Optical waveguide junction substrate and method for manufacturing optical waveguide junction substrate

Abstract

Provided is an optical waveguide junction substrate which enables suppression of the occurrence of cracks and the like even when heated. An optical waveguide junction substrate (1) according to an aspect of the present disclosure comprises: a support substrate (10); and a functional substrate (20) that is joined to a junction surface (12) of the support substrate (10) and that is composed of a material identical to that of the support substrate (10). A metal diffusion region (11) in which a metal element is diffused is formed on the junction surface (12) side of the support substrate (10). The refractive index of the support substrate (10) in the metal diffusion region (11) is lower than the refractive index of the functional substrate (20). Light is reflected on the junction surface (12) of the support substrate (10) and the functional substrate (20) to thereby be guided in the functional substrate (20).

Description

Optical waveguide bonded substrate and method for manufacturing optical waveguide bonded substrate

[0001] The present disclosure relates to an optical waveguide bonded substrate and a method for manufacturing an optical waveguide bonded substrate.

[0002] In recent years, development of an optical waveguide bonded substrate in which an electro-optic crystal such as lithium niobate is bonded to a support substrate has been progressing. Patent Document 1 discloses a technology relating to a bonded body for an optical modulator in which an optical waveguide material made of lithium niobate or the like is bonded to a support substrate.

[0003] Japanese Patent Application Laid-Open No. 2021-60479

[0004] In the technology disclosed in Patent Document 1, an optical waveguide bonded substrate is constructed by bonding an optical waveguide substrate to a support substrate. However, in the technology disclosed in Patent Document 1, the support substrate and the optical waveguide substrate are made of different materials, which causes a difference in thermal expansion between the support substrate and the optical waveguide substrate. This causes a problem in that stress is generated at the bonded interface when the optical waveguide bonded substrate is heated, which can cause cracks and other problems. Other issues and novel features will become apparent from the description of this specification and the accompanying drawings.

[0005] An optical waveguide bonded substrate according to one aspect of the present disclosure includes a support substrate and a functional substrate made of the same material as the support substrate and bonded to the bonding surface of the support substrate. A metal diffusion region in which a metal element is diffused is formed on the bonding surface side of the support substrate, and the refractive index of the metal diffusion region of the support substrate is lower than the refractive index of the functional substrate. Light is reflected at the bonding surface between the support substrate and the functional substrate, thereby being guided within the functional substrate.

[0006] A method for manufacturing an optical waveguide bonded substrate according to one aspect of the present disclosure includes a diffusion step of diffusing a metal element onto a bonding surface side of a support substrate, and a bonding step of bonding a functional substrate made of the same material as the support substrate onto the bonding surface side of the support substrate, wherein the refractive index of the bonding surface side of the support substrate into which the metal element has been diffused is lower than the refractive index of the functional substrate.

[0007] The present disclosure provides an optical waveguide bonded substrate that can suppress the occurrence of cracks and the like even when heated, and a method for manufacturing the optical waveguide bonded substrate.

[0008] FIG. 6 is a cross-sectional view illustrating an optical waveguide bonded substrate according to a first embodiment. FIG. 7 is a cross-sectional view illustrating an optical waveguide bonded substrate according to the first embodiment. FIG. 8 is a flowchart illustrating a method for manufacturing an optical waveguide bonded substrate according to the first embodiment. FIG. 9 is a cross-sectional view illustrating a method for manufacturing an optical waveguide bonded substrate according to the first embodiment. FIG. 10 is a plan view illustrating an optical waveguide bonded substrate according to a second embodiment. FIG. 11 is a cross-sectional view taken along line VII-VII in FIG. 6. FIG. 12 is a flowchart illustrating a diffusion process in a method for manufacturing an optical waveguide bonded substrate according to the second embodiment. FIG. 13 is a cross-sectional view illustrating a diffusion process in a method for manufacturing an optical waveguide bonded substrate according to the second embodiment.

