JP2006208548A - Optical waveguide and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide in which propagation loss is reduced by reducing roughness on the side face of a core and to provide a method of manufacturing the optical waveguide. <P>SOLUTION: The optical waveguide 10 comprises a substrate 11, a clad 12 formed on the substrate 11, and a core 13 arranged in the clad 12. The core 13 is composed of a first core 14 and a second core 15 which is formed on a side face which is parallel to the propagation direction of light in the first core 14, the surface roughness of the second core 15 appearing at the boundary face between the second core 15 and the clad 12 is smaller than the surface roughness of the first core 14 appearing at the boundary face between the first core 14 and the second core 15. In this case, the refractive index of the second core 15 is approximately equal to the refractive index of the first core 14, the second core is formed only on the side face of the first core by an anisotropic etching. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical waveguide having a planar core-clad waveguide shape and a method for manufacturing the same, and more particularly to an optical waveguide having a core structure for reducing the propagation loss and a method for manufacturing the same.

  In an optical waveguide device having a planar core / cladding waveguide shape, a technique for reducing light propagation loss is an important technique for improving device performance. Factors that hinder the reduction of propagation loss include the impurities contained in the core and the core shape, and one of them is the roughness (unevenness) of the interface between the core and the clad. The presence of roughness at the interface between the core and the cladding increases the propagation loss due to light scattering at the interface, and the propagation loss is smaller for optical waveguides with a smaller core cross-sectional area, such as small optical waveguides with strong optical confinement. Tends to increase.

  FIG. 4 is a process diagram showing a typical manufacturing method of a conventional optical waveguide. The optical waveguide is manufactured by forming a core on a lower clad and forming an upper clad thereon. For example, the lower clad 32a is formed on the substrate 31 (FIG. 4A), then the core layer 33 is formed (FIG. 4B), and a resist pattern is formed on the core layer 33 using photolithography. 34 (FIG. 4C), and using the resist pattern 34 as a mask, the exposed core layer 33 is etched by a dry etching method to form a core 35 (FIG. 4D). An upper waveguide 32b is formed so as to cover the core 35 formed in this way, and an optical waveguide is obtained (FIG. 4E). The lower clad 32a and the upper clad 32b function as clads surrounding the core, and light propagates through the core while being repeatedly reflected at the interface between the core and the clad.

  In such a conventional manufacturing method, the roughness (unevenness) at the interface between the core and the clad described above mainly occurs in the step of forming the core 35 by dry etching the core layer 33 (FIG. 4D). Appears on the side of the core 35. This roughness (unevenness) is considered to be caused by unevenness on the side surface of the resist pattern that becomes a mask for etching the core 35. That is, during the dry etching of the core layer 33 using the resist pattern as a mask, the resist is struck in the plasma to roughen the side surface of the resist, and the roughness is transferred to the core 35 below, so that the core This is thought to be caused by roughening the sides.

  If the roughness of the side surface of the core caused by the conventional manufacturing method is expressed by, for example, the maximum height Rz (the difference in height between the lowest and highest portions of the unevenness) that is one of the indicators of surface roughness, it is usually 0. For example, in the case of an optical waveguide having a refractive index difference of several percent or more between the core and the clad having a side of the core of 2 to 3 μm, the propagation loss due to the influence of the roughness becomes 0.1 dB / cm or more. Can be ignored. In the Si wire waveguide to be developed in the future, one side of the core is further reduced to about 0.5 μm, so that the propagation loss due to the roughness of the side surface of the core is further increased. On the other hand, it is difficult to reduce the unevenness on the side surface of the resist pattern that causes the roughness on the side surface of the core, and a technique for reducing the unevenness on the side surface of the resist pattern to the extent that the influence on the propagation loss can be ignored has not been proposed.

As a conventional example for the purpose of reducing propagation loss, the following Patent Document 1 discloses light caused by unevenness on the side surface of a core by forming an organic nonlinear optical material having a refractive index smaller than that of the core around the core. It has been proposed to suppress scattering to reduce propagation loss and non-linear optical effect. Patent Document 2 and Non-Patent Document 1 propose a method in which Si is used as a core material, and the core is formed by dry etching and then thermally oxidized, thereby reducing the irregularities on the side surface of the core. .
JP-A-3-103803 JP 2004-151700 A Kevin K. Lee et al., “Fabrication of ultralow-loss Si / SiO2 waveguides by roughness reduction”, Optics Letters. , Vol. 12, No. 10, pp. 1771-1776 (October 1994)

However, the method described in Patent Document 1 is not sufficient in reducing the propagation loss, and the methods described in Patent Document 2 and Non-Patent Document 1 can be applied only when Si is used as the core material. In the case of using the above material, there is a problem that it cannot be applied, and since SiO ₂ having a significantly different refractive index is formed on the side surface of the core, the SiO ₂ does not function sufficiently as a part of the core, It is considered that the effect of reducing the propagation loss due to the rough side of the core is still insufficient.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical waveguide capable of reducing the roughness of the side surface of the core and reducing the propagation loss, and a method for manufacturing the same. .

