JPH07333452A - Production of optical waveguide - Google Patents

Abstract

PURPOSE:To produce an optical waveguide ensuring a small loss in the wavelength range of 0.6-1.6mum. CONSTITUTION:A buffer layer 2 of Sin. and a core layer 3 of SiOxNyHz are successively formed on a substrate 1 by low temp. plasma CVD at <=450 deg.C, the core layer 3 is patterned by photolithography to form a core pattern 3a and the surface of the substrate 1 with the formed core pattern 3a is coated with a cladding layer 6 of SiO2 by low temp. plasma CVD at <=450 deg.C to produce an optical waveguide. At this time, the substrate 1 with the formed core pattern 3a is heat-treated at a high temp. of 1,000-1,300 deg.C for at least 1hr in a nitrogen- contg. atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an optical waveguide having improved loss wavelength characteristics over a wavelength range of 0.6 μm to 1.6 μm.

[0002]

2. Description of the Related Art FIG. 7 is a sectional view of an optical waveguide previously proposed by the present inventor (JP-A-5-181031).

In the figure, the optical waveguide is the substrate 1 (SiO 2
₂ or Si) on the buffer layer (SiO ₂ or S
iO ₂ to which at least one kind of refractive index control additive is added) 2 is formed, and a substantially rectangular core pattern (SixOyNz or SixOyNz to which at least one kind of refractive index control additive is added) 3a, and the entire core pattern 3a is covered with a clad layer (buffer layer 2
The same type of material) 6 is used. Since SixOyNz in which nitrogen is added to the core pattern 3a is used in this configuration, by adjusting the content of nitrogen, the relative refractive index difference Δ (= ((( n _w −n _c ) / n _w ) × 100
%, N _w = refractive index of core, n _c = refractive index of clad) can be increased to a maximum of about 7%. By increasing the relative refractive index difference Δ, the size of the waveguide type optical component (Mach-Zehnder type optical filter, optical ring resonator, optical directional coupler, etc.) can be significantly reduced.

FIG. 8 is a diagram showing a manufacturing process of the optical waveguide shown in FIG. 7, and FIG. 9 is a diagram showing a cross section of the optical waveguide in each manufacturing process shown in FIG.

In FIGS. 8 and 9, the buffer layer 2 is formed on the substrate 1. The buffer layer 2 is formed by a low temperature plasma CVD method at 270 ° C. (S1, FIG. 9).
(A)).

A core layer 3 is formed on the buffer layer 2.
This core layer 3 is also formed by the low temperature plasma CVD method at 270 ° C. (S2).

A WSi layer 4 for a mask is formed on the core layer 3 by a sputtering method. This WSi layer 4
Is also formed at a low temperature of 300 ° C. or lower (S3, FIG. 9).
(B)).

A photoresist pattern (not shown) is formed on the WSi layer 4 by photolithography,
The WSi layer is patterned by dry etching using this photoresist pattern as a mask to form a WSi pattern 4a (S4, FIG. 9C).

This dry etching is carried out by placing the semi-finished product 5 in a reaction container (not shown) in a plasma atmosphere at 300 ° C. or lower kept in a vacuum of 10 ⁻² torr or lower and flowing NF ₃ gas. Next, the core layer 3 is dry-etched using the WSi pattern 4a described above as a mask and patterned to form the core pattern 3a. CHF ₃ is used as the gas for dry etching (S5,
FIG. 9D).

After that, the WSi mask pattern 4a on the core pattern 3a is removed by dry etching (this step is not shown). Finally, the cladding layer 6 is formed so as to cover the core pattern 3a. This clad layer 6 is also formed by the above-mentioned low temperature plasma CVD method (S6, FIG. 9).
(E)).

The optical waveguide shown in FIG. 7 is characterized in that it can be formed at a low temperature of 300 ° C. or lower. This means that when an electronic circuit element or an optically active element is previously formed on the upper surface of the substrate 1, the inside of the substrate 1, or the lower surface of the substrate 1, an optical waveguide is formed without damaging these elements. There is an advantage that can be done.

FIG. 10 is a diagram showing the loss wavelength characteristic of the optical waveguide formed by the method shown in FIG. 8, in which the horizontal axis shows the wavelength and the vertical axis shows the loss.

