Composite substrate and method for manufacturing the same

Abstract

To provide a composite substrate capable of contributing to improving the performance of a SAW filter.SOLUTION: A composite substrate according to an embodiment of the present invention includes a support substrate and a piezoelectric layer disposed on one side of the support substrate, and the amplitude of waviness of the shape of the support substrate having a spatial frequency exceeding 0.045 cyc / mm is 10 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a composite substrate and a method for manufacturing the composite substrate. [Background technology]

[0002] In communication devices such as mobile phones, filters that utilize surface acoustic waves (SAW filters) are used to extract electrical signals of any frequency. These SAW filters have a structure in which electrodes and the like are formed on a composite substrate having a piezoelectric layer (see, for example, Patent Document 1).

[0003] In recent years, in the field of information and communication devices, for example, communication volume has increased dramatically, and there is a demand for higher performance SAW filters. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-150488 Summary of the Invention [Problem to be solved by the invention]

[0005] A primary object of the present invention is to provide a composite substrate that can contribute to improving the performance of SAW filters. [Means for solving the problem]

[0006] A composite substrate according to an embodiment of the present invention includes a support substrate and a piezoelectric layer disposed on one side of the support substrate, and the amplitude of waviness of the shape of the support substrate exceeding a spatial frequency of 0.045 cyc / mm is 10 nm or less. In one embodiment, the absolute value of the difference between the thickness T1 at the first point and the thickness T2 at the second point of the piezoelectric layer is 100 nm or less. In one embodiment, the piezoelectric layer has a thickness of 5 μm or less. A surface acoustic wave element according to another embodiment of the present invention includes the above-described composite substrate.

[0007] A method for manufacturing a composite substrate according to another embodiment of the present invention includes bonding a support substrate to a first main surface side of a piezoelectric substrate having first and second main surfaces facing each other, and polishing a surface on the second main surface side of the piezoelectric substrate, wherein the amplitude of waviness of the shape of the support substrate exceeding a spatial frequency of 0.045 cyc / mm is 10 nm or less. In one embodiment, the absolute value of the difference between the thickness T1 at the first point and the thickness T2 at the second point of the piezoelectric layer obtained by polishing the piezoelectric substrate is 100 nm or less. In one embodiment, the thickness of the piezoelectric layer obtained by polishing the piezoelectric substrate is 5 μm or less. [Effects of the Invention]

[0008] The embodiments of the present invention can contribute to improving the performance of SAW filters, for example. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view showing an outline of the configuration of a composite substrate according to one embodiment of the present invention. [Figure 2] FIG. 1 is a diagram illustrating an example of the appearance of a composite substrate. [Figure 3A] 1A and 1B are diagrams illustrating an example of the outline of the shape of a support substrate and the shape of a piezoelectric layer. [Figure 3B] 10A and 10B are diagrams illustrating another example of the outline of the shape of the support substrate and the shape of the piezoelectric layer. [Figure 4A] 1A to 1C are diagrams illustrating an example of a manufacturing process for a composite substrate according to one embodiment. [Figure 4B] This is a continuation of Figure 4A. [Figure 4C] This is a continuation of Figure 4B. [Figure 4D] This is a continuation of Figure 4C. [Figure 5A] FIG. 2 is a diagram showing the shape of a silicon substrate according to an embodiment of the present invention. [Figure 5B]FIG. 10 is a diagram showing the film thickness distribution of the LT layer in the example. [Figure 5C] FIG. 10 is a diagram showing the results of FFT analysis of the shape of a silicon substrate and the film thickness distribution of an LT layer in an example. [Figure 5D] FIG. 5D is an enlarged view of the vertical and horizontal axes of FIG. 5C. [Figure 6A] FIG. 10 is a diagram showing the shape of a silicon substrate of a comparative example. [Figure 6B] FIG. 10 is a diagram showing the film thickness distribution of the LT layer of the comparative example. [Figure 6C] FIG. 10 is a diagram showing the results of FFT analysis of the shape of a silicon substrate and the film thickness distribution of an LT layer in a comparative example. [Figure 6D] FIG. 6D is an enlarged view of the vertical and horizontal axes of FIG. 6C. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In addition, in order to clarify the description, the drawings may show the width, thickness, shape, etc. of each part more schematically than in the embodiments, but these are merely examples and do not limit the interpretation of the present invention.

