JPH10104559A - Optical waveguide device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device with excellent reliability capable of suppressing a temp. drift with simple constitution by forming a conductive layer with specific resistance smaller than a specified value on a surface of a pyroelectric substrate and electrically connecting the conductive layer to an electrode layer. SOLUTION: A light modulator S1 consists of the pyroelectric substrate 1, a dielectric layer 3, the conductive layer 5 and electrodes (GND electrode 10a, signal electrode 10b, GND electrode 10c). The pyroelectric substrate 1 on whose one main surface an optical waveguide 2 is formed, and the dielectric layer 3 is formed on one area containing this optical waveguide 2. The conductive layer 5 is formed on a belt like groove 9 formed on the dielectric layer 3 along the optical waveguide 2, and whose specific resistance is smaller than 10<6> Ω.cm. The conductive layer 5 is connected to the electrode layer electrically. This conductive layer 5 forms the groove 9 arriving at the surface of the pyroelectric substrate 1 between the signal electrode 10b and the GND electrode 10a, and between the signal electrode 10b and the GND electrode 10c to be formed on the pyroelectric substrate 1 surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device such as an optical modulator and an optical switch used in the optical communication field.

[0002]

2. Description of the Related Art With the practical use of next-generation large-capacity optical communication, optical control devices such as high-speed optical modulators and optical switches are required. In particular, lithium niobate (L
A waveguide-type optical control device (hereinafter, referred to as an optical waveguide device) using the electro-optic effect of a ferroelectric substrate made of iNbO ₃ ) or the like is very promising because of its low insertion loss and high-speed operation. I have.

Conventionally, in an optical waveguide device using a ferroelectric substrate of this type, Z-cut has been mainly employed as a substrate orientation in which an electric field acts most effectively on an optical waveguide formed on the surface of the substrate.

However, since the ferroelectric material has pyroelectricity, static electricity is generated on the c-plane, which is the direction of polarization, that is, the (001) plane of the substrate due to a temperature change, and the characteristics change over time. there were. That is, in a portion where a plurality of electrodes for applying an electric field to the optical waveguide cover the substrate surface, a charge corresponding to the polarization of the substrate is induced, whereas in a portion where the electrode does not cover the substrate surface, a portion of the substrate is covered. The charge corresponding to the polarization is not easily supplied to the substrate surface.
For this reason, when the polarization of the substrate changes due to a temperature change, non-uniformity of charge occurs in a portion where the electrode does not cover the surface of the substrate, and an electric field due to such non-uniform charge is applied to the optical waveguide, thereby causing a temperature drift. (For example,
See IEICE Technical Report, OQE86-44, p115-121, Sawaki et al.).

Therefore, as a measure for solving this problem,
The following are proposed: FIG. 5 is a cross-sectional view of the optical waveguide device J1 cut in a direction orthogonal to the incident direction of the optical waveguide 52 formed on the surface of the pyroelectric substrate 51. It has a structure in which a layer 53, a semiconductive film 54, and an electrode 55 are sequentially laminated. Here, the portion where the electrode 55 does not cover the surface of the substrate 51 is covered with a semiconductive film 54 such as ITO having a certain degree of conductivity and having a resistivity sufficient to apply an electric field between the electrodes. By doing so, the surface charge of the substrate 51 generated by the temperature change is made uniform (for example, see Japanese Patent Publication No. 5-78016).

[0006]

However, although such a semiconductive film has a resistance value for preventing substantial conduction, it connects between electrodes to which a voltage is applied by a conductor, so that a direct current to a low frequency is applied. There has been a problem that voltage is not applied effectively when voltage is applied. In addition, it is necessary to control inconsistent conditions of providing resistance for preventing conduction and conduction for quickly uniformizing the surface charge of the substrate, so that it is strict to control the resistance value during fabrication, There was a problem that the characteristics fluctuated due to the change. Further, since the semiconductive film is close to the region where the microwave electric field acts, there is a problem that a dielectric loss of the microwave transmission line is caused, and the frequency characteristics of the electrode are deteriorated.

