JP2738121B2 - Method for manufacturing silica-based optical waveguide - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a silica-based optical waveguide, and more particularly to a method for compensating for a shape deviation in a patterning step of the optical waveguide.

[Prior Art] In recent years, a so-called quartz optical waveguide is formed on a Si or quartz substrate, and optical circuits such as an optical multiplexer / demultiplexer, an optical directional coupler, an optical star coupler, and an optical filter are planarly configured. Research on optical integrated circuits is active. When configuring this optical integrated circuit, the thickness and width of the core through which light propagates are usually set to improve the matching with the silica-based single mode optical fiber.
It is designed to a value of around 10 μm.

FIG. 3 shows a process chart of a method of manufacturing a silica-based optical waveguide previously proposed by the present inventors. As shown in the figure, first, a buffer layer 2 having a low refractive index and a core layer 3 having a desired thickness T (about 10 μm) are formed on a substrate 1 (SiO ₂ glass) ((a) in the figure). A (WSi) film 4 is formed (FIG. 3B), a photoresist 5 is applied thereon, and patterning is performed by photolithography (FIG. 3C). The metal film 4 is patterned using the photoresist 5 as a mask. Next, based on the mask of the photoresist film 5 and the metal film 4, the film of the core layer 3 having a thickness T of about 10 μm formed on the low refractive index layer is patterned by dry etching to have a width W ( It is processed into a rectangular pattern of about 10 μm) (FIG. 2D). Lastly, after removing the photoresist film 5 and the metal film 4 to obtain two cores 31 and 32 (FIG. 10E), the cladding film 6 having a low refractive index is obtained.
(FIG. 1F), a buried quartz optical waveguide is realized.

[Problems to be Solved by the Invention] However, when a directional coupler type optical duplexer having a structure as shown in FIG. Spacing S of about 2 μm over the length
When trying to realize a structure arranged in parallel with maintaining
It has been found that the following problems occur.

That is, using the pattern of the photoresist film 5 and the metal film 4 as a mask, a film of the core layer 3 having a thickness T of about 10 μm is formed.
When processed into a rectangular shape by the dry etching process,
Width W of the core 31 and 32 than the mask width W _0, each time the creation
The width was reduced in the range of 0.2 to 1.0 μm.

This width reduction is due to an undercut at the time of dry etching, and is a phenomenon that cannot be avoided if the core thickness T is as large as about 10 μm. And
This reduction in width has led to the problem of causing a shift in the center wavelength of the optical demultiplexer and a decrease in isolation in the stop band.

FIGS. 5A to 5C show the center wavelength shift characteristics. This shows the results of the sensitivity analysis characteristics calculated by the present inventor for the basic optical multiplexer / demultiplexer shown in FIG. 5 (d), where (a) shows the deviation of the core width W. (B) is the relative refractive index difference Δn of the cladding relative to the core.
(B) shows the deviation characteristic of the thickness T, and (b) shows the deviation characteristic of the thickness T.

By considering the width reduction in advance when designing a photolithography mask, some improvement is possible, but no significant improvement can be expected. Because the core interval S is originally about 2 μm, when the width reduction is taken into consideration for a photolithographic mask, the interval is 0.5 to 1.5 μm.
m, which is a value that is difficult to realize depending on the mask production accuracy and the resolution of photolithography.

The object of the present invention is to solve the above-mentioned problems of the prior art,
An object of the present invention is to provide a method for realizing a silica-based optical waveguide having desired mask accuracy.

[Means for Solving the Problems] The method of manufacturing a quartz-based optical waveguide according to the present invention includes a method in which a metal and photoresist film patterned mask is formed on a core glass film formed on a substrate having a low refractive index layer. Make up,
Based on the two masks, the core glass film is patterned into a rectangular shape by dry etching, and then a thin film of the core glass for compensation is formed on the surface of the pattern from which the photoresist film has been removed by a low-pressure plasma CVD method. After the etching of the compensating core glass film and the etching of the metal film until the surface of the metal film is exposed by dry etching, the cladding film is coated. As the compensating core glass thin film, one having a refractive index equal to or lower than that of the core glass film or a softening temperature equal to or lower than that of the core glass film can be used. However, instead of the compensating core glass thin film, a two-layer film of a compensating core glass thin film and a cladding glass thin film may be formed.