The length of the optical waveguide used for measuring the loss wavelength characteristic is 5 cm, and the relative refractive index difference Δ is 2%. From the figure, it can be seen that the loss is extremely low in the wavelength range of 0.6 μm to 1.34 μm. In particular, the loss at an oscillation wavelength of 1.3 μm of a semiconductor laser element often used as a light source of an optical waveguide is 0.12 dB / cm, which is an extremely low value.

[0014]

However, the conventional optical waveguide has the following problems.

(1) The optical waveguide contains a large amount of OH groups, and the absorption loss due to the OH groups (wavelength 1.39 μm
It was found that the absorption peak in 1) was very large.
This absorption loss has been a factor of increasing the loss from the wavelength range of 1.35 μm to 1.5 μm.

(2) The optical waveguide also contains a large amount of Si-H groups, which causes a large absorption loss near the wavelength of 1.49 μm. Moreover, this absorption loss is caused by the wavelength 1.
The tail is drawn in the range of 4 μm to 1.6 μm, and the loss in this wavelength range is greatly increased.

(3) The absorption loss due to the above (1) and (2) is
Since the core layer 3 is formed by the low temperature (270 ° C.) plasma CVD method using SiH ₄ , N ₂ O and N ₂ gas, it is caused by the result that OH groups and Si—H groups remain in the core layer 3. Was considered to be.

Therefore, the temperature at which the core layer 3 is formed is set to 45
I tried raising the temperature to 0 ° C, but the loss did not decrease so much. As a next measure, an attempt was made to perform high temperature heat treatment after forming the core layer 3 shown in FIG. As a result of evaluating three kinds of samples (hereinafter referred to as “semi-finished products”) having a high temperature heat treatment temperature of 500 ° C., 1000 ° C. and 1200 ° C.,
Absorption loss due to OH groups is about 8 in high temperature heat treatment at 500 ℃.
It was possible to reduce it by about dB, and the loss at 1.39 μm could be lowered to about 7 dB.

However, it was found that the OH group still remained in the core layer 3. The absorption loss due to the Si—H group could be reduced by about 10 dB at the wavelength of 1.49 μm, but a large amount of Si—H group still remained in the core layer 3. Next, when the semi-finished product which was heat-treated at a temperature of 1000 ° C. and 1200 ° C. was taken out from the electric furnace, it was found that many cracks were generated in the core layer 3,
It has been found that high temperature heat treatment at ℃ or higher is difficult. This is due to the difference in the coefficient of thermal expansion between the substrate 1, the buffer layer (both of which is SiO ₂ ) 2 and the core layer (SixNyHz) 3, and the thick buffer layer (about 6 μm) and the core layer (about 6 μm) 3. It is thought that this is due to the effect of residual stress inside.

As another measure, a semi-finished product was produced by forming the cladding layer 6 shown in FIG. 9 (e) and then subjecting it to the same high temperature heat treatment as described above. In this case as well, the temperature was 1000 ° C. and 1200 ° C. Cracking occurred in the heat-treated semi-finished product. Moreover, more cracks were generated than the cracks in the semi-finished product that had been subjected to the high temperature heat treatment described above. Further, the substrate 1 itself has warped. It is considered that this is because the clad layer (SiO ₂ ) 6 was formed to have a thickness of 10 μm or more, so that cracks were more significantly generated due to the difference in stress.

Thus, from the wavelength of 1.35 μm to the wavelength of 1.
It was difficult to reduce the loss of the optical waveguide over the range of 6 μm.

Therefore, an object of the present invention is to solve the above problems and to provide a method of manufacturing an optical waveguide having a small loss in the wavelength range of 0.6 μm to 1.6 μm.

[0023]

In order to achieve the above object, the present invention provides a buffer layer made of SiO ₂ and S on a substrate.
A core layer made of iOxNyHz is sequentially formed by a low temperature plasma CVD method at 450 ° C. or lower, and photolithography is performed to pattern the core layer to form a core pattern.
In the method for producing an optical waveguide which is subjected to the following low temperature plasma CVD method and covered with a cladding layer made of SiO ₂ , the substrate on which the core pattern is formed is heated at least at a temperature of 1000 ° C. to 1300 ° C. in an atmosphere containing nitrogen. It was heat treated at high temperature for 1 hour.

