JPH0731288B2 - Method of forming optical element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for forming an optical element used for an optical waveguide and a microlens used in the field of optical information processing using coherent light, or in the fields of optical communication and optical application measurement control. It is about.

Conventional technology Conventionally, a LiNbO ₃ substrate, which is a ferroelectric material, is heat-treated in a benzoic acid solution at about 160 ° C to 250 ° C to exchange Li of the LiNbO ₃ substrate with H in benzoic acid, and thus a high refractive index layer Rate difference Δn = 0.
13) was formed as an optical waveguide. "Ju-El-Jackel, CIE Rice and J-Ju Beselka" Proton Exchange for High-Index Waveguide in LiNbO ₃ "Applied Physics Letters, Vol. 41, No. 7, pp. 607-608 (198).
2) (JLJackel, CE, Rice and JJVeselka, “Proton
exchange for high−index weveguides in LiNbO ₃ , "App
L. Phys. Lett, Vol 41, No. 7, PP 607-608 (1982)) ". An optical waveguide will be taken as an example of the optical element, and a method of forming the optical waveguide will be described below. FIG. 5 shows a concrete configuration diagram of an optical waveguide forming method using a conventional proton exchange method in a solution. 1 is a LiNbO ₃ substrate which is a ferroelectric substrate, 2'is a protective mask made of Al, and 4 is a slit formed on the protective mask 2'by a photo process and etching. The LiNbO ₃ substrate 1 on which the protective mask 2 ′ and the slit 4 are formed is dipped in a benzoic acid solution 5 ′. Solution 5'is heater 9
A constant temperature of 200 ° C. is maintained through the beaker 10 heated by. The LiNbO ₃ substrate 1 is heat-treated in this solution 5 ′ for 60 minutes and then washed with methanol. In this way, H in the benzoic acid solution 5'is exchanged with Li in the LiNbO ₃ substrate 1 to form the high refractive index layer 8. This high refractive index layer 8 has a thickness of 0.
It becomes an optical waveguide of about 5 μm.

Problems to be Solved by the Invention The method for forming an optical element in a solution as described above has the following problems. (1) Workability deteriorates due to the vapor generated by the solution. (2) Work such as stirring is required to make the temperature of the solution uniform. (3) Cracks are likely to occur when a substrate is moved in and out due to a rapid temperature change.
(4) The surface other than the surface to be proton-exchanged is immersed in the solution, so that the proton is exchanged. (5) Since the whole sample holder is immersed in the solution, the life of the solution is short and a large amount of it must be used.

Means for Solving the Problems The present invention solves the above-mentioned problems, and improves workability, controllability, and mass productivity significantly by adding a new device to the method for forming an optical element. . That is, the method of forming the optical element of the present _{invention, LiNbxTa (1- x) O 3} (0 ≦ x ≦ 1)
It comprises a step of applying an acid containing pyrophosphoric acid as a main component to the surface of the substrate, and a step of heat-treating the substrate to form a high refractive index portion on the substrate immediately below the applied acid. .

Action The present invention allows proton exchange by the above-mentioned means in the same process as ordinary diffusion, annealing, etc., and greatly improves workability, controllability, and mass productivity.

Example 1 A first example of the method for forming an optical element of the present invention is shown in FIG. In the first embodiment, LiNbO _{3 is used} as a method for forming an optical element.
A method for forming an optical waveguide by coating pyrophosphoric acid (H ₄ P ₂ O ₇ ) on a substrate will be described. In FIG. 1a, 1 is a + Z plate (+ Z
Substrate cut out perpendicular to the axis) LiNbO ₃ substrate, 2 is LiNb
It is a protective mask of Ta formed on the O ₃ substrate 1 + Z surface 3. The thickness of the protective mask 2 is 300Å. 4 is a protective mask 2 by photo process and lift-off process
It is a slit formed in. First, as shown in FIG. 3B, pyrophosphoric acid 5 having a purity of 95% is used on a LiNbO ₃ substrate 1 using a spinner.
The coating was performed on the + Z surface 3 of. Specifically, the rotation speed is 300 rpm
It was spin coated for 20 seconds. Pyrophosphoric acid 5 having a purity of 95% is a liquid at room temperature, and surface coating such as spin coating is possible.

Next, as shown in Fig. 6c, 6 in an oven 6 heated to 200 ° C.
Heat treatment was performed for 0 minutes. 7 is A for equalizing the temperature
It is a plate made of l. Finally, the plate 7 is taken out from the oven 6 and cooled, and then the protective mask 2 made of Ta is removed with a sodium hydroxide-based solution.
Is. The high refractive index layer 8 was formed immediately below the slit 4 by proton exchange. The thickness of the high refractive index layer 8 is 0.5 μ
m, which functions as an optical waveguide. The maximum refractive index difference Δn between the high refractive index layer 8 formed of pyrophosphoric acid and the LiNbO ₃ substrate 1 is 0.145, which is higher than that in the solution treatment of benzoic acid, and it is possible to manufacture an optical waveguide having a larger light confinement.
By the way, since pyrophosphoric acid 5 has an extremely small amount of evaporation compared to benzoic acid, etc., and the amount used for making an optical element at one time is very small, safety and maintenance such as maintenance of a solution exchange oven are performed. Considering the aspect, workability is dramatically improved.

