JP2003215379A - Method for producing optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce an optical waveguide element which suppresses optical damage due to impurities and has high reliability by diminishing impurities present in a crystal substrate. <P>SOLUTION: An ion exchange layer 11 is formed on the surface of an optical crystal substrate 10 and the layer 11 is removed by wet etching with hydrofluoric acid. An optical waveguide 18 and a plurality of polarizing reversal layers 15 which cut periodically across the optical waveguide are then formed on the surface of the substrate 10. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical waveguide device and a method for manufacturing the same.

[0002]

2. Description of the Related Art Optical waveguides are widely used for optical control in fields such as optical communication, optical information processing, and optical applied measurement. In particular, the application of the optical waveguide to the optical wavelength conversion element is
It is being actively studied as a material for realizing a short wavelength light source by coupling with a semiconductor laser.

As such an optical waveguide type optical wavelength conversion element, quasi-phase matching (Quasi-Phase matching;
It is called "QPM". ) Method second harmonic generation (Second H
armonic Generation; hereinafter referred to as "SHG". There is an element. In this QPM method, the greatest conversion efficiency can be realized when the sign of the nonlinear optical constant of the waveguide is inverted at the cycle in which the fundamental wave and the generated second harmonic wave are phase-matched. Therefore, by selecting the inversion period, it is possible to obtain the second harmonic of an arbitrary fundamental wavelength. The sign reversal of the non-linear optical constant can be realized by so-called polarization reversal, in which the direction of spontaneous polarization is reversed because the non-linear optical crystal used in the optical wavelength conversion element of the QPM system is mainly a ferroelectric substance.

FIG. 2 is a perspective view showing the structure of an optical wavelength conversion device to which the QPM method is applied. In this light wavelength conversion element, on the surface of the optical crystal plate 10 such as LiNbO ₃ ,
A striped optical waveguide 18 and a plurality of polarization inversion layers 15 that periodically cross the optical waveguide 18 are formed. Further, a high refractive index layer 19 is formed on the optical waveguide 18 of the optical crystal substrate 10. Thus, by forming the high refractive index layer 19 on the optical waveguide 18, the fundamental wave is strongly confined, and the efficiency of the optical wavelength conversion element is improved. The light wavelength conversion element having such a configuration is, for example,
It is described in JP-A-4-254834.

A conventional method for manufacturing such a light wavelength conversion element will be described. First, the surface of the optical crystal plate is washed with water or an organic solvent. After that, a comb-shaped electrode having a shape corresponding to the periodic pattern of the domain-inverted layer and a rod-shaped electrode facing the comb-shaped electrode are formed on the surface of the substrate, and a voltage is applied between both electrodes to form the domain-inverted layer. Subsequently, after forming a mask having an opening having a shape corresponding to the stripe-shaped pattern of the optical waveguide on the surface of the substrate, the non-masked portion of the substrate is subjected to proton exchange and further annealed to form the optical waveguide. Form. Furthermore, a high refractive index layer is formed on the surface of the substrate to obtain a light wavelength conversion element.

[0006]

As described above, the conventional optical waveguide device is manufactured by cleaning the surface of the optical crystal plate with water or an organic solvent and then forming an optical waveguide on the surface of the substrate. However, the optical waveguide device manufactured by such a manufacturing method has low optical damage resistance,
There was a problem of poor reliability.

In particular, in the case of the light wavelength conversion element as described above, there is a problem that it is difficult to obtain high-output SHG light because of poor light damage resistance in a short wavelength region of blue light or less. For example, when a light wavelength conversion element using LiNbO ₃ is combined with an infrared semiconductor laser having a wavelength of 820 nm to obtain output light of blue violet (wavelength 410 nm), there is only a few mW of light damage resistance. It was not obtained. Further, even when LiTaO ₃ having a higher light damage resistance than LiNbO ₃ was used as the optical crystal substrate, the light damage resistance was about 10 mW. Further, there is a problem that the output becomes unstable for the SHG light of higher output.

Therefore, it is an object of the present invention to provide an optical waveguide device having high optical damage resistance and excellent reliability, and a manufacturing method thereof.

[0009]

In order to achieve the above object, the manufacturing method of the present invention is a method of manufacturing an optical waveguide device comprising an optical crystal substrate and an optical waveguide formed on the surface of the substrate. And a step of forming the optical waveguide on the surface of the substrate, and a step of etching at least a surface of a region of the substrate where the optical waveguide is formed.

In the optical waveguide device using the optical crystal substrate, the cause of low light damage resistance to short wavelength light is considered to be generation of charge carriers and existence of impurities such as Fe as a supply source thereof. For example, when the above-described optical wavelength conversion element is manufactured using a crystal substrate containing a large amount of such impurities, electrons trapped in the impurities in the crystal substrate are excited by SHG light, and the electrons are polarized. Due to the distribution, an electric field is generated, the refractive index changes via the electro-optic effect, and the output of SHG light becomes unstable.

However, according to the manufacturing method of the present invention, impurities in the crystal substrate can be effectively reduced at the same time when the surface of the crystal substrate is etched. Therefore, optical damage caused by impurities can be suppressed, and an optical waveguide device with high reliability can be formed.

