Composite substrate and method for manufacturing a composite substrate

Abstract

Provided is a composite substrate that contributes to the high performance of a SAW filter. The 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, wherein the amplitude of a relief having a spatial frequency of the shape of the support substrate is 10 nm or less, and the absolute value of the difference between the thickness T1 at a first point of the piezoelectric layer and the thickness T2 at a second point of the piezoelectric layer is 100 nm or less. The thickness of the piezoelectric layer can be 5 μm or less.

Description

Technical Field

[0001] This invention relates to composite substrates and methods for manufacturing composite substrates. Background Technology

[0002] In communication devices such as mobile phones, filters utilizing surface wave (SAW) are used, for example, to extract electrical signals of arbitrary frequencies. These SAW filters have a structure in which electrodes are formed on a composite substrate having a piezoelectric layer (see, for example, Patent Document 1).

[0003] In recent years, the field of information and communication equipment has seen a dramatic increase in communication volume, demanding higher performance from the aforementioned SAW filters.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-150488 Summary of the Invention

[0007] The main objective of this invention is to provide a composite substrate that can contribute to the high performance of SAW filters.

[0008] The composite substrate according to the embodiments of the present invention has: a support substrate and a piezoelectric layer disposed on one side of the support substrate, wherein the amplitude of the undulation of the shape of the support substrate having a spatial frequency exceeding 0.045 cyc / mm is less than 10 nm.

[0009] In one embodiment, the absolute value of the difference between the thickness T1 at the first location and the thickness T2 at the second location of the piezoelectric layer is less than 100 nm.

[0010] In one embodiment, the thickness of the piezoelectric layer is 5 μm or less.

[0011] Another embodiment of the present invention relates to an elastic surface wave element having the above-described composite substrate.

[0012] Another embodiment of the present invention relates to a method for manufacturing a composite substrate, comprising: bonding a support substrate to the first main surface side of a piezoelectric substrate having a first main surface and a second main surface opposite to each other; and grinding the surface of the second main surface side of the piezoelectric substrate, wherein the amplitude of the undulation of the shape of the support substrate having a spatial frequency exceeding 0.045 cyc / mm is less than 10 nm.

[0013] In one embodiment, the absolute value of the difference between the thickness T1 at the first location and the thickness T2 at the second location of the piezoelectric layer obtained by grinding the piezoelectric substrate is less than 100 nm.

[0014] In one embodiment, the thickness of the piezoelectric layer obtained by grinding the piezoelectric substrate is less than 5 μm.

[0015] Invention Effects

[0016] According to embodiments of the present invention, for example, it can contribute to the high performance of SAW filters. Attached Figure Description

[0017] Figure 1 This is a schematic cross-sectional view showing the general structure of a composite substrate according to one embodiment of the present invention.

[0018] Figure 2 This is a diagram showing an example of the appearance of a composite substrate.

[0019] Figure 3A This is an example diagram showing a general outline of the shape of the supporting substrate and the shape of the piezoelectric layer.

[0020] Figure 3B This is another example of a diagram showing the outline of the shape of the supporting substrate and the piezoelectric layer.

[0021] Figure 4A This is a diagram illustrating an example of the manufacturing process of a composite substrate according to one embodiment.

[0022] Figure 4B It was immediately afterwards Figure 4A The image.

[0023] Figure 4C It was immediately afterwards Figure 4B The image.

[0024] Figure 4D It was immediately afterwards Figure 4C The image.

[0025] Figure 5A This is a diagram showing the shape of the silicon substrate in the embodiment.

[0026] Figure 5B This is a diagram showing the film thickness distribution of the LT layer in the embodiment.

[0027] Figure 5C This is a graph showing the FFT analysis results of the shape of the silicon substrate and the film thickness distribution of the LT layer in the embodiment.

[0028] Figure 5D It is Figure 5C The diagram is shown in magnification along the vertical and horizontal axes.

[0029] Figure 6A This is a diagram showing the shape of the silicon substrate of the comparative example.