Therefore, the present invention overcomes the above-mentioned problems and has a simple structure and a simple manufacturing process.
Moreover, an excellent optical waveguide device capable of minimizing temperature drift is provided.

[0008]

An optical waveguide device for solving the above problems is an optical waveguide device comprising a dielectric layer and an electrode layer sequentially laminated on an optical waveguide formed on a surface layer of a pyroelectric substrate. And the specific resistance value is 1 on the surface of the pyroelectric substrate.
Together to form 0 ⁶ Ω · cm smaller conductive layer, characterized in that is electrically connected to the conductive layer on the electrode layer.

Further, a part of the conductive layer is substantially in the same plane as the dielectric layer, and may be formed at a predetermined distance from a side end face of the electrode layer. It may be formed on the inner wall of a groove formed in one region of the surface layer of the conductive substrate. In addition, the method for manufacturing the optical waveguide device includes:
A step of forming an optical waveguide on the pyroelectric substrate, a step of simultaneously patterning a dielectric layer and a conductive layer made of a semiconductor or a conductor on the pyroelectric substrate, and sequentially laminating a base electrode and a thick film electrode. And forming a pattern.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment according to the present invention will be described with reference to the drawings for an optical modulator which is one of optical waveguide devices.

First, a schematic configuration of the optical modulator will be described. As shown in FIGS. 1A to 1C, the optical modulator S1
Is formed on a pyroelectric substrate (hereinafter, referred to as a substrate) 1 made of, for example, lithium niobate or lithium tantalate having an optical waveguide 2 formed on one principal surface, and formed on one region of the substrate 1 including the optical waveguide 2. A dielectric layer 3 as a buffer layer made of SiO ₂ , Al ₂ O _3, etc., and a strip-shaped groove 9 formed in the dielectric layer 3 along the optical waveguide 2. (Rich silicon oxide) or Cr, Ti, Ni, Mo, Au, Pt,
Ag, Cu, Al, Nb and alloys containing these elements,
A conductive layer 5 made of an oxide or a conductive polymer and having a specific resistance of less than 10 ⁶ Ω · cm; and a base electrode layer 6 formed on one region of the dielectric layer 3 along the optical waveguide 2. 7
A CPW (coplanar waveguide) electrode 10 which is an electrode layer formed by laminating a plating layer 8 as a thick film electrode layer thereon.
(Ground (hereinafter referred to as GND) electrode 10a, signal electrode 1
0b, GND electrode 10c).

Here, the conductive layer 5 is formed between the signal electrode 10b and the G electrode.
Between the ND electrode 10a and the signal electrode 10b and the GN
A groove reaching the surface of the substrate 1 is formed between the electrode and the D electrode 10c and formed on the substrate surface. Also,
At both ends of the conductive layer 5, the conductive layer 5 is
b, thereby supplying electric charge to the conductive layer 5 and further suppressing the temperature drift. However, when the specific resistance value is 10 ⁶ Ω · cm or more, it is difficult to suppress the temperature drift. Become.

As described above, the effect of dielectric loss on the microwave electric field propagating in the CPW electrode 10 can be reduced by a simple manufacturing process, and the high-frequency transmission characteristics can be improved.

Generally, in a CPW type electrode, microwave
The light is confined and propagated between the ND electrode 10a and the GND electrode 10c. At this time, the conductivity in the vicinity of the electrode directly affects the equivalent tan δ of the medium, which causes a dielectric loss. In the present invention, the dielectric loss can be further reduced by moving the conductive layer 5 for suppressing temperature drift away from the microwave electromagnetic field region as compared with the conventional example. The degree of the effect depends on the electrode design (signal electrode 10
b width: electrode width w, distance g between electrodes, GND electrode 10a,
(Thickness of signal electrode 10b and GND electrode 10c; indicated by electrode layer thickness d and dielectric layer thickness t).
This becomes more remarkable when the thickness is increased or the electrode gap g is designed to be narrow, and the degree of freedom in selecting the material used for the conductive layer 5 can be increased.

In this embodiment, an optical modulator has been described as an optical waveguide device. However, the present invention is not limited to this. An optical waveguide, an electrode, and the like are formed on various pyroelectric substrates such as an optical switch. The present invention can be applied to various optical waveguide devices, and can be appropriately modified and implemented without departing from the gist of the present invention.

