Method for producing composite substrate

Abstract

Provided is a method for producing a composite substrate which can form a modified layer of a thickness that is a prescribed value or greater and which can suppress peeling of an active layer. A method for producing a composite substrate according to an embodiment of the present invention comprises: forming an active layer on at least one surface of a support substrate that has crystallinity; and forming a modified layer on a support substrate surface side by irradiating a surface of the support substrate with a pulse laser from the active layer side. The method comprises performing irradiation with the pulse laser such that the thickness of the modified layer becomes 10 nm or greater.

Description

Manufacturing method for composite substrates

[0001] This invention relates to a method for manufacturing a composite substrate.

[0002] Information and communication equipment uses functional elements such as surface acoustic wave elements (e.g., SAW filters) that utilize surface acoustic waves, and electro-optic elements (e.g., optical modulators) that can change the phase of light, in order to extract electrical signals of arbitrary frequencies. In recent years, the amount of data transmitted in the field of information and communication equipment has been rapidly increasing, and there is a demand for higher performance composite substrates that can be used for functional elements. To improve the performance of composite substrates, for example, Patent Document 1 proposes a composite substrate in which a charge trapping layer, such as polycrystalline silicon, is provided as an intermediate layer between the piezoelectric layer and the support substrate. In the composite substrate of Patent Document 1, the charge trapping layer is provided as a trap layer to capture free charges (charge carriers) in the support substrate.

[0003] Patent documents 2 and 3 describe a composite substrate comprising a base layer, a damaged layer, a silicon dioxide layer, and a single-crystal piezoelectric layer in that order, and both documents describe forming the damaged layer by laser etching. However, the methods described in patent documents 2 and 3 have the problem that the silicon dioxide layer peels off. Furthermore, the methods described in patent documents 2 and 3 make it difficult to form a damaged layer with a thickness greater than a predetermined value.

[0004] Japanese Patent No. 6612872, Chinese Patent No. 113541626 Specification, Chinese Patent No. 111755588 Specification

[0005] The main object of the present invention is to provide a method for manufacturing a composite substrate that can form a modified layer with a thickness greater than or equal to a predetermined value and can suppress peeling of the active layer.

[0006] [1] A method for manufacturing a composite substrate according to an embodiment of the present invention includes: forming an active layer on at least one surface of a crystalline support substrate; forming a modified layer on the surface side of the support substrate by irradiating the surface of the support substrate with a pulsed laser from the side of the active layer; and irradiating the modified layer with the pulsed laser such that the thickness of the modified layer is 10 nm or more. [2] In the method for manufacturing a composite substrate according to [1] above, the pulse width of the pulsed laser may be less than 100 ps. [3] In the method for manufacturing a composite substrate according to [1] or [2] above, the energy density of the pulsed laser is 10 mJ / cm². ２ ~5000mJ / cm ２ It may be within the range of [1] to [3] above. [4] In the method for manufacturing a composite substrate according to any of [1] to [3] above, the wavelength of the pulse laser λ ａ [nm] may satisfy the following equation (1). λ ａ ≥ 1240 / (E ｇ１ +2.1) ... (1) In equation (1), E ｇ１∫ is the band gap of the support substrate. [5] In the method for manufacturing a composite substrate according to any one of [1] to [4] above, the pulsed laser may be irradiated at intervals in at least one direction within the plane of the support substrate. [6] In the method for manufacturing a composite substrate according to any one of [1] to [5] above, forming the active layer and irradiating with the pulsed laser may be included in this order. [7] In the method for manufacturing a composite substrate according to any one of [1] to [6] above, a modified layer may be formed at the interface between the support substrate and the active layer. [8] In the method for manufacturing a composite substrate according to any one of [1] to [7] above, before forming the active layer, the surface roughness Ra of the support substrate may be smoothed to 10 nm or less. [9] In the method for manufacturing a composite substrate according to any one of [1] to [8] above, the active layer may include an oxide film layer, and the oxide film layer may be formed by oxidizing the support substrate.

[10] In the method for manufacturing a composite substrate according to [9] above, the oxide film layer may be formed by thermal oxidation of the support substrate.

[11] In the method for manufacturing a composite substrate according to any one of [1] to

[10] above, the active layer may be smoothed until the surface roughness Ra of the active layer is 1 nm or less.

[12] In the method for manufacturing a composite substrate according to [9] or

[10] above, a functional substrate may be bonded to the oxide film layer on the side opposite to the support substrate.

[13] In the method for manufacturing a composite substrate according to

[12] above, after bonding the functional substrate to the oxide film layer, the functional substrate may be thinned to a thickness of 1000 nm or less to form a functional layer.

[14] In the method for manufacturing a composite substrate according to

[12] or

[13] above, an intermediate layer may be formed between the oxide film layer and the functional substrate before bonding the oxide film layer and the functional substrate.

[15] In the method for manufacturing a composite substrate according to any one of [1] to

[14] above, the pulsed laser may be irradiated such that the area ratio of the modified layer to the total area in a plan view is 40% or more.

[16] The method for manufacturing a composite substrate according to any one of [1] to

[15] above may also include forming electrodes on the active layer.

[0007] According to embodiments of the present invention, a composite substrate can be realized that can form a modified layer with a thickness greater than or equal to a predetermined value and that can suppress peeling of the active layer.

[0008] This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to one embodiment. the general configuration of a composite substrate according to one embodiment of the present invention. This is a schematic cross-sectional view illustrating the general configuration when the composite substrate of Figure 2A is equipped with electrodes. This is a TEM image showing an example of a partial cross-section of a composite substrate according to one embodiment. This is a TEM image showing a partial cross-section of a composite substrate in Example 1. This is a TEM image showing a partial cross-section of a composite substrate in Example 2. This is a TEM image showing a partial cross-section of a composite substrate in Example 4. This is a TEM image showing a partial cross-section of a composite substrate in Comparative Example 1.

[0009] Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Note that the drawings are schematic for clarity, and the thickness, length, width, shape, proportions, etc., do not accurately reflect the actual shape.

[0010] A. Overview of the Method for Manufacturing a Composite Substrate The method for manufacturing a composite substrate according to an embodiment of the present invention includes: forming an active layer on at least one surface of a crystalline support substrate; and forming a modified layer on the surface side of the support substrate (typically, at the interface between the support substrate and the active layer) by irradiating the surface of the support substrate with a pulsed laser from the active layer side. Figures 1A to 1D are explanatory diagrams illustrating the general method for manufacturing a composite substrate according to one embodiment of the present invention. Figure 2A is a schematic cross-sectional view of a composite substrate that can be obtained by the method for manufacturing a composite substrate according to the embodiments shown in Figures 1A to 1D. In the method for manufacturing a composite substrate according to one embodiment, as shown in Figure 1A, a support substrate 10 is first prepared. The support substrate 10 is typically crystalline. The support substrate 10 may be, for example, a single-crystal substrate or a polycrystalline substrate. Details of the support substrate will be described in section B-1. Next, as shown in Figure 1B, an active layer 20 is formed on at least one surface (10a and / or 10b) of the support substrate 10. In the illustrated example, an oxide film layer 21 is formed as the active layer 20. The oxide film layer 21 is one layer that can constitute the active layer. The active layer 20 includes any suitable chemical and / or physically active layer. The active layer may consist of a single layer or multiple layers. The active layer can be provided in any suitable way. Details of the active layer are described in sections B-2 and B-4 to B-6. Next, as shown in Figure 1C, a pulsed laser is irradiated from the active layer 20 side to form a modified layer 30 at the interface between the support substrate 10 and the active layer 20, for example, as shown in Figure 1D. The modified layer 30 is typically a layer in which the crystallinity of the surface of the support substrate 10 has been modified. In the illustrated example, the modified layer 30 is composed of a plurality of modified portions 301. Details of the modified layer are described in section B-3. In this way, a composite substrate 100 comprising the support substrate 10, the modified layer 30 and the active layer 20 in this order can be manufactured (see Figures 1D and 2A).

[0011] A method for manufacturing a composite substrate according to an embodiment of the present invention typically involves irradiating the modified layer with a pulsed laser so that the thickness of the modified layer is 10 nm or more. The composite substrate obtained in this way has a modified layer with a thickness of a predetermined value or more, and peeling of the active layer is suppressed. The inventors have found that even when attempting to form a modified layer on a support substrate by forming an active layer (e.g., an oxide film layer) on the support substrate and irradiating it with a laser, it may not be possible to form an active layer of the desired thickness, and / or the active layer may peel off from the support substrate. More specifically, they have found that when a laser is irradiated from the active layer side at the interface between the support substrate and the active layer (typically an oxide film layer), the laser may not be able to reach the desired depth inside the support substrate, and the support substrate may not be modified to have a thickness of a predetermined value or more, and a thermal reaction may occur between the support substrate and the oxide film layer, making ablation (such as instantaneous evaporation) more likely, which may cause the active layer to peel off from the support substrate. Therefore, after diligent investigation, the inventors discovered that by irradiating the surface side of the support substrate, more specifically the surface side of the support substrate (typically the interface between the support substrate and the active layer) with a pulsed laser so that the thickness of the modified layer becomes 10 nm or more, it is possible to make the laser light reach a desired depth inside the support substrate while minimizing ablation. Accordingly, the inventors found that the manufacturing method of the composite substrate according to the embodiment of the present invention can produce a composite substrate in which a modified layer with a thickness of a predetermined value or more can be formed and peeling of the active layer can be suppressed. The irradiation conditions of the pulsed laser will be described in detail in Section B. Furthermore, the manufacturing method of the composite substrate according to the embodiment of the present invention has the advantage that a modified layer that can function as a charge trapping layer can be formed by irradiation with a pulsed laser. That is, the manufacturing method of the composite substrate according to this embodiment does not require special equipment to ensure safety as in the case of formation by the CVD method, so the increase in cost can be suppressed. Accordingly, this embodiment has the advantage that composite substrates can be manufactured at low cost.