In the above manufacturing method, in the step of etching the substrate surface, the etching amount is 100 to
It is preferably 1000 nm.

In the manufacturing method, the step of etching the surface of the substrate may include forming an ion exchange layer on the surface of at least the region of the substrate where the optical waveguide is formed, and then etching the ion exchange layer. It is preferable to include a step of removing at least the surface layer portion.

In this preferable example, the ion exchange layer is preferably formed by proton exchange of the substrate.

Further, in a preferred example, the ion exchange layer is preferably removed by wet etching. In this case, it is preferable to use an etching solution containing hydrofluoric acid.

In the manufacturing method, the step of etching the surface of the substrate includes a step of removing the damaged layer by etching after forming a damaged layer on at least a surface of the substrate where the optical waveguide is formed. It is preferable to include.

In this preferred example, the damaged layer is
It can be formed by exposing the surface of the substrate to plasma.

Further, in this preferable example, it is preferable that the damaged layer is removed by wet etching. In this case, it is preferable to use an etching solution containing hydrofluoric acid.

In the above manufacturing method, the step of etching the surface of the substrate can be performed after the step of forming the optical waveguide, but it is performed before the step of forming the optical waveguide. Is preferred.

In the above manufacturing method, it is preferable that the substrate has a non-linear optical effect. By using such a substrate, it becomes possible to manufacture active elements such as a light wavelength conversion element and a light modulation element.

Further, it is preferable that the method further comprises the step of forming two or more domain-inverted layers on the surface of the substrate so as to periodically traverse the optical waveguide. By forming the domain inversion layer,
It is possible to manufacture a QPS type optical wavelength conversion element.

In this case, the step of etching the surface of the substrate can be performed after the step of forming the domain-inverted layer, but is preferably performed before the step of forming the domain-inverted layer.

The optical waveguide device of the present invention is manufactured by the manufacturing method of the present invention. This optical waveguide device has the advantages of excellent light damage resistance and high reliability. Therefore, when the optical waveguide device of the present invention is applied to a QPS type optical wavelength conversion device, it becomes possible to obtain high-output SHG light. For example, when the optical waveguide device of the present invention is applied to a light wavelength conversion device and 160 mW of incident light (wavelength 820 nm) is input thereto, Li as a crystal substrate is used.
When NbO ₃ is used, it is 60mW or more blue-violet (wavelength 4
10 nm) output light can be obtained.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIG. 2 is a perspective view showing an example of an optical waveguide device manufactured by the manufacturing method of the present invention. This optical waveguide element is an optical wavelength conversion element using SHG by a non-linear optical effect. In this optical waveguide device, stripe-shaped embedded optical waveguides 18 are formed on the surface of the optical crystal substrate 10. A plurality of polarization inversion layers 15 are formed on the optical crystal substrate 10 so as to periodically cross the optical waveguide 18 for phase matching. Further, a high refractive index layer 19 is laminated on at least the optical waveguide 18 on the surface of the optical crystal substrate 10.

A nonlinear optical material is used for the optical crystal substrate 10. Examples of the nonlinear optical material include LiNbO ₃ , LiTaO ₃ , and Li _x Ta _(1-x) NbO.
₃ (0 <x <1), KTiOPO ₄ and Mg
A material doped with O, Nb, Nd or the like can be used. Further, as the optical crystal substrate 10, any of an X plate, a Y plate, and a Z plate can be used, and an off substrate having a tilted crystal axis can also be used.

The optical waveguide 18 is a portion in which some of the constituent atoms of the crystal substrate 10 are exchanged with ions such as protons. For example, when the crystal substrate 10 is a LiNbO ₃ crystal, the optical waveguide 18 is a portion in which a part of Li in the LiNbO ₃ crystal is replaced with H. Further, the optical waveguide 18 is not limited to the one by proton exchange but may be one by other ion exchange such as Mg. Further, it may be a ridge-type waveguide instead of ion exchange.

The polarization inversion layer 15 is a portion in which the spontaneous polarization is inverted from the other portions of the crystal substrate.

The high refractive index layer 19 is incident on the optical waveguide 18.
It is a layer for confining the
It is a layer with a high refractive index. The high refractive index layer 19 includes, for example,
Nb ₂O_Five, Ta₂O_Five, TiO₂, SbTiO₃Using
You can

Next, the method of manufacturing the optical waveguide device according to this embodiment will be described by taking the case of manufacturing the above-mentioned optical wavelength conversion device as an example. 1A to 1C are process cross-sectional views for explaining an example of the manufacturing method according to the present embodiment.

First, the surface of the optical crystal substrate is etched. As a result, the surface layer portion of the crystal substrate is removed and the impurities inherent in the surface layer portion of the crystal substrate are removed.
Therefore, impurities on the surface of the substrate after etching are reduced, so that an optical waveguide device having excellent light damage resistance can be manufactured. Even if the etching amount is small, the effect of reducing impurities can be obtained, but in order to achieve a more sufficient effect, the etching amount is preferably 100 nm or more. Further, although the upper limit of the etching amount is not particularly limited, when the etching amount exceeds 1000 nm, the impurity reducing effect is not significantly changed even if the etching amount is increased. Therefore, the etching amount is 100
˜1000 nm, more preferably 200˜
The thickness is preferably 500 nm.