  An optical waveguide of the present invention for solving the above problem is an optical waveguide having a substrate, a clad formed on the substrate, and a core disposed in the clad. And a surface of the second core that appears at the interface between the second core and the clad, and a second core formed on a side surface parallel to the light propagation direction of the first core. The roughness is smaller than the surface roughness of the first core appearing at the interface between the first core and the second core.

  According to this invention, the surface roughness of the second core that appears at the interface between the second core and the cladding is smaller than the surface roughness of the first core that appears at the interface between the first core and the second core. Therefore, the roughness of the side surface of the core is reduced, and light scattering caused by the roughness is reduced. As a result, the propagation loss of the optical waveguide can be reduced.

When the optical waveguide of the present invention, the relative refractive index difference between the cladding and the second core and delta _2, the relative refractive index difference between the cladding and the first core and the delta _1, | It is preferable that Δ ₂ −Δ ₁ | ≦ 0.5.

  According to the present invention, since the refractive index of the first core and the refractive index of the second core are substantially equal, the interface between the core and the clad becomes the interface between the second core and the clad. As described above, the interface between the second core and the clad is small in roughness, and light scattering due to the roughness is small, so that the propagation loss of the optical waveguide can be reduced.

  In the optical waveguide of the present invention, it is preferable that the softening temperature of the second core is lower than the softening temperature of the first core.

  According to the present invention, since the softening temperature of the second core is lower than the softening temperature of the first core, the optical waveguide having such an aspect can be obtained by adding a heat treatment process to the manufacturing process. It is advantageous to smooth the surface and reduce its surface roughness.

  An optical waveguide manufacturing method of the present invention for solving the above-described problems includes a substrate, a clad formed of a lower clad and an upper clad formed on the substrate, a first core disposed in the clad, and a first core A method of manufacturing an optical waveguide having a core composed of two cores, the step of forming a lower clad on the substrate, the step of forming a first core layer on the lower clad, and the first Selectively etching the core layer to form a first core, forming a second core layer so as to cover the lower cladding and the first core, and the second core layer The second core layer formed on the side of the first core other than the side parallel to the light propagation direction is removed by anisotropic etching to form the second core on the side of the first core. The lower cladding, the first core, and the Characterized by a step of forming on the clad so as to cover the second core.

  According to the present invention, the second core layer formed on the second core layer other than the side surface parallel to the light propagation direction of the first core is removed by anisotropic etching, and the first core layer is removed. Since it has the process of forming the 2nd core in the side of a core, the 2nd core formed by the process has the surface roughness of the 2nd core which appears in the interface of the 2nd core and a clad 1st. It becomes smaller than the surface roughness of the first core appearing at the interface between the core and the second core. As a result, the roughness of the side surface of the core that forms the boundary with the cladding is reduced, and light scattering caused by the roughness is reduced, so that an optical waveguide with a small propagation loss can be easily manufactured. In the manufacturing method of the present invention, since the second core formed on the side surface of the core is slightly etched back by anisotropic etching, the second core formed on the side surface of the first core is etched back. The surface roughness tends to be smoother, and as a result, the manufacturing method of the present invention can be preferably applied to a small-sized optical waveguide. The second core layer formed on the first core other than the side surface parallel to the light propagation direction is the second core layer formed on the upper surface of the first core, and the first core layer It is a second core layer formed on the upper surface of the lower clad where no core is formed.

In the method of this invention, the relative refractive index difference between the upper cladding and the second core and delta _4, and the first core and ₃ the relative refractive index difference delta between the upper cladding In this case, it is preferable that | Δ ₄ −Δ ₃ | ≦ 0.5.

  According to the present invention, since the refractive index of the first core and the refractive index of the second core are substantially equal, the interface between the core and the clad becomes the interface between the second core and the clad. As described above, the interface between the second core and the clad is manufactured so as to reduce the roughness. As a result, light scattering due to the roughness is reduced, so that an optical waveguide with small propagation loss can be easily manufactured.

  In the optical waveguide manufacturing method of the present invention, the softening temperature of the second core layer is lower than the softening temperature of the first core layer, and the step of forming the second core layer and forming the upper cladding It is preferable to have a heat treatment step between the steps to be performed.

  According to this invention, since the softening temperature of the second core layer is configured to be lower than the softening temperature of the first core layer, the step of forming the second core layer and the step of forming the upper cladding, By providing a heat treatment step between them, the surface of the second core is softened, which is advantageous for smoothing the surface and reducing its surface roughness.

  In the method for manufacturing an optical waveguide according to the present invention, it is preferable that the second core layer is formed by any one of a chemical vapor deposition method, a sputtering method, and a spin coating method.

  In the method for manufacturing an optical waveguide of the present invention, the anisotropic etching is preferably performed by a dry etching method.

  According to the present invention, since the dry etching method is used as the anisotropic etching, it is easy to leave the second core layer only on the side surface of the first core, and the second core can be easily formed. Can do.

