Method for manufacturing optical waveguide element

Abstract

The invention provides a method for manufacturing an optical waveguide element. A method for manufacturing an optical waveguide element includes: preparing a structure including a substrate and a ridge-shaped optical waveguide layer that is provided on the substrate and is composed of a crystal material having an electro-optical effect; forming a cladding entirety covering the optical waveguide layer by using a formed film; and performing a heat treatment on the structure after the formation of the cladding layer.

Description

[0001] Cross-reference of related applications

[0002] This application claims priority to Japanese Patent Application No. 2024-101110 filed with the Japan Patent Office on June 24, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to a method for manufacturing an optical waveguide element. Background Technology

[0004] An optical waveguide element is known, comprising: a substrate; an optical waveguide layer disposed on the substrate, which is composed of a crystalline material with electro-optic effect, such as a lithium niobate film; and a cladding layer that covers the optical waveguide layer. As a method for manufacturing such an optical waveguide element, for example, Japanese Patent No. 6136666 describes a method for obtaining a low-loss optical waveguide by etching a lithium niobate film formed on a substrate to form ridges and then performing heat treatment. Summary of the Invention

[0005] The problem the invention aims to solve

[0006] When forming a cladding layer on an optical waveguide, oxygen defects sometimes occur on the surface of the waveguide layer, resulting in the loss of the oxygen self-crystalline material from its crystal structure. This raises concerns about increased transmission loss in the fabricated optical waveguide element.

[0007] This disclosure describes a method for manufacturing an optical waveguide element capable of producing an optical waveguide element with low transmission loss.

[0008] One aspect of the present disclosure discloses a method for manufacturing an optical waveguide element, comprising the following steps: preparing a structure including a substrate and a ridge-shaped optical waveguide layer disposed on the substrate and made of a crystal material having an electro-optic effect; forming a cladding over the optical waveguide layer by film deposition; and performing heat treatment on the structure after the film deposition of the cladding is completed.

[0009] In this method for manufacturing an optical waveguide element, heat treatment is performed after a cladding layer covering the optical waveguide layer is formed using film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer during cladding formation, oxygen is supplied to the optical waveguide layer via the cladding layer through heat treatment. This compensates for the oxygen defects, thereby reducing the transmission loss of the optical waveguide element. Based on the above, an optical waveguide element with low transmission loss can be manufactured according to the above method for manufacturing an optical waveguide element.

[0010] Alternatively, the heat treatment temperature can be above 400 degrees Celsius and below 700 degrees Celsius. In this case, the heat treatment time can be shortened while reducing the possibility of cracks forming in the optical waveguide layer.

[0011] Another aspect of the method for manufacturing an optical waveguide element disclosed herein includes the following steps: preparing a structure comprising a substrate and a ridge-shaped optical waveguide layer disposed on the substrate and composed of a crystal material having an electro-optic effect; forming a portion of a cladding covering the optical waveguide layer by film deposition; performing heat treatment on the structure after the portion of the cladding has been formed by film deposition; and forming the remaining portion of the cladding by film deposition after the heat treatment, in a manner covering a portion of the cladding.

[0012] In this method for manufacturing an optical waveguide element, a cladding covering the optical waveguide layer is formed in two stages: a portion and the remainder. Heat treatment is performed after a portion of the cladding has been formed using film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer during the formation of the portion of the cladding using film deposition, oxygen is supplied to the optical waveguide layer via the portion of the cladding through heat treatment. This compensates for the oxygen defects, thereby reducing the transmission loss of the optical waveguide element. Because heat treatment is performed after a portion of the cladding has been formed using film deposition, oxygen can more easily permeate through the portion of the cladding compared to the case where heat treatment is performed after the entire cladding has been formed using film deposition. In other words, oxygen is supplied to the optical waveguide layer efficiently, thus shortening the time required for heat treatment. Based on the above, according to the above method for manufacturing an optical waveguide element, it is possible to manufacture an optical waveguide element with low transmission loss while improving manufacturing efficiency.

[0013] Alternatively, a portion of the cladding layer may be thinner than the remaining portion. In this case, oxygen can more easily permeate through a portion of the cladding layer. This means oxygen is supplied to the optical waveguide layer more efficiently, thus further improving the manufacturing efficiency of low-loss optical waveguide devices.

[0014] Alternatively, a portion of the cladding layer may have a film thickness of 10 nm or more but less than 100 nm. In this case, it is possible to increase the proportion (permeability) of oxygen permeating a portion of the cladding layer while forming a portion of the cladding layer with a uniform film thickness.

[0015] Alternatively, the heat treatment performed on the structure after a portion of the cladding has been formed using film deposition can be carried out at a temperature of 400 degrees Celsius or higher but below 700 degrees Celsius. In this case, the possibility of cracks forming in the optical waveguide layer can be reduced while shortening the time required for heat treatment.

[0016] Alternatively, the manufacturing method of the aforementioned optical waveguide element may further include the following step: performing heat treatment on the structure after the cladding film is formed by utilizing the remaining portion of the film to form the cladding. In this case, the heat treatment is performed after the cladding is formed using the film. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer during the formation of the remaining portion of the cladding using the film, oxygen is supplied to the optical waveguide layer via the cladding through the heat treatment. This compensates for the oxygen defects, thereby further reducing the transmission loss of the optical waveguide element. Consequently, it is possible to manufacture an optical waveguide element with lower transmission loss.

[0017] Alternatively, the heat treatment performed on the structure after the cladding film is formed can be carried out at a temperature between 550°C and 650°C. In this case, the possibility of cracks forming in the optical waveguide layer can be reduced, and the time required for the heat treatment can be shortened.

[0018] Alternatively, the process of preparing the structure may include the following steps: forming a crystalline film made of a crystalline material on a substrate; forming an optical waveguide layer by etching the crystalline film; and performing heat treatment on the optical waveguide layer. In this case, heat treatment is performed after the optical waveguide layer has been formed. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer due to etching, oxygen is supplied to the optical waveguide layer through heat treatment. This compensates for the oxygen defects, thereby further reducing the transmission loss of the optical waveguide element. Consequently, it is possible to manufacture an optical waveguide element with lower transmission loss.

