Composite substrate and method for manufacturing the same

Abstract

The present invention provides a composite substrate that can maintain charge trapping performance, contribute to high performance when used in devices for high-frequency and / or harmonic applications, and can be manufactured at low cost. The composite substrate according to an embodiment of the present invention comprises, in this order, a crystalline support substrate, a first modified layer, a second modified layer, and an active 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 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.

Description

【Technical Field】 【0001】 The present invention relates to a composite substrate and a method for manufacturing the same. 【Background Art】 【0002】 In information and communication devices, in order to extract an electrical signal of an arbitrary frequency, for example, functional elements such as an acoustic surface wave element (e.g., SAW filter) using an acoustic surface wave, an electro-optic element (e.g., optical modulator, etc.) capable of changing the phase of light, etc. are used. In recent years, in the field of information and communication devices, the communication volume has been increasing rapidly, and higher performance of functional elements has been demanded. For functional elements, for example, a composite substrate having a piezoelectric layer and a support substrate is used, and in order to ensure good performance of an RF device, a composite substrate provided with a charge trapping layer (trap-rich layer) such as polycrystalline silicon as an intermediate layer between the piezoelectric layer and the support substrate has been proposed (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 a function of suppressing insertion loss in a high-frequency signal and / or formation of harmonic components. However, when a piezoelectric layer as an active layer is further provided on the charge trapping layer after providing the charge trapping layer on the 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 degraded. Further, in Patent Document 1, when providing a charge trapping layer on a support substrate, a chemical vapor deposition (CVD) method is used for forming the charge trapping layer, and since it is necessary to use a highly active and reactive gas such as silane (SiH4), special equipment for ensuring safety is required, resulting in an increase in cost. 【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 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. [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 first modified layer, a second modified layer, and an active 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 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. [2] In the above [1], the first modified layer includes an amorphous structure and a crystalline structure. [3] In the above [2], the composite substrate includes a region in the thickness direction in which the amorphous structure of the first modified layer is located on the support substrate side than the crystalline structure. [4] In any of the above [1] to [3], the first modified layer includes a region having strain in part. [5] In any of the above [1] to [4], the active layer includes a layer having a band gap of 2.5 eV or more. [6] In any of the above [1] to [5], the active layer contains an oxide. [7] In any of the above [1] to [6], the active layer has an oxide film layer. [8] In the above [7], the oxide film layer is a thermal oxide film. [9] In any of the above [1] to [8], the active layer includes the thermal oxide film of the support substrate.

[10] In any of the above [1] to [9], the active layer includes a functional layer.

[11] In any of the above [1] to [9], the active layer includes an oxide film layer and a functional layer, and the oxide film layer and the functional layer are stacked in order from the support substrate side.

[12] In the above

[11] , the composite substrate further comprises a bonding layer between the oxide film layer and the functional layer in the active layer.

[13] In the above

[11] or

[12] , the composite substrate further includes an intermediate layer between the oxide film layer and the functional layer in the active layer.

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

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

[15] 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

[14] 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 first modified layer and a second modified layer at the interface between the support substrate and the active layer, in order from the side of the support substrate.

[16] In the above

[15] , the manufacturing method includes the active layer comprising an oxide film layer, and forming the oxide film layer by oxidizing the support substrate.

[17] In the above

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

[18] In any of the above

[15] to

[17] , the manufacturing method includes smoothing the surface of the support substrate until the surface roughness Ra is 10 nm or less before forming the active layer.

[19] In any of the above

[15] to

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

[20] In any of the above

[16] to

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

[21] In

[20] above, the manufacturing method includes forming a functional layer by thinning the thickness of the functional substrate to 1000 nm or less after bonding the functional substrate to the oxide film layer.