  As described above, according to the optical waveguide of the present invention, the roughness of the side surface of the core composed of the first core and the second core is reduced, and light scattering due to the roughness is reduced. The propagation loss of the waveguide can be reduced. Furthermore, according to the optical waveguide of the present invention, since the refractive index of the first core and the refractive index of the second core are substantially equal, the interface between the core and the clad becomes the interface between the second core and the clad. Since light scattering on the second core surface (that is, the interface between the second core and the clad) with low roughness is reduced, the propagation loss of the optical waveguide can be reduced.

  In addition, according to the method of manufacturing an optical waveguide of the present invention, the surface roughness of the second core that appears at the interface between the second core and the cladding is the first roughness that appears at the interface between the first core and the second core. It can be made smaller than the surface roughness of the core. As a result, the roughness of the side surface of the core that forms the boundary with the cladding is reduced, and light scattering caused by the roughness is reduced, so that an optical waveguide with a small propagation loss can be easily manufactured. In the manufacturing method of the present invention, since the second core formed on the side surface of the core is slightly etched back by anisotropic etching, the second core formed on the side surface of the first core is etched back. The surface roughness tends to be smoother, and as a result, the manufacturing method of the present invention can be preferably applied to a small-sized optical waveguide.

  Furthermore, according to the method of manufacturing an optical waveguide of the present invention, since the refractive index of the first core and the refractive index of the second core are substantially equal, the interface between the core and the cladding becomes the interface between the second core and the cladding, Therefore, it can be manufactured so as to reduce the roughness of the interface between the second core and the clad. As a result, light scattering due to the roughness is reduced, so that an optical waveguide with small propagation loss can be easily manufactured. Furthermore, according to the method of manufacturing an optical waveguide of the present invention, the surface of the second core is smoothed by providing a heat treatment step between the step of forming the second core layer and the step of forming the upper cladding. It is advantageous for reducing the surface roughness.

  Below, the optical waveguide of this invention and its manufacturing method are demonstrated with reference to drawings.

(Optical waveguide)
1A and 1B are schematic views showing an example of an optical waveguide according to the present invention. FIG. 1A is a plan view and FIG. 1B is a cross-sectional view thereof. The optical waveguide 10 of the present invention is an optical waveguide having a substrate 11, a clad 12 formed on the substrate 11, and a core 13 disposed in the clad 12. In this optical waveguide 10, a feature of the present invention is that the core 13 includes a first core 14 and a second core 15 formed on a side surface parallel to the light propagation direction of the first core 14. Furthermore, the surface roughness of the second core 15 that appears at the interface between the second core 15 and the clad 12 is the first roughness that appears at the interface between the first core 14 and the second core 15. This is because the surface roughness of the core 14 is smaller.

  The substrate 11 is not particularly limited as long as it is a substrate for an optical waveguide and has sufficient mechanical strength, but a Si substrate, a quartz substrate, other glass substrates, etc. can be usually used. In particular, a Si substrate, a quartz substrate and the like are preferably used. Since these substrates 11 are materials to which semiconductor process technology can be applied, the clad 12 (lower clad 12a and upper clad 12b) and the core 13 (first core 14 and second core 15) are formed on the substrate 11. Can be easily formed. The thickness of the substrate is not particularly limited, but is usually designed with a thickness of about 0.7 to 1.5 mm.

  As shown in FIG. 1, the clad 12 is formed on the base material 11 and the core 13 is disposed therein. Such a clad 12 includes a lower clad 12a and an upper clad 12b. In addition, as long as the core 13 is arrange | positioned in the clad 12, the form of the clad 12 is not specifically limited.

  As a constituent material of the clad 12, quartz, polymer, or the like is used, and quartz is particularly preferably used. In the case where the clad 12 is composed of the lower clad 12a and the upper clad 12b, both have the same or substantially the same refractive index but are made of different materials even if they are made of the same material. There may be. The thicknesses of the lower clad 12a and the upper clad 12b are not particularly limited, but the thickness of both is usually about 8 to 20 μm.

  As shown in FIG. 1, the core 13 is a portion through which light propagates. In the present invention, the core 13 includes a first core 14 and a second core 15, and has a rectangular shape. The second core 15 is formed on a side surface parallel to the light propagation direction of the first core 14. That is, the second core 15 is formed on the left and right side surfaces in the longitudinal direction of the core 13 having a rectangular cross section. The core 13 having such a configuration is disposed in the above-described clad 12, and is formed, for example, in such a manner that it is sandwiched between the lower clad 12a and the upper clad 12b as shown in FIG.

