JP2005241915A - Optical waveguide and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an optical waveguide which allows a structure for reducing branching loss and a structure of ordinary optical waveguides to be simultaneously and easily manufactured with high reproducibility. <P>SOLUTION: When a structure wherein the thickness of a core layer 3 in a thickness direction is varied is formed in a gap part between branching optical waveguides in the vicinity of a branch structure, a mask pattern and an exposure condition are selected so as to cause a resist 4 to remain in the gap part between waveguides of a branching waveguide pattern when forming a resist pattern by using an exposure mask 5, and the resist pattern is so formed that the thickness of the remaining resist is reduced according to a distance away from the branch part, and then the resist pattern is taken as a dry etching mask to dry-etch a core layer 3 and a clad layer 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical waveguide having an optical branch used in the fields of optical communication, optical signal processing, sensors, and the like, and a method for manufacturing the same.

  In recent years, an optical waveguide formed on a planar substrate has been actively used in the fields of optical communication, optical signal processing, sensors, and the like. In these optical circuits, the optical branching and multiplexing functions are one of the most important basic elements constituting the circuit and are indispensable. Conventionally, techniques and design techniques for forming such optical branching and multiplexing with low loss have been studied. This is because these elements are basic elements of the circuit, and even a slight loss accumulates the loss, which hinders the realization of a low-loss optical circuit.

  Now, it is well known that the loss of these branch circuits is usually caused by the signal light dissipating from the gap between the branched cores. There are a number of solutions to this solution, but the cause is due to signal light scattering due to the discontinuity of the branch part, so the loss is reduced by increasing the continuity of the branch part. be able to.

  FIG. 10 shows a basic structure for reducing the branch loss of the branch optical waveguide 101 by taking a 1 × 2 branch circuit as an example. FIG. 10A shows a top view of the branch pattern. Here, a very important part in reducing the loss of the branch part is the transition region 102 in the gap part between the branch waveguides in the vicinity of the branch structure of the waveguide. An example of the A-A ′ cross section of the transition region 102 is shown in FIGS. As shown in both figures, the core film thickness in the gap portion between the waveguides gradually decreases according to the distance from the branching portion. FIGS. 10D and 10E show examples of the B-B ′ cross section of the transition region 102. Here, the difference between FIG. 10 (d) and FIG. 10 (e) is that FIG. 10 (d) has a concave and square cross-sectional shape, whereas FIG. 10 (e) has a U-shaped central portion. It is a point that is rounded. Regarding the cross-sectional direction, the core film thickness of the gap portion may be thinner than the core film thickness of the waveguide portion.

In the figure, 1 is a substrate, 2 is a cladding layer, 3 is a core layer, and 6 is an overcladding layer.

  As described above, by adopting a continuous or almost continuous structure, there is light leaking in the transition region 102 in the vicinity of the bifurcation, but the light moves to each waveguide as it propagates. Therefore, it is possible to reduce the propagation loss at the branching portion.

  When the field distribution in the horizontal direction of light in each region is shown, the field distribution of incident light is 107, the field distribution in the transition region is 108, and the field distribution in the output waveguide is 109, which initially leaks into the transition region 102. It is important to design so that the light that has been drawn is drawn into the output waveguide side. Although only the horizontal distribution is shown in this figure, the continuity of the field distribution in the film thickness direction is also important from the viewpoint of the continuity of the waveguide.

  If the propagating light does not feel optical discontinuity (the length is about several μm), it does not cause a large increase in loss. Therefore, as shown in FIG. The core film thickness of the gap portion may have some undulations.

  In addition, in multi-branch splitters and arrayed waveguide gratings (AWG), the number of branch waveguides is increased, but the shape of the gap portion uses a transition region structure similar to that of a 1 × 2 splitter to reduce branch loss. I can do it.

  As described above, the basic structure for reducing the branching loss of the branched optical waveguide 101 is already known as disclosed in Japanese Patent Laid-Open No. 3-190422. However, in practice, such a structure is simplified and stabilized. It is not easy to manufacture. In many cases, it is necessary to use complicated manufacturing processes or to use unstable manufacturing conditions that are difficult to manage and differ from normal waveguide manufacturing conditions. As a result, the desired characteristics cannot be stably obtained, and the device cost is increased.

  Below, the typical manufacturing method reported conventionally is demonstrated.

  First, the literature (C. van Dam, at al., “Loss reduction for phased-array demultiplexers using a double etch technology”, IPR 1996, pp. 52-55) uses two-step etching for InGaAsP-based AWGs. Propagation loss is reduced by providing two regions, a transition region for shallow etching and a normal waveguide region for deep etching.

  However, this method requires two steps of etching with different etching depths and has a problem that the process becomes complicated. In addition, there is a problem that it is difficult to produce a precise pattern in a stepped structure. As a result, it is effective for a semiconductor with a small film thickness, but there is a problem that the effect of reducing the loss is small with a thick film structure such as a glass waveguide.