[22] In

[20] above, the manufacturing method includes forming an intermediate layer between the oxide film layer and the functional substrate before bonding the oxide film layer and the functional substrate. 【Effect of the Invention】 【0007】 According to an embodiment of the present invention, it is possible to maintain the performance of charge trapping, contribute to the high performance when used in devices for high frequency and / or harmonic applications, and realize a composite substrate that can be manufactured at a low cost. 【Brief Description of the Drawings】 【0008】 [Figure 1A] It is a schematic cross-sectional view showing a schematic configuration of a composite substrate according to one embodiment of the present invention. [Figure 1B] It is a schematic cross-sectional view showing a schematic configuration of a modified example of the composite substrate according to the above embodiment. [Figure 1C] It is a schematic cross-sectional view showing a schematic configuration of a composite substrate according to another embodiment of the present invention. [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 1E] It is a schematic cross-sectional view showing a schematic configuration of a composite substrate when provided with an electrode. [Figure 2A] It is a schematic cross-sectional view explaining one step in the manufacturing method of a composite substrate according to one embodiment. [Figure 2B] It is a schematic cross-sectional view explaining one step in the manufacturing method of a composite substrate according to one embodiment. [Figure 2C] It is a schematic cross-sectional view explaining one step in the manufacturing method of a composite substrate according to one embodiment. [Figure 2D] It is a schematic cross-sectional view explaining one step in the manufacturing method of a composite substrate according to one embodiment. [Figure 2E] It is a schematic cross-sectional view explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2F] It is a schematic cross-sectional view explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2G] It is a schematic cross-sectional view explaining one step in a method for manufacturing a composite substrate according to one embodiment. [Figure 2H] It is a schematic cross-sectional view 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 evaluation of high-frequency characteristics. [Figure 3B] It is a diagram showing an enlarged view of a portion surrounded by a broken line in FIG. 3A. [Figure 4A] It is a TEM image showing a cross-section of a part of the composite substrate in Example 1. [Figure 4B] It is a TEM image showing a cross-section obtained by enlarging a part of FIG. 4A. [Figure 4C] It is a diagram showing an example of a region composed of a crystalline structure in a modified layer in a TEM image showing a cross-section obtained by enlarging a part (near the modified layer) of FIG. 4B. [Figure 4D] It is a TEM image showing a cross-section obtained by further enlarging a part of FIG. 4C. 【Embodiments for Carrying Out the Invention】 【0009】 Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. The drawings are schematically drawn for easy viewing, and the thickness, length, width, shape, ratio, etc. do not accurately reflect the actual shape. 【0010】 A. Composite Substrate A-1. Overview of the Composite Substrate Figure 1A is a schematic cross-sectional view showing the general configuration of a composite substrate according to one embodiment of the present invention. The illustrated example composite substrate 100 has a support substrate 10, a modified layer 30, and an active layer 20 in that 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, a modified layer 30 is formed, in which the crystallinity of the surface of the support substrate 10 has been modified. The modified layer 30 has a first modified layer 31 and a second modified layer 32, in that order from the support substrate 10 side. 【0011】 A "modified layer" refers to a layer having regions in which the density, refractive index, mechanical strength, physical properties, etc., have been 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 that have different properties from the support substrate in the above-mentioned characteristics. Typically, a modified layer is a layer in which the crystallinity of the surface of the support substrate has been modified, as described above. The first modified layer 31 contains the same elements as those constituting the support substrate 10. The second modified layer 32 contains an amorphous structure of the same elements as those constituting the support substrate 10. Furthermore, the first modified layer and the second modified layer in the modified layer according to the embodiment of the present invention have different regularities in their atomic arrangement. The regularity of the atomic arrangement is a crystallographic indicator for determining crystalline and / or amorphous structures. Note that the regularity of a crystalline structure can differ depending on the combination and state of the atomic arrangement, so "crystalline structure" may include multiple crystalline states. Similarly, "amorphous structure" may include multiple amorphous states. The crystallinity and / or amorphous nature of the support substrate, the first modified layer, and the second modified layer in a 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 microscopy). Differences in the regularity of the amorphous structure can also be confirmed by measuring Raman scattering spectroscopy using Raman spectroscopy and observing the short-range order. The possibility that the first modified layer and the second modified layer contain the same elements as those constituting the support substrate can be confirmed by elemental and compositional analysis using EDX (energy-dispersive X-ray fluorescence). 