As shown in FIG. 1, the first core 14 is provided so as to be parallel to the substrate surface, and the cross-sectional shape orthogonal to the direction in which the first core 14 extends long is as shown in FIG. Usually, it has a rectangular shape. Such a first core 14 is a main part of the core 13 and is composed of a material used as a core in a conventional optical waveguide, for example, quartz doped with impurities, silicon oxynitride (SiON), polymer. In particular, it is preferable to use quartz doped with impurities. The refractive index of the first core 14 made of such a material is larger than the refractive index of the clad 12 made of the lower clad 12a and the upper clad 12b. Relative refractive index difference Δ ₁ = 2% between the first core and the clad). Here, the relative refractive index difference Δ ₁ between the first core and the clad is Δ ₁ = (the refractive index of the first core−the refractive index of the clad) / the refractive index of the first core × 100 (%), It is an amount defined by For example, when the refractive index of the core 14 made of quartz doped with impurities is 1.48, the refractive index of the clad 12 made of the lower cladding 12a and the upper cladding 12b made of quartz is 1.45, and the first core And the relative refractive index difference between the clad 12 and the clad 12 is about 2%. Note that Si or polymer can be used as the material of the first core 14, but also in this case, the difference in refractive index needs to be the design value of the optical waveguide.

  The composition of each material can be specified by, for example, a plasma emission analysis (ICP) method or a fluorescent X-ray analysis (XRF) method. The refractive index of each of the above materials can be measured by a method in which a prism is made of the same material as that of each of the materials having the composition specified thereby, and an interference spectrum using the prism is measured. By this method, it is possible to measure the lower cladding 12a, the first core 14, the second core 15, and the upper cladding 12b.

  The refractive index of the first core 14 can be adjusted by various methods, and can be controlled by, for example, the type and amount of impurities doped into quartz. In order to increase the refractive index of the first core 14, one or more selected from germanium, phosphorus, nitrogen, boron, aluminum, and lanthanum can be used as a dopant.

  As a method of forming the first core 14, after forming the constituent material on the substrate, it can be formed by, for example, a dry etching method using a photoresist as a mask. The side surface of the formed first core 14 has a rough surface with irregularities, and the surface roughness is measured, for example, from the maximum height Rz (measured from the surface roughness curve) according to JIS B0601-2001. For example, the height is about 0.2 to 0.4 μm.

  The surface roughness is generally calculated from the result of measuring a roughness curve at a predetermined reference length with a stylus type surface roughness meter, a scanning tunneling microscope, or the like. In the optical waveguide of the present invention, since the second core 15 is laminated on the side surface of the first core 14, the surface roughness of the first core 14 cannot be directly measured. Therefore, in the present invention, as a surface roughness measurement of the first core 14, the side surface of the first core 14 is cut in the longitudinal direction so as to be parallel to the substrate surface, and the first surface appearing on the cut surface is obtained. Several interfaces of the core 14 and the second core 15 are extracted, the interfaces are observed with an electron microscope or the like, a roughness curve at a predetermined reference length is measured, and based on the results, for example, in accordance with JIS B0601-2001 The calculated maximum height Rz is calculated.

As shown in FIG. 1, the second core 15 is formed on a side surface parallel to the light propagation direction of the first core 14. The side surface on which the second core 15 is formed is specifically the left and right side surfaces of the first core 14 when FIG. 1 is viewed in plan. Since the second core 15 constitutes the core 13 together with the first core 14, the refractive index thereof is the same as the refractive index of the first core 14, or the relative refraction of the second core and the clad. If the rate difference was delta _2, wherein the delta ₁ relationship with the | Δ ₂ -Δ ₁ | is desirably ≦ 0.5. Here, the relative refractive index difference Δ ₂ between the second core and the clad is Δ ₂ = (refractive index of the second core−refractive index of the clad) / refractive index of the second core × 100 (%), It is an amount defined by As the constituent material of the second core 15, a material having such a refractive index can be used. For example, it is preferable to use the same material as that of the first core 14. The second core 15 having such a refractive index functions as the core 13 together with the first core 14. On the side surface of the core 13, light propagates while being reflected at the interface between the second core 15 and the clad 12. .

  The surface roughness of the second core 15 is smaller than the surface roughness of the side surface of the first core 14 and is a smooth surface with small irregularities. The surface roughness of the second core 15 can be measured by the same method as that of the first core 14, and the maximum height Rz is smaller than the maximum height Rz of the side surface of the first core 14. Specifically, the surface of the second core 15 has, for example, a maximum height Rz (difference in height between the lowest portion and the highest portion measured from the surface roughness curve) according to JIS B0601-2001. For example, it is about 0.1 to 0.3 μm. The second core 15 can be formed with a thin average thickness of about 0.2 to 1.0 μm, for example.

  As described above, in the optical waveguide of the present invention, since the roughness of the side surface of the core 13 composed of the first core 14 and the second core 15 is small, light scattering due to the roughness is small. As a result, the propagation loss of the optical waveguide can be reduced. Note that the optical waveguide of the present invention can reduce propagation loss, and therefore can be preferably applied to a small-sized optical waveguide that is easily affected by roughness of the core side surface.

  Next, another embodiment of the optical waveguide of the present invention will be described.