[0012] In one embodiment, the method for manufacturing a composite substrate further includes providing a functional substrate on a support substrate or an oxide film layer. In the method for manufacturing a composite substrate of this embodiment, a functional layer can be formed by providing a functional substrate and, if necessary, thinning the functional substrate. The functional layer is typically a separate layer from the oxide film layer that can constitute an active layer. Figures 1E to 1H show an embodiment in which a functional substrate 41 is further provided on the oxide film layer 21 of the composite substrate 100 shown in Figures 1D and 2A. In the method for manufacturing a composite substrate of one embodiment, as shown in Figure 1E, the functional substrate 41 is first prepared. The functional substrate 41 can be made of any suitable material depending on the desired function of the functional layer. Details of the functional substrate will be described in section B-4-1. Next, as shown in Figure 1F, the oxide film layer 21 of the composite substrate 100 (composite 100') shown in Figure 1D (and Figure 2A) and the functional substrate 41 are joined together. By joining in this manner, the joined body 102' shown in Figure 1G can be obtained. Next, the functional substrate 41 of the bonded body 102' shown in Figure 1G is thinned to a desired thickness. By thinning, a functional layer 22 can be formed as shown in Figure 1H. Preferably, the functional substrate is thinned until its thickness is 1000 nm or less. Details of the functional layer will be explained in section B-4. In this way, a composite substrate 102 can be obtained, which has a support substrate 10, a modified layer 30, and an active layer 20 (oxide film layer 21 and functional layer 22) in this order, as shown in Figure 1H. The manufacturing method of the composite substrate according to this embodiment, similar to the above embodiment shown in Figures 1A to 1D, typically includes irradiating with a pulsed laser so that the thickness of the modified layer is 10 nm or more. Therefore, the composite substrate that can be obtained has a modified layer with a thickness of a predetermined value or more, and peeling of the active layer is suppressed. Furthermore, the composite substrate that can be obtained according to this embodiment has the advantage that the modified layer can function more effectively as a charge trapping layer.

[0013] In one embodiment, the method for manufacturing a composite substrate includes irradiating a support substrate with pulsed lasers at intervals in at least one direction within its plane. In this way, a plurality of modified portions can be formed at intervals from one another, as shown in the illustrated example. Thus, the modified layer 30 may be formed partially. Specifically, the modified layer may be composed of a plurality of modified portions. In the illustrated example, the modified layer 30 is composed of a plurality of modified portions 301. The thickness, shape, regularity, etc., of the modified layer and the plurality of modified portions will be described in detail in section B.

[0014] In one embodiment, the method for manufacturing a composite substrate includes forming electrodes on the active layer. Multiple electrodes 50 may be provided on the active layer 20 at intervals (see Figure 2B). In this way, a composite substrate having electrodes can be obtained. In the illustrated example, the composite substrate 101 has multiple (three in the drawing) electrodes 50 formed on the active layer 20 at intervals. The number of electrodes and the spacing between the multiple electrodes can be appropriately set depending on the purpose. However, electrodes are not an essential component of a composite substrate and may be omitted if necessary.

[0015] In the illustrated example, the modified layer 30 and the active layer 20 are formed in this order only on one surface of the support substrate 10 (the upper surface in the illustrated example). However, the modified layer and the active layer may be formed in this order only on the other surface of the support substrate (the lower surface), or the modified layer and the active layer may be formed in this order on both the upper and lower surfaces of the support substrate. In the illustrated example, the pulsed laser irradiation is performed with the active layer formed on the support substrate. However, as long as the objective of the present invention can be achieved, the pulsed laser may be irradiated, for example, before the active layer is formed. For example, the pulsed laser may be irradiated on the support substrate alone, or the pulsed laser may be irradiated on the support substrate with the oxide film layer and the functional layer provided as the active layer.

[0016] The composite substrate can be manufactured in any suitable shape. In one embodiment, the composite substrate can be manufactured in the form of a so-called wafer. The size of the composite substrate can be appropriately set according to the purpose. The diameter of the wafer is, for example, 100 mm to 200 mm. The total thickness of the composite substrate (without a functional layer) may be, for example, 100 μm to 1500 μm. The total thickness of the composite substrate (with a functional layer) may be, for example, 100 μm to 1500 μm. Note that the thickness of the electrodes (if present) is not included in the total thickness of the composite substrate.

[0017] In this specification, the active layer may typically include at least one of an oxide layer, a functional layer, and an intermediate layer. The active layer preferably includes either or both of the oxide layer and / or the functional layer. In the illustrated examples, in Figures 1A to 1D and 2A, the active layer 20 includes an oxide layer 21, and in Figures 1E to 1H, the active layer 20 includes an oxide layer 21 and a functional layer 22. Specifically, the active layer may consist of, for example, only an oxide layer, or only a functional layer, or it may consist of an oxide layer and a functional layer in that order from the support substrate side, or for example, the active layer may further have an intermediate layer between the oxide layer and the functional layer, or it may consist of an oxide layer, an intermediate layer and a functional layer in that order from the support substrate side. Also, for example, there may be bonding layers between each of the layers that can constitute the active layer. The bonding layer may be, for example, a layer provided when bonding an oxide film layer to a functional layer, a layer provided when bonding an oxide film to an intermediate layer, and / or a layer provided when bonding a functional layer to an intermediate layer. The type, function, number, combination, arrangement, etc., of such layers can be appropriately set according to the purpose.

[0018] In this specification, "modified layer" means a layer having regions in which the density, refractive index, mechanical strength, physical properties, etc., have been modified relative to the support substrate. For example, the modified layer is composed of the same elements as the support substrate, but is modified to have regions in which it has different properties from the support substrate. The modified layer can be distinguished from the support substrate and the active layer by the presence or absence of crystallinity, differences in crystallinity, etc.

[0019] B. Details of the Manufacturing Method for Composite Substrates Below, typical examples of manufacturing methods for composite substrates will be described with reference to Figures 1A to 1H. Figure 1D is identical to Figure 2A. Sections B-1 to B-7 below describe the manufacturing methods for cases where the active layer of the composite substrate comprises only an oxide film layer, and cases where it comprises both an oxide film layer and a functional layer. However, the manufacturing methods according to embodiments of the present invention are not limited to these. For example, as shown in the modified examples, any appropriate configuration, process, conditions, etc., can be adopted as long as the objective of the present invention is achieved.

[0020] B-1. Preparation of the support substrate As shown in Figure 1A, in a manufacturing method of a composite substrate according to one embodiment, typically, first, a support substrate 10 to be used for forming the active layer 20 is prepared.

[0021] B-1-1. Support Substrate Any suitable substrate can be used as the support substrate. Typically, the support substrate is crystalline. The support substrate may consist only of a single crystal structure, only of a polycrystalline structure, or a combination of a single crystal structure and a polycrystalline structure. Typically, the support substrate may be composed of a semiconductor material. Preferably, silicon or germanium can be used as the material constituting the support substrate. Preferably, the support substrate may be single-crystal silicon, polycrystalline silicon, single-crystal germanium, or polycrystalline germanium. If the support substrate is single-crystal silicon or single-crystal germanium, a polycrystalline layer may be formed on the surface. A single-crystal substrate manufactured by any suitable manufacturing method may be used as the support substrate. Examples of methods for manufacturing the support substrate include the CZ method (Czochralski method), FZ method (float zone method), and MCZ method (magnetic field applied Czochralski method). Among these, a support substrate that can be manufactured by the FZ method is preferred.

[0022] The surface orientation of the support substrate may have a surface with an appropriate surface orientation according to the purpose (for example, the function of the device to which the composite substrate is applied). The support substrate may have, for example, a surface with a surface orientation of (100), (111), or (110). Preferably, the support substrate has a surface with a surface orientation of (100). With such a configuration, when forming an active layer (typically an oxide film layer) by thermally oxidizing the support substrate, the oxidation rate in the surface orientation of (100) is faster than the oxidation rates of the surface orientations of (110) and (111), so there is an advantage of excellent productivity.

[0023] The electrical resistivity of the support substrate is preferably greater than 3 kΩ·cm, preferably 3.5 kΩ·cm or more, and preferably 4 kΩ·cm or more. The upper limit of the electrical resistivity of the support substrate is, for example, preferably 20 kΩ·cm. If the electrical resistivity of the support substrate is within such a range, electrical loss is less likely to occur, so the composite substrate can have better electrical characteristics. The electrical resistivity of the support substrate can be obtained by spreading resistance measurement (SRP). Note that the electrical resistivity of the modified layer can be measured in the same way.

[0024] The support substrate may have an n-type region or a p-type region. The p-type region is a region that contains a large amount of positive charges (holes) as carriers (charge carriers). The p-type region can be formed by adding any appropriate acceptor impurity to the support substrate. When single crystal silicon is used as the support substrate, examples of the acceptor impurity include boron, aluminum, gallium, and indium. On the other hand, the n-type region is a region that contains a large amount of negative charges (free electrons) as carriers. The n-type region can be formed by adding any appropriate donor impurity to the support substrate. When single crystal silicon is used as the support substrate, examples of the donor impurity include phosphorus, arsenic, and antimony. The carrier type of the support substrate can be measured and confirmed by the hot probe method.

[0025] If the support substrate is a silicon substrate or a germanium substrate, it is preferable from the viewpoint of realizing good thermal expansion coefficient and thermal conductivity. When the support substrate includes a functional layer described later, the thermal expansion coefficient of the semiconductor material constituting the support substrate is preferably smaller than that of the functional substrate constituting the functional layer. According to such a support substrate, changes in the shape and size of the active layer (for example, the functional layer) when the temperature changes can be suppressed, and for example, changes (losses) in the frequency characteristics of a functional device manufactured by applying a composite substrate can be suppressed. For example, if the material constituting the support substrate is silicon, such a relationship of the thermal expansion coefficient can be satisfied well.

[0026] As the thickness of the support substrate, any appropriate thickness can be adopted. The thickness of the support substrate is, for example, 100 μm to 1000 μm (1 mm). If the thickness of the support substrate is within such a range, sufficient mechanical strength can be imparted to the composite substrate on which the support substrate and the modified layer are formed. In this case, for example, thinning of the functional layer can be facilitated.

