Composite substrate and method for manufacturing the same

Abstract

The present invention provides a composite substrate that can contribute to improved performance when used in devices for harmonic applications. The composite substrate according to an embodiment of the present invention comprises, in this order, a crystalline support substrate, a modified layer in which the crystallineity of the support substrate has been modified, and an active layer. The area ratio of the modified layer to the total area in a plan view is 40% or more.

Description

[Technical Field] 【0001】 The present invention relates to a composite substrate and a method for manufacturing the same. [Background technology] 【0002】 Information and communication equipment utilizes functional elements such as surface acoustic wave elements (e.g., SAW filters) that use surface acoustic waves to extract electrical signals of arbitrary frequencies, and electro-optic elements (e.g., optical modulators) that can change the phase of light. In recent years, the volume of data transmitted in the field of information and communication equipment has increased dramatically, and there is a growing demand for higher performance functional elements. Functional elements include, for example, composite substrates having a piezoelectric layer and a support substrate. To ensure good performance of RF devices, composite substrates have been proposed in which a charge trapping layer (trap-rich layer) made of polycrystalline silicon or the like is provided as an intermediate layer between the piezoelectric layer and the support substrate (for example, Patent Document 1). 【0003】 The charge trapping layer used in the composite substrate of Patent Document 1 is provided as a trap layer for capturing free charges (charge carriers) in the support substrate and may have the function of suppressing insertion loss and / or the formation of harmonic components in high-frequency signals. However, when a piezoelectric layer is further provided on a charge trapping layer as an active layer after a charge trapping layer has been provided on a support substrate, as in Patent Document 1, the performance of the charge trap may be impaired. As a result, when such a composite substrate is applied to a device for high-frequency or harmonic applications, there is a problem that the performance of the device may be reduced. In addition, in Patent Document 1, chemical vapor deposition (CVD) is used to form the charge trapping layer on the support substrate, and since it requires the use of highly active and reactive gases such as silane (SiH4), special equipment is required to ensure safety, resulting in increased costs. [Prior art documents] [Patent Documents] 【0004】 [Patent Document 1] Patent No. 6612872 [Overview of the Initiative] [Problems that the invention aims to solve] 【0005】 In view of the above, the main object of the present invention is to provide a composite substrate that can contribute to improving the performance when used in devices for harmonic applications. [Means for solving the problem] 【0006】 [1] A composite substrate according to an embodiment of the present invention comprises, in this order, a crystalline support substrate, a modified layer in which the crystalline properties of the support substrate have been modified, and an active layer. In the composite substrate, the area ratio of the modified layer to the total area in a plan view is 40% or more. [2] In the above [1], the area ratio of the modified layer is less than 100%. [3] In the above [1] or [2], the modified layer is composed of a plurality of modified portions, each having a thickness of 10 nm or more. [4] In the above [3], each of the plurality of modified portions is formed such that the thickness decreases in the in-plane direction from the center of the modified portion toward the edge. [5] In any of the above [1] to [4], the plurality of modified parts are formed with space between them. [6] In any of the above [1] to [5], a plurality of electrodes are formed on the active layer at intervals from each other. [7] In any of the above [1] to [6], the modified layer comprises, in order from the support substrate side, a first modified layer and a second modified layer. The second modified layer contains an amorphous structure of the same type of elements as those constituting the support substrate, and the first modified layer contains the same type of elements as those constituting the support substrate. The regularity of the atomic arrangement in the first modified layer and the regularity of the atomic arrangement in the second modified layer are different from each other. [8] In any of the above [1] to [7], the first modified layer includes an amorphous structure and a crystalline structure. [9] In the above [7] or [8], the amorphous structure of the first modified layer includes a region that is located on the support substrate side of the crystalline structure in the thickness direction.

[10] In any of the above [7] to [9], the first modified layer includes a region having strain in part.

[11] In any of the above [1] to

[10] , the band gap of the active layer is 2.5 eV or greater.

[12] In any of the above [1] to

[11] , the active layer has an oxide film layer.

[13] In the above

[12] , the oxide film layer is a thermal oxide film.

[14] In any of the above [1] to

[13] , the active layer includes a functional layer.

[15] In any of the above [1] to

[14] , the active layer includes an oxide film layer and a functional layer in order from the support substrate side.

[16] In the above

[15] , the active layer further includes a bonding layer between the oxide film layer and the functional layer.

[17] In the above

[15] or

[16] , the active layer further includes an intermediate layer between the oxide film layer and the functional layer.

[18] In any of the above [1] to

[17] , the surface roughness Ra of the surface of the active layer opposite to the modified layer is 1 nm or less.

[19] According to another aspect of the present invention, a method for manufacturing a composite substrate is provided. The manufacturing method is a method for manufacturing a composite substrate according to any of [1] to

[18] above, and comprises, in this order, forming an active layer on at least one surface of a crystalline support substrate, and irradiating the surface of the support substrate with a laser from the side of the active layer to form a modified layer at the interface between the support substrate and the active layer. The irradiation with the laser includes irradiating with the laser such that the area ratio of the modified layer to the total area in a plan view is 40% or more.

[20] In the above

[19] , the method for manufacturing the composite substrate includes irradiating the support substrate with the laser at intervals in at least one direction within the plane of the support substrate.

[21] In the above

[19] or

[20] , the active layer includes an oxide film layer, and the method for manufacturing the composite substrate includes forming the oxide film layer by oxidizing the support substrate.

[22] In the above

[21] , the method for manufacturing the composite substrate includes forming the oxide film layer by thermal oxidation of the support substrate.

[23] In any of the above

[19] to

[22] , the method for manufacturing the composite substrate includes smoothing the active layer until the surface roughness Ra of the active layer is 1 nm or less.

[24] In any of the above

[21] to

[23] , the method for manufacturing the composite substrate includes bonding a functional substrate to the side of the oxide film layer opposite to the support substrate.

[25] In the above

[24] , the method for manufacturing the composite substrate includes bonding the functional substrate to the oxide film layer, and then thinning the functional substrate to a thickness of 1000 nm or less to form a functional layer.

