Composite substrate for photonic crystal device

Abstract

To provide a composite substrate that can achieve a photonic crystal element having excellent properties.SOLUTION: A composite substrate for photonic crystal elements comprises: an electro-optic crystal substrate having an electro-optic effect; an optical-loss-reduced and cavity-machined layer provided on one surface of the electro-optic crystal substrate; and a support substrate which is integrated with the electro-optic crystal substrate through the optical-loss-reduced and cavity-machined layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a composite substrate for a photonic crystal device.

Background Art

[0002] Various electro-optical devices are known. An electro-optical device can convert an electrical signal into an optical signal by utilizing the electro-optical effect. Electro-optical devices are employed, for example, in photonic integrated circuits, and their development is underway to achieve high-speed and high-capacity communication, low power consumption (low driving voltage), and a small footprint. Electro-optical devices are typically configured using a composite substrate. A composite substrate typically includes an electro-optical crystal substrate having an electro-optical effect and a support substrate bonded to the electro-optical crystal substrate. This enables the electro-optical crystal substrate to be made thinner, and application development for realizing the above-described various functions has been actively carried out. In a conventional composite substrate, the electro-optical crystal substrate and the support substrate were bonded together with an adhesive. According to such a configuration, peeling may occur in the composite substrate due to the aging deterioration of the adhesive, and further, damage (e.g., cracks) may occur in the electro-optical crystal substrate due to such peeling.

[0003] In order to solve the above problems, a technique for directly bonding an electro-optical crystal substrate and a support substrate without using an adhesive has been developed. However, when the electro-optical crystal substrate and the support substrate are directly bonded, an amorphous layer composed of elements of the electro-optical crystal substrate and elements of the support substrate is formed between the electro-optical crystal substrate and the support substrate. This amorphous layer has no crystallinity, and its optical properties are also different from those of the electro-optical crystal substrate and the support substrate. Moreover, the interface between the electro-optical crystal substrate and the amorphous layer is not flat. Such a non-flat interface may cause light transmitted through the electro-optical crystal substrate to be scattered (e.g., diffuse reflection, leakage) and / or absorbed. Furthermore, the electro-optical effect of the electro-optical crystal deteriorates due to this amorphization, and the desired reduction in driving voltage may not be achieved. For such problems, techniques such as interposing a low refractive index layer between the electro-optical crystal substrate and the support substrate have been proposed, for example.

[0004] By the way, as one of electro-optical elements, the development of photonic crystal elements is in progress. Photonic crystal elements are expected to be applied and developed in a wide range of fields such as optical waveguides, next-generation high-speed communication, sensors, laser processing, and solar power generation. Along with the development of such photonic crystal elements, a composite substrate suitable for photonic crystal elements is desired.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The main object of the present invention is to provide a composite substrate capable of realizing a photonic crystal element having excellent characteristics.

Means for Solving the Problems

[0007] The composite substrate for a photonic crystal element according to an embodiment of the present invention includes an electro-optic crystal substrate having an electro-optic effect, an optical loss suppression and cavity processing layer provided on one surface of the electro-optic crystal substrate, and a support substrate integrated with the electro-optic crystal substrate through the optical loss suppression and cavity processing layer. In one embodiment, the optical loss suppression and cavity processing layer is a single layer. In this embodiment, the composite substrate for a photonic crystal element may further include an anti-peeling layer between the electro-optic crystal substrate and the optical loss suppression and cavity processing layer. In this embodiment, the composite substrate for a photonic crystal element may further include a bonding layer between the optical loss suppression and cavity processing layer and the support substrate. In this embodiment, the composite substrate for a photonic crystal element may have a patterned sacrificial layer formed on the optical loss suppression and cavity processing layer. In this embodiment, the composite substrate for a photonic crystal element may further include an overcoat layer between the optical loss suppression and cavity processing layer and the support substrate. In another embodiment, the optical loss suppression and cavity processing layer includes an optical loss suppression layer provided on the electro-optic crystal substrate and a cavity processing layer provided on the support substrate, and the optical loss suppression layer and the cavity processing layer are directly bonded. In this embodiment, the composite substrate for a photonic crystal element may further include a bonding layer between the optical loss suppression layer and the support substrate. In this embodiment, the composite substrate for a photonic crystal element may have a patterned sacrificial layer formed on the optical loss suppression layer or the cavity processing layer. In this embodiment, the composite substrate for a photonic crystal element may further include an overcoat layer between the optical loss suppression layer and the cavity processing layer. According to another aspect of the present invention, a photonic crystal device is provided. The photonic crystal device is a photonic crystal device using the above composite substrate for a photonic crystal device. The photonic device includes a photonic crystal layer in which holes are periodically formed in the electro-optic crystal substrate; a bonding portion provided below the photonic crystal layer for integrating the photonic crystal layer and the support substrate; and a cavity defined by the lower surface of the photonic crystal layer, the upper surface of the support substrate, and the bonding portion. In one embodiment, the photonic crystal device is configured using the above composite substrate for a photonic crystal device. In one embodiment, an etching through-hole is formed in the photonic crystal layer. In this case, the size of the etching through-hole may be larger than the size of the hole.

Advantages of the Invention

[0008] According to an embodiment of the present invention, in a composite substrate for a photonic crystal device having an electro-optic crystal substrate and a support substrate, an optical loss suppression and cavity processing layer is provided on one surface of the electro-optic crystal substrate, and the electro-optic crystal substrate and the support substrate are integrated through the optical loss suppression and cavity processing layer, whereby a photonic crystal device having excellent characteristics can be realized.

Brief Description of the Drawings

[0009]

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Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0011] A. Composite Substrate for Photonic Crystal Element A-1. Overall Configuration and Modification Example FIG. 1 is a schematic perspective view of a composite substrate for a photonic crystal device (hereinafter sometimes simply referred to as a composite substrate) according to one embodiment of the present invention; FIG. 2 is a schematic cross-sectional view of the composite substrate of FIG. 1. The composite substrate according to the embodiment of the present invention can typically be manufactured in the form of a so-called wafer as shown in FIG. 1. The composite substrate may be provided to the manufacturer of the photonic crystal device in the form of a wafer as shown in FIG. 1, or may be provided to the manufacturer in the form of a wafer on which a photonic crystal layer is formed (photonic crystal wafer) as described later. In this specification, the photonic crystal wafer may sometimes be referred to as a photonic crystal device. That is, in this specification, the "photonic crystal device" includes both the photonic crystal wafer and the chip obtained by cutting the photonic crystal wafer.

