JP3145529B2 - Method for manufacturing optical waveguide substrate - Google Patents

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 A single crystal of lithium niobate (LiNbO ₃ )
It is expected as an optoelectronic material. As a method for forming an optical waveguide on a substrate material made of 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, ion exchange between H ⁺ and Li ⁺ is caused by putting the above substrate material into a melt of benzoic acid (C ₆ H ₅ COOH), and the surface of the substrate is consisting Hx Li _1-x Nb O ₃ to form a high refractive index layer. The substrate is then annealed at a 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.

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

[0005]

However, a disadvantage of the above-mentioned proton exchange method is that a large amount of benzoic acid, which is a proton exchange source, evaporates, 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 ^{+ in} the optical waveguide was uneven, and the diffusion of impurities was observed locally. For this reason,
The variation in the insertion loss of light is large, and an optical waveguide of constant quality cannot be manufactured stably.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent unevenness of H ⁺ diffusion state and local attachment and diffusion of impurities in the above-mentioned proton exchange step, to reduce variation in insertion loss in an optical waveguide, and to reduce the variation. It is an object of the present invention to stably manufacture an optical waveguide having a high quality.

[0007]

According to the present invention, a material to be treated including a substrate material made of an electro-optic crystal is brought into contact with heated benzoic acid in a liquid state, and the material to be treated is treated with respect to benzoic acid. The substrate is rotated or shaken at a relative speed of 10-200 rpm to form an optical waveguide in the substrate material by a proton exchange process.

[0008]

As described above, the present inventors have studied in detail the cause of the increase or variation in insertion loss in the optical waveguide throughout the manufacturing process. As a result, when the material to be treated is rotated or shaken at the stage of performing the proton exchange by immersing the material to be treated in a melt of benzoic acid,
Insertion loss is reduced in the optical waveguide after annealing,
Further, it has been found that the variation in insertion loss is very small.

The reason for this is not necessarily clear, but it is considered that the rotation or shaking of the material to be processed removes impurities in the liquid adhering to the surface of the material to be processed, so that the characteristics do not deteriorate. Can 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 Probably because it does not occur.

Further, the inventor of the present invention fixes the material to be treated in a fixed position in a liquid acid, stirs the liquid acid in a direction parallel to the surface of the material to be treated, and removes the liquid acid from the material to be treated. I tried to rotate or shake. As a result, it was found that substantially the same operation and effect as described above were 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. I was However, also by pouring the acid on the material to be treated,
The above effects were obtained. That is, the material to be treated only needs to dynamically contact 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.
m, 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
If it exceeds rpm, the insertion loss of the optical waveguide tends to increase.

In the present invention, a melt of benzoic acid is used as a proton exchange source.

[0014]

EXAMPLES First, the embodiments of the present invention will be further described.

In the present invention, the material to be subjected to the proton exchange treatment includes a substrate material made of an 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 an opening along the optical waveguide pattern is provided on the surface of the substrate material, and this is used as a material to be processed.

The time during which the material to be treated is fixed in the heated steam of the acid depends on the heat capacity of the material to be treated and the heating temperature of the acid.
Preferably, the heated steam of the acid is refluxed (refluxed) in the closed vessel so that the heated steam is always circulated in the closed vessel.

The form of the material to be treated can be variously changed. In one method, a substantially rectangular chip-shaped substrate material in plan view can be treated according to the present invention. Also, wafer-shaped substrate material 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 opening corresponding to the optical waveguide is provided in the photoresist layer. At this time, for example, a large number of Mach-Zehnder or linear patterns are formed.

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

When a three-dimensional optical waveguide is formed on the above-mentioned chip-shaped substrate material, a masking layer is first 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. Thereafter, the processing is performed substantially 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 single crystal of lithium niobate having a thickness of 1 mm, which had been cut on the X-plane, by resistance heating and vacuum evaporation. A photoresist layer was provided on the surface of the aluminum film, an opening was formed 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 on the surface of the substrate material by the aluminum film.

Using the material to be treated thus obtained, proton exchange was carried out using an 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.
Cylindrical shape with a bottom. A glass lid 8 was placed over the glass container 10 to seal the inside of the container 10. The lid 8 is provided with cylindrical projections 8a, 8b, 8c. Protrusion 8
A capacitor 9 is attached to b, and the opening of the projection 8c is sealed with a silicon stopper.

A 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 rotation shaft of the motor 6. The space between the shaft 7 and the opening of the projection 8a is sealed by a sealing member made of Teflon or the like.

When the material to be processed is shaken, the opening of the projection 8a is made so that the shaft 7 can be shaken.
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.