[26] In the above

[24] or

[25] , the method for manufacturing the composite substrate includes forming an intermediate layer between the oxide film layer and the functional substrate before joining the oxide film layer and the functional substrate. [Effects of the Invention] 【0007】 According to embodiments of the present invention, a composite substrate can be realized that can contribute to improved performance when used in devices for harmonic applications. [Brief explanation of the drawing] 【0008】 [Figure 1A] This is a schematic plan view cross-sectional view showing the general configuration of a composite substrate according to one embodiment of the present invention. [Figure 1B] Figure 1A is a cross-sectional view of the composite substrate along the BB line. [Figure 1C] Figure 1A is a schematic cross-sectional view showing the general configuration of the composite substrate when it is equipped with electrodes. [Figure 1D] It is a schematic cross-sectional view showing a schematic configuration of a composite substrate according to another embodiment of the present invention. [Figure 2A] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2B] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2C] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2D] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2E] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2F] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2G] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2H] It is a schematic cross-sectional view for explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 3A] In the examples, it is an explanatory diagram of a coplanar waveguide pattern used for evaluating high-frequency characteristics. [Figure 3B] It is a view showing an enlarged view of the portion surrounded by the broken line in FIG. 3A. [Figure 4A] It is a TEM image showing a partial cross-section of a composite substrate in Example 1. [Figure 4B] It is a TEM image showing a part of a cross-section of a modified portion in a composite substrate in Example 1. [Figure 4C] It is a TEM image showing a partial cross-section of a composite substrate in Example 3. [Figure 4D] It is a TEM image showing an enlarged cross-section of a part of FIG. 4C. [Figure 4E]This TEM image shows an enlarged cross-section of a portion of Figure 4D (near the modified layer), illustrating an example of a region composed of crystalline structures within the modified layer. [Figure 4F] This is a TEM image showing a further enlarged cross-section of a portion of Figure 4E. [Figure 4G] This is a TEM image showing a cross-section of a portion of the composite substrate in Example 6. [Figure 4H] This is a TEM image showing a further enlarged cross-section of a portion of Figure 4G. [Figure 5A] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Example 1. [Figure 5B] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Example 3. [Figure 5C] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Example 4. [Figure 5D] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Comparative Example 1. [Figure 5E] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Example 7. [Figure 5F] This is an optical microscope image showing a plan view cross-section of the composite substrate (modified layer) in Example 8. [Modes for carrying out the invention] 【0009】 Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Note that the drawings are schematic for clarity, and the thickness, length, width, shape, proportions, etc., do not accurately reflect the actual shape. 【0010】 A. Composite substrate A-1. Overview of composite substrates Figure 1A is a schematic plan view cross-sectional view showing the general configuration of a composite substrate according to one embodiment of the present invention. Figure 1B is a side view cross-sectional view of the composite substrate of Figure 1A along line BB. The illustrated example composite substrate 100 has a support substrate 10, a modified layer 30, and an active layer 20 in this order. The support substrate 10 is crystalline. The support substrate 10 can typically be a single-crystal substrate or a polycrystalline substrate. Between the support substrate 10 and the active layer 20, the modified layer 30 is formed, which is obtained by modifying the crystallinity of the surface of the support substrate 10. In the illustrated example, the modified layer 30 and the active layer 20 are formed in this order only on one surface of the support substrate 10 (the top surface in the drawing), but the modified layer and the active layer may be formed in this order only on the other surface of the support substrate (the bottom surface), or the modified layer and the active layer may be formed in this order on both the top and bottom surfaces of the support substrate. 【0011】 A modified layer can typically be composed of multiple modified sections. In the illustrated example, the modified layer 30 is composed of multiple modified sections 301. A "modified layer" refers to a layer having regions in which the density, refractive index, mechanical strength, physical properties, etc., are modified relative to the support substrate. For example, a modified layer is composed of the same elements as the support substrate, but is modified to have regions in which it has different properties from the support substrate in the above-mentioned characteristics. A modified layer is typically a layer in which the crystallinity of the surface of the support substrate is modified, as described above. In the composite substrate according to this embodiment, the area ratio of the modified layer to the total area in a plan view from the thickness direction is typically 40% or more. In other words, 40% or more of the surface of the support substrate is modified in a plan view. The area ratio of the modified layer is preferably 45% or more, more preferably 50% or more, even more preferably 80% or more, and particularly preferably 95% or more. In this specification, the modified layer is the sum of the modified portions (modified parts) of the support substrate that exist on the same plane when viewed from the thickness direction. For example, even if the modified parts are scattered on the same plane, the modified layer only needs to have a ratio of 40% or more of the total area of ​​the modified parts to the total area in a plan view from the thickness direction. On the other hand, for example, in multiple modified parts, unmodified parts (i.e., parts of the support substrate that have not been modified) that exist between adjacent modified parts are not included in the modified layer. Therefore, the modified layer does not have to be formed in a strictly layered manner. The area ratio of the modified layer is typically less than 100%, preferably 99.5% or less, and more preferably 99.0% or less. If the area ratio of the modified layer is within the above range, the charge trapping performance of the composite substrate can be maintained, and this can contribute to improved performance when the composite substrate is used in devices for high-frequency and / or harmonic applications. As described later, the modified layer can be formed by irradiating the surface of the support substrate with a laser from the active layer side after the active layer has been formed on the support substrate. Normally, it is difficult to create a new layer (region) between the support substrate and the active layer after the active layer has been formed on the support substrate (e.g., formation of an oxide film layer by an oxide film and / or arrangement (bonding) of a functional layer). In contrast, in the composite substrate according to the embodiment of the present invention, after the active layer has been formed on the support substrate, a modified layer can be provided between the support substrate and the active layer at a predetermined area ratio. In this case, since the active layer is not formed on the modified layer after the modified layer has been provided on the support substrate, it is possible to suppress the impairment of the performance of the modified layer due to the formation of the active layer. Thus, the modified layer in the composite substrate according to the embodiment of the present invention can maintain its function as a charge trapping layer. According to the embodiment of the present invention, when forming the active layer and the modified layer, special equipment for ensuring safety is not required, as is the case when forming them by CVD, so an increase in costs can be suppressed.Therefore, according to embodiments of the present invention, a composite substrate can be realized that can maintain the performance of charge traps, contribute to improved performance when used in devices for high-frequency and / or harmonic applications, and can be manufactured at low cost. 【0012】 The modified layer 30 may be formed partially, as shown in Figures 1A and 1B, or it may be formed substantially over the entire support substrate, as long as the area ratio of the modified portion in plan view is typically less than 100%. In the illustrated example, the modified layer 30 is composed of a plurality of modified portions 301. The plurality of modified portions 301 may have any shape in plan view. The plurality of modified portions 301 may be formed randomly or in a regular manner in plan view, for example. Also, the thickness of the plurality of modified portions may be formed randomly or uniformly. The thickness, shape, regularity, etc. of the modified layer and the plurality of modified portions will be described in detail in Section A-3. Note that "formed substantially over the entire surface" includes the fact that it appears to be formed over the entire surface, except when the entire surface of the support substrate is the modified portion (i.e., when the area of ​​the modified portion is 100%). 【0013】 The active layer 20 includes a layer made of any suitable material. The active layer may consist of a single layer or multiple layers. Typically, the active layer may include at least one of an oxide layer, a functional layer, and an intermediate layer. Preferably, the active layer includes either an oxide layer or a functional layer, or both. The active layer may consist of only an oxide layer, only a functional layer, or an oxide layer and a functional layer in this order from the support substrate side, or an oxide layer, an intermediate layer, and a functional layer in this order from the support substrate side. For example, when the active layer has an oxide layer and a functional layer, the oxide layer and the functional layer are typically bonded together. Such a composite substrate has the advantage that the modified layer can function better as a trap-rich layer. The oxide layer, functional layer, and intermediate layer will be described in detail in section A-4. 【0014】 A composite substrate may have multiple electrodes formed on the active layer at intervals (see Figure 1C). In the illustrated example, the composite substrate 101 has multiple (three in the drawing) electrodes 50 formed on the active layer 20 at intervals. The electrodes 50 may be formed on, for example, an oxide film layer, a functional layer, or a layer constituting another active layer (e.g., an intermediate layer). The number of electrodes and the spacing between multiple electrodes can be appropriately set depending on the purpose. However, electrodes are not an essential component of a composite substrate and may be omitted if necessary. 【0015】 Figure 1D is a schematic side cross-sectional view showing the general configuration of a composite substrate according to another embodiment of the present invention. In the illustrated example, the composite substrate 102 has a modified layer 30 which includes a first modified layer 31 and a second modified layer 32. The support substrate 10 and the active layer 20 may have the same configuration as the support substrate 10 and the active layer 20 in the composite substrate 100 of Figures 1A and 1B, respectively. In this embodiment as well, the area ratio of the modified layer 30 (substantially the first modified layer 31 and the second modified layer 32) to the total area in a plan view is 40% or more. The first modified layer 31 and the second modified layer 32 can typically function as charge trapping layers. The first modified layer 31 typically contains elements of the same type as those constituting the support substrate 10. The second modified layer 32 typically contains an amorphous structure of elements of the same type as those constituting the support substrate 10. Furthermore, the first and second modified layers have different atomic arrangement regularities. The regularity of atomic arrangement is a crystallographic indicator for determining crystalline and / or amorphous structures. Note that the regularity of crystalline structures can vary depending on the combination and state of atomic arrangements, so "crystalline structure" can include multiple crystalline states. Similarly, "amorphous structure" can also include multiple amorphous states. The crystallinity and / or amorphous nature of the support substrate and modified layers (first and second modified layers) in the composite substrate can be confirmed by examining the atomic arrangement using X-ray diffraction and / or by observing the cross-section of the composite substrate using TEM (transmission electron microscope). Differences in the regularity of the amorphous structure can be confirmed by measuring Raman scattering spectroscopy using Raman spectroscopy and observing short-range order. The fact that the first and second modified layers may contain the same elements as those constituting the supporting substrate can be confirmed by elemental and compositional analysis using EDX (energy-dispersive X-ray fluorescence). As described above, the composite substrate according to this embodiment has a first modified layer and a second modified layer between the support substrate and the active layer, the crystallinity of which differs from that of the support substrate, and the regularity of the atomic arrangement in the first modified layer and the regularity of the second modified layer are different from each other. In this embodiment as well, similar to the above embodiment, the modified layers (first modified layer and second modified layer) can be formed by irradiating the surface of the support substrate with a laser from the active layer side after the active layer has been formed on the support substrate. Therefore, according to this embodiment, it is possible to realize a composite substrate that can maintain better charge trap performance, contribute to high performance when used in devices for high frequency and / or harmonic applications, and can be manufactured at low cost. 