Composite substrate and production method therefor

Abstract

Provided is a composite substrate capable of exhibiting superior electrical properties. A composite substrate according to an embodiment of the present invention comprises, in the given order: a crystalline support substrate; a modified layer in which the crystallinity of the support substrate has been modified; and an active layer. The oxygen concentration of the support substrate is less than 1.0×1018 atoms / cm3.

Description

Composite substrate and method for manufacturing the same 【0001】 The present invention relates to a composite substrate and a method for manufacturing the same. 【0002】 Information and communication equipment utilizes functional elements such as surface acoustic wave elements (e.g., SAW filters) that utilize surface acoustic waves, and electro-optic elements (e.g., optical modulators) that can change the phase of light, in order to extract electrical signals of arbitrary frequencies. In recent years, the amount of data transmitted in the field of information and communication equipment has increased rapidly, and there is a demand for higher performance functional elements. Functional elements include, for example, composite substrates having a piezoelectric layer and a support substrate, and in order 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 described in Patent Document 1 is provided as a trap layer to capture 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 charge trapping layer is provided on the support substrate as in Patent Document 1, the electrical resistivity in the support substrate may decrease when forming the charge trapping layer and / or after formation during a predetermined process. 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. Furthermore, in Patent Document 1, when providing a charge trapping layer on the support substrate, chemical vapor deposition (CVD) is used for the formation of the charge trapping layer, and silane (SiH) is used. ４ Because it requires the use of highly active and reactive gases such as those mentioned above, special equipment is needed to ensure safety, which results in increased costs. 【0004】 Patent No. 6612872 【0005】 In view of the above, the main object of the present invention is to provide a composite substrate that can have excellent electrical properties. 【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 are modified, and an active layer. In the composite substrate, the oxygen concentration in the support substrate is 1.0 × 10 １８ atoms / cm ３[1] The composite substrate described in [1] above may have an electrical resistivity of more than 3 kΩ·cm. [2] The composite substrate described in [1] above may have an electrical resistivity of more than 3 kΩ·cm. [3] The composite substrate described in [1] or [2] above may have a p-type region. [4] The composite substrate described in any of [1] to [3] above may have a surface with a plane orientation (100). [5] The composite substrate described in any of [1] to [4] above may have an area ratio of the modified layer to the total area in a plan view of 40% or more. [6] The composite substrate described in any of [1] to [5] above may have an area ratio of the modified layer of less than 100%. [7] The composite substrate described in any of [1] to [5] above may have an area ratio of the modified layer to the total area in a plan view of 40% or more. [8] The composite substrate described in [7] above may have an area ratio of less than 100%. [9] The composite substrate described in [1] to [5] above may have an area ratio of less than 100%.

[10] The composite substrate described in [7] above may have an area ratio of less than 10 nm.

[11] The composite substrate described in [7] above may have an area ratio of less than 100%.

[12] The composite substrate described in [1] above may have an area ratio of less than 100%.

[13] The composite substrate described in [7] above may have an area ratio of less than 100%.

[14] The composite substrate described in [7] above may have an area ratio of less than 100%.

[15] The composite substrate described in any of [1] above may have an area ratio of less than 100%.

[16] The composite substrate described in [7] above may have an area ratio of less than 100%.

[17] The composite substrate described in any of [1] above may have an area ratio of less than 100%. [18 [9] In the composite substrate described in [7] or [8] above, the plurality of modified portions may be formed at intervals from each other.

[10] In the composite substrate described in any of [5] to [9] above, a plurality of electrodes may be formed at intervals on the active layer.

[11] In the composite substrate described in any of [1] to

[10] above, the modified layer may comprise, in order from the support substrate side, a first modified layer and a second modified layer. The second modified layer may include an amorphous structure of the same type of element as the elements constituting the support substrate, and the first modified layer may include the same type of element as the elements 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 may be different from each other.

[12] In the composite substrate described in

[11] above, the first modified layer may include an amorphous structure and a crystalline structure.

[13] In the composite substrate described in

[11] or

[12] above, the amorphous structure of the first modified layer may include a region that is located on the support substrate side of the crystalline structure in the thickness direction.

[14] In the composite substrate described in any of

[11] to

[13] above, the first modified layer may include a region that is partially strained.

[15] In the composite substrate according to any of [1] to

[10] above, the band gap of the active layer may be 2.5 eV or more.

[16] In the composite substrate according to any of [1] to

[15] above, the active layer may have an oxide film layer.

[17] In the composite substrate according to

[16] above, the oxide film layer may be a thermal oxide film.

[18] In the composite substrate according to any of [1] to

[17] above, the active layer may include a functional layer.

[19] In the composite substrate according to any of [1] to

[18] above, the active layer may include an oxide film layer and a functional layer in that order from the support substrate side.

[20] In the composite substrate according to

[19] above, the active layer may further include a bonding layer between the oxide film layer and the functional layer.

[21] In the composite substrate according to

[19] or

[20] above, the active layer may further include an intermediate layer between the oxide film layer and the functional layer.

[22] In the composite substrate described in any of [1] to

[21] above, the surface roughness Ra of the surface of the active layer opposite to the modified layer may be 1 nm or less.

[23] 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 described in any of [1] to