[0011] A. Composite substrate FIG. 1 is a schematic cross-sectional view showing the overall configuration of a composite substrate according to one embodiment of the present invention. The composite substrate 100 includes a support substrate 10 and a piezoelectric layer 20 disposed on one side of the support substrate 10. Although not shown, the composite substrate 100 may further include any optional layers. The type, function, number, combination, and arrangement of such layers may be appropriately determined depending on the purpose. For example, the composite substrate 100 may include an intermediate layer (e.g., an inorganic material layer) disposed between the piezoelectric layer 20 and the support substrate 10. Furthermore, for example, the composite substrate 100 may include a bonding layer disposed between the piezoelectric layer 20 or the intermediate layer (not shown) and the support substrate 10.

[0012] The composite substrate 100 can be manufactured in any suitable shape. In one embodiment, it can be manufactured in the form of a so-called wafer, as shown in FIG. 2. The size of the composite substrate 100 can be appropriately set depending on the purpose. For example, the diameter of the wafer is 50 mm to 150 mm.

[0013] A-1.Support board Any appropriate thickness can be adopted as the thickness of the support substrate 10. The thickness of the support substrate is, for example, 100 μm to 1000 μm.

[0014] The amplitude of the waviness of the support substrate, whose spatial frequency exceeds 0.045 cyc / mm, is 0 nm to 10 nm, preferably 5 nm or less. By using such a support substrate, the thickness precision of the piezoelectric layer, which will be described later, can be achieved satisfactorily.

[0015] Any suitable substrate can be used as the support substrate. The support substrate may be composed of a single crystal or a polycrystalline body. The material constituting the support substrate is preferably selected from the group consisting of silicon, sapphire, glass, quartz, crystal, and alumina.

[0016] The silicon may be single crystal silicon, polycrystalline silicon, or high-resistivity silicon.

[0017] Typically, the sapphire is a single crystal having a composition of Al2O3, and the alumina is a polycrystalline having a composition of Al2O3.

[0018] The thermal expansion coefficient of the material constituting the support substrate is preferably smaller than that of the material constituting the piezoelectric layer (described later). Such a support substrate can suppress changes in the shape and size of the piezoelectric layer when the temperature changes, thereby suppressing changes in the frequency characteristics of the resulting surface acoustic wave element.

[0019] A-2. Piezoelectric layer Any appropriate piezoelectric material can be used as the material constituting the piezoelectric layer. A single crystal having the composition LiAO3 is preferably used as the piezoelectric material. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO3 may be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a lithium niobate-lithium tantalate solid solution.

[0020] When the piezoelectric material is lithium tantalate, it is preferable to use a piezoelectric layer whose normal direction is rotated 32° to 55° (for example, 42°) from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, or (180°, 58° to 35°, 180°) in Euler angle representation, because this reduces propagation loss.

[0021] When the piezoelectric substrate is made of lithium niobate, it is preferable to use a piezoelectric layer whose normal is rotated 37.8° from the Z axis to the -Y axis around the X axis, which is the propagation direction of the surface acoustic wave, or (0°, 37.8°, 0°) in Euler angles, because this has a large electromechanical coupling coefficient.Also, when the piezoelectric substrate is made of lithium niobate, it is preferable to use a piezoelectric layer whose normal is rotated 40° to 65° from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, or (180°, 50° to 25°, 180°) in Euler angles, because this has a high acoustic velocity.

[0022] The thickness of the piezoelectric layer is preferably 5 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less. On the other hand, the thickness of the piezoelectric layer is, for example, 0.2 μm or more. With such a thickness, a high-performance surface acoustic wave element can be obtained. Specifically, effects such as improved temperature coefficient of friction (TCF) and improved Q value can be expected.

