JP2001255426A - Optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of cracks due to mismatching of hardening and shrinking between a polymer optical material and a substrate in an optical waveguide using a polymer optical material. SOLUTION: A first polymer optical film to be used as a lower clad 2 is formed on a substrate 1, and a second polymer optical film is formed on the first polymer optical film. Gaps 11 are formed by etching or the like in the second polymer optical film and the residual part is used as a core ridge 5 and a buffer 10. With the presence of the buffer 10, release of the compressive stress in the substrate 1 is suppressed to the minimum to prevent decrease in the warpage of the substrate 1, and therefore, to prevent generation of cracks. Further a third polymer optical film to be used as an upper clad 6 is formed on the core ridge 5 and the buffer 10 to manufacture the optical waveguide. The gap 11 is formed to have the minimum width which can prevent leaking of light from the core ridge 5 to the buffer 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure for preventing a crack caused by stress in an optical waveguide using, for example, a polymer optical material.

[0002]

2. Description of the Related Art An optical waveguide using a polymer optical material is different from an optical waveguide using an optical glass material or an inorganic optical crystal material in that application by a spin coater without using an ultra-high vacuum apparatus, and at most 200 There is an advantage that a polymer optical film can be formed by curing by baking at a relatively low temperature of about 400 ° C. to 400 ° C., and that it can be easily processed by oxygen plasma or the like. It is expected to be applied to waveguide components and is being studied.

[0003] However, although only the advantages are emphasized, there are many problems to be solved. Above all, due to the difference in the coefficient of thermal expansion between the polymer optical material and the substrate material, in most cases when the polymer optical film is cooled to room temperature after firing and curing, a strong tensile stress is generated in the polymer optical film, For this reason, no fundamental solution has been found so far for the problem that cracks occur in the polymer optical film in the subsequent optical waveguide fabrication process and component fabrication process.

The fabrication of an optical waveguide using a polymer optical material is performed, for example, ideally as shown in FIG.

First, as shown in FIG. 4A, a polymer optical material to be a polymer optical film is polymerized on a silicon or glass substrate 1 by a solution in which a polymer optical material is dissolved in a solvent or the like or by heat or light irradiation. Then, a liquid polymer optical material to be a polymer optical film is applied by, for example, a spin coater. Thereafter, the solvent is evaporated or polymerized by heating in an oven, light irradiation, or the like, and cured to form a first polymer optical film to be the lower clad 2 of the optical waveguide.

Next, as shown in FIG. 4B, a substrate 1 on which a first polymer optical film to be a lower clad 2 is formed.
A solution adjusted so as to become a polymer optical film having a refractive index slightly larger than that of the lower clad 2 after curing is applied by a spin coater on the upper side, and the solution is placed in an oven in the same manner as the first polymer optical film. The second polymer optical film which is to be cured by heating, light irradiation, or the like in the step (a) to become the core layer film 3 of the optical waveguide is formed.

Next, as shown in FIG. 4C, a metal film or a resist film 4 is formed on the second polymer optical film, and a portion other than the core ridge 5 of the channel waveguide is formed by a photo process and etching. The metal film and the resist film 4 are removed. Then, using the metal film and the resist film 4 as a mask,
The second polymer optical film is processed by etching or the like to obtain the core ridge 5 of the channel waveguide.

Finally, as shown in FIG. 4D, the metal film and the resist film remaining on the core ridge 5 of the channel waveguide are removed to form the upper clad 6 having the same refractive index as the lower clad 2. The third polymer optical film is formed in the same manner as the first or second polymer optical film, and the formation of the polymer optical waveguide is completed.

Here, it should be recognized that FIG. 4 is a schematic diagram of an ideal process, and that generation of cracks and the like are not considered at all.

[0010]

A polymer optical material has a thermal expansion coefficient of 10 ⁻⁶ / of silicon or glass as a substrate material.
℃ or 10 ^-7 / ℃, whereas 10 ^-5 / ℃
Or it is as large as 10 ^-4 / ° C. When the polymer optical film made of such a polymer optical material is heated to about 300 ° C. and cured, and then cooled to room temperature, a mismatch of about 1% occurs between the polymer optical film and the substrate 1.

