity or equivalent shear wave acoustic velocity of the support substrate is in the range of 2000 to 3000 m / s or 6000 to 8000 m / s, and the thickness of the piezoelectric substrate is in the range of 0.2 wavelengths to 20 wavelengths.

17. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: forming a support substrate; On the support substrate, LiTaO ３ Crystal or LiNbO ３ forming a piezoelectric substrate including a crystal; forming an electrode groove on one surface of the piezoelectric substrate; forming an interdigital electrode embedded in the electrode groove; Including, the support substrate is provided in contact with a surface of the piezoelectric substrate opposite to the surface on which the interdigital transducer is provided, The method, wherein the shear wave acoustic velocity or equivalent shear wave acoustic velocity of the support substrate is in the range of 3000 to 6000 m / s, and the thickness of the piezoelectric substrate is in the range of 2 to 20 wavelengths.

18. 1. A method for fabricating a device utilizing higher-order mode surface acoustic waves, comprising: forming a support substrate; On the support substrate, LiTaO ３ Crystal or LiNbO ３ forming a piezoelectric substrate including a crystal; forming an electrode groove on one surface of the piezoelectric substrate; forming an interdigital electrode embedded in the electrode groove; Including, the support substrate is provided in contact with a surface of the piezoelectric substrate opposite to the surface on which the interdigital transducer is provided, The linear expansion coefficient of the support substrate is 10.4×10 －６ / °C or less, and the thickness ratio of the support substrate to the piezoelectric substrate is equal to or greater than the value of TR defined by the following formula (1), where α is a linear expansion coefficient: TR=α×0.55×10 ６ + 2.18 (1)

19. The method of claim 1 , wherein the interdigital transducers are formed to protrude from the one surface of the piezoelectric substrate.

20. 20. The method of claim 1, further comprising forming a thin film or substrate in contact with the piezoelectric substrate.

21. Further comprising forming a support substrate and / or a multilayer film; The method according to claim 1 , wherein the support substrate and / or the multilayer film is provided on a surface of the piezoelectric substrate opposite to the one surface on which the interdigital transducer is provided.

22. The method of claim 21 , wherein the support substrate comprises a material other than a metal.

23. 23. The method of claim 22, wherein the support substrate comprises at least one of Si, quartz, sapphire, glass, quartz, germanium, and alumina.

24. 24. The method according to claim 21, wherein the multilayer film includes an acoustic multilayer film formed by stacking a plurality of layers having different acoustic impedances.

25. 25. The method of claim 1, wherein the metallization ratio of the interdigital transducer is 0.45 or greater.

26. 25. The method of claim 1, wherein the metallization ratio of the interdigital transducer is 0.63 or greater.

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