Composite substrate

Abstract

Provided is a composite substrate capable of exhibiting superior electrical properties. A composite substrate according to an embodiment of the present invention comprises a support substrate, a charge trapping layer, an intermediate layer, and a functional layer in the given order. The oxygen concentration of the support substrate is 1.0×1018 atoms / cm3 or less. The charge trapping layer has a crystal structure different from that of the support substrate, or has an amorphous structure.

Description

Composite substrate 【0001】 The present invention relates to a composite substrate. 【0002】 In information communication devices, in order to extract electrical signals of arbitrary frequencies, for example, functional elements such as surface acoustic wave elements (for example, SAW filters) using surface acoustic waves, electro-optic elements (for example, optical modulators, etc.) that can change the phase of light, etc. are used. In recent years, in the field of information communication devices, the communication volume has been increasing rapidly, and higher performance of functional elements is required. As a functional element, a composite substrate applicable to high-frequency devices such as optical modulators has been proposed (for example, Patent Documents 1 and 2). 【0003】 Patent Document 1 describes an optical device having a thin film layer such as LN (lithium niobate) and an optical waveguide, with a signal electrode disposed on the optical waveguide. Patent Document 2 proposes forming a defect layer between a substrate layer and an isolation layer in a composite substrate having a substrate layer such as silicon, an isolation layer such as silicon dioxide, and a thin film layer such as LN. 【0004】 Japanese Unexamined Patent Application Publication No. 2023-8095 Chinese Patent Specification No. 118091993 【0005】 Further improvement in high-frequency characteristics may be required for composite substrates applied to high-frequency devices. However, in composite substrates such as those in Patent Documents 1 and 2, it may be difficult to suppress electrical losses. In view of the above, the main object of the present invention is to provide a composite substrate that can have excellent electrical characteristics. 【0006】 [1] The composite substrate according to an embodiment of the present invention includes a support substrate, a charge trapping layer, an intermediate layer, and a functional layer in this order. The oxygen concentration of the support substrate is 1.0×10 １８ atoms / cm ３The following applies: The charge trapping layer has a different crystalline structure or amorphous structure from the support substrate. [2] In the composite substrate described in [1] above, a warping suppression layer may be further provided on the side of the support substrate opposite to the charge trapping layer. [3] In the composite substrate described in [1] or [2] above, the thickness of the charge trapping layer may be 0.1 μm or more and 3 μm or less. [4] In the composite substrate described in any of [1] to [3] above, the charge trapping layer may have a polycrystalline structure. [5] In the composite substrate described in any of [1] to [4] above, the support substrate may have a surface with a plane orientation (100). [6] In the composite substrate described in any of [1] to [5] above, the electrical resistivity of the support substrate may be 3 kΩ·cm or more. [7] In the composite substrate described in any of [1] to [6] above, the support substrate may have an n-type region. [8] In the composite substrate described in any of [1] to [7] above, the thickness of the intermediate layer may be 2.0 μm or more and 30 μm or less. [9] In the composite substrate described in any of [1] to [8] above, the surface roughness Ra of the first main surface on the charge trapping layer side of the support substrate and the second main surface on the opposite side of the first main surface may be less than the surface roughness Ra of the second main surface.

[10] In the composite substrate described in any of [1] to [9] above, the surface roughness Ra of the surface of the functional layer on the intermediate layer side may be 1.0 nm or less.

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

[10] above, the surface roughness of the surface of the functional layer on the opposite side of the intermediate layer may be 0.5 nm or less.

[12] The composite substrate described in any of [1] to

[11] above may be used for high-frequency applications.

[13] In the composite substrate according to any one of [1] to

[12] above, a coating layer may be further provided on the functional layer to cover the surface of the functional layer opposite to the intermediate layer.

[14] In the composite substrate according to any one of [1] to

[13] above, the composite substrate may have an optical waveguide on the functional layer.

[15] In the composite substrate according to

[14] above, the optical waveguide may be a ridge waveguide.

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

[15] above, a plurality of electrodes may be further provided on the functional layer.

[17] In the composite substrate described in

[16] above, the electrodes may be spaced apart in a direction perpendicular to the optical waveguide in a plan view.

[18] In the composite substrate described in

[14] above, the optical waveguide may have an input section into which light is input and an output section that outputs modulated light obtained by modulating the light by receiving an electric field.

[19] In the composite substrate described in

[18] above, the optical waveguide may have: a first waveguide and a second waveguide provided between the input section and the output section, respectively; a branching section between the input section and the first waveguide and the second waveguide that branches the light from the input section toward the first waveguide and the light from the input section toward the second waveguide; and a coupling section between the output section and the first waveguide and the second waveguide that couples the modulated light from the first waveguide toward the output section and the modulated light from the second waveguide toward the output section.

