Composite substrate and production method for same

Abstract

Provided is a composite substrate capable of having excellent bonding strength. A composite substrate according to an embodiment of the present invention comprises a support substrate and a functional layer. The composite substrate has a bonding interface, and an oxygen reduction region in which the proportion of oxygen atoms is lower than that of at least one of layers defining the bonding interface is formed in the bonding interface.

Description

Composite substrate and method for manufacturing the same

[0001] The present invention relates to a composite substrate and a method for manufacturing the same.

[0002] Composite substrates, which consist of a support substrate and a functional substrate joined together, are used in various applications. To obtain such composite substrates, the support substrate and the functional substrate are directly bonded together with an oxide film in between. Known direct bonding methods include the SAB (Surface Activated Bonding) method and the PAB (Plasma Activated Bonding) method (for example, Patent Document 1).

[0003] Japanese Patent Publication No. 2016-225537

[0004] According to Patent Document 1, a composite substrate can be manufactured by directly bonding a single-crystal functional substrate and a support substrate via an oxide film using the PAB method. However, such a composite substrate may not be able to secure sufficient bonding strength.

[0005] The primary objective of the present invention is to provide a composite substrate that can have excellent bonding strength.

[0006] [1] A composite substrate according to one embodiment of the present invention comprises a support substrate and a functional layer. The composite substrate has a bonding interface, and an oxygen-reduced region is formed at the bonding interface in which the proportion of oxygen atoms is lower than that of at least one of the layers defining the bonding interface. [2] In the composite substrate according to [1] above, an intermediate layer is further provided between the support substrate and the functional layer, and the bonding interface may be formed between the support substrate and the intermediate layer, between the functional layer and the intermediate layer, or within the intermediate layer. [3] In the composite substrate according to [2] above, the thickness of the intermediate layer may be 0.1 μm or more and 10 μm or less. [4] In the composite substrate according to any one of [1] to [3] above, the thickness of the functional layer may be 0.1 μm or more and 30 μm or less. [5] In the composite substrate according to any one of [1] to [4] above, the thickness of the oxygen-reduced region may be 1 nm or more and 100 nm or less. [6] In the composite substrate described in any of [1] to [5] above, the functional layer may contain at least one material selected from the group consisting of lithium niobate, lithium tantalate, silicon carbide, quartz, aluminum oxide, gallium nitride, indium phosphide, and silicon. [7] In the composite substrate described in any of [1] to [6] above, the support substrate may contain at least one material selected from the group consisting of silicon, silicon carbide, gallium nitride, and glass. [8] According to another aspect of the present invention, a method for manufacturing a composite substrate is provided. The method for manufacturing a composite substrate is the method for manufacturing a composite substrate described in any of [1] or [4] to [7] above, and comprises, in this order, plasma-treating the surfaces of the support substrate and the functional substrate with a plasma of a gas containing oxygen and hydrogen; and joining the plasma-treated surfaces together. [9] According to another aspect of the present invention, a method for manufacturing a composite substrate is provided.The method for manufacturing the composite substrate is the method for manufacturing a composite substrate described in any of [2] to [7] above, and includes in this order: forming a silicon oxide layer on one of the support substrate or the functional substrate; plasma treating the surface of the silicon oxide layer and the surface of the support substrate or the functional substrate on which the silicon oxide layer is not formed with a plasma of a gas containing oxygen and hydrogen; and joining the plasma treated surfaces together.

[10] According to another aspect of the present invention, a method for manufacturing a composite substrate is provided. The method for manufacturing the composite substrate is the method for manufacturing a composite substrate described in any of [2] to [7] above, and includes in this order: forming a silicon oxide layer on the support substrate and the functional substrate, respectively; plasma treating the surface of the silicon oxide layer with a plasma of a gas containing oxygen and hydrogen; and joining the plasma treated surfaces together.

[11] In the method for manufacturing the composite substrate described in any of [8] to

[10] above, the gas may be water vapor.

[12] In the method for manufacturing the composite substrate described in [9] or

[10] above, the surface of the silicon oxide layer is planarized before the plasma treatment.

[13] The manufacturing method according to any one of [8] to

[12] above, further comprising placing the bonded material in an environment with a temperature of 200°C to 250°C for 10 hours or more.

[14] The manufacturing method according to any one of [8] to

[13] above, further comprising thinning the functional substrate after bonding.

[15] The manufacturing method according to any one of [8] to

[14] above, further comprising thinning the functional substrate to a thickness of 100 nm to 2 μm to form a functional layer.

[0007] According to embodiments of the present invention, a composite substrate having excellent bonding strength can be obtained.

[0008] This is a schematic cross-sectional view of a composite substrate according to one embodiment of the present invention. This is a schematic cross-sectional view of a composite substrate according to another embodiment of the present invention. This is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. This is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to one a modified example of a composite substrate according to an embodiment of the present invention. This is a schematic cross-sectional view illustrating another modified example of a composite substrate according to an embodiment of the present invention. This is a schematic diagram for illustrating the bonding strength by the blade test method in the example. This is a graph showing the results of the EDX measurement in Example 1.

[0009] A. Overall Structure of the Composite Substrate The outline of a composite substrate according to an embodiment of the present invention will be described. Figure 1A is a schematic cross-sectional view of a composite substrate according to one embodiment of the present invention. The illustrated example composite substrate 100 comprises a support substrate 10 and a functional layer 30. The support substrate 10 and the functional layer 30 are typically directly bonded together. Therefore, the bond interface of the composite substrate 100 is defined by the support substrate 10 and the functional layer 30. The bond interface includes the bond surface and its vicinity. In this specification, the vicinity region refers to the region within a thickness range of 100 nm from the bond surface, with the bond surface as the reference. The bond interface can be confirmed based on the results of EDX (energy-dispersive X-ray spectroscopy) measurements (distribution map based on spectral data) or RBS (Rutherford backscatter spectroscopy) and / or HFS (hydrogen forward scatter spectroscopy) measurements (distribution map based on spectral data). The support substrate and / or functional layer may contain oxygen atoms, and the oxygen-depleted region is a region in which the proportion of oxygen atoms is lower than that of at least one of these. Specifically, in the embodiment shown in Figure 1A, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the support substrate 10 and / or the functional layer 30. For example, if the support substrate 10 and the functional layer 30 contain oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the support substrate 10 and the functional layer 30. Also, for example, if only the support substrate 10 contains oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the support substrate 10. Also, for example, if only the functional layer 30 contains oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the functional layer 30.

[0010] Conventional composite substrates are manufactured by joining a support substrate and a functional substrate via an intermediate layer, such as a silicon oxide layer. However, when joining using the SAB method, the silicon oxide layer can act as the bonding interface, making it difficult to obtain sufficient bonding strength. Furthermore, if the silicon oxide layer is used as the interface, it is necessary to deposit additional silicon. In addition, when joining using the PAB method with plasma treatment, although heating after joining can improve bonding strength, it is difficult to perform the heat treatment because a large difference in thermal expansion coefficients between the support substrate and the functional substrate may cause delamination. As a result, it is difficult to increase the bonding strength of composite substrates.

[0011] As a result of diligent research, the inventors have found that by employing a plasma containing oxygen and hydrogen in the plasma treatment using the PAB method, as described later, the bonding strength of the composite substrate can be improved, and excellent bonding strength can be achieved. The reason why the composite substrate according to the embodiment of the present invention has superior bonding strength is not entirely clear, but the following reasons may be considered. In the composite substrate according to the embodiment of the present invention, when the surface of the object to be bonded is treated with a plasma containing oxygen and hydrogen during the bonding process, an oxygen-reduced region is formed on the treated surface (activated surface) due to the reducing action of activated hydrogen, and the number of hydroxyl groups (OH groups) can increase. As a result, it can be inferred that when heating is performed after bonding, more OH groups than in the conventional method can be converted into stronger covalent bonds, thereby increasing the bonding strength of the composite substrate. However, this mechanism is merely a conjecture and does not limit the present invention, nor is it constrained by this mechanism.