  The optical waveguide of the present invention can be such that the softening temperature of the second core 15 is lower than the softening temperature of the first core 14 in the optical waveguide of the above embodiment. By heat-treating the core 13 formed of the first core 14 and the second core 15 having such a relationship at a predetermined temperature, the second core 15 is softened and the surface thereof becomes smooth and the surface roughness is increased. Becomes smaller. As a result, the surface of the core 13 becomes smooth, and light scattering due to the roughness of the core surface is suppressed, so that the propagation loss of the optical waveguide can be reduced. The softening temperature here is the same as the softening point defined and measured according to JIS R3103-1: 2001 (viscosity of glass and viscosity fixed point-Part 1: measurement method of softening point). Defined.

  For example, when the first core 14 is made of germanium-doped quartz (softening temperature 950 ° C.), phosphorus-doped quartz (softening temperature 950 ° C.), lanthanum-doped quartz (softening temperature 1000 ° C.), etc. A second core 15 made of BPSG (Boro-Phospho Silicated Glass) having a temperature of 800 ° C. can be used. The core 13 having such a configuration can reduce the surface roughness Rz of the second core 14 to, for example, about 0.2 μm to 0.1 μm by performing a heat treatment at 800 ° C.

(Optical waveguide manufacturing method)
Next, the manufacturing method of the optical waveguide of this invention is demonstrated. FIG. 2 is a process diagram showing an embodiment of a method for producing an optical waveguide of the present invention.

  The optical waveguide manufacturing method of the present invention includes a substrate 11, a clad 12 made of a lower clad 12 a and an upper clad 12 b formed on the substrate 11, a first core 14 disposed in the clad 12, and a first core 14. A method of manufacturing an optical waveguide 10 having a core 13 composed of two cores 15, the step of forming a lower cladding 12 a on a substrate 11, and the formation of a first core layer 14 a on the lower cladding 12 a A step of selectively etching the first core layer 14 a to form the first core 14, and a second core layer 15 a so as to cover the lower cladding 12 a and the first core 14. And the second core layer 15a formed on the second core layer 15a other than the side surface parallel to the light propagation direction of the first core 14 is removed by anisotropic etching. The And a step of forming a second core 15 on both sides of the first core 14, the lower clad 12a, and forming the upper clad 12b so as to cover the first core 14 and second core 15.

  In particular, the present invention is characterized by removing the second core layer 15a formed on the second core layer 15a other than the side surface parallel to the light propagation direction of the first core 14 by anisotropic etching. The method includes the step of forming the second core 15 on the side surface of the first core 14. Here, the second core layer 15a formed on a side other than the side surface parallel to the light propagation direction of the first core 14 is, for example, when the first core 14 is viewed in a plan view of FIG. A second core layer formed on the upper surface of the lower clad, and a second core layer formed on the upper surface of the lower cladding where the first core is not formed.

  Hereafter, the manufacturing method of the optical waveguide of this invention is demonstrated in order.

  First, as shown in FIG. 2A, a lower clad 12a is formed on a substrate 11. The lower clad 12a can be formed of, for example, quartz having a thickness of about 8 to 20 μm. As a film formation method, a chemical vapor deposition method (hereinafter referred to as a CVD method) such as a low pressure CVD method or an atmospheric pressure CVD method is used. ), A sputtering method, an ion plating method, or the like. The lower cladding 12a can also be formed of a polymer or a silica-based coating material, and examples of the film forming method include a spin coating method, a spray coating method, and an embossing method.

  Next, as shown in FIG. 2B, a first core layer 14a is formed on the lower cladding 12a. The first core layer 14a can be formed of, for example, germanium or phosphorus-doped quartz, or Si, and the film formation method is the same as in the case of the lower cladding 12a described above. And an ion plating method. Similarly to the case of the lower clad 12a, it can also be formed of a polymer or a silica-based coating material, and examples of the film forming method include a spin coating method, a spray coating method, and an embossing method. it can.

  Next, as shown in FIG. 2C, a resist pattern 21 for selectively etching the first core layer 14a is formed. The resist pattern 21 can be formed by forming a resist film on the first core layer 14a, and exposing and developing the resist film using a mask.

Next, as shown in FIG. 2 (d), the first core layer 14 a is selectively etched to form the first core 14. The first core 14 can be formed by etching the first core layer 14a using the resist pattern 21 as a mask. As the etching, it is preferable to apply a dry etching method using, for example, a fluorocarbon gas (for example, CF ₄ ) as a main component. In particular, dry etching capable of anisotropic etching is desirable, and the side surface of the first core 14 obtained after the etching can be formed into a substantially vertical shape by such anisotropic etching. After the first core 14 is formed, the resist pattern 21 serving as a mask is removed by a method such as plasma ashing using oxygen.

