JPH06250033A - Production of optical waveguide substrate - Google Patents

Abstract

PURPOSE:To stably produce the optical waveguide having consistent quality by preventing the nonuniformity in a diffused state of H<+> and the local adhesion and diffusion of impurities in the proton exchange stage for forming the optical waveguide, and decreasing the variations in the insertion loss of the optical waveguide. CONSTITUTION:The optical wveguide is formed in a substrate material consisting of an electrooptical crystal by dynamically bringing a material 21, which is to be treated and includes the substrate material, into contact with a heated liquid acid 18. The material 21 to be treated is rotated or shaken volatively to the acid 18 in the state of immersing the material 21 to be treated in the acid 18. The material 21 to be treated is rotated or shaken or the liquid acid 18 is rotated or shaken velatively to the material 21 to be treated by stirring the acid.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an optical waveguide on a substrate made of an electro-optic crystal such as lithium niobate by a so-called proton exchange method.

[0002]

2. Description of the Related Art Lithium niobate (LiNbO ₃ ) single crystals are
It is expected as an optoelectronic material. As a method of forming an optical waveguide on a substrate material made of a lithium niobate single crystal, a titanium diffusion method and a proton exchange method are currently practical.

In a typical example of such a proton exchange method, the above substrate material is put into a molten solution of benzoic acid (C ₆ H ₅ COOH) to cause ion exchange between H ⁺ and Li ^+, and this substrate surface is A high refractive index layer made of Hx Li _1-x Nb O ₃ is formed. The substrate is then annealed at high temperature to promote the diffusion of protons into the crystal. The optical waveguide substrate thus obtained is expected to be applied to various optical components.

Benzoic acid has a melting point of 121 ° C. and a boiling point of 250 ° C. at atmospheric pressure. Then, specifically, benzoic acid is melted in a glass container, a substrate material made of lithium niobate is immersed in the melt, and proton exchange is performed at about 195 ° C., for example. At this stage, an optical waveguide having a stepwise refractive index distribution is formed. The substrate is then annealed to promote intracrystalline diffusion of protons.

[0005]

However, as a drawback of the above-mentioned proton exchange method, the amount of benzoic acid that is a proton exchange source is large, and it is very difficult to control the process when forming an optical waveguide. As a particularly serious problem, after the proton exchange, the diffusion state of H ⁺ was uneven in the optical waveguide, or the diffusion of impurities was locally observed. For this reason,
The variation in the insertion loss of light was large, and it was not possible to stably manufacture an optical waveguide of constant quality.

An object of the present invention is to prevent non-uniformity of H ⁺ diffusion state and local adhesion and diffusion of impurities in the above-mentioned proton exchange step, and to reduce the variation of insertion loss in the optical waveguide to a constant value. It is to make it possible to stably manufacture an optical waveguide of the above quality.

[0007]

According to the present invention, an optical waveguide is formed in a substrate material by dynamically contacting a material to be treated containing the substrate material made of an electro-optic crystal with a heated liquid acid. The present invention relates to a method for manufacturing an optical waveguide substrate. Preferably, the material to be treated is rotated or shaken relative to the acid while the material to be treated is in contact with the acid.

[0008]

The present inventor has studied in detail the cause of the increase or variation of the insertion loss in the optical waveguide throughout the manufacturing process, as described above. As a result, when the material to be treated is rotated or shaken at the stage of immersing the material to be treated in a melt of benzoic acid and performing proton exchange,
Insertion loss decreases in the optical waveguide after annealing,
Moreover, it has been found that the variation in insertion loss is extremely small.

The reason for this is not clear, but it is considered that the impurities in the liquid adhering to the surface of the material to be treated are released by the rotation or shaking of the material to be treated, so that the characteristic deterioration due to this does not occur. To be Further, since no uneven temperature of the surface of the object to be treated, and that does not cause localized non-uniform diffusion of H ^+, the concentration unevenness of H ⁺ in the liquid near the surface of the material to be treated It is considered that it does not occur.

