JPH085854A - Forming method of ridge - Google Patents

Abstract

PURPOSE:To form a ridge structure at a practically enough forming rate on a substrate or thin film comprising at least one of lithium niobate, lithium tantalate and sapphire. CONSTITUTION:A noble metal layer is formed and patterned on an optical crystal substrate or an optical crystal thin film comprising at least one of lithium niobate, lithium tantalate and sapphire. The substrate or the thin film is brought into contact with a fused liquid of at least one compd. selected from metal oxides or metal halides so that the substrate or the thin film where the noble metal layer is not formed is etched to form a ridge structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide which is a basic element for optical communication, and more particularly to a method for manufacturing a ridge type optical waveguide.

[0002]

2. Description of the Related Art Optical crystals are widely used in fields such as optical fiber communication capable of transmitting a large amount of information. Especially, when they are selectively thinned, a ridge-type waveguide (research technique, OQE92- 139) and a ridge type optical modulator (JP-A-H
No. 4-288518), an optical switch (JP-A-H4-288)
531), an optical integrated circuit (Japanese Patent Laid-Open No. H4-268765).
Such a ridge processing technique for an optical crystal has been desired because it can be used as an optical device.

Techniques for forming a ridge structure on such a substrate are as follows: 1) RIE after performing a step of forming a mask pattern on the substrate using a metal such as Ta, Al, Cr, or Ti or a photoresist. Method, a method of obtaining a ridge structure by dry etching by RIBE method, ion beam etching method, etc., 2) Wet etching with an alkaline solution or an acid solution after patterning a photoresist or SiO ₂ on the substrate by a photo process After that, photoresist and SiO
There is a method of peeling ₂ to obtain a ridge structure.

[0004]

However, these methods have the following problems. In the example of 1) above, the substrate is damaged by the dry etching process. Since the etching rate of dry etching is slow, it takes time to obtain a deep groove. Furthermore, dry etching is a powerful etching, and when there is no suitable mask material, the mask material is etched at the same time as the etching of lithium niobate or lithium tantalate or a mixture thereof or sapphire. The thickness of the mask material is 10 μm to 50 μm, and there are problems that it is difficult to manufacture the mask film, it is difficult to perform accurate patterning, and it is uneconomical because many materials are used. In the case of 2), there is a problem that lithium niobate or lithium tantalate or a mixture thereof or sapphire does not have a suitable acid or alkali solution that can be etched at a rate sufficient for practical use. The object of the present application is at least one selected from lithium niobate, lithium tantalate or sapphire at a speed sufficient for practical use.
Forming a ridge structure on the seed substrate or thin film.

[0005]

Means for Solving the Problems As a result of intensive studies to solve these problems, the inventors of the present invention have found that even if a precious metal is brought into contact with a metal oxide such as V ₂ O ₅ or a metal halide melt. , Noble metal is not attacked by melt, lithium niobate,
It was found that at least one substrate or thin film selected from lithium tantalate or sapphire is sufficiently dissolved in these melts, and by utilizing this,
A mask layer made of a noble metal is provided on the substrate, and by immersing this in a melt, only the portion where the mask layer is not formed is selectively etched, and the ridge structure is formed on the optical crystal substrate surface or the optical crystal thin film on the substrate. Has been realized.

[0006]

According to the present invention, an optical crystal substrate made of at least one selected from lithium niobate, lithium tantalate, and sapphire on which a patterned noble metal layer is formed, or an optical crystal thin film thereof is formed of a metal oxide or a metal halogen. Optical crystal substrate characterized by forming a ridge portion by etching a portion of the optical crystal substrate on which the noble metal layer is not formed by bringing the optical crystal substrate into contact with a melt composed of at least one compound selected from the following: This is a method of forming a ridge type on the upper thin film. The noble metal mask formed on the substrate is not etched by the metal oxide or metal halide melt, and the noble metal does not oxidize under high temperature oxidation conditions and does not react with the substrate, so the function as a mask layer is severe. It is maintained under various conditions and does not contaminate the melt. An optical crystal substrate made of lithium niobate, lithium tantalate, a mixture thereof, or sapphire or a thin film thereof is hardly etched by a general acid or alkaline aqueous solution, but a metal oxide containing V ₂ O ₅ or a metal halogen. The compound melt etches very quickly. Therefore, a portion covered with the noble metal film is not etched, and a portion not covered with the noble metal film is promptly etched, so that the ridge structure can be quickly formed on the optical crystal substrate or the thin films thereof.

