JP3932242B2 - Method for forming optical waveguide element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming an optical waveguide element used in a long-distance, large-capacity optical fiber communication system, an optical measuring instrument, and the like, and more particularly, metal loading mounted for the purpose of stabilizing device characteristics against incident polarization fluctuations The present invention relates to a method for forming an optical waveguide device having a type waveguide polarizer.
[0002]
[Prior art]
An optical waveguide device in which an optical waveguide, a buffer layer, an electrode, and the like are integrated on a substrate having an electro-optic effect such as lithium niobate (LN) is a long-distance, large-capacity optical fiber communication system, an optical measuring instrument, Has come to be widely used.
Such an optical waveguide element is formed by forming an optical waveguide on a substrate such as LN using a Ti thermal diffusion method, and then SiO 2 ₂ A wafer is manufactured by sequentially forming a buffer layer made of the above and the like, a modulation electrode for controlling guided light, and the like. Thereafter, the wafer is cut into elements using a dicer or the like, and each element is mounted on a case, and then an optical fiber is connected.
[0003]
For the optical waveguide device manufactured in this manner, a light wave having a polarization component that vibrates only in the vertical or horizontal direction with respect to the main surface of the substrate is usually used. For this reason, in these optical waveguide elements, a polarization-maintaining fiber is connected to the incident side so that so-called linearly polarized light having the polarization component is incident.
However, when stress is applied to the polarization maintaining fiber from the outside, or the incident light wave is not linearly polarized light, there arises a problem that the extinction ratio of the light wave is deteriorated on the output side of the optical waveguide element. Therefore, it is impossible to accurately turn on / off the signal in the optical waveguide device, and the S / N ratio in signal transmission is deteriorated.
[0004]
In order to solve such problems, in Japanese Patent Laid-Open Nos. 4-282608 and 62-299913, a thin polarizer is attached to the end face of the optical waveguide, or a transition metal is provided in the vicinity of the optical waveguide. A technique for incorporating a so-called waveguide-type polarizer by thermally diffusing and the like is disclosed. Then, unnecessary polarization components are removed by this waveguide type polarizer to stabilize the device characteristics against incident polarization fluctuations.
[0005]
In Suematsu et. Al: Appl. Phys. Lett., Vol. 21, No. 6 (1972), a metal layer is loaded on the optical waveguide, and the difference in the electric field component absorption of each polarization by this metal layer is measured. A metal-loaded waveguide polarizer is described that uses it to provide a polarizing action. Specifically, polarized light that vibrates in the horizontal direction (referred to as TE mode light) with respect to the main surface of the substrate is transmitted, and polarized light that vibrates in the vertical direction (referred to as TM mode light) is absorbed. This metal-loaded waveguide polarizer is characterized in that a relatively high extinction ratio can be obtained for a simple structure.
[0006]
The metal-loaded waveguide polarizer is formed as follows. That is, after applying a photoresist on the LN substrate on which the optical waveguide is formed, a polarizer pattern is formed by photolithography. Next, a metal layer for acting as a polarizer, for example, an Al layer is formed on the polarizer pattern by vapor deposition or the like. Thereafter, an excess Al layer on the resist is removed in an organic solvent to form an Al-loaded waveguide polarizer on the optical waveguide.
[0007]
On the other hand, for the purpose of improving the speed matching between the substrate and the modulation electrode for modulating the light wave guided in the optical waveguide, SiO is used. ₂ In some cases, a buffer layer made of the above is formed. In addition, the metal-loaded waveguide polarizer needs to be formed so as to be in contact with the optical waveguide as described above. Therefore, when forming the buffer layer as described above, an opening is formed in the buffer layer to expose the main surface of the substrate, and a waveguide-type polarizer is formed in the opening, whereby the waveguide type The polarizer and the optical waveguide are in contact with each other.
The opening is made of the SiO mainly used for the buffer layer. ₂ Because of the high etching rate of CF ₄ , CHF ₃ It was formed by reactive dry etching using chlorofluorocarbon gas plasma.
[0008]
[Problems to be solved by the invention]
However, when the opening is formed by using the dry etching method as described above, the buffer layer is over-etched due to variations in the buffer layer thickness or variations in the etching rate between etching batches. Even the surface was sometimes etched.
When an Al-loaded waveguide polarizer as described above is formed in the opening formed by such over-etching, the polarization characteristics of the waveguide polarizer vary, and the optical waveguide element itself In some cases, the extinction ratio was significantly deteriorated.
