JP3207876B2 - Manufacturing method of optical waveguide device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical waveguide device using a lithium niobate crystal having high performance and high stability.

[0002]

2. Description of the Related Art Conventionally, a buffer layer which is indispensable for improving the performance and stability of a lithium niobate optical element used for an optical switch or an optical modulator, that is, SiO ₂ or A
The thin film layer made of l ₂ O _{3 is formed} by a sputtering method, a CVD method,
Techniques such as the electron beam evaporation method have been used, but have the following problems.

That is, for example, in the case of sputtering or CVD processing, the substrate temperature rises due to substrate heating or plasma, and is not suitable for a proton exchange waveguide whose characteristics are degraded by the temperature rise. In addition, there is a problem that impurities are easily mixed by plasma. In addition, in the case of the electron beam evaporation treatment, although it is homogeneous and contains little impurities, the buffer layer has problems such as difficulty in obtaining denseness and mechanical strength of the thin film.

Further, in the above method, a heat treatment at a relatively high temperature (up to 600 ° C. or more) (ie, at least 600 ° C.) is used in order to reduce a DC drift phenomenon caused by oxygen defects in the thin film.
Although an annealing process is necessary, the heat treatment is not possible in the case where an insulating thin film such as SiO ₂ is formed on a proton exchange waveguide which is fatally deteriorated by a rise in temperature.

[0005] In addition, since a waveguide generally has a wide formation surface, it is difficult to obtain a uniform and uniform film over the entire surface. In the case of the sputtering method, the substrate surface temperature increases due to the plasma. In addition, when formed by a normal electron beam evaporation method, a uniform and uniform film with few impurities can be obtained, but in order to obtain mechanical adhesion strength and denseness of the film, heating of the substrate is required. In any case, it is unsuitable for those requiring low-temperature treatment, particularly for forming a proton exchange waveguide.

Further, in the above-mentioned method, as a common problem, an annealing treatment is required to reduce a DC drift caused by oxygen defects in the buffer layer, and the number of steps is increased, resulting in poor productivity. In particular, for a sample whose temperature cannot be raised, for example, a sample in which an SiO ₂ film is formed on a proton exchange waveguide, a heat treatment cannot be performed, so that a buffer layer for high performance and stabilization cannot be formed. There was a problem.

[0007]

SUMMARY OF THE INVENTION The present invention provides an electron beam evaporation method and an ion assist method in order to solve the above-mentioned drawbacks of the conventional method for producing a lithium niobate optical waveguide. An object of the present invention is to provide a method for producing a stable and high-performance optical waveguide by forming a waveguide thin film by using the same together. That is, an object of the present invention is to provide a method for producing a LiNbO ₃ device having a buffer layer which does not require heat treatment (annealing treatment) and solves the problem of insufficient mechanical strength between a DC drift and a thin film. And

[0008]

In order to solve the above-mentioned technical problems, the present invention provides a method for manufacturing an optical waveguide device, comprising the steps of: forming an optical waveguide device on a main surface of a substrate made of LiNbO ₃ crystal; When an insulating layer or a buffer layer is formed by electron beam evaporation, O ₂ and Ar
Wherein the insulating layer or the buffer layer is formed while irradiating ions or molecules of at least one of the above or a mixture thereof. In a preferred aspect of the present invention, the main surface of the substrate comprises a Z-cut surface of the LiNbO ₃ crystal.

A method for producing a titanium-diffused LiNbO ₃ optical waveguide device using a LiNbO ₃ crystal substrate includes a wafer process for forming an optical waveguide and an electrode, and cutting, processing and shaping a chip from the wafer to form an optical fiber. And a mounting process of fixing and bonding.

First, in the optical waveguide forming step, T
An optical waveguide is formed by forming a waveguide pattern of an i film and thermally diffusing it into a substrate. The thickness of the Ti film is
Determine the amount of increase in the refractive index of the optical waveguide, usually about 100 to 10
A value of about 00 is taken. Next, a buffer layer as described below is coated, and then an electrode pattern is formed. Further, as a mounting process, end face processing (cutting, polishing or cleavage) is performed, device characteristics are checked, and then, coupling and mounting of fibers, LDs, and the like are performed.

The diffusion of Ti follows a normal diffusion theory, and the shape of the Ti concentration distribution after diffusion, ie, the refractive index distribution, is determined by the diffusion temperature and time. Generally, when the substrate is heated to 1000 ° C. or higher, Li ₂ O is diffused outside. Therefore, in order to prevent the diffusion, Li ₂ O is diffused in an atmosphere of Ar, O ₂ , air or the like containing water vapor. The concentration of Ti in the substrate after diffusion is very high, about 1%, unlike impurity diffusion in a Si device or the like. The process of Ti diffusion is as follows.
It becomes O ₂ and then reacts with the substrate material to form a compound.