[0012] The composite substrate 100 in the illustrated example includes an electro-optic crystal substrate 10 having an electro-optic effect, an optical loss suppression and cavity processing layer 20 provided on one surface of the electro-optic crystal substrate, and a support substrate 30 integrated with the electro-optic crystal substrate 10 via the optical loss suppression and cavity processing layer 20. In the illustrated embodiment, the optical loss suppression and cavity processing layer 20 and the support substrate 30 are directly bonded to integrate the electro-optic crystal substrate 10 and the support substrate 30. Note that an amorphous layer (not shown) is typically formed at the bonding interface of the direct bonding. In the illustrated embodiment, the amorphous layer is a layer formed at the bonding interface by directly bonding the optical loss suppression and cavity processing layer 20 and the support substrate 30. The amorphous layer has an amorphous structure as the name implies and is composed of the elements constituting the optical loss suppression and cavity processing layer 20 and the elements constituting the support substrate 30. Note that in the embodiment of the present invention, not limited to the embodiments shown in FIGS. 1 and 2, an amorphous layer can typically be formed at the bonding interface of the direct bonding. The amorphous layer is composed of the constituent elements of the mutually directly bonded layers or substrates.

[0013] In the electro - optical crystal substrate 10, as will be described later, vacancies are formed in a predetermined pattern and become a photonic crystal layer in the photonic crystal element. The optical loss suppression and cavity processing layer 20 prevents the formation of an amorphous layer on the electro - optical crystal substrate during direct bonding, suppressing the optical loss of the electro - optical crystal substrate; and, after fulfilling the optical loss suppression function during direct bonding, it can be removed by etching to form a cavity in the photonic crystal element. Further, by adjusting the constituent material, thickness, etc. of the optical loss suppression and cavity processing layer 20, etching (typically, dry etching) can be stopped at an appropriate level.

[0014] By integrally joining the electro - optical crystal substrate 10 and the support substrate 30 by direct bonding, delamination of the composite substrate can be favorably suppressed. As a result, damage (e.g., cracks) to the electro - optical crystal substrate due to such delamination can be favorably suppressed. Further, by directly bonding the optical loss suppression and cavity processing layer 20 and the support substrate 30, direct bonding between the electro - optical crystal substrate and the support substrate can be avoided. Therefore, the formation of an amorphous layer on the electro - optical crystal substrate can be prevented. As a result, a decrease in the optical characteristics or optical loss of the electro - optical crystal substrate can be suppressed.

[0015] As used herein, "direct bonding" means that the components of the composite substrate (the optical loss suppression and cavity processing layer 20 and the support substrate 30 in the examples of FIGS. 1 and 2) are joined without an intervening adhesive. The form of direct bonding can be appropriately set according to the structure of the layers or substrates to be joined to each other. For example, direct bonding can be realized by the following procedure. Inside a high - vacuum chamber (e.g., 1×10 -6At the Pa level, a neutralized beam is irradiated onto each bonding surface of the components (layers or substrates) to be joined. As a result, each bonding surface is activated. Next, in a vacuum atmosphere, the activated bonding surfaces are brought into contact with each other and joined at room temperature. The load during this joining can be, for example, 100 N to 20,000 N. In one embodiment, when performing surface activation by a neutralized beam, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power supply to an electrode disposed in the chamber. With such a configuration, electrons move due to the electric field generated between the electrode (positive electrode) and the chamber (negative electrode), and a beam of atoms and ions by the inert gas is generated. Among the beams reaching the grid, the ion beam is neutralized by the grid, so a beam of neutral atoms is emitted from the high-speed atomic beam source. The atomic species constituting the beam is preferably an inert gas element (for example, argon (Ar), nitrogen (N)). The voltage during activation by beam irradiation is, for example, 0.5 kV to 2.0 kV, and the current is, for example, 50 mA to 200 mA. Note that the direct bonding method is not limited to this, and a surface activation method using an ion gun, an atomic diffusion method, a plasma bonding method, etc. can also be applied.

[0016] The optical loss suppression and cavity processing layer may be a single layer as described above, or may have an optical loss suppression layer and a cavity processing layer as described later. That is, the optical loss suppression and cavity processing layer may have both an optical loss suppression function and a cavity formation function as a single layer, or may be separated into two layers as an optical loss suppression layer and a cavity processing layer to share the functions.

[0017] Hereinafter, modified examples of the composite substrate will be described. Note that the specific configuration of the components (layers or substrates) of the composite substrate will be described later in Sections A-2 to A-8.

[0018] FIG. 3 is a schematic cross-sectional view of a composite substrate according to another embodiment of the present invention. In the composite substrate 100a shown in the figure, an anti-peeling layer 40 is provided between the electro-optical crystal substrate 10 and the optical loss suppression and cavity processing layer 20, and an overcoat layer 50 and a bonding layer 60 are provided between the optical loss suppression and cavity processing layer 20 and the support substrate 30. The bonding layer 60 can be directly bonded to the support substrate 30 and / or an adjacent layer on the side opposite to the support substrate 30 (the overcoat layer 50 or the optical loss suppression and cavity processing layer 20 in the illustrated example). A bonding layer may be provided on each of the support substrate 30 and the overcoat layer 50 or the optical loss suppression and cavity processing layer 20, and the respective bonding layers may be directly bonded. By providing the anti-peeling layer 40, peeling between the electro-optical crystal substrate 10 and the adjacent layer (the optical loss suppression and cavity processing layer 20 in the illustrated example) can be suppressed. The overcoat layer 50 can be provided as a layer for planarization when the optical loss suppression and cavity processing layer 20 has irregularities. Specifically, when a sacrificial layer 70 is formed as shown in FIG. 4 described later, since the sacrificial layer 70 and the optical loss suppression and cavity processing layer 20 are formed in separate processes, irregularities may occur on the lower surface in the illustrated example. At this time, by forming the overcoat layer 50, a surface as a single layer can be formed, so that planarization processing can be easily performed. Further, by providing the bonding layer 60, strong integration between the electro-optical crystal substrate 10 and the support substrate 30 can be realized. The anti-peeling layer 40, the overcoat layer 50, and the bonding layer 60 are optional layers provided as needed, and at least one of them may be omitted. In the illustrated example, for example, the overcoat layer 50, the anti-peeling layer 40 and the overcoat layer 50, or the overcoat layer 50 and the bonding layer 60 may be omitted. When a bonding layer is present, an amorphous layer can be formed at the interface of direct bonding between the bonding layer and the adjacent layer (including the interface of direct bonding between the bonding layers). When the overcoat layer and the bonding layer are omitted, as in FIG. 2, the optical loss suppression and cavity processing layer 20 and the support substrate 30 are directly bonded, and an amorphous layer can be formed at the bonding interface.