This etching step can be carried out by wet etching the surface of the crystal substrate. In this case, an aqueous solution containing hydrofluoric acid can be used as the etching solution. As such an etching solution,
For example, an aqueous solution containing hydrofluoric acid, hydrofluoric acid and nitric acid, an aqueous solution containing hydrofluoric acid and ammonium hydrofluoride and the like can be mentioned. Further, various conditions such as the concentration of the etching solution and the etching time are not particularly limited, and can be appropriately set according to the desired etching amount and the like. For example, the hydrofluoric acid concentration of the etching solution is 10 to 20%, and the etching time is 30 to 60 minutes.

However, the mere wet etching as described above may take a long time because the etching rate for the substrate is small. Therefore, in the present embodiment, it is preferable to adopt the following method in order to improve the etching rate.

First, the ion exchange layer 11 is previously formed on the surface of the crystal substrate 10 (FIG. 1 (a)). The ion exchange layer 1
1 can be formed by, for example, proton exchange of the crystal substrate 10. In this case, in the ion exchange layer 11, some of the atoms forming the crystal are exchanged with protons.

The layer thickness of the ion exchange layer 11 is not particularly limited, but it is preferably set according to the desired etching amount. If the layer thickness of the ion exchange layer 11 is too small with respect to the desired etching amount, it may take a long time to achieve the desired etching amount in the subsequent etching step. Further, if the layer thickness of the ion exchange layer 11 is too large with respect to the desired etching amount, the ion exchange layer will remain after etching. When the domain inversion layer is formed after etching as in the present embodiment, if the ion exchange layer remains, it may be difficult to form the domain inversion layer. Therefore, the layer thickness of the ion exchange layer 11 is, for example, 1
The thickness is from 00 to 1000 nm, preferably from 200 to 500 nm.

The proton exchange can be carried out by heat-treating the crystal substrate 10 in an appropriate solution. As the solution, for example, an aqueous solution of pyrophosphoric acid, orthophosphoric acid, benzoic acid, sulfuric acid or the like can be used. Further, the concentration of the solution, the treatment temperature and the time can be appropriately set according to the desired proton concentration and layer thickness of the ion exchange layer. For example, the proton concentration can be increased by increasing the solution concentration or the treatment temperature. Further, by increasing the treatment time, the layer thickness of the ion exchange layer can be increased. As the solution, for example, a solution containing 85 to 100% of a saturated solution can be used. The processing temperature is, for example, 100 to
The temperature is 300 ° C., and the processing time is, for example, 10 minutes to 20 hours.

After the ion exchange layer 11 is formed, the crystal substrate 10 is washed if necessary. The washing method is not particularly limited, and for example, washing with water, washing with an organic solvent such as acetone, or the like can be adopted.

After that, the ion exchange layer 11 is removed by etching (FIG. 1 (b)). At this time, at least the surface layer portion of the ion exchange layer 11 needs to be removed, and does not need to be completely removed. However, when the formation and etching of the ion exchange layer on the surface of the substrate are performed before the formation of the polarization inversion layer as in the present embodiment, the formation of the polarization inversion layer may be difficult if the ion exchange layer remains. It is preferable to completely remove the ion exchange layer.

The ion exchange layer 11 can be etched by wet etching. in this case,
As the etching liquid, for example, an aqueous solution containing hydrofluoric acid, such as an aqueous solution containing hydrofluoric acid, hydrofluoric acid and nitric acid, an aqueous solution containing hydrofluoric acid and ammonium hydrofluoride, can be used. Further, various conditions such as the concentration of the etching solution and the etching time are not particularly limited, and can be appropriately set according to the desired etching amount and the like.
Suitable conditions are, for example, that the hydrofluoric acid concentration of the etching solution is 1
The etching time is 0 to 20% and the etching time is 15 to 30 minutes.

Next, a domain-inverted layer is formed on the etched surface of the crystal substrate. The method of forming the domain-inverted layer is not particularly limited, and for example, the following method can be adopted.

First, the comb-shaped electrode 12 and the rod-shaped electrode 13 are formed on the etched surface of the crystal substrate 10 (FIG. 1).
(C)). The comb-shaped electrode 12 is an electrode having a plurality of electrode fingers (corresponding to comb teeth) corresponding to the periodic pattern of the domain-inverted layer to be formed, and the rod-shaped electrode 13 is spaced apart from the comb-shaped electrode 12. The electrodes are provided so as to face each other. The electrode material is not particularly limited as long as it has good adhesion to the crystal substrate 10, and for example,
Metals such as Ta, Al, Ni, Cr and Ti can be used. As a method of forming the electrodes, a method of forming a metal film on the surface of the crystal substrate by sputtering or the like and then patterning the metal film by photolithography and etching can be adopted.

Subsequently, a voltage is applied between the comb-shaped electrode 12 and the rod-shaped electrode 13. Thereby, the polarization inversion layer 15 is
It can be formed by extending each comb-shaped pattern of the comb-shaped electrode (FIG. 1D). Then, the comb-shaped electrode 12 and the rod-shaped electrode 13 are removed by etching.

