Surface acoustic wave device

Abstract

To provide a surface acoustic wave device having a high Q in a fundamental mode and having a high sound velocity and capable of exciting a higher-frequency surface acoustic wave in a higher-order mode by using a quartz thin plate.SOLUTION: A surface acoustic wave device includes: a quartz thin plate 11: an electrode 12 provided on one surface of the quartz thin plate 11; and a support substrate 13 provided on the other surface side of the quartz thin plate 11 and having a sound velocity higher than that of the surface acoustic wave in the quartz thin plate 11 or having a relative dielectric constant lower than that of quartz. A short-circuit electrode 15 may be provided between the quartz thin plate 11 and the support substrate 13. In addition, the support substrate 13 may have a thin film 14 made of SiO2 or AlN on the surface on the quartz thin plate 11 side.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a surface acoustic wave device. [Background technology]

[0002] In recent years, the global spread of the Internet and mobile communications has led to a significant increase in the amount of data transmitted and received over communication lines and the Internet. In particular, mobile communications, such as those for smartphones and tablet devices, are using higher frequencies to achieve higher speeds and larger capacities, and higher frequencies are also required for base stations. Meanwhile, the increased speed and capacity of optical communications in data centers and backbone networks requires higher speeds and larger capacities in the optical transceivers used there, which in turn requires high-frequency oscillators. Furthermore, atomic clocks require high-frequency oscillators, such as those with frequencies of 3.4 and 4.6 GHz.

[0003] In network communications and related fields, crystal oscillators are used to provide processor clock heartbeat signals, data link bit timing, data conversion sampling times, and master frequencies for tuners and synthesizers. These crystal oscillators require temperature stability as a reference oscillator, so acoustic wave elements such as bulk acoustic wave resonators (BAWR) using an AT-cut quartz substrate or surface acoustic wave resonators (SAWR) using an ST-cut quartz substrate are commonly used. Furthermore, crystal reference oscillators are required to have stable temperature characteristics, excellent oscillation stability, low noise, and low jitter, which necessitates a crystal resonator with a high Q.

[0004] Conventionally, high-frequency oscillators have been made by multiplying the fundamental frequency of a typical BAWR or SAWR in a circuit, but multiplication generates noise, jitter, etc., which limits how high the frequency can be increased. Therefore, in order to suppress the generation of noise, etc., it is thought that an acoustic wave element with a high Q, or an acoustic wave element with a small multiplication factor or that can be used without multiplication is needed.

[0005] In elastic wave devices using bulk acoustic waves (BAW), the frequency is determined by [sound velocity in the substrate divided by twice the substrate thickness]. Due to limitations in thinning the quartz substrate, limitations in microfabrication of electrodes, and the precision of stable thickness, the upper limit for practically used BAWR is 54 MHz (see, for example, Non-Patent Document 1), making it difficult to achieve higher frequencies.

[0006] In devices (resonators, filters) using surface acoustic waves (SAW), the surface acoustic waves are excited by an interdigital transducer (IDT) formed on the substrate, and the center frequency of the surface acoustic waves is determined by [sound velocity in the substrate / period λ of the IDT]. From this equation, it can be seen that there are two ways to increase the frequency of SAW devices: to use a substrate with a high sound velocity, or to shorten the wavelength of the IDT. Meanwhile, to achieve a high Q factor, the only solution available until now has been to use epitaxial aluminum electrodes as the electrodes forming the IDT (see, for example, Non-Patent Document 2).

[0007] Among quartz substrates, there is a moderate electromechanical coupling coefficient k for Rayleigh waves. 2 (k 2 The ST-cut X-propagation (ST-X or ST-cut X) quartz substrate is known as a substrate with a stable temperature characteristic and a sound velocity of 3,158 m / s, and an electromechanical coupling coefficient k 2 is 0.16% (see, for example, Non-Patent Document 3), and the upper frequency limit of a quartz SAW resonator is 2 GHz (see, for example, Non-Patent Document 4).

[0008] In addition to the ST cut, LST cut quartz substrates also exist (see, for example, Non-Patent Document 3), but they are rarely used due to the presence of a small amount of leakage component.The present inventors have also reported that a resonator having an interdigital transducer (IDT) made of a heavy metal such as Au, Ta, or W formed on a quartz substrate can excite a leaky surface acoustic wave (LSAW) containing only the SH component, which has a large electromechanical coupling coefficient, a large reflection coefficient, and excellent temperature stability (see, for example, Non-Patent Document 5). [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] Teruhisa Miyazawa, "Crystal Device Basics Course 4: How is the Oscillation Frequency of a Crystal Oscillator Determined?" [online], April 16, 2011, EE Times Japan, [Retrieved May 11, 2023], Internet: https: / / eetimes.itmedia.co.jp / ee / articles / 1104 / 05 / news111.html [Non-patent document 2] Eiji Ieki and Atsushi Sakurai, "SAW Resonator Using Epitaxial Al Electrodes," IEICE Transactions on Electronics, Information and Communication Engineers, Vol. J76-A, No. 2, February 1993, pp. 145-152 [Non-patent document 3] CS Lam and DE Hol, “A COMPARISON OF TEMPERATURE PERFORMANCE OF SAW RLTERS MADE ON ST- AND LST-CUT QUARTZ”, Proc. IEEE Ultrason. Symp, 1988, p.274 [Non-patent document 4] Seiko Epson Corporation, Technical Notes, "Methods for Obtaining High-Frequency Output (Part 2: SAW Oscillators)," [online], [Retrieved June 20, 2023], Internet: https: / / www5.epsondevice.com / ja / information / technical_info / pdf / tech_notes2013_06saw.pdf [Non-patent document 5] M. Kadota et al., “Resonator filters using shear horizontal-type leaky surface acoustic wave consisting of heavy-metal electrode and quartz substrate”, IEEE trans. Ultrason. Ferroelec. Freq. Contr., Feb. 2004, vol.51, No.2, p.202-210 [Non-patent document 6] JJ Campbell and WR Jone, “A Method for Estimating Optimal Crystal Cuts and Propagation Directions for Excitation of Piezoelectric Surface Waves”, IEEE Trans. Sonic. & Ultrason., Oct. 1968, Vol.SU-15, No.4, p.209-217 Summary of the Invention [Problem to be solved by the invention]

[0010] SAW devices using quartz substrates have a structure in which an IDT made of Al is configured on a quartz substrate such as ST-X, and as described in Non-Patent Document 2, there was a problem in that the only way to increase the Q factor was to reduce the resistance of the Al-IDT.

[0011] On the other hand, in order to increase the frequency of SAW devices that use quartz substrates, it is sufficient to use quartz substrates with a high acoustic velocity. However, as described in Non-Patent Document 3, an ST-cut quartz substrate with a coupling coefficient of 0.1% or more and stable temperature characteristics has a Rayleigh wave acoustic velocity of 3,158 m / s. This poses a problem in that it is difficult to realize SAW devices that exceed 2 GHz, as described in Non-Patent Document 4, for example.

[0012] The present invention has been made in response to these problems, and aims to provide a surface acoustic wave device that uses a thin quartz crystal plate and has a high Q in the fundamental mode, as well as a surface acoustic wave device that can excite surface acoustic waves at higher frequencies and with faster acoustic velocities in higher-order modes. [Means for solving the problem]

[0013] In order to achieve the above object, the surface acoustic wave device of the present invention is a surface acoustic wave device that utilizes surface acoustic waves, and is characterized by having a quartz crystal thin plate, an electrode provided on one surface of the quartz crystal thin plate, and a support substrate provided on the other surface of the quartz crystal thin plate, which has a sound velocity faster than the sound velocity of the surface acoustic wave in the quartz crystal thin plate or has a dielectric constant smaller than the dielectric constant of quartz.

[0014] When the support substrate provided on the other surface of the quartz crystal plate has a faster acoustic velocity than the surface acoustic wave in the quartz crystal plate, the surface acoustic wave device according to the present invention can excite surface acoustic waves with faster acoustic velocities and higher frequencies than devices consisting only of a quartz crystal plate without a support substrate. In this case, to achieve higher frequencies, the shear wave acoustic velocity of the excited surface acoustic waves is preferably 5000 m / s or higher, and more preferably 5385 m / s or higher. Thus, by using a quartz crystal plate and a support substrate with a higher acoustic velocity, it is possible to excite surface acoustic waves with higher frequencies, making it possible to realize, for example, a high-frequency resonator.

[0015] Furthermore, when the dielectric constant of the support substrate is smaller than that of quartz, the surface acoustic wave device according to the present invention can achieve a higher Q factor than a device consisting only of a quartz substrate without a support substrate. In this way, by using a thin quartz plate and a support substrate with a lower dielectric constant, it is possible to excite surface acoustic waves with a higher Q factor.