A LiNbO ₃ device using a Z plate is:
In order to utilize the change of the electro-optic constant by applying an electric field in the depth direction, electrodes are provided on the optical waveguide. In this case, if a metal film is directly provided on the waveguide, very large light absorption occurs in the TM mode used as the guided light.
It becomes impossible to insert a transparent insulating film having a smaller refractive index than the waveguide between the electrode and the optical waveguide as a buffer layer. Usually, a SiO ₂ film or an Al ₂ O ₃ film is used,
In order to avoid instability of device operation (for example, due to DC drift), a high-quality film having particularly good insulating properties is required. For example, in the case of a SiO ₂ film, a CVD film is superior to a sputtered film,
Is performed. The required film thickness of the buffer layer is about 1000 ° or more in the case of a SiO ₂ film.

Conventionally, such a buffer layer is formed by:
Heat treatment is required to suppress the DC drift phenomenon caused by oxygen vacancies in the film. However, in the case of a proton exchange waveguide, the temperature at which the waveguide is formed is 200 ° C. to 30 ° C.
Since the temperature is 0 ° C., annealing at a high temperature of 600 ° C. or more cannot be applied.

On the other hand, the method of forming a buffer layer according to the present invention forms a thin film while irradiating ions or molecules such as O ₂ or Ar simultaneously with electron beam evaporation. As a result, a thin film having high adhesion strength can be manufactured without heating the substrate to a high temperature.

According to the present invention, the irradiation of O ₂ or the like also reduces oxygen defects in the thin film, suppresses DC drift, and eliminates the conventional problem of high-temperature annealing of the film after forming the thin film. No longer needed. In addition, since electron beam evaporation is used, it is possible to form a film with extremely high productivity. Furthermore,
It is possible to form a film with extremely high productivity by eliminating the problem of unevenness and uneven film thickness over a wide area of the film, which has been a problem in sputtering and CVD, and the problem of contamination with impurities. Can be.

Further, since a dense film can be formed even at a low temperature, a high-quality buffer layer can be formed even on a sample to which a high temperature (heat treatment) cannot be applied, such as a buffer layer of a proton exchange waveguide. be able to. As a result, the heat treatment (annealing treatment) which has been conventionally required becomes unnecessary, and the buffer layer can be formed even in the proton exchange waveguide element where the formation of an effective buffer layer was impossible. Became.

The method for producing the LiNbO ₃ waveguide device of the present invention is applicable to the production of optical switches, phase modulators and other buffer layers regardless of whether the titanium diffusion method or the proton exchange method is used. This is a commonly used technology.

Next, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[0019]

FIG. 1 is a schematic view of an electron beam evaporation apparatus for carrying out the method of the present invention. As shown in the schematic plan view of FIG. 1A, this apparatus simultaneously uses an electron gun 1 for depositing a deposition material such as SiO ₂ and Al ₂ O ₃ and an electron gun 2 of an ion assist type. And SiO ₂ and Al
_While irradiating ₂ O ₃ with ions or molecules of O ₂ or a mixed gas of Ar and O ₂ , LiN placed on the substrate holding means 8 is irradiated.
By irradiating the surface of the bO ₃ substrate 9, an insulating layer (buffer layer) 23 (see FIG. 2) is formed at a predetermined position. Note that these processes are performed in the vacuum chamber-5. Then, the formed substrate 9 and the formed buffer layer 23 are monitored by the optical monitor-3 and the crystal monitor 4, and controlled by the computer 7 to form a precise buffer layer. FIG. 1B is a schematic plan view showing details of the electron gun. Cathode 11, anode 12, magnet 13,
It has a mesh 14.

FIG. 2 shows an optical waveguide 22 formed by titanium diffusion and proton exchange on a lithium niobate substrate 21 according to the manufacturing method of the present invention, and a SiO ₂ film buffer formed thereon. FIG. 4 is a cross-sectional view showing a structure of an optical modulator in which a layer 23 is formed and an electrode pattern 24 is formed.

The substrate 21 shown in FIG.
_An optical waveguide 22 was formed by thermally diffusing Ti onto the substrate crystal, which was a 3Z-cut crystal. The formed Ti diffusion film 22 has a thickness of 700 °, a width of 8 μm, a diffusion temperature of 1000 ° C., and a diffusion time of 20 hours.

[0022] on top of such a substrate, the SiO _2, O ₂
The buffer layer 13 was formed to a thickness of 8000 ° by electron beam evaporation while irradiating the molecules. At this time,
Drive voltage is 700V and drive current is 400mA
And the beam current was 32 mA. This buffer layer 1
Electrode 14 was formed on 3 to manufacture a light intensity modulator.
This modulator changes the light intensity when a voltage is applied to the electrodes, but does not change under a constant voltage. However, if oxygen defects or impurities are present in the SiO ₂ buffer layer, charge transfer occurs in the buffer layer even under a constant voltage, the effective electric field applied to the device decreases, and the light intensity fluctuates. I do. That is, a so-called DC drift of the buffer layer occurs.