[0016]

【Example】

[Embodiment 1] Next, a specific and representative manufacturing method of the optical modulator S1 will be described with reference to the drawings.

As shown in FIG. 2A, a lift-off method is used on a substrate 1 of an optical grade lithium niobate single crystal (Z cut; cut surface, ie, surface is (001) surface) which has been mirror-polished on both sides. After the formation of the Ti thin film pattern 21, the optical waveguide 2 was formed by thermally diffusing the Ti thin film pattern 21 from the Ti thin film pattern 21 to the substrate 1 at about 1050 ° C. as shown in FIG.

Next, as shown in FIG. 2C, a dielectric layer 3 of a SiO ₂ thin film serving as a buffer layer is formed on the substrate 1 by about 2 μm.
An m-thick layer was formed by sputtering, and a heat treatment at about 700 ° C. was performed for the purpose of improving the resistivity. Further, FIG.
As shown in (d), photolithography for etching the dielectric layer 3 on the pattern to be the conductive layer 5 was performed. At this time, the cross-sectional shape of the photoresist pattern 22 was formed into an inversely tapered shape at the pattern edge. And
After etching the SiO ₂ thin film as shown in FIG. 2E, a Ti thin film 23 is vacuum-deposited to a thickness of about 0.02 μm as shown in FIG. The pattern formation of the dielectric layer 3 and the conductive layer 5 was simultaneously performed at a predetermined position.

Next, as shown in FIG. 2G, photolithography for lift-off of the CPW electrode 10 is performed.
After the photoresist 24 is laminated on the conductive layer 5, FIG.
As shown in (h), chromium (Cr) is deposited as the first base electrode layer 6 to a thickness of 0.05 μm and gold (Au) is deposited to a thickness of 0.5 μm as the second base electrode layer 7 in order. FIG.
As shown in (i), by lifting off, CPW
A base electrode pattern of the electrode 10 was formed. Further, a lead portion for making electrical contact between the strip-shaped conductive layer 5 and the electrode layer 10b was formed in the electrode lead portion of the first base electrode layer 6.

Next, as shown in FIG. 2 (j), a photoresist 25 except for the electrode portion is formed by photolithography.
, And as shown in FIG.
A gold plating layer 8 was laminated thereon, and finally a Mach-Zehnder interferometer type light (intensity) modulator S1 was manufactured as shown in FIG.

Next, the result of the temperature drift characteristic of the manufactured optical modulator S1 will be described.

Here, the temperature drift ΔD is defined by ΔD = (ΔV / Vπ) × 100 as shown in FIG. It is considered that the reason why the relaxation time is shorter than in the conventional example is that the electric charge generated by the pyroelectric effect could be rapidly uniformed because the conductive layer 5 was made of Ti, which is a conductor. On the other hand, when the S21 characteristic of the electrode is compared with that of the conventional example, the band is somewhat limited as compared with the conventional example, but a characteristic that is sufficient and has no practical problem is obtained.

In this embodiment, a Ti thin film formed by vacuum deposition on the conductive layer 5 is used. However, the same effect can be obtained by using other film formation methods such as sputtering other than vacuum deposition. can get. The material is not limited to Ti, but may be a semiconductor made of Si or SiOx (silicon-rich silicon oxide) or Cr, Ti, Ni, Mo, Au, Pt, A
Similar effects can be obtained by using g, Cu, Al, Nb, an alloy containing these elements, an oxide, or a conductive polymer. Depending on the electrode design, various materials having an optimum resistivity may be selected from the conductor or the semiconductor material. As a design policy in that case, the thickness of the buffer layer is 1.5
μm or more, electrode width 8 μm or less, resistivity of conductive layer 10 ⁶ Ω
Cm or less, and the thickness and pattern width of the conductive layer are desirably 1/10 or less of the thickness of the buffer layer due to high frequency characteristics, regardless of which film forming method or material is used. Further, the pattern needs to occupy at least half of the electrode interval. If an optical modulator is manufactured by the above-described method, favorable characteristics can be obtained for both temperature drift characteristics and high-frequency characteristics.

