JP2503880B2 - Light control device manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control device for modulating a light wave and switching an optical path, and in particular, an optical control device using an optical waveguide formed in a substrate of LiNbO ₃ or LiTaO ₃ having an electro-optical effect. The present invention relates to a method for manufacturing a corrugated optical switch.

[0002]

2. Description of the Related Art With the practical use of optical communication systems, higher capacity and multifunctional advanced systems are required, and new functions such as generation of higher-speed optical signals, switching of optical transmission lines, and exchanges are required. Is required. In the current practical system, an optical signal is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode. However, direct modulation has a high speed of several GHz or more due to the effect of relaxation oscillation and the occurrence of wavelength chirping. There are drawbacks such as difficulty in modulation and difficulty in application to the coherent optical transmission system due to wavelength variation. As a means to solve this, there is a method of using an external modulator, and in particular, a waveguide type optical modulator composed of an optical waveguide formed in an electro-optic crystal substrate has features of small size, high efficiency and high speed. There is.

On the other hand, an optical switch is used as a means for obtaining the function of switching the optical transmission line and switching the network. The optical switch currently in practical use is a prism,
Since the optical path is switched by mechanically moving a mirror, a fiber, etc., there are drawbacks such as low speed, large shape, and unsuitable for matrix formation. As a means for solving this, a waveguide type optical switch using an optical waveguide is being developed, and it has features such as high speed, multi-element integration, and high reliability. In particular, a material using a ferroelectric material such as lithium niobate (LiNbO ₃ ) crystal is characterized by low light absorption and low loss and high efficiency because it has a large electro-optical effect. , Directional coupler type modulators or optical switches, total reflection type optical switches, Mach-Zehnder type optical modulators and other various types of optical control devices have been reported.

In recent years, research and development of high-density integration of this waveguide type optical switch have been actively carried out. According to a document by Yu Nishimoto et al., IEICE OQE 38-147, a LiNbO ₃ substrate is used. An 8 × 8 matrix optical switch in which 64 elements of directional coupler type optical switches are integrated is obtained. On the other hand, research and development of a device including a single optical switch element such as an external optical modulator has been actively pursued. The characteristics of such an optical switch device include stability of operation against environment such as switching voltage (power), crosstalk, extinction ratio, loss, switching speed and temperature / humidity, and operation of continuous voltage application. Stability etc.

[0005]

Among the above-mentioned characteristic items, stable operation is the most important point in practical use.

The conventional technique will be described with reference to the drawings. FIG. 3 is a sectional view showing the structure of a conventional optical switch using the directional coupler 5 disclosed in Japanese Patent Laid-Open No. 4-195115. In FIG. 3, LiNb having an electro-optical effect
A buffer layer 3a is loaded on a substrate 1 including a directional coupler 5 composed of two optical waveguides 2 formed on an O ₃ or LiTaO ₃ substrate (hereinafter referred to as a substrate), and the buffer layer 3a is interposed therebetween. Electrode 4a mainly made of metal material,
4b is formed on the optical waveguide 2. Then, the conductive film 6 is further loaded. This conductive film 6 is described by Sawaki et al.
According to CLEO '86 MF-2, page 46, it has an action of preventing characteristic destabilization due to movement of charges generated in the substrate 1 due to the pyroelectric effect of the ferroelectric substance when temperature fluctuation occurs. . That is, it is considered that there is an effect of stabilizing the switch operation with respect to the temperature change, and the Si film is used. The buffer layer 3a is used to prevent the light propagating through the optical waveguide 2 from being absorbed by the electrodes 4a and 4b and the conductive film 6, and is an insulator that absorbs very little ordinary light, particularly SiO ₂ or Al ₂ O
₃ is commonly used. Because of their refractive index, LiNbO ₃ or LiTaO ₃ having an electro-optical effect is used.
This is because the refractive index of the substrate 1 is smaller than about 2.2 and there is almost no absorption of light. When the refractive index is small, the thickness of the buffer layer 3a required to prevent light absorption by the electrodes 4a and 4b and the conductive film 6 can be made thinner than when the refractive index is large. Considering the switching voltage, the electrodes 4a,
When a voltage is applied to 4b, since the dielectric constant of the buffer layer 3a is usually smaller than that of the substrate 1, the electric field concentrates on the buffer layer 3a, so that the thicker the buffer layer 3a, the higher the switching voltage. Therefore, as the buffer layer 3a, Si having a small refractive index and an extremely small light absorption is used.
O ₂ is used. However, as the buffer layer 3a, Si
When a direct current voltage is continuously applied to the electrode 4b with O ₂ using the electrode 4a as the ground, the ions in the buffer layer move due to the electric field, and the ions below the electrodes 4a and 4b have the opposite signs of the voltage applied from the outside. Gather. Therefore, the electrodes 4a, 4
During b, a de-electric field is generated with respect to the electric field in the substrate generated by an externally applied voltage. The magnitude of this demagnetizing field increases as the total amount of movement of ions increases with time. This is because the electrical insulation of SiO ₂ is generally high,
This is because ionic conduction is dominant. This phenomenon is generally called DC drift. When the voltage applied from the outside is kept constant, when an anti-electric field is generated, the electric field applied to the optical waveguide is reduced, resulting in characteristic deterioration. That is, when a demagnetizing field that increases with time is generated, a voltage for canceling the demagnetizing field must be applied from the outside in order to restore the characteristics before the demagnetizing field is generated. This means a shift of the operating voltage point of the switch operation, which is a major obstacle to practical use. Further, this ion is mixed in the buffer layer 3a as an impurity in the manufacturing process of the buffer layer 3a. N as the type of impurity ion
There are alkali metals such as a, Li and K, and it is known that these alkali metals easily ionize and move in the buffer layer 3b as mobile ions. In particular, Li, which is an element constituting the substrate 1, diffuses from the substrate 1 into the buffer layer 3a by a thermal process during or after the formation of the buffer layer 3a, and becomes a mobile ion that causes a DC drift. The buffer layer 3a is formed on the substrate 1 by sputtering, CVD, vapor deposition or the like, and is formed into a dense film by annealing after the film formation. Li ions are used for the substrate 1 in the annealing process.
To the buffer layer 3a.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of manufacturing an optical control device that can obtain stable operation over a long period of time as compared with a conventional optical control device.