[0012] Figure 1B is a schematic cross-sectional view of a composite substrate according to another embodiment of the present invention. The illustrated composite substrate 101 comprises a support substrate 10, an intermediate layer 20, and a functional layer 30 in this order. In the illustrated example, the intermediate layer 20 is formed on the support substrate 10, and the intermediate layer 20 and the functional layer 30 are directly bonded. Therefore, the bonding interface of the composite substrate 101 is defined by the intermediate layer 20 and the functional layer 30. The intermediate layer and / or the functional layer may contain oxygen atoms, and the oxygen-reduced region is a region in which the proportion of oxygen atoms is lower than that of at least one of them. Specifically, in the embodiment shown in Figure 1B, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the intermediate layer 20 and / or the functional layer 30. For example, if the intermediate layer 20 and the functional layer 30 contain oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the intermediate layer 20 and the functional layer 30. Also, for example, if only the intermediate layer 20 contains oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the intermediate layer 20. For example, if only the functional layer 30 contains oxygen atoms, the oxygen reduction region 22 will have a lower proportion of oxygen atoms than the functional layer 30.

[0013] Figure 1C is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. The illustrated composite substrate 102 comprises a support substrate 10, an intermediate layer 22, and a functional layer 30 in this order. In the illustrated example, the intermediate layer 20 is formed on the functional layer 30, and the support substrate 10 and the intermediate layer 20 are typically directly bonded. Therefore, the bonding interface of the composite substrate 102 is defined by the support substrate 10 and the intermediate layer 20. The support substrate 10 and / or the intermediate layer 20 may contain oxygen atoms, and the oxygen-reduced region 22 is a region in which the proportion of oxygen atoms is lower than that of at least one of them. Specifically, in the embodiment shown in Figure 1C, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the support substrate 10 and / or the intermediate layer 20. For example, if the support substrate 10 and the intermediate layer 20 contain oxygen atoms, the oxygen-reduced region 22 has a lower proportion of oxygen atoms than the support substrate 10 and the intermediate layer 20. For example, if only the support substrate 10 contains oxygen atoms, the oxygen-reduced region 22 will have a lower proportion of oxygen atoms than the support substrate 10. Also, for example, if only the intermediate layer 20 contains oxygen atoms, the oxygen-reduced region 22 will have a lower proportion of oxygen atoms than the intermediate layer 20.

[0014] Figure 1D is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. The composite substrate 103 in the illustrated example comprises a support substrate 10, an intermediate layer 20, and a functional layer 30 in this order. In the illustrated example, the intermediate layer 20 is formed by directly joining a silicon oxide layer 201 formed on the support substrate 10 and a silicon oxide layer 202 formed on the functional layer 30 (substantially the functional substrate 300 described later). In this specification, the silicon oxide layer 201 formed on the support substrate may be referred to as the "first silicon oxide layer," and the silicon oxide layer 202 formed on the functional substrate may be referred to as the "second silicon oxide layer." Therefore, the bonding interface of the composite substrate 103 is defined by the first silicon oxide layer 201 and the second silicon oxide layer 202. In other words, the intermediate layer 20 is composed of the first silicon oxide layer 201 and the second silicon oxide layer 202, and the bonding interface is formed within the intermediate layer 20. The first silicon oxide layer 201 and the second silicon oxide layer 202 constituting the intermediate layer 20 may contain oxygen atoms, and the oxygen-reduced region 22 is a region in which the proportion of oxygen atoms is lower than that of these layers. In the embodiment shown in Figure 1D, the proportion of oxygen atoms in the oxygen-reduced region 22 is lower than that of the first silicon oxide layer 201 and the second silicon oxide layer 202.

[0015] The composite substrates according to the embodiments shown in Figures 1B to 1D have an oxygen-reduced region formed at the bonding interface of the composite substrate, and therefore exhibit excellent bonding strength, similar to the embodiment shown in Figure 1A. In the embodiments shown in Figures 1B to 1D, the intermediate layer 20 can be formed by any suitable method. A detailed explanation of the intermediate layer will be provided in Section B-3.

[0016] The composite substrate may have one or more suitable layers provided between the support substrate and the intermediate layer, and / or between the intermediate layer and the functional layer, as needed. Examples of such layers include dielectric layers. The dielectric layer may preferably be provided between the support substrate and the intermediate layer. For example, as shown in Figure 1E, in the composite substrate 104, the dielectric layer 40 may be provided between the support substrate 10 and the intermediate layer 20.

[0017] In one embodiment, the bonding interface may have a hydrogen-containing region. The hydrogen-containing region, like the oxygen-depleted region, is a region containing hydrogen that may be formed at the bonding interface during the manufacturing process of the composite substrate according to an embodiment of the present invention. The hydrogen-containing region and the oxygen-depleted region can coexist. If the composite substrate has a hydrogen-containing region at the bonding interface, the bonding strength of the composite substrate may be further improved. The presence and thickness of the hydrogen-containing region can be confirmed, for example, by RBS (Rutherford backscatter spectroscopy) and / or HFS (hydrogen forward scatter spectroscopy) measurements.

[0018] B. Details of the composite substrate Next, the components of the composite substrate will be explained in detail.

[0019] B-1. Support Substrate Any suitable substrate can be used as the support substrate 10. The support substrate may be composed of a single crystal, a polycrystalline material, or a combination thereof. Examples of materials that make up the support substrate include silicon, sapphire, sialon, cordierite, mullite, glass, quartz, crystal, alumina, germanium, silicon carbide, gallium nitride, indium phosphide, and aluminum nitride. Preferably, the support substrate contains at least one material from silicon, silicon carbide, gallium nitride, and glass. With such a configuration, the bonding strength can be further increased when the support substrate and the functional substrate are directly bonded, or when an intermediate layer (a silicon oxide layer that can substantially constitute an intermediate layer) is provided between the support substrate and the functional substrate and they are directly bonded.

[0020] It is preferable that the thermal expansion coefficient of the material constituting the support substrate is smaller than that of the material constituting the functional layer (functional substrate). With such a support substrate, changes in the shape and / or size of the functional layer when the temperature changes can be suppressed. As a result, changes in frequency characteristics can be suppressed when, for example, a composite substrate according to an embodiment of the present invention is applied to a surface acoustic wave element. In one embodiment of the present invention, when the thermal expansion coefficients of the material constituting the support substrate and the material constituting the functional layer are relatively large (for example, when the difference in thermal expansion coefficients is 9.0 × 10⁻⁶) －６Even in the above cases, the influence of temperature changes on changes in the shape and / or size of the functional layer can be suppressed. That is, the composite substrate according to the embodiment of the present invention not only maintains high bonding strength, but even if the functional layer deforms due to temperature changes such as heating, the influence on the support substrate can be mitigated, and deformation of the support substrate can be suppressed.

[0021] Any appropriate thickness can be used for the support substrate. For example, the thickness of the support substrate may be between 100 μm and 1000 μm (1 mm).

[0022] The arithmetic mean roughness Ra of the functional layer side or the intermediate layer side of the support substrate may be, for example, 0.1 nm to 1.0 nm, or for example, 0.2 nm to 0.8 nm, or for example, 0.3 nm to 0.7 nm, or for example, 0.4 nm to 0.6 nm. With such a support substrate, for example, a high-performance (for example, one with a high Q factor) surface acoustic wave element can be obtained. Note that the arithmetic mean roughness Ra is a value measured by an atomic force microscope (AFM) in a field of view of 10 μm × 10 μm.

