JP3158203B2 - Manufacturing method of ridge type three-dimensional waveguide - Google Patents

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 method for manufacturing a ridge-type three-dimensional waveguide, in which the side surface of the ridge has less roughness.
The present invention relates to a method of manufacturing an optical waveguide having low propagation loss and good light confinement.

[0002]

2. Description of the Related Art Conventionally, a ridge-type ternary waveguide has been manufactured by Nishihara, Haruna, and Suhara.
As shown in 5), a mask of Ti or the like is deposited only on the portion desired to be a ridge-type waveguide on the two-dimensional waveguide by photolithography, vapor deposition, and lift-off, and then a physical layer is formed by plasma of Ar or the like. Was manufactured by performing dry etching. The material under Ti is not etched until the Ti has completely disappeared, resulting in a ridge-type three-dimensional waveguide.

[0003] Another three-dimensional method by physical etching is to use Ni instead of Ti as a mask, CCl ₂ F ₂ , Ar, and O ₂ as an etching gas to add a chemical etching effect to physical etching to obtain an optical material. Reactive ion etching to reduce the damage to crystallinity of the crystal and increase the etching ratio between the mask and the optical material (etching speed of the optical material / etching speed of the mask) is disclosed in Appl. PhysLett. Vol. 38 No11
(1981). An attempt to further increase the etching ratio by irradiating the sample with a reactive gas beam has been attempted in Journal of Lig.
htwave tech. Vol. LT-2, No4
(1984).

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

[0005]

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

When fabricating a ridge type three-dimensional waveguide by a physical dry etching method, it is necessary to increase the thickness of the mask material because the etching ratio between the etching mask and the optical material is small. In general, a lift-off method is used for forming an etching mask. When a thick etching mask is formed 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. It will be rough. The roughness of the side wall of the ridge causes scattering loss and increases propagation loss.

[0008] In addition, reactive ion etching and reactive ion beam etching are performed. However, there are problems that the apparatus is very expensive, that it is difficult to increase the area, and that 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 occurs to form a fine shape. It is not practical because there is a problem that it cannot be done.

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

[0011]

Means for Solving the Problems In order to achieve the object, the inventors of the present invention have conducted intensive studies and as a result, have taken advantage of the fact that the etching rate ratio between the polarized surface, the + surface and the-surface of the dielectric material is greatly different, and It has been found that a ridge-type three-dimensional waveguide can be formed without using a mask. SUMMARY OF THE INVENTION The present invention provides a method for etching a two-dimensional waveguide comprising a single-polarized dielectric material.
A method of manufacturing a ridge-type three-dimensional waveguide, characterized by forming a three-dimensional waveguide by simultaneously etching a boundary where different polarization directions are in contact with each other and a part in both polarization directions without using a mask. After inverting the polarization direction of at least a portion of the converted dielectric material, a ridge structure is formed by simultaneously etching the boundary where the different polarization directions are in contact and both polarization direction portions without using an etching mask. And a method of manufacturing a ridge-type three-dimensional waveguide characterized by forming an optical waveguide. In the waveguide having the ridge portion, it is preferable 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, a method for producing a three-dimensional waveguide
This will be described with reference to FIG. 1 taking iNbO ₃ as an example. The material targeted by the present invention is not limited to lithium niobate, but lithium tantalate, quartz, strontium titanate, lithium gallate, (Pb, La) (Z
Any dielectric material such as (r, Ti) O ₃ can be used.

First, the LiNbO ₃ two-dimensional waveguide on the + C plane is turned into the −C plane after processing, except for the portion that becomes the ridge type three-dimensional waveguide. A method for inverting the + C plane of LiNbO ₃ to a −C plane is to deposit a substance capable of forming polarization inversion on a LiNbO ₃ two-dimensional waveguide by a vacuum evaporation method, a sputtering method, or the like,
When a heat treatment is performed at a predetermined temperature, for a predetermined time, the area immediately below the substance or the area immediately below the part where the substance is not deposited is inverted to the -C plane.