[0009]
For this reason, the present inventors tried to detect an etching end point by performing a process monitor using mass spectrum analysis, emission analysis, or the like in the step of forming the opening, thereby preventing over-etching. However, even when these techniques are used, overetching of the substrate cannot be completely prevented.
[0010]
The present invention provides an optical waveguide device comprising a metal-loaded waveguide polarizer, wherein the optical waveguide device can prevent a fluctuation in polarization characteristics of the waveguide polarizer and can stably obtain a high extinction ratio. An object of the present invention is to provide a method for forming a waveguide element.
[0011]
[Means for Solving the Problems]
The method for forming an optical waveguide device of the present invention is a method for forming an optical waveguide device comprising an optical waveguide, a buffer layer, a metal-loaded waveguide polarizer, and a modulation electrode on a substrate having an electro-optic effect. There, The substrate is made of an oxide having lithium as a constituent element species, A first step of forming an optical waveguide on the substrate; a second step of forming a buffer layer on the substrate; and a first opening formed by partially removing the buffer layer by non-reactive dry etching A third step of exposing at least a portion of the main surface of the substrate on which the optical waveguide is formed, and the metal-loaded waveguide on the substrate in the first opening. And a fourth step of forming a mold polarizer.
[0012]
When the metal-loaded waveguide polarizer is formed in the opening formed by over-etching the buffer layer, the inventors of the present invention change the polarization characteristics of the waveguide polarizer. Detailed examination was performed using an LN substrate.
As a result, when a metal-loaded waveguide polarizer was formed directly on the substrate without forming a buffer layer, no corrosion was observed on the metal layer constituting the waveguide polarizer. On the other hand, when a metal-loaded waveguide polarizer was formed in the opening, it was found that the metal layer constituting the waveguide polarizer was corroded.
[0013]
Then, when the cause of such corrosion was further explored, it was found that excessive free oxygen in the main surface of the substrate exposed in the opening was the cause.
And since such generation | occurrence | production of excess free oxygen respond | corresponds with the over-etching at the time of forming an opening part, the present inventors assumed the mechanism of the following excess oxygen generation | occurrence | production. That is, by reactive dry etching mainly using chemical action using plasma of chlorofluorocarbon gas, fluorine radical as an etching species selectively reacts with lithium atoms constituting the LN substrate to form fluoride. It was assumed that oxygen is excessively present in the vicinity of the substrate surface by being discharged from the substrate.
[0014]
Based on such a mechanism, the present inventors have partially removed the buffer layer by non-reactive dry etching that performs physical etching using argon gas plasma to form an opening. When a metal-loaded waveguide polarizer was formed in this opening, no corrosion was observed on the metal layer constituting this waveguide polarizer, and the polarization characteristics of the waveguide polarizer were I found that it does not fluctuate. As a result, it was found that an optical waveguide device having a high extinction ratio can be stably obtained.
The present invention has been made as a result of such an enormous research search by the present inventors.
[0015]
According to the present invention, when the buffer layer is partially dry-etched to form the opening, even if the main surface of the substrate is etched by overetching, the surface layer portion of the main surface of the substrate is excessive. There is no free oxygen present. Therefore, even when a metal-loaded waveguide polarizer is formed in the opening, the metal layer constituting the waveguide polarizer is not oxidized and corroded.
Therefore, the polarization characteristics of the waveguide polarizer are stable, and an optical waveguide device having a high extinction ratio can be stably obtained.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments of the invention.
FIG. 1 is a perspective view showing an example of an optical waveguide device produced by the method for forming an optical waveguide device of the present invention. In FIG. 1, detailed portions are not shown in order to clarify the features of the present invention.
Hereinafter, the forming method of the present invention will be described with reference to FIG.
[0017]
An optical waveguide element 10 shown in FIG. 1 includes a substrate 1 having an electro-optic effect, an optical waveguide 2 formed on the substrate 1, a signal electrode 5 and a ground electrode 6 constituting a modulation electrode. Further, a buffer layer 3 is formed between the substrate 1 and the signal electrode 5 and the ground electrode 6. A metal-loaded waveguide polarizer 4 is formed at the left end of the substrate 1 so as to be in contact with the main surface 1A of the substrate 1. Further, a DC electrode 7 is formed at the right end of the substrate 1 so as to be in contact with the main surface 1A of the substrate 1.
