JP2001311844A - Method for manufacturing optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive method for manufacturing an optical waveguide in which generation of voids is securely prevented, the degree of freedom for selecting materials and dopants to be used is made greater and moreover the loss of signal light is reduced by removing residual internal stresses in respective film forming layers. SOLUTION: For example, glass to which dopant is not added is accumulated by forming a core and a clad layer by a flame deposition method and applying heating/pressurizing treatment. Residual internal stresses in the core and an upper clad layer are removed and moreover voids are closed by applying heating/pressurizing treatment even if the voids are generated because glass does not penetrate into the recessed part or the like. Thus, usable kind of materials for the core and the upper clad layer and dopant is increased and the degree of freedom for selection is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical waveguide applied to a device for transmitting signal light in, for example, optical communication.

[0002]

2. Description of the Related Art Conventionally, as a planar lightwave circuit (PLC) used for optical communication or the like, an optical waveguide such as a so-called optical star coupler has been used as a branching device, for example. This optical waveguide generally has a step index type refractive index distribution in a buried waveguide structure.

As such an optical waveguide, a buffer layer (or a lower cladding layer) on a substrate made of quartz or silicon wafer, a core having a desired shape formed on the buffer layer, and a core having a desired shape are formed. A substrate having an upper cladding layer to cover the substrate, a substrate formed of quartz or the like which also functions as a buffer layer in consideration of the refractive index, and omitting the buffer layer.
As disclosed in Japanese Patent Application Laid-Open No. 04-2004, a recess is formed so as to form a desired core shape on the surface of the substrate or the surface of the buffer layer, a film is formed so as to bury the core in the recess, and a cladding layer is formed thereon. And the like.

For example, to manufacture an optical waveguide having the above structure, first, SiO ₂ is formed on the surface of a substrate by various film forming methods.
After forming a buffer layer and a core layer mainly composed of, and forming a predetermined waveguide pattern on the surface of the core layer with a photoresist, RIE (Reactive Ion) is performed.
A waveguide core having a predetermined pattern is formed by etching or the like. After that, an upper clad layer having a low refractive index containing SiO ₂ as a main component is formed. Finally, it is cut into a predetermined shape by a dicing device. Then, by polishing the light input / output end faces, each optical waveguide is completed.

It is necessary that the refractive index of the core be higher than that of the buffer layer and the upper cladding layer by about 0.2% to 0.8%. For this reason, when forming the core, a dopant for increasing the refractive index is added, or when forming the buffer layer and the upper cladding layer, a dopant for lowering the refractive index is added.

In addition, the manufacturing process of the optical waveguide having the above structure can be said to omit the step of forming the buffer layer from the manufacturing process of the optical waveguide having the above structure. Further, in the manufacturing process of the optical waveguide having the structure described above, in the manufacturing process of the optical waveguide having the structure described above, a step of forming a concave portion by RIE or the like on the surface portion of the substrate or the surface portion of the buffer layer before forming the core. It can be said that the polishing or etching step is added and the core patterning step is omitted.

As a method for forming the buffer layer, the core, and the upper clad layer, for example, Japanese Patent Application Laid-Open No. 58-10511
No. 1, JP-A-61-259205, JP-A-6-259205
A flame deposition method (FHD method) as described in JP-A-2-288802, JP-A-1-189614, JP-A-1-192732, etc., and various chemical vapor deposition methods (CVD)
Methods, such as a plasma CVD method), various physical vapor deposition methods (PVD methods, such as a vacuum vapor deposition method), or vapor deposition methods combining these vapor deposition methods (hereinafter, these vapor deposition methods are collectively referred to as a low-temperature film forming method). ).

The low-temperature film formation method is more advantageous than the flame deposition method described later in controlling the film thickness and uniformly distributing the dopant, but for example, a plurality of cores are arranged close to each other in an optical waveguide having a structure such as described above. In such a case, the glass is not sufficiently filled between the adjacent cores, and a gap (void) may be generated. This void increases the loss of the signal light when the optical waveguide transmits the signal light. is there. Also,
In the optical waveguide having the structure described above, when the core is formed in the concave portion on the surface of the substrate or the buffer layer, the glass is not sufficiently filled as in the above, and the void may be generated as in the above.