[22] above, wherein the composite substrate is crystalline and has a density of 1.0 × 10. １８ atoms / cm ３ The process includes, in this order, preparing a support substrate that is less than [a certain value], forming an active layer on at least one surface of the 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. 【0007】 According to embodiments of the present invention, a composite substrate with excellent electrical properties can be realized. 【0008】This is a schematic cross-sectional view showing the general configuration of a composite substrate according to one embodiment of the present invention. This is a plan view cross-sectional view of the composite substrate of Figure 1A along the line B-B. This is a schematic cross-sectional view showing the general configuration of the composite substrate of Figure 1A when electrodes are provided. This is a schematic cross-sectional view showing the general configuration of a composite substrate according to another embodiment of the present invention. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to one embodiment an explanatory diagram of the pattern of the coplanar waveguide used to evaluate the high-frequency characteristics in the embodiment. This is a diagram showing an enlarged view of the area enclosed by the dashed line in Figure 3A. This is a TEM image showing a cross-section of a part of the modified portion of a composite substrate that can be obtained in one embodiment. This is a TEM image showing a cross-section of a portion of the composite substrate in Figure 4A. This is a TEM image showing a further magnified cross-section of a portion of Figure 4A (near the modified layer), illustrating an example of a region composed of crystalline structure in the modified layer. This is a TEM image showing a further magnified cross-section of a portion of Figure 4C. 【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 the Composite Substrate FIG. 1A is a schematic side cross-sectional view showing the schematic configuration of a composite substrate according to one embodiment of the present invention. The composite substrate 100 in the illustrated example has a support substrate 10, a modified layer 30, and an active layer 20 in this order. The support substrate 10 typically has crystallinity. The support substrate is typically a single crystal substrate or a polycrystalline substrate. A modified layer 30 in which the crystallinity of the surface of the support substrate 10 is modified is formed between the support substrate 10 and the active layer 20. As will be described later, the modified layer 30 can typically be formed by irradiating the surface of the support substrate with a laser from the active layer side after the active layer is formed on the support substrate. The "modified layer" means a layer having a region in which density, refractive index, mechanical strength, physical properties, etc. are modified with respect to the support substrate. For example, the modified layer is composed of the same kind of elements as the support substrate, but is modified so as to have properties different from those of the support substrate in the above characteristics. The modified layer is typically a layer in which the crystallinity of the surface of the support substrate is modified. In the illustrated example, the modified layer 30 and the active layer 20 are formed in this order only on one surface (the upper surface in the illustrated example) of the support substrate 10, but the modified layer and the active layer may be formed in this order only on the other surface (the lower surface) of the support substrate, or the modified layer and the active layer may be formed in this order on both the upper and lower surfaces of the support substrate. Note that the modified layer may be formed before the active layer is formed. Further, the method for forming the modified layer is not limited to the method by laser irradiation as long as the object of the present invention can be achieved, and an appropriate method as the method for forming the modified layer can be adopted. 【0011】 In the composite substrate of the embodiment of the present invention, the oxygen concentration of the support substrate is typically 1.0×10 １８ atoms / cm ３ or less, preferably 9.0×10 １７ atoms / cm ３ or less, more preferably 8.0×10 １７ atoms / cm ３ or less, still more preferably 5.0×10 １７ atoms / cm ３ or less, even more preferably 1.0×10 １７ atoms / cm ３The following, and particularly preferably 5.0 × 10 １６ atoms / cm ３ The following applies: The lower limit of the oxygen concentration in the support substrate is, for example, 1.0 × 10⁻⁶. １５ atoms / cm ３ If the oxygen concentration of the support substrate in a composite substrate is below the above upper limit, the electrical properties of the composite substrate can be significantly improved. If the oxygen concentration of the support substrate in a composite substrate is above the above lower limit, the mechanical strength (typically toughness and impact resistance) of the support substrate in the composite substrate can be maintained well. For example, if it is above this lower limit, cracking and / or chipping of the support substrate can be suppressed. When a laser is irradiated onto the support substrate, the crystals constituting the support substrate melt, the atomic arrangement of the crystal structure collapses (changes), and then an amorphous structure can be formed by cooling. In this process, heat may be generated in the irradiated part of the support substrate due to laser irradiation. Also, when oxidation is performed to form an oxide film on the support substrate, the support substrate may be heated. Thus, since providing a charge trapping layer on the support substrate involves the generation of heat within the support substrate or heating of the support substrate, a thermal load may be applied, and donors (carriers of negative charge caused by oxygen; also called oxygen donors) may be released from the oxygen in the support substrate. In this case, the electrical resistivity of the support substrate may decrease. As a result, even if the charge trapping layer has the desired electrical resistivity, when the electric field reaches the interior of the support substrate, electrical losses due to the support substrate may become large. In the case of a composite substrate, the oxygen concentration in the support substrate is 1.0 × 10⁻⁶. １８ atoms / cm ３ If the values ​​are above this level, such electrical losses tend to occur significantly, making it difficult to obtain the desired electrical characteristics of the composite substrate. In contrast, in the composite substrate according to the embodiment of the present invention, the oxygen concentration in the support substrate is 1.0 × 10⁻⁶. １８ atoms / cm ３By keeping the value below a certain level, the amount of oxygen donor released from the support substrate can be reduced. Therefore, the phenomenon of excessive increase in conductivity (i.e., electrical loss) in the support substrate of the composite substrate can be suppressed. As a result, the composite substrate according to the embodiment of the present invention may have significantly superior electrical properties. According to the composite substrate according to the embodiment of the present invention, the conductivity in the support substrate is maintained, and the charge trapping effect of the modified layer can be effectively exhibited. 【0012】 In embodiments of the present invention, 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, as described above. 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 (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 embodiments of the present invention, the modified layer can be provided between the support substrate and the active layer after the active layer has been formed on the support substrate. 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, the performance of the modified layer is less likely to be impaired by the formation of the active layer. Thus, the modified layer in the composite substrate according to embodiments of the present invention can maintain its function as a charge trapping layer more effectively. Furthermore, according to embodiments 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 the CVD method, so cost increases 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. 【0013】The modified layer can typically be composed of multiple modified sections. In the illustrated example of the composite substrate 100, the modified layer 30 is composed of multiple modified sections 301. The modified layer does not necessarily have to be formed in a strictly layered manner. The modified layer can have an area ratio of the modified layer to the total area in a plan view (hereinafter sometimes simply referred to as the area ratio of the modified layer) insofar as it can achieve the objectives of the present invention. Specifically, the area ratio of the modified layer is preferably 40% or more. In other words, it is preferable that the support substrate is modified by 40% or more of the entire surface of the support substrate in a plan view. The area ratio of the modified layer is more preferably 50% or more, even more preferably 60% or more, and may be, for example, 80% or more, or 90% or more. If the area ratio of the modified layer is 40% or more, the effect of the modified layer as a charge trapping layer can be more favorably obtained. On the other hand, the area ratio of the modified layer may be, for example, 100% or less, less than 100%, or 99.0% or less. If the area ratio of the modified layer is less than 100%, the bonding strength between the support substrate and the active layer, between the support substrate and the modified layer, and between the modified layer and the active layer can be increased. In the case where the area ratio of the modified portion is less than 100%, the modified layer may be formed over substantially the entire surface of the support substrate. "Formed over substantially the entire surface" includes the case where the entire surface of the support substrate is the modified portion (i.e., the area of ​​the modified portion is 100%), but otherwise it is formed over the entire surface as far as it is concerned. The effects of the present invention become more pronounced when the area ratio of the modified layer is within the above range. 【0014】The modified layer 30 may be formed partially, for example, as shown in Figures 1A and 1B, or it may be formed substantially over the entire support substrate, for example, as shown in Figure 1D (described later). For example, in Figure 1A, as described above, 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. Also, for example, the thickness of the plurality of modified portions may be the same, some may be different, or all of the modified portions may have random thicknesses. The thickness, shape, regularity, etc. of the modified layer and the plurality of modified portions will be described in detail in Section A-3. 