The DC electrode 7 is for applying a predetermined bias voltage to the optical waveguide 2 in order to control the operating point of the optical waveguide element 10.
[0018]
The light wave is incident on the optical waveguide 2 of the optical waveguide element 10 from the direction of the arrow shown in the figure, and only the polarization component parallel to the main surface 1A of the substrate 1 is selected by the metal-loaded waveguide polarizer 4. . The light wave is extinguished and non-quenched by being modulated by a predetermined modulation signal applied to the signal electrode 5 and the ground electrode 6, and the electric signal is replaced with on / off of light.
[0019]
In the present invention, the optical waveguide 2 is first formed on the substrate 1 having the electro-optic effect. The optical waveguide 2 can use any method such as a Ti thermal diffusion method, a proton exchange method, an epitaxial growth method, and an ion implantation method.
[0020]
Next, in the present invention, the buffer layer 3 is formed on the substrate 1. The buffer layer 3 can be arbitrarily selected according to the type of material constituting the buffer layer 3 by a known film forming method such as an evaporation method, a sputtering method, an ion plating method, or a CVD method.
In addition, the buffer layer 3 aims to achieve speed matching between the light wave guided through the optical waveguide and the microwave that is the modulation signal applied to the signal electrode, and prevents absorption loss of the light wave by the modulation electrode. For the purpose, it is preferably formed to a thickness of 0.2 to 2.0 μm.
[0021]
Next, in the present invention, the buffer layer 3 is partially removed by non-reactive dry etching to form the first opening 8.
Non-reactive dry etching is performed as follows, for example.
First, a chromium mask is formed on the buffer layer 3 by a vapor deposition method or the like so as to be slightly thicker than the buffer layer, for example, to a thickness of 0.3 to 2.1 μm. Then, after forming a photoresist on the chromium mask to a thickness of 0.7 to 1.0 μm, the photoresist is patterned by photolithography. Next, a portion corresponding to the first opening 8 of the chrome mask is removed by chemical etching. The remaining photoresist is removed with an organic solvent.
[0022]
Thereafter, for example, the wafer obtained as described above is placed in a dry etching apparatus using an ECR plasma source, and the buffer layer 3 is dry etched. Then, only the portion of the buffer layer 3 corresponding to the first opening 8 where the chromium mask is not formed is removed by etching, and the first opening 8 is formed. The remaining chromium mask is removed by chemical etching or the like.
[0023]
The etching gas that can be used for non-reactive dry etching is not particularly limited as long as it forms non-reactive plasma ion species. However, it is preferable to use an inert gas because it has a relatively high etching rate and is chemically stable and easy to handle. In particular, it is preferable to use argon gas from the viewpoint of easy availability and low price and easy control of the etching rate.
[0024]
When the buffer layer 3 is formed to be relatively thick as 1.0 μm or more, it is preferable to form the first opening 8 by using reactive dry etching and non-reactive dry etching in combination.
That is, first, the buffer layer 3 is etched by reactive dry etching. Then, the reactive dry etching is switched to the non-reactive dry etching immediately before the main surface 1A of the substrate 1 is exposed as the etching progresses.
[0025]
Since reactive dry etching etches the buffer layer mainly by a chemical action, a high etching rate can be obtained. Therefore, the formation time of the first opening 8 can be shortened. Furthermore, since the selection ratio with the chrome mask or the like is relatively high, even when the buffer layer is formed thick, it is not necessary to form the chrome mask or the like thick. For this reason, it leads also to the saving of mask material.
Then, by switching to non-reactive dry etching immediately before the main surface 1A of the substrate 1 is exposed, even if the main surface 1A of the substrate 1 is over-etched, excess free oxygen remains in the surface layer portion of the main surface of the substrate 1. Does not occur.
[0026]
It is preferable to use a chlorofluorocarbon gas for the reactive dry etching. Fluorocarbon gas is the main material of the buffer layer, SiO ₂ Therefore, it is extremely effective in improving the etching rate as described above.
As the fluorocarbon gas, the CF as described above is used. ₄ , CHF ₃ In addition to gas etc., C ₂ F ₆ And C ₃ F ₈ Can be illustrated.
[0027]
Next, in the present invention, the metal-loaded waveguide polarizer 4 is formed on the main surface 1 </ b> A of the substrate 1 in the first opening 8.
