JPH0843778A - Waveguide type optical device - Google Patents

Abstract

PURPOSE:To suppress a dC-drift by containing an essential component consisting of amorphous silicon oxide and a doping element component contg. lithium and niobium into a dielectric layer and forming this dielectric layer having the refractive index lower than the refractive index of the undoped amorphous silicon oxide. CONSTITUTION:This waveguide type optical device has a substrate formed of a lithiun niobate an optical waveguide formed in a substrate surface part, electrodes arranged apart from the optical waveguide surface and the dielectric layer arranged in the parting parts of the optical waveguide and the electrodes for controlling optical signals propagating in the optical waveguide. The dielectric layer contains the essential component consisting of the amorphous silicon oxide and the doping element component contg. at least the lithium and niobium. The refractive index of the dielectric layer is lower than the refractive index of the undoped amorphous silicon oxide. The total content of the doping element component contained in the dielectric layer is >=10<15>atom/cm<3> and more preferably < 10<20>atom/cm<3>.

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 device. More specifically, the present invention relates to a waveguide type optical device, particularly a modulator element, which has a substrate containing lithium niobate as a main component and an optical waveguide and can suppress dc-drift. Such a waveguide type optical device is useful as one of the key devices of an optical communication system.

[0002]

2. Description of the Related Art In a waveguide type optical element having a lithium niobate substrate, the occurrence of dc-drift has been a problem, and it is known that this generation mechanism can be explained by an RC equivalent circuit model. S. Yamada and M. Minakata, Jpn. J. Appl. Phys., Vo
l. 20 (1981), 733. CM Gee, GD Thurmond, H. Blauvelt and HM Yen,
Appl. Phys. Lett., Vol. 47 (1985), 211 RA Becker, Opt. Lett., Vol. 10 (1985), 417

That is, the direction and the magnitude of the dc-drift are determined by the lithium niobate substrate which constitutes the optical element.
It is known that the electric resistance (R) and the electric capacitance (C) of the dielectric layer formed on the substrate largely depend on the relaxation time τ defined as the product of R and C. Has been.

From the above fact, R of the substrate and the dielectric layer,
By defining C and τ, dc-
It can be seen that the drift can be controlled. The following techniques have already been reported as specific means for defining such material parameters.

By reducing the hydrogen content in the substrate, the magnitude of dc-drift can be suppressed (H. Nagata,
J. Ichikawa, M. Kobayashi, J. Hidaka, H. Honda,
K. Kuchi, and T. Sugamata, Appl. Phys. Lett., Vol.
64 (1994) 1180). Specifically, since hydrogen in the lithium niobate substrate is introduced during the poling treatment of the crystal (substrate), in the steps after this treatment step,
By heat-treating the substrate in a dry atmosphere, the amount of hydrogen can be easily reduced. Although the mechanism of the relationship between the amount of hydrogen in the substrate and the dc-drift is not clear, the R of the low hydrogen substrate is generally larger than the R of the high hydrogen substrate, and this may contribute to the reduction of the dc-drift. Is high.

Silicon oxide is generally used for the dielectric layer, and in the silicon oxide, there are III to VIII groups and I.
A method of reducing dc-drift by adding elements of Group B and Group IIb is known (Japanese Patent Laid-Open No. 257/257).
105). This method seems to correspond to manipulating R, C and τ of the dielectric layer.

It has also been reported that the dc-drift can be reduced by modifying the surface of the lithium niobate substrate by an ion implantation method or the like prior to forming the dielectric layer. Furthermore, it has been reported that the compositional metamorphic layer formed during the process of forming a waveguide on the surface of a lithium niobate substrate is removed by etching to reduce the dc-drift (Mitakata, The Institute of Electronics, Information and Communication Engineers. Poetry, vo
l. J77-C-1 (1994) 194).

[0008]

Although various methods as described above have been reported for reducing dc-drift, a sufficiently satisfactory means has not been established yet. Each of the above-mentioned conventional methods has also been found experimentally, and the numerical correspondence with the RC equivalent circuit model proposed earlier has not been sufficiently taken. As described above, each of the conventional methods is also effective in reducing the dc-drift, and it cannot be said that any of the methods is excellent. However, from the standpoint of the manufacturing process, the simplest possible means is preferable because it is easy to manage. For example, the method of forcibly adding the second and third elements into the dielectric layer causes a problem that process control is required for quantitatively and reproducibly adding these elements.