[20] The composite substrate described in

[19] above, comprising: a first electrode provided on the functional layer between the first waveguide and the second waveguide; a second electrode provided on the functional layer opposite to the first electrode and facing the first electrode on the side of the first electrode opposite to the second waveguide; and a third electrode provided on the functional layer opposite to the first electrode and facing the second electrode on the side of the first electrode opposite to the first waveguide; each of the first electrode, the second electrode and the third electrode may not overlap with either the first waveguide or the second waveguide, and may be provided parallel to the first waveguide and the second waveguide in a plan view. 【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 schematic cross-sectional view showing the general configuration of a composite substrate according to another embodiment of the present invention. This is a schematic plan view showing the general configuration of a composite substrate according to the above-mentioned other embodiment. This is a schematic cross-sectional view showing the general configuration of a composite substrate according to yet another embodiment of the present invention. This is a schematic plan view showing the general configuration of a composite substrate according to yet another embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the first embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the third embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the fourth embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to the above embodiment.In the embodiment, it is an explanatory diagram of a coplanar waveguide pattern used for evaluating high-frequency characteristics. It is a diagram showing an enlarged view of a portion surrounded by a broken line in FIG. 8A. 【0009】 Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. The drawings are schematically drawn for ease of viewing, and the thickness, length, width, shape, ratio, etc. do not accurately reflect the actual shape. 【0010】 A. Overview of the composite substrate FIG. 1 is a schematic cross-sectional view showing a 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 charge trapping layer 20, an intermediate layer 30, and a functional layer 40 in this order. The support substrate 10 typically has crystallinity. The support substrate 10 is typically a single crystal substrate or a polycrystalline substrate. The charge trapping layer 20 typically has a crystal structure different from that of the support substrate 10 or an amorphous structure. The intermediate layer 30 is typically an insulator and may be, for example, a dielectric. The functional layer 40 typically has a refractive index higher than that of the intermediate layer 30. The composite substrate may have an optical waveguide 400 on the functional layer 40 as in the illustrated example. Light can be transmitted through the composite substrate 100 via the functional layer 40 and the optical waveguide 400. 【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 ３ or less, particularly preferably 5.0×10 １６ atoms / cm ３ or less. The lower limit of the oxygen concentration in the support substrate is, for example, 1.0×10 １５ atoms / cm３ The following applies. If the oxygen concentration of the support substrate in a composite substrate is below the above upper limit, electrical losses in the composite substrate can be suppressed. As a result, the electrical characteristics when the composite substrate is applied to a high-frequency device 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 the above lower limit, cracking and / or chipping of the support substrate can be suppressed. More specifically, the following applies. For example, in the heating process when using a composite substrate for manufacturing and / or a high-frequency device, donors (carriers of negative charge caused by oxygen; also called oxygen donors) may be released from the oxygen in the support substrate, and the electrical resistivity of the support substrate may decrease. For example, when a composite substrate is applied to a high-frequency device and a voltage is applied to the composite substrate from an electrode, if the electric field reaches inside the support substrate, electrical losses caused by the support substrate may increase. １８ atoms / cm ３ If the oxygen concentration is excessive, such electrical losses are likely to occur, 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 ３ The following conditions can reduce the amount of oxygen donor released from the support substrate. 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. Furthermore, according to the composite substrate according to the embodiment of the present invention, conductivity in the support substrate is maintained, and the effect of trapping charge in the charge trapping layer can be effectively exhibited. In this specification, "high frequency" means a high frequency band (millimeter wave to subterahertz wave) that can be used in communication systems. In high frequency applications, the operating frequency is preferably 75 GHz or higher, and more preferably 100 GHz or higher. The upper limit of the operating frequency may be, for example, 500 GHz. 【0012】As described above, the charge trapping layer 20 typically has a different crystalline structure or amorphous structure from the support substrate 10. A crystalline structure refers to a structure that possesses crystalline properties. Specifically, a crystalline structure may include single-crystal structures and / or polycrystalline structures. Note that the regularity of a crystalline structure can differ depending on the combination and state of atomic arrangement, so "crystalline structure" may include multiple different crystalline states. Similarly, "amorphous structure" may also include multiple different amorphous states. The crystalline and / or amorphous nature of the support substrate and the charge trapping 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 elements constituting the support substrate and / or the charge trapping layer can be confirmed by elemental analysis and compositional analysis using EDX (energy-dispersive X-ray fluorescence). Thus, the charge trapping layer can be distinguished from the support substrate by the presence or absence of crystallinity, differences in crystallinity, etc. Details of the charge trapping layer will be described in Section B-2. 【0013】 The intermediate layer 30 can be composed of any suitable material. Typically, the intermediate layer may be a layer formed of a material with a lower refractive index than the functional layer. The intermediate layer 30 may consist of a single layer or multiple layers. Details of the intermediate layer will be described later in section B-3. 【0014】 The functional layer 40 can be formed from any suitable functional substrate. Typically, the functional layer may have an electro-optic effect. With such a configuration, a composite substrate having an optical waveguide can be suitably used for high-frequency applications. Details of the functional layer will be described later in section B-4. 【0015】 The optical waveguide 400 can be formed on the functional layer 40 as described above. The optical waveguide 400 may be formed integrally with the functional layer 40, for example, as shown in the illustrated example, or it may be formed embedded inside the functional layer, that is, it may be formed as a part of the functional layer, or it may be provided on the functional layer using a material different from the functional layer. 【0016】 The optical waveguide 400 may have an appropriate shape depending on the purpose (for example, the function of the device to which the composite substrate is applied). The optical waveguide 400 may be, for example, a ridge waveguide, a rib waveguide, a planar waveguide (slab waveguide), or an embedded waveguide, or a combination of two or more of these. In a composite substrate, the optical waveguide is preferably a ridge waveguide. With such a configuration, the composite substrate can be used particularly suitably for high-frequency devices (for example, optical modulators). 【0017】 In a composite substrate according to an embodiment of the present invention, the optical waveguide 400 is formed, for example, along one direction parallel to the plane direction in a plan view. The optical waveguide 400 typically has an input section and an output section. The input section may be provided on one end face in one direction in the plane direction in a plan view; the output section may be provided on the other end face in the same direction in a plan view. For example, when the composite substrate is applied to a high-frequency device such as an optical modulator, and a voltage is applied to the composite substrate (substantially the device), the input section may receive the unmodulated light before modulation by the optical modulator. The output section may output modulated light that can be generated by the optical modulator modulating the unmodulated light. Modulated light means light whose phase has been changed (modulated) by the electric field generated by the applied voltage. For example, when a voltage is applied to the composite substrate and an electric field is generated, the refractive index in the optical waveguide changes. As a result, the phase of the light guiding through the optical waveguide changes. As a result, a difference occurs in the propagation speed of the guided light (modulated light) relative to the input light (unmodulated light). The guidance direction of the optical waveguide 400 can be appropriately set according to the purpose. The guidance direction of the optical waveguide 400 is, for example, one direction parallel to the plane direction in a plan view. The modulated light and the modulated light can travel along the guidance direction of the optical waveguide 400. 【0018】Figure 2A is a schematic diagram illustrating a composite substrate according to another embodiment. Figure 2B is a schematic diagram illustrating a plan view of Figure 2A. Figure 2A is a cross-sectional view taken along the A-A line in Figure 2B. In the illustrated example, the optical waveguide 400 is provided with one waveguide formed in the shape of a ridge protruding in the direction opposite to the intermediate layer 30 of the functional layer 40 (see Figure 2A). The optical waveguide 400 is provided along a direction parallel to one direction in the planar direction (left-right direction in the drawing) (see Figure 2B). As shown in the illustrated example, the optical waveguide 400 has an input section 400a and an output section 400b. The input section 400a is provided on one end face in the direction of the planar direction, and the output section 400b is provided on the other end face in the direction of the planar direction. The waveguide direction of the optical waveguide 400 may be, for example, from the input section 400a to the output section 400b. 【0019】 As shown in Figures 2A and 2B, the composite substrate 110 may further include a plurality of electrodes 80. Typically, the plurality of electrodes 80 are provided above the functional layer 40. Specifically, the electrodes 80 may be provided directly on the functional layer 40 (i.e., on the first main surface 40a), as shown in the illustrated example, or on an intervening layer (for example, a coating layer 70 provided on the functional layer 40). For example, in the composite substrate 110 shown in Figure 2A, two electrodes 80 are provided on a coating layer 70 provided on the functional layer 40. 【0020】 Multiple electrodes 80 (a pair of electrodes 80, 80 in the drawing) are spaced apart in one direction in the plan view. Specifically, the multiple electrodes 80 are spaced apart in a direction perpendicular to the guidance direction of the optical waveguide 400 in the plan view. In Figure 2B, the pair of electrodes 80, 80 are positioned with the optical waveguide 400, which is positioned in one direction in the plan view, in between them. The pair of electrodes 80, 80 are positioned facing each other in a direction parallel to the guidance direction of the optical waveguide 400. The number of electrodes, the spacing between the electrodes, and the distance between the optical waveguide and the electrodes can be appropriately set according to the purpose. 【0021】Figure 3A is a schematic diagram illustrating a composite substrate according to yet another embodiment. Figure 3B is a schematic diagram illustrating a plan view of Figure 3A. Figure 3A is a cross-sectional view taken along the A-A line in Figure 3B. In the illustrated example, the optical waveguide 400 is provided with two waveguides (a first waveguide 401 and a second waveguide 402) formed in a ridge shape that protrudes in the direction opposite to the intermediate layer 30 of the functional layer 40 (see Figure 3A). In a plan view, the optical waveguide 400 is formed from one end face to the other end face in one direction in the planar direction (left-right direction in the drawing) (see Figure 3B). 【0022】 In the illustrated example composite substrate 111, the optical waveguide 400 has a first waveguide 401 and a second waveguide 402. The first waveguide 401 and the second waveguide 402 are spaced apart in one direction parallel to the plane direction in a plan view. The spacing between the first waveguide 401 and the second waveguide 402 can be appropriately set according to the purpose. As shown in the illustrated example, the optical waveguide 400 has an input section 400a and an output section 400b. Similar to the composite substrate 110 shown in Figure 2B, the input section 400a is provided on one end face in one direction of the plane direction, and the output section 400b is provided on the other end face in the same direction. The waveguide direction of the optical waveguide 400 may be, for example, from the input section 400a to the output section 400b. In the illustrated example composite substrate 111, the optical waveguide 400 further includes a branching section 400c and a coupling section 400d. The optical waveguide 400 is configured such that a first waveguide 401 and a second waveguide 402 are branched at the branching section 400c and coupled at the coupling section 400d. At the branching section 400c, the light (modulated light) that can be input from the input section 400a is branched into light going from the input section 400a to the first waveguide 401 and light going from the input section 400a to the second waveguide 402. At the coupling section 400d, the modulated light going from the first waveguide 401 to the output section 400b and the modulated light going from the second waveguide 402 to the output section 400b are coupled. 