  Next, as an example of the manufacturing method according to the prior art, there is a method described in “Optical Waveguide Circuit” in Japanese Patent Application Laid-Open No. 2000-147283. That is, as an example of a conventional manufacturing method, first, a glass film is formed as a lower cladding layer and a core layer. The glass film can be formed by a flame deposition method, a sputtering method, a CVD method, or the like. Next, a resist pattern is formed on the formed glass layer by photolithography. Then, using the resist pattern as a mask, the glass layer is selectively etched by, for example, reactive ion etching to form each waveguide core. At this time, it is stated that if a combination of conditions of each process is optimized, a tapered structure can be realized in the gap portion between the waveguides of the branch waveguide.

  However, in the above manufacturing method, an extremely large number of parameters are involved, and it is not easy to perform all these optimizations. For example, in the above manufacturing method, it is described that a resist pattern exposure condition and a gas type used for etching, a gas mixture ratio, a gas pressure during etching, and a combination of high-frequency power for generating plasma during etching are used. Yes.

  More specifically, it is described that an etching depth (remaining film ratio) varies depending on a pattern width of a gap pattern between waveguides, particularly depending on dry etching conditions. For this reason, as described herein, parameters relating to etching show a very important function for forming a tapered shape. Although there is no detailed description of the factors that cause the etching depth to vary with respect to the pattern width, it is generally known that a phenomenon called a loading effect occurs depending on the pattern density.

  In dry etching, etching by activated etching gas and suppression of etching by deposition of polymer by etching gas decomposition products occur simultaneously and the etching rate is determined by the balance. The ratio of etching to polymer deposition depends on the composition and concentration of the activated gas, so gas mixing ratio, gas pressure during etching, high frequency power for plasma generation during etching, activation by substrate condition It depends on parameters such as the consumption rate of the gas.

  As a result, it is presumed that a desired structure is obtained by using the effect of changing the composition and concentration of the gas in a microscopic region by changing the pattern density. However, the balance between etching and deposition is very fragile, and it has been difficult to perform stable production. Furthermore, since it is necessary to use conditions different from the manufacturing conditions of normal waveguides, it is difficult to produce a normal waveguide structure at the same time. There is a problem that the manufacturing process is complicated, and a manufacturing method capable of forming a tapered structure in the gap portion between the waveguides more easily is required.

  Another method is to reduce the discontinuity by manufacturing the branching waveguide and then irradiating light mainly in the region before branching to change the refractive index distribution of the portion before branching. Has also been reported. However, since it is different from the structure and manufacturing method targeted by the present invention, it will not be described in detail.

  Furthermore, although not the method of manufacturing the structure of interest this time, there are other methods for forming the tapered structure of the waveguide, such as cutting, polishing, and selective selection often used in semiconductor waveguide manufacturing. Using film deposition (growth), shadow mask method "J.of Lightwave Technology vol.8, no.4, pp.587-593, 1990", etc. There are various techniques such as those that have been used and combinations thereof (for example, “JP-A-9-152528”). However, it is easy to apply any of these methods to a wide area, but it is difficult to apply to the formation of the tapered structure of the gap portion between the waveguides to be applied this time, or a complicated manufacturing process is required. It was a problem.

  There are also many reported examples of methods for forming a resist taper structure. It has been described that a resist taper structure can be produced using electron beam exposure or focused ion beam irradiation. However, although these methods are also suitable for forming a taper structure having a somewhat wide region, it is difficult to produce a taper structure in a narrow region such as a branching portion of a waveguide. Actually, it is necessary to carry out exposure while changing the exposure dose depending on the location, but a small amount can be produced, but there is a problem that cannot be said to be a technique suitable for mass production.

  In addition, as a prior art relevant to this invention, the following patent document 1 and patent document 2 can be mentioned.

Japanese Patent Laid-Open No. 03-190422 JP 2000-147283 A

  In view of the above-described problems of the prior art, the present invention provides an optical waveguide capable of simultaneously manufacturing a structure for reducing branch loss and a structure of a normal optical waveguide with simple and high reproducibility, and a method for manufacturing the same. The purpose is to provide.

  The configuration of the present invention that achieves the above object is characterized by the following points.

1) In an optical waveguide partially including a branched structure,
Assuming that the increase rate of the gap width between the branched optical waveguides is changed sequentially from the vicinity of the branch portion to kc, kd, and kb, it is configured to satisfy kc <kd and kd> kb, The gap width increasing rate kc is 0.5 μm or less per 100 μm, and the branched portion has a tapered portion where the thickness of the core layer decreases starting from the branched portion, and the length of the tapered portion is 100 μm or larger. There is.

2) An arrayed waveguide diffraction grating having the optical waveguide described in 1) above is formed at the coupling portion between the arrayed waveguide and the slab waveguide.