[0030] Figure 6BThis is a graph showing the film thickness distribution of the LT layer in the comparative example.

[0031] Figure 6C This is a graph showing the FFT analysis results of the shape of the silicon substrate and the film thickness distribution of the LT layer in the comparative example.

[0032] Figure 6D It is Figure 6C The diagram is shown in magnification along the vertical and horizontal axes. Detailed Implementation

[0033] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings; however, the present invention is not limited to these embodiments. Furthermore, the accompanying drawings are used to make the description clearer, and sometimes the width, thickness, shape, etc., of each part are schematically shown compared to the embodiments, but this is merely an example and does not limit the interpretation of the present invention.

[0034] A. Composite substrate

[0035] Figure 1 This is a schematic cross-sectional view showing the general structure 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 layers. The type, function, number, combination, and arrangement of these 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. In addition, 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.

[0036] The composite substrate 100 can be manufactured in any suitable shape. In one embodiment, such as... Figure 2 As shown, it can be manufactured in the form of a so-called wafer. The size of the composite substrate 100 can be appropriately set according to the purpose. For example, the diameter of the wafer is 50 mm to 150 mm.

[0037] A-1. Supporting substrate

[0038] The thickness of the support substrate 10 can be any suitable thickness. For example, the thickness of the support substrate is 100 μm to 1000 μm.

[0039] For the support substrate, the amplitude of the undulation with a spatial frequency exceeding 0.045 cyc / mm is between 0 nm and 10 nm, preferably less than 5 nm. By using such a support substrate, the thickness accuracy of the piezoelectric layer described later can be well achieved.

[0040] Any suitable substrate can be used as the support substrate. The support substrate can be made of single crystal or polycrystalline. The preferred materials for the support substrate are those made of silicon, sapphire, glass, quartz, crystal, and alumina.

[0041] The silicon mentioned above can be monocrystalline silicon, polycrystalline silicon, or high-resistivity silicon.

[0042] Typically, the sapphire described above is a single crystal with an Al2O3 composition, and the aluminum oxide described above is a polycrystalline material with an Al2O3 composition.

[0043] The coefficient of thermal expansion of the material constituting the support substrate is preferably less than that of the material constituting the piezoelectric layer described later. Based on this support substrate, changes in the shape and size of the piezoelectric layer due to temperature variations can be suppressed, for example, suppressing changes in the frequency characteristics of the resulting elastic surface wave element.

[0044] A-2. Piezoelectric layer

[0045] Any suitable piezoelectric material can be used as the material constituting the piezoelectric layer. Preferably, a single crystal with the composition LiAO3 is used. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO3 can be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a lithium niobate-lithium tantalate solid solution.

[0046] When the piezoelectric material is lithium tantalate, the piezoelectric layer is preferably one with a direction centered on the propagation direction of the elastic surface wave, i.e., the X-axis, and whose normal direction is rotated from the Y-axis to the Z-axis by 32° to 55° (e.g., 42°), i.e., expressed in Euler angles as (180°, 58° to 35°, 180°), resulting in smaller propagation loss.

[0047] When the piezoelectric material substrate is lithium niobate, a piezoelectric layer with a direction centered on the X-axis (the direction of surface wave propagation) and its normal direction rotated 37.8° from the Z-axis to the Y-axis (i.e., a piezoelectric layer with Euler angles of (0°, 37.8°, 0°)) is preferred because it has a larger electromechanical coupling coefficient. Furthermore, when the piezoelectric material substrate is lithium niobate, a piezoelectric layer with a direction centered on the X-axis (the direction of surface wave propagation) and its normal direction rotated 40° to 65° from the Y-axis to the Z-axis (i.e., a piezoelectric layer with Euler angles of (180°, 50° to 25°, 180°) is preferred because it produces high-speed sound.

[0048] 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 elastic surface wave element can be obtained. Specifically, it is expected to improve the temperature coefficient of performance (TCF) and increase the Q value.