Prior to the voltage application, it is preferable to form an insulating film 14 such as a silicon oxide film on the crystal substrate so as to cover the comb-shaped electrode 12 and the rod-shaped electrode 13. By covering the electrodes with the insulating film 14, the movement of free charges to the periphery of the electrodes can be suppressed, and as a result, the lateral inversion of the domain-inverted layer spreading in the periphery of the electrodes can be suppressed when a voltage is applied. This makes it possible to form a domain-inverted layer having a uniform periodic structure. The insulating film 14 is removed by etching after the domain inversion layer is formed.

Next, an optical waveguide is formed on the surface of the crystal substrate. The method for forming the optical waveguide is not particularly limited, and it can be carried out by ion exchange such as proton exchange. An example of the forming method will be described.

First, an ion exchange resistant mask 16 having a stripe-shaped waveguide pattern opening is formed on the surface of the crystal substrate 10 (FIG. 1E). At this time, the openings of the mask are formed so as to cross the plurality of polarization inversion layers 15 formed in the previous step. As the ion exchange resistant mask 16, for example, a material having acid resistance such as Ta, Ta ₂ O ₅ , Pt, or Au can be used. As a forming method thereof, for example, a method of forming Ta or the like by a sputtering method and then forming an opening in the formed film by photolithography and etching can be adopted.

Subsequently, the crystal substrate 10 provided with the mask 16 is heat-treated in an appropriate solution. As a result, the crystal substrate is ion-exchanged in the opening of the mask to form the optical waveguide precursor 17 (FIG. 1 (f)). As the solution used for ion exchange, the same solution as that illustrated in the ion exchange layer forming step (step of FIG. 1A) can be used. After the ion exchange, a heat treatment is performed to diffuse the ions (for example, protons) in the crystal substrate to increase the thickness of the ion exchange layer that is the optical waveguide precursor 17 to form the optical waveguide 18 (FIG. 1).
(G)).

When proton exchange is carried out as ion exchange, the proton-exchanged portion of the crystal substrate has a higher refractive index than the other portions, so that this portion can function as an optical waveguide. Further, the heat treatment after the ion exchange makes it possible to thermally diffuse the protons and reduce the proton concentration in the optical waveguide. The higher the proton concentration, the higher the refractive index. Therefore, by adjusting the heat treatment time, the refractive index of the optical waveguide can be adjusted in addition to the dimensions of the optical waveguide.

After forming the optical waveguide, a high refractive index layer is formed on the surface of the crystal substrate by a sputtering method or the like. After that, the crystal substrate is cut into a desired size, the incident end face and the emitting end face are polished, and the reflection films are formed on both end faces of the optical waveguide element.

In the above description, the case where the crystal substrate surface is etched by wet etching has been described, but the manufacturing method of the present invention is not limited to this.

The etching of the surface of the crystal substrate can also be carried out by dry etching. If dry etching is adopted, a relatively large etching rate can be obtained for the crystal substrate. As the dry etching, for example, reactive ion etching, sputter etching or the like can be adopted. In particular, it is preferable to employ reactive ion etching. In this case,
As the etching gas, for example, it can be used a gas containing fluorocarbon such as CF _4, CHF _3, a chlorine-based gas such as BCl _3. Further, various conditions such as input power, gas flow rate, pressure, temperature and time are not particularly limited, and can be appropriately set according to a desired etching amount and the like. For example, input power 100 to 30
0 W, pressure 2 to 5 Pa, temperature 20 to 100 ° C., time 10
~ 60 minutes.

Further, in the etching process of the surface of the crystal substrate, it is possible to carry out dry etching and then wet etching. According to this mode, the damaged layer formed on the surface of the crystal substrate by dry etching can be removed by wet etching. The damaged layer is a region where damage such as crystal defects is formed or increased by exposing the substrate surface to plasma during dry etching. By removing such a damaged layer, the waveguide loss of the optical waveguide device can be further suppressed.

In the case of such a form, the total etching amount of dry etching and wet etching is preferably within the above-mentioned range. Furthermore, in order to completely remove the damaged layer, the etching amount in wet etching is preferably 5 to 100 nm,
More preferably, it is 10 to 50 nm.

In this case, in the dry etching, it is possible to adopt the same etching gas and conditions as those in the embodiment in which only the dry etching as described above is carried out. Further, also in the wet etching performed thereafter, the same etching solution and conditions as those in the above-described embodiment can be adopted. However, in the damaged layer, the bond between the atoms constituting the crystal is weak, so that a relatively large etching rate can be secured. Therefore, as compared with the above-described form in which the substrate surface is simply wet-etched, or the form in which the ion-exchange layer is formed and then wet-etched, the etching time can be shortened or the etching solution concentration can be reduced. It is possible.

(Second Embodiment) In the first embodiment, the case where the surface of the crystal substrate is etched is described as an example, but the manufacturing method of the present invention is not limited to this. It is not limited to. The etching of the surface of the crystal substrate may be performed on at least the region where the optical waveguide is formed.