An object of the present invention is to provide a waveguide type optical device capable of suppressing dc-drift without newly adding special processing steps and control items.

[0010]

A waveguide type optical device of the present invention comprises a substrate made of lithium niobate, an optical waveguide formed on a surface portion of the substrate, and an optical signal propagating through the optical waveguide. An electrode for controlling the optical waveguide, the electrode being spaced apart from the optical waveguide surface, and a dielectric layer disposed in a space between the optical waveguide and the electrode, wherein the dielectric layer is amorphous. It is characterized in that it contains a main component made of silicon oxide and a doped element component containing at least lithium and niobium, and the refractive index of this dielectric layer is lower than that of undoped amorphous silicon oxide. is there.

In the waveguide type optical device of the present invention, the total amount of the doping element components contained in the dielectric layer is 10 ¹⁵ atoms / cm ³ or more, provided that it is less than 10 ²⁰ atoms / cm ^3. preferable.

In the waveguide type optical device of the present invention, the undoped amorphous silicon oxide has a refractive index of 0.63.
It is 1.457 for the light wave of 3 μm, and the refractive property of the dielectric layer is 1.44 for the light wave of the same wavelength as the above.
The following is preferred.

In the waveguide type optical device of the present invention, it is preferable that the derivative layer further contains a compound or ion containing hydrogen.

In the waveguide type optical device of the present invention, the total amount of the hydrogen-containing compound and ions contained in the derivative layer is 10 ²¹ atoms / converted into hydrogen atoms.
It is preferably cm ³ or more.

In the waveguide type optical device of the present invention, at least one of the doping element components in the dielectric layer may be diffused from the lithium niobate substrate into the dielectric layer.

[0016]

ACTION AND EXAMPLES The dc-drift reducing means according to the present invention is not numerically equivalent to the RC equivalent circuit model as in the conventional method, but its effect has been confirmed experimentally.

In the waveguide type optical device of the present invention, lithium niobate is used as the material for forming the substrate, and silicon oxide is used as the material for forming the dielectric layer. Lithium niobate has a large electro-optical effect and is being put into practical use as a material for elements such as high-speed modulators. Silicon oxide has a low dielectric constant and is generally used as a base of a high frequency line.

In the waveguide type optical device of the present invention, consideration is given to R, C and τ of the dielectric (silicon oxide) layer formed on the lithium niobate substrate as a factor that influences the magnitude of the dc-drift. To do. These parameters are greatly affected by impurities in silicon oxide, the density of silicon oxide, and the like. However, the impurity / film density depends on the film forming method. Therefore, the present inventors
The materials of the silicon oxide layers of the two types of devices having greatly different dc-drift behaviors were examined, and the materials necessary for reducing the dc-drift were experimentally obtained. And based on the result, the present invention was completed. These devices have the same processing steps regarding the substrate, for example, the substrate manufacturing conditions, pretreatment, and titanium diffusion waveguide forming conditions, and the vacuum deposition method or the RF-sputtering method is used in the method for forming the silicon oxide layer. It was different in that it was. However, there is no limitation regarding the film forming method. This is because the quality of the obtained film can be changed by changing the film forming conditions. In other words, it is not necessary to specify the film forming method because the film quality close to that obtained by the vacuum deposition film can be obtained by the sputtering method, for example.

FIG. 1 shows dc-drift measured for an optical device (light intensity modulator) sample VE according to the present invention and another optical device (light intensity modulator) sample SP. The temperature is 80 ° C., the applied dc bias is 5 V,
It went on condition of. The sample SP has a silicon oxide layer formed by a sputtering method.
Is formed by a vacuum evaporation method. All samples were heat-treated at 600 ° C. in an oxygen stream after forming the film. The measurement was performed at 80 ° C. for about 80 days,
It can be seen that the drift of the sample VE is stable and small.

FIG. 2 shows the composition of the silicon oxide films of the samples SP and VE of FIG. 1 measured by the secondary ion mass spectrometry (SIM).
The result measured by S) is shown. (A) and (b) are sample SP
(C) and (d) are the results before and after the heat treatment of Sample VE. Both samples contained lithium and niobium on the order of 10 ¹⁵ to 10 ¹⁹ atoms / cm ³ . The contents of lithium and niobium are very small, and it is sufficient that they are diffused from the lithium niobate substrate during film formation and heat treatment, and it is not particularly necessary to forcibly add them. Specifically, 4
The film may be formed using a silicon oxide raw material of N to 5N (a part of Li in the film is an impurity in the raw material, and most of Nb is due to diffusion from the substrate).