3) A method of manufacturing an optical waveguide including a branched structure in part, the step of forming a cladding layer and a core layer on a substrate, the step of forming a resist pattern on the core layer using an exposure mask, In the manufacturing method including the step of processing the core and the cladding layer by dry etching,
As a means for forming a structure in which the core film thickness in the thickness direction changes in the gap portion between the branching waveguides in the vicinity of the branching structure of the waveguide, in the step of forming the resist pattern using the exposure mask, the branching waveguide pattern is guided. The mask pattern and the exposure conditions are selected so that the resist remains in the gap portion between the waveguides, and the remaining resist film thickness is reduced according to the distance from the branch portion, and the formation A step of dry etching the core layer and the clad layer using the resist pattern as a tri-etching mask.

4) The method for producing an optical waveguide according to 3) above, wherein the thickness of the core layer is t, and the ratio of the etching rate of the core layer to the resist in the dry etching process is k (resist etching rate is 1). If the etching rate of the core layer is k), and if the residual resist film thickness in the gap between the waveguides at the branch start point of the branch waveguide pattern is ts, then ts ≧ t / k is satisfied and the taper of the residual resist is satisfied. Including a step of selecting an exposure mask shape and exposure conditions so that the length Lt of the portion is 100 μm or more.

5) The method for producing an optical waveguide according to 3) or 4) above, wherein when the gap width between the waveguides of the exposure mask is 0.5 μm or more, the optical waveguide is dry with respect to the gap width between the waveguides of the exposure mask. Including a step of selecting conditions under which the variation in etching rate during etching is 10% or less.

6) The method for manufacturing an optical waveguide according to 3) to 5) above, wherein a gap pattern width between waveguides at a branch start point of a gap portion between the branch waveguides in an exposure mask is 0.5 μm or more and 1 μm or less. Including a process using the designed mask.

7) The method for producing an optical waveguide according to 3) to 6) above, which comprises a step of using a negative resist pattern formed by exposure as an etching mask for dry etching. Method.

8) The method for producing an optical waveguide according to 7) above, wherein in the step of forming a resist pattern using the optical mask, in addition to exposure using a mask including a branched waveguide pattern at the time of exposure, at least the waveguide Including a step of using exposure having no pattern in the vicinity of the branching portion.

  In the present invention as described above, it has been confirmed that the problems of the prior art can be remarkably improved by improving the resist forming process and the dry etching process at the branch portion. In addition, since the process steps of the present invention have a very high affinity with the conventionally used process procedures, not only the branch portion of the waveguide but also a normal waveguide structure can be formed at the same time.

  According to the method for manufacturing an optical waveguide of the present invention, it is possible to manufacture a structure for reducing the branching loss of a branched optical waveguide and a structure of a normal optical waveguide at the same time with a simple and high reproducibility. An optical circuit including a branched optical waveguide can be manufactured with a low loss and high reproducibility by a simple process. Thereby, cost reduction of the planar optical circuit can be realized.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, FIG. 1 shows a basic manufacturing procedure. In this embodiment, it is necessary to manufacture the structure for reducing the branching loss of the branch optical waveguide and the structure of the normal optical waveguide simultaneously and easily. The procedure is almost the same as the conventional manufacturing procedure. For this reason, the characteristic steps are mainly described in the following examples. In the following description, a 1 × 2 branch circuit will be described for the sake of simplicity. However, as described in the prior art, the present invention can also be applied to circuits such as a multi-branch splitter and an arrayed waveguide grating. Further, the same portions in the following drawings are denoted by the same reference numerals, and redundant description is omitted.

  Hereinafter, the manufacturing method according to the first embodiment of the present invention will be described by taking a 1 × 2 branch circuit as an example. Also in this example, manufacturing was performed according to the basic manufacturing procedure shown in FIG. First, the lower clad layer 2 was formed in order on the substrate 1, and a glass film was formed as the core layer 3. For forming the glass film, a flame deposition method was used, and the refractive index of the core layer 3 was set to be about 1.5% higher than that of the lower cladding layer 2. The film thickness of the core layer 3 was 5 μm. Next, a resist 4 was applied by spin coating, and exposure was performed using an exposure mask 5.

  Here, the point of this example is a method of manufacturing an optical waveguide partially including a branched structure, in which a step of forming a clad layer 2 and a core layer 3 on a substrate 1, In a manufacturing method including a step of forming a resist pattern using an exposure mask 5 and a step of processing the core layer 3 and the cladding layer 2 by dry etching, a gap portion between branch waveguides in the vicinity of the branch structure of the waveguide As a means for forming a structure in which the thickness of the core in the thickness direction changes, a step of forming a resist pattern using the exposure mask 5 is included. In this step, a resist is formed in a gap portion between the waveguides of the branched waveguide pattern. The mask pattern of the exposure mask 5 and the exposure conditions are selected so that 4 remains, and the film thickness of the resist 4 remaining at this time is the distance from the vicinity of the branching portion. Flip formed so as to decrease.

