JP2565002B2 - Signal electrode for optical waveguide device and method of forming the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal electrode for an optical waveguide device and a method for forming the signal electrode. Specifically, when a substrate having an electro-optical effect is used to configure an optical waveguide device such as an optical modulator or an optical switch in a high frequency band, the adhesion of the signal electrode used for the device, for example, the traveling wave signal electrode to the substrate. The present invention relates to an electrode configuration and a method for forming the same that improve the heat dissipation and improve the performance and reliability of the optical waveguide device.

[0002]

2. Description of the Related Art Recent advances in optical fibers and laser light sources
With the development, various systems and devices applying optical technology such as optical communication have been put into practical use and widely used. On the other hand, due to the development of advanced technology, especially the recent demand for high speed optical communication systems. A technique for controlling an optical signal at high speed by using an optical waveguide type device, for example, a high-speed optical modulation technique has been required.

For example, in a fast optical communication system with a transmission rate of up to about 1.6 Gbps, a laser diode (L
D) has been used for direct modulation, but when the modulation frequency becomes higher, the speed and long-distance communication increase due to the temporal minute fluctuations of the modulated light wavelength, so-called chirping phenomenon and optical fiber dispersion characteristics. Limits to.

On the other hand, as a high speed optical modulation method, an external modulation method in which semiconductor laser light is externally modulated is well known. In particular, a Mach-Zehnder external modulator, which is provided with a branch optical waveguide on a substrate having an electro-optical effect and is driven using a signal electrode, for example, a traveling wave signal electrode, is considered to be promising.

FIG. 4 is a diagram showing a basic configuration example of an optical modulator.
The same figure (a) is a plan view and the same figure (b) is a YY sectional view.
In the figure, 1 is a substrate having an electro-optical effect processed into a plane,
For example, LiNbO ₃ or LiTaO ₃ substrates. Reference numeral 2 is an optical waveguide, and branch optical waveguides 2a and 2b are formed in the middle. In this optical waveguide, a metal such as Ti is usually diffused only on the optical waveguide portion on the surface of the substrate so that the refractive index of that portion is slightly larger than the surrounding portions. Reference numeral 3 is a signal electrode, for example, a traveling wave signal electrode, and 4 is a ground electrode. Reference numeral 5 is a buffer layer for reducing the absorption of light into the metal electrode layer on the optical waveguide, and a thin film such as SiO _{2 is} usually used.

The signal electrode 3 and the ground electrode 4 are formed on the optical waveguide via the buffer layer 5 by vapor deposition or plating of metal such as Au. Now, for example, DC light emitted from a semiconductor laser (not shown) enters from the optical waveguide 2 on the left side and is divided into two by the branched optical waveguides 2a and 2b. In the meantime, the signal electrode 3 is supplied with a signal power source for high frequency modulation. When a signal voltage is applied from 6, the branched optical waveguide provided on the substrate
A phase difference occurs between the two lights branched by the electro-optic effect in 2a and 2b. These two lights are combined again, and the modulated optical signal output is extracted from the single optical waveguide 2 on the right side,
A photodetector (not shown) is configured to convert into an electric signal. If a signal voltage is applied so that the phase difference between the two lights in the branched optical waveguides 2a and 2b becomes π or 0, for example, the optical signal output is obtained as an ON-OFF pulse signal. Note that _RT is a terminating resistor.

Usually, in order to pass a high-speed microwave through an electrode for such an optical waveguide device, gold (Au), which has a low electric resistance, is used as the electrode material, and the thickness of the electrode should be made as thick as possible. I have to. Therefore, it is general to apply the plating technique to the formation. An example is shown below.

FIG. 5 is a diagram showing an example of a conventional electrode forming method, showing the main steps in order. In addition,
The same figure (a) is a top view (electrode and waveguide arrangement on the substrate), and the same figure (b) is a YY sectional view of the same figure (a).

