Method for manufacturing optical waveguide element

Abstract

To provide a method for manufacturing an optical waveguide element by which an optical waveguide element having low propagation loss can be manufactured.SOLUTION: The method M1 for manufacturing an optical wave guide device includes a step S1 of preparing a structural body including a substrate and a ridge-shaped optical wave guide layer provided on the substrate and made of a crystalline material having an electro-optic effect, a step S2 of forming all of a cladding layer covering the optical wave guide layer, and a step S3 of performing heat treatment on the structural body on which the cladding layer has been formed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing an optical waveguide element. [Background technology]

[0002] An optical waveguide element is known that includes a substrate, an optical waveguide layer formed on the substrate and made of a crystalline material having an electro-optic effect, such as a lithium niobate film, and a clad layer formed to cover the optical waveguide layer. Patent Document 1, for example, describes a method for manufacturing such an optical waveguide element, in which a ridge is formed by etching a lithium niobate film formed on a substrate, and a low-loss optical waveguide is obtained by performing a heat treatment. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6136666 Summary of the Invention [Problem to be solved by the invention]

[0004] When forming a cladding layer on an optical waveguide layer, oxygen defects, which are caused by oxygen being removed from the crystalline structure of the crystalline material, may occur on the surface of the optical waveguide layer, which may increase the propagation loss of the manufactured optical waveguide element.

[0005] The present disclosure describes a method for manufacturing an optical waveguide element that can produce an optical waveguide element with low propagation loss. [Means for solving the problem]

[0006] A method for manufacturing an optical waveguide element according to one aspect of the present disclosure includes the steps of preparing a structure including a substrate and a ridge-shaped optical waveguide layer provided on the substrate and made of a crystalline material having an electro-optic effect; depositing all of the cladding layer covering the optical waveguide layer; and heat treating the structure after the deposition of the cladding layer has been completed.

[0007] In this method for manufacturing an optical waveguide element, a heat treatment is performed after the cladding layer covering the optical waveguide layer is formed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer when the cladding layer is formed, oxygen is supplied to the optical waveguide layer through the cladding layer by the heat treatment. This compensates for the oxygen defects, thereby reducing the propagation loss of the optical waveguide element. As a result, the above-mentioned method for manufacturing an optical waveguide element makes it possible to manufacture an optical waveguide element with low propagation loss.

[0008] The temperature for the heat treatment may be 400° C. or higher and 700° C. or lower. In this case, the time required for the heat treatment can be shortened while reducing the possibility of cracks occurring in the optical waveguide layer.

[0009] A method for manufacturing an optical waveguide element according to another aspect of the present disclosure includes the steps of preparing a structure including a substrate and a ridge-shaped optical waveguide layer provided on the substrate and composed of a crystalline material having an electro-optic effect; depositing a portion of a cladding layer covering the optical waveguide layer; subjecting the structure on which the portion of the cladding layer has been deposited to a heat treatment; and, after the heat treatment, depositing the remainder of the cladding layer so as to cover the portion of the cladding layer.

[0010] In this method for manufacturing an optical waveguide element, a cladding layer covering an optical waveguide layer is formed in two stages: a portion and a remainder. After the portion of the cladding layer is formed, a heat treatment is performed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer when forming the portion of the cladding layer, oxygen is supplied to the optical waveguide layer through the portion of the cladding layer by the heat treatment. This compensates for the oxygen defects, thereby reducing the propagation loss of the optical waveguide element. Because the heat treatment is performed after the portion of the cladding layer is formed, oxygen is more likely to pass through the portion of the cladding layer than when the heat treatment is performed after the entire cladding layer is formed. In other words, because oxygen is efficiently supplied to the optical waveguide layer, the time required for the heat treatment can be shortened. As described above, the above method for manufacturing an optical waveguide element makes it possible to manufacture an optical waveguide element with low propagation loss while improving manufacturing efficiency.

[0011] The thickness of a portion of the cladding layer may be smaller than the thickness of the remaining portion of the cladding layer. In this case, oxygen can more easily pass through the portion of the cladding layer. In other words, oxygen is more efficiently supplied to the optical waveguide layer. Therefore, the manufacturing efficiency of low propagation loss optical waveguide elements can be further improved.

[0012] The thickness of a portion of the cladding layer may be 10 nm or more and 100 nm or less, which allows the portion of the cladding layer to be formed with a uniform thickness and increases the rate at which oxygen permeates the portion of the cladding layer (transmittance).

[0013] The temperature of the heat treatment performed on the structure on which part of the cladding layer has been formed may be 400° C. or more and 700° C. or less. In this case, the time required for the heat treatment can be shortened while reducing the possibility of cracks occurring in the optical waveguide layer.

[0014] The method for manufacturing an optical waveguide element may further include a step of performing a heat treatment on the structure in which the formation of the clad layer is completed by depositing the remaining portion of the clad layer. In this case, the heat treatment is performed after the clad layer is deposited. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer when depositing the remaining portion of the clad layer, oxygen is supplied to the optical waveguide layer through the clad layer by the heat treatment. This compensates for the oxygen defects, thereby further reducing the propagation loss of the optical waveguide element. Therefore, it is possible to manufacture an optical waveguide element with even lower propagation loss.

[0015] The temperature of the heat treatment performed on the structure after the cladding layer has been formed may be 550° C. or higher and 650° C. or lower, which reduces the possibility of cracks occurring in the optical waveguide layer and shortens the time required for the heat treatment.

[0016] The process of preparing the structure may include a process of forming a crystalline film made of a crystalline material on a substrate, a process of forming an optical waveguide layer by etching the crystalline film, and a process of applying heat treatment to the optical waveguide layer. In this case, the heat treatment is applied after the optical waveguide layer is formed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer due to etching, oxygen is supplied to the optical waveguide layer by the heat treatment. This compensates for the oxygen defects, thereby further reducing the propagation loss of the optical waveguide element. Therefore, it is possible to manufacture an optical waveguide element with even lower propagation loss.