【0015】 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. 【0016】 The composite substrate may have multiple electrodes formed at intervals on the active layer (see Figure 1C). In the illustrated example, the composite substrate 101 has multiple (three in the drawing) electrodes 50 formed at intervals on the active layer 20. The electrodes 50 may be formed on, for example, an oxide film layer, a functional layer, or another layer constituting the active layer (e.g., an intermediate layer). The number of electrodes and the spacing between multiple electrodes can be appropriately set according to the purpose. However, electrodes are not an essential component of the composite substrate and may be omitted if necessary. 【0017】 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 comprising a first modified layer 31 and a second modified layer 32. The support substrate 10 and the active layer 20 may have similar configurations to the support substrate 10 and the active layer 20 in the composite substrates 100 to 101 of Figures 1A to 1C, respectively. The first modified layer 31 and the second modified layer 32 can typically function as charge trapping layers. The first modified layer 31 preferably contains elements of the same type as those constituting the support substrate 10. The second modified layer 32 preferably contains an amorphous structure of elements of the same type as those constituting the support substrate 10. Furthermore, the first modified layer and the second modified layer preferably have different atomic arrangement regularities. The atomic arrangement regularity is a crystallographic indicator for determining crystalline and / or amorphous structures. Furthermore, since the regularity of crystalline structures can differ depending on the combination and state of atomic arrangement, "crystalline structure" may include multiple crystalline states. Similarly, "amorphous structure" may include multiple amorphous states. The crystalline and / or amorphous nature of the support substrate and the modified layers (first modified layer and 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 microscope). Differences in regularity in amorphous structures can also be confirmed by measuring Raman scattering spectroscopy using Raman spectroscopy and observing the short-range order. The fact that the first and second modified layers may contain the same elements as those constituting the support substrate can be confirmed by elemental analysis and compositional analysis using EDX (energy-dispersive X-ray fluorescence). Similar to Figures 1A to 1C, 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 the performance of charge traps better, contribute to improved performance when used in devices for high-frequency and / or harmonic applications, and can be manufactured at low cost. 【0018】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 as an active layer, for example, between the oxide film layer and the functional 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 layers 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. 【0019】 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 the composite substrate (without a functional layer) may be, for example, 100 μm to 1500 μm. The total thickness of the composite substrate with a functional layer may be, for example, 100 μm to 1500 μm. 【0020】 The components of the composite substrate will be described in detail below. A-2. Support Substrate As the support substrate, any suitable substrate can be used insofar as it can achieve the objectives of the present invention. Typically, the support substrate is crystalline. The support substrate may be composed of, for example, only a single crystal structure, or only a polycrystalline structure, or a combination of a single crystal structure and a polycrystalline structure. Typically, the support substrate may be composed of a semiconductor material. Preferably, the material constituting the support substrate is silicon or germanium. Preferably, the support substrate is single crystal silicon, polycrystalline silicon, single crystal germanium, or polycrystalline germanium. When the support substrate is single crystal silicon or single crystal germanium, a polycrystalline layer may be formed on the surface. Therefore, the support substrate may be, for example, a single crystal substrate or a polycrystalline substrate. 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. 【0021】 As described above, the oxygen concentration in the support substrate is typically 1.0 × 10⁻⁶. １８ atoms / cm ３ It is less than 9.0 × 10 １７ atoms / cm ３ The following, and more preferably 8.0 × 10 １７ atoms / cm ３ The following, and more preferably 1.0 × 10 １７ atoms / cm ３ The following, and more preferably 1.0 × 10 １７ atoms / cm ３ The following, and particularly preferably 5.0 × 10 １６ atoms / cm ３ The following applies: The oxygen concentration of the support substrate is, for example, 1.0 × 10⁻⁶. １５ atoms / cm ３ The above, preferably 5.0 × 10 １５ atoms / cm ３ The above is more preferable, and more preferably 1.0 × 10 １６ atoms / cm ３ The above is a summary. The oxygen concentration of the support substrate in a composite substrate can be measured by secondary ion mass spectrometry (SIMS). When the effect of knock-on effects is anticipated in the SIMS measurement of the oxygen concentration of the support substrate in a composite substrate, the measurement may be performed after removing the active layer (oxide film layer and / or functional layer) from the composite substrate, or it may be measured by backside SIMS. The support substrate can be manufactured by any suitable method. Examples of methods for manufacturing the support substrate include the CZ method (Czochralski method), FZ method (float zone method), and MCZ method (magnetic field applied Czochralski method). From the viewpoint of controlling the oxygen concentration to a desired concentration, the support substrate can preferably be manufactured by the FZ method or the MCZ method. The oxygen concentration of the support substrate (single layer) can be measured by the FT-IR method, for example, in accordance with ASTM F121-1979. 【0022】The surface orientation of the support substrate may be appropriate depending on the purpose (e.g., the function of the device to which the composite substrate is applied). In one embodiment, the support substrate preferably has a surface with surface orientation (100). With such a configuration, electrical losses in the composite substrate can be further suppressed. Specifically, the amount of fixed charge on the support substrate may differ depending on the surface orientation, and the amount of fixed charge is less with surface orientation (100) compared to surface orientations (110) and (111). Therefore, if the support substrate has a surface with surface orientation (100), electrical losses can be further reduced. As a result, the electrical characteristics of the composite substrate can be further improved. Also, when an active layer (typically an oxide film layer) is formed by thermal oxidation of the support substrate, the oxidation rate with surface orientation (100) is faster than the oxidation rate with surface orientations (110) and (111), which has the advantage of superior productivity. 【0023】 The electrical resistivity of the support substrate is preferably greater than 3 kΩ·cm, preferably 3.5 kΩ·cm or more, and preferably 4 kΩ·cm or more. The upper limit of the electrical resistivity of the support substrate is, for example, preferably 20 kΩ·cm. If the electrical resistivity of the support substrate is within this range, electrical losses will be less likely to occur, and the composite substrate may have better electrical characteristics. The electrical resistivity of the support substrate can be obtained by spreading resistance measurement (SRP). The electrical resistivity of the modified layer can also be measured in the same manner. 【0024】The support substrate preferably has a p-type region. A p-type region is a region that contains many positive charges (holes) as carriers (charge carriers). Near the interface between the support substrate and the active layer, and / or near the interface between the active layer and the modified layer, fixed charges with positive charges due to unbonded hands near such interfaces are formed, and carriers may concentrate from the support substrate side toward the interface to cancel out such charges. As a result, the effective electrical resistivity of the support substrate decreases, which may increase electrical losses. In contrast, if the support substrate has a p-type region, a depletion layer with almost no charge can be formed in the support substrate through bonding between holes and free electrons (charge recombination). As a result, further charge recombination cannot occur in the depletion layer, and electrical losses in the support substrate can be further suppressed. Therefore, if the support substrate has a p-type region, the electrical properties of the composite substrate can be further improved. The above effect can be particularly pronounced when the area ratio of the modified layer is less than 100%. The modified layer may also have a p-type region. The effects described above can also occur when the modified layer has a p-type region. Therefore, the presence of a p-type region in the modified layer can contribute to further improvement of the electrical properties of the composite substrate. The p-type region can be formed by adding any appropriate acceptor impurity to the support substrate. When silicon is used as the support substrate, examples of acceptor impurities include boron, aluminum, gallium, and indium. The n-type region is a region that contains many negatively charged (free electrons) as carriers. The n-type region can be formed by adding any appropriate donor impurity to the support substrate. Examples of donor impurities include phosphorus, arsenic, and antimony. The carrier type of the support substrate can be measured and confirmed by the hot probe method. The carrier type of the modified layer can also be measured in the same way. 【0025】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. 【0026】 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. 【0027】 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" means arithmetic mean roughness (Ra). The arithmetic mean roughness (Ra) can be obtained by measuring with an atomic force microscope (AFM) in a 10 μm × 10 μm field of view, in accordance with JIS B0601:2013. 