In the present invention, a buffer layer made of SiO ₂ and a core layer made of SiOxNyHz are formed on the substrate.
Sequentially formed by a low temperature plasma CVD method of 0 ° C. or lower, photolithography is applied to pattern the core layer to form a core pattern, and the surface of the substrate on which the core pattern is formed is subjected to a low temperature plasma CVD method of 450 ° C. or lower. S
In a method of manufacturing an optical waveguide covered with a cladding layer made of io ₂ , a protective layer is formed in advance on the core layer, and the protective layer and the core layer are patterned to form a core pattern. The substrate is at least 1 at a temperature of 1000 ° C. to 1300 ° C. in an atmosphere containing nitrogen.
It was heat-treated for a long time at high temperature.

In the present invention, in addition to the above construction, any one of a low pressure CVD method, an atmospheric pressure CVD method, an electron beam evaporation method and a sputtering method may be used instead of the low temperature plasma CVD method.

According to the present invention, in addition to the above structure, a buffer layer,
At least one additive for controlling the refractive index may be contained in the core layer, the protective layer and the clad layer.

In the present invention, in addition to the above structure, at least one rare earth element may be contained in the core layer.

[0028]

According to the above structure, since the substrate on which the core pattern is exposed is subjected to the high temperature heat treatment, the OH in the core pattern is reduced.
The group and the Si-OH group can be easily diffused outside the core pattern, and the absorption loss due to the OH group and the Si-OH group can be significantly reduced. This results in a wavelength of 0.6
It is possible to realize an optical waveguide with low loss over a wide range from μm to 1.6 μm.

Further, even if the optical waveguide is subjected to high temperature heat treatment, the core pattern is not cracked, and the warp of the substrate is extremely small. This is because the core pattern width is from a few μm to 20 μm.
Since the core pattern is narrow in the range of about μm and the core patterns are sparsely present in the buffer layer on the substrate, cracks due to the difference in thermal expansion coefficient and residual stress are unlikely to occur, and can be ignored. Further, the core pattern of SiOxNyHz is heat-treated in a nitrogen atmosphere, and since the diffusion of nitrogen in the core pattern is small, the change in the refractive index of the core pattern is also small.

When a SiO ₂ protective layer is provided on the core pattern, OH groups and S by high temperature heat treatment are applied.
The diffusion and release of the i-H group to the outside of the core layer is performed on both side surfaces of the patterned core layer. However, since the thickness of the protective layer made of SiO ₂ is thin, some OH groups and S may pass through the protective layer.
The i-H group is diffused and released. With this protective layer,
The scattering loss due to the structural irregularity on the upper part of the core pattern can be significantly reduced. Further, it has an effect of suppressing the diffusion of nitrogen in the core pattern to the outside of the core pattern, and can reduce the change in the refractive index of the core pattern before and after the heat treatment.

Instead of the low temperature plasma CVD method, various methods can be used for film formation, and the flexibility of the manufacturing method is high, so that the optical waveguide can be economically manufactured. For example, the low pressure CVD method and the atmospheric pressure CVD method are economical because they are performed using a cheaper apparatus. Further, in the electron beam evaporation method and the sputtering method, Si ₃ N ₄ and SiO are used.
Since the film is formed using ₂ , the OH group and the Si—H group hardly enter the core pattern. In addition, since the film can be formed in a high vacuum, a denser and more uniform layer can be formed.

Since the materials for the buffer layer, the core layer, the protective layer, and the clad layer and the selection range of the refractive index can be widened, it becomes easy to suppress the warp of the substrate and increase the relative refractive index difference Δ.

[0033]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a process drawing showing an embodiment of the method of manufacturing an optical waveguide of the present invention, and FIG. 2 is a view showing a cross section of the optical waveguide in each manufacturing process shown in FIG. FIG. 3 shows FIG.
FIG. 6 is a plan view of an optical waveguide manufactured by the method shown in FIG. The same members as those in the conventional example are designated by the same reference numerals.

1 and 2, steps S10 to S13 are the same as steps S1 to S5 shown in FIG. The difference from the conventional example is that high-temperature heat treatment is performed after patterning the core layer.

First, the buffer layer 2 is formed on the substrate 1 by using the plasma CVD method (S10, FIG. 2A).