Further, the inventor of the present invention fixed the material to be treated in a fixed position in a liquid acid, stirred the liquid acid in a direction parallel to the surface of the material to be treated, and added the liquid acid to the material to be treated. I tried to rotate or shake it. As a result, it has been found that almost the same effects as the above can be obtained.

As described above, if the material to be treated is rotated or shaken relative to the acid, the above-mentioned effects can be obtained. In the above case, however, the material to be treated is immersed in the acid. Was there. However, even by the method of pouring acid on the material to be treated,
The above-mentioned effects are obtained. That is, the material to be treated may be brought into dynamic contact with the acid.

When the material to be treated is rotated relative to the acid according to the present invention, the rotation speed is 10 to 200 rp.
It is preferably m, and more preferably 20 to 100 rpm. If the rotation speed is less than 10 rpm, the above-mentioned effects are not so remarkable. This rotation speed is 200
Above rpm, the insertion loss of the optical waveguide tends to increase.

Although the case where the molten liquid of benzoic acid is used as the proton exchange source has been described above, almost the same action and effect can be obtained with other proton exchange sources.

[0014]

EXAMPLES First, the aspect of the present invention will be further described. The heated liquid acid includes a melt of an acid that is solid at room temperature and a heated product of the acid that is liquid at room temperature. Examples of the solid acid at room temperature include benzoic acid. Examples of acids that are liquid at room temperature include phosphoric acid, pyrophosphoric acid, phosphorous acid, and nonanoic acid.

In the present invention, the material to be treated for the proton exchange treatment includes a substrate material made of electro-optic crystal.
At this time, when forming a slab optical waveguide, a substrate material as a material to be processed is immersed in an acid. When forming a three-dimensional optical waveguide, a masking layer having openings along the optical waveguide pattern is provided on the surface of the substrate material, and this is used as the material to be treated.

The time for fixing the material to be treated in the heated steam of acid varies depending on the heat capacity of the material to be treated and the heating temperature of the acid.
Further, preferably, the heated steam of the acid is refluxed in the closed container so that the heated steam is always circulated in the closed container.

The form of the material to be treated can be variously changed. As one method, a chip-shaped substrate material having a substantially rectangular shape in plan view can be treated according to the present invention. Also, wafer-shaped substrate materials can be processed according to the present invention. In any case, when forming a slab optical waveguide, it is preferable to immerse each substrate material in an acid.

When a three-dimensional optical waveguide is formed on the above-mentioned wafer-shaped substrate material, the following is performed. First,
A masking layer is provided on the surface of the substrate material, a photoresist layer is provided on the surface of the masking layer, and a planar pattern of openings corresponding to the optical waveguide is provided in the photoresist layer. At this time, for example, many Mach-Zehnder type or linear patterns are formed.

Then, the masking layer is etched and the photoresist layer is removed to obtain a material to be treated. Then
A three-dimensional optical waveguide is formed in the substrate material by contacting the exposed portion of the substrate material with acid. A large number of chip-shaped optical waveguide substrates are cut out from the wafer thus obtained.

Also in the case of forming a three-dimensional optical waveguide on the above-mentioned chip-shaped substrate material, first, a masking layer is provided on the surface of the substrate material, and a photoresist layer is provided on the surface of the masking layer. An opening having a pattern such as an I shape is provided in the photoresist layer. After that, the processing is performed as described above.

Hereinafter, more specific experimental results will be described. An aluminum film was formed on a wafer-shaped substrate material made of a lithium niobate single crystal having a thickness of 1 mm, which was cut in the X plane, by resistance heating and vacuum deposition. A photoresist layer was provided on the surface of the aluminum film, an opening was provided in the photoresist layer by exposure, the aluminum film was etched, and the photoresist layer was removed. In this state, a large number of linear waveguide patterns are formed by the aluminum film on the surface of the substrate material.