[0049] The thickness of the piezoelectric layer is preferably uniform. Figure 3A and Figure 3B These are examples of diagrams showing an outline of the shape of the supporting substrate (e.g., in the X-axis direction) and the shape of the piezoelectric layer. Figure 3A As shown, when the undulation frequency of the support substrate 10 is low (e.g., when the spatial frequency is 0.045 cyc / mm or less), it is easy to make the shape of the piezoelectric layer 20 correspond to the undulation of the support substrate 10, and excellent film thickness accuracy of the piezoelectric layer 20 can be achieved. 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 location and the thickness (second thickness) T2 of the piezoelectric layer at the second location is preferably 100 nm or less, more preferably 50 nm or less. By having such film thickness accuracy, a high-performance surface wave element can be obtained. Specifically, effects such as improved Q value are expected. In addition, a surface wave element with small characteristic deviation can be obtained. Figure 3B As shown, when the undulation frequency of the support substrate 10 is high (for example, when the spatial frequency exceeds 0.045 cyc / mm), there is a tendency to make it 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 thickness accuracy of the piezoelectric layer 20 can be made excellent.

[0050] A-3. Other

[0051] 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.

[0052] The aforementioned intermediate layer can be formed using any suitable method. For example, it can be formed using physical vapor deposition such as sputtering and ion beam assisted evaporation (IAD), chemical vapor deposition, or atomic layer deposition (ALD).

[0053] Furthermore, 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.

[0054] The bonding layer can be formed using any suitable method. Specifically, it can be formed using the same method as the intermediate layer described above.

[0055] A-4. Manufacturing Method

[0056] One embodiment of the present invention relates to a method for manufacturing a composite substrate, comprising: bonding a support substrate to the first main surface side of a piezoelectric substrate having a first main surface and a second main surface opposite to each other; and grinding the surface of the second main surface side of the piezoelectric substrate. Typically, grinding is performed after bonding.

[0057] Figures 4A to 4D This is a diagram illustrating an example of the manufacturing process of a composite substrate according to one embodiment.

[0058] Figure 4A This indicates the state in which the grinding of the two opposing main surfaces of the support substrate 10 is completed. Figure 4A In the example shown, the lower surface 10a of the support substrate 10 is ground flat, and the upper surface 10b has a downwardly curved convex shape. The amplitude of the undulation of the shape of the support substrate 10 with a spatial frequency exceeding 0.045 cyc / mm is 0 nm to 10 nm, preferably 5 nm or less. For example, this value can be well achieved using the shape shown in the figure. Although not shown, the upper surface 10b can have an upwardly curved convex shape.

[0059] Figure 4B This indicates a direct bonding state between the support substrate 10 and the piezoelectric substrate 22. The piezoelectric substrate 22 has a first main surface 22a and a second main surface 22b facing each other. After grinding the first main surface 22a, it is bonded to the support substrate 10. During direct bonding, the bonding surfaces are preferably activated using any suitable activation treatment. For example, the upper 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, and pressure is applied, thereby achieving direct bonding. Based on this, a... Figure 4B The assembly shown is 90.

[0060] The second main surface 22b of the piezoelectric substrate 22 of the obtained bonding body 90 is subjected to grinding, polishing and other processing to make it a piezoelectric layer of the desired thickness. Figure 4C This indicates the state of the second main surface 22b after grinding is complete. Figure 4D This indicates the state where the grinding of the second main surface 22b is completed. A piezoelectric layer 20 is formed by grinding to obtain a composite substrate 100. The shape of the upper surface 20a of the piezoelectric layer 20 can correspond to, for example, the shape of the upper surface 10b of the support substrate 10.

[0061] Examples of the aforementioned polishing methods include mirror polishing using chemical mechanical polishing (CMP) and lap polishing. Chemical mechanical polishing is preferred. Specifically, chemical mechanical polishing employs an polishing slurry (e.g., colloidal silica) and a polishing pad.