【0016】 Although not shown in the diagram, the composite substrate may further have arbitrary layers. As described above, the composite substrate may further have an intermediate layer between, for example, the oxide film layer and the functional layer as the active layer. Alternatively, for example, junction layers may be present between each layer that can constitute the active layer. The intermediate layer may be, for example, a dielectric layer. The junction layer may be, for example, a layer provided when joining the oxide film layer and the functional layer, a layer provided when joining the oxide film and the intermediate layer, and / or a layer provided when joining the functional layer and the intermediate layer. The type, function, number, combination, arrangement, etc., of such layers can be appropriately set according to the purpose. 【0017】 The composite substrate can be manufactured in any suitable shape. In one embodiment, the composite substrate can be manufactured in the form of a so-called wafer. The size of the composite substrate can be appropriately set according to the purpose. The diameter of the wafer is, for example, 100 mm to 200 mm. The total thickness of a composite substrate (without a functional layer) can be, for example, 100 μm to 1500 μm. The total thickness of a composite substrate (with a functional layer) can also be, for example, 100 μm to 1500 μm. 【0018】 The components of the composite substrate will be explained in detail below. A-2.Support board Any suitable substrate can be used as the support substrate. Typically, the support substrate is crystalline. For example, the support substrate may consist solely of a single-crystal structure, solely of a polycrystalline structure, or a combination of single-crystal and polycrystalline structures. Typically, the support substrate may be composed of a semiconductor material. Preferred materials for the support substrate include silicon or germanium. The support substrate may preferably be single-crystal silicon, polycrystalline silicon, single-crystal germanium, or polycrystalline germanium. When the support substrate is composed of a single crystal, the orientation of the support substrate may be the (111) plane. When the support substrate is single-crystal silicon or single-crystal germanium, a polycrystalline layer may be formed on the surface. When the support substrate is a silicon substrate, it is preferable from the viewpoint of achieving a good coefficient of thermal expansion and thermal conductivity. When the support substrate is a germanium substrate, it is preferable from the viewpoint of achieving a good coefficient of thermal expansion and thermal conductivity. 【0019】 The thermal expansion coefficient of the semiconductor material constituting the support substrate is preferably smaller than that of the functional substrate constituting the functional layer, if the support substrate includes a functional layer as described later. Such a support substrate can suppress changes in the shape and size of the active layer (e.g., the functional layer) when the temperature changes, and can suppress changes (losses) in the frequency characteristics of the functional element when a composite substrate is used for fabrication. For example, if the material constituting the support substrate is silicon, this relationship of thermal expansion coefficients can be satisfied. 【0020】 Any appropriate thickness can be used for the support substrate. For example, the thickness of the support substrate is 100 μm to 1000 μm (1 mm). If the thickness of the support substrate is within this range, sufficient mechanical strength can be provided to the composite substrate in which the support substrate and the modified layer are formed. In this case, for example, it may be easier to thin the functional layer. 【0021】 The surface roughness Ra of the support substrate may be, for example, 0.1 nm to 10 nm. The surface roughness Ra is preferably 5.0 nm or less, more preferably 1.0 nm or less, and even more preferably 0.5 nm or less. In this specification, "surface roughness Ra" refers to the arithmetic mean roughness (Ra). The arithmetic mean roughness (Ra) can be obtained by measuring it in a 10 μm × 10 μm field of view using an atomic force microscope (AFM) in accordance with JIS B0601:2013. 【0022】 A-3. Modified layer In the composite substrate according to embodiments of the present invention, the modified layer is typically formed between the support substrate and the active layer (e.g., an oxide film layer and / or a functional layer) such that its area in plan view is 40% or more, as described above. The modified layer can be formed on the surface of the support substrate (i.e., between the support substrate and the active layer) after the active layer has been formed on the support substrate, as described later. As described above, the modified layer can be distinguished from the support substrate and the active layer by the presence or absence of crystallinity, differences in crystallinity, etc. 【0023】 As described above, the modified layer may consist of multiple modified sections. The plan view shape of the multiple modified sections can be appropriately chosen so that the area ratio of the modified layer to the total area in plan view falls within a desired range. The plan view shape of the modified section can be set to any shape, for example, by appropriately adjusting the laser beam profile. The plan view shape of the modified section may be, for example, circular or elliptical, or a polygon such as a triangle, square, or rectangle, or a combination thereof. For example, if the modified layer consists of multiple modified sections, the plan view shapes of each of the multiple modified sections may be the same or different. 【0024】 In one embodiment, multiple modified sections are formed with spacing between them. Such a configuration can further contribute to improved performance when the composite substrate is used in devices for high-frequency and / or harmonic applications. The spacing between adjacent modified sections can be adjusted by appropriately adjusting the laser irradiation pitch and the shape of the modified sections (e.g., the diameter of the modified sections), as described later, insofar as the objectives of the present invention can be achieved. "Spacing between adjacent modified sections" means the distance between the centers of adjacent modified sections. The spacing between adjacent modified sections may be, for example, 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 50 μm or more. The spacing between adjacent modified sections may be, for example, less than 100 μm, preferably 90 μm or less, more preferably 80 μm or less, and even more preferably 75 μm or less. By appropriately controlling the shape of the modified sections and the spacing between adjacent modified sections, a desired area ratio of the modified layer can be achieved. 【0025】 Multiple modification sections may be arranged in parallel, for example, in a first direction and in a second direction intersecting the first direction. The intersection angle between the first direction and the second direction can be any suitable angle, as long as it achieves the objectives of the present invention. The intersection angle between the first direction and the second direction may be, for example, 90° or 60°. 【0026】 Preferably, each of the multiple modified portions is formed such that its thickness decreases from the center to the edge in the in-plane direction. As an example, Figure 4B shows a schematic diagram of a TEM image observing the center to the edge of a modified portion (part) in the modified layer. Specifically, in the illustrated example, the left end is the center side of the modified portion, and the right end in the figure is the edge side of the modified portion. As shown in the illustrated example, it is preferable that the thickness dc of the center of the modified portion 301 decreases from the center toward the edge of the modified portion 301. In this case, the thickness dr on the edge side of the modified portion 301 can be formed to be smaller than the thickness dc on the center side of the modified portion 301. With such a configuration, the charge trapping performance of the composite substrate can be maintained better, and this can further contribute to improving the performance when the composite substrate is used in devices for high-frequency and / or harmonic applications. 【0027】 The thickness of the modified layer is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. The thickness of the modified layer may preferably be 1000 nm or less, more preferably 750 nm or less, and even more preferably 500 nm or less. If the modified layer has a first modified layer and a second modified layer as described later, the sum of the thicknesses of the first modified layer and the second modified layer is the thickness of the modified layer. If the thickness of the modified layer is 10 nm or more, the insertion loss of the composite substrate can be improved well, and the harmonic components can be reduced well. If the thickness of the modified layer is 1000 nm or less, it can contribute to reducing the cost in manufacturing the composite substrate. The thickness of the modified part can be confirmed by an image obtained by an electron microscope (e.g., a transmission electron microscope (TEM)), and can be determined by checking the cross-section (substantially a side view cross-section) of the image. If the modified layer is composed of multiple modified parts, the thickness of the modified layer is the arithmetic mean of the thicknesses of the multiple modified parts. "Insertion loss" is one of the indicators that can be used to evaluate high-frequency characteristics. Insertion loss can be measured and evaluated using the methods and conditions described in the embodiments below. 【0028】 (First and second reforming layers) In one embodiment, the modified layer may comprise a first modified layer and a second modified layer, in that order from the support substrate side. The first modified layer and the second modified layer can be distinguished by the fact that the regularity of the atomic arrangement in the first modified layer and the regularity of the atomic arrangement in the second modified layer are different from each other, as described above. The first modified layer and the second modified layer include the modified parts (multiple modified parts) described above. 【0029】 The first modified layer 31 contains the same elements as those that make up the support substrate 10, as described above. In one embodiment, the first modified layer 31 includes an amorphous structure and a crystalline structure. The crystalline structure means a structure that has crystallinity and may include a single-crystal structure and / or a polycrystalline structure. As described above, having crystallinity means having regularity in the arrangement of atoms. With such a configuration, the modified layer can function particularly well as a trap-rich layer. 【0030】 In another embodiment, the first modified layer 31 includes a region that is partially strained. In this embodiment, the atomic arrangement of the crystalline support substrate is disrupted (altered) by laser irradiation, forming an amorphous second modified layer. The stress resulting from the disruption of the regularity of the atomic arrangement may concentrate at the interface between the second modified layer and the unmodified support substrate. It is presumed that the deformation that may occur at the interface due to this stress becomes "strain". Therefore, a region with strain may be formed in at least a part of the first modified layer. The strain in the first modified layer can be confirmed as a darkened region by examining the TEM image of the modified layer. With such a configuration, the modified layer can function particularly well as a trap-rich layer. 【0031】 As described above, the first modified layer contains elements of the same type as those constituting the support substrate. As described above, the second modified layer contains an amorphous structure of elements of the same type as those constituting the support substrate. The first modified layer may contain an amorphous structure. For example, if the support substrate is a silicon substrate, the first modified layer preferably contains amorphous silicon and polycrystalline silicon, and the second modified layer contains amorphous silicon. Also, for example, if the support substrate is a germanium substrate, the first modified layer preferably contains amorphous germanium and polycrystalline germanium, and the second modified layer contains amorphous germanium. If the first modified layer contains only an amorphous structure, the atomic arrangement of the amorphous structure will differ from the atomic arrangement of the amorphous structure of the second modified layer. In this case, the difference between the atomic arrangement of the amorphous structure of the first modified layer and the atomic arrangement of the amorphous structure of the second modified layer can be determined by the strain at the interface between the first modified layer and the second modified layer. 【0032】 The amorphous structure in the first modified layer preferably includes a region located on the support substrate side in the thickness direction compared to the crystalline structure. Specifically, it is preferable that the first modified layer has a region composed of an amorphous structure on the support substrate side (opposite side from the second modified layer) in the thickness direction, and a region composed of a crystalline structure on the second modified layer side compared to that region. 【0033】 The distribution of amorphous structures in the first and second modified layers can be any suitable distribution. This distribution may be, for example, a Gaussian distribution, a top-hat distribution, or a combination thereof. This distribution can be confirmed, for example, by a laser beam profile or by a TEM image. For example, by confirming this distribution, the thickness distribution of the amorphous structures in the first and second modified layers can be determined. 【0034】 The thickness of the first modified layer is preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 7 nm or more. thickness The upper limit is, for example, 50 nm. If the thickness of the first modified layer is within the above range, the stress and strain of the modified layer (as a whole) can be reduced, and as a result, warping of the composite substrate can be suppressed. The thickness of the second modified layer is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. thickness The upper limit is, for example, 500 nm. If the thickness of the second modified layer is 10 nm or more, the insertion loss of the composite substrate can be improved and the harmonic components can be reduced effectively. If the thickness of the second modified layer is 1000 nm or less, it can contribute to reducing the cost of manufacturing the composite substrate. 【0035】 A-4.Active layer The active layer comprises any suitable chemical and / or physically active layer. The active layer comprises one or more layers, which may be composed of any suitable material as described above. 