On the other hand, in the flame deposition method, glass fine particles are sprayed and deposited on the substrate surface while being heated by a torch or the like. If the viscosity of the glass is reduced during the deposition, the above-mentioned or other structure of the optical waveguide can be obtained. Glass flows into the concave portion of the optical waveguide having a structure between adjacent cores or between the adjacent cores, and the inside thereof is sufficiently filled.

[0010]

However, it is necessary to add a dopant to lower the viscosity at the time of deposition, that is, the softening point of the glass to be deposited, but the addition of the dopant changes the refractive index. Accordingly, it is necessary to separately add a dopant to each layer to adjust the refractive index of each layer, and in practice, the materials to be used, the dopant to be added are limited, and the degree of freedom in selection is low. . In particular, for example, when an optical waveguide having a structure in which a dopant is added to the core and a dopant is not added to the buffer layer and the upper cladding layer is to be manufactured, the softening point of the glass forming the upper cladding layer is high. Because of its high viscosity, glass is not sufficiently filled between adjacent cores at the time of film formation, and voids are generated, which causes the same problem as the low-temperature film formation method.
In addition, since the flame deposition method mixes and sprays the glass particles with the dopant, it is troublesome to evenly distribute the dopant.

Here, it is conceivable to raise the temperature at the time of deposition and lower the viscosity without adding a dopant.
If the temperature is too high, it is not practical because adverse effects such as softening of another layer already laminated and diffusion of a dopant added to a certain layer to another layer or a substrate become a problem. It is also conceivable that the recess is sufficiently filled by heat treatment after film formation. However, in this case, the heat treatment generally needs to be higher than the temperature at the time of deposition. An adverse effect such as dopant diffusion becomes a problem.

Further, for example, it is conceivable to use a low-temperature film forming method only when manufacturing an optical waveguide having a structure in which a dopant is added to the core and no dopant is added to the buffer layer and the upper cladding layer. In addition to the outbreak problem,
A plurality of expensive film forming apparatuses must be prepared according to the optical waveguide to be manufactured, and the cost increases significantly.

On the other hand, apart from the above, the internal stress remains in the layer formed by any method, and the polarization dependent loss (PDL)
May be increased, thereby increasing the loss of signal light.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its main objects to prevent the generation of voids and to increase the degree of freedom in selecting a material and a dopant to be used. It is an object of the present invention to provide an inexpensive method for manufacturing an optical waveguide which can improve and reduce the loss of signal light by removing the residual internal stress of each film formation layer.

[0015]

According to the present invention, there is provided a method for manufacturing an optical waveguide having a core covered by a cladding layer, the method comprising: directly on a surface of a substrate or via a buffer layer; The core layer is formed by a flame deposition method,
After patterning, a cladding layer is formed thereon by a flame deposition method, and then a heating and pressure treatment is performed, or a desired core is formed on the surface of the substrate or the surface of the buffer layer. A concave portion is formed so as to form a shape, a core is formed in the concave portion by a flame deposition method, a heating / pressing process is performed, and a clad layer is formed on the upper portion by a flame deposition method. This is achieved by providing a method of manufacturing an optical waveguide. For example, by depositing glass to which a dopant has not been added, and even if voids are generated without the glass wrapping around the concave portions, the residual internal stress of the core and the upper cladding layer can be obtained by applying heat and pressure. Can be removed and the void can be closed. Here, by performing the treatment at a high pressure, the voids can be closed even if the temperature is lower than that at the time of deposition, and there is no fear of adverse effects such as diffusion of the dopant into another layer.

[0016]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG.
1 is a perspective view of an optical waveguide 1 manufactured by the method of the present invention. The optical waveguide 1 has a buried waveguide structure, and a core 4 is formed in a pattern for branching an optical signal, and is used as a branching device in optical communication, for example. For example, signal light is input from one end 6 (input end) of the core 4 located on the left side in the drawing, and signal light is output from the other end 7 (output end) of the plurality of cores 4 located on the right side in the drawing. ing.