The metal loaded waveguide type polarizer 4 forms a predetermined photoresist pattern in the first opening 8 and then deposits a metal on the pattern by, for example, vapor deposition to form a predetermined metal layer. To make it. The loading length L of the metal-loaded waveguide polarizer 4 is generally 0.5 to 5.0 mm.
[0028]
Next, in the optical waveguide device 10 shown in FIG. 1 which is a preferred embodiment of the present invention, the signal electrode 5 and the ground electrode 6 which are modulation electrodes are formed on the buffer layer 3. These electrodes are formed by vapor deposition and plating.
[0029]
Further, in the optical waveguide device 10 shown in FIG. 1, the DC electrode 7 is formed on the right end of the substrate 1. In general, the DC electrode 7 is formed directly on the main surface 1A of the substrate 1 as shown in FIG. 1 because there is no problem such as speed matching and the drive voltage can be reduced. preferable. Therefore, in the present invention, the buffer layer 3 is removed by etching in the same manner as in the case of the metal-loaded waveguide polarizer 4, and the second opening 9 is formed. A DC electrode 7 is formed.
[0030]
The formation of the second opening 9 is generally preferably performed simultaneously with the formation of the first opening 8 for the purpose of reducing the number of steps and reducing the manufacturing cost. In the present invention, the first opening 8 is formed using non-reactive dry etching. For this reason, depending on the non-reactive dry etching conditions and the degree of overetching, oxygen in the surface layer portion of the main surface 1A of the substrate 1 may be depleted.
When a DC electrode is formed on a substrate deficient in oxygen as described above, carriers may be generated in the substrate due to oxygen defects in the surface layer portion of the substrate. For this reason, a large DC drift may occur in the DC electrode.
[0031]
Therefore, when the DC electrode is formed as described above, it is preferable to anneal the substrate in an oxygen-containing atmosphere after dry etching.
Specifically, after forming the first opening 8 and the second opening 9 in FIG. 1, the substrate 1 is placed in an electric furnace such as a tubular furnace, and in an oxygen-containing atmosphere at 100 to 900 ° C. The heat treatment is performed for 1 to 20 hours.
Here, the oxygen-containing atmosphere refers to an atmosphere containing a predetermined amount of oxygen such as synthetic air or air in addition to an oxygen gas atmosphere.
[0032]
Annealing in an oxygen-containing atmosphere prevents the oxidation of the metal layer constituting the metal-loaded waveguide polarizer 4 after the first opening 8 and the second opening 9 are formed. Therefore, it is preferable to carry out before forming the metal loaded waveguide type polarizer 4.
[0033]
When the buffer layer 3 is thick, the second opening 9 may be formed by using both reactive dry etching and non-reactive dry etching in the same manner as in the case of the first opening 8. it can.
[0034]
In FIG. 1, only the DC electrode is formed in the second opening 9. However, when the DC electrode is not formed, the second opening may be formed in a portion where the modulation electrode is provided, and the modulation electrode may be formed directly on the main surface of the substrate in the second opening. it can. As a result, the optical waveguide device can be driven with a lower driving voltage.
Further, the second opening can be formed simply in a concave shape by leaving a part of the buffer layer in the film thickness direction without exposing the main surface of the substrate. However, when a DC electrode is formed in the recess, the DC electrode is preferably formed directly on the main surface of the substrate. Therefore, the recess is preferably formed as an opening.
When the modulation electrode is formed in the recess, it is preferable that only the signal electrode constituting the modulation electrode is formed in the recess. Thereby, reduction of the driving voltage, improvement of velocity matching between the light wave guided through the optical waveguide and the modulation signal applied to the modulation electrode, and suppression of absorption of the light wave by the electrode can be achieved. it can.
[0035]
In addition, the recess is formed separately from the second opening, the DC electrode is formed in the second opening as described above, and the modulation electrode is formed in the recess as described above. You can also. Thereby, the effect of forming the DC electrode as described above and the effect of directly forming the modulation electrode on the main surface of the substrate can be obtained at the same time.
In this case as well, the concave portion can be formed so that the main surface of the substrate is exposed and configured as an opening (hereinafter referred to as “third opening”).
[0036]
The formation of the third opening is preferably performed simultaneously with the formation of the first opening and the second opening, for the purpose of reducing the number of steps and reducing the manufacturing cost.
Accordingly, in this case, the formation of the third opening is performed by non-reactive dry etching in the same manner as in the case of the first opening and the second opening. Also, reactive dry etching can be used in combination.