【0036】 The active layer preferably includes a layer with a band gap of 2.5 eV or more. The band gap of the active layer can be calculated by measuring and analyzing the spectrum using any suitable measurement method. For example, optical measurement methods such as ultraviolet-visible absorption spectroscopy, transmission spectroscopy, and diffuse reflectance spectroscopy, or X-ray-based measurement methods such as X-ray absorption spectroscopy (e.g., XAFS spectroscopy) and X-ray photoelectron measurement (e.g., XPS, ESCA) can be employed. The band gap of the active layer may more preferably be 3.0 eV or more, even more preferably 3.5 eV or more, and particularly preferably 4.0 eV or more. The upper limit of the band gap of the active layer may be, for example, 20 eV. The band gap of the active layer can be adjusted by appropriately selecting the material that can be used for the active layer. Specific materials that constitute a layer with a band gap of 2.5 eV or greater include, for example, silicon dioxide (approx. 9.0 eV), silicon carbide (approx. 2.9 eV), aluminum nitride (approx. 6.3 eV), gallium nitride (approx. 3.4 eV), gallium oxide (approx. 4.5-4.9 eV), gallium sulfide (approx. 2.5 eV), beryllium oxide (10.6 eV), magnesium oxide (approx. 7.8 eV), zinc oxide (approx. 3.4 eV), and zinc sulfide (approx. 3.6 eV). The values ​​in parentheses for the above materials represent their band gaps. 【0037】 The active layer may preferably contain an oxide. The oxide can be any suitable oxide, as long as it does not hinder the objectives of the present invention. Typical oxides may include oxides that can constitute the oxide film layer, oxides that can constitute the functional layer, and oxides that can constitute the intermediate layer, as described later. Examples of oxides include silicon dioxide, germanium dioxide, lithium niobate, lithium tantalate, and other oxides that can be included in the piezoelectric materials listed later (see Section A-4-2). 【0038】 As described above, the active layer may consist of a single layer or multiple layers. Typically, the active layer may include at least one of an oxide layer, a functional layer, and an intermediate layer. Representative examples of active layers are described in detail below. 【0039】 A-4-1. Oxide film layer The oxide layer may be a layer composed of any suitable oxide. For example, the oxide layer may be composed of an oxide of the semiconductor material constituting the support substrate. For example, if the semiconductor material is silicon or germanium, the oxide layer may contain silicon oxide or germanium oxide. 【0040】 The oxide film layer can be formed by any suitable method. Examples of methods for forming the oxide film layer include oxidation, sputtering, atomic layer deposition (ALD), physical vapor deposition (PVD) such as ion beam-assisted deposition (IAD), and chemical vapor deposition (CVD). Preferably, the oxide film layer can be formed by oxidizing the support substrate. Any suitable method can be used for oxidation. The oxidation method is preferably thermal oxidation. Examples of thermal oxidation include wet oxidation (pyrogenic oxidation) and dry oxidation. Thermal oxidation will be explained in detail in section B-2. In an oxide film layer formed by thermal oxidation of the support substrate, defects in the oxide film can be significantly suppressed. As a result, when a composite substrate having an oxide film layer obtained by thermal oxidation is applied to a SAW filter or a device using a SAW filter, it can contribute to improving the yield. 【0041】 Any appropriate thickness can be used for the oxide film layer. For example, the thickness of the oxide film layer may be between 0.05 μm (50 nm) and 30 μm. Preferably, the thickness of the oxide film layer may be between 0.1 nm and 25 μm, and more preferably between 1 nm and 20 μm. The thickness of the oxide film layer can be adjusted, for example, by the conditions when oxidizing the support substrate (heating temperature during oxidation, etc.), the type of gas constituting the oxidizing atmosphere, and the smoothing treatment by polishing after oxidizing the support substrate. 【0042】 The surface roughness Ra of the oxide film layer on the side opposite the modified layer is preferably 1 nm or less, more preferably 0.5 nm or less, and even more preferably 0.2 nm or less. The lower limit of Ra may be, for example, 0.1 nm. Having Ra within this range can increase the bonding strength when another layer (e.g., a functional layer, an intermediate layer) is laminated on the oxide film layer in a composite substrate. 【0043】 A-4-2. Functional Layer The functional layer is a layer that can constitute the active layer and may be provided as needed. The functional layer may be composed of any material having appropriate functionality. Examples of functional materials include piezoelectric materials, materials with electro-optical effects, and semiconductor materials. 【0044】 In one embodiment, the functional layer may be a piezoelectric layer. By providing a piezoelectric layer in the active layer of the composite substrate, a functional element capable of achieving excellent high-frequency characteristics can be obtained. As a result, the composite substrate according to the embodiment of the present invention can be particularly suitable for use in surface acoustic wave elements such as SAW filters. Any piezoelectric material can be used as the material constituting the piezoelectric layer. 【0045】 Preferably, a single crystal having the composition LiAO3 can be used as the piezoelectric material. Here, A is one or more elements selected from niobium and tantalum. Specifically, LiAO3 may be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a lithium niobate-lithium tantalate solid solution. When lithium niobate and / or lithium tantalate are used, crystals doped with MgO or with a stoichiometric composition may be used to suppress photodamage. 【0046】 Other examples of piezoelectric materials include potassium titanate phosphate (KTiOPO4:KTP) and potassium lithium niobate (K x Li (1-x) NbO2 (0≦x≦1:KLN), potassium niobate (KNbO3:KN), potassium tantalate-niobate (KNb x Ta (1-x) Examples include O3 (0≦x≦1:KTN), silicon, quartz, silica, silicon carbide, gallium nitride, indium phosphide, and lead zirconate titanate (PZT). 【0047】 When the piezoelectric material is lithium tantalate, the functional layer is preferably such that, for example, when the X-axis (crystal axis) of the piezoelectric material is taken as the propagation direction of the surface acoustic wave (X1), the direction rotated 32° to 55° (e.g., 42°) from the Y-axis toward the Z-axis corresponds to the direction perpendicular to the main surface of the functional layer (X3), specifically, (180°, 58° to 35°, 180°) in Euler angle notation. 【0048】 When the piezoelectric material is lithium niobate, it is preferable that the functional layer is such that, for example, the direction obtained by rotating the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X1) from the Z-axis to the -Y-axis by 0° to 40° (e.g., 37.8°) corresponds to the direction perpendicular to the main surface of the functional layer (X3), specifically in Euler angle notation (0°, 0° to 40°, 0°). When the piezoelectric material is lithium niobate, it is also preferable that the piezoelectric layer is such that, for example, the direction obtained by rotating the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X1) from the Y-axis to the Z-axis by 40° to 65° corresponds to the direction perpendicular to the main surface of the functional layer (X3), specifically in Euler angle notation (180°, 50° to 25°, 180°). 【0049】 In another embodiment, the functional layer may be an electro-optic layer having an electro-optic effect. By providing an electro-optic layer in the active layer of the composite substrate, a functional element capable of achieving excellent harmonic characteristics can be obtained. As a result, the composite substrate according to the embodiment of the present invention can be particularly suitable for use in electrical engineering elements (such as optical waveguide devices) such as optical modulators. Any material having arbitrary electro-optic properties can be used as the material constituting the electrical functional layer. 【0050】 When composite substrates are used in electro-optic devices (e.g., thin-film LN optical modulators), materials exhibiting electro-optic effects may preferably include lithium niobate, lithium tantalate, lithium niobate-lithium tantalate, KTP (potassium titanate phosphate), and PZT (lead zirconate titanate). Specifically, X-cut and / or Z-cut lithium niobate may be used as materials exhibiting electro-optic effects. When using lithium niobate and / or lithium tantalate, MgO-doped or stoichiometric crystals may be used to suppress photodamage. 【0051】 The functional layer may be composed of any appropriate functional material depending on the functions and performance required of the composite substrate. As mentioned above, semiconductor materials can be used as functional materials. Examples of semiconductor materials include materials similar to the semiconductor materials (silicon, germanium) described in A-2 above, and silicon carbide (SiC). 【0052】 The thickness of the functional layer can be set to any appropriate thickness depending on the usage and application of the composite substrate. For example, the thickness of the functional layer is 0.05 μm or more and 30 μm or less, preferably 0.10 μm or more and 20 μm or less. 【0053】 The surface roughness Ra of the functional layer surface (bonding surface side) may be, for example, 1.0 nm or less, 0.8 nm or less, 0.6 nm or less, or 0.4 nm or less. On the other hand, the surface roughness Ra may be 0.1 nm or more. With such a surface roughness Ra, for example, when the functional layer is a piezoelectric layer, the composite substrate can be applied particularly well to devices (functional elements) for high-frequency or harmonic applications. 【0054】 A-4-3. Middle Class The intermediate layer is an optional layer provided as needed. For example, a composite substrate may further include an intermediate layer between the oxide film layer and the functional layer. The intermediate layer may be composed of a material having any suitable function depending on the purpose. The intermediate layer may be, for example, a dielectric layer. By providing a dielectric layer as an intermediate layer, for example, the stability of the temperature characteristics of the composite substrate may be improved. 【0055】 When the intermediate layer is a dielectric layer, the dielectric layer can be composed of any suitable dielectric material. Examples of dielectric materials include silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, and aluminum oxynitride. The dielectric layer may be a single layer or may have a laminated structure consisting of multiple layers composed of different dielectric materials. 【0056】 The thickness of the dielectric layer may be, for example, 100 nm to 1000 nm, or for example, 200 nm to 800 nm, or for example, 300 nm to 700 nm, or for example, 400 nm to 600 nm. 【0057】 The dielectric layer can be formed by any suitable method. Specific examples of methods for forming the dielectric layer include sputtering and ion-assisted deposition. 【0058】 A-5. Electrode In one embodiment, the composite substrate may comprise a plurality of electrodes. The plurality of electrodes may be formed at intervals on the active layer of the composite substrate. Specifically, the electrodes may be provided on, for example, an oxide layer, a functional layer, or an intermediate layer. For example, if the active layer includes an oxide layer and a functional layer in that order from the support substrate side, the electrodes may be provided on the functional layer. The electrodes may be, for example, coplanar waveguides (CPWs). The electrodes may be formed from any suitable conductive material. Since configurations that are well known and common in the industry may be used for the electrodes, a detailed explanation is omitted. 【0059】 B. Manufacturing method of composite substrates B-1. Overview of the manufacturing method for composite substrates A method for manufacturing a composite substrate according to an embodiment of the present invention includes, in this order: forming an active layer on at least one surface of a crystalline support substrate; and forming a modified layer at the interface between the support substrate and the active layer by irradiating the surface of the support substrate with a laser from the active layer side. In the manufacturing method according to an embodiment of the present invention, a modified layer can be formed at the interface between the support substrate and the active layer by irradiating the surface of the support substrate with a laser, thereby altering the crystallinity of the surface of the support substrate. The manufacturing method of a composite substrate according to an embodiment of the present invention includes irradiating with a laser such that the area ratio of the modified layer to the total area in a plan view is 40% or more. According to the manufacturing method of a composite substrate according to an embodiment of the present invention, a modified layer capable of functioning as a charge trapping layer can be formed by laser irradiation. As a result, since special equipment for ensuring safety is not required, as in the case of formation by the CVD method, an increase in cost can be suppressed. Therefore, according to an embodiment of the present invention, a composite substrate can be manufactured at low cost. 【0060】 In one embodiment, a modified layer having a first modified layer and a second modified layer can be formed by irradiating the surface of the support substrate with a laser to modify its crystallinity. The first and second modified layers are as described in Section A-3 above. In one embodiment, the reason why the first and second modified layers are formed on the surface of the support substrate by laser irradiation is not entirely clear, but the following reasons may be considered. By irradiating the surface of the support substrate with a laser through an active layer formed on the support substrate, the atoms constituting the support substrate vibrate, and the bonds between atoms can be broken from the surface side of the support substrate. As a result, the regularity of the atomic arrangement on the surface side of the support substrate is disrupted, and the second modified layer can be formed. On the other hand, as the laser intensity decreases as it propagates further inward (inside) from the surface of the support substrate, it can be inferred that a first modified layer may be formed with a different regularity of atomic arrangement from the second modified layer, consisting of areas where bonds are partially broken and / or areas where crystallinity can be maintained at a certain depth (the part away from the surface of the support substrate). However, this is merely speculation and does not limit the present invention, nor does it restrict the present invention by this mechanism. 