[0019] Alternatively, the crystal material can be lithium niobate or lithium tantalate. Excellent electro-optic effects are obtained by using these crystal materials.

[0020] Alternatively, the optical waveguide layer can 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. Therefore, it is possible to bend the optical waveguide, etc., which can increase the degree of freedom in the design of the optical waveguide layer.

[0021] Alternatively, the cladding can be made of silicon oxide. Because silicon oxide has a relatively low refractive index, it increases the likelihood of confining light within the optical waveguide layer. This further reduces the transmission loss of the optical waveguide element.

[0022] Alternatively, the manufacturing method of the aforementioned optical waveguide element may further include the following steps: planarizing the cladding; performing heat treatment on the structure after planarizing the cladding; forming a buffer layer on the planarized cladding using film deposition; performing heat treatment on the structure after forming the buffer layer using film deposition; and forming electrodes on the buffer layer. In this case, heat treatment is performed after planarizing the cladding and after forming the buffer layer using film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer due to the planarization of the cladding and the film deposition of the buffer layer, oxygen is supplied to the optical waveguide layer through heat treatment. This compensates for the oxygen defects, thereby further reducing the transmission loss of the optical waveguide element. Therefore, it is possible to manufacture an optical waveguide element with lower transmission loss.

[0023] According to various aspects and embodiments of this disclosure, it is possible to manufacture optical waveguide elements with low transmission loss. Attached Figure Description

[0024] Figure 1 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to one embodiment.

[0025] Figure 2 (a) is a diagram illustrating the process of forming a crystal film.

[0026] Figure 2 (b) is a diagram illustrating the process of forming the optical waveguide layer.

[0027] Figure 2 (c) is a diagram illustrating the process of forming a coating using film formation.

[0028] Figure 3 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to another embodiment.

[0029] Figure 4 (a) is a diagram illustrating the process of forming a portion of the coating using film formation.

[0030] Figure 4 (b) is a diagram illustrating the process of forming the remaining portion of the coating using film formation.

[0031] Figure 5 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to another embodiment.

[0032] Figure 6 (a) is a diagram illustrating the process of planarizing the cladding.

[0033] Figure 6 (b) is a diagram illustrating the process of forming a buffer layer using film deposition.

[0034] Figure 6(c) is a diagram illustrating the process of forming the electrode.

[0035] Figure 7 (a) means Figure 1 The graph shows the relationship between annealing time and insertion loss when the annealing temperature of process S3 is 400 degrees.

[0036] Figure 7 (b) means Figure 1 The graph shows the relationship between annealing time and insertion loss when the annealing temperature of process S3 is 500 degrees.

[0037] Figure 8 It means Figure 1 The graph shows the transmission loss when the annealing temperature of process S3 is 500 degrees.

[0038] Figure 9 It means Figure 1 The graph shows the transmission loss when the annealing temperature of process S3 is 700 degrees.

[0039] Figure 10 This is a diagram representing the Arrhenius curve.

[0040] Figure 11 This is a graph showing the calculated results of the annealing time. Detailed Implementation

[0041] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings. Furthermore, in the description of the drawings, the same reference numerals are used to denote the same elements, and repeated descriptions are omitted. An XYZ coordinate system is sometimes shown in the figures. The Y-axis direction is the direction that intersects (e.g., is orthogonal) the X-axis and Z-axis directions. The Z-axis direction is the direction that intersects (e.g., is orthogonal) the X-axis and Y-axis directions. In this specification, the numerical range indicated by “~” represents the range within which the values ​​described before and after the “~” are included as the minimum and maximum values, respectively. The separately described upper and lower limits can be arbitrarily combined.

[0042] Reference Figure 1 and Figure 2 (a)~ Figure 2 (c) describes a method for manufacturing an optical waveguide element according to one embodiment. Figure 1 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to one embodiment. Figure 2 (a) is a diagram illustrating the process of forming a crystal film. Figure 2 (b) is a diagram illustrating the process of forming the optical waveguide layer. Figure 2 (c) is a diagram illustrating the process of forming a coating using film formation. Figure 1The method M1 shown is a method for manufacturing optical waveguide components. Method M1 includes steps S1 to S3.

[0043] <Process S1>

[0044] Process S1 is the process of preparing the structure 10. The structure 10 includes a substrate 11 and an optical waveguide layer 12 (see reference). Figure 2 (b)). The substrate 11 functions as a lower cladding layer. The substrate 11 is made of a material having a refractive index lower than that of the constituent material of the optical waveguide layer 12. Examples of constituent materials of the substrate 11 include sapphire and silicon oxide. Silicon can also be used as a constituent material of the substrate 11. In this case, a buffer layer is formed in the silicon, which has a refractive index lower than that of the constituent material of the optical waveguide layer 12. 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 surfaces defined by the X-axis direction and the Y-axis direction, intersecting the Z-axis direction (orthogonal in this embodiment).

[0045] The optical waveguide layer 12 is a ridge-shaped optical waveguide disposed on the substrate 11. Specifically, the optical waveguide layer 12 is disposed on the main surface 11a of the substrate 11. The optical waveguide layer 12 is made of a crystalline material exhibiting electro-optic effects. Examples of crystalline materials exhibiting electro-optic effects include lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). For example, in the case of Li... x NbO z When representing the composition of lithium niobate, x can be 0.9–1.05 and z can be 2.8–3.2. For lithium (Li) and niobium (Nb), up to 10% can be replaced with other elements. Examples of other elements used for substitution 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). Combinations of two or more of these elements can also be used for substitution. The same applies to the composition of lithium tantalate. The optical waveguide layer 12 can have either a c-axis orientation or an a-axis orientation. In other words, the optical waveguide layer 12 can be made of either a Z-cut or an X-cut crystal material. The optical waveguide layer 12 includes a convex ridge 12a and a flat plate 12b.