【0028】A-3. Modified Layer In the composite substrate according to the embodiment of the present invention, the modified layer is formed between the support substrate and the active layer (e.g., an oxide film layer and / or a functional layer) such that the area ratio of the modified layer is an appropriate value depending on the purpose. The area ratio of the modified layer is preferably 40% or more, more preferably 50% or more, even more preferably 60% or more, and may be, for example, 80% or more, or 90% or more. The area ratio of the modified layer is, for example, 100% or less. That is, the modified layer may be formed over the entire surface of the support substrate. The area ratio of the modified layer may be, for example, less than 100%, or for example, 99.0% or less. As described below, 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 may be distinguished from the support substrate and the active layer by the presence or absence of crystallinity, differences in crystallinity, etc. 【0029】 As described above, the modified layer may preferably consist of multiple modified sections. The plan view shape of the multiple modified sections may be appropriately chosen, for example, so that the area ratio of the modified layer to the total area in plan view is within a desired range. The plan view shape of the modified section may be set to any shape by appropriately adjusting the laser beam profile, for example. The plan view shape of the modified section may be, for example, circular or elliptical, or polygonal, 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. 【0030】The multiple modified sections may preferably be formed with spacing between them. With such a configuration, electrical losses in the composite substrate can be suppressed more effectively. Therefore, this 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. 【0031】 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 appropriate 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°. 【0032】 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 4A shows a schematic explanatory diagram of a TEM image observing the center to the edge of a modified portion (part) in the modified layer of a composite substrate that can be obtained by one embodiment of the present invention. 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, preferably, each of the multiple modified portions 301 is formed such that the thickness dr on the edge side of the modified portion 301 is 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. 【0033】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, 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 with 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. 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 using the methods and conditions described in the embodiments below. 【0034】 (First Modified Layer and Second Modified Layer) 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 portions (multiple modified portions) described above. As an example, Figure 4B shows a schematic diagram of a partial TEM image observing the modified layer in a composite substrate that can be obtained by one embodiment of the present invention. In the illustrated example, at least an amorphous region (modified layer 30) is formed between the active layer 20 and the support substrate 10, and the modified layer 30 has a first modified layer 31 and a second modified layer 32, and each of the first modified layer 31 and the second modified layer 32 is composed of multiple modified portions 301. 【0035】The first modified layer 31 preferably contains elements of the same type as those constituting the support substrate 10. In one embodiment, the first modified layer 31 preferably includes an amorphous structure and a crystalline structure. A 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. As an example, Figure 4C shows a schematic diagram based on a TEM image of the composite substrate (near the modified layer) of Figure 4B, observed in a more magnified view. Figure 4D shows a schematic diagram based on a TEM image of a part of Figure 4C, observed in a more magnified view. In the illustrated example, regions of crystalline structure and regions of amorphous structure are mixed, and there are multiple regions of amorphous structure closer to the support substrate 10 than regions of crystalline structure. 【0036】 The first modified layer 31 preferably includes a region having strain in part. 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 caused by 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 having 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. In the illustrated example (see Figures 4C and 4D), darkened regions (i.e., regions having strain) are formed in the area on the support substrate 10 side (first modified layer 31) and in the area indicated by the dashed line. 【0037】As described above, the first modified layer preferably contains elements of the same type as those constituting the support substrate. The second modified layer preferably 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. 【0038】 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. 【0039】 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. 【0040】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. The upper limit of the first modified layer 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 (overall) 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. The upper limit of the second modified layer 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 well, and harmonic components can be reduced well. If the thickness of the second modified layer is 1000 nm or less, it can contribute to reducing the cost in manufacturing the composite substrate. 【0041】 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. 【0042】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 appropriate 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 (approximately 9.0 eV), silicon carbide (approximately 2.9 eV), aluminum nitride (approximately 6.3 eV), gallium nitride (approximately 3.4 eV), gallium oxide (approximately 4.5–4.9 eV), gallium sulfide (approximately 2.5 eV), beryllium oxide (10.6 eV), magnesium oxide (approximately 7.8 eV), zinc oxide (approximately 3.4 eV), and zinc sulfide (approximately 3.6 eV). The values ​​in parentheses for the above materials represent the band gap. 【0043】 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 contained in piezoelectric materials listed later (see Section A-4-2). 【0044】 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. 【0045】A-4-1. Oxide film layer The oxide film layer may be a layer composed of any suitable oxide. The oxide film layer may be composed of, for example, an oxide of the semiconductor material constituting the support substrate. For example, if the semiconductor material is silicon or germanium, the oxide film layer may contain silicon oxide or germanium oxide. 【0046】 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), ion beam-assisted deposition (IAD), and other physical vapor deposition (PVD), as well as 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. 【0047】 Any appropriate thickness can be used for the oxide film layer. For example, the thickness of the oxide film layer may be 0.05 μm (50 nm) or more and 30 μm or less. Preferably, the thickness of the oxide film layer may be 0.1 nm or more and 25 μm or less, and preferably 1 nm or more and 20 μm or less. 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. 【0048】 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 the composite substrate. 【0049】A-4-2. Functional Layer The functional layer is a layer that can constitute the active layer and can be provided as needed. The functional layer can be composed of any material having appropriate functionality. Examples of functional materials include piezoelectric materials, materials with electro-optical effects, and semiconductor materials. 【0050】 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 suitably used in surface acoustic wave elements such as SAW filters. Any piezoelectric material can be used as the material constituting the piezoelectric layer. 【0051】 Preferably, LiAO is used as the piezoelectric material. ３ A single crystal having the following composition may be used. Here, A is one or more elements selected from niobium and tantalum. Specifically, LiAO ３ Lithium niobate (LiNbO) ３ ) may also be lithium tantalate (LiTaO ３ ) may be lithium niobate and / or lithium tantalate solid solution. When lithium niobate and / or lithium tantalate are used, MgO-doped or stoichiometric crystals may be used to suppress photodamage. 【0052】 Another example of a piezoelectric material is potassium titanate phosphate (KTiOPO). ４ :KTP), potassium niobate lithium (K ｘ Li （１－ｘ） NboO ２ , 0 ≤ x ≤ 1: KLN), potassium niobate (KNbO ３ :KN), potassium tantalate / niobate (KNb ｘ Ta （１－ｘ） O ３ Examples include 0≦x≦1: KTN), silicon, quartz, silica, silicon carbide, gallium nitride, indium phosphide, and lead zirconate titanate (PZT). 【0053】If the piezoelectric material is lithium tantalate, the functional layer is, for example, aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is the case, the direction rotated 32° to 55° (for example, 42°) from the Y-axis toward the Z-axis is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the angle corresponds to (180°, 58° to 35°, 180°) in Euler angle notation. 【0054】 When the piezoelectric material is lithium niobate, the functional layer is, for example, aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is the case, the direction rotated from the Z-axis toward the -Y-axis by 0° to 40° (for example, 37.8°) is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the X-axis (crystal axis) of the piezoelectric material is aligned with the propagation direction of the surface acoustic wave (X １ When this is the case, the direction rotated 40° to 65° from the Y-axis toward the Z-axis is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the angle corresponds to (180°, 50° to 25°, 180°) in Euler angle notation. 【0055】 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 electro-optic 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. 【0056】When a composite substrate is used in an electro-optical device (e.g., a thin-film LN optical modulator), the materials that exhibit electro-optical effects are preferably lithium niobate, lithium tantalate, lithium niobate-lithium tantalate, KTP (potassium titanate phosphate), and PZT (lead zirconate titanate). Specifically, as the material that exhibits electro-optical effects, for example, X-cut and / or Z-cut lithium niobate can be used. When using lithium niobate and / or lithium tantalate, MgO-doped or stoichiometric crystals can be used to suppress photodamage. 【0057】 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). 