[0023] B-2. Functional Layer The functional layer 30 may be, for example, a piezoelectric layer, or an electro-optic layer having an electro-optic effect. The functional layer 30 can be formed from any suitable functional substrate. Examples of functional substrates include piezoelectric substrates, electro-optic crystal substrates, and semiconductor substrates. Preferably, the specific material constituting the functional substrate is lithium niobate (LiNbO ３ ), lithium tantalate (LiTaO ３ ), silicon carbide, quartz, aluminum oxide, gallium nitride, indium phosphide, and silicon may be used. Below, piezoelectric layers and electro-optic layers will be described as representative examples of functional layers. However, it will be apparent to those skilled in the art that the effects of the embodiments of the present invention do not depend on the type of functional layer.

[0024] Any suitable piezoelectric material can be used as the material constituting the piezoelectric layer. Preferably, LiAO is used as the piezoelectric material. ３A single crystal having the composition can be used. Here, A is one or more elements selected from niobium and tantalum. Specifically, LiAO ３ may be lithium niobate (LiNbO ３ : LN), lithium tantalate (LiTaO ３ : LT), or a lithium niobate-lithium tantalate solid solution.

[0025] Examples of piezoelectric materials include lithium niobate, lithium tantalate, potassium titanyl phosphate (KTiOPO ４ : KTP), potassium lithium niobate (K ｘ Li （１－ｘ） NbO ２ , 0≦x≦1: KLN), potassium niobate (KNbO ３ : KN), potassium tantalate niobate (KNb ｘ Ta （１－ｘ） O ３ , 0≦x≦1: KTN), silicon, crystal, quartz, silicon carbide, gallium nitride, indium phosphide, and lead zirconate titanate (PZT).

[0026] When the piezoelectric material is lithium tantalate, the cut angle can be appropriately set according to the purpose. For example, when the X-axis (crystal axis) of the piezoelectric material is defined as the propagation direction of surface acoustic waves (X1), it is preferable that the direction rotated 32° to 55° (for example, 42°) from the Y-axis toward the Z-axis corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), and specifically, the Euler angle representation is (180°, 58° to 35°, 180°).

[0027] When the piezoelectric material is lithium niobate, the cut angle can be appropriately set depending on the purpose. For example, it is preferable that the piezoelectric layer is positioned such that the direction rotated from the Z-axis to the -Y-axis by 0° to 40° (e.g., 37.8°) when the X-axis (crystal axis) of the piezoelectric material is the propagation direction of surface acoustic waves (X1) corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), specifically in Euler angle notation (0°, 0° to 40°, 0°). When the piezoelectric material is lithium niobate, it is also preferable that the piezoelectric layer is positioned such that the direction rotated from the Y-axis to the Z-axis by 40° to 65° when the X-axis (crystal axis) of the piezoelectric material is the propagation direction of surface acoustic waves (X1) corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), specifically in Euler angle notation (180°, 50° to 25°, 180°).

[0028] The thickness of the piezoelectric layer is, for example, 30 μm (30,000 nm) or less, preferably 10 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, and particularly preferably 1.5 μm or less. With such a thickness, for example, a high-performance (for example, having good temperature characteristics and / or a high Q value) surface acoustic wave element can be obtained. On the other hand, the thickness of the piezoelectric layer may be, for example, 0.05 μm (50 nm) or more, or for example, 0.2 μm or more.

[0029] The arithmetic mean roughness Ra of the surface on the support substrate side of the piezoelectric layer, or on the intermediate layer side of the piezoelectric layer, may be, for example, 1.0 nm or less, 0.8 nm or less, 0.6 nm or less, 0.4 nm or less, or 0.2 nm or less. With such a piezoelectric layer, for example, a high-performance (for example, having a high Q factor) surface acoustic wave element can be obtained.

[0030] The electro-optic layer can be fabricated from any suitable electro-optic crystal substrate. Any appropriate electro-optic crystal material may be used as the material constituting the electro-optic crystal substrate (a material having an electro-optic effect). Examples of such materials include lithium niobate, lithium tantalate, potassium titanyl phosphate, potassium lithium niobate, potassium niobate, potassium tantalate niobate, solid solutions of lithium niobate and lithium tantalate, silicon carbide (SiC), and quartz. Note that the electro-optic layer may be the electro-optic crystal substrate itself.

[0031] Part or all of the electro-optic layer can serve as an optical waveguide that transmits light in an electro-optic element. The electro-optic layer may have an upper surface exposed to the outside and a lower surface located within the composite substrate. The optical constant (for example, refractive index) of the electro-optic layer can change when an electric field is applied thereto. The c-axis of the electro-optic crystal substrate constituting the electro-optic layer may be parallel to the electro-optic crystal substrate (that is, the electro-optic crystal substrate may be an X-cut substrate or a Y-cut substrate), or the c-axis of the electro-optic crystal substrate may be perpendicular to the electro-optic crystal substrate (that is, the electro-optic crystal substrate may be a Z-cut substrate).

[0032] The thickness of the electro-optic layer can be set to any appropriate thickness depending on the purpose. The thickness of the electro-optic layer may be, for example, 0.1 μm to 10 μm, preferably 0.2 μm or more, and more preferably 0.3 μm or more. When the lower limit of the thickness of the electro-optic layer falls within the above range, the light propagation loss in the electro-optic element can be reduced. On the other hand, the thickness of the electro-optic layer is preferably 5.0 μm or less, more preferably 2.5 μm or less, and even more preferably 1.0 μm or less. When the upper limit of the thickness of the electro-optic layer falls within the above range, the high-speed and low-voltage driving performance of the electro-optic element can be improved. Furthermore, when the thickness of the electro-optic layer falls within the above range, the effect of using a dielectric layer becomes significant. Accordingly, driving at higher speed and lower voltage can be achieved while suppressing an increase in light propagation loss.

[0033] B-3. Intermediate Layer The intermediate layer is a layer that can be provided as needed. The intermediate layer may be formed of any appropriate material. Examples of materials for forming the intermediate layer include silicon oxide, amorphous silicon (a-Si), silicon nitride, silicon oxynitride (a compound of silicon, oxygen and nitrogen), aluminum oxide, aluminum nitride, and aluminum oxynitride (a compound of aluminum, oxygen and nitrogen).

[0034] Any appropriate method can be adopted as a method for forming the intermediate layer. Examples of the forming method include oxidation, sputtering, vapor deposition, and ion plating. The intermediate layer may be formed, for example, by oxidizing a support substrate, or may be formed on a substrate (the support substrate and / or a functional substrate) by sputtering, for example.

[0035] The intermediate layer may be a single layer, or may have a laminated structure including a plurality of layers. The intermediate layer can preferably be composed of silicon oxide. The intermediate layer may be, for example, a single layer of silicon oxide, or may have a laminated structure of one silicon oxide layer and another different silicon oxide layer. When the intermediate layer is a single layer, the intermediate layer can be composed of, for example, a silicon oxide layer formed on the surface of either the support substrate or the functional substrate. When the intermediate layer has a laminated structure (for example, two silicon oxide layers), the intermediate layer can be composed of silicon oxide layers formed on the surfaces of both the support substrate and the functional substrate. For example, as shown in FIG. 1D, in the composite substrate 103, a first silicon oxide layer 201 formed on the support substrate 10 and a second silicon oxide layer 202 formed on the functional layer 30 are directly bonded, the bonding interface is defined by the first silicon oxide layer 201 and the second silicon oxide layer 202, and the intermediate layer 20 is constituted by the same. In such an intermediate layer, an oxidation-reduced region 22 can be formed at the bonding interface between the first silicon oxide layer 201 and the second silicon oxide layer 202.

