JP2000275592A - Waveguide type optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid such a problem that a metal film is damaged during chemical etching of an electrode base layer and that the metal film does not function as a polarizer, by coating the metal film with a protective layer. SOLUTION: The waveguide type optical modulator consists of an optical waveguide 12 formed on the surface of a lithium niobate substrate 10, a SiO2 buffer layer 14a covering the substrate surface except for the region where a metal film is to be formed, a travelling wave type electrode 18 formed on the optical waveguide 12 and a metal film 20 formed on the substrate surface to be directly in contact with the optical waveguide 12. The metal film 20 is covered with a SiO2 film 16 as a dielectric material. The SiO2 film 16 is formed just after the metal film 20 is formed, and acts as a protective layer to prevent the metal film 20 from etching with an etching liquid in the process of forming an electrode 18 after the metal film 20 is formed. It is not necessary to form the metal film 20 into excessively thick, but the film is formed to the thickness necessary enough to act as a polarizer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a waveguide type optical device, and more particularly to a waveguide type optical device such as a waveguide type optical modulator in which a polarizer and an electrode are integrated.

[0002]

2. Description of the Related Art A waveguide-type light is formed by connecting an optical fiber to a chip formed by forming an optical waveguide, a buffer layer, and a traveling-wave-type electrode on a substrate having an electro-optic effect such as lithium niobate. Waveguide-type optical devices such as modulators have been developed as key devices in long-distance, wide-band optical communication systems, and have been introduced into actual systems.

In a waveguide type optical modulator used as such an optical device, usually, only one of the oscillating wave components oscillating only vertically or horizontally with respect to the substrate among the light waves propagating through the optical waveguide. Select and use. Therefore, in many cases, a polarization maintaining fiber is provided on the incident side of the waveguide type optical modulator to selectively input linearly polarized light oscillating vertically or horizontally to the substrate.

However, when stress is applied to the polarization maintaining fiber from the outside, or when the incident light is not linearly polarized light, there arises a problem that the extinction ratio in the fiber on the output side decreases.

To solve such a problem, Japanese Unexamined Patent Publication No. 4-282608 discloses a method in which a superpolarizer is attached to an end face of an input waveguide or both end faces of an input / output waveguide to reduce an extra polarization component. It has been proposed to cut to prevent the extinction ratio from decreasing.

In Japanese Patent Application Laid-Open No. 62-99913, a film having a refractive index equal to (or substantially equal to) the transmission refractive index of guided light is arranged near an optical waveguide, and this film is used as an optical waveguide. It is disclosed to operate and selectively take out one of two types of light waves propagating through the optical waveguide.

By the way, as a waveguide type polarizer for selecting one of two types of light waves propagating through an optical waveguide, a rectangular metal film is loaded above an optical waveguide formed on a substrate as the simplest configuration. There is a metal-loaded waveguide polarizer having a configuration.

This metal-loaded waveguide polarizer utilizes the difference in the absorption efficiency of a metal film with respect to the polarization direction of light. Suematsu et. Al: Appl. Phys. Lett., Vol. , No
6 (1972) illustrates the principle.

Briefly, a metal film loaded on an optical waveguide vibrates horizontally (hereinafter referred to as T
It is called E-mode light. ) And selectively absorbs a vertically oscillating polarized wave (hereinafter, referred to as TM mode light), and has a simple structure and a relatively high extinction ratio. It has the feature of.

[0010] This metal-loaded waveguide polarizer is generally constructed by loading a rectangular metal film above an optical waveguide parallel to the surface of a substrate. Since a loaded waveguide polarizer always absorbs TM-mode light, it is used for a waveguide-type optical modulator or the like using X-cut lithium niobate as a substrate using TE-mode light.

Generally, such a waveguide type polarizer is provided on the same substrate together with an electrode for modulating the guided light to form a waveguide type optical device. Such a waveguide-type optical device is, for example, forming a polarizer pattern by photolithography, forming a metal film to act as a polarizer such as Al on the entire surface of the substrate, and lifting off the polarizer pattern to form a metal. It is obtained by forming an electrode layer after forming a film.

In recent years, a waveguide-type optical device such as a waveguide-type optical modulator in which a substance having a dielectric constant lower than that of the substrate is formed as a buffer layer on the surface of the substrate to increase the speed of microwaves propagating through the electrodes. Has been proposed.

In general, in such a waveguide type optical device, it is desirable to form the electrodes thicker by electrolytic plating or the like in order to further increase the speed of the microwave propagating through the traveling wave type electrodes. Since a dielectric is used for the layer, an underlayer made of a metal layer is required as a conductive layer for electrolytic plating.