【0058】 The thickness of the functional layer can be set to any appropriate thickness depending on the method of use 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. 【0059】 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. 【0060】A-4-3. Intermediate Layer The intermediate layer is an optional layer that may be 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 appropriate 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. 【0061】 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. 【0062】 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. 【0063】 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. 【0064】 A-5. Electrodes In one embodiment, the composite substrate may have 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 film layer, a functional layer, or an intermediate layer. For example, if the active layer includes an oxide film 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 will be omitted. 【0065】 B. Method for Manufacturing Composite Substrates B-1. Overview of the Method for Manufacturing Composite Substrates The method for manufacturing composite substrates according to the embodiment of the present invention has crystalline properties and an oxygen concentration of 1.0 × 10 １８ atoms / cm３ The method includes, in this order: preparing a support substrate having an oxygen concentration of less than 1.0 × 10⁻¹⁰; forming an active layer on at least one surface of the 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 the 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). As described above, in the manufacturing method of a composite substrate according to the embodiment of the present invention, the support substrate is crystalline and has an oxygen concentration of 1.0 × 10⁻¹⁰ １８ atoms / cm ３ A support substrate of less than 100 mm is used. By using such a support substrate, even when a modified layer is formed by laser irradiation, the amount of heat generation and / or the release of oxygen donors in the support substrate due to heating can be reduced, thereby suppressing excessive changes in the conductivity of the support substrate (i.e., electrical loss). Therefore, the composite substrate that can be obtained by the manufacturing method of the composite substrate according to the embodiment of the present invention may have significantly superior electrical properties. Furthermore, as described above, according to the manufacturing method of the 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, 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 the embodiment of the present invention, a composite substrate can be manufactured at low cost. 【0066】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 this 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 with a different regularity of atomic arrangement from the second modified layer can be formed, which consists 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. 【0067】 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. Sections B-2 to B-6 describe manufacturing methods for cases where the active layer in 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. 【0068】 B-2. Formation of the Oxide Film Layer First, as shown in Figure 2A, a suitable substrate is prepared as the support substrate 10, insofar as it can achieve the objective of the present invention. In the embodiment of the present invention, as described above, the support substrate is crystalline and has an oxygen concentration of 1.0 × 10 １８ atoms / cm ３ Prepare a support substrate with an oxygen concentration of less than 9.0 × 10. １７atoms / cm ３ The following, and more preferably 8.0 × 10 １７ atoms / cm ３ The following, and more preferably 1.0 × 10 １７ atoms / cm ３ The following, and more preferably 1.0 × 10 １７ atoms / cm ３ The following, and particularly preferably 5.0 × 10 １６ atoms / cm ３ The following applies: The oxygen concentration of the support substrate is, for example, 1.0 × 10⁻⁶. １５ atoms / cm ３ The above, preferably 5.0 × 10 １５ atoms / cm ３ The above is more preferable, and more preferably 1.0 × 10 １６ atoms / cm ３ That concludes the explanation. The substrate and materials that can be used as the support substrate 10 are as described in section A-2 above. 【0069】 The support substrate 10 may, if necessary, have one side (10a or 10b) or both sides (10a and 10b) subjected to any suitable smoothing treatment. 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. 【0070】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. 【0071】 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 to oxidize the support substrate. The oxidizing atmosphere can be prepared 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. 【0072】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 or more and 30 μm or less, 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. 【0073】 The oxide film layer may be subjected to a smoothing treatment as needed. When smoothing is performed, for example, as described above, the oxide film layer may be polished until its surface roughness Ra is 1 nm or less. The smoothing treatment 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 of the oxide film layers may be smoothed. Note that the smoothing treatment of the oxide film layer is optional, and it may be used as is in the next step without any smoothing treatment. 【0074】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, as described above, a modified layer is typically formed between the support substrate and the active layer, and preferably, a first modified layer and a second modified layer can be formed in order from the support substrate. 【0075】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 micrometers) 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 millimeters). Furthermore, by shifting the laser focus in this way (i.e., not focusing it), ablation of the target object (e.g., support substrate, active layer) (thermal decomposition by laser irradiation, etc.) 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 or more and 100 ps or less. The laser frequency may be, for example, 1 kHz or more and 1 MHz or less. 【0076】 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 supporting substrate. Specifically, the laser wavelength is λ [nm] and the band gap of the oxide film layer is E ｇ１ [eV], the band gap of the support substrate is E ｇ２ If we set it to [eV], then E ｇ１ ≥ 1240 / λ, and E ｇ２ It is preferable that the wavelength satisfies the relationship -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 preferably used as the laser wavelength. 【0077】 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: 【0078】 When irradiating with a laser, it is preferable to irradiate at intervals in at least one direction within the plane of the support substrate. The laser irradiation pitch (interval) when irradiating with a laser can be any appropriate interval, for example, depending on the area of ​​the target modified layer. 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 interval of the modified portion (modified portion) of the support substrate surface in a plan view. The minimum value of the interval of the modified portion 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 interval of the region may be, for example, 0.1 μm. Within such a range, the effects of the present invention may become more pronounced. When irradiating with a laser with a small irradiation area (typically a pulsed laser), it is not necessary to irradiate the entire surface of the support substrate. For example, depending on the purpose and type of the device (functional element) to which the composite substrate is applied, the laser may be irradiated only to the parts of the device where electrical loss may occur. More specifically, in a high-frequency device, for example, the laser may be irradiated only in the area where high frequency is to be applied and where electrodes are to be installed (the area where electrodes are to be installed). In this way, the processing time due to laser irradiation can be reduced. However, the type of laser, wavelength, pulse width, frequency, etc., are not limited to those mentioned above. 【0079】 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. 【0080】 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 the method 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. In this way, a composite substrate 100 can be obtained, as shown in Figure 2D (Figure 1A), comprising the support substrate 10, the modified layer 30, and the active layer 20 (oxide film layer 21) in this order. 【0081】 B-4. The composite substrate for forming the functional layer may have a functional layer on the oxide layer, as another layer that can constitute the active layer, in addition to the support substrate, the modified layer, and the oxide layer that can constitute the active layer. The functional layer can be fabricated, for example, by bonding a functional substrate to the oxide layer and, if necessary, thinning the functional substrate to any appropriate thickness. 【0082】 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 described in Section A-5. 【0083】 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 can 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. 【0084】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. 【0085】 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. 【0086】 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. 【0087】The functional layer can be formed, for example, by polishing a functional substrate to make it thinner. The functional layer can preferably be formed by thinning the functional substrate until its thickness becomes 1000 nm or less after bonding the functional substrate to the oxide film layer. Note that the above thinning process can be appropriately omitted according to the type of the functional substrate, the conditions of laser irradiation, etc. As described above, a composite substrate 103 having a functional layer 22 on an oxide film layer 21 as an active layer 20 as shown in FIG. 2H can be fabricated. When fabricating a composite substrate provided with a functional layer in this way, for example, even if the functional substrate has absorbability to laser light, there is an advantage that a modified layer can be formed between the oxide film layer and the support substrate before providing the functional layer. 