[0046] Process S1 includes processes S11 to S13.

[0047] <Process S11>

[0048] Process S11 is the process of forming a crystal film 20 on the substrate 11, which serves as the precursor to the optical waveguide layer 12. For example... Figure 2 As shown in (a), in process S11, a substrate 11 is first prepared, and a crystal film 20 is formed on the main surface 11a of the substrate 11. The crystal film 20 is composed of the aforementioned crystal material and is formed by epitaxial growth of the crystal material. Examples of methods for forming the crystal film 20 include sputtering and CVD (chemical vapor deposition).

[0049] <Process S12>

[0050] Next, step S12 is performed after step S11. Step S12 is the process of forming the optical waveguide layer 12 by etching the crystal film 20. Specifically, in step S12, a mask pattern corresponding to the ridge 12a is first formed on the crystal film 20. Then, the portion of the crystal film 20 not covered by the mask pattern is etched to a depth corresponding to the height of the ridge 12a by dry etching. After that, the mask pattern is removed. Thus, as... Figure 2 As shown in (b), an optical waveguide layer 12 is formed on the main surface 11a of the substrate 11.

[0051] <Process S13>

[0052] Next, step S13 is performed after step S12. Step S13 is a heat treatment (annealing) process for the optical waveguide layer 12. Due to the etching in step S12, oxygen defects sometimes occur on the surface of the optical waveguide layer 12, resulting in the loss of oxygen from the crystal structure of the crystalline material. Therefore, by performing heat treatment on the optical waveguide layer 12, oxygen is supplied to the optical waveguide layer 12 to compensate for the oxygen defects.

[0053] From the viewpoint of reducing the possibility of cracks forming in the optical waveguide layer 12, heat treatment of process S13 is performed in the atmosphere at a temperature below 700 degrees Celsius. From the viewpoint of suppressing the time required for heat treatment, heat treatment of process S13 is performed in the atmosphere at a temperature above 200 degrees Celsius. In heat treatment below 200 degrees Celsius, the reaction rate is slow, and oxygen cannot be adequately supplied to the optical waveguide layer 12. The following may refer to the heat treatment temperature as the "annealing temperature" and the heat treatment time as the "annealing time." Unless otherwise specified, temperature refers to Celsius (°C). The annealing time of process S13 is appropriately set according to 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.

[0054] Through the above processes S11 to S13, the structure 10 is manufactured and prepared. Heat treatment of the crystal film 20 may also be performed between processes S11 and S12. Process S13 may also be omitted. Furthermore, since oxygen is supplied to the optical waveguide layer 12 through heat treatment to compensate for oxygen defects, there may be cases where the optical waveguide layer 12 after heat treatment differs from the optical waveguide layer 12 before heat treatment in composition, etc. However, for ease of explanation, the same reference numerals are used to denote the optical waveguide layer 12 before and after heat treatment. The same applies in the following description.

[0055] <Process S2>

[0056] Next, step S2 is performed after step S1. Step S2 is the process of forming the entire coating 13 using film deposition. The coating 13 functions as the upper coating layer. Figure 2 As shown in (c), the cladding 13 is formed over the entire upper surface of the optical waveguide layer 12, covering it. The cladding 13 is made of a material having a refractive index lower than that of the constituent material of the optical waveguide layer 12. Examples of the constituent material of the cladding 13 include silicon oxide (e.g., SiO2, LaAlSiInO, and SiInO). The film thickness of the cladding 13 is substantially uniform throughout its entirety. The film thickness of the cladding 13 is, for example, 0.5 μm to 1.0 μm.

[0057] <Process S3>

[0058] Next, step S2 proceeds to step S3. Step S3 is a process of heat treatment (annealing) performed on the structure 10 after the cladding 13 has been formed. Due to the formation of the cladding 13, oxygen defects sometimes occur on the surface of the optical waveguide layer 12. Therefore, by performing heat treatment on the structure 10 after the cladding 13 has been formed, oxygen is supplied to the optical waveguide layer 12 through the cladding 13 to compensate for the oxygen defects.

[0059] From the viewpoint of reducing the possibility of cracks forming in the optical waveguide layer 12, the heat treatment of process S3 is performed, for example, at a temperature below 700 degrees Celsius in the atmosphere. From the viewpoint of suppressing the time required for heat treatment, the heat treatment of process S3 is performed, for example, at a temperature above 400 degrees Celsius in the atmosphere. Furthermore, in process S3, since the optical waveguide layer 12 is covered by the cladding layer 13, the oxygen supply efficiency to the optical waveguide layer 12 decreases. Therefore, the lower limit of the annealing temperature for process S3 is higher than the lower limit of the annealing temperature for process S13. The annealing time for process S3 is appropriately set according to 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.

[0060] The above processes are used to manufacture optical waveguide element 1.

[0061] In method M1 described above, heat treatment is performed after the cladding 13 covering the optical waveguide layer 12 is formed by film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer 12 during the formation of the cladding 13 by film deposition, oxygen is supplied to the optical waveguide layer 12 via the cladding 13 through heat treatment. This compensates for the oxygen defects, thereby reducing the transmission loss of the optical waveguide element 1. In view of the above, according to method M1, it is possible to manufacture an optical waveguide element 1 with low transmission loss.

[0062] The heat treatment temperature (annealing temperature) performed on the structure 10 after the cladding 13 is formed by film deposition is 400 to 700 degrees Celsius. In this case, the possibility of cracking in the optical waveguide layer 12 can be reduced while the heat treatment time can be shortened.

[0063] In process S1, heat treatment is performed after the optical waveguide layer 12 is formed. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer 12 due to etching, oxygen is supplied to the optical waveguide layer 12 through heat treatment. This compensates for the oxygen defects, thereby further reducing the transmission loss of the optical waveguide element 1. As a result, it is possible to manufacture an optical waveguide element 1 with lower transmission loss.