[Operation] A specific embodiment of the present invention is, for example, a method in which a photoresist and a metal film are patterned on a core glass film formed on a substrate, and the core glass film is subjected to a dry etching process using the patterned film as a mask. After patterning into a rectangular shape, a compensating core film having a thickness of 0.1 μm to 1 μm is formed on the patterned surface from which only the photoresist has been removed by a reduced pressure plasma CVD method. By etching, the compensating core film is etched until the metal film surface is exposed, and thereafter, the metal film is etched and then the clad film is covered, thereby compensating for the decrease in the core width due to dry etching. Things. The compensation core film is made of a material having a refractive index equal to or slightly lower than the refractive index of the core.

The reason why the core width can be reduced by the compensating core film is as follows.

When a glass film is formed by a low-pressure plasma CVD method, a uniform thickness can be formed even on a pattern surface having a large unevenness with a large aspect ratio. Therefore, first,
A compensation core film having a thickness corresponding to the decrease in the core width of the rectangular core is formed by a low-pressure plasma CVD method. Next, the metal film and the compensation core film on the buffer tank are etched by dry etching. However, in this dry etching, the etching rate in the thickness direction is much higher than the etching rate in the direction perpendicular to the thickness direction (5 times or more).
Therefore, the compensation core film on the metal film and the buffer layer is etched, but the compensation core film on the side surface of the rectangular core is hardly etched. As a result, a decrease in the core width can be compensated.

The method of the present invention also has another feature that light scattering loss due to surface roughness on the side surface of the core can be significantly reduced by applying the compensation core film.

Embodiment FIG. 1 shows an embodiment of a method for compensating a shape of a quartz optical waveguide according to the present invention.

FIG. 3A shows a schematic cross section when the photoresist film 5 on the metal film 4 is removed after the dry etching shown in FIG. 3D.

Here, quartz glass, multi-component glass, sapphire, Si, or the like is used for the substrate 1. The buffer layer 2, SiO _2, or a B to _{SiO 2, P, Ti, Ge} , Ta, Al, the refractive index control additives such as F which contains at least one, the refractive index thereof is n _b . The cores 31 and 32 have a refractive index of n _w (n _w
> N _b) of Ti to _{SiO 2, P, Ge, Al} , B, Ta, F, Er, Nd, Na, Yb, K, Sm, Z
A material containing at least one kind of additive for controlling the refractive index such as n, or SiO ₂ is used. For the metal film 4, W, WS
i, MoSi, Mo, Cr, αSi and the like are used.

In order to construct a single mode optical waveguide, the thicknesses To of the cores 31 and 32 are set to about 8 μm, the width Wo is set to about 10 μm, and the refractive index difference between the core and the buffer layer is set to about 0.1%. Also, when trying to construct a directional coupler, a multiplexer / demultiplexer, etc.,
The interval So between the first and second 32 is set to about 2 μm.

In the stage of FIG. 1A, for example, Wo = 10 μm,
When dry etching was performed with So = 2 μm and To = 8 μm, W ₁ was about 9 μm and S ₁ was about 3 μm.

Therefore, as shown in FIG. 1 (b), a compensation core film 8 having a thickness equal to or slightly lower than the refractive index of the cores 31 and 32 was formed to a thickness of 0.5 μm by a low pressure plasma CVD method.

Here, the low pressure plasma CVD method was performed as follows. That is, the substrate of FIG. 1A was placed on the lower electrode of the reaction vessel, heated to 350 ° C., and the inside of the reaction vessel was kept at 10 ⁻³ Torr. Next, a vapor of Si (OC ₂ H ₅ ) ₄ and PO (OC ₂ H ₅ ) ₃ is sprayed onto the above-mentioned substrate together with a carrier gas of O _{2 in} the form of a shower from the upper electrode side facing the lower electrode. Was. By applying high-frequency power between both electrodes, plasma was generated, and the compensation core film 8 was formed.

The refractive index of the compensating core film 8 formed in this state is lower than those of the cores 31 and 32. That is, since the compensating core film 8 is formed at a low temperature, the refractive index has a slightly low value, but before or after the final (e) cladding film formation, annealing is performed at a high temperature (> 1200 ° C.). By doing so, the refractive index changes to a value substantially equal to the refractive index of the cores 31 and 32.

Next, as shown in (c), the metal film 8 and the compensating core film 8 on the buffer layer 2 are etched using a dry etching process. This dry etching is CHF ₃
This is performed using a gas and a reactive ion etching apparatus.

Thereafter, as shown in FIG. 1D, the metal film 4 is removed by dry etching using NF ₃ gas.

Then, as finally shown in (e), the refractive index n _o (n _o <
The cladding 6 of n _w ) is formed. Before or after this process (e), the substrate is annealed at a high temperature.