Even when the third opening is formed by non-reactive dry etching, the substrate is annealed in an oxygen-containing atmosphere as described above. The annealing conditions are the same as those for forming the first opening and the second opening.
[0037]
Further, when the first opening 8 or the second opening 9 is formed, the buffer layer portion located between the signal electrode 5 and the ground electrode 6 can also be removed simultaneously by dry etching. By removing the buffer layer portion in this way, the rate at which the modulation signal applied to the signal electrode 5 leaks to the buffer layer side is reduced. Therefore, the modulation signal is concentrated on the optical waveguide, and the modulation efficiency of the optical waveguide element is improved.
[0038]
The formation method of the present invention exhibits its effect remarkably particularly on a substrate made of a ferroelectric material containing lithium. That is, in the conventional reactive dry etching using a chlorofluorocarbon gas, as described above, fluorine radicals react with lithium on the substrate to form fluoride, and are discharged from the substrate, so that the main surface of the substrate is Oxygen excess in the surface layer becomes significant.
On the other hand, in the forming method of the present invention, the above-described fluoride is not formed, so that oxygen does not become excessive in the surface layer portion of the main surface of the substrate. For this reason, when a metal-loaded waveguide polarizer is formed on the main surface of the substrate in the opening, the difference in the degree of corrosion contained lithium between the conventional method and the method of the present invention. This becomes prominent when a substrate made of a ferroelectric material is used.
[0039]
Examples of the ferroelectric material containing lithium include the above-described lithium niobate (LiNbO ₃ ) And lithium tantalate (LiTaO) ₃ ). In particular, lithium niobate is generally used as a substrate material because high-quality crystals can be obtained at a relatively low cost and it is easy to fabricate an optical waveguide by a Ti thermal diffusion method or the like.
[0040]
【Example】
The present invention will be specifically described below with reference to examples.
Example 1
In this example, an optical waveguide device 10 as shown in FIG. 1 was formed by using the forming method of the present invention. However, in the present example, the second opening 9 and the DC electrode 7 were not formed.
As the substrate 1, an X-cut plate of lithium niobate single crystal was used. Next, an optical waveguide 2 was formed in the substrate 1 by a Ti thermal diffusion method. Next, SiO 2 is deposited on the substrate 1 by vapor deposition. ₂ The buffer layer 3 made of was formed to a thickness of 0.5 μm. Next, the substrate 1 on which the buffer layer 3 was formed was annealed in an oxygen atmosphere at 600 ° C. for 5 hours for the purpose of improving the mechanical strength of the buffer layer and supplying oxygen to the oxygen deficient portion.
[0041]
Next, while using a dry etching apparatus having an ECR plasma source, argon gas was used as an etching gas, and the first opening 8 was formed according to the procedure described in the “Embodiment of the Invention”.
Next, according to the procedure described in the “Embodiment of the invention”, a metal-loaded waveguide polarizer 4 having a loading length of 5 mm in the first opening 8 and a thickness of 1000 mm of Al layer is used. Formed.
Thereafter, a Ti layer and an Au layer were formed as a base layer on the buffer layer 3 by vapor deposition, and then an Au layer was formed thick as an electrode layer by plating. Further, the Ti layer and the Au layer were separated by chemical etching, and the signal electrode 5 and the ground electrode 6 were formed to a thickness of 20 μm.
[0042]
An optical fiber was connected to the optical waveguide element 10 thus obtained, and its polarization extinction ratio was examined. As a result, a high polarization extinction ratio of about 21 dB was obtained.
[0043]
(Example 2)
Also in this example, an optical waveguide device 10 as shown in FIG. 1 was formed by using the forming method of the present invention.
The process up to the annealing after forming the buffer layer 3 was performed in the same manner as in Example 1. However, unlike Example 1, in this example, the thickness of the buffer layer 3 was 1.0 μm.
Next, using a dry etching apparatus having an ECR plasma source, the first opening 8 and the second opening 9 were formed according to the procedure described in the “Embodiments of the Invention”. However, when forming the opening, first CF ₄ The buffer layer 3 was formed by removing about 0.9 μm in the thickness direction by reactive dry etching using gas and then removing the remaining about 0.1 μm in the same manner as in Example 1.
[0044]
Next, the substrate 1 was annealed in an oxygen stream at 600 ° C. for 5 hours, and then a metal-loaded waveguide polarizer 4 was formed in the same manner as in Example 1.