The present invention will be described in detail below. The substrate used in the present invention needs to be lithium niobate, lithium tantalate or a mixture thereof, or a sapphire substrate or a thin film thereof. Lithium niobate, lithium tantalate, or a mixture thereof has a large electro-optic constant and is very promising as a light control element. Sapphire is extremely hard, stable, and has a small dielectric constant, and therefore is very important as a substrate for forming a thin film for a light control element. Next, a noble metal mask portion is formed on this substrate. As a method of forming the mask portion, a method using a photo process is accurate and simple. There are an etching method and a lift-off method as a method of forming a mask portion using a photo process, but the lift-off method is advantageous because it can be applied to any material.

In the lift-off method, a substrate is coated with a photoresist, then exposed to ultraviolet rays and then developed.
A resist pattern is formed. After that, a thin film made of a noble metal is formed by a vacuum deposition method, a sputtering method, or the like.
After that, the photoresist is dissolved by an organic solvent to remove the noble metal on the resist and form a noble metal pattern on the substrate.

The noble metal includes Ru, Rh, Pd,
Ag, Os, Ir, Pt, Au, etc. are desirable. These noble metals are suitable as a mask material because they do not react with the optical crystal in the melt and do not oxidize under high temperature oxidation conditions. The thickness of the noble metal layer is preferably 0.01 μm to 10 μm. The reason is that the precious metal layer is 0.01
If the thickness is less than μm, it will diffuse into the substrate at the melt deposition temperature, or the precious metal will sinter to form a uniform film, or it will dissolve in the melt to a small extent but the effect of the present invention will not be obtained. On the other hand, if the thickness is more than 10 μm, the difference in thermal expansion between the noble metal and the substrate at the melt deposition temperature is too different, and the noble metal layer is peeled off from the substrate during melt deposition.

Examples of the metal oxide or metal halide melt include V ₂ O ₅ , B ₂ O ₃ , PbO and Bi ₂ O.
Oxides such as ₃ , Na ₂ O, Li ₂ O, K ₂ O, Rb ₂ O
Alkali metal oxides such as LiF, NaF, KF, Rb
Alkali metal halides such as F, LiCl, NaCl, KCl, and RbCl, or compounds containing these elements and having a melting point of 1000 ° C. or lower, such as LiVO ₃ , are preferable. The reason why the melting point is 1000 ° C. or lower is that if the melting point is higher than 1000 ° C., the noble metal layer may be melted, and for example, the melting point of lithium niobate crystal is 12
This is because the temperature is 53 ° C. and the melting point of the lithium niobate crystal and the melting point of the melt are close to each other, so that it is difficult to control etching. If the melting point is 1000 ° C or lower, the temperature is low and it is easy to use. The contact time and temperature are appropriately set depending on the type and depth of the substrate or film to be etched, but especially 500
It is desirable to be ~ 1200 ° C and 3 minutes to 20 hours.
This is because if the temperature is lower than 500 ° C., the etching rate is slow, and if the temperature is higher than 1200 ° C., the lithium niobate substrate or the thin films thereof are softened. Further, if the growing time is shorter than 3 minutes, it is difficult to control the film thickness, and if it is longer than 20 hours, industrial productivity is lowered. In the present invention, any location on the substrate can be etched, but if necessary, the film surface is polished or the mask material is etched to be used as a ridge-type waveguide.

The ridge type optical waveguide is selectively formed by etching the portion of the substrate on which the noble metal mask layer is not formed by the above method.

[0012]

【Example】

(Example 1) An example in which a ridge type waveguide is formed by subjecting a lithium niobate substrate, which is one of the present invention, to a ridge process will be shown below. A commercially available Z-cut lithium niobate substrate (A in FIG. 1) was used as the substrate. A Ti film of 600 A was formed on this substrate by sputtering (B in FIG. 1) and then heat-treated at 1020 ° C. for 10 hours in a humidified atmosphere to diffuse Ti into the substrate (2 in FIG. 1). A commercially available positive photoresist (OFPR-800 manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied as a photoresist to the Ti-diffused substrate by spin coating (3 in FIG. 1). The film thickness at this time was 1 μm. Thereafter, a linear pattern having a width of 10 μm was exposed and developed to transfer the resist (4 in FIG. 1). Next, a platinum film having a thickness of 0.2 μm was formed by sputtering and then lifted off to form a platinum pattern on the substrate (5 in FIG. 1).
Next, the substrate on which the platinum pattern is formed is brought into contact with the melt to etch the Ti diffused portion by about 5 μm, and then platinum is removed by etching with aqua regia to obtain a ridge type channel waveguide on the lithium niobate single crystal. (6 in FIG. 1). The melt is LiVO3 and the etching temperature is 800 ° C.
For 10 minutes. Next, when the end face of this element was polished and the light of a semiconductor laser with a wavelength of 1.55 μm was coupled in the so-called TM mode by a microscope lens, it was confirmed that the light was emitted from the end face on the opposite side. It was found that this guided light. When the propagation loss of this device was measured by the cutback method at a wavelength of 1.55 μm, it was found to be a practical channel waveguide with a low loss of 1.5 dB / cm.

