JP3681947B2 - Manufacturing method of optical waveguide - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a manufacturing method of the optical waveguide path capable of preventing cracks and peeling due to stress.
[0002]
[Prior art]
Compared to optical waveguides using optical glass materials and inorganic optical crystal materials, optical waveguides using polymer materials are coated with a spin coater and compared to about 200 ° C to 400 ° C at the most without using an ultra-high vacuum device. There are advantages such as being able to form an optical film by being cured by baking at a low temperature and being easily processed by oxygen plasma, etc., and expected to be applied to low-cost and high-performance optical waveguide components, Consideration is ongoing.
[0003]
However, although only the benefits are emphasized, there are many problems to be solved. In particular, due to the difference in thermal expansion coefficient between the polymer material and the substrate material, a strong tensile stress is generated in the polymer film in most cases when it is cooled to room temperature after firing and curing. At present, no fundamental solution has been found for the problem that cracks and peeling occur in the polymer film in the subsequent waveguide fabrication process and component fabrication process.
[0004]
The production of the polymer optical waveguide is ideally performed by the following steps shown in FIG.
[0005]
First, on a silicon or glass substrate 11, a solution in which a material to be a polymer optical film is dissolved in a solvent or the like, or a liquid material that is polymerized by heat or light irradiation to become a polymer optical film is formed by, for example, a spin coater. Apply. Thereafter, the first polymer optical film 12 (hereinafter referred to as polymer film) to be cured by evaporating or polymerizing the solvent by heating in an oven, light irradiation, or the like to be the lower clad of the optical waveguide. Alternatively, it is referred to as an optical film) (FIG. 1A).
[0006]
Next, on the substrate 11 on which the first optical film 12 to be the lower clad is formed, a solution adjusted so as to become an optical film having a slightly higher refractive index than the lower clad after coating is applied by a spin coater. Then, similarly to the first optical film, the second optical film 13 to be the core of the optical waveguide is formed by being cured by heating in an oven, light irradiation, or the like (FIG. 1B).
[0007]
Next, a metal film, a resist film, or the like is formed on the second optical film 13, and the metal film or the resist film other than the core of the channel waveguide is removed by a photo process, etching, or the like. Thereafter, the second optical film 13 is processed by etching or the like using the metal film or resist film 14 as a mask to obtain the core 15 of the channel waveguide (FIG. 1C).
[0008]
Finally, the metal film or resist film 14 remaining on the core 15 of the channel waveguide is removed, and the third optical film 16 to be the upper cladding having the same refractive index as the lower cladding is replaced with the first or first optical film. 2 is completed by the same method as the optical film 2 (FIG. 1D).
[0009]
Here, FIG. 1 is a schematic diagram of an ideal process, and it should be recognized that warpage of a substrate and a film due to stress, occurrence of cracks and peeling, and the like are not taken into consideration at all.
[0010]
[Problems to be solved by the invention]
The thermal expansion coefficient of silicon or glass as the substrate material is in the order of 10 ⁻⁶ / ° C. or 10 ⁻⁷ / ° C., whereas the thermal expansion coefficient of the polymer material is 10 ⁻⁵ / ° C. or 10 ⁻⁴ / ° C. Big with stand. When an optical film made of such a polymer material is cured by heating at about 300 ° C. and then cooled to room temperature, a mismatch of about 1% occurs between the substrate and the substrate.
[0011]
Due to this mismatch, a tensile stress is generated in the polymer film, and the substrate is compressed in a form of compressing the substrate in an attempt to alleviate the stress, and the polymer film is warped concavely. That is, the substrate and the polymer film hardly warp at a high temperature immediately after firing, but warp occurs when cooled to room temperature. FIG. 2 schematically illustrates this. 2 in FIG. 2A is a polymer film immediately after baking, and 18 in FIG. 2B is a polymer film cooled to room temperature after baking.
[0012]
Also, not only polymerization / curing by heating, but also when polymerizing / curing by light irradiation, for example, the polymer film contracts due to the polymerization reaction. It should be appreciated that under tensile stress, the substrate is subjected to compressive stress.