[0016] In the surface acoustic wave device according to the present invention, the surface acoustic waves excited are, for example, fundamental modes of Rayleigh waves and LSAWs (leaky surface acoustic waves), and higher modes thereof including Sezawa waves. In the surface acoustic wave device according to the present invention, the electrode provided on one surface of the quartz thin plate is preferably an interdigital transducer (IDT).

[0017] In the surface acoustic wave device according to the present invention, the quartz crystal thin plate is preferably made of an ST-cut X quartz crystal thin plate, an LST-cut X quartz crystal thin plate, an AT-cut 90°X propagation quartz crystal thin plate (rotated 90° counterclockwise from the X axis; hereinafter, also referred to as AT-90°X or AT-cut 90°X) quartz crystal thin plate, or an ST-cut 90°X propagation quartz crystal thin plate. In this case, the impedance ratio can be increased, and a high Q can be achieved. In addition, the electromechanical coupling coefficient k 2 This allows for a large frequency, resulting in good energy conversion efficiency. In addition, the frequency temperature dependency can be reduced, resulting in excellent temperature stability.

[0018] In the surface acoustic wave device according to the present invention, when the supporting substrate is high speed, the supporting substrate may be made of any material as long as it has a sound velocity faster than the sound velocity of the surface acoustic wave in the quartz thin plate, and for example, Si, sapphire, spinel, SiC, Si x N y(x and y are composition ratios), TiO2, AlN, or diamond. Furthermore, when the support substrate has a low dielectric constant, the support substrate may be made of any material as long as it has a dielectric constant lower than that of the quartz crystal thin plate, such as synthetic quartz, fused quartz, quartz glass, or low-dielectric-constant glass. For example, the relative dielectric constant of fused quartz is 3.75, which is lower than the relative dielectric constant of quartz crystal, 4.52.

[0019] In the surface acoustic wave device according to the present invention, the support substrate may have a thin film made of a low dielectric constant material, such as SiO2, SiOF, ScAlN, or AlN, on the surface facing the quartz crystal thin plate. In this case, a high Q can be achieved. Furthermore, when the support substrate is used at high speed, the Q can be increased and the sound velocity in the support substrate can be made faster, thereby exciting a high Q and high frequency surface acoustic wave.

[0020] The surface acoustic wave device according to the present invention may have a short-circuit electrode disposed between the quartz crystal thin plate and the support substrate. Furthermore, if the support substrate has a thin film made of a low-dielectric-constant material, a short-circuit electrode may be disposed on the side of the thin film opposite the quartz crystal substrate. In these cases, surface acoustic waves with high acoustic velocities and high frequencies can be excited. The short-circuit electrode (also called a floating electrode) may be made of the same material as the electrode disposed on one surface of the quartz crystal thin plate, or may be made of a different material. When the electrode disposed on one surface of the quartz crystal thin plate is an interdigital transducer, the short-circuit electrode may be connected to one of the interdigital transducers, or may be connected to a common electrode other than that of the surface acoustic wave device according to the present invention.

[0021] In the surface acoustic wave device according to the present invention, in order to excite a surface acoustic wave with a high acoustic velocity and a high frequency, the support substrate may have Euler angles (φ, θ, ψ), where φ and θ are predetermined angles, and the ψ angle may be ±10°, the angle at which the power flow angle of the propagation of the surface acoustic wave becomes zero, or the ψ angle may be ±5°, the angle at which the power flow angle of the propagation of the surface acoustic wave becomes zero. Here, the power flow angle (PFA) is defined as tan -1 It is the angle defined by (1 / V ∂ψ / ∂V), and the ψ at which ∂ψ / ∂V is zero is the angle at which PFA = 0. In other words, the angle ψ at which the velocity of the surface acoustic wave does not change when the angle of ψ is changed is the angle at which PFA = 0, and when the change in the velocity of the surface acoustic wave against the angle of ψ is plotted, it is the angle of ψ at which the velocity of the surface acoustic wave gives local maximum and minimum values.

[0022] In the surface acoustic wave device according to the present invention, when the support substrate has a low dielectric constant, to excite a high-Q acoustic device, for example, the surface acoustic wave is in the fundamental mode of a Rayleigh wave, the quartz thin plate is made of an ST-cut X-shaped quartz thin plate, and has a thickness of 0.01λ to 3λ, where λ is the wavelength of the surface acoustic wave, and the support substrate may be made of synthetic quartz, fused silica, or quartz glass. Alternatively, the surface acoustic wave is in the fundamental mode of a LSAW, the quartz thin plate is made of an LST-cut X-shaped quartz thin plate, and has a thickness of 0.01λ to 3λ, and the support substrate may be made of synthetic quartz, fused silica, or quartz glass. Alternatively, the surface acoustic wave is in the fundamental mode of a LSAW, and the quartz thin plate is made of an AT-cut 90°X-shaped quartz thin plate or an ST-cut 90°X-shaped quartz thin plate, and has a thickness of 0.01λ to 3λ, and may be made of synthetic quartz, fused silica, or quartz glass.

[0023] In a surface acoustic wave device according to the present invention, in consideration of the temperature coefficient of frequency (TCF), to set the TCF to zero or a value close to zero, the surface acoustic wave may be a Rayleigh wave or LSAW in zeroth or higher mode, the quartz crystal thin plate may have Euler angles of (0°, 132.75° to 180°, 0°), and the support substrate may have a positive temperature coefficient of frequency (TCF). Alternatively, the surface acoustic wave may be a Rayleigh wave or LSAW in zeroth or higher mode, the quartz crystal thin plate may have Euler angles of (0°, 15° to 45°, 0°), (0°, 60° to 132.75°, 0°), or (0°, 40° to 125°, 90°), and the support substrate may have a positive temperature coefficient of frequency (TCF). The surface acoustic wave may be a Rayleigh wave or LSAW in zeroth mode or a higher mode, the quartz crystal thin plate may have Euler angles of (0°, 0° to 132.75°, 0°), and the support substrate may have a negative temperature coefficient of frequency (TCF).The surface acoustic wave may be a Rayleigh wave or LSAW in zeroth mode or a higher mode, the quartz crystal thin plate may have Euler angles of (0°, 0° to 15°, 0°), (0°, 45° to 60°, 0°), (0°, 132.75° to 180°, 0°), (0°, 0° to 40°, 90°), or (0°, 125° to 180°, 90°), and the support substrate may have a negative temperature coefficient of frequency (TCF).

[0024] Furthermore, when considering this TCF, in order to achieve a high Q, for example, the surface acoustic wave may be in the zeroth mode, the quartz crystal thin plate may have a thickness of 0.09λ to 1.5λ, where λ is the wavelength of the surface acoustic wave, and the support substrate may have a relative dielectric constant of less than 4.52. Furthermore, in order to achieve a high Q and excite high-frequency surface acoustic waves, for example, the surface acoustic wave may be in the zeroth mode, the quartz crystal thin plate may have a thickness of 0.09λ to 1.5λ, and the support substrate may have a shear wave acoustic velocity of 5385 m / s or higher, or may have a thin film made of SiO2, SiOF, ScAlN, or AlN on the surface facing the quartz crystal thin plate.

[0025] Furthermore, when considering this TCF, in order to achieve a high Q and excite high-frequency surface acoustic waves, for example, the surface acoustic waves may be in first-order mode, the quartz crystal thin plate may have a thickness of 0.12λ to 1.5λ, where λ is the wavelength of the surface acoustic waves, and the support substrate may have a shear wave acoustic velocity of 5385 m / s or higher, or may have a thin film made of SiO2, SiOF, ScAlN, or AlN on the surface facing the quartz crystal thin plate.Also, the surface acoustic waves may be in second-order mode of Rayleigh waves, the quartz crystal thin plate may have a thickness of 0.5λ to 0.73λ, and the support substrate may have a shear wave acoustic velocity of 5385 m / s or higher, or may have a thin film made of SiO2, SiOF, ScAlN, or AlN on the surface facing the quartz crystal thin plate. Furthermore, the surface acoustic wave may be a third-order Rayleigh wave mode, the quartz crystal thin plate may have a thickness of 0.6λ to 1λ, and the support substrate may have a shear wave sound velocity of 5385 m / s or more, or may have a thin film made of SiO2, SiOF, ScAlN, or AlN on the surface facing the quartz crystal thin plate.