Next, as shown in FIG. 2E, a second core layer 15a is formed so as to cover the lower clad 12a and the first core. The second core layer 15a is preferably formed of a material having the same refractive index as that of the first core layer or a material having substantially the same refractive index. For example, the material forming the second core layer 15a is , clad on its refractive index is the same as the refractive index of the first core 14, or, and the second core, and ₄ the relative refractive index difference between the upper clad Δ, which will be described later, the first core, which will be described later the relative refractive index difference when the delta ₃ and, | it is preferred that a ≦ 0.5 _| Δ ₄ -Δ _3. Here, the relative refractive index difference Δ ₄ between the second core and the upper clad is Δ ₄ = (refractive index of the second core−refractive index of the upper clad) / refractive index of the second core × 100 (% ), And the relative refractive index difference Δ ₃ between the first core and the upper clad is Δ ₃ = (the refractive index of the first core−the refractive index of the upper clad) / the refractive index of the first core × It is an amount defined by 100 (%).

  As a film forming method for the second core layer 15a, it is preferable to form the second core layer 15a by an isotropic film forming method such as a CVD method such as an atmospheric pressure CVD method or a low pressure CVD method, or a sputtering method. Are formed with a substantially uniform thickness on the top surface and the top surface and side surfaces of the first core 14. In particular, in the CVD method, surface migration occurs during film formation, so that a thick film is formed on the concave portion of the first core 14 and a thin film is formed on the convex portion. As a result, the surface of the second core layer 15a is a relatively smooth surface with small irregularities. In particular, as an example of a film forming method capable of obtaining a smooth surface by surface migration, an atmospheric pressure CVD method using tetraethoxysilane (TEOS) as a Si raw material can be preferably exemplified. Further, when a polymer or silica-based coating material is used as the material of the first core 14, it is preferable to use the same material as the material of the second core layer 15. In that case, a spin coating method or the like is used. Can be used to form a film. The thickness of the second core 15 to be formed is preferably larger than the surface roughness (maximum height Rz) on the side surface of the first core 14, for example, the maximum height Rz on the side surface of the first core 14. Is 0.2 μm, the thickness of the second core 15 is preferably about 0.4 μm.

  Next, as shown in FIG. 2 (f), the second core layer 15 a formed on the second core layer 15 a other than the side surface parallel to the light propagation direction of the first core 14 is anisotropic. The second core 15 is formed on the side surface of the first core 14 by removing by etching.

The anisotropic etching in the present invention refers to the second core layer 15a formed on the upper surface of the first core 14 and the second cladding layer 12a formed on the upper surface of the lower cladding 12a where the first core 14 is not formed. This is a means for selectively etching only the second core layer 15a, and an etching means capable of leaving the second core layer 15a formed on the side surface of the first core 14. That is, the anisotropic etching in the present invention is weak in the direction parallel to the substrate surface and strong in the direction perpendicular to the substrate surface. This anisotropic etching is performed until the second core layer 15a formed on the lower cladding 12a and the second core layer 15a formed on the upper surface of the first core 14 are removed. preferable. By such anisotropic etching, the second core layer 15 a can be left only on the side surface of the first core 14. At this time, if the second core layer 15a remains on the lower clad 12a, it becomes an unnecessary light propagation path, which causes an increase in propagation loss of the optical waveguide. Therefore, it is particularly important to remove the second core layer 15a on the lower cladding 12a. As anisotropic etching, a dry etching method can be preferably used. For example, for a quartz-based material, it is preferable to use a dry etching method using a fluorocarbon gas (for example, CF ₄ ) as a main component.

  As a result of this anisotropic etching, the second core layer 15a formed on the side surface of the first core 14 undergoes so-called etch back, and the second core layer 15a becomes slightly thin and the surface thereof becomes smooth. And a smooth surface. In the etch back, unlike the etching for forming the first core 14, there is no resist pattern 21 (see FIG. 2C) serving as a mask. The surface of the formed second core 15 is not roughened. For example, as described above, when the maximum height Rz on the side surface of the first core 14 is 0.2 μm and the thickness of the second core 15 is about 0.4 μm, the second after the etch back is performed. The average thickness of the core 15 can be about 0.2 μm. This can also be applied to a small-sized optical waveguide having a rectangular core having a side of 2 μm or less, for example.

  Next, as shown in FIG. 2G, the upper clad 12 b is formed so as to cover the lower clad 12 a, the first core 14, and the second core 15. The upper clad 12b is preferably formed of the same material as the lower clad 12a described above, or a material having the same or substantially the same refractive index as the lower clad. As a method for forming the upper clad 12b, it is preferable to manufacture the film by the same method as that for the lower clad 12a. The thickness of the upper clad 12b is usually 8 to 20 μm. Thus, the optical waveguide according to the present invention is manufactured.

  FIG. 3 is a schematic plan view for explaining the surface roughness of the side surface of the core constituting the optical waveguide of the present invention.

  FIG. 3A is a plan view of the form shown in FIG. 2D described above, and is a schematic plan view illustrating the degree of roughness of the side surface of the first core. After the first core 14 is formed by etching, the degree of roughness of the side surface of the first core 14 is large, and the roughness of the resist pattern 21 (FIG. 2 (c) when the first core layer 14a is etched. It is considered that the unevenness present on the side surface of (1) is formed by the phenomenon of being transferred to the side surface of the first core 14. The unevenness present on the side surface of the resist pattern is generated in a photolithography process when forming the resist pattern, or is generated by plasma irradiation when the second core layer 15a is formed by a dry etching method.

