JPH0720336A - Structure of optical waveguide and its production - Google Patents

Abstract

PURPOSE:To obtain stable characteristics without receiving the influence of stresses with an external environmental change, such as temp. change by forming an optical waveguide in a groove formed on the surface of a waveguide substrate. CONSTITUTION:The groove 5 is worked by excimer laser on the front surface of the substrate 1 composed of silicone. For example, in he case of forming the clad layer on the upper part and the side face part of the core part 3 as well, a bottom clad part 2 consisting of an SiO2 film is formed by a CVD method over the entire part of the front surface of the substrate 1 including the groove 5. Further, a core part 3 consisting of an SiO2 film doped with germanium is formed in the upper part of the bottom clad part 2 in order to confine light by increasing the refractive index. The core part 3 is patterned by photolithography to such a width at which the guided light attains a single mode with the optical wavelength pattern along the groove 5. The optical waveguide is formed in the groove 5 formed on the substrate 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical device, and more particularly to a structure of an optical waveguide comprising a core portion and a cladding portion laminated on the surface of a substrate and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a waveguide type optical device, an optical waveguide is formed on a substrate, and the optical waveguide itself can be provided with optical switching, optical coupling, and optical modulation functions, or can be coupled with an optical semiconductor element on the substrate. . Therefore, it can be mass-produced as well as semiconductor technology such as electronic devices, and at the same time, it can be highly integrated and highly functional, and has many excellent features that conventional micro-optics type devices do not have. .

An example of such an optical waveguide is 19
Paper C-189 "Silica-based Waveguide Broadband Coupler" published in the 90th IEICE Spring National Congress (Reference 1)
The structures described are known. The optical waveguide described in this paper has a flame deposition method and a CVD (Chemic) method on the surface of a substrate.
A silicon dioxide film (SiO ₂ film) is deposited by using a method such as al vapor deposition method, and only a portion to be an optical waveguide is doped with impurities such as phosphorus and germanium to form a portion having a high refractive index to be a core portion. The formed optical waveguide is called a silica optical waveguide (or quartz optical waveguide).

However, in the case of a silica optical waveguide having a simple structure in which a core portion and a cladding portion are laminated as in the conventional case, a large thermal expansion coefficient difference of one digit or more is generated between the substrate material and silicon. Therefore, when the SiO ₂ film is coated at a high temperature of 100 ° C. or higher, stress is generated between the substrate and the coating when the temperature is returned to room temperature. Even if the optical waveguide is formed in consideration of the temperature change in the manufacturing process so that the desired characteristics can be obtained at room temperature, when the temperature change is applied to the device in the normal use state, the inside of the film is similarly heated. Stress will occur due to the difference in expansion coefficient. If stress is generated inside the coating, the refractive index of the optical waveguide core and the shape of the waveguide may change even in a simple straight waveguide or a curved portion of the waveguide, which may adversely affect the propagation loss. There is. In the optical branching section and optical demultiplexing section that use a directional coupler,
Even a slight change in the refractive index or shape near the optical waveguide core causes a large change in characteristics, and there is a problem in that stable characteristics of the device cannot be secured.

Conventionally, in order to remove or reduce the stress generated inside the film of the above-mentioned optical waveguide, the paper 372 “Quartz-based with bridge structure” published in the National Conference of Semiconductor and Materials Division of the Institute of Electronics, Information and Communication Engineers in 1987. Optical waveguide "
As shown in (Reference 2), a structure has been proposed in which a groove for removing stress is provided near the optical waveguide core.

[0006]

[Patent Document 2]
In the structure described, a groove pattern must be formed near the optical waveguide and must be formed by etching or the like, which complicates the groove manufacturing process. Even if stress due to temperature change can be alleviated by forming the groove, the physical strength is lowered because the optical waveguide is in a state similar to that in which it is hollow, and it is vulnerable to mechanical external forces such as vibration and shock. Become. Further, it has been difficult to form such a groove in the light directional coupling portion.

It is an object of the present invention to remove the influence of the stress generated in the optical waveguide due to the above-mentioned temperature change around the device, to obtain stable characteristics, and to achieve high productivity and high integration. An object is to provide an optical waveguide structure for an optical device.

