JPH05215928A - Production of ridge type three-dimensional waveguide - Google Patents

Abstract

PURPOSE:To obviate roughening of the side walls of a three-dimensional waveguide and to decrease propagation losses by inverting the polarization direction in a part on the two-dimensional waveguide consisting of a dielectric material subjected to single polarization and forming the three-dimensional waveguide by etching. CONSTITUTION:After the two-dimensional waveguide of the +C face of the dielectric material subjected to the single polarization is worked, this face is inverted to the -C face exclusive of the part where the ridge type three- dimensional waveguide is to be formed. Namely, a material 5 which can be inverted in polarization is deposited by evaporation on the two-dimensional waveguide 2. The part right under this material or the part right under the part where this material is not deposited by evaporation is inverted to the -C phase when the material is heat treated for a prescribed period of time at a prescribed temp. The etching rates of the +C face and the -C face for an etching liquid vary greatly (+C phase <<-C phase) and, therefore, the two-dimensional waveguide 2 is etched by utilizing the difference in the etching rate, by which the ridge type three-dimensional waveguide having the ridge part in the part of the +C phase is obtd.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a ridge-type three-dimensional waveguide, and more particularly to a ridge-shaped side surface with less roughness.
The present invention relates to a method for manufacturing an optical waveguide having a low propagation loss and a good light confining property.

[0002]

2. Description of the Related Art Hitherto, a ridge type ternary waveguide has been disclosed by Nishihara, Haruna, and Suhara in an optical integrated circuit P195 Ohmsha (198).
As shown in 5), a mask of Ti or the like is vapor-deposited on the two-dimensional waveguide at a portion to be formed into a ridge-type waveguide by photolithography technique, vapor deposition, and lift-off. It was produced by dry etching. Since the material under Ti is not etched until Ti is completely disappeared, it becomes a ridge type three-dimensional waveguide.

As another three-dimensional method by physical etching, Ni is used as a mask instead of Ti, and CCl ₂ F ₂ , Ar, and O ₂ are used as an etching gas, and a chemical etching action is added to the physical etching to add an optical material. Reactive ion etching for reducing damage to crystallinity and increasing the etching ratio between the mask and the optical material (optical material etching rate / mask etching rate) is described in Appl. PhysLett. Vol. 38 No11
(1981). An attempt to further increase the etching ratio by irradiating the sample with a reactive gas as a beam is a Journal of Lig.
www.tech.com. Vol. LT-2, No4
(1984).

On the other hand, a method of forming a three-dimensional waveguide by chemical etching using hydrofluoric acid, nitric acid / hydrofluoric acid mixed solution, or the like has been attempted.

[0005]

However, when the three-dimensional waveguide is manufactured by the conventional physical or chemical etching method, there are the following problems.

When a ridge type three-dimensional waveguide is manufactured by a physical dry etching method, the film thickness of the mask material needs to be large because the etching ratio of the etching mask and the optical material is small. The lift-off method is generally adopted for forming the etching mask. If a thick etching mask is created by the lift-off method, the side wall becomes a very rough surface.

In the physical dry etching method, since the etching mechanism is only physical etching, the shape of the side wall of the mask is directly transferred to the ridge portion of the waveguide, so that the side wall of the ridge-type three-dimensional waveguide is very large. It will be rough. Roughness of the side wall of the ridge causes scattering loss, resulting in large propagation loss.

In addition, reactive ion etching and reactive ion beam etching are performed, but there are problems that the apparatus is very expensive, it is difficult to increase the area, and a large etching ratio cannot be obtained. ..

On the other hand, in the method of forming a three-dimensional waveguide by chemical etching, there is no suitable etching mask, and since it is isotropic etching, side etching may occur to form a fine shape. There is a problem that it can not be done and it is not practical.

An object of the present invention is to solve the above-mentioned problems and to propose a method for producing a ridge-type three-dimensional waveguide with low propagation loss by eliminating the roughness of the side wall of the ridge-type three-dimensional waveguide.

