Bonded substrate, surface acoustic wave element, surface acoustic wave element device, and bonded substrate manufacturing method

Abstract

A bonded substrate includes a quartz substrate and a piezoelectric substrate which is bonded on the quartz substrate and on which a surface acoustic wave propagates, wherein the quartz substrate and the piezoelectric substrate are bonded by covalently coupling at a bonding interface, and an orientation of the quartz substrate and an orientation of the piezoelectric substrate intersect with each other on an orthogonal direction side or in the range of 65 degrees to 115 degrees in a bonding surface direction.

Claims

1. A bonded substrate comprising: a quartz substrate; and a piezoelectric substrate which is bonded on the quartz substrate and on which a surface acoustic wave propagates, wherein the quartz substrate and the piezoelectric substrate are bonded by covalently coupling at a bonding interface; and a crystal orientation of the quartz substrate and a crystal orientation of the piezoelectric substrate intersect with each other on an orthogonal direction side in a bonding surface direction.

2. The bonded substrate according to claim 1, wherein the piezoelectric substrate is used for exciting a leaky surface acoustic wave.

3. The bonded substrate according to claim 1, comprising an amorphous layer between the quartz substrate and the piezoelectric substrate, wherein an interface between the amorphous layer and each of the quartz substrate and the piezoelectric substrate becomes the bonding interface.

4. The bonded substrate according to claim 3, wherein the amorphous layer has a thickness of 100 nm or less.

5. The bonded substrate according to claim 3, wherein the amorphous layer is composed of silicon dioxide or aluminum oxide.

6. The bonded substrate according to claim 1, wherein a thickness of the piezoelectric substrate corresponds to 0.05 to 1.0 wavelengths based on a wavelength of the surface acoustic wave.

7. The bonded substrate according to claim 1, wherein the quartz substrate has a thickness of 150 to 500 μm.

8. The bonded substrate according to claim 1, wherein the quartz substrate is an AT-cut quartz substrate or an ST-cut quartz substrate.

9. The bonded substrate according to claim 1, wherein the piezoelectric substrate is composed of lithium tantalate or lithium niobate.

10. The bonded substrate according to claim 9, wherein the piezoelectric substrate has a thickness of 0.1 to 100 μm.

11. The bonded substrate according to claim 1, wherein the bonded substrate has a temperature characteristic of frequency (TCF) of −20 to +5 ppm/° C. and a coupling factor (K.sup.2) of 5% or more.

12. A surface acoustic wave element comprising at least one interdigital electrode on a principal surface of the piezoelectric substrate in the bonded substrate according to claim 1.

13. A surface acoustic wave element device, wherein the surface acoustic wave element according to claim 12 is sealed in a package.

14. A method for manufacturing the bonded substrate of claim 1, the bonded substrate including the quartz substrate and the piezoelectric substrate that are bonded to each other, the method comprising: irradiating a bonding surface of the quartz substrate and a bonding surface of the piezoelectric substrate with ultraviolet light under a reduced pressure; arranging the quartz substrate and the piezoelectric substrate such that the bonding surface of the quartz substrate and the bonding surface of the piezoelectric substrate contact with each other after the irradiation in a state where the crystal orientation of the quartz substrate and the crystal orientation of the piezoelectric substrate intersect with each other on the orthogonal direction side in the bonding surface direction; and pressurizing the quartz substrate and the piezoelectric substrate in a thickness direction with the quartz substrate and the piezoelectric substrate arranged in said state to bond the bonding surfaces with each other.

15. The method for manufacturing a bonded substrate according to claim 14, wherein heating at a predetermined temperature is performed in the pressurization.

16. The method for manufacturing a bonded substrate according to claim 14, wherein the quartz substrate is obtained by growing a crystal by a hydrothermal synthesis method and cutting out the crystal in any direction.

17. The method for manufacturing a bonded substrate according to claim 14, wherein an amorphous layer is interposed on one or both of the bonding surfaces of the quartz substrate and the piezoelectric substrate.

18. The method for manufacturing a bonded substrate according to claim 17, wherein the amorphous layer is attached by a thin film forming method.

