Multi-phase readout of signal charge to vertical CCD

Details Multi-phase readout of signal charge to vertical CCD Multi-phase readout of signal charge to vertical CCD

1999-07-01

2004-07-06

Christensen, Andrew (Department: 2615)

Television

Camera, system and detail

Solid-state image sensor

C348S317000, C257S242000

Reexamination Certificate

active

06760072

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of driving a solid-state image sensor, and more particularly to a method of reading out signal charges from photoelectric transfer devices to vertical CCDs (charge coupled devices) through signal readers when a pulse is applied to the signal readers,
2. Description of the Related Art
These days, there has been developed a camera to be used for a personal computer in order to input images to the computer. An image sensor incorporated in a conventional camera employing a television system such as NTSC and PAL has been conventionally designed to have an interlace system in which a frame is displayed with two fields. For instance, such an image sensor has been suggested in Japanese Unexamined Patent Publications Nos. 62-154891 and 9-275206.
However, an image sensor with an interlace system is accompanied with a problem of low resolution of images. Hence, an image sensor incorporated in a camera is recently designed to have a non-interlace system in order to enhance resolution of images. In a non-interlace system, signals running through horizontal scanning lines constituting a frame are output in time series. A non-interlace system has an advantage that images can be displayed with ease on a screen such as a personal computer. Hence, an image sensor associated with a non-interlace system, that is, a progressive scan type image sensor has been in demand, and thus, has been researched and developed.
FIG. 1
is a plan view of an interline type charge coupled device (CCD) image sensor associated with a progressive scan system. The illustrated CCD image sensor is comprised of an image sensing region
1
, a horizontal CCD
2
, an output section or a charge detector
3
, a plurality of photodiodes
4
arranged in the image sensing region
1
in a two-dimensional matrix, and a plurality of vertical CCDs
5
each located adjacent to each row of photodiodes.
Each of the photodiodes
4
converts a light into a signal electric charge, and accumulates the thus converted electric charge therein. Each of the vertical CCDs
5
vertically transfers signal electric charges having been transferred from the photodiodes
4
. An electric charge reader
6
positioned between each of the photodiodes
4
and each of the vertical CCDs
5
reads a signal electric charge out of each of the photodiodes
4
into each of the vertical CCDs
5
. The image sensing region
1
except the photodiodes
4
, the vertical CCDs
5
, and each of the electric charge readers
6
defines an insulating region for insulating a photodiode from another photodiode. The image-sensing region
1
except the photodiodes, the vertical CCDs
5
, and the electric charge readers
6
defines a device isolation region
7
.
In operation, a light is converted into an electric charge for a certain period of time in each of the photodiodes
4
, and the thus generated electric charge is accumulated in each of the photodiodes
4
. The electric charges accumulated in the photodiodes
4
are read out into the vertical CCDs
5
through the electric charge readers
6
by applying a certain voltage to the electric charge readers
6
. The electric charges having been read out into the vertical CCDs
5
are transferred towards the horizontal CCD
2
line by line. The electric charges having been transferred to the horizontal CCD
2
are horizontally transferred in the horizontal CCD
2
, and then, detected at the output section
3
as an output voltage.
FIG. 2
is an enlarged view of a part of the image sensing region
1
of the image sensor associated with the progressive scan system and capable of reading out signals in every three vertical pixels. A part of the image sensing region
1
, illustrated in
FIG. 2
, is defined by vertical five pixels x horizontal three pixels.
Each of the vertical CCDs
5
includes four vertical transfer electrodes
8
a,
8
b,
8
c,
and
8
d
for each of the photodiodes
4
. At least one of the vertical transfer electrodes
8
a,
8
b,
8
c,
and
8
d
doubles as a read-out electrode for reading out a signal electric charge from the photodiode
4
to the vertical CCD
5
. For instance, the vertical transfer electrode
8
b
doubles as such a read-out electrode in FIG.
2
.
In the image sensor illustrated in
FIG. 2
, transfer of electric charges in the vertical CCD
5
is carried out by four-phase drive pulses. Specifically, four-phase pulses &phgr;V
1
to &phgr;V
4
are applied to the vertical transfer electrodes
8
a
to
8
d,
respectively, in a four-electrode cycle.
Though the four-phase pulses &phgr;V
1
to &phgr;V
4
are illustrated as applied only to the vertical transfer electrodes
8
a,
8
b,
8
c
and
8
d
in the rightmost vertical CCD
5
for the purpose of simplification, the same phase pulse is applied to the electrodes located in a common line in the vertical CCDs
5
. For instance, the drive pule &phgr;V
1
is applied to all the vertical transfer electrodes
8
situated uppermost in the three vertical CCDs
5
illustrated in FIG.
2
.
In order to make it possible to read out signals at every three pixel lines, drive pulses &phgr;V
3
A, &phgr;V
3
B, and &phgr;V
3
C can be separately applied to the vertical transfer electrode
8
b
acting as a read-out electrode at every three pixel lines.
FIG. 3
is an enlarged plan view of a couple of pixels illustrated in FIG.
2
. As illustrated in
FIG. 3
, a transfer channel of the vertical CCD
5
vertically extends adjacently to the photodiode
4
. As illustrated in
FIG. 4
, each of the vertical transfer electrodes
8
a
to
8
d
to which an electric charge transfer pulse is to be applied is composed of three-layered polysilicon. One of the vertical transfer electrodes
8
a
to
8
d,
specifically, the vertical transfer electrode
8
b
doubles as a read-out electrode for transferring signal electric charges to the vertical CCDs
5
from the photodiodes
4
.
FIG. 4
is a cross-sectional view taken along the line IV—IV in
FIG. 3
, and illustrates a structure of the vertical transfer electrode
8
. As illustrated in
FIG. 4
, the drive pule &phgr;V
1
is applied to a vertical transfer electrode
8
-
1
constituted of a first polysilicon layer, the drive pulse &phgr;V
2
is applied to a vertical transfer electrode
8
-
2
a
constituted of a second polysilicon layer deposited over the first polysilicon layer, and the drive pulse &phgr;V
3
is applied to a vertical transfer electrode
8
-
2
b
constituted of the second polysilicon layer. Since the vertical transfer electrodes
8
-
2
a
and
8
-
2
b
are electrically insulated from each other, it is possible to apply separate pulses to the vertical transfer electrodes
8
-
2
a
and
8
-
2
b,
although they are constituted of the common polysilicon layer. The drive pule &phgr;V
3
is applied to a vertical transfer electrode
8
-
3
constituted of a third polysilicon layer deposited over the second polysilicon layer.
FIG. 5
is a cross-sectional view taken along the line V—V in FIG.
3
. As illustrated in
FIG. 5
, an image sensor is comprised of an n-type substrate
9
, a p-type well layer
10
formed in the substrate
9
, including an n-type photodiode layer
11
accomplishing photoelectric transfer and accumulating generated signal electric charges, n-type vertical CCD buried layers
12
for vertically transferring electric charges, and an electric charge reader
6
(not illustrated) for reading out electric charges from the photodiode layer
11
into the vertical CCD buried layers
12
, p-type vertical CCD well layers
13
located just below the vertical CCD buried layers
12
, a heavily doped p-type impurity layer
14
formed above the photodiode layer
11
and between the photodiode layer
11
and the layers
12
,
13
, an insulating film
15
formed on the p-type well layer
10
, vertical CCD transfer electrodes
8
composed of polysilicon and formed within the insulating film
15
above the vertical CCD buried layers
12
, and a light-impermeable film
16
formed on the insulating film
15
. The light-impermeable film
16
is for

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