[0064] When the crystal material constituting the optical waveguide layer 12 is lithium niobate or lithium tantalate, an optical waveguide layer 12 with excellent electro-optic effect can be obtained.

[0065] When the optical waveguide layer 12 has a c-axis orientation, an electric field is applied to the ridge 12a in the stacking direction (Z-axis direction) of the optical waveguide layer 12 relative to the substrate 11. Therefore, the ridge 12a can be bent, etc., increasing the design freedom of the optical waveguide layer 12. A crystal film 20 with a c-axis orientation is formed by sputtering onto the sapphire substrate 11. Therefore, the fabrication of the optical waveguide element 1 can be simplified.

[0066] Silicon oxide has a relatively low refractive index. Therefore, when the cladding 13 is made of silicon oxide, the possibility of confining light within the optical waveguide layer 12 can be increased. As a result, the transmission loss of the optical waveguide element 1 can be further reduced.

[0067] Next, refer to Figure 3 and Figure 4 (a)~ Figure 4 (b) describes a method for manufacturing an optical waveguide element according to another embodiment. Figure 3 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to another embodiment. Figure 4 (a) is a diagram illustrating the process of forming a portion of the coating using film formation. Figure 4 (b) is a diagram illustrating the process of forming the remaining portion of the coating using film formation. Figure 3Method M2, as shown, is a method for manufacturing optical waveguide components. Its main difference from method M1 lies in the fact that the cladding 13 is divided into a portion 13a and the remaining portion 13b for film formation. Method M2 includes steps S21 to S25.

[0068] <Process S21>

[0069] Process S21 is the process for preparing the structure 10. Since process S21 is the same as process S1, its detailed description is omitted.

[0070] <Process S22>

[0071] Next, step S22 is performed after step S21. Step S22 is the process of forming a portion 13a of the coating 13 using film formation. For example... Figure 4 As shown in (a), a portion 13a is formed on the entire upper surface of the optical waveguide layer 12 in a manner that covers the optical waveguide layer 12. The constituent material of portion 13a is the same as that of the cladding layer 13 described above. The film thickness of portion 13a is substantially uniform over its entire surface. 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 proportion of oxygen passing through 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 the cladding layer 13.

[0072] <Process S23>

[0073] Next, step S23 is performed after step S22. Step S23 is a process of heat treatment (annealing) performed on the structure 10 after a portion 13a of the cladding 13 has been formed by film deposition. Due to the film deposition of the portion 13a of the cladding 13, oxygen defects sometimes occur on the surface of the optical waveguide layer 12. Therefore, by performing heat treatment on the structure 10 after the portion 13a has been formed by film deposition, oxygen is supplied to the optical waveguide layer 12 via the portion 13a to compensate for the oxygen defects.

[0074] From the viewpoint of reducing the possibility of cracks forming in the optical waveguide layer 12, heat treatment of process S23 is performed, for example, at a temperature below 700 degrees Celsius in the atmosphere. From the viewpoint of suppressing the time required for heat treatment, heat treatment of process S23 is performed, for example, at a temperature above 400 degrees Celsius in the atmosphere. The annealing time of process S23 is appropriately set according to the annealing temperature. The annealing time relative to each annealing temperature is determined in advance through experiments, etc. The higher the annealing temperature, the shorter the annealing time.

[0075] <Process S24>

[0076] Next, step S24 is performed after step S23. Step S24 is the step of forming the remaining portion 13b of the coating 13 using film formation. For example... Figure 4 As shown in (b), after the heat treatment in step S23, the remaining portion 13b is formed on the entire upper surface of the portion 13a in a manner that covers the portion 13a. The constituent material of the remaining portion 13b is the same as that of the cladding layer 13 described above. The film thickness of the remaining portion 13b is substantially uniform over its entire area. The film thickness of the remaining portion 13b is greater than that of the portion 13a. The film thickness of the remaining portion 13b is, for example, 0.4 μm to 0.99 μm.

[0077] <Process S25>

[0078] Next, step S25 is performed after step S24. Step S25 is a process of heat treatment (annealing) performed on the structure 10 after the film formation of the remaining portion 13b by film formation is completed. Since a portion 13a is relatively thin, oxygen defects sometimes occur on the surface of the optical waveguide layer 12 due to the film formation of the remaining portion 13b. Therefore, by performing heat treatment on the structure 10 after the remaining portion 13b is formed by film formation, oxygen is supplied to the optical waveguide layer 12 through the cladding layer 13 to compensate for the oxygen defects.

[0079] To reduce the likelihood of crack formation in the optical waveguide layer 12 while minimizing the time required for heat treatment, process S25 is heat-treated at a temperature of 550°C to 650°C in air. The annealing time for process S25 is appropriately set based on the annealing temperature. The annealing time relative to each annealing temperature is determined in advance through experiments, etc. The higher the annealing temperature, the shorter the annealing time. For example, at an annealing temperature of 550°C, the annealing time is approximately 7 hours. At an annealing temperature of 650°C, the annealing time is approximately 5 hours.

[0080] The optical waveguide element 1 is manufactured through the above processes. Process S25 can also be omitted.

[0081] In method M2 described above, the same effect as method M1 can be achieved in the processing (structure) common to method M1. In method M2, the cladding 13 covering the optical waveguide layer 12 is formed in two stages: a portion 13a and the remaining portion 13b. Heat treatment is performed after the portion 13a is formed by film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer 12 during the formation of the portion 13a by film deposition, oxygen is supplied to the optical waveguide layer 12 via the portion 13a through heat treatment. This compensates for the oxygen defects, thereby reducing the transmission loss of the optical waveguide element 1. In method M2, since heat treatment is performed after the portion 13a is formed by film deposition, oxygen is more easily transmitted through the portion 13a compared to the case where heat treatment is performed after the entire cladding 13 is formed by film deposition. In other words, oxygen is supplied to the optical waveguide layer 12 efficiently, thus shortening the time required for heat treatment. In view of the above, according to method M2, it is possible to manufacture an optical waveguide element with low transmission loss while improving manufacturing efficiency.