[0027] B-1-2. Smoothing treatment of the support substrate The support substrate 10 can be subjected to an optional appropriate smoothing treatment on one surface (10a or 10b) or both surfaces (10a and 10b) as necessary. In one embodiment, the method for manufacturing a composite substrate includes smoothing the oxide film layer until the surface roughness Ra of the support substrate is 10 nm or less before forming the active layer (typically an oxide film layer). The smoothing treatment can be performed, for example, by polishing the surface of the material. As the polishing method, any appropriate method can be adopted. Examples of the polishing method include lapping and chemical mechanical polishing (CMP). Note that the support substrate 10 may be used as it is without being subjected to treatments such as smoothing treatment.

[0028] The surface roughness Ra of the support substrate can be, for example, 0.1 nm to 10 nm. The surface roughness Ra is preferably 5.0 nm or less, more preferably 1.0 nm or less, and still more preferably 0.5 nm or less. In this specification, "surface roughness Ra" means arithmetic mean roughness (Ra). The arithmetic mean roughness (Ra) can be obtained by measuring in a 10 μm × 10 μm field of view with an atomic force microscope (AFM) in accordance with JIS B0601:2013.

[0029] B-2. Formation of the Oxide Film Layer Next, as shown in Figure 1B, in the manufacturing method of a composite substrate according to one embodiment, an oxide film layer 21 is formed on the support substrate 10. The oxide film layer 21 is a layer that can constitute the active layer 20 as described above.

[0030] B-2-1. Method for preparing an oxide film layer. The oxide film layer can be formed by any suitable method. Examples of methods for forming an oxide film layer include oxidation, sputtering, atomic layer deposition (ALD), vapor deposition (physical vapor deposition such as ion beam-assisted vapor deposition (IAD) or chemical vapor deposition (CVD)), and ion plating.

[0031] In one embodiment, the formation of the oxide film layer includes forming the oxide film layer by oxidizing the support substrate. Any suitable method can be used as the oxidation method. In the manufacturing method of the composite substrate according to one embodiment, the oxide film layer is formed by thermal oxidation of the support substrate. When thermal oxidation is used, a remarkable effect can be obtained in which defects in the oxide film (oxide film layer) of the formed support substrate can be greatly suppressed. As a result, the yield in manufacturing a composite substrate having an oxide film layer in the active layer can be improved. However, when forming an oxide film by deposition, if heated to a temperature above the deposition temperature, outgassing of hydrogen, moisture, etc. may occur from within the formed film. For example, in the sputtering method, the deposition temperature can be around 200°C. In this case, outgassing may adversely affect the reliability of the device to which it is applied. In contrast, according to the manufacturing method of the composite substrate according to one embodiment, the thermal oxide film can usually be formed by oxidation at a temperature of 700°C or higher. As a result, thermally oxidized films are less likely to contain components that could cause outgassing, and therefore have the advantage of superior thermal stability of the film quality in composite substrates when produced by thermal oxidation.

[0032] Any suitable method and conditions can be used for thermal oxidation. Typically, thermal oxidation can be carried out under heating conditions of 700°C to 1200°C in an oxidizing atmosphere. Specifically, this is done as follows: A support substrate is placed in a chamber, the inside of the chamber is heated to 700°C to 1200°C, and then any suitable gas is supplied into the chamber to create an oxidizing atmosphere, thereby oxidizing the support substrate. The oxidizing atmosphere can be prepared by supplying, for example, oxygen, hydrogen, water vapor, hydrochloric acid (hydrogen chloride), or a mixture of two or more of these gases. With such thermal oxidation, oxidation can proceed from the surface of the support substrate, and an oxide film layer can be formed.

[0033] Examples of thermal oxidation methods include wet oxidation, pyrogenic oxidation, steam oxidation, dry oxidation, and hydrochloric acid oxidation. Among these, thermal oxidation is preferably carried out by wet oxidation or pyrogenic oxidation. In wet oxidation, for example, the oxidation of the target material can proceed by supplying oxygen and water vapor. In pyrogenic oxidation, for example, a mixed gas of hydrogen and oxygen can be supplied, and the oxidation of the target material can proceed by the water vapor produced by the combustion of the mixed gas. The thickness of the oxide film layer can be adjusted by setting the processing temperature, processing time, and other conditions to any appropriate conditions so that it reaches the desired thickness. The thickness of the oxide film layer after thermal oxidation can be, for example, 0.05 μm or more and 30 μm or less, as described above. In this way, an oxide film layer 21 can be formed by oxidizing at least one surface of the support substrate 10.

[0034] The oxide layer may be a layer composed of any suitable oxide. As described above, the oxide layer may be composed of, for example, the oxide of the supporting substrate. Specifically, for example, if the semiconductor material is silicon or germanium, the oxide layer may include silicon oxide or germanium oxide.

[0035] B-2-2. Smoothing treatment of the oxide film layer The oxide film layer may be subjected to a smoothing treatment as needed. When smoothing is performed, for example, the oxide film layer may be polished until the surface roughness Ra of the oxide film layer is 1 nm or less. The method of smoothing treatment may be the same as the method used to smooth the support substrate. If the oxide film layer is formed on both sides of the support substrate, the surfaces of both oxide film layers may be smoothed, or only the surface of one oxide film layer may be smoothed. Note that the smoothing treatment of the oxide film layer is optional, and it may be used as is in the next step without performing any smoothing treatment.

[0036] The surface roughness Ra of the oxide film layer on the side opposite the modified layer is preferably 1 nm or less, more preferably 0.5 nm or less, and even more preferably 0.2 nm or less. The lower limit of Ra may be, for example, 0.1 nm. Having Ra within this range can increase the bonding strength when another layer (for example, a functional layer or intermediate layer described later) is provided in the oxide film layer of the composite substrate.

[0037] Any appropriate thickness can be used for the oxide film layer. For example, the thickness of the oxide film layer may be 0.05 μm (50 nm) or more and 30 μm or less. Preferably, the thickness of the oxide film layer may be 0.1 nm or more and 25 μm or less, and preferably 1 nm or more and 20 μm or less. The thickness of the oxide film layer can be adjusted by, for example, the conditions when oxidizing the support substrate (heating temperature during oxidation, etc.), the type of gas constituting the oxidizing atmosphere, and the smoothing treatment after oxidizing the support substrate.

[0038] B-3. ​​Formation of the Modified Layer Next, as shown in Figure 1C, in the manufacturing method of a composite substrate according to one embodiment, a pulsed laser is irradiated onto the surface of the support substrate 10 from the oxide film layer 21 side. The oxide film layer 21 typically has laser light transmittance (laser light transmission). When a pulsed laser is irradiated onto a laminate comprising the oxide film layer 21 and the support substrate 10 from the oxide film layer 21 side, the pulsed laser can penetrate the oxide film layer 21 and reach the surface of the support substrate 10. In the manufacturing method of a composite substrate according to an embodiment of the present invention, as described above, the surface of the support substrate 10 can be modified by irradiating it with a pulsed laser, and a modified layer 30 (including the modified portion 301) can be formed (see Figure 1D). Normally, it is difficult to create a new layer (region) between the support substrate and the active layer after an active layer has been formed on the support substrate (for example, the formation of an oxide film layer by an oxide film and / or the arrangement (bonding) of a functional layer). Furthermore, even when attempting to modify the support substrate by irradiating it with a laser, the laser light may not reach the desired depth inside the support substrate, preventing the support substrate from being modified to have a thickness greater than a predetermined value, and / or a thermal reaction may occur between the support substrate and the oxide film layer, making ablation more likely and causing the active layer to peel off from the support substrate. In contrast, the composite substrate obtainable according to the embodiment of the present invention, as described above, can form a modified layer with a thickness greater than a predetermined value, and peeling of the active layer is suppressed. Also, in this embodiment, the active layer, such as an oxide film layer, is not formed on the modified layer after the modified layer has been formed on the support substrate. Therefore, the deterioration of the performance of the modified layer due to the formation of the active layer can be suppressed. That is, the modified layer in the composite substrate obtainable according to this embodiment can maintain its function as a charge trapping layer well. Furthermore, according to this embodiment, when forming the active layer and the modified layer, special equipment for ensuring safety is not required, as in the case of formation by the CVD method, so an increase in cost can be suppressed.

[0039] B-3-1. Irradiation with a pulsed laser In the method for manufacturing a composite substrate according to an embodiment of the present invention, as described above, typically, a pulsed laser is used as the laser (laser light). The irradiation with the pulsed laser can be performed by an appropriate method as long as the object of the present invention can be achieved. Examples of the pulsed laser include femtosecond, picosecond or nanosecond pulsed lasers, and preferably, a femtosecond or picosecond pulsed laser can be used.

[0040] The pulse width of the pulsed laser is preferably less than 100 ps, more preferably 80 ps or less, still more preferably 50 ps or less, even more preferably 10 ps or less, and particularly preferably 1 ps or less. The pulse width of the pulsed laser is preferably 1 fs or more, more preferably 10 fs or more, and still more preferably 100 fs or more. Within such a range, the thermal reaction on the surface of the support substrate can be suppressed, and ablation can be more effectively suppressed. Therefore, even when a laser is irradiated after forming an active layer on the support substrate, delamination at the interface between the support substrate and the active layer can be more effectively suppressed.

[0041] The frequency of the laser can be, for example, 1 kHz or more and 1 MHz or less.

