JPH03284707A - Method for compensating shape of quartz group optical waveguide - Google Patents

Abstract

PURPOSE:To prevent the increment of light dispersion loss due to surface roughness by constituting a metal film on a core glass film formed on a base having a low refractive index layer, forming a compensating glass thin film on the surface of the metal film, exposing the metal film by dry etching, and then coating the etched surface with a clad film. CONSTITUTION:When the metal films 4 respectively having width W0 are formed on cores 31, 32 and then dry-etched, the sizes of the cores 31, 32 are slightly thinned. After forming a compensating core film 8 having a refractive index slightly smaller than that of the cores 31, 32 by pressure-reduced plasma CVD method, the film 8 formed on the film 4 and a buffer layer 2 is etched by means of CHF3 gas in a dry etching process. Then, the film 4 is dry-etched by NF3 gas and removed, a clad 6 with a refractive index nc (nc is smaller than the refractive index nw of the cores 31, 32) is formed and the whole block including a base 1 is annealed at a high temperature. Since the rough part of the surface is coated with the compensating core films 8, light dispersion loss can sharply reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to the manufacture of silica-based optical waveguides, and particularly to a method for compensating for deviations in shape during the patterning process of optical waveguides.

[Conventional technology: In recent years, so-called so-called optical circuits such as optical multiplexers/demultiplexers, optical directional couplers, optical star couplers, optical filters, etc. are configured in a planar manner by forming quartz-based optical waveguides on Si or quartz substrates. Research on optical integrated circuits is becoming more active. When constructing this optical integrated circuit, the thickness and width of the core through which light propagates is usually designed to be around 10 μm in order to improve the compatibility with a silica-based single mode optical fiber.

FIG. 3 shows a process diagram of a method for manufacturing a silica-based optical waveguide previously proposed by the present inventor. As shown in the figure,
First, a buffer layer 2 with a low refractive index and a core layer 3 with a desired thickness T (approx. 10 μm) are formed on a substrate 1 (SiO□ glass) (FIG. 1(a)), and then a metal (WS i ) film 4 is formed. The photoresist 5 is applied thereon, and patterning is performed by photolithography (see figure (b)).
C)) Then, using the photoresist film 5 as a mask, the metal film 4 is patterned. Next, based on the photoresist film 5 and the metal film 4 mask, a pattern is formed on the low refractive index layer. The film of the core layer 3 having a thickness T of about 10 μm was buttered by dry etching, and
(approximately 10μre) into a rectangular pattern (see the same figure).
d)) Finally, after removing the photoresist film 5 and the metal film 4 to obtain two cores 31 and 32 (see (e) in the same figure).
), by coating with a low refractive index cladding layer 6 (
As shown in (f) of the same figure, a buried silica-based optical waveguide is realized.

[Problem to be solved by the invention] However, when manufacturing a directional coupler type optical demultiplexer having the structure shown in FIG. 4 using the above manufacturing method, the two cores 31 and 32 are It has been found that when attempting to realize a structure in which the elements are arranged in parallel with an interval S of about 2 μm maintained over the length, the following problem occurs.

Now, using the photoresist film 5 and the pattern of the metal M4 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 dry etching process,
Compared to the width W of the cores 31 and 32 or the mask width WO, the width was reduced in the range of 0.2 to 1.0 μm each time the cores were manufactured.

This reduction is due to undercutting during dry etching, and is an unavoidable phenomenon when the core thickness T is as thick as about 10 μf. and,
This percentage of 4 led to problems such as a shift in the center wavelength of the optical demultiplexer and a decrease in isolation in the stop band.

Figure 5 ra) to (C) show this center wavelength deviation characteristic.
This shows the results of the sensitivity analysis characteristics calculated by the inventor for the basic optical multiplexer/demultiplexer shown in Figure 5(d). In the figure, fa) is the deviation characteristic of the core Fuku W. ,
(b) is the relative refractive index difference Δn (%) of the cladding with respect to the core
(b) shows the difference characteristic of the thickness T.

- By considering the above reduction in advance when designing the photolithography mask, some improvement is possible, but a major improvement cannot be expected because the core spacing S is originally around 2 μm. Considering the width reduction mentioned above for photolithography masks, the opening distance is 0.5 to 1
．． This is because the value is 5 μt, which is a value that can be realized in many countries depending on the precision of mask creation and the resolution of photolithography.

An object of the present invention is to provide a method for solving the problems of the prior art described above and realizing a silica-based optical waveguide with desired mask precision.