[0008]

In order to eliminate the above-mentioned drawbacks, the optical waveguide structure of the present invention comprises a core part for guiding light and a clad part having a small refractive index at least under the core part. A waveguide type device having an optical waveguide on a substrate is characterized in that the optical waveguide is formed in a groove formed on the surface of the waveguide substrate.

The present invention is also directed to a waveguide type optical device having an optical waveguide on the substrate, which has not only the lower portion of the core portion but also the side surface and the upper surface of the core portion. It is characterized in that these optical waveguides are formed in the groove formed in. Here, the clad part under the core part of the optical waveguide is formed in contact with the bottom surface of the groove on the substrate surface, and the core part is formed on this, and the width of the groove of the substrate is smaller than the width of the core part. It has a width of at least three times, and is characterized in that the bottom surface of the core portion is formed at a position lower than the surface of the region of the substrate where the groove is not formed.

As a method of forming a groove on the surface of the substrate,
It is characterized by a method using ablation processing by an excimer laser.

The present invention is particularly directed to a waveguide type optical device using an optical waveguide formed by laminating a core portion and a clad portion on a substrate as typified by a quartz optical waveguide. This is different from the conventional technique in that the core portion and the clad portion are formed inside the groove formed in advance on the substrate surface. With such a structure, even if a temperature change occurs around the device, the substrate is not warped due to the difference in the thermal expansion coefficient between the substrate and the film, and an optical waveguide near the optical waveguide is configured. Stress is less likely to occur in the coating, and stable device characteristics can be obtained.

Further, as a concrete means capable of efficiently and accurately performing the groove processing of the waveguide type optical device having the above-mentioned optical waveguide structure, the groove processing is characterized by excimer laser processing. As a method for forming a groove on a silicon substrate, conventionally, a chemical etching utilizing the anisotropy of silicon has been used as a V groove (not a groove for forming an optical waveguide described in the present invention, but an optical fiber for arranging optical fibers). groove)
It was generally used as a forming means. However, it is difficult to form the above-mentioned groove by chemical etching because a smooth bent portion or a branched portion is also required to form a waveguide type optical device. Therefore, by using the excimer laser processing means of the present invention, it becomes possible to easily form the above-mentioned groove for constructing the optical waveguide in an arbitrary pattern in accordance with the pattern of the optical waveguide.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of the optical waveguide of the present invention. In the figure, a groove 5 is processed by an excimer laser on the surface of a substrate 1 made of silicon. A bottom clad portion 2 made of a SiO ₂ film is formed on the entire surface of the substrate 1 including the groove 5 by CVD (Chemical Vapor Depo).
It is formed by the method). Further, a core portion 3 made of a SiO ₂ film doped with germanium for increasing the refractive index and confining light is formed above the bottom clad portion 2. The core portion 3 is patterned by photolithography so as to have an optical waveguide pattern along the groove 5 and a width such that the guided light has a single mode. The embodiment shown in FIG. 1 shows an example in which a clad layer is formed on the upper portion and the side surface portion of the core portion in the present invention.

Next, the evaluation results of the characteristics of the optical waveguide using the optical waveguide structure of the present invention will be described.

First, in order to compare and evaluate the propagation characteristics of the optical waveguide structure of the present invention and the conventional ridge type optical waveguide, optical waveguides each having a radius of 20 mm and a bending pattern of an angle of 90 degrees were manufactured.

In the groove processing of the optical waveguide structure of the present invention, first, a groove 5 having a width of about 30 μm and a depth of about 20 μm is processed on the surface of the substrate 1 by an excimer laser ablation processing so as to have a desired optical waveguide pattern. As shown in FIG. 2, in the groove processing by the ablation processing using the excimer laser, the laser beam irradiation energy density is about 10 J.
Processing was performed while irradiating the substrate for a certain time in a state of being narrowed down by a lens so as to be / cm ² , and processing was performed while moving the XY stage on which the substrate was installed according to a program set to a desired optical waveguide pattern. The energy density can be lower than this, and the lower the energy density, the larger the processing area that can be machined at one time, but the machining speed in the depth direction of the groove decreases, so the energy density is set to this value. .