[0023] The thickness of the piezoelectric layer is preferably uniform. FIGS. 3A and 3B are diagrams showing examples of the shape of the support substrate (e.g., in the X-axis direction) and the shape of the piezoelectric layer, respectively. As shown in FIG. 3A, when the undulation frequency of the support substrate 10 is low (e.g., when the spatial frequency is 0.045 cyc / mm or less), the shape of the piezoelectric layer 20 can be easily adapted to the undulation of the support substrate 10, resulting in excellent film thickness accuracy of the piezoelectric layer 20. For example, in the X-axis direction, the absolute value of the difference between the thickness (first thickness) T1 of the piezoelectric layer 20 at a first point and the thickness (second thickness) T2 of the piezoelectric layer 20 at a second point is preferably 100 nm or less, more preferably 50 nm or less. Such film thickness accuracy allows for a high-performance surface acoustic wave element. Specifically, effects such as an improved Q value can be expected. Furthermore, a surface acoustic wave element with minimal variation in characteristics can be obtained. As shown in FIG. 3B, when the undulation frequency of the support substrate 10 is high (for example, when the spatial frequency exceeds 0.045 cyc / mm), it tends to be difficult to make the shape of the piezoelectric layer 20 correspond to the undulation of the support substrate 10. However, by satisfying the amplitude of the undulation of the support substrate, the film thickness precision of the piezoelectric layer 20 can be excellent.

[0024] A-3.Other As described above, the composite substrate may have an intermediate layer. Examples of materials that can form the intermediate layer include silicon oxide, hafnium oxide, tantalum oxide, zirconium oxide, and aluminum oxide. The thickness of the intermediate layer is, for example, 0.1 μm to 2 μm.

[0025] The intermediate layer can be formed by any suitable method, such as sputtering, physical vapor deposition such as ion beam assisted deposition (IAD), chemical vapor deposition, or atomic layer deposition (ALD).

[0026] As described above, the composite substrate may have a bonding layer. Examples of materials that can form the bonding layer include silicon oxide, silicon, tantalum oxide, niobium oxide, aluminum oxide, titanium oxide, and hafnium oxide. The thickness of the bonding layer is, for example, 0.005 μm to 1 μm.

[0027] The bonding layer can be formed by any suitable method, specifically, by the same method as the method for forming the intermediate layer.

[0028] A-4. Manufacturing method A method for manufacturing a composite substrate according to one embodiment of the present invention includes bonding a support substrate to a first principal surface side of a piezoelectric substrate having first and second principal surfaces facing each other, and polishing the surface of the second principal surface side of the piezoelectric substrate. Typically, polishing is performed after bonding.

[0029] 4A to 4D are diagrams showing an example of a manufacturing process for a composite substrate according to one embodiment.

[0030] FIG. 4A shows a state in which polishing of both opposing principal surfaces of support substrate 10 has been completed. In the example shown in FIG. 4A, lower surface 10a of support substrate 10 is polished flat, and upper surface 10b has a downwardly convex curved shape. The amplitude of the waviness of the shape of support substrate 10, which exceeds a spatial frequency of 0.045 cyc / mm, is 0 nm or more and 10 nm or less, and preferably 5 nm or less. Such a value can be satisfactorily achieved, for example, by the shape of the illustrated example. Although not shown, upper surface 10b may have an upwardly convex curved shape.

[0031] FIG. 4B shows the support substrate 10 and the piezoelectric substrate 22 directly bonded together. The piezoelectric substrate 22 has a first main surface 22a and a second main surface 22b facing each other. The first main surface 22a is polished before bonding to the support substrate 10. For direct bonding, the bonding surfaces are preferably activated by any appropriate activation process. For example, the top surface 10b of the support substrate 10 is activated, and the first main surface 22a of the piezoelectric substrate 22 is activated. Then, the activated surfaces of the support substrate 10 and the piezoelectric substrate 22 are brought into contact with each other and pressure is applied to bond them directly. In this way, a bonded body 90 shown in FIG. 4B is obtained.

[0032] The second main surface 22b of the piezoelectric substrate 22 of the resulting bonded body 90 is subjected to processing such as grinding and polishing to form a piezoelectric layer of the desired thickness. FIG. 4C shows the second main surface 22b after grinding is complete, and FIG. 4D shows the second main surface 22b after polishing is complete. The piezoelectric layer 20 is formed by polishing, and the composite substrate 100 is obtained. The shape of the upper surface 20a of the piezoelectric layer 20 can correspond to the shape of the upper surface 10b of the support substrate 10, for example.

[0033] Examples of the polishing method include mirror polishing by chemical mechanical polishing (CMP) and lap polishing. Preferably, chemical mechanical polishing is used. Specifically, chemical mechanical polishing using a polishing pad with a polishing slurry (e.g., colloidal silica) is used.