[0011]

As a result of intensive studies by the present inventors in order to achieve the object, as a result of utilizing the fact that the etching rate ratios of the polarized surface, + surface and − surface of the dielectric material are greatly different, It has been found that a ridge type three-dimensional waveguide may be created without using a mask. The present invention comprises a step of inverting the polarization direction of at least a part of a two-dimensional waveguide made of a single-polarized dielectric material, and a step of forming a three-dimensional waveguide by etching. A method of manufacturing a three-dimensional waveguide, and a step of inverting the polarization direction of at least a part of the single-polarized dielectric material, a step of forming a ridge structure by etching, and a step of forming an optical waveguide. Ridge type three-dimensional waveguide manufacturing method for
A ridge-type three-dimensional waveguide formed of a waveguide material having a ridge portion, which is made of a single-polarized dielectric material, and the surface of the ridge portion is a positive polarization surface. It is characterized by a ridge-type three-dimensional waveguide. In the waveguide having the ridge portion, it is desirable that the surface of the ridge portion has a positive polarization surface and the non-ridge portion has a negative polarization surface.

[0012]

According to the present invention, the method for producing a three-dimensional waveguide is
iNbO ₃ will be described as an example with reference to FIG. 1. The material targeted by the present invention is not limited to lithium niobate, but may be lithium tantalate, quartz, strontium titanate, lithium gallium oxide, (Pb, La) (Z).
Any dielectric such as r, Ti) O ₃ can be used.

First, the + C plane LiNbO ₃ two-dimensional waveguide is inverted to the −C plane except for the portion which becomes a ridge type three-dimensional waveguide after processing. The method of inverting the + C plane of LiNbO ₃ to the −C plane is to deposit a material capable of forming polarization inversion on the LiNbO ₃ two-dimensional waveguide by vacuum vapor deposition or sputtering.
When heat treatment is performed for a predetermined temperature and time, the portion directly below the substance or immediately below the portion where the substance is not vapor-deposited is inverted to the -C plane.

Here, as a method for forming a material capable of forming polarization inversion on a desired portion, photolithography (FIG. 1, steps a to c), vapor deposition techniques such as vacuum vapor deposition and sputtering (FIG. 1, step d; Here, Ti can be used and the lift-off method (FIG. 1, step f) can be used. Since the mask and vapor deposition film that are created are very thin, fine processing is possible.

Examples of substances capable of forming polarization inversion include Ti,
There are Cr, Fe, SiO ₂ , Ta, NiO, MgO, etc. Among them, Ti, SiO ₂ , MgO are preferable. +
If depositing Ti and SiO ₂ in LiNbO ₃ dimensional waveguide on the C plane, directly below the Ti and SiO ₂ is changed to LiNbO ₃ dimensional waveguide -C surface by heat treatment. this is,
L due to internal diffusion of Ti and external diffusion of Li into SiO _2.
The Curie point of iNbO ₃ is lowered, and the diffusion portion causes polarization reversal.

On the other hand, in the case of MgO, contrary to the case of Ti and SiO ₂ , the portion where MgO is not vapor-deposited is changed to LiNbO ₃ on the −C plane. This is due to the diffusion of MgO
The Curie point of iNbO ₃ rises, and as a result, the non-diffused portion having a relatively lower Curie point causes polarization reversal.

Further, when MgO is used as the polarization inversion forming substance, it is diffused in the LiNbO ₃ substrate by heat treatment. MgO improves the light damage resistance of LiNbO ₃ and is therefore suitable for use in optical devices.

Since the thickness of the polarization inversion layer corresponds to the maximum step of the ridge type three-dimensional waveguide, considering the light confining property,
At least half the thickness of the two-dimensional waveguide needs to be polarization-inverted.

The etching rates of the LiNbO _{3 on} the + C plane and the −C plane differ greatly with respect to the corrosive liquid, and there is a relationship of + C plane <<-C plane. If the difference in the etching rate between the + C plane and the −C plane is used, the inside of the LiNbO ₃ two-dimensional waveguide is +
When the C-plane and the -C plane exist, the two-dimensional waveguide is etched to form a ridge-type three-dimensional waveguide in which the + C plane is a ridge portion (FIG. 1, step g).

In this method, since the processing from the two-dimensional waveguide to the ridge-type three-dimensional waveguide is carried out by wet etching, the ridge-type three-dimensional waveguide has very little roughness on the side wall of the ridge portion. At this time, the etching thickness of the -C plane may be the whole or a part of the formed -C plane.

According to the present invention, the optical waveguide can be formed before the polarization inversion as described above, but the waveguide can be formed during the polarization inversion operation or after the ridge shape is formed. it can.

[0022]

EFFECTS OF THE INVENTION Since a three-dimensional waveguide can be manufactured by utilizing anisotropic chemical etching having a large selectivity, the roughness of the side wall of the ridge, which has been a problem in the past, is eliminated and the propagation loss is reduced. Fine processing is possible, which is economically advantageous.
This ridge-type three-dimensional waveguide has low propagation loss and good light confinement, and can be widely used in optical devices in general.