[0009] First Embodiment Hereinafter, an embodiment will be described with reference to the drawings. Fig. 1 is a cross-sectional view for explaining an optical waveguide bonded substrate according to the first embodiment. As shown in Fig. 1, the optical waveguide bonded substrate 1 according to the present embodiment includes a support substrate 10 and a functional substrate 20.

[0010] The support substrate 10 and the functional substrate 20 are made of the same material. Here, "homogeneous material" typically means a material whose main component is the same compound. For example, the support substrate 10 and the functional substrate 20 can be made using a lithium niobate substrate. Alternatively, the support substrate 10 and the functional substrate 20 may be made using a lithium tantalate substrate. In this embodiment, other electro-optic crystals (e.g., barium titanate, potassium niobate, etc.) may be used as the support substrate 10 and the functional substrate 20. In addition, the electro-optic crystal used in the functional substrate 20 may be doped with an impurity element for the purpose of improving optical damage resistance.

[0011] 1 , the functional substrate 20 is bonded to a bonding surface 12 of the support substrate 10. Specifically, the support substrate 10 and the functional substrate 20 are directly bonded via the bonding surface 12. Here, "directly bonded" means that the support substrate 10 and the functional substrate 20 are bonded without an intermediate layer or the like therebetween. For example, in this embodiment, the support substrate 10 and the functional substrate 20 are directly bonded using a plasma activation method. That is, the bonding surfaces of the support substrate 10 and the functional substrate 20 are activated by plasma treatment, and the activated support substrate 10 and the functional substrate 20 are bonded together by being attached to each other.

[0012] A metal diffusion region 11 in which a metal element is diffused is formed on the bonding surface 12 side of the support substrate 10. The refractive index of the metal diffusion region 11 is lower than the refractive index of the functional substrate 20. That is, in this embodiment, by diffusing a metal element on the bonding surface 12 side of the support substrate 10, the refractive index of the bonding surface 12 side of the support substrate 10, i.e., the refractive index of the metal diffusion region 11, is made lower than the refractive index of other parts of the support substrate 10. Therefore, the refractive index of the metal diffusion region 11 can be made lower than the refractive index of the functional substrate 20.

[0013] The metal element diffused toward the bonding surface 12 of the support substrate 10 may be any material that reduces the refractive index of the metal diffusion region 11. In this embodiment, at least one of magnesium, nickel, and vanadium may be used as the metal element diffused into the support substrate 10. For example, it is preferable to use magnesium as the metal material. For example, when forming the metal diffusion region 11, a film of a material containing the metal element may be formed on the bonding surface 12 of the support substrate 10, and the support substrate 10 on which the film of the material containing the metal element has been formed may be heated to diffuse the metal element toward the bonding surface 12 of the support substrate 10. The material containing the metal element to be formed in this case may be a metal or a metal oxide.

[0014] As described above, in this embodiment, the refractive index of the metal diffusion region 11 of the support substrate 10 is configured to be lower than the refractive index of the functional substrate 20. Therefore, light is reflected at (or in the vicinity of) the bonding surface 12 between the support substrate 10 and the functional substrate 20, and the light is guided within the functional substrate 20.

[0015] Furthermore, in this embodiment, since the support substrate 10 and the functional substrate 20 are made of the same material, it is possible to substantially eliminate the difference in thermal expansion between the support substrate 10 and the functional substrate 20. Therefore, when the optical waveguide junction substrate 1 is heated, it is possible to suppress the occurrence of stress at the bonding interface between the support substrate 10 and the functional substrate 20, and to suppress the occurrence of cracks and the like.