[0082] Because the film thickness of a portion 13a is smaller than that of the remaining portion 13b, oxygen can easily pass through that portion 13a. In other words, oxygen is supplied to the optical waveguide layer 12 more efficiently. Therefore, the manufacturing efficiency of the low-transmission-loss optical waveguide element 1 can be further improved.

[0083] When the film thickness of a portion of 13a is 10 nm to 100 nm, it is possible to increase the proportion (transmittance) of oxygen permeating the portion of 13a while forming a portion of 13a with a uniform film thickness.

[0084] When the heat treatment temperature of the structure 10 after a portion 13a has been formed by film deposition is 400 to 700 degrees, the time required for heat treatment can be shortened while reducing the possibility of cracks in the optical waveguide layer 12.

[0085] When forming the remaining portion 13b using film deposition, oxygen defects sometimes occur on the surface of the optical waveguide layer 12. In method M2, the structure 10 after the film deposition of the cladding 13 by forming the remaining portion 13b using film deposition is performed is subjected to heat treatment. Therefore, even if oxygen defects occur on the surface of the optical waveguide layer 12 when forming the remaining portion 13b using film deposition, oxygen is supplied to the optical waveguide layer 12 via the cladding 13 through heat treatment. This compensates for the oxygen defects, thereby further reducing the transmission loss of the optical waveguide element 1. As a result, it is possible to manufacture an optical waveguide element 1 with lower transmission loss.

[0086] When the heat treatment of the structure 10 after the cladding 13 is formed is performed at a temperature of 550°C to 650°C, the possibility of cracks in the optical waveguide layer 12 can be reduced, and the time required for the heat treatment can be shortened.

[0087] Next, refer to Figure 5 and Figure 6 (a)~ Figure 6 (c) describes a method for manufacturing an optical waveguide element according to another embodiment. Figure 5 This is a process diagram illustrating a method for manufacturing an optical waveguide element according to another embodiment. Figure 6 (a) is a diagram illustrating the process of planarizing the cladding. Figure 6 (b) is a diagram illustrating the process of forming a buffer layer using film deposition. Figure 6 (c) is a diagram illustrating the process of forming the electrode. Figure 5 Method M3 shown is a method for manufacturing optical waveguide element 1A. Optical waveguide element 1A is an optical modulation element with c-axis orientation. Therefore, optical waveguide layer 12 has 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.

[0088] <Process S26>

[0089] Next, step S24 proceeds to step S26. Step S26 is the step that flattens the cladding layer 13. For example... Figure 6 As shown in (a), the cladding 13 is planarized by exposing the top surface of the ridge 12a through processes such as ion milling and chemical mechanical polishing (CMP). Cladding 13A is thus obtained.

[0090] <Process S27>

[0091] Next, step S27 is performed after step S26. Step S27 is a process of heat treatment (annealing) of the structure 10 after planarizing the cladding 13. Due to the film formation of the remaining portion 13b and the planarization of the cladding 13, oxygen defects sometimes occur on the surface of the optical waveguide layer 12. Therefore, oxygen is supplied to the optical waveguide layer 12 by performing heat treatment on the structure 10.

[0092] From the viewpoint of reducing the possibility of cracks forming in the optical waveguide layer 12, heat treatment of process S27 is performed in the atmosphere at a temperature below 700 degrees Celsius, for example. From the viewpoint of suppressing the time required for heat treatment, heat treatment of process S27 is performed in the atmosphere at a temperature above 400 degrees Celsius, for example. The annealing time of process S27 is appropriately set according to the annealing temperature. The annealing time relative to each annealing temperature is determined in advance through experiments, etc. The higher the annealing temperature, the shorter the annealing time.

[0093] <Process S28>

[0094] Next, step S27 proceeds to step S28. Step S28 is the process of forming a buffer layer 14 on the cladding 13A using film deposition. The buffer layer 14 is provided to suppress light absorption by the electrodes 15-17, which will be described later. Figure 6 As shown in (b), the buffer layer 14 is formed on the upper surface of the cladding 13A and the entire top surface of the ridge 12a, covering the cladding 13A and the ridge 12a. The buffer layer 14 is made of a material having a refractive index lower than that of the constituent material of the optical waveguide layer 12. Examples of the constituent material of the buffer layer 14 include silicon oxide (e.g., SiO2, LaAlSiInO, and SiInO). The film thickness of the buffer layer 14 is substantially uniform throughout the buffer layer 14. The film thickness of the buffer layer 14 is, for example, 0.4 μm to 1.0 μm.

[0095] <Process S29>

[0096] Next, step S29 is performed after step S28. Step S29 is a step of heat treatment (annealing) of the structure 10 after the buffer layer 14 has been formed by film deposition. Due to the film deposition of the buffer layer 14, oxygen defects sometimes occur on the surface of the optical waveguide layer 12. Therefore, by performing heat treatment on the structure 10, oxygen is supplied to the optical waveguide layer 12 through the buffer layer 14 and the cladding 13A.

[0097] From the viewpoint of reducing the possibility of cracks forming in the optical waveguide layer 12, heat treatment of process S29 is performed in the atmosphere at a temperature below 700 degrees Celsius, for example. From the viewpoint of suppressing the time required for heat treatment, heat treatment of process S29 is performed in the atmosphere at a temperature above 400 degrees Celsius, for example. The annealing time of process S29 is appropriately set according to the annealing temperature. The annealing time relative to each annealing temperature is determined in advance through experiments, etc. The higher the annealing temperature, the shorter the annealing time.