Another feature of this method is that it does not require a pre-annealing step and a device for preventing cracking of the LiNbO ₃ substrate 1.

Further, Ta can be used as a protective mask which is not etched by phosphoric acid such as pyrophosphoric acid 5 and the pattern formation seems to be simple and effective.

A second embodiment of the optical element forming method of the present invention will be described. The second embodiment shows a method of forming a microlens as an optical element. FIG. 2 is a perspective view of a microlens forming method. 1 is LiNbO ₃ substrate 5 is 1 mg of pyrophosphoric acid. Pyrophosphoric acid 5 does not spread due to surface tension and maintains a constant size. By performing proton exchange in a quartz tube at 260 ° C. for 6 hours, the liquid gradually spreads, and a high refractive index layer 8 having a convex shape at the center as shown in FIG. 3 is formed. This high refractive index layer 8 becomes a microlens.

As described above, when this method is used, proton exchange can be performed even at a temperature of 260 ° C or higher, which cannot be realized with benzoic acid, the diffusion constant can be increased, and the process time can be shortened. Further, the protective mask can be applied only to a desired portion, and the protective mask manufacturing process can be omitted.

Regarding the third embodiment of the method for forming an optical element of the present invention, LiTa
The case of forming an optical waveguide on the O ₃ substrate 1'will be described. FIG. 4 is a sectional view of the method for forming the optical element.
1 'is LiTaO ₃ substrate, 2 is LiTaO ₃ substrate 1' is a protective mask of Ta formed on the -Z surface 3 'of. Reference numeral 4 is a slit formed on the protective mask 2 by a photo process and a lift-off process. The LiTaO ₃ substrate 1 ′ was heated to 70 ° C. and 100% pure pyrophosphoric acid 5 was applied by a syringe. Next, heat treatment was performed for 10 hours in an oven heated to 280 ° C. Finally, the protective mask was gradually removed. In this way, a high-refractive index layer (Δn = 0.1) similar to that in the above-mentioned example was formed by means of proton exchange immediately below the slit 4. This high refractive index layer serves as an optical waveguide.

As described above, according to the present invention, since the high temperature treatment is easy, the optical waveguide can be formed on the LiTaO ₃ substrate 1 '.

Although heat treatment was performed at 200 ° C. or higher in the examples, an optical element can be formed at 100 ° C. or higher if it takes time. Also, coating at 90 ° C or higher is difficult in terms of work, and it is desirable to coat at 90 ° C or lower. Further, although the optical element is formed on the + Z plate, a high-quality optical element can be formed even on a -Z plate without chemical damage. In particular, when a + Z plate or a -Z plate is used, other X
Or surface roughening due to acid (chemical damage) compared to Y plate
Is preferable because it is less.

Effects of the Invention As described above, according to the method for forming an optical element of the present invention, Li
NbxTa _(1- x ₎ O (0 ≤ x ≤ 1) The surface of the substrate is coated with an acid containing pyrophosphoric acid as a main component, and the substrate is heat-treated.
Since the high-refractive-index portion can be formed immediately below the acid-coated region, the optical element can be easily manufactured with good controllability.
In addition, since proton exchange can be performed by the same heat treatment as diffusion and annealing used in a normal semiconductor process, workability and mass productivity are greatly improved.

[Brief description of drawings]

FIG. 1 is a process diagram of a method for forming an optical element in a first embodiment of the present invention, FIG. 2 is a perspective view for explaining the method of the second embodiment, and FIG. 3 is a second embodiment. FIG. 4 is a sectional view for explaining an example method, FIG. 4 is a sectional view for explaining the method of the third embodiment, and FIG. 5 is a configuration diagram for explaining a conventional optical element forming method. 1 …… LiNbO ₃ substrate, 2 …… Protective mask, 3 …… + Z surface,
4 ... Slit, 5 ... Pyrophosphoric acid, 8 ... High refractive index layer.

Claims (3)

[Claims] 1. A step of applying an acid containing pyrophosphoric acid as a main component on the surface of a LiNbxTa _(1- x ₎ O ₃ (0 ≦ x ≦ 1) substrate, and heat treating the substrate to remove the applied acid. And a step of forming a high refractive index portion on the substrate immediately below. 2. A LiNbxTa _(1- x ₎ O ₃ (0 ≦ x ≦ 1) substrate is + z.
The method for forming an optical element according to claim 1, which is a plate or a -z plate. 3. A step of forming a protective mask pattern of Ta on the surface of the LiNbxTa _(1- x ₎ O ₃ (0 ≦ x ≦ 1) substrate prior to the step of applying an acid to the substrate. The method for forming an optical element according to claim 1.