[0034] During the above bonding, it is preferable to clean the surfaces of each layer, for example, to remove abrasive residues, processing-affected layers, etc. Cleaning methods include, for example, wet cleaning, dry cleaning, and scrub cleaning. Among these, scrub cleaning is preferred because it allows for simple and efficient cleaning. A specific example of scrub cleaning is a method in which a cleaning agent (for example, the Sun Wash series manufactured by Lion Corporation) is used, followed by cleaning in a scrub cleaning machine using a solvent (for example, a mixed solution of acetone and isopropyl alcohol (IPA)).

[0035] The activation process is typically performed by irradiating a neutralizing beam. Preferably, a neutralizing beam is generated using an apparatus such as that described in JP 2014-086400 A, and the activation process is performed by irradiating this beam. Specifically, a saddlefield-type fast atom beam source is used as the beam source, an inert gas such as argon or nitrogen is introduced into a chamber, and a high voltage is applied to the electrodes from a DC power supply. This generates a saddlefield-type electric field between the electrode (positive electrode) and the housing (negative electrode), causing electrons to move, generating a beam of atoms and ions from the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, and a beam of neutral atoms is emitted from the fast atom beam source. The voltage during the activation process by beam irradiation is preferably 0.5 kV to 2.0 kV, and the current during the activation process by beam irradiation is preferably 50 mA to 200 mA.

[0036] The contact and pressure application of the bonding surfaces is preferably carried out in a vacuum atmosphere. The temperature at this time is typically room temperature. Specifically, the temperature is preferably 20°C or higher and 40°C or lower, and more preferably 25°C or higher and 30°C or lower. The pressure applied is preferably 100N to 20,000N.

[0037] B. Surface acoustic wave element A surface acoustic wave element according to an embodiment of the present invention includes the composite substrate. The surface acoustic wave element typically includes the composite substrate and an electrode (comb-shaped electrode) provided on the piezoelectric layer side of the composite substrate. Such a surface acoustic wave element is suitable for use as a SAW filter in communication devices such as mobile phones. [Example]

[0038] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0039] [Example] A lithium tantalate (LT) substrate with a diameter of 4 inches and a thickness of 500 μm (a 42° Y-cut X-propagation LT substrate with a rotational Y-cut angle, where the propagation direction of the surface acoustic wave (SAW) is X) was prepared.

[0040] In addition, a silicon substrate with a diameter of 4 inches and a thickness of 500 μm was prepared, and the surfaces (both sides) of this silicon substrate were polished. Specifically, the silicon substrate was placed on a SUS carrier of a CMP polishing machine, and double-sided polishing was performed using a hard urethane pad and colloidal silica as an abrasive.

[0041] Next, the LT substrate and the silicon substrate were directly bonded. Specifically, after cleaning the surfaces of the LT substrate and the silicon substrate, both substrates were placed in a vacuum chamber and heated for 10 minutes. -6 After evacuation to the Pa range, the surfaces of both substrates were irradiated with a fast atom beam (accelerating voltage 1 kV, Ar flow rate 27 sccm) for 80 seconds. After irradiation, the beam-irradiated surfaces of both substrates were placed together and pressed together at 1200 kgf for 2 minutes to bond the two substrates, obtaining a bonded assembly.

[0042] Next, the back surface of the LT substrate of the above bonded body (composite substrate) was ground from the initial thickness of 500 μm to 3 μm using a grinder processing machine, and further mirror-polished to a thickness of 1 μm using a CMP polisher with a hard urethane pad and colloidal silica as an abrasive, to obtain a composite substrate having a silicon substrate and an LT layer.

[0043] [Comparative Example] A composite substrate was obtained in the same manner as in Example, except that a nonwoven fabric was used instead of a hard urethane pad when polishing both sides of the silicon substrate.

