JP3096183B2 - 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, cracks often occur in the substrate when the substrate is pulled up from the melt, and the yield of the substrate is low. In addition, when a three-dimensional optical waveguide is formed, there is considerable variation in light insertion loss, and it has not been possible to stably produce an optical waveguide having a constant quality.

An object of the present invention is to prevent the occurrence of cracks in the substrate material in the proton exchange step, thereby improving the yield of the substrate and stably producing an optical waveguide having a constant quality. It is.

[0007]

SUMMARY OF THE INVENTION The present invention includes a substrate material made of an electro-optic crystal by heating an acid to a liquid state in a closed vessel equipped with a condenser, refluxing the heated vapor of the acid in the closed vessel. The material to be treated is housed in a closed container while being held by the holder, and the material to be treated is exposed to heated steam by holding the material without contacting the acid, and then the material to be treated is immersed in a liquid acid. An optical waveguide is formed in the substrate material by immersion, the holder is provided with a through hole for flowing down the liquid acid, a tray for supporting the material to be processed, and a frame for supporting the tray. It is characterized by having.

[0008]

The present inventor has studied in detail the cracks in the substrate material as described above and the causes of the variation and increase in insertion loss throughout the manufacturing process. As a result, it was considered that there was a problem in the method of introducing the substrate material into the benzoic acid melt at the stage of immersing the substrate material in the melt. Prior to immersing the substrate material in the melt, the substrate material was fixed on the melt for a predetermined time and exposed to benzoic acid vapor. Then, it was found that cracks were less likely to occur in the substrate material, and finally, the insertion loss of the optical waveguide became almost constant.

The reason why such an effect is obtained is probably that the substrate material is heated by exposing the substrate material to the vapor of benzoic acid, and the thermal shock when the substrate material is injected into the melt is reduced. This prevents cracks. In addition, it is considered that lattice distortion or the like occurs near the surface of the substrate material due to the thermal shock, and proton exchange is inhibited at this portion compared to the surroundings.

Further, it is considered that the surface of the substrate material is wetted with the vapor of benzoic acid, thereby eliminating the adverse effects of water droplets on the surface, and the proton exchange occurs uniformly on the surface. Furthermore, it is considered that proton exchange gradually progresses due to benzoic acid condensed on the substrate surface, whereby a sharp reaction is prevented when the benzoic acid is injected into the melt.

It is effective to reflux (reflux) the heated steam of the acid in the closed vessel so that the heated steam is constantly circulated in the closed vessel. By holding the material to be processed with the holder having the above, it is effective to allow the liquid acid to flow down efficiently. As described above, the case where the melt of benzoic acid is used as the proton exchange source has been described, but substantially the same operation and effect can be obtained with other proton exchange sources.

[0012]

EXAMPLES First, the embodiments 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 an acid that is liquid at room temperature. Benzoic acid can be exemplified as an acid that is solid at room temperature. Examples of the acid that is liquid at room temperature include phosphoric acid, pyrophosphoric acid, phosphorous acid, and nonanoic acid.

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 for fixing the material to be treated in the heating steam of the acid depends on the heat capacity of the material to be treated and the heating temperature of the acid.

The form of the material to be processed 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, each substrate material is immersed in an acid.

In the case where 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. (Experiment 1) An aluminum film was formed on a wafer-like substrate material made of lithium niobate single crystal having a thickness of 1 mm, which was 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 set to a diameter of 120 mm and a height of 130 mm.
Round bottom shape. A glass lid 8 was placed over the glass container 10 to seal the inside of the container 10. On the lid 8,
Cylindrical projections 8a, 8b, 8c are provided. The condenser 9 is attached to the projection 8b, and the opening of the projection 8c is
Sealed with silicone 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.

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 planar 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 a 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 vessel 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.

