Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-05-25
2002-09-10
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S348000, C257S431000, C257S439000, C257S507000, C257S517000, C257S592000, C257S616000
Reexamination Certificate
active
06448614
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit-incorporating photosensitive device using an SOI (Silicon on Insulator) wafer, and especially to a circuit-incorporating photosensitive device which has high sensibility and low power consumption.
2. Description of the Related Art
A circuit-incorporating photosensitive device is widely used as an optical pickup, optical communication, or a photosensor, e.g., a photocoupler. In recent years, there has been intense demand for the higher sensibility, faster operation, and lower power consumption of circuit-incorporating photosensitive devices in all such applications.
FIG. 8
is a cross-sectional view illustrating the structure of a conventional circuit-incorporating photosensitive device
400
. The conventional circuit-incorporating photosensitive device
400
shown in
FIG. 8
has a laminated structure of a P-type silicon substrate
1
and an N-type silicon substrate
4
epitaxially grown on the P-type silicon substrate
1
. In this laminated structure, a photodiode
270
, and a bipolar transistor
280
which is a circuit for processing signals output from the photodiode
270
are integrally provided. The N-type silicon substrate
4
is separated into plural regions by P-type embedded diffusion layers
13
. The photodiode
270
and the bipolar transistor
280
are respectively provided in the regions separated by the P-type embedded diffusion layers
13
.
The photodiode
270
is of a PN junction type, formed with the laminated structure of the P-type silicon substrate
1
and the N-type silicon substrate
4
.
The bipolar transistor
280
has a P-type diffusion layer
7
formed in the N-type silicon substrate
4
near the surface thereof. An N-type diffusion layer
8
is formed in the P-type diffusion layer
7
. Furthermore, the N-type silicon substrate
4
includes an N-type diffusion layer
6
which extends from the surface of the N-type silicon substrate
4
to an N-type diffusion layer
12
.
An oxide film layer
9
is provided on the entire surface of the N-type silicon substrate
4
. In the bipolar transistor
280
region, the oxide film layer
9
is provided with wiring
10
a
connected to the N-type diffusion layer
6
, wiring
10
b
connected to the P-type diffusion layer
7
, and wiring
10
c
connected to the N-type diffusion layer
8
(which is embedded near the surface of the P-type diffusion layer
7
).
In the circuit-incorporating photosensitive device
400
having such a structure, the photosensitivity of the photosensitive portion of the photodiode
270
depends on the photosensitivity at the PN junction, as well as the amount of the light absorption corresponding to the size and thickness of the photodiode
270
.
In a circuit-incorporating photosensitive device used as an optical pickup, light having a wavelength of about 635 nm for DVD applications, about 780 nm for CD applications, about 850 nm for space optical transmission, or about 950 nm for a photosensor (e.g., a photocoupler) is normally used. The light absorption coefficients of silicon (Si) and light penetration depths into silicon for these wavelengths are shown in Table 1.
TABLE 1
Light
Light
absorptance
absorption
Penetration
in an active
Wavelength
coefficient
depth
layer of 1 &mgr;m
650 nm
2500
cm
−1
4
&mgr;m
22%
780 nm
1200
cm
−1
8.5
&mgr;m
11%
850 nm
800
cm
−1
12.5
&mgr;m
8%
950 nm
400
cm
−1
25
&mgr;m
4%
As shown in Table 1, the depths to which these light wavelengths penetrate into silicon are no less than 4 &mgr;m. Normally, the depths are greater than the thickness of the N-type silicon substrate
4
which forms the circuit-incorporating photosensitive device
400
. Thus, the PN junction between the N-type silicon substrate
4
and the P-type silicon substrate
1
is used to improve the photosensitivity of the photodiode
270
and to improve the absorptance at these light wavelengths.
On the other hand, for faster operation and lower power consumption, it is effective to use a SiGe layer (which has a higher light absorptance) as a base layer, as well as an SOI (Silicon on Insulator) wafer, as shown in, for example, Japanese Laid-Open Publication No. 6-61434.
FIG. 9
is a cross-sectional view illustrating a circuit-incorporating photosensitive device
410
in which an SOI wafer
290
is used. The SOI wafer
290
includes a silicon substrate
1
and an N-type silicon substrate
4
, with an N-type diffusion layer
3
formed on a lower surface thereof and an oxide film
2
interposed therebetween.
The N-type silicon substrate
4
of the SOI wafer
290
is separated into plural regions by trench-type separation layers
5
. A photodiode
270
and a bipolar transistor
280
are respectively provided in the regions separated by the trench-type separation layers
5
. The trench-type separation layers
5
extend from the surface of the N-type silicon substrate
4
, through the N-type diffusion layer
3
, so as to reach the oxide film
2
.
In the photodiode
270
, a P-type diffusion layer
7
a,
which serves as an active layer, is formed near the surface of the N-type silicon substrate
4
. An N-type diffusion layer
6
is provided so as to extend from the surface of the N-type silicon substrate
4
to the N-type diffusion layer
3
.
In an NPN-type bipolar transistor
280
which is a signal processing circuit of the photodiode
270
, a base layer
7
b
formed of SiGe is embedded as a P-type diffusion layer near the surface of the N-type silicon substrate
4
. An N-type diffusion layer
8
is provided near the surface of the base layer
7
b.
Furthermore, in the N-type silicon substrate
4
, an N-type diffusion layer
6
is provided so as to extend from the N-type silicon substrate
4
to the N-type diffusion layer
3
.
An oxide film
9
is provided on the entire surface of the N-type silicon substrate
4
. In the NPN-type bipolar transistor
280
region, the oxide film
9
is provided with an electrode
10
a
connected to the N-type diffusion layer
6
, a base electrode
10
b
connected to the base layer
7
b,
and an electrode
10
c
connected to the N-type diffusion layer
8
(which is embedded near the surface of the base layer
7
b
).
In the circuit-incorporating photosensitive device
410
having such a structure, the thickness of the silicon layer
7
a,
which serves as an active layer forming the photosensitive portion of the photodiode
270
, is normally about 1 &mgr;m, so that there is a problem in that the amount of light absorption is small. Table 1 also shows the light absorptance at different wavelengths in the case where the thickness of the silicon active layer
7
a
is 1 &mgr;m. The light absorptance is 22% for a light wavelength of 650 nm, 11% for a light wavelength of 780 nm, 8% for a light wavelength of 850 nm, and 4% for a light wavelength of 950 nm.
The photodiode
270
has a low photosensitivity because the amount of the light absorption of each of the silicon photosensitive layers
3
a,
4
a,
and
7
a
is small.
The output of the photodiode
270
having such a low photosensitivity could be subjected to gain compensation by the signal processing circuit. However, when the gain of the output is compensated, the response speed of the signal processing circuit and the signal-to-noise ratio (S/N ratio) may decrease.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a circuit-incorporating photosensitive device comprising: an SOI wafer comprising a first silicon substrate, a second silicon substrate, and an oxide film; a photodiode formed in a first region of the SOI wafer; and a signal processing circuit formed in a second region of the SOI wafer, wherein the photodiode comprises a photosensitive layer comprising an SiGe layer.
In one embodiment of the invention, the photosensitive layer is formed after the signal processing circuit is formed.
In one embodiment of the invention, the photosensitive layer is provided in a recess formed in the SOI wafer.
In one embodiment of the invention, the signal processing circuit co
Fukushima Toshihiko
Kubo Masaru
Tani Zenpei
Nixon & Vanderhye P.C .
Sharp Kabushiki Kaisha
Wojciechowicz Edward
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