  FIG. 3B is a plan view of the form shown in FIG. 2E described above, and is a schematic plan view illustrating the degree of roughness of the side surface of the core 13 after the second core layer 15a is formed. is there. The second core layer 15 a is formed with a thickness larger than the surface roughness of the side surface of the first core 14. The second core layer 15a is preferably formed by an isotropic film forming means, and the surface of the second core layer 15a is a relatively smooth surface.

  FIG. 3C is a plan view of the form shown in FIG. 2F described above, and is a schematic plan view for explaining the degree of roughness of the side surface of the core 13 after anisotropic etching. Since the etch back is slightly caused by the anisotropic etching, the surface roughness of the second core layer 15a becomes smaller and the surface becomes smooth.

(Other embodiments)
Another embodiment of the optical waveguide manufacturing method of the present invention will be described.

  In the optical waveguide of the present invention, the softening temperature of the second core layer 15a is made lower than the softening temperature of the first core layer 14a, and the step of forming the second core layer 15a and the upper cladding 12b are formed. It can manufacture by providing a heat treatment process between processes. Therefore, the heat treatment process may be performed between the formation process of the second core layer 15a and the anisotropic etching process, or may be performed between the anisotropic etching process and the formation process of the upper clad 12b. By doing so, the surface of the second core layer 15a can be softened, and the surface energy can be reduced, that is, a smooth surface can be obtained.

  The heat treatment conditions in the heat treatment step can be arbitrarily set depending on the material for forming the first core layer and the material for forming the second core layer. For example, quartz (phosphorus doped) having a softening temperature of 950 ° C. is used as a constituent material of the first core layer, and BPSG (boron phosphorus silicate glass: Boro-phosphosilicate glass having a softening temperature of 800 ° C. is used as a constituent material of the second core layer. When Silicated Glass is used, it is preferable to perform a heat treatment at a temperature at which the second core layer BPSG is softened, for example, about 800 ° C. The upper limit temperature is preferably less than the softening temperature of the first core, but even if the temperature is lower than the softening temperature of the first core, the first core and the lower cladding are adversely affected. Therefore, it is desirable to perform heat treatment at a temperature at which such a problem does not occur.

  The heat treatment atmosphere is preferably nitrogen, and the heat treatment time is arbitrarily set so that the second core layer softens and the surface becomes smooth.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
A Si substrate having a thickness of 0.8 mm was used as the substrate, and a lower cladding having a thickness of 10 μm was formed on the substrate. The lower clad was quartz having a refractive index of 1.45, and was formed by an atmospheric pressure CVD method. Next, a first core layer having a thickness of 3 μm was formed on the lower cladding. The first core layer was made of quartz having a refractive index of 1.5 doped with 4 mol% of lanthanum, and was formed by the atmospheric pressure CVD method similar to the lower clad.

Next, a resist pattern for forming a core having a width of 3 μm and a light propagation direction length of 10 cm is formed on the first core layer by photolithography, and then dry etching is performed using the resist pattern as a mask. The first core layer was etched by this method to form a first core having a width of 3 μm and a length of 10 cm in the light propagation direction. The dry etching method in this case was performed by plasma etching using a gas containing CF ₄ as a main component. Note that the resist pattern after the etching was removed by plasma ashing using oxygen.

  Next, a second core layer having a thickness of 0.5 μm was formed so as to cover the lower cladding and the first core. The second core layer is made of quartz doped with phosphorus and having a refractive index of 1.5, and was formed by the atmospheric pressure CVD method similar to the first core layer. The average film thickness of the second core layer formed on the side surface (side wall) of the first core was 0.4 μm. Next, the second core layer is anisotropically etched to remove the second core layer formed on the first core and the second core layer formed on the lower cladding. Thus, the second core layer was left only on the side surface of the first core. The anisotropic etching at this time was performed under the same conditions as the dry etching of the first core layer. By the anisotropic etching, the second core layer formed on the side surface of the first core was slightly etched back, and the average film thickness of the second core after the etch back was 0.2 μm.

  Next, an upper clad having a thickness of 10 μm is formed by the same method as the lower clad so as to cover the lower clad and the core composed of the first core and the second core, and the optical section having a core cross section of 3 μm × 3 μm is formed. A waveguide was manufactured.

  About the obtained optical waveguide, the surface roughness of each core was measured. First, the side surface of the first core is cut in the longitudinal direction so as to be parallel to the substrate surface, and 10 interfaces between the first core and the second core appearing on the cut surface are extracted, and the interface is defined as an electron. It observed with a microscope etc., the roughness curve in predetermined | prescribed reference | standard length (500 micrometers) was measured, and the maximum height Rz based on JISB0601-2001 was computed and measured from the result. Thus, the surface roughness of the first core appearing at the interface between the first core and the second core was measured. Next, the surface roughness of the second core appearing at the interface between the second core and the clad was measured by the same method. As a result, the surface roughness of the first core that appears at the interface between the first core and the second core is 0.4 μm at the maximum height Rz, and the first surface that appears at the interface between the second core and the clad. The surface roughness of the core No. 2 was 0.2 μm at the maximum height Rz.