According to this method, the width of the cores 31 and 32 can be made to be the metal mask width Wo. Furthermore, since the mask width can be set to the initial mask design, the optical characteristics deteriorate due to the decrease in the core width (for example, the center wavelength shift of the optical multiplexer / demultiplexer, the engagement characteristics of the 3 dB directional coupler). Deviation) can be compensated.

In order to dry-etch the core layer 3 having a thickness To of about 8 μm in the process of FIGS. 2C to 2D over 1 hour or more, the etched side surfaces of the cores 31 and 32 are formed of a photoresist 5. In addition, surface roughness proportional to the mask edge roughness of the metal 4 occurs. This surface roughness induces scattering loss of the optical waveguide.

In the method of the present invention, the roughened surface is covered with the compensating core film 8, so that the scattering loss can be significantly reduced. Further, the compensating core film 8 is made of a glass having a softening temperature lower than the softening temperature of the glass of the cores 31 and 32, for example, Si.
_{O 2 -P 2 O 5 -B 2} O 3 based _{glass, SiO 2 -TiO 2 -P 2 O} 5 -B 2 O 3 based glass, the use of such SiO ₂ -GeO ₂ -B ₂ O ₃ based glass, First
In the annealing at a high temperature before or after the formation of the clad film shown in FIG. 9E, the compensation core film 8 is buried without gaps in the roughened surfaces of the cores 31 and 32, so that the scattering loss is greatly reduced.

FIG. 2 shows another embodiment of the method for compensating the shape of a silica-based optical waveguide according to the present invention.

This is because, after the processes of dry etching and photoresist film removal shown in FIG. 2A, a compensating core film 8 is first formed in a thickness of 0.1 μm to 1 μm using a low-pressure plasma CVD method, and then (b) ), The clad film 9 having a refractive index equal to or lower than the refractive index of the clad 6 (thickness: about 0.1 μm to 1 μm) is formed, and the core film 3 and the compensation core film are formed. 8 from process contamination. The subsequent processes (c) to (e) are the same as those in FIG.

As described above, in the film forming process of FIGS. 1B and 2B, the films may be formed of a plurality of layers having different physical characteristics such as a refractive index, a composition, and a softening temperature.

 The present invention is not limited to the above embodiment.

For example, the optical waveguide may be a ridge type in addition to the buried type. When a quartz glass substrate is used as the substrate 1, the buffer layer 2 may not be provided.

[Effects of the Invention] As described above, according to the present invention, a decrease in the core width due to the dry etching process can be compensated. For example, an optical multiplexer / demultiplexer, an optical filter, a 3 dB optical coupler, and the like are manufactured. In this case, the center wavelength shift, the isolation characteristic, the deterioration of the coupling ratio, and the like can be suppressed. It is also possible to reduce an increase in light scattering loss due to surface roughness on the core side.

[Brief description of the drawings]

1 and 2 are views showing an embodiment of a method for compensating the shape of a silica-based optical waveguide according to the present invention, and FIG. 3 is a process diagram of a method for manufacturing a silica-based optical waveguide previously proposed by the present inventors. Figure showing
FIG. 4 schematically shows a conventional directional coupler type optical demultiplexer, wherein FIG. 4 (a) is a plan view, FIG.
FIG. 5 shows the results of sensitivity analysis characteristics of the optical multiplexer / demultiplexer calculated by the inventor. In the drawing, 1 is a substrate, 2 is a buffer layer, 3 is a core layer, 4 is a metal film, 5 is a photoresist film, 6 is a clad, 8 is a compensation core film, 9 is a thin film clad, and 31 and 32 are cores. .

Claims (4)

(57) [Claims] 1. A metal and photoresist film-patterned mask is formed on a core glass film formed on a substrate having a low refractive index layer, and the core glass film is formed based on the two masks. Patterned into a rectangular shape by dry etching, and then a thin film of core glass for compensation was applied on the surface of the pattern from which the photoresist film was removed under reduced pressure plasma CVD.
A quartz-based optical waveguide formed by etching the compensating core glass film until the surface of the metal film is exposed by dry etching, then etching the metal film, and then covering the clad film. Manufacturing method. 2. The method according to claim 1, wherein the compensation core glass thin film has a refractive index equal to or lower than that of the core glass film. 3. The method according to claim 1, wherein the compensation core glass thin film has a softening temperature equal to or lower than that of the core glass film. 4. A method for manufacturing a silica-based optical waveguide according to claim 1, wherein a two-layer film of a compensating core glass thin film and a cladding glass thin film is formed instead of the compensating core glass thin film. .