JP2812652B2 - Manufacturing method of waveguide type optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical element used in the fields of optical communication, optical information processing, optical applied measurement, etc., and more particularly to a waveguide type electro-optic such as a high speed optical modulator and a high speed optical switch. The present invention relates to a method for manufacturing an element. The waveguide type optical device manufactured by the method of the present invention has a feature that it exhibits stable operation characteristics even when a control DC voltage is applied.

[0002]

2. Description of the Related Art As an example of the prior art relating to the present invention, a Mach-Zehnder type optical modulator using a lithium niobate substrate will be described and described. In this optical device, a waveguide pattern having a Y-branch / combination form is formed on a lithium niobate (LiNbO ₃ ) substrate by a Ti thermal diffusion method, and a buffer layer made of a dielectric material is formed thereon. And
Further, an electrode corresponding to the waveguide pattern is provided thereon (for example, "Optical Integrated Circuit", co-authored by Nishihara, Haruna, and Suhara, Ohmsha, 1985). If necessary, the charge generated by the pyroelectric effect of LiNbO ₃ when the ambient temperature of the optical device is changed is uniformly dispersed on the device surface without concentrating on the lower portion of the electrode, and the device characteristics (modulation phase ), A thin layer having a lower electric resistance than the buffer layer may be inserted above or below the buffer layer (K. Seino, T. Na
kazawa, Y. Kubota, M. Doi, T. Yamane and H. Hakogi;
Proc. OFC'92, San Jose, Feb. 8-11, 1992
(Optical Soc. Am., Washington, 1992) pp.32
5, cross letters, Nozawa; IEICE Transactions C-1, J
75-C (1992) 17, JP-A-5-66428, etc.)
.

[0003] Thermal diffusion treatment of titanium for forming an optical waveguide in a lithium niobate substrate is performed at a temperature of about 1000 ° C, and external diffusion (evaporation) of lithium at a high temperature.
It is common practice to use a humid gas, that is, a gas atmosphere in which water is bubbled, in order to suppress the occurrence of light (for example, Nishihara, Haruna, and Suhara, “Optical Integrated Circuit”, Ohmsha, 1985). As a gas for forming such a thermal diffusion atmosphere, pure oxygen, an argon / oxygen mixed gas, air, or the like is often used.

The dielectric buffer layer, which is an important component of the optical device, is generally made of SiO ₂ or Al ₂ O.
Oxide dielectric ₃ is used, a vacuum deposition method, ion-assisted vacuum deposition, sputtering, or pre-arranged in the waveguide using a chemical vapor deposition a generic thin film deposition method such as (CVD) Formed on the substrate.

Oxides such as SiO ₂ and Al ₂ O ₃ have high electrical insulation and a small dielectric constant (refractive index), so that an optical signal propagating in a waveguide and a high-frequency signal propagating in an electrode are used. Since the electric signal is less likely to be scattered and lost in these substances and is the same oxide as the substrate (LiNbO ₃ ) substance, the chemical bonding at the interface between both layers is strong, that is, the adhesion strength is high. That is, a buffer layer having many advantages can be formed.

After the dielectric buffer layer film is formed by the above-described method such as the vacuum evaporation method or the sputtering method,
It is known that heat treatment is performed in an oxidizing atmosphere at about 600 ° C. to compensate for oxygen deficiency in the film (Japanese Patent Application Laid-Open No. 58-181318). At this time, in order to suppress the outward diffusion (evaporation) of lithium and to promote the oxidation reaction of the film, it is general to use a wet oxygen gas, for example, an oxygen gas obtained by bubbling water.

[0007]

In the conventional method, the thermal diffusion at the time of forming the waveguide and the heat treatment of the buffer layer are generally performed in a wet gas as described above. There is a problem to be solved in the characteristics of optical elements, especially in a DC-drift phenomenon which is a problem in a Mach-Zehnder type optical modulator using a lithium niobate substrate.

[0008] The DC-drift phenomenon is a phenomenon in which an optical modulation phase adjusted by a DC voltage in an optical element changes with time and deviates from an adjustment point. Although the cause of this DC-drift is still being discussed at various stages, the view that it is strongly related to the electrical properties (resistivity, dielectric constant, etc.) of the substrate and the buffer layer is influential. It is difficult to completely prevent this. (Tomoji, Nozawa; IEICE Transactions C-1, J75-
C-1 (1992) 17, JP-A-5-66428, JP-A-4-346310, and the like. An object of the present invention is to solve the above-mentioned problems of the conventional method, improve the characteristics of the obtained optical device, and particularly provide a method of efficiently manufacturing a waveguide type optical device having a remarkably small DC-drift phenomenon. It is.