FIG. 2 is a sectional view of the optical waveguide 1 taken along line II-II. A core 4 is formed on a substrate 2 made of glass or ceramic such as quartz via a buffer layer (lower clad layer) 3, and an upper clad layer 5 is formed so as to cover the core 4 and the buffer layer 3 without the core 4. Is formed.

The buffer layer 3 is formed on the substrate 2 into a film having a thickness of, for example, about 25 μm. In addition, core 4
Has a square cross section with a side of about 8 μm, for example. The core 4 is formed to have a slightly higher refractive index than the buffer layer 3 and the upper cladding layer 5, so that signal light used for optical communication or the like passes therethrough. The upper clad layer 5 is formed on the buffer layer 3 and the core 4 in a film shape having a thickness of, for example, about 25 μm so as to cover the core 4 together with the buffer layer 3.

The procedure for manufacturing the optical waveguide 1 will be described below with reference to FIGS. 3 (a) to 3 (e) and FIG. First, SiO _{2 was} deposited on the surface of the substrate 2 by flame deposition.
A buffer layer 3 mainly composed of (FIG. 3
(A)). Here, as shown in FIG. 4, in the present configuration, as a flame deposition method, for example, glass fine particles are supplied into a flame generated by a torch 20 to which oxygen and hydrogen are supplied, heated and synthesized, and the substrate 2 is synthesized. Sprayed and deposited on the surface of
Then, a method of heating to form a film is adopted. Actually, it is not limited to the flame deposition method using the oxyhydrogen burner method,
Any material may be used as long as glass fine particles are supplied into the flame, synthesized, and sprayed onto the substrate surface to form a film.

Next, a core layer 4 'mainly composed of SiO ₂ is formed on the surface of the buffer layer 3 by the same flame deposition method as described above.
Is formed (FIG. 3B). At this time, the refractive index is higher by about 0.2% to 0.8% than that of the buffer layer 3,
Phosphorus (P), titanium (Ti), germanium (Ge),
Aluminum (Al), boron (B) and fluorine (F)
Is added to the core layer 4 '. Where phosphorus (P),
Titanium (Ti), germanium (Ge), and aluminum (Al) are dopants for increasing the refractive index, and boron (B) and fluorine (F) are dopants for decreasing the refractive index. A desired refractive index can be obtained by adding these alone or in combination. In this state, if necessary, heat is applied from the surroundings at a high pressure or a normal pressure to perform preliminary heat treatment.

Here, generally, the refractive index of the core layer 4 'is preferably the same as the refractive index of the core of an optical fiber used for optical communication. For this reason, when only the refractive index of the core layer 4 'is adjusted, the refractive index between the glass substrate 2 and the upper cladding layer 5 cannot be secured, and the same refractive index as that of the core of the optical fiber cannot be obtained. Alternatively, a doping agent for lowering the refractive index may be added during the formation of the buffer layer 3 and a doping agent for lowering the refractive index may also be added during the formation of the upper cladding layer 5 to adjust the respective refractive indices.

Next, a predetermined waveguide pattern is formed on the surface of the core layer 5 by using a photoresist, and etching such as RIE is performed, so that the waveguide core 4 having the predetermined pattern is formed.
Is formed (FIG. 3C).

Thereafter, by the same flame deposition method as described above,
An upper cladding layer 5 having SiO ₂ as a main component and having a low refractive index is formed (FIG. 3D). At this time, if a plurality of cores 4 are formed close to each other, the voids 8 may be generated between the adjacent cores 4 because the SiO ₂ forming the upper cladding layer 5 is not sufficiently filled. In particular, when the dopant for lowering the softening point is not added to the upper cladding layer 5, the tendency becomes remarkable.