Here, as a method of forming a substance capable of forming domain inversion at a desired portion, a deposition technique such as photolithography (FIG. 1, steps a to c), a vacuum deposition method or a sputtering method (FIG. 1, step d; Here, Ti) and a lift-off method (FIG. 1, step f) can be used. Since a mask or a deposited film to be formed is very thin, fine processing can be performed.

Examples of the substance capable of forming domain inversion include Ti,
There are Cr, Fe, SiO ₂ , Ta, NiO, MgO and the like, and among them, Ti, SiO ₂ and 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 the internal diffusion of Ti and the external diffusion of Li into SiO ₂
The Curie point of iNbO ₃ is lowered, and the diffusion portion causes polarization inversion.

On the other hand, in the case of MgO, the portion where MgO is not deposited changes to LiNbO ₃ on the -C plane, contrary to the case of Ti and SiO ₂ . This is due to the diffusion of MgO
The Curie point of iNbO ₃ increases, and as a result, the non-diffusion portion having a relatively low Curie point causes polarization inversion.

Further, when MgO is used as the polarization inversion forming material, it diffuses into the LiNbO ₃ substrate by heat treatment. MgO is convenient when used for an optical device in order to improve the light damage resistance of LiNbO ₃ .

Since the thickness of the domain-inverted layer corresponds to the maximum step of the ridge type three-dimensional waveguide, considering the light confinement,
At least half of the thickness of the two-dimensional waveguide needs to be reversed.

The etching rates of the LiNbO ₃ + C face and the −C face with respect to the etchant greatly differ from each other, and there is a relation of + C face ≪−C face. By utilizing the difference in the etching rate between the + C plane and the −C plane, a + N plane can be formed in the LiNbO ₃ two-dimensional waveguide.
When the C-plane and the -C-plane are present, the two-dimensional waveguide is etched to form a ridge-type three-dimensional waveguide in which the + C-plane portion becomes a ridge (FIG. 1, step g).

In this method, since the processing from the two-dimensional waveguide to the ridge-type three-dimensional waveguide is performed by wet etching, a ridge-type three-dimensional waveguide having extremely small roughness on the side walls of the ridge portion is obtained. At this time, the etching thickness of the −C plane may be all or part of the formed −C plane.

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

[0022]

According to the present invention, a three-dimensional waveguide can be manufactured by using anisotropic chemical etching having a high selectivity, so that the side wall of the ridge portion, which has been a problem in the prior art, is eliminated and the propagation loss is reduced. Fine processing is possible, which is economically advantageous.
The ridge-type three-dimensional waveguide has low propagation loss, good light confinement, and can be widely used in general optical devices.

[0023]

【Example】

Example 1 (1) A positive resist was applied to a thickness of 1 μm on a 2-inch LiNbO ₃ two-dimensional waveguide on the + C plane using 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 which can be formed by polarization inversion is
蒸 着 Evaporated by sputtering. (4) Subsequently, the resist was dissolved and removed by immersion 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 air 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 above (5) was chemically etched in a boiling liquid of HF: HNO ₃ = 1: 2 for 5 minutes. As a result, the portion of the C-plane was etched more and the LiNbO ₃ ridge type having a width of 5 μm and a ridge height of 1 μm was obtained. It became a three-dimensional waveguide. (7) The roughness of the side wall of the ridge of the LiNbO ₃ three-dimensional waveguide fabricated in (6) was measured using a non-contact type surface roughness meter and a scanning electron microscope.
It was 04 μm. (8) 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 manufactured LiNbO ₃ three-dimensional waveguide was measured by a rocking curve method and found to be 20 seconds.

