Composite substrate and method for manufacturing the same

Abstract

A composite substrate and a method for manufacturing the composite substrate that can contribute to improving the performance of SAW filters are provided. [Solution] The composite substrate 100 includes a support substrate 10 and a piezoelectric layer 20 disposed on one side of the support substrate 10. The composite substrate 100 may further include any layer, for example, the composite substrate 100 may include an intermediate layer disposed between the piezoelectric layer 20 and the support substrate 10. The support substrate 10 has a shape whose undulation amplitude, with a spatial frequency exceeding 0.045 cyc / mm, is 10 nm or less, preferably 5 nm or less. By using such a support substrate, good film thickness accuracy of the piezoelectric layer 20 can be achieved.

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, for example, filters that utilize surface acoustic waves (SAW filters) are used to extract electrical signals of any frequency. This SAW filter has 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] JP 2020-150488 A 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 capable of contributing 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 having a spatial frequency exceeding 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 opposing each other, and polishing a surface of 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 piezoelectric layer obtained by polishing the piezoelectric substrate has a thickness of 5 μm or less. Effect of the Invention

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

[0009] [Figure 1] 1 is a schematic cross-sectional view showing an outline of a configuration of a composite substrate according to one embodiment of the present invention. [Diagram 2] FIG. 1 is a diagram showing an example of the appearance of a composite substrate. [Figure 3A] 1A and 1B are diagrams illustrating an example of an outline of the shape of a support substrate and the shape of a piezoelectric layer. [Figure 3B] 13A and 13B are diagrams showing another example of the outline of the shape of a support substrate and the shape of a 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. 13 is a diagram showing the film thickness distribution of the LT layer in the embodiment. [Figure 5C] FIG. 13 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. 1 is a diagram showing the shape of a silicon substrate of a comparative example. [Figure 6B] FIG. 13 is a diagram showing the film thickness distribution of the LT layer of the comparative example. [Figure 6C] FIG. 13 is a diagram showing the shape of a silicon substrate and the results of FFT analysis of 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 PREFERRED EMBODIMENTS

[0010] Hereinafter, the 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 width, thickness, shape, etc. of each part may be shown more diagrammatically than in the embodiment, but these are merely examples and do not limit the interpretation of the present invention.

[0011] A. Composite board FIG. 1 is a schematic cross-sectional view showing an outline of a composite substrate according to one embodiment of the present invention. The composite substrate 100 has 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 have any layer. The type, function, number, combination, arrangement, etc. of such layers may be appropriately determined according to the purpose. For example, the composite substrate 100 may have an intermediate layer (e.g., an inorganic material layer) disposed between the piezoelectric layer 20 and the support substrate 10. Also, for example, the composite substrate 100 may have a bonding layer disposed between the piezoelectric layer 20 or an intermediate layer (not shown) and the support substrate 10.

[0012] The composite substrate 100 may be manufactured in any appropriate shape. In one embodiment, the composite substrate 100 may be manufactured in the form of a so-called wafer, as shown in FIG. 2. The size of the composite substrate 100 may 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, which exceeds a spatial frequency of 0.045 cyc / mm, is 0 to 10 nm, preferably 5 nm or less. By using such a support substrate, the thickness precision of the piezoelectric layer described below can be achieved satisfactorily.

[0015] Any suitable substrate may be used as the support substrate. The support substrate may be made of a single crystal or a polycrystalline material. 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 resistance silicon.

[0017] Typically, the sapphire is a single crystal having the composition Al2O3, and the alumina is a polycrystalline material having the composition 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 below. Such a support substrate can suppress changes in the shape and size of the piezoelectric layer when the temperature changes, and can suppress changes in the frequency characteristics of the resulting surface acoustic wave element, for example.

[0019] A-2. Piezoelectric layer Any suitable piezoelectric material may be used as the material constituting the piezoelectric layer. As the piezoelectric material, a single crystal having a composition of LiAO3 is preferably used. 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, in order to reduce propagation loss.

[0021] When the piezoelectric substrate is made of lithium niobate, it is preferable to use a piezoelectric layer having a normal direction 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, and an Euler angle of (0°, 37.8°, 0°) in order to obtain a large electromechanical coupling coefficient. Also, when the piezoelectric substrate is made of lithium niobate, it is preferable to use a piezoelectric layer having a normal direction 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, and an Euler angle of (180°, 50° to 25°, 180°) in order to obtain a high sound velocity.

[0022] The thickness of the piezoelectric layer is preferably 5 μm or less, more preferably 3 μm or less, and further 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. FIG. 3A and FIG. 3B are diagrams showing an example of the outline of the shape of the support substrate (for example, 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 (for example, when the spatial frequency is 0.045 cyc / mm or less), the shape of the piezoelectric layer 20 can be easily made to correspond to the undulation of the support substrate 10, and the thickness accuracy of the piezoelectric layer 20 can be excellent. 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 the first point and the thickness (second thickness) T2 of the piezoelectric layer at the second point is preferably 100 nm or less, more preferably 50 nm or less. By having such thickness accuracy, a high-performance surface acoustic wave element can be obtained. Specifically, effects such as the possibility of improving the Q value can be expected. In addition, a surface acoustic wave element with small 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 above-mentioned amplitude of the undulation of the support substrate, the piezoelectric layer 20 can have excellent film thickness accuracy.