  Next, the core layer 3 and the cladding layer 2 are dry-etched using the resist pattern thus formed as a dry etching mask.

  In a process that has been reported in the past, the resist 4 is produced under the condition that the resist 4 does not remain in the gap portion between the branched optical waveguides 101 or is not controlled during exposure. Further, when the resist 4 is composed of a plurality of layers, the resist 4 is manufactured so that at least the upper layer having high photosensitivity does not remain, and the tapered shape of the resist 4 is not formed.

  In the process according to this embodiment, the structure for reducing the branching loss of the branched optical waveguide 101 can be stably manufactured by intentionally controlling the shape of the resist 4 remaining in the gap portion between the branched optical waveguides 101. Is a feature.

  In order to realize the shapes of these resists 4, the mask pattern is set so that the gap width between the waveguides is about 0.5 to 1.5 μm as a starting point of the branching portion, and the gap width increases as the distance from the branching portion increases. Setting was made to increase. Here, it is important to set the cap width between the waveguides at the starting point of the branching portion to be equal to or smaller than the minimum resolution width of the exposure apparatus. Here, the mask width at which resist residual film does not occur even in part of the gap due to exposure is defined as the minimum resolution width.

  Moreover, as the film thickness of the resist 4, a thick resist 4 having a thickness of 5 μm was used. By using a thick resist having a thickness of 3 μm or more, the minimum resolution width is widened (deteriorated), so that it is difficult to expose a narrow gap pattern, and a good taper shape can be formed. Furthermore, it was confirmed that a normal waveguide pattern portion can maintain a good shape even when a thick resist is used. Since the taper length of the resist can be easily measured by an optical observation method such as a microscope, an exposure amount that provides a desired taper length can be selected.

  FIG. 2 shows a schematic view and a cross-sectional view of the resist pattern that has been prepared. In the produced pattern, the film thickness of the resist pattern for forming the transition region 102 between the gaps between the branched optical waveguides 101 gradually decreases as the distance from the branch start point in the AA ′ cross section is about 200 μm from the branch start point. Thus, the film thickness of the resist 4 is set to zero. Further, the BB ′ cross-sectional shape of the resist 4 was a slightly rounded V shape. Since the shape of the resist 4 is V-shaped, there is an effect of narrowing the relative gap width compared to the concave-shaped resist pattern, so that a high loss reduction effect can be obtained. . Moreover, since the shape of the resist 4 is almost the same as the normal one except for the vicinity of the branch portion of the branch optical waveguide 101, it is possible to simultaneously manufacture a normal waveguide structure.

  In order to form such a shape, the resist 4 may be composed of two or more layers. However, as shown in the figure, it is desirable that the layer is composed of a single layer or the upper layer is thinner than the lower layer resist from the viewpoint of stability of conditions.

  After the resist pattern formation step, the core layer 3 and the cladding layer 2 were etched using the formed resist pattern as a dry etching mask. Finally, the over cladding layer 6 was formed to complete the waveguide manufacturing process.

  Hereinafter, the manufacturing method according to the second embodiment of the present invention will be described by taking a 1 × 2 branch circuit as an example. In this embodiment, it is possible to obtain a further low-loss branching characteristic by selecting specific conditions in the optical waveguide manufacturing method of the first embodiment.

  In the resist formation process of Example 1, as shown in FIG. 3, the thickness of the core layer 3 is t, and the ratio of the etching rate of the core layer 3 and the resist 4 in the dry etching process is k (the etching rate of the resist 4). If 1 is 1, the etching rate of the core layer 3 is k.) If the residual resist film thickness in the gap between the waveguides at the branch start point of the pattern of the branched optical waveguide 101 is ts, then ts ≧ t / and a step of selecting an exposure mask shape and an exposure condition so that the length Lt of the taper portion of the remaining resist is 100 μm or more. Specifically, the mask pattern is set so that the gap width increases as the distance from the branch portion increases. Although depending on the exposure conditions, when a 5 μm thick resist is used, the minimum resolution width can be set to about 1 to 2 μm. Therefore, in order to form a taper of 100 μm or more, the gap width at the branch portion starting point is used. Is set to be about 0.5 μm less than the minimum resolution width, and the mask width at the taper end is set to be the minimum resolution width.

  In the manufacturing method of the low-loss branch structure according to the present embodiment, the remaining amount of the resist 4 before branching and the length of the taper portion of the remaining resist have particularly important functions for obtaining good characteristics. . As a result of examining the branch characteristics of the element using the remaining amount of the resist 4 in the branch portion as a parameter, it was found that the loss increases rapidly when the residual resist film thickness ts <t / k at the branch start point is set. . Further, it was found that when ts ≧ t / k, the loss gradually increases. However, it was found that the increase in loss with respect to the change in the film thickness ts is significantly smaller when ts ≧ t / k than when ts <t / k.