[0062] During the aforementioned bonding process, it is preferable to clean the surfaces of each layer to remove abrasive residues, processed or altered layers, etc. Examples of cleaning methods include wet cleaning, dry cleaning, and brushing. Among these, brushing is preferred for its ease of use and high efficiency. A specific example of brushing is a method in which a cleaning agent (e.g., manufactured by Lion Corporation, Sun Wash series) is used, followed by cleaning with a solvent (e.g., a mixture of acetone and isopropanol (IPA)) using a brushing machine.

[0063] Typically, the activation process described above is performed by irradiating a neutral beam. Preferably, a device similar to the one described in Japanese Patent Application Publication No. 2014-086400 is used to generate a neutral beam and irradiate the beam to perform the activation process. Specifically, a saddle-field type high-speed atomic beam source is used as the beam source, and inert gases such as argon and nitrogen are introduced into the chamber. A high voltage is applied to the electrodes from a DC power supply. The saddle-field type electric field generated between the electrodes (positive electrode) and the shell (negative electrode) causes electrons to move, generating a beam of atoms and ions of inert gas. In the beam reaching the grid, the ion beam is neutralized at the grid, and thus, a beam of neutral atoms is emitted from the high-speed atomic beam source. The voltage for activation processing using beam irradiation is preferably 0.5 kV to 2.0 kV, and the current for activation processing using beam irradiation is preferably 50 mA to 200 mA.

[0064] The contact and pressurization of the aforementioned mating surfaces are preferably carried out in a vacuum atmosphere. Typically, the temperature at this time is room temperature. Specifically, it is preferably 20°C to 40°C, more preferably 25°C to 30°C. The applied pressure is preferably 100N to 20000N.

[0065] B. Elastic surface wave element

[0066] The surface acoustic wave (SAW) element according to embodiments of the present invention includes the aforementioned composite substrate. Typically, the SAW element includes the aforementioned composite substrate and electrodes (comb electrodes) disposed on the piezoelectric layer side of the aforementioned composite substrate. This SAW element is preferably used as, for example, a SAW filter in communication devices such as mobile phones.

[0067] Example

[0068] The present invention will be specifically described below through embodiments; however, the present invention is not limited to these embodiments.

[0069] [Example]

[0070] Prepare a lithium tantalate (LT) substrate with a diameter of 4 inches and a thickness of 500 μm (a cutting plate with the propagation direction of the elastic surface wave (SAW) set as X and the cutting angle as rotation Y is used to cut the LT substrate with X propagation at 42° Y).

[0071] In addition, a silicon substrate with a diameter of 4 inches and a thickness of 500 μm is prepared, and the surface (both sides) of the silicon substrate is polished. Specifically, the silicon substrate is placed on the SUS carrier of a CMP polisher, a rigid polyurethane pad is used, and colloidal silica is used as the polishing agent to polish both sides.

[0072] Next, the LT substrate and the silicon substrate are directly bonded. Specifically, after cleaning the surfaces of both the LT substrate and the silicon substrate, the two substrates are placed in a vacuum chamber and evacuated to 10 °C. -6 After reaching the Pa level, the surfaces of the two substrates were irradiated with a high-speed atomic beam (accelerating voltage 1 kV, Ar flow rate 27 sccm) for 80 seconds. After irradiation, the irradiated surfaces of the two substrates were aligned, and a pressure of 1200 kgf was applied for 2 minutes to bond the two substrates together to obtain a bonded body.

[0073] Next, the back side of the LT substrate of the above-mentioned bonding body (composite substrate) is ground using a grinding machine to reduce its thickness from 500 μm to 3 μm. Then, using a CMP polishing machine, a hard polyurethane pad, and colloidal silica as the polishing agent, it is mirror polished to a thickness of 1 μm to obtain a composite substrate with a silicon substrate and an LT layer.

[0074] [Comparative Example]

[0075] During the grinding of both sides of the silicon substrate, a non-woven fabric was used instead of a rigid polyurethane pad. Otherwise, a composite substrate was obtained in the same manner as in the example.