[0024] A-3.Other As described above, the composite substrate may have an intermediate layer. Examples of materials constituting 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 deposited 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 constituting 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 may be formed by any suitable method. Specifically, the bonding layer may be formed 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 a first principal surface and a second principal surface facing each other, and polishing a surface of the second principal surface side of the piezoelectric substrate. Typically, polishing is performed after bonding.

[0029] 4A to 4D are diagrams illustrating 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 main surfaces of the support substrate 10 has been completed. In the example shown in FIG. 4A, the lower surface 10a of the support substrate 10 is polished flat, and the upper surface 10b has a downwardly convex curved shape. The amplitude of the waviness of the shape of the support substrate 10 exceeding 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, the upper surface 10b may have an upwardly convex curved shape.

[0031] FIG. 4B shows a state in which the support substrate 10 and the piezoelectric substrate 22 are directly bonded to each other. The piezoelectric substrate 22 has a first main surface 22a and a second main surface 22b facing each other, and after the first main surface 22a is polished, it is bonded to the support substrate 10. In the direct bonding, it is preferable that the bonding surfaces are activated by any appropriate activation process. For example, after activating the upper surface 10b of the support substrate 10 and activating the first main surface 22a of the piezoelectric substrate 22, the activated surfaces of the support substrate 10 and the piezoelectric substrate 22 are brought into contact with each other and pressed to be directly bonded. 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 so as to form a piezoelectric layer having the desired thickness. Fig. 4C shows the second main surface 22b in a state where grinding has been completed, and Fig. 4D shows the second main surface 22b in a state where polishing has been completed. 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 may 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 is used with a polishing pad using a polishing slurry (e.g., colloidal silica).

[0034] During the above bonding, it is preferable to clean the surface of each layer, for example, to remove abrasive residues, processing-degraded layers, etc. Examples of cleaning methods include wet cleaning, dry cleaning, and scrub cleaning. Among these, scrub cleaning is preferable because it is simple and efficient. A specific example of scrub cleaning is a method in which a cleaning agent (e.g., Sun Wash series manufactured by Lion Corporation) is used, followed by cleaning with a scrub cleaner using a solvent (e.g., 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 the neutralizing beam. Specifically, a saddle field 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 from a DC power source to the electrodes. As a result, a saddle field type electric field generated between the electrode (positive electrode) and the housing (negative electrode) moves electrons, and a beam of atoms and ions is generated by 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 suitably used as a SAW filter in communication devices such as mobile phones. EXAMPLES

[0038] The present invention will be described in detail 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 the propagation direction of the surface acoustic wave (SAW) set to X, and the cut angle being a rotated Y-cut plate) 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 both sides were polished 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 -6 After evacuation to the Pa range, the surfaces of both substrates were irradiated with a fast atomic beam (accelerating voltage 1 kV, Ar flow rate 27 sccm) for 80 seconds. After irradiation, the beam-irradiated surfaces of both substrates were overlapped and pressed at 1200 kgf for 2 minutes to bond the two substrates, obtaining a bonded body.

[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 by a grinder processing machine, and further mirror-polished to a thickness of 1 μm by a CMP polisher using 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 the 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 in the X-axis direction of the silicon substrate after double-side polishing was measured using an oblique incidence interference flatness tester ("FT-17" manufactured by NIDEK Corporation). In addition, the thickness in the X-axis direction of the LT layer of the obtained composite substrate was measured using a microspectrophotometric film thickness meter ("OPTM" manufactured by Otsuka Electronics Co., Ltd.). Specifically, measurements were taken at 32 points at 2.8 mm pitch in the range of -43.4 mm to +43.4 mm when the center of the wafer was taken as the origin. The results for the silicon substrate of the embodiment are shown in Figure 5A, and the results for the LT layer of the embodiment 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. When performing the calculations, the slope and DC components were removed, and the analysis tool in Microsoft Excel was used. For FFT conversion and amplitude calculations, 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 on which 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 having excellent 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 for 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 opposed to each other; a piezoelectric layer disposed on an upper surface side of the support substrate; an upper surface of the support substrate is a curved surface and a lower surface of the support substrate is a flat surface; The upper surface of the support substrate has a downwardly convex curved shape, When the piezoelectric material constituting the piezoelectric layer is lithium tantalate, the Euler angles of the piezoelectric layer are (180°, 58° to 35°, 180°), and when the piezoelectric material constituting the piezoelectric layer is lithium niobate, the Euler angles are (0°, 37.8°, 0°) or (180°, 50° to 25°, 180°); the piezoelectric layer has a lower surface located on the support substrate side and an upper surface opposite to the lower surface, the upper surface of the piezoelectric layer having a downwardly convex curved shape, In the X-axis, which is the propagation direction of the surface acoustic wave, the absolute value of the 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. Composite board.

2. The composite substrate according to claim 1 , wherein the piezoelectric layer has a thickness of 5 μm or less.

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