【0012】 As described above, the composite substrate according to the embodiment of the present invention has a first modified layer and a second modified layer between the support substrate and the active layer, the modified layers having different crystallinity from the support substrate. The regularity of the atomic arrangement in the first modified layer and the regularity of the second modified layer are different from each other. The first modified layer and the second modified layer can typically function as charge trapping layers. The modified layers 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, as described later. Normally, it is difficult to provide a new layer (region) between the support substrate and the active layer after the active layer has been formed on the support substrate (for example, the formation of an oxide film layer by an oxide film and / or the 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, modified layers (first modified layer and second modified layer) can be provided between the support substrate and the active layer. In this case, since the modified layer is not provided on the support substrate and then the active layer is not formed on the modified layer, 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. As a result, according to the embodiment of the present invention, a composite substrate with a well-functioning charge trapping layer can be realized. 【0013】 In the illustrated example, the modified layer 30 and the activated layer 20 are formed in this order only on one surface (upper surface 10a) of the support substrate 10. However, the modified layer and the activated layer may be formed in this order only on the other surface (lower surface 10b) of the support substrate 10, or the modified layer and the activated layer may be formed in this order on both the upper surface 10a and the lower surface 10b of the support substrate 10. 【0014】 The modified layer 30 may be formed substantially over the entire support substrate 10, as shown in Figure 1A, or it may be formed partially, as shown in Figure 1B. When the modified layer is formed partially, it may have any shape in plan view. When the modified layer is formed partially, it may be formed randomly or in a regular manner, for example, in plan view. Also, for example, the thickness of the modified layer in cross-sectional view may be formed randomly or uniformly. The thickness of the modified layer will be discussed later. 【0015】 The active layer 20 is located on at least one side of the support substrate 10. In the illustrated example, the active layer 20 is located on the upper surface 10a side of the support substrate 10, but the active layer 20 may also be located on the lower surface 10b side of the support substrate 10, or on both sides of the support substrate 10. When the active layer is located on both sides of the support substrate, the modified layer may be formed on the upper side of the support substrate, on the lower side, or on both sides. 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 oxide layer, functional layer, and intermediate layer will be described in detail later. Figures 1A and 1B show an example where the active layer 20 consists only of an oxide layer 21, but as shown in Figure 1C, in the composite substrate 101, the active layer 20 may consist only of, for example, a functional layer 22. Alternatively, for example, the oxide layer and the functional layer may be laminated on the modified layer in this order (see Figure 1D below). Alternatively, for example, the active layer may be constructed by laminating an oxide layer, an intermediate layer, and a functional layer on the modified layer in this order (not shown). 【0016】 Figure 1D is a schematic cross-sectional view showing the general configuration of a composite substrate according to another embodiment of the present invention. The illustrated example composite substrate 102 comprises a support substrate 10, a modified layer 30, and an active layer 20 (oxide film layer 21 and functional layer 22) in this order. The support substrate 10 and the modified layer 30 may each have the same configuration as in the embodiments of Figures 1A and 1B. That is, the composite substrate 102 according to this embodiment may include an oxide film layer 21 and a functional layer 22 as the active layer 20 in the composite substrate according to the above embodiment, and further comprises a functional layer 22 on the side of the oxide film layer 21 opposite to the modified layer 30. Note that the modified layer 30 in the illustrated example composite substrate 102 may be formed substantially over the entire support substrate 10 as in Figure 1A, or it may be formed partially as in Figure 1B. In the illustrated example composite substrate, the oxide film layer 21 and the functional layer 22 are typically bonded together. Such a composite substrate has the advantage that the modified layer can function more effectively as a trap-rich layer. In the illustrated example, the active layer 20 is located on the upper side of the support substrate 10, but, as with the composite substrates in the embodiments of Figures 1A and 1B, the active layer may be located on the lower side of the support substrate, or on both sides. 【0017】 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. 【0018】 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. 【0019】 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. 【0020】 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. 【0021】 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. 【0022】 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. 【0023】 A-3. Modified layers (first modified layer and second modified layer) In the composite substrate according to an embodiment 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), and comprises a first modified layer and a second modified layer. As described later, the modified layer may 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 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. 【0024】 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. 【0025】 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. 【0026】 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. 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. 【0027】 In the first modified layer, the amorphous structure 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. 【0028】 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. When 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. When the thickness of the modified layer is 1000 nm or less, it can contribute to reducing the cost in manufacturing the composite substrate. Note that "insertion loss" is one of the indicators that can be used to evaluate high-frequency characteristics. Insertion loss can be measured and evaluated by the method and conditions described in the examples below. 