[0036] Any appropriate thickness can be adopted for the intermediate layer. The thickness of the intermediate layer may be, for example, 10 nm to 20,000 nm (20 μm), preferably 100 nm to 15,000 nm (15 μm), more preferably 100 nm to 10,000 nm (10 μm), and even more preferably 1,000 nm (1 μm) to 10,000 nm (10 μm). If the intermediate layer is composed of multiple silicon oxide layers, the thickness of the intermediate layer is the total thickness of the multiple silicon oxide layers. If an oxygen-reduced region is formed within the intermediate layer, the thickness of the intermediate layer includes the thickness of the oxygen-reduced region.

[0037] B-4. Oxygen-reduced regions The oxygen-reduced regions 22 may be regions formed at the bonding interface during the manufacturing process of the composite substrate according to the embodiment of the present invention. Typically, the oxygen-reduced regions are regions in which the proportion of oxygen atoms is lower than that of at least one of the layers defining the bonding interface (substantially, the support substrate, the functional layer, or the intermediate layer). In one embodiment, the oxygen-reduced regions may be formed in layers over the entire bonding interface or in part thereof, or multiple oxygen-reduced regions may be formed scattered throughout the interface.

[0038] The thickness of the oxygen-depleted region is preferably 1 nm to 100 nm, more preferably 2 nm to 80 nm, and even more preferably 3 nm to 50 nm. When the thickness of the oxygen-depleted region is within the above range, the high bonding strength of the composite substrate can be well maintained. "Thickness of the oxygen-depleted region" refers to the average thickness when it is formed in layers. When multiple oxygen-depleted regions are formed in a scattered manner, the thickness of the oxygen-depleted region is the weighted average thickness of the thickness in each region. The presence of the oxygen-depleted region can be confirmed by EDX measurement as described above. The proportion of oxygen atoms in the oxygen-depleted region and the thickness of the oxygen-depleted region can typically be calculated from a distribution map based on spectral data obtained by EDX measurement. Specifically, the above distribution diagram shows the relationship between the distance from the thickness direction ("thickness direction distance") on the horizontal axis and the atomic ratio ("atomic content ratio") on the vertical axis. The thickness of the oxygen reduction region at the bonding interface can be confirmed by reading the interval from the "thickness direction distance" at the beginning of the falling edge of the peak of the oxygen atom "atomic ratio" in the distribution diagram to the "thickness direction distance" at the end of the rising edge (see, for example, Figure 5). The specific measurement method and conditions for EDX measurement are as described in the examples below. The oxygen reduction region may also be calculated from a distribution diagram based on spectral data obtained by RBS measurement and / or HFS measurement.

[0039] The oxygen content relative to the total atoms in the oxygen-depleted region at the bonding interface is, for example, 55 at% to 70 at%, preferably 65 at% or less, and more preferably 60 at% or less. With such a configuration, the bonding strength of the composite substrate can be further improved.

[0040] The amount of oxygen atom reduction in the oxygen reduction region relative to the layer defining the bonding interface is, for example, 1 at% or more, preferably 2 at% or more, more preferably 3 at% or more, and even more preferably 5 at% or more. The upper limit of the above reduction is, for example, 20 at%. The amount of oxygen atom reduction in the oxygen reduction region means the degree of reduction from the oxygen atom content (content ratio) of the layer defining the bonding interface. For example, if the oxygen atom content ratio of the intermediate layer is 65 at% and the oxygen atom content ratio of the oxygen reduction region is 64 at%, the reduction is 1 at%. That is, the amount of oxygen atom reduction (degree of reduction) in the above oxygen reduction region is calculated by the difference between the oxygen atom content ratio of the layer defining the bonding interface and the oxygen atom content ratio of the oxygen reduction region.

[0041] B-5. Other layers may be provided between the support substrate and the intermediate layer and / or between the intermediate layer and the functional layer, as needed. As described above, examples of such layers include dielectric layers. Dielectric layers can improve the stability of the temperature characteristics of the composite substrate. Dielectric layers are preferably provided between the support substrate and the intermediate layer. For example, when the functional layer includes an electro-optic layer, the dielectric layer may preferably be provided between the support substrate and the intermediate layer.

[0042] Any suitable dielectric material can be used as the material constituting the dielectric layer. Examples of dielectric materials include silicon oxide, amorphous silicon (a-Si), silicon nitride, silicon oxynitride (a compound of silicon, oxygen, and nitrogen), aluminum oxide, aluminum nitride, and aluminum oxynitride (a compound of aluminum, oxygen, and nitrogen). The dielectric layer may be a single layer or may have a laminated structure consisting of multiple layers composed of different dielectric materials. For example, the dielectric may be a single layer of silicon oxide, or it may have a laminated structure of silicon oxide and silicon oxynitride, or silicon oxide and silicon nitride.

[0043] The thickness of the dielectric layer may be, for example, 100 nm or more and 1000 nm or less, for example, 200 nm or more and 800 nm or less, for example, 300 nm or more and 700 nm or less, or for example, 400 nm or more and 600 nm or less.

[0044] The dielectric layer can be formed by any suitable method. Specific examples of methods for forming the dielectric layer include sputtering, CVD, and ion-assisted deposition.

[0045] C. Method for Manufacturing a Composite Substrate A method for manufacturing a composite substrate according to one embodiment includes, in this order: forming a silicon oxide layer on a support substrate and a functional substrate, plasma treating the surface of the silicon oxide layer with a plasma of oxygen and hydrogen gas, and joining the plasma treated surfaces together. In this embodiment, as long as the above order is followed, the other steps may be performed in any order as necessary, as long as it does not hinder the objective of the present invention.

[0046] A specific example of a method for manufacturing a composite substrate according to one embodiment will be described below with reference to Figures 2A to 2F. Note that Figure 2F is identical to Figure 1D.

[0047] C-1. Formation of the Intermediate Layer First, a support substrate 10 is prepared as shown in Figure 2A. At least one main surface of the support substrate may be subjected to a planarization treatment. The planarization treatment includes polishing the main surface so that the arithmetic mean roughness Ra is, for example, 0.1 nm or more and 1.0 nm or less. Examples of polishing methods include lapping and chemical mechanical polishing (CMP). Note that the planarization treatment may be omitted.

[0048] As shown in Figure 2A, a silicon oxide layer (first silicon oxide layer 201) that can constitute an intermediate layer 20 may be formed on at least one main surface of the support substrate 10 (first main surface 10a in the illustrated example) as needed. Any suitable method can be used to form the silicon oxide layer that can constitute an intermediate layer on the support substrate. Representative examples of formation methods include oxidation, sputtering, vapor deposition, and ion plating.

[0049] When the material constituting the support substrate contains silicon, the method for forming the silicon oxide layer is preferably thermal oxidation. Thermal oxidation has the remarkable effect of significantly suppressing defects in the oxide film (silicon oxide layer) of the formed support substrate. As a result, the yield when manufacturing a composite substrate having a silicon oxide layer as an intermediate layer can be improved. 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. In contrast, when the oxide film (silicon oxide layer) is formed by thermal oxidation as described above, the silicon oxide layer can usually be formed by oxidation at a temperature of 700°C or higher. As a result, the thermal oxide film (silicon oxide layer) is less likely to contain components that can cause outgassing, and therefore has the advantage of having excellent thermal stability of the film quality in the composite substrate.

