JP3602928B2 - Manufacturing method of optical functional device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing an optical functional element as an optical waveguide type optical control device used in various fields of optical communication and optical information processing.
[0002]
[Prior art]
In recent years, optical communication systems and optical information processing systems have been put into practical use, and systems capable of processing even larger optical signals have been developed and studied.
[0003]
In these systems, an optical integrated circuit in which optical functional elements are integrated is indispensable. For example, using a planar technology, a substrate of a single crystal material such as lithium niobate LiNbO ₃ (lithium niobate is abbreviated as LN) is used. This is realized by integrating (optical integrated circuit) a straight waveguide, a bent waveguide, a branch waveguide, a control electrode, and the like.
[0004]
In order to provide such an optical integrated circuit with functionality, a path in which light is confined by forming a region having a large refractive index, that is, an optical waveguide, is generally formed on the surface layer of the substrate having an electro-optical effect, and a control section is formed on the upper part. Techniques for providing electrodes and controlling the intensity or phase of emitted light by an applied electric field have been further studied.
[0005]
For example, the light is split by a Y-branch waveguide in which light is incident from one end, and the refractive index of each waveguide is changed by applying an asymmetric electric field to the parallel waveguide. When the light is multiplexed again by the waveguide, the output light intensity can be controlled by controlling the phase difference of the light, and the light can be modulated. An optical modulator provided with a control electrode therefor, so-called interference Type LN intensity light modulators have been proposed.
[0006]
In such an interferometric LN intensity optical modulator, if the control electrode is provided directly on the waveguide, the energy of the guided light leaks into the control electrode, and the guided light is not effectively propagated in the waveguide. is there.
[0007]
Therefore, in order to solve this problem, a thin-film insulator made of SiO ₂ (hereinafter, this thin-film insulator is called a buffer layer) is usually provided on the surface layer on which the optical waveguide is formed by using a thin-film forming technique. In addition, a technique of forming a control electrode thereon is employed.
[0008]
However, when a DC component is included in the electric signal applied to the control electrode, electric polarization occurs in the buffer layer, and the electric polarization changes with time over a long period of time, usually several tens of seconds or more. Therefore, there is a problem that it is difficult to effectively apply an electric field by the control electrode. Such a phenomenon is called DC drift.
[0009]
The DC drift will be described using an interferometric LN intensity modulator as an example. FIG. 6 shows an optical output characteristic when a sine wave voltage signal is applied to the interferometric LN intensity modulator. Ideally, when the applied voltage is zero, the optical output intensity reaches a maximum value. The optical output maximum value with respect to the voltage is slightly shifted. Therefore, in practice, the optical modulation operation is realized by complementing the input electric signal by adding the DC bias voltage. However, since the effect of the applied DC bias is offset with time by the DC drift phenomenon, it becomes difficult to effectively apply the DC bias.
[0010]
By the way, although the clear mechanism of DC drift is still discussed, it is known that there is a strong correlation with the film quality of SiO ₂ forming the buffer layer, and it is said that the cause is ion conduction in the buffer layer. I have. This buffer layer is formed by a sputtering method, a plasma CVD method, or the like, but DC drift cannot be suppressed by any of these methods.
[0011]
Therefore, the present invention has been completed in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing an optical functional device capable of suppressing DC drift as much as possible.
[0012]
[Means for Solving the Problems]
The method of manufacturing an optical functional device of the present invention is to form an optical waveguide having an increased refractive index on a surface layer of a substrate made of lithium niobate single crystal or lithium tantalate single crystal, and to form a perhydropolysilazane on the surface layer of the substrate. Coating and sintering the perhydropolysilazane to form an electrical insulating layer; and forming an electrode for controlling an electric field in the substrate on the electrical insulating layer.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described with reference to FIGS. FIG. 1 is a perspective view of an interferometric LN light intensity modulator as an optical functional device of the present invention, FIG. 2 is a cross-sectional view taken along a cutting line AA ′ in FIG. 1, FIG. 3 is a chemical formula of perhydropolysilazane, FIG. 4 is a diagram showing the etching resistance of the buffer layer as the electrical insulating layer, and FIG. 5 is a diagram showing the amount of DC drift.
[0014]
1 and 2, the linear waveguide 3, the Y-branch waveguide 4, the parallel waveguide 5, and the Y-branch waveguide 6 are provided on the surface of the LN substrate 2 having the electro-optical substrate. And a linear waveguide 7 are sequentially formed, and a buffer layer 8 made of SiO ₂ and a control electrode 9 as the electrode are provided on the surface layer.
[0015]