[0016] On the other hand, if the support substrate 10 and the functional substrate 20 are made of the same material, the refractive index of the support substrate 10 and the functional substrate 20 is the same, and therefore the functional substrate 20 cannot be used as an optical waveguide. Therefore, in this embodiment, a metal element is diffused on the bonding surface 12 side of the support substrate 10 to partially lower the refractive index on the bonding surface 12 side of the support substrate 10. This configuration allows light to be reflected at the bonding surface 12 (or its vicinity) between the support substrate 10 and the functional substrate 20, allowing the functional substrate 20 to be used as an optical waveguide. Furthermore, since the support substrate 10 and the functional substrate 20 are made of the same material, the difference in thermal expansion between the support substrate 10 and the functional substrate 20 can be substantially zero. Therefore, when the optical waveguide bonded substrate 1 is heated, stress is prevented from occurring at the bonding interface between the support substrate 10 and the functional substrate 20, and cracks and other problems are prevented.

[0017] Furthermore, in this embodiment, since the support substrate 10 and the functional substrate 20 are made of the same material, it is possible to prevent steps from occurring on the side surfaces of the optical waveguide bonded substrate 1 when the optical waveguide bonded substrate 1 is diced. Therefore, it is possible to reduce optical loss on the side surfaces of the optical waveguide bonded substrate 1.

[0018] 2 is a cross-sectional view illustrating the optical waveguide bonded substrate according to the first embodiment. Fig. 2 shows a configuration example in which the functional substrate 20 shown in Fig. 1 is processed to form a ridge waveguide 21 in the functional substrate 20. Light is guided within the ridge waveguide 21. The shape of the ridge waveguide 21 can be any shape depending on the route along which the light is guided.

[0019] Next, a method for manufacturing an optical waveguide bonded substrate according to this embodiment will be described. Fig. 3 is a flowchart for explaining the method for manufacturing an optical waveguide bonded substrate according to the first embodiment. Fig. 4 and Fig. 5 are cross-sectional views for explaining the method for manufacturing an optical waveguide bonded substrate according to the first embodiment.

[0020] 3, when manufacturing the optical waveguide bonded substrate 1 according to this embodiment, first, a support substrate 10 and a functional substrate 20 are prepared (steps S1 and S5). At this time, substrates made of the same material are prepared as the support substrate 10 and the functional substrate 20. For example, a lithium niobate substrate or a lithium tantalate substrate is prepared as the support substrate 10 and the functional substrate 20.

[0021] Next, as shown in FIGS. 3 and 4 , a material containing a metal element (metal film 15) is formed on the bonding surface 12 side of the support substrate 10 (step S2). In this embodiment, at least one of magnesium, nickel, and vanadium can be used as the metal film 15 (metal element). The metal film 15 may be a metal or a metal oxide film. For example, when magnesium is used as the metal element, metallic magnesium or magnesium oxide is formed as the metal film 15. The metal film 15 can be formed using, for example, a sputtering method or an electron beam evaporation method. The thickness of the metal film 15 is, for example, about 5 μm.

[0022] Next, the support substrate 10 on which the metal film 15 is formed is heated to diffuse the metal elements from the metal film 15 toward the bonding surface 12 of the support substrate 10 (step S3). This forms a metal diffusion region 11 in which the metal elements are diffused toward the bonding surface 12 of the support substrate 10. The refractive index of the metal diffusion region 11 is lower than that of the other portions of the support substrate 10. For example, the amount of diffusion of the metal elements can be adjusted by adjusting the heat treatment temperature and heat treatment time. This allows the refractive index of the metal diffusion region 11 to be controlled. For example, the heat treatment temperature is approximately 850 to 1000°C, and the heat treatment time is approximately 10 hours. Furthermore, when the metal element is magnesium, the magnesium concentration in the metal diffusion region 11 is approximately 0.2 to 5 mol%. Steps S2 and S3 are collectively referred to as the diffusion process.

[0023] Next, the metal film 15 formed on the bonding surface 12 side of the support substrate 10 is removed (step S4). The metal film 15 can be removed by cleaning with a chemical solution or by polishing.