【0023】The composite substrate 111 in Figures 3A and 3B further comprises a plurality of electrodes 80 (81, 82, 83). The plurality of electrodes 80 (81, 82, 83) are typically provided above the functional layer 40. In the illustrated example, the plurality of electrodes 80 are provided on a coating layer 70 provided on the functional layer 40. The plurality of electrodes 80 (81, 82, 83) are provided at intervals in one direction in the planar direction. Specifically, as electrodes 80, three electrodes (first electrode 81, second electrode 82, and third electrode 83) are provided at intervals from each other in a direction perpendicular to the waveguide direction of the optical waveguide 400 (first waveguide 401 and second waveguide 402) in a planar view. The first electrode 81, second electrode 82, and third electrode 83 are provided parallel to each other in one direction in the planar direction. In the illustrated example, the first electrode 81 is provided between the first waveguide 401 and the second waveguide 402; the second electrode 82 is provided opposite the first electrode 81 to the second waveguide 402 and facing the first electrode 81; and the third electrode 83 is provided opposite the first electrode 81 to the first waveguide 401 and facing the second electrode 82. In other words, the first electrode 81 and the second electrode 82 are provided with the first waveguide 401 in between, and the second electrode 82 and the third electrode 83 are provided with the second waveguide 402 in between. Each of the first electrode 81, the second electrode 82, and the third electrode 83 does not overlap with either the first waveguide 401 or the second waveguide 402, and is provided parallel to the first waveguide 401 and the second waveguide 402 in a plan view. In this embodiment, for example, the first electrode 81 can function as a signal electrode, and the second electrode 82 and the third electrode 83 can function as ground electrodes. 【0024】 Although not shown in the diagram, the first waveguide and the second waveguide may be arranged parallel to each other on the functional layer in a plan view. In other words, the optical waveguide may not have branching or coupling sections and may have multiple waveguides. 【0025】In one embodiment, the composite substrate further comprises a cladding layer. For example, in the composite substrate 110 shown in Figure 2A, the cladding layer 70 covers the main surface (first main surface 40a) of the functional layer 40. The cladding layer 70 may be configured to cover the optical waveguide 400 of the functional layer 40. The cladding layer 70 has a lower refractive index than, for example, the functional layer and the optical waveguide. When the composite substrate is equipped with a cladding layer, the optical confinement efficiency can be further improved when the composite substrate is applied to high-frequency applications (e.g., optical modulators). That is, the cladding layer can function as a cladding layer to the core (core layer), such as the optical waveguide and / or the functional layer, and can contribute to improving the optical properties of the composite substrate. 【0026】 In one embodiment, the composite substrate further comprises a warp suppression layer. For example, in the composite substrate 100 shown in Figure 1, a warp suppression layer 60 is provided on the side opposite to the charge trapping layer 20 of the support substrate 10. When a composite substrate is provided with a warp suppression layer, it is possible to reduce the stress (sometimes referred to as residual stress, compressive stress, etc.) that may occur within the composite substrate. As a result, the yield when manufacturing the composite substrate and / or when using the composite substrate for the manufacture of high-frequency devices can be improved. 【0027】 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 may be, for example, 100 μm to 1500 μm. The total thickness of the composite substrate when a functional layer is included may be, for example, 100 μm to 1500 μm. 【0028】B. Details of the Composite Substrate The components of the composite substrate will be described in detail below. B-1. Support Substrate As the support substrate, any suitable substrate can be used insofar as it can achieve the objectives of the present invention. As described above, the support substrate is typically 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. The support substrate may typically be composed of a semiconductor material. Preferably, the material constituting the support substrate is silicon or germanium. The support substrate may preferably be single-crystal silicon, polycrystalline silicon, single-crystal germanium, or polycrystalline germanium. When the support substrate is single-crystal silicon or single-crystal germanium, a polycrystalline layer may be formed on the surface. 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 preferred from the viewpoint of achieving a good coefficient of thermal expansion and thermal conductivity. When the support substrate is a germanium substrate, it is preferred from the viewpoint of achieving a good coefficient of thermal expansion and thermal conductivity. 【0029】 As described above, the oxygen concentration in the support substrate is typically 1.0 × 10⁻⁶. １８ atoms / cm ３ The following, preferably 9.0 × 10 １７ atoms / cm ３ The following, and more preferably 8.0 × 10 １７ atoms / cm ３ The following, and more preferably 5.0 × 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 １６atoms / cm ３ This concludes the explanation. Within this range, insertion loss can be suppressed even when the composite substrate is applied to high-frequency devices such as optical modulators. "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. The oxygen concentration of the support substrate in the composite substrate can be measured by secondary ion mass spectrometry (SIMS). In the SIMS measurement of the oxygen concentration of the support substrate in the composite substrate, if the effect of the knock-on effect is anticipated, the measurement may be performed after removing the intermediate layer and / or functional layer from the composite substrate, or it may be measured by backside SIMS. 【0030】 The support substrate can be manufactured by any suitable method. Examples of methods for manufacturing the support substrate include the CZ method (Czochralski method), the FZ method (Float Zone method), and the MCZ method (magnetic field applied Czochralski method). From the viewpoint of controlling the oxygen concentration to a desired level, the support substrate can preferably be manufactured by the FZ method or the MCZ method. The oxygen concentration of the support substrate (single sheet) can be measured by the FT-IR method, for example, in accordance with ASTM F121-1979. 【0031】 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). The support substrate may have a surface with surface orientation (100), (110), or (111), for example. 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, with less fixed charge at 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. Furthermore, when an oxide film is formed by thermal oxidation of the support substrate, the oxidation rate at surface orientation (100) is faster than the oxidation rates at surface orientations (110) and (111), which has the advantage of potentially superior productivity. 【0032】The electrical resistivity of the support substrate is preferably 3 kΩ·cm or more, 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 charge trapping layer can be measured in the same manner. 【0033】 The support substrate may have, for example, an n-type region, or a p-type region, or it may have both an n-type and a p-type region. Preferably, the support substrate has an n-type region. With such a configuration, the electrical properties can be further improved. An n-type region is a region that contains many negative charges (free electrons) as carriers (charge carriers). An n-type region can be formed by adding any appropriate donor impurities to the support substrate. Examples of donor impurities include phosphorus, arsenic, and antimony. A p-type region is a region that contains many positive charges (holes) as carriers. A p-type region can be formed by adding any appropriate acceptor impurities to the support substrate. When silicon is used as the support substrate, examples of acceptor impurities include boron, aluminum, gallium, and indium. A higher electrical resistivity of the support substrate tends to contribute to reducing electrical losses. It is preferable that the support substrate is substantially free of donor impurities or acceptor impurities (dopants), such as boron and phosphorus in the case of a silicon substrate. As the amount of such dopants is reduced, there is a tendency for the carrier type to become n-type. The carrier type of the support substrate can be measured and confirmed by the hot probe method. The carrier type of the charge trapping layer can also be measured in the same manner. 【0034】It is preferable that the thermal expansion coefficient of the semiconductor material constituting the support substrate is smaller than that of the functional substrate constituting the functional layer. With such a support substrate, changes in the shape and size of the intermediate layer and / or functional layer when the temperature changes can be suppressed, and changes in the frequency characteristics (losses) of the functional element when a composite substrate is used can be suppressed, for example. For example, if the material constituting the support substrate is silicon, this relationship of thermal expansion coefficients can be suitably satisfied. 【0035】 Any appropriate thickness can be adopted 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 this case, for example, it may be easier to thin the functional layer. Any appropriate diameter can be adopted for the support substrate. For example, the diameter of the support substrate may be 10 mm to 500 mm. 【0036】 The surface roughness Ra of the support substrate can have any appropriate surface roughness Ra. The surface roughness Ra of the main surface (first main surface 10a) of the support substrate 10 on the charge trapping layer 20 side may be, for example, 0.1 nm to 10 nm. The surface roughness Ra of the first main surface 10a of the support substrate 10 is preferably 5.0 nm or less, more preferably 1.0 nm or less, and even more preferably 0.5 nm or less. The surface roughness Ra of the main surface (second main surface 10b) of the support substrate 10 opposite to the first main surface 10a may be, for example, 1 nm or more, 10 nm or more, or 50 nm or more. It is preferable that the surface roughness Ra of the first main surface 10a of the support substrate 10 is smaller than the surface roughness Ra of the second main surface 10b. With such a configuration, the yield when manufacturing the composite substrate and / or when using the composite substrate for manufacturing high-frequency devices can be improved. In this specification, "surface roughness Ra" means arithmetic mean roughness (Ra). The arithmetic mean roughness (Ra) can be obtained by measuring it in a 10 μm × 10 μm field of view using an atomic force microscope (AFM) in accordance with JIS B0601:2013. Unless otherwise specified, in this specification, surface roughness Ra is measured by the method described above. 【0037】B-2. Charge Trap Layer As described above, the charge trap layer typically has a crystalline structure or an amorphous structure different from that of the support substrate. The charge trap layer may have, for example, only a polycrystalline structure, only an amorphous structure, or both an amorphous and a polycrystalline structure. The charge trap layer is preferably an amorphous or polycrystalline structure, and more preferably a polycrystalline structure. Such a configuration can function particularly well to trap charges. Therefore, such a configuration can particularly suitably suppress electrical losses in devices using a composite substrate. Furthermore, if the charge trap layer has a polycrystalline structure, there is an advantage in that dielectric losses can be more suitably suppressed when applied to devices for high-frequency applications of, for example, 75 GHz or higher. 【0038】 The charge trapping layer 20 may contain the same elements as those constituting the support substrate 10. For example, if the support substrate is a silicon substrate, the charge trapping layer may contain amorphous silicon and polycrystalline silicon, amorphous silicon only, or polycrystalline silicon only. For example, if the support substrate is a germanium substrate, the charge trapping layer may contain amorphous germanium and polycrystalline germanium, amorphous germanium only, or polycrystalline germanium only. The charge trapping layer may be, for example, amorphous silicon (a-Si) or polycrystalline silicon (polysilicon: poly-Si). The charge trapping layer is preferably polycrystalline silicon. The polycrystalline silicon is preferably undoped polycrystalline silicon. With such a configuration, dielectric loss in the composite substrate can be more effectively suppressed. 【0039】The charge trapping layer can be formed by any suitable method. Examples of methods for forming the charge trapping layer include physical vapor deposition (PVD) such as sputtering, atomic layer deposition (ALD), and ion beam-assisted deposition (IAD), and chemical vapor deposition (CVD). The charge trapping layer may also be formed by laser irradiation, for example. For example, by irradiating the support substrate with a desired laser, the crystal structure of the surface of the support substrate may be altered (modified), and a charge trapping layer having a different crystal structure or amorphous structure from the support substrate may be formed on the support substrate. When using a laser, the charge trapping layer can be formed between the support substrate 10 and the intermediate layer 30 even after the intermediate layer 30 has been formed on the support substrate 10. For example, there is the advantage that the charge trapping layer can be formed even after the support substrate has been oxidized and an oxide film has been formed. 【0040】 The charge trapping layer may have an appropriate thickness insofar as it can achieve the objectives of the present invention. The thickness of the charge trapping layer is, for example, 100 nm to 3000 nm (3 μm), preferably 200 nm or more, more preferably 300 nm or more, and even more preferably 1000 nm or more; on the other hand, it may be 2000 nm or less, or 1000 nm or less. Such a thickness can increase the amount of charge that can be trapped by the charge trapping layer, and can better maintain the charge trapping performance of the composite substrate. Therefore, such a thickness can further contribute to the performance improvement when the composite substrate is used in devices for high-frequency and / or harmonic applications. The thickness of the charge trapping layer can be confirmed by imaging with an electron microscope (e.g., a scanning electron microscope (SEM) or a transmission electron microscope (TEM)), and can be determined by examining a cross-section (substantially a side view) of the image. 【0041】 B-3. ​​Interlayer The interlayer may typically have insulating properties. The interlayer typically has a lower refractive index than the functional layer. The interlayer may also have dielectric properties, for example. Any suitable dielectric material can be used as the material constituting the interlayer. Examples of dielectric materials include silicon oxide (SiO₂). ２Examples include silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, and aluminum oxynitride. The intermediate layer may contain an oxide. The oxide can be any suitable oxide insofar as it can achieve the objectives of the present invention. Examples of oxides include silicon oxide (SiO₂). ２ ), germanium oxide (GeO ２ Examples include the following. The intermediate layer may preferably be composed of silicon oxide. The intermediate layer may be doped with any suitable atom. Preferred atoms for doping include fluorine atoms and carbon atoms. 【0042】 The intermediate layer can be formed by any suitable method. Examples of methods for forming the intermediate layer include physical vapor deposition (PVD) such as oxidation, sputtering, atomic layer deposition (ALD), and ion beam-assisted deposition (IAD), as well as chemical vapor deposition (CVD). 