JP5718215B2 - Optical waveguide and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical waveguide manufacturing process of a high-performance PLC device that mainly plays a central role in an optical communication system and an optical signal processing system.

  Silica-based waveguide optical circuit (PLC) devices have been commercialized as a practical waveguide-type optical component because they have good compatibility with silica-based optical fibers. In order to manufacture a PLC device, a technique for depositing a silica glass film having a thickness of several μm to several tens of μm and a technique for processing into a pattern shape with a width of several μm using photolithography are combined (Non-patent Document 1). . Therefore, the optical characteristics of the optical circuit are determined by a combination of the deposition technique of the core film and the cladding layer and the microfabrication technique. In recent years, development of high performance and high functionality of existing devices has been promoted.

The core film deposition technology of the present silica-based optical waveguide that forms the core of the PLC device mainly includes flame deposition (FHD) method, low pressure chemical vapor deposition (LP-CVD) method, atmospheric pressure chemical vapor deposition (AP-). CVD) method, plasma enhanced chemical vapor deposition (P-CVD) method, etc., each of which is a deposition technique having features. A conventional general silica-based optical waveguide manufacturing process is shown in FIGS. In many cases, the underclad layer 2 is deposited on the silicon substrate 1 before the core film is deposited (FIG. 1A). Next, a core film 3 is deposited on the undercladding layer 2 (FIG. 1B), and after the core film 3 is deposited, an optical waveguide pattern is processed to form a rectangular core 4 from the core film 3 (FIG. 1C). . Subsequently, an over cladding layer 5 is deposited on the under cladding layer 2 and the core 4 (FIG. 1D). In particular, an ultra-high Δ optical waveguide having a high relative refractive index difference (Δ) required for high integration of PLC devices uses a material system having a high refractive index such as SiO ₂ —Ta ₂ O ₅ or SiON as a core. In recent years, active research and development has been carried out because device size can be expected to be reduced.

  In the conventional process for producing a silica-based optical waveguide, heat treatment is generally performed after deposition of the core film and the overcladding layer in order to stabilize the optical film quality of the deposited film. However, when heat-treating the over clad layer for film quality stabilization, the core glass already deposited may crystallize depending on the material system. Since the crystal inside the core becomes a scatterer for the guided light, the propagation loss of the optical waveguide increases, and crystallization of the core glass is an undesirable phenomenon for the optical waveguide. In order to avoid this, in the over clad heat treatment process, a method of performing heat treatment in a temperature range sufficiently lower than the crystallization temperature of the core material or a method of using a core material having a crystallization temperature sufficiently higher than the heat treatment temperature is used. It has been. However, there is a problem that it cannot be produced in a specific material system because the process is complicated, sufficient film quality stabilization cannot be realized, or the material freedom of the core is limited.

Further, it is known that when thick SiO ₂ having no dopant is used as the overcladding layer, since it is a very hard deposited film, there are cases where cracks and gaps may occur, resulting in poor embedding characteristics. In order to solve this problem, when a dopant is added to the over clad layer, such as using SiO ₂ (BPSG) containing B and P as the over clad layer, the deposited film becomes soft and the embedding characteristics are improved. However, when a dopant is present in the overcladding layer, the overcladding heat treatment process diffuses the dopant in the overcladding layer into the core, which clearly promotes crystallization inside the core. It has become. Therefore, as an example of a device for suppressing the diffusion of the dopant into the core, there is a multi-layer over clad structure in which the over clad is deposited not in a single layer but in a multilayer. The feature of this structure is that the layer in contact with the core in the multilayer is a layer having a very low dopant concentration, thereby having a function of suppressing movement of the dopant to be uniformly diffused. However, in this multi-layer overclad structure, the layer mainly in contact with the core causes an increase in propagation loss when the film is not sufficiently densified as a glass film, that is, when the density is not sufficiently increased. Therefore, in this case, the densification of the film is an important factor.

  As for the conventional multilayer clad technology, as in Patent Document 1, the multilayer overclad of materials having different reflow temperatures is used to improve the adhesion between layers and improve the burying characteristics of the optical waveguide, In order to improve the polarization characteristics, and as in Patent Document 2, a two-layer undercladding structure is introduced for the purpose of strengthening the undercladding layer, and thermal deformation that the optical waveguide is subjected to by heat treatment in the manufacturing process is prevented. There have been reports of improvements in reproducibility and characteristics in optical waveguide fabrication.