19. A bonded substrate comprising: a quartz substrate; and a piezoelectric substrate which is bonded on the quartz substrate and on which a surface acoustic wave propagates, wherein the quartz substrate and the piezoelectric substrate are bonded by covalently coupling at a bonding interface; and a crystal orientation of the quartz substrate and crystal orientation of the piezoelectric substrate intersect with each other in the range of 65 degrees to 115 degrees in a bonding surface direction.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating a bonded state of a bonded substrate of an embodiment of present invention.

(2) FIG. 2 is a schematic diagram illustrating the bonded substrate and a surface acoustic wave element in the same embodiment.

(3) FIG. 3 is a schematic diagram illustrating a bonded substrate and a surface acoustic wave element in another embodiment.

(4) FIG. 4 is a schematic diagram illustrating a bonding processing apparatus used for manufacturing a bonded substrate in an embodiment of the present invention.

(5) FIG. 5 is a diagram for explaining a bonding manner of the quartz substrate and the piezoelectric substrate in the same embodiment.

(6) FIG. 6 is a schematic diagram illustrating a surface acoustic wave device in the same embodiment.

(7) FIG. 7 is a graph illustrating the relationship between the thickness of a piezoelectric substrate and each of a phase velocity, propagation attenuation, TCF, and K.sup.2 in an LSAW of LN/AT90° X-Quartz in the same embodiment.

(8) FIG. 8 is a graph illustrating the relationship between the thickness of a piezoelectric substrate and each of a phase velocity, propagation attenuation, TCF, and K.sup.2 in an LSAW of LT/AT90° X-Quartz in the same embodiment.

(9) FIG. 9 is a graph illustrating the particle displacement distribution of an LSAW of 36° YX-LT/AT90° X-Quartz in the same embodiment.

(10) FIG. 10 is a graph illustrating LSAW resonance characteristics (infinite periodic structure) on 36° YX-LT/AT90° X-Quartz based on FEM analysis in the same embodiment.

(11) FIG. 11 is a diagram illustrating the relationship between an intersecting angle and a phase velocity in the same embodiment.

(12) FIG. 12 is a diagram illustrating the relationship between an intersecting angle and a phase velocity, and the relationship between an intersecting angle and an electromechanical coupling factor in the same embodiment.

(13) FIG. 13 is a graph illustrating the relationship between an intersecting angle and TCF, and the relationship between an intersecting angle and an electromechanical coupling factor in the same embodiment.

(14) FIG. 14 is a diagram illustrating the relationship between the thickness of a piezoelectric substrate and each of TCF and an electromechanical coupling factor in the same embodiment.

DESCRIPTION OF EMBODIMENTS

(15) Hereinafter, a bonded substrate and a surface acoustic wave element according to an embodiment of the present invention will be described with reference to the accompanying drawings.

(16) A bonded substrate 5 includes a quartz substrate 2 and a piezoelectric substrate 3 bonded by covalently coupling via a bonding interface 4.

(17) The quartz substrate 2 suitably has a thickness of 150 to 500 μm. The piezoelectric substrate 3 suitably has a thickness corresponding to 0.05 to 1.0 wavelengths based on the wavelengths of a surface acoustic wave. As the present invention, the thickness of the piezoelectric substrate more desirably is 0.05 to 0.8 wavelengths based on the wavelengths of the surface acoustic wave, and still more desirably is 0.05 to 0.25 wavelengths.

(18) As the quartz substrate 2, it can use, for example, the quartz substrate which is obtained by growing a crystal by a hydrothermal synthesis method and cutting out the crystal in any direction. The piezoelectric substrate 3 can use a proper material, and be composed, for example, of lithium tantalate or lithium niobate. In particular, it can use lithium tantalate which is Y-cut at 36° in a plane orientation and X-propagating, or lithium niobate which is Y-cut at 41° and X-propagating.