  As a result of a more detailed study, when ts ≧ t / k, the start point of the branch structure formed after dry etching deviates from the position of the designed mask, but the step at the branch start point is small. all right. On the other hand, when the remaining resist film thickness ts <t / k at the branch start point was set, the step at the branch start point became large. That is, it has been found that the level difference at the branch start point causes the loss more than the deviation of the branch start point.

  It should be noted that a higher ratio k of the etching rate of the object to be etched and the etching mask is preferred because there is no deformation of the pattern, but in this processing, the value of k is 1.5-3. It has been found desirable to select a resist.

When k is 1.5 or less, the thickness of the object to be etched and the etching mask are almost the same, so that it is considered that a good taper shape cannot be formed.

  In addition, when k is 3 or more, it is considered that a slight change in the resist remaining amount after the exposure is enlarged when transferred to the workpiece, so that the scattering loss is increased.

  This tendency was more prominent in the 1 × multiple branch circuit used in the AWG or the like than in the 1 × 2 branch circuit. When the length Lt of the taper portion of the remaining resist was 100 μm or less, the loss increased rapidly.

  FIG. 4 is a graph showing the relationship between the taper length and the excess loss of the AWG. With reference to FIG. 4, the loss increases sharply at 100 μm or less, whereas the loss decrease is notable at 100 μm or more. You can clearly see that there is no. Here, the taper length is preferably 200 μm or more. However, since it is not preferable to increase the taper length unnecessarily from the viewpoint of production, 300 μm or less is preferable.

  In addition, since the length of the taper portion of the finally formed core layer is shorter than the length Lt of the taper portion of the remaining resist, the production variation of loss is increased. As a result, it has been found that the length of the taper portion of the remaining resist is preferably more than 100 μm.

  Further, as a mask pattern, a linear pattern 301 can be realized as shown by the dotted line in FIG. 3, but the increase rate of the gap between the waveguides becomes small as shown by the solid line in FIG. Designing with the pattern 302 is particularly effective in forming a longer tapered region. As a result, a longer tapered portion was formed in the vicinity of the branch portion than in the conventional design, and a branch waveguide with a smaller branch loss could be fabricated.

  Here, as a method of forming a long tapered portion, the same effect as in FIG. 3 can be obtained by reducing the branch angle between the two waveguides, but by using the mask pattern of the pattern 302 in FIG. Even if the branching angle is increased, a low-loss branching waveguide can be manufactured, and thus the size can be reduced.

  The effect of this design is particularly high when the angle between the waveguides is fixed due to element design factors, as in the arrayed waveguide grating of FIG.

An optical waveguide formed by this example is shown in FIG. As shown by the dotted line in the figure, the conventional pattern 401 has a structure in which the gap width in the vicinity of the branch increases rapidly in order to separate the arrayed waveguides 404 more quickly. On the contrary, the improved pattern 402 according to the example has a structure in which the spread of the gap in the vicinity of the branch is suppressed as indicated by the solid line in FIG. In addition, the gap is now more rapidly widened away from the branch. That is, in the conventional pattern, when the increase rate of the gap width in the vicinity of the branch portion is ka and the increase rate of the distant portion is kb, ka ≧ kb is satisfied. If the increase rate of the gap width is changed to kc, kd, kb from the vicinity of the branch part, kc <kd and kd> kb are satisfied. More specifically, as shown in FIG. 6, the region length is 100 μm or more as the condition of the gap width increase rate k _{c in} the vicinity of the branch portion, and the gap width at the branch portion starting point is reduced by about 0.5 μm from the minimum resolution width. It is desirable to perform setting so that the mask width at the taper end is the minimum resolution width.

  As a result, the gap width increase rate is small near the branch, and a long taper can be formed. Further, since the gap width suddenly widens away from the branch, it can be avoided that the taper length becomes too long. The shape and length of the tapered part could be controlled stably. As a result, it has become possible to stably produce an arrayed waveguide grating with low loss and little variation in loss characteristics.

  Hereinafter, the manufacturing method according to Example 3 of the present invention will be described. In the manufacturing method of the optical waveguide of Example 1 or Example 2, the dry etching conditions for processing the core layer 3 and the cladding layer 2 were not described in particular, but in this example, specific dry etching conditions were set. By selecting, the reproducibility of loss characteristics at the time of creation and the ease of processing are greatly improved. That is, when the gap width between the waveguides in the exposure mask 5 is 0.5 μm or more, the etching rate variation during dry etching is 10% or less with respect to the gap width between the waveguides in the exposure mask 5. It includes a dry etching process to be selected. However, when a reduced projection exposure apparatus is used, a mask enlarged according to the reduction ratio is used in order to form the same pattern.