[0013]
In the structure of the conventional polymer optical waveguide described above, at the stage where the second polymer film is formed and cured and cooled to room temperature (in the polymerization and curing by light irradiation, after the completion of the polymerization reaction), FIG. As shown in FIG. 4, in fact, on the side of the substrate 11 on which the polymer film is formed as described above, the first polymer film 12 serving as the lower cladding and the second polymer film 13 serving as the core after processing are formed. It receives a compressive stress from both and warps in a concave shape.
[0014]
Conventionally, the second polymer film 13 other than the core 15 of the channel waveguide has been removed as being an unnecessary part except for a marker or the like. That is, the ratio of the area of the second polymer film remaining after processing, including the core, to the area before processing was at most several percent or less. By removing this unnecessary portion, the compressive stress on the substrate 11 is relieved, and the warpage of the substrate 11 is reduced as shown in FIG.
[0015]
When the warpage of the substrate is reduced, it means that a stronger tensile stress is generated when viewed from the polymer film side. The first polymer film 12 after processing is further stretched compared to before processing. As shown in FIG. 3C, cracks 19 and peeling often occur at the boundary between the lower part of the core 15 where stress is concentrated and the lower clad 12 as shown in FIG.
[0016]
Cracks may occur during processing of the core, after processing, and even after the fabrication of the waveguide. When a crack occurs, problems such as an increase in propagation loss of the waveguide, an optical confinement defect, and a decrease in reliability are also caused. FIG. 3D schematically shows the cross-sectional structure of the waveguide in which cracks occur. In addition, peeling often occurs between the lower cladding and the substrate.
[0017]
The object of the present invention is to prevent the occurrence of cracks and delamination by suppressing the increase in stress on the optical waveguide material in view of the problem of the occurrence of such cracks and delamination, and thus has low loss and high reliability. and to provide a manufacturing method of the optical waveguide path.
[0018]
[Means for Solving the Problems]
The gist of the present invention is to solve the problem of occurrence of cracks and peeling in the optical waveguide described above, leaving only a small part of the lower cladding layer near the core including the core on the substrate for forming the optical waveguide. Most of the lower clad layer on the substrate surface is removed, a core is formed on a part of the lower clad layer that is only partially left on the substrate, and the lower clad layer is left on the remaining lower clad layer. By forming the upper clad layer so as to cover the core, the stress between the lower clad, the core and the upper clad and the substrate is released as much as possible, thereby preventing the occurrence of cracks and peeling in the waveguide.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The embodiment of the manufacturing method of the optical waveguide path of the present invention will be described with reference to FIG.
[0020]
First, for example, when the adhesion between the substrate and the optical film is poor, the adhesion to the lower clad layer is selectively performed on the portion where the lower clad is to be left on the substrate (the portion where the core is to be formed and its vicinity). A region with improved properties (adhesion improving portion) is formed. This improvement in adhesiveness may be performed by applying an adhesive or by surface treatment with plasma or the like. Or when the adhesiveness of a board | substrate and an optical film is favorable, the surface treatment for peeling a lower clad selectively is performed to the part (peeling part) which does not leave the lower clad on the said board | substrate. The surface treatment for peeling can be performed by, for example, forming a metal film or a dielectric film that can be selectively removed by chemical etching, or forming a film such as a fluorinated resin or a dielectric that originally has poor adhesion to the substrate. good.
[0021]
In addition, as a method for selectively forming the adhesion improving portion or the peeling portion, for example, a resist film mask is formed by a photo process to remove the adhesive that is not covered with the mask, or the mask is covered with the mask. It may be applicable to remove the resist mask by treating the unexposed portion with plasma or subjecting the non-covered portion to treatment for peeling.
[0022]
On the substrate 21 (FIG. 4A) on which the adhesion improving portion 22 or the peeling portion 23 is formed in this way, a solution in which a material to be an optical film functioning as a lower clad is dissolved in a solvent, A liquid material that is polymerized by light irradiation to form an optical film is applied by, for example, a spin coater, and cured by evaporating the solvent or polymerizing by heating in an oven, light irradiation, etc. As shown in FIG. 2, the lower cladding layer film 24 of the optical waveguide is used.
[0023]
Here, even if the region having poor adhesion is mostly closed, the lower clad layer film 24 shrinks while being cooled from the firing temperature to room temperature and is concaved on the substrate 21 as long as it does not trigger peeling. It should be noted that this warpage occurs, that is, tensile stress is applied to the lower cladding layer film 24.