Step (1): For example, a substrate 1 made of LiNbO ₃
To the optical waveguide 2 (2a, 2b) of the predetermined size and shape,
After forming by the Ti diffusion method, as the buffer layer 4, for example, a SiO ₂ film having a thickness of 500 nm is formed by sputtering. On top of that, as a metal underlayer 34, for example, a gold (A
u) is vapor-deposited.

Step (2): A resist pattern 7 having a thickness of about 10 μm is formed on a portion of the processing substrate other than the region where the signal electrode 3 and the ground electrode 4 are formed as shown in the figure. Step (3): To the extent that the metal plating layer electrodes 3b, 4b made of, for example, a gold plating layer are aligned with the upper surface of the resist pattern 7 on the metal underlayer 34 on which the resist pattern 7 of the processing substrate is not formed. Is formed as shown in the figure. Such gold plating is formed, for example, by electroplating for 30 minutes at a current density of 3 mA / cm ² using a cyan gold plating solution having a liquid temperature of 65 ° C.

Step (4): The resist pattern 7 on the treated substrate is removed with an appropriate stripping solution. Step (5): The treated substrate is etched in, for example, a mixed aqueous solution of iodine and potassium iodide for about 30 seconds, and Au of the metal underlayer 34 other than the region where the signal electrode 3 and the ground electrode 4 are formed is formed. Is dissolved and removed to form an optical waveguide device, for example, a Mach-Zehnder type optical modulator.

[0012]

However, since the electrode width of the electrode of the optical waveguide device according to the above-mentioned conventional electrode forming method, especially the signal electrode 3, is usually as narrow as 10 μm or less, the metal base layer 34 is formed in the final step of forming the electrode. When etching the
As shown in FIG. 5B in the step (5) in FIG. 5, underetching 34 ′ is generated in the portion of the metal underlayer 34, and the adhesion with the substrate 1 is deteriorated. In addition, there is a serious problem that the peeling of the signal electrode 3 is further accelerated and the reliability of the optical waveguide device is impaired due to the distortion due to the difference in thermal expansion between the substrate and the electrode due to the influence of heat generation due to the signal current. It has occurred and its solution is required.

[0013]

The above-mentioned problems are solved in the signal electrode for the optical waveguide device arranged so as to control the light propagating through the optical waveguide 2 formed on the substrate 1 having the electro-optical effect. This can be solved by a signal electrode for an optical waveguide device in which a protruding metal pattern 30 is formed on at least one side of the signal electrode 3. Specifically, the portion to be the protruding metal pattern 30 is formed on the thin metal underlayer electrode 3a, and the thick metal plating layer electrode 3b is formed on the metal underlying layer electrode 3a excluding the portion to be the protruding metal pattern 30. This can be effectively solved by the signal electrode for the optical waveguide device forming the.

[0014]

According to the present invention, the signal electrode 3 having a narrow width is patterned at the stage of the metal underlayer 34 so that it is not exposed to the etching solution when the unnecessary portion is etched in a later step. Since the protruding metal pattern 30 is formed on at least one side of the narrow signal electrode 3 while preventing the etching, the area of the metal underlayer 34 is increased, the adhesion strength is increased, and the signal current is increased. Since it functions as a heat radiation fin against the heat generation of the signal electrode 3 due to the flow, thermal distortion is also reduced, and the signal electrode 3 is prevented from peeling from the substrate 1.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an embodiment of the present invention, in which FIG. 1 (a) is a plan view, FIG. 1 (b) is a sectional view taken along line XX, and FIG.
Is a Y ₁ -Y ₁ cross-sectional view, and (d) is a Y _2- Y ₂ cross-sectional view. In the figure, 30 is a protruding metal pattern.

The same parts as those described in the above drawings are designated by the same reference numerals, and the description of the same parts is omitted. For example, a branched optical waveguide formed on a substrate 1 in which the surface of a LiNbO ₃ Z plate is mirror-polished.
A signal electrode 3, for example, a traveling wave signal electrode and a ground electrode 4 are formed on the optical waveguide 2 including 2a and 2b via a buffer layer 5 as shown in the figure. Signal electrode 3 and ground electrode 4
Both are composed of thin metal underlayer electrodes 3a, 4a and thick metal plating layer electrodes 3b, 4b.