【0043】 The intermediate layer may be composed of an oxide film of an oxidizing material (oxidizing substrate), for example. Specifically, the intermediate layer may be formed by oxidizing an oxidizing substrate, for example. More specifically, the intermediate layer may be composed of an oxide (oxide film) of the semiconductor substrate, using a semiconductor material such as a silicon substrate or a germanium substrate as the oxidizing substrate. In such a case, the intermediate layer may contain silicon oxide or germanium oxide. 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 C. In an oxide film (layer) formed by thermal oxidation of an oxidizing 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 SAW filters, devices using SAW filters, optical modulators, and other high-frequency devices, it can contribute to improving yield. 【0044】 The intermediate layer may be a single layer, or it may have a laminated structure consisting of multiple layers made of different dielectric materials. 【0045】The thickness of the intermediate layer can be any appropriate thickness, as long as it achieves the objectives of the present invention. For example, the thickness of the intermediate layer is 1 μm or more and 30 μm or less, preferably 2.0 μm or more, more preferably 4.0 μm or more, even more preferably 7.0 μm or more, and even more preferably 10 μm or more. If the thickness of the intermediate layer is above the lower limit, the effect of light confinement in the composite substrate can be more favorably exhibited. If the thickness of the intermediate layer is below the upper limit, it is possible to reduce production costs while ensuring that the composite substrate has the desired optical properties. 【0046】 The surface roughness Ra of the surface of the intermediate layer opposite the charge trapping 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. If Ra is within this range, the bonding strength can be increased when another layer, such as a functional substrate or functional layer, is laminated (e.g., bonded) to the surface of the intermediate layer in the composite substrate. 【0047】 B-4. Functional Layer The functional layer can be composed of any appropriate functional material (functional material). Examples of functional materials include materials with electro-optic effects (electro-optic materials), piezoelectric materials, and semiconductor materials. Typical functional materials are electro-optic materials. Typical electro-optic crystals are a typical example of electro-optic materials. 【0048】 In one embodiment, the functional layer may be an electro-optic layer having an electro-optic effect. By providing an electro-optic layer as the functional layer of the composite substrate, a functional element capable of achieving excellent high-frequency and / or harmonic characteristics can be obtained. As a result, the composite substrate according to the embodiment of the present invention can be particularly suitably used for electro-optic elements such as optical modulators (e.g., electro-optic devices such as optical waveguide devices). 【0049】 When a composite substrate is used in high-frequency devices such as thin-film optical modulators, the electro-optical material is preferably lithium niobate (LiNbO). ３ ), lithium tantalate (LiTaO ３ ), lithium niobate - lithium tantalate, KTP (KTiOPO ４Potassium titanate phosphate and PZT (lead zirconate titanate) can be used. Specifically, as the electro-optic material, for example, X-cut and / or Z-cut lithium niobate can be used. When lithium niobate and / or lithium tantalate are used, MgO-doped or stoichiometric crystals can be used to suppress photodamage. Preferably, MgO-doped functional materials can be used as such functional substrates. With such functional substrates, photodamage can be suppressed and changes in the optical constants of the functional substrate can be suppressed. 【0050】 The functional layer may be composed of appropriate functional materials depending on the functions and performance required of the composite substrate. As for functionality, semiconductor materials can be used as described above. Examples of semiconductor materials include materials similar to those described in B-1 above (silicon, germanium), silicon carbide (SiC), indium phosphide, lead zirconate titanate (PZT), and silicon nitride. 【0051】 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 3 μm or less, preferably 0.10 μm or more and 1 μm or less. 【0052】 The surface roughness Ra of the first main surface 40a of the functional layer 40 is preferably 0.5 nm or less, more preferably 0.4 nm or less, and even more preferably 0.3 nm or less. On the other hand, the surface roughness Ra of the first main surface 40a of the functional layer 40 may be 0.1 nm or more. With such a surface roughness Ra, for example, the composite substrate can be particularly well applied to devices (functional elements) for high-frequency or harmonic applications. 【0053】The surface roughness Ra of the second main surface 40b of the functional layer 40 is preferably 1.0 nm or less, more preferably 0.5 nm or less, even more preferably 0.4 nm or less, and particularly preferably 0.3 nm or less. On the other hand, the surface roughness Ra of the second main surface 40b of the functional layer 40 may be 0.1 nm or more. With such a surface roughness Ra, for example, the composite substrate can be particularly well applied to devices (functional elements) for high-frequency or harmonic applications. In one embodiment, the functional layer and the intermediate layer are bonded together. A composite substrate in which the functional layer and the intermediate layer are bonded in this way has the advantage that the charge trapping layer as a trap-rich layer can function more well. Also, with such a configuration, a bonding surface may exist between the functional layer 40 and the intermediate layer 30. The surface roughness of the intermediate layer side of the functional layer may be the surface roughness of the bonding surface between the functional layer and the intermediate layer. The surface roughness Ra of the bonding surface between the functional layer and the intermediate layer is calculated as follows. First, the composite substrate is cut by a focused ion beam (FIB). The cut surface of the cut composite substrate is observed using an electron microscope (e.g., a scanning electron microscope (SEM) or a transmission electron microscope (TEM)) and a microscopic image is obtained. From the obtained image, the cutoff value (λc) can be calculated as 50 nm (evaluation length 50 nm × 5 = 250 nm) based on the calculation method described in JIS B0601:2013. 【0054】 B-5. Optical Waveguides A composite substrate according to one embodiment of the present invention has optical waveguides on a functional layer. As described above, the optical waveguide 400 can have an appropriate shape depending on the purpose (for example, the function of the device to which the composite substrate is applied). The optical waveguide 400 may be, for example, a ridge waveguide, a rib waveguide, a planar waveguide (slab waveguide), or an embedded waveguide, or a combination of two or more of these. In the composite substrate, the optical waveguide is preferably a ridge waveguide. With such a configuration, the composite substrate can be particularly suitable for use in high-frequency devices (for example, optical modulators). 【0055】One or more optical waveguides 400 can be formed on the functional layer 40. For example, an optical waveguide has multiple waveguides, which may be arranged parallel to each other, or they may be arranged to intersect at predetermined locations. As described above, an optical waveguide 400 typically has an input section 400a and an output section 400b. When there are multiple waveguides, in one embodiment, an optical waveguide may have a first waveguide and a second waveguide between the input section and the output section. The first waveguide and the second waveguide may be arranged parallel to each other (without intersecting) on ​​the functional layer. In another embodiment, an optical waveguide has a branch section and a coupling section, and the first waveguide and the second waveguide may intersect at the branch section and the coupling section. 【0056】 The optical waveguide can be made from a material similar to the material that can be used as a functional substrate as described in section B-4 above. Preferably, the optical waveguide can be made from the electro-optic material described above. 【0057】 In one embodiment, an optical waveguide may be formed by processing a functional layer or a functional substrate. For example, an etching process or a lift-off process may be used as a method for forming the optical waveguide. The processing method will be described in detail in section C of the manufacturing method. In another embodiment, an optical waveguide may be formed, for example, by forming a functional layer and then further providing an electro-optic material of an appropriate shape on the functional layer, or by further providing an electro-optic material of an appropriate shape on a functional substrate. 【0058】 The number, shape, and dimensions of optical waveguides can be appropriately determined according to the purpose. For example, the number, shape, and dimensions of optical waveguides can be appropriately determined according to the shape of the etching (etching process) or lift-off (lift-off process) pattern. 【0059】B-6. Warpage Suppression Layer The warpage suppression layer is typically formed on the side of the support substrate opposite to the charge trapping layer (i.e., the second main surface of the support substrate). When a warpage suppression layer is formed, warpage in the composite substrate can be suppressed. Specifically, the warpage suppression layer can reduce stress that may occur inside the composite substrate when manufacturing the composite substrate and / or when using the composite substrate for manufacturing devices, thereby suppressing warpage of the composite substrate. 【0060】 The warpage suppression layer can be formed from a suitable material, insofar as it achieves the objectives of the present invention. The material forming the warpage suppression layer may be, for example, the same material as the support substrate, or it may be a different material from the support substrate. The warpage suppression layer may be composed of a material containing silicon, for example. The different material from the support substrate may be, for example, a metallic material. The warpage suppression layer may be a single layer, or it may have a laminated structure consisting of multiple layers. 【0061】 The warpage suppression layer may be formed, for example, by polishing the support substrate, by depositing a film on the support substrate by sputtering, or by oxidizing the support substrate to form an oxide film layer. When the support substrate is polished, the crystal structure of the support substrate surface (the surface opposite to the charge trapping layer) changes, and a layer with a different crystal structure from the support substrate may be formed. 【0062】 The thickness of the warp-suppressing layer is, for example, 0.1 μm to 100 μm, preferably 1 μm or more, and more preferably 10 μm or more. 【0063】 B-7. Coating Layer The coating layer is typically a layer that can cover the main surface of the functional layer (the surface opposite to the intermediate layer). Preferably, in addition to the functional layer, the coating layer may also cover the circumferential surface of the optical waveguide. 【0064】 The coating layer can be made of a suitable material insofar as it can achieve the objectives of the present invention. The coating layer is typically an insulating layer. The coating layer may have a similar structure to the intermediate layer described above, or it may have a different structure. The coating layer may be, for example, silicon dioxide (SiO₂ ２ ) It may also be a layer. 【0065】The coating layer preferably has a lower refractive index than the functional layer and the optical waveguide. For example, the refractive index of the coating layer for light with a wavelength of 500 nm is, for example, 1.3 to 1.6, preferably 1.4 or higher, and more preferably 1.5 or higher. 【0066】 The thickness of the coating layer is, for example, 0.3 μm to 5.0 μm, preferably 0.5 μm or more, more preferably 0.7 μm or more; it may also be, for example, 3.0 μm or less. 【0067】 B-8. The electrode composite substrate may have multiple electrodes as described above. The multiple electrodes may be provided at intervals above the functional layer of the composite substrate. Specifically, the electrodes may be provided directly on the functional layer, for example, or on an intervening layer (for example, an optical waveguide, a coating layer, or other component other than the functional layer). Typically, the electrodes may be provided on the functional layer or the coating layer. The electrodes may be formed by an appropriate method depending on the purpose. For example, they may be formed by an etching process or a lift-off process. 【0068】 The electrodes can be formed from any suitable conductive material. For example, the electrodes may be capable of forming the optical waveguide and coplanar waveguide (CPW) described above. Since commonly accepted configurations in the industry can be used for the electrodes, a detailed explanation is omitted. However, electrodes are not an essential component in the composite substrate and may be omitted if necessary. 【0069】 B-9. Other Layers Although not shown in the diagram, the composite substrate may further have any other layers as long as the objectives of the present invention can be achieved. For example, the composite substrate may further have another dielectric layer between the intermediate layer and the functional layer. Alternatively, for example, the composite substrate may have bonding layers between each layer that have a different composition from the other layers. The bonding layer may be, for example, a layer formed during bonding. The type, function, number, combination, arrangement, etc., of such layers can be appropriately set according to the purpose. 【0070】In the illustrated example, the charge trapping layer 20, the intermediate layer 30, and the functional layer 40 are provided on only one side of the support substrate 10. However, each layer may be provided on only the other side of the support substrate, or on both sides of the support substrate. 【0071】 C. Method for Manufacturing a Composite Substrate Hereinafter, typical examples of methods for manufacturing a composite substrate will be described with reference to the drawings (Figures 4A to 7C). Figures 4H, 5D, 6E, and 7C are identical in content to Figure 1. However, the methods for manufacturing a composite substrate 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 embodiments of the present invention. 【0072】 C-1. First Embodiment A method for manufacturing a composite substrate according to one embodiment of the present invention (referred to as the first embodiment) will be described with reference to Figures 4A to 4H. 【0073】 (Preparation of the support substrate) As shown in Figure 4A, first, the support substrate 10 is prepared. As the support substrate, any suitable substrate can be used insofar as it can achieve the objectives of the present invention. The substrates that can be used as the support substrate and the materials that make them up are as described in Section B-1 above. 【0074】 The support substrate 10 may be subjected to any suitable smoothing treatment on one or both sides as needed. The smoothing treatment may be performed, for example, by polishing the surface of the material. Any suitable polishing method may be employed. Examples of polishing methods include lapping and chemical mechanical polishing (CMP). The support substrate may be smoothed until the surface roughness Ra of the surface forming the charge trapping layer (first main surface) is preferably 10 nm or less. 