[0050] The method and conditions for thermal oxidation can be any suitable method and conditions. Typically, thermal oxidation can be carried out under heating conditions of 700°C to 1200°C in an oxidizing atmosphere. Specifically, this is done as follows: A support substrate is placed in a chamber, the chamber is heated to 700°C to 1200°C, and then an oxidizing atmosphere is created by supplying any suitable gas into the chamber to oxidize the support substrate. The oxidizing atmosphere can be created by supplying, for example, oxygen, hydrogen, water vapor, hydrochloric acid (hydrogen chloride), or a mixture of two or more of these gases. With such thermal oxidation, oxidation can proceed from the surface of the support substrate and an oxide film (silicon oxide layer) can be formed.

[0051] 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 (first silicon oxide layer) can be adjusted by setting the processing temperature, processing time, and other conditions to any appropriate conditions so that it reaches a desired thickness. The thickness of the oxide film after thermal oxidation is preferably 0.05 μm or more and 30 μm or less. In this way, one or both sides of the support substrate 10 can be oxidized, and the first silicon oxide layer 201 can be formed.

[0052] On the other hand, as shown in Figure 2B, a functional substrate 300 is prepared. The functional substrate is a substrate that is thinned to form a functional layer, as will be described later. Therefore, the materials constituting the functional substrate are as described in Section B-2 above with respect to the functional layer. At least one main surface of the functional substrate may be subjected to a planarization treatment. The planarization treatment includes polishing the main surface so that the arithmetic mean roughness Ra is, for example, 0.1 nm or more and 1.0 nm or less. As for the polishing method, similar to that of the support substrate described above, examples include lapping and chemical mechanical polishing (CMP). The first main surface 300a of the functional substrate 300 may be polished so that the arithmetic mean roughness Ra is 0.1 μm or more and 1.0 μm or less, or for example, 0.2 μm or more and 0.4 μm or less. Note that the planarization treatment may be omitted.

[0053] The thickness of the functional substrate may be, for example, 100 μm or more and 1000 μm (1 mm) or less, or for example, 200 μm or more and 500 μm or less.

[0054] If necessary, a silicon oxide layer (second silicon oxide layer 202) that can constitute an intermediate layer 20 may be formed on the first main surface 300a of the functional substrate. Any suitable method can be used to form the silicon oxide layer on the functional substrate. A typical example of such a method is sputtering.

[0055] The thickness of the first silicon oxide layer and the second silicon oxide layer are preferably 5 nm to 3000 nm, more preferably 10 nm to 500 nm, and even more preferably 50 nm to 300 nm. The thickness of the first silicon oxide layer 201 and the thickness of the second silicon oxide layer 202 may be the same or different. For example, the thickness of the first silicon oxide layer 201 may be greater or less than the thickness of the second silicon oxide layer 202.

[0056] C-2. Planarization Treatment of Joining Surfaces In one embodiment, before plasma treatment, the surfaces to be joined (i.e., exposed surfaces) are subjected to a planarization (smoothing) treatment. Specifically, for example, the first main surface 201a of the first silicon oxide layer 201 formed on the support substrate 10 and / or the first main surface 202a of the second silicon oxide layer 202 formed on the functional substrate 300 are subjected to a planarization treatment.

[0057] By planarization treatment, the first main surface 201a of the first silicon oxide layer 201 and the first main surface 202a of the second silicon oxide layer 202 can be polished so that their arithmetic mean roughness Ra is, for example, 0.1 nm or more and 10 nm or less. Planarization treatment can be performed in the same manner as the polishing treatment described above. Note that planarization treatment may be omitted. If no intermediate layer (e.g., silicon oxide layer) is formed on the support substrate and functional substrate, the same treatment as the planarization treatment for the support substrate and functional substrate described in section C-1 above may be used.

[0058] C-3. Plasma Treatment (Activation Treatment) Next, as shown in Figures 2C and 2D, when joining silicon oxide layers (first silicon oxide layer 201 and second silicon oxide layer 202), plasma treatment is performed on the first main surface 201a of the first silicon oxide layer 201 and the first main surface 202a of the second silicon oxide layer 202, respectively. In the method for manufacturing a composite substrate according to the embodiment of the present invention, the plasma used is a plasma of a gas containing oxygen and hydrogen, as described above. By plasma treatment (activation treatment) with a gas containing oxygen and hydrogen, the first main surface 201a of the first silicon oxide layer 201 and the first main surface 202a of the second silicon oxide layer 202, which form the joining surface, can be made hydrophilic. In this specification, "gas containing oxygen and hydrogen" means that the gas is a gas containing at least oxygen atoms and hydrogen atoms. Furthermore, the surface state resulting from plasma treatment (for example, a hydrophilic state) can be confirmed by measuring the spectrum of the surface after plasma treatment using solid-state emission spectroscopy (OES: Optical Emission Spectrometer) and examining the resulting emission spectrum. When plasma treatment is performed using a gas containing oxygen and hydrogen, the presence of many hydroxyl groups (OH groups) on the surface can be confirmed from the emission spectrum.

[0059] The gas containing oxygen and hydrogen may preferably be water vapor. Using water vapor (water vapor plasma) in the plasma treatment can make the effects of the present invention even more pronounced.

[0060] Plasma treatment can be performed, for example, in any suitable chamber. The conditions for plasma treatment can be appropriately set according to the gas used for the plasma, the type, thickness and surface roughness Ra of the support substrate, the type, thickness and surface roughness Ra of the functional substrate, and the thickness and surface roughness Ra of the silicon oxide layer, etc.

[0061] The flow rate of the gas used for the plasma during plasma processing is preferably 10 sccm or more, more preferably 20 sccm or more, and even more preferably 50 sccm or more. The upper limit of the gas flow rate may be, for example, 100 sccm.

[0062] The ambient pressure during plasma treatment is preferably 2 Pa or more, more preferably 5 Pa or more, and even more preferably 10 Pa or more. On the other hand, the ambient pressure is preferably 100 Pa or less, more preferably 80 Pa or less, and even more preferably 50 Pa or less.

[0063] The temperature during plasma treatment is preferably 30°C or higher, more preferably 35°C or higher, and even more preferably 40°C or higher. On the other hand, the temperature during plasma treatment is preferably 100°C or lower, more preferably 90°C or lower, and even more preferably 80°C or lower. At such temperatures, for example, the deterioration of the functional substrate due to plasma irradiation can be suppressed.

[0064] The plasma irradiation energy may preferably be 100 W or more, more preferably 200 W or more. On the other hand, the plasma irradiation energy may be, for example, 1000 W or less, preferably 500 W or less, more preferably 400 W or less.

[0065] The plasma irradiation time may preferably be 60 seconds or more, more preferably 80 seconds or more, and even more preferably 120 seconds or more. The upper limit of the plasma irradiation time may be, for example, 600 seconds.

[0066] After plasma treatment, the bonding surfaces may be cleaned before joining. Cleaning the bonding surfaces can remove impurities such as particles that may be generated by the plasma treatment. The bonding surfaces can be cleaned by any suitable method. For example, ultrasonic cleaning using pure water can be used to clean the bonding surfaces.

[0067] C-4. Bonding Next, the plasma-treated surfaces are joined together. As shown in Figure 2E, for example, the bonding surfaces (substantially the first main surfaces 201a and 202a) of the support substrate 10 having the first silicon oxide layer 201 after plasma treatment and the functional substrate 300 having the second silicon oxide layer 202 after plasma treatment are superimposed. Due to the predetermined plasma treatment described above, each of the first silicon oxide layer and the second silicon oxide layer may have hydroxyl groups. In one embodiment, each of the first silicon oxide layer and the second silicon oxide layer may have silanol groups. An intermediate layer 20 may be formed by bonding the first silicon oxide layer 201 and the second silicon oxide layer 202. The intermediate layer 20 may consist of the first silicon oxide layer 201 and the second silicon oxide layer 202. The intermediate layer 20 may contain silicon oxide.