In the first embodiment, the surface of the crystal substrate is etched before the domain inversion layer and the optical waveguide are formed, but the manufacturing method of the present invention is not limited to this. Absent. For example, the etching of the surface of the crystal substrate may be performed after the polarization inversion layer is formed and before the optical waveguide is formed, or after the polarization inversion layer and the optical waveguide are formed. Also, etching of the crystal substrate surface may be incorporated during the process of forming the domain inversion layer and the optical waveguide.

In this embodiment, as an example of such a mode, the surface of the crystal substrate is etched not on the entire surface of the substrate but on the region where the optical waveguide is formed, and the domain inversion layer and the optical waveguide are formed. The case of incorporation in the process will be described.

The structure of the optical waveguide device manufactured by the manufacturing method according to the present embodiment is the same as that of the first embodiment, and detailed description thereof will be omitted.

FIG. 3 is a process sectional view for explaining an example of the manufacturing method according to the present embodiment. In this figure, the same members as those in FIG. 1 are designated by the same reference numerals.

First, the crystal substrate 10 is washed if necessary, and then a domain-inverted layer is formed on the surface of the crystal substrate. The washing method is not particularly limited, and for example, washing with water, washing with an organic solvent such as acetone, or the like can be adopted.

The polarization inversion layer can be formed by forming the comb-shaped electrode 12 and the rod-shaped electrode 13 on the surface of the crystal substrate 10 and applying a voltage between both electrodes, as in the first embodiment. it can.

As in the first embodiment, the comb-shaped electrode 12 and the rod-shaped electrode 13 are formed by forming a metal film on the surface of the crystal substrate 10 by a sputtering method or the like and then patterning the metal film by photolithography and etching. The method can be adopted. However, in the present embodiment, when the metal film is patterned, the etching is continued even after the metal film is removed and the comb-shaped electrode 12 and the rod-shaped electrode 13 are formed, that is, by performing over-etching, the crystal The surface of the substrate 10 is etched (FIG. 3A).

At this time, the etching amount of the crystal substrate 10 by overetching is not particularly limited, but the total amount of the etching amount of the crystal substrate in the optical waveguide forming step to be performed later is 100 to 1000 n.
The thickness is preferably m, and more preferably 200 to 500 nm. Specifically, the etching amount of the crystal substrate in this step is, for example, 50 to 100 nm, preferably 100 to 250 nm.

For etching, it is preferable to adopt dry etching. As the dry etching, for example, reactive ion etching, sputter etching or the like can be adopted. In particular, it is preferable to employ reactive ion etching, and in this case, a gas containing a fluorocarbon such as CF ₄ or CHF ₃ can be used as an etching gas. Further, various conditions such as etching time are not particularly limited, and may be appropriately set according to a desired etching amount and the like.

After the comb-shaped electrode 12 and the rod-shaped electrode 13 are formed, the insulating film 14 that covers these electrodes is preferably formed. Then, a voltage is applied between the both electrodes to form the polarization inversion layer 15 (FIG. 3B). Polarization inversion layer 15
After the formation, the comb-shaped electrode 12 and the rod-shaped electrode 13 are removed by immersing the substrate in, for example, hydrofluoric nitric acid. These steps can be carried out in the same manner as in the first embodiment.

After that, an optical waveguide is formed. The optical waveguide can be formed by ion exchange such as proton exchange as in the first embodiment.

First, an ion exchange resistant mask 16 having stripe-shaped waveguide pattern openings is formed on the surface of the crystal substrate 10. The first method for forming this mask is
As in the above embodiment, a method of forming an acid resistant material by a sputtering method and then forming an opening in the formed film by photolithography and etching can be adopted.
However, in the present embodiment, when forming the opening of the mask, the etching is continued even after the acid resistant material is removed and the opening of the mask is formed, that is, by performing over-etching, The surface of the substrate 10 is etched (FIG. 3C).

At this time, the etching amount of the crystal substrate 10 due to overetching needs to be smaller than the thickness of the polarization inversion layer 15 in order to avoid the disappearance of the polarization inversion layer 15. Specifically, the etching amount of the crystal substrate 10 by overetching is, for example, 100 nm or less, preferably 20 to 50 nm.

It is preferable to adopt dry etching for this etching. For dry etching,
For example, the same method as the above-described etching in the polarization inversion layer forming step (step of FIG. 3A) such as reactive ion etching using a gas containing fluorocarbon can be adopted. Further, various conditions such as etching time are not particularly limited, and may be appropriately set according to a desired etching amount and the like.

Furthermore, it is preferable to wet-etch the surface of the non-mask portion of the crystal substrate 10. According to such a form, it is possible to remove the damaged layer formed on the surface of the crystal substrate 10 by dry etching and further suppress the waveguide loss of the optical waveguide device. It is also possible to remove by-products attached to the substrate surface during dry etching.

In this wet etching, the etching amount is preferably 10 nm or more in order to completely remove the damaged layer. Further, the upper limit of the etching amount in the wet etching is smaller than the thickness of the polarization inversion layer 15 and is, for example, 100 nm or less, in order to avoid the disappearance of the domain inversion layer 15, the etching amount by the dry etching performed before is smaller than the total thickness. To adjust. Specifically, the etching amount in wet etching is
For example, it is 10 to 30 nm. In the wet etching, it is possible to use the same etching solution as that exemplified in the first embodiment, such as an aqueous solution containing hydrofluoric acid. Further, the etching solution concentration and time are not particularly limited, and may be set appropriately according to the desired etching amount and the like.