【0075】(Formation of charge trapping layer) Next, as shown in Figure 4B, a charge trapping layer 20 is formed on the surface of the support substrate 10. The charge trapping layer 20 can be formed by an appropriate method depending on the purpose. For example, the charge trapping layer 20 can be formed by depositing a material for forming the charge trapping layer 20 onto the surface of the support substrate 10. Examples of deposition methods include sputtering, CVD, and ion-assisted deposition. In this embodiment, the charge trapping layer 20 can be formed by sputtering the material described in section B-2 onto the surface of the target object (support substrate 10). 【0076】 The surface of the charge trapping layer 20 may be smoothed as needed. When the charge trapping layer is smoothed, it may be polished until the surface roughness Ra on the side of the charge trapping layer opposite the support substrate is, for example, 1 nm or less. The polishing method may be the same as the method used to polish the support substrate described above. 【0077】 In this way, a laminate 50 having a support substrate 10 and a charge trapping layer 20 can be obtained, as shown in Figure 4B. 【0078】 (Formation of the intermediate layer) Next, as shown in Figure 4C, an intermediate layer 30 is formed on the charge trapping layer 20 of the laminate 50. Any suitable method can be used to form the intermediate layer 30 on the charge trapping layer 20. For example, the intermediate layer 30 can be formed by depositing a material for forming the intermediate layer 30 on the surface of the charge trapping layer 20. The film deposition method can be the same as described above. In this embodiment, the intermediate layer 30 can be formed by sputtering the surface of the charge trapping layer 20 with the dielectric material described in Section B-3 as the target. 【0079】 The surface of the intermediate layer 30 may be smoothed as needed. When the intermediate layer is smoothed, it may be polished until the surface roughness Ra on the side of the intermediate layer opposite the charge trapping layer is, for example, 1 nm or less. The polishing method may be the same as the method for polishing the support substrate described above. 【0080】 In this way, a laminate 51 having a support substrate 10, a charge trapping layer 20, and an intermediate layer 30 can be obtained, as shown in Figure 4C. 【0081】(Bonding of functional substrate) Next, as shown in Figure 4D, the intermediate layer 30 of the laminate 51 and the functional substrate 41 are bonded together. The functional materials that can be used as the functional substrate are as described in Section B-4 above. In this embodiment, electro-optical materials can be used as the functional material. 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. 【0082】 The surface of the functional substrate 41 (for example, the first main surface 41a (see Figure 1)) may be smoothed as needed. When the functional substrate is smoothed, it may be polished until the surface roughness Ra of the functional substrate is, for example, 1 nm or less. The polishing method may be the same as the method for polishing the support substrate described above. 【0083】 The intermediate layer 30 and the functional substrate 41 can be joined by any suitable method. Examples of joining methods include bonding with an adhesive; and direct bonding without an adhesive, such as surface activation bonding, plasma activation bonding, and atomic diffusion bonding. The joining method is preferably direct bonding. Direct bonding allows for thinning of the composite substrate and prevents adverse effects from adhesives. In this embodiment, the intermediate layer 30 and the functional substrate 41 are joined by plasma activation bonding. 【0084】 Direct bonding by plasma-activated bonding can be achieved by activating the bonding surfaces (intended bonding surfaces) of the intermediate layer and the functional substrate by plasma irradiation, then bringing these intended bonding surfaces into contact, and performing heat treatment as necessary. 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. 【0085】In this way, a bonded body 91 having a support substrate 10, a charge trapping layer 20, an intermediate layer 30, and a functional substrate 41 in that order can be obtained, as shown in Figure 4E. 【0086】 (Formation of Warpage Suppression Layer) If necessary, a warpage suppression layer may be formed on the surface of the support substrate 10 opposite to the charge trapping layer 20 in the laminate 51 (or bonded body 91). The warpage suppression layer can be formed at any appropriate timing. In this embodiment, the warpage suppression layer 60 is formed after obtaining the bonded body 91. The warpage suppression layer can be formed by any appropriate method. The warpage suppression layer 60 may be formed, for example, by depositing a film on the surface (second main surface) of the support substrate 10 opposite to the charge trapping layer 20 by sputtering, or by grinding the surface of the support substrate 10 opposite to the charge trapping layer 20. In this embodiment, the warpage suppression layer 60 is formed by grinding the surface (second main surface) of the support substrate 10 opposite to the charge trapping layer 20. Examples of grinding methods include the polishing methods described above, namely lapping and chemical mechanical polishing (CMP). 【0087】 In this way, a bonded body 92 having a warp suppression layer 60, a support substrate 10, a charge trapping layer 20, an intermediate layer 30, and a functional substrate 41 in this order can be obtained, as shown in Figure 4F. 【0088】(Formation of functional layer and optical waveguide) Next, the functional substrate 41 in the bonded body 91 (or bonded body 92) is thinned. The functional layer can be made by thinning the functional substrate to an appropriate thickness depending on the purpose. For example, the functional layer 40 can be formed by thinning the functional substrate 41 until its thickness is 1000 nm or less (see Figure 4G). The optical waveguide 400 may also be formed when thinning the functional substrate 41. Specifically, for example, the functional substrate 41 may be thinned to form the functional layer 40 and then the optical waveguide 400 may be provided on the functional layer 40, or the optical waveguide 400 may be provided on the functional substrate 41 and then the functional substrate 41 may be thinned to form the functional layer 40, or the optical waveguide 400 may be formed when thinning the functional substrate 41 and the functional layer 40 may be formed. Alternatively, for example, the optical waveguide 400 may be formed by bonding or other means using the same material as the functional substrate 41 or a different material after thinning the functional substrate 41. Any suitable method can be used for the thinning process. Examples of thinning processes include polishing, etching, and lift-off processes. 【0089】In this embodiment, the functional layer and optical waveguides are formed by an etching process. The etching process can be carried out, for example, as follows. First, the functional substrate 41 is polished to a predetermined thickness against the bonded body 92. Next, a resist is applied to the surface of the polished functional substrate 41 to form a resist film. Then, the resist film is exposed to and developed to form the pattern of the optical waveguide 400. As a result, a resist pattern corresponding to the optical waveguide 400 is formed on the surface of the functional substrate 41 in the bonded body 92. Next, etching is performed on the bonded body on which the resist pattern has been formed. Any suitable etching method can be used, and it may be dry etching or wet etching. As an example of a dry etching method, reactive ion etching (RIE) using Ar plasma can be used. By RIE, the portion of the functional substrate 41 other than the portion corresponding to the resist pattern is thinned and formed as a functional layer 40, and the portion of the functional substrate 41 corresponding to the resist pattern can be formed as a ridge-shaped optical waveguide 400. In this way, by forming a portion of the functional substrate 41 as an optical waveguide 400, a functional layer 40 having an optical waveguide 400 can be formed from the functional substrate 41. Therefore, this method has the advantage that the thinning of the functional substrate and the formation of the optical waveguide can be performed in the same process. 【0090】 In this way, a composite substrate 100 (101) can be obtained, as shown in Figure 4H, which comprises a warp suppression layer 60, a support substrate 10, a charge trapping layer 20, an intermediate layer 30, and a functional layer 40 in that order, with an optical waveguide 400 on the functional layer 40. 【0091】(Formation of coating layer, formation of electrodes) If necessary, a coating layer may be formed to cover the surface of the composite substrate opposite to the intermediate layer of the functional layer. As described above, the coating layer can be formed from any suitable material, insofar as the effects according to the embodiments of the present invention can be obtained. The coating layer can be formed, for example, by depositing a material for forming the coating layer so as to cover the functional layer (and optical waveguide). The material is as described in Section B-7. The deposition method may be sputtering or a different method. When forming the coating layer, it is preferable to cover not only the functional layer but also the optical waveguide with the coating layer. Typically, the coating layer can be configured to cover the functional layer and the optical waveguide. 【0092】 Electrodes may be provided on the functional layer of the composite substrate as needed. The electrodes may be provided directly on the functional layer or on the coating layer (if present). The electrodes may be formed, for example, by an etching process. Any suitable etching method may be employed, and it may be dry etching or wet etching. For example, reactive ion etching (RIE) using the Ar plasma described above can be used as an etching method. Specifically, the electrodes may be formed, for example, as follows: First, a material for forming electrodes is deposited on the functional layer or coating layer (if present) having an optical waveguide by any suitable film deposition method to form a metal film. A resist is applied to the surface of the metal film on the bonded body with the metal film deposited, in the same manner as the etching process for the functional substrate described above, to form a resist film. The resist film may be formed so that the electrodes have an appropriate shape according to the shape of the optical waveguide. Next, the electrode pattern is exposed to and developed on the resist film. This forms a resist pattern corresponding to the electrode on the surface of the metal film in the bonded body. Next, the bonded structure on which the resist pattern is formed is etched using RIE with Ar plasma, similar to the etching process for the functional substrate described above. RIE removes the metal film portions other than those corresponding to the resist pattern, and the metal film portions corresponding to the resist pattern are formed as electrodes on the surface of the functional layer or coating layer. 【0093】 By forming electrodes in this manner, a composite substrate 110 or 111 having electrodes 80 on a coating layer 70, as shown in Figure 2A or Figure 3A, can be obtained. 【0094】 (Other) The order of the above steps may be changed depending on the purpose, and each process may be performed accordingly. For example, forming the warp suppression layer on one surface of the support substrate (the surface opposite to the charge trapping layer) and joining the support substrate and the charge trapping layer may be performed in reverse order. For example, the warp suppression layer may be formed before forming the intermediate layer, or before joining the layers (for example, joining the intermediate layer and the functional substrate in the above embodiment). If the warp suppression layer is formed before forming the intermediate layer, the stress caused by the intermediate layer when forming the intermediate layer can be effectively reduced. If the warp suppression layer is formed before joining the layers, the stress when heated during joining can be effectively reduced. 【0095】In the manufacturing method according to the above embodiment, 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 intermediate 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 structure of the charge trapping 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. 【0096】 The support substrate (including semiconductor material) and the functional substrate (including functional material) may be cleaned using any suitable solvent before processing. Examples of cleaning methods include wet cleaning, dry cleaning, and scrubbing. Among these, scrubbing is preferred because it is simple and efficient. A specific example of scrubbing is a method in which a cleaning agent (e.g., Lion Corporation's Sunwash series) is used, followed by cleaning with a solvent (e.g., a mixed solution of acetone and isopropyl alcohol (IPA)) using a scrubbing machine. The cleaning process can remove contaminants (e.g., fine particles, metal impurities, organic matter, etc.) adhering to the surface. Furthermore, when performing the above-mentioned film formation, bonding, etc., it is preferable to clean the surface of each layer to remove, for example, abrasive residue, unwanted layers generated by processing, etc. 【0097】 C-2. Method for Manufacturing a Composite Substrate According to a Second Embodiment A method for manufacturing a composite substrate according to another embodiment of the present invention (hereinafter referred to as the second embodiment) will be described with reference to Figures 5A to 5D (and Figures 4E and 4F as necessary). Details that overlap with those described in the first embodiment will be omitted as appropriate. 【0098】 (Preparation of functional substrate) As shown in Figure 5A, first prepare the functional substrate 41. The functional substrate is the same as in the first embodiment described above. The surface of the functional substrate 41 may be smoothed as needed. The functional substrate may be polished until the surface roughness Ra is, for example, 1 nm or less. The same polishing method as described above may be used. 【0099】 (Formation of Intermediate Layer and Charge Trap Layer) Next, an intermediate layer 30 is formed on the functional substrate 41. Any suitable method can be used to form the intermediate layer 40. In this embodiment, the intermediate layer 30 can be formed on the functional substrate 41 by sputtering the material to be formed. The sputtering is the same as described in the first embodiment. The surface of the intermediate layer 30 may be smoothed as needed. When the intermediate layer 30 is smoothed, the surface roughness Ra of the surface of the intermediate layer 30 opposite to the functional substrate 41 can be polished to, for example, 1 nm or less. The polishing method can be the same as the polishing method described in the first embodiment. 【0100】 Next, as shown in Figure 5B, a charge trapping layer 20 is formed on the side of the intermediate layer 30 opposite to the functional substrate 41. The formation of the charge trapping layer 20 may be the same as described in the first embodiment. In this embodiment, the charge trapping layer 20 can be formed by sputtering the surface of the intermediate layer 30 using a material for forming the charge trapping layer 20 as a target. The surface of the charge trapping layer 20 may be smoothed as needed. The surface roughness Ra on the side of the charge trapping layer opposite to the support substrate may be polished to, for example, 1 nm or less. The polishing method may be the same as described above. 