Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2002-11-15
2004-12-14
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C257S186000
Reexamination Certificate
active
06831265
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to optical semiconductor devices and more particularly to a high-speed photodetection device used in a high-speed and large-capacity optical fiber telecommunication system having a transmission rate of 10 Gbps or more, or 40 Gbps or more.
FIG. 1
shows a schematic cross-sectional diagram of a conventional avalanche photodiode
20
of back-illuminated type receiving an incident optical beam at a substrate and designed for flip-chip mounting.
FIG. 2
, on the other hand, is an enlarged cross-sectional diagram showing the circled region with an enlarged scale.
Referring to
FIGS. 1 and 2
, the avalanche photodiode
20
is constructed on an n-type InP substrate
1
and includes an n-type InP buffer layer
2
formed on the substrate
1
, an InGaAs optical absorption layer
3
of low doping concentration formed on the buffer layer
2
, and an n-type InP layer
4
is formed on the InGaAs optical absorption layer
3
with an intervening n-type InGaAsP graded layer
13
interposed therebetween, wherein the graded layer
13
fills the energy band discontinuity between the InGaAs layer
3
and the InP layer
4
.
On the InP avalanche layer
4
, there is provided an insulation film such as an SiN film not illustrated, and an ion implantation process of a p-type impurity element is conducted into the n-type InP layer
4
selectively through a ring-shaped window formed in such an insulation film. As a result, there is formed a guard ring
14
in the n-type InP layer
4
.
Further, the SiN film is removed and another SiN film
5
-
1
shown in
FIG. 2
is formed, and a p-type impurity element is introduced into the n-type InP layer
4
through a window formed in the SiN film
5
-
1
. Thereby, there is formed a p-type InP region
6
inside the n-type InP layer
4
in a manner surrounded by the guard ring
14
, and is also formed a multiplication region
4
-
1
under the p-type InP region in the n-type InP layer.
A detailed description of a p-side electrode will be made from hereon with reference to
FIGS. 1 and 2
.
Referring to
FIGS. 1 and 2
, there is formed a p-side ohmic electrode
7
of an Au/Zn alloy on the InP region
6
, wherein the p-side electrode
7
has a peripheral part covered by another SiN film
5
-
2
provided so as to cover a ring-shaped region, in which the InP layer
4
is exposed between the SiN film
5
-
1
and the p-side electrode
7
.
Further, the p-side ohmic electrode
7
is covered with a barrier metal layer
9
having a Ti/Pt laminated structure such that the barrier metal layer
9
makes a contact with the p-side ohmic electrode at the contact window formed in the SiN film
5
-
2
so as to expose the p-side ohmic electrode
7
.
The barrier metal layer
9
carries thereon an Au pillar
10
, and the Au pillar
10
carries thereon a solder bump
11
. Thereby, the barrier metal layer
9
blocks the diffusion of Au between the p-side ohmic electrode
7
and the Au pillar
10
so as to prevent Au from diffusing to the InP region
6
through the contact window.
Further, as represented in
FIG. 1
, there is formed a groove exposing the n-type InP buffer layer
2
around the guard ring
14
, and an n-type ohmic electrode
8
is provided in contact with the buffer layer
2
such that the n-type ohmic electrode
8
extends along the sidewall of the groove and reaches the surface of the n-type InP layer
4
. Thereby, a barrier metal layer
9
A similar to the barrier metal layer
9
, an Au pillar
10
A similar to the Au pillar
10
and a solder bump
11
A similar to the solder bump
11
are formed on the n-side ohmic electrode
8
on the InP layer
4
. Further, the exposed sidewall of the grove and the n-side ohmic electrode
8
are covered by an insulation film
5
formed of the SiN film
5
-
1
and the SiN film
5
-
2
.
Further, a microlens
15
is formed on the bottom surface of the substrate
1
in the photodetector
20
of
FIG. 1
, wherein it can be seen that an antireflection coating
12
is provided further on the bottom surface of the substrate
1
.
Next, the operation of the photodetector
20
of
FIGS. 1-2
will be explained.
In operation, a reverse bias voltage is applied across the p-side ohmic electrode
7
and the n-side ohmic electrode
8
, and a signal light having a wavelength near 1300 nm or a wavelength of 1450-1650 nm is injected into the bottom side of the substrate
1
.
It should be noted that the InP crystal constituting the substrate
1
is transparent to the incident light of the foregoing wavelength, and the signal light thus injected reaches the optical absorption layer
3
without being absorbed. Thereby, the absorption of the signal light takes place exclusively in the optical absorption layer
3
.
In such a photodetector, the frequency response is determined by the CR time constant given by a product of the device capacitance C and the load resistance R and further by the carrier transit time.
Thus, when an attempt is made to improve the frequency response of the photodetector in view of the fast transmission rate of 10 Gbps or 40 Gbps, it is necessary to reduce the carrier transit time in addition to the CR time constant.
Because the carrier transit time increases in proportion with the thickness of the optical absorption layer
3
, it is necessary to reduce the thickness of the optical absorption layer
3
in order to achieve the improvement of frequency response by reducing the carrier transit time. On the other hand, such a decrease of thickness of the optical absorption layer
3
causes the problem of decreased quantum efficiency because of the incomplete absorption of the incident light by the optical absorption layer
3
. In such a case, the responsitivity of photodetection is degraded.
In this way, frequency response and quantum efficiency (photoresponsitivity) are in the relationship of tradeoff, and it has been difficult to design a high-speed photodetector having an optimum optical absorption layer thickness.
In view of the situation noted above, the inventor of the present invention has conceived a high-speed photodetector
30
having a mirror on the n-type InP layer
4
of
FIG. 1
or
2
so as to reflect back the signal light not absorbed by the optical absorption layer again to the optical absorption layer.
FIG. 3
shows the construction of the photodetector
30
noted above, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. It should be noted that
FIG. 3
is an enlarged cross-sectional view corresponding to the part shown in FIG.
2
.
Referring to
FIG. 3
, an SiN pattern
5
-
2
A is formed on the p-type region
6
in the photodetector
30
as a result patterning of the SiN film
5
-
2
, and a ring-shaped p-side electrode
7
A of the Au/Zn alloy is formed in the ring-shaped opening between the SiN pattern
5
-
2
A and the SiN film
5
-
2
in place of the p-side electrode
7
.
On the p-side ring-shaped electrode
7
A, there is formed a ring-shaped barrier metal pattern
9
A of the Ti/Pt stacked structure so as to cover the ring-shaped p-side ohmic electrode
7
A and so as to cover the peripheral part of the SiN pattern
5
-
2
A as well as the peripheral part of the SiN pattern
5
-
2
along the ring-shaped opening. Further, the Au pillar
10
is provided on the barrier metal pattern
9
A so as to make a contact with the SiN pattern
5
-
2
A at the central opening. The Au pillar
10
thus formed carries thereon the solder bump
11
similarly to the construction of
FIGS. 1 and 2
.
In the photodetector
30
of
FIG. 3
, it should be noted that the signal light injected into the substrate
1
is reflected by a high-reflectivity mirror formed from the SiN pattern
5
-
2
A and the Au pillar
10
and is returned to the optical absorption layer
13
. Thus, it becomes possible to realize sufficient quantum efficiency in the device
30
of
FIG. 3
even in the case the thickness of the optical absorption layer
3
is reduced.
In the photodetector
20
or
30
of
FIGS
Hanawa Ikuo
Yoneda Yoshihiro
Allen Stephone B.
Armstrong Kratz Quintos Hanson & Brooks, LLP
Fujitsu Quantum Devices Limited
Lee Patrick J.
LandOfFree
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