JPH05164932A - Glass waveguide formed by using si substrate and production thereof - Google Patents

Abstract

(57) [Summary] [Purpose] To realize a glass waveguide with almost no warpage, a large difference in refractive index between the core and the clad, and low loss. [Structure] A groove 10 formed on the surface of a Si substrate 1, a Si oxide film 2 formed by subjecting the inner surface of the groove 10 to a thermal oxidation treatment, and a high-refractive index provided so as to be embedded in the groove 10. It is characterized in that it is provided with a core 3 made of glass, and a clad layer 4 provided on the substrate 1 so as to cover the core 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a glass waveguide using a Si substrate and a manufacturing method thereof.

[0002]

2. Description of the Related Art With the progress of optical fiber communication, optical devices have (1) mass productivity, (2) high reliability (3) no adjustment of coupling, (4) automatic assembly, (5) low loss As a result of increasing demands, waveguide type optical devices have come to the spotlight in order to solve these problems. Among the optical waveguides, the silica glass optical waveguide has a low loss and the connection loss with the optical fiber is very small, so that it is regarded as a promising optical waveguide in the future.

Conventionally, the flame deposition method shown in FIG. 5 is known as a method of manufacturing a silica glass optical waveguide. this is,
(a) Formation of a silica glass porous film (buffer layer) 15 on a Si substrate 1 and a silica glass porous film containing a dopant (Ti or Ge etc.) for controlling the refractive index on the buffer layer 15 (Core glass film) 16 is formed, (b) Planar optical waveguide film is formed by heating transparent quartz glass, (c) Core mask 17 is formed on the core glass film 16, (d) Core mask 17 is masked The three-dimensional optical waveguide (core) 18 is formed by the etching described above, (e) the silica glass porous film 19 to be the clad is formed, and (f) the clad 20 is formed by heating and transparentizing the same (Miyashita : Optical waveguide technology, 1. Recent optical waveguide technology, Opl
usE, NO.78, PP.59-67).

As another method for forming an optical waveguide on a Si substrate, an oxide film is formed on the surface of the Si substrate by thermal oxidation to form a clad layer, and a substantially rectangular core pattern is formed on the clad layer. A method has been proposed in which a cladding layer is formed and finally the whole is covered with a cladding layer (Japanese Patent Laid-Open No. 57-176006).
issue). Further, a method of forming a core in the oxide film after forming the oxide film is also proposed (JP-A-3-10205).
No. 3-13907).

[0005]

However, the conventional method for manufacturing a silica glass optical waveguide has the following problems.

(1) In order to manufacture a glass waveguide having a large refractive index difference between the core 18 and the clad 20, a core glass film 16 having a high refractive index is formed on the buffer layer (or clad layer) 15
If it is formed on the substrate, the whole substrate is warped due to the difference in the coefficient of thermal expansion, and it is difficult to pattern an optical circuit with high dimensional accuracy.

(2) In the above manufacturing method, it was found that the difference in the refractive index between the core 18 and the cladding 20 is limited due to the following reasons in addition to the reason (1). That is, even if the core porous film 16 having a high refractive index is deposited, the dopant for controlling the refractive index is volatilized in the sintering process of FIG. 5 (b) to realize the core 18 having a high refractive index. Is difficult.

(3) When the core layer containing a large amount of the dopant for controlling the refractive index is patterned by the dry etching process as shown in FIG. 5 (d), the difference in etching rate between SiO ₂ and the above dopant is observed. As a result, the etched side surface becomes rough, and scattering loss increases.

(4) Since the sintering process is performed twice and takes time, it is difficult to reduce the cost because of the utility cost.

(5) The core cannot be formed in a rectangular shape by the method of forming a core having a high refractive index in the Si oxide film by diffusion. Therefore, it is difficult to design an optical circuit such as an optical directional coupler, an optical demultiplexer, and an optical filter in which two cores are arranged in parallel, and it is difficult to realize it with good reproducibility. Further, since the refractive index distribution in the core is non-uniform, the light is poorly confined in the core and it is difficult to reduce the loss.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide a glass waveguide using a Si substrate, having almost no warpage, a large refractive index difference between the core and the clad, and low loss. It is in.

[0012]

In order to achieve the above object, a glass waveguide using a Si substrate of the present invention is formed by a groove formed on the surface of the Si substrate and a thermal oxidation treatment on the inner surface of the groove. The Si oxide film thus formed, a high-refractive-index glass core provided so as to be embedded in the groove, and a clad layer provided on the substrate so as to cover the core. The clad layer may be composed of a plurality of layers having different refractive indexes. Further, it is desirable that a dopant such as B or F for controlling the refractive index is added to the Si oxide film.