[0017] The crystalline material may be lithium niobate or lithium tantalate, which provides excellent electro-optical effects.

[0018] The optical waveguide layer may have a c-axis orientation. In this case, an electric field is applied to the ridge-shaped optical waveguide in the stacking direction of the optical waveguide layer relative to the substrate. This allows the optical waveguide to be curved, thereby improving the degree of freedom in designing the optical waveguide layer.

[0019] The cladding layer may be made of silicon oxide, which has a relatively low refractive index and therefore increases the possibility of light being confined in the optical waveguide layer, thereby further reducing the propagation loss of the optical waveguide element.

[0020] The method for manufacturing the optical waveguide element may further include the steps of planarizing the cladding layer, applying heat treatment to the structure after the cladding layer has been planarized, depositing a buffer layer on the planarized cladding layer, applying heat treatment to the structure after the buffer layer has been deposited, and forming an electrode on the buffer layer. In this case, the heat treatment is performed after the cladding layer has been planarized, and after the buffer layer has been deposited. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer due to the planarization of the cladding layer and the deposition of the buffer layer, oxygen is supplied to the optical waveguide layer by the heat treatment. This compensates for the oxygen defects, thereby further reducing the propagation loss of the optical waveguide element. Therefore, it is possible to manufacture an optical waveguide element with even lower propagation loss. [Effects of the Invention]

[0021] According to each aspect and embodiment of the present disclosure, an optical waveguide element with low propagation loss can be manufactured. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a process diagram showing a method for manufacturing an optical waveguide element according to an embodiment. [Figure 2] Fig. 2(a) is a diagram for explaining the process of forming a crystal film, Fig. 2(b) is a diagram for explaining the process of forming an optical waveguide layer, and Fig. 2(c) is a diagram for explaining the process of forming a cladding layer. [Figure 3] FIG. 3 is a process diagram showing a method for manufacturing an optical waveguide element according to another embodiment. [Figure 4]Fig. 4(a) is a diagram illustrating a step of depositing a portion of the cladding layer, and Fig. 4(b) is a diagram illustrating a step of depositing the remaining portion of the cladding layer. [Figure 5] FIG. 5 is a process chart showing a method for manufacturing an optical waveguide element according to still another embodiment. [Figure 6] Fig. 6(a) is a diagram illustrating a step of planarizing a cladding layer, Fig. 6(b) is a diagram illustrating a step of forming a buffer layer, and Fig. 6(c) is a diagram illustrating a step of forming an electrode. [Figure 7] Fig. 7(a) is a diagram showing the relationship between the annealing time and the insertion loss when the annealing temperature in step S3 shown in Fig. 1 is 400°C. Fig. 7(b) is a diagram showing the relationship between the annealing time and the insertion loss when the annealing temperature in step S3 shown in Fig. 1 is 500°C. [Figure 8] FIG. 8 is a diagram showing the propagation loss when the annealing temperature in step S3 shown in FIG. 1 is 500 degrees. [Figure 9] FIG. 9 is a diagram showing the propagation loss when the annealing temperature in step S3 shown in FIG. 1 is 700 degrees. [Figure 10] FIG. 10 is a diagram showing an Arrhenius plot. [Figure 11] FIG. 11 shows the calculation results of the annealing time. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are given the same symbols, and duplicate explanations will be omitted. An XYZ coordinate system may be shown in each drawing. The Y-axis direction is a direction that intersects (e.g., is perpendicular to) the X-axis and Z-axis directions. The Z-axis direction is a direction that intersects (e.g., is perpendicular to) the X-axis and Y-axis directions. In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​written before and after "to" as the minimum and maximum values, respectively. Individually written upper and lower limit values ​​can be combined in any way.

[0024] A method for manufacturing an optical waveguide element according to one embodiment will be described with reference to Fig. 1 and (a) to (c) of Fig. 2. Fig. 1 is a process diagram showing a method for manufacturing an optical waveguide element according to one embodiment. (a) of Fig. 2 is a diagram for explaining a step of forming a crystal film. (b) of Fig. 2 is a diagram for explaining a step of forming an optical waveguide layer. (c) of Fig. 2 is a diagram for explaining a step of forming a cladding layer. Method M1 shown in Fig. 1 is a method for manufacturing an optical waveguide element. Method M1 includes steps S1 to S3.

[0025] <Process S1> Step S1 is a step of preparing a structure 10. The structure 10 includes a substrate 11 and an optical waveguide layer 12 (see FIG. 2(b)). The substrate 11 functions as a lower cladding layer. The substrate 11 is made of a material having a lower refractive index than the material of the optical waveguide layer 12. Examples of materials for the substrate 11 include sapphire and silicon oxide. Silicon may also be used as the material for the substrate 11. In this case, a buffer layer having a lower refractive index than the material of the optical waveguide layer 12 is formed on the silicon. The substrate 11 has a main surface 11a and a back surface 11b opposite to the main surface 11a. The main surface 11a and the back surface 11b are planes defined by the X-axis direction and the Y-axis direction, and intersect with the Z-axis direction (orthogonal in this embodiment).