[0009]

According to the present invention, there is provided a method of manufacturing a waveguide type optical device, comprising the steps of thermally diffusing titanium on a lithium niobate substrate to form an optical waveguide having a desired shape; A step of depositing and forming a dielectric buffer layer immediately above, and a step of arranging an electrode on the dielectric buffer layer. Performing in a dry gas atmosphere having a dew point of not more than 0 ° C., and subjecting the dielectric buffer layer to heat treatment in a dry gas atmosphere containing at least oxygen and having a dew point of 0 ° C. or less. . In the method of the present invention, the heat treatment of the dielectric buffer layer is preferably performed at a temperature of about 600 ° C. In the method of the present invention, each of the drying gases in the titanium thermal diffusion step and the heat treatment of the dielectric buffer layer has a dew point of -70 ° C or less.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, an optical waveguide is formed by thermally diffusing titanium into a predetermined shape on a substrate made of lithium niobate (LiNbO ₃ ). This heat diffusion step is performed in a dry gas atmosphere containing at least oxygen, and the heat diffusion temperature is preferably 900 to 1000 ° C. As the oxygen-containing dry gas, it is preferable to use a gas containing at least oxygen such as oxygen gas, an argon / oxygen mixed gas, or air, but generally it is more preferable to use oxygen gas. The water vapor content in the dry gas atmosphere is preferably such that the gas exhibits a dew point of 0 ° C. or less, but the dew point is −70.
It is more preferable that the temperature is not higher than ° C.

[0011] A dielectric buffer layer is formed immediately above the optical waveguide formed as described above. This dielectric buffer layer is formed by depositing a dielectric oxide such as SiO ₂ , Al ₂ O ₃ , or ITO by a vacuum deposition method, an ion-assisted vacuum deposition method,
It is formed by deposition by a thin film deposition method such as a sputtering method or a chemical vapor deposition method (CVD). The dielectric buffer layer thus formed is subjected to a heat treatment in a dry gas atmosphere containing at least oxygen. The heat treatment temperature is preferably about 600 ° C. As the atmosphere gas, oxygen gas, argon /
It is preferable to use an oxygen-containing gas or an oxygen-containing dry gas such as air, and it is particularly preferable to use an oxygen gas. The moisture content of the dry gas in the heat treatment atmosphere is preferably an amount showing a gas dew point of 0 ° C. or less, and more preferably the dew point is −70 ° C. or less.

In the method of the present invention, an electrode for adjusting characteristics of a light wave passing through the optical waveguide is provided on the dielectric buffer layer. This electrode is provided by forming an Au electrode pattern (thickness: several hundred mm) using photolithography technology and thin film forming technology, and then increasing the thickness of the electrode pattern to approximately several μm using gold plating technology. It is done by doing.

The characteristics of the optical device obtained by the method of the present invention will be described in comparison with the optical device obtained by the conventional method. FIG. 1 shows Ti: LiNbO ₃ prepared by a conventional method.
3 shows the results of DC-drift measurement of a Mach-Zehnder type optical intensity modulator (the results of measurements on two samples are shown). The preparation of the Ti diffusion waveguide is performed at 980 ° C. while flowing in wet synthetic air (bubbling in ultrapure water at room temperature, water content: the amount of gas introduced into the furnace indicating the final dew point of 40 ° C.). went. A buffer layer made of silicon oxide is deposited and formed thereon by a sputtering method.
Heat treatment was performed in a stream of moist oxygen. DC of the obtained optical waveguide
-Drift measurement is performed by applying a DC
A bias voltage of 5 V is applied, and an AC voltage (± 10
V) was superimposed and applied, and by observing the optical modulation waveform output from the optical element on an oscilloscope, tracking of a temporal shift of an arbitrary light intensity peak position was performed.
The DC-drift is a phenomenon in which the temperature is accelerated, and the measurement result at 80 ° C. shown in this figure is observed with the drift speed accelerated by about 1000 times compared to the measurement at room temperature (25 ° C.). As is clear from FIG. 1, in the optical modulator manufactured by the conventional method, the applied bias (5 V) was applied for about several hours.
Was observed.

On the other hand, for comparison, in the above-mentioned manufacturing process, the thermal diffusion treatment of the waveguide was performed by using dry oxygen (water content: -7
As shown in FIG. 2, in the sample subjected to the buffer layer heat treatment in a wet oxygen stream, the same measurement conditions (a bias voltage of 5 V and a measurement temperature of 80 ° C.) were used.
C.) showed only a drift of 2-3 V (half the value of the modulator corresponding to FIG. 1).

FIG. 3 shows an example (Example 1) in which the buffer layer is also heat-treated in a dry oxygen atmosphere in the above method. As is clear from FIG. 3, the DC-drift was further improved by changing the heat treatment atmosphere from a wet gas to a dry gas (water content: an amount indicating a gas dew point at −70 ° C.). In the measurement performed using a plurality of different LiNbO ₃ substrates (all of which are z-cut), the same as in FIG.
It is clear that only half the drift of the conventional method is observed, so that the processing method according to the invention significantly improves the DC-drift of at least LiNbO ₃ -based electro-optical elements. It can be easily analogized that the same effect can be obtained with another electro-optical crystal (for example, LiTaO ₃ ) having the same crystal structure as LiNbO ₃ .