[0008]

A first invention is an optical waveguide formed on a substrate of LiNbO ₃ or LiTaO ₃ having an electro-optical effect, a buffer layer formed on the optical waveguide, and an optical waveguide of the optical waveguide. In a method of manufacturing a waveguide type optical control device, which comprises a plurality of electrodes formed on the buffer layer in the vicinity and a conductive film formed on the electrodes or between the electrodes and the buffer layer, a metal is used. A method of manufacturing an optical control device, characterized in that the buffer layer is formed by one of a method of oxidizing or nitriding a film.

The second invention is Li having an electro-optical effect.
An optical waveguide formed on a substrate of NbO ₃ or LiTaO ₃ , a buffer layer formed on the optical waveguide, an electrode composed of a plurality of electrodes formed on the buffer layer near the optical waveguide, and the electrode or In the method of manufacturing a waveguide type optical control device comprising a conductive film formed between the electrode and the buffer layer, before the step of forming the buffer layer on the optical waveguide, the optical waveguide is formed on the optical waveguide. A method for manufacturing an optical control device, comprising: a step of forming a metal film; and a step of subjecting the metal film to an oxidation or nitriding treatment after the step of forming the buffer layer.

[0010]

In the method of manufacturing a light control device according to the first aspect of the invention, since a buffer layer is formed by oxidizing or nitriding a metal film having a lower ionic conductivity than a dielectric film such as SiO ₂ , the contamination of Li from the substrate is caused. Can be suppressed. Although the metal film is electrically conductive and has a large light absorption, it cannot be used as it is as a buffer layer, but the metal film is subjected to an oxidation or nitriding treatment such as annealing to improve the insulating property and suppress the light absorption. Therefore, a good buffer layer can be formed by the method of the present invention.

In the method for manufacturing a light control device according to the second aspect of the invention, since the metal film is inserted between the substrate and the buffer layer, the same effect as the first aspect is exerted on the buffer layer and the metal film. Mixing of Li from the substrate can be suppressed. Further, since the buffer layer is loaded on the upper portion, this metal film can be thinned, and the time for oxidizing or nitriding the metal film can be shortened.

Due to the above-described operation, the optical control device manufactured by the method of the present invention can reduce the DC drift and can be stably operated.

[0013]

The present invention will be described below with reference to the drawings. 1 and 2 are cross-sectional views showing the structure of an optical control device according to an embodiment of the first and second inventions, respectively, and (a) and (b) in FIG. 1 and FIG. Each represents the final form. FIG. 1A is a sectional view showing an initial structure in a method of manufacturing an optical switch according to the first invention using a directional coupler 5. In FIG. 1A, the substrate 1 is LiNbO ₃ or L having an electro-optical effect.
The substrate 1 is made of iTaO ₃ , and two optical waveguides 2 are formed on the substrate 1. The metal film 3 is loaded on the substrate 1 including the directional coupler 5 composed of these two optical waveguides 2. Electrodes 4a, 4 mainly made of a metal material through the film 3
b is formed on the optical waveguide 2. Further, the conductive film 6 is further loaded. Here, when Si is used for the metal film 3, since the ion conductivity of Si is lower than that of a dielectric such as SiO ₂ , Si is annealed in an oxygen atmosphere to form SiO.
Substrate 1 even if heat treatment is applied to change the buffer layer of ₂ films
Therefore, Li becomes difficult to be mixed. However, Si oxidizes and Si
Annealing is completed after reaching O ₂ . in this way,
In this embodiment, Si metal is heat-treated in oxygen to oxidize Si and form a buffer layer of SiO ₂ having a low dielectric constant. FIG. 1B shows a final form of the light control device according to the first embodiment of the present invention. As described above, according to the embodiment of the first invention, it is possible to manufacture the light control device in which the amount of Li ions that can move in the buffer layer is reduced and the DC drift is reduced.