For this reason, the optical waveguide 1 is placed in a state where the upper cladding layer 5 is exposed, by a hot isostatic pressing method (HIP method: HIP method).
o Isostatic Pressing,
By heating and pressurizing from the surroundings by simply using the HIP method (heating and pressurizing treatment), the void 8 is reduced to a size having no practical problem or eliminated, and the substrate 2, the layers 3 and 4 are removed. The internal stress generated during the film formation is removed (FIG. 3E). In this HIP method, an inert gas such as argon (Ar) is used and 1500 kgf is used.
/ Cm ² and maintained at a temperature of 1100 ° C. for about 2 hours. Here, when heating and pressing are performed by the HIP method, the entire optical waveguide 1 is uniformly pressed from the surroundings, so that there is no concern that the core shape or the like changes.

The present inventors have observed the disappearance of the void 8 when the temperature condition and the pressure condition in the HIP method were changed parametrically as shown in Table 1. still,
In Table 1, the temperatures are 800 ° C, 1000 ° C and 110 ° C.
0 ° C and the pressure was 300 kgf / cm ² , 100
^{0kgf / cm 2, 1500kgf / cm} 2 and 170
It is changed to 0 kgf / cm ² .

[0026]

[Table 1]

According to Table 1, the pressure is 300 kgf /
cm ² and 1000 kgf / cm ² and the temperature is 80
At 0 ° C., it became clear that it was difficult to eliminate the void 8. The pressure is 1500kgf / cm
^{In the case of 2} , the void 8 can be eliminated when the temperature is 1100 ° C. or higher, and when the pressure is 1700 kgf / cm ² , the void 8 can be eliminated when the temperature is 1000 ° C. or higher. .

Therefore, in the present embodiment, the heating and pressurizing conditions by the HIP method are set such that the pressure is 1500 kgf / cm.
² and the temperature is 1100 ° C, but the pressure is 170
The temperature may be 0 kgf / cm ² and the temperature may be 1000 ° C. or higher. However, in consideration of the diffusion of the dopant into other layers, it is desirable that the processing temperature and the pressure are minimized as long as the voids can be filled, and that heating and pressing more than necessary are avoided. .

Here, when the heating and pressurizing treatment is performed by the HIP method, there is a concern that argon (Ar) may enter the upper clad layer 5 to lower the durability and the like. In the range, that is, when the temperature and pressure are such that the voids 8 are reduced to a size that does not cause a practical problem or are eliminated and the internal stress generated at the time of film formation on the substrate 2, the layers 3, 4, and 5 is removed, It was found by durability evaluation that there was no problem in durability even if argon (Ar) penetrated into the cladding layer 5. Therefore, usually, a protective film temporarily provided in the case of performing the heating and pressurizing treatment by the HIP method can be omitted, and the process is simplified.

Finally, the substrate 2 is cut by a dicing apparatus.
Is cut into a predetermined shape, and the end faces of the input and output ends 6 and 7 are polished to be optically flattened, thereby obtaining individual optical waveguides 1 such as an optical star coupler as shown in FIG.

FIGS. 5A to 5F are process explanatory views showing another embodiment of the present invention. Optical waveguide 1 of this configuration
Reference numeral 1 denotes a recess 13 in the buffer layer 13 formed on the substrate 12.
a is formed, a core 14 is formed in the recess 13a,
The upper clad layer 15 is formed on the surfaces of the buffer layer 13 and the core 14 which are on the same plane.
(F)).

As a manufacturing procedure of the optical waveguide 11, first, a buffer layer 13 is formed on the surface of a glass substrate 12 made of quartz or the like by the same flame deposition method as described above (FIG. 5).
(A)). Then, a predetermined waveguide pattern is formed on the buffer layer 13 by using a photoresist, and etching such as RIE is performed.
a is formed (FIG. 5B).

Next, the core layer 14 'is formed by the same flame deposition method as shown above (FIG. 5C), and the refractive index is about 0.2% to 0.8% higher than that of the glass substrate 12 as described above. Is added to the core layer 1 so as to increase the refractive index.
Add to 4 '. At this time, the core layer 1 is placed in the recess 13a.
Voids 18 may be formed due to insufficient filling of the SiO ₂ forming 4 ′.

Therefore, the optical waveguide 11 is connected to the core layer 14 '.
In the state where is exposed, it is heated from the surroundings by the HIP method,
By applying pressure (heating / pressing treatment), void 1
8 is reduced to a size that does not cause a practical problem, or is eliminated, and the internal stress generated during the film formation on the substrate 12 and the core layer 14 'is removed (FIG. 5D).