  The effect of the present embodiment will be described with reference to FIG. First, in dry etching, it is well known that the etching rate changes according to the density of the pattern. This is thought to be because the activated etching gas and the deposition of the decomposition product of the etching gas proceed simultaneously in parallel, and the etching rate is determined by the balance. For example, according to the conventional manufacturing technique, when the resist gap pattern width at the time of exposure is changed as shown in FIG. 7C, the dry etching process changes the pattern width as shown in FIG. The etching depth can be changed.

  However, as described above, etching and deposition proceed simultaneously in parallel, and as a result of determining the etching rate based on the balance, the shape produced by slight variations in conditions such as the state of the sample may cause a large variation. become.

  On the other hand, as shown in the exposure process of FIG. 7A and the dry etching process of FIG. 7B, if etching conditions that suppress deposition by the manufacturing method according to the present embodiment are used, the gap pattern width of the resist 4 is increased. Variation in etching rate can be reduced. This is because the etching can basically proceed only under the condition that the deposition is suppressed, so that the variation of the etching shape due to the state of the sample is extremely small. As a result, if the resist shape is formed in a tapered shape with good reproducibility according to the exposure conditions, the conditions for dry etching can be fixed, so that the determination of process conditions can be greatly simplified.

  The maintenance of exposure conditions can be ascertained collectively on a single substrate, and it can be ascertained in a much shorter time than the maintenance of dry etching conditions that require confirmation of each individual condition. Therefore, high reproducibility can be obtained.

  Here, when the mask width of the gap portion is less than 0.5 μm, it is difficult to obtain reproducibility because the reproducibility of the pattern of the resist 4 is mainly low.

  When the variation in the etching rate with respect to the gap width is 10% or more, there is a tendency that the etching rate in the region where the gap pattern width is narrow changes rapidly depending on the state of the sample. This region with a narrow gap pattern width is particularly important as a branch start point, but due to fluctuations in the etching rate, the position of the branch start point shifts or a step outside the design occurs at the branch start point, causing loss.

Specifically, the means for setting the fluctuation of the etching rate to 10% or less can be realized by changing the mixing ratio of the etching gas. For example, in a CF-based gas that is common in glass etching, a gas containing a large amount of C or H (for example, CH ₄ or the like) is mixed in addition to a gas (CF ₄ or the like) that mainly performs etching. It is known that a gas containing a large amount of C or H promotes formation of a CF-based or CHF-based polymer. For this reason, what is necessary is just to reduce the ratio of the gas which accelerates | stimulates deposition of these polymers.

  Hereinafter, the manufacturing method according to Example 4 of the present invention will be described. In this embodiment, in the optical waveguide manufacturing methods of Embodiments 1 to 3, by defining the mask design conditions, a lower loss characteristic can be obtained. That is, the loss can be remarkably reduced by using a process using a mask designed so that the width of the waveguide gap pattern at the branch start point of the exposure mask 5 in the tapered portion is 0.5 μm or more and 1 μm or less. It was. However, when a reduced projection exposure apparatus is used, a mask enlarged according to the reduction ratio is used in order to form the same pattern.

  The reason why the loss can be remarkably reduced as described above is that, as the narrow gap is used, it becomes easier to control the film thickness of the remaining resist formed by diffraction of light leaking from the exposure mask 5. It is. Next, it is because the principle loss can be reduced as the width of the starting point of the waveguide gap pattern in the transition region 102 to be fabricated is narrower. For this reason, the gap pattern width is preferably closer to 0 μm in principle. Also, in the production, if a thin resist is used and exposure is performed with an exposure apparatus having a high resolving power, a pattern of 0.5 μm or less can be easily manufactured. However, since it is desirable to use a thick resist from the viewpoint of etching controllability in this manufacturing process, it is not easy to stably manufacture a pattern of 0.5 μm or less. In addition, it has been found that even when the fabrication is performed, it is not desirable in terms of the characteristics of the branch circuit, such as causing loss and disturbance of the waveguide field. Further, when the width of the waveguide gap pattern was made larger than 1 μm, the increase in loss became significant.

  Here, as an application example of this manufacturing method, an arrayed waveguide grating element was manufactured. The core layer 3 was 5 μm thick and Δ1.5%. Although it is set higher than the value of Δ normally used in quartz glass, it is more difficult to reduce the loss as the value of Δ is higher. I can get it.

  FIG. 8 shows changes in loss (minimum loss) at the transmission peak wavelength of the AWG with respect to the mask gap width. The horizontal axis represents the gap width on the mask between the waveguides at the branch start point, and the vertical axis represents the excess loss of the AWG circuit with respect to the reference waveguide. When the starting point of the gap width was 0.5 μm, a very low loss characteristic of 0.6 dB with a loss of 1/2 or less was obtained compared to other conditions (exposure condition 1). In addition, when the starting point of the gap width is 1 μm, by changing the exposure conditions and increasing the residual amount of the resist from the exposure conditions 1, the gap width at which the loss is minimized can be expanded to 1 μm. (Exposure condition 2).