[0042] As the wavelength of the laser, an appropriate wavelength can be adopted according to the bandgap of the support substrate and / or the bandgap of the active layer (substantially the oxide film layer). In one embodiment, the wavelength λ ａ [nm] of the pulsed laser satisfies the following formula (1). λ ａ ≧1240 / (E ｇ１ + 2.1) ... (1) In formula (1), E ｇ１ [eV] is the bandgap of the support substrate. For example, when the support substrate is silicon, E ｇ１ ≒1.1 to 1.2 eV, and for example, when the support substrate is germanium, E ｇ１ ≒0.67. The wavelength λ ａThe upper limit is preferably 3000 nm or less, more preferably 2500 nm or less, and even more preferably 2000 nm or less. Within this wavelength range, the effects of the present invention can be more significantly obtained. In particular, generally, when modifying a support substrate by irradiating it with laser light, energy greater than the band gap of the support substrate is required. However, if the energy of the laser light (photon energy) is increased, the light can only be absorbed near the surface of the support substrate, making it difficult to allow the laser light to reach the desired depth inside the support substrate. As a result, it may be difficult to form a modified layer with the desired thickness. In contrast, by irradiating with a pulsed laser with a wavelength that satisfies the above formula (1), it is possible to reach the desired depth inside the support substrate. Furthermore, the wavelength λ ａ Even if the wavelength is relatively long (e.g., 1250-3000 nm), multiphoton absorption can occur by using a pulsed laser. As a result, even if the wavelength of the laser light has an energy below the band gap of the support substrate, the laser light can reach the desired depth inside the support substrate more effectively. Therefore, the wavelength λ of the pulsed laser ａ If the above range is maintained, a modified layer having a thickness of a predetermined value (e.g., 10 nm) or more can be formed more effectively. The above multiphoton absorption is preferably two-photon absorption. Furthermore, as the wavelength of the laser light increases, the light can penetrate deeper into the interior of the support substrate, so the light energy density per unit volume in the region that has reached the interior of the support substrate may decrease. In order to obtain the light energy per unit volume necessary for modifying the support substrate material, it may be necessary to increase the laser energy density per unit area, as described later, as necessary as the wavelength of the laser light increases. The band gap of the support substrate can be calculated by measuring and analyzing the spectrum using any appropriate measurement method. For example, optical measurement methods such as ultraviolet-visible absorption spectroscopy, transmission spectroscopy, and diffuse reflectance spectroscopy, or X-ray measurement methods such as X-ray absorption spectroscopy (e.g., XAFS spectroscopy) and X-ray photoelectron measurement (e.g., XPS, ESCA) can be employed.

[0043] The laser energy density can be appropriately adjusted depending on the purpose. In one embodiment, the laser energy density is preferably 10 mJ / cm². ２ The above is true, and more preferably 100 mJ / cm². ２ The above is preferable, and more preferably 150 mJ / cm². ２ The above is preferable, and more preferably 450 mJ / cm². ２ That concludes the explanation. The laser energy density is preferably 5000 mJ / cm². ２ The following, preferably 3000 mJ / cm² ２ The following, preferably 2000 mJ / cm² ２ The following applies: Within this range, a pulsed laser can be used to deliver the desired laser light to the desired depth inside the support substrate while suppressing thermal reactions, and as a result, ablation can be effectively suppressed. Consequently, the effects of the present invention may become more pronounced.

[0044] The laser irradiation pitch (spacing) during laser irradiation can be any appropriate interval, as long as the objective of the present invention can be achieved. The laser irradiation pitch can be appropriately set, for example, taking into account the effects of ablation. Specifically, the laser irradiation pitch can be set, for example, according to the minimum spacing of the modified portions (modified portions) on the surface of the support substrate in a plan view. The minimum spacing of the modified portions is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 10 μm or less. The lower limit of the minimum spacing of the region may be, for example, 0.1 μm. Within such a range, the effects of the present invention can be more pronounced.

[0045] The pulsed laser may be irradiated without focusing on the surface of the support substrate, for example. In this way, for example, when forming a modified layer, the focal position of the support substrate can be shifted by an appropriate distance (e.g., several millimeters), eliminating the need to focus on the support substrate each time. Furthermore, by shifting the focus of the pulsed laser (i.e., not focusing it), ablation of the target object (e.g., support substrate, active layer) can be more effectively suppressed. When irradiating with a pulsed laser with a small irradiation area, it is not necessary to irradiate the entire surface of the support substrate. For example, depending on the purpose and type of the device (functional element) to which the composite substrate is applied, the laser may be irradiated only to the areas where electrical loss of the device may occur. More specifically, in a high-frequency device, for example, the laser may be irradiated only to the area where high frequency is to be applied and where electrodes are to be installed (the area where electrodes are planned to be installed). In this way, the processing time due to laser irradiation can be reduced.

[0046] (Area ratio of modified layer) In one embodiment, the method for manufacturing a composite substrate includes irradiating with a pulsed laser such that the area ratio of the modified layer to the total area in a plan view (hereinafter sometimes simply referred to as the area ratio of the modified layer) is 40% or more. The area ratio of the modified layer can be achieved by appropriately controlling the shape of the modified portion and the spacing between adjacent modified portions. The area ratio of the modified layer is preferably 45% or more, more preferably 50% or more, even more preferably 80% or more, and particularly preferably 95% or more. The area ratio of the modified layer may be, for example, 100% or less, or less than 100%, or 99.0% or less. If the area ratio of the modified layer is within the above range, the effect of the modified layer as a charge trapping layer can be more favorably obtained. Therefore, the charge trapping performance of the composite substrate can be maintained, and this can contribute to the high performance when the composite substrate is used in devices for high-frequency and / or harmonic applications. If the area ratio of the modified layer is less than 100%, the bonding strength between the support substrate and the active layer, between the support substrate and the modified layer, and between the modified layer and the active layer can be increased. Note that if the area ratio of the modified layer is less than 100%, the modified layer may be formed substantially over the entire surface of the support substrate. "Substantially over the entire surface" includes the case where the modified portion completely covers the entire surface of the support substrate (i.e., the modified portion covers 100%), but not the case where the modified portion covers the entire surface. Furthermore, the modified layer does not necessarily have to be formed in a strictly layered manner.

[0047] (Multiple Modified Sections) In one embodiment, the method for manufacturing a composite substrate includes irradiating a support substrate with pulsed lasers spaced apart in at least one direction within the plane of the substrate. In this way, multiple modified sections can be formed spaced apart from each other. As a result, a modified section 30 may be composed of multiple modified sections 301. Such a configuration can further contribute to improving the performance when the composite substrate is used in devices for high-frequency and / or harmonic applications. The spacing between adjacent modified sections can be adjusted by appropriately adjusting the laser irradiation pitch and the planar shape of the modified sections (e.g., the diameter of the modified sections). "Spacing between adjacent modified sections" means the distance between the centers of adjacent modified sections. The spacing between adjacent modified sections may be, for example, 5 μm or more, for example, 10 μm or more, for example, 20 μm or more, or for example, 50 μm or more. The spacing between adjacent modified sections may be, for example, less than 100 μm, for example, 90 μm or less, for example, 80 μm or less, or for example, 75 μm or less.

[0048] The plan view shape of the modified layer (including multiple modified sections) can be any suitable shape, as long as it achieves the objectives of the present invention. The plan view shape of the modified section can be set to any shape, for example, by appropriately adjusting the laser beam profile. The plan view shape of the modified section may be, for example, circular or elliptical, or a polygon such as a triangle, square, or rectangle, or a combination thereof. For example, if the modified layer is composed of multiple modified sections, the plan view shapes of each of the multiple modified sections may be the same or different. The multiple modified sections 301 may be formed randomly or in a regular manner in plan view, for example. Also, for example, the thicknesses of the multiple modified sections may be the same, some may be different, or all modified sections may have random thicknesses.

[0049] Multiple modification sections may be arranged in parallel, for example, in a first direction and in a second direction intersecting the first direction. The intersection angle between the first direction and the second direction can be any appropriate angle, as long as the objective of the present invention is achieved. The intersection angle between the first direction and the second direction may be, for example, 90° or 60°.

[0050] In one embodiment of the manufacturing method for a composite substrate, each of the multiple modified portions is formed such that its thickness decreases from the center to the edge in the in-plane direction. As an example, Figure 3 shows a schematic explanatory diagram of a TEM image observing the center to the edge of a modified portion (part) in the modified layer of a composite substrate that can be obtained by one embodiment of the present invention. Specifically, in the illustrated example, the left end is the center side of the modified portion, and the right end in the figure is the edge side of the modified portion. As shown in the illustrated example, each of the multiple modified portions 301 is preferably formed such that the thickness dr on the edge side of the modified portion 301 is smaller than the thickness dc on the center side of the modified portion 301. With such a configuration, the charge trapping performance of the composite substrate can be maintained better, and this can further contribute to improving the performance when the composite substrate is used in devices for high-frequency and / or harmonic applications.

[0051] (Thickness of the modified layer) The thickness of the portion of the support substrate modified by pulsed laser irradiation corresponds to the thickness of the modified layer (overall). The thickness of the modified layer is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. The thickness of the modified layer may preferably be 1000 nm or less, more preferably 750 nm or less, and even more preferably 500 nm or less. If the modified layer has a first modified layer and a second modified layer as described later, the sum of the thicknesses of the first modified layer and the second modified layer is the thickness of the modified layer. If the thickness of the modified layer is 10 nm or more, the insertion loss of the composite substrate can be improved and the harmonic components can be reduced effectively. Note that "insertion loss" is one of the indicators that can be used to evaluate high-frequency characteristics. If the thickness of the modified layer is 1000 nm or less, it can contribute to reducing the cost in manufacturing the composite substrate. The thickness of the modified portion can be confirmed by imaging using an electron microscope (e.g., a transmission electron microscope (TEM)), and determined by examining the cross-section (essentially a side view) of the image. If the modified layer consists of multiple modified portions, the thickness of the modified layer is the arithmetic mean of the thicknesses of the multiple modified portions.

[0052] (First Modified Layer and Second Modified Layer) In one embodiment, the method for manufacturing a composite substrate includes irradiating it with a pulsed laser to form a modified layer 30 having a first modified layer 31 and a second modified layer 32 in order from the support substrate side 10 (see Figure 3). The first modified layer 31 and the second modified layer 32 may each include the above-mentioned modified portion (multiple modified portions) 301. The first modified layer 31 and the second modified layer 32 can typically function as charge trapping layers. The reason why the above-mentioned first modified layer and second modified layer are formed on the surface of the support substrate by irradiation with a pulsed laser is not necessarily clear, but the following reasons may be considered. By irradiating the surface of the support substrate with a pulsed laser through an active layer formed on the support substrate, the atoms constituting the support substrate vibrate, and the interatomic bonds can be broken from the surface side of the support substrate. As a result, the regularity of the atomic arrangement on the surface side of the support substrate is disrupted, and it can be formed as the second modified layer. On the other hand, as the laser intensity decreases as it propagates further inward (into) from the surface of the support substrate, it can be inferred that a first modified layer may be formed with a different atomic arrangement regularity from the second modified layer, which consists of areas where the bonds are partially broken and / or areas where crystallinity can be maintained at a certain depth (the portion away from the surface of the support substrate). However, this is merely a conjecture and does not limit the present invention, nor does it constrain the present invention by this mechanism.