[0026] The optical waveguide layer 12 is a ridge-type optical waveguide provided on the substrate 11. Specifically, the optical waveguide layer 12 is provided on the main surface 11a of the substrate 11. The optical waveguide layer 12 is made of a crystalline material having an electro-optic effect. Examples of crystalline materials having an electro-optic effect include lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). For example, when the composition of lithium niobate is Li x NbO z In this case, x may be 0.9 to 1.05, and z may be 2.8 to 3.2. Up to 10% of lithium (Li) and niobium (Nb) may be replaced with other elements. Examples of the other elements used for replacement include potassium (K), sodium (Na), rubidium (Rb), cesium (Cs), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), chromium (Cr), molybdenum (Mo), tungsten (W), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), scandium (Sc), and cerium (Ce). A combination of two or more of these elements may be used for replacement. The same applies to the composition of lithium tantalate. The optical waveguide layer 12 may have a c-axis orientation or an a-axis orientation. In other words, the optical waveguide layer 12 may be made of a Z-cut crystal material or an X-cut crystal material. The optical waveguide layer 12 includes a convex ridge portion 12a and a flat slab 12b.

[0027] Step S1 includes steps S11 to S13.

[0028] <Process S11> Step S11 is a step of forming a crystalline film 20, which will be the basis of the optical waveguide layer 12, on a substrate 11. As shown in FIG. 2(a), in step S11, the substrate 11 is first prepared, and the crystalline film 20 is formed on the main surface 11a of the substrate 11. The crystalline film 20 is made of the above-mentioned crystalline material and is formed by epitaxially growing the crystalline material. Examples of methods for forming the crystalline film 20 include a sputtering method and a CVD (Chemical Vapor Deposition) method.

[0029] <Process S12> Step S11 is followed by step S12. Step S12 is a step of forming the optical waveguide layer 12 by etching the crystal film 20. Specifically, in step S12, a mask pattern corresponding to the ridge portion 12a is first formed on the crystal film 20. Next, by dry etching, the portion of the crystal film 20 not covered by the mask pattern is removed to a depth corresponding to the height of the ridge portion 12a. Thereafter, the mask pattern is removed. As a result, the optical waveguide layer 12 is formed on the main surface 11a of the substrate 11, as shown in FIG. 2(b).

[0030] <Process S13> Step S12 is followed by step S13, which is a step of subjecting the optical waveguide layer 12 to heat treatment (annealing). The etching in step S12 may cause oxygen defects on the surface of the optical waveguide layer 12, where oxygen is removed from the crystal structure of the crystalline material. Therefore, by subjecting the optical waveguide layer 12 to heat treatment, oxygen is supplied to the optical waveguide layer 12, and the oxygen defects are compensated for.

[0031] To reduce the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S13 is performed in the atmosphere at a temperature of 700°C or less. To reduce the time required for the heat treatment, the heat treatment in step S13 is performed in the atmosphere at a temperature of 200°C or more. Heat treatment at temperatures below 200°C results in a slow reaction rate, and oxygen is not sufficiently supplied to the optical waveguide layer 12. Hereinafter, the heat treatment temperature may be referred to as the "annealing temperature," and the heat treatment time may be referred to as the "annealing time." Unless otherwise specified, temperatures are expressed in Celsius degrees (°C). The annealing time in step S13 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance by experiment, etc. The higher the annealing temperature, the shorter the annealing time.

[0032] The structure 10 is manufactured and prepared through the above steps S11 to S13. A heat treatment may be applied to the crystal film 20 between steps S11 and S12. Step S13 may be omitted. Note that the heat treatment supplies oxygen to the optical waveguide layer 12 and compensates for oxygen defects, so the optical waveguide layer 12 after the heat treatment may differ in composition from the optical waveguide layer 12 before the heat treatment. However, for ease of explanation, the same reference numerals are used for the optical waveguide layer 12 before and after the heat treatment. This also applies to the following explanation.

[0033] <Process S2> Step S2 follows step S1. Step S2 is a step of depositing the entire cladding layer 13. The cladding layer 13 functions as an upper cladding layer. As shown in FIG. 2(c), the cladding layer 13 is formed over the entire upper surface of the optical waveguide layer 12 so as to cover the optical waveguide layer 12. The cladding layer 13 is made of a material having a lower refractive index than the material constituting the optical waveguide layer 12. Examples of materials constituting the cladding layer 13 include silicon oxides (e.g., SiO2, LaAlSiInO, and SiInO). The thickness of the cladding layer 13 is substantially uniform throughout the entire cladding layer 13. The thickness of the cladding layer 13 is, for example, 0.5 μm to 1.0 μm.

[0034] <Process S3> Step S2 is followed by step S3, which is a step of subjecting the structure 10, on which the formation of the cladding layer 13 has been completed, to a heat treatment (annealing). The formation of the cladding layer 13 may cause oxygen defects on the surface of the optical waveguide layer 12. For this reason, by subjecting the structure 10, on which the formation of the cladding layer 13 has been completed, to a heat treatment, oxygen is supplied to the optical waveguide layer 12 through the cladding layer 13, and the oxygen defects are compensated for.

[0035] To reduce the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S3 is performed, for example, in air at a temperature of 700°C or less. To reduce the time required for the heat treatment, the heat treatment in step S3 is performed, for example, in air at a temperature of 400°C or more. Note that in step S3, the optical waveguide layer 12 is covered with the cladding layer 13, which reduces the efficiency of oxygen supply to the optical waveguide layer 12. For this reason, the lower limit of the annealing temperature in step S3 is higher than the lower limit of the annealing temperature in step S13. The annealing time in step S3 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance by experiment or the like. The higher the annealing temperature, the shorter the annealing time.

[0036] In this way, the optical waveguide element 1 is manufactured.

[0037] In the method M1 described above, a heat treatment is performed after the cladding layer 13 covering the optical waveguide layer 12 is formed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 when forming the cladding layer 13, oxygen is supplied to the optical waveguide layer 12 through the cladding layer 13 by the heat treatment. This compensates for the oxygen defects, thereby reducing the propagation loss of the optical waveguide element 1. As described above, the method M1 makes it possible to manufacture an optical waveguide element 1 with low propagation loss.