[0098] <Process S30>

[0099] Next, step S29 proceeds to step S30. Step S30 is the process of forming electrodes 15-17 on the buffer layer 14. Electrode 15 is a signal electrode. Electrode 15 is formed on the ridge 12a, separated from the buffer layer 14. Electrode 15 extends along the ridge 12a. Electrodes 16 and 17 are ground electrodes. Electrodes 16 and 17 are formed on the buffer layer 14 separately from electrode 15, with electrode 15 located between electrodes 16 and 17. Electrodes 15-17 are made of metallic material.

[0100] The optical waveguide element 1A is manufactured through the above processes. Method M3 may also include processes S1 to S3 of method M1 instead of processes S21 to S24.

[0101] In method M3 described above, the same effect as method M2 can be achieved in the same processing (structure) as method M2. In method M3, heat treatment is performed after planarizing the cladding 13 and after forming the buffer layer 14 by film deposition. Therefore, even if oxygen defects are generated on the surface of the optical waveguide layer 12 due to the film deposition of the remaining portion 13b, the planarization of the cladding 13, and the film deposition of the buffer layer 14, oxygen is supplied to the optical waveguide layer 12 through heat treatment. This compensates for the oxygen defects, thereby reducing the transmission loss of the optical waveguide element 1A. Thus, it is possible to manufacture an optical waveguide element 1A with low transmission loss.

[0102] Furthermore, the method for manufacturing the optical waveguide element disclosed herein is not limited to the above-described embodiments.

[0103] For example, the annealing temperature of process S13 can be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12, and can be lower than 200 degrees or higher than 700 degrees. The annealing temperature of process S3 can also be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12 and the cladding 13, and can be lower than 400 degrees or higher than 700 degrees.

[0104] The annealing temperature of process S23 can be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12 and the structure (film thickness) and constituent materials of a portion 13a of the cladding layer 13, etc. It can be lower than 400 degrees or higher than 700 degrees. The annealing temperature of process S25 can also be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12 and the structure and constituent materials of the cladding layer 13, etc. It can be lower than 550 degrees or higher than 650 degrees.

[0105] The annealing temperature of process S27 can also be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12 and the cladding layer 13A, etc., and can be lower than 400 degrees or higher than 700 degrees. The annealing temperature of process S29 can also be appropriately changed according to the structure and constituent materials of the optical waveguide layer 12, the cladding layer 13, and the structure (film thickness) and constituent materials of the buffer layer 14, etc., and can be lower than 400 degrees or higher than 700 degrees.

[0106] To illustrate the above effects, the present disclosure is described in more detail below through embodiments. The present disclosure is not limited to these embodiments.

[0107] <Evaluation of Insertion Loss>

[0108] Reference Figure 7 (a) and Figure 7 (b) describes the evaluation of insertion loss. Figure 7 (a) means Figure 1The graph shows the relationship between annealing time and insertion loss when the annealing temperature of process S3 is 400 degrees. Figure 7 (b) means Figure 1 The graph shows the relationship between annealing time and insertion loss when the annealing temperature of process S3 is 500 degrees. Figure 7 (a) and Figure 7 The horizontal axis of (b) represents the annealing time (in hours). Figure 7 (a) and Figure 7 The vertical axis of (b) represents the insertion loss (unit: dB).

[0109] Use via Figure 1 Insertion loss was evaluated for three optical waveguide elements with waveguide lengths of 5.6 mm, 9.4 mm, and 12.4 mm manufactured using method M1. The waveguide width and height of these three waveguide elements were both 0.7 μm and 0.7 μm respectively. The waveguide length is the length along the Y-axis of the ridge 12a. The waveguide width is the length along the X-axis of the ridge 12a, referred to below as "waveguide width W". The waveguide height is the length along the Z-axis of the ridge 12a. In these three optical waveguide elements, sapphire was used as the substrate material 11, lithium niobate (LiNbO3) was used as the optical waveguide layer material 12, and silicon dioxide (SiO2) was used as the cladding material 13. Insertion loss was measured by incidenting light with a wavelength of 637 nm onto the optical waveguide elements obtained by heat treatment (annealing) at 400°C and 500°C for 6 hours, 12 hours, and 18 hours respectively.

[0110] like Figure 7 (a) and Figure 7 As shown in (b), it can be seen that at any annealing temperature and with the same annealing time, the insertion loss increases with increasing waveguide length. It can also be seen that at any annealing temperature and with the same waveguide length, the insertion loss decreases with increasing annealing time, but the rate of decrease in insertion loss per unit time decreases.

[0111] <Evaluation of transmission loss>

[0112] Reference Figure 8 and Figure 9 Explain the evaluation of transmission loss. Figure 8 It means Figure 1 The graph shows the transmission loss when the annealing temperature of process S3 is 500 degrees. Figure 9 It means Figure 1 The graph shows the transmission loss when the annealing temperature of process S3 is 700 degrees. Figure 8 and Figure 9The horizontal axis represents the waveguide length (unit: cm). Figure 8 and Figure 9 The vertical axis represents insertion loss (unit: dB).

[0113] The transmission loss is calculated as the slope of a linear function approximating the insertion loss as a linear function of the waveguide length. Figure 8 The function Fr shown is obtained by approximating the insertion loss measured when light with a wavelength of 637 nm is incident on three optical waveguide elements, using a linear function of the waveguide length: these three optical waveguide elements are... Figure 7 The three optical waveguide elements used in the evaluation of insertion loss in (b) were those subjected to a heat treatment at an annealing temperature of 500 degrees Celsius for two hours. The waveguide length was set as x, and the insertion loss as y, expressed as the function Fr by y = 9.95x + 9.16. Therefore, the transmission loss at the time of heat treatment at 500 degrees Celsius for two hours was approximately 10 dB / cm. Furthermore, before the heat treatment in step S3, none of the optical waveguide elements transmitted light; therefore, the transmission loss at the time of heat treatment at 500 degrees Celsius for two hours was used as the comparison object.