【0029】 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. When 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. 【0030】 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. 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 the band gap. 【0031】 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). 【0032】 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. 【0033】 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. 【0034】 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. 【0035】 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. 【0036】 The surface roughness Ra of the oxide film layer on the side opposite to the second 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 the composite substrate. 【0037】 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. 【0038】 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. 【0039】 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. 【0040】 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). 【0041】 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. 【0042】 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°). 【0043】 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. 【0044】 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. 【0045】 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). 【0046】 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. 【0047】 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. 【0048】 As shown in Figure 1E, electrodes 50 may be provided on the surface of the functional layer 22 of the composite substrate 103. The electrodes may be, for example, coplanar waveguides (CPWs). The electrodes may also be provided on the surface of the oxide film layer 21 in the composite substrate 100, as shown in Figures 1A and 1B. 【0049】 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. 【0050】 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. 【0051】 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. 【0052】 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. 【0053】 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 irradiating the surface of the support substrate with a laser from the active layer side to form a first modified layer and a second modified layer at the interface between the support substrate and the active layer, in that order from the support substrate side. In the manufacturing method according to an embodiment of the present invention, by irradiating the surface of the support substrate with a laser, a modified layer can be formed at the interface between the support substrate and the active layer, in which the crystallinity of the surface of the support substrate is modified (modified). By modifying the crystallinity of the surface of the support substrate, a first modified layer is formed which contains the same elements as those constituting the support substrate, but has different crystallinity (i.e., different atomic arrangement regularity) from that of the support substrate, and a second modified layer is formed which contains an amorphous structure of the same elements as those constituting the support substrate. As described above, the atomic arrangement regularity in the first modified layer and the atomic arrangement regularity in the second modified layer are different from each other. According to the manufacturing method of a composite substrate according to an embodiment of the present invention, modified layers (first modified layer and second modified layer) capable of functioning as charge trapping layers can be formed by laser irradiation. As a result, since special equipment for ensuring safety is not required, as is the case when forming by the CVD method, cost increases can be suppressed. Therefore, according to the embodiment of the present invention, a composite substrate can be manufactured at low cost. 【0054】 The reason why the first and second modified layers described above are formed on the surface of the support substrate by laser irradiation is not entirely clear, but the following reasons are possible. By irradiating the surface of the support substrate with a laser through the 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 (into) from the surface of the support substrate, it can be inferred that the first modified layer may be formed at a certain depth (the part away from the surface of the support substrate) in areas where the bonds are partially broken and / or in areas where crystallinity can be maintained, resulting in a different regularity of atomic arrangement from the second modified layer. However, this is merely a conjecture and does not limit the present invention, nor does it restrict the present invention by this mechanism. 【0055】 The following describes typical examples of manufacturing methods for composite substrates with reference to Figures 2A to 2H. Figure 2D is identical to Figure 1A, and Figure 2H is identical to Figure 1C. Sections B-2 to B-6 below describe 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. 【0056】 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. 【0057】 The support substrate 10 may be subjected to any suitable smoothing treatment on one or both sides as needed. 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. 【0058】 Next, as shown in Figure 2B, an oxide film layer 21 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. 【0059】 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. 【0060】 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. 【0061】 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. 【0062】 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 the first modified layer 31 and the second modified layer 32 can be formed in order from the support substrate 10. As described above, in the method for manufacturing a composite substrate according to the embodiment of the present invention, modified layers (first modified layer and second modified layer) that can function as charge trapping layers 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. 