Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2001-12-21
2003-06-24
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S098000, C257S184000, C257S190000, C385S008000
Reexamination Certificate
active
06583480
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electro-optic element and manufacturing method therefor. In particular, the present invention relates to an electro-optic element which can be suitably used in optical waveguide modulators for optical communications or optical measurement, and to a manufacturing method therefore.
BACKGROUND ART
FIG. 7
is a cross-section showing an example of a conventional optical waveguide modulator.
This optical waveguide modulator comprises a ferroelectric substrate made of lithium niobate (LiNbO
3
) which is most generally and practically used in ferroelectric substrates for optical waveguide modulators.
In
FIG. 7
, reference
10
indicates a ferroelectric substrate comprising an X-cut lithium niobate. When this ferroelectric substrate
10
is cut along the X-axis direction (the crystallographic c-axis), which forms the principal optical axis, the Z-cut ferroelectric substrate
10
exhibits the pyroelectric effect (electric-optic effect). As shown in
FIG. 7
, the axis which exhibits the pyroelectric effect (electric-optic effect) of this ferroelectric substrate
10
is the Z-axis direction (the crystallographic c-axis) which forms the principal optical axis, and as shown in
FIG. 7
, it is a direction which is parallel to the face (in this specification this is referred to as the “main face”) of the ferroelectric substrate
10
in which optical wave guides
2
and
2
are formed.
In the vicinity of the main face of the ferroelectric substrate
10
, the optical waveguides
2
and
2
in which Ti has been thermally diffused are formed. Above that, a buffer layer
3
comprising SiO
2
is formed. In addition, on to the buffer layer
3
, electrodes
4
comprising Au are formed so that they lie parallel to the optical wave guides
2
and
2
. A transition metal layer
5
comprising a transition metal such as Ti, Cr, or Ni is provided between these electrodes
4
and the buffer layer
3
.
To manufacture this type of optical waveguide modulator, a method is used in which, first, optical waveguides
2
and
2
are formed in the main face of the ferroelectric substrate
10
by a thermal diffusion method. And the buffer layer
3
is formed by means of a vacuum deposition method, a sputtering method, or the like on the side of ferroelectric substrate
10
in which the optical waveguides
2
and
2
are formed. Next, a transition metal film and an Au film are successively formed across the entire surface of the buffer layer
3
by means of a vacuum deposition method. In addition, electrodes
4
are formed on this Au film by using an electrolytic plating method onto only the electrode formation regions which are regions on which the electrodes
4
and
4
are formed. Thereafter, the Au film and the transition metal film which remain between the electrodes
4
and
4
are removed by chemical etching, and the transition metal layer
5
is completed.
As described above, in this optical waveguide modulator, the direction of the Z-axis of the ferroelectric substrate
10
is parallel to the main face of the ferroelectric substrate
10
.
In the optical waveguide modulator shown in
FIG. 8
, the direction of the Z-axis of the ferroelectric substrate
11
is orthogonal to the main face of the ferroelectric substrate
11
. In this optical waveguide modulator, as the surrounding temperature changes, the generation of an electric charge due to the pyroelectric effect between the electrodes
4
and
4
is tend to occur. If the charge due to the pyroelectric effect accumulates between the electrodes
4
and
4
, due to a random discharge phenomenon, or the like, the interaction between the electrodes
4
and
4
and the optical waveguides
2
and
2
becomes disturbed, and the modulation conditions of signals of the optical waveguide modulator become noticeably unstable.
Since the direction of the Z-axis of the ferroelectric substrate
10
of the optical waveguide modulator shown in
FIG. 7
is parallel to the main face of the ferroelectric substrate
10
, the charge which is generated by the pyroelectric effect does not substantially accumulate between the electrodes
4
and
4
, and it has the advantage that problems due to the pyroelectric effect do not occur.
However, in this type of optical waveguide modulator, the buffer layer
3
is exposed between the electrodes
4
and
4
. Therefore, there is the problem that the surface
3
a
at which the buffer layer
3
is exposed and the inner part of buffer layer
3
are easily contaminated by contaminants such as K, Ti, and Cr.
In particular, when the density of the buffer layer
3
is lowered by forming the buffer layer
3
using vacuum deposition method in order to regulate the properties of the optical waveguide modulator, it is easy for contaminants to penetrate into the buffer layer
3
through the exposed parts of the buffer layer
3
, and this is a problem.
When the surface
3
a
of the buffer layer
3
and the inner part of the buffer layer
3
of the optical waveguide modulator become contaminated, dc drift may be generated. This dc drift is a phenomenon in which the electric current being applied to the electrodes
4
and
4
leaks through the buffer layer
3
due to the presence of mobile ions such as alkali ions, such as K and Na, and protons, the desired voltage is not applied effectively on the device, and this has a negative impact on the properties of the optical waveguide modulator.
Furthermore, when the contaminants of the buffer layer
3
reach the interface with the ferroelectric substrate
10
due to thermal treatments in the mounting process and the like, due to the contaminants, the chemical bonds of the buffer layer
3
comprising SiO
2
are broken, the bonds which bond the ferroelectric substrate
10
comprising lithium niobate and the buffer layer
3
are reduced, and the problem results that the bonding strength between the two is remarkably weakened.
In addition, as another example of a conventional optical waveguide modulator, there is the optical waveguide modulator described in Japanese Unexamined Patent Application, Application No. Sho 60-214024 shown in FIG.
8
.
In the same way as the optical waveguide modulator shown in
FIG. 7
, this optical waveguide modulator uses a ferroelectric substrate comprising lithium niobate (LiNbO
3
), however, the direction of the Z-axis which exhibits the pyroelectric effect of the ferroelectric substrate is different to that of the optical waveguide modulator shown in FIG.
7
.
In
FIG. 8
, reference
11
indicates a ferroelectric substrate comprising lithium niobate having a Z-cut. The direction of the Z-axis which exhibits the pyroelectric effect of this ferroelectric substrate
11
is orthogonal to the main face of the ferroelectric substrate
11
in which the optical wave guides
2
and
2
are formed.
In the vicinity of the main face of the ferroelectric substrate
11
, optical waveguides
2
and
2
comprising Ti are formed, and above that, a buffer layer
3
comprising SiO
2
is formed. In addition, onto the buffer layer
3
, a semi-conductive film
6
comprising an Si thin film or the like is provided. On this semi-conductive film
6
, electrodes
4
comprising Au are formed so that they lie parallel to the optical waveguides
2
and
2
.
In this type of optical waveguide modulator, the direction of the Z-axis of the ferroelectric substrate
11
is orthogonal to the main face of the ferroelectric substrate
11
. Therefore, when the surrounding temperature changes, an electric charge is readily generated due to the pyroelectric effect between the electrodes
4
and
4
. If the charge due to the pyroelectric effect accumulates between the electrodes
4
and
4
, due to a random discharge phenomenon, or the like, the interaction between the electrodes
4
and
4
and the optical waveguides
2
and
2
becomes disordered, and the modulation conditions of signals of the optical waveguide modulator become noticeably unstable.
In the optical waveguide modulator shown in
FIG. 8
, a semi-conductive film
6
is provided on the buffer layer
3
. Thi
Miyama Yasuyuki
Nagata Hirotoshi
Baker & Botts LLP
Ho Tu-Tu
Nelms David
Sumitomo Osaka Cement Co. Ltd.
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