As described above, the mechanism relating to the effect of the present invention is not yet clear, but it has been found from a related experimental result that the hydrogen content in the optical element chip changes by using a heat treatment atmosphere ( For example,
L. Kovacs, M. Wohlecke, A. Jovanovic, K. Polgar and
S. Kapphan, “Infrared Absorption Study of the OH
Vibrational Band in LiNbO ₃ Crystal _, "J. Phys. Che
m. Solids Vol. 52 (1991) pp. 797-803 describes many research results). Table 1 shows the relative change in the amount of hydrogen due to the difference in the heat treatment atmosphere actually measured by the Fourier transform infrared (IR) absorption spectrum method similar to the above-mentioned literature, as the absorption coefficient value in the OH stretching mode. In the following description, according to Table 1, the sample names are “WET / WET” (conventional method), WET / DRT (conventional method), “DRY / WET” (conventional method), and “DRY / DR”.
Y "(the method of the present invention).

[0017]

[Table 1]

As is clear from Table 1, the "DRY / DRY" sample according to the method of the present invention has a smaller size than the conventional method.
The amount of hydrogen (OH) in the chip has been significantly reduced, and this phenomenon is considered to have greatly contributed to the reduction of DC-drift.

[0019]

The present invention is further described by the following examples. Example 1 Z-cut T prepared in a dry oxygen gas stream (water content: amount indicating a gas dew point of -70 ° C., temperature: 980 ° C.)
i: An SiO ₂ buffer layer was formed on a LiNbO ₃ waveguide substrate by a sputtering method, and dried oxygen (dew point: −
Mach-Zehnder type optical modulator (“DRY / DRY” sample) that has undergone a heat treatment (600 ° C.) in 70 ° C.)
FIG. 3 shows the results of the DC-drift measurement. The conditions for measuring the drift are exactly the same as those described above. The DC-drift in Example 1 was reduced to about half that of the conventional method (FIG. 1).

Comparative Example 1 and Example 2 In order to confirm the reproducibility of the results of the present invention, the Z-cut L used in FIGS.
A Mach-Zehnder modulator was manufactured using an iNbO ₃ substrate and a substrate manufactured by a different manufacturer. By changing the Ti thermal diffusion atmosphere and the buffer layer heat treatment atmosphere, "DRY /
Wet "(Comparative Example 1) and" DRY / DRY "(Example 2) samples were prepared. The DC-drift measurement results are shown in FIG. 4 (" DRY / WET ", Comparative Example 1) and FIG.
("DRY / DRY", Example 2). Both are D
The C-drift was reduced to about half of the conventional method (FIG. 1, the substrate maker is the same as that of the present embodiment), and the reproducibility of the invention was confirmed.

[0021]

According to the process of the present invention, there is a problem that DC-DC becomes a problem in applying the waveguide type electro-optical element to high-speed optical communication or the like.
It has become possible to efficiently manufacture an optical element with significantly reduced drift.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a Ti-Li manufactured by a conventional method.
9 is a graph showing DC-drift measurement results of an example of the NbO ₃ optical modulator.

FIG. 2 is a diagram showing a Ti—Li manufactured by a conventional method.
9 is a graph showing DC-drift measurement results of an example of the NbO ₃ optical modulator.

FIG. 3 shows Ti-L produced by the method of the present invention.
9 is a graph showing DC-drift measurement results of another example of the iNbO ₃ optical modulator.

FIG. 4 is a diagram showing Ti—Li manufactured by a conventional method.
9 is a graph showing DC-drift measurement results of still another example of the NbO ₃ optical modulator.

FIG. 5 shows Ti-L manufactured by the method of the present invention.
9 is a graph showing DC-drift measurement results of still another example of the iNbO ₃ optical modulator.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideki Honda 585 Tomicho, Funabashi-shi, Chiba Sumitomo Cement Co., Ltd.Sumitomo Central Research Institute (72) Inventor Masanobu Kobayashi 585, Tomiyoshi-cho, Funabashi-shi, Chiba Sumitomo Cement Co., Ltd. Within the Optoelectronics Division (72) Inventor Kazumasa Kiuchi 585 Toyotomi-cho, Funabashi-shi, Chiba Sumitomo Cement Co., Ltd. Central Research Laboratory (56) References JP, A) JP-A-4-249215 (JP, A) JP-A-3-259204 (JP, A) JP-A-2-196213 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02F 1/00-1/055 505 G02B 6/12-6/14 G02F 1/29-1/313

Claims (3)

(57) [Claims] A step of thermally diffusing titanium on a lithium niobate substrate to form an optical waveguide having a desired shape; a step of depositing and forming a dielectric buffer layer immediately above the optical waveguide; Disposing a titanium heat diffusion step for forming the optical waveguide in an atmosphere of a dry gas containing at least oxygen and having a dew point of 0 ° C. or less, and A method for manufacturing a waveguide-type optical element, comprising: performing a heat treatment in a dry gas atmosphere containing at least oxygen and having a dew point of 0 ° C. or less in a buffer layer. 2. A heat treatment of the dielectric buffer layer is performed for about 60 hours.
The method according to claim 1, which is performed at a temperature of 0 ° C. 3. The method of claim 1, wherein each of the drying gas in the titanium thermal diffusion step and the heat treatment of the dielectric buffer layer has a dew point of −70 ° C. or less.