[0023]

【Example】

Example 1 (1) A positive resist of 1 μm was applied by a spinner on a 2-inch LiNbO ₃ two-dimensional waveguide on the + C plane. (2) The resist pattern was exposed by a photolithography technique using a mask having a black portion of 5 μm. (3) Next, Ti that can form polarization inversion is formed on the entire surface of the substrate by 150
Å It was deposited by sputtering. (4) Subsequently, the resist was dissolved and removed by immersing it in an organic solvent. At this time, Ti on the resist was also removed at the same time and the mask pattern was transferred. (5) By heat-treating the above (4) in the atmosphere at 1015 ° C. for 15 minutes, the portion immediately below the Ti film was inverted to the −C plane. The inversion depth at this time was about 1.2 μm. (6) above the _{(5) HF: HNO 3 =} 1: 2 boiling liquid more etched width 5μm five minutes chemical etched at -C face portion C. in, LiNbO ₃ ridge type ridge height 1μm It became a three-dimensional waveguide. (7) When the roughness of the side wall of the ridge portion of the LiNbO ₃ three-dimensional waveguide prepared in (6) above was measured using a non-contact surface roughness meter and a scanning electron microscope, the maximum surface roughness was found to be 0.
It was 04 μm. (8) Further, when the propagation loss of the three-dimensional waveguide formed by using the cutback method and the prism moving method was measured, the propagation loss was 1.5 dB / cm. (9) The crystallinity of the prepared LiNbO ₃ three-dimensional waveguide was 20 sec as measured by the rocking curve method.

Example 2 (1) A film thickness of 0.1 μm was formed on a 3-inch LiNbO ₃ two-dimensional waveguide by photolithography and RF sputtering.
A SiO ₂ film having a window with a width of 5 μm was formed. (2) The above (1) was heat-treated at 1020 ° C. for 10 minutes.
By this treatment, a domain-inverted phase having a depth of 0.6μ was formed. (3) After the heat treatment, the SiO ₂ film was dissolved and removed by chemical etching in a boiling solution of HF. (4) Next, chemical etching was performed for 3 minutes in a boiling liquid of HF: HNO ₃ = 1: 2, width 5 μm, ridge height 0.5 μ.
A three-dimensional waveguide of m was obtained. (5) When the state of the LiNbO ₃ three-dimensional waveguide produced in steps (1) to (4) was evaluated in the same manner as in Example 1,
The maximum surface roughness of the side wall of the ridge portion was 0.03 μm, and the propagation loss was 1.3 dB / cm.

[0025] Example 3 (1) 1 inch LiNbO ₃ film thickness by the two-dimensional waveguide on photolithography and RF sputtering 150 Å, to form an MgO film of width 4 [mu] m. (2) The above (1) was heat-treated at 1000 ° C. for 20 minutes. (3) Heat treatment was further performed at 1100 ° C. for 10 minutes. (4) After the heat treatment, chemical etching is performed for 5 minutes in a boiling liquid of HF: HNO ₃ = 1: 2, the width is 5 μm, and the ridge height is 1 μm.
A three-dimensional waveguide of (5) When the state of the LiNbO ₃ three-dimensional waveguide produced in steps (1) to (4) was evaluated in the same manner as in Example 1,
The maximum surface roughness of the ridge side wall was 0.06 μm, and the propagation loss was 1.7 dB / cm.

Example 4 (1) A film thickness of 200 Å and a width of 4.5 were formed on a 3-inch LiTaO ₃ single crystal by photolithography and RF sputtering.
A μm MgO film was formed. (2) The above (1) was heat-treated at 1000 ° C. for 30 minutes. (3) Heat treatment was further performed at 1100 ° C. for 10 minutes. By this treatment, a domain-inverted phase having a depth of 1.3 μm was formed. (3) After the heat treatment, chemical etching was performed for 3 minutes in a boiling liquid of HF: HNO ₃ = 1: 2, the width was 5 μm, and the ridge height was 0.5.
A ridge shape of μm was obtained. (4) The sample prepared in steps (1) to (3) was heat-treated in pyrophosphoric acid at 250 ° C. for 60 minutes to prepare a proton exchange waveguide. (5) When the state of the LiTaO ₃ three-dimensional waveguide produced in steps (1) to (4) was evaluated in the same manner as in Example 1,
The maximum surface roughness of the side wall of the ridge portion was 0.05 μm, and the propagation loss was 1.9 dB / cm.