For example, a traveling-wave electrode to be formed on a high-speed modulator or the like having a buffer layer made of SiO ₂ is formed by first forming a base metal layer on the buffer layer over the entire surface of the substrate and then patterning the electrode with a photoresist. Do
After the plating is grown, the photoresist is removed, the exposed underlying metal layer is removed by chemical etching, and the signal electrode and the ground electrode are separated from each other.

[0015]

The base metal layer must be formed of a material having good adhesion to both the buffer layer and the electrode. In general, a combination of a material having a high value and a material of the same type as a metal to be plated and grown, such as Au, is used.

However, in chemical etching for removing a base metal layer used for separating a signal electrode and a ground electrode, a metal film constituting a polarizer formed in advance on the same substrate may be used for removing Au, for example. The metal film is etched and damaged due to exposure to an etching solution such as an aqueous solution of potassium iodide or an aqueous solution of ammonia and hydrogen peroxide for removing Ti, and the function as a polarizer is deteriorated. There's a problem.

Conventionally, the thickness of the metal film itself has been set so that the metal film acting as a polarizer can function as a polarizer even if the metal film is exposed to an etching solution during etching of the base metal layer and is slightly etched. Is made thicker.

However, in practice, variations in film thickness, film quality, and the like that occur during the formation of the metal film cause variations in damage to the metal film due to etching of the base metal layer, and the function as a polarizer is lost. It has deteriorated, lowering the yield.

In addition, since the metal film must be formed thicker than the thickness of the polarizer, the film forming time of the metal film constituting the polarizer becomes longer.

From the above, according to the present invention, the metal film can be formed to a thickness enough to sufficiently function as a polarizer, and the metal film previously formed on the same substrate when the electrode underlayer is chemically etched. It is an object of the present invention to obtain a waveguide-type optical device that is not likely to be damaged and no longer function as a polarizer.

[0021]

According to one aspect of the present invention, an optical waveguide, an electrode for controlling guided light, and an optical waveguide are provided on a substrate having an electro-optic effect. A waveguide-type optical device comprising: a waveguide-type polarizer having a metal film that removes an unnecessary polarization component of the propagating guided light, wherein the metal film is covered with a protective layer. .

That is, in the waveguide type optical device according to the first aspect of the present invention, the metal film constituting the polarizer previously formed on the same substrate is covered with the protective layer, so that the metal film is not etched. , Function deterioration can be prevented. Also,
Since it is not necessary to form the metal film unnecessarily thick, the metal film can be configured to have a thickness sufficient to function as a polarizer, and the time required for forming the metal film can be reduced, and the metal film can be formed thin. Material costs can be reduced.

The protective layer may be made of a material that does not have corrosion resistance to an etchant, and may serve to protect the coated metal film by being etched instead of the metal film. As described above, it may be made of a material having corrosion resistance to the etchant, and may be configured not to be etched by the etchant itself.

In the case where the protective layer is made of a material having no corrosion resistance to the etching solution, if the protective layer is made of the same material as the electrode,
There is no need to buy new materials for forming the protective layer,
Also, since a protective layer can be formed simultaneously with the formation of the electrodes,
It is efficient.

[0025]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIG. As shown in FIG. 1, the waveguide type optical modulator according to the present embodiment includes an optical waveguide 12 formed on a substrate surface, a buffer layer 14 made of SiO ₂ covering the substrate surface excluding a metal film formation region, and a buffer layer. 1
A traveling-wave-type electrode 18 formed on the optical waveguide 12 on the substrate 4 and a metal film 20 formed on the substrate surface so as to be in direct contact with the optical waveguide 12.
Are covered with a SiO ₂ film 16 which is a dielectric material.

The SiO ₂ film 16 is provided immediately after the metal film 20 is formed, and functions as a protective layer for preventing the metal film 20 from being etched by an etchant when the electrodes are formed after the metal film 20 is formed.

Since the SiO ₂ film 16 prevents the metal film 20 from being etched, it is not necessary to form the metal film 20 in an excessively large thickness. Is formed.

Here, a method of manufacturing the waveguide type optical modulator shown in FIG. 1 will be briefly described below with reference to FIGS. First, a lithium niobate substrate (hereinafter, LN
It is called a substrate. 10) An optical waveguide 12 is formed by a Ti diffusion method. Then, the LN substrate 1 is formed by a sputtering method.
A buffer layer 14 of SiO ₂ having a thickness of 0.5
After forming to a thickness of μm, annealing is performed at 600 ° C. Further, after a Cr film 22 is formed on the entire surface of the substrate, a photoresist 24 is applied on the entire surface (see FIG. 2).