[0076] <Evaluation>

[0077] The following evaluation is made based on the above embodiments and comparative examples.

[0078] 1. Measurement of flatness (shape) and thickness (film thickness distribution)

[0079] The flatness of the silicon substrate after grinding on both sides was measured along the X-axis using a flat nestester method (FT-17 manufactured by NIDEK). Furthermore, the thickness of the LT layer of the resulting composite substrate was measured along the X-axis using a microspectral thickness gauge (OPTM manufactured by Otsuka Electronics). Specifically, measurements were taken at 32 locations within a range of -43.4 mm to +43.4 mm, with the wafer center as the origin, at intervals of 2.8 mm.

[0080] The results of the silicon substrate in the embodiment are shown below. Figure 5A The results of the LT layer of the embodiment are shown in Figure 5B In addition, the results for the comparative silicon substrate are shown below. Figure 6A The results of the LT layer of the comparative example are shown in Figure 6B It should be explained that... Figure 5A , Figure 5B and Figure 6A , Figure 6B The graph shown illustrates the value obtained by subtracting the average thickness.

[0081] 2. FFT (Fast Fourier Transform) Analysis

[0082] The results obtained in step 1 above are analyzed using FFT. Specifically, the shape and film thickness distribution obtained in step 1 are set as window functions, multiplied by a Hanning window, and then analyzed using FFT. During the calculation, after excluding tilt components and DC components, the analysis tools of Microsoft Excel are used. For FFT transformation and amplitude calculation, the absolute value obtained is divided by 16 (32 data points ÷ 2), and then the effect of the Hanning window is considered, making it twice.

[0083] It should be noted that when the silicon substrate is convex downwards (on the side without the LT substrate), unwanted frequency components are generated when multiplying by the Hanning window. Therefore, the data is inverted by subtracting each data from the maximum value, as if the silicon substrate were convex upwards, and the calculation is performed.

[0084] The results of the embodiments are shown in Figure 5C and Figure 5D The results of the comparative examples are shown in Figure 6C and Figure 6D .

[0085] like Figure 5B As shown, in this embodiment, a composite substrate with excellent film thickness accuracy of the LT layer is obtained.

[0086] Industrial availability

[0087] Typically, the composite substrate involved in the embodiments of the present invention can be preferably used for elastic surface wave elements.

[0088] Symbol Explanation

[0089] 10 Supporting substrate

[0090] 20 piezoelectric layers

[0091] 100 composite substrate

Claims

1. A composite substrate, wherein a support substrate having upper and lower surfaces opposing each other, and a piezoelectric layer disposed on the upper surface side of the support substrate, the upper surface of the support substrate is a curved surface, and the lower surface of the support substrate is a flat surface, an amplitude of a fluctuation of a spatial frequency of a shape of the support substrate exceeding 0.045 cyc / mm is 10 nm or less, an absolute value of a difference between a thickness Tl at a first site and a thickness T2 at a second site of the piezoelectric layer is 100 nm or less.

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

3. A method for manufacturing a composite substrate, wherein, including: bonding a support substrate on a first main surface side of a piezoelectric substrate having first and second main surfaces opposing each other, and polishing a surface on the second main surface side of the piezoelectric substrate, polishing both main surfaces of the support substrate opposing each other before the bonding, the upper surface of the support substrate on the piezoelectric substrate bonding side being a curved surface, the lower surface of the support substrate being a flat surface, an amplitude of a fluctuation of a spatial frequency of a shape of the support substrate exceeding 0.045 cyc / mm being 10 nm or less, an absolute value of a difference between a thickness Tl at a first site and a thickness T2 at a second site of the piezoelectric layer obtained by polishing the piezoelectric substrate is 100 nm or less.

4. The manufacturing method according to claim 3, wherein a thickness of the piezoelectric layer obtained by polishing the piezoelectric substrate is 5 μm or less.

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