[0114] Figure 8 The function F1 shown is obtained by approximating the insertion loss measured when light with a wavelength of 637 nm is incident on three optical waveguide elements as a linear function of the waveguide length. These three optical waveguide elements are obtained by subjecting the aforementioned three optical waveguide elements to an annealing temperature of 500 degrees Celsius for an additional 16 hours of heat treatment (i.e., at the time of the 18-hour heat treatment). Let the waveguide length be x and the insertion loss be y, then function F1 is expressed as y = 2.43x + 7.75. Therefore, the transmission loss after heat treatment is approximately 2.4 dB / cm. Based on the above, it can be seen that the transmission loss is reduced through the heat treatment in step S3.

[0115] Figure 9 The function F2 shown is obtained by approximating the insertion loss measured when light with a wavelength of 637 nm is incident on three optical waveguide elements as a linear function of the waveguide length. These three optical waveguide elements were obtained by heat treatment at an annealing temperature of 700 degrees Celsius for 6 hours. The three optical waveguide elements used to obtain function F2 are... Figure 8The three optical waveguide elements used in the evaluation differ only in the annealing temperature and time in process S3; the constituent materials and structures of each layer are identical. Let the waveguide length be x and the insertion loss be y, then the function F2 is expressed as y = 2.82x + 9.86. Therefore, the transmission loss after heat treatment is approximately 2.8 dB / cm. Since this transmission loss is comparable to that after 18 hours of heat treatment at 500 degrees Celsius, it can be concluded that the heat treatment in process S3 reduces the transmission loss.

[0116] Figure 9 The function F3 shown is obtained by approximating the insertion loss measured when light with a wavelength of 637 nm is incident on three optical waveguide elements, which are obtained by heat-treating them at 700 degrees Celsius for 6 hours. These three waveguide elements differ from the three waveguide elements used to obtain function F2 only in their waveguide width W, and are different in waveguide width W from the three waveguide elements used to obtain function F2. The waveguide width W of these three waveguide elements is 1 μm. Let the waveguide length be x and the insertion loss be y, then function F3 is expressed as y = 3.53x + 7.03. Therefore, the transmission loss after heat treatment is approximately 3.5 dB / cm. Since this transmission loss is similar to that in the case where the waveguide width W is 0.7 μm, it is considered that the transmission loss is reduced by the heat treatment in step S3.

[0117] <Method for Calculating Annealing Time>

[0118] Reference Figure 7 of (a), Figure 7 (b) Figure 10 and Figure 11 An example illustrating the method for calculating annealing time. Figure 10 This is a diagram representing the Arrhenius curve. Figure 11 This is a graph showing the calculated results of the annealing time. Figure 10 The horizontal axis represents the reciprocal of the absolute temperature T (unit: K). -1 ). Figure 10 The vertical axis represents the natural logarithm of time τ. Figure 11 The horizontal axis represents the annealing temperature (unit: deg.C). Figure 11 The vertical axis represents the annealing time (in hours).

[0119] like Figure 7 (a) and Figure 7As shown in (b), the insertion loss IL decreases monotonically as a function of exponential function with respect to 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. Through least squares fitting, it is expressed by equation (1) respectively. Figure 7 (a) and Figure 7 Given the insertion loss shown in (b), calculate the time τ. Time τ is the annealing time t during which the decrease (rate of decrease) of the insertion loss IL per hour is less than 0.1 dB.

[0120] [Formula 1]

[0121] IL=Aexp(Bt)+C Equation (1)

[0122] Calculate the time τ for annealing at 400 degrees Celsius and the time τ for annealing at 500 degrees Celsius, and use these times to produce... Figure 10 The Arrhenius curve is shown here. (Using a specific method for...) Figure 7 The average of the time τ calculated for the three waveguide lengths shown in (a) is taken as the time τ when the annealing temperature is 400 degrees. Similarly, using the values ​​for... Figure 7 The average time τ calculated for the three waveguide lengths shown in (b) is taken as the time τ when the annealing temperature is 500 degrees.

[0123] Figure 10 The Arrhenius curve shown is represented by the activation energy Ea, Boltzmann constant k, and absolute temperature T using equation (2). Therefore, the activation energy Ea is obtained by multiplying the slope of the curve by the Boltzmann constant k. Here, the reciprocal of the absolute temperature is set as x, and the natural logarithm of time τ is set as y, represented by y = 4863.9x - 4.3. Figure 10 The curve shown is an example of this. Therefore, the activation energy Ea is 0.42 eV (9.66 kcal / mol).

[0124] [Formula 2]

[0125]

[0126] like Figure 11 As shown, the time τ at any absolute temperature T is obtained by substituting the activation energy Ea into equation (2). The time τ obtained at each absolute temperature T can be used as the annealing time when that absolute temperature T is set as the annealing temperature. Figure 11 At an annealing temperature of 400 degrees Celsius, the annealing time is approximately 20 hours, but at an annealing temperature of 300 degrees Celsius, the annealing time is approximately 100 hours. This shows that if the annealing temperature is lower than 400 degrees Celsius, the annealing time increases dramatically.

[0127] (Postscript)

[0128] [Project 1]

[0129] A method for manufacturing an optical waveguide element, wherein,

[0130] The manufacturing method includes the following steps:

[0131] Prepare a structure including a substrate and a ridge-shaped optical waveguide layer disposed on the substrate, the optical waveguide layer being made of a crystalline material with electro-optic effect;

[0132] The cladding layer covering the optical waveguide layer is formed by film deposition; and

[0133] The structure is subjected to heat treatment after the coating film is formed.

[0134] [Project 2]

[0135] According to the manufacturing method of the optical waveguide element described in Project 1, wherein,

[0136] The heat treatment temperature is above 400 degrees and below 700 degrees.

[0137] [Project 3]

[0138] A method for manufacturing an optical waveguide element, wherein,

[0139] The manufacturing method includes the following steps:

[0140] Prepare a structure including a substrate and a ridge-shaped optical waveguide layer disposed on the substrate, the optical waveguide layer being made of a crystalline material with electro-optic effect;

[0141] A portion of the cladding layer covering the optical waveguide layer is formed by film deposition.