[0019] FIG. 4 is a schematic cross-sectional view of a composite substrate according to still another embodiment of the present invention. In the composite substrate 100b of the illustrated example, a sacrificial layer 70 is formed on the optical loss suppression and cavity processing layer 20. By providing the sacrificial layer 70, a cavity for effectively expressing the function of the photonic crystal can be formed in a desired shape and easily. The cavity preferably has a sufficient thickness over the entire area directly below the pores of the photonic crystal. For this reason, the sacrificial layer 70 is formed in a predetermined pattern according to the purpose. In the embodiment of the illustrated example, the sacrificial layer 70 is typically formed in a pattern and shape corresponding to the cavity in the photonic crystal element. In the embodiment of the illustrated example, if necessary, an overcoat layer 50 and / or a bonding layer 60 may be further provided between the optical loss suppression and cavity processing layer 20 (sacrificial layer 70) and the support substrate 30. When the bonding layer 60 is provided alone, the bonding layer 60 can be directly bonded to the optical loss suppression and cavity processing layer 20 (sacrificial layer 70) and / or the support substrate 30. When the overcoat layer 50 and the bonding layer 60 are provided, typically, the overcoat layer 50 is provided on the sacrificial layer 70 side. The bonding layer 60 can be directly bonded to the overcoat layer 50 and / or the support substrate 30. Similar to the above embodiment, a bonding layer may be provided on each of the layers or substrates to be bonded, and the bonding layers may be directly bonded to each other.

[0020] FIG. 5 is a schematic cross-sectional view of a composite substrate according to still another embodiment of the present invention. In the composite substrate 100c of the illustrated example, the optical loss suppression and cavity processing layer has an optical loss suppression layer 21 and a cavity processing layer 22. Typically, the optical loss suppression layer 21 is formed on the electro-optic crystal substrate 10, and the cavity processing layer 22 is formed on the support substrate 30. Typically, as shown in FIG. 6, the optical loss suppression layer 21 of the laminate of the optical loss suppression layer 21 / electro-optic crystal substrate 10 and the cavity processing layer 22 of the laminate of the cavity processing layer 22 / support substrate 30 are directly joined to form a laminated structure of the optical loss suppression layer 21 and the cavity processing layer 22. By adopting the laminated structure of the optical loss suppression layer 21 and the cavity processing layer 22, damage to the electro-optic crystal layer due to etching of the cavity processing layer can be suppressed, and / or it is possible to prevent pores from penetrating the cavity processing layer and reaching the support substrate during the production of the photonic crystal structure. Furthermore, it is possible to prevent diffusion of foreign elements into the electro-optic crystal substrate during processes such as bonding and etching.

[0021] FIG. 7 is a schematic cross-sectional view of a composite substrate according to still another embodiment of the present invention. In the composite substrate 100d of the illustrated example, an overcoat layer 50 and a bonding layer 60 are provided between the optical loss suppression layer 21 and the cavity processing layer 22. The overcoat layer 50 and the bonding layer 60 are optional layers provided as needed, and at least one of them may be omitted. In the illustrated embodiment, often only the bonding layer 60 may be provided. The bonding layer 60 can be directly bonded to the cavity processing layer 22 and / or an adjacent layer on the opposite side of the cavity processing layer 22 (in the illustrated example, the overcoat layer 50 or the optical loss suppression layer 21). Similar to the above embodiment, a bonding layer may be provided for each of the layers to be joined (in the illustrated example, the cavity processing layer 22, and the overcoat layer 50 or the optical loss suppression layer 21), and the respective bonding layers may be directly joined.

[0022] FIG. 8 is a schematic cross-sectional view of a composite substrate according to still another embodiment of the present invention. In the composite substrate 100e shown in the figure, a sacrificial layer 70 is formed in the cavity processing layer 22. By adopting a laminated structure of the optical loss suppression layer 21 and the cavity processing layer 22, and forming the sacrificial layer 70 in the cavity processing layer 22, there is an advantage that a cavity can be formed at a designed position and in a designed shape. In the embodiment shown in the figure, if necessary, a bonding layer 60 may be further provided between the optical loss suppression layer 21 and the cavity processing layer 22 (sacrificial layer 70), or between the cavity processing layer 22 (sacrificial layer 70) and the support substrate 30. Similar to the case of the above embodiment, the bonding layer 60 may be directly bonded to at least one adjacent layer, or a bonding layer may be provided for each of the layers to be bonded, and the respective bonding layers may be directly bonded to each other.

[0023] FIG. 9 is a schematic cross-sectional view of a composite substrate according to still another embodiment of the present invention. In the composite substrate 100f shown in the figure, a sacrificial layer 70 is formed in the optical loss suppression layer 21. In the composite substrate 100f, the cavity processing layer 22 and the support substrate 30 may be directly bonded. By adopting a laminated structure of the optical loss suppression layer 21 and the cavity processing layer 22, and forming the sacrificial layer 70 in the optical loss suppression layer 21, there is an advantage that a cavity can be formed at a designed position and in a designed shape. This structure is effective when it is difficult to pattern the cavity processing layer 22. In the embodiment shown in the figure, if necessary, an overcoat layer 50 and / or a bonding layer 60 may be further provided between the optical loss suppression layer 21 (sacrificial layer 70) and the cavity processing layer 22, or between the cavity processing layer 22 and the support substrate 30.

[0024] The above embodiments may be appropriately combined according to the purpose. Further / Alternatively, modifications well known in the art may be added to the above embodiments.