Using the material to be treated thus obtained, proton exchange was carried out using the apparatus schematically shown in FIG. A round bottom glass container 10 was used as a reaction container. In this example, the dimensions of the glass container 10 are 150 mm in diameter and 130 mm in height.
It has a bottomed cylindrical shape. A glass lid 8 was put on the glass container 10 to seal the inside of the container 10. The lid 8 is provided with cylindrical protrusions 8a, 8b, 8c. Protrusion 8
A condenser 9 is attached to b, and the opening of the protrusion 8c is sealed with a silicon stopper.

The shaft 7 made of a corrosion resistant material such as Teflon is inserted into the container 10 through the opening of the projection 8a,
The upper end of the shaft 7 is connected to the rotating shaft of the motor 6. A seal member made of Teflon or the like seals between the shaft 7 and the opening of the protrusion 8a.

When the material to be treated is shaken, the shaft 7 can be shaken through the opening of the protrusion 8a.
A shaking mechanism was arranged in place of the motor 6. Hereinafter, the case of rotating the material to be processed shown in FIG. 1 will be described in detail.

The oil 14 is contained in the container 13, and the throw-in heater 15a of the heating device 15 is put into the oil 14. The lower part of the container 10 is immersed in the oil 14.

A frame 24 is attached to the lower end of the shaft 7, and a tray 20 is attached to the lower end of the frame 24.
FIG. 2A is a plan view showing the tray 20.
(B) is a front view of the tray 20.

The planar contour of the tray 20 is substantially circular,
An arcuate side wall 20b is provided so as to surround the circular bottom surface 20a. In this example, the notches 22 are formed at four locations, and the notches 22 are deeper than the bottom surface 20a. A predetermined number of circular through holes 23 are provided on the bottom surface 20a. In this example, the bottom surface 20a has a diameter of 100 mm, and the circular through hole 23 has a diameter of 4 mm.
The material to be processed 21 was placed on the bottom surface 20a. However, Figure 2
In (a), the material 21 to be treated is omitted from the drawing.

As shown in the sectional view of FIG. 3, a flat plate-shaped base portion 24a of the frame 24 is fixed to the lower end of the shaft 7, and flat plate-shaped arm portions 24b are provided at both ends of the base portion 24a. . The tray 2 is provided on the pair of arms 24b facing each other.
0 is sandwiched and fixed by bolts or the like.

When carrying out the proton exchange, in this embodiment, 750 g of benzoic acid is put into the container 10 and the temperature of the oil 14 is raised to 195 ° C. to completely melt the benzoic acid. The pan 20 and the frame 24 are allowed to stand in the melt 18 for about 2 hours while being rotated to make the temperature of the melt 18 uniform. During this time, the entire container 10 is kept warm and the condenser 9
Cooling water is circulated in the inside as indicated by arrow B so that the benzoic acid is constantly refluxed. With such an apparatus, there is almost no decrease in the amount of liquid due to evaporation of benzoic acid, and the proton exchange step can be carried out at normal pressure for a long time.

The material 21 to be processed after the photoresist layer is removed as described above is ultrasonically cleaned with acetone, isopropyl alcohol and pure water. Raise the shaft 7,
The saucer 20 is located inside the lid 8. Container 1
A partition plate made of Teflon is sandwiched between the container 0 and the lid 8, and the container 1
The inner space between 0 and the lid 8 is separated. The lid 8 is lifted, and the processed material 21 is placed on the saucer 20. At this time, since the container 10 is covered with the partition plate made of Teflon, the vapor of benzoic acid remains sealed. Then, the lid 8 is closed, the partition plate made of Teflon is pulled out, and the saucer 20 is exposed to the vapor of benzoic acid.

Then, the shaft 7 is slowly lowered to immerse the pan 20 in the melt 18. Then 1
Proton exchange was carried out at 95 ° C for 20 minutes. At this time, by rotating the shaft 7 in the direction of arrow A, the tray 2
The material to be treated 21 in 0 was rotated according to the present invention. The rotation speed was variously changed as shown in FIG. However, the horizontal axis in FIG. 4 is represented by a logarithmic scale.