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

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

The flat outline of the tray 20 is substantially circular.
An arc-shaped side wall 20b is provided so as to surround the circular bottom surface 20a. In this example, four notches 22 are formed, and the notches 22 are deeper than the bottom surface 20a. A predetermined number of circular through holes 23 are provided in the bottom surface 20a. In this example, the size of the bottom surface 20a is, for example, 100 mm in diameter, and the diameter of the circular through hole 23 is 4 mm,
The workpiece 21 was placed on the bottom surface 20a. However, FIG.
In (a), the workpiece 21 is not shown.

As shown in the sectional view of FIG. 3, a flat base 24a of the frame 24 is fixed to the lower end of the shaft 7, and flat arms 24b are provided at both ends of the base 24a. . A pair of saucer 2 is provided on a pair of opposite arm portions 24b.
0 is sandwiched and fixed by bolts or the like.

In performing the proton exchange, in this embodiment, 750 g of benzoic acid is charged into the container 10, the temperature of the oil 14 is raised to 195 ° C., and the benzoic acid is completely melted. The tray 20 and the frame 24 are left rotating for about 2 hours in the melt 18 to make the temperature of the melt 18 uniform. During this time, the entire container 10 is kept warm and the condenser 9
Inside, cooling water is circulated as indicated by arrow B so that 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 performed at normal pressure for a long time.

The workpiece 21 from which the photoresist layer has been removed as described above is ultrasonically cleaned with acetone, isopropyl alcohol and pure water. Raise the shaft 7,
The tray 20 is positioned inside the lid 8. Container 1
A container made of Teflon is sandwiched between the cover
Separate the inner space between 0 and the lid 8. The lid 8 is lifted, and the workpiece 21 is placed on the tray 20. At this time, since the container 10 is covered with a partition plate made of Teflon, the vapor of benzoic acid remains sealed. Then, the lid 8 is closed, the Teflon partition plate is pulled out, and the tray 20 is exposed to the vapor of benzoic acid.

Then, the shaft 7 is slowly lowered, and the tray 20 is immersed in the melt 18. Then 1
Proton exchange was performed at 95 ° C. for 20 minutes. At this time, by rotating the shaft 7 in the direction of arrow A,
The workpiece 21 within 0 was rotated according to the invention. This 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 raised, and the tray 20 is taken out of the melt 18 and positioned in the space 19. At this time, the melt remaining in the tray 20 flows down from the circular through hole 23.

Then, the shaft 7 was rotated as shown by the arrow A, and the melt adhered to the surface of the workpiece 21 was scattered toward the side wall 20b by centrifugal force and removed.

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

Next, the wafer was annealed to promote H ⁺ diffusion in the single crystal and obtain a stable optical waveguide with low loss. Specifically, a jig made of platinum wire is placed at the center of a glass petri dish, a wafer is placed on the jig, the substrate is prevented from touching the glass, and a glass lid is placed. This petri dish was placed in an electric furnace, heated from room temperature to 340 ° C. at a rate of 10 ° C./min, and kept at 340 ° C. for 7 hours. Then, the wafer 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 with a thermocouple attached to each place in the electric furnace that the temperature distribution in the electric furnace was uniform within the space containing the petri dish. Further, the temperature was controlled so that the temperature fluctuation during the temperature holding at 340 ° C. was ± 0.5 ° C.

The 3-inch wafer after annealing is cut,
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 the proton exchange while rotating the material to be treated 21 according to the present invention, the variation in the insertion loss of the optical waveguide is reduced, and the insertion loss is also reduced. In particular, the rotation speed of the processing target material 21 is set to 1
The effect was greatest at 0 to 200 rpm, and further at 20 to 100 rpm.

FIG. 5 shows the 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 No. 1. However, in FIG. 5, a mark “○” indicates a case where the reciprocating shaking was performed with a shaking width of 20 mm. The mark “x” indicates a case where the figure was shaken in the shape of a figure 8, and the shake width was 20 mm × 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 in the insertion loss of the optical waveguide is reduced, the insertion loss is reduced, and the optical waveguide substrate having a constant quality is obtained. Can be manufactured stably.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an apparatus for proton exchange.

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

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

FIG. 4 is a graph showing a relationship between a rotation speed of a wafer-like processing target material 21 and an insertion loss of an 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 processed 21;

[Explanation of symbols]

 18 Melt of benzoic acid 20 Receiving tray 21 Material to be treated A Rotation direction of shaft 7

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12-6/14 JICST file (JOIS)

Claims (1)

(57) [Claims] In a state in which a material to be treated including a substrate material made of an electro-optic crystal is brought into contact with heated benzoic acid in a liquid state, the material to be treated is relatively heated to 10-2 with respect to benzoic acid.
A method for manufacturing an optical waveguide substrate, wherein the optical waveguide is rotated or shaken at a speed of 00 rpm to form an optical waveguide on the substrate material by a proton exchange process.