In this example, a protruding metal pattern 30 is formed on one side of the metal underlayer electrode 3a of the thin signal electrode 3, for example, on the outer side as shown in the figure. Protruding metal pattern
The size and arrangement of 30 are, for example, 100 to 200 nm in thickness,
Width 10 to 20 μm, protruding length 20 to 30 μm, spacing 100 μm
It may be formed to some extent.

Since the protruding metal pattern 30 is formed, the portion has a function of preventing under-etching during processing during the manufacturing process, for example, etching of the metal underlayer, and has a narrow width. When a signal current flows through the signal electrode 3 to generate heat, it functions as a heat radiation fin, and thermal distortion is reduced, and the signal electrode 3 is prevented from peeling off from the substrate 1, which is larger than that of a conventional element structure. Width improves reliability. If the protruding metal pattern 30 has a size as in the above example, it does not affect the characteristics of the optical waveguide device, for example, the optical modulator.

In the above embodiment, the protruding metal pattern is used.
30 is one side of the metal underlayer electrode 3a of the signal electrode 3, that is,
Although it is formed only on the outer side, it goes without saying that the effect is further enhanced if it is formed on both sides, that is, on the inner and outer sides. Further, not only the metal underlayer electrode 3a but also the thick metal plating layer electrode 3b thereon may be continuously formed. Further, it is needless to say that it may be formed on the inner end surface of the ground electrode 4 if necessary.

FIGS. 2 and 3 are views (No. 1) and (No. 2) showing an embodiment of the electrode forming method of the present invention, in which the main steps are shown in order. The figure (a)
Is a top view (arrangement of electrodes and waveguides on the substrate), and FIG. 6B is a YY cross-sectional view at the position shown in FIG.

Step (1): The substrate 1 has, for example, a size of 40
The surface of a Z plate made of LiNbO ₃ having a size of mm × 2 mm and a thickness of 1 mm is mirror-polished and used. After vacuum-depositing Ti to a thickness of about 100 nm on this substrate and performing a general photo-etching method so that Ti remains on the portion corresponding to the optical waveguide 2 including the branched optical waveguides 2a and 2b, about 1050 ^After heating at ⁰ C for 10 hours, Ti is changed to LiNb.
The optical waveguide 2 is formed by thermal diffusion in O ₃ . The length of the branch optical waveguide is adjusted to 25 mm and the width of the optical waveguide is adjusted to 7 μm. The distance between the branch optical waveguides 2a and 2b is about 15 μm.
m, and the angle of the branch portion is formed to be about 2 °. Then
A SiO ₂ film is formed as a buffer layer to a thickness of 500 nm by a vacuum evaporation method. Further, a gold (Au) film having a thickness of 150 nm, for example, is vapor-deposited thereon as a metal underlayer 34.

Step (2): On the metal base layer 34 of the treated substrate,
As shown in the figure, an etch hole 31 is formed by photoetching, leaving a region to be the signal electrode 3 and the ground electrode 4, an outer peripheral portion of the substrate, and a bridge portion 30 'that is a portion that connects them and becomes the protruding metal pattern 30. Form.

When such a bridge portion 30 'does not exist, the signal electrode 3 is thin, and thus patterning causes non-negligible electrical resistance, which causes nonuniformity of the thickness of the plating film. However, by providing the bridge portion 30 ', it is possible to externally supply a current necessary for plating to the signal electrode 3 having a narrow width, and the metal plating layer electrode 3b having a uniform thickness can be obtained.

Step (3): A resist having a thickness of about 10 μm is spin-coated on the treated substrate, for example, and then a thickness of 10 μm is applied to a portion other than the region to be the signal electrode 3 and the ground electrode 4.
A resist pattern 7 of about m is formed as shown.

Step (4): Metal plating layer electrodes 3b and 4b made of, for example, a gold plating layer are formed on the upper surface of the resist pattern 7 on the metal underlayer 34 of the processing substrate on which the resist pattern 7 is not formed. It is formed as illustrated so as to have a thickness that matches. Such gold plating is formed, for example, by electroplating for 30 minutes at a current density of 3 mA / cm ² using a cyan gold plating solution having a liquid temperature of 65 ° C.