Thereafter, the crystal substrate 10 provided with the mask
Is heat treated in a suitable solution. Thereby, the mask 16
The crystal substrate 10 is ion-exchanged in the opening of the above to form the optical waveguide precursor 17 (FIG. 3D). Further, by performing heat treatment, the thickness of the optical waveguide precursor 17 is increased to form the optical waveguide 18 (FIG. 3 (e)). After that, a high refractive index layer is formed on the surface of the crystal substrate, the crystal substrate is cut out, the incident end face and the emitting end face are polished, and the reflective film is formed. These steps can be carried out in the same manner as in the first embodiment.

In the above description, the crystal substrate is etched in both the polarization inversion layer forming step and the optical waveguide forming step, but the present embodiment is not limited to this, and the polarization inversion layer is not limited to this. The crystal substrate may be etched in at least one of the forming step and the optical waveguide forming step.

In the above description, the crystal substrate is etched only by dry etching in the domain inversion layer forming step, but wet etching may be further carried out after dry etching.

Further, in the above description, the crystal substrate is etched by overetching in the etching for forming the electrode or the ion-exchange resistant mask, but the present embodiment is not limited to this. is not. For example, after forming an electrode or an ion exchange resistant mask by etching, another new etching may be performed using these as a mask.

[0074]

EXAMPLES Example 1 In this example, LiNb is used.
Taking an O ₃ substrate as an example, the etching of the optical crystal substrate,
Detailed studies were conducted on the impurity concentration distribution in the optical crystal substrate before and after etching.

First, a secondary ion mass spectrum (SIMS) measurement was performed on a commercially available LiNbO ₃ substrate (manufactured by Yamajusha Co., Ltd .; hereinafter the same). As shown in FIG. It was confirmed that the Fe concentration of was maximum on the substrate surface and gradually decreased from the substrate surface toward the deep portion. It was also confirmed that when the depth from the substrate surface exceeds about 200 nm, the Fe concentration does not change significantly even if the depth increases. The origin of Fe is not clear, but it is considered that Fe was mixed during the surface polishing.

Next, for a commercially available LiNbO ₃ substrate,
Wet etching was performed with a 10% HF aqueous solution for about 30 minutes to remove the substrate surface by about 60 nm, and then SIMS measurement was performed on this substrate. As a result, it was confirmed that the Fe concentration was lower than that of the substrate before etching. This is because Fe existing near the substrate surface was removed at the same time when the substrate surface was etched by the HF aqueous solution.

Thus, it was confirmed that the Fe concentration in the substrate can be reduced by wet etching the surface of the LiNbO ₃ substrate. As mentioned above, LiNb
The etching of the O ₃ substrate can be performed using an HF aqueous solution. However, if the LiNbO ₃ substrate is HF
When etching with an aqueous solution, the etching rate is small, for example, about 2n when using a 10% HF aqueous solution.
It was m / min.

Therefore, when a method of increasing the etching rate was examined, when the LiNbO ₃ substrate was subjected to ion exchange, the etching rate of the formed ion exchange layer was higher than that of the non-ion-exchanged substrate. I found that. For example, Li
Proton exchange was performed on the NbO ₃ substrate, and the thickness was 200 nm.
When the ion exchange layer is formed, the H
The etching rate for F10% aqueous solution is about 20 nm
It was confirmed that the rate could rise to / min. Furthermore, it was confirmed that when wet etching was performed after ion exchange, the generation of pits was suppressed on the substrate surface. Such suppression of pits is preferable because it can suppress the waveguide loss in the optical waveguide.

Next, for a commercially available LiNbO ₃ substrate,
After dry etching using CF ₄ gas for 3 minutes to remove the surface of the substrate by about 60 nm, SI is applied to this substrate.
MS measurement was performed. In addition, the etching was performed by a reactive ion etching apparatus with an input power of 200 W and a gas flow rate of 20 m
It was carried out under the conditions of 1 / min, pressure 3 Pa and temperature 20 ° C. As a result, as shown in FIG. 4B, it was confirmed that the Fe concentration was lower than that of the substrate before etching. this is,
This is because Fe existing near the substrate surface was removed at the same time when the substrate surface was etched.

As described above, it was confirmed that the Fe concentration in the substrate can also be reduced by dry etching the surface of the LiNbO ₃ substrate. When a LiNbO ₃ substrate is dry-etched, its etching rate is relatively high. For example, when etching was performed under the same conditions as above using CF ₄ gas, the etching rate was about 20 nm / min.

Further, for a commercially available LiNbO ₃ substrate,
Dry etching was performed under the same conditions as above to remove the substrate surface by about 60 nm. Then, this substrate is HF: N
The substrate surface was wet-etched by about 20 nm by immersing it in an aqueous solution of H ₄ F = 1: 10 for 10 minutes. As a result of performing SIMS measurement on this substrate, as shown in FIG. 4C, it was confirmed that the Fe concentration was further reduced as compared with the substrate subjected to only dry etching.