The core layer 3 is formed on the buffer layer 2.
A WSi layer 4 for a mask is formed on the core layer 3 by a sputtering method (S11, FIG. 2B).

A photoresist pattern (not shown) is formed on the WSi layer 4 by photolithography,
Using this photoresist pattern as a mask, the WSi layer is patterned by dry etching to form a WSi pattern 4a (S12, FIG. 2C).

The core layer 3 is dry-etched by using the WSi pattern 4a as a mask and patterned to form the core pattern 3a (S13, FIG. 2D).

After removing the WSi mask pattern 4a on the core pattern 3a by dry etching, a high temperature heat treatment is performed (S14, FIG. 2E).

After the high temperature heat treatment is completed, the optical waveguide is obtained by performing the plasma CVD method on the core pattern 3a to form the cladding layer 6 (S15, FIG. 2 (f)).

Next, the operation will be described.

Three faces of the core pattern 3a (top face, left face,
Since the high temperature heat treatment is performed with the right side surface exposed, it is easy to diffuse and release the components of the OH group and the Si—H group remaining in the core pattern 3a to the outside of the core pattern 3a. Therefore, the absorption loss due to the OH group and the Si-OH group can be significantly reduced.

By the way, when the high temperature heat treatment is performed at the stage where the step S11 is completed, the core pattern 3a is formed.
There is concern that cracks may occur inside. However, in the optical waveguide, as shown in FIG. 3, the width W of the core pattern 3a is narrow (about several μm to several tens of μm), and the core pattern 3a
Exist sparsely on the surface of the buffer layer 2, and the area where the core pattern 3a and the buffer layer 2 are in contact with each other is extremely small. Therefore, a crack may occur due to a difference in thermal expansion coefficient or residual stress. Absent.

After completion of the high temperature heat treatment step of step S14,
The clad layer 6 is formed, but since the core pattern 3a and the buffer layer 2 are changed into dense layers by high temperature heat treatment,
The adhesion between the cladding layer 6 and the core pattern 3a and the buffer layer 2 can be improved. This is also one of the effects of the high temperature heat treatment.

Next, specific numerical values will be described, but the numerical values are not limited.

The substrate 1 has a diameter of 3 inches (about 7.6c).
m), quartz glass having a thickness of 1 mm is used, and SiH ₄ , N ₂ O gas, and O ₂ gas are used for the buffer layer 2 at about 270 ° C.
An SiO ₂ film was formed by the low temperature plasma CVD method described above.
The thickness of this SiO ₂ film is about 6 μm.

For the core layer 3, SiH ₄ , N ₂ O and N ₂ gases are used to form Si by a low temperature plasma CVD method at 270 ° C.
An OxNyHz film was formed. The refractive index of the film is about 1.4
At 85 (at a wavelength of 0.63 μm), the film thickness was about 5 μm. A WSi film 4 having a thickness of about 1 μm was formed on the core pattern 3a by using a sputtering device. After that, a film thickness of about 1 μm is formed on the WSi film 4 by photolithography.
A photoresist pattern was formed. Next, using this photoresist pattern as a mask, dry etching was performed to pattern the WSi film 4. Then, the core layer 3 was patterned using the WSi pattern 4a as a mask to form the core pattern 3a. After that, the WSi film 4 on the core pattern 3a was removed by dry etching. Next, the first semi-finished product 5a was put in the electric furnace, and while flowing N ₂ gas in the electric furnace (N ₂ gas flow rate of about 5 liters / minute), the temperature was raised to 500 ° C. in about 1.5 hours, and thereafter, The temperature was kept at 500 ° C. for 3 hours, and the temperature was lowered to room temperature over 3 hours. For the second semi-finished product 5b, 100
The temperature was raised to 0 ° C., and after keeping the temperature for 3 hours, the temperature was lowered to room temperature over about 4 hours. Furthermore, as the third semi-finished product 5c, the temperature was raised to 1200 ° C over about 1.5 hours, and then 3 hours 1
After keeping the temperature at 200 ° C., the temperature was lowered to room temperature over about 4 hours. SiO on the above 3 types of semi-finished products 5a, 5b, 5c
_An optical waveguide was obtained by forming the _second cladding layer 6 by a low temperature (270 ° C.) plasma CVD method with a thickness of about 10 μm.