Step (5): If the resist pattern 7 on the treated substrate is removed by a suitable stripper, a metal underlayer having bridge portions 31 'and etch holes 31 formed on the outer and inner sides of the signal electrode 3 as shown in the figure. The formation 34, that is, the vapor deposition film of gold (Au) is exposed.

Step (6): On the metal plating layer electrodes 3b, 4b made of the gold plating layer on the above-mentioned treated substrate, for example, slightly wider than the electrode pattern width of the metal plating layer electrodes 3b, 4b, that is, on the side. A resist pattern 8 having a thickness of, for example, about 1 μm is formed in such a width that the end portion is covered with the resist as shown in the drawing.

Step (7): The exposed portion of the metal underlayer 34 made of the gold-plated layer of the treated substrate is etched for about 30 seconds in a mixed aqueous solution of iodine and potassium iodide to form the metal underlayer 34. After removing Au by dissolution, resist pattern 8
Is removed with an appropriate stripping solution, the signal electrode 3 of the present invention,
At least an optical waveguide device in which the protruding metal pattern 30 is formed on one side, for example, a Mach-Zehnder type optical modulator is manufactured.

The optical waveguide type device manufactured in this manner does not cause peeling of the signal electrode 3, and
Since the heat dissipation is good, the operating characteristics are stable and the reliability is improved. In the above embodiments, the case of the Mach-Zehnder type optical modulator has been described as an example, but it goes without saying that the present invention can be applied to various other optical waveguide devices.

Further, the above-mentioned embodiment shows an example, and it is needless to say that other materials such as a material to be used and a detailed process may be appropriately selected and used without departing from the spirit of the present invention.

[0031]

As described above, according to the present invention, since the protruding metal pattern 30 is formed on at least one side of the signal electrode 3 having a narrow width, that portion is the metal underlayer.
When etching 34, under-etching is suppressed, a plating film having a uniform thickness can be obtained, and a heat radiation fin acts against heat generation of the signal electrode 3 due to a signal current, so that thermal strain is reduced. The signal electrode 3 is prevented from peeling off from the substrate 1, and it greatly contributes to improvement of performance / quality and reliability of the optical waveguide device.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram (No. 1) showing an example of an electrode forming method of the present invention.

FIG. 3 is a diagram (No. 2) showing an example of the electrode forming method of the present invention.

FIG. 4 is a diagram showing a basic configuration example of an optical modulator.

FIG. 5 is a diagram showing an example of a conventional electrode forming method.

[Explanation of symbols]

 1 is a substrate, 2 is an optical waveguide, 2a and 2b are branched optical waveguides, 3 is a signal electrode, 4 is a ground electrode, 3a and 4a are metal base layer electrodes, 3b and 4b are metal plating layer electrodes, 5 is a buffer layer, 7 and 8 are resist patterns, 30 is a protruding metal pattern, 30 'is a bridge part 31, etch holes, 34 is a metal underlayer,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kunio Sugada Kunio Sugada 1015 Kamiodanaka, Nakahara-ku, Kawasaki, Kanagawa, Fujitsu Limited (72) Inventor Tadao Nakazawa 1015 Kamedota, Nakahara-ku, Kawasaki, Kanagawa

Claims (2)

(57) [Claims] 1. A signal electrode for an optical waveguide device arranged to control light propagating through an optical waveguide (2) formed on a substrate (1) having an electro-optic effect, wherein the signal electrode (3 (3) A signal electrode for an optical waveguide device, characterized in that a protruding metal pattern (30) is formed on at least one side thereof. 2. The metal underlayer electrode (3a), wherein a portion to be the protruding metal pattern (30) is formed on a thin metal underlayer electrode (3a), and the portion to be the protruding metal pattern (30) is excluded. 2. The method for forming a signal electrode for an optical waveguide device according to claim 1, wherein a thick metal plating layer electrode (3b) is formed on the top.