[0025] A-2. Electro-optical crystal substrate The electro-optic crystal substrate 10 has an upper surface exposed to the outside and a lower surface located within the composite substrate. In an embodiment of the present invention, part or all of the electro-optic crystal substrate 10 serves as an optical waveguide that transmits light in a photonic crystal element manufactured from the composite substrate. The electro-optic crystal substrate 10 is composed of a crystal of a material having an electro-optic effect. Specifically, when an electric field is applied to the electro-optic crystal substrate 10, the optical constants (e.g., refractive index) can change. In one embodiment, the c-axis of the electro-optic crystal substrate 10 can be parallel to the electro-optic crystal substrate 10. That is, the electro-optic crystal substrate 10 may be an X-cut substrate or a Y-cut substrate. In another embodiment, the c-axis of the electro-optic crystal substrate 10 can be perpendicular to the electro-optic crystal substrate 10. That is, the electro-optic crystal substrate 10 may be a Z-cut substrate. The thickness of the electro-optic crystal substrate 10 can be set to any appropriate thickness according to the frequency and wavelength of the electromagnetic wave used. The thickness of the electro-optic crystal substrate 10 can be, for example, 0.1 μm to 10 μm, or for example, 0.1 μm to 3 μm. As will be described later, since the composite substrate is reinforced by the support substrate, the thickness of the electro-optic crystal substrate can be reduced.

[0026] As the material constituting the electro-optic crystal substrate 10, any appropriate material can be used as long as the effects according to the embodiments of the present invention can be obtained. Such materials typically include dielectrics (e.g., ceramics). Specific examples include lithium niobate (LiNbO3:LN), lithium tantalate (LiTaO3:LT), potassium titanyl phosphate (KTiOPO4:KTP), potassium lithium niobate (K x Li (1-x) NbO2:KLM), potassium niobate (KNbO3:KN), potassium tantalate niobate (KNb x Ta (1-x) O3:KTN), and a solid solution of lithium niobate and lithium tantalate.

[0027] A-3. Support Substrate The support substrate 30 has an upper surface positioned within the composite substrate and a lower surface exposed to the outside. The support substrate 30 is provided to enhance the strength of the composite substrate, thereby enabling the reduction of the thickness of the electro-optical crystal substrate. Any suitable configuration may be adopted for the support substrate 30. Specific examples of the material constituting the support substrate 30 include silicon (Si), glass, sialon (Si3N4 - Al2O3), mullite (3Al2O3·2SiO2, 2Al2O3·3SiO2), aluminum nitride (AlN), silicon nitride (Si3N4), magnesium oxide (MgO), sapphire, quartz, quartz crystal, gallium nitride (GaN), silicon carbide (SiC), and gallium oxide (Ga2O3). Note that the closer the linear expansion coefficient of the material constituting the support substrate 30 is to the linear expansion coefficient of the material constituting the electro-optical crystal substrate 10, the more preferable it is. With such a configuration, thermal deformation (typically, warping) of the composite substrate can be suppressed. Preferably, the linear expansion coefficient of the material constituting the support substrate 30 is within the range of 50% to 150% with respect to the linear expansion coefficient of the material constituting the electro-optical crystal substrate 10. From this perspective, the support substrate may be made of the same material as the electro-optical crystal substrate 10.

[0028] A-4. Optical Loss Suppression and Cavity Processing Layer A-4-1. Single Layer As described above, the optical loss suppression and cavity processing layer (single layer) 20 has an optical loss suppression function, a cavity processing function, and an etching stop function. As long as it has such functions, any suitable configuration may be adopted for the optical loss suppression and cavity processing layer. Examples of the material constituting the optical loss suppression and cavity processing layer (single layer) include silicon oxide (SiO2), amorphous silicon (a-Si), polycrystalline silicon (i.e., excluding single crystal silicon), molybdenum, aluminum oxide (Al2O3), compounds of these materials, and mixtures of these materials. The thickness of the optical loss suppression and cavity processing layer (single layer) is, for example, 0.1 μm to 1.0 μm, and also, for example, 0.5 μm to 1.0 μm.

[0029] A-4-2. Laminated Structure of Optical Loss Suppression Layer and Cavity Processing Layer When the optical loss suppression and cavity processing layer has an optical loss suppression layer 21 and a cavity processing layer 22, any appropriate configuration can be adopted as long as it has an optical loss suppression function for the optical loss suppression layer. Examples of materials constituting the optical loss suppression layer include amorphous silicon, polycrystalline silicon (i.e., excluding single-crystalline silicon), molybdenum, aluminum oxide, compounds of these materials, and mixtures of these materials. The thickness of the optical loss suppression layer is, for example, 0.01 μm (10 nm) to 0.1 μm (100 nm), and also, for example, 0.01 μm (10 nm) to 0.05 μm (50 nm).

[0030] Any appropriate configuration can be adopted for the cavity processing layer as long as it has a cavity processing function and an etching stop function. Examples of materials constituting the cavity processing layer include silicon oxide, amorphous silicon, polycrystalline silicon, single-crystalline silicon, molybdenum, aluminum oxide, compounds of these materials, and mixtures of these materials. The thickness of the cavity processing layer is, for example, 0.1 μm to 1.0 μm, and also, for example, 0.3 μm to 0.7 μm.

[0031] A-5. Anti-peeling layer As described above, the anti-peeling layer 40 is provided to prevent or suppress peeling between the electro-optic crystal substrate 10 and the adjacent layer (typically, the optical loss suppression and cavity processing layer 20). Any appropriate configuration can be adopted for the anti-peeling layer according to the configurations of the electro-optic crystal substrate and the adjacent layer. Examples of materials constituting the anti-peeling layer include amorphous silicon, tantalum oxide (Ta2O5), niobium oxide (Nb2O5), titanium oxide (TiO2), aluminum oxide, and hafnium oxide (HfO2). The thickness of the anti-peeling layer is, for example, 0.01 μm to 0.1 μm.

[0032] A-6. Overcoat layer As described above, the overcoat layer 50 is provided for suppressing optical loss and flattening when the cavity processing layer 20 has irregularities. As the overcoat layer, any appropriate configuration can be adopted according to the purpose and the configuration of the adjacent layer (for example, the sacrificial layer). Examples of the material constituting the overcoat layer include amorphous silicon, niobium oxide, tantalum oxide, silicon oxide, titanium oxide, aluminum oxide, and hafnium oxide. The thickness of the overcoat layer is, for example, 0.01 μm to 1 μm.

[0033] A-7. Bonding layer As described above, the bonding layer 60 is provided to enhance the bonding strength and realize a strong integration of the electro-optic crystal substrate and the support substrate. As the bonding layer, any appropriate configuration can be adopted according to the configuration of the substrate or layer to be bonded. Examples of the material constituting the bonding layer include silicon oxide, amorphous silicon, tantalum oxide, alumina (Al2O3), hafnia (HfO2), Cr / Au, and Cr / Cu. The thickness of the bonding layer is, for example, 0.01 μm to 0.1 μm, and also for example, 0.01 μm to 0.05 μm.