Embodiment 2 Next, another embodiment according to the present invention will be described with reference to FIGS. Also in this embodiment, FIG.
As shown in (l), a pyroelectric substrate (hereinafter, referred to as lithium niobate or lithium tantalate) having an optical waveguide 2 and a strip-shaped groove 9 formed along the optical waveguide 2 on one principal surface is formed.
A dielectric layer 3 made of SiO2, Al2 O3 or the like formed on one area including the optical waveguide 2 of the substrate 1 and Si formed on another area.
Or a semiconductor made of SiOx (silicon-rich silicon oxide) or Cr, Ti, Ni, Mo, Au, P
a conductive layer 5 made of t, Ag, Cu, Al, Nb, an alloy, an oxide or a conductive polymer containing these elements and having a specific resistance of less than 10 ⁶ Ω · cm; and one region of the dielectric layer 3 A CPW electrode 10 which is an electrode layer obtained by laminating a thick film electrode layer on a base layer formed on the optical waveguide 2
(GND electrode 10a, signal electrode 10b, GND electrode 10
c).

The conductive layer 5 is formed on the surface of the strip-shaped groove 9 formed along the optical waveguide 2, and is electrically connected to the signal electrode 10b. Thus, electric charges are supplied to the conductive layer 5, the temperature drift is suppressed, and the CPW electrode 1 is supplied.
The effect of dielectric loss on the microwave electric field propagating inside 0 can be reduced as much as possible, and the high-frequency transmission characteristics can be significantly improved.

As described in the first embodiment, the conductive layer 5 formed near the CPW electrode 10 to improve the temperature drift
Is a factor that gives a dielectric loss near the electrodes. This embodiment can further reduce the dielectric loss by separating the conductive layer 5 from the microwave electromagnetic field region as compared with the first embodiment. The effect is electrode design (the width of the signal electrode 10b; the electrode width w, the width of the electrode 11a; the electrode gap g, the electrode 11b
Width; electrode spacing g, GND electrode 10a, signal electrode 10
b, the thickness of the GND electrode 10c; indicated by the electrode layer thickness d, the buffer layer thickness T1, and the depth T2 of the groove 9), whether the buffer layer thickness d is increased or the depth T2 of the groove 9 is increased. Alternatively, when the electrode spacing g is designed to be narrow, it becomes more remarkable, and the degree of freedom in selecting the material used for the conductive layer 5 can be increased.

Next, a specific and typical manufacturing method of the optical modulator S2 will be described. As shown in FIG.
Optical grade lithium niobate single crystal polished on both sides (Z cut;
1) After forming a Ti thin film pattern on the surface 1 of the substrate 1 by a lift-off method, and as shown in FIG. 4B, the Ti thin film pattern is thermally diffused to the substrate 1 at about 1050 ° C. The optical waveguide 2 was formed.

Next, as shown in FIG. 4C, a dielectric layer 3 of an SiO ₂ thin film is laminated on the substrate 1 by sputtering to a thickness of about 2 μm, and the dielectric layer 3 is formed to a thickness of about 700 μm for the purpose of improving the resistivity.
The heat treatment of ° C was performed.

Next, as shown in FIG. 4D, a Ti thin film pattern 26 is formed along the optical waveguide 2 by using a lift-off method as in the method of forming the optical waveguide 2, and the Ti thin film pattern 26 is formed. By using RIE (reactive ion etching) as a mask, as shown in FIG.
A strip-shaped groove 9 was formed in the dielectric layer 3 and the substrate 1. Then, as shown in FIG. 4 (f), after the Ti thin film 26 used for RIE is removed by etching, as shown in FIG. 4 (g), a photoresist is formed on a portion other than the strip-shaped groove 9, and
As shown in FIG. 4H, a Ti thin film pattern was formed on the inner surface of the strip-shaped groove 9 to a thickness of about 0.02 μm by a lift-off method, and the conductive layer 5 was formed. According to this method, the pattern formation of the strip-shaped groove 9, the dielectric layer 3, and the conductive layer 5 can be performed accurately with a simple process.