The results in FIG. 4 show the intensity (S1) of the signal corresponding to Fe in the SIMS measurement as a ratio (S1 / S2) to the intensity (S2) of the signal corresponding to Li.

Further, although Fe was taken as an impurity in this embodiment, it is considered that other metal impurities can be similarly removed.

Example 2 In this example, a light wavelength conversion element was manufactured by the same manufacturing method as that shown in FIG.

First, as a crystal substrate having a nonlinear optical effect, an X-plate LiNbO ₃ substrate having a thickness of 500 μm was used. First, a LiNbO ₃ substrate was treated in a 95% pyrophosphoric acid solution at 120 ° C. for 15 hours to obtain L
By exchanging Li of the iNbO ₃ substrate for H ^{+ in} pyrophosphoric acid, a proton exchange layer having a thickness of 1 μm was formed on the surface of the LiNbO ₃ substrate. After washing with water and washing with acetone, wet etching was performed with a 47% HF aqueous solution for 1 hour to completely remove the proton exchange layer. In this case, no pit was observed on the surface of the substrate after removal, and a flat surface was obtained.

Next, a domain-inverted layer was prepared by the following method. First, a Ta film having a thickness of 200 nm was formed on the surface of the LiNbO ₃ substrate by the sputtering method. A comb-shaped pattern and a rod-shaped pattern were formed of a resist by photolithography, and the Ta film was dry-etched using this resist as a mask to form a comb-shaped electrode and a rod-shaped electrode. Dry etching uses a reactive ion etching device.
It was performed using CF ₄ gas. A 300 nm thick SiO ₂ film was formed on the LiNbO ₃ substrate by a chemical vapor deposition (CVD) method so as to cover the comb-shaped electrodes and the rod-shaped electrodes. Then, a pulse voltage was applied between the both electrodes to form a plurality of periodically poled layers. After forming the domain-inverted layer, the SiO ₂ film, the comb-shaped electrode and the rod-shaped electrode were removed by immersing in the hydrofluoric nitric acid (HF: HNO ₃ = 2: 1 (capacity ratio)) for 5 seconds.

Next, an optical waveguide was manufactured by the following method. First, a Ta film having a thickness of 50 nm was formed on the surface of the LiNbO ₃ substrate by the sputtering method. A stripe-shaped waveguide pattern is formed with a resist by photolithography, and the Ta film is dry-etched using CF ₄ gas using this resist as a mask to form an ion exchange-resistant mask having an opening width of 4 μm. Formed. next,
The LiNbO ₃ substrate was heat treated in pyrophosphoric acid at 120 ° C.,
A proton exchange layer was formed on the unmasked portion of the LiNbO ₃ substrate. The thickness of the proton exchange layer was about 0.2 μm. After the ion exchange, the ion exchange resistant mask was removed using hydrofluoric nitric acid. Then, in an oxygen atmosphere at 300 ° C, 60
By performing annealing treatment for minutes, the thickness of the proton exchange layer was increased to about 2 μm to form an optical waveguide.

After forming the optical waveguide, a 170 nm thick Nb ₂ O ₅ film was formed by sputtering. After that, the substrate is cut into a length (waveguide length) of 10 mm and a width of 0.6 mm, the entrance end face and the exit end face are polished, and then a SiO ₂ reflective film is formed on each end face to obtain a light wavelength conversion element. Got

The manufactured light wavelength conversion element and wavelength 820 nm
The characteristics of this optical wavelength conversion element were evaluated by combining with an infrared semiconductor laser of. As a result, when 160 mW of light was incident, 50 mW of blue-violet (wavelength 410 nm) output light was obtained. The output was stable, and output fluctuation due to optical damage was not substantially observed.

Example 3 In this example, an optical wavelength conversion element was manufactured by the same manufacturing method as that shown in FIG.

First, a crystal substrate having a nonlinear optical effect and
Then, the same LiNbO as in Example 2 was used. ₃A substrate was used. A
LiNbO by washing with seton₃Removal of impurities on the substrate surface
After that, the domain-inverted layer was produced by the following method.

First, by the same method as in Example 2, Li
After forming a Ta film with a thickness of 200 nm on the surface of the NbO ₃ substrate, a comb-shaped pattern and a rod-shaped pattern were formed with a resist by photolithography. Subsequently, the Ta film was dry-etched using this resist as a mask to form a comb-shaped electrode and a rod-shaped electrode. The dry etching was performed by using a reactive ion etching apparatus with CF ₄ gas, input power 200 W, gas flow rate 20 ml / min, pressure 3
It was performed under the conditions of Pa and a temperature of 20 ° C. At this time, the surface of the LiNbO ₃ substrate was etched by about 20 nm by setting the over-etching time after removing the Ta film to 1 minute. Then, as in Example 2, a SiO ₂ film was formed to cover the comb-shaped electrodes and the rod-shaped electrodes, and a pulse voltage was applied between the electrodes to form a plurality of periodically inverted domain-inverted layers. . After forming the domain inversion layer, hydrofluoric nitric acid (HF: HN
By immersing in O ₃ = 2: 1 (volume ratio) for 5 seconds, S
The iO ₂ film, the comb-shaped electrode and the rod-shaped electrode were removed.