[0034] A-8. Sacrificial layer As described above, the sacrificial layer 70 is provided to form a cavity in the designed position and with the designed shape. As the sacrificial layer, any appropriate configuration can be adopted according to the purpose. Examples of the material constituting the sacrificial layer include amorphous silicon, silicon, molybdenum, silicon oxide, aluminum oxide, compounds of these materials, and mixtures of these materials. The thickness of the sacrificial layer is, for example, 0.1 μm to 1.0 μm, and also for example, 0.2 μm to 0.7 μm.

[0035] B. Photonic crystal element B-1. Configuration of the photonic crystal element FIG. 10 is a schematic perspective view of a photonic crystal device according to one embodiment of the present invention. The photonic crystal device 200 in the illustrated example includes a photonic crystal layer 10a in which holes 12 are periodically formed in an electro-optic crystal substrate 10; a joint portion 20a provided below the photonic crystal layer 10a for integrating the photonic crystal layer 10a and a support substrate 30; and a cavity 80 defined by the lower surface of the photonic crystal layer 10a, the upper surface of the support substrate 30, and the inner surface of the joint portion 20a. The cavity 80 is formed by suppressing optical loss of the composite substrate described in item A above and removing the cavity processing layer by etching, and the joint portion 20a is formed by the remainder of the optical loss suppression and the cavity processing layer.

[0036] The photonic crystal that constitutes the photonic crystal layer 10a is a multi-dimensional periodic structure in which a medium with a high refractive index and a medium with a low refractive index are arranged at a period comparable to the wavelength of light, and has a light band structure similar to the band structure of electrons. Therefore, by appropriately designing the periodic structure, a predetermined light forbidden band (photonic band gap) can be expressed. A photonic crystal having a forbidden band functions as an object that does not cause light reflection or transmission for light of a predetermined wavelength. When a line defect that disturbs the periodicity is introduced into a photonic crystal having a photonic band gap, a waveguide mode is formed within the frequency range of the band gap, and an optical waveguide that propagates light with low loss can be realized.

[0037] The photonic crystal in the illustrated example is a so-called slab-type two-dimensional photonic crystal. A slab-type two-dimensional photonic crystal is a thin slab of a dielectric or semiconductor in which cylindrical or polygonal low-refractive-index pillars having a refractive index lower than that of the material constituting the thin slab are provided at appropriate two-dimensional periodic intervals according to the purpose and desired photonic bandgap, and the upper and lower portions of the thin slab are sandwiched between an upper cladding and a lower cladding having a refractive index lower than that of the thin slab. In the illustrated example, the holes 12 function as low-refractive-index pillars, the portion 14 between the holes 12, 12 of the electro-optic crystal substrate 10 functions as a high-refractive-index portion, the cavity 80 functions as a lower cladding, and the outer environment (air portion) above the photonic crystal element 200 functions as an upper cladding. The portion where the periodic pattern of the holes 12 is not formed in the electro-optic crystal substrate 10 becomes a line defect, and the line defect portion constitutes the optical waveguide 16.

[0038] The holes 12 can be formed as a periodic pattern as described above. The holes 12 are typically arranged to form a regular lattice. As the form of the lattice, any appropriate form can be adopted as long as a predetermined photonic bandgap can be realized. Representative examples include a triangular lattice and a square lattice. In one embodiment, the holes 12 can be through holes. Through holes are easy to form, and as a result, it is easy to adjust the refractive index. As the planar shape of the hole (through hole), any appropriate shape can be adopted. Specific examples include equilateral polygons (e.g., equilateral triangle, square, regular pentagon, regular hexagon, regular octagon), substantially circular, and elliptical. A substantially circular shape is preferred. The aspect ratio of the major axis / minor axis of the substantially circular shape is preferably 0.90 to 1.10, more preferably 0.95 to 1.05. Note that the through hole 12 may be a low-refractive-index pillar (a columnar portion made of a low-refractive-index material) as described above. However, through holes are easier to form, and since through holes are composed of air with the lowest refractive index, the refractive index difference from the optical waveguide can be increased. Also, the hole diameter may be partially different from other hole diameters.

[0039] The lattice pattern of the air holes can be appropriately set according to the purpose and the desired photonic band gap. In the illustrated example, air holes with a diameter d1 form a square lattice with a period P. The square lattice pattern is formed on both sides of the photonic crystal element, and an optical waveguide 16 is formed in the central portion where no lattice pattern is formed. The width of the optical waveguide 16 can be, for example, 1.01P to 3P (2P in the illustrated example) with respect to the air hole period P. The number of rows of air holes (hereinafter sometimes referred to as lattice rows) in the direction of the optical waveguide can be 3 to 10 rows (5 rows in the illustrated example) on each side of the optical waveguide. The air hole period P can satisfy, for example, the following relationship. (1 / 7)×(λ / n)≦P≦1.4×(λ / n) Here, λ is the wavelength (nm) of the light introduced into the optical waveguide, and n is the refractive index of the electro-optic crystal substrate. Specifically, the air hole period P can be 0.1 μm to 1 μm. In one embodiment, the air hole period P can be equal to the thickness of the photonic crystal layer (electro-optic crystal substrate). The diameter d1 of the air holes can be, for example, 0.1P to 0.9P with respect to the air hole period P. By appropriately combining and adjusting the diameter d1 of the air holes, the air hole period P, the number of lattice rows, the number of air holes in one lattice row, the thickness of the photonic crystal layer, the constituent material (substantially the refractive index) of the electro-optic crystal substrate, the width of the line defect portion, the width and height of the cavity described later, etc., a desired photonic band gap can be obtained. Furthermore, the same effect can be obtained for electromagnetic waves other than light waves. Specific examples of electromagnetic waves include millimeter waves, microwaves, and terahertz waves.

[0040] The cavity 80 is formed by suppressing the optical loss of the composite substrate as described above and removing the cavity processing layer 20 by etching, and can function as a lower cladding. The width of the cavity is preferably larger than the width of the optical waveguide. For example, the cavity 80 may extend from the optical waveguide 16 to the third row of lattice rows. In the illustrated example, the cavity 80 extends from the optical waveguide 16 to the third row of lattice rows. Since light not only propagates within the optical waveguide portion but also a part of the light energy may diffuse to the lattice rows near the optical waveguide portion, by providing a cavity directly below such lattice rows, the propagation loss due to light leakage can be suppressed. From this viewpoint, the cavity may be formed over the entire region of the hole formation portion. The height of the cavity is preferably 0.1 μm or more, and more preferably 1 / 5 or more of the wavelength of the propagating light. With such a height, the thin plate slab functions as a photonic crystal, and a lower-loss optical waveguide with higher wavelength selectivity can be realized. The height of the cavity can be controlled by adjusting the thickness of components (layers) other than the electro-optic crystal substrate and the support substrate in the composite substrate.