Next, as shown in FIG. 4I, in order to perform photolithography for lift-off of the CPW electrode 10 on the dielectric layer 3, a photoresist 2
Then, as shown in FIG. 4 (j), chromium (Cr) was used as the first under electrode 6 to a thickness of 0.05 μm,
A second underlayer electrode 7 is formed by sequentially depositing gold (Au) with a thickness of 0.5 μm and lifting off the CPW electrode 10.
Was formed. Further, a lead portion for making electrical contact between the strip-shaped conductive layer 5 and the electrode layer 10b was formed in the electrode lead portion of the base electrode 6.

Next, as shown in FIG. 4K, a portion other than the CPW electrode 10 is covered with a photoresist 30 by photolithography, and a gold plating layer 8 is formed on the underlying electrode layer 7.
And finally a Mach-Zehnder interferometer type light (intensity) modulator was manufactured as shown in FIG. 4 (l).

Also in this embodiment, the relaxation time is shorter than in the conventional example. This is considered to be because the charge generated due to the pyroelectric effect could be rapidly uniformed because Ti, which is a conductor, was used for the conductive layer 5. On the other hand, the electrode S21
When the characteristics are compared with those of the conventional example, the band is somewhat limited compared to the conventional example, but characteristics that are practically satisfactory are obtained. In this embodiment, a Ti thin film formed by vacuum evaporation is used for the conductive layer 5, but the same effect can be obtained by using other film formation methods such as sputtering other than vacuum evaporation. In addition to Ti, the material is Si or Si.
Ox (silicon-rich silicon oxide) semiconductor or Cr, Ti, Ni, Mo, Au, Pt, Ag,
Cu, Al, Nb, alloys containing these elements, oxides,
Alternatively, the same effect can be obtained by using a conductive polymer. Depending on the electrode design, various materials having an optimum resistivity may be selected from the conductor or the semiconductor material. In this case, the thickness of the buffer layer is 1.5 μm as a design policy.
m, electrode width 8 μm or less, resistivity of conductive layer 10 ⁶ Ω ·
cm or less, and the thickness and pattern width of the conductive layer are desirably 1/10 or less of the thickness of the buffer layer in terms of high frequency characteristics, regardless of which film formation method or material is used. Further, the pattern needs to occupy at least half of the electrode interval. If the optical modulator is manufactured by the above method, the temperature drift characteristic,
Good characteristics can be obtained together with high-frequency characteristics.

[0033]

As is evident from the above description, according to the optical waveguide device of the present invention, the signal electrode can be formed with a very simple configuration and a simple manufacturing process even if there is a sudden change in the surrounding temperature. It is possible to prevent an electric field generated between the ground electrodes from acting on the optical waveguide as much as possible, to suppress the temperature drift to a level that causes substantially no problem, and to provide an optical waveguide device with extremely excellent reliability.

[Brief description of the drawings]

1A is a plan view showing an example of an optical waveguide device according to the present invention, FIG. 1B is an enlarged cross-sectional view taken along line bb of FIG. 1A, and FIG. 'Line sectional view.

FIGS. 2A to 2L are cross-sectional views illustrating manufacturing steps of Example 1. FIGS.

FIG. 3 is a diagram illustrating the definition of temperature drift.

FIGS. 4 (a) to (l) are cross-sectional views illustrating manufacturing steps in Example 2. FIGS.

FIG. 5 is a sectional view showing an example of a conventional optical waveguide device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Optical waveguide 3 ... Dielectric layer 4,8 ... Photoresist 5 ... Conductive layer 9 ... Groove S1, S2 ... Optical waveguide device 10 ... CPW electrode (electrode layer)

Claims (1)

[Claims] 1. An optical waveguide device comprising a dielectric layer and an electrode layer sequentially laminated on an optical waveguide formed on a surface layer of a pyroelectric substrate, wherein the surface of the pyroelectric substrate has a specific resistance value. To form a conductive layer smaller than 10 ⁶ Ω · cm,
An optical waveguide device, wherein the conductive layer is electrically connected to the electrode layer.