  Hereinafter, the manufacturing method according to Embodiment 5 of the present invention will be described. The present embodiment is a manufacturing method characterized by including a step of using an exposure pattern with a negative resist as an etching mask for dry etching in the first to fourth embodiments.

  In this manufacturing process, comparing the case of using a negative resist with the case of using a positive resist, it is possible to form almost the same pattern if the normally used mask pattern is reversed. It was found that the use of a negative resist is more suitable for manufacturing a structure that reduces the branching loss of the branching optical waveguide 101, and better characteristics can be obtained. In the positive resist pattern, the waveguide portion is a light shielding pattern as a mask, and the gap portion is a pattern through which light is transmitted. For this reason, in order to form a taper structure in a gap part, it is necessary to reduce the amount of exposure from usual. As a result, the stability of the pattern and the like also decreases in the normal pattern.

  On the other hand, in the negative resist, the waveguide portion is a transmission pattern, and the gap portion is shielded from light. For this reason, in order to form a taper structure in the waveguide gap portion, the exposure amount is increased. However, as the exposure amount increases, the stability of the pattern width increases. As a result, it is possible to perform exposure with a stable light amount, and reproducibility is improved. As a result, by making a pattern with a negative resist, it was possible to increase the tolerance of the production conditions as compared with the case of making with a positive resist, and the reproducibility of the loss of the branch circuit could be improved.

  Hereinafter, the manufacturing method according to Example 6 of the present invention will be described. In the manufacturing method according to the fifth embodiment, the manufacturing method according to the fifth embodiment includes a step of using, in addition to exposure using a mask including a branching waveguide pattern at the time of exposure, at least an exposure having no pattern near the branching portion of the waveguide. It is characterized by.

  The effect of the present invention will be described with reference to FIG. First, exposure is performed using an exposure mask 5 having a branched waveguide pattern based on a normal process (FIG. 9A). Next, exposure without a pattern is performed at least in the vicinity of the branching portion of the waveguide (FIG. 9B). At this time, an exposure mask 5 that does not have the light shielding pattern 5 a that shields light in the vicinity of the waveguide branching portion may be used, or exposure may be performed without using the exposure mask 5. In exposure using only the normal exposure mask 5, the gap portion between the waveguides in the vicinity of the branching portion is slightly exposed to light leaking from the light shielding pattern of the waveguide gap portion arranged on the exposure mask 5. Thereby, the thickness of the remaining resist changes depending on the width of the gap portion (FIG. 9C). On the other hand, in the manufacturing method according to the present embodiment, light is evenly exposed even to a light-shielded region by further performing exposure without a pattern. As a result, there is an effect of increasing the exposure amount of the light-shielded region, so that it is possible to increase the remaining amount of resist with respect to the gap width between the waveguides even if the same mask is used. (FIG. 9 (d)). In the description so far, the tapered shape is formed by setting the gap width at the branch start point to be equal to or less than the minimum resolution. However, by using the manufacturing method according to the present embodiment, the taper is tapered even with the mask width exceeding the minimum resolution. It became possible to form a shape. In addition, when the dependency of the residual thickness of the resist on the gap width between the waveguides was examined, it was also possible to reduce the dependency. As a result, a longer tapered portion can be formed even when the same mask is used.

  Furthermore, the length of the taper can be controlled by changing the exposure amount of exposure without a pattern. As a result, even if the same exposure mask 5 is used, it is possible to form a longer taper, thereby reducing branch loss. In addition, in exposure without a pattern, if the exposure amount is excessively large, the entire surface will be exposed, which will adversely affect the normal waveguide pattern, and therefore less than half of the exposure amount that the negative resist resolves. It is preferable for the formation of a good pattern. Further, adjusting the exposure amount having a pattern is effective because it is possible to control both the length of the taper and the resist thickness at the taper starting point.

  Here, the vicinity of the branching portion indicates a region of about 500 μm or less from the starting point of the branching optical waveguide 101 because it is only necessary to irradiate light to the region where the taper is formed. In consideration of the alignment accuracy at the time of exposure, if the exposure mask 5 that performs exposure is also applied to about 10 μm before the branch start point of the branch optical waveguide 101, it is possible to suppress the influence when the positional deviation at the time of exposure occurs. it can. Further, if a gray mask having a high transmittance at the taper branch point and decreasing in transmittance as it moves away from the branch point, the tolerance to the exposure condition can be further increased. By using this manufacturing method, it becomes possible to obtain a desired resist taper shape in the waveguide gap portion more reliably, and therefore it is possible to stably propose the branching loss of the branching optical waveguide.