[0038] The temperature (annealing temperature) of the heat treatment performed on the structure 10 on which the cladding layer 13 is formed is 400 to 700 degrees C. In this case, the time required for the heat treatment can be shortened while reducing the possibility of cracks occurring in the optical waveguide layer 12.

[0039] In step S1, a heat treatment is performed after the optical waveguide layer 12 is formed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 due to etching, oxygen is supplied to the optical waveguide layer 12 by the heat treatment. This compensates for the oxygen defects, thereby further reducing the propagation loss of the optical waveguide element 1. Therefore, it is possible to manufacture an optical waveguide element 1 with even lower propagation loss.

[0040] When the crystalline material constituting the optical waveguide layer 12 is lithium niobate or lithium tantalate, the optical waveguide layer 12 has an excellent electro-optic effect.

[0041] When the optical waveguide layer 12 has a c-axis orientation, an electric field is applied to the ridge portion 12a of the ridge shape in the stacking direction (Z-axis direction) of the optical waveguide layer 12 relative to the substrate 11. This allows the ridge portion 12a to be curved, improving the degree of freedom in designing the optical waveguide layer 12. By forming the crystal film 20 by sputtering on the substrate 11 made of sapphire, the crystal film 20 having a c-axis orientation is formed. This simplifies the manufacture of the optical waveguide element 1.

[0042] Silicon oxide has a relatively low refractive index. Therefore, when the cladding layer 13 is made of silicon oxide, the possibility of confining light in the optical waveguide layer 12 can be increased. As a result, the propagation loss of the optical waveguide element 1 can be further reduced.

[0043] Next, a method for manufacturing an optical waveguide element according to another embodiment will be described with reference to Fig. 3 and Fig. 4(a) and (b). Fig. 3 is a process diagram showing a method for manufacturing an optical waveguide element according to another embodiment. Fig. 4(a) is a diagram illustrating a process for depositing a portion of the cladding layer. Fig. 4(b) is a diagram illustrating a process for depositing the remainder of the cladding layer. Method M2 shown in Fig. 3 is a method for manufacturing an optical waveguide element, and differs from method M1 mainly in that cladding layer 13 is deposited separately into portion 13a and remainder 13b. Method M2 includes steps S21 to S25.

[0044] <Process S21> Step S21 is a step of preparing the structure 10. Step S21 is the same as step S1, so a detailed description thereof will be omitted.

[0045] <Process S22> Step S22 follows step S21. Step S22 is a step of depositing portion 13a of cladding layer 13. As shown in FIG. 4(a), portion 13a is formed over the entire upper surface of optical waveguide layer 12 so as to cover optical waveguide layer 12. The constituent material of portion 13a is the same as the constituent material of cladding layer 13 described above. The film thickness of portion 13a is substantially uniform over the entire portion 13a. From the viewpoint of making the film thickness of portion 13a uniform, the film thickness of portion 13a is, for example, 10 nm or more. From the viewpoint of increasing the rate at which oxygen permeates portion 13a (transmittance), the film thickness of portion 13a is, for example, 100 nm or less. The film thickness of portion 13a is, for example, 1% or more and 20% or less of the film thickness of cladding layer 13.

[0046] <Process S23> Step S22 is followed by step S23. Step S23 is a step of subjecting the structure 10, on which the portion 13a of the cladding layer 13 has been formed, to a heat treatment (annealing). The formation of the portion 13a of the cladding layer 13 may cause oxygen defects on the surface of the optical waveguide layer 12. Therefore, by subjecting the structure 10, on which the portion 13a has been formed, to a heat treatment, oxygen is supplied to the optical waveguide layer 12 via the portion 13a, and the oxygen defects are compensated for.

[0047] To reduce the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S23 is performed in the atmosphere at a temperature of 700°C or less. To reduce the time required for the heat treatment, the heat treatment in step S23 is performed in the atmosphere at a temperature of 400°C or more. The annealing time in step S23 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance by experiment or the like. The higher the annealing temperature, the shorter the annealing time.

[0048] <Process S24> Step S23 is followed by step S24. Step S24 is a step of depositing the remaining portion 13b of the cladding layer 13. As shown in FIG. 4(b), the remaining portion 13b is formed over the entire upper surface of the portion 13a so as to cover the portion 13a after the heat treatment in step S23. The constituent material of the remaining portion 13b is the same as the constituent material of the cladding layer 13 described above. The film thickness of the remaining portion 13b is substantially uniform over the entire portion 13b. The film thickness of the remaining portion 13b is greater than the film thickness of the portion 13a. The film thickness of the remaining portion 13b is, for example, 0.4 μm to 0.99 μm.

[0049] <Process S25> Step S24 is followed by step S25. Step S25 is a step of subjecting the structure 10, on which the formation of the cladding layer 13 is completed by depositing the remaining portion 13b, to a heat treatment (annealing). Because the portion 13a is thin, the deposition of the remaining portion 13b may cause oxygen defects on the surface of the optical waveguide layer 12. Therefore, by subjecting the structure 10 on which the remaining portion 13b has been deposited to a heat treatment, oxygen is supplied to the optical waveguide layer 12 through the cladding layer 13, and the oxygen defects are compensated for.

[0050] In order to reduce the time required for the heat treatment while reducing the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S25 is performed, for example, in the atmosphere at a temperature of 550 to 650 degrees. The annealing time in step S25 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance through experiments, etc. The higher the annealing temperature, the shorter the annealing time. For example, when the annealing temperature is 550 degrees, the annealing time is about 7 hours. When the annealing temperature is 650 degrees, the annealing time is about 5 hours.

[0051] This completes the manufacturing process of the optical waveguide device 1. Step S25 may be omitted.