A method of manufacturing the interference type LN light intensity modulator 1 having the above configuration will be described. The LN substrate 2 is an optical-grade LN single-crystal substrate (thickness: about 1.0 mm) whose both main surfaces are optically polished, and further has a Z-cut (the cut surface is (0, 0, 1) plane). ). Then, a Ti thin film serving as a diffusion source is formed on the main surface of the LN substrate 2 by a lift-off method or an etching method, and each of the waveguides 3 to 7 having a high Ti concentration is formed by using a thermal diffusion method. . Thereafter, the buffer layer 8 and the control electrode 9 are sequentially laminated. The control electrode 9 is also provided by a lift-off method or an etching method as in the case of forming the Ti thin film pattern.
[0016]
In order to produce the buffer layer 8, perhydropolysilazane as a precursor inorganic polymer (hereinafter, perhydropolysilazane is referred to as polysilazane) is sintered. Thereby, a dense SiO ₂ thin film can be obtained. Polysilazane is an inorganic polymer having the chemical formula shown in FIG. 3. When polysilazane is fired in the air, the reaction of oxidation by oxygen and hydrolysis by water vapor in the air proceeds, and amorphous Si is generated, and A SiO ₂ thin film having a high density can be obtained.
[0017]
That is, on the LN substrate 2 on which the respective waveguides 3 to 7 are formed on the surface layer, polysilazane is applied and formed with a thickness of about 5 μm by using a spin coating method, and then, in an oven at a temperature of about 100 ° C. Heating for about 1 hour, thereby removing the organic solvent as a solvent, and then heating and sintering at a temperature of about 500 ° C. in an atmosphere for about 1 hour to obtain a SiO ₂ thin film.
[0018]
According to the interference type LN light intensity modulator 1 of the present invention thus obtained, since the buffer layer 8 is baked while oxidizing the polysilazane to produce amorphous Si, the buffer layer 8 is more dense than the conventional one. An SiO ₂ thin film having a high density was obtained.
[0019]
Next, in order to compare the SiO ₂ film formed by the polysilazane sintering according to the present invention, the thermal oxide film, and the SiO ₂ film obtained by the conventional sputtering method, a comparative evaluation was performed using an etching rate by buffer hydrofluoric acid immersion. As a result, the result as shown in FIG. 4 was obtained. In the figure, the horizontal axis is the etching time (minute), and the vertical axis is the etching amount (μm).
[0020]
The thermal oxide film is the highest quality layer obtained by thermally oxidizing the surface of a silicon single crystal, and is used as an index in this example.
[0021]
The etching method was used to evaluate the denseness of the film. The phenomenon that etching is difficult indicates that the potential is physically low or chemically low and stable (that is, the film is dense and can be said to be a good film).
[0022]
As is clear from the results in FIG. 4, it is understood that the SiO ₂ film obtained by the polysilazane sintering according to the present invention has excellent denseness.
[0023]
Next, when the DC drift amounts of the interferometric LN light intensity modulator of the present invention and the conventional interferometric LN light intensity modulator were measured, the results shown in FIG. 5 were obtained. According to the figure, a DC voltage of 5 V is applied for DC drift measurement, and the voltage is saturated at 5 V. However, since DC drift is a characteristic relating to long-term reliability, the slope of the curve with respect to time is measured. Indicates that the smaller is, the better. Therefore, as shown in FIG. 5, it can be seen that the DC drift of the interferometric LN light intensity modulator of the present invention is well suppressed.
[0024]
In the embodiment of the present invention, the LN single crystal material is used as the substrate having the electro-optical effect. However, a lithium tantalate single crystal material may be used instead.
[0025]
【The invention's effect】
As described above, according to the method for manufacturing an optical functional device of the present invention, an optical waveguide having an increased refractive index is formed on the surface layer of a substrate made of lithium niobate single crystal or lithium tantalate single crystal, and the light waveguide is formed on the surface layer of the substrate. Peel hydropolysilazane was applied and formed, and the hydrohydropolysilazane was sintered to form an electrical insulating layer, and an electrode for controlling an electric field in the base was formed on the electrical insulating layer. As a result, a good optical functional element with extremely small DC drift could be provided.
[Brief description of the drawings]
FIG. 1 is a perspective view of an interferometric LN light intensity modulator of the present invention.
FIG. 2 is a cross-sectional view taken along a cutting line AA ′ in FIG.
FIG. 3 is a chemical formula of perhydropolysilazane.
FIG. 4 is a diagram showing the etching resistance of a buffer layer.
FIG. 5 is a diagram illustrating a DC drift amount.
FIG. 6 is a diagram illustrating optical output characteristics of an interference type LN intensity modulator.
[Explanation of symbols]
1: Interferometric LN light intensity modulator 2: LN substrate 3: linear waveguide 4: Y branch waveguide 5: parallel waveguide 6: Y branch waveguide 7: linear waveguide 8: buffer layer (electrically insulating layer)
9: Control electrode

Claims (1)

Forming an optical waveguide having an increased refractive index in the table layer substrate ing from lithium niobate single crystal or lithium tantalate single crystal, a pair El perhydropolysilazane coating formed on the front layer before Symbol substrate, said Peeru the perhydropolysilazane form an air insulation layer conductive by sintering, a method of manufacturing an optical functional device and forming an electrode for controlling an electric field in said substrate in the electrically insulating layer.

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