【0063】 Laser irradiation can be performed in any suitable manner. For example, the laser may be irradiated without focusing on the surface of the support substrate. Specifically, for example, the support substrate may be positioned at a predetermined distance (e.g., several hundred μm) from the laser beam. Even in such a case, as will be described later, the laser beam may be absorbed by the surface of the support substrate. Therefore, when forming the modified layer, the effort of focusing on the support substrate each time can be eliminated by shifting the focal position of the support substrate by an appropriate distance (e.g., several mm). Furthermore, by shifting the laser focus in this way (i.e., not focusing it), ablation (thermal decomposition by laser irradiation, etc.) of the target object (e.g., support substrate, active layer) can be suppressed. Any suitable method and conditions for laser irradiation can be adopted as long as the surface of the support substrate can be modified. Typically, pulsed lasers can be used as the laser. For example, femtosecond, picosecond, or nanosecond pulsed lasers can be used, taking into consideration the effect of ablation on the support substrate and active layer. Preferably, a femtosecond or picosecond pulsed laser may be used. When a pulsed laser is used, the pulse width may be, for example, 1 fs to 100 ps. The laser frequency may be, for example, 1 kHz to 1 MHz. 【0064】 The laser wavelength can be any appropriate wavelength depending on the band gap of the active layer (essentially the oxide film layer) and the band gap of the support substrate. Specifically, when the laser wavelength is λ [nm], the band gap of the oxide film layer is Eg1 [eV], and the band gap of the support substrate is Eg2 [eV], it is preferable that the wavelength satisfies the relationship Eg1 ≥ 1240 / λ and Eg2 - 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), Eg1 ≈ 9.0 eV and Eg2 ≈ 1.1~1.2 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. As a laser wavelength, for example, a wavelength of 1030 nm can be suitably used. 【0065】 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: 【0066】 The laser irradiation pitch (spacing) during laser irradiation can be any appropriate interval 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. Preferably, it may be 10 μm or more, more preferably 20 μm or more, and even more preferably 50 μm or more. The upper limit of the laser irradiation pitch may be, for example, 100 μm. However, the type of laser, wavelength, pulse width, frequency, etc., are not limited to those listed above. 【0067】 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 formed by laser irradiation is as explained in Section A-3. 【0068】 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 (first modified layer 31 and second modified layer 32), and an oxide film layer 21 (active layer 20) in this order, as shown in Figure 2D (Figure 1A). 【0069】 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. 【0070】 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. 【0071】 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. 【0072】 Next, as shown in Figure 2F, the oxide film layer 21 of the composite substrate 100 (composite) having the support substrate 10, the modified layer 30, and the oxide film layer 21 in that order is bonded to the functional substrate 41. By bonding in this way, a composite 102' 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. 【0073】 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. 【0074】 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. 【0075】 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 102, as shown in Figure 2H (Figure 1D), 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. 【0076】 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. 【0077】 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. 【0078】 B-7. Variations Sections B-2 to B-6 above describe, as described above, a specific example of a method for manufacturing a composite substrate according to an embodiment of the present invention, in which a modified layer is formed by irradiating the surface of a support substrate with a laser, and then a functional layer is fabricated by 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 onto the surface of the support substrate from the functional substrate side, the first modified layer and the second 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. 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. 【0079】 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. 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 layers (first modified layer and second modified layer). 【0080】 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. 【0081】 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] 【0082】 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. 【0083】 (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. 【0084】 (2) Evaluation: High frequency characteristics For the composite substrates of Example 1 and Comparative Example 1, a coplanar waveguide (CPW) was formed on the surface of the oxide film layer to prepare samples for evaluating high-frequency characteristics. The CPW was formed using the following procedure. A 0.7 μm thick Au film was deposited on the oxide film layer of the obtained composite substrate by sputtering. Then, a coplanar waveguide was formed on the obtained Au film by lithography and ion trimming. Details of the coplanar waveguide are shown in Figures 3A and 3B, where L1 is 8000 μm, L2 is 8060 μm, L3 is 9060 μm, W1 is 80 μm, W2 is 3000 μm, and G1 is 140 μm. High-frequency probes (ACP110-A-GSG-150) were placed in contact with both ends of the coplanar waveguide in the longitudinal direction, and the insertion loss (S21) was measured using a high-frequency extender (N5295AX53) and network analyzer (N5227B) manufactured by Keysight Technologies. The insertion loss was measured at five points within the plane of each substrate. The measurement frequency ranged from 0.01 GHz to 80 GHz, and the insertion loss was measured at 0.01 GHz intervals within this range. The results obtained from the measurements were evaluated according to the following criteria. ◎(Good): Insertion loss of 2.5dB or less at 80GHz. × (Defective): Insertion loss greater than 2.5dB at 80GHz 【0085】 For the composite substrates of Example 2 and Comparative Example 2, a coplanar waveguide (CPW) was formed on the surface of the functional layer, and samples were prepared to evaluate the high-frequency characteristics. The CPW was formed using the following procedure. A 1.0 μm thick Au film was deposited on the functional layer of the obtained composite substrate by sputtering. Then, a coplanar waveguide was formed on the obtained Au film by lithography and ion trimming. Details of the coplanar waveguide are shown in Figures 3A and 3B, where L1 is 15000 μm, L2 is 15080 μm, L3 is 16080 μm, 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 insertion loss (S21) was measured using a network analyzer (Agilent E5071C). The insertion loss was measured at five points within the plane of each substrate. The measurement frequency ranged from 0.1 GHz to 20 GHz, and the insertion loss was measured at 0.1 GHz intervals within this range. The results obtained from the measurements were evaluated according to the following criteria. ◎(Good): Insertion loss is 6.0dB or less at 20GHz. × (Defective): Insertion loss greater than 6.0 dB at 20 GHz. 