JP2538161B2 - Method for manufacturing optical waveguide and lens - 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 used for manufacturing optical waveguides and lenses used in the field of optical information processing utilizing coherent light or in the field of optical measurement and control.

[0002]

2. Description of the Related Art LiNbO ₃ substrates, which are conventional ferroelectric substrates,
A high refractive index layer (refractive index difference Δn = 0.13) was formed by vapor-depositing a Cr or Al film, etc., and opening a slit having a width of several μm by photoprocess and etching, and heat-treating it in benzoic acid. (J-El Jacquel, CIE Rice and J-Je Beselka, Proton Exchange Fore High Indices Waves In-LiNbO3 ", Affixes Letters, 41, No. 7, 607
−608 (1982) (JLJackel, CE Rice and JJVese
lka, Proton Exchange for High-index Waveguidesin
LiNbO ₃ ", Appl.Phys.Lett, Vol 41, No.7.pp 607−608 (198
2) , refer to JP-A-60-7403 )

The formation of an optical waveguide as an optical element will be described below. FIG. 4 shows a specific structure for forming an optical waveguide using a conventional proton exchange method. 1 is a ferroelectric substrate
A LiNbO ₃ substrate, 2 is an Al film that serves as a protective mask, and 3 is a slit formed on the protective mask 2 by photoprocess and etching. The LiNbO ₃ substrate 1 is immersed in benzoic acid 4. By the way, the dissociation index of benzoic acid 4 is 4.12, which has a melting point of 122 ° C and a boiling point of 248 ° C. The heat treatment temperature Tc of the quartz beaker 5 containing the benzoic acid 4 is kept at 200 ° C. by the heater 6, and the benzoic acid 4 is liquid at this temperature. The LiNbO ₃ substrate 1 is heat-treated in this benzoic acid 4 for 60 minutes, and then washed with methanol. In this way, the hydrogen in the benzoic acid 4 and the lithium in the LiNbO ₃ substrate 1 are exchanged, and the high refractive index layer 7 is formed. This high refractive index layer 7 becomes an optical waveguide of about 0.5 μm.

[0004]

In the method for forming an optical element using benzoic acid as an acid as described above, the dissociation index PKa of benzoic acid is as large as 4.12 and the hydrogen concentration in the solution is low, so that L
It is difficult to form the high refractive index layer 7 having a refractive index difference Δn of 0.13 or more on the iNbO ₃ substrate 1. Further, since the boiling point is 248 ° C., it is difficult to raise the diffusion constant to a higher temperature and it takes time to manufacture the device.

Therefore, in the present invention, LiNb _x Ta _1-x O ₃ (0≤x≤
1) Optical waveguide and lens forming high refractive index layer on substrate
It aims at providing the manufacturing method of.

[0006]

The optical waveguide of the present invention and
In order to solve the above problems, the lens manufacturing method has a dissociation index PKa in the range of 1.0 ≦ PKa ≦ 2.2, a liquid at a temperature of 250 ° C. or higher, and a water-soluble acid (phosphoric acid
Except) , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate,
Heat treatment is performed at a temperature T in the range of 250 ° C. ≦ T ≦ 300 ° C. to form a high refractive index layer on the LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate.

[0007]

According to the present invention, since the hydrogen concentration is high during the heat treatment in an acid by the above means, a high refractive index layer having a refractive index difference Δn of 0.13 or more can be easily formed on a LiNbO ₃ substrate. LiNbO ₃
Not only the substrate but also the composition of LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) substrate can form a high refractive index layer having a large amount of change in refractive index more than that obtained by the conventional method . Moreover, since high-temperature treatment is possible, the time required to manufacture the device can be greatly reduced. Furthermore, since the acid is soluble in water, it is very easy to handle and the workability is improved.

[0008]

EXAMPLE A first example of the present invention will be described with reference to FIG. In this first embodiment, formation of an optical waveguide as an optical element will be described. Reference numeral 1 is a LiNbO ₃ substrate which is a ferroelectric substrate, and 2 is a Ti-Au film which is a protective mask formed on the + Z plane of the LiNbO ₃ substrate 1 and has a Ti film thickness of 500Å and an Au film thickness of 500Å. Reference numeral 3 is a slit formed on the protective mask 2 by a photo process and etching. The LiNbO ₃ substrate 1 is immersed in phosphoric acid 8. Phosphoric acid is a general term for orthophosphoric acid (H ₃ PO ₄ ), pyrophosphoric acid (H ₄ P ₂ O ₇ ), and phosphoric acid 8 is orthophosphoric acid (PKa =
2.2) and pyrophosphoric acid (PKa = 1.7) are mixed. Phosphoric acid 8
The quartz beaker 5 containing is always kept at a temperature of Tc = 250 ° C. by a heater 6 with a temperature controller. LiNbO
₃ Substrate 1 is heat-treated for 4 minutes, pulled out, and washed with running water. In this way, the high refractive index layer 7 having a thickness of 0.5 μm is formed, and this high refractive index layer 7 becomes an optical waveguide.