[0142] The structure after the cladding layer has been formed by film deposition is subjected to heat treatment; and

[0143] The remaining portion of the cladding is formed by film formation after the heat treatment, in a manner that covers the portion of the cladding.

[0144] [Project 4]

[0145] According to the manufacturing method of the optical waveguide element described in Project 3, wherein,

[0146] The thickness of a portion of the cladding is smaller than the thickness of the remaining portion of the cladding.

[0147] [Project 5]

[0148] According to the manufacturing method of the optical waveguide element described in Project 3 or Project 4, wherein,

[0149] The thickness of the portion of the cladding is greater than 10 nm and less than 100 nm.

[0150] [Project 6]

[0151] The method for manufacturing an optical waveguide element according to any one of items 3 to 5, wherein,

[0152] The heat treatment performed on the structure after the cladding layer has been formed by film formation is performed at a temperature of 400 degrees or higher and 700 degrees or lower.

[0153] [Project 7]

[0154] The method for manufacturing an optical waveguide element according to any one of items 3 to 6, wherein,

[0155] The manufacturing method further includes the following steps: performing heat treatment on the structure after the film formation of the cladding is completed by forming the remaining portion of the cladding using film formation.

[0156] [Project 8]

[0157] According to the manufacturing method of the optical waveguide element described in Project 7, wherein,

[0158] The heat treatment performed on the structure after the coating film is formed is at a temperature of 550 degrees or higher and 650 degrees or lower.

[0159] [Project 9]

[0160] The method for manufacturing an optical waveguide element according to any one of items 1 to 8, wherein,

[0161] The process of preparing the structure includes the following steps:

[0162] A crystalline film made of the crystalline material is formed on the substrate;

[0163] The optical waveguide layer is formed by etching the crystal film; and

[0164] The optical waveguide layer is subjected to heat treatment.

[0165] [Project 10]

[0166] The method for manufacturing an optical waveguide element according to any one of items 1 to 9, wherein,

[0167] The crystal material is lithium niobate or lithium tantalate.

[0168] [Project 11]

[0169] The method for manufacturing an optical waveguide element according to any one of items 1 to 10, wherein,

[0170] The optical waveguide layer has a c-axis orientation.

[0171] [Project 12]

[0172] The method for manufacturing an optical waveguide element according to any one of items 1 to 11, wherein the cladding is composed of silicon oxide.

[0173] [Project 13]

[0174] The manufacturing method of the optical waveguide element according to any one of items 1 to 12, wherein the manufacturing method further includes the following steps:

[0175] To flatten the cladding layer;

[0176] After planarizing the cladding, the structure is subjected to heat treatment;

[0177] A buffer layer is formed on the planarized cladding by film deposition;

[0178] After the buffer layer is formed by film deposition, the structure is subjected to heat treatment; and electrodes are formed on the buffer layer.

Claims

1. A method for manufacturing an optical waveguide element, wherein, The manufacturing method includes the following steps: Prepare a structure including a substrate and a ridge-shaped optical waveguide layer disposed on the substrate, the optical waveguide layer being made of a crystalline material with electro-optic effect; The cladding layer covering the optical waveguide layer is formed by film deposition; and The structure is subjected to heat treatment after the coating film is formed.

2. The method for manufacturing an optical waveguide element according to claim 1, wherein, The heat treatment temperature is above 400 degrees and below 700 degrees.

3. A method for manufacturing an optical waveguide element, wherein, The manufacturing method includes the following steps: Prepare a structure including a substrate and a ridge-shaped optical waveguide layer disposed on the substrate, the optical waveguide layer being made of a crystalline material with electro-optic effect; A portion of the cladding layer covering the optical waveguide layer is formed by film deposition. The structure after the cladding layer has been formed by film formation is subjected to heat treatment; as well as The remaining portion of the cladding is formed by film formation after the heat treatment, in a manner that covers the portion of the cladding.

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

5. The method for manufacturing an optical waveguide element according to claim 3 or 4, wherein, The thickness of the portion of the cladding is greater than 10 nm and less than 100 nm.

6. The method for manufacturing the optical waveguide element according to any one of claims 3 to 5, wherein, The heat treatment performed on the structure after the cladding layer has been formed by film formation is performed at a temperature of 400 degrees or higher and 700 degrees or lower.

7. The method for manufacturing an optical waveguide element according to any one of claims 3 to 6, wherein, The manufacturing method further includes the following steps: performing heat treatment on the structure after the film formation of the cladding is completed by forming the remaining portion of the cladding using film formation.

8. The method for manufacturing an optical waveguide element according to claim 7, wherein, The heat treatment performed on the structure after the coating film is formed is at a temperature of 550 degrees or higher and 650 degrees or lower.

9. A method for manufacturing an optical waveguide element according to any one of claims 1 to 8, wherein, The process of preparing the structure includes the following steps: A crystalline film made of the crystalline material is formed on the substrate; The optical waveguide layer is formed by etching the crystal film; and The optical waveguide layer is subjected to heat treatment.

10. A method for manufacturing an optical waveguide element according to any one of claims 1 to 9, wherein, The crystal material is lithium niobate or lithium tantalate.

11. A method for manufacturing an optical waveguide element according to any one of claims 1 to 10, wherein, The optical waveguide layer has a c-axis orientation.

12. A method for manufacturing an optical waveguide element according to any one of claims 1 to 11, wherein, The cladding is made of silicon oxide.

13. A method for manufacturing an optical waveguide element according to any one of claims 1 to 12, wherein, The manufacturing method also includes the following steps: To flatten the cladding layer; After planarizing the cladding, the structure is subjected to heat treatment; A buffer layer is formed on the planarized cladding by film deposition; After the buffer layer is formed by film deposition, the structure is subjected to heat treatment; as well as Electrodes are formed on the buffer layer.

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