[0024]
Next, a part of the lower clad layer film 24 is scratched, or a part of the lower clad layer 24 is dry-etched, for example, to cause peeling, or the lower clad layer 24 other than the adhesion improving portion 22 is dry-etched. If the removal or removal treatment is removal by chemical etching of the metal film of the underlying layer of the lower cladding layer 24 to be peeled off, as shown in FIG. The lower clad 25 is left only on a desired part of the substrate, that is, on the adhesion improving portion 22.
[0025]
The lower clad 25 occupies a very small area on the substrate 1, and the warpage of the substrate 1 is almost eliminated. Furthermore, it seems that the tensile stress to the lower clad 25 is seemingly stronger due to the disappearance of the substrate warp. However, the lower clad layer 25 contracts toward the adhesion improving portion 22 at the time of peeling, so that the stress is reduced. Can be considered to be slightly relaxed.
[0026]
Further, when using, for example, a photo-curing resin (LTV curable resin or the like) as the cladding material, light is irradiated through the mask only to the portion where the adhesion is improved, and the resin in the region where the adhesion is poor is a solvent. The method of removing by is also considered.
[0027]
Next, when the adhesion between the substrate and the optical film is poor on the substrate formed up to the lower cladding 25, the optical film having a refractive index slightly higher than that of the lower cladding after curing is left as it is. The prepared solution is applied by a spin coater and cured by heating in the oven, light irradiation, or the like in the same manner as the lower clad layer, and as shown in FIG. A layer film 26 is formed. When the adhesion between the substrate and the optical film is good, the core layer film is formed after performing a peeling process on the region other than the lower clad 25.
[0028]
Next, as shown in FIG. 4E, if the core layer film in the region other than the lower clad 25 is peeled in advance in the same manner as the lower clad, the stress on the core is relieved as in the case of the lower clad. It is the same.
[0029]
Thereafter, a metal film, a resist film, or the like is formed on the remaining core layer film 27, and the metal film or the resist film other than the core of the channel waveguide is removed by a photo process and etching. Thereafter, using the metal film or resist film as a mask, the core layer film is processed by etching or the like to obtain the core (core ridge portion) 28 of the channel waveguide as shown in FIG.
[0030]
Next, on the substrate formed up to the core 28, when the adhesion between the substrate and the optical film is poor, the optical film having the same refractive index as that of the lower clad after the curing is adjusted as it is. The upper clad to be the upper clad of the optical waveguide, as shown in FIG. 4 (g), is applied by a spin coater and cured by heating in the oven, light irradiation or the like in the same manner as the lower clad layer. A layer film 29 is formed. If the adhesion between the substrate and the optical film is good, the upper clad layer film is formed after a treatment for peeling is performed on a region other than the lower clad 25.
[0031]
Finally, as shown in FIG. 4H, when the upper cladding layer film 29 in the region other than the lower cladding 25 is peeled in the same manner as the lower cladding, the stress on the upper cladding 30 is relieved. It is the same as the case of. The upper clad 30 may be processed simply by peeling as described above. If necessary, a part of the remaining upper clad may be trimmed by etching.
[0032]
In the optical waveguide of the present invention, the lower cladding layer film and the upper cladding layer differ from the conventional structure in which only a part of the core ridge portion and the marker are removed from the core layer film while leaving only a part of the optical waveguide. Even for the film, only the lower and upper clads in the minimum necessary area remain by intentional peeling, and the shrinkage toward the adhesive part of the optical film during peeling is used to remove the film into the optical waveguide. Since the increase in stress can be mitigated, there is an advantage that peeling and cracking in the waveguide can be prevented.
[0033]
In the above structure, as to what kind of lower clad is to be left, it is basically preferable to leave it only in the smallest possible area as long as no light leaks from the waveguide core. The optical waveguide of the present invention has the greatest advantage of preventing the occurrence of cracks and peeling, particularly in polymer waveguides. However, since the increase in stress is suppressed, the polarization dependent loss associated with stress can be improved. Don't forget to contribute. This is also true for, for example, a quartz waveguide.