Next, for the film deposited on the surface of the substrate 1, the bottom clad portion 5 is formed by SiO ₂ by the CVD method.
The film was deposited about 10 μm. Further, as the core portion 3, a SiO ₂ film doped with germanium was deposited to a thickness of about 8 μm. The film thickness of the SiO ₂ film deposited as the upper clad portion 4 on the entire surface of the substrate 1 was also about 10 μm. The width of the core portion 3 is 8 μm in order to make the guided light a single mode.
And

The width of the groove 5 formed first is larger than the width of the core portion 3 so that the upper clad portion 4 is surely attached to this side surface after the SiO ₂ film is evenly attached to the bottom surface to form the core portion 3. Also, it is set to 30 μm, which is three times or more. When the width of the groove is about 3 times or less as large as the width of the core portion, stress is generated in the film due to the presence of the groove when the layer to be the core portion is formed, or a portion to be the core portion There may be defects such as the formation of a depression on the surface of the film and difficulty in forming a core portion having a rectangular cross section. The width of the groove 5 is set to be three times or more than the width of the core portion 3 in order to prevent these. In the photolithography process, since there is a step due to the groove on the surface of the substrate, a stepper that can be used without exposing the mask to the substrate was used for all exposure.

FIG. 3 shows the result of measuring the temperature dissimilarity of the loss in the bent portion of the optical waveguide having the above-mentioned optical waveguide structure. As is clear from FIG. 3, the loss due to the conventional optical waveguide structure described in Document 1 is about 0.2d at room temperature.
Even if it can be made as small as B, the loss increases to 0.6 dB or more at a high temperature of 65 ° C. or a low temperature of 0 ° C.
This is because when there is a change in temperature, stress is concentrated near the optical waveguide due to the difference in thermal expansion coefficient between the silicon substrate 1 and SiO ₂ which is the optical waveguide material, and as a result, the optical confinement state of the optical waveguide weakens. It is thought that this is due to the fact that is emitted. On the other hand, in the loss evaluation result in the bent portion using the optical waveguide structure of the present invention, even if there is a temperature change, the stress is not concentrated in the optical waveguide portion, so there is almost no loss fluctuation with respect to temperature change and it is stable. ing.

Next, the stability of the wavelength division multiplexing / demultiplexing characteristics of the directional coupling type optical multiplexer / demultiplexer using the optical waveguide structure of the present invention and the conventional optical waveguide structure described in Document 1 will be described. Comparative evaluation was performed. FIG. 4 shows a sectional structure of a directional coupling type optical multiplexer / demultiplexer having the optical waveguide structure of the present invention. Graphs showing branching characteristics (wavelength dependence of branching ratio) of the respective optical multiplexers / demultiplexers are shown in FIGS. 5 and 6, respectively. The branching characteristics were evaluated at room temperature and 65 ° C., respectively. It can be seen that the branching characteristics of the conventional optical waveguide structure change from the initial (room temperature) state at high temperature as shown in FIG. . On the other hand, it can be seen that the optical waveguide structure of the present invention is stable with almost no change even if the temperature changes. It is considered that the difference in the result of this evaluation is due to the difference in the effect of stress as well as the cause of the above-described loss evaluation result in the curved portion.

In the above-mentioned embodiment, the depth of the groove 5 is set to 20 μm so that the whole core portion 3 is lower than the surface of the region of the substrate 1 where the groove is not formed. The stress due to the difference in thermal expansion coefficient between the substrate and the film cannot be sufficiently relaxed and removed. That is, in order to sufficiently relax the stress, it is necessary that the lower slad portion 4 is almost completely separated from the groove 5 and the region where the groove is not formed, and the surface of the lower clad portion 4, that is, at least the core portion 3 is separated. It is better to determine the depth of the groove 5 so that the bottom surface of the groove is lower than the surface of the substrate 1. On the contrary, if the groove is too deep, not only it takes time to process the groove, but also the patterning of the core portion 3 becomes difficult. Therefore, in the optical waveguide having the lower clad portion 4 having a thickness of 10 μm and the core portion 3 having a thickness of 8 μm as in this embodiment, the groove depth is preferably about 15 to 20 μm.