Due to this mismatch, a tensile stress is generated in the polymer optical film, and the substrate 1 is bent in a concave shape toward the side where the polymer optical film is formed in a manner to compress the substrate 1 in an attempt to alleviate the stress. That is, when the temperature is high immediately after firing, the substrate 1 and the polymer optical film hardly warp. Upon cooling to room temperature, warpage occurs. FIGS. 2 (a) and 2 (b) schematically show this, where 7 is a polymer optical film immediately after baking, and 8 is a polymer optical film cooled to room temperature after baking.

In addition, when the polymer optical film is not polymerized and cured by heating but is polymerized and cured by light irradiation, for example, the polymer optical film shrinks due to the polymerization reaction, so that stress is similarly generated between the polymer optical film and the substrate 1. It should be recognized that the polymer optical film receives a tensile stress and the substrate 1 receives a compressive stress.

In the structure of the optical waveguide using the conventional polymer optical material described above, the step of forming and curing up to the second polymer optical film and cooling it to room temperature (in the polymerization and curing by light irradiation, the polymerization reaction At the end), actually, on the side of the substrate 1 where the polymer optical film is formed as described above, the first polymer optical film which is the lower clad 2 and the second high layer which becomes the core layer film 3 after processing. It receives a compressive stress from both of the molecular optical films and warps concavely (see FIG. 3A).

Conventionally, a core ridge 5 of a channel waveguide is used.
Most of the second polymer optical film except for the marker was removed as an unnecessary portion except for a marker and the like. That is, the ratio of the area of the second polymer optical film remaining after processing including the core portion to the area before processing was at most about several percent or less. By removing the unnecessary portion, the compressive stress on the substrate 1 is reduced, and the warpage of the substrate 1 is reduced (see FIG. 3B).

The fact that the warpage of the substrate 1 is reduced means that
When viewed from the polymer optical film side, it means that a stronger tensile stress is generated. The second polymer optical film after the processing is further elongated as compared with before the processing. Cracks 9 are formed around the boundary between the lower part of the core ridge 5 and the lower clad 2 where the stress is concentrated without being able to withstand further elongation.
Often occurs (see FIG. 3C).

The crack 9 may be generated during the processing of the core ridge 5, after the processing, or even after the fabrication of the optical waveguide. When the crack 9 occurs, problems such as an increase in the propagation loss of the optical waveguide, poor light confinement, and a decrease in reliability occur. FIG. 3 (d)
1 schematically shows a cross-sectional structure of an optical waveguide in which cracks 9 have occurred.

An object of the present invention is to prevent the occurrence of cracks by suppressing an increase in stress on the polymer optical film in view of the problem of the occurrence of cracks, thereby achieving low loss and low reliability. It is to provide a high optical waveguide.

[0018]

SUMMARY OF THE INVENTION The gist of the present invention is to solve the above-mentioned problem of crack generation by determining the area of an unnecessary polymer optical film to be removed at the time of core ridge processing by crosstalk or the like. By minimizing the release of the compressive stress as seen from the substrate to prevent the substrate from warping, the core ridge after processing is prevented from being further extended by minimizing the release of the compressive stress as viewed from the substrate. Thus, an increase in stress on the core ridge is minimized, thereby preventing the occurrence of cracks below the core ridge.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and examples of the present invention will be described below in detail.

In order to achieve the above object, in the embodiment of the present invention, for example, up to the second polymer optical film is formed in the same manner as in the above-mentioned prior art. That is, first,
A solution in which a polymer optical material to be a polymer optical film is dissolved in a solvent or the like or a liquid polymer optical material to be polymerized by heat or light irradiation to be a polymer optical film is formed on a silicon or glass substrate 1. For example, by applying with a spin coater, and evaporating the solvent by heating or light irradiation in an oven,
The first polymer optical film which is to be cured by polymerization to become the lower clad 2 of the optical waveguide is obtained.

Next, on the substrate 1 on which the first polymer optical film to be the lower clad 2 is formed, after curing, a polymer optical film having a refractive index slightly larger than that of the lower clad 2 is formed. The prepared polymer solution is applied by a spin coater and cured by heating in an oven or light irradiation in the same manner as the first polymer optical film to form a second polymer optical film to become a core layer film of the optical waveguide. Form a film. Up to this point, there is no difference from the prior art.