(19) However, as shown in FIG. 1, in the bonding, the quartz substrate 2 and the piezoelectric substrate 3 are bonded to each other in a state where an orientation 2D of the quartz substrate and an orientation 3D of the piezoelectric substrate 3 intersect on a right angle direction side in a plane direction. The phrase “intersecting on a right angle direction side” means that an intersecting angle between the quartz substrate and the piezoelectric substrate exceeds 45 degrees. Suitably, the intersecting angle is desirably 65 degrees to 115 degrees. The bonded substrate in which the quartz substrate and the piezoelectric substrate are bonded to each other at the aforementioned intersecting angle has an excellent temperature characteristic of frequency and a high electromechanical coupling coefficient. When the intersecting angle is out of the aforementioned angle range, the results satisfied in both the temperature characteristic of frequency and the electromechanical coupling factor are not obtained. The bonded substrate having the aforementioned intersecting angle has TCF of −20 to +5 ppm/° C. and an electromechanical coupling factor K.sup.2 of 5% or more.

(20) As shown in FIG. 2, a surface acoustic wave element 1 is obtained by providing an interdigital electrode 10 on the bonded substrate 5. In the present invention, an orientation represents a plane orientation. In this embodiment, the orientation represents an X direction of a surface Y-cut at 36° in LT, and an X direction of a surface Y-cut at 41° in LN, and the orientation represents an X direction of an AT-cut or ST-cut surface in the quartz substrate.

(21) As shown in FIG. 3, an amorphous layer 6 can be interposed between the quartz substrate 2 and the piezoelectric substrate 3 to provide a surface acoustic wave element 1A. The same configurations as those in the aforementioned embodiment are given the same reference numerals, and their description is omitted. Also in this embodiment, the quartz substrate 2 and the piezoelectric substrate 3 are bonded to each other in a state where an orientation of the quartz substrate 2 and an orientation of the piezoelectric substrate 3 intersect with each other on a right angle direction side in a plane direction.

(22) In this embodiment, when the amorphous layer 6 is interposed therebetween, a bonding interface exists between the amorphous layer 6 and the quartz substrate 2, and on the other side of the amorphous layer 6, a bonding interface exists between the amorphous layer 6 and the piezoelectric substrate 3. The material of the amorphous layer 6 is not particularly limited in the present invention, but SiO.sub.2 and Al.sub.2O.sub.3 and the like can be used. The thickness of the amorphous layer is desirably 100 nm or less.

(23) In forming the amorphous layer 6, the amorphous layer 6 can be formed in a manner of forming a thin film on the surface of the quartz substrate 2 or the piezoelectric substrate 3. Amorphous layers can also be formed on both the surface of the quartz substrate 2 and the surface of the piezoelectric substrate 3.

(24) The amorphous layer can be formed by a known method, and chemical vapor deposition or physical vapor deposition such as sputtering can be utilized.

(25) Next, manufacturing of the bonded substrate and the surface acoustic wave element will be described with reference to FIG. 4.

(26) A quartz substrate and a piezoelectric element of predetermined materials are prepared. When an amorphous layer is formed on a bonding surface, with respect to one or both of the quartz substrate and the piezoelectric element which are the target to be formed, deposition processing is performed on the bonding surface side. A method for the deposition processing is not particularly limited, but a thin film forming technique such as a vacuum vapor deposition method or a sputtering method can be used. For example, an amorphous layer which has a thickness of 100 nm or less can be formed on the bonding surface by Electron Cyclotron Resonance plasma deposition. This amorphous film can be formed to have a very high film density, and hence, the degree of activation of the bonding surface is high, which results in generation of more OH groups.

(27) The prepared quartz substrate 2 and piezoelectric substrate 3 are disposed in a processing apparatus 20 having a tightly-sealed structure. The figure presents only the quartz substrate 2.

(28) A vacuum pump 21 is connected to the processing apparatus 20, and the processing apparatus 20 is depressurized to, for example a pressure of 10 Pa or less. Discharge gas is introduced into the processing apparatus 20, and discharge is performed by a discharge apparatus 22 in the processing apparatus 20 to generate ultraviolet light. The discharge can be performed by using a method for applying a high frequency voltage or the similar method.

(29) The quartz substrate 2 and the piezoelectric substrate 3 are disposed in a state where they can be irradiated with ultraviolet light, and the bonding surfaces of the quartz substrate 2 and the piezoelectric substrate 3 are irradiated with ultraviolet light to be activated. When an amorphous layer is formed on one or both of the quartz substrate 2 and the piezoelectric substrate 3, the irradiation with ultraviolet light is performed by taking the surface of the amorphous layer as the bonding surface.