【0086】 [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. 【0087】 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. 【0088】 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. 【0089】 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): 50 μm Laser pulse width: 1.0 ps Frequency: 40kHz Defocus: 2.0mm A composite having an oxide film layer / modified layer / supporting substrate was obtained by laser irradiation. 【0090】 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 / modified layer (second modified layer / first modified layer) / support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. The insertion loss of the composite substrate, measured at 80 GHz after forming a CPW for evaluation, was 2.18 dB. It was also confirmed that the reflection loss at this time was 20 dB or less. The results are shown in Table 1. 【0091】 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 200 kV and a total magnification of 1,000,000x, it was confirmed that at least an amorphous region (second modified layer) was formed between the active layer (oxide film layer) and the silicon substrate, as shown in Figure 4A. Furthermore, when the area near the boundary between the amorphous region and the region with a different atomic arrangement regularity was observed under conditions of an acceleration voltage of 200 kV and a total magnification of 2,000,000x, and the image was magnified to confirm the atomic arrangement, as shown in Figures 4B to 4D, it was found that crystalline and amorphous regions were mixed, and multiple areas (first modified layer) of the amorphous region were found to be located closer to the silicon substrate than the crystalline region (see the area enclosed by the dashed line in Figure 4C). In addition, observation of the TEM image revealed the presence of darkened regions, i.e., regions with distortion. The total depth (distance) of the modified layer was approximately 60 nm. The depth of the first modified layer was approximately 10 nm. Furthermore, by observing the composite substrate from above (i.e., the oxide film layer side) with an optical microscope and processing the images, the presence of modified layers (first and second modified layers) and an unmodified layer (i.e., the oxide film layer) was confirmed. Elemental and compositional analysis was 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 layers were formed of silicon. The crystals (including the types of elements) constituting the support substrate, modified layer, and oxide film layer are shown in the respective columns of Table 1. 【0092】 [Comparative Example 1] A composite substrate was fabricated in the same manner as in Example 1, except that laser irradiation was not performed. The obtained composite substrate was subjected to the evaluation of the high-frequency characteristics described in (2) above, in the same manner as in Example 1. The insertion loss at 80 GHz of the composite substrate, which was measured by forming a CPW for evaluation, was 2.86 dB. It was also confirmed that the reflection loss at this time was 20 dB or less. Furthermore, the insertion loss was not smaller than that of Example 1 in the measurement range from 0.01 GHz to 80 GHz. The results are shown in Table 1. Upon examining the TEM image of the composite substrate in Comparative Example 1, it was found that no modified layer had formed between the support substrate and the oxide film layer. 【0093】 [Example 2] A composite substrate with a functional layer was fabricated. Specifically, the composite substrate was fabricated as follows. Note that explanations common to Example 1 will be 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. 【0094】 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. 【0095】 Next, the oxide film layer after cleaning and the LN substrate were directly bonded using a plasma activation method to obtain a bonded body. 【0096】 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. 【0097】 Next, the bonded structure was subjected to laser irradiation under the same conditions as in Example 1, starting from the LN substrate side and passing through the LN substrate and oxide film layer. 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. 【0098】 The LN substrate of the above composite was subjected to a 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 insertion loss of the composite substrate at 20 GHz, measured after forming a CPW for evaluation, was 5.25 dB. The results are shown in Table 1. 【0099】 [Comparative Example 2] A composite substrate was fabricated in the same manner as in Example 2, except that laser irradiation was not performed. The obtained composite substrate was subjected to the evaluation described in (2) above. The insertion loss at 20 GHz of the composite substrate, which was measured after forming a CPW for evaluation, was 6.75 dB. The results are shown in Table 1. Furthermore, the insertion loss was not lower than that of Example 2 in the measurement range from 0.1 GHz to 20 GHz. 【0100】 [Table 1] 【0101】 In each embodiment, it can be seen that the insertion loss is reduced. Therefore, it is suggested that the composite substrates of each embodiment can be applied to applications that require excellent high-frequency characteristics. [Industrial applicability] 【0102】 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] 【0103】 10 Support substrate 20 Active layer 21 Oxide layer 22 Functional Layers 30 Modified layer 31. First Modified Layer 32 Second Modified Layer 41 Functional substrates 100 Composite substrates 101 Composite substrate 102 Composite substrate

