Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-02-08
2001-08-28
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C438S031000, C438S039000
Reexamination Certificate
active
06282009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a light modulator for modulating a laser beam and, more particularly, to a high speed light modulator of a kind used in a high speed optical fiber communication system. The present invention also relates to a method of manufacturing such light modulator.
2. Description of the Prior Art
In the high speed optical fiber communication system, a considerable amount of data is transmitted by the use of semiconductor laser beams and optical fibers. In order to cope with this feature, the semiconductor laser beams are required to be modulated at a high speed. With the conventional direct modulation system in which the electrical current injected into a single-mode semiconductor laser is modulated to provide the modulated output laser beam, the change in wavelength resulting from change in density of injected carriers (i.e., wavelength chirping) is so substantial that the conventional direct modulation system cannot be used in high-speed modulation of
10
Gbps or higher.
In view of the foregoing, as an alternative to the direct modulation system, the external modulation system has come to be the cynosure of those concerned, in which a light modulator having a low chirping and disposed externally of a semiconductor laser is utilized to modulate the laser beam while the current injected into the semiconductor laser is fixed. The combined modulator and laser assembly in which a light modulator, a single-mode semiconductor laser and an isolator separating the light modulator and the semiconductor laser from each other are integrated together on a single chip is shown by
60
in FIG.
7
. Since no circuit is required between the modulator and the laser, the combined modulator and laser assembly
60
shown therein has a high practical utility and is extremely important as a key device for optical fiber communication of a large amount of data.
The light modulator will now be described. As shown in
FIG. 8A
, the light modulator
70
includes an InP semiconductor substrate
52
on which a semiconductor mesa layer
56
of a predetermined width containing a light absorption layer
51
and a semiconductor bonding pad layer
55
are formed. The laser beam inputted to the light modulator
70
is modulated by the light absorption layer
51
. More specifically, by applying a voltage to the bonding pad electrode
55
a
, an electric field is applied from an electrode
54
, covering the semiconductor mesa layer
56
, to the light absorbing layer
51
, and by shifting the absorption wavelength of the light absorbing layer
51
, the input laser beam is modulated.
As shown in
FIG. 8B
, a groove
57
is formed between the semiconductor mesa layer
56
and the semiconductor bonding pad layer
55
for separating the semiconductor layers
55
and
56
from other semiconductor layers. The semiconductor mesa layer
56
, the semiconductor bonding pad layer
55
and the groove
57
have their respective surfaces covered by a continuous insulating film
53
. The bonding pad electrode
55
a
and the electrode
54
are formed by a metallic film continuously covering the insulating film
53
while the electrode
54
is held in ohmic contact with the semiconductor mesa layer
56
through an opening in the insulating film
53
.
The conventional method of manufacturing the conventional light modulator is shown in
FIGS. 9A
to
9
C. Referring first to
FIG. 9A
, a predetermined crystalline layer is epitaxially grown on the InP substrate
52
to form the semiconductor mesa layer
56
of the predetermined width including the light absorption layer
51
, the groove
57
and the semiconductor bonding pad layer
55
. Then, as shown in
FIG. 9B
, the insulating film
53
of SiO
2
having a film thickness of about 4000 Å is formed so as to cover the entire surface of the InP substrate
52
. After a window for the ohmic contact has been formed in an upper surface of the semiconductor mesa layer
54
including the light absorption layer
51
, the metallic film is formed at a predetermined location as shown in
FIG. 9C
to complete the bonding pad electrode
55
a
and the electrode
54
.
In order for the light modulator to be used for high-speed modulation, it is necessary to reduce the static capacitance (hereinafter referred to as a “parasitic static capacitance”) between surface electrodes (the bonding pad electrode
55
a
and the electrode
54
) and a rear surface electrode. The parasitic static capacitance of the light modulator is expressed by the sum of the parasitic static capacitance of the mesa layer
56
plus the parasitic static capacitance of the bonding pad layer
55
. In order to reduce the parasitic static capacitance of the light modulator, attempts have currently been made to minimize the surface area of each of the mesa layer
54
and the bonding pad layer
55
by forming the groove
57
therebetween.
It has, however, been found that considering the chirping of light that is propagated by the light absorption layer
51
, the width of the mesa layer
56
can only be reduced to a certain limited dimension. Also, considering the bonding surface area of the bonding wire, the size of the bonding pad layer
55
is limited to about 50×50 &mgr;m. Thus, the approach to reduce the surface area of the mesa layer
56
and the bonding pad layer
55
in an attempt to reduce the parasitic static capacitance is limited and, therefore, a sufficiently high-speed modulation characteristic has been difficult to accomplish.
SUMMARY OF THE INVENTION
The present invention has therefore been developed in view of the foregoing problems and is intended to provide an improved light modulator capable of high-speed light modulation in which the parasitic static capacitance is reduced and also to provide an improved method of manufacturing such light modulator.
The light modulator of the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to substantially eliminate the above discussed problems, and is therefore effective to achieve the high-speed modulation. More specifically, the light modulator of the present invention includes a semiconductor substrate having first and second surfaces opposite to each other with a grounding conductor formed on the second surface thereof. A mesa section of a predetermined width laminated with a semiconductor layer including a light absorption layer and a bonding pad forming section adjacent the mesa section are formed on the semiconductor substrate. An insulating layer continuing from the mesa section to the bonding pad section is formed with an opening defined in a portion of the insulating film above the mesa section, and an electrode contacting an upper surface of the mesa section through the opening and extending to the bonding pad forming section is formed over the insulating layer. Accordance with the present invention, the light modulator is featured in that a portion of the insulating layer the bonding pad forming section has a thickness greater than that of the remaining portion of the insulating layer to reduce the parasitic static capacitance of the bonding pad section.
The portion of the insulating layer immediately above the bonding pad forming section comprises a multi layered structure containing at least insulating films laminated one above other. The remaining portion of the insulating layer comprises a single or multi layered structure containing a insulating films, in which a number of the insulating film is less than that of the bonding pad forming section.
The insulating films are two insulating films, one of the two insulating films is made of SiO
2
and the other is made of SiN.
The upper-layer insulating film of remaining portion of the insulating layer is same as the 2nd upper-layer insulating film of the bonding pad forming section.
The first method of manufacturing the light modulator according to the present invention is such that the parasitic static capacitance of the bonding pad section has been reduced to subs
Kadowaki Tomoko
Tada Hitoshi
Epps Georgia
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Thompson Tim
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