(30) As for the quartz substrate 2 and the piezoelectric substrate 3 which have undergone the irradiation with ultraviolet light, in a state where the orientation of the quartz substrate 2 and the orientation of the piezoelectric substrate 3 intersect with each other on a right angle direction side in a plane direction, the bonding surfaces of the quartz substrate 2 and the piezoelectric substrate 3 are contacted with each other and heated at ambient temperature or a temperature of 200° C. or less, and a pressure is applied across both of them to perform bonding. The applied pressure can be set at 10 Pa and the processing time can be set to be approximately from 5 minutes to 4 hours. However, the pressure and the processing time are not particularly limited in the present invention.

(31) By the aforementioned processing, the quartz substrate 2 and the piezoelectric substrate 3 are securely bonded by covalently coupling at the bonding interface, and are bonded in a state where the orientations of them intersect with each other on a right angle direction side.

(32) FIG. 5 shows states of the bonding surfaces of the quartz substrate 2 and the piezoelectric substrate 3.

(33) Portion A of the FIG. 5 shows a state where the bonding surfaces are activated by irradiation with ultraviolet light and OH groups are formed on the surfaces. Portion B of the FIG. 5 shows a state where the substrates are contacted with each other, and pressurized and heated to perform bonding. In the bonding, the OH groups react so that the substrates are covalently coupled with each other. Extra H.sub.2O is release to the outside in heating.

(34) The aforementioned processes provide the bonded substrate. With respect to the bonded substrate, as shown in FIG. 1, patterns of interdigital electrodes 10 are formed on the principal surface of the piezoelectric substrate 3. A method for forming the interdigital electrodes 10 is not particularly limited, but a proper method can be used. For the shape of the interdigital electrode 10, a proper shape can be employed. The aforementioned processes provide the surface acoustic wave element 1. The propagation direction of the acoustic wave is along the orientation of the piezoelectric substrate 3.

(35) As shown in FIG. 6, the surface acoustic wave element 1 is disposed in a packaging 31 and connected to not-shown electrodes and sealed with a lid 32, thereby can be provided as a surface acoustic wave device 30.

Example 1

(36) Hereinafter, Example of the present invention are described. A bonded substrate was obtained based on the aforementioned embodiment. A SAW resonator was provided on the principal surface of a piezoelectric substrate such that the propagation direction of an LLSAW becomes the X-direction.

(37) In this example, as the piezoelectric substrate, lithium tantalate which was Y-cut at 36° in a plane orientation and X-propagating, and lithium niobate which was Y-cut at 41° in a plane orientation and X-propagating were used. As a quartz substrate in which crystal was grown by a hydrothermal synthesis method, a quartz substrate having a thickness of 250 μm and cut out in an AT-cut direction or a ST-cut direction was used.

(38) The bonded sample was polished on the lithium tantalate side to be thin.

(39) For the obtained bonded substrate, its bonding strength was measured by a method of tensile testing (tension is applied perpendicular to the wafer plane). As a result, it was found that 5 MPa or more (converted into the value per unit area) of bonding strength was obtained, furthermore, excellent bonding strength which leads to the bulk destruction was obtained.

(40) As for the sample material obtained by making the piezoelectric substrate thin after bonding the quartz substrate and the piezoelectric substrate to each other, a phase velocity, an electromechanical coupling factor, and a temperature characteristic of frequency of the LSAW were calculated. Quartz constant of Kushibiki et al. (p. 83), lithium niobate (hereinafter, referred to as LN) constant of Kushibiki et al., and lithium tantalate (hereinafter, referred to as LT) constant (p. 377) described in “Acoustic Wave Device Technique” edited by the Japan Society for the Promotion of Science, the 150th committee of acoustic wave element technique, were used for calculating.

(41) The LSAW having propagation attenuation was analyzed based on the method of Yamanouchi et al., and the analysis of a layer structure was performed by using the methods of Farnell and Adler. In these analyses, the phase velocity and the propagation attenuation of the LSAW which propagates on the layer structure are analyzed by numerically solving the acoustic wave motion equation and the charge conservation equation under a boundary condition.