Dissociation index PKa of phosphoric acid 8 is as low as about 2,
Since the hydrogen concentration is high, a high refractive index layer with Δn = 0.14 can be formed. Further, since phosphoric acid 8 is soluble in water, it is easy to wash it after the proton exchange. Furthermore, phosphoric acid 4 is inexpensive. Further, even at a high temperature of 300 ° C. or higher, phosphoric acid 4 has little evaporation and its melting point is room temperature or less, so even if it evaporates, it solidifies and adheres to the surroundings to prevent work. On the other hand, benzoic acid not only hinders the work by solidifying the evaporate and adhering to the surroundings, but it is insoluble in water and cannot be washed with water, which is a problem.

The + Z plane or the −Z plane of LiNbO ₃ 1 is particularly resistant to chemical damage by acid and is advantageous for long-term proton exchange. Next, a second embodiment of the present invention will be described with reference to FIG. The second embodiment shows a method of forming a microlens as an optical element. FIG. 2 is a cross-sectional view showing a microlens forming step, which is similar to
The slit 3 on the LiNbO ₃ substrate 1 is changed to a circular portion 9 which becomes a lens. Proton exchange is carried out in phosphoric acid 8 as in the first embodiment.

FIG. 3 shows the relationship between the proton exchange time and the proton exchange depth. From Fig. 3, at the temperature of T = 280 ℃ 6.
By performing the heat treatment for 3 hours, the high refractive index layer 7 having a depth of 10 μm can be obtained. On the other hand, it takes almost 100 hours to obtain a depth of 10 μm with benzoic acid at T = 230 ° C, and it can be seen that the high temperature treatment greatly contributes to shortening the manufacturing time. It can also be used to form Fresnel lenses and grating lenses.

In the embodiment, a LiNbO ₃ substrate which is easy to obtain a good quality crystal is used as the ferroelectric substrate, but LiTaO ₃ or the like can be used. Since LiTaO ₃ has a smaller diffusion constant than LiNbO ₃ , high temperature treatment is extremely effective. The dissociation index PKa is 1.0
Phosphoric acid was used as a water-soluble acid that was liquid at a temperature of 250 ° C. or higher with ≦ PKa ≦ 4.0, but other arsenic acids (dissociation index 2.2) and the like are also possible. However, dissociation index PKa is 1
The following acids are prone to chemical damage and cannot be used. Also
By using a liquid acid at 250 ° C or higher, there is little vapor, and stable and reproducible proton exchange can be performed at high temperature. Further, in order to obtain the high refractive index layer (Δne = 0.14 or more) with almost no chemical damage, the acid dissociation index PKa is 1.0 ≦ PKa ≦
A range of 4.0 is desirable. In particular, an acid having a dissociation index of 2.2 or less is effective because the refractive index layer produced has a large change in the refractive index.

By the way, if the temperature for heat treatment is 170 ° C. or lower, the diffusion constant is low, which is an industrial problem. Further, when the temperature for heat treatment is 300 ° C. or higher, the acid state is apt to change greatly and the reproducibility deteriorates.

[0014]

EFFECT OF THE INVENTION Manufacturing Method of Optical Waveguide and Lens of the Present Invention
According to the method , when the dissociation index PKa is 1.0 ≦ PKa ≦ 2.2 , 25
Acid that is liquid at 0 ° C or higher and soluble in water (phosphoric acid
By performing the proton exchange with the exception), LiNb _x Ta
_1-x O ₃ (0 ≦ x ≦ 1) Obtained by conventional method for each composition of substrate
In addition to being able to form a high-refractive-index layer having a large amount of change in the refractive index, the high-temperature treatment can be performed and the proton exchange time can be greatly shortened. Furthermore, since the acid is soluble in water, it is very easy to handle and the workability is improved.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

FIG. 2 is a diagram for explaining a microlens forming process using a second embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between proton exchange time and proton exchange depth.

FIG. 4 is a diagram for explaining a conventional optical element forming method.

[Explanation of symbols]

1 LiNbO ₃ 2 Protective mask 3 Slit 7 High refractive index layer 8 Phosphoric acid

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-7403 (JP, A) "Chemical Handbook Basic Edition Revised 2nd Edition" by The Chemical Society of Japan, Basic Edition 2, Showa 50, 6, 20 Maruzen, P. 993-999

Claims (2)

(57) [Claims] _1. LiNb _x Ta _{1 in a} dissociation index PKa in the range of 1.0 ≦ PKa ≦ 2.2, a liquid at a temperature of 250 ° C. or higher, and a water-soluble acid (excluding phosphoric acid). _-x O
₃ (0 ≤ x ≤ 1) Temperature T is 250 ℃ ≤ T ≤ 300 ℃
And heat treatment is performed in the range of LiNb _x Ta _1-x O ₃ (0 ≦
x ≦ 1) light characterized by forming a high refractive index layer on the substrate
Method of manufacturing waveguide . 2. LiNb _x Ta _{1 in a} dissociation index PKa in the range of 1.0 ≦ PKa ≦ 2.2, a liquid at a temperature of 250 ° C. or higher, and a water-soluble acid (excluding phosphoric acid). _-x O
₃ (0 ≤ x ≤ 1) Temperature T is 250 ℃ ≤ T ≤ 300 ℃
And heat treatment is performed in the range of LiNb _x Ta _1-x O ₃ (0 ≦
Les characterized by forming the high refractive index layer x ≦ 1) substrate
Manufacturing method .