FIG. 4 is a diagram showing the loss wavelength characteristic of the optical waveguide manufactured by the method shown in FIG. 1 and the loss wavelength characteristic of the optical waveguide manufactured by the conventional manufacturing method.
The vertical axis shows the loss. Further, in the same figure, the broken line shows the loss wavelength characteristic of the conventional optical waveguide, and the dashed-dotted line shows 500
The loss wavelength characteristic of the optical waveguide heat-treated at 1200C is shown, and the solid line shows the loss wavelength characteristic of the optical waveguide heat-treated at 1200 ° C.

As is clear from the figure, the optical waveguide manufactured by the manufacturing method of the present invention has less loss than the loss wavelength characteristic of the conventional optical waveguide. That is, 500 ° C
By heat treatment at
It was possible to reduce even near dB. Also, the absorption loss due to the Si-H group could be reduced by about 10 dB. However, the reduction of absorption loss due to the Si-H group is not yet sufficient. On the other hand, the heat treatment at 1200 ° C. can almost completely eliminate the absorption loss due to the OH group, and the Si-
The absorption loss due to the H group could be reduced by 20 dB or more. Although the result of heat treatment at 1000 ° C. is not shown in FIG. 4, the absorption loss due to the OH group can be almost completely removed in this case, and the absorption loss due to the Si—H group can be reduced by about 18 dB. We were able to. From this, the heat treatment temperature is 1000 ° C to 1300 ° C.
It can be said that the range is preferable. As atmospheric gas during heat treatment has been tried with O ₂ in addition to N _2, it was found that undesirable end up with different refractive index is large core layer in this case. As for the heat treatment time, the optical waveguide was prototyped by changing the heat retention time to a high temperature in the range of 1 to 5 hours, and the loss wavelength characteristics were evaluated. It turns out that the loss can be reduced. However, if the temperature is kept longer than the above-mentioned time, the refractive index of the core layer is likely to change, which causes inconvenience. Therefore, it is considered that the above-mentioned heat-retention time range is appropriate. The same conclusions can be made regarding the rate of temperature increase and the rate of temperature decrease as the heat retention time.

FIG. 5 is a diagram showing the manufacturing process of another embodiment of the method for manufacturing an optical waveguide of the present invention, and FIG. 6 is a diagram showing a cross section of the optical waveguide in each manufacturing process shown in FIG.

In the figure, the steps shown in steps S20 to S22 are the same as the steps shown in steps S10 to S13 shown in FIG.

The difference from the embodiment shown in FIG. 1 is that a step of forming the protective layer 7 (7a) on the core pattern 3a and a step S23 of patterning the protective layer 7 are added. This protective layer 7 (7a) is made of SiO ₂ or Si.
O ₂ containing an additive for controlling the refractive index such as B, P, Ti, Al, and F, and the refractive index of the core layer 3 is (n
value lower than _w = 1.480 to 1.50 (1.450)
˜1.475) SiOxNyHz is used. The thickness of the protective layer 7 (7a) is selected from the range of 0.1 μm to 2 μm. However, if the protective layer 7 becomes too thick, the OH groups and Si-H in the core pattern 3a are subjected to high temperature heat treatment.
It becomes difficult for the base to diffuse and be released to the outside of the core pattern 3a.
The protective layer 7 can act as a part of the cladding layer 6 of the optical waveguide prototyped as shown in the step S25. The effect of the protective layer 7 (7a) has already been described above, but in addition to that, the following effect is also provided. That is, in the process shown in step S23, the protective layer 7 (7a) and the core layer 3 are patterned by dry etching using the WSi mask pattern as a mask. After that, WSi on the protective layer 7 (7a) is removed by dry etching. At this time, the upper surface of the protective layer 7 (7a) is etched by the WSi etching gas (N
F ₃ ) will make you rough. If the protective layer 7 (7a) is not provided, the upper surface of the core pattern 3a will be roughened and scattering loss of the optical waveguide will increase.
Since the upper surface of the core pattern 3a is protected by a), the increase in scattering loss as described above does not occur.