[0024] Next, as shown in FIGS. 3 and 5 , the functional substrate 20 is bonded to the bonding surface 12 of the support substrate 10 (step S6). For example, the bonding surfaces of the support substrate 10 and the functional substrate 20 are activated by plasma treatment, and the activated support substrate 10 and the functional substrate 20 are then bonded together to directly bond the support substrate 10 and the functional substrate 20. An annealing process is then performed (step S7). The annealing temperature is preferably 300° C. or less to prevent the metal elements contained in the metal diffusion region 11 from diffusing toward the functional substrate 20. The support substrate 10 and the functional substrate 20 are bonded together by the processes of steps S6 and S7. Steps S6 and S7 are also referred to as bonding processes.

[0025] Next, as shown in Figures 3 and 5, the surface of the functional substrate 20 is polished (step S8). In Figure 5, the polished functional substrate 20 is indicated by a dashed line. The polishing process can be performed using, for example, chemical mechanical polishing (CMP). For example, the functional substrate 20 is polished to a thickness of 0.1 μm to 10 μm.

[0026] Next, the functional substrate 20 is subjected to ridge processing to form a ridge waveguide 21 in the functional substrate 20 (step S9). For example, the ridge waveguide 21 can be formed by removing the region of the functional substrate 20 other than the ridge waveguide 21 using RIE (reactive ion etching) or dicing. The ridge waveguide 21 is a waveguide through which light is guided. Note that step S9 (waveguide formation step) may be omitted. In other words, when the steps up to step S8 are performed, the optical waveguide bonded substrate 1 shown in FIG. 1 can be manufactured, and when the steps up to step S9 are performed, the optical waveguide bonded substrate 2 shown in FIG. 2 can be manufactured.

[0027] The optical waveguide bonded substrate according to the present embodiment can be manufactured using the manufacturing method described above. In the optical waveguide bonded substrate manufactured using the manufacturing method described above, the refractive index of the metal diffusion region 11 of the support substrate 10 is lower than the refractive index of the functional substrate 20. Therefore, light is reflected at the bonding surface 12 between the support substrate 10 and the functional substrate 20, and the light is guided within the functional substrate 20.

[0028] In this embodiment, the support substrate 10 and the functional substrate 20 are made of the same material. Therefore, the difference in thermal expansion between the support substrate 10 and the functional substrate 20 can be made substantially zero, which can prevent stress from occurring at the bonding interface when the optical waveguide junction substrate 1 is heated, thereby preventing cracks and other problems from occurring.

[0029] <Second Embodiment> Next, a second embodiment will be described. Fig. 6 is a plan view for explaining an optical waveguide bonded substrate according to the second embodiment. As shown in Fig. 6, in an optical waveguide bonded substrate 3 according to the second embodiment, a ridge waveguide 21 has a branch point 25a and a coupling point 25b when the optical waveguide bonded substrate 3 is viewed in plan. Note that apart from the waveguide structure of the ridge waveguide 21, the optical waveguide bonded substrate 3 is similar to the optical waveguide bonded substrates 1 and 2 described in the first embodiment, and therefore the same components are designated by the same reference numerals and redundant description will be omitted.

[0030] 6, the optical waveguide bonded substrate 3 according to this embodiment includes a support substrate 10 and a ridge waveguide 21 (functional substrate 20) formed on the support substrate 10. The ridge waveguide 21 includes a branching point 25a where light branches and a multiplexing point 25b where the light is multiplexed. Specifically, light propagating through the ridge waveguide 21 branches into a first route 31 and a second route 32 at the branching point 25a, and then multiplexes at the multiplexing point 25b. Two bends 26a and 26b are formed in the first route 31. Two bends 26c and 26d are formed in the second route 32.

[0031] In other words, the ridge waveguide 21 is composed of a straight structure and a bent structure, and in this embodiment, the branching point 25 a, the combining point 25 b, and the bent portions 26 a to 26 d are bent structures, and the remaining portions are straight structures. In this embodiment, when the optical waveguide bonded substrate 3 is viewed in plan, the amount of diffusion of metal elements in the support substrate 10 (metal diffusion region 11) at positions corresponding to the bent structures is configured to be greater than the amount of diffusion of metal elements in the support substrate 10 (metal diffusion region 11) at positions corresponding to the straight structures.