[0026] More specifically, for example, the surface acoustic wave may be a second-order Rayleigh wave mode, the quartz crystal thin plate may be an ST-cut X-type quartz crystal thin plate, and have a thickness of 0.45λ to 0.7λ, where λ is the wavelength of the surface acoustic wave, and the support substrate may be made of sapphire or Si. Alternatively, the surface acoustic wave may be a third-order Rayleigh wave mode, the quartz crystal thin plate may be an ST-cut X-type quartz crystal thin plate, and have a thickness of 0.9λ to 2.1λ, and the support substrate may be made of sapphire. Furthermore, the support substrate may have a thin film made of SiO2 on the other surface. Alternatively, the surface acoustic wave may be a higher-order LSAW mode, the quartz crystal thin plate may be an AT-cut 90°X-type quartz crystal thin plate or an ST-cut 90°X-type quartz crystal thin plate, and have a thickness of 0.3λ to 0.5λ, and the support substrate may be made of sapphire. The surface acoustic wave may be a first-order Rayleigh wave mode, the quartz crystal thin plate may be made of an ST-cut X-type quartz crystal thin plate and have a thickness of 0.05λ to 0.35λ, the support substrate may be made of Si and have a thin film of SiO2 on the other surface. The surface acoustic wave may be a first-order LSAW mode, the quartz crystal thin plate may be made of an LST-cut X-type quartz crystal thin plate and have a thickness of 0.4λ to 0.7λ, the support substrate may be made of Si and have a thin film of SiO2 on the other surface. The surface acoustic wave may be a second-order LSAW mode, the quartz crystal thin plate may be made of an LST-cut quartz crystal thin plate and have a thickness of 0.4λ to 0.7λ, and the support substrate may be made of sapphire. Furthermore, the surface acoustic wave may be a first-order mode LSAW, the quartz crystal thin plate may be an AT-cut 90°X quartz crystal thin plate or an ST-cut 90°X quartz crystal thin plate with a thickness of 0.05λ to 0.4λ, and the support substrate may be made of Si and have a thin film of SiO2 on the other surface. [Effects of the Invention]

[0027] According to the present invention, it is possible to provide a surface acoustic wave device using a quartz crystal thin plate, which has a high Q in the fundamental mode, and a surface acoustic wave device that can excite surface acoustic waves at higher frequencies and with faster acoustic velocities in higher-order modes. [Brief explanation of the drawings]

[0028] [Figure 1] (a) A cross-sectional view showing a basic surface acoustic wave device of an embodiment of the surface acoustic wave device of the present invention; (b) A cross-sectional view showing a modified example in which the support substrate has a thin film on the surface facing the thin quartz plate; (c) A cross-sectional view showing a modified example in which a short-circuit electrode is provided between the thin quartz plate and the support substrate; (d) A cross-sectional view showing a modified example in which the thin film of (b) and the short-circuit electrode of (c) are provided; and (e) A cross-sectional view showing a modified example in which the support substrate has a short-circuit electrode on the side opposite the thin quartz plate. [Figure 2] 10 is a graph showing the change in sound velocity (phase velocity) of each mode with respect to the thickness of a quartz crystal thin plate in a surface acoustic wave device according to an embodiment of the present invention, in which an ST-cut X-type quartz crystal thin plate is provided on the surface of an R sapphire 73° X-propagation substrate (0°, 122.39°, 73°). [Figure 3] 1 is a graph showing the change in electromechanical coupling factor of the fundamental mode (zeroth mode), (b) first mode, (c) second mode, and (d) third mode with respect to the thickness of the quartz crystal thin plate for surface acoustic wave devices according to embodiments of the present invention, including (A) a structure in which an interdigital electrode is formed on the surface of a quartz crystal thin plate, (B) a structure in which an interdigital electrode is formed on the surface of a support substrate and a quartz crystal thin plate is formed to cover the interdigital electrode, (C) a structure in which a short-circuiting electrode is provided between the quartz crystal thin plate and the support substrate and an interdigital electrode is formed on the surface of the quartz crystal thin plate, and (D) a structure in which a short-circuiting electrode is provided on the surface of the quartz crystal thin plate of the structure (B). [Figure 4] 10A and 10B are graphs showing the Euler angle θ dependence of (a) the electromechanical coupling coefficient k2 and (b) the temperature coefficient of frequency (TCF) of the excited Rayleigh waves and LSAW waves of a quartz crystal thin plate with Euler angles of (0°, θ, 0°) for a surface acoustic wave device according to an embodiment of the present invention. [Figure 5] 10A and 10B are graphs showing the Euler angle θ dependence of (a) the electromechanical coupling coefficient k2 and (b) the temperature coefficient of frequency (TCF) of the LSAW excited in a quartz crystal thin plate with Euler angles of (0°, θ, 90°) for a surface acoustic wave device according to an embodiment of the present invention. [Figure 6] 1(a) and 1(b) of the surface acoustic wave device according to an embodiment of the present invention, when the quartz crystal thin plate is an ST-X quartz crystal thin plate, is shown in a graph showing the dependence of the fundamental mode (a) velocity, (b) bandwidth, (c) impedance ratio, and (d) mechanical Q (Q) on the thickness of the quartz crystal thin plate. [Figure 7] 1( a) is a graph showing the fundamental mode frequency characteristics of a surface acoustic wave device according to an embodiment of the present invention, shown in FIG. 1(a), in which an Al IDT with an MR of 0.5 and a thickness of 0.04λ is formed on an ST-X quartz / support substrate with a thickness of 0.2λ, using three types of support substrates: synthetic quartz, fused quartz, and quartz glass, and a reference example in which there is no support substrate. [Figure 8] 8 is a graph showing the dependence of the fundamental mode impedance ratio on quartz crystal thickness for structures using the three types of support substrates shown in FIG. 7, as well as for a structure using 42°YX LiTaO3 as a support substrate as a reference example, and a structure without a support substrate (single quartz crystal plate). [Figure 9] 1(a) and 1(b) of the surface acoustic wave device according to the embodiment of the present invention, when the quartz crystal thin plate is an LST-X quartz crystal thin plate, is shown in a graph showing the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate. [Figure 10] 1(a) and 1(b) are graphs showing the dependence of the thickness of the quartz crystal thin plate on the (a) velocity, (b) bandwidth, (c) impedance ratio, and (d) mechanical Q (Qr at the resonance point and Qa at the anti-resonance point) when the quartz crystal thin plate is an AT90°X quartz crystal thin plate and an ST90°X quartz crystal thin plate, respectively, of the surface acoustic wave device according to an embodiment of the present invention. [Figure 11]1(c) is a graph showing the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate of the surface acoustic wave device shown in FIG. 1(c) according to an embodiment of the present invention, when the quartz crystal thin plate is an ST-X quartz crystal thin plate and the support substrate is a sapphire substrate (c-sapphire). [Figure 12] 1(d) shows graphs illustrating the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate when the quartz crystal thin plate is an ST-X quartz crystal thin plate and the support substrate is SiO2 / Si, in the surface acoustic wave device according to an embodiment of the present invention. [Figure 13] 1(c) is a graph showing the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate of the surface acoustic wave device shown in FIG. 1(c) according to an embodiment of the present invention, when the quartz crystal thin plate is an LST-X quartz crystal thin plate and the support substrate is a sapphire substrate (c-sapphire). [Figure 14] 1(d) shows graphs illustrating the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate when the quartz crystal thin plate is an LST-X quartz crystal thin plate and the support substrate is SiO2 / Si, in the surface acoustic wave device according to an embodiment of the present invention. [Figure 15] 1(c) is a graph showing the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate of the surface acoustic wave device shown in FIG. 1(c) according to an embodiment of the present invention, when the quartz crystal thin plate is an AT90°X quartz crystal thin plate and the support substrate is a sapphire substrate (c-sapphire). [Figure 16]1(d) shows graphs illustrating the dependence of (a) velocity, (b) bandwidth, and (c) impedance ratio on the thickness of the quartz crystal thin plate when the quartz crystal thin plate is an AT90°X quartz crystal thin plate and the support substrate is SiO2 / Si, in accordance with an embodiment of the present invention. [Figure 17] 1 is a graph showing the change in acoustic velocity with respect to the propagation direction ψ of the surface acoustic wave for (a) (111) Si, (b) (100) Si, (c) R sapphire, and (d) c sapphire used as a support substrate for a surface acoustic wave device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, embodiments of the present invention will be described with reference to the drawings and examples. 1 to 17 show a surface acoustic wave device according to an embodiment of the present invention. As shown in FIG. 1( a ), a surface acoustic wave device 10 includes a quartz crystal thin plate 11 , an electrode 12 , and a support substrate 13 .