Thereafter, the metal film for the polarizer is patterned by photolithography to expose the Cr film 22 in the region where the metal film for the polarizer is formed.
After forming (not shown), the exposed Cr film 22 is etched by chemical etching. After the etching of the Cr film 22, the resist pattern is removed, so that the Cr film having the opening 11a in the polarizer metal film forming region is formed.
A mask 22a is formed on the surface of the buffer layer 14 (see FIG. 3).

Then, after removing the buffer layer exposed in the opening of the Cr mask 22a by dry etching, the Cr mask 22a is removed by chemical etching, and the buffer layer 14a having the opening 11b in the polarizer metal film forming region is exposed. (FIG. 4).

After patterning the buffer layer 14 in this manner, a photoresist 24 is applied to the entire surface of the LN substrate 10, and the polarizer metal film portion is patterned by photolithography, thereby forming one of the optical waveguides 12. After forming a resist pattern 24a exposing the surface of the LN substrate 10 including the portion, an Al film 20 having a thickness of 50 nm and a SiO ₂ film 16 having a thickness of 50 nm are continuously formed on the entire surface of the substrate by a sputtering method (FIG. 5).

Thereafter, the LN substrate 10 is immersed in an organic solvent, and the resist pattern 24a is lifted off to form a two-layer film of the Al film 20 and the SiO ₂ film 16 in the opening 11b of the buffer layer 14a. LN substrate 10
(See FIG. 6).

Next, a 50 nm-thick Ti film 15 and a 50 nm-thick
The Au film 17 having a thickness of 10 nm is sequentially formed by vapor deposition (see FIG. 7). The Ti film 15 is for improving the adhesion to the buffer layer, and the Au film 17 is for improving the adhesion to the traveling wave electrode 18 (hereinafter, referred to as the electrode 18) formed on the upper layer. It is for.

Thereafter, a photoresist 24 is applied to the entire surface of the substrate, and the electrode forming portion is patterned by photolithography to form a resist pattern 24b having an opening in the electrode forming region (see FIG. 8). Further, an Au film having a thickness of 10 μm is formed as an electrode layer by an electrolytic plating method (see FIG. 9).

Thereafter, the resist pattern 24b is removed, and the entire substrate is immersed in an aqueous solution of potassium iodide and Au is provided as a base exposed between the signal electrode and the ground electrode.
The film 17 is removed. At this time, the electrode 18 is also etched, but there is no problem because the Au film provided as the electrode layer is much thicker than the Au film 17 provided as the base.
After the removal of the Au film 17, the etching solution is changed to an ammonia-hydrogen peroxide mixture to etch the Ti film under the Au film 17 to separate the signal electrode from the ground electrode, as shown in FIG. A waveguide optical modulator is obtained.

In this waveguide type optical modulator, since the metal film 20 acting as a polarizer is covered with the SiO ₂ film 16, the Au film 17 constituting the underlayer can be etched even with an aqueous solution of potassium iodide and potassium iodide. , Ti film 15
Also, in the etching with an aqueous solution of potassium iodide, the metal film 20 is etched and damaged.
The function as a polarizer does not deteriorate. Further, since the metal film 20 is not etched, the metal film 20 can be formed thin enough to function as a polarizer.

In the above-described embodiment, the metal film 20 acting as a polarizer and the SiO ₂ film 16 provided as a protective layer on the metal film 20 have the same dimensions. 20 as a protective layer provided on top of
The size of the iO ₂ film 16 may be larger than that of the metal film 20.

The metal film 20 which functions as a polarizer
The protective layer was formed by forming SiO ₂ as a dielectric material on the upper layer, but the material constituting the protective layer is not limited to SiO ₂ ,
Materials having good corrosion resistance to chemical etching of the electrode underlayer, such as metals such as r, semiconductors such as Si, and organic materials such as polyimide, can be used.

The protective layer is not formed of a member separate from the metal film 20 acting as a polarizer, but is formed by oxidizing the surface of the metal film 20 acting as a polarizer by anodic oxidation. It can also be composed of layers.

Further, in the above-described embodiment, the protective film for covering the metal film 20 acting as a polarizer is formed separately. However, the electrode layer is more formed than the underlying layer which is removed by etching in a later step. Since the electrode layer is not completely etched when the underlying layer is etched because the thickness is much larger, it is possible to form a protective layer by leaving a part of the electrode layer on the metal film 20. It is.