(42) A phase velocity vf of the free surface (Free), and a phase velocity v.sub.m when the surface of the thin plate was electrically shorted (metallized) were obtained, and K.sup.2 was obtained from K.sup.2=2×(v.sub.f−v.sub.n)/v.sub.f. A linear expansion coefficient in a propagation direction was assumed to the linear expansion coefficient of a quartz supporting substrate, and a temperature coefficient of frequency (TCF) of the electrically shorted surface was calculated.

(43) It is considered that, since the quartz has large anisotropy, propagation characteristics during bonding largely depend on the propagation direction of the quartz. As a result of calculating the phase velocity of the LSAW with respect to a propagation angle from an X-axis on AT-cut quartz, it was found that this phase velocity was the highest speed in X-propagating at 0° and X-propagating at 90° in the LSAW. In these propagation orientations, the maximum phase velocity difference from the LN/LT simplex provides an expectable concentration effect of particle displacement.

(44) FIG. 7 and FIG. 8 show the calculated values of (a) phase velocity, (b) propagation attenuation, and (c) TCF and K.sup.2 of the LSAW when the 41° YX-LN thin plate and the 36° YX-LT thin plate obtained by the aforementioned analysis were bonded to AT-cut 90° X-quartz (intersecting angle of 90 degrees, hereinafter, the same shall apply), respectively. A horizontal axis represents a plate thickness h/λ of LN and LT thin plates normalized by a wavelength λ. In both of the cases, it is found that the increase in the plate thickness causes the phase velocity of quartz simplex to approach asymptotically to the phase velocity of the LN/LT simplex. When attention is paid to the K.sup.2 calculated value, a plate thickness in which K.sup.2 greater than the value of the simplex is obtained exists in both the cases. In the present invention, as the suitable range of the plate thickness, the plate thickness has a thickness corresponding to 0.05 to 1.0 wavelengths based on the wavelength of the surface acoustic wave. In this range, an excellent value than that of the TCF of the LT/LN simplex is shown. K.sup.2 is higher than that of the simplex LN when h/λ is 0.08 or more, and higher than that of the simplex LT when h/λ is 0.4 or more. Taking these into consideration, the plate thickness h/λ is more desirably set to 0.05 to 0.8.

(45) The 41° YX-LN thin plate exhibited K.sup.2 which was 1.5 times (23.9%) of that of the simplex when the plate thickness h/λ was 0.19, and the propagation attenuation of the plate thickness and the TCF of the electrically shorted surface were respectively calculated to be 0.002 dB/λ or less and −55 ppm/° C. Meanwhile, the 36° YX-LT thin plate simultaneously exhibited TCF of 0 and K.sup.2 of 11.9% (2.3 times of the simplex) when the plate thickness h/λ was 0.17. It was found that the propagation attenuation of the plate thickness was 0.0002 dB/λ or less, and a substrate structure having high stability, high coupling, and low loss is obtained.

(46) In order to examine the factor of the high coupling as described above, a particle displacement distribution with respect to the depth direction of the LSAW was calculated. The aforementioned analysis was used in the calculation. As for the LSAW on 36° YX-LT/AT90° X-quartz, the displacement distribution of SH ingredient (u.sub.2) in the electrically shorted surface is shown in FIG. 9. The displacement is normalized by a surface value. It was found that the displacement distribution of the bonding structure concentrated near the surface as compared with that of the LT simplex, and a thinner normalized plate thickness provided a higher concentration effect.

(47) Next, the resonance characteristics of the LSAW of an IDT type resonator (λ=8.0 μm, intersection width W=25λ) formed on an LT/quartz bonding structure were analyzed by using the Finite Element Method (FEM). Femtet (manufactured by Murata Software Co., Ltd.) was used as analysis software. As an analysis model, the plate thickness of the supporting substrate was set to 10λ. A periodic boundary condition (infinite periodic structure) was assumed on both sides of the IDT for one cycle, and a completely-matched layer was assumed on the bottom face.

(48) FIG. 10 shows the analysis example of the LSAW on the 36° YX-LT/AT90° X-Quartz structure. An LT plate thickness is 0.15λ, and an electrode A1 film thickness is 0.09λ. In the bonding structure, an admittance ratio of 126 dB was obtained, which was markedly increased as compared with 72 dB of the LT simplex. Resonance Q was increased by an order of magnitude to 12050 from 1350 of the LT simplex. A specific bandwidth was also increased to 5.7% from 4.4% of the LT simplex.