After the proton exchange is completed, the shaft 7 is lifted, the pan 20 is taken out of the melt 18, and is placed in the space 19. At this time, the melt remaining in the pan 20 flows down from the circular through hole 23.

Then, the shaft 7 was rotated as shown by the arrow A, and the molten liquid adhering to the surface of the material to be treated 21 was scattered and removed toward the side wall 20b by the centrifugal force to be removed.

Then, the wafer is taken out from the container 10 by using a partition plate made of Teflon according to the procedure which is completely opposite to the above. The wafer is ultrasonically cleaned with ethanol, acetone, isopropyl alcohol, and pure water. Then, the aluminum film is removed by a usual etching technique.

Next, this wafer was annealed in order to promote diffusion of H ^{+ in} the single crystal and obtain a stable optical waveguide with low loss. Specifically, a platinum wire jig is placed in the center of a glass petri dish, a wafer is placed on the jig so that the substrate does not touch the glass, and a glass lid is closed. This petri dish was placed in an electric furnace, heated from room temperature to 340 ° C. at 10 ° C./min, and kept at 340 ° C. for 7 hours. Then, it was naturally cooled to 100 ° C. or lower, and the wafer was taken out.

While the temperature was maintained at 340 ° C., it was confirmed by thermocouples attached to various places in the electric furnace that the temperature distribution in the electric furnace was uniform within the space containing the petri dish. The temperature fluctuation was controlled to be ± 0.5 ° C while the temperature was kept at 340 ° C.

After cutting the 3 inch wafer after annealing,
A chip-shaped optical waveguide substrate was cut out. On this surface,
A linear optical waveguide is formed. Next, the end face of the optical waveguide is optically polished by high-speed lapping and mechanochemical polishing. Next, the optical waveguide substrate whose end face was optically polished was set in a normal optical system, and the insertion loss was evaluated. The result is shown in FIG.

As can be seen from FIG. 4, by performing proton exchange while rotating the material to be treated 21 according to the present invention, the variation in insertion loss of the optical waveguide was reduced and the insertion loss was reduced. Especially, the rotation speed of the processed material 21 is set to 1
At 0 to 200 rpm, and further at 20 to 100 rpm, the greatest effect was obtained.

Further, FIG. 5 shows a material 2 to be treated according to the present invention.
1 shows the relationship between the shaking speed and the insertion loss of the optical waveguide when proton exchange is performed while shaking 1. However, in FIG. 5, the mark “◯” indicates the case where the plate is shaken back and forth with a shaking width of 20 mm. The "x" mark indicates the case of shaking in a figure 8 shape, and the shaking width is 20 mm x 25 mm.

[0040]

As described above, when the optical waveguide substrate is manufactured by the proton exchange method according to the present invention, the variation of the insertion loss of the optical waveguide is reduced, the insertion loss is reduced, and the optical waveguide substrate of constant quality is obtained. Can be manufactured stably.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a device for proton exchange.

2 (a) is a plan view showing a tray 20 and FIG. 2 (b) is a front view showing the tray 20. FIG.

FIG. 3 is a cross-sectional view schematically showing a state in which a material to be treated 21 in a tray 20 is exposed to benzoic acid vapor.

FIG. 4 is a graph showing the relationship between the rotation speed of the wafer-shaped material to be processed 21 and the insertion loss of the optical waveguide.

FIG. 5 is a graph showing the relationship between the shaking speed and the insertion loss of the optical waveguide when proton exchange is performed while shaking the material to be treated 21.

[Explanation of symbols]

 18 Melted solution of benzoic acid 20 Sauce 21 Material to be processed A Rotating direction of shaft 7

Claims (2)

[Claims] 1. A method of manufacturing an optical waveguide substrate, wherein an optical waveguide is formed on the substrate material by dynamically contacting a material to be treated containing the substrate material made of an electro-optic crystal with a heated liquid acid. 2. The optical waveguide substrate according to claim 1, wherein the material to be processed is rotated or shaken relative to the acid while the material to be processed is in contact with the acid. Production method.