【0088】 B-5. Bonding of the intermediate layer The intermediate layer is an arbitrary layer provided as a layer that can constitute an active layer as necessary in the composite substrate as described in Item A-6 above. The intermediate layer (when 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 forming an arbitrary appropriate material on the surface of the oxide film layer and / or the surface of the functional substrate. Any appropriate method can be adopted as the film formation method of the intermediate layer. Examples of the film formation method include a sputtering method, a CVD method, and an ion assist evaporation method. For example, the intermediate layer can be fabricated by forming the dielectric material described in Item A-6 on an object (oxide film layer and / or functional substrate) by the sputtering method. The surface (bonding surface) of the intermediate layer may be subjected to a smoothing process as necessary. When the intermediate layer is subjected to a smoothing process, it can be polished until the surface roughness Ra of the bonding surface of the intermediate layer becomes, for example, 1 nm or less. An intermediate layer (for example, a SiO ２ layer) can be formed on the functional substrate, and a bonded body can be formed by bonding the intermediate layer and the oxide film layer. The bonding method can be the same as the bonding method of the oxide film layer and the functional substrate described in Item B-4 above. 【0089】B-6. Other support substrates (including semiconductor materials) and functional substrates (including functional materials) 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. 【0090】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 depending on the purpose. 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 adopted 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 10 hours. The heat treatment may involve holding the heating temperature at the highest temperature (e.g., 600°C) for 10 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. The atmosphere for the heat treatment may be, for example, an atmospheric atmosphere, an inert gas atmosphere such as helium, nitrogen and / or argon, a hydrogen atmosphere, or a vacuum atmosphere. Preferably, the atmosphere for the heat treatment may be an inert gas atmosphere such as helium, nitrogen and / or argon, a hydrogen atmosphere, or a vacuum atmosphere. If an interface exists between the support substrate and the active layer, the increase in fixed charge can be suppressed under the above atmospheres. Therefore, the electrical properties of the composite substrate can be maintained well. 【0091】B-7. Modifications In sections B-2 to B-6 above, as described above, a specific example of a method for manufacturing a composite substrate according to an embodiment of the present invention was explained 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 it with a laser from the functional layer side. Alternatively, a modified layer may be formed by irradiating it with a laser from the functional substrate side, and then a functional layer may be formed by thinning the functional substrate. 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. 【0092】 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. 【0093】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 subjected to a smoothing treatment as needed, a modified layer (preferably a first modified layer and a 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 (preferably a first modified layer and a second modified layer), and a functional layer can be obtained. 【0094】 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. 【0095】C. Functional Elements The composite substrate according to the embodiment of the present invention has improved electrical characteristics as described above. Therefore, the composite substrate according to the embodiment of the present invention can maintain charge trapping performance and improve high-frequency and / or harmonic characteristics, and can therefore 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 the composite substrate, the composite substrate can be used as a surface acoustic wave element. A surface acoustic wave element typically has the above-mentioned composite substrate and electrodes (comb-type electrodes) provided on the piezoelectric layer side of the composite substrate. Such a surface acoustic wave element can be suitably used, for example, as a SAW filter in communication equipment such as mobile phones. Also, for example, when an electro-optic layer is provided as a functional layer in the active layer of the 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. The optical modulation device is, for example, a Mach-Zehnder type optical modulator, which modulates light propagating through an optical waveguide by applying a voltage to a Mach-Zehnder interferometer formed by 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. 【0096】 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. 【0097】 (1) Surface roughness Ra (arithmetic mean roughness (Ra)) Surface roughness Ra was measured in accordance with JIS B0601:2013 using an atomic force microscope (AFM) in a field of view of 10 μm × 10 μm. 【0098】(2) Evaluation: High-frequency characteristics Samples for evaluating the high-frequency characteristics of the composite substrates of Examples 1 to 3 and Comparative Example 1 were prepared by forming a coplanar waveguide (CPW) on the surface of the oxide film layer. The CPW was formed by the following procedure. A coplanar waveguide (CPW) was formed as an electrode by forming 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. A high-frequency probe (ACP40-GSG-150) was brought into contact with both ends of the coplanar waveguide in the longitudinal direction, and the S21 of the S-parameter was measured using a network analyzer (Agilent E5071C) manufactured by Keysight Technologies. Measurement S21 was performed at five points within the plane of each substrate. The measurement frequency was in the range of 0.01 GHz to 20 GHz, and the insertion loss was measured at 0.01 GHz intervals within that 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): At 20 GHz, the insertion loss is 0.45 dB / mm or less. B (Poor): At 20 GHz, the insertion loss is greater than 0.45 dB / mm. 【0099】For the composite substrates of Examples 4-5 and Comparative Example 2, 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 1.0 μm thick Au film was formed on the functional layer of the obtained composite substrate by a lift-off process to form a coplanar waveguide (CPW) as an electrode. 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. A high-frequency probe (ACP40-GSG-150) was brought into contact with both ends of the coplanar waveguide in the longitudinal direction, and the S21 of the S-parameter was measured using a network analyzer (Agilent E5071C) manufactured by Keysight Technologies. Measurement S21 was performed at five points within the plane of each substrate. The measurement frequency was in the range of 0.01 GHz to 20 GHz, and the insertion loss was measured at 0.01 GHz intervals within that 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): At 20 GHz, the insertion loss is 0.45 dB / mm or less. B (Poor): At 20 GHz, the insertion loss is greater than 0.45 dB / mm. 【0100】 [Examples 1-3 and Comparative Example 1] (Example 1) As a support substrate, a single-crystal silicon substrate (hereinafter simply referred to as silicon substrate) having an orientation flat portion (OF portion), a diameter of 4 inches, and a thickness of 230 μm was prepared. The oxygen concentration of the silicon substrate was 8.0 × 10⁻⁶ １７ atoms / cm ３ The carbon concentration is 3.0 × 10⁻⁶. １６ atoms / cm ３ The silicon substrate had an electrical resistivity of 3.0 kΩ·cm and an n-type carrier. The surface orientation was (100). The surface roughness Ra of the silicon substrate 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 about 1 nm had formed on the surface of the silicon substrate. 【0101】 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: The silicon substrate was placed in a chamber capable of supplying oxygen and water vapor, and while the inside of the chamber was heated to 700°C to 1200°C, an oxidizing atmosphere was created by supplying oxygen and water vapor, and wet oxidation was performed on the silicon substrate. Subsequently, the silicon substrate with oxide film layers formed on both sides was removed. The thickness of the oxide film layer was 4.5 μm on each side. 【0102】 Next, one side of the support substrate, which is used to form electrodes in the oxide film layer formed on both sides (the electrode formation surface), was smoothed by polishing using CMP (Chemical Polishing Machine) until the surface roughness Ra reached 0.5 nm. 【0103】 Next, laser irradiation was performed from the oxide film layer side of the electrode formation surface toward the silicon substrate. The laser irradiation conditions were as follows: Wavelength: 1030 nm Laser energy: 25 μJ (Laser density: 500 mJ / cm²) ２ Irradiation pitch (feed width): 50 μm Laser pulse width: 900 fs Frequency: 40 kHz Defocus: 2.0 mm A composite having an oxide film layer / modified layer / support substrate configuration was obtained by laser irradiation. 【0104】 Next, the surface of the oxide film layer of the composite was smoothed by polishing with CMP until the thickness of the oxide film layer was approximately 100 nm (Ra: approximately 0.2 nm). In this way, 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. The oxygen concentration and carbon concentration of the support substrate in the obtained composite substrate were measured and evaluated using Dynamic SIMS. The evaluation was performed at a depth of 2.0 μm from the surface of the support substrate (evaluation depth), and areas affected by the knock-on effect were excluded from the evaluation. The results are shown in Table 1. 【0105】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). Optical microscopy observation yielded a plan view image of the cross-section of the composite substrate including the modified layers. 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. Using image processing software, noise reduction was performed as needed, followed by monochrome processing and binarization to extract the modified region, and the area ratio (%) of the modified layer to the total area 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 200 kV and a total magnification of 1,000,000x, it was confirmed that at least an amorphous region (modified layer) was formed between the active layer (oxide film layer) and the silicon substrate, and that regions of crystalline and amorphous structures were mixed. The total depth (distance) of the modified layer was approximately 50 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. Furthermore, it was confirmed that the thickness dc in the center of the modified region was greater than the thickness dr at the edges, and that the thickness decreased from the center to the edges of the modified region. Furthermore, 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 200 kV and a total magnification of 2,000,000x. Upon magnification of the image to confirm the atomic arrangement, it was found that crystalline and amorphous structures coexisted, and multiple locations where the amorphous structure region was located closer to the silicon substrate than the crystalline structure region were confirmed. In addition, observation of the TEM image revealed the presence of darkened regions, i.e., regions with strain. Furthermore, 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 layer was formed of silicon. Table 1 shows the crystals (including the types of elements) constituting the support substrate, modified layer, and oxide film layer.Regarding the fact that the modified layer has amorphous structures with different regularities of atomic arrangements, in the table, the first modified layer is denoted as "amorphous Si(1)" and the second modified layer is denoted as "amorphous Si(2)" for distinction. 