Example 5 (1) A positive resist of 1 μm was coated on a 2-inch LiNbO ₃ single crystal by a spinner. (2) The resist pattern was exposed by a photolithography technique using a mask having a black portion of 5 μm. (3) Next, Ti that can form polarization inversion is formed on the entire surface of the substrate by 250.
Å It was deposited by sputtering. (4) Subsequently, the resist was dissolved and removed by immersing it in an organic solvent. At this time, Ti on the resist was also removed at the same time, and a mask pattern was formed in which the Ti film was not formed only on the upper portion where the waveguide should be finally formed. (5) By heat-treating the above (4) in the atmosphere at 1015 ° C. for 15 minutes, the portion immediately below the Ti film was inverted to the −C plane. The inversion depth at this time was about 1.2 μm. (6) The sample prepared in the steps (1) to (5) was heat-treated in pyrophosphoric acid at 250 ° C. for 60 minutes to prepare a proton exchange waveguide using the remaining Ti film as a resist. 350 ° C
Stabilized by heat treatment for 40 minutes. (7) When the above (6) was chemically etched in a boiling solution of HF: HNO ₃ = 1: 2 for 5 minutes, the Ti film was first dissolved and removed, and then the −C plane was more etched and the width was 5 μm. , The ridge height was 1 μm, and it became a LiNbO ₃ ridge type three-dimensional waveguide. (8) When the state of the LiNbO ₃ three-dimensional waveguide prepared in (7) above was evaluated in the same manner as in Example 1, the maximum surface roughness of the ridge sidewall was 0.04 μm, and the propagation loss was
It was 1.5 dB / cm. (9) The crystallinity of the produced LiNbO ₃ three-dimensional waveguide was 23 sec as measured by the rocking curve method.

Comparative Example 1 (1) A 1-inch LiNbO ₃ substrate was two-dimensionally processed. (2) LiNbO _{3 A} two-dimensional waveguide is coated with a resist, and the photolithography technique is applied to the resist to a thickness of 10 μm.
I opened the window. (3) Ti was vapor-deposited on the entire surface of the substrate by 1 μm sputtering. Then, the substrate was immersed in a solvent to remove (lift off) the mask on the resist. (4) Dry etching the entire surface of the substrate after lift-off,
A ridge-type three-dimensional waveguide is used. (5) When the roughness of the side wall of the ridge portion of the LiNbO ₃ three-dimensional waveguide prepared in (4) above was measured using a non-contact surface roughness meter and a scanning electron microscope, the maximum surface roughness was found to be 0.
It was 5 μm. (6) Further, when the propagation loss of the three-dimensional waveguide formed by using the cutback method and the prism moving method was measured, the propagation loss was 7.0 dB / cm.

Comparative Example 2 (1) A one-inch LiNbO ₃ substrate was two-dimensionally processed. (2) LiNbO _{3 A} two-dimensional waveguide was coated with a hydrofluoric acid resistant resist, and 5 μm was applied to the resist by photolithography.
The pattern of m was produced. (3) The entire substrate was chemically etched in a boiling liquid of HF: HNO ₃ = 1: 2 for 7 minutes to obtain a three-dimensional waveguide having a width of 4.5 μm and a ridge height of 1.2 μm. (4) When the resist was removed and the characteristics of the three-dimensional waveguide were examined, the waveguide width was narrower than that of the mask due to side etching. When the state of the three-dimensional waveguide was evaluated in the same manner as in Comparative Example 1, the maximum surface roughness of the sidewall of the ridge portion was 0.9 μm and the propagation loss was 6 dB / cm.

[Brief description of drawings]

FIG. 1 is a method for manufacturing a ridge-type three-dimensional waveguide according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive resist 2 Two-dimensional waveguide 3 Substrate 4 Mask 5 Material capable of reversing polarization 6 Reversal region

Claims (2)

[Claims] 1. A ridge comprising the steps of reversing the polarization direction of at least a portion of a two-dimensional waveguide made of a single-polarized dielectric material and forming a three-dimensional waveguide by etching. Type three-dimensional waveguide manufacturing method. 2. A ridge-type tertiary structure comprising the steps of inverting the polarization direction of at least a part of a single-polarized dielectric material, forming a ridge structure by etching, and forming an optical waveguide. Original waveguide manufacturing method. [0001]