[0052] The above-described method M2 also achieves the same effects as method M1 in the same process (configuration) as method M1. In method M2, the cladding layer 13 covering the optical waveguide layer 12 is formed in two stages: a portion 13a and a remaining portion 13b. After the portion 13a is formed, a heat treatment is performed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 when the portion 13a is formed, oxygen is supplied to the optical waveguide layer 12 through the portion 13a by the heat treatment. This compensates for the oxygen defects, thereby reducing the propagation loss of the optical waveguide device 1. In method M2, since the heat treatment is performed after the portion 13a is formed, oxygen is more likely to pass through the portion 13a than when the heat treatment is performed after the entire cladding layer 13 is formed. In other words, oxygen is efficiently supplied to the optical waveguide layer 12, thereby shortening the time required for the heat treatment. As described above, method M2 enables the manufacture of optical waveguide devices with low propagation loss while improving manufacturing efficiency.

[0053] Since the film thickness of the portion 13a is smaller than the film thickness of the remaining portion 13b, oxygen can easily pass through the portion 13a. In other words, oxygen is more efficiently supplied to the optical waveguide layer 12. This further improves the manufacturing efficiency of the optical waveguide element 1 with low propagation loss.

[0054] When the film thickness of the portions 13a is 10 nm to 100 nm, the portions 13a can be formed to a uniform film thickness, and the rate at which oxygen passes through the portions 13a (transmittance) can be increased.

[0055] When the temperature of the heat treatment performed on the structure 10 on which the portion 13a has been formed is 400 to 700 degrees, the time required for the heat treatment can be shortened while reducing the possibility of cracks occurring in the optical waveguide layer 12.

[0056] When depositing the remaining portion 13b, oxygen defects may occur on the surface of the optical waveguide layer 12. In method M2, a heat treatment is performed on the structure 10 after the formation of the cladding layer 13 is completed by depositing the remaining portion 13b. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 when depositing the remaining portion 13b, oxygen is supplied to the optical waveguide layer 12 through the cladding layer 13 by the heat treatment. This compensates for the oxygen defects, thereby further reducing the propagation loss of the optical waveguide element 1. Therefore, it is possible to manufacture an optical waveguide element 1 with even lower propagation loss.

[0057] When the temperature of the heat treatment applied to the structure 10 after the deposition of the cladding layer 13 is 550 to 650 degrees, the possibility of cracks occurring in the optical waveguide layer 12 can be reduced and the time required for the heat treatment can be shortened.

[0058] Next, a method for manufacturing an optical waveguide element according to yet another embodiment will be described with reference to FIGS. 5 and 6(a) to 6(c). FIG. 5 is a process diagram illustrating a method for manufacturing an optical waveguide element according to yet another embodiment. FIG. 6(a) is a diagram illustrating a process for planarizing a cladding layer. FIG. 6(b) is a diagram illustrating a process for forming a buffer layer. FIG. 6(c) is a diagram illustrating a process for forming an electrode. Method M3 shown in FIG. 5 is a method for manufacturing an optical waveguide element 1A. The optical waveguide element 1A is an optical modulation element having a c-axis orientation. Therefore, the optical waveguide layer 12 has a c-axis orientation. Method M3 differs from Method M2 mainly in that it includes steps S26 to S30 instead of step S25. Steps S26 to S30 will be described here.

[0059] <Process S26> Step S24 is followed by step S26, which is a step of planarizing the cladding layer 13. As shown in FIG. 6(a), the cladding layer 13 (the upper surface of the cladding layer 13) is planarized by ion milling, chemical mechanical polishing (CMP), or the like, until the top surface of the ridge portion 12a is exposed. This results in the cladding layer 13A.

[0060] <Process S27> Step S26 is followed by step S27. Step S27 is a step of subjecting the structure 10 to a heat treatment (annealing) after the cladding layer 13 has been planarized. Oxygen defects may occur on the surface of the optical waveguide layer 12 due to the deposition of the remaining portion 13b and the planarization of the cladding layer 13. Therefore, by subjecting the structure 10 to a heat treatment, oxygen is supplied to the optical waveguide layer 12.

[0061] To reduce the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S27 is performed in the atmosphere at a temperature of 700°C or less. To reduce the time required for the heat treatment, the heat treatment in step S27 is performed in the atmosphere at a temperature of 400°C or more. The annealing time in step S27 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance by experiment or the like. The higher the annealing temperature, the shorter the annealing time.

[0062] <Process S28> Step S27 is followed by step S28. Step S28 is a step of depositing a buffer layer 14 on the cladding layer 13A. The buffer layer 14 is provided to suppress light absorption by the electrodes 15 to 17, which will be described later. As shown in FIG. 6(b), the buffer layer 14 is formed over the entire upper surface of the cladding layer 13A and the top surface of the ridge portion 12a so as to cover the cladding layer 13A and the ridge portion 12a. The buffer layer 14 is made of a material having a refractive index lower than that of the material constituting the optical waveguide layer 12. Examples of materials constituting the buffer layer 14 include silicon oxides (e.g., SiO2, LaAlSiInO, and SiInO). The thickness of the buffer layer 14 is substantially uniform throughout the buffer layer 14. The thickness of the buffer layer 14 is, for example, 0.4 μm to 1.0 μm.

[0063] <Process S29> Step S28 is followed by step S29. Step S29 is a step of subjecting the structure 10 to a heat treatment (annealing) after the buffer layer 14 has been formed. The formation of the buffer layer 14 may cause oxygen defects on the surface of the optical waveguide layer 12. Therefore, by subjecting the structure 10 to a heat treatment, oxygen is supplied to the optical waveguide layer 12 via the buffer layer 14 and the cladding layer 13A.