[0041] In one embodiment, an etching through-hole 90 may be formed in the photonic crystal layer 10a. By forming the etching through-hole 90, the etching solution can be evenly distributed over the entire area to be etched. As a result, a desired cavity can be formed more precisely. In the illustrated example, a single etching through-hole is formed, but a plurality (for example, two, three, or four) of etching through-holes may be formed. The etching through-hole is formed, for example, at a position more than three rows away from the lattice row in the optical waveguide. With such a configuration, the etching solution can be evenly distributed over the entire area to be etched without adversely affecting the photonic bandgap. The etching through-hole may also be formed, for example, on the input portion side and / or the output portion side (i.e., the corner of the photonic crystal layer) at the end opposite to the optical waveguide of the lattice pattern. With such a configuration, the adverse effect on the photonic bandgap can be further preferably prevented. For example, when four etching through-holes are formed, they may be formed at the four corners of the photonic crystal layer. The size of the etching through-hole 90 is typically larger than the size of the hole 12. For example, the diameter d2 of the etching through-hole is preferably at least five times, more preferably at least 50 times, and even more preferably at least 100 times the diameter d1 of the hole. On the other hand, d2 is preferably at most 1000 times d1. If d2 is too small, the etching solution may not be evenly distributed over the entire area to be etched. If d2 is too large, it may adversely affect the photonic bandgap.

[0042] B-2. Method for manufacturing a photonic crystal element With reference to FIGS. 11 to 13, a representative example of a method for manufacturing a photonic crystal device will be briefly described. FIGS. 11(a) to 11(c) are schematic cross-sectional views illustrating an example of a process for fabricating a photonic crystal device from a composite substrate. This example is a process for fabricating a photonic crystal device from a composite substrate similar to the composite substrate of FIG. 4 as shown in FIG. 11(a). This composite substrate is provided with an additional bonding layer 60 between the optical loss suppression and the cavity processing layer 20 (sacrificial layer 70) and the support substrate 30 as compared with the composite substrate of FIG. 4. First, as shown in FIG. 11(b), holes 12 are formed in the electro-optic crystal substrate 10 by etching through a predetermined mask. The etching is typically dry etching (e.g., reactive ion etching). The holes 12 can be formed, for example, in a pattern as shown in FIG. 10. Note that the formation of through-holes for etching is omitted in the drawings. Next, the composite substrate with holes formed in the electro-optic crystal substrate is brought into contact (e.g., immersed) with a predetermined etching solution to etch the sacrificial layer 70. As a result, as shown in FIG. 11(c), a cavity 80 is formed and a photonic crystal device is obtained. If the etching mask for hole formation and the sacrificial layer are made of the same material, the remaining mask and the sacrificial layer can be removed simultaneously by a single contact (e.g., immersion).

[0043] Figs. 12(a) to 12(d) are schematic cross-sectional views for explaining another example of a process for fabricating a photonic crystal element from a composite substrate. This example is a process for fabricating a photonic crystal element from a composite substrate similar to the composite substrate of Fig. 5 as shown in Fig. 12(a). This composite substrate is further provided with a bonding layer 60 between the optical loss suppression layer 21 and the cavity processing layer 22 as compared with the composite substrate of Fig. 5. First, as shown in Fig. 12(b), holes 12 are formed in the electro-optic crystal substrate 10, the optical loss suppression layer 21, and the bonding layer 60 by dry etching (for example, reactive ion etching) through a predetermined mask. Next, as shown in Fig. 12(c), a predetermined portion of the cavity processing layer 22 is removed by wet etching (for example, immersion in an etching solution). Finally, as shown in Fig. 12(d), the remaining optical loss suppression layer 21 and the bonding layer 60 are removed by wet etching (for example, immersion in an etching solution). As a result, a cavity 80 is formed and a photonic crystal element is obtained.

[0044] Figs. 13(a) to 13(d) are schematic cross-sectional views for explaining yet another example of a process for fabricating a photonic crystal element from a composite substrate. This example is a process for fabricating a photonic crystal element from a composite substrate similar to the composite substrate of Fig. 9 as shown in Fig. 13(a). This composite substrate is further provided with a bonding layer 60 between the cavity processing layer 22 and the support substrate 30 as compared with the composite substrate of Fig. 9. First, as shown in Fig. 13(b), holes 12 are formed in the electro-optic crystal substrate 10 by dry etching (for example, reactive ion etching) through a predetermined mask. Next, as shown in Fig. 13(c), the sacrificial layer 70 is removed by wet etching (for example, immersion in an etching solution), and then, as shown in Fig. 13(d), the cavity processing layer 22 is removed by wet etching (for example, immersion in an etching solution). As a result, a cavity 80 is formed and a photonic crystal element is obtained. If the sacrificial layer and the cavity processing layer are made of the same material, the remaining sacrificial layer and the cavity processing layer can be removed simultaneously by a single contact (for example, immersion).

[0045] Needless to say, a process different from the illustrated example may be adopted in manufacturing the photonic crystal element. By appropriately combining the overall configuration of the composite substrate, the constituent materials of each layer of the composite substrate, the mask, the etching pattern, etc., holes and cavities can be formed with high precision and in an efficient procedure, and the photonic crystal element can be manufactured.

Example

[0046] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples.

