Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-03-24
2002-03-26
Chaudhari, Chandra (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S078000, C438S546000
Reexamination Certificate
active
06362019
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a MOS (metal oxide semiconductor) solid-state imaging device using MOS transistors for reading a signal and to a method of manufacturing the same.
BACKGROUND OF THE INVENTION
Solid-state imaging devices are generally classified into two categories according to the method by which a signal is read: CCD devices, which use CCDs (charged coupled devices) to transfer signal charge and read a signal simultaneously from a plurality of pixels, and MOS devices, which use read-out circuits comprising MOS transistors, formed for each pixel, to read a signal by selecting one pixel after the other.
In recent years, MOS solid-state imaging devices, especially CMOS devices that are produced with a CMOS (complementary MOS) process, have received great attention as image input elements for portable imaging apparatus such as small PC cameras. Because they can be driven with low voltage and low power consumption, and they can be integrated on one chip together with peripheral circuits.
MOS solid-state imaging devices in turn are classified into two categories according to the read-out circuit that is formed for each pixel: passive devices, which directly read the signal charge that accumulates in a photo-receiving portion into an output line, and active devices, which amplify the potential difference that occurs due to the accumulation of the signal charge with an amplifying circuit before giving it out.
FIGS. 9 and 10
are cross-sectional drawings showing structures of pixels in conventional MOS solid-state imaging devices.
FIG. 9
shows a pixel in an active solid-state imaging device. The signal charge is transferred from the photo-receiving portion to a detecting portion. The potential difference occurring at the detecting portion is given out. Each pixel comprises a photo-receiving portion and four transistors: a charge transfer transistor, an amplify transistor, a reset transistor and a select transistor. The charge transfer transistor is a MOS transistor consisting of a photo-receiving portion
73
a
and a detecting portion
74
a
formed in a silicon substrate
70
, an insulating film
71
formed on the silicon substrate, and a gate electrode
72
formed on the insulating film
71
at least between the photo-receiving portion
73
a
and the detecting portion
74
a.
The photo-receiving portion
73
a
corresponds to the source and the detecting portion
74
a
corresponds to the drain of the charge transfer transistor.
FIG. 10
shows a pixel in an active solid-state imaging device giving out the potential difference occurring at the photo-receiving portion. Each pixel comprises a photo-receiving portion and three transistors: an amplify transistor, a reset transistor and a select transistor. The reset transistor is a MOS transistor consisting of a photo-receiving portion
83
a
and a charge drain portion
84
a
formed in a silicon substrate
80
, an insulating film
81
formed on the silicon substrate, and a gate electrode
82
formed on the insulating film
81
at least between the photo-receiving portion
83
a
and the charge drain portion
84
a.
The photo-receiving portion
83
a
corresponds to the source and the charge drain portion
84
a
corresponds to the drain of the reset transistor.
As in regular MOS transistors, the MOS transistors in these MOS solid-state imaging devices have a lightly doped drain (LDD) structure, comprising a diffusion region with a low impurity concentration at the end of the drain region near the gate electrode (referred to as “LDD portion” in the following), to suppress deterioration of the element properties due to a concentration of the electric field near the drain. This LDD structure is also used in MOS transistors taking the photo-receiving portion for the source. For example, in the solid-state imaging device shown in
FIG. 9
, an LDD portion
74
b
is formed at the end of the detecting portion
74
a,
which corresponds to the drain of the charge transfer transistor, near the gate electrode. And in the solid-state imaging device shown in
FIG. 10
, an LDD portion
84
b
is formed at the end of the charge drain portion
84
a,
which corresponds to the drain of the reset transistor, near the gate electrode.
Furthermore, in a conventional MOS solid-state imaging device such as the one shown in
FIG. 9
or
FIG. 10
, a diffusion region with a low impurity concentration (in
FIG. 9
, this is the region
73
b,
and in
FIG. 10
, this is the region
83
b
) is formed at the end of the photo-receiving portion near the gate electrode.
FIG. 11
illustrates a method for manufacturing a conventional solid-state imaging device with LDD structure such as the one shown in FIG.
9
. First, impurity ions are implanted into the silicon substrate
70
, whereon the insulating film
71
and the gate electrode
72
have been formed, to form the photo-receiving portion
73
a,
the LDD portions, and the detecting portion (FIG.
11
(
a
)). However, at this stage the impurity concentration in the detecting portion is low and roughly equal to the impurity concentration in the LDD portions. Then, a silicon oxide film
75
is deposited (FIG.
11
(
b
)). A portion of this silicon oxide film
75
is then removed by plasma etching (FIG.
11
(
c
)). Silicon oxide films
75
a
and
75
b
remain on both sides of the gate electrode. Then, using the remaining silicon oxide films
75
a
and
75
b
as masks, ions are implanted again, to increase the impurity concentration in the photo-receiving portion
73
a
and the detecting portion
74
a
(FIG.
11
(
d
)).
However, in solid-state imaging devices that are manufactured with the method explained above, the quality of the output image can deteriorate due to crystal defects in the photo-receiving portion, appearing as white marks for example.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a solid-state imaging device with superior quality of the output image and a method for manufacturing such a solid-state imaging device.
In order to achieve this object, a solid-state imaging device in accordance with the present invention comprises a plurality of pixels, each pixel comprising a semiconductor substrate of a first conductivity type; a photo-receiving portion of a second conductivity type formed in the semiconductor substrate; a first diffusion region of the second conductivity type formed in the semiconductor substrate; a first insulating film formed on the semiconductor substrate; a gate electrode formed on the first insulating film at least between the photo-receiving portion and the first diffusion region; a read-out circuit, which is electrically connected to one of the photo-receiving portion and the first diffusion region; and a second diffusion region of the second conductivity type formed in the semiconductor substrate, which is adjacent to an end of the first diffusion region near the gate electrode and separate from the photo-receiving portion. An impurity concentration in the photo-receiving portion and an impurity concentration in the second diffusion region are lower than an impurity concentration in the first diffusion region.
In this specification, the term of “impurity concentration” means the concentration of the impurities that give the desired conductivity characteristic to the region of the substrate where the impurities have been implanted.
In a conventional solid-state imaging device, the impurity concentration in the photo-receiving portion is equal to the impurity concentration in the drain region of the transistor having the photo-receiving portion as the source (in
FIG. 9
, this is the detecting portion
74
a
and in
FIG. 10
, this is the charge drain portion
84
a
). Consequently, the photo-receiving portion may be damaged by the ion implantation for achieving such a high impurity concentration, which can deteriorate the quality of the output image.
On the other hand, in a solid-state imaging device according to the present invention, the impurity concentration in the photo-receiving portion is low, so that the damage inflicte
Chaudhari Chandra
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
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