Japanese Patent No. 2667751 Japanese Patent No. 3070018

“Silica waveguides on silicon and their application to integrated-optic components” M. Kawachi, Opt. Quantum Electron. Vol. 22, pp391-416, 1990.

  For the purpose of reducing propagation loss, a major concern when adopting a multi-layer overcladding structure that prevents dopant diffusion into the core to avoid crystallization of the core glass is mainly the denseness of the layer in contact with the core. If the density is insufficient, there is a problem in that the scattering loss increases because the guided light feels a region where the density is insufficient. In order for the first overcladding layer in contact with the core to function efficiently as a dopant diffusion preventing layer, the dopant in this layer is first reduced as much as possible, and the overlayer after the second layer deposited after the core and this layer is first. It is required that the softening temperature is higher than that of the cladding layer, and this layer needs to be sufficiently densified prior to the second and subsequent overcladding layers. However, in the conventional heat treatment process for forming a multi-layer overclad structure, the first layer in contact with the core and the layer deposited thereafter are not necessarily sufficiently dense.

A conventional 10% -ΔSiO ₂ -Ta ₂ O ₅ optical waveguide will be described below as a specific example. Figure 2 is conventional, after the core layer is deposited, the propagation loss spectrum of the waveguide of the single-layer structure and the two-layer overclad structures 10% -ΔSiO ₂ -Ta ₂ O ₅ optical waveguide was heat-treated at 1100 ° C. After the over cladding layer deposition It is. When the overcladding layer has a single layer structure, a dopant is added to the overcladding layer, and heat treatment is performed after depositing the overcladding layer. When the overclad layer has a two-layer structure, the first overclad layer (diffusion prevention layer) in contact with the core film has a dopant concentration as low as possible, and the second overclad layer is more dopant than the first overclad layer. The concentration is high. The heat treatment is performed once after the second overcladding layer is deposited. In the 1100 ° C. heat treatment with a single layer structure, the propagation loss is remarkably increased, and shows a large increasing tendency especially on the short wavelength side. This can be attributed to an increase in scattering loss due to crystallization of the core glass because the dopant contained in the overcladding layer diffused into the core. In addition, although the increase in loss on the short wavelength side is suppressed in the two-layer structure as compared with the single-layer structure, the propagation loss at the wavelength of 1300 nm is still about 0.6 dB / cm, which is still higher overall, and this is in contact with the core. It is considered that the scattering loss is increased because the overcladding layer is not sufficiently densified.

  The object of the present invention is to avoid the crystallization of the core glass by the heat treatment in the optical waveguide manufacturing process of the high-performance PLC device that plays a central role in the optical communication system and the optical signal processing system, An object of the present invention is to provide a low-loss optical waveguide characterized by suppressing an increase in scattering loss due to insufficient densification of subsequent layers in the vicinity of the core, and a method for manufacturing the same.

In order to achieve the above object, the present invention is configured as described in the claims. That is, as described in claim 1, a substrate, an undercladding layer made of silica-based glass deposited on the substrate, a core made of quartz-based glass and deposited on the undercladding layer and having a light propagation action, the first overclad becomes the first overclad layer of a single layer of SiO ₂ having no dopant deposited on the core and the under-cladding layer, B, and P of SiO ₂ (BPSG) comprising A silica-based optical waveguide including one or a plurality of second overclad layers deposited on the layer, and after forming the first overclad layer, before forming the second overclad layer The first over clad layer is subjected to heat treatment at 1100 ° C. or higher and 1400 ° C. or lower.

Further, as in claim 2, in the optical waveguide according to claim 1, the second over clad layer is composed of a plurality of layers , and at least of each step of forming the second over clad layer. A heat treatment at 1100 ° C. or higher and 1400 ° C. or lower is further performed between one step and the next step after at least one step.

According to a third aspect of the present invention, there is provided a method for producing an optical waveguide, the step of forming an underclad layer made of quartz glass on a substrate, and a light propagation action made of quartz glass on the underclad layer. forming a core having, on the under-cladding layer and the core, forming a first overcladding layer having a single layer of SiO ₂ with no dopant to embed core, first over clad layer And a heat treatment at 1100 ° C. or higher and 1400 or lower , and one or a plurality of second layers made of SiO ₂ (BPSG) quartz-based glass containing B and P on the heat-treated first overcladding layer . And a step of further forming an overcladding layer.