[0030] Electrode 12 is provided on one surface of quartz crystal thin plate 11. Electrode 12 is preferably made of an interdigital transducer (IDT) to utilize surface acoustic waves. Support substrate 13 is provided on the other surface of quartz crystal thin plate 11, and has a relative dielectric constant smaller than the relative dielectric constant of 4.52 of quartz crystal, or an acoustic velocity faster than the acoustic velocity of surface acoustic waves in quartz crystal thin plate 11. In the latter case, support substrate 13 preferably has a relative dielectric constant smaller than the relative dielectric constant of 4.52 of quartz crystal. In the case of high speed, support substrate 13 may be made of any material as long as it has an acoustic velocity faster than the acoustic velocity of surface acoustic waves in quartz crystal thin plate 11, such as Si, sapphire, spinel, SiC, Si x N y , TiO2, AlN, or diamond. In the case of a low dielectric constant, the support substrate 13 may be made of any material having a relative dielectric constant of less than 4.52, such as synthetic quartz, fused silica, quartz glass, or low dielectric constant glass.

[0031] In addition to the basic structure shown in FIG. 1( a), the surface acoustic wave device 10 may have a support substrate 13 with a thin film 14 made of a low-dielectric-constant material, such as SiO or AlN, on the surface of the support substrate 13 facing the quartz crystal thin plate 11, as shown in FIGS. 1( b) and 1( e). The surface acoustic wave device 10 may have a short-circuit electrode 15 between the quartz crystal thin plate 11 and the support substrate 13, in contact with the quartz crystal substrate 11, as shown in FIGS. 1( c) and 1( d). The support substrate 13 may also have a short-circuit electrode 15 on the side opposite the quartz crystal substrate 11 from the thin film 14, as shown in FIG. 1( e). The short-circuit electrode 15 may be made of the same material as the electrode 12, or a different material. When the electrode 12 is an interdigital transducer, the short-circuit electrode 15 may be connected to one of the electrodes 12 of the interdigital transducer, or to the common electrode 12 of another device.

[0032] Next, the operation will be described. When the support substrate 13 has a sound velocity faster than that of the surface acoustic wave in the thin quartz crystal plate 11, the surface acoustic wave device 10 can excite surface acoustic waves with a faster sound velocity and higher frequency than a device made only of the thin quartz crystal plate 11 (quartz crystal substrate) without the support substrate 13. Furthermore, when the relative dielectric constant of the support substrate 13 is smaller than the relative dielectric constant of 4.52 of quartz, the impedance ratio can be increased and a higher Q can be achieved compared to a device made only of the thin quartz crystal plate 11 (quartz crystal substrate) without the support substrate 13. The surface acoustic waves that can be excited are, for example, fundamental modes of Rayleigh waves and leaky surface acoustic waves (LSAWs), as well as higher modes thereof, including Sezawa waves.

[0033] In this way, the surface acoustic wave device 10 can achieve a high Q by using the quartz crystal thin plate 11 and the support substrate 13 with a smaller dielectric constant. Also, the surface acoustic wave device 10 can excite higher frequency surface acoustic waves by using the quartz crystal thin plate 11 and the support substrate 13 with a higher speed, and can realize, for example, a high frequency resonator.

[0034] 1(b) and 1(e), if the surface acoustic wave device 10 has a thin film 14 made of a low-dielectric-constant material, it can excite high-Q and high-frequency surface acoustic waves when the supporting substrate 13 has a high speed. Also, as shown in FIGS. 1(c), 1(d), and 1(e), even when the short-circuiting electrode 15 is provided, if the supporting substrate 13 has a low dielectric constant or a thin film 14 made of a low-dielectric-constant material, it can excite high-Q and high-frequency surface acoustic waves.

[0035] The structure shown in FIG. 1 and the reasons for selecting the azimuth angle of the quartz substrate 11 will be described below as an example of the surface acoustic wave device 10.

[0036] [Relationship between the thickness of the quartz crystal thin plate 11 and the sound velocity] Figure 2 shows the change in sound velocity (phase velocity) versus thickness of the quartz crystal thin plate 11 for a structure in which the support substrate 13 is made of an R sapphire 73°X propagation substrate (0°, 122.39°, 73°) and an ST-cut X quartz crystal thin plate (ST-X quartz crystal) is provided on its surface. The thickness of the quartz crystal thin plate 11 is expressed using the wavelength λ of the excited surface acoustic wave (the same applies below).

[0037] As shown in Figure 2, when the thickness of the quartz crystal thin plate 11 is in the range of up to 1.0λ, the sound velocity of the fundamental mode (zeroth mode) of the surface acoustic wave is in the range of 3,300 m / s to 5,700 m / s. However, the sound velocity increases as the mode becomes higher, with the sound velocity of the first mode being 5,200 m / s or more, the sound velocity of the second mode being 5,500 m / s or more, the sound velocity of the third mode being 5,800 m / s or more, and the sound velocity of the fourth mode being 5,900 m / s or more.

[0038] Next, for four known structures of surface acoustic wave devices with interdigital transducers of wavelength λ, the electromechanical coupling coefficient k 2The coupling factor (coupling factor) was calculated for the four structures: (A) a structure in which an interdigital electrode is formed on the surface of the quartz crystal thin plate 11 (corresponding to Figure 1(a)), (B) a structure in which an interdigital electrode is formed on the surface of the support substrate 13 and the quartz crystal thin plate 11 is formed to cover the interdigital electrode, (C) a structure in which a short-circuit electrode 15 is formed between the quartz crystal thin plate 11 and the support substrate 13 and an interdigital electrode is formed on the surface of the quartz crystal thin plate 11 (corresponding to Figure 1(c)), and (D) a structure in which a short-circuit electrode 15 is formed on the surface of the quartz crystal thin plate 11 of the structure (B).

[0039] In addition, the electromechanical coupling coefficient k 2 is the acoustic velocity V when the surface of the quartz crystal thin plate 11 where the IDT exists is electrically released, according to the Campbell-John method (see, for example, Non-Patent Document 6). f and the speed of sound V when short-circuited s and k 2 =2×(V f -V s ) / V f was calculated by

[0040] For each structure, the changes in the electromechanical coupling coefficient for the fundamental mode (zeroth mode), first mode, second mode, and third mode versus the thickness of the quartz crystal thin plate 11 were determined, and are shown in Figures 3(a) to 3(d). As shown in Figures 3(a) to 3(d), it was confirmed that for each mode, structure (C) had the highest overall electromechanical coupling coefficient, followed by structure (A). It was also confirmed that structure (D) had a high electromechanical coupling coefficient for the fundamental mode and first mode, but was slightly lower in the second mode.

[0041] More specifically, as shown in Figure 3(a), in the 0th order (fundamental) mode, the k value is 0.03% or more for the structure (C) when the quartz crystal thickness is 0.09λ to 1λ, and for the structure (A) when the quartz crystal thickness is 0.11λ to 1λ. 2 (Although not shown, both the (C) and (A) structures have k 2(k is greater than 0.03%), and in the structure (D), when the crystal thickness is 0.3λ to 0.9λ, and in the structure (B), when the crystal thickness is 0.38λ to 0.9λ, the k is 0.03% or more. 2 It was confirmed that this could be obtained.

[0042] As shown in Figure 3(b), in the first mode, the k value is 0.03% or more for the structure (C) with a quartz crystal thickness of 0.12λ to 1λ, and for the structure (A) with a quartz crystal thickness of 0.15λ to 1λ. 2 (Although not shown, both the (C) and (A) structures have k 2 (k is greater than 0.03%), and in the structure (D), when the crystal thickness is 0.12λ to 0.6λ, and in the structure (B), when the crystal thickness is 0.17λ to 0.53λ, the k is 0.03% or more. 2 It was confirmed that this could be obtained.

[0043] As shown in Figure 3(c), in the second-order mode, the k values ​​of structures (C) and (A) are 0.03% or more when the quartz crystal thickness is between 0.5λ and 0.73λ. 2 In the structures (D) and (B), when the quartz crystal thickness is 0.5λ to 0.6λ, k of 0.03% or more is obtained. 2 As shown in Figure 3(d), in the third-order mode, the structures (C) and (A) have k values ​​of 0.03% or more when the quartz crystal thickness is between 0.6λ and 1λ. 2 Although it is not shown in the figure, even with 3λ, k 2 (D) and (B) structures have k values ​​of 0.03% or more. 2 It was confirmed that this was not possible.

[0044] Based on the results shown in Fig. 3, the following study was carried out on the surface acoustic wave device 10 in Fig. 1(a), which corresponds to the structure (A), and the surface acoustic wave device 10 in Fig. 1(c), which corresponds to the structure (C). Furthermore, the following study was also carried out on the structures having a thin film 14 made of a low dielectric constant material on the surface of the support substrate 13 (the structures in Fig. 1(b) and (d)), as it is believed that the sound velocity will be faster.