At this time, as shown in FIG. 3, the vapor of benzoic acid is refluxed in the container 10 as shown by the arrow C. Immediately after the target material 21 was placed on the tray 20, the benzoic acid vapor cooled and solidified on the surface of the target material 21, and the surface of the aluminum film became white. However, the material to be treated 21 was gradually heated by the benzoic acid vapor, and the benzoic acid film solidified on the surface was melted. In this state, it is considered that the temperature of the material to be treated is higher than the melting point of benzoic acid (121 ° C.). In this example, preheating was performed until the benzoic acid film was completely dissolved. And shaft 7
Is slowly lowered, and the tray 20 is immersed in the melt 18.

On the other hand, in the comparative example, the workpiece 21 is
Immediately after placing on the tray, the tray 20 was lowered and the material to be treated 21 was immersed in the melt 18.

Next, proton exchange is performed at 195 ° C. for 20 minutes. At this time, the shaft 7 (the tray 20) is rotated. After the proton exchange is completed, the shaft 7 is raised, the tray 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
It 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 material 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 the diffusion of H ^{+ 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 in which the petri dish was accommodated. Further, the temperature was controlled so that the temperature fluctuation during the temperature holding at 340 ° C. was ± 0.5 ° C.

The annealed 3-inch wafer is cut,
Ten chip-shaped optical waveguide substrates were cut out per wafer. 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. Then
The optical waveguide substrate whose end face was optically polished was set in a normal optical system, and the insertion loss was evaluated. Also, about the wafer,
The presence or absence of cracks was checked.

For the example of the present invention and the comparative example, five wafers were manufactured, and the above-described experiment was performed. As a result, in the example of the present invention, no crack occurred in all five wafers. And
When a total of 50 optical waveguide substrates were cut out from these wafers, the distribution of insertion loss was as shown in Table 1.

[0037]

[Table 1]

On the other hand, cracks occurred in all of the wafers of the comparative examples, so that the insertion loss of the optical waveguide substrate could not be measured.

(Experiment 2) 10 mm × 35 mm
A substantially rectangular chip-shaped substrate material having the following dimensions was used, and a linear pattern was provided on the surface thereof. Other than that, a chip-shaped optical waveguide substrate was manufactured in the same manner as in Experiment 1. At this time, for each of the example of the present invention and the comparative example, 6
Each sample was prepared, and the presence or absence of cracks and the insertion loss were measured. Table 2 shows the results of Examples of the present invention, and Table 3 shows the results of Comparative Examples.

[0040]

[Table 2]

[0041]

[Table 3]

As can be seen from the above results, in the embodiment of the present invention in which the workpiece 21 is exposed to the vapor in the space 19, no crack occurs and the insertion loss is in the range of 4.0 to 4.6 dB. Further, in the example of the present invention, no optical damage occurred even with the output light power of 2 mW.

On the other hand, in the comparative example, cracks occurred in three samples. Also, in sample numbers 8, 9, and 12 in which no crack occurred, there was a large variation in the insertion loss.

[0044]

As described above, according to the present invention, when the substrate material is subjected to the proton exchange treatment, cracks are less likely to occur in the substrate material, and finally, the insertion loss of the optical waveguide becomes almost constant. An optical waveguide having stable quality 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.

[Explanation of symbols]

 10 Container 18 Melt of benzoic acid 20 Receiving tray 21 Material to be processed 23 Circular through hole A Rotating direction of shaft C Vapor of benzoic acid

Claims (1)

(57) [Claims] An acid is heated to a liquid state in a closed vessel provided with a condenser, and the heated vapor of the acid is refluxed in the closed vessel to hold a material to be processed including a substrate material made of an electro-optic crystal. Housed in the closed container in a state of being held by, exposing the material to be heated to the heated steam by holding the material to be processed without contacting the acid,
Next, an optical waveguide is formed in the substrate material by immersing the material to be treated in the liquid acid, and the holder is provided with a through hole for allowing the liquid acid to flow down. A method for manufacturing an optical waveguide substrate, comprising: a tray for supporting a material; and a frame for supporting the tray.