[0047] <Example 1> 1. Fabrication of a composite substrate for a photonic crystal element An X-cut lithium niobate substrate with a diameter of 4 inches was prepared as an electro-optic crystal substrate, and a silicon substrate with a diameter of 4 inches was prepared as a support substrate. First, amorphous silicon (a-Si) was sputtered on the electro-optic crystal substrate to form an optical loss suppression layer with a thickness of 20 nm. On the other hand, silicon oxide was sputtered on the support substrate to form a cavity processing layer with a thickness of 0.5 μm, and a-Si was sputtered on the cavity processing layer to form a bonding layer with a thickness of 20 nm. Next, the surfaces of the optical loss suppression layer and the bonding layer were polished by CMP respectively so that the arithmetic mean roughness Ra of the surfaces of the optical loss suppression layer and the bonding layer was 0.3 nm or less. Next, after washing the surfaces of the optical loss suppression layer and the bonding layer, the electro-optic crystal substrate and the support substrate were integrated by directly bonding the optical loss suppression layer and the bonding layer. The direct bonding was performed as follows. 10 -6In a vacuum of the PA stage, a high-speed Ar neutral atom beam (acceleration voltage: 1 kV, Ar flow rate: 60 sccm) was irradiated onto the bonding surfaces (the surfaces of the optical loss suppression layer and the bonding layer) of the electro-optic crystal substrate and the support substrate for 70 seconds. After the irradiation, the electro-optic crystal substrate and the support substrate were left to cool for 10 minutes, and then the bonding surfaces of the electro-optic crystal substrate and the support substrate were brought into contact and pressed at 4.90 kN for 2 minutes to bond the electro-optic crystal substrate and the support substrate. After the bonding, polishing was performed until the thickness of the electro-optic crystal substrate reached 0.5 μm, and a composite substrate for a photonic crystal element similar to FIG. 5 (however, there is a bonding layer between the optical loss suppression layer and the cavity processing layer) was obtained. In the obtained composite substrate for a photonic crystal element, no defects such as peeling were observed at the bonding interface.

[0048] 2. Fabrication of Photonic Crystal Element A photonic crystal element was fabricated from the composite substrate for the photonic crystal element obtained above by a method corresponding to the manufacturing method shown in FIG. 12. Specifically, the photonic crystal element was fabricated by the following procedure. First, molybdenum (Mo) was deposited on the electro-optic crystal substrate as a metal mask. Next, a resin pattern having holes in a predetermined arrangement was formed on the metal mask by nanoimprint lithography. Specifically, as a hole pattern corresponding to the holes of the photonic crystal, 10 lattice rows each having holes with a diameter of 444 nm at a period (pitch) of 550 nm in the optical waveguide direction and in a direction perpendicular to the optical waveguide direction were formed on the left and right sides when viewed in plan. Holes were not formed in the central part when viewed in plan (finally, this part becomes the optical waveguide). Further, four holes (patterns of etching through-holes) with a diameter of 200 μm were formed at the corners when viewed in plan (the input side and output side of the opposite ends of the portions that become the optical waveguides in the left and right lattice row portions). Next, holes corresponding to the above pattern were formed in the Mo mask by etching with an Mo etching solution (a mixed solution of nitric acid:acetic acid:phosphoric acid with a mixing ratio of 10:15:1). Next, a hole pattern and an etching through-hole were formed in the composite substrate by fluorine-based reactive ion etching through the patterned Mo mask. Next, the composite substrate was immersed in a BHF (buffered hydrofluoric acid) etching solution to remove the cavity processing layer and form cavities. Further, the remaining Mo mask was removed with the Mo etching solution. Finally, the composite substrate was immersed in tetramethylammonium hydroxide (TMAH) diluted to about 10% to etch the optical loss suppression layer and the bonding layer, and a photonic crystal wafer was fabricated. The obtained photonic crystal wafer was diced to cut the chips, and photonic crystal elements were obtained. The optical waveguide length of the photonic crystal element was set to 10 mm. After dicing the chips, the input-side end face and the output-side end face of the optical waveguide were polished.

[0049] The obtained photonic crystal element (chip) was cut in the thickness direction, and as a result of observing the cross section with a microscope, cavities were well formed directly under the photonic crystal layer. The yield of the chips in which the cavities were formed as designed was 100%. Furthermore, for the obtained chip, the optical insertion loss was measured. Specifically, light with a wavelength of 1.55 μm was introduced into the chip (substantially, the optical waveguide of the photonic crystal layer) through the input-side ball lens fiber coupled to the optical fiber, and the amount of light output through the output-side ball lens fiber was measured with a photodetector to calculate the propagation loss. The propagation loss of the optical waveguide was 0.5 dB / cm.

[0050] <Example 2> 1. Fabrication of a composite substrate for a photonic crystal device An electro-optic crystal substrate and a support substrate similar to those in Example 1 were prepared. Next, an Mo film (thickness: 0.5 μm) as a sacrificial layer was formed on the electro-optic crystal substrate by sputtering. Note that Mo is not diffused into the electro-optic crystal substrate (lithium niobate substrate), and thus is not considered to cause optical degradation of the electro-optic crystal substrate. Further, the sacrificial layer was patterned by photolithography. Specifically, the portion of the Mo film that would become the sacrificial layer was covered with a resist mask pattern, and the exposed portion was removed with an Mo etching solution. Next, silicon oxide was sputtered on the surface where the Mo pattern was formed to form an optical loss suppression and cavity processing layer with a thickness of 1 μm, and the arithmetic mean roughness Ra of the surface of this layer was made 0.3 nm or less by CMP polishing. Further, a-Si was sputtered on the surface of the polished layer to form a bonding layer with a thickness of 20 nm, and the arithmetic mean roughness Ra of the surface of the bonding layer was made 0.3 nm or less by CMP polishing. Next, after cleaning the surfaces of the bonding layer and the support substrate, the electro-optic crystal substrate and the support substrate were integrated by directly bonding the bonding layer and the support substrate. The conditions for direct bonding were the same as those in Example 1. After bonding, polishing was performed until the thickness of the electro-optic crystal substrate became 0.5 μm, and a composite substrate for a photonic crystal device similar to that in FIG. 4 (however, with a bonding layer between the electro-optic crystal substrate and the support substrate) was obtained. No defects such as delamination were observed at the bonding interface in the obtained composite substrate for a photonic crystal device.

[0051] 2. Fabrication of a photonic crystal device A photonic crystal element was fabricated from the composite substrate for the photonic crystal element obtained above by a method corresponding to the manufacturing method shown in FIG. 11. Specifically, the photonic crystal element was fabricated by the following procedure. First, a hole pattern and a through-hole for etching were formed in the same manner as in Example 1. Next, the composite substrate was immersed in a Mo etching solution to etch the remaining Mo mask and the sacrificial layer, and a photonic crystal wafer was fabricated. The obtained photonic crystal wafer was diced into chips in the same manner as in Example 1 to obtain a photonic crystal element. The optical waveguide length of the photonic crystal element was set to 10 mm as in Example 1. After dicing the chips, end face polishing was performed in the same manner as in Example 1.