[0053] In one embodiment, the first modified layer 31 includes a region having strain in part. In this embodiment, as described above, the atomic arrangement of the crystalline support substrate is disrupted (altered) by pulse laser irradiation, thereby forming an amorphous second modified layer. The stress caused by the disruption of the regularity of the atomic arrangement can concentrate at the interface between the second modified layer and the unmodified support substrate. It is presumed that the deformation that can occur at the interface due to this stress becomes "strain". Therefore, a region having strain can be formed in at least a part of the first modified layer. The strain in the first modified layer can be confirmed as a darkened region by examining the TEM image of the modified layer. With such a configuration, the modified layer can function particularly well as a trap-rich layer.

[0054] In one embodiment, the first modified layer 31 contains elements of the same type as those constituting the support substrate 10. The first modified layer 31 may include both an amorphous and a crystalline structure. In one embodiment, the second modified layer 32 contains an amorphous structure of elements of the same type as those constituting the support substrate 10. The first and second modified layers can typically be distinguished by the fact that the regularity of the atomic arrangement in the first modified layer and the regularity of the atomic arrangement in the second modified layer are different from each other. The regularity of the atomic arrangement is a crystallographic indicator for determining whether a structure is crystalline and / or amorphous. Note that the regularity of a crystalline structure can differ depending on the combination and state of the atomic arrangement, so "crystalline structure" may include multiple crystalline states. A crystalline structure means a structure that has crystallinity, and may include single-crystal structures and / or polycrystalline structures. As described above, having crystallinity means having regularity in the atomic arrangement. Similarly, "amorphous structure" may also include multiple amorphous states. For example, when the support substrate is a silicon substrate, preferably the first modified layer contains amorphous silicon and polycrystalline silicon, and the second modified layer contains amorphous silicon. Also, for example, when the support substrate is a germanium substrate, preferably the first modified layer contains amorphous germanium and polycrystalline germanium, and the second modified layer contains amorphous germanium. If the first modified layer contains only an amorphous structure, the atomic arrangement of the amorphous structure will differ from the atomic arrangement of the amorphous structure of the second modified layer. In this case, the difference between the atomic arrangement of the amorphous structure of the first modified layer and the amorphous atomic arrangement of the second modified layer can be determined by the strain at the interface between the first modified layer and the second modified layer. With such a configuration, the modified layer can function particularly well as a trap-rich layer. The crystallinity and / or amorphous nature of the support substrate and the modified layers (first modified layer and second modified layer) in a composite substrate can be confirmed by examining the atomic arrangement using X-ray diffraction and / or by observing the cross-section of the composite substrate using TEM (transmission electron microscope). Differences in the regularity of the amorphous structure can also be confirmed by measuring Raman scattering spectroscopy using Raman spectroscopy and observing the short-range order.The fact that the first and second modified layers may contain the same elements as those constituting the support substrate can be confirmed by elemental and compositional analysis using EDX (energy-dispersive X-ray fluorescence).

[0055] In one embodiment, the amorphous structure in the first modified layer includes a region located on the support substrate side in the thickness direction compared to the crystalline structure. Specifically, the first modified layer may have a region composed of an amorphous structure on the support substrate side (opposite side from the second modified layer) in the thickness direction, and a region composed of a crystalline structure on the second modified layer side compared to that region.

[0056] The distribution of amorphous structures in the first and second modified layers can be any suitable distribution. This distribution may be, for example, a Gaussian distribution, a top-hat distribution, or a combination thereof. This distribution can be confirmed, for example, by a laser beam profile or by a TEM image. Furthermore, the thickness distribution of the amorphous structures in the first and second modified layers can be determined by examining this distribution.

[0057] The thickness of the first modified layer is preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 7 nm or more. The upper limit of the first modified layer is, for example, 50 nm. If the thickness of the first modified layer is within the above range, the stress and strain of the modified layer (overall) can be reduced, and as a result, warping of the composite substrate can be suppressed. The thickness of the second modified layer is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. The upper limit of the second modified layer is, for example, 500 nm. If the thickness of the second modified layer is 10 nm or more, the insertion loss of the composite substrate can be improved well, and harmonic components can be reduced well. If the thickness of the second modified layer is 1000 nm or less, it can contribute to reducing the cost in manufacturing the composite substrate.

[0058] B-3-2. Smoothing treatment of the oxide film layer after formation of the modified layer After forming the modified layer by pulsed laser irradiation, the surface of the oxide film layer may be smoothed as needed. In one embodiment, the method for manufacturing the composite substrate includes smoothing the active layer (typically the oxide film layer) until the surface roughness Ra of the oxide film layer becomes 1 nm or less. The method of smoothing can be the same as the method for smoothing the surface of the support substrate. Note that even if the oxide film layer has been smoothed before laser irradiation, the oxide film layer may be smoothed again.

[0059] As described above, a composite substrate 100 can be obtained, which comprises a support substrate 10, a modified layer 30, and an active layer 20 (oxide film layer 21) in this order, as shown in Figure 1D (Figure 2A).

[0060] B-4. Formation of a Functional Layer In the method for manufacturing a composite substrate according to an embodiment of the present invention, a functional layer may be further formed on the oxide film layer. The functional layer is a separate layer from the oxide film layer that can constitute the active layer as described above. The functional layer may be provided as needed. Specifically, a method for manufacturing a composite substrate according to one embodiment includes preparing a functional substrate 41 as shown in Figure 1E, and bonding the functional substrate 41 to the side of the oxide film layer 21 opposite to the support substrate 10 as shown in Figure 1F. The functional layer can be manufactured, for example, by bonding a functional substrate to the oxide film layer and, if necessary, thinning the functional substrate to any appropriate thickness. The functional substrate may be bonded to the oxide film layer before forming the modified layer (i.e., before irradiating with a pulsed laser), as will be described later (see Section B-8).

[0061] B-4-1. Functional Substrates Functional substrates can be composed of any suitable material depending on the desired function of the functional layer. Functional substrates can be used as materials with functionality. Examples of functional substrates include piezoelectric materials and materials with electro-optical effects.

[0062] Preferably, LiAO is used as the piezoelectric material. ３ A single crystal having the following composition may be used. Here, A is one or more elements selected from niobium and tantalum. Specifically, LiAO ３ Lithium niobate (LiNbO)３ ) may also be lithium tantalate (LiTaO ３ ) may be lithium niobate and / or lithium tantalate solid solution. When lithium niobate and / or lithium tantalate are used, MgO-doped or stoichiometric crystals may be used to suppress photodamage.

[0063] Another example of a piezoelectric material is potassium titanate phosphate (KTiOPO). ４ :KTP), potassium niobate lithium (K ｘ Li （１－ｘ） NboO ２ , 0 ≤ x ≤ 1: KLN), potassium niobate (KNbO ３ :KN), potassium tantalate / niobate (KNb ｘ Ta （１－ｘ） O ３ Examples include 0≦x≦1: KTN), silicon, quartz, silica, silicon carbide, gallium nitride, indium phosphide, and lead zirconate titanate (PZT).

[0064] If the piezoelectric material is lithium tantalate, the functional layer is, for example, aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is the case, the direction rotated 32° to 55° (for example, 42°) from the Y-axis toward the Z-axis is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the angle corresponds to (180°, 58° to 35°, 180°) in Euler angle notation.

[0065] When the piezoelectric material is lithium niobate, the functional layer is, for example, aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is the case, the direction rotated from the Z-axis toward the -Y-axis by 0° to 40° (for example, 37.8°) is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the X-axis (crystal axis) of the piezoelectric material is aligned with the propagation direction of the surface acoustic wave (X １When this is the case, the direction rotated 40° to 65° from the Y-axis toward the Z-axis is the direction perpendicular to the main surface of the functional layer (X ３ It is preferable that the angle corresponds to (180°, 50° to 25°, 180°) in Euler angle notation.

[0066] In one embodiment, a piezoelectric layer can be formed as a functional layer by using a piezoelectric material as a functional substrate. By providing a piezoelectric layer in the active layer of the composite substrate, a functional element capable of achieving excellent high-frequency characteristics can be obtained. As a result, the composite substrate according to the embodiment of the present invention can be particularly suitably used in surface acoustic wave elements such as SAW filters.

[0067] Preferably, lithium niobate, lithium tantalate, lithium niobate-lithium tantalate, KTP (potassium titanate phosphate), and PZT (lead zirconate titanate) can be used as materials exhibiting electro-optic effects. Specifically, for example, X-cut and / or Z-cut lithium niobate can be used as materials exhibiting electro-optic effects. When using lithium niobate and / or lithium tantalate, MgO-doped or stoichiometric crystals can be used to suppress photodamage.

[0068] In one embodiment, by using a material having an electro-optic effect as a functional substrate, an electro-optic layer can be formed as a functional layer. By providing an electro-optic layer in the active layer of the composite substrate, a functional element capable of achieving excellent harmonic characteristics can be obtained. As a result, the composite substrate according to the embodiment of the present invention can be particularly suitably used in electro-optic elements (such as optical waveguide devices) such as optical modulators.

[0069] The functional layer may be composed of any other suitable material having functional properties, depending on the functions and performance required of the composite substrate. For example, semiconductor materials may be used as materials having functional properties other than those mentioned above. Examples of semiconductor materials include materials similar to those described for the support substrate (silicon, germanium), and silicon carbide (SiC).