[0068] The ambient temperature during joining can be any suitable temperature. For example, the ambient temperature during joining is room temperature.

[0069] In this way, a bonded body 90 is obtained comprising a support substrate 10, an intermediate layer 20 (first silicon oxide layer 201 / second silicon oxide layer 202), and a functional substrate 300 (see Figure 2E). In the bonded body 90, an oxygen-reducing region 22 is formed within the intermediate layer 20 (substantially, at the bonding interface between the first silicon oxide layer 201 and the second silicon oxide layer 202).

[0070] C-5. Heat Treatment In one embodiment, the above manufacturing method preferably includes heating the bonded body (heat treatment) after bonding. The heat treatment may be performed at any appropriate point after bonding. Specifically, the heat treatment may be performed after bonding the support substrate and the functional substrate (substantially via the silicon oxide layer) and before thinning of the functional substrate, during thinning of the functional substrate, or after thinning of the functional substrate (after formation of the functional layer). The heat treatment may preferably be performed after bonding as described above, and more preferably after bonding and before thinning of the functional substrate.

[0071] Heat treatment can be carried out under any suitable conditions. Typically, heat treatment can be carried out in air or an inert gas atmosphere.

[0072] The heating temperature is, for example, 200°C to 600°C, preferably 300°C to 590°C, more preferably 400°C to 580°C, and even more preferably 500°C to 550°C. If the heating temperature is within this range, the bonding strength in the composite substrate can be particularly increased.

[0073] The heating rate and the cooling rate after heating can be any appropriate rate. The heating rate and cooling rate may be, for example, 10°C / min or less, or for example, 0.5°C / min or more and 5°C / min or less. Cooling after heating may be done by natural cooling.

[0074] The heating time (heating and holding time) is preferably 10 hours or more, more preferably 15 hours or more, and even more preferably 20 hours or more. The upper limit of the heating time may be, for example, 50 hours. In one embodiment, the heat treatment may preferably be carried out by placing the material in an environment of 200°C or higher for 10 hours or more. The above temperature is more preferably 200°C or higher and 250°C or lower. When the heat treatment is carried out under such conditions, the effects of the present invention may be particularly pronounced.

[0075] C-6. Thinning of the functional substrate (formation of the functional layer) Next, as shown in Figures 2E to 2F, the functional substrate 300 of the bonded body 90 is thinned to form a functional layer 30. Specifically, as described above, a functional substrate with a thickness of 100 μm or more and 1000 μm or less is polished to form a thin functional layer 30. In one embodiment, the thickness of the functional substrate is preferably thinned to 100 nm or more and 2 μm or less to form the functional layer. The thickness after polishing (i.e., the thickness of the functional layer) can be, for example, 50 nm or more and 30000 nm or less, or for example, 200 nm or more and 1500 nm or less, as explained in Section B-2 above.

[0076] As described above, a composite substrate 103 having a support substrate 10, an intermediate layer 20, and a functional layer 30 in this order can be obtained as shown in Figure 2F. In the composite substrate 103, the bonding interface is defined by the first silicon oxide layer 201 and the second silicon oxide layer 202. The composite substrate 103 has an oxygen reduction region 22 at the bonding interface within the intermediate layer 20.

[0077] C-7. Modified Examples In the above, a case was described in which a silicon oxide layer that can constitute an intermediate layer is formed on both the support substrate and the functional substrate. However, an intermediate layer (for example, a silicon oxide layer that can constitute an intermediate layer) may be formed on only one of the support substrate or the functional substrate (Modified Example 1). Such a composite substrate can be manufactured by a method that includes, in this order, forming a silicon oxide layer on one of the support substrate or the functional substrate, plasma-treating the surface of the silicon oxide layer and the surface of the support substrate or functional substrate where the silicon oxide layer is not formed with a plasma of oxygen and hydrogen gas, and joining the plasma-treated surfaces together. Specifically, the embodiment according to Modified Example 1 differs from the above in that the target of forming the silicon oxide layer is only one of either the support substrate or the functional substrate, and the targets of plasma treatment are the surface of the silicon oxide layer and the surface of one of the substrates (the substrate where the silicon oxide layer is not formed) of the support substrate or the functional substrate. Therefore, the explanations in sections C-1 to C-6 above may be applied to other processing methods, conditions, etc. The following explanation of the modified example will omit any explanations that overlap with the above.

[0078] In the embodiment according to Modification 1, for example, first, a first silicon oxide layer 201 (or a second silicon oxide layer 202) is formed on the support substrate 10 (or functional substrate 300). Next, the first main surface 201a of the first silicon oxide layer (or the first main surface 202a of the second silicon oxide layer 202) and the first main surface 300a of the functional substrate 300 (or the first main surface 10a of the support substrate 10), which does not have a silicon oxide layer formed on it, are subjected to plasma treatment using a plasma of a gas containing oxygen and hydrogen. Next, the plasma-treated bonding surfaces are overlapped and bonded to obtain a bonded body. Then, a composite substrate can be obtained by thinning the functional substrate and forming a functional layer. When the first silicon oxide layer 201 is formed on the support substrate 10, a composite substrate 101 as shown in Figure 3A can be obtained. In the composite substrate 101, the intermediate layer 20 may be composed of the first silicon oxide layer 201. An oxygen-reduced region 22 may be formed at the junction interface between the first silicon oxide layer 201 and the functional layer 30. Figures 3A and 1B represent the same content. When a second silicon oxide layer 202 is formed on the functional substrate 300, a composite substrate 102 as shown in Figure 3B may be obtained. In the composite substrate 102, the intermediate layer 20 may be composed of the second silicon oxide layer 202. An oxygen-reduced region 22 may be formed at the junction interface between the support substrate 10 and the second silicon oxide layer 202. Figures 3B and 1C represent the same content.

[0079] When manufacturing a composite substrate having a dielectric layer, the dielectric layer may be formed before joining the support substrate and the functional substrate. Specifically, for example, in section C-1, before forming the silicon oxide layer (first silicon oxide layer 201), a dielectric layer may be formed on the surface of the support substrate 10 using any appropriate method. Alternatively, for example, in section C-1, before forming the silicon oxide layer (second silicon oxide layer 202), a dielectric layer may be formed on the surface of the functional substrate 300 using any appropriate method. In this case, it is preferable not to smooth the functional substrate before forming the dielectric layer. After forming the dielectric layer, a silicon oxide layer may be formed on the dielectric layer. A typical example of a method for forming the dielectric layer is sputtering, as described above. Next, a silicon oxide layer is formed on the dielectric layer. As for the method of forming the silicon oxide layer on the dielectric layer, any appropriate method may be used, as described above.

[0080] Alternatively, a composite substrate may be manufactured by directly joining a support substrate and a functional substrate (Modification 2). Such a composite substrate can be manufactured by a method that includes, in this order, plasma treatment of the surfaces of the support substrate and the functional substrate with a plasma of a gas containing oxygen and hydrogen, and joining the plasma-treated surfaces together. That is, the method for manufacturing the composite substrate in the embodiment according to Modification 2 may be the same as the above embodiment except that an intermediate layer (for example, a silicon oxide layer that can constitute an intermediate layer) is not formed. Therefore, the explanations in sections C-1 to C-6 above can be used for other processing methods, conditions, etc. In the embodiment according to Modification 2, for example, first, the first main surface 10a of the support substrate 10 and the first main surface 300a of the functional substrate 300 are subjected to plasma treatment with a plasma of a gas containing oxygen and hydrogen. Next, the plasma-treated joining surfaces are overlapped and joined, and heated as necessary to obtain a joined body. Next, the functional substrate 300 is thinned to form a functional layer 30. By doing so, a composite substrate 100 as shown in Figure 1A can be obtained. In the composite substrate 100, an oxygen-reducing region 22 may be present at the bonding interface between the support substrate 10 and the functional layer 30.

