Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2003-05-16
2004-05-25
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S239000
Reexamination Certificate
active
06740861
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photodetector and more particularly relates to a high-speed photodetector with the capacitance at its pad portion reduced by forming a mesa-shaped light-absorbing layer on part of a semi-insulating semiconductor substrate and an electrode pad an another part thereof, respectively.
A photodetector for use in fiber optics communication, which exhibits a photosensitivity to some incident radiation with a long wavelength ranging from 1.3 &mgr;m to 1.55 &mgr;m, may be implemented typically as a pin photodiode made of InGaAs and InP compound semiconductors. A pin photodiode of this type often has its response speed restricted by a CR time constant, which is a product of the capacitance of the photodiode and a load resistance. Accordingly, to increase the response speed of a pin photodiode, the photodiode needs to have a reduced capacitance.
And to reduce the capacitance of a photodiode, not only the junction capacitance but also the capacitance associated with its electrode pad should be reduced as well. In high-speed photodetectors (or photodiodes) of today, in particular, the photodiode section thereof has a much smaller size. Accordingly, a ring electrode (typically with a diameter of about 35 &mgr;m) formed on the photodiode section is now smaller in size than an electrode pad (typically with a diameter of about 80 &mgr;m) extended from, and disposed near, the ring electrode. For that reason, the capacitance of the electrode pad has a considerable effect on the response speed of the photodiode. In a known structure specially designed to reduce the pad capacitance, a thick insulating film of polyimide is interposed between the electrode pad and a semiconductor layer. However, to further reduce and almost eliminate the pad capacitance, another known structure includes: a mesa-shaped light-absorbing layer on part of a semi-insulating semiconductor substrate; and an electrode pad on another part thereof on which the light-absorbing layer does not exist.
A photodetector with such a structure is disclosed, for example, in Japanese Laid-Open Publication No. 5-82829.
FIG. 9
schematically illustrates the structure of the photodetector disclosed in this publication.
The photodetector
500
shown in
FIG. 9
includes photodiode mesa
505
and pad mesa
506
that are formed on a semi-insulating InP substrate
501
. More specifically, the photodiode mesa
505
includes n
+
-InP, n
−
-InGaAs and n-InP layers
502
,
503
and
504
, which are stacked in this order on part of the substrate
501
. On the other hand, the pad mesa
506
also includes the n
+
-InP, n
−
-InGaAs and n-InP layers
502
,
503
and
504
, which are stacked in this order on another part of the substrate
501
where the photodiode mesa
505
does not exist. A pad electrode
511
is formed on the upper surface of the pad mesa
506
.
The photodiode mesa
505
further includes a p
+
-type doped region
507
that has been formed by heavily doping part of the n-InP layer
504
with a p-type dopant so that the dopant reaches the InGaAs layer
503
as a light-absorbing layer. And an insulating film
510
of SiN has been deposited over the substrate
501
. A p-side electrode
508
is formed on the insulating film
510
and is electrically connected to part of the doped region
507
. An n-side electrode
509
is also formed on the insulating film
510
but is electrically connected to part of the n-InP layer
504
where the doped region
507
does not exist. The p-side electrode
508
on the photodiode mesa
505
is connected to the pad electrode
511
on the pad mesa
506
by way of an interconnect
512
that has been formed on the insulating film
510
.
In the photodetector
500
shown in
FIG. 9
, part of the n
+
-InP layer
502
, which existed between the photodiode and pad mesas
505
and
506
originally, has been removed completely to electrically isolate the photodiode and pad mesas
505
and
506
from each other. The n
+
-InP layer
502
will be herein called a “semiconductor conductive layer”. However, the semiconductor conductive layer
502
and semiconductor substrate
501
are both made of InP, and it is difficult to etch away that part alone as intended. For that reason, in the known photodiode
500
, the semiconductor conductive layer
502
is etched rather deep and the surface of the substrate
501
is also etched away partially to remove that part of the semiconductor conductive layer
502
located between the photodiode and pad mesas
505
and
506
completely. As a result, the photodiode and pad mesas
505
and
506
can be isolated electrically, but the respective heights of the mesas
505
and
506
as measured from the surface of the substrate
501
are higher than the originally designed ones.
The higher the photodiode and pad mesas
505
and
506
, the harder it is to form the interconnect
512
and bridge these mesas
505
and
506
together as designed. This is because where the mesas
505
and
506
are so high, part of the interconnect
512
located around the corner between the photodiode or pad mesa
505
and
506
and the substrate
501
easily peels off. That is to say, to form the interconnect
512
more reliably, the mesas
505
and
506
should preferably have their heights reduced. In the known photodetector
500
, however, the heights of the photodiode and pad mesas
505
and
506
exceed the minimum required ones to completely isolate these mesas
505
and
506
electrically from each other.
In addition, the photodetector
500
shown in
FIG. 9
also has a non-negligibly large interconnect capacitance. In the photodetector
500
, the pad electrode
511
and part of the interconnect
512
on the substrate
501
create no parasitic capacitance. However, another part of the interconnect
512
on the photodiode mesa
505
does create some interconnect capacitance. Where the photodetector
500
should operate at a high speed with the area of the doped region
507
minimized, this interconnect capacitance is non-negligibly large compared to the junction capacitance thereof. Particularly when the insulating film
510
located between the interconnect
512
and the n-InP layer
504
(which will be herein sometimes called a “window layer”) is made of a single SiN layer, the interconnect capacitance increases noticeably. The reason is as follows. Firstly, the SiN layer should be thin enough because cracks would be formed easily otherwise. Also, an SiN film has a higher dielectric constant than that of any other insulating film (e.g., SiO
2
film).
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a photodetector that can be mass-produced easily.
Another object of this invention is to provide a high-performance photodetector with an optical filtering function.
Still another object of the invention is to provide a high-speed photodetector with a reduced interconnect capacitance.
An inventive photodetector includes semi-insulating semiconductor substrate, semiconductor conductive layer, light-absorbing layer, wide bandgap layer and doped region. The conductive layer has been formed on a surface region of the substrate and has electrical conductivity. The light-absorbing layer has been formed on the conductive layer and absorbs light that has been incident on the photodetector. The wide bandgap layer has been formed on the light-absorbing layer and has a bandgap wider than that of the light-absorbing to layer. And the doped region has been defined in the wide bandgap layer by doping part of the wide bandgap layer with a dopant that reaches the light-absorbing layer. In this photodetector, the conductive layer has etch susceptibility different from that of the substrate.
In the photodetector according to a first aspect of the present invention, the conductive layer has etch susceptibility different from that of the substrate. Accordingly, by using an etchant (e.g., an etchant containing hydrochloric acid) that etches the conductive layer selectively with respect to the
Le Que T.
Matsushita Electric Industrial Co. LTD
Nixon & Peabody LLP
Studebaker Donald R.
LandOfFree
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