Furthermore, as according to claim 4, a method of manufacturing an optical waveguide according to claim 3, the second over-cladding layer consists of a plurality of layers, each forming a second overclad layer Among these, the method further includes a step of performing a heat treatment at 1100 ° C. or higher and 1400 ° C. or lower between at least one step and a step subsequent to at least one step.

  That is, the present invention is a problem in the conventional heat treatment process in the optical waveguide manufacturing process having a multi-layer overclad structure that prevents dopant diffusion into the core for manufacturing the optical waveguide that avoids crystallization of the core glass. The object is to suppress an increase in scattering loss due to insufficient densification of the first layer in contact with the core and the subsequent layers near the core. As a result, the loss of the optical waveguide can be reduced, and the role of the optical waveguide of the present invention in the PLC device industry is extremely large.

  As described above, according to the present invention, an optical waveguide manufacturing process of a high-performance PLC device that plays a central role in an optical communication system and an optical signal processing system, particularly, an optical waveguide that avoids crystallization of the core glass is manufactured. In the optical waveguide fabrication process with a multi-layer overclad structure that prevents dopant diffusion into the core, the increase in scattering loss is suppressed by sufficiently densifying the first layer in contact with the core and subsequent layers near the core Therefore, it is possible to provide a low-loss optical waveguide.

It is a figure which shows the conventional optical waveguide production process. Conventional illustrates a propagation loss spectrum of the waveguide of the single-layer structure and the two-layer over-cladding structure having a 10% -ΔSiO ₂ -Ta ₂ O ₅ core was heat-treated at 1100 ° C.. It is a figure which shows the two layer overclad embedded optical waveguide of this invention. It is a propagation loss spectrum of the waveguide of the present invention which has a 10% -ΔSiO ₂ -Ta ₂ O ₅ core and the conventional two-layer overclad structure waveguide obtained by heat-treating the first layer in contact with the core at 1100 ° C.

As an example of an embodiment of the present invention, a method for manufacturing a 10% -ΔSiO ₂ -Ta ₂ O ₅ optical waveguide having a two-layer overclad structure will be described. Hereinafter, an outline of a manufacturing process of the present embodiment will be described.

The structure according to Example 1 is shown in FIG. First, an under cladding layer 2 of 15 μm was deposited on the Si substrate 1 by the FHD method, and then a 10% -ΔSiO ₂ -Ta ₂ O ₅ film was deposited as a core film by a sputtering film forming method by 1.5 μm. Subsequently, heat treatment was performed at 900 ° C. for 3 hours in an oxygen atmosphere in order to stabilize the film quality of the core. Next, core processing 4 of the optical waveguide pattern was performed by photolithography and dry etching. The overclad was made into a two-layer overclad structure by the AP-CVD method. First, 0.5 μm of an SiO ₂ layer was deposited as a first layer overclad 6 in contact with the core 4, followed by heat treatment at 1100 ° C. for 3 hours in an oxygen atmosphere. Next, 10 μm of a BPSG layer was deposited as the second overclad 7 and subsequently heat-treated at 1100 ° C. for 3 hours in an oxygen atmosphere. Here, the temperature of the two-layer overcladding film formed by the AP-CVD method is about 450 ° C. When the propagation loss of the optical waveguide having a width of 2.0 μm produced by the above process is evaluated, it becomes about 0.07 dB / cm 2 in the communication wavelength band of 1550 nm, and is a loss that is one standard as a practical PLC device, 0.1 dB. An optical waveguide with a very low loss of / cm 2 or less could be realized. FIG. 4 shows the propagation loss spectrum of the waveguide of the present invention and a conventional two-layer overclad structure waveguide. It can be seen that in the waveguide according to the present invention, an increase in propagation loss on the short wavelength side is suppressed, that is, the first layer in contact with the core is sufficiently densified. Furthermore, when this optical waveguide was made into a cross-sectional shape and observed with a transmission electron microscope (TEM), it was confirmed that crystallization did not proceed inside the core.