【0101】In this way, a laminate 52 having a functional substrate 41, an intermediate layer 30, and a charge trapping layer 20 in that order can be obtained, as shown in Figure 5B. 【0102】 (Bonding of support substrate and charge trapping layer) Prepare a support substrate. The same support substrate as the one prepared in the first embodiment may be used. Next, as shown in Figure 5C, the support substrate 10 and the charge trapping layer 20 of the laminate 52 are bonded together. Any suitable bonding method may be used. In this embodiment, as described in the first embodiment above, the support substrate 10 and the charge trapping layer 20 are directly bonded by a plasma-activated bonding method. 【0103】 In this way, a bonded body 91 (see Figure 4E) can be obtained having the support substrate 10, the charge trapping layer 20, the intermediate layer 30, and the functional substrate 41 in this order. 【0104】 (Formation of warp suppression layer) If necessary, a warp suppression layer may be formed on the surface of the support substrate 10 opposite to the charge trapping layer 20 in the bonded body 91. The warp suppression layer can be formed at any appropriate timing. In this embodiment, as in the first embodiment, the warp suppression layer 60 is formed after the bonded body 91 is obtained. 【0105】 In this way, a bonded body 92 (see Figure 4F) can be obtained having the warp suppression layer 60, the support substrate 10, the charge trapping layer 20, the intermediate layer 30, and the functional substrate 41 in this order. 【0106】 (Formation of functional layer and optical waveguide) Next, the functional substrate 41 in the bonded body 91 (or bonded body 92) is thinned to form a functional layer 40 (and an optical waveguide 400 if necessary). The method for forming the functional layer 40 and the optical waveguide 400 is the same as in the first embodiment described above. 【0107】 In this way, a composite substrate 100 (102) can be obtained, as shown in Figure 5D, which comprises a warp suppression layer 60, a support substrate 10, a charge trapping layer 20, an intermediate layer 30, and a functional layer 40 in that order, with an optical waveguide 400 on the functional layer 40. 【0108】In this embodiment as well, a coating layer and / or electrodes may be formed, similar to the first embodiment. Furthermore, the other steps described in the first embodiment may also be performed in the same manner. 【0109】 C-3. Method for Manufacturing a Composite Substrate According to a Third Embodiment A method for manufacturing a composite substrate according to yet another embodiment of the present invention (hereinafter referred to as the third embodiment) will be described with reference to Figures 6A to 6D (and Figures 4B and 4F as necessary). Details that overlap with those described in the first and second embodiments will be omitted as appropriate. 【0110】 (Preparation of support substrate and formation of charge trapping layer) As described in the first embodiment, the support substrate 10 is prepared and a laminate 50 having the support substrate 10 and the charge trapping layer 20 is fabricated (see Figure 4B). 【0111】 (Formation of oxide film on oxidizing substrate) Meanwhile, an oxidizing substrate 11 is prepared. Any suitable substrate that can be oxidized can be used as the oxidizing substrate. For example, the same material as the support substrate described above may be used as the oxidizing substrate 11. The oxidizing substrate is as described in section B-3. Specifically, for example, a silicon substrate can be used as the oxidizing substrate. Note that the oxidizing substrate is removed as described later, so it is not substantially an element that constitutes the composite substrate. 【0112】Next, as shown in Figure 6A, oxide films 31 (31a, 31b) are formed on both sides of the oxidizable substrate 11. The oxide films can be formed by any suitable method. For example, oxidation, sputtering, vapor deposition, and ion plating can be used to form the oxide films. Thermal oxidation is preferred as the oxidation method. That is, the oxide films are preferably formed by thermal oxidation of the oxidizable substrate. In this embodiment, the oxide films are formed by subjecting the oxidizable substrate to thermal oxidation. Thermal oxidation can have the remarkable effect of significantly suppressing defects in the formed oxide film. As a result, using the oxide film as an intermediate layer can contribute to improving the yield when manufacturing composite substrates. Thermal oxide films can usually be formed by oxidation at a temperature of 700°C or higher. As a result, thermal oxide films are less likely to contain components that can cause outgassing (such as hydrogen and moisture from within the formed film), and therefore have the advantage of having excellent thermal stability of the film quality in composite substrates. 【0113】 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, it is as follows: An oxidizing substrate is placed in a chamber, the inside of the chamber is heated to 700°C to 1200°C, and then an oxidizing atmosphere is created by supplying any suitable gas into the chamber, thereby oxidizing the oxidizing 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 oxidizing substrate and an oxide film can be formed. 【0114】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 can be adjusted by setting the processing temperature, processing time, and other conditions to any appropriate conditions so that the desired thickness is achieved. The thickness of the oxide film after thermal oxidation can be, for example, 0.05 μm or more and 30 μm or less, as described above. In this way, oxide films 31 (31a, 31b) can be formed on both sides of the oxidizable substrate 11 (see Figure 6A). 【0115】 The surface of the oxide film 31 may be smoothed as needed. When the surface of the oxide film is smoothed, the surface of the oxide film may be polished, for example, until the surface roughness Ra is preferably 1 nm or less. The polishing method may be the same as described above. One of the oxide films may be removed as described later and therefore does not need to be polished, and this one of the oxide films is not substantially an element constituting the composite substrate. 【0116】 In this way, a laminate 53 having an oxide film 31 (31b), an oxidizing substrate 11, and an oxide film 31 (31a) in that order can be obtained, as shown in Figure 6A. 【0117】 (Bonding of oxide film and functional substrate) Next, the oxide film 31a (or 31b) of the laminate 53 and the functional substrate 41 are bonded. Any suitable bonding method can be used. In this embodiment, as described in the first and second embodiments above, the functional substrate 41 and the oxide film 31 (oxide film 31a in the drawing) are directly bonded by a plasma-activated bonding method. 【0118】 In this way, a bonded body 94 having an oxide film 31 (31b), an oxidizing substrate 11, an oxide film 31 (31a), and a functional substrate 41 in this order can be obtained, as shown in Figure 6B. 【0119】(Removal of oxide film and oxidizing substrate) Next, the oxidizing substrate 11 and one of the oxide films 31 (oxide film 31b in the drawing) are removed from the bonded body 94. Removal can be done, for example, by grinding the bonded body 94 from the oxide film 31b down to the portion of the oxidizing substrate 11. Alternatively, one of the oxide films 31b and the oxidizing substrate 11, and a portion of the other oxide film 31a may be ground down to a thickness smaller than that of the original oxide film 31a. 【0120】 In this way, a bonded body 95 having a functional substrate 41 and an oxide film 31 (31a), as shown in Figure 6C, can be obtained. The oxide film 31a functions as an intermediate layer 30. 【0121】 (Joining of the laminate and the bonded body) Next, the oxide film 31a of the bonded body 95 and the charge trapping layer 20 of the laminate 50 are joined. The joining method is the same as described above. In this embodiment, the oxide film 31a and the charge trapping layer 20 are directly joined by a plasma-activated bonding method. 【0122】 In this way, a bonded body 96 having a support substrate 10, a charge trapping layer 20, an oxide film 31a, and a functional substrate 41 in this order can be obtained, as shown in Figure 6D. 【0123】 (Formation of warp suppression layer) If necessary, a warp suppression layer may be formed on the surface of the joint 95 (or joint 96) opposite to the charge trapping layer 20 of the support substrate 10. The warp suppression layer can be formed at any appropriate time, as in the first and second embodiments. In this embodiment, the warp suppression layer 60 is formed after obtaining the joint 96, in the same manner as in the first embodiment. 【0124】 In this way, a bonded body 92 (see Figure 4F) can be obtained having the warp suppression layer 60, the support substrate 10, the charge trapping layer 20, the intermediate layer 30, and the functional substrate 41 in this order. 【0125】 (Formation of functional layer and optical waveguide) Next, the functional substrate 41 in the bonded body 92 is thinned to form a functional layer 40 (and an optical waveguide 400 if necessary). The method for forming the functional layer 40 and the optical waveguide 400 is the same as in the first and second embodiments described above. 【0126】 In this way, a composite substrate 100 (103) can be obtained, as shown in Figure 6E, which comprises a warp suppression layer 60, a support substrate 10, a charge trapping layer 20, an intermediate layer 30 (31a), and a functional layer 40 in that order, with an optical waveguide 400 on the functional layer 40. 【0127】 In this embodiment as well, a coating layer and / or electrodes may be formed, similar to the first embodiment. Furthermore, the other steps described in the first embodiment may also be performed in the same manner. 【0128】 C-4. Method for Manufacturing a Composite Substrate According to the Fourth Embodiment A method for manufacturing a composite substrate according to yet another embodiment of the present invention (hereinafter referred to as the fourth embodiment) will be described with reference to Figures 7A to 7C (and Figure 4B as necessary). Details of explanations that overlap with those described in the first to third embodiments will be omitted. 【0129】 (Preparation of support substrate and formation of charge trapping layer) As described in the first or third embodiment, a support substrate 10 is prepared and a laminate 50 having the support substrate 10 and the charge trapping layer 20 is fabricated (see Figure 4B). In this embodiment, the thickness of the charge trapping layer may be adjusted to be greater than that of the charge trapping layer in the first and third embodiments and then sputtered. 【0130】(Oxidation of the Laminate) Next, oxide films 32 (32a, 32b) are formed on both sides of the laminate 50 by oxidizing both sides of the laminate 50. In the third embodiment, the method for forming the oxide films may be the same as the method for oxidizing an oxidizable substrate or forming a film on an oxidizable substrate. Preferably, thermal oxidation can be used as the method for oxidizing the laminate 50. By subjecting the laminate 50 to thermal oxidation, the surfaces of the support substrate 10 and the charge trapping layer 20 can be oxidized. In this way, an oxide film 32a can be formed on the charge trapping layer 20, where the surface of the charge trapping layer 20 is oxidized, and an oxide film 32b can be formed on the support substrate 10, where the surface opposite to the charge trapping layer 20 is oxidized. The oxide film 32a can function as an intermediate layer 30, and the oxide film 32b can function as a warpage suppression layer 60. In this embodiment, the warpage suppression layer 60 can be formed together with the intermediate layer 30, which has the advantage of contributing to improved productivity of the composite substrate. 【0131】 In this way, a laminate 54 can be obtained having a warp suppression layer 60 (32b), a support substrate 10, a charge trapping layer 20, and an intermediate layer 30 (oxide film 32a) in this order, as shown in Figure 7A. 【0132】 (Bonding of functional substrate) Next, the functional substrate 41 is bonded to the intermediate layer 30 (32a) of the laminate 54. Any suitable bonding method can be used. In this embodiment, the intermediate layer 30 and the functional substrate 41 are directly bonded by a plasma-activated bonding method, similar to the first embodiment described above. 【0133】 In this way, a bonded body 97 can be obtained having, in this order, a warp suppression layer 60 (32b), a support substrate 10, a charge trapping layer 20, an intermediate layer 30 (32a), and a functional substrate 41, as shown in Figure 7B. 【0134】 (Formation of functional layer and optical waveguide) Next, the functional substrate 41 in the bonded body 97 is thinned to form a functional layer 40 (and an optical waveguide 400 if necessary). The method for forming the functional layer 40 and the optical waveguide 400 is the same as in the first to third embodiments described above. 【0135】In this way, a composite substrate 100 (104) can be obtained, as shown in Figure 7C, which comprises a warp suppression layer 60, a support substrate 10, a charge trapping layer 20, an intermediate layer 30, and a functional layer 40 in that order, with an optical waveguide 400 on the functional layer 40. 【0136】 In this embodiment as well, a coating layer and / or electrodes may be formed, similar to the first embodiment. Furthermore, the other steps described in the first embodiment may also be performed in the same manner. 【0137】 D. 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 trap performance and improve high-frequency and / or harmonic characteristics. Therefore, the composite substrate according to the embodiment of the present invention can be particularly suitable for use in devices for high-frequency applications. 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. An optical modulation device is, for example, a Mach-Zehnder type optical modulator, which modulates light propagating in an optical waveguide by applying a voltage to a Mach-Zehnder interferometer formed with an optical waveguide having an electro-optic effect. Such an electro-optic element can be suitably used, for example, as an optical modulator in optical communication systems. 【0138】 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. 【0139】(1) Surface roughness Ra (arithmetic mean roughness (Ra)) - Surface roughness Ra of materials used in each process, laminates and joints 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. - Surface roughness Ra of the joint surface of the joint The composite substrate was cut with a focused ion beam (FIB), and the cut surface of the cut sample was observed with a TEM (transmission electron microscope: Hitachi High-Technologies Corporation, "H-9500") under conditions of acceleration voltage of 200 kV and total magnification of 1,000,000 times, and a TEM image was obtained. Based on the obtained image, the surface roughness Ra of the joint surface was calculated according to the calculation method described in JIS B0601:2013. Here, the cutoff value (λc) was set to 50 nm, and the evaluation length was set to 50 nm × 5 = 250 nm for the calculation. 【0140】 (2) Concentration Measurement (Oxygen Concentration and Carbon Concentration) The oxygen and carbon concentrations of the support substrate in the composite substrates of each example and comparative example 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. 