【0106】 (Example 2) A composite substrate was fabricated in the same manner as in Example 1, except that a single crystal silicon substrate with an oxygen concentration of 5.0 × 10 １６ atoms / cm ３ was used. The carbon concentration of the single crystal silicon substrate was 3.0 × 10 １６ atoms / cm ３ , the electrical resistivity was 3.0 kΩ·cm, the carrier type was n-type, and the plane orientation was (100). (Example 3) A composite substrate was fabricated in the same manner as in Example 2, except that a single crystal silicon substrate with a p-type carrier type was used. The carbon concentration of the single crystal silicon substrate was 5.0 × 10 １６ atoms / cm ３ , the carbon concentration was 3.0 × 10 １６ atoms / cm ３ , the electrical resistivity was 3.0 kΩ·cm, and the plane orientation was (100). (Comparative Example 1) A composite substrate was fabricated in the same manner as in Example 1, except that a single crystal silicon substrate with an oxygen concentration of 3.0 × 10 １８ atoms / cm ３ was used. The carbon concentration of the single crystal silicon substrate was 3.0 × 10 １６ atoms / cm ３The electrical resistivity was 3.0 kΩ·cm, the carrier type was n-type, and the surface orientation was (100). (Evaluation) Similar to Example 1, the oxygen and carbon concentrations of the support substrate in the composite substrates of Examples 2-3 and Comparative Example 1 were measured by Dynamic SIMS and evaluated at an evaluation depth of 2.0 μm. Similar to Example 1, areas affected by the knock-on effect were excluded from the evaluation. The results are shown in Table 1. The obtained composite substrates were subjected to the evaluation in (2) above. The results are shown in Table 1. For Examples 2-3, it was also confirmed that the reflection loss (S11) at this time was -20 dB or less (reflection 1% or less). On the other hand, for Comparative Example 1, 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 including the modified layer in a plan view of the obtained composite substrate was observed with an optical microscope and the area ratio of the modified layer (ratio of the area of ​​the modified layer to the total cross-sectional area) was calculated. The results are shown in Table 1. In Examples 2 and 3, the shape and area of ​​the modified layer were confirmed from optical microscope images, similar to Example 1. 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. As a result, similar to Example 1, regions of crystalline structure and regions of amorphous structure were mixed, and multiple areas of amorphous structure (first modified layer) were confirmed to be located closer to the silicon substrate than the regions of crystalline structure. Furthermore, by observing the TEM images, it was found that there were darkened regions, i.e., regions with strain. The overall depth (distance) of the modified layer was approximately 50 nm, and the depth of the first modified layer was approximately 10 nm. In addition, 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. 【0107】 [Example 4] A composite substrate was prepared with an oxide film layer and a functional layer as the active layer. In this example, a laser was irradiated before bonding the oxide film layer and the functional substrate. Specifically, the composite substrate was prepared as follows. Note that explanations common to Example 1 will be omitted as appropriate. As the support substrate, a single-crystal silicon substrate (with an oxygen concentration of 8.0 × 10¹⁶) was used, similar to that in Example 1. １７atoms / cm ３ A solution was prepared and the silicon substrate was cleaned. The carbon concentration of the single-crystal silicon substrate was 3.0 × 10⁻⁶. １６ atoms / cm ３ The electrical resistivity was 3.0 kΩ·cm, the carrier type was n-type, and the surface orientation was (100). Subsequently, similar to 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 4.5 μm. Next, one side of the oxide film layer formed on both sides of the support substrate (electrode formation surface) was smoothed by CMP processing until the surface roughness Ra was 0.5 nm. 【0108】 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 1. 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 1. 【0109】 As a functional substrate, a lithium niobate substrate (hereinafter referred to as LN substrate), which is a piezoelectric substrate having an OF (Optical Field) 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. 【0110】 Next, the oxide film layer after cleaning and the LN substrate 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. 【0111】Next, the LN substrate of the bonded body was subjected to a thinning treatment. Specifically, the bonded body was placed in a nitrogen atmosphere oven (120°C) and heated for 10 hours. After removing the LN substrate from the oven, grinding and lapping were performed, and then CMP processing was carried out to obtain an LN layer (functional layer) with a thickness of 500 nm. In this way, 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. Similar to Example 1, the oxygen concentration and carbon concentration of the support substrate in the obtained composite substrate were evaluated at an evaluation depth of 2.0 μm using Dynamic SIMS. Similar to Example 1, regions affected by the knock-on effect were excluded from the evaluation. The results are shown in Table 1. The obtained composite substrate was subjected to the evaluation in (2) above. The results are shown in Table 1. 【0112】 [Example 5] A composite substrate was fabricated in the same manner as in Example 4, except that laser irradiation was performed after bonding the oxide film layer and the functional substrate. A composite substrate was fabricated with an oxide film layer and a functional layer as the active layer. Specifically, the composite substrate was fabricated as follows. Explanations common to Example 4 will be omitted as appropriate. 【0113】 As the support substrate, a single-crystal silicon substrate (with an oxygen concentration of 8.0 × 10) similar to that in Example 4 was used. １７ atoms / cm ３ A solution was prepared and the silicon substrate was cleaned. Next, similar to Example 4, 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 4.5 μm. Subsequently, one side of the oxide film layer formed on both sides of the support substrate (electrode formation surface) was smoothed by polishing with CMP processing until the surface roughness Ra was 0.5 nm. 【0114】 As a functional substrate, an LN substrate similar to that in Example 4 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 4 to remove impurities and other contaminants from the surface of the LN substrate. 【0115】 Next, the oxide film layer after cleaning and the LN substrate were directly bonded using a plasma activation method to obtain a bonded body. 【0116】 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. 【0117】 Next, the composite 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 modified layer between the silicon substrate and the oxide film layer. The presence of the modified layer was confirmed by TEM imaging, as in Example 1. In this way, a composite comprising an LN substrate, an oxide film layer, modified layers (second modified layer / first modified layer), and a silicon substrate was obtained. 【0118】 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, grinding and lapping were performed, and further CMP processing was carried out to obtain an LN layer (functional layer) with a thickness of 500 nm. In this way, 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. Similar to Example 1, the oxygen concentration and carbon concentration of the support substrate in the obtained composite substrate were evaluated at an evaluation depth of 2.0 μm using Dynamic SIMS. Similar to Example 1, regions affected by the knock-on effect were excluded from the evaluation. The results are shown in Table 1. The obtained composite substrate was subjected to the evaluation in (2) above. The results are shown in Table 1. 【0119】 [Comparative Example 2] As the support substrate, a single-crystal silicon substrate similar to that in Comparative Example 1 (oxygen concentration of 3.0 × 10 １８ atoms / cm ３A composite substrate comprising a functional layer / oxide film layer / modified layer (second modified layer / first modified layer) / support substrate was obtained in the same manner as in Example 4, except that the ) was prepared. As in Example 1, the oxygen concentration and carbon concentration of the support substrate in the obtained composite substrate were evaluated at an evaluation depth of 2.0 μm using Dynamic SIMS. As in Example 1, regions affected by the knock-on effect were excluded from the evaluation. The results are shown in Table 1. The obtained composite substrate was subjected to the evaluation in (2) above. The results are shown in Table 1. 【0120】 【0121】 As is clear from the above examples and comparative examples, in each example, the oxygen concentration in the support substrate of the composite substrate is 1.0 × 10⁻⁶. １８ atoms / cm ３ The fact that it is less than indicates that the insertion loss is reduced. Therefore, it is suggested that the composite substrates of each embodiment have a modified layer formed that can function as a charge trapping layer and can be applied to applications with excellent electrical properties. Furthermore, in each embodiment, the charge trapping layer (modified layer) can be formed by laser irradiation by forming an active layer (oxide film layer and / or functional layer) on the support substrate and then irradiating it with a laser. In other words, it has been shown that in each embodiment, 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. 【0122】 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. 【0123】 10 Support substrate 20 Active layer 21 Oxide film layer 22 Functional layer 30 Modified layer 301 Modified section 31 First modified layer 32 Second modified layer 41 Functional substrate 100, 101, 102, 103 Composite substrate