Means for Solving Two Problems] The shape compensation method for a silica-based optical waveguide of the present invention includes a mask patterned with a metal and photoresist film on a core glass film formed on a substrate having a low refractive index layer. The core glass film is patterned into a rectangular shape by dry etching based on the two masks, and then a thin film of compensating core glass is formed on the patterned surface from which the photoresist film has been removed. After dry etching the compensating core glass film until the surface of the metal film is exposed, and then etching the metal film,
It covers the cladding film.

Low-pressure plasma CV is used to form the thin film of the compensation core glass.
D method is suitable. As the compensating core glass thin film, a film 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, it is also possible to form a two-layer film of a compensating core glass thin film and a cladding glass thin film.

[Two functions] In a specific embodiment of the present invention, for example, 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 it into a rectangular shape, a compensating core film is formed to a thickness of 0.5 mm by low pressure plasma CVD on the patterned surface from which only the photoresist has been removed. Formed in the range of several μm to 1 μm,
Next, the compensating core film is etched by dry etching until the metal film surface is exposed, and then the metal film is etched and then covered with a cladding film to compensate for the reduction in core width due to dry etching. This is how it was done.

The compensating core film is made of a material whose refractive index is the same as or slightly lower than that of the core.

The compensation core film can compensate for the decrease in core width because:
Due to the following reasons.

By forming a glass film by the low pressure plasma CVD method, it is possible to form a glass film with a uniform thickness even on a pattern surface having a large unevenness in aspect ratio.

Therefore, first, a core film for compensating the thickness of the rectangular core by the reduction in core width is formed by low pressure plasma CVD. Next, the metal film and the compensation core film on the buffer tank are etched by dry etching.
In this dry etching, the etching rate in the thickness direction is much higher (more than 5 times) than the etching rate in the direction perpendicular to the thickness direction, so the compensation core film on the metal film and buffer layer is etched. The compensating core film on the side surface of the rectangular core is hardly etched.

This makes it possible to compensate for the decrease in core width.

Note that another feature of the method of the present invention is that light scattering loss due to surface roughness on the side surface of the core can be significantly reduced by depositing a compensating core film.

[Example] FIG. 1 shows an example of the method for compensating the shape of a quartz optical waveguide according to the present invention.

Figure fa) shows a schematic cross section when the photoresist M5 on the metal film 4 is removed after the dry etching of Figure fd).

Here, the substrate 1 includes quartz glass, multi-component glass,
Sapphire, Si, etc. are used. The buffing layer 2 contains SiO2 or Sin, B, P, Ti, Ge,
It contains at least one type of refractive index controlling additive such as Ta, Affl, F, etc., and its refractive index is nb. The cores 31 and 32 have a refractive index of n, (n=>nb> and S i O2 and Ti.

P, Ge, AJ B, Ta, F Er, Nd.

A material containing at least one kind of refractive index controlling additive such as Sm or Zn in addition to Na, Yb, or SiO□ is used. The metal film 4 includes W, WSi, MoSi, Mo
, Cr, αsi, etc. are used.

When attempting to configure a single mode optical waveguide, the thickness TO of the cores 31 and 32 is 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, several percent.

Furthermore, when constructing a directional coupler, multiplexer/demultiplexer, etc., the gap S between the cores 31 and 32 is set to about 2 μm.

At the stage of FIG. 1(a), for example, W.

10 μm, 5o=2 μll, To=8 μl1
When dry etching is performed as
m, Sl was about 3μ river.

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

Here, the low pressure plasma CVD method was performed as follows. That is, the substrate shown in FIG. 1(a) is placed on the lower electrode of a reaction vessel, heated to 350'C, and the inside of the reaction vessel is heated to 10-
Next, the vapors of S1(OC2Hs)4 and PO(OC2Hl)s were blown onto the above substrate along with the carrier gas of 02 in a shower form from the upper electrode side opposite to the lower electrode. Ta. Then, by applying high frequency power between both electrodes, plasma was generated and the compensating core film 8 was formed.

Compensation core II formed in this state! The refractive index of 8 is
It is lower than that of cores 31 and 32. That is, since the compensating core film 8 is formed at a low temperature, its refractive index is slightly low, but it is formed at a high temperature (>1200°C) before or after the final cladding film formation in (e). By annealing, the refractive index changes to a value approximately equal to that of the cores 31 and 32.

Next, as shown in fc), the compensation core film 8 on the metal film 8 and buffer layer 2 is etched using a dry etching process. This dry etching is CHF
This is done using reactive ion etching equipment using s gas.

After that, as shown in (d), the metal 1114 is NF,
Remove by dry etching using gas.

Finally, as shown in (e), the refractive index nc(n,
<n, ) cladding 6 is formed. This process (
Before or after step e), the substrate is annealed at high temperature.

According to this method, the width of the cores 31 and 32 can be made to match the metal mask width Wo.