  Next, the propagation loss per unit length of light having a wavelength of 1550 nm was measured. The propagation loss was measured by the cutback method, and a result of propagation loss of 0.12 dB / cm was obtained. Here, the cutback method is a method of calculating propagation loss from a difference in output caused by changing the length of the optical waveguide.

(Example 2)
The material for forming the second core layer in Example 1 was changed to quartz having a refractive index of 1.5 (BPSG) doped with boron and phosphorus, and the second core layer forming step and anisotropic etching step The optical waveguide of Example 2 was manufactured in the same manner as the optical waveguide of Example 1, except that a heat treatment step was performed between the two.

  The softening temperature of the second core layer forming material used in Example 2 is 800 ° C., which is lower than the softening temperature (1000 ° C.) of the first core layer forming material. As the heat treatment step, heat treatment was performed at 800 ° C. for 2 hours in a nitrogen atmosphere to smooth the surface of the second core layer.

  For the obtained optical waveguide, the surface roughness of each core was measured in the same manner as in Example 1. As a result, the surface roughness of the first core that appears at the interface between the first core and the second core is 0.4 μm at the maximum height Rz, and the first surface that appears at the interface between the second core and the clad. The surface roughness of the core No. 2 was 0.15 μm at the maximum height Rz. Next, the propagation loss per unit length of light having a wavelength of 1550 nm was measured by the same method as in Example 1, and a result of propagation loss of 0.10 dB / cm was obtained.

(Comparative Example 1)
An optical waveguide of Comparative Example 1, which is a conventional optical waveguide, was manufactured in the same manner as in Example 1 except that the second core in Example 1 was not formed.

  For the obtained optical waveguide, the surface roughness of each core was measured in the same manner as in Example 1. As a result, the surface roughness of the first core appearing at the interface between the first core and the clad was 0.4 μm at the maximum height Rz. Next, the propagation loss per unit length of light having a wavelength of 1550 nm was measured by the same method as in Example 1, and a result of propagation loss of 0.20 dB / cm was obtained.

It is a form figure which shows an example of the optical waveguide of this invention. It is process drawing which shows one Embodiment of the manufacturing method of the optical waveguide of this invention. It is a schematic plan view for demonstrating the surface roughness of the side surface of the core which comprises the optical waveguide of this invention. It is process drawing which shows the typical manufacturing method of the conventional optical waveguide.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical waveguide 11, 31 Board | substrate 12, 32 Cladding 12a, 32a Lower cladding 12b, 32b Upper cladding 13, 35 Core 14 1st core 14a 1st core layer 15 2nd core 15a 2nd core layer 21, 34 Resist pattern 33 Core layer

Claims (8)

An optical waveguide having a substrate, a clad formed on the substrate, and a core disposed in the clad,
The core includes a first core and a second core formed on a side surface parallel to the light propagation direction of the first core,
The surface roughness of the second core appearing at the interface between the second core and the clad is larger than the surface roughness of the first core appearing at the interface between the first core and the second core. An optical waveguide characterized by being small. When the second core relative refractive index difference between the cladding and delta _2, the relative refractive index difference between the cladding and the first core and _{Δ 1, | Δ 2 -Δ 1} | ≦ 0 The optical waveguide according to claim 1, wherein the optical waveguide is .5.   The optical waveguide according to claim 1, wherein a softening temperature of the second core is lower than a softening temperature of the first core. An optical waveguide manufacturing method comprising: a substrate; a clad formed of a lower clad and an upper clad formed on the substrate; and a core composed of a first core and a second core disposed in the clad. ,
Forming a lower clad on the substrate; forming a first core layer on the lower clad; and selectively etching the first core layer to form a first core. And a step of forming a second core layer so as to cover the lower cladding and the first core, and a side other than the side surface of the second core layer parallel to the light propagation direction of the first core. Removing the second core layer formed on the first core by anisotropic etching to form a second core on a side surface of the first core, the lower cladding, the first core, and the second core And a step of forming an upper cladding so as to cover the core. Wherein the second core relative refractive index difference between the upper cladding and delta _4, the relative refractive index difference between the upper cladding and the first core when the _{Δ 3, | Δ 4 -Δ 3} | The optical waveguide manufacturing method according to claim 4, wherein ≦ 0.5.   The softening temperature of the second core layer is lower than the softening temperature of the first core layer, and there is a heat treatment step between the step of forming the second core layer and the step of forming the upper cladding. The method for manufacturing an optical waveguide according to claim 4, wherein:   The optical waveguide according to claim 4, wherein the second core layer is formed by any one of a chemical vapor deposition method, a sputtering method, and a spin coating method. Manufacturing method. The method for manufacturing an optical waveguide according to claim 4, wherein the anisotropic etching is performed by a dry etching method.

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