Then, an optical waveguide was manufactured by the following method. First, in the same manner as in Example 2, a Ta film and a resist were formed on a LiNbO ₃ substrate, and the Ta film was dry-etched using this resist as a mask to form an ion exchange resistant mask. The dry etching is performed by using a reactive ion etching apparatus with CF ₄ gas.
Input power 200W, gas flow 20ml / min, pressure 3P
a, the temperature was 20 ° C. At this time, by setting the overetching time after removing the Ta film to 1 minute,
The surface of the LiNbO ₃ substrate was etched by about 20 nm. Further, the substrate is buffered with hydrofluoric acid (HF: HNO ₃ : H ₂
The substrate surface including the damaged layer formed by dry etching was etched by about 20 nm by immersing it in O = 1: 5: 50 (capacity ratio) for 10 minutes.

Then, in the same manner as in Example 2, LiNbO
After forming a proton exchange layer on the non-masked portion of the _three substrates, an annealing process was performed to form an optical waveguide. Further, after forming an Nb ₂ O ₅ film in the same manner as in Example 2,
The substrate was cut into a predetermined size, the incident end face and the emitting end face were polished, and a reflective film was formed to obtain a light wavelength conversion element.

The manufactured light wavelength conversion element and a wavelength of 820 nm
The characteristics of this optical wavelength conversion element were evaluated by combining with an infrared semiconductor laser of. As a result, when 160 mW of light was incident, 50 mW of blue-violet (wavelength 410 nm) output light was obtained. The output was stable, and output fluctuation due to optical damage was not substantially observed.

[0096]

As described above, according to the method of manufacturing an optical waveguide device of the present invention, it is included in the crystal substrate by including the step of etching at least a region of the crystal substrate where the optical waveguide is formed. It is possible to provide an optical waveguide device having excellent reliability by reducing impurities and improving light damage resistance.

[Brief description of drawings]

FIG. 1 is a process sectional view for explaining an example of a manufacturing method according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing an example of a light wavelength conversion element manufactured by the manufacturing method of the present invention.

FIG. 3 is a process sectional view for explaining an example of the manufacturing method according to the second embodiment of the present invention.

FIG. 4 is a diagram showing the results of measuring the Fe distribution in a LiNbO ₃ substrate by SIMS.

[Explanation of symbols]

10 Optical crystal substrate 11 Ion exchange layer 12 comb electrodes 13 Rod electrode 14 Insulating film 15 Polarization inversion layer 16 Ion exchange resistant mask 17 Optical Waveguide Precursor 18 Optical Waveguide 19 High refractive index layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor, Kiminori             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F term (reference) 2H047 KA04 PA13 PA24 QA01 QA03                       TA11                 2K002 AB12 CA03 DA06 FA17 FA24                       FA26 FA27 HA20

Claims (15)

[Claims] 1. A method of manufacturing an optical waveguide device comprising an optical crystal substrate and an optical waveguide formed on the surface of the substrate, the method comprising the step of forming the optical waveguide on the surface of the substrate, and at least the substrate. And a step of etching a surface of a region where the optical waveguide is formed. 2. The method of manufacturing an optical waveguide device according to claim 1, wherein an etching amount is 100 to 1000 nm in the step of etching the substrate surface. 3. The step of etching the substrate surface comprises:
The optical waveguide device according to claim 1 or 2, further comprising a step of removing at least a surface layer portion of the ion exchange layer by etching after forming an ion exchange layer on a surface of at least a region of the substrate where the optical waveguide is formed. Manufacturing method. 4. The method of manufacturing an optical waveguide device according to claim 3, wherein the ion exchange layer is formed by proton exchange of the substrate. 5. The method for manufacturing an optical waveguide device according to claim 3, wherein the ion exchange layer is removed by wet etching. 6. The method for manufacturing an optical waveguide device according to claim 5, wherein the etching solution used for the wet etching contains hydrofluoric acid. 7. The step of etching the substrate surface comprises:
3. The method of manufacturing an optical waveguide element according to claim 1, further comprising the step of removing the damaged layer by etching after forming a damaged layer on at least the surface of the substrate where the optical waveguide is formed. 8. The method of manufacturing an optical waveguide device according to claim 7, wherein the damaged layer is formed by exposing the surface of the substrate to plasma. 9. The method of manufacturing an optical waveguide device according to claim 7, wherein the damaged layer is removed by wet etching. 10. The method for manufacturing an optical waveguide device according to claim 9, wherein the etching solution used for the wet etching contains hydrofluoric acid. 11. The method of manufacturing an optical waveguide element according to claim 1, wherein the step of etching the surface of the substrate is performed before the step of forming the optical waveguide. 12. The method for manufacturing an optical waveguide device according to claim 1, wherein the substrate has a nonlinear optical effect. 13. The method of manufacturing an optical waveguide device according to claim 1, further comprising the step of forming two or more domain-inverted layers that periodically cross the optical waveguide on the surface of the substrate. . 14. The method of manufacturing an optical waveguide element according to claim 13, wherein the step of etching the substrate surface is performed before the step of forming the domain-inverted layer. 15. An optical waveguide device manufactured by the manufacturing method according to claim 1.