Furthermore, since the mask width can be made to match the original mask width, optical characteristics may deteriorate due to a decrease in core width (for example, the center wavelength of an optical multiplexer/demultiplexer may be shifted, or the engagement characteristics of a 3 dB directional device may be affected). etc.) can be compensated.

In addition, in order to dry-etch the core layer 3 having a thickness To of about 8 μt for more than one hour in the process shown in FIGS. 2(C) to (d), the etched cores 31 and 32 are
Surface roughness occurs on the side surface in proportion to the mask edge roughness of the photoresist 5 and metal 4. This surface roughness induces scattering loss in the optical waveguide.

In the method of the present invention, the surface roughness is removed by the compensating core film 8.
Since the scattering loss is covered with , the above-mentioned scattering loss can be greatly reduced. In addition, core 31 is added to this compensation core WA8.
and a glass whose softening temperature is lower than that of glass No. 32, such as SiO□-P 205B20. system glass, SiO□-
If TiO□P2O5B2O3-based glass, SiO□GeO2B2O3-based glass, etc. are used,
) During the high temperature annealing before or after the formation of the cladding film, the compensating core film 8 is embedded without any gaps in the roughened portions 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.

After the process of dry etching and photoresist film removal shown in FIG. 5(a), a compensating core film 8 is first formed using a low pressure plasma CVD method to have a film thickness of O1 in the range of several μm to 1 μm, and then (b) A thin L! having a refractive index equal to or lower than the refractive index of the cladding 6! A cladding film 9 (ff: 0°, several μm to 1 μm) is formed to protect the core film 3 and the compensating core 118 from contamination during the process. Subsequent (C) to (e)
The process is similar to that in FIG.

In this manner, in the film forming process shown in FIG. 1(b) and FIG. 2(b), the film may be formed into a plurality of layers having different physical properties such as a composition with a refractive index of 1 and a softening temperature.

The present invention is not limited to the above embodiments.

For example, the optical waveguide may be a ridge type instead of a buried type. Furthermore, when a quartz glass substrate is used as the substrate 1,
Buffer layer 2 may be omitted.

[Effects of the Invention] As described above, according to the present invention, it is possible to compensate for the decrease in core width due to the dretching process.
For example, it is possible to suppress center wavelength shift, deterioration of isolation characteristics, coupling ratio, etc. when optical multiplexers/demultiplexers, optical filters, 3 dB optical couplers, etc. are manufactured. It is also possible to reduce the increase in light scattering loss due to surface roughness on the core side.

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing an embodiment of the method for compensating the shape of a silica-based optical waveguide according to the present invention, and Figure 3 is a process diagram of a method for manufacturing a silica-based optical waveguide previously proposed by the present inventor. A diagram showing
FIG. 4 shows an outline of a conventional directional coupler type optical demultiplexer, in which [a] is a plan view, (b) is a sectional view taken along line A-A,
FIG. 5 shows the results of the sensitivity analysis characteristics of the optical multiplexer/demultiplexer calculated by the inventor. In the figure, 1 is the substrate, 2 is the buffer layer, 3 is the core layer, 4 is the metal film, 5 is the photoresist film, 6 is the cladding, 8 is the compensation core film, 9 is the thin film cladding, and 31° and 32 are the cores. . Patent applicant Hitachi Cable Co., Ltd.

Claims (1)

[Claims] 1. A patterned mask of a metal and a photoresist film is formed on a core glass film formed on a substrate having a low refractive index layer, and based on these two masks, a core glass film is formed. The glass film is patterned into a rectangular shape by dry etching,
Next, a thin film of compensating core glass is formed on the pattern surface from which the photoresist film has been removed, and then the compensating core glass film is etched by dry etching until the metal film surface is exposed, and then the metal film is etched. A method for compensating the shape of a silica-based optical waveguide, the method comprising: covering the silica-based optical waveguide with a cladding film. 2. Low-pressure plasma C for forming a thin film of the above-mentioned compensating core glass
2. The method for compensating the shape of a silica-based optical waveguide according to claim 1, characterized in that a VD method is used. 3. The method for compensating the shape of a silica-based optical waveguide according to claim 1, wherein the compensating core glass thin film is made of a material whose refractive index is equal to or lower than that of the core glass film. 4. The method for compensating the shape of a silica-based optical waveguide according to claim 1, wherein the compensating core glass thin film has a softening temperature equal to or lower than that of the core glass film. 5. The method for compensating the shape of a silica-based optical waveguide according to claim 1, characterized in that, instead of the compensating core glass thin film, a two-layer film consisting of a compensating core glass thin film and a cladding glass thin film is formed.