In this case, as shown in FIG. 10, an electrode 18 forming a ground electrode is used as a metal film 20 acting as a polarizer.
The protective layer may be formed by patterning so as to cover the upper layer. In this case, the protective layer is formed by covering the upper layer of the metal film 20 acting as a polarizer when the electrode 18 is patterned. Is also good. With this configuration, it is not necessary to form a layer made of a metal, a dielectric, or the like as a separate member as a protective layer on the metal film 20 acting as a polarizer. Since it can be formed, the number of steps is small, which is preferable.

In the present embodiment, the metal film 20 acting as a polarizer and the SiO ₂ film 16 as a protective layer covering the metal film 20 are formed by sputtering. 20 and SiO ₂ film 16
The method of forming is not limited to this, and may be formed by another film forming method such as a vapor deposition method.

Further, the description has been given of the case where an LN substrate is used as the substrate. However, the present invention is not limited to the LN substrate, and other materials such as a substrate using another dielectric material or semiconductor material can be used. Similarly, the present invention is not limited to the materials described in the above embodiment.

[0044]

As described above, in the waveguide type optical device of the present invention, the metal film can be formed to a thickness enough to sufficiently function as a polarizer. The effect is obtained that there is no possibility that the metal film formed in advance on the same substrate is damaged and does not function as a polarizer.

[Brief description of the drawings]

FIG. 1A is a top view of a waveguide type optical modulator according to an embodiment of the present invention, and FIG. 1B is an end view taken along line XX of FIG. 1A.

FIG. 2A shows an LN substrate 1 on which an optical waveguide 12 is formed.
Buffer layer 14, Cr film 22, photoresist 2
It is a top view which shows the state which formed 4 in order, and (B) is an XX line end view of (A).

FIG. 3 (A) shows pattern exposure of the substrate shown in FIG. 2;
It is a top view showing the state where it developed and formed Cr mask 22a, and (B) is an XX line end view of (A).

4A is a view showing a Cr mask 22 of the substrate shown in FIG. 3;
FIG. 6B is a plan view showing a state in which the buffer layer 14 is patterned by using a to remove the Cr mask 22a, and FIG.
FIG. 2 is an end view taken along line XX of FIG.

[5] (A) After forming a resist pattern 24a on the substrate shown in FIG. 4, the Al film 20, SiO ₂ film 16
(B) is an end view taken along the line XX of (A). FIG.

FIG. 6A is a plan view showing a state where the resist pattern 24a of the substrate shown in FIG. 5 is lifted off;
(B) is an XX line end view of (A).

7 (A) shows a Ti film 1 on the entire surface of the substrate shown in FIG. 6;
5A is a plan view showing a state in which the Au film 5 and the Au film 17 are sequentially formed, and FIG. 5B is an end view taken along line XX of FIG.

8A is a plan view showing a state in which a resist pattern 24b having an opening in an electrode formation region is formed on the substrate shown in FIG. 7, and FIG. 8B is an end view taken along line XX of FIG. It is.

9A is a plan view showing a state in which an Au film as an electrode layer is formed on the substrate shown in FIG. 8, and FIG. 9B is an end view taken along line XX of FIG. 9A.

FIGS. 10A and 10B are explanatory views showing another embodiment of the present invention, in which FIG. 10A is a top view and FIG. 10B is an end view taken along line XX of FIG.

[Explanation of symbols]

Reference Signs List 10 LN substrate 11a, 11b opening 12 optical waveguide 14 buffer layer 14a patterned buffer layer 15 Ti film 16 SiO ₂ film 17 Au film 18 traveling wave electrode (electrode) 20 metal film 22a Cr mask 22 Cr film 24 photoresist 24a , 24b resist pattern

Continued on the front page (72) Inventor Hirotoshi Nagata 585 Tomimachi, Funabashi-shi, Chiba F-term in the New Technology Research Laboratory, Sumitomo Osaka Cement Co., Ltd. 2H047 KA02 KA06 LA21 NA02 PA24 QA03 RA08 TA44 2H079 AA02 AA12 BA01 BA02 BA03 CA05 CA08 DA03 DA22 EA02 EB05 JA07 KA05 KA20

Claims (3)

[Claims] 1. A waveguide type comprising: a substrate having an electro-optic effect, an optical waveguide, an electrode for controlling the guided light, and a metal film for removing an unnecessary polarized component of the guided light propagating through the optical waveguide. A waveguide type optical device comprising a polarizer, wherein the metal film is covered with a protective layer. 2. The waveguide light according to claim 1, wherein the protective layer is made of a material having corrosion resistance to an etchant used for chemical etching performed after the formation of the waveguide polarizer. device. 3. The optical device according to claim 1, wherein the protective layer is made of the same material as the electrode.