Claims

[Claim 1] The structure comprises a crystalline support substrate, a first modified layer, a second modified layer, and an active layer, in this order. 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 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. The first modified layer includes an amorphous structure and a crystalline structure. Composite circuit board. [Claim 2] The composite substrate according to claim 1, 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 3] The composite substrate according to claim 1, wherein the first modified layer includes a region having strain in part. [Claim 4] The composite substrate according to claim 1, wherein the active layer includes a layer having a band gap of 2.5 eV or more. [Claim 5] The composite substrate according to claim 1, wherein the active layer contains an oxide. [Claim 6] The composite substrate according to claim 1, wherein the active layer has an oxide film layer. [Claim 7] The composite substrate according to claim 6, wherein the oxide film layer is a thermal oxide film. [Claim 8] The composite substrate according to claim 1, wherein the active layer includes the thermal oxide film of the support substrate. [Claim 9] The composite substrate according to claim 1, wherein the active layer includes a functional layer. [Claim 10] The active layer includes an oxide film layer and a functional layer. The composite substrate according to claim 1, wherein the oxide film layer and the functional layer are laminated in order from the support substrate side. [Claim 11] The composite substrate according to claim 10, wherein the active layer further comprises a bonding layer between the oxide film layer and the functional layer. [Claim 12] The composite substrate according to claim 10, wherein the active layer further includes an intermediate layer between the oxide film layer and the functional layer. [Claim 13] The composite substrate according to claim 1, wherein the surface roughness Ra of the surface of the active layer opposite to the second modified layer is 1 nm or less. [Claim 14] A method for manufacturing a composite substrate according to any one of claims 1 to 13, Forming an active layer on at least one surface of a crystalline support substrate, This includes, in this order, irradiating the surface of the support substrate with a laser from the side of the active layer, thereby forming a first modified layer and a second modified layer at the interface between the support substrate and the active layer, starting from the side of the support substrate. A method for manufacturing composite substrates. [Claim 15] The active layer includes an oxide film layer, A method for manufacturing a composite substrate according to claim 14, comprising forming the oxide film layer by oxidizing the support substrate. [Claim 16] A method for manufacturing a composite substrate according to claim 15, comprising forming the oxide film layer by thermal oxidation of the support substrate. [Claim 17] A method for manufacturing a composite substrate according to claim 14, comprising smoothing the surface of the support substrate until the surface roughness Ra of the support substrate is 10 nm or less, before forming the active layer. [Claim 18] A method for manufacturing a composite substrate according to claim 14, comprising smoothing the active layer until the surface roughness Ra of the active layer is 1 nm or less. [Claim 19] A method for manufacturing a composite substrate according to claim 15, comprising bonding a functional substrate to the side of the oxide film layer opposite to the support substrate. [Claim 20] A method for manufacturing a composite substrate according to claim 19, 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 21] A method for manufacturing a composite substrate according to claim 19, 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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