[0064] To reduce the possibility of cracks occurring in the optical waveguide layer 12, the heat treatment in step S29 is performed in the atmosphere at a temperature of 700°C or less. To reduce the time required for the heat treatment, the heat treatment in step S29 is performed in the atmosphere at a temperature of 400°C or more. The annealing time in step S29 is set appropriately depending on the annealing temperature. The annealing time for each annealing temperature is determined in advance by experiment or the like. The higher the annealing temperature, the shorter the annealing time.

[0065] <Process S30> Step S29 is followed by step S30. Step S30 is a step of forming electrodes 15 to 17 on the buffer layer 14. The electrode 15 is a signal electrode. The electrode 15 is formed on the ridge portion 12a via the buffer layer 14. The electrode 15 extends along the ridge portion 12a. The electrodes 16 and 17 are ground electrodes. The electrodes 16 and 17 are formed on the buffer layer 14 so as to be spaced apart from the electrode 15 and so that the electrode 15 is located between the electrodes 16 and 17. The electrodes 15 to 17 are made of a metal material.

[0066] In this way, the optical waveguide element 1A is manufactured. Method M3 may include steps S1 to S3 of method M1 instead of steps S21 to S24.

[0067] The method M3 described above also achieves the same effects as the method M2 in the processes (configurations) common to the method M2. In the method M3, the heat treatment is performed after the cladding layer 13 is planarized, and the heat treatment is performed after the buffer layer 14 is formed. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 due to the formation of the remaining portion 13b, the planarization of the cladding layer 13, and the formation of the buffer layer 14, oxygen is supplied to the optical waveguide layer 12 by the heat treatment. This compensates for the oxygen defects, thereby reducing the propagation loss of the optical waveguide element 1A. Therefore, it is possible to manufacture an optical waveguide element 1A with low propagation loss.

[0068] The method for manufacturing an optical waveguide element according to the present disclosure is not limited to the above embodiment.

[0069] For example, the annealing temperature in step S13 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12, and may be lower than 200°C or higher than 700°C. The annealing temperature in step S3 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12 and the structure and constituent materials of the cladding layer 13, and may be lower than 400°C or higher than 700°C.

[0070] The annealing temperature in step S23 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12 and the structure (film thickness) and constituent materials of the part 13a of the cladding layer 13, and may be lower than 400°C or higher than 700°C. The annealing temperature in step S25 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12 and the structure and constituent materials of the cladding layer 13, and may be lower than 550°C or higher than 650°C.

[0071] The annealing temperature in step S27 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12 and the structure and constituent materials of the cladding layer 13A, and may be lower than 400° C. or higher than 700° C. The annealing temperature in step S29 may be changed as appropriate depending on the structure and constituent materials of the optical waveguide layer 12, the structure and constituent materials of the cladding layer 13, and the structure (film thickness) and constituent materials of the buffer layer 14, and may be lower than 400° C. or higher than 700° C.

[0072] The present disclosure will be described in more detail below with reference to examples to illustrate the above-mentioned effects, but the present disclosure is not limited to these examples.

[0073] <Insertion loss evaluation> The evaluation of insertion loss will be described with reference to Figures 7(a) and 7(b). Figure 7(a) is a diagram showing the relationship between annealing time and insertion loss when the annealing temperature in step S3 shown in Figure 1 is 400 degrees. Figure 7(b) is a diagram showing the relationship between annealing time and insertion loss when the annealing temperature in step S3 shown in Figure 1 is 500 degrees. The horizontal axis of Figures 7(a) and 7(b) represents annealing time (unit: hours). The vertical axis of Figures 7(a) and 7(b) represents insertion loss (unit: dB).

[0074] Three optical waveguide elements fabricated by method M1 shown in FIG. 1, with waveguide lengths of 5.6 mm, 9.4 mm, and 12.4 mm, were used to evaluate insertion loss. The waveguide width and height of these three optical waveguide elements were 0.7 μm and 0.7 μm, respectively. The waveguide length is the length of the ridge portion 12 a in the Y-axis direction. The waveguide width is the length of the ridge portion 12 a in the X-axis direction, and may be referred to as the "waveguide width W" below. The waveguide height is the length of the ridge portion 12 a in the Z-axis direction. In these three optical waveguide elements, sapphire was used as the material for the substrate 11, lithium niobate (LiNbO) was used as the material for the optical waveguide layer 12, and silicon dioxide (SiO) was used as the material for the cladding layer 13. The insertion loss of each of the optical waveguide elements obtained by heat treatment (annealing) at annealing temperatures of 400°C and 500°C for 6, 12, and 18 hours was measured by irradiating light having a wavelength of 637 nm onto each element.

[0075] As shown in Figures 7(a) and 7(b), it can be seen that at both annealing temperatures, the insertion loss increases as the waveguide length increases for the same annealing time. At both annealing temperatures, it can be seen that at the same waveguide length, the insertion loss decreases as the annealing time increases, but the decrease in insertion loss per unit time (decrease rate) decreases.

[0076] <Propagation loss evaluation> The evaluation of propagation loss will be described with reference to Figures 8 and 9. Figure 8 is a diagram showing the propagation loss when the annealing temperature in step S3 shown in Figure 1 is 500 degrees. Figure 9 is a diagram showing the propagation loss when the annealing temperature in step S3 shown in Figure 1 is 700 degrees. The horizontal axis in Figures 8 and 9 represents the waveguide length (unit: cm). The vertical axis in Figures 8 and 9 represents the insertion loss (unit: dB).

[0077] The propagation loss is calculated as the slope of a linear function obtained by approximating the insertion loss with a linear function of the waveguide length. The function Fr shown in Figure 8 is a function obtained by approximating the insertion loss measured when light having a wavelength of 637 nm is incident on the three optical waveguide elements used in the insertion loss evaluation of Figure 7(b) after two hours of heat treatment at an annealing temperature of 500°C, using a linear function of the waveguide length. The function Fr is expressed as y = 9.95x + 9.16, where x is the waveguide length and y is the insertion loss. Therefore, the propagation loss after two hours of heat treatment at an annealing temperature of 500°C is approximately 10 dB / cm. Note that none of the optical waveguide elements propagated light before the heat treatment in step S3. Therefore, the propagation loss after two hours of heat treatment at an annealing temperature of 500°C is used for comparison.