[0032] 7 is a cross-sectional view taken along line VII-VII in FIG. 6 , showing a position where the ridge waveguide 21 has a bent structure. In this embodiment, the amount of metal element diffusion in the support substrate 10 at the position where the ridge waveguide 21 has a bent structure is greater than the amount of metal element diffusion in the support substrate 10 at the position where the ridge waveguide 21 has a straight structure. That is, as shown in FIG. 7 , at the position where the ridge waveguide 21 has a bent structure, the metal diffusion region 11 of the support substrate 10 includes a region 13 where the amount of metal element diffusion is greater. This region 13 has a greater amount of metal element diffusion than the surrounding area, and therefore has an even lower refractive index.

[0033] As described above, in this embodiment, by increasing the amount of metal element diffusion into the support substrate 10 at the position where the ridge waveguide 21 has a bent structure, a region 13 is formed where the amount of metal element diffusion is greater than in the surrounding area. Because this region 13 has an even lower refractive index than the surrounding area, the difference in refractive index between the ridge waveguide 21 and the support substrate 10 (region 13) can be made larger than in other positions (positions of the straight structure). Therefore, the radius of curvature of the bent structure can be made smaller, thereby realizing miniaturization of devices using the optical waveguide junction substrate 3.

[0034] Next, a method for manufacturing an optical waveguide bonded substrate according to this embodiment will be described. Fig. 8 is a flowchart for explaining the diffusion process in the method for manufacturing an optical waveguide bonded substrate according to embodiment 2. Fig. 9 is a cross-sectional view for explaining the diffusion process in the method for manufacturing an optical waveguide bonded substrate according to embodiment 2. In the method for manufacturing an optical waveguide bonded substrate according to this embodiment, the diffusion process S3 is different from the diffusion process S3 described in embodiment 1. Other than this, the method for manufacturing an optical waveguide bonded substrate is the same as the method for manufacturing an optical waveguide bonded substrate described in embodiment 1.

[0035] 8 and 9, in this embodiment, after the metal film 15 is formed on the bonding surface 12 side of the support substrate 10 in step S2 (see FIG. 3), the support substrate 10 on which the metal film 15 is formed is heated to diffuse metal elements from the metal film 15 to the bonding surface 12 side of the support substrate 10 (step S3_1: overall heat treatment). As a result, a metal diffusion region 11 in which the metal elements are diffused is formed on the bonding surface 12 side of the support substrate 10.

[0036] Thereafter, a position corresponding to the bent structure of the ridge waveguide 21 is locally heated (step S3_2: local heating treatment). Specifically, as shown in FIG. 9, a laser beam 35 is used to locally heat a position corresponding to the bent structure of the ridge waveguide 21. This allows a region 13 in which a large amount of metal element has diffused to be formed in the metal diffusion region 11 of the support substrate 10. This region 13 has a larger amount of metal element diffused therein than the surrounding area, and therefore has an even lower refractive index. Note that while FIG. 9 shows an example of local heating using laser beam 35, means other than laser beam may be used to achieve local heating.

[0037] As described above, in this embodiment, the amount of metal element diffused in the support substrate 10 is increased at a position corresponding to the bent structure of the ridge waveguide 21. This region (region 13) with a larger amount of metal element diffused has a lower refractive index than the surrounding area. Therefore, the difference in refractive index between the ridge waveguide 21 and the support substrate 10 (region 13) can be made larger than at other positions (positions of the linear structure). Therefore, the radius of curvature of the bent structure can be made smaller, thereby enabling miniaturization of devices using the optical waveguide junction substrate 3.

[0038] The present invention has been described above in accordance with the above-mentioned embodiment, but the present invention is not limited to the configuration of the above-mentioned embodiment, and naturally includes various modifications, alterations, and combinations that a person skilled in the art can make within the scope of the invention as defined in the claims of this application.

