KR930010830B1 - Wave guide - Google Patents

Abstract

spin coating a photoresist on a LiNbO3 substrate; placing the substrate close to a mask, exposing it to ultraviolet light, and developing the sample; forming the desired waveguide by depositing Ti on the developed substrate, immersing the substrate in acetone and removing residual photoresist on it; depositing SiO2 on the substrate before thermal diffusion of Ti in the electric furnace so as to prevent out-diffusion of Li2O in the thermal diffusion process.

Description

Method for manufacturing optical waveguide which prevents surface leach of lithium oxide during thermal diffusion

1 is a schematic diagram showing a manufacturing process of an optical waveguide.

Figure 1a is a conventional method,

Figure 1b shows the method of the present invention,

2 is a photograph showing the waveguide mode of the manufactured optical waveguide,

2a and b show the waveguide mode of an optical waveguide manufactured by a conventional method,

2c and d show the waveguide mode of the optical waveguide of the present invention prepared by coating a silicon dioxide thin film having a thickness of 250 Å,

3 shows the waveguide mode of the optical waveguide of the present invention prepared by coating a silicon dioxide thin film having a thickness of 1100 Å.

The present invention provides a method for producing an improved optical waveguide that prevents out-diffusion of lithium oxide (Li ₂ O) during thermal diffusion, and more specifically, titanium lithium niobate (Ti) by a diffusion process. In the method for producing an optical waveguide using LiNbO ₃ ), the present invention relates to a method for producing an optical waveguide in which exudation of lithium oxide (Li ₂ O) is prevented.

Lithium niobate crystals have good characteristics in terms of their optical and electro-optical properties, and also have a relatively easy manufacturing process. Therefore, they are widely used as active devices in devices using thin film optical waveguides.

A method of forming an optical waveguide using lithium niobate crystals is generally a method of diffusing titanium (Ti) at a high temperature, typically 900 ° C to 1150 ° C (MN Armenise, IEE Proceedings, No. 2, 85). Page, 1988), there is one problem with this approach. In the process of thermally diffusing another material such as titanium (Ti) into the lithium niobate crystal to form an optical waveguide, an exudation phenomenon occurs in which lithium oxide escapes from the lithium niobate crystal. The device manufactured in this way becomes relatively larger than the refractive index inside the crystal and serves as a kind of optical waveguide when an extra-ordinary wave is incident (IP Kaminow and JR Carruthers). , Appl. Phys. Lett. 22, p. 326, 1973). In addition to the desired optical waveguide, an unexpected surface optical waveguide is formed on the surface, where the depth of the out-diffusion layer is similar to the depth of the desired optical waveguide, between the surface optical waveguide and the desired optical waveguide. Coupling is induced and loss of waveguides results in poor waveguide performance (J. Noda, M. Fukuma and YI to J. Appl. Phys. No. 51, page 1379). , 1980). Therefore, there is a need for solving the problem of improving the waveguide performance by reducing the loss of the optical waveguide due to the surface optical waveguide.

To date, various methods have been proposed to prevent or attenuate such problems, namely surface optical waveguides. These methods can be classified into several categories (see J. L, Jackel, J. Opt. Commun, No. 3, page 82, 1982).

First, a method of controlling the partial pressure of lithium oxide on a sample to prevent lithium exudation or to increase the amount of lithium (RL Holman, PJ Cressman, and JF Revelli, Appl. Phys. Lett. , Page 280, 1978).

Second, a method of compensating for an increase in refractive index caused by loss of lithium by simultaneously diffusing a material having a lower refractive index than that of lithium niobate crystals (J. Noda, m. Fukuma and A. Saito, Appl. Phys. Lett, 27, page 19, 1975).

Third, the method of selecting an atmospheric gas or a suitable temperature during diffusion (see J. L. Jackel, V. Ramaswamy, and S. P. Lyman, Appl. Phys. Lett., 38, 509, 1981).