FIG. 3 shows a conventional method for producing SiO _2.
Are graphs showing the results of manufacturing an optical modulator on which an electron beam was deposited and an optical modulator on which an SiO ₂ film was formed while irradiating O ₂ molecules, and measuring and comparing the characteristics of both.
That is, FIG. 3 shows S formed by a normal electron beam evaporation process.
iO ₂ film (A) and its annealed film (B)
And optical response characteristics of an optical modulator (C) formed by forming a SiO ₂ film while irradiating O ₂ with respect to an applied electric signal. In FIG. 3, the vertical axis indicates
The applied voltage is measured at 5 mV intervals, and the time is plotted on the horizontal axis at 20 μsec intervals, and the response light signal to the applied electric signal is measured. That is, when an electric signal having a frequency of 20 kHz is applied, an optical signal that changes in response is measured. However, the measurement was performed by adjusting the bias so that the signal was at the center.

From the results shown in FIG. 3, the light responsiveness of the optical modulator manufactured by the conventional electron beam evaporation shows, as shown in A, the charge in the buffer layer which is considered to be caused by oxygen defects. It can be seen that the electric signal has deteriorated due to the movement. In the case where this was annealed, oxygen vacancies in the buffer layer were reduced, and as shown in B,
It can be seen that there is no deterioration in the signal response characteristics. In addition, SiO
The _second refractive index is O ₂ irradiation, densified, it has increased from 1.45 to 1.47, the conventional web - ha - from the SiO ₂ film was generated when cutting out the chip peeling is It has been found that the lack of the resin has improved the density and the lack of mechanical strength. In addition, it was confirmed that not only oxygen molecules but also Ar and their ions have similar effects.

FIG. 3A shows the optical response characteristics of an optical waveguide device manufactured without performing an annealing process by forming a buffer layer by the conventional method, and FIG. FIG. 3C shows the optical response characteristics of an optical waveguide manufactured by forming a layer and performing an annealing process, and FIG. 3C shows the optical response characteristics of an optical waveguide manufactured by the method for manufacturing a waveguide of the present invention.

[0026]

As described above, the following remarkable technical effects were obtained by the method of manufacturing an optical waveguide of the present invention. First, when the buffer layer is formed, the buffer layer is formed by irradiating O ₂ molecules or O ions, thereby forming a dense lithium niobate-based crystal with improved mechanical strength. A method for fabricating a waveguide is provided.

Second, there has been provided a method for forming a lithium niobate-based waveguide capable of suppressing the DC drift of the buffer layer. Third, the present invention provides a method for manufacturing an optical waveguide which can form a lithium niobate-based waveguide without heat treatment and can improve productivity.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an apparatus for performing a method of manufacturing an optical waveguide according to the present invention.

FIG. 2 shows Li produced by the method for producing an optical waveguide of the present invention.
FIG. ₃ is a cross-sectional view illustrating a structure of an NbO ₃ optical modulator.

FIG. 3 shows an example of Li manufactured by the method of manufacturing an optical waveguide according to the present invention.
FIGS. 4A and 4B are graphs showing results of measurement of an applied electric signal and a response light signal, respectively, for comparison between an NbO ₃ optical modulator and a similar optical modulator manufactured by a conventional manufacturing method. FIGS.

[Explanation of symbols]

Reference Signs List 21 LiNbO ₃ substrate 22 Ti diffusion waveguide 23 Buffer layer 24 Electrode

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Shoichi Shimotsu 585 Tomicho-cho, Funabashi-shi, Chiba Sumitomo Cement Co., Ltd. (72) Inventor Seiichiro Ishihara 585 Toyotomi-cho, Funabashi-shi, Chiba Sumitomo Cement Co., Ltd. Central Research Laboratory (56) References JP-A-2-196213 (JP, A) JP-A-1-244402 ( JP, A) JP-A-3-20457 (JP, A) JP-A-63-166960 (JP, A)

Claims (2)

(57) [Claims] In a method of manufacturing an optical waveguide device, when an insulating layer or a buffer layer is formed by electron beam evaporation on a main surface of a substrate made of LiNbO ₃ crystal constituting the optical waveguide device, _A method for manufacturing an optical waveguide element, comprising forming the insulating layer or the buffer layer while irradiating ions or molecules of at least one of Ar and Ar or a mixture thereof. 2. The method according to claim 1, wherein a main surface of the substrate is a Z-cut surface of the LiNbO ₃ crystal.