[0039] This application claims priority based on Japanese Patent Application No. 2024-100334, filed June 21, 2024, the disclosure of which is incorporated herein in its entirety by reference.

[0040] REFERENCE SIGNS LIST 1, 2, 3 Optical waveguide bonded substrate 10 Support substrate 11 Metal diffusion region 12 Bonding surface 15 Metal film 20 Functional substrate 21 Ridge waveguide 25a Branch point 25b Combining point 26a, 26b, 26c, 26d Bent portion 31 First route 32 Second route 35 Laser light

Claims

1. An optical waveguide bonded substrate comprising: a support substrate; and a functional substrate made of the same material as said support substrate and bonded to the bonding surface of said support substrate, wherein a metal diffusion region in which a metal element is diffused is formed on the bonding surface side of said support substrate, the refractive index of said metal diffusion region of said support substrate being lower than the refractive index of said functional substrate, and wherein light is reflected at the bonding surface between said support substrate and said functional substrate, thereby being guided within said functional substrate.

2. The optical waveguide junction substrate according to claim 1, wherein the support substrate and the functional substrate are lithium niobate substrates.

3. The optical waveguide junction substrate according to claim 1, wherein the support substrate and the functional substrate are lithium tantalate substrates.

4. The optical waveguide bonded substrate according to claim 1, wherein the support substrate and the functional substrate are directly bonded via the bonding surface.

5. The optical waveguide junction substrate according to claim 1, wherein the metal element diffused into the support substrate is at least one of magnesium, nickel, and vanadium.

6. The optical waveguide junction substrate according to claim 1, wherein a ridge waveguide is formed in the functional substrate, and the light is configured to be guided within the ridge waveguide.

7. The optical waveguide junction substrate according to claim 6, wherein the ridge waveguide has a straight structure and a bent structure, and when the optical waveguide junction substrate is viewed in a plane, the amount of diffusion of the metal element in the metal diffusion region at a position corresponding to the bent structure is greater than the amount of diffusion of the metal element in the metal diffusion region at a position corresponding to the straight structure.

8. A method for manufacturing an optical waveguide bonded substrate, comprising: a diffusion step of diffusing a metal element into a bonding surface side of a support substrate; and a bonding step of bonding a functional substrate made of the same material as the support substrate to the bonding surface side of the support substrate, wherein the refractive index of the bonding surface side of the support substrate into which the metal element has been diffused is lower than the refractive index of the functional substrate.

9. The method for manufacturing an optical waveguide junction substrate according to claim 8, wherein the support substrate and the functional substrate are lithium niobate substrates.

10. The method for manufacturing an optical waveguide junction substrate according to claim 8, wherein the support substrate and the functional substrate are lithium tantalate substrates.

11. The method for manufacturing an optical waveguide bonded substrate according to claim 8, wherein the support substrate and the functional substrate are directly bonded via the bonding surface.

12. The method for producing an optical waveguide junction substrate according to claim 8, wherein the metal element diffused into the support substrate is at least one of magnesium, nickel, and vanadium.

13. A method for manufacturing an optical waveguide junction substrate according to claim 8, wherein the diffusion step comprises: a step of depositing a material containing the metal element on the bonding surface side of the support substrate; and a step of heating the support substrate on which the material containing the metal element has been deposited, thereby diffusing the metal element on the bonding surface side of the support substrate.

14. The method for producing an optical waveguide junction substrate according to claim 8, further comprising a waveguide forming step of forming a ridge waveguide for guiding light in said functional substrate.

15. A method for manufacturing an optical waveguide junction substrate according to claim 14, wherein the ridge waveguide has a straight structure and a bent structure, and in the diffusion step, a position of the support substrate corresponding to the bent structure is locally heated, thereby making the amount of diffusion of the metal element at the position of the support substrate corresponding to the bent structure greater than the amount of diffusion of the metal element at the position of the support substrate corresponding to the straight structure.

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