[0070] B-4-2. Smoothing treatment of the functional substrate The surface (bonding surface) of the functional substrate 41 may be subjected to a smoothing treatment as needed. When the functional substrate is subjected to a smoothing treatment, the functional substrate may be polished until the surface roughness Ra of the bonding surface of the functional substrate becomes, for example, 1 nm or less, as described above. The thickness of the functional substrate may be, for example, 100 μm to 1000 μm (1 mm), or for example, 200 μm to 500 μm.

[0071] B-4-3. Bonding of the Oxide Film Layer and the Functional Substrate Next, as shown in Figure 1F, in a manufacturing method of a composite substrate according to one embodiment, the oxide film layer 21 of a composite substrate 100 (composite 100') having a support substrate 10, a modified layer 30, and an active layer 20 (oxide film layer 21) in that order is bonded to the functional substrate 41. By bonding in this manner, a composite 102' having a support substrate 10, a modified layer 30, an oxide film layer 21, and a functional substrate 41 in that order can be obtained, as shown in Figure 1G.

[0072] In one embodiment, any suitable method can be used to bond the oxide film layer and the functional substrate. Examples of bonding methods include bonding with adhesives, surface activation bonding, plasma activation bonding, and atomic diffusion bonding. Preferably, the bonding method is so-called direct bonding, which does not involve an adhesive. Direct bonding allows for thinning of the composite substrate and prevents adverse effects from adhesives.

[0073] Direct bonding by plasma-activated bonding can be achieved by activating the bonding surfaces of the oxide film layer and the functional substrate by plasma irradiation, then bringing these bonding surfaces into contact, and, if necessary, performing heat treatment. Examples of gases included in the atmosphere during the activation treatment include oxygen, nitrogen, hydrogen, and argon. These may be used individually or in combination of two or more (as a mixed gas). Nitrogen is preferably used. The atmospheric pressure during the plasma irradiation activation treatment is preferably 10 Pa to 80 Pa, more preferably 30 Pa to 80 Pa. The energy during plasma irradiation is preferably 30 W to 150 W, more preferably 60 W to 120 W. The plasma irradiation time is preferably 5 seconds to 30 seconds.

[0074] B-4-4. Thinning Treatment of Functional Substrate Next, as shown in Figure 1G, in the manufacturing method of a composite substrate according to one embodiment, a functional layer 22 is formed by thinning the functional substrate 41 in the composite 102'. Specifically, the functional layer 22 can be formed, for example, by polishing and thinning the functional substrate 41. Preferably, the functional layer 22 can be formed by bonding the functional substrate 41 to the oxide film layer 21 and then thinning the functional substrate 41 until its thickness is 1000 nm or less. Note that the above thinning treatment may be omitted as appropriate depending on the type of functional substrate, the laser irradiation conditions, etc.

[0075] The thickness of the functional layer can be set to any appropriate thickness depending on the method of use and application of the composite substrate. For example, the thickness of the functional layer is 0.05 μm or more and 30 μm or less, preferably 0.10 μm or more and 20 μm or less.

[0076] The surface roughness Ra of the functional layer surface (bonding surface side) may be, for example, 1.0 nm or less, 0.8 nm or less, 0.6 nm or less, or 0.4 nm or less. On the other hand, the surface roughness Ra may be 0.1 nm or more. With such a surface roughness Ra, for example, when the functional layer is a piezoelectric layer, the composite substrate can be applied particularly well to devices (functional elements) for high-frequency or harmonic applications.

[0077] Based on the above, a composite substrate 102 having a functional layer 22 on an oxide film layer 21 as an active layer 20 can be fabricated, as shown in Figure 1H. Fabricating a composite substrate with a functional layer in this way has the advantage that, for example, even if the functional substrate absorbs laser light, a modified layer can be formed between the oxide film layer and the support substrate before the functional layer is provided.

[0078] B-5. Formation of an Intermediate Layer In one embodiment, the method for manufacturing a composite substrate may include further forming an intermediate layer between the oxide film layer and the functional substrate before bonding the functional substrate and the oxide film layer. The intermediate layer is an arbitrary layer provided as one layer that can constitute the active layer, as described above, and may be provided as needed. The intermediate layer may be composed of any suitable material having any appropriate function depending on the purpose. The intermediate layer may be, for example, a dielectric layer. The dielectric layer may be composed of any suitable dielectric material. Examples of dielectric materials include silicon oxide, silicon nitride, oxysilicon nitride, aluminum oxide, aluminum nitride, and oxyaluminum nitride. The dielectric layer may be a single layer or may have a laminated structure consisting of multiple layers composed of different dielectric materials. By providing a dielectric layer as an intermediate layer, for example, the stability of the temperature characteristics of the composite substrate may be improved.

[0079] The intermediate layer is formed, for example, by depositing any suitable material onto the surface of the oxide film layer and / or the surface of the functional substrate. Any suitable method can be used to deposit the intermediate layer. Examples of deposition methods include sputtering, CVD, and ion-assisted evaporation. The intermediate layer can be fabricated, for example, by forming a dielectric material on the target object (oxide film layer and / or functional substrate) by sputtering.

[0080] The surface (bonding surface) of the intermediate layer may be smoothed as needed. In one embodiment, when the intermediate layer is smoothed, the method for manufacturing the composite substrate may include polishing until the surface roughness Ra of the bonding surface of the intermediate layer is, for example, 1 nm or less. In one embodiment, an intermediate layer (e.g., SiO) is placed on a functional substrate. ２ A bonded body may be formed by forming an intermediate layer and joining the oxide film layer. The joining method may be the same as the joining method for the oxide film layer and the functional substrate described in Section B-4-3 above.

[0081] The thickness of the intermediate layer may be, for example, 100 nm to 1000 nm, or for example, 200 nm to 800 nm, or for example, 300 nm to 700 nm, or for example, 400 nm to 600 nm.

[0082] B-6. Characteristics of the Active Layer As described above, the active layer is any suitable chemically and / or physically active layer. The active layer may typically include at least one of an oxide layer, a functional layer, and an intermediate layer. The active layer preferably includes either or both of the oxide layer and / or functional layer.

[0083] In one embodiment, the active layer typically contains an oxide. The oxide may be any suitable oxide, as long as it does not hinder the objectives of the present invention. The oxide typically includes oxides that can constitute an oxide film layer, oxides that can constitute a functional layer, and oxides that can constitute an intermediate layer. Examples of oxides include oxides that can be contained in a support substrate (see Section B-1-1), such as silicon dioxide and germanium dioxide, and oxides that can be contained in a functional substrate (see Section B-4-1), such as lithium niobate and lithium tantalate.

[0084] The band gap of the active layer may be, for example, 2.5 eV or higher. The band gap of the active layer may be, for example, 3.0 eV or higher, 3.5 eV or higher, or 4.0 eV or higher. The upper limit of the band gap of the active layer may be, for example, 20 eV. The band gap of the active layer can be adjusted by appropriately selecting the material that can be used for the active layer. The band gap of the active layer can be measured by the same method as the method for measuring the band gap of the support substrate described in Section B-1. Specific materials that constitute a layer with a band gap of 2.5 eV or greater include, for example, silicon dioxide (approximately 9.0 eV), silicon carbide (approximately 2.9 eV), aluminum nitride (approximately 6.3 eV), gallium nitride (approximately 3.4 eV), gallium oxide (approximately 4.5–4.9 eV), gallium sulfide (approximately 2.5 eV), beryllium oxide (10.6 eV), magnesium oxide (approximately 7.8 eV), zinc oxide (approximately 3.4 eV), and zinc sulfide (approximately 3.6 eV). The values ​​in parentheses for the above materials represent the band gap.

[0085] In Figures 1A to 1H, the active layer 20 is formed only on the upper surface of the support substrate 10, but the active layer may be formed only on the lower surface of the support substrate, or on both sides of the support substrate. Specifically, for example, the oxide film layer 21 may be formed on only one surface of the support substrate 10 (e.g., the upper surface 10a), or on only the other surface of the support substrate 10 (e.g., the lower surface 10b), or on both the upper surface 10a and the lower surface 10b of the support substrate 10, or the oxide film layer 21 may be formed on the entire outer circumferential surface of the support substrate 10. For example, when oxidizing the support substrate, oxide film layers can be formed on both sides of the support substrate (substantially the entire outer circumferential surface of the support substrate). Also, for example, a functional layer and / or an intermediate layer may be provided on at least one side of the oxide film layer to constitute the active layer. For example, when oxide film layers are formed on both sides of the support substrate, the modified layer can be formed by irradiating the support substrate with a laser from at least one side of the active layer. Therefore, the modified layer may also be formed on the upper surface, lower surface, or both sides of the support substrate. B-7. Formation of electrodes In one embodiment, the method for manufacturing the composite substrate includes forming electrodes on the active layer. Multiple electrodes may be provided at intervals. The electrodes may be formed on, for example, an oxide film layer, a functional layer, or an intermediate layer. The electrodes may be formed from any suitable conductive material. The electrodes may be, for example, coplanar waveguides (CPWs). Since well-known and common configurations in the industry may be used for the electrodes, a detailed explanation is omitted.

[0086] In this embodiment, the area ratio of the modified layer may be 40% or more (see Section B-3). By forming electrodes with a modified layer having an area ratio of 40% or more, the resulting composite substrate can be particularly suitable for use as a functional element.

[0087] By doing so, a composite substrate having electrodes can be obtained (see Figure 2B).

[0088] B-8. Modified Forms Sections B-1 to B-7 above describe a specific example of a method for manufacturing a composite substrate according to embodiments of the present invention, in which a modified layer is formed by irradiating the surface of a support substrate with a pulsed laser from the oxide film layer side, and then, if necessary, a functional layer is created by providing a functional substrate on the oxide film layer and thinning it. On the other hand, the method for manufacturing a composite substrate according to embodiments of the present invention may include embodiments shown in the following modified forms. In the method for manufacturing a composite substrate according to one embodiment, for example, after providing a functional substrate on the oxide film layer (typically, after joining the oxide film layer and the functional substrate), a modified layer may be formed by irradiating with a pulsed laser from the functional substrate side, and if necessary, the functional substrate may be thinned to form a functional layer. In the method for manufacturing a composite substrate according to another embodiment, for example, after thinning the functional substrate to form a functional layer, a modified layer may be formed by irradiating with a pulsed laser from the functional layer side. When the functional substrate is thinned before laser irradiation, the transmittance of the laser light due to pulsed laser irradiation may be improved. As a result, even when laser light is irradiated onto the surface of the support substrate from the functional substrate side, a modified layer can be formed well. In yet another embodiment of the manufacturing method for a composite substrate, for example, an oxide film layer and a functional substrate may be provided on a support substrate, a pulsed laser may be irradiated from the functional substrate side to form a modified layer, and then the functional substrate may be thinned to form a functional layer. The thinning treatment of the functional substrate may be performed before the pulsed laser irradiation, after the irradiation, or both before and after the irradiation.