Although the relationship between the inclusions in the film and the magnitude of the dc-drift is not clear, the factors relating to the reduction of the dc-drift become clear by defining the refractive index of the film shown in FIG. FIG. 3 shows samples SP and VE.
Shows the wavelength dependence of the refractive index after heat treatment. The measurement was performed with an ellipsometer. With sample SP and VE,
In the sample VE having a clearly different refractive index and a small dc-drift, the refractive index is a value of a typical pure amorphous silicon oxide (for example, n = 1.4 at a wavelength of 0.633 μm).
57), which is sufficiently smaller than, for example, a wavelength of 0.633 μm
And n ≦ 1.44.

The difference in the refractive index depends on the density of the film unless the film has significant defects such as oxygen vacancies.
It can be said that E has a lower density than the sample SP (it is difficult to quantitatively evaluate the density of the thin film, so the refractive index is used as an index here). In connection with this, the amount of hydrogen in the silicon oxide film of the sample VE is larger than that of the sample SP as shown in FIG.
More than the order of 10 ²¹ atom / cm ³ . The measurement was based on the hydrogen forward scattering method (HFS). It is not clear how hydrogen is contained, but it is probably H ₂
It is highly possible that it is incorporated into the void portion in the silicon oxide film in the form of O and OH groups.

It is considered that the density of the silicon oxide layer and the incorporation of H ₂ O.OH groups greatly contribute to the electrical characteristics of the film, especially C, which is a factor that reduces the dc-drift of the sample VE. It's easy to imagine being there.

[0024]

According to the present invention, a waveguide type optical device having a small dc-drift (having a silicon oxide dielectric layer).
Will be provided.

[Brief description of drawings]

FIG. 1 is a graph showing dc-drift measurement results of optical device samples SP (comparative example) and VE (present invention).

FIG. 2 is a graph showing the results of measuring the silicon oxide film composition of the sample of FIG. 1 by secondary ion mass spectrometry (SIMS). In the figure, (a) and (b) show the results before and after the heat treatment of the sample SP (comparative example) (c) and (d).
Shows the results before and after heat treatment of sample VE (invention).

FIG. 3 is a graph showing the wavelength dependence of the refractive index of samples SP and VE after heat treatment.

FIG. 4 is a graph showing the amount of hydrogen in a silicon oxide layer of samples SP and VE (after heat treatment) measured by hydrogen forward scattering method (HFS).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Manabu Yamada 585 Tomicho, Funabashi, Chiba Sumitomo Cement Corporation Central Research Laboratory

Claims (6)

[Claims] 1. A substrate formed of lithium niobate, an optical waveguide formed on a surface portion of the substrate, and an optical waveguide surface for controlling an optical signal propagating through the optical waveguide, the optical waveguide being disposed apart from the optical waveguide surface. Electrode and a dielectric layer disposed in a space between the optical waveguide and the electrode, the dielectric layer containing a main component made of amorphous silicon oxide and at least lithium and niobium. A waveguide type optical device comprising a doped element component and having a refractive index of the dielectric layer lower than that of undoped amorphous silicon oxide. 2. The total amount of the doping element component contained in the dielectric layer is 10 ¹⁵ atoms / cm ³ or more, provided that 10 ²⁰ atoms / cm ³ or more.
The waveguide type optical device according to claim 1, which has an atomic number of less than atom / cm ³ . 3. The refractive index of the undoped amorphous silicon oxide is 1.457 for a light wave having a wavelength of 0.633 μm, and the refractive property of the dielectric layer is 1 for the light wave having the same wavelength as the above. The waveguide type optical device according to claim 1 or 2, which is less than or equal to 0.44. 4. The waveguide type optical device according to claim 1, wherein the derivative layer further contains a compound or ion containing hydrogen. 5. The waveguide type according to claim 4, wherein the total amount of the compound containing hydrogen and the ions contained in the derivative layer is 10 ²¹ atoms / cm ³ or more in terms of hydrogen atoms. Optical device. 6. The waveguide type according to claim 1, wherein at least one of the doping element components in the dielectric layer is diffused from the lithium niobate substrate into the dielectric layer. Optical device.