Next, the gist of the present invention will be described with reference to FIG. A metal film, a resist film, or the like is formed on the second polymer optical film, and a limited portion of the channel waveguide near the core ridge 5 (FIG.
Only the metal film and the resist film (corresponding to the gap 11 in (a)) are removed. That is, only the metal film and the resist film in the limited portions on both sides of the core ridge 5 are removed. Thereafter, using the metal film or the resist film as a mask, the second polymer optical film is processed by etching or the like to obtain the core ridge 5 of the channel waveguide and the buffer portion 10 for alleviating an increase in stress on the core ridge.

Finally, the core ridge 5 of the channel waveguide
And removing the metal film and the resist film remaining on the buffer section 10 and forming the third polymer optical film to be the upper clad 6 having the same refractive index as the lower clad 2 with the first or second polymer. The formation of the polymer optical waveguide of the present invention having a buffering portion formed by the same method as that of the optical film to reduce the increase in stress is completed (see FIG. 1B).

The structure of the polymer optical waveguide of the present invention is different from the conventional structure in which the core ridge 5 and most of the marker and the like are removed and almost all portions are removed in processing the second polymer optical film. Then, only a very small portion near the core ridge 5 (part of both sides of the core ridge 5) is removed, and most of the second polymer optical film remains. There is an advantage that the stress does not change and the stress on the core ridge 5 can be reduced.

In the above structure, what kind of buffer 10
Is preferably left over as large an area as possible unless light leaks from the core ridge 5 of the optical waveguide to the buffer portion 10. Further, if there is no problem in confining light without etching to the depth of the first polymer optical film when etching the second polymer optical film, the second portion of the portion to be etched away is removed. Leaving a part of the lower part of the polymer optical film of 2,
It is more preferable from the viewpoint of relaxing stress.

The structure of the optical waveguide according to the present invention has the greatest advantage of preventing the occurrence of cracks, particularly in a polymer optical waveguide. However, since the increase in stress is suppressed, the polarization dependent loss accompanying the stress is reduced. Don't forget to contribute to improvement. This is also true for, for example, quartz waveguides.

[0027]

Next, an embodiment will be described. 0.5 mm thick 4 with thermal oxide film about 0.5 μm thick by single side polishing
An inch silicon substrate ((100) orientation) was prepared, and a three-dimensional embedded optical waveguide made of fluorinated polyimide was manufactured. First, it is preferable to previously modify the surface of the thermal oxide film by plasma or the like, or to improve the adhesiveness with the fluorinated polyimide by applying and curing a polyimide varnish or the like.

Approximately 7 μm of the fluorinated polyamic acid solution
Was coated on the silicon substrate by a spin coater and heated in an oven to form a lower clad layer. The heating temperature is preferably 300 ° C. or higher, which is sufficient for the polyamic acid to be polymerized into a polyimide, and more preferably, while introducing an inert gas such as nitrogen.

The thickness of the lower cladding is not particularly required to be 7 μm, and there is no problem as long as it is necessary and sufficient to confine light. For example, about 5 μm and 10 μm
There may be more.

Next, on the lower clad, a fluorinated substance adjusted so that the refractive index in the communication wavelength region (1.3 to 1.55 μm wavelength) becomes slightly larger than that of the lower clad after heat polymerization. A polyamic acid solution was spin-coated to a thickness of about 7 μm, heated and cured in an oven to form a core layer film. Needless to say, the thickness of the core layer film needs to be determined in consideration of the refractive index difference from the cladding.

A metal mask for processing a core ridge was formed on the core layer film thus formed. The material of the metal mask may be any material as long as the selectivity of dry etching with the polyimide material is large,
Titanium, niobium, molybdenum, tantalum, etc., which can easily process the metal mask itself by dry etching, are particularly preferred, but copper, chromium, aluminum, magnesium, gold, silver, platinum and the like do not pose any problem.

In the processing of the metal mask, a resist pattern is formed on the metal film by a photo process, and unnecessary portions of the metal film are removed by reactive ion etching or ion beam etching using the resist pattern as a mask. It can be done by doing.

In the present invention, as shown in FIG. 1, only the core layer film around the core ridge 5 is removed, but the core ridge 5 and the buffer 1 near the core ridge 5 are removed.
The distance from zero, so-called gap 11, may be determined in consideration of the following conditions.