[0045] [Azimuth angle of quartz crystal thin plate 11] The azimuth angle of the quartz crystal thin plate 11 was examined. First, for the quartz crystal thin plate 11 with Euler angles of (0°, θ, 0°), the electromechanical coupling coefficient k 2 The dependence of the temperature coefficient of frequency (TCF) on the Euler angle θ was calculated and shown in Figures 4(a) and 4(b). The temperature coefficient of frequency is also calculated by the linear expansion coefficient and the sound velocity V when the surface of the quartz crystal thin plate 11 is electrically released. f was calculated from the temperature dependence of

[0046] As shown in Figures 4(a) and (b), it was confirmed that the TCF is low (close to 0) and the electromechanical coupling coefficient is high when using an ST-cut X quartz thin plate (ST-X quartz thin plate) with Euler angles of (0°, 132°45', 0°) for Rayleigh waves, and when using an LST-cut X quartz thin plate (LST-X quartz thin plate) with Euler angles of (0°, 15°, 0°) for LSAW. Based on these results, we investigated the quartz thin plate 11 with Euler angles of (0°, θ, 0°) using these two azimuth angles as representative examples.

[0047] For the quartz crystal thin plate 11 with Euler angles of (0°, θ, 90°), the electromechanical coupling coefficient of the excited LSAW and the TCF as a function of the Euler angle θ were determined and are shown in Figures 5(a) and (b), respectively. Note that the electromechanical coupling coefficient for Rayleigh waves was almost zero at this azimuth angle, so it is not shown. As shown in Figures 5(a) and (b), it was confirmed that the AT-cut 90°X quartz crystal thin plate (AT90°X quartz crystal thin plate) with Euler angles of (0°, 125°, 90°) provided good TCF and a certain level of electromechanical coupling coefficient. Based on these results, we used the quartz crystal thin plate 11 with this azimuth angle as a representative for our investigation of the quartz crystal thin plate 11 with Euler angles of (0°, θ, 90°).

[0048] We also investigated the thin quartz crystal plate 11 with Euler angles of (90°, 90°, ψ), but found that there were no Rayleigh waves or LSAWs with good TCFs. For this reason, we excluded the thin quartz crystal plate 11 with Euler angles of (90°, 90°, ψ) from our investigation.

[0049] When considering the temperature coefficient of frequency (TCF), it is affected by the TCF of the support substrate 13. For a support substrate 13 made primarily of SiO2, such as quartz or glass, which has a positive TCF, it is necessary to combine a quartz crystal thin plate 11 with an orientation that has a negative TCF, and for a support substrate 13 made of sapphire or Si, which has a negative TCF, it is necessary to combine a quartz crystal thin plate 11 with an orientation that has a positive TCF.

[0050] Specifically, for a support substrate 13 having a positive TCF, such as synthetic quartz, fused silica, or quartz glass, the azimuth angles of suitable quartz thin plate 11 having a negative TCF are (0°, 132.75° to 180°, 0°) for the 0th and higher modes of Rayleigh SAW, and (0°, 15° to 45°, 0°), (0°, 60° to 132.75°, 0°), and (0°, 40° to 125°, 90°) for the 0th and higher modes of LSAW, as shown in Figures 4(b) and 5(b).

[0051] On the other hand, for a support substrate 13 with a negative TCF, the azimuth angles of a suitable quartz crystal thin plate 11 with a positive TCF are (0°, 0° to 132.75°, 0°) for the 0th and higher modes of Rayleigh SAW, and (0°, 0° to 15°, 0°), (0°, 45° to 60°, 0°), (0°, 132.75° to 180°, 0°), (0°, 0° to 40°, 90°), and (0°, 125° to 180°, 90°) for the 0th and higher modes of LSAW.

[0052] 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 elastic wave. Specifically, with respect to the X, Y, and Z crystal axes of the quartz that make up the piezoelectric substrate, the X axis is rotated counterclockwise by φ around the Z axis to obtain the X' axis. Next, the Z axis is rotated counterclockwise by θ around the X' axis to obtain the Z' axis. In this case, the Z' axis is 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 the propagation direction of the elastic 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.

[0053] By defining Euler angles in this way, for example, propagation in the X direction through a 40° rotated Y-plate is expressed in Euler angles as (0°, -50°, 0°), and propagation in the X direction through a 40° rotated Y-plate at 90° is expressed in Euler angles as (0°, -50°, 90°).

[0054] When cutting a piezoelectric substrate at the desired Euler angles, there is a possibility of an error of up to ±0.5° for each Euler angle component. Regarding elastic wave characteristics, a deviation of ±5° for φ and ψ, among the Euler angles (φ, θ, ψ), causes almost no difference in characteristics. Furthermore, the planes (0°, θ + 360°, 0°) are equivalent to the Euler angles (0°, θ, 0°). On the other hand, the planes (0°, θ + 180°, 0°) are not equivalent to the planes (0°, θ, 0°) in terms of crystallography, since they are the difference between the + and - planes, i.e., the front and back surfaces of the substrate. However, because acoustic wave devices exhibit the same frequency characteristics on both the front and back surfaces, the orientations of the front and back surfaces of the substrate are also considered equivalent from an acoustical perspective.

[0055] [Study of various surface acoustic wave devices 10] For each of the surface acoustic wave devices 10 shown in Figures 1(a) to 1(e), analysis was performed using the finite element method (FEM) for structures using various quartz crystal thin plates 11 and support substrates 13 under the conditions of the azimuth angle of the recommended thin plates 11 described above. In the analysis, the electrodes 12 were made of aluminum interdigital electrodes, and the metallization ratio MR was set to 0.5. In the analysis, the resonant frequency f r , resonant impedance Z r , anti-resonant frequency f a , anti-resonance impedance Z a Also, the speed of sound was calculated as f a × wavelength, and the bandwidth (BW) is calculated as (f a -f r ) / f r (≒ 2×k 2 ) and calculate the impedance ratio as 20×log(Z a / Z r) where Q is proportional to the square root of the impedance ratio. The metallization ratio MR is MR=F / (F+G) where F is the width of the electrode fingers of the interdigital transducer and G is the gap between the electrode fingers. r When the frequency at which the imaginary part (1 / Z) before and after becomes maximum is f1 and the frequency at which it becomes minimum is f2, the resonance Q is r I also found it as / (f2-f1).

[0056] For the surface acoustic wave device 10 shown in Figures 1(a) and 1(b), the acoustic velocity, bandwidth, impedance ratio, and mechanical Q (Q) were measured and shown in Figures 6(a) through 6(d), respectively, when the quartz crystal thin plate 11 was an ST-X quartz crystal thin plate. The thickness of the electrode 12 was set to 0.06λ. Four types of support substrates 13 were used: a 42° Y-cut X-propagating LT (LiTaO3) substrate, a Si substrate, a quartz glass substrate, and a Si substrate with a thin SiO2 film 14 on the surface (SiO2 / Si). For comparison, the acoustic velocity, bandwidth, and impedance ratio were measured without the support substrate 13. The acoustic velocity, bandwidth, and impedance ratio were measured as 3,019 m / s, 0.07%, and 89 dB, respectively.

[0057] The results shown in Figure 6 indicate that the surface acoustic waves excited were in the fundamental mode. As shown in Figure 6(c), when the support substrate 13 was Si, quartz glass, or SiO2 / Si, a higher impedance ratio was obtained than the 89 dB obtained without the support substrate 13. In particular, when the support substrate 13 was Si, the impedance ratio was 93 dB when the quartz thin plate 11 was 0.3λ thick, confirming that an impedance ratio higher than 89 dB was obtained for thicknesses of 0.2λ or greater. Furthermore, when the support substrate 13 was quartz glass, the impedance ratio was 100 dB when the quartz thin plate 11 was 0.2λ thick, confirming that an impedance ratio higher than 89 dB was obtained for thicknesses of 0.15λ or greater. Furthermore, when the support substrate 13 was SiO2 / Si, the impedance ratio was 99 dB when the quartz thin plate 11 was 0.2λ thick, confirming that an impedance ratio higher than 89 dB was obtained for thicknesses of 0.15λ or greater.

[0058] As shown in Figures 6(b) and (d), it was confirmed that a good frequency band and a high Q can be obtained in the range of these high impedance ratios. Furthermore, as shown in Figure 6(a), it was confirmed that when the support substrate 13 is Si, a sound velocity of 3,500 m / s to 3,900 m / s can be obtained when the thickness of the quartz crystal thin plate 11 is 0.3λ or less. It was also confirmed that when the support substrate 13 is LT, quartz glass, or SiO2 / Si, the sound velocity is approximately the same as when there is no support substrate 13.

[0059] As shown in Figures 6(a) and (c), silica glass has a large impedance ratio despite the fact that the sound velocity is slower than that of quartz. Here, we investigated the difference in dielectric constant, which is thought to be one of the reasons for this. Table 1 shows the dielectric constants of materials that are thought to be usable as the support substrate 13. In Table 1, if the substrate structure is isotropic, there is only one dielectric constant ε, but if the substrate structure is anisotropic, there are two dielectric constants, ε11 and ε33. Therefore, the square root of (ε11 × ε33) is listed in the ε column of Table 1.