[0044] <Evaluation> The above examples and comparative examples were evaluated as follows. 1. Measurement of flatness (shape) and thickness (film thickness distribution) The flatness of the silicon substrate in the X-axis direction after double-side polishing was measured using an oblique incidence interferometric flatness tester ("FT-17" manufactured by NIDEK Corporation). The thickness of the LT layer of the resulting composite substrate in the X-axis direction was also measured using a microspectrophotometer ("OPTM" manufactured by Otsuka Electronics Co., Ltd.). Specifically, measurements were taken at 32 points with a 2.8 mm pitch in the range of -43.4 mm to +43.4 mm, with the center of the wafer as the origin. The results for the silicon substrate of the example are shown in Figure 5A, and the results for the LT layer of the example are shown in Figure 5B. The results for the silicon substrate of the comparative example are shown in Figure 6A, and the results for the LT layer of the comparative example are shown in Figure 6B. Note that the graphs shown in Figures 5A, 5B, 6A, and 6B show values ​​after subtracting the average thickness value. 2.FFT (Fast Fourier Transform) analysis An FFT analysis was performed on the results obtained in 1 above. Specifically, the shape and film thickness distribution obtained in 1 above were multiplied by a Hanning window as a window function, and an FFT analysis was performed. The slope and DC components were removed during the calculation, and the analysis tool in Microsoft Excel was used. For FFT conversion and amplitude calculation, the obtained absolute value was divided by 16 (32 data points divided by 2), and then doubled to take into account the effects of processing using the Hanning window. In addition, when the silicon substrate had a downward convex shape (the side where the LT substrate was not placed), unnecessary frequency components were generated when the Hanning window was multiplied. Therefore, the data was inverted by subtracting each data point from the maximum value so that the silicon substrate had an upward convex shape. The results of the Example are shown in FIGS. 5C and 5D, and the results of the Comparative Example are shown in FIGS. 6C and 6D.

[0045] As shown in FIG. 5B, in the example, a composite substrate with excellent film thickness accuracy of the LT layer was obtained. [Industrial Applicability]

[0046] The composite substrate according to the embodiment of the present invention can be suitably used, typically, in a surface acoustic wave element. [Explanation of symbols]

[0047] 10 Support substrate 20 Piezoelectric layer 100 Composite Board

Claims

1. a support substrate having upper and lower surfaces facing each other; a piezoelectric layer disposed on an upper surface side of the support substrate; an intermediate layer disposed between the support substrate and the piezoelectric layer; an upper surface of the support substrate is a curved surface and a lower surface of the support substrate is a flat surface; an absolute value of a difference between a thickness T1 at an arbitrary first point and a thickness T2 at an arbitrary second point of the piezoelectric layer is 100 nm or less; The amplitude of waviness of the shape of the support substrate exceeding a spatial frequency of 0.045 cyc / mm is 10 nm or less. Composite board.

2. 2. The composite substrate according to claim 1, wherein the material constituting the intermediate layer is selected from the group consisting of silicon oxide, hafnium oxide, tantalum oxide, zirconium oxide and aluminum oxide.

3. 2. The composite substrate according to claim 1, wherein the thickness of the intermediate layer is 0.1 μm to 2 μm.

4. The composite substrate according to claim 1 , wherein the thickness of the piezoelectric layer is 5 μm or less.

5. The composite substrate according to claim 1 , wherein the thermal expansion coefficient of the material constituting the support substrate is smaller than the thermal expansion coefficient of the material constituting the piezoelectric layer.

6. The composite substrate according to claim 1 , further comprising a bonding layer disposed between the support substrate and the intermediate layer.

7. A method for manufacturing a composite substrate having a support substrate, an intermediate layer, and a piezoelectric substrate, comprising: polishing both opposing main surfaces of the support substrate; forming the intermediate layer on the piezoelectric substrate or the support substrate; bonding the support substrate to a first principal surface side of the piezoelectric substrate having a first principal surface and a second principal surface facing each other; and polishing the surface of the piezoelectric substrate on the second main surface side; polishing both opposing main surfaces of the support substrate to form a curved upper surface of the support substrate on the side to which the piezoelectric substrate is bonded and a flat lower surface of the support substrate; an absolute value of a difference between a thickness T1 at an arbitrary first point and a thickness T2 at an arbitrary second point of the piezoelectric layer obtained by polishing the piezoelectric substrate is 100 nm or less; By polishing both opposing main surfaces of the support substrate, the amplitude of waviness of the shape of the support substrate, which has a spatial frequency exceeding 0.045 cyc / mm, is set to 10 nm or less. A method for manufacturing a composite substrate.

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