[0089] In the above modified example, when a pulsed laser is irradiated after bonding the oxide film layer and the functional substrate, it is preferable to smooth the functional substrate so that its surface roughness Sa is 20 nm or less before irradiating it with the pulsed laser. That is, in one embodiment, the method for manufacturing the composite substrate includes smoothing the functional substrate so that its surface roughness Sa is 20 nm or less before irradiating it with the pulsed laser. With such a configuration, scattering of laser light can be suppressed. As a result, the modified layer can be formed more efficiently. When a pulsed laser is irradiated after bonding the oxide film layer and the functional substrate, a functional material having MgO doping can preferably be used as the functional substrate. If such a material is used, photodamage to the functional substrate can be suppressed, and changes in the optical constants of the functional substrate material can be suppressed by irradiation with a pulsed laser.

[0090] Furthermore, for example, a composite substrate may be manufactured that does not have an oxide film layer as an active layer, but has a functional layer. For example, in the manufacturing method of a composite substrate described in sections B-1 to B-7 above, a composite substrate can be manufactured by omitting the formation of the oxide film layer described in section B-2 and instead using a functional substrate (resulting in a functional layer) described in section B-4-1. Specifically, for example, a functional substrate can be bonded to a support substrate as described in section B-1 (see section B-4-3), the functional substrate can be smoothed as needed (see section B-4-2), a modified layer (typically a first modified layer and a second modified layer) can be formed by irradiating the surface of the support substrate from the functional substrate side with a pulsed laser (see section B-3-1), and the functional substrate can be thinned as needed (section B-4-4), thereby obtaining a composite substrate comprising a support substrate, a modified layer, and a functional layer.

[0091] Although not shown in the diagram, for example, bonding layers may be formed between each layer that can constitute the active layer. The bonding layers may be, for example, a layer provided when bonding an oxide film layer to a functional layer, a layer provided when bonding an oxide film to an intermediate layer, and / or a layer provided when bonding a functional layer to an intermediate layer. The type, function, number, combination, arrangement, etc., of such layers can be appropriately set according to the purpose.

[0092] Furthermore, in a manufacturing method for a composite substrate according to one embodiment, a heat treatment may be performed at any appropriate point in time. The heat treatment may be performed, for example, after bonding the oxide film layer and the functional substrate. Alternatively, the heat treatment may be performed before, during, or after the thinning treatment of the functional substrate after bonding. Alternatively, the thinning treatment and the heat treatment of the functional substrate may be repeated alternately multiple times. Any appropriate heating conditions can be used for the heat treatment. The heating temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 500°C or lower. The lower limit of the heating temperature may be, for example, 100°C. The heating time is, 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 a heat treatment, high bonding strength of the composite substrate can be achieved while maintaining the amorphous structure of the modified layer. The atmosphere for the heat treatment may be, for example, an atmospheric atmosphere, an inert gas atmosphere such as helium, nitrogen and / or argon, a hydrogen atmosphere, or a vacuum atmosphere. Preferably, the atmosphere for the heat treatment may be an inert gas atmosphere such as helium, nitrogen and / or argon, a hydrogen atmosphere, or a vacuum atmosphere. If an interface exists between the support substrate and the active layer, the increase in fixed charge can be suppressed under the above atmospheres. Therefore, the electrical properties of the composite substrate can be maintained well.

[0093] B-9. Other support substrates (including semiconductor materials) and functional substrates (including functional materials) may be cleaned using any suitable solvent before processing. Examples of cleaning methods include wet cleaning, dry cleaning, and scrubbing. Among these, scrubbing is preferred because it is simple and efficient. A specific example of scrubbing is a method in which a cleaning agent (e.g., Lion Corporation's Sunwash series) is used, followed by cleaning with a solvent (e.g., a mixed solution of acetone and isopropyl alcohol (IPA)) using a scrubbing machine. The cleaning process can remove contaminants (e.g., fine particles, metal impurities, organic matter, etc.) adhering to the surface. Furthermore, when performing the above-mentioned film formation, bonding, etc., it is preferable to clean the surface of each layer to remove, for example, abrasive residue, unwanted layers generated by processing, etc.

[0094] C. Functional Elements As described above, the composite substrate according to the embodiment of the present invention can maintain charge trapping performance and improve high-frequency and / or harmonic characteristics, and can therefore be suitably used as a functional element for high-frequency and / or harmonic applications. For example, when a piezoelectric layer is provided as a functional layer in the active layer of the composite substrate, the composite substrate can be used as a surface acoustic wave element. A surface acoustic wave element typically comprises the composite substrate and electrodes (comb-type electrodes) provided on the piezoelectric layer side of the composite substrate. Such a surface acoustic wave element is suitably used, for example, as a SAW filter in communication equipment such as mobile phones. Also, for example, when an electro-optic layer is provided as a functional layer in the active layer of the composite substrate (typically when a functional material having an electro-optic effect such as lithium niobate (LN) or lithium tantalate is used), the composite substrate can be used as an electro-optic element. An electro-optic element can typically be an optical modulation device. The optical modulation device is, for example, a Mach-Zehnder type optical modulator, which modulates light propagating through an optical waveguide by applying a voltage to a Mach-Zehnder interferometer formed by an optical waveguide having an electro-optic effect. Such an electro-optic element is suitably used, for example, as an optical modulator in optical communication systems.

[0095] 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.

[0096] (1) Surface roughness Ra (arithmetic mean roughness (Ra)) Surface roughness Ra was measured in accordance with JIS B0601:2013 using an atomic force microscope (AFM) in a field of view of 10 μm × 10 μm.

[0097] (2) Thickness of the modified layer and presence or absence of delamination The cross-section of the obtained composite substrate was observed using a transmission electron microscope (Hitachi High-Technologies Corporation, "H-9500") under the conditions of an acceleration voltage of 200 kV and a total magnification of 2,000,000 times, and a TEM image was obtained. Based on the TEM image, the depth (distance) of the entire modified layer was confirmed and this was defined as the thickness of the modified layer. The results are shown in Table 1. In addition, based on the acquired TEM image, it was confirmed whether or not the oxide film layer had delaminated from the support substrate. If there was no delamination, it was evaluated as "none," and if there was delamination, it was evaluated as "present." The results are shown in Table 1.

[0098] [Example 1] As a support substrate, a single-crystal silicon substrate with an orientation flat (OF) portion, a diameter of 4 inches, a thickness of 230 μm, and high resistance (>3 kΩ·cm) was prepared (hereinafter simply referred to as the silicon substrate). The silicon substrate was manufactured by the FZ method, and the electrical resistivity of the silicon substrate was 3.0 kΩ·cm, the carrier type was n-type, and the plane orientation was (100). The surface roughness Ra of the silicon substrate surface was 0.5 nm. Next, the silicon substrate was washed with a mixed solution of acetone and isopropyl alcohol (IPA) to remove impurities from the surface of the silicon substrate. It was confirmed that a native oxide film of about 1 nm was formed on the surface of the silicon substrate after washing.

[0099] Next, an oxide film layer was formed on the surface (top and bottom) of the silicon substrate by thermal oxidation. Thermal oxidation was carried out as follows: The silicon substrate was placed in a chamber capable of supplying oxygen and water vapor, and while the inside of the chamber was heated to 700°C to 1200°C, an oxidizing atmosphere was created by supplying oxygen and water vapor, and wet oxidation was performed on the silicon substrate. Subsequently, the silicon substrate with oxide film layers formed on both sides was removed. The thickness of the oxide film layer was 4.5 μm on each side.

[0100] Next, one side of the support substrate, which is used to form electrodes in the oxide film layer formed on both sides (the electrode formation surface), was smoothed by polishing using CMP (Chemical Polishing Machine) until the surface roughness Ra reached 0.5 nm.

[0101] Next, a pulsed laser was irradiated from the oxide film layer side of the electrode formation surface toward the silicon substrate. The pulsed laser irradiation conditions were as follows: Wavelength: 400 nm Laser energy (laser density): 100 mJ / cm ２ Irradiation pitch (feed width): 75 μm Laser pulse width: 80 ps Frequency: 40 kHz Defocus: 2.0 mm A composite having an oxide film layer / modified layer / support substrate configuration was obtained by laser irradiation.

[0102] Next, the surface of the oxide film layer of the composite was smoothed by polishing with CMP until the thickness of the oxide film layer was approximately 100 nm (Ra: approximately 0.2 nm). In this way, a composite substrate comprising an oxide film layer, a modified layer, and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. Figure 4A shows the TEM image of Example 1. The results are shown in Table 1. According to the TEM image of the obtained composite substrate, the total depth (distance) of the modified layer was 13 nm. Furthermore, by observing the TEM with magnification, it was confirmed that the thickness of the center of the modified portion was greater than the thickness of the edges, and that the thickness decreased from the center to the edges of the modified portion. In addition, when the area near the boundary between the region with an amorphous structure containing a-Si and the region with a different atomic arrangement regularity was observed with further magnification, and the atomic arrangement was confirmed by magnifying the image, it was found that regions with crystalline structures and regions with amorphous structures were mixed, and multiple locations were confirmed where the regions with amorphous structures were located closer to the silicon substrate than the regions with crystalline structures. Furthermore, observation of the TEM images revealed the presence of darkened regions, i.e., regions exhibiting distortion. Table 1 shows the modified layer in which the presence of an amorphous structure was confirmed, indicated as "containing a-Si". Elemental and compositional analysis was performed on the obtained composite substrate by EDX (energy-dispersive X-ray spectroscopy) to identify the constituent elements of each layer, and it was determined that the modified layer was formed of silicon.