In order to manufacture the glass waveguide using the above Si substrate, in the manufacturing method of the present invention, a groove serving as a core portion is formed on the surface of the glass waveguide using the Si substrate, and the inner surface of the groove is formed. After subjecting the substrate to a thermal oxidation treatment, a high-refractive-index glass was embedded in the groove to planarize the substrate surface, and a clad glass film was formed on the planarized substrate surface.

Further, another manufacturing method according to the present invention is to form a groove to be a core portion on the surfaces of a pair of Si substrates,
After heat-oxidizing the inner surface of each groove, high-refractive-index glass is embedded in the groove to flatten the substrate surface, and then the two substrates are joined by abutting their cores. is there.

In these manufacturing methods according to the present invention, it is desirable to use, as the glass having a high refractive index, a solution obtained by drying and solidifying an aqueous solution of an alkaline silicate. The groove may be formed by etching using the Si oxide film formed on the surface of the Si substrate as a mask.

[0016]

In the glass waveguide of the present invention, most of the core surface (lower surface and both side surfaces) is surrounded and covered with the low-refractive-index Si oxide film, and the remaining core surface portion (upper surface) is covered with the cladding layer. It has a covered structure. Si
Since the oxide film is formed by subjecting the inner surface of the groove on the surface of the Si substrate to the thermal oxidation treatment, the portion surrounding the core is provided so as to swell inside the substrate, and the inside of the core when light propagates. Contributes to uniform electric field distribution. Therefore, according to the glass waveguide structure of the present invention, it is possible to construct a low-loss optical waveguide having a small difference in the propagation characteristics between the TE mode and the TM mode, and the optical directional coupler and the optical demultiplexer. A wavelength-dependent optical circuit such as an optical device and an optical filter can be realized with little polarization dependence.

Next, according to the manufacturing method of the present invention, a core layer containing a large amount of dopant for controlling the refractive index is formed on the buffer layer as in the prior art, and a rectangular pattern is formed by a dry etching process. -Since it is not necessary to
It is possible to obtain a waveguide with low scattering loss without the roughness of the unevenness on the etched side surface caused by the difference in etching rate between O ₂ and the dopant. Also, since the core shape can be realized in a rectangular shape with good reproducibility, for example,
It is possible to reproducibly obtain the characteristics of an optical circuit in which two core waveguides such as an optical directional coupler and an optical multiplexer / demultiplexer are coupled.

Further, it is only necessary to embed the high-refractive-index glass film for core in the groove, and then to cover the cladding glass film without sintering the glass film for core by a sintering process. Therefore, unlike the prior art, there is no inconvenience such as the warpage phenomenon of the entire substrate caused by the difference in the coefficient of thermal expansion due to the sintering process and the volatilization of the dopant for controlling the refractive index. In addition, since the sintering process only needs to be performed once after the clad film is formed, the utility cost is lower than that of the conventional method.

[0019]

EXAMPLES Next, examples of the present invention will be described.

FIG. 1 shows a sectional structure of the glass waveguide according to the present invention. The figure (a) shows the case where the clad layer is one layer, and the figure (b) shows the case where the clad layer is two layers. In either case, the Si oxide film 2 is formed on the surface of the Si substrate 1, and a core 3 having a substantially rectangular cross section made of high-refractive index glass is formed in the Si oxide film 2, and the lower part of the core 3 is formed. The feature is that the Si oxide film 2 of No. 7 bulges inside the substrate 1. By adopting this glass waveguide structure, the electric field distribution of the core can be kept uniform and an optical circuit with little polarization dependence can be realized. S
The thickness of the i-oxide film 2 is set within the range of several μm to 20 μm, and the thicker it is, the more the propagation loss of the waveguide can be reduced. This Si oxide film 2 is obtained by high temperature thermal oxidation of the surface of the Si substrate 1, as described later. Refractive index of the core 3 n _w is the refractive index n _s of the Si oxide film, the refractive index n ₂ of the refractive index n ₁ second clad 6 cladding layer 4 and the first cladding 5
Is set to a value higher than any of The high refractive index glass of the core 3 is made of SiO ₂ , P, Ge, Ti, Al, Ta,
It is composed of at least one dopant such as Zn, Zr, Li, K, Na, Ln, Er, and Nd. The width W and the thickness T of the core 3 are set within the range of several .mu.m to 20 .mu.m for the single mode transmission, and within the range of several tens .mu.m to 100 .mu.m for the multi mode transmission. The cladding layer 4 and the first cladding layer 5 are made of SiO ₂ , or SiO ₂ is made of B, F, P, Ge and T.
i, Al, Li, K, de such as Na - is one that contains at least one kind of dopant used, the refractive index n _s is equal to the refractive index n _s of the Si oxide film 2, a slightly low or slightly Can be chosen to be a high value. The thickness is selected from the range of several μm to several tens of μm. Second
The clad layer 6 is a layer for adjusting the confined state of light in the core 3, and the confined state of the light is changed by the refractive index n ₂ of the second clad layer 6. The value of n ₂ is set to be a value lower than or slightly higher than the refractive index n ₁ of the first cladding layer 5. The bulging portion 2a of the Si oxide film 2 is important for reducing the propagation loss of the optical signal propagating in the core 3. That is, it is provided in order to prevent the seeping light from the inside of the core 3 from reaching the inside of the Si substrate 1. Thus n _s and n around the core 3
By enclosing it in a layer of ₁ (or n ₂ ) isotropically,
The loss due to the seeping light is suppressed.