【0141】(3) Evaluation: High-frequency characteristics For each example and comparative example of composite substrate, a coplanar waveguide (CPW) was formed on the functional layer by a lift-off process to form a coating layer and electrodes (Au film with a thickness of 1.0 μm) to create a coplanar waveguide (CPW) and prepare samples for evaluating high-frequency characteristics. The CPW was formed by the following procedure. Details of the coplanar waveguide are shown in Figures 8A and 8B, 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 (FormFactor I110-A-GSG-100) was brought into contact with both ends of the coplanar waveguide in the longitudinal direction, and the insertion loss (S21) from 1 GHz to 120 GHz was measured using a network analyzer (keyseeight N5291A). Measurement S21 was performed at five points within the plane of each substrate. Insertion loss was measured at 0.01 GHz intervals within the above range. Based on the results obtained from the measurements, the absolute value of S21 was used as the insertion loss and evaluated according to the following criteria: A (Good): Insertion loss of 0.90 dB / mm or less at 100 GHz B (Poor): Insertion loss greater than 0.90 dB / mm at 100 GHz 【0142】 [Example 1] As a support substrate, a single-crystal silicon substrate (hereinafter simply referred to as the silicon substrate) having an orientation flat portion (OF portion), a diameter of 6 inches (150 mm), and a thickness of 675 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 a carrier type of p-type. The surface orientation was (111). The surface roughness Ra of both surfaces of the silicon substrate was 0.5 nm. Next, the silicon substrate was cleaned with 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. 【0143】As a functional substrate, a lithium niobate single crystal substrate (hereinafter referred to as LN substrate) was prepared, which is an electro-optic substrate with an OF (Optical Field) portion, a diameter of 6 inches (150 mm), and a thickness of 250 μm. An 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 to remove impurities and other contaminants from the surface of the LN substrate. 【0144】 Next, SiO ２ Using Ar gas as the target, silicon oxide (SiO) is sputtered onto the surface of the LN substrate. ２ A film was formed using SiO as an intermediate layer on the surface of the LN substrate. ２ A film was formed. SiO ２ The film thickness was 2.5 μm. 【0145】 Next, SiO ２ An a-Si film was deposited on the surface of the intermediate layer by sputtering with Ar gas, using amorphous silicon (a-Si) as the target. This formed an a-Si film as a charge trapping layer on the surface of the intermediate layer. The thickness of the a-Si film was 100 nm. This resulted in a laminate having the configuration of LN substrate / intermediate layer / charge trapping layer. 【0146】 Next, the surface of the charge trapping layer of the laminate (the surface intended for bonding) was smoothed by CMP processing until the surface roughness Ra was 0.5 nm. Then, the surface of the charge trapping layer was cleaned in the same manner as the silicon substrate to remove impurities and other contaminants from the surface of the charge trapping layer. 【0147】 Next, the charge trapping layer of the cleaned laminate and the silicon substrate were directly bonded by a plasma activation method to obtain a bonded body having the configuration of LN substrate / intermediate layer / charge trapping layer / silicon substrate. 【0148】 Next, the side of the silicon substrate of the laminate opposite to the charge trapping layer was smoothed by CMP processing until the surface roughness Ra was 0.5 nm. Then, SiO was applied to the polished surface of the silicon substrate of the bonded structure. ２Using Ar gas as the target, silicon oxide (SiO) is sputtered onto the surface of the support substrate. ２ A film was formed using SiO as a warpage suppression layer on the side of the support substrate opposite to the charge trapping layer. ２ A film was formed. SiO ２ The film thickness was 2.5 μm. 【0149】 Next, the LN substrate of the bonded assembly was subjected to a thinning treatment. Specifically, the thinning treatment involved placing the bonded assembly in a nitrogen atmosphere oven (120°C) and heating it for 10 hours. After removing the LN substrate from the oven, grinding and lapping were performed, followed by CMP processing to create an LN layer (functional layer) with a thickness of 500 nm. Subsequently, the surface of the LN layer was smoothed to achieve a surface roughness Ra of 1.0 μm on the surface of the LN layer (the side opposite the intermediate layer). 【0150】 As described above, a composite substrate having the configuration of LN layer / intermediate layer / charge trapping layer / silicon substrate / warpage suppression layer was obtained. The obtained composite substrate was subjected to the evaluations described in (1) to (3) above. The results are shown in Table 1. 【0151】 [Comparative Example 1 and Examples 2-4] (Comparative Example 1) As a support substrate, the oxygen concentration was 3.0 × 10 １８ atoms / cm ３ A composite substrate having a functional layer / intermediate layer / charge trapping layer / support substrate / warpage suppression layer was fabricated in the same manner as in Example 1, except that a single-crystal silicon substrate was used. The carbon concentration of the single-crystal silicon substrate used was 3.0 × 10⁻¹⁴. １６ atoms / cm ３ The electrical resistivity was 3.0 kΩ·cm, and the carrier type was p-type. 【0152】 (Example 2) A composite substrate was fabricated in the same manner as in Example 1, except that a single-crystal silicon substrate with a crystal orientation of (100) was used as the support substrate. 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, and the carrier type was p-type. 【0153】(Example 3) As a support substrate, an oxygen concentration of 5.0 × 10 １６ atoms / cm ３ A composite substrate was fabricated in the same manner as in Example 2, except that a single-crystal silicon substrate was used. 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 p-type, and the surface orientation was (100). 【0154】 (Example 4) A composite substrate was fabricated in the same manner as in Example 3, except that a single-crystal silicon substrate with an n-type carrier was used as the support substrate. The carbon concentration of the single-crystal silicon substrate was 3.0 × 10⁻¹⁶. １６ atoms / cm ３ The electrical resistivity was 3.0 kΩ·cm, and the surface orientation was (100). 【0155】 As described above, composite substrates having the configuration of LN layer / intermediate layer / charge trapping layer / silicon substrate / warpage suppression layer were obtained in Comparative Example 1 and Examples 2 to 4, respectively. The obtained composite substrates were subjected to the evaluations described in (1) to (3) above. In addition, as in Example 1, regions affected by the knock-on effect were excluded from the evaluation when measuring the concentration. The results are shown in Table 1. 【0156】 [Example 5] As the support substrate, a single-crystal silicon substrate (with an oxygen concentration of 8.0 × 10) similar to that in Example 2 was used. １７ atoms / cm ３ , surface orientation (100), p-type, electrical resistivity 3.0 kΩ·cm, carbon concentration 3.0 × 10 １６ atoms / cm ３ A cleaning solution was prepared and the silicon substrate was cleaned. 【0157】 As the functional substrate, an LN substrate (X-cut LN) similar to that in Example 1 was prepared. The surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm, as in Example 1, and then cleaned in the same manner as the silicon substrate to remove impurities and other contaminants from the surface of the LN substrate. 【0158】Next, sputtering was performed on one surface of the silicon substrate using Ar gas, with amorphous silicon (a-Si) as the target. This deposited an a-Si film on the silicon substrate surface, forming a charge trapping layer. The thickness of the a-Si film was 100 nm. Subsequently, SiO was applied to the charge trapping layer. ２ Using Ar gas as the target, silicon oxide (SiO) is sputtered onto the surface of the LN substrate. ２ A film was formed using SiO as an intermediate layer on the surface of the LN substrate. ２ A film was formed. SiO ２ The film thickness was 2.5 μm. This resulted in a laminate having the configuration of an intermediate layer / charge trapping layer / silicon substrate. 【0159】 Next, the surface of the intermediate layer of the laminate (the surface to be bonded) was smoothed by polishing using CMP (Chemical Polishing Machine) until the surface roughness Ra was 0.5 nm. Then, the surface of the intermediate layer was cleaned in the same manner as the silicon substrate to remove impurities and other contaminants from the surface of the intermediate layer. 【0160】 Next, the intermediate layer of the cleaned laminate and the LN substrate were directly bonded by a plasma activation method. In this way, a bonded body having the configuration of LN substrate / intermediate layer / charge trapping layer / silicon substrate was obtained. 【0161】 Next, in the same manner as in Example 1, the surface of the silicon substrate of the bonded body opposite to the charge trapping layer was smoothed by CMP processing until the surface roughness Ra was 0.5 nm, and then SiO ２ Using Ar gas as the target, sputtering is performed on the surface of the support substrate to obtain SiO ２ A thin film is formed, and SiO is used as a warpage suppression layer. ２ A film was formed. SiO ２ The film thickness was 2.5 μm. 【0162】 Next, a functional layer (LN layer) was formed in the same manner as in Example 1. In this way, a composite substrate comprising an LN layer, an intermediate layer, a charge trapping layer, a silicon substrate, and a warp suppression layer was obtained. The obtained composite substrate was subjected to the evaluations described in (1) to (3) above. The results are shown in Table 1. 【0163】 [Example 6] As the support substrate, a single-crystal silicon substrate (with an oxygen concentration of 8.0 × 10) similar to that in Example 2 was used. １７ atoms / cm ３ , surface orientation (100), p-type, electrical resistivity 3.0 kΩ·cm, carbon concentration 3.0 × 10 １６ atoms / cm ３ A silicon substrate was prepared and cleaned. Next, an a-Si film was formed on the silicon substrate as a charge trapping layer by sputtering, in the same manner as in Example 1. This resulted in obtaining a laminate having a silicon substrate / charge trapping layer configuration. 【0164】 As the functional substrate, an LN substrate (X-cut LN) similar to that in Example 1 was prepared. The surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm, as in Example 1, and then cleaned in the same manner as the silicon substrate to remove impurities and other contaminants from the surface of the LN substrate. 【0165】 As the oxidizing substrate, the same single-crystal silicon substrate as the support substrate described above was prepared. A thermal oxide film was formed on both sides of the oxidizing substrate by thermal oxidation. Thermal oxidation was carried out as follows: The oxidizing 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, oxygen and water vapor were supplied to create an oxidizing atmosphere, and wet oxidation was performed on the silicon substrate. Subsequently, the laminate with oxide films formed on both sides of the oxidizing substrate was removed. The thickness of the oxide film on each side was 4.5 μm. 【0166】 Next, the oxide film surface of one of the laminates having the oxidizing substrate on which the oxide film was formed was smoothed by CMP processing until the surface roughness Ra was 0.5 nm. Then, after cleaning the laminate and the LN substrate, the LN substrate and the polished oxide film were directly bonded by a plasma activation method. In this way, a bonded body having the configuration of LN substrate / oxide film / oxidizing substrate / oxide film was obtained. 【0167】Subsequently, the outermost oxide film on the opposite side of the LN substrate of the above laminate and the oxidizing substrate were sequentially removed by grinding. Thus, a joined body having a structure of LN substrate / oxide film was obtained. 【0168】 Subsequently, in the above laminate (silicon substrate / charge trapping layer) and the above joined body (LN substrate / oxide film), the surfaces of the charge trapping layer and the oxide film were smoothed by polishing by CMP processing until the surface roughness Ra became 0.5 nm. Next, the charge trapping layer of the laminate and the oxide film of the joined body were directly joined by a plasma activation method. 【0169】 Subsequently, the intermediate layer of the above laminate after washing and the LN substrate were directly joined by a plasma activation method. Thus, a joined body having a structure of LN substrate / intermediate layer / charge trapping layer / silicon substrate was obtained. 【0170】 Subsequently, in the same manner as in Example 1, the surface of the silicon substrate of the joined body on the side opposite to the charge trapping layer was smoothed by polishing by CMP processing until the surface roughness Ra became 0.5 nm, and then SiO ２ was used as a target, and Ar gas was used to perform sputtering to form SiO ２ on the surface of the support substrate, and an SiO ２ film was formed as a warpage suppression layer. The thickness of the SiO ２ film was 2.5 μm. 【0171】 Subsequently, in the same manner as in Example 1, a functional layer (LN layer) was formed. As described above, a composite substrate including an LN layer / intermediate layer / charge trapping layer / silicon substrate / warpage suppression layer was obtained. The obtained composite substrate was subjected to the evaluations (1) to (3) above. The results are shown in Table 1. 【0172】 [Example 7] A composite substrate was produced in the same manner as in Example 1, except that a polycrystalline silicon (poly-Si) film was formed as the charge trapping layer instead of a-Si. The poly-Si was formed on the Si substrate of the support substrate by the LP-CVD (low-pressure CVD) method to a thickness of 100 nm. SiO ２After forming the layers (intermediate layers), a functional layer was formed in the same manner as in Example 1 to obtain a composite substrate. 【0173】 The obtained composite substrates were subjected to the evaluations described in (1) to (3) above. The results are shown in Table 1. 【0174】 【0175】 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 following results demonstrate that the insertion loss is reduced. Furthermore, it was confirmed that the reflection loss (S11) for each embodiment was -20 dB or less (reflection of 1% or less). On the other hand, for Comparative Example 1, the insertion loss was not smaller than that of any of the embodiments in the measurement range from 1 GHz to 120 GHz. Thus, it is suggested that the composite substrates of each embodiment have a charge trapping layer that can function as a charge trapping layer and can be applied to applications requiring excellent electrical characteristics. 【0176】 The composite substrate according to the embodiment of the present invention can be suitably used in high-frequency devices such as optical modulators. 【0177】 10 Support substrate 20 Charge trapping layer 30 Intermediate layer 40 Functional layer 41 Functional substrate 400 Optical waveguide 400a Input section 400b Output section 400c Branch section 400d Coupling section 401 First waveguide 402 Second waveguide 60 Warping suppression layer 70 Coating layer 80 Electrode 81 First electrode 82 Second electrode 83 Third electrode 100, 101, 102, 103, 104, 110, 111 Composite substrate