[0078] The function F1 shown in Figure 8 is a function obtained by approximating the insertion loss measured when light having a wavelength of 637 nm is incident on the three optical waveguide elements obtained by additionally subjecting the three optical waveguide elements to an annealing temperature of 500 degrees for 16 hours (i.e., after 18 hours of heat treatment), using a linear function of the waveguide length. The function F1 is expressed as y = 2.43x + 7.75, where x is the waveguide length and y is the insertion loss. Therefore, the propagation loss after the heat treatment is approximately 2.4 dB / cm. From the above, it can be seen that the propagation loss is reduced by the heat treatment in step S3.

[0079] The function F2 shown in Figure 9 was obtained by approximating the insertion loss measured when 637 nm light was incident on three optical waveguide devices obtained by annealing at 700°C for six hours with a linear function of the waveguide length. The three optical waveguide devices used to obtain function F2 differed from the three optical waveguide devices used for the evaluation in Figure 8 only in the annealing temperature and annealing time in step S3; the materials and structure of each layer were the same. Function F2 is expressed as y = 2.82x + 9.86, where x is the waveguide length and y is the insertion loss. Therefore, the propagation loss after the heat treatment was approximately 2.8 dB / cm. This propagation loss is similar to the propagation loss after 18 hours of heat treatment at 500°C, indicating that the heat treatment in step S3 reduces the propagation loss.

[0080] The function F3 shown in Figure 9 was obtained by approximating the insertion loss measured when light having a wavelength of 637 nm was incident on three optical waveguide elements obtained by heat-treating three optical waveguide elements, each differing only in its waveguide width W from the three optical waveguide elements used to obtain function F2, with a linear function of the waveguide length. The waveguide width W of these three optical waveguide elements was 1 μm. The function F3 is expressed as y = 3.53x + 7.03, where x is the waveguide length and y is the insertion loss. Therefore, the propagation loss after the heat treatment is approximately 3.5 dB / cm. This propagation loss is similar to that when the waveguide width W is 0.7 μm, and therefore it is believed that the heat treatment in step S3 reduces the propagation loss.

[0081] <How to calculate annealing time> An example of a method for calculating the annealing time will be described with reference to Fig. 7(a), Fig. 7(b), Fig. 10, and Fig. 11. Fig. 10 is a diagram showing an Arrhenius plot. Fig. 11 is a diagram showing the calculation results of the annealing time. The horizontal axis of Fig. 10 represents the reciprocal of the absolute temperature T (unit: K -1) The vertical axis of FIG. 10 represents the natural logarithm of time τ. The horizontal axis of FIG. 11 represents the annealing temperature (unit: deg. C). The vertical axis of FIG. 11 represents the annealing time (unit: hour).

[0082] As shown in Figures 7(a) and 7(b), the insertion loss IL monotonically decreases exponentially with the annealing time t. Therefore, the insertion loss IL can be expressed by equation (1) using the initial optical loss A, the time constant B, the optical loss unaffected by annealing C, and the annealing time t. By least-squares fitting, the insertion losses shown in Figures 7(a) and 7(b) can be expressed by equation (1), and the time τ can be calculated. The time τ is the annealing time t at which the decrease in the insertion loss IL per hour (decrease rate) falls below 0.1 dB.

number

[0083] The time τ when the annealing temperature is 400°C and the time τ when the annealing temperature is 500°C are determined, and these are used to create the Arrhenius plot shown in Figure 10. Here, the average of the times τ determined for each of the three types of waveguide lengths shown in Figure 7(a) is used as the time τ when the annealing temperature is 400°C. Similarly, the average of the times τ determined for each of the three types of waveguide lengths shown in Figure 7(b) is used as the time τ when the annealing temperature is 500°C.

[0084] The Arrhenius plot graph shown in FIG. 10 is expressed by equation (2) using activation energy Ea, Boltzmann constant k, and absolute temperature T. Therefore, activation energy Ea can be obtained by multiplying the slope of the graph by the Boltzmann constant k. Here, the graph shown in FIG. 10 is expressed as y = 4863.9x - 4.3, where x is the reciprocal of absolute temperature and y is the natural logarithm of time τ. Therefore, activation energy Ea is 0.42 eV (9.66 kcal / mol).

number

[0085] As shown in FIG. 11, the time τ at any absolute temperature T can be calculated by substituting the activation energy Ea into equation (2). The time τ calculated at each absolute temperature T can be used as the annealing time when that absolute temperature T is the annealing temperature. According to FIG. 11, when the annealing temperature is 400°C, the annealing time is approximately 20 hours, but when the annealing temperature is 300°C, the annealing time is approximately 100 hours. It can be seen that when the annealing temperature is lowered below 400°C, the annealing time increases sharply.

[0086] (Addendum) [Article 1] A step of preparing a structure including a substrate and a ridge-shaped optical waveguide layer formed on the substrate and made of a crystalline material having an electro-optic effect; forming a cladding layer covering the optical waveguide layer; a step of subjecting the structure on which the cladding layer has been formed to a heat treatment; A method for manufacturing an optical waveguide element, comprising:

[0087] [Clause 2] Item 1, wherein the temperature of the heat treatment is 400 degrees or more and 700 degrees or less.