Next, the signal electrode 5 and the ground electrode 6 were formed in the same manner as in Example 1, and the DC electrode 7 was formed by using a predetermined mask simultaneously with the signal electrode 5 and the like.
[0045]
An optical fiber was connected to the optical waveguide element 10 thus obtained, and its polarization extinction ratio was examined. As a result, a high polarization extinction ratio of about 20 dB was obtained.
Further, as a result of examining the DC drift of the optical waveguide device 10, a curve as shown in FIG. 2A was obtained. In this case, the DC bias drift value was found to be about 1.5V.
[0046]
(Comparative Example 1)
In the first embodiment, the first opening 8 is CF ₄ An optical waveguide element was formed in the same manner as in Example 1 except that it was formed by reactive dry etching using gas. When the polarization extinction ratio of the optical waveguide element 10 was measured in the same manner as in Example 1, it was about 2 dB.
[0047]
(Comparative Example 2)
In Example 2, it implemented like Example 2 except not having annealed in oxygen-containing atmosphere after forming the 1st opening part 8 and the 2nd opening part 9. FIG.
When the polarization extinction ratio of the obtained optical waveguide device was examined in the same manner as in Example 1, a value of about 18 dB was obtained.
Further, when the DC drift of this optical waveguide device was examined, a curve as shown in FIG. 2B was obtained. In this case, the voltage drift value was found to be about 10.5V.
[0048]
As apparent from Examples 1 and 2 and Comparative Example 2 and Comparative Example 1, an opening is formed by non-reactive dry etching according to the present invention, and a metal-loaded waveguide polarizer is formed in the opening. When formed, it can be seen that an optical waveguide device having a high extinction ratio can be obtained.
Further, as is clear from Example 2 and Comparative Example 2, it can be seen that DC drift is extremely small when the substrate is annealed in an oxygen-containing atmosphere before the DC electrode is formed according to the present invention.
[0049]
As described above, the present invention has been described based on the embodiments of the invention with specific examples. However, the present invention is not limited to the above-described contents, and various changes and modifications are possible without departing from the scope of the present invention. Is possible.
[0050]
【The invention's effect】
According to the forming method of the present invention, even when the buffer layer formed on the substrate is dry etched to form the opening, the surface layer portion of the main surface of the substrate exposed in the opening is in an oxygen-excess state. Never become. Therefore, even when a metal-loaded waveguide polarizer is formed on the main surface of the substrate, corrosion of the waveguide polarizer can be prevented, and an optical waveguide element having a high extinction ratio can be obtained. Can do.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an example of an optical waveguide device manufactured by the method for forming an optical waveguide device of the present invention.
FIG. 2 is a diagram showing a DC drift curve of an optical waveguide element.
[Explanation of symbols]
1 Substrate
2 Optical waveguide
3 Buffer layer
4 Metal-loaded waveguide polarizer
5 Signal electrodes
6 Ground electrode
7 DC electrode
8 First opening
9 Second opening
10 Optical waveguide device

Claims (6)

A method of forming an optical waveguide element comprising an optical waveguide, a buffer layer, a metal-loaded waveguide polarizer, and a modulation electrode on a substrate having an electro-optic effect,
The substrate is made of an oxide having lithium as a constituent element species,
A first step of forming an optical waveguide on the substrate;
A second step of forming a buffer layer on the substrate;
A third step of partially removing the buffer layer by non-reactive dry etching to form a first opening and exposing at least a part of a portion of the main surface of the substrate where the optical waveguide is formed. When,
And a fourth step of forming the metal-loaded waveguide polarizer on the substrate in the first opening. A method of forming an optical waveguide device, comprising:   2. The method of forming an optical waveguide element according to claim 1, wherein the non-reactive dry etching is performed using an inert gas.   The method for forming an optical waveguide element according to claim 2, wherein the inert gas is an argon gas. In the third step, the buffer layer is partially removed in the thickness direction by reactive dry etching, and then the remaining buffer layer in the thickness direction is removed by non-reactive dry etching. The method for forming an optical waveguide element according to claim 1, wherein the method is a step of forming an opening .   The method for forming an optical waveguide element according to claim 4, wherein the reactive dry etching is performed using a chlorofluorocarbon gas.   The method for forming an optical waveguide element according to claim 1, wherein the substrate is made of lithium niobate.

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