【0061】 The following describes typical examples of manufacturing methods for composite substrates with reference to Figures 2A to 2H. Figure 2D is identical to Figure 1B. Sections B-2 to B-6 describe the manufacturing methods for cases where the active layer of the composite substrate comprises only an oxide film layer, or where it comprises both an oxide film layer and a functional layer. However, the manufacturing methods according to embodiments of the present invention are not limited to these. Any appropriate configuration, process, conditions, etc., can be adopted as long as they do not hinder the effects of the present invention. 【0062】 B-2. Formation of the oxide film layer First, as shown in Figure 2A, a suitable substrate is prepared as the support substrate 10. The substrate used as the support substrate 10 and the materials that make it up are as described in Section A-2 above. 【0063】 The support substrate 10 may, if necessary, be subjected to any suitable smoothing treatment on one side (10a or 10b) or both sides (10a and 10b). The smoothing treatment may be performed, for example, by polishing the surface of the material. Any suitable polishing method may be used. Examples of polishing methods include lapping and chemical mechanical polishing (CMP). Preferably, the support substrate may be smoothed until the surface roughness Ra of the support substrate is 10 nm or less, as described above, before forming the oxide film layer described later. The support substrate 10 may also be used as is without undergoing any treatment such as smoothing. 【0064】 Next, as shown in Figure 2B, an oxide film layer 21 that can constitute the active layer 20 is formed on the support substrate 10. The oxide film layer can be formed by any suitable method. For example, oxidation, sputtering, vapor deposition, and ion plating can be used to form the oxide film layer. Preferably, the oxide film layer is formed by thermal oxidation of the support substrate as described above. When thermal oxidation is used, a remarkable effect can be obtained in which defects in the oxide film (oxide film layer) of the formed support substrate can be greatly suppressed. As a result, it can contribute to improving the yield when manufacturing a composite substrate having an oxide film layer in the active layer. When forming an oxide film by deposition, if heated to a temperature above the deposition temperature, outgassing of hydrogen, water, etc. may occur from within the formed film. For example, in the sputtering method, the temperature during deposition can be around 200°C. In this case, outgassing may adversely affect the reliability of the device to which it is applied. In contrast, as in the manufacturing method according to one embodiment of the present invention, the thermal oxide film can usually be formed by oxidation at a temperature of 700°C or higher. As a result, the thermal oxide film is less likely to contain components that could cause outgassing, and therefore has the advantage of excellent thermal stability of the film quality in composite substrates. 【0065】 The method and conditions for thermal oxidation can be any suitable method and conditions. Typically, thermal oxidation can be carried out under heating conditions of 700°C to 1200°C in an oxidizing atmosphere. Specifically, this is done as follows: A support substrate is placed in a chamber, the inside of the chamber is heated to 700°C to 1200°C, and then an oxidizing atmosphere is created by supplying any suitable gas into the chamber, thereby oxidizing the support substrate. The oxidizing atmosphere can be created by supplying, for example, oxygen, hydrogen, water vapor, hydrochloric acid (hydrogen chloride), or a mixture of two or more of these gases. With such thermal oxidation, oxidation can proceed from the surface of the support substrate, and an oxide film layer can be formed. 【0066】 Examples of thermal oxidation methods include wet oxidation, pyrogenic oxidation, steam oxidation, dry oxidation, and hydrochloric acid oxidation. Among these, thermal oxidation is preferably carried out by wet oxidation or pyrogenic oxidation. In wet oxidation, for example, the oxidation of the target material can proceed by supplying oxygen and water vapor. In pyrogenic oxidation, for example, a mixed gas of hydrogen and oxygen can be supplied, and the oxidation of the target material can proceed by the water vapor produced by the combustion of the mixed gas. The thickness of the oxide film layer can be adjusted by setting the processing temperature, processing time, and other conditions to any appropriate conditions so that it reaches the desired thickness. The thickness of the oxide film layer after thermal oxidation can be, for example, 0.05 μm to 30 μm, as described above. In this way, an oxide film layer 21 can be formed by oxidizing at least one surface of the support substrate 10. 【0067】 The oxide film layer may be subjected to a smoothing treatment as needed. If smoothing is performed, for example, the oxide film layer may be polished until its surface roughness Ra is 1 nm or less, as described above. The smoothing method may be the same as the method used to smooth the support substrate. If the oxide film layer is formed on both sides of the support substrate, the surfaces of both oxide film layers may be smoothed, or only the surface of one oxide film layer may be smoothed. Note that smoothing the oxide film layer is optional, and it may be used as is in the next step without any smoothing treatment. 【0068】 B-3. ​​Formation of the modified layer Next, as shown in Figure 2C, a laser is irradiated onto the surface of the support substrate 10 from the oxide film layer 21 side. The oxide film layer typically has laser light transmittance (laser light transmission). Therefore, when a laser is irradiated onto a laminate comprising the oxide film layer and the support substrate from the oxide film layer side, the laser light can pass through the oxide film layer and reach the surface of the support substrate. By irradiating with a laser, the surface of the support substrate 10 is modified, and a modified layer 30 (including the modified portion 301) can be formed (see Figure 2D). Therefore, in the method for manufacturing a composite substrate according to the embodiment of the present invention, a modified layer that can function as a charge trapping layer can be formed by laser irradiation. As a result, special equipment for ensuring safety is not required as in the case of formation by the CVD method, cost increases can be suppressed, and composite substrates can be manufactured at low cost. By irradiating with a laser, preferably, a first modified layer and a second modified layer can be formed in order from the support substrate. 【0069】 Laser irradiation can be performed by any suitable method. The laser may be irradiated, for example, without focusing on the surface of the support substrate. Specifically, for example, the support substrate may be disposed in a state where it is warped by a predetermined distance (for example, several hundred μm) with respect to the laser light. Even in such a case, as described later, the laser light can be absorbed by the surface of the support substrate. Therefore, in forming the modified layer, by shifting the focal position of the support substrate by an appropriate distance (for example, several mm), the labor of focusing on the support substrate each time can be omitted. Furthermore, by shifting the focus of the laser in this way (that is, not condensing), ablation (such as thermal decomposition by laser irradiation) of the object (for example, the support substrate, the active layer) can be suppressed. The laser irradiation method and irradiation conditions can adopt any suitable method and conditions as long as the surface of the support substrate can be modified. Typically, a pulsed laser can be used as the laser. For example, considering the influence of ablation on the support substrate and the active layer, a femtosecond, picosecond or nanosecond pulsed laser can be used. Preferably, a femtosecond or picosecond pulsed laser can be used. When using a pulsed laser, the pulse width can be, for example, 1 fs or more and 100 ps or less. The frequency of the laser can be, for example, 1 kHz or more and 1 MHz or less. 【0070】 As the wavelength of the laser, any suitable wavelength can be adopted according to the band gap of the active layer (substantially the oxide film layer) and the band gap of the support substrate. Specifically, when the wavelength of the laser is λ [nm], the band gap of the oxide film layer is E g1 [eV], and the band gap of the support substrate is E g2 [eV], it is preferable that the wavelength satisfies the relationship of E g1 ≧1240 / λ and E g2 -0.1≦1240 / λ. More specifically, for example, when the semiconductor material constituting the support substrate is silicon and the oxide film layer is silicon oxide (silicon dioxide), E g1 ≒9.0 eV, E g2The voltage is approximately 1.1 to 1.2 eV, and a laser with λ = 1030 nm can be used. In this case, the laser light is not absorbed by the oxide film layer, but can be absorbed by the support substrate. For example, a wavelength of 1030 nm can be suitably used as the laser wavelength. 【0071】 The laser energy density can be adjusted according to the area of ​​the object being irradiated (essentially the support substrate), for example, 10 mJ / cm². 2 More than 10000mJ / cm 2 The following are possible: 【0072】 When irradiating with a laser, it is preferable to irradiate with a gap in at least one direction within the plane of the support substrate. The laser irradiation pitch (gap) when irradiating with a laser can be any appropriate gap depending on the area of ​​the target modified layer in plan view. The laser irradiation pitch may be set considering, for example, the effect of ablation. Specifically, the laser irradiation pitch may be set according to the minimum value of the gap between the modified parts (modified areas) on the surface of the support substrate in plan view, and the minimum value of the gap between the modified areas is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 10 μm or less. The lower limit of the minimum value of the gap in this area may be, for example, 0.1 μm. Within such a range, the effects of the present invention may become more pronounced. However, the type of laser, wavelength, pulse width, frequency, etc., are not limited to those listed above. 【0073】 The thickness of the portion of the support substrate altered by laser irradiation corresponds to the thickness of the modified layer (overall). The thickness of the modified layer is as explained in section A-3. 【0074】 After forming the modified layer by laser irradiation, the surface of the oxide film layer may be smoothed as needed. When the surface of the oxide film layer is smoothed, the surface of the oxide film layer may be polished until the surface roughness Ra is preferably 1 nm or less, as described above. The method of smoothing may be the same as that used to smooth the surface of the support substrate. Even if the oxide film layer has been smoothed before laser irradiation, it may be smoothed again. As described above, a composite substrate 100 can be obtained, comprising a support substrate 10, a modified layer 30, and an active layer 20 (oxide film layer 21) in this order, as shown in Figure 2D (Figure 1A). 【0075】 B-4. Formation of the functional layer The composite substrate may have a support substrate, a modified layer, and an oxide film layer that can constitute an active layer, as well as a functional layer on the oxide film layer, which may constitute another active layer. The functional layer can be fabricated, for example, by bonding a functional substrate to the oxide film layer and, if necessary, thinning the functional substrate to any appropriate thickness. 【0076】 The functional substrate can be composed of any suitable material depending on the desired function of the functional layer. For example, if the functional layer is a piezoelectric layer, the functional substrate can be composed of a piezoelectric material. Details of piezoelectric materials are shown in A- 4-2 As explained in the previous section. 【0077】 Specifically, to provide a functional layer, a functional substrate 41 is prepared as shown in Figure 2E. The surface (bonding surface) of the functional substrate 41 may be smoothed as needed. When the functional substrate is smoothed, it may be polished until the surface roughness Ra of the bonding surface of the functional substrate becomes, for example, 1 nm or less, as described above. The thickness of the functional substrate may be, for example, 100 μm to 1000 μm (1 mm), or for example, 200 μm to 500 μm. 【0078】 Next, as shown in Figure 2F, the oxide film layer 21 of the composite substrate 100 (composite 100'), which has the support substrate 10, the modified layer 30, and the active layer 20 (oxide film layer 21) in that order, is joined to the functional substrate 41. By joining in this way, a composite 103' having the support substrate 10, the modified layer 30, the oxide film layer 21, and the functional substrate 41 in that order can be obtained, as shown in Figure 2G. 【0079】 Any suitable method can be used to bond the oxide film layer and the functional substrate. Examples of bonding methods include bonding with adhesives, surface activation bonding, plasma activation bonding, and atomic diffusion bonding. Preferably, the bonding method is so-called direct bonding, which does not involve an adhesive. Direct bonding allows for the thinning of the composite substrate and prevents adverse effects from adhesives. 【0080】 Direct bonding by plasma-activated bonding can be achieved by activating the bonding surfaces of the oxide film layer and the functional substrate by plasma irradiation, then bringing these bonding surfaces into contact, and, if necessary, performing heat treatment. Examples of gases included in the atmosphere during the activation treatment include oxygen, nitrogen, hydrogen, and argon. These may be used individually or in combination of two or more (as a mixed gas). Nitrogen is preferably used. The atmospheric pressure during the plasma irradiation activation treatment is preferably 10 Pa to 80 Pa, more preferably 30 Pa to 80 Pa. The energy during plasma irradiation is preferably 30 W to 150 W, more preferably 60 W to 120 W. The plasma irradiation time is preferably 5 seconds to 30 seconds. 【0081】 The functional layer can be formed, for example, by polishing and thinning a functional substrate. Preferably, the functional layer can be formed by bonding a functional substrate to an oxide film layer and then thinning the functional substrate until its thickness is 1000 nm or less. The thinning process may be omitted as appropriate depending on the type of functional substrate, laser irradiation conditions, etc. Based on the above, a composite substrate 103, as shown in Figure 2H, can be fabricated, which has a functional layer 22 on an oxide film layer 21 as an active layer 20. When fabricating a composite substrate with a functional layer in this manner, there is an advantage in that, for example, even if the functional substrate is absorbent of laser light, a modified layer can be formed between the oxide film layer and the support substrate before the functional layer is added. 