[0052] The obtained photonic crystal element (chip) was subjected to the same evaluation as in Example 1. As a result, a cavity was well formed directly under the photonic crystal layer, and the yield of the chips in which the cavity was formed as designed was 100%. Furthermore, the propagation loss of the optical waveguide of the obtained chip was 0.5 dB / cm.

[0053] <Example 3> 1. Fabrication of a composite substrate for a photonic crystal element An electro-optic crystal substrate and a support substrate similar to those in Example 1 were prepared. Next, a Mo film (thickness: 0.215 μm) as an optical loss suppression layer was formed on the electro-optic crystal substrate by sputtering. Further, the optical loss suppression layer was patterned by photolithography. Specifically, the portion of the Mo film that becomes the optical loss suppression layer was covered with a resist mask pattern, and the exposed portion was removed with a Mo etching solution. Next, silicon oxide was sputtered on the surface where the Mo pattern was formed to form a sacrificial layer with a thickness of 0.25 μm, and by CMP polishing, the arithmetic mean roughness Ra of the surface of the sacrificial layer was made 0.3 nm or less. Next, silicon oxide was sputtered on the polished surface of the sacrificial layer to form a cavity processing layer with a thickness of 0.5 μm, and by CMP polishing, the arithmetic mean roughness Ra of the surface of the cavity processing layer was made 0.3 nm or less. Further, a-Si was sputtered to form a bonding layer with a thickness of 20 nm, and by CMP polishing, the arithmetic mean roughness Ra of the surface of the bonding layer was made 0.3 nm or less. The following procedure was the same as in Example 1, and a composite substrate for a photonic crystal element similar to that in FIG. 9 (however, there is a bonding layer between the electro-optic crystal substrate and the support substrate) was obtained. In the obtained composite substrate for a photonic crystal element, no defects such as peeling were observed at the bonding interface.

[0054] 2. Fabrication of Photonic Crystal Element A photonic crystal element was fabricated from the composite substrate for a photonic crystal element obtained above by a method corresponding to the manufacturing method shown in FIG. 13. Specifically, a photonic crystal element was fabricated by the following procedure. First, a hole pattern and a through hole for etching were formed in the same manner as in Example 1. Next, the composite substrate was immersed in a Mo etching solution to etch the remaining Mo mask. Further, the composite substrate was immersed in a BHF etching solution to remove the sacrificial layer and the cavity processing layer to form a cavity, and a photonic crystal wafer was fabricated. The obtained photonic crystal wafer was diced in the same manner as in Example 1 to obtain a photonic crystal element. The optical waveguide length of the photonic crystal element was 10 mm as in Example 1. After dicing, end face polishing was performed in the same manner as in Example 1.

[0055] The obtained photonic crystal device (chip) was subjected to the same evaluation as in Example 1. As a result, a cavity was well formed directly under the photonic crystal layer, and the yield of the chips in which the cavity was formed as designed was 100%. Furthermore, the propagation loss of the optical waveguide of the obtained chip was 0.5 dB / cm.

[0056] <Example 4> A composite substrate for a photonic crystal device was fabricated in the same manner as in Example 1 except that through-holes for etching were not formed, and a photonic crystal wafer and a photonic crystal device (chip) were fabricated from the composite substrate. The obtained photonic crystal device (chip) was subjected to the same evaluation as in Example 1. As a result, chips in which no cavity was formed directly under the photonic crystal layer were observed. The yield of the chips in which the cavity was formed as designed was approximately 50%. Furthermore, the propagation loss of the optical waveguide of the chips in which the cavity was formed was 0.5 dB / cm, but the propagation loss of the optical waveguide of the chips in which no cavity was formed was 2 dB / cm or more.

Industrial Applicability

[0057] The composite substrate according to the embodiment of the present invention can be suitably used for a photonic crystal device. The photonic crystal device according to the embodiment of the present invention can be suitably used in a wide range of fields such as optical waveguides, next-generation high-speed communication, sensors, laser processing, and solar power generation.

Explanation of Signs

[0058] 10 Electro-optic crystal substrate 12 Vacancy 14 Thin plate slab 16 Optical waveguide 20 Optical loss suppression and cavity processing layer 21 Optical loss suppression layer 22 Cavity processing layer 30 Support substrate 40 Anti-peeling layer 50 Overcoat layer 60 Bonding layer 70 Sacrificial layer 80 Cavity Through-holes for 90 etching Composite substrates for 100 photonic crystal elements Composite substrates for 100a photonic crystal elements Composite substrates for 100b photonic crystal elements Composite substrates for 100c photonic crystal elements Composite substrates for 100d photonic crystal elements Composite substrates for 100e photonic crystal elements Composite substrates for 100f photonic crystal elements 200 Photonic crystal elements

Claims

1. An electro-optic crystal substrate having an electro-optic effect, An optical loss suppression and cavity processing layer provided on one surface of the electro-optic crystal substrate, A support substrate integrated with the electro-optic crystal substrate through the optical loss suppression and cavity processing layer, and having, The optical loss suppression and cavity processing layer has an optical loss suppression layer provided on the electro-optic crystal substrate and a cavity processing layer laminated on the optical loss suppression layer, The cavity processing layer and the support substrate are directly bonded, and an amorphous layer is formed between the cavity processing layer and the support substrate, A composite substrate for a photonic crystal element, wherein the thickness of the optical loss suppression layer is 0.01 μm to 0.05 μm.

2. An electro-optic crystal substrate having an electro-optic effect, An optical loss suppression and cavity processing layer provided on one surface of the electro-optic crystal substrate, A support substrate integrated with the electro-optic crystal substrate through the optical loss suppression and cavity processing layer, and having, The optical loss suppression and cavity processing layer has an optical loss suppression layer provided on the electro-optic crystal substrate and a cavity processing layer laminated on the optical loss suppression layer, The cavity processing layer and the support substrate are directly bonded, and an amorphous layer is formed between the cavity processing layer and the support substrate, A composite substrate for a photonic crystal element, wherein the material constituting the optical loss suppression layer is amorphous silicon, polycrystalline silicon, molybdenum, or a mixture of these materials.

3. The composite substrate for a photonic crystal element according to claim 1 or 2, wherein a patterned sacrificial layer is formed in the cavity processing layer.

4. The composite substrate for a photonic crystal element according to claim 2, wherein the thickness of the optical loss suppression layer is 0.01 μm to 0.1 μm.

5. The composite substrate for a photonic crystal element according to any one of claims 1 to 4, further having a bonding layer between the cavity processing layer and the support substrate.

Images