Claims

1. The device comprises a support substrate, a charge trapping layer, an intermediate layer, and a functional layer in this order, and the oxygen concentration of the support substrate is 1.0 × 10⁻⁶. １８ atoms / cm ３ A composite substrate wherein the charge trapping layer has a crystalline structure or amorphous structure different from that of the support substrate.

2. The composite substrate according to claim 1, further comprising a warp suppression layer on the side of the support substrate opposite to the charge trapping layer for suppressing warping of the composite substrate.

3. The composite substrate according to claim 1, wherein the thickness of the charge trapping layer is 0.1 μm or more and 3 μm or less.

4. The composite substrate according to claim 1, wherein the charge trapping layer has a polycrystalline structure.

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

6. The composite substrate according to claim 1, wherein the electrical resistivity of the support substrate is 3 kΩ·cm or more.

7. The composite substrate according to claim 1, wherein the support substrate has an n-type region.

8. The composite substrate according to claim 1, wherein the thickness of the intermediate layer is 2.0 μm or more and 30 μm or less.

9. The composite substrate according to claim 1, wherein, on the first main surface of the support substrate on the charge trapping layer side and on the second main surface opposite to the first main surface, the surface roughness Ra of the first main surface is smaller than the surface roughness Ra of the second main surface.

10. The composite substrate according to claim 1, wherein the surface roughness Ra of the functional layer on the intermediate layer side is 1.0 nm or less.

11. The composite substrate according to claim 1, wherein the surface roughness Ra of the functional layer on the side opposite to the intermediate layer is 0.5 nm or less.

12. The composite substrate according to claim 1, for high-frequency applications.

13. The composite substrate according to claim 1, further comprising a coating layer on the functional layer that covers the surface of the functional layer opposite to the intermediate layer.

14. The composite substrate according to claim 1, further comprising an optical waveguide on the functional layer.

15. The composite substrate according to claim 14, wherein the optical waveguide is a ridge waveguide.

16. The composite substrate according to claim 14, further comprising a plurality of electrodes on the functional layer.

17. The composite substrate according to claim 16, wherein the electrodes are provided spaced apart in a direction perpendicular to the optical waveguide in a plan view.

18. The composite substrate according to claim 14, wherein the optical waveguide has an input section into which light is input and an output section that outputs modulated light obtained by modulating the light by an electric field.

19. The composite substrate according to claim 18, wherein the optical waveguide comprises: a first waveguide and a second waveguide provided between the input section and the output section, respectively; a branching section between the input section and the first waveguide and the second waveguide, which branches the light from the input section toward the first waveguide and the light from the input section toward the second waveguide; and a coupling section between the output section and the first waveguide and the second waveguide, which couples the modulated light from the first waveguide toward the output section and the modulated light from the second waveguide toward the output section.

20. The composite substrate according to claim 19, comprising: a first electrode provided on the functional layer between the first waveguide and the second waveguide; a second electrode provided on the functional layer opposite to the first electrode and facing the first electrode on the side of the first electrode opposite to the second waveguide; and a third electrode provided on the functional layer opposite to the first electrode and facing the second electrode on the side of the first electrode opposite to the first waveguide, wherein each of the first electrode, the second electrode and the third electrode does not overlap with either the first waveguide or the second waveguide and is provided parallel to the first waveguide and the second waveguide in a plan view.

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