【0082】 B-5. Intermediate layer joining The middle layer is the above A- 4-3 As described in the section, it is an arbitrary layer provided as one layer that can constitute the active layer in a composite substrate as needed. The intermediate layer (if provided) can be formed, for example, before bonding the functional substrate and the oxide film layer. The intermediate layer can be formed, for example, by depositing any suitable material on the surface of the oxide film layer and / or the surface of the functional substrate. Any suitable method can be used to deposit the intermediate layer. Examples of deposition methods include sputtering, CVD, and ion-assisted deposition. For example, the intermediate layer may be A- 4-3 The dielectric material described in the section can be fabricated by forming it on the target object (oxide film layer and / or functional substrate) by sputtering. The surface (bonding surface) of the intermediate layer may be smoothed as needed. When the intermediate layer is smoothed, it may be polished until the surface roughness Ra of the bonding surface of the intermediate layer is, for example, 1 nm or less. An intermediate layer (e.g., an SiO2 layer) may be formed on a functional substrate, and the bonded body may be formed by bonding the intermediate layer and the oxide film layer. The bonding method may be the same as the bonding method for the oxide film layer and the functional substrate described in Section B-4 above. 【0083】 B-6. Others The support substrate (including semiconductor material) and the functional substrate (including functional material) may be cleaned using any suitable solvent before processing. Examples of cleaning methods include wet cleaning, dry cleaning, and scrubbing. Among these, scrubbing is preferred because it is simple and efficient. A specific example of scrubbing is a method in which a cleaning agent (e.g., Lion Corporation's Sunwash series) is used, followed by cleaning with a solvent (e.g., a mixed solution of acetone and isopropyl alcohol (IPA)) using a scrubbing machine. The cleaning process can remove contaminants (e.g., fine particles, metal impurities, organic matter, etc.) adhering to the surface. Furthermore, when performing the above-mentioned film formation, bonding, etc., it is preferable to clean the surface of each layer to remove, for example, abrasive residue, unwanted layers generated by processing, etc. 【0084】 B-7. Variations Sections B-2 to B-6 above describe, as a specific example of a method for manufacturing a composite substrate according to an embodiment of the present invention, in which a functional layer is fabricated by irradiating the surface of a support substrate with a laser to form a modified layer, and then providing a functional substrate on an oxide film layer and thinning it. However, a composite substrate with a functional layer may also be fabricated by providing a functional substrate on an oxide film layer and then forming a modified layer. For example, a functional layer may be formed by thinning the functional substrate, and then a modified layer may be formed by irradiating the functional layer with a laser from the functional layer side. Alternatively, a modified layer may be formed by irradiating the functional substrate with a laser from the functional substrate side, and then a functional layer may be formed by thinning the functional substrate with a laser. When the functional substrate is thinned before laser irradiation, the transmittance of the laser light due to laser irradiation can be improved. As a result, even when laser light is irradiated from the functional substrate side onto the surface of the support substrate, a modified layer can be formed well. Note that the thinning treatment of the functional substrate may be performed before laser irradiation, after laser irradiation, or both before and after laser irradiation. 【0085】 In the above modified example, when irradiating with a laser after bonding the oxide film layer and the functional substrate, it is preferable to smooth the functional substrate so that its surface roughness Sa is 20 nm or less before irradiating with the laser. That is, it is preferable to smooth the functional substrate so that its surface roughness Sa is 20 nm or less before irradiating with the laser. With such a configuration, scattering of laser light can be suppressed. As a result, the modified layer can be formed more efficiently. When irradiating with a laser after bonding the oxide film layer and the functional substrate, a functional material having MgO doping can preferably be used as the functional substrate. If such a material is used, photodamage to the functional substrate can be suppressed, and changes in the optical constants of the functional substrate material can be suppressed by laser irradiation. 【0086】 Alternatively, for example, a composite substrate may be manufactured that does not have an oxide film layer as an active layer, but has a functional layer. For example, in the manufacturing method described in sections B-2 to B-6 above, a composite substrate can be obtained by omitting the formation of the oxide film layer in section B-2 and using a functional material (functional layer) instead of the oxide film layer in sections B-3 to B-6. Specifically, a functional material is bonded to the support substrate in section B-2, then the functional material is smoothed as needed, a modified layer (first modified layer and second modified layer) is formed by irradiating the surface of the support substrate with a laser from the functional material side, and the functional material is polished to make a thin film as needed, thereby making the functional material a functional layer. In this way, a composite substrate comprising a support substrate, a modified layer (first modified layer and second modified layer), and a functional layer can be obtained. 【0087】 In the method for manufacturing a composite substrate according to embodiments of the present invention, heat treatment may be performed at any appropriate point in time. For example, the heat treatment may be performed after bonding the oxide film layer and the functional substrate. Alternatively, for example, the heat treatment may be performed before, during, or after the thinning treatment of the functional substrate after bonding. Alternatively, for example, the thinning treatment and heat treatment of the functional substrate may be repeated alternately multiple times. Any appropriate heating conditions can be used for the heat treatment. The heating temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 500°C or lower. The lower limit of the heating temperature may be, for example, 100°C. The heating time may be, for example, in the range of 5 minutes to 5 hours. By performing such heat treatment, high bonding strength of the composite substrate can be achieved while maintaining the amorphous structure of the modified layer. 【0088】 In the illustrated example, the active layer 20 is formed only on the upper surface of the support substrate 10, but as described above, the active layer may also be formed on the lower surface of the support substrate, or on both sides of the support substrate. When oxidizing the support substrate, for example, it can be formed on both sides of the support substrate. When oxide film layers are formed on both sides of the support substrate, a modified layer can be formed on the surface of the support substrate by irradiating it with a laser from at least one of the oxide film layer sides. 【0089】 C. Functional elements As described above, the composite substrate according to the embodiment of the present invention can maintain charge trap performance and improve high-frequency and / or harmonic characteristics, and therefore can be suitably used as a functional element for high-frequency and / or harmonic applications. For example, when a piezoelectric layer is provided as a functional layer in the active layer of a composite substrate, the composite substrate can be used as a surface acoustic wave (SWA) element. A typical SWA element comprises the composite substrate and electrodes (comb-type electrodes) provided on the piezoelectric layer side of the composite substrate. Such an SWA element is suitably used, for example, as a SAW filter in communication devices such as mobile phones. Furthermore, for example, when an electro-optic layer is provided as a functional layer in the active layer of a composite substrate (typically, when a functional material having an electro-optic effect such as lithium niobate (LN) or lithium tantalate is used), the composite substrate can be used as an electro-optic element. An electro-optic element can typically be an optical modulation device. An optical modulation device is, for example, a Mach-Zehnder type optical modulator, which modulates light propagating in an optical waveguide by applying a voltage to a Mach-Zehnder interferometer formed with an optical waveguide having an electro-optic effect. Such an electro-optic element is suitably used, for example, as an optical modulator in optical communication systems. [Examples] 【0090】 The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods for each characteristic in the examples are as follows. Unless otherwise specified, "%" in the examples is based on weight. 【0091】 (1) Surface roughness Ra (arithmetic mean roughness (Ra)) Surface roughness Ra was measured using an atomic force microscope (AFM) in accordance with JIS B0601:2013, within a 10 μm × 10 μm field of view. 【0092】 (2) Evaluation: High frequency characteristics For the composite substrates of Examples 1-5 and Comparative Examples 1-2, coplanar waveguides (CPWs) were formed on the surface of the oxide film layer to prepare samples for evaluating high-frequency characteristics. The CPWs were formed using the following procedure. A coplanar waveguide (CPW) was formed as an electrode by creating a 1.0 μm thick Au film on the oxide film layer of the obtained composite substrate using a lift-off process. Details of the coplanar waveguide are shown in Figures 3A and 3B, where L1 is 10.00 mm, L2 is 10.08 mm, L3 is 11.60 mm, W1 is 25 μm, W2 is 800 μm, and G1 is 41 μm. High-frequency probes (ACP40-GSG-150) were placed in contact with both ends of the coplanar waveguide in the longitudinal direction, and the S21 parameter of the S-parameter was measured using a Keysight Technologies network analyzer (Agilent E5071C). S21 was measured at five points within the plane of each substrate. The measurement frequency ranged from 0.01 GHz to 20 GHz, and the insertion loss was measured at 0.01 GHz intervals within this range. Based on the results obtained from the measurements, the absolute value of S21 was used as the insertion loss and evaluated according to the following criteria. A (Good): Insertion loss is 0.45 dB / mm or less at 20 GHz. B (Defective): Insertion loss greater than 0.45 dB / mm at 20 GHz. 【0093】 For the composite substrates of Examples 6-7 and Comparative Example 3, coplanar waveguides (CPWs) were formed on the surface of the functional layer to prepare samples for evaluating high-frequency characteristics. The CPWs were formed using the following procedure. A coplanar waveguide (CPW) was formed as an electrode by creating a 1.0 μm thick Au film on the functional layer of the obtained composite substrate using a lift-off process. Details of the coplanar waveguide are shown in Figures 3A and 3B, where L1 is 10.00 mm, L2 is 10.08 mm, L3 is 11.60 mm, W1 is 21 μm, W2 is 800 μm, and G1 is 39 μm. High-frequency probes (ACP40-GSG-150) were placed in contact with both ends of the coplanar waveguide in the longitudinal direction, and the S21 parameter of the S-parameter was measured using a Keysight Technologies network analyzer (Agilent E5071C). S21 was measured at five points within the plane of each substrate. The measurement frequency ranged from 0.01 GHz to 20 GHz, and the insertion loss was measured at 0.01 GHz intervals within this range. Based on the results obtained from the measurements, the absolute value of S21 was used as the insertion loss and evaluated according to the following criteria. A (Good): Insertion loss is 0.45 dB / mm or less at 20 GHz. B (Defective): Insertion loss greater than 0.45 dB / mm at 20 GHz. 【0094】 [Example 1] As a support substrate, a single-crystal silicon substrate with an orientation flat (OF) portion, a diameter of 4 inches, a thickness of 230 μm, and high resistance (>3 kΩ·cm) was prepared (hereinafter simply referred to as the silicon substrate). The surface roughness Ra of the silicon substrate surface was 0.5 nm. Next, the silicon substrate was cleaned using a mixed solution of acetone and isopropyl alcohol (IPA) to remove impurities from the surface of the silicon substrate. After cleaning, it was confirmed that a native oxide film of approximately 1 nm was formed on the surface of the silicon substrate. 【0095】 Next, an oxide film layer was formed on the surface (top and bottom) of the silicon substrate by thermal oxidation. Thermal oxidation was carried out as follows. A silicon substrate was placed in a chamber capable of supplying oxygen and water vapor. While heating the inside of the chamber to 700°C to 1200°C, oxygen and water vapor were supplied to create an oxidizing atmosphere, and the silicon substrate was wet-oxidized. Subsequently, the silicon substrate with oxide film layers formed on both sides was removed. The thickness of the oxide film layer was 4.0 μm on each side. 【0096】 Next, one side of the support substrate, which is the electrode-forming surface for the oxide film layer formed on both sides, was smoothed by CMP (Chemical Polishing) until the surface roughness Ra reached 0.5 nm. 【0097】 Next, laser irradiation was performed from the oxide film layer side of the electrode formation surface toward the silicon substrate. A laser irradiation device (model number TruMicro 5050) manufactured by TRUMPF Corporation was used for the laser irradiation. The laser irradiation conditions were as follows: Wavelength: 1030nm Laser energy: 30 μJ (Laser density: 500 mJ / cm²) 2 ) Irradiation pitch (feed width): 75 μm Laser pulse width: 900 fs Frequency: 40kHz Defocus: 2.0mm A composite having an oxide film layer / modified layer / supporting substrate was obtained by laser irradiation. 【0098】 Next, the surface of the oxide film layer of the composite was smoothed by polishing using CMP (Chemical Polishing) until the thickness of the oxide film layer was approximately 100 nm (Ra: approximately 0.2 nm). As described above, a composite substrate comprising an oxide film layer, a modified layer, and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. It was also confirmed that the reflection loss (S11) at this time was -20 dB or less (reflection 1% or less). The results are shown in Table 1. 