Next, by performing etching or polishing, the unnecessary core layer 14 ′ is removed to expose the surface of the substrate 12, and the surface of the core 14 and the surface of the substrate 12 are made flat to make them flat ( FIG. 5 (e).

Then, by the same flame deposition method as described above,
An upper clad layer 5 having a low refractive index mainly composed of SiO ₂ is formed (FIG. 5F). Thereafter, the same cutting, polishing, and the like are performed as described above, and the optical waveguide 11 is completed.

Although the core and the upper clad layer are formed on the substrate via the buffer layer in each of the above-described configurations, the substrate may be a glass substrate of low refractive index such as quartz or the like if the buffer is also used as the buffer layer. It is not necessary to provide a layer, and the core and the upper clad layer can be formed directly on the substrate, and the steps of forming the buffer layer shown in FIG. 3A and FIG. Is done.

[0038]

As is apparent from the above description, according to the method for manufacturing an optical waveguide according to the present invention, the core and the clad layer are formed by the flame deposition method, and the heat and pressure treatment is performed. Even if glass without any dopant added is deposited and the voids are generated without the glass wrapping around the concave parts, the residual internal stress of the core and the upper cladding layer can be reduced by applying heat and pressure treatment. The voids can be removed and the voids can be closed, and the types of materials and dopants used for the core and the upper cladding layer can be increased, and the degree of freedom in selection can be increased. By performing the heat and pressure treatment at a high pressure, the voids can be closed even if the temperature is lower than that at the time of deposition, and there is no fear of adverse effects such as diffusion of the dopant into another layer.

Here, the residual internal stress of the core and the upper clad layer is removed and the void is closed, for example, by HIP.
Even if the heating / pressing treatment by the method is performed in a state where the upper clad layer or the core is exposed on the surface, a small amount of Ar or the like used therein penetrates each layer and does not affect the durability and the like. In addition, the heat and pressure treatment can be performed without a protective film, and the film formation and removal steps are not required, and the process is simplified.

[Brief description of the drawings]

FIG. 1 is a perspective view of an optical star coupler as an optical waveguide to which the present invention is applied.

FIG. 2 is a sectional view taken along line II-II in FIG. 1;

FIGS. 3 (a) to 3 (e) are process explanatory views showing a manufacturing process of the optical waveguide of FIG. 1;

FIG. 4 is a conceptual diagram illustrating a flame deposition method.

FIGS. 5A to 5F are process explanatory views similar to FIG. 3, illustrating another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 optical waveguide 2 glass substrate 3 buffer layer 4 core 4 'core layer 5 upper cladding layer 6, 7 input / output end 8 void 11 optical waveguide 12 glass substrate 13 buffer layer 14 core 14' core layer 15 upper cladding layer 18 void 20 torch

Claims (5)

[Claims] 1. A method for manufacturing an optical waveguide having a core covered by a cladding layer, comprising: forming a core layer on a surface of a substrate directly or through a buffer layer by a flame deposition method, and patterning the core layer. A method for manufacturing an optical waveguide, comprising forming a cladding layer on the upper portion by a flame deposition method, and then performing a heating and pressure treatment. 2. The method of manufacturing an optical waveguide according to claim 1, wherein the heat / pressure treatment is performed in a state where the upper clad layer is exposed on the surface. 3. A method of manufacturing an optical waveguide having a core covered by a cladding layer, wherein a concave portion is formed on the surface of the substrate or the surface of the buffer layer so as to form a desired core shape, and the core is formed in the concave portion. Is formed by a flame deposition method, and then subjected to a heat and pressure treatment, and a cladding layer is formed thereon by a flame deposition method. 4. The method of manufacturing an optical waveguide according to claim 3, wherein the heat / pressure treatment is performed in a state where the core is exposed on the surface. 5. The method of manufacturing an optical waveguide according to claim 1, wherein the heating / pressing treatment is performed by a hot isostatic pressing method.