Fourth, a method of manufacturing an optical waveguide by selecting a sample having a composition ratio of lithium niobate crystals close to stoichiometrically stable composition ratio (Van E. Wood, J. Appl. Phys., No. 52, No. 1118). Page, 1981).

Among these methods, the first method has the best characteristics as a method of preventing the exudation of lithium, but this method has a disadvantage in that cumbersome work such as special preparation of the crucible is required. The second method is a method of diffusing magnesium oxide (MgO) at the same time, when magnesium oxide is diffused into the lithium niobate crystal, the refractive index is lowered while the electrical insulation is lowered.

The third and fourth methods do not have a clear effect on preventing effusion.

Accordingly, an object of the present invention is an improvement in preventing lithium oxides from leaching in the surface direction during coating and diffusion of silicon dioxide on titanium during fabrication of a titanium lithium niobate (Ti: LiNbO ₃ ) optical waveguide by a diffusion method. It is to provide a method for producing an optical waveguide.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

The object of the present invention described above is to spin-coated a photoresist on a lithium niobate substrate, to enclose the photoresist spin-coated substrate in close contact with a mask, and then to engrav the substrate by developing ultraviolet rays, and then the engraved substrate. Titanium is coated on the substrate, and the titanium-coated substrate is immersed in acetone to dissolve and remove the remaining photoresist to form a desired optical waveguide on the substrate, and then the substrate is placed in an electric furnace to thermally diffuse titanium. A method of manufacturing a waveguide is achieved by adding a process of depositing silicon dioxide on a substrate before placing the substrate on which the optical waveguide is formed and thermally diffusing titanium.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1A is a schematic diagram of a process showing a conventional photo-lisography method and a thermal diffusion method, which will be described below. Spin coating a photoresist on the lithium niobate crystal, and then attaching it to a mask, irradiating with ultraviolet rays of 365 to 436 nm, and then developing to produce an intaglio form. After coating titanium using a vacuum evaporator, and then depositing the desired optical waveguide-type titanium on the lithium nitrate crystals by a so-called lift-off process of immersing in acetone to melt the remaining photoresist. Subsequently, the sample was placed in an electric furnace at 1050 ° C to thermally diffuse titanium to produce an optical waveguide.

Figure 1b schematically shows a method of manufacturing an optical waveguide according to the present invention. It is noteworthy in the method of FIG. B according to the present invention that titanium remaining after the lift-off process and silicon dioxide are coated on the entire surface of the sample to thermally diffuse together. When the optical waveguide is manufactured by this method, since the silicon dioxide is coated on the entire surface of the sample, the depth of the exudation layer can be reduced by effectively delaying the rate of exhalation in the surface direction even if lithium oxide cannot be exuded or exuded.

2A and 2B show the waveguide mode of the optical waveguide manufactured by the conventional method. FIG. a shows the case where the polarization of the incident light source is an ordinary wave, and can clearly distinguish the desired waveguide mode, but in FIG. b shows the case where the polarization of the incident light source is an extra-ordinary wave. It is difficult to distinguish between the desired waveguide mode and the surface waveguide by the exudation.

2c and d show the waveguide mode of the optical waveguide prepared by coating the silicon dioxide thin film to 250 Å thickness according to the present invention. Fig. c shows the case where the polarization of the incident light source is normal light, and can clearly distinguish the waveguide mode, and d shows the impairment of the polarization of the incident light source as an abnormal light beam. have. Further details regarding FIG. 2 are described in Example 1 below.

3 shows the waveguide mode of an optical waveguide made by coating a silicon dioxide thin film with a thickness of 1100 에 according to the present invention, in which a and c are the case where the polarization of the incident light is an ordinary light, and b and d are the incident light sources. It shows the case where the polarization of is an abnormal light ray, respectively. The shape of the waveguide mode is almost circular, not only in normal light, but also in abnormal light, which shows that almost no exudation occurs. Related to FIG. 3 is described in more detail in Example 2 below.

The following examples are intended to illustrate the invention, but not to limit or limit the invention.