【0099】 The obtained composite substrate was observed from above (oxide film layer side) with an optical microscope, and image processing confirmed the presence of modified layers (first modified layer and second modified layer) and an unmodified layer (i.e., oxide film layer). Figure 5A shows a plan view cross-sectional image of the composite substrate including the modified layers, obtained from optical microscope observation. Based on this image, the area ratio of the modified layers was calculated using image processing software (WinROOF 2021). Specifically, a rectangular optical microscope image with a side length of 300 μm or more was prepared, and after noise reduction processing as necessary, monochrome processing was performed using image analysis software, and the modified region was extracted by binarization processing, and the area ratio (%) of the modified layer to the total area in plan view was calculated. The results are shown in Table 1. When the cross-section of the obtained composite substrate was observed using a transmission electron microscope (Hitachi High-Technologies Corporation, "H-9500") under conditions of an acceleration voltage of 200kV and a total magnification of 1,000,000x, as shown in Figure 4A, it was confirmed that at least an amorphous region (modified layer 30) was formed between the active layer 20 (oxide film layer 21) and the silicon substrate 10, and that regions of crystalline structure and amorphous structure were mixed. The total depth (distance) of the modified layer was approximately 70 nm. This distance was defined as the thickness of the modified layer. The depth of the first modified layer was approximately 10 nm. This distance was defined as the thickness of the first modified layer. Figure 4B shows a TEM image of a portion of several modified parts 301 in the modified layer 30, observed from the center to the edge. As shown in Figure 4B, it was confirmed that the thickness dc at the center of the modified part 301 was greater than the thickness dr at the edge, and that the thickness decreased from the center to the edge of the modified part. Furthermore, when the region near the boundary between the amorphous structure and the region with a different atomic arrangement regularity was observed under conditions of an acceleration voltage of 200kV and a total magnification of 2,000,000x, and the atomic arrangement was examined by magnifying the image, it was found that regions of crystalline and amorphous structures were mixed, and multiple locations where the amorphous structure was located closer to the silicon substrate than the crystalline structure were confirmed. In addition, by observing the TEM image, it was found that there were darkened regions, i.e., regions with strain. Furthermore, elemental and compositional analyses were performed on the obtained composite substrate by EDX (energy-dispersive X-ray spectroscopy) to confirm the constituent elements of each layer, and it was determined that the modified layer was formed of silicon. Table 1 shows the crystals (including the types of elements) that make up the support substrate, modified layer, and oxide film layer. Note that the modified layer has an amorphous structure with a different regularity of atomic arrangement, which is distinguished in the table by being labeled "amorphous Si(1)" for the first modified layer and "amorphous Si(2)" for the second modified layer. 【0100】 [Examples 2-5 and Comparative Examples 1-2] A composite substrate was fabricated in the same manner as in Example 1, except that the laser energy, irradiation pitch, and the shape of the modified layer (modified section) were changed to those shown in Table 1. The shape of the modified section was adjusted by setting the shape of the laser beam profile. In Comparative Example 1, laser irradiation was not performed, so the laser energy, irradiation pitch, and shape of the modified section are indicated with "-" in the table. The obtained composite substrates were subjected to the evaluation described in (2) above. The results are shown in Table 1. For Examples 2 to 5, it was also confirmed that the reflection loss (S11) was -20 dB or less (reflection 1% or less). For Comparative Examples 1 and 2, the insertion loss was not smaller than that of any of the examples in the measurement range from 0.01 GHz to 20 GHz. Similar to Example 1, the cross-section of the obtained composite substrate, including the modified layer in a plan view, was observed using an optical microscope, and the area ratio of the modified layer (the ratio of the area of ​​the modified layer to the total cross-sectional area) was calculated. The results are shown in Table 1. Figures 5B to 5D show optical microscope images of the cross-section of the modified layer in Examples 3 and 4 and Comparative Example 2. In Examples 2 and 5, the shape and area of ​​the modified layer were similarly confirmed from optical microscope images. Similar to Example 1, TEM images of the thickness-direction cross-section of the obtained composite substrate were acquired to confirm the formation of the modified layer, the overall thickness of the modified layer, and the thicknesses of the first and second modified layers. For the composite substrate of Example 3, similar to Example 1, the area near the boundary between the amorphous structure region and the region with a different atomic arrangement regularity was observed under conditions of an acceleration voltage of 200kV and a total magnification of 2,000,000x, and the image was further magnified to confirm the atomic arrangement. The magnified view is shown in Figures 4C to 4F. As shown in Figures 4C to 4F, similar to Example 1, regions with crystalline structures and regions with amorphous structures are mixed, and multiple areas (first modified layer 31) with amorphous structures were found to be located on the silicon substrate side than the crystalline structure region (see the area enclosed by the dashed line in Figure 4E). Furthermore, by observing the TEM images, it was found that there were darkened regions, i.e., regions with strain. The total depth (distance) of the modified layer was approximately 60 nm, and the depth of the first modified layer was approximately 10 nm. Furthermore, the composition of the active layer, modified layer, and support substrate of the obtained composite substrate was confirmed by EDX measurement, similar to Example 1. 【0101】 [Example 6] A composite substrate was fabricated in the same manner as in Example 3, except that a heat treatment was performed after laser irradiation. Specifically, laser irradiation was performed under the conditions described in Table 1 to obtain a composite having an oxide film layer / modified layer / support substrate, and then the composite was heated at a temperature of 500°C for 5 hours. After 5 hours, it was cooled (allowed to cool) and smoothed to obtain a composite substrate comprising an oxide film layer / modified layer / support substrate. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 1. It was also confirmed that the reflection loss (S11) at this time was -20 dB or less (reflection 1% or less). Similar to Example 1, the cross-section of the obtained composite substrate, including the modified layer in a plan view, was observed using an optical microscope, and the shape and area ratio (ratio of the area of ​​the modified layer to the total cross-sectional area) of the modified layer were calculated. The results are shown in Table 1. In Example 6, as in Example 1, the area near the boundary between the region with an amorphous structure and the region with a different atomic arrangement regularity was observed under conditions of an acceleration voltage of 200kV and a total magnification of 1,000,000x, and the atomic arrangement was confirmed by further magnifying the image. Figure 4G shows a TEM image of the interface between the support substrate and the modified layer (amorphous Si(1) of the first modified layer). Figure 4H shows a TEM image of Figure 4G further magnified (observed under conditions of a total magnification of 2,000,000x). As shown in Figures 4G and 4H, which show the interface between the amorphous structure of the modified layer and the silicon substrate, it was confirmed that even after heat treatment (500°C, 5 hours), crystallization of the amorphous structure (amorphous Si) of the modified layer did not occur, and the amorphous structure was maintained. 【0102】 [Table 1] 【0103】 [Example 7] A composite substrate was fabricated, comprising an oxide film layer and a functional layer as the active layer. Specifically, the composite substrate was fabricated as follows. Descriptions common to Example 1 are omitted as appropriate. As with Example 1, a silicon substrate was prepared as the support substrate and cleaned. Subsequently, as with Example 1, the silicon substrate was thermally oxidized (wet oxidation) to form oxide film layers on both sides of the silicon substrate. The thickness of each oxide film layer was 2.5 μm. Next, one side of the support substrate, which is the electrode-forming surface for the oxide film layer formed on both sides, was smoothed by CMP (Chemical Polishing) until the surface roughness Ra reached 0.5 nm. 【0104】 As a functional substrate, a lithium niobate substrate (hereinafter referred to as LN substrate), which is a piezoelectric substrate with an OF (Oxygen-Free) portion, a diameter of 4 inches, and a thickness of 250 μm, was prepared. X-cut LN substrate was used. The surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. Subsequently, the LN substrate was cleaned in the same manner as the silicon substrate in Example 1 to remove impurities and other contaminants from the surface of the LN substrate. 【0105】 Next, the oxide film layer after cleaning and the LN substrate were directly bonded using a plasma activation method to obtain a bonded body. 【0106】 Next, the bonded structure was placed in a nitrogen-filled oven (120°C) and heated for 10 hours. After that, the LN substrate of the bonded structure was removed from the oven and subjected to grinding and lapping, and then the thickness of the LN substrate was reduced to 3.0 μm by CMP processing. 【0107】 Next, the bonded structure was irradiated with a laser from the LN substrate side, through the LN substrate and the oxide film layer, under the same laser irradiation conditions as in Example 1. Laser irradiation formed a first modified layer and a second modified layer between the silicon substrate and the oxide film layer, starting from the silicon substrate side. The presence of the modified layers was confirmed by TEM imaging, as in Example 1. In this way, a composite material comprising an LN substrate, an oxide film layer, a modified layer (second modified layer / first modified layer), and a silicon substrate was obtained. 【0108】 The LN substrate of the above composite was subjected to thinning treatment. Specifically, the composite was placed in a nitrogen atmosphere oven (120°C) and heated for 10 hours. After removing the LN substrate from the oven, it was ground and lapped, and then subjected to CMP processing to obtain a 500 nm thick LN layer (functional layer). As described above, a composite substrate comprising a functional layer, an oxide film layer, a modified layer (second modified layer / first modified layer), and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 2. 【0109】 [Example 8] A composite substrate was fabricated in the same manner as in Example 7, except that laser irradiation was performed before bonding the oxide film layer and the functional substrate. Specifically, the composite substrate was fabricated as follows. Explanations common to Example 7 will be omitted as appropriate. 【0110】 As with Example 7, a silicon substrate was prepared as the support substrate and cleaned. Subsequently, as with Example 7, the silicon substrate was thermally oxidized (wet oxidation) to form oxide film layers on both sides of the silicon substrate. The thickness of each oxide film layer was 2.5 μm. Next, one side (the electrode formation surface) of the oxide film layer formed on both sides of the support substrate was smoothed by CMP (Chemical Polishing) until the surface roughness Ra reached 0.5 nm. 【0111】 Next, a laser was irradiated onto the silicon substrate surface from the polished oxide layer side of the laminate of the silicon substrate and the oxide film layer, under the same laser irradiation conditions as in Example 7. Laser irradiation formed a modified layer between the silicon substrate and the oxide film layer, starting from the silicon substrate side. The presence of the modified layer was confirmed by TEM imaging, as in Example 7. 【0112】 As a functional substrate, an LN substrate similar to that in Example 7 was prepared, and the surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. Subsequently, the LN substrate was cleaned in the same manner as in Example 7 to remove impurities and other contaminants from the surface of the LN substrate. 【0113】 Next, the oxide film layer and the LN substrate after cleaning were directly bonded by a plasma activation method to obtain a bonded body comprising an LN substrate, an oxide film layer, a modified layer (second modified layer / first modified layer), and a silicon substrate. 【0114】 Next, the LN substrate of the bonded structure was subjected to a thinning treatment. Specifically, the bonded structure was placed in a nitrogen atmosphere oven (120°C) and heated for 10 hours. After removing the LN substrate from the oven, it was ground and lapped, and then subjected to CMP processing to create a 500 nm thick LN layer (functional layer). As described above, a composite substrate comprising a functional layer, an oxide film layer, a modified layer (second modified layer / first modified layer), and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 2. 【0115】 [Comparative Example 3] A composite substrate was fabricated in the same manner as in Example 7, except that laser irradiation was not performed. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 2. Furthermore, Comparative Example 3 did not exhibit lower insertion loss than Examples 7 and 8 in the measurement range from 0.01 GHz to 20 GHz. 【0116】 [Table 2] 【0117】 As is clear from the above examples and comparative examples, in each example, a modified layer of a predetermined area (40%) or more is formed, and insertion loss is reduced. Therefore, it is suggested that the composite substrates of each example have a modified layer that can function as a charge trapping layer and can be applied to applications with excellent electrical properties (especially high-frequency properties). Furthermore, in each example, the charge trapping layer (modified layer) can be formed by laser irradiation after forming an active layer (oxide film layer and / or functional layer) on the support substrate. In other words, in each example, it has been shown that special equipment for ensuring safety is not required as in the case of formation by the CVD method, the increase in cost can be suppressed, and composite substrates can be manufactured at low cost. [Industrial applicability] 【0118】 The composite substrate according to the embodiment of the present invention can be suitably used in functional elements such as elastic wave devices and optical modulation devices such as thin-film LN optical modulators. [Explanation of symbols] 【0119】 10 Support substrate 20 Active layer 21 Oxide layer 22 Functional Layers 30 Modified layer 301 Modification section 31. First Modified Layer 32 Second Modified Layer 41 Functional substrates 100, 101, 102, 103 Composite substrate