[0088] [Article 3] A step of preparing a structure including a substrate and a ridge-shaped optical waveguide layer formed on the substrate and made of a crystalline material having an electro-optic effect; forming a portion of a cladding layer covering the optical waveguide layer; a step of subjecting the structure on which the portion of the cladding layer has been deposited to a heat treatment; After the heat treatment, depositing a remainder of the cladding layer so as to cover the portion of the cladding layer; A method for manufacturing an optical waveguide element, comprising:

[0089] [Article 4] 4. The method for manufacturing an optical waveguide element according to claim 3, wherein the thickness of the portion of the cladding layer is smaller than the thickness of the remaining portion of the cladding layer.

[0090] [Article 5] Item 3 or Item 4. The method for manufacturing an optical waveguide element according to item 3 or 4, wherein the thickness of the portion of the cladding layer is 10 nm or more and 100 nm or less.

[0091] [Article 6] The method for manufacturing an optical waveguide element according to any one of clauses 3 to 5, wherein the temperature of the heat treatment applied to the structure on which the portion of the cladding layer has been deposited is 400 degrees or higher and 700 degrees or lower.

[0092] [Article 7] The method for manufacturing an optical waveguide element according to any one of clauses 3 to 6, further comprising a step of subjecting the structure, after the deposition of the cladding layer has been completed by depositing the remaining portion of the cladding layer, to a heat treatment.

[0093] [Article 8] 8. The method for manufacturing an optical waveguide element according to claim 7, wherein the temperature of the heat treatment performed on the structure after the deposition of the cladding layer is 550 degrees or more and 650 degrees or less.

[0094] [Article 9] The step of preparing the structure includes: forming a crystalline film made of the crystalline material on the substrate; forming the optical waveguide layer by etching the crystal film; a step of subjecting the optical waveguide layer to a heat treatment; Item 10. A method for producing the optical waveguide element according to any one of items 1 to 8, comprising:

[0095] [Article 10] 10. The method for manufacturing an optical waveguide element according to any one of claims 1 to 9, wherein the crystalline material is lithium niobate or lithium tantalate.

[0096] [Article 11] 11. The method for producing an optical waveguide element according to any one of items 1 to 10, wherein the optical waveguide layer has a c-axis orientation.

[0097] [Article 12] 12. The method for manufacturing an optical waveguide element according to any one of claims 1 to 11, wherein the cladding layer is made of silicon oxide.

[0098] [Article 13] planarizing the cladding layer; subjecting the structure to a heat treatment after the cladding layer is planarized; depositing a buffer layer on the planarized cladding layer; subjecting the structure to a heat treatment after the buffer layer is deposited; forming an electrode on the buffer layer; 13. The method for producing an optical waveguide element according to any one of claims 1 to 12, further comprising: [Explanation of symbols]

[0099] 1, 1A...optical waveguide element, 10...structure, 11...substrate, 12...optical waveguide layer, 12a...ridge portion, 13, 13A...cladding layer, 13a...part, 13b...remaining portion, 14...buffer layer, 15, 16, 17...electrodes.

Claims

1. A step of preparing a structure including a substrate and a ridge-shaped optical waveguide layer formed on the substrate and made of a crystalline material having an electro-optic effect; forming a cladding layer covering the optical waveguide layer; a step of subjecting the structure on which the cladding layer has been formed to a heat treatment; A method for manufacturing an optical waveguide element, comprising:

2. 2. The method for manufacturing an optical waveguide element according to claim 1, wherein the temperature of the heat treatment is 400 degrees or more and 700 degrees or less.

3. A step of preparing a structure including a substrate and a ridge-shaped optical waveguide layer formed on the substrate and made of a crystalline material having an electro-optic effect; forming a portion of a cladding layer covering the optical waveguide layer; a step of subjecting the structure on which the portion of the cladding layer has been deposited to a heat treatment; After the heat treatment, depositing a remainder of the cladding layer so as to cover the portion of the cladding layer; A method for manufacturing an optical waveguide element, comprising:

4. The method for manufacturing an optical waveguide element according to claim 3 , wherein the thickness of the portion of the cladding layer is smaller than the thickness of the remaining portion of the cladding layer.

5. The method for manufacturing an optical waveguide element according to claim 3 , wherein the thickness of the portion of the cladding layer is 10 nm or more and 100 nm or less.

6. The method for manufacturing an optical waveguide element according to any one of claims 3 to 5, wherein the temperature of the heat treatment applied to the structure on which the portion of the cladding layer has been formed is 400 degrees or more and 700 degrees or less.

7. The method for manufacturing an optical waveguide element according to any one of claims 3 to 5, further comprising a step of performing a heat treatment on the structure after the deposition of the clad layer has been completed by depositing the remaining portion of the clad layer.

8. The method for manufacturing an optical waveguide element according to claim 7, wherein the temperature of the heat treatment performed on the structure after the formation of the cladding layer is 550°C or more and 650°C or less.

9. The step of preparing the structure includes: forming a crystalline film made of the crystalline material on the substrate; forming the optical waveguide layer by etching the crystal film; a step of subjecting the optical waveguide layer to a heat treatment; A method for producing the optical waveguide element according to any one of claims 1 to 5, comprising:

10. 6. The method for manufacturing an optical waveguide element according to claim 1, wherein the crystalline material is lithium niobate or lithium tantalate.

11. 6. The method for manufacturing an optical waveguide element according to claim 1, wherein the optical waveguide layer has a c-axis orientation.

12. 6. The method for manufacturing an optical waveguide element according to claim 1, wherein the cladding layer is made of silicon oxide.

13. planarizing the cladding layer; subjecting the structure to a heat treatment after the cladding layer is planarized; depositing a buffer layer on the planarized cladding layer; subjecting the structure to a heat treatment after the buffer layer is deposited; forming an electrode on the buffer layer; The method for producing an optical waveguide element according to any one of claims 1 to 5, further comprising:

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