In the post-process of manufacturing the PLC device, an optical waveguide was manufactured by the following process when flattening of the over clad layer was required. As in Example 1, first, an under cladding layer of 15 μm is deposited on a Si substrate by FHD, and then a 10% -ΔSiO ₂ -Ta ₂ O ₅ film is formed as a core film by sputtering. μm was deposited. Subsequently, heat treatment was performed at 900 ° C. for 3 hours in an oxygen atmosphere in order to stabilize the film quality of the core. Next, the core processing of the optical waveguide pattern was performed by photolithography and dry etching. The overclad was made into a two-layer overclad structure by the AP-CVD method. First, an SiO ₂ layer having a thickness of 1.5 μm was deposited as a first overcladding layer in contact with the core, and subsequently heat treatment was performed at 1100 ° C. for 3 hours in an oxygen atmosphere. Next, 10 μm of a BPSG layer was deposited as a second layer, and then heat treatment was performed at 1150 ° C. for 3 hours in an oxygen atmosphere. Here, the film formation temperature of the two-layer overclad by the AP-CVD method is about 450 ° C. as in the first embodiment. Here, in order to realize flattening of the over clad layer, the heat treatment temperature after deposition was set to a higher temperature, and the thickness of the first over clad in contact with the core was increased. When the propagation loss of the 2.0 μm-wide optical waveguide produced by the above steps is evaluated, the result is about 0.09 dB / cm 2 in the communication wavelength band of 1550 nm, as in Example 1, and the loss is 0.1 dB / cm or less. It was also confirmed by TEM observation that crystallization did not proceed inside the core.

In this example, for the purpose of manufacturing a 10% -ΔSiO ₂ -Ta ₂ O ₅ optical waveguide having a two-layer overclad structure as an example, the FHD method is used for the underclad layer, the sputter film forming method is used for the core film, and the overclad layer is used. Although deposited using the AP-CVD method, the film forming method of each step is not limited as long as it is a structure that avoids crystallization of the core glass by the heat treatment process having the characteristics of the present invention. In addition, you may combine freely. The heat treatment temperature between the layers was 1100 ° C. or 1150 ° C. at this time. However, if the heat treatment effect is significant at 1400 ° C. or less that does not exceed the Si melting temperature, an appropriate gas atmosphere and temperature depending on the material system are used. Can be selected.

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Under clad layer 3 Deposited core film 4 Processed rectangular core 5 Over clad layer 6 First over clad layer 7 Second over clad layer

Claims (4)

A substrate,
An undercladding layer made of quartz-based glass deposited on the substrate;
A core made of quartz glass and deposited on the under-cladding layer and having a light propagation action;
A first overclad layer of a single layer of SiO ₂ having no deposited dopant over said before and SL core under-cladding layer,
A silica-based optical waveguide comprising SiO ₂ (BPSG) containing B and P, and including one or a plurality of second over-cladding layers deposited on the first over-cladding layer, After forming the over clad layer, the first over clad layer is heat-treated at 1100 ° C. or higher and 1400 ° C. or lower before forming the second over clad layer. An optical waveguide according to claim 1, wherein the second over-cladding layer consists of a plurality of layers, one of each of the steps of forming a pre-Symbol second over-cladding layer, and at least one step, the An optical waveguide, wherein a heat treatment at 1100 ° C. or higher and 1400 ° C. or lower is further performed between at least one step and the next step. Forming an undercladding layer made of quartz glass on a substrate;
Forming a core made of quartz glass and having a light propagation action on the under-cladding layer;
Forming a first overcladding layer of the on the under-cladding layer and the core, before SL single layer of SiO ₂ having no dopant embedding the core,
Applying heat treatment to the first overcladding layer at 1100 ° C. or higher and 1400 or lower;
And further forming one or more second overcladding layers made of SiO ₂ (BPSG) silica-based glass containing B and P on the heat-treated first overcladding layer. An optical waveguide manufacturing method characterized by the above. A manufacturing method of an optical waveguide according to claim 3, wherein the second over-cladding layer consists of a plurality of layers, one of each of the steps of forming a pre-Symbol second overclad layer, at least one step If the between the next step of the at least one step further method for manufacturing a optical waveguide comprising the step of applying a heat treatment at 1100 ° C. or higher 1400 ° C. or less.

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