[0060] [Table 1]

[0061] Figure 7 shows the fundamental mode frequency characteristics of the surface acoustic wave device 10 shown in Figure 1(a) in which an Al IDT with an MR of 0.5 and a thickness of 0.04λ is formed on an ST-X quartz / support substrate 13 with a thickness of 0.2λ. The support substrate 13 is made of one of three materials: synthetic quartz, fused silica, or quartz glass, all of which have a lower dielectric constant than quartz. For reference, the figure also shows the frequency characteristics of a structure in which an IDT of the same shape is formed on an ST-X quartz substrate. As shown in Figure 7, the impedance ratios of the other three types of structures were confirmed to be approximately 14 dB higher than when using only a quartz substrate.

[0062] Figure 8 shows the dependence of the impedance ratio of these three fundamental modes on quartz crystal thickness. For reference, the figure also shows the impedance ratios of a structure using a 42°YXLiTaO3 support substrate 13 with a high dielectric constant, and a structure using only a single-layer quartz crystal without a support substrate 13. While a high impedance ratio cannot be achieved with a support substrate 13 with a high dielectric constant, the three support substrates 13 with low dielectric constants all achieved a higher impedance ratio than the single-layer quartz crystal structure when the quartz crystal thickness was 0.01 to 3λ, especially when the quartz crystal thickness was 0.01 to 1.5λ. These results demonstrate that a high impedance ratio can be achieved in the fundamental mode by using a support substrate 13 with a low dielectric constant in addition to a high acoustic velocity.

[0063] For the surface acoustic wave device 10 shown in Figures 1(a) and 1(b), the thickness dependence of the acoustic velocity, bandwidth, and impedance ratio of the quartz crystal thin plate 11 was determined when the quartz crystal thin plate 11 was an LST-X quartz crystal thin plate. These results are shown in Figures 9(a) to 9(c), respectively. The thickness of the electrode 12 was set to 0.13λ. Four types of support substrates 13 were used: a 42° Y-cut X-propagation LT (LiTaO3) substrate, a Si substrate, a quartz glass substrate, and a SiO2 / Si substrate. For comparison, the acoustic velocity and impedance ratio were determined without the support substrate 13. The acoustic velocity and impedance ratio were found to be 2,677 m / s and 88 dB, respectively.

[0064] The results shown in Figure 9 indicate that the surface acoustic waves excited were in the fundamental mode. As shown in Figure 9(c), when the support substrate 13 was made of Si, quartz glass, or SiO2 / Si, a higher impedance ratio was obtained than the 89 dB impedance ratio obtained without the support substrate 13. In particular, when the support substrate 13 was made of Si, the impedance ratio was 94 dB when the quartz thin plate 11 was 0.2λ thick, confirming that an impedance ratio higher than 88 dB was obtained at thicknesses of 0.1λ or greater. Furthermore, when the support substrate 13 was made of quartz glass or SiO2 / Si, the impedance ratio was 102 dB when the quartz thin plate 11 was 0.15λ thick, confirming that an impedance ratio higher than 88 dB was obtained at thicknesses of 0.15λ or greater. Furthermore, in the case of the fundamental mode of LSAW, a high impedance ratio was not obtained when the support substrate 13 was made of LT, which has a high dielectric constant. However, a high impedance ratio was obtained when the support substrate 13 was made of quartz glass, which has a low dielectric constant.

[0065] As shown in Figure 9(b), it was confirmed that a good frequency band can be obtained when these impedance ratios are high. Furthermore, as shown in Figure 9(a), it was confirmed that when the support substrate 13 is Si, a sound velocity of 2,800 m / s to 3,000 m / s can be obtained when the thickness of the quartz crystal thin plate 11 is 0.2λ or less. Furthermore, when the support substrate 13 is LT, quartz glass, or SiO2 / Si, it was confirmed that the sound velocity is approximately the same as or lower than the sound velocity when there is no support substrate 13. Note that here, instead of Q, a graph of the impedance ratio, which is correlated with Q, is shown.

[0066] Regarding the surface acoustic wave device 10 shown in Figures 1(a) and (b), when the quartz crystal thin plate 11 is an AT90°X quartz crystal thin plate, the acoustic velocity, bandwidth, impedance ratio, and mechanical Q (Q at the resonance point) are as follows: r , and Q at the anti-resonance point a The dependence of the impedance on the thickness of the quartz crystal thin plate 11 was calculated and shown in Figures 10(a) to 10(d). The thickness of the electrode 12 was set to 0.14λ. Four types of support substrates 13 were used: a 42° Y-cut X-propagation LT (LiTaO3) substrate, a Si substrate, a quartz glass substrate, and SiO2 / Si. For comparison, the sound velocity and impedance ratio were calculated without the support substrate 13. The sound velocity was 3,391 m / s and the impedance ratio was 98 dB. For comparison, the results when the quartz crystal thin plate 11 was an ST90°X quartz crystal thin plate 11 and the support substrate 13 was quartz glass are also shown in Figures 10(a) to 10(d).

[0067] The results shown in Figure 10 indicate that the excited surface acoustic waves were in the fundamental mode. As shown in Figure 10(c), when the support substrate 13 was made of LT, Si, quartz glass, or SiO2 / Si, it was confirmed that an impedance ratio higher than 98 dB was obtained when there was no support substrate 13. In particular, when the support substrate 13 was made of Si, the impedance ratio was 108 dB when the thickness of the quartz crystal thin plate 11 was 0.2λ, confirming that an impedance ratio higher than 98 dB was obtained at 1.5λ or less. Furthermore, when the support substrate 13 was made of quartz glass, the impedance ratio was 107 dB when the thickness of the quartz crystal thin plate 11 was 0.6λ, confirming that an impedance ratio higher than 98 dB was obtained at 1.5λ or less. Furthermore, when the support substrate 13 was made of SiO2 / Si, the impedance ratio was 107 dB when the thickness of the quartz crystal thin plate 11 was 0.5λ, confirming that an impedance ratio higher than 98 dB was obtained at 1.5λ or less.

[0068] As shown in Figures 10(b) and (d), it was confirmed that a good frequency band and a high Q can be obtained in these high impedance ratio ranges. Furthermore, as shown in Figure 10(a), when the support substrate 13 is LT or Si, a sound velocity of 3,400 m / s to 3,500 m / s can be obtained when the thickness of the quartz crystal thin plate 11 is 0.2λ or less. Furthermore, when the support substrate 13 is quartz glass or SiO2 / Si, the sound velocity is approximately the same as or lower than the sound velocity without the support substrate 13. Furthermore, in the case of LSAW with an AT90°X quartz crystal thin plate, a large impedance ratio cannot be obtained in the fundamental mode when the support substrate 13 is LT, which has a high dielectric constant. However, a high impedance ratio can be obtained when the support substrate 13 is quartz glass, which has a low dielectric constant.

[0069] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(c), the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined when the quartz crystal thin plate 11 was an ST-X quartz crystal thin plate, and these results are shown in FIGS. 11(a) to 11(c). The thickness of the electrode 12 was set to 0.06λ. A sapphire substrate (face orientation: C-plane, c-sapphire) was used as the support substrate 13. FIG. 11 shows the results for the fundamental mode (0th), first mode (1st), second mode (2nd), and third mode (3rd) of the Rayleigh wave as the excited surface acoustic wave.

[0070] As shown in Figures 11(a) and 11(c), in the fundamental mode, when the thickness of the thin quartz crystal plate 11 was 1.0λ, the impedance ratio reached a maximum of 90 dB and the sound velocity was confirmed to be 3,110 m / s. In contrast, in the first-order mode, when the thickness of the thin quartz crystal plate 11 was 0.5λ, the sound velocity was 5,130 m / s, but the impedance ratio was confirmed to be small at a maximum of 69 dB. In the second-order mode, when the thickness of the thin quartz crystal plate 11 was 0.45λ to 0.7λ, the impedance ratio was confirmed to be 90 dB or more and the sound velocity was confirmed to be fast, at 5,400 m / s or more. In particular, when the thickness of the thin quartz crystal plate 11 was 0.6λ, the impedance ratio reached a maximum of 91 dB and the sound velocity was confirmed to be fast, at 5,540 m / s. In the third mode, when the thickness of the quartz crystal thin plate 11 was between 0.9λ and 2.1λ, the impedance ratio was 89 dB or more, and the sound velocity was confirmed to be fast, at 5,400 m / s or more. In particular, when the thickness of the quartz crystal thin plate 11 was 1.0λ, the impedance ratio reached its maximum of 90 dB, and the sound velocity was confirmed to be fast, at 5,700 m / s. As a result, it can be said that in these second and third mode ranges, the impedance ratio is high and stable, and it is possible to excite high-frequency surface acoustic waves. Furthermore, as shown in Figure 11(b), it was confirmed that a good frequency band can be obtained in these high impedance ratio ranges.