Further, the optical waveguide having the clad formed on the upper and lower portions and the side surface of the core portion has been described. However, the optical waveguide having the clad portion formed only on the lower portion of the core portion, or the clad only on the lower portion and the side surface portion. It goes without saying that the structure of the present invention can be applied to the optical waveguide in which the portion is formed, because it has stress relieving and removing effects.

The above steps other than forming the groove 5 are not particularly different from those of the conventional optical waveguide manufacturing method. The method using excimer laser ablation processing was used to form the groove. However, with the conventional method using scientific etching that utilizes the anisotropy of the silicon substrate, the groove depth is approximately 10 μm or more, which is extremely long (usually Tens of minutes) will be required. Moreover, a clean groove can be formed only in a certain direction with respect to the groove crystal orientation, and the optical waveguide pattern usually has an arbitrary pattern including a curve. Further, it is necessary to change the groove width at the coupling portion of the directional coupler, and this point is not necessarily suitable for the groove processing method of the optical waveguide structure of the present invention. Further, the method of forming a groove by chemical etching is as shown in “Quartz-based optical waveguide on sapphire substrate” described in article number 373 of the National Conference on Semiconductor and Materials Division of the Institute of Electronics, Information and Communication Engineers in 1987. It cannot be applied to an optical waveguide using a substrate material other than silicon. On the other hand, the processing by the excimer laser can be processing other than silicon such as quartz glass if the substrate material is a material that absorbs ultraviolet rays, or if it absorbs ultraviolet rays, the structure of the optical waveguide of the present invention is applied. can do.

[0024]

As described above, by using the optical waveguide structure of the waveguide type optical device of the present invention, stable characteristics can be obtained without stress in the vicinity of the optical waveguide even when the temperature around the device changes. It becomes possible to fabricate a waveguide type optical device having the same with high productivity. Moreover, since it is not necessary to provide a groove having a wide width in the vicinity of the optical waveguide in order to relieve or remove stress, there is also a feature that high integration is not hindered. Further, by processing the groove on the surface of the substrate, which is necessary for manufacturing the structure of the optical waveguide of the present invention, with an excimer laser, the groove of an arbitrary pattern can be easily processed, and a Y-branch or a directional coupler can be formed. It can also be used for various patterns.

[Brief description of drawings]

FIG. 1 is a sectional view showing an optical waveguide structure of a waveguide type optical device of the present invention.

FIG. 2 is a perspective view showing a processed state of an optical waveguide substrate with an excimer laser.

FIG. 3 is a diagram showing an evaluation result of loss fluctuation with respect to temperature change of an optical waveguide in a bent portion having a radius of 20 mm and an angle of 90 ° C.

FIG. 4 is a sectional structure of a directional coupling type optical multiplexer / demultiplexer using the optical waveguide structure of the present invention.

5 is a diagram showing a change in branching ratio with respect to a temperature change in the optical multiplexer / demultiplexer shown in FIG.

FIG. 6 is a diagram showing a change in branching ratio with respect to a temperature change in an optical multiplexer / demultiplexer having a conventional optical waveguide structure.

[Explanation of symbols]

 1 Silicon substrate 2 Bottom clad part 3 Core part 4 Upper clad part 5 Groove

Claims (6)

[Claims] 1. A structure of an optical waveguide of a waveguide type optical device having a core part for guiding light and an optical waveguide having a clad part having a refractive index smaller than that of the core part at least under the core part on a substrate. 2. The optical waveguide structure according to claim 1, wherein the optical waveguide is formed in a groove formed on the surface of the substrate, with the clad portion being in contact with the bottom surface of the groove. 2. The structure of the optical waveguide according to claim 1, wherein the optical waveguide is provided with a clad portion having a refractive index smaller than that of the core portion on a lower portion and a side surface portion of the core portion. 3. The structure of the optical waveguide according to claim 1, wherein the optical waveguide includes clad portions having a smaller refractive index than the core portion at upper and lower portions and side surface portions of the core portion. 4. The structure of the optical waveguide according to claim 2, wherein the groove of the substrate has a width that is at least three times as large as that of the core portion. 5. The structure of the optical waveguide according to claim 1, wherein a bottom surface of the core portion is located below a surface of a region of the substrate where the groove is not formed. 6. The method of manufacturing an optical waveguide according to claim 1, wherein the groove on the surface of the substrate is formed by ablation processing using an excimer laser.