[0034]
【Example】
A three-dimensional embedded light made of fluorinated polyimide based on the present invention was prepared by preparing a 0.5 mm thick 4-inch silicon substrate ((100) orientation) with a thermal oxide film of about 0.5 μm thickness by single-side polishing. A waveguide was produced. The adhesion between the fluorinated polyimide and the thermally oxidized silicon film is very poor. First, the surface of the thermally oxidized film is modified by plasma or the like for the region where the lower cladding is to be formed, or the silane coupling material, polyimide varnish, etc. The adhesiveness with the fluorinated polyimide must be improved by applying and curing the adhesive.
[0035]
When plasma processing is performed, it is necessary to cover a region other than a preselected region with a photoresist or the like. In addition, when applying varnish or the like, it is necessary to remove unnecessary varnish or the like after curing by a photo process and etching.
[0036]
For the substrate having such a region having improved adhesiveness selectively, a fluorinated polyamic acid solution is applied on the silicon substrate by a spin coater to a thickness of about 7 μm and heated in an oven to form a lower portion. A clad layer film was obtained. The heating temperature is preferably 300 ° C. or higher, sufficient for polymerizing the polyamic acid to form a polyimide, and more preferably, while introducing an inert gas such as nitrogen. The thickness of the lower clad layer is not particularly required to be 7 μm, and there is no problem as long as the thickness is sufficient to confine light. For example, it may be about 5 μm or 10 μm or more.
[0037]
For the lower clad layer film formed in this way, use a peripheral tooth or a laser trimming device to make a slight cut near the boundary between the adhesive improvement portion and the poor adhesion portion, for example, slightly outside the boundary. The lower clad layer can be easily peeled off by pre-selected by scratching with a sharp cutter knife from several places on the edge of the substrate and picking up the peeled film with tweezers and lifting it. The lower clad layer film other than the adhesive improvement part was removed.
[0038]
Note that there is no problem in performing etching for trimming the end portion of the lower clad after removal by peeling. Further, in addition to the peeling with scratches, an investigation was made to leave only the lower clad layer film of the adhesion improved portion by a normal photo process and etching, but there was no difference between the two. For curved waveguides and the like, it is natural that the etching method can easily control the area occupied by the lower cladding.
[0039]
Next, on the selectively formed lower clad, the refractive index is adjusted to be slightly higher in the communication wavelength region (1.31 to 1.55 μm wavelength) than the lower clad after heat polymerization. The fluorinated polyamic acid solution was spin-coated to a thickness of about 7 μm and heated and cured in an oven to form a core layer film. Needless to say, the thickness of the core layer film must be determined in consideration of the difference in refractive index from the cladding.
[0040]
For the core layer film thus formed, in the same manner as the lower clad, a cut is made slightly outside the lower clad region, the substrate end is peeled off with a knife, and further, A metal mask for processing the core ridge portion was formed on the core layer film left on the lower clad. The metal mask material may be any material as long as the selection ratio of the dry etching with the polyimide material is large, and titanium, niobium, molybdenum, tantalum, etc. are particularly preferable because the metal mask itself can be easily processed by dry etching. Steel, chrome, aluminum, magnesium, gold, silver, platinum, etc. have no problem.
[0041]
The metal mask is processed by forming a resist pattern on the metal film by a photo process, and removing unnecessary portions of the metal film by reactive ion etching or ion beam etching using the resist pattern as a mask. be able to.
[0042]
The core ridge portion was processed by etching using the metal mask thus formed. Since the adhesion between the core layer film and the lower clad is good, there is no need to improve the adhesion, but there is no problem in treating the surface of the lower clad with plasma.
[0043]
By the way, in the present invention, as shown in FIG. 4, the lower clad 25 is left only around the core 28, but the distance between the core and the end of the lower clad is the refractive index between the core and the clad of the waveguide. The minimum value differs depending on the difference, but it should be determined to be as small as possible based on at least no leakage of guided light from the core to the outside of the cladding. If this can be satisfied, it is preferable that the distance is small in order to relieve the stress on the film, and if it is possible to prevent the cracking of the polymer material, it should be as small as 5 μm or less.
[0044]
However, in reality, from the viewpoints of technology and yield, mask alignment, etc., and even if the effect of relaxing the stress on the polymer film is sufficient, it may be 10 μm or more, for example, about 50 μm or more. No problem. Needless to say, the distance need not be uniform on the waveguide chip. For example, where the cores are brought close to each other, priority must be given.