[0071] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(d), when the quartz crystal thin plate 11 is an ST-X quartz crystal thin plate, the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined and is shown in FIGS. 12(a) to 12(c). Here, the thickness of the electrode 12 was set to 0.06λ. SiO2 / Si was used as the support substrate 13. FIG. 12 shows the results for the fundamental mode (0th), first mode (1st), second mode (2nd), and third mode (3rd) of the Rayleigh wave as the excited surface acoustic wave.

[0072] As shown in Figures 12(a) and 12(c), in the fundamental mode, when the thickness of the thin quartz crystal plate 11 was 0.3λ, the impedance ratio reached a maximum of 102 dB and the sound velocity was confirmed to be 3,040 m / s. In contrast, in the first-order mode, when the thickness of the thin quartz crystal plate 11 was 0.05λ to 0.35λ, the impedance ratio reached 90 dB or more and the sound velocity was confirmed to be fast, at 4,200 m / s or more. In particular, when the thickness of the thin quartz crystal plate 11 was 0.15λ, the impedance ratio reached a maximum of 110 dB and the sound velocity was confirmed to be fast, at 4,256 m / s. In the second-order mode, the impedance ratio was confirmed to be 50 dB or less. In the third-order mode, when the thickness of the thin quartz crystal plate 11 was 0.4λ, the sound velocity was fast, at 5,704 m / s, but the impedance ratio was confirmed to be small, at a maximum of 81 dB. As a result, it can be said that in these ranges of the first mode, the impedance ratio is high and stable, and it is possible to excite high-frequency surface acoustic waves. Furthermore, as shown in Figure 12(b), it was confirmed that a good bandwidth can be obtained in these ranges of high impedance ratio.

[0073] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(c), the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined when the quartz crystal thin plate 11 was an LST-X quartz crystal thin plate, and these results are shown in FIGS. 13(a) to 13(c). The thickness of the electrode 12 was set to 0.13λ. A sapphire substrate (face orientation: C-plane, c-sapphire) was used as the support substrate 13. FIG. 13 shows the results for the fundamental mode (0th), first-order mode (1st), and second-order mode (2nd) of Rayleigh waves as the excited surface acoustic waves.

[0074] As shown in Figures 13(a) and 13(c), in the fundamental mode, when the thickness of the quartz crystal thin plate 11 was 0.3λ, the impedance ratio reached a maximum of 90 dB and the sound velocity was 2,900 m / s. In contrast, in the first-order mode, when the thickness of the quartz crystal thin plate 11 was 0.6λ, the sound velocity was 3,816 m / s, but the impedance ratio was a small 68 dB. In the second-order mode, when the thickness of the quartz crystal thin plate 11 was 0.4λ to 0.7λ, the impedance ratio was 80 dB or more and the sound velocity was fast, at 5,000 m / s or more. In particular, when the thickness of the quartz crystal thin plate 11 was 0.5λ, the impedance ratio reached a maximum of 87 dB and the sound velocity was fast, at 5,100 m / s. As a result, it can be said that within these second-order mode ranges, the impedance ratio is high and stable, and high-frequency surface acoustic waves can be excited. Furthermore, as shown in FIG. 13(b), it was confirmed that a good band was obtained in the range where the impedance ratio was high.

[0075] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(d), when the quartz crystal thin plate 11 is an LST-X quartz crystal thin plate, the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined and is shown in FIGS. 14(a) to 14(c). Here, the thickness of the electrode 12 was set to 0.13λ. SiO2 / Si was used as the support substrate 13. FIG. 14 shows the results for the fundamental mode (0th), first-order mode (1st), and second-order mode (2nd) of Rayleigh waves as the excited surface acoustic waves.

[0076] As shown in Figures 14(a) and 14(c), in the fundamental mode, when the thickness of the quartz crystal thin plate 11 was 0.15λ, the impedance ratio reached a maximum of 79 dB and the sound velocity was 3,893 m / s. In contrast, in the first-order mode, when the thickness of the quartz crystal thin plate 11 was 0.4λ to 0.7λ, the impedance ratio reached 80 dB or more and the sound velocity was also fast, at 4,500 m / s or more. In particular, when the thickness of the quartz crystal thin plate 11 was 0.6λ, the impedance ratio reached a maximum of 85 dB and the sound velocity was also fast, at 4,563 m / s. In the second-order mode, when the thickness of the quartz crystal thin plate 11 was 0.2λ, the sound velocity was fast, at 4,985 m / s, but the impedance ratio was only a small maximum of 64 dB. As a result, it can be said that within these first-order mode ranges, the impedance ratio is high and stable, and high-frequency surface acoustic waves can be excited. Furthermore, as shown in FIG. 14(b), it was confirmed that a good band was obtained in the range where the impedance ratio was high.

[0077] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(c), the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined when the quartz crystal thin plate 11 was an AT90°X quartz crystal thin plate, and these results are shown in FIGS. 15(a) to 15(c). The thickness of the electrode 12 was set to 0.14λ. A sapphire substrate (face orientation: C-plane, c-sapphire) was used as the support substrate 13. FIG. 15 shows the results for the fundamental mode (0th), first-order mode (1st), and second-order mode (2nd) of Rayleigh waves as the excited surface acoustic waves.

[0078] As shown in Figures 15(a) and 15(c), in the fundamental mode, when the thickness of the quartz crystal plate 11 was 0.15λ, the impedance ratio reached a maximum of 111 dB and the sound velocity was confirmed to be 3,511 m / s. In contrast, for the first-order Sezawa wave mode, when the thickness of the quartz crystal plate 11 was 0.3λ to 0.5λ, the impedance ratio reached 85 dB or more and the sound velocity was confirmed to be fast, at 5,400 m / s or more. In particular, when the thickness of the quartz crystal plate 11 was 0.4λ, the impedance ratio reached a maximum of 89 dB and the sound velocity was confirmed to be fast, at 5,483 m / s. In the second-order mode, when the thickness of the quartz crystal plate 11 was 3λ (not shown), the sound velocity was 5,176 m / s, but the impedance ratio was confirmed to be small, at 65 dB. As a result, it can be said that within these first-order mode ranges, the impedance ratio is high and stable, and it is possible to excite high-frequency surface acoustic waves. Furthermore, as shown in FIG. 15(b), it was confirmed that a good band was obtained in the range where the impedance ratio was high.

[0079] For a surface acoustic wave device 10 having a short-circuit electrode 15 shown in FIG. 1(d), the dependence of the sound velocity, bandwidth, and impedance ratio on the thickness of the quartz crystal thin plate 11 was determined when the quartz crystal thin plate 11 was an AT90°X quartz crystal thin plate, and these results are shown in FIGS. 16(a) to 16(c). Here, the thickness of the electrode 12 was set to 0.14λ. SiO2 / Si was used as the support substrate 13. FIG. 16 shows the results for the fundamental mode (0th) and first mode (1st) of Rayleigh waves as the excited surface acoustic waves.

[0080] As shown in Figures 16(a) and 16(c), in the fundamental mode, when the thickness of the quartz crystal thin plate 11 was 0.4λ, the impedance ratio reached a maximum of 107 dB and the sound velocity was 3,381 m / s. In contrast, in the first-order mode, when the thickness of the quartz crystal thin plate 11 was between 0.05λ and 0.4λ, the impedance ratio reached 80 dB or more and the sound velocity was also fast, at 4,300 m / s or more. In particular, when the thickness of the quartz crystal thin plate 11 was 0.1λ, the impedance ratio reached a maximum of 107 dB and the sound velocity was also fast, at 4,311 m / s. As a result, it can be said that within these first-order mode ranges, the impedance ratio is high and stable, and it is possible to excite high-frequency surface acoustic waves. Furthermore, as shown in Figure 16(b), it was confirmed that a good bandwidth can be obtained within these high impedance ratio ranges.

[0081] The change in the sound velocity with respect to the propagation direction ψ of the surface acoustic wave on various substrates was calculated and is shown in Figures 17(a) to 17(d). As shown in Figure 17(a), when the substrate is (111)Si, even if the propagation direction ψ of the SAW is changed, the sound velocity remains constant at 4,759 m / s, and it was confirmed that the power flow angle (PFA) is zero for any propagation direction. Therefore, it can be seen that when the substrate is (111)Si, the propagation direction of the SAW does not matter.