[0103] [Examples 2-5 and Comparative Example 1] Composite substrates were fabricated in the same manner as in Example 1, except that the laser irradiation conditions (pulse width, energy density, and wavelength) were changed to those shown in Table 1. The obtained composite substrates were subjected to the evaluation described in (2) above. The results are shown in Table 1. Figure 4B shows the TEM image of the composite substrate of Example 2, and Figure 4C shows the TEM image of the composite substrate of Example 4. Similar to Example 1, it was confirmed that a modified layer was formed between the silicon substrate and the support substrate. Similarly, it was confirmed that a modified layer was formed in Examples 3 and 5. The thickness is shown in Table 1. On the other hand, in Comparative Example 1, as shown in Figure 4D, the oxide film layer 21 was peeled off from the silicon substrate 10 (see "peeled oxide film 21p" shown by the dashed line), and no modified layer was formed. Therefore, the thickness of the modified layer for Comparative Example 1 is indicated by "-". Furthermore, for the obtained composite substrates, in Examples 2-5, the composition of the active layer, modified layer, and support substrate was confirmed by EDX measurement, similar to Example 1.

[0104] [Example 6] A composite substrate was prepared with an oxide film layer and a functional layer as the active layer. Specifically, the composite substrate was prepared as follows. Descriptions common to Examples 1 and 4 will be omitted as appropriate. A silicon substrate similar to that in Example 1 was prepared as the support substrate, and the silicon substrate was cleaned in the same manner as in Example 1. Subsequently, the silicon substrate was thermally oxidized (wet oxidation) in the same manner as in Example 1, and oxide film layers were formed on both sides of the silicon substrate. The thickness of each oxide film layer was 4.5 μm. Subsequently, one side of the oxide film layer formed on both sides of the support substrate (electrode formation surface) for forming electrodes was smoothed by CMP processing until the surface roughness Ra was 0.5 nm.

[0105] As a functional substrate, a lithium niobate substrate (hereinafter referred to as LN substrate), which is a piezoelectric substrate having an OF (Optical Field) portion, a diameter of 4 inches, and a thickness of 250 μm, was prepared. X-cut LN substrate was used. The surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. Subsequently, the LN substrate was cleaned in the same manner as the silicon substrate in Example 1 to remove impurities and other contaminants from the surface of the LN substrate.

[0106] Next, the oxide film layer after cleaning and the LN substrate were directly bonded using a plasma activation method to obtain a bonded body.

[0107] Next, the bonded structure was placed in a nitrogen-filled oven (120°C) and heated for 10 hours. After that, the LN substrate of the bonded structure was removed from the oven and subjected to grinding and lapping, and then the thickness of the LN substrate was reduced to 3.0 μm by CMP processing.

[0108] Next, the composite was irradiated with a laser from the LN substrate side, through the LN substrate and the oxide film layer, under the same laser irradiation conditions as in Example 4 (see Table 1). Laser irradiation formed a first modified layer and a second modified layer between the silicon substrate and the oxide film layer, starting from the silicon substrate side. The presence of the modified layers was confirmed by TEM imaging, as in Example 1. In this way, a composite comprising an LN substrate / oxide film layer / modified layers (second modified layer / first modified layer) / silicon substrate was obtained.

[0109] The LN substrate of the above composite was subjected to thinning treatment. Specifically, the composite was placed in a nitrogen atmosphere oven (120°C) and heated for 10 hours. After removing the LN substrate from the oven, it was ground and lapped, and then subjected to CMP processing to obtain an LN layer (functional layer) with a thickness of 500 nm. In this way, a composite substrate comprising a functional layer, an oxide film layer, a modified layer (second modified layer / first modified layer), and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 1.

[0110] [Example 7] A composite substrate was fabricated in the same manner as in Example 6, except that laser irradiation was performed before bonding the oxide film layer and the functional substrate. Specifically, the composite substrate was fabricated as follows. Explanations common to Example 6 will be omitted as appropriate.

[0111] A silicon substrate similar to that in Example 6 was prepared as the support substrate, and the silicon substrate was cleaned in the same manner as in Example 6. Subsequently, the silicon substrate was thermally oxidized (wet oxidation) in the same manner as in Example 6, forming oxide film layers on both sides of the silicon substrate. The thickness of each oxide film layer was 4.5 μm. Next, one side of the oxide film layer formed on both sides of the support substrate (the electrode formation surface) was smoothed by polishing using CMP (Chemical Polishing) until the surface roughness Ra was 0.5 nm.

[0112] Next, a laser was irradiated onto the silicon substrate surface from the polished oxide layer side of the laminate of the silicon substrate and the oxide film layer, under the same laser irradiation conditions as in Example 6. Laser irradiation formed a modified layer between the silicon substrate and the oxide film layer, starting from the silicon substrate side. The presence of the modified layer was confirmed by TEM imaging, as in Example 6.

[0113] As a functional substrate, an LN substrate similar to that in Example 6 was prepared, and the surface of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. Subsequently, the LN substrate was cleaned in the same manner as in Example 6 to remove impurities and other contaminants from the surface of the LN substrate.

[0114] Next, the oxide film layer after cleaning and the LN substrate were directly bonded by a plasma activation method to obtain a bonded body comprising an LN substrate, an oxide film layer, a modified layer (second modified layer / first modified layer), and a silicon substrate.

[0115] Next, the LN substrate of the bonded structure was subjected to a thinning treatment. Specifically, the bonded structure was placed in a nitrogen atmosphere oven (120°C) and heated for 10 hours. After removing the LN substrate from the oven, grinding and lapping were performed, and then CMP processing was carried out to obtain an LN layer (functional layer) with a thickness of 500 nm. In this way, a composite substrate comprising a functional layer, an oxide film layer, a modified layer (second modified layer / first modified layer), and a support substrate was obtained. The obtained composite substrate was subjected to the evaluation described in (2) above. The results are shown in Table 1.

[0116]

[0117] As is clear from the above examples and comparative examples, in each example, a modified layer with a thickness of 10 nm or more was formed, and it was found that a composite substrate was obtained in which the oxide film layer did not peel off from the support substrate. On the other hand, in the comparative example, it was found that the modified layer was not formed even when irradiated with a pulsed laser, and the oxide film layer peeled off from the support substrate. Therefore, it was shown that the manufacturing method of the composite substrate in each example can form a modified layer with a thickness of a predetermined value or more, and a composite substrate can be obtained in which peeling of the active layer can be suppressed. Furthermore, it is suggested that the composite substrate in each example has a modified layer that can function as a charge trapping layer and can be applied to applications with excellent electrical properties (especially high-frequency properties). In addition, it was shown that in each example, special equipment for ensuring safety is not required as in the case of formation by the CVD method, the increase in cost can be suppressed, and composite substrates can be manufactured at low cost.

[0118] In the manufacturing method of the composite substrate according to the embodiment of the present invention, a composite substrate that can be suitably used in functional elements such as elastic wave devices and optical modulation devices such as thin-film LN optical modulators can be obtained.

[0119] 10 Support substrate 20 Active layer 21 Oxide film layer 22 Functional layer 30 Modified layer 301 Modified section 41 Functional substrate 100, 101, 102 Composite substrate

Claims

1. A method for manufacturing a composite substrate, comprising: forming an active layer on at least one surface of a crystalline support substrate; and forming a modified layer on the surface side of the support substrate by irradiating the surface of the support substrate with a pulsed laser from the side of the active layer, wherein the pulsed laser is irradiated such that the thickness of the modified layer is 10 nm or more.

2. The method for manufacturing a composite substrate according to claim 1, wherein the pulse width of the pulsed laser is less than 100 ps.

3. The energy density of the pulsed laser is 10 mJ / cm². ２ ~5000mJ / cm ２ A method for manufacturing a composite substrate according to claim 1, which is within the range.

4. The wavelength λ of the pulsed laser ａ A method for manufacturing a composite substrate according to claim 1, wherein [nm] satisfies the following formula (1): λ ａ ≥ 1240 / (E ｇ１ +2.1) ... (1) In equation (1), E ｇ１ is the band gap of the support substrate.

5. A method for manufacturing a composite substrate according to claim 1, comprising irradiating the support substrate with the pulsed laser at intervals in at least one direction within the plane of the support substrate.

6. A method for manufacturing a composite substrate according to claim 1, comprising in this order: forming the active layer and irradiating it with the pulsed laser.

7. A method for manufacturing a composite substrate according to claim 6, comprising forming a modified layer at the interface between the support substrate and the active layer.

8. The method for manufacturing a composite substrate according to claim 1, comprising smoothing the surface of the support substrate until the surface roughness Ra is 10 nm or less before forming the active layer.

9. The method for manufacturing a composite substrate according to claim 1, wherein the active layer includes an oxide film layer, and the oxide film layer is formed by oxidizing the support substrate.

10. A method for manufacturing a composite substrate according to claim 9, comprising forming the oxide film layer by thermal oxidation of the support substrate.

11. A method for manufacturing a composite substrate according to claim 1, comprising smoothing the active layer until the surface roughness Ra of the active layer is 1 nm or less.

12. A method for manufacturing a composite substrate according to claim 9, comprising bonding a functional substrate to the side of the oxide film layer opposite to the support substrate.

13. A method for manufacturing a composite substrate according to claim 12, comprising bonding the functional substrate to the oxide film layer, and then thinning the functional substrate to a thickness of 1000 nm or less to form a functional layer.

14. A method for manufacturing a composite substrate according to claim 12, comprising forming an intermediate layer between the oxide film layer and the functional substrate before joining the oxide film layer and the functional substrate.

15. A method for manufacturing a composite substrate according to claim 1, comprising irradiating the pulsed laser such that the area ratio of the modified layer to the total area in a plan view is 40% or more.

16. A method for manufacturing a composite substrate according to claim 15, further comprising forming an electrode on the active layer.

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