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

Example 3 (1) An MgO film having a thickness of 150 ° and a width of 4 μm was formed on a 1-inch LiNbO ₃ two-dimensional waveguide by photolithography and RF sputtering. (2) The above (1) was heat-treated at 1000 ° C. for 20 minutes. (3) Further heat treatment was performed at 1100 ° C. for 10 minutes. (4) After the heat treatment, the film is chemically etched in a boiling solution of HF: HNO ₃ = 1: 2 for 5 minutes, and the width is 5 μm and the ridge height is 1 μm.
Was obtained. (5) The state of the LiNbO ₃ three-dimensional waveguide fabricated in the 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 ° 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) Further heat treatment was 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, the substrate is chemically etched in a boiling liquid of HF: HNO ₃ = 1: 2 for 3 minutes, and has a width of 5 μm and a ridge height of 0.5.
A ridge shape of μm was obtained. (4) The samples prepared in the steps (1) to (3) were 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 fabricated in the steps (1) to (4) was evaluated in the same manner as in Example 1,
The maximum surface roughness of the ridge side wall was 0.05 μm, and the propagation loss was 1.9 dB / cm.

Example 5 (1) A positive resist was applied to a thickness of 1 μm on a 2-inch LiNbO ₃ single crystal using a spinner. (2) The resist pattern was exposed by a photolithography technique using a mask having a black portion of 5 μm. (3) Next, 250 nm of Ti capable of forming domain inversion is formed on the entire surface of the substrate.
蒸 着 Evaporated by sputtering. (4) Subsequently, the resist was dissolved and removed by immersion in an organic solvent. At this time, the 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 at the upper portion where the waveguide was to be finally formed. (5) By heat-treating the above (4) in the air 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 samples prepared in the steps (1) to (5) were heat-treated in pyrophosphoric acid at 250 ° C. for 60 minutes to form 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 portion was etched more and the width was 5 μm. And a LiNbO ₃ ridge type three-dimensional waveguide having a ridge height of 1 μm. (8) When the state of the LiNbO ₃ three-dimensional waveguide fabricated in (7) above was evaluated in the same manner as in Example 1, the maximum surface roughness of the ridge side wall was 0.04 μm, and the propagation loss was:
It was 1.5 dB / cm. (9) The crystallinity of the manufactured LiNbO ₃ three-dimensional waveguide was measured by the rocking curve method and found to be 23 seconds.

Comparative Example 1 (1) A 1-inch LiNbO ₃ substrate was processed two-dimensionally. (2) LiNbO _{3 A} resist is applied on a two-dimensional waveguide, and the resist is applied to the resist by photolithography to a thickness of 10 μm.
Window opened. (3) Ti was deposited on the entire surface of the substrate by sputtering at 1 μm. Subsequently, the substrate was immersed in a solvent, and the mask on the resist was removed (lift-off). (4) dry etching the entire surface of the substrate after lift-off,
A ridge type three-dimensional waveguide was used. (5) The roughness of the side wall of the ridge of the LiNbO ₃ three-dimensional waveguide fabricated in (4) was measured using a non-contact type surface roughness meter and a scanning electron microscope.
It was 5 μm. (6) 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 1-inch LiNbO ₃ substrate was processed two-dimensionally. (2) LiNbO _{3 A} two-dimensional waveguide is coated with a hydrofluoric acid resist, and the resist is applied to the resist by photolithography at 5 μm.
m pattern 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 width of the waveguide 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 side wall of the ridge portion was 0.9 μm, and the propagation loss was 6 dB / cm.

[Brief description of the drawings]

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

[Explanation of symbols]

 1 Positive resist 2 Two-dimensional waveguide 3 Substrate 4 Mask 5 Polarizable substance 6 Polarized area

Claims (2)

(57) [Claims] 1. A method according to claim 1, wherein after inverting the polarization direction of at least a part of the two-dimensional waveguide made of a single-polarized dielectric material , the boundary between the different polarization directions and the two polarization directions are used without using an etching mask. A method for manufacturing a ridge-type three-dimensional waveguide, wherein a three-dimensional waveguide is formed by simultaneously etching a part and a part. 2. An etching mask after reversing the polarization direction of at least a part of a single-polarized dielectric material.
Forming a ridge structure by simultaneously etching a boundary where different polarization directions are in contact with each other and a portion in both polarization directions without using an optical waveguide, and thereafter forming an optical waveguide. . [0001]