Example 1

The photoresist was coated on a lithium niobate crystal whose crystal plane was cut parallel to the x-axis, irradiated with ultraviolet rays (wavelength range: 365-436nm) to the photoresist, and then photosensitive. After the waveguide was formed, a waveguide was formed by using a vacuum evaporator (electron gun method), coated with titanium to a thickness of 300 μm, and then immersed in acetone to melt the photoresist to form a waveguide. Then, the silicon dioxide was coated with 250 Å of thickness on it, and then placed in an electric furnace and thermally diffused at 1050 ° C. for 5 hours.

The waveguide characteristics of the optical waveguide thus produced were investigated for the case where normal light and abnormal light were used as polarization of the incident light source. The results are as shown in Figures 2c and d.

Figure 2c shows the good waveguide in the case of transverse magnetic (TM) mode, and d shows the waveguide mode is quite large in the case of transverse electric (TE) mode. Surface waveguide by out-diffusion is shown, not the waveguide mode of the waveguide. The shape in this picture is not easy to recognize, The waveguide mode is very small and difficult to see. The part marked C in the picture is the shape of the actual waveguide mode. It looks like this. The reason for this appearance is that the thickness of the silicon dioxide (SiO ₂ ) coated on the titanium (Ti) is so thin that the exudation occurs mainly next to the titanium, causing the waveguide loss of the optical waveguide so large that the waveguide mode cannot be recognized. Because I made it. However, comparing the results of FIG. 2b which shows the waveguide mode of the conventional optical waveguide and FIG. 2d which shows the waveguide mode of the optical waveguide manufactured in this embodiment, the possibility that silicon dioxide can prevent the leakage of lithium oxide during the diffusion process Suggests.

Example 2

In this embodiment, the experiment was performed by making the thickness of the silicon dioxide thin film thicker than that of titanium. In this example, the same method as in Example 1 was carried out except that silicon dioxide (SiO ₂ ) was coated on the titanium with a thickness of 1100 GPa and diffused. The results are shown in FIG. 3a and c show a transverse electric mode of a sample prepared by coating a silicon dioxide thin film on titanium at a thickness of 1100 μs, and b and d show a transverse magnetic mode, showing very satisfactory waveguide characteristics. have. No out-diffusion is seen in this picture. The shape of the waveguide mode (ie point A) is close to circular. Therefore, it can be seen that the waveguide property is improved by not coupling between the exudation layer and the optical waveguide. In addition, portions other than the waveguide mode (ie, point A) (ie, point B) are substrate modes, which can be removed by adjusting the incident conditions.

Comparing the experimental results of the optical waveguide manufactured in this example with the experimental results (FIG. 2b) of the optical waveguide manufactured without covering the silicon dioxide by the conventional method, the silicon dioxide thin film exuded lithium oxide in the surface direction. You can see that it prevents you from doing.

According to the present invention, it is possible to prevent the lithium oxide from exuding in the surface direction by thermal diffusion during the manufacture of the optical waveguide. Therefore, the waveguide characteristics of the optical waveguide manufactured by the method of the present invention can be significantly improved.

Claims (2)

Spin coating a photoresist on a lithium niobate substrate, then adhering the substrate to a mask and then developing by irradiating with ultraviolet light to engrav the substrate, then coating titanium on the engraved substrate, and applying the titanium coated substrate to acetone In the optical waveguide manufacturing method in which a desired optical waveguide shape is formed on a substrate by immersing and dissolving and removing the remaining photoresist, and then placing the substrate in an electric furnace to thermally diffuse titanium, the substrate in which the optical waveguide shape is formed is put in an electric furnace. A method of manufacturing an optical waveguide, characterized in that the silicon dioxide is deposited on a substrate prior to thermal diffusion to prevent exudation of the lithium oxide in the surface direction during thermal diffusion. The method of claim 1 wherein the silicon dioxide is deposited to a thickness of 250 microns to 1,100 microns.