In this embodiment, the core layer 3 is made of SiOxNyHz.
However, the present invention is not limited to this, and SiOxN is used.
yHz may include at least one additive for controlling the refractive index such as Ge, P, F, and B. In addition, the buffer layer 2,
The protective layer 7 (7a) and the clad layer 6 are also made of SiO ₂ ,
SiO ₂ may contain at least one kind of refractive index controlling additive as described above. Further, the substrate 1 is made of SiO
_In addition to the glass substrate such as ₂ , a semiconductor substrate such as Si may be used. In addition, the surface of the clad layer 6 is not necessarily shown in FIG.
It may have irregularities instead of being flat as shown in FIG.

[0055]

In summary, according to the present invention, the following excellent effects are exhibited.

Since the substrate on which the core pattern is exposed is subjected to a high temperature heat treatment, the OH group and the Si--H group contained in the optical waveguide are greatly reduced without cracks or warping of the substrate. Wavelength from 0.6 μm to 1.6
Low loss characteristics can be obtained over a wide range of μm.

[Brief description of drawings]

FIG. 1 is a process chart showing an embodiment of a method for manufacturing an optical waveguide of the present invention.

FIG. 2 is a diagram showing a cross section of an optical waveguide in each manufacturing step shown in FIG.

FIG. 3 is a plan view of an optical waveguide manufactured by the method shown in FIG.

4 is a diagram showing a loss wavelength characteristic of an optical waveguide manufactured by the method shown in FIG. 1 and a loss wavelength characteristic of an optical waveguide manufactured by a conventional manufacturing method.

FIG. 5 is a diagram showing a manufacturing process of another embodiment of the method of manufacturing an optical waveguide of the present invention.

6 is a diagram showing a cross section of an optical waveguide in each manufacturing step shown in FIG.

FIG. 7 is a sectional view of an optical waveguide previously proposed by the present inventor.

FIG. 8 is a diagram showing a manufacturing process of the optical waveguide shown in FIG.

FIG. 9 is a view showing a cross section of the optical waveguide in each manufacturing step shown in FIG. 7.

10 is a diagram showing loss wavelength characteristics of the optical waveguide formed by the method shown in FIG.

[Explanation of symbols]

 1 base plate 2 buffer layer 3 core layer 3a core pattern 6 clad layer

Claims (5)

[Claims] 1. A buffer layer made of SiO ₂ and a core layer made of SiOxNyHz are sequentially formed on a substrate by a low temperature plasma CVD method at 450 ° C. or lower, and photolithography is performed to pattern the core layer to form a core pattern. On the surface of the substrate on which the core pattern is formed.
In a method of manufacturing an optical waveguide in which a low temperature plasma CVD method at a temperature equal to or lower than 0 ° C. is performed and a cladding layer made of SiO ₂ is covered, the substrate on which the core pattern is formed is heated to a temperature of 1000 ° C. to 1300 ° C. in an atmosphere containing nitrogen. 1. A method for manufacturing an optical waveguide, characterized in that the high temperature heat treatment is performed for at least 1 hour. _{2. A} buffer layer made of SiO ₂ and a core layer made of SiOxNyHz are sequentially formed on a substrate by a low temperature plasma CVD method at 450 ° C. or lower, and photolithography is performed to pattern the core layer to form a core pattern. On the surface of the substrate on which the core pattern is formed.
In a method for producing an optical waveguide which is subjected to a low temperature plasma CVD method at a temperature of ℃ or less and covered with a cladding layer made of SiO ₂ , a protective layer is formed in advance on the core layer, and the protective layer and the core layer are patterned. A method of manufacturing an optical waveguide, comprising forming a core pattern and subjecting the substrate on which the core pattern is formed to a high temperature heat treatment at a temperature of 1000 ° C. to 1300 ° C. for at least 1 hour in an atmosphere containing nitrogen. 3. The method of manufacturing an optical waveguide according to claim 1, wherein any one of a low pressure CVD method, an atmospheric pressure CVD method, an electron beam evaporation method and a sputtering method is used instead of the low temperature plasma CVD method. 4. The optical waveguide according to claim 1, wherein the buffer layer, the core layer, the protective layer, and the cladding layer contain at least one additive for controlling the refractive index. Manufacturing method. 5. The method of manufacturing an optical waveguide according to claim 1, wherein at least one kind of rare earth element is contained in the core layer.