[0082] In contrast, when the substrate is (100)Si (Fig. 17(b)), R-sapphire (Fig. 17(c)), or c-sapphire (Fig. 17(d)), it was confirmed that the sound velocity changes depending on the SAW propagation direction. To excite a high-frequency SAW, a propagation direction where the sound velocity is fast and the PFA is zero is considered desirable. For (100)Si (Fig. 17(b)), the sound velocity is 4915 m / s at ψ = 0°, 90°, and 180°, where PFA = 0. At (90°, 90°, 45°±10°) and (90°, 90°, 135°±10°), the PFA is zero and the sound velocity is high at 5,275 m / s, which is more desirable.

[0083] In addition, for R sapphire shown in Figure 17(c), when ψ = 0°, 90°, and 180°, where PFA = 0, the sound speed is 5,760 m / s; when (0°, 112.39°, 13° ± 5°) and (0°, 112.39°, 167° ± 5°), PFA is zero and the sound speed is 5,965 m / s; and when (0°, 112.39°, 73° ± 5°) and (0°, 112.39°, 107° ± 5°), PFA is zero and the sound speed is 6,020 m / s. In addition, for c-sapphire shown in Figure 17(d), when ψ = 0°, 60°, 120°, and 180°, where PFA = 0, the sound speed is 5,572 m / s, and when (90°, 90°, 30° ± 5°), (90°, 90°, 90° ± 5°), and (90°, 90°, 150° ± 5°), PFA becomes zero and the sound speed is 5,715 m / s.

[0084] Other high acoustic velocity substrates include Si x N y , SiC, TiO2, spinel, AlN, C, etc. The shear wave acoustic velocities of these are shown in Table 2. In order to excite a high frequency SAW, it is particularly preferable that the support substrate 13 has a shear wave acoustic velocity of 5,385 m / s or more.

[0085] [Table 2]

[0086] The results of Figures 6 to 16 are summarized in Table 3. The table shows the results for the fundamental mode (0th), first mode (1st), second mode (2nd), and third mode (3rd) of the SAW Rayleigh wave or LSAW. The table also shows the optimal crystal thickness, Va, and BW when the Z ratio is largest. Note that "Al electrode" in the table represents the thickness of the Al electrode, and the "Al electrode" and "optimal crystal thickness" are expressed as a magnification with respect to the wavelength λ of the excited surface acoustic wave. Also, "Va" in the table represents the speed of sound (m / s), "BW" represents the bandwidth, and "Z ratio" represents the impedance ratio. In the "Substrate boundary" column in the table, "Open" represents the structure shown in Figure 1(a) (without a short-circuit electrode), and "Short" represents the structure shown in Figure 1(c) (with a short-circuit electrode). In addition, "holding substrate" in the table represents the support substrate 13, and "42YXLT" represents a 42° Y-cut X-propagation LT (LiTaO3) substrate.

[0087] [Table 3]

[0088] The values ​​in the table for ST-X quartz, LST-X quartz, and AT-90°X quartz without a support substrate are typical values ​​for each quartz. Among these, the values ​​for ST-X quartz (with an Al electrode / ST-X quartz structure) are the standard values ​​for quartz resonators, with the sound velocity, bandwidth, and impedance ratio being 3,019 m / s, 0.0007, and 89 dB, respectively.

[0089] As shown in Table 3, the surface acoustic wave device 10, due to its structure, exhibits a sound velocity similar to that of the standard Al electrode / ST-X quartz crystal, but it can also achieve a higher impedance ratio and a higher Q. Furthermore, the above results indicate that a surface acoustic wave device with a low TCF can be realized by fine-tuning the azimuth angle of the quartz crystal. [Explanation of symbols]

[0090] 10 Surface acoustic wave devices 11 Crystal thin plate 12 electrodes 13 Support substrate 14 Thin Films 15 Short-circuiting electrode

Claims

1. A surface acoustic wave device that utilizes surface acoustic waves, A thin quartz crystal plate, an electrode provided on one surface of the quartz crystal thin plate; a support substrate provided on the other surface side of the quartz crystal thin plate, the support substrate having a sound velocity faster than the sound velocity of the surface acoustic wave in the quartz crystal thin plate or a relative dielectric constant smaller than the relative dielectric constant of quartz crystal; A surface acoustic wave device comprising:

2. 2. The surface acoustic wave device according to claim 1, further comprising a short-circuit electrode provided between the quartz crystal thin plate and the support substrate.

3. The support substrate has a shear wave acoustic velocity of 5385 m / s or more, and is made of Si, sapphire, spinel, SiC, Si ｘ N ｙ , TiO ２ 3. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is made of AlN or diamond.

4. 4. The surface acoustic wave device according to claim 3, wherein the support substrate has Euler angles (φ, θ, ψ), and when φ and θ are each a predetermined angle, the angle of ψ is ±10°, an angle at which the power flow angle of the propagation of the surface acoustic wave is zero.

5. 3. The surface acoustic wave device according to claim 1, wherein the support substrate has a dielectric constant smaller than that of quartz, and is made of synthetic quartz, fused silica, quartz glass, or low-dielectric-constant glass.

6. The support substrate has a SiO ２ 4. The surface acoustic wave device according to claim 3, further comprising a thin film made of SiOF, ScAlN, or AlN.

7. 3. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave is a Rayleigh wave, a leaky surface acoustic wave (LSAW), a Sezawa wave, or a higher mode thereof.

8. 3. The surface acoustic wave device according to claim 1, wherein the quartz crystal thin plate is made of an ST-cut X-shaped quartz crystal thin plate, an LST-cut X-shaped quartz crystal thin plate, an AT-cut 90° X-shaped quartz crystal thin plate, or an ST-cut 90° X-shaped quartz crystal thin plate.

9. the surface acoustic wave is a Rayleigh wave or a LSAW in zero-order mode or a higher-order mode, The quartz crystal thin plate has Euler angles of (0°, 132.75° to 180°, 0°), The support substrate has a positive temperature coefficient of frequency (TCF).

2. The surface acoustic wave device according to claim 1.

10. the surface acoustic wave is a Rayleigh wave or a LSAW in zero-order mode or a higher-order mode, The quartz crystal thin plate has Euler angles of (0°, 15° to 45°, 0°), (0°, 60° to 132.75°, 0°), or (0°, 40° to 125°, 90°), The support substrate has a positive temperature coefficient of frequency (TCF).

2. The surface acoustic wave device according to claim 1.

11. the surface acoustic wave is a Rayleigh wave or a LSAW in zero-order mode or a higher-order mode, The quartz crystal thin plate has Euler angles of (0°, 0° to 132.75°, 0°), The support substrate has a negative temperature coefficient of frequency (TCF).

2. The surface acoustic wave device according to claim 1.

12. the surface acoustic wave is a Rayleigh wave or a LSAW in zero-order mode or a higher-order mode, The quartz crystal thin plate has Euler angles of (0°, 0° to 15°, 0°), (0°, 45° to 60°, 0°), (0°, 132.75° to 180°, 0°), (0°, 0° to 40°, 90°), or (0°, 125° to 180°, 90°), The support substrate has a negative temperature coefficient of frequency (TCF).

2. The surface acoustic wave device according to claim 1.

13. the surface acoustic wave is in a zeroth mode, The quartz crystal thin plate has a thickness of 0.09λ to 1.5λ, where λ is the wavelength of the surface acoustic wave, The support substrate has a shear wave acoustic velocity of 5385 m / s or more, or has SiO ２ SiOF, ScAlN, or AlN thin film.

13. The surface acoustic wave device according to claim 9, wherein:

14. the surface acoustic wave is in a zeroth mode, The quartz crystal thin plate has a thickness of 0.09λ to 1.5λ, where λ is the wavelength of the surface acoustic wave, The support substrate has a relative dielectric constant of less than 4.

52.

6. The surface acoustic wave device according to claim 5.

15. the surface acoustic wave is in a first mode, The quartz crystal thin plate has a thickness of 0.12λ to 1.5λ, where λ is the wavelength of the surface acoustic wave, The support substrate has a shear wave acoustic velocity of 5385 m / s or more, or has SiO ２ SiOF, ScAlN, or AlN thin film.

13. The surface acoustic wave device according to claim 9, wherein:

16. the surface acoustic wave is a second mode Rayleigh wave, The quartz crystal thin plate has a thickness of 0.5λ to 0.73λ, where λ is the wavelength of the surface acoustic wave, The support substrate has a shear wave acoustic velocity of 5385 m / s or more, or has SiO ２ SiOF, ScAlN, or AlN thin film.

13. The surface acoustic wave device according to claim 9, wherein:

17. the surface acoustic wave is a third mode Rayleigh wave, The quartz crystal thin plate has a thickness of 0.6λ to 1λ, where λ is the wavelength of the surface acoustic wave, The support substrate has a shear wave acoustic velocity of 5385 m / s or more, or has SiO ２ SiOF, ScAlN, or AlN thin film.

13. The surface acoustic wave device according to claim 9, wherein:

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