[0081] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement and evaluation methods in the examples are as follows. Unless otherwise specified, "parts" and "%" in the examples are based on weight.

[0082] (1) Bonding Strength The composite substrates obtained in the examples and comparative examples were subjected to bonding strength measurement using the blade test method. Specifically, the bonding strength measurement using the blade test method was performed as follows. As shown in Figure 4, a blade 7B was inserted between one layer (first layer 1A) and the other layer (second layer 2A) that were bonded via the bonding surface of the composite substrate test piece 100S, and the amount of peeling (peeling extension length) between the first layer 1A and the second layer 2A was measured. Based on the obtained results, the bonding strength γ was calculated from the following formula (1). The bonding surface between the first layer 1A and the second layer 2A is the same surface that was bonded in each example and comparative example. In formula (1), E １ t is the Young's modulus of the first layer 1A. ｗ１ is the thickness of the first layer 1A, E ２ t is the Young's modulus of the second layer 2A. ｗ２ γ is the thickness of the second layer 2A, and L is the peel extension length. Based on the obtained results (bonding strength γ), the following criteria were used for evaluation: ◎ (Good): Bonding strength γ is 1.5 J / m ２ The above is correct. ○ (Acceptable): Joint strength γ is 1.0 J / m ２ 1.5J / m or more ２ Less than × (Poor): Bonding strength γ is 1.0 J / m ２ Furthermore, the joint strength is less than 1.5 J / m ２ Based on the above, it can be determined that no delamination occurs during the thin-film deposition process (of the functional substrate), and the yield is approximately 100%. The bonding strength is 1.0 J / m ２ 1.5J / m or more ２ If the value is less than 1.0 J / m, it can be determined that some peeling occurs during the thin-film formation process, resulting in a yield of approximately 60%. ２ If the value is less than this, peeling will occur in most cases, and the yield can be judged to be approximately 10% or less.

[0083] (2) Oxygen Atom Content and Thickness of Oxygen Depletion Region The composite substrates obtained in the examples and comparative examples were further CMP polished until the thickness of the functional layer was 0.1 μm (100 nm). EDX (energy dispersive X-ray spectroscopy) measurements were performed on the polished composite substrates using an EDX (energy dispersive X-ray spectroscopy) apparatus under the conditions of an acceleration voltage of 200 kV and a beam diameter of approximately 0.2 nm. Based on the spectral data obtained from the above measurements, a distribution map showing the relationship between these was obtained, with the horizontal axis representing the distance in the thickness direction and the vertical axis representing the atomic content. The thickness of the oxygen depletion region was defined as the distance from the starting position of the falling edge of the peak of the oxygen (O) atomic content at the bonding interface to the ending position of the rising edge in the thickness direction. Note that "distance in the thickness direction" is the distance in the thickness direction from the reference point (0 nm) where the measurement start position at the bonding interface is the reference point. In addition, the oxygen atom content of the layer defining the bonding interface and the oxygen atom content (at%) in the nearby region (approximately 1 to 5 nm from the bonding surface) were confirmed in the distribution map.

[0084] [Example 1] A silicon substrate with an orientation flat (OF) portion, a diameter of 4 inches, and a thickness of 500 μm was prepared as the support substrate. The arithmetic mean roughness Ra of the surface (first main surface) of the silicon substrate was 0.2 nm.

[0085] Next, an oxide film was formed on the surface (first main surface and second main surface) 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, oxygen and water vapor were supplied to create an oxidizing atmosphere, and wet oxidation was performed on the silicon substrate. Next, the silicon substrate with silicon oxide layers formed on both sides was removed. The thickness of the silicon oxide layer was 3 μm (3000 nm). One of the silicon oxide layers formed on both sides of the silicon substrate was designated as the first silicon oxide layer, and this was used as the target surface for bonding described later (the other silicon oxide layer is referred to as the other silicon oxide layer).

[0086] As the functional substrate, a lithium niobate substrate (LN substrate) with an OF (Optical Fiber) portion, a diameter of 4 inches, and a thickness of 250 μm was used. An X-cut LN substrate was used, with the surface acoustic wave (SAW) propagation direction set to X. The surface (first main surface) of the LN substrate was mirror-polished to an arithmetic mean roughness Ra of 0.2 nm or less. The arithmetic mean roughness was evaluated using an atomic force microscope (AFM) over a 10 μm x 10 μm square field of view.

[0087] Next, the first main surface of the first silicon oxide layer formed on the silicon substrate and the first main surface of the LN substrate were mirror-polished to achieve an arithmetic mean roughness Ra of 0.2 nm or less. This planarized each surface (first main surface).

[0088] Next, the silicon substrate having the smoothed first silicon oxide layer and the LN substrate were placed in a plasma chamber, and a water vapor plasma was generated in the chamber. This irradiated the first main surface of the first silicon oxide layer and the first main surface of the LN substrate with the plasma, performing a plasma activation treatment. The plasma treatment conditions were as follows: <Plasma Treatment Conditions> Gas type: Water vapor Gas flow rate: 20 sccm Chamber pressure: 5 Pa Treatment time: 480 seconds Output: 900 W

[0089] Next, the activated surface (the bonding surface of the first silicon oxide layer and the LN substrate that had undergone plasma treatment) was ultrasonically cleaned with pure water and then spin-dried to remove particles adhering to the activated surface.

[0090] Next, the substrates were aligned, and at room temperature, the bonding surfaces (activated surfaces) of both substrates were overlapped and bonded so that the LN substrate was on top. This resulted in a bonded body having a different silicon oxide layer / support substrate / intermediate layer (first silicon oxide layer) / functional substrate configuration.

[0091] Next, the bonded body was subjected to heat treatment (primary annealing). The heat treatment was carried out in an atmospheric environment. The heating was increased from room temperature to 150°C at a rate of 1°C / min, held at 150°C for 10 hours, and then allowed to cool naturally. Subsequently, the surface of the LN substrate (the outermost layer surface opposite the support substrate of the bonded body) was polished using a grinding machine until the thickness of the LN substrate was 3 μm, thereby forming a functional layer by thinning it.

[0092] Next, the polished bonded body was subjected to further heat treatment (secondary annealing). The heat treatment was carried out in an atmospheric environment. The heating was increased from room temperature to 250°C at a rate of 1°C / min, held at 250°C for 10 hours, and then allowed to cool naturally. Finally, the surface of the functional layer was mirror-polished using a CMP polishing machine until the thickness reached 0.6 μm (600 nm).

[0093] A composite substrate was obtained as described above. The obtained composite substrate was subjected to the evaluations described in (1) and (2) above. The results are shown in Table 1. Figure 5 shows the results (distribution map) obtained based on the EDX measurement described in (2) above. As is clear from Figure 5, the existence of an oxygen-depleted region was confirmed at the bonding interface of the composite substrate, where the oxygen atom ratio is lower than that of the first silicon oxide layer (intermediate layer) and the functional layer (LN layer) that define the bonding interface. Specifically, in the distribution map, it was confirmed that at the bonding interface, oxygen atoms are locally present from approximately 17 nm to approximately 20 nm (between the thickness direction distance at the start of the falling edge and the thickness direction distance at the end of the rising edge), with reference to 0 nm on the functional layer side of the bonding interface of the composite substrate. The thickness of the oxygen-depleted region was approximately 3 nm. The oxygen atom ratios of the layer defining the bonding interface and the oxygen-depleted region are shown in Table 1.