First, the minimum value of the gap 11 differs depending on the refractive index difference between the core and the clad of the optical waveguide.
At least there is no leakage of the guided light from the core ridge 5 to the buffer portion 10. Next, the gap 11 should be such that the processed shape of the core ridge side wall and the gap bottom surface due to the presence of the gap 11 is good, that is, the gap 11 should not cause excessive loss. Further, the gap 11 is such that bubbles and voids are not generated when the upper clad 6 is formed. It is necessary to determine at least the above three points.

If these conditions can be satisfied, the gap 11 is preferably small in order to alleviate the stress on the film. For polymer optical materials in which cracks are difficult to prevent, the gap 11 should be reduced to 3 μm or less if possible.

However, in practice, from the viewpoints of technology, yield, mask alignment, etc., if the effect of alleviating the stress on the polymer optical film is sufficient, it is 5 μm or more, for example, about 50 μm or more. There is no problem. Needless to say, the gap does not need to be uniform on the optical waveguide chip. For example, where the core portions are brought close to each other, the buffer portion may be set outside the core portions with priority.

The processing of the core layer film having the core part and the buffer part having the structure separated by the gap determined in this way was performed by dry etching through the above-mentioned metal mask. As described in the section of the embodiment of the invention, the etching depth does not necessarily need to reach the lower clad. After the processing of the core layer film, the metal mask was removed by dry etching, wet etching, or the like. Thereafter, an upper clad having a thickness of about 10 μm was formed by spin coating using a fluorinated polyamic acid solution equivalent to that of the lower clad and heating to complete the production of the polymer optical waveguide of the present invention.

No crack was observed in the optical waveguide thus manufactured. On the other hand, when an optical waveguide having a conventional structure without a buffer portion was manufactured in the same manner except for a mask pattern for processing a core layer film, a portion where cracks occurred between a lower portion of a core ridge and a lower clad was found. Many were seen.

Further, similar optical waveguides were manufactured using various polymer optical materials other than fluorinated polyimide, for example, silicone resin, PMMA, UV curable resin and the like. No cracks were found in at least a plurality of locations in the conventional structure.

[0040]

As described above, according to the present invention,
By minimizing the area of unnecessary polymer optical film that should be removed during core ridge processing as long as problems such as crosstalk do not occur, release of compressive stress seen from the substrate is minimized. Providing an optical waveguide free from cracks under the core ridge by preventing the warpage of the substrate from becoming small and preventing the core ridge after processing from being further stretched, minimizing the increase in stress on the core ridge. can do.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a manufacturing process of an optical waveguide according to the present invention.

FIG. 2 is a schematic view showing a state in which the substrate is warped due to a mismatch between the substrate and the polymer optical film.

FIG. 3 is a schematic view showing generation of cracks due to stress.

FIG. 4 is a schematic view showing a process for manufacturing a conventional polymer optical waveguide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Lower clad 3 Core layer film 4 Resist or metal mask 5 Core ridge 6 Upper clad 7 Polymer optical film immediately after baking 8 Polymer optical film cooled to room temperature after baking 9 Crack 10 Buffer section 11 Gap

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Makoto Hikita 2-3-1 Otemachi, Chiyoda-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (72) Inventor Koji Enbutsu 2-3-3, Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation (72) Inventor Toru Maruno 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Shoichi Hayashida Otemachi, Chiyoda-ku, Tokyo 2-3-1, Nippon Telegraph and Telephone Corporation (72) Inventor Takashi Kurihara 2-3-1, Otemachi, Chiyoda-ku, Tokyo F-term within Nippon Telegraph and Telephone Corporation 2H047 KA04 PA02 PA24 PA24 PA28 TA00

Claims (1)

[Claims] 1. A substrate, a lower polymer cladding that is a first polymer optical film formed by applying and curing a first polymer optical material on the substrate, and a second clad on the lower clad. The light from the core ridge is applied to both sides of the portion serving as the core ridge for the second polymer optical film having a higher refractive index than the first polymer optical film, which is formed by applying and curing the polymer optical material. By forming a gap to prevent leakage of
A core ridge and a buffer portion formed by the remaining portion of the second polymer optical film; and a third polymer optical material applied on the second polymer optical film after the removal processing, followed by curing. An optical waveguide comprising a first polymer optical film and an upper clad as a third polymer optical film having the same refractive index.