  In the above description, for simplification of explanation, the value when the reduction is not used as the mask pattern width is shown. However, when the reduction projection exposure apparatus is used, the mask pattern width depends on the reduction ratio. And set to an enlarged value.

  The present invention can be effectively used in industrial fields such as optical communication, optical signal processing, or sensors.

It is explanatory drawing which shows notionally the basic manufacturing process which concerns on embodiment of this invention. It is the schematic diagram and sectional drawing which show the resist pattern obtained by the manufacturing method which concerns on Example 1 of this invention. It is the schematic diagram and sectional drawing which show the resist pattern obtained by the manufacturing method which concerns on Example 2 of this invention. In the said Example 2, it is a graph which shows the AWG excess loss characteristic with respect to a taper length. It is explanatory drawing which shows the aspect at the time of applying the manufacturing method which concerns on Example 2 of this invention to an arrayed-waveguide grating | lattice. In Example 2 of this invention, it is a graph which shows the increase rate of a gap width in comparison with a prior art. It is explanatory drawing which shows the manufacturing method which concerns on Example 3 of this invention in comparison with a prior art. It is a characteristic view which shows the loss characteristic with respect to the mask gap width in the arrayed-waveguide grating | lattice obtained by the manufacturing method which concerns on Example 4 of this invention. It is explanatory drawing which shows the manufacturing method which concerns on the 6th Example of this invention. It is explanatory drawing which shows the structure which reduces the branching loss of the branching optical waveguide based on a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Clad layer 3 Core layer 4 Resist 5 Exposure mask 6 Overclad layer 101 Branch optical waveguide 102 Transition region 401 Conventional pattern 402 Improved pattern 403 Slab waveguide 404 Array waveguide 405 Input waveguide 406 Output waveguide 401 Linear pattern

Claims (8)

In an optical waveguide including a branched structure in part,
Assuming that the increase rate of the gap width between the branched and adjacent optical waveguides changes from kb, kd, and kb in order from the vicinity of the branching portion, kc <kd and kd> kb are satisfied. The gap width increasing rate kc is 0.5 μm or less per 100 μm, and the branched portion has a tapered portion where the thickness of the core layer decreases starting from the branched portion, and the length of the tapered portion is 100 μm or larger. An optical waveguide characterized by being.   An arrayed waveguide diffraction grating comprising the optical waveguide according to claim 1 at a coupling portion between the arrayed waveguide and the slab waveguide. A method of manufacturing an optical waveguide partially including a branched structure, the step of forming a cladding layer and a core layer on a substrate, the step of forming a resist pattern on the core layer using an exposure mask, and the core And a manufacturing method including a step of processing the cladding layer by dry etching,
As a means for forming a structure in which the core film thickness in the thickness direction changes in the gap portion between the branching waveguides in the vicinity of the branching structure of the waveguide, in the step of forming the resist pattern using the exposure mask, the branching waveguide pattern is guided. The mask pattern and the exposure conditions are selected so that the resist remains in the gap portion between the waveguides, and the remaining resist film thickness is reduced according to the distance from the branch portion, and the formation A method of manufacturing an optical waveguide, comprising a step of dry etching the core layer and the cladding layer using the resist pattern as a tri-etching mask.   4. The method of manufacturing an optical waveguide according to claim 3, wherein the thickness of the core layer is t and the ratio of the etching rate of the core layer to the resist in the dry etching process is k (the resist etching rate is 1). The etching rate of the layer is k), and when the residual resist film thickness in the gap between the waveguides at the branch start point of the branching waveguide pattern is ts, ts ≧ t / k is satisfied and the taper portion of the residual resist A method of manufacturing an optical waveguide, comprising a step of selecting an exposure mask shape and exposure conditions so that the length Lt of the film becomes 100 μm or more.   5. The method of manufacturing an optical waveguide according to claim 3, wherein when the gap width between the waveguides of the exposure mask is 0.5 μm or more, the dry mask is used for the gap width between the waveguides of the exposure mask. The manufacturing method of the optical waveguide characterized by including the process which selected the conditions from which the fluctuation | variation of the etching rate becomes 10% or less.   6. The method of manufacturing an optical waveguide according to claim 3, wherein the gap pattern width between the waveguides at the branch start point of the gap portion between the branch waveguides in the exposure mask is designed to be 0.5 μm or more and 1 μm or less. The manufacturing method of the optical waveguide characterized by including the process using a mask.   7. The method of manufacturing an optical waveguide according to claim 3, comprising a step of using a negative resist pattern formed by exposure as an etching mask for dry etching.   8. The method of manufacturing an optical waveguide according to claim 7, wherein in the step of forming a resist pattern using the optical mask, at least branching of the waveguide is performed in addition to exposure using a mask including a branching waveguide pattern during exposure. The manufacturing method of the optical waveguide characterized by including the process of using together the exposure which does not have a pattern in the part vicinity.

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