Example 2 A commercially available Z-cut sapphire substrate was used as a substrate. A commercially available positive photoresist (OFPR-800 manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied as a photoresist to this substrate by spin coating. The film thickness at this time was 1 μm. Thereafter, a linear pattern having a width of 10 μm was exposed and developed to transfer the resist. Next, a platinum film having a thickness of 0.2 μm was formed by sputtering and then lifted off to form a platinum pattern on the substrate. Next, the substrate on which the platinum pattern was formed was brought into contact with the melt. The melt is Li3AlF6 and the etching temperature is 900 ° C.
For 10 minutes. When the amount of etching was measured with a surface roughness meter, it was found to be about 5 μm. Then, platinum was removed by etching with aqua regia to obtain a ridge type on the sapphire single crystal substrate. Next, when the end face of this element was polished and the light of a semiconductor laser having a wavelength of 1.55 μm was coupled by the so-called TM mode with a microscope lens, it was confirmed that the light was emitted from the end face on the opposite side. It was found that this guided light. When the propagation loss of this device was measured by the cutback method at a wavelength of 1.55 μm, it was found to be a practical channel waveguide with a low loss of 1.5 dB / cm.

(Comparative Example) A commercially available Z-cut lithium niobate substrate was used as the substrate. A Ti film of 600 A was formed on this substrate by sputtering and then heat-treated at 1020 ° C. for 10 hours in a humidified atmosphere to diffuse Ti into the substrate. A commercially available positive photoresist (OFPR-800 manufactured by Tokyo Ohka Kogyo Co., Ltd.) was applied as a photoresist to the substrate having Ti diffused by spin coating. The film thickness at this time was 1 μm. Thereafter, a linear pattern having a width of 10 μm was exposed and developed to transfer the resist. Next, a Ti film having a thickness of 1 μm was formed on this substrate by sputtering. The Ti pattern was transferred to the lithium niobate substrate by the lift-off method. The lithium niobate substrate on which the Ti pattern was transferred was subjected to Ar / O2 = by ion beam etching.
When etching was attempted with a mixed gas of 1/1, the etching rate was extremely slow at 0.04 μm / min, and the Ti film disappeared when the lithium niobate substrate was ground to 3 μm, and further ridge processing could not be performed. It was

[0015]

INDUSTRIAL APPLICABILITY According to the manufacturing method of the present invention, a ridge structure can be formed in lithium niobate, lithium tantalate, a mixture thereof, or sapphire very easily and in a short time, when used as a waveguide, in producing an optical device. Therefore, an optical modulator, an optical switch, etc. can be manufactured extremely easily. Further, the present invention can be applied not only to the production of single crystals but also to the production of grooves such as polycrystalline grooves such as ceramics.

[Brief description of drawings]

1A to 1C of FIG. 1 are schematic views of a forming process of the present invention.

[Explanation of symbols]

 A Lithium niobate single crystal substrate B Ti thin film C Ti diffusion part D Photoresist E Platinum thin film

Claims (4)

[Claims] 1. A patterned noble metal layer is formed, and at least one optical crystal substrate or optical crystal thin film selected from lithium niobate, lithium tantalate, or sapphire is selected from metal oxides or metal halides. A method for forming a ridge type, which comprises forming a ridge type by etching the optical crystal substrate or the optical crystal thin film in a portion where the noble metal layer is not formed in contact with a melt comprising at least one of the above. 2. The noble metal is Ru, Rh, Pd, A
The ridge-type forming method according to claim 1, comprising g, Os, Ir, Pt, and Au. 3. The noble metal layer has a thickness of 0.01 μm to
The ridge type forming method according to claim 1, wherein the ridge type has a thickness of 10 μm. 4. The melt comprises at least V ₂ O ₅ , B ₂ O
₃ , PbO, Na ₂ O, Li ₂ O, K ₂ O, Rb ₂ O,
Bi ₂ O ₃ , LiF, NaF, KF, RbF, LiC
The ridge-type forming method according to claim 1, wherein the ridge-type forming material contains 1, NaCl, KCl, and RbCl.