(49) Next, the change in the phase velocity due to the misalignment of the orientations of the 41° YX-LN or 36° YX-LT and the AT-cut quartz substrate was obtained by the analysis described in Paragraph 0047, and shown in FIG. 11.

(50) As is apparent from FIG. 11, the phase velocity reaches a maximum when the intersecting angle between the AT-cut quartz substrate and the LN or the LT is 0 degree and 90 degree, and the phase velocity is smaller as the intersecting angle is away from 0 degrees and 90 degrees.

(51) Next, changes in a phase velocity and an electromechanical coupling factor when an orientation angle difference in bonding was changed in the bonded substrate of the 36° YX-LT having h/λ of 0.15 and the ST-cut quartz substrate were calculated by the aforementioned analysis, and shown in FIG. 12. It is found that, when the intersecting angle of the orientations is 0 degrees or 90 degrees, changes in a phase velocity and an electromechanical coupling factor show the maximum numerical value, but the intersecting angle is desirably 30 degrees or less or 65 degrees to 115 degrees in order to obtain an electromechanical coupling factor K.sup.2 of 5% or more. The electromechanical coupling factor of 5% is K.sup.2 obtained in the LT simplex, and the combination of the piezoelectric substrate with the quartz has an advantage that the electromechanical coupling factor is 5% or more.

(52) Next, as for the bonded substrate obtained by bonding the 36° YX-LT and the AT-cut quartz substrate to each other at an intersecting angle of 0 degrees and the bonded substrate obtained by coupling the 36° YX-LT and the AT-cut quartz substrate at an intersecting angle of 90 degrees, TCF and an electromechanical coupling factor were calculated by the aforementioned analysis, and the results were shown in FIG. 13.

(53) As is apparent from the right drawing of FIG. 13, both of the bonded substrates having intersecting angles of 0 degrees and 90 degrees have a high electromechanical coupling factor in a suitable thickness. As for the thickness h/λ of the piezoelectric substrate, examples of the range in which TCF exceeds 0° include 0.05 to 0.25.

(54) Meanwhile, as for the TCF, by appropriately setting the thickness of the bonded substrate having an intersecting angle of 90 degrees, the result that the TCF becomes 0 ppm/° C. is obtained. However, even when the TCF is the smallest, the bonded substrate having an intersecting angle of 0 degrees has a temperature characteristic of frequency of about −10 ppm/° C., which is clearly poorer than the bonded substrate having an intersecting angle of 90 degrees in the aspect of temperature characteristic of frequency. Therefore, in order to obtain excellent characteristics in both the TCF and the electromechanical coupling factor, the quartz substrate and the piezoelectric substrate are required to intersect with each other on an orthogonal direction side in a bonding surface direction.

(55) In the bonded substrate having an intersecting angle of 90 degrees, the TCF and the electromechanical coupling factor were collectively shown in FIG. 14. As is apparent from this figure, in order to obtain a better TCF and a higher electromechanical coupling factor, it is found that h/λ of the piezoelectric substrate is desirably set properly. In the case of this figure, h/λ is set to the range of 0.05 to 1.0, whereby more desirable results are obtained in both the temperature characteristic of frequency and the coupling characteristic.

(56) As above, the present invention has been described based on the aforementioned embodiment and Example. The scope of the present invention is not limited to the contents of the aforementioned description, but proper modifications of the aforementioned embodiment and Example can occur without departing from the scope of the present invention.

INDUSTRIAL APPLICABILITY

(57) The present invention can be utilized for a SAW resonator, a SAW filter, a highly-functional piezoelectric sensor, and a BAW device and the like.

REFERENCE SIGNS LIST

(58) 1: surface acoustic wave element 1A: surface acoustic wave element 2: quartz substrate 2D: quartz substrate orientation 3: piezoelectric substrate 3D: piezoelectric substrate orientation 4: bonding interface 5: bonded substrate 10: interdigital electrode 20: processing apparatus 30: surface acoustic wave device