[0094] [Example 2] A composite substrate was fabricated in the same manner as in Example 1, except that a silicon oxide layer was not formed on the support substrate (silicon substrate), and a silicon oxide layer was formed on the functional substrate (LN substrate). The silicon oxide layer on the functional substrate was formed as follows: On the LN substrate, SiO was formed by RF sputtering. ２A silicon oxide layer was formed by depositing a film with a thickness of 200 nm. RF sputtering was performed using a boron-doped Si target. Oxygen gas was introduced as the oxygen source, and the total pressure and partial pressure of oxygen in the chamber atmosphere were adjusted by controlling the amount of oxygen gas introduced. Next, the laminate of the LN substrate (functional substrate) / (second) silicon oxide layer was subjected to heat treatment. The heat treatment was performed under a nitrogen atmosphere. Heating was performed by raising the temperature from room temperature to 200°C at a rate of 1°C / min, holding at 200°C for 5 hours, and then cooling down to room temperature at a rate of 1°C / min. The thickness of the second silicon oxide layer was 200 nm. The obtained composite substrate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In Example 2, high bonding strength was demonstrated, similar to Example 1. In addition, EDX measurement results confirmed the existence and depth (thickness) of the oxygen reduction region, similar to Example 1. The oxygen atom ratio of the oxygen reduction region in Example 2 is shown in Table 1.

[0095] [Example 3] A composite substrate was fabricated in the same manner as in Example 1, except that a silicon oxide layer was formed on a support substrate (silicon substrate) by RF sputtering, and a silicon oxide layer was formed on a functional substrate (LN substrate) by RF sputtering. The RF sputtering method was the same as the method applied to the LN substrate in Example 2. In the RF sputtering method for the silicon substrate, a 200 nm silicon oxide layer was formed on the silicon substrate. The obtained composite substrate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In Example 3, high bonding strength was demonstrated, similar to Example 1. In addition, the presence and depth (thickness) of an oxygen-reduced region were confirmed from the EDX measurement results, similar to Example 1. The oxygen atom ratio of the oxygen-reduced region in Example 3 is shown in Table 1.

[0096] [Example 4] A composite substrate was fabricated in the same manner as in Example 1, except that the support substrate was changed to a quartz substrate and no silicon oxide layer was formed on either the support substrate or the functional substrate (LN substrate) (i.e., the support substrate and the functional substrate were directly bonded). The quartz substrate used was a quartz (SiO₂) having an orientation flat (OF) portion, a diameter of 4 inches, and a thickness of 500 μm. ２A crystalline substrate was prepared. The arithmetic mean roughness Ra of the surface (first main surface) of the quartz substrate was 0.2 nm. As the functional substrate, a lithium niobate substrate (LN substrate; 4 inches in diameter, 250 μm thick, X-cut) similar to that used in Example 1 was used. The surface (first main surface) of the LN substrate was mirror-polished so that the arithmetic mean roughness Ra was 0.2 nm or less. The arithmetic mean roughness was evaluated using an atomic force microscope (AFM) in a square field of view of 10 μm x 10 μm. The obtained composite substrate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In Example 4, high bonding strength was demonstrated, similar to Example 1. Furthermore, EDX measurement results confirmed the existence and depth (thickness) of the oxygen-reduced region, similar to Example 1. Specifically, in Example 4, it was confirmed that an oxygen-reduced region was formed at the bonding interface by direct bonding between the support substrate and the functional substrate (functional layer).

[0097] [Comparative Example 1] A composite substrate was fabricated in the same manner as in Example 1, except that the plasma treatment conditions were changed as follows: <Plasma Treatment Conditions> Gas type: Oxygen Gas flow rate: 20 sccm Chamber pressure: 5 Pa Treatment time: 240 seconds Output: 900 W The obtained composite substrate was subjected to the same evaluation as in Example 1. The results are shown in Table 1. In Comparative Example 1, the bonding strength could not be improved. In addition, the EDX measurement results showed that no oxygen-reduced region like those in Examples 1 to 4 was formed.

[0098]

[0099] The composite substrate obtained according to the embodiment of the present invention can be suitably used to fabricate functional elements such as surface acoustic wave elements that can be used in high-frequency communication devices and electro-optic elements that can be used in optical communication devices.

[0100] 10 Support substrate 20 Intermediate layer 201 Silicon oxide layer (first silicon oxide layer) 202 Silicon oxide layer (second silicon oxide layer) 22 Oxygen reduction region 30 Functional layer 40 Dielectric layer 300 Functional substrate 100 Composite substrate 101 Composite substrate 102 Composite substrate 103 Composite substrate 104 Composite substrate

Claims

1. A composite substrate comprising a support substrate and a functional layer, wherein the composite substrate has a bonding interface, and an oxygen-reduced region is formed at the bonding interface in which the proportion of oxygen atoms is lower than that of at least one of the layers defining the bonding interface.

2. The composite substrate according to claim 1, further comprising an intermediate layer between the support substrate and the functional layer, wherein the bonding interface is formed between the support substrate and the intermediate layer, between the functional layer and the intermediate layer, or within the intermediate layer.

3. The composite substrate according to claim 2, wherein the thickness of the intermediate layer is 0.1 μm or more and 10 μm or less.

4. The composite substrate according to claim 1 or 2, wherein the thickness of the functional layer is 0.1 μm or more and 30 μm or less.

5. The composite substrate according to claim 1 or 2, wherein the thickness of the oxygen-reduced region is 1 nm or more and 100 nm or less.

6. The composite substrate according to claim 1 or 2, wherein the functional layer contains at least one material selected from the group consisting of lithium niobate, lithium tantalate, silicon carbide, quartz, aluminum oxide, gallium nitride, indium phosphide, and silicon.

7. The composite substrate according to claim 1 or 2, wherein the support substrate contains at least one material selected from the group consisting of silicon, silicon carbide, gallium nitride, and glass.

8. A method for manufacturing a composite substrate according to claim 1, comprising, in this order: plasma-treating the surfaces of a support substrate and a functional substrate with a plasma of a gas containing oxygen and hydrogen; and joining the plasma-treated surfaces together.

9. A method for manufacturing a composite substrate according to claim 2, comprising in this order: forming a silicon oxide layer on one of a support substrate or a functional substrate; plasma treating the surface of the silicon oxide layer and the surface of the support substrate or the functional substrate on which the silicon oxide layer is not formed with a plasma of a gas containing oxygen and hydrogen; and joining the plasma treated surfaces together.

10. A method for manufacturing a composite substrate according to claim 2, comprising in this order: forming a silicon oxide layer on a support substrate and a functional substrate; plasma treating the surface of the silicon oxide layer with a plasma of a gas containing oxygen and hydrogen; and joining the plasma treated surfaces together.

11. The method for manufacturing a composite substrate according to any one of claims 8 to 10, wherein the gas is water vapor.

12. A method for manufacturing a composite substrate according to claim 9 or 10, comprising planarizing the surface of the silicon oxide layer before performing the plasma treatment.

13. A method for manufacturing a composite substrate according to any one of claims 8 to 10, comprising placing the bonded substrate in an environment with a temperature of 200°C or higher and 250°C or lower for 10 hours or more.

14. A method for manufacturing a composite substrate according to any one of claims 8 to 10, comprising thinning the functional substrate after bonding.

15. A method for manufacturing a composite substrate according to any one of claims 8 to 10, comprising thinning the functional substrate to a thickness of 100 nm or more and 2 μm or less to form a functional layer.

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