Claims

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, wherein the oxygen concentration in the support substrate is 1.0 × 10⁻⁶. １８ atoms / cm ３ A composite substrate that is less than [a certain value].

2. The composite substrate according to claim 1, wherein the electrical resistivity of the support substrate is greater than 3 kΩ·cm.

3. The composite substrate according to claim 1, wherein the support substrate has a p-type region.

4. The composite substrate according to claim 1, wherein the support substrate has a surface orientation (100).

5. The composite substrate according to claim 1, wherein the area ratio of the modified layer to the total area in a plan view is 40% or more.

6. The composite substrate according to claim 5, wherein the area ratio of the modified layer is less than 100%.

7. 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.

8. The composite substrate according to claim 7, 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.

9. The composite substrate according to claim 7, wherein the plurality of modified portions are formed at intervals from one another.

10. The composite substrate according to claim 5, wherein a plurality of electrodes are formed on the active layer at intervals.

11. The composite substrate according to claim 1, wherein the modified layer comprises, in order from the support substrate side, a first modified layer and a second modified layer, the second modified layer containing an amorphous structure of the same type of elements as those constituting the support substrate, the first modified layer containing the same type of elements as those constituting the support substrate, and 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.

12. The composite substrate according to claim 11, wherein the first modified layer includes an amorphous structure and a crystalline structure.

13. The composite substrate according to claim 12, wherein in the thickness direction, the amorphous structure of the first modified layer is located on the support substrate side than the crystalline structure.

14. The composite substrate according to claim 11, wherein the first modified layer includes a region having strain in part.

15. The composite substrate according to claim 1, wherein the band gap of the active layer is 2.5 eV or greater.

16. The composite substrate according to claim 1, wherein the active layer has an oxide film layer.

17. The composite substrate according to claim 16, wherein the oxide film layer is a thermal oxide film.

18. The composite substrate according to claim 1, wherein the active layer includes a functional layer.

19. The composite substrate according to claim 1, wherein the active layer comprises an oxide film layer and a functional layer in that order from the support substrate side.

20. The composite substrate according to claim 19, wherein the active layer further includes a bonding layer between the oxide film layer and the functional layer.

21. The composite substrate according to claim 19, wherein the active layer further includes an intermediate layer between the oxide film layer and the functional layer.

22. 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.

23. A method for manufacturing a composite substrate according to any one of claims 1 to 22, wherein the substrate is crystalline and has an oxygen concentration of 1.0 × 10 １８ atoms / cm ３ A method for manufacturing a composite substrate, comprising: preparing a support substrate having a value less than; forming an active layer on at least one surface of the 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 side of the active layer.

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