Claims

[Claim 1] The device comprises, in this order, a crystalline support substrate, a modified layer in which the crystalline properties of the support substrate have been modified, and an active layer. The area ratio of the modified layer to the total area in a plan view is 40% or more and less than 100%. Composite circuit board. [Claim 2] The composite substrate according to claim 1, wherein the modified layer is composed of a plurality of modified portions, each having a thickness of 10 nm or more. [Claim 3] The composite substrate according to claim 2, wherein each of the plurality of modified portions is formed such that its thickness decreases in the in-plane direction from the center to the edge of the modified portion. [Claim 4] The composite substrate according to claim 2, wherein the plurality of modified portions are formed at intervals from one another. [Claim 5] The composite substrate according to claim 1, wherein a plurality of electrodes are formed on the active layer at intervals. [Claim 6] The modified layer comprises, in order from the support substrate side, a first modified layer and a second modified layer. The second modified layer contains an amorphous structure of the same elements as those constituting the support substrate, The first modified layer contains the same elements as those constituting the support substrate, The composite substrate according to claim 1, wherein the regularity of the atomic arrangement in the first modified layer and the regularity of the atomic arrangement in the second modified layer are different from each other. [Claim 7] The composite substrate according to claim 6, wherein the first modified layer includes an amorphous structure and a crystalline structure. [Claim 8] The composite substrate according to claim 7, wherein in the thickness direction, the amorphous structure of the first modified layer includes a region where it is located closer to the support substrate than the crystalline structure. [Claim 9] The composite substrate according to claim 6, wherein the first modified layer includes a region having strain in part. [Claim 10] The composite substrate according to claim 1, wherein the band gap of the active layer is 2.5 eV or greater. [Claim 11] The composite substrate according to claim 1, wherein the active layer has an oxide film layer. [Claim 12] The composite substrate according to claim 11, wherein the oxide film layer is a thermal oxide film. [Claim 13] The composite substrate according to claim 1, wherein the active layer includes a functional layer. [Claim 14] The composite substrate according to claim 1, wherein the active layer includes an oxide film layer and a functional layer in that order from the support substrate side. [Claim 15] The composite substrate according to claim 14, wherein the active layer further includes a bonding layer between the oxide film layer and the functional layer. [Claim 16] The composite substrate according to claim 14, wherein the active layer further includes an intermediate layer between the oxide film layer and the functional layer. [Claim 17] The composite substrate according to claim 1, wherein the surface roughness Ra of the surface of the active layer opposite to the modified layer is 1 nm or less. [Claim 18] A method for manufacturing a composite substrate according to any one of claims 1 to 17, Forming an active layer on at least one surface of a crystalline support substrate, The process includes, in this order, irradiating the surface of the support substrate with a laser from the active layer side to form a modified layer at the interface between the support substrate and the active layer, This includes irradiating the laser such that the area ratio of the modified layer to the total area in a plan view is 40% or more. A method for manufacturing composite substrates. [Claim 19] The method for manufacturing a composite substrate according to claim 18, comprising irradiating the support substrate with the laser at intervals in at least one direction within the plane of the support substrate. [Claim 20] The active layer includes an oxide film layer, A method for manufacturing a composite substrate according to claim 18, comprising forming the oxide film layer by oxidizing the support substrate. [Claim 21] A method for manufacturing a composite substrate according to claim 20, comprising forming the oxide film layer by thermal oxidation of the support substrate. [Claim 22] A method for manufacturing a composite substrate according to claim 18, comprising smoothing the active layer until the surface roughness Ra of the active layer is 1 nm or less. [Claim 23] A method for manufacturing a composite substrate according to claim 20, comprising bonding a functional substrate to the side of the oxide film layer opposite to the support substrate. [Claim 24] A method for manufacturing a composite substrate according to claim 23, comprising bonding the functional substrate to the oxide film layer, and then thinning the functional substrate to a thickness of 1000 nm or less to form a functional layer. [Claim 25] A method for manufacturing a composite substrate according to claim 23, comprising forming an intermediate layer between the oxide film layer and the functional substrate before joining the oxide film layer and the functional substrate.

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