[0045]
Then, after removing the metal mask on the core ridge, an upper clad layer film having a thickness of about 10 μm was formed by spin coating and heating using a fluorinated polyamic acid solution having a refractive index equivalent to that of the lower clad. . In the same way as the lower cladding, the upper cladding layer film formed in this way is cut slightly outside the lower cladding region, and the substrate edge is peeled off with a knife and left. An optical waveguide according to the present invention was produced using the above-mentioned portion as an upper cladding.
[0046]
In the waveguide thus produced, no cracks or peeling occurred. On the other hand, an optical waveguide having a conventional structure in which the lower clad and the upper clad are present almost on the entire surface of the substrate is the same except that the adhesive improvement treatment is performed on the entire surface of the substrate and the peeling process is not performed. As a result, many cracks were observed between the core lower part and the lower clad, and film peeling occurred from the end of the waveguide chip.
[0047]
In addition to this, various polymer materials other than fluorinated polyimide, such as silicone resin, PMMA, UV curing and peeling were not observed, and cracks and peeling were observed at multiple locations for the conventional structure. .
[0048]
【The invention's effect】
As described above, according to the present invention, in order to solve the problem of occurrence of cracks and peeling in the optical waveguide, a very small portion near the core including the core on the substrate for forming the optical waveguide. Most of the lower clad layer on the substrate surface is removed, leaving only the clad layer, a core is formed on a part of the lower clad layer that is left on the substrate, and the remaining lower part is formed. By forming the upper clad layer on the clad layer so as to cover the core, the stress between the lower clad, the core and the upper clad and the substrate is released as much as possible, thereby preventing the occurrence of cracks and delamination in the waveguide. There is an effect that can be prevented.
[Brief description of the drawings]
FIG. 1 is a process diagram showing an example of a conventional method of manufacturing an optical waveguide. FIG. 2 is a schematic diagram showing the occurrence of warpage of a substrate due to a mismatch between the substrate and an optical film. FIG. process diagram showing an example of embodiment of the manufacturing method of the optical waveguide path of the schematic diagram FIG. 4 the invention [description of symbols]
21: substrate, 22: adhesion improving part, 23: peeling part, 24: lower cladding layer film, 25: lower cladding, 26: core layer film, 27: core layer film left after peeling, 28: core, 29 : Upper clad layer film, 30: upper clad.

Claims (5)

In an optical waveguide manufacturing method for forming an optical waveguide having a lower clad, a core and an upper clad on a substrate,
Of the surface of the substrate, only the region where the core is to be formed and the vicinity thereof is a region having good adhesion to the optical waveguide material, and the other region is a region having poor adhesion,
Forming a lower cladding layer on the substrate;
Of the formed lower cladding layer, the lower cladding layer is removed by removing the lower cladding layer on the poorly adhesive region so as to leave only the lower cladding layer on the region having good adhesiveness with the optical waveguide material. Form the
Forming a core layer on the substrate on which the lower cladding is formed;
Of the core layer thus formed, the remaining part is removed except for only the lower cladding,
Only the selected portion of the remaining core layer is processed as a core,
Forming an upper cladding layer on the substrate on which the lower cladding and the core are formed;
An upper clad is formed by removing the upper clad layer on the region having poor adhesion so as to leave only the lower clad including the core among the formed upper clad layer. Production method.   In the case where the adhesion between the substrate surface itself and the optical waveguide material is poor, the region on the substrate surface where the core is to be formed and the vicinity thereof are surface-treated so that the adhesion with the optical waveguide material is good. The method of manufacturing an optical waveguide according to claim 1, wherein:   When the adhesion between the substrate surface itself and the optical waveguide material is good, the region of the substrate surface that has poor adhesion to the optical waveguide material by surface treatment other than the region where the core is to be formed and the vicinity thereof The method of manufacturing an optical waveguide according to claim 1, wherein:   4. The method of manufacturing an optical waveguide according to any one of claims 1 to 3, wherein a silicon or glass substrate is used as the substrate and a polymer material is used as the optical waveguide material.   At least one of the removal of the lower clad layer, the core layer, and the upper clad layer is performed by making a cut near the boundary between the area to be removed and the area to be removed, and peeling the area to be removed. The method for producing an optical waveguide according to claim 4.

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