FIG. 2 shows an embodiment of a method of manufacturing the glass waveguide shown in FIG. 1 (a) having a single-layered cladding layer. First, as shown in (a), a metal film (for example, WSi, W, Mo, etc.) 8 is formed on the surface of the Si substrate 1, and then a metal film 8 is formed thereon.
A pattern of the photoresist film 9 for forming a groove pattern is formed. The metal film 8 is formed by a sputtering method, a vacuum evaporation method, an electron beam evaporation method or the like which is commonly used. The patterning of the photoresist film 9 can be realized by a photolithography process. Next, as shown in (b), the photoresist film 9
The metal film 8 is patterned by the etching process using the pattern of No. 2 as a mask, and then the Si film is etched by the etching process using these two films 8 and 9 as masks.
A groove 10 to be a core portion is formed on the surface of the substrate 1. Dry or wet etching can be used for the etching process. Next, as shown in (c), Si
The photoresist film 9 and the metal film 8 on the surface of the substrate 1 are removed. Thereafter, as shown in (d), the Si substrate 1 is heated at a high temperature (about 1200 ° C.) to form the Si oxide film 2 on the substrate surface in which the groove 4 is formed. For this thermal oxidation process, a method often used in manufacturing a semiconductor integrated circuit can be used. Next, as shown in (e), a high-refractive index glass film 11 is formed on the surface of the Si oxide film 2, and the groove 4 is filled with high-refractive index glass. This high refractive index glass film 1
1 can be formed by a plasma CVD method, an electron beam evaporation method, an ion plating method, or the like. Furthermore, an aqueous solution of alkali metal silicate Me ₂ O.nSi
O ₂ · xH ₂ O (Me: alkali metal) is added to Si oxide film 2
A method of forming a glass film by applying it to the surface of and then drying it may be used. Here, as Me, those containing at least one of Na, K, Li, N ⁺ R4 and the like are used. Further, an aqueous solution containing P, B, Ti, Ge, Al or the like or an aqueous solution containing a rare earth element may be added to the above aqueous solution as a dopant for controlling the refractive index. For example, P ₂ O ₅ , Na ₂ C
An aqueous solution of O ₃ , Li ₂ CO ₃ , H ₃ BO ₃ , BCl ₃ or the like can be used. Since the process for forming the high refractive index glass film 11 is a low temperature (800 ° C. or lower) process, the dopant for controlling the refractive index is hardly vaporized. Then, unnecessary high-refractive-index glass on the flat surface of the Si oxide film 2 is removed as shown in (f) so that the high-refractive-index glass film 11 exists only in the groove 4. That is, the high refractive index glass left in the groove 4 by this step constitutes the core 3. The removal of the unnecessary high-refractive index glass is performed by dry or wet etching, and further by surface polishing. Finally, as shown in (g),
The clad layer 4 is formed on the substrate so as to cover the entire surfaces of the Si oxide film 2 and the core 3. This clad layer 4
Is the plasma CVD method, the electron beam evaporation method,
It can be formed by an ion plating method, a flame deposition method, or the like. In addition, after forming the clad layer 4, a sintering process may be finally performed so that the glass becomes a denser glass.

