Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1999-08-25
2003-09-30
Vu, Ngoc-Yen (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S316000, C348S320000, C348S298000, C257S222000, C257S232000
Reexamination Certificate
active
06628332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid imaging device incorporating charge coupled devices (CCD) or the like. In particular, the present invention relates to a solid imaging device of an interline transfer type suitable for the imaging of still images.
2. Description of the Related Art
Methods for obtaining high-resolution still images are generally known which employ an interline transfer type solid imaging device (e.g., a CCD) in combination with an optical shutter.
FIG. 5
is a schematic diagram illustrating two adjoining pixels
1
a
and
1
b
and a vertical transfer section
2
of a conventional interline transfer type solid imaging device. Specifically,
FIG. 5
shows chronologically-occurring states ((a) to (e)) of the pixels
1
a
and
1
b
. The dots in
FIG. 5
represent signal charges.
It is assumed that the pixel
1
a
constitutes a (2n−1)
th
(or “odd-numbered ”) photosensitive section and that the pixel
1
b
constitutes a (2n)
th
(or “even-numbered ”) photosensitive section, where n is a natural number. The information in the odd-numbered photosensitive section
1
a
and the information in the even-numbered photosensitive section
1
b
together correspond to one bit in the vertical transfer section
2
.
State (a) represents a state immediately after an optical shutter (not shown) has been shut following a period for obtaining sufficient exposure. It is assumed that the optical shutter remains shut throughout states (a) to (e).
In state (a), the photosensitive sections
1
a
and
1
b
store signal charges. In state (b), the signal charges in the odd-numbered photosensitive section
1
a
are all read into the vertical transfer section
2
. In state (c), the signal charges which have been read into the vertical transfer section
2
are sequentially read to the outside, thereby providing a first field image signal corresponding to the odd-numbered photosensitive section
1
a.
Next, in state (d), the signal charges in the remaining even-numbered photosensitive section
1
b
are all read into the vertical transfer section
2
. In state (e), the signal charges which have been read into the vertical transfer section
2
are sequentially read to the outside, thereby providing a second field image signal corresponding to the even-numbered photosensitive section
1
b.
FIG. 6
illustrates a plane pattern implementing the conventional photosensitive sections
1
a
and
1
b
and the vertical transfer section
2
schematically shown in FIG.
5
.
In
FIG. 6
, broken lines depict lower electrodes
20
; solid lines depict upper electrodes
21
; a hatched portion represents a channel interruption region
5
; and a dotted portion represents a surface channel region
4
. In
FIG. 6
, the component elements which also appear in
FIG. 5
are denoted by the same reference numerals as used therein.
The photosensitive sections
1
a
and
1
b
are formed on a low-concentration P layer. In general, the photosensitive sections
1
a
and
1
b
(PD) are implemented by using an embedded photodiode structure. The surface of the photosensitive sections
1
a
and
1
b
is a high-concentration P
+
layer (FIG.
8
A). The underlying one of the photosensitive sections
1
a
and
1
b
is an N layer (storage layer) for storing signal charges (electrons). The N storage layer is formed on the low-concentration P layer. The structure of the photosensitive sections
1
a
and
1
b
along the depth direction will be described in greater detail below with reference to FIG.
8
B.
In general, the vertical transfer section
2
(VCCD) is implemented as an embedded channel composed essentially of an N surface layer overlying the low-concentration P layer. Clock signals &phgr;
v2
and &phgr;
v4
are applied to the lower electrodes
20
. Clock signals &phgr;
v1
and &phgr;
v3
are applied to the upper electrodes
21
.
FIG. 7
illustrates an operation of the interline transfer type solid imaging device shown in FIG.
6
.
At time t
0
, an optical shutter (not shown) is shut after a period for obtaining sufficient exposure, corresponding to state (a) in FIG.
5
.
During period t
1
, the vertical transfer section
2
is driven at a high speed so as to drain unnecessary charges within the vertical transfer section
2
. At time t
2
, the signal charges in the odd-numbered photosensitive section
1
a
are read into the vertical transfer section
2
, corresponding to state (b) in FIG.
5
.
During period t
4
, the vertical transfer section
2
is driven at a normal speed so that the signal charges within the vertical transfer section
2
are read to the outside to give a first field image signal corresponding to the odd-numbered photosensitive section
1
a
, corresponding to state (c) in FIG.
5
.
Next, at time t
5
, the signal charges in the even-numbered photosensitive section
1
b
are read into the vertical transfer section
2
, corresponding to state (d) in FIG.
5
.
Finally, during period t
7
, the vertical transfer section
2
is driven at a normal speed so that the signal charges within the vertical transfer section
2
are read to the outside to give a second field image signal corresponding to the even-numbered photosensitive section
1
b
, corresponding to state (e) in FIG.
5
.
In recent years, a vertical overflow drain structure is generally adopted for the photosensitive sections
1
a
and
1
b.
FIG. 8A
shows an exemplary vertical overflow drain structure. The vertical overflow drain structure shown in
FIG. 8A
includes a surface potential stabilization layer (high-concentration P
+
layer)
13
, a signal charge storage layer(N layer)
12
, a potential barrier layer (low-concentration P layer)
11
, and a substrate (low-concentration N layer)
10
underlying the potential barrier layer
11
.
FIG. 8B
is a graph illustrating a potential distribution of the vertical overflow drain structure shown in FIG.
8
A.
As shown in
FIG. 8B
, if a large amount of signal charges are generated under excessive light and flow into the signal charge storage layer
12
, a portion which cannot be stored in the layer
12
may overflow into the substrate
10
. This is because the substrate
10
functions as a drain with respect to the signal charges so that the low-concentration P layer
11
forms a potential barrier. As a result, the signal charges will be stored up to a certain saturation value Q
sat
(defined below), past which the signal charges will be drained to the substrate
10
.
The following problems may arise when the above-described vertical overflow drain structure is combined with an optical shutter.
For the sake of illustration, it is assumed that an amount (Q
sat
) of signal charges are stored in the signal charge storage layer
12
at time t
0
in
FIG. 7
(i.e., immediately after the optical shutter has been shut). The amount Q
sat
defines the saturation level, or the upper limit, of the amount of signal charges which can be stored in the signal charge storage layer
12
.
Since no signal charges are generated in the photosensitive sections
1
a
and
1
b
between time to and the next time the shutter is opened, the saturation level Q
sat
of signal charges continuously decreases due to heat emission effects. This relationship between the saturation level Q
sat
and lapse of time can be expressed as follows (C. H. Sequin and M. F. Tompsett, translated by Takeishi and Kayama, “CHARGE TRANSFER DEVICES ”, p.85, Kindai Kagakusha 1978):
Q
(
t
)=
Q
0
−C·kT·In
[1+(
t−t
0
)/&tgr;] eq. 1
In eq. 1, t and t
0
represent points in time; C represents the capacity of the charge storage layer; k represents the Boltzmann constant; T represents absolute temperature; and &tgr; represents a time constant which is determined in accordance with the structure of the charge storage layer.
The following values are illustrative of actual measurements that may be obtained in connection with the relationship shown in eq. 1:
C·kT/Q
0
≈0.05, (
t−t
0
)/&tgr;≈1 to 500 eq. 2
Accordingly, Q(t)/Q
0
≈
Conlin David G.
Edwards & Angell
Sharp Kabushiki Kaisha
Vu Ngoc-Yen
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