FIG. 2A shows a second example using the directional coupler 5.
3 is a cross-sectional view showing an initial structure of an optical switch that is an embodiment of the invention of FIG. In FIG. 2A, the substrate 1 is made of LiNbO ₃ or LiTaO ₃ having an electro-optical effect, two optical waveguides 2 are formed on the substrate 1, and the directionality of these two optical waveguides 2 is made. The metal film 3 and the buffer layer 3a are loaded on the substrate 1 including the coupler 5,
Electrodes 4a and 4b mainly made of a metal material are formed on the optical waveguide 2 via a. Further, the conductive film 6 is further loaded. Here, the metal film 3 is made of Si,
It functions as a barrier layer for Li ions that are about to be mixed into the buffer layer 3a from the substrate 1, and can suppress mixing of Li from the substrate 1 even during the heat treatment process. The metal film 3 is formed as a buffer layer having a low dielectric constant by heat treatment in oxygen as in the embodiment of FIG. 1, and the buffer layer 3a is a SiO ₂ film formed on the metal film 3 by sputtering. FIG. 2B shows the final form of the light control device of one embodiment according to the second invention. Buffer layer 3 immediately below the electrodes 4a and 4b
Since a is made of a conventional material, the thickness of the metal film 3 that prevents the mixing of ions is made thin, so that it has almost no influence on the electric field strength in the substrate 1 when a voltage is applied to the electrodes 4a and 4b. Since it does not exist, the rise in switching voltage can be suppressed.

As described above, the directional coupler shown in FIGS. 1 (b) and 2 (b) has a sectional structure which is seemingly the same as that of the conventional directional coupler shown in FIG. Then, since a metal film is first formed on the substrate 1 and then the metal film is oxidized by heat treatment to form a buffer layer, Li ions transferred from the substrate 1 into the buffer layer during the heat treatment are generated in the conventional case of FIG. Remarkably less than the structure.

Examples of the oxidation or nitriding method include a heat treatment in oxygen or nitrogen, as well as a chemical reaction in a liquid phase or a solid phase.

Further, as the material used for the metal film 3, Ti, Al and the like can be mentioned in addition to Si, and an insulating film having a low dielectric constant such as TiO ₂ and Al ₂ O ₃ is formed by oxidizing each.

These inventions are effective not only for the optical control device composed of a directional coupler, but also for all the optical control devices of the Mach-Zehnder type, the total reflection type using a crossed waveguide, and the like. Is clear.

[0019]

In the light control device manufactured by the method of the present invention, the amount of Li ions mixed in the buffer layer from the substrate and moving in the buffer layer can be reduced,
DC drift can be reduced and stable operation can always be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view (a) of an initial form of a light control device according to a first invention and a sectional view (b) of a final form of the light control device.

FIG. 2 is a sectional view (a) of an initial form of a light control device according to a second invention and a sectional view (b) of a final form of the light control device.

FIG. 3 is a cross-sectional view showing the structure of a conventional light control device.

[Explanation of symbols]

1 LiNbO ₃ or Li having electro-optic effect
TaO ₃ substrate 2 optical waveguide 3 metal film 3a buffer layer 4a, 4b electrode 5 directional coupler 6 conductive film

Claims (2)

(57) [Claims] 1. An optical waveguide formed on a substrate of LiNbO ₃ or LiTaO ₃ having an electro-optical effect, a buffer layer formed on the optical waveguide, and formed on the buffer layer near the optical waveguide. In a method of manufacturing a waveguide type optical control device comprising an electrode composed of a plurality of electrodes and a conductive film formed on the electrode or between the electrode and the buffer layer, one of oxidation or nitridation of a metal film is performed. A method for manufacturing a light control device, characterized in that the buffer layer is formed by a method. 2. An optical waveguide formed on a substrate of LiNbO ₃ or LiTaO ₃ having an electro-optical effect, a buffer layer formed on the optical waveguide, and formed on the buffer layer near the optical waveguide. In a method of manufacturing a waveguide type optical control device comprising a plurality of electrodes and a conductive film formed on the electrodes or between the electrodes and the buffer layer, the buffer layer is formed on the optical waveguide. Optical control, characterized by including a step of forming a metal film on the optical waveguide before the step, and a step of subjecting the metal film to an oxidation or nitriding treatment after the step of forming the buffer layer. Device manufacturing method.