FIG. 3 shows another embodiment of the manufacturing method according to the present invention. This is SiO ₂ instead of the metal film 8 of FIG.
This is a configuration using the film 12. First, as shown in (a), S
A SiO ₂ film 12 is formed on the surface of the i substrate 1, and a pattern of the photoresist film 9 is formed thereon. Next, as shown in (b), using the photoresist film 9 as a mask, SiO ₂
The membrane 12 is patterned and then these two membranes 9,
12 is used as a mask to etch the surface of the Si substrate 1,
The groove 10 which will be the core portion is formed. Next, as shown in (c), the photoresist film 9 is removed. Then, as shown in (d), thermal oxidation is applied to the inner surface of the groove 4 of the Si substrate 1 in a high temperature atmosphere to form a Si oxide film 13 having a U-shaped cross section.
The U-shaped Si oxide film 13 is continuous with the flat Si oxide film 12 previously formed on the surface of the substrate 1 and corresponds to the bulging portion 7 of the Si oxide film 2 described in FIG. .. next,
As shown in (e), a high refractive index glass film 11 is formed on the substrate 1, and the groove 10 is filled with high refractive index glass. afterwards
As shown in (f), unnecessary high-refractive-index glass is removed by etching or surface polishing so that only the high-refractive-index glass that becomes the core 3 remains in the groove 10. Finally
As shown in (g), a clad glass film 14 is formed to cover the core 3. In this structure, the SiO ₂ film 12 acts not only as a mask but also as a second cladding layer.

The present invention is not limited to the above embodiment. The core 3 made of high refractive index glass may be formed not only on the front surface of the Si substrate 1 but also on the back surface thereof. By laminating two substrates shown in FIG. 3 (f), it is also possible to realize a waveguide having a structure in which the core 3 is surrounded by the Si oxide film 13 in a substantially symmetrical manner as shown in FIG. The cross-sectional shape of the core is not only rectangular but also triangular, inverted triangular, elliptical,
And so on. Further, when forming the Si oxide films 2, 12 and 13, a dopant such as B or F may be included therein. By doing so, the difference between the refractive index n _{w of the} core and the refractive index n _s of the Si oxide film can be further increased. Further, in FIG. 3, instead of the Si oxide film 12,
If the Si oxide film containing the refractive index control dopant such as B and F is used, the refractive index difference between the core 3 and the Si oxide film 12 can be controlled also by the dopant. further,
The clad layer may have a multi-layered structure including three or more layers.

[0024]

In summary, according to the glass waveguide and the method for manufacturing the same of the present invention, the following excellent effects can be exhibited.

(1) The side surface roughness of the core is extremely small, and a waveguide with low scattering loss can be obtained, and as a result, a low loss waveguide can be realized.

(2) Even if a glass having a high refractive index is used for the core, there is almost no warp phenomenon of the substrate, and the evaporation of the dopant for controlling the refractive index is very small.

(3) Since only one sintering process is required and the manufacturing process can be simplified, the cost of the waveguide can be reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a glass waveguide according to the present invention, where (a) is a case where the cladding layer is a single layer and (b) is a case where the cladding layer is a double layer.

FIG. 2 is a series of process drawings showing an embodiment of the manufacturing method according to the present invention.

FIG. 3 is a series of process drawings showing another embodiment of the manufacturing method according to the present invention.

FIG. 4 is a sectional view showing another embodiment of the glass waveguide according to the present invention.

FIG. 5 is a series of process drawings showing a conventional manufacturing method.

[Explanation of symbols]

 1 Si substrate 2 Si oxide film 3 Core 4 Cladding layer 5 First cladding layer 7 Lower part of core 8 Metal film 9 Photoresist film 10 Groove 11 High refractive index glass film 12 Si oxide film 13 Si oxide film

Claims (8)

[Claims] 1. A groove formed on the surface of a Si substrate, and a Si oxide film formed by subjecting the inner surface of the groove to a thermal oxidation treatment.
A high-refractive-index glass core provided so as to be embedded in the groove and a clad layer provided on the substrate so as to cover the core were used. Glass waveguide. 2. The glass waveguide according to claim 1, wherein the cladding layer comprises a plurality of layers having different refractive indexes. 3. The glass waveguide according to claim 1, wherein a dopant for controlling a refractive index such as B or F is added to the Si oxide film. 4. A groove to be a core portion is formed on the surface of a Si substrate, the inner surface of the groove is subjected to a thermal oxidation treatment, and high refractive index glass is embedded in the groove to flatten the surface of the substrate and flatten the surface. A method of manufacturing a glass waveguide using a Si substrate, characterized in that a clad glass film is formed on the surface of the formed substrate. 5. A pair of Si substrates are each provided with a groove serving as a core portion, the inner surface of each groove is subjected to a thermal oxidation treatment, and a high refractive index glass is embedded in the groove to flatten the substrate surface. Then, after that, the two substrates are joined by butting the core portions of each other together.
A method for manufacturing a glass waveguide using a substrate. 6. The method for producing a glass waveguide according to claim 5, wherein the glass having a high refractive index is obtained by drying and solidifying an aqueous solution of an alkaline silicate. 7. The manufacturing method according to claim 5, wherein the groove is formed by etching using a Si oxide film formed on the surface of the Si substrate as a mask. 8. The method according to claim 6, wherein a dopant for controlling the refractive index such as B or F is added to the Si oxide film.