Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1998-06-23
2002-08-27
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C257S232000
Reexamination Certificate
active
06441853
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to an interline charge-coupled device (CCD) image sensor of a simultaneous reading type having transfer electrodes formed by three-layer conductor. The present invention also relates to a method for fabrication thereof.
(b) Description of the Related Art
Recently, video cameras comprising solid-state image sensor are widely used for business or home use. These conventional video cameras have generally adopted an interlace scanning system which scans every other horizontal signal line during a single frame corresponding to a television system (for instance, NTSC system and PAL system). On the other hand, image pick-up cameras for personal computers, developed actively in recent years, use a simultaneous reading scheme which simultaneously reads all the pixels of the image sensor, from the viewpoint of obtaining a high resolution still picture and easiness of output to the displays. This system is called simultaneous reading system, sequential scanning system, non-interlacing scanning system or progressive scanning system. The solid-state image sensor of the simultaneous reading type reads out signal charge from all of the pixels simultaneously and independently. A typical solid-state image sensor of the simultaneous reading type is more specifically described in the followings.
FIG. 1
is a top plan view of an active area of a conventional interline CCD image sensor of the simultaneous reading type. The active area of the image sensor comprises a imaging region
11
, a horizontal CCD
12
, and an output section (or charge detecting section)
13
. Imaging region
11
comprises a plurality of photodiodes
14
arranged in a two-dimensional matrix for accumulating therein signal charge obtained by photoelectric conversion, and a vertical CCD
15
disposed between adjacent columns of photodiodes
14
for transferring the signal charge in the vertical direction. A transfer gate area
16
is also disposed between a column of photodiodes
14
and a corresponding vertical CCD
15
for reading the signal charge from the column of photodiodes
14
to the vertical CCD
15
. The rest of the imaging region
11
is formed as an element isolation region
17
.
In operation, the signal charge, obtained by photoelectric conversion in each photodiode
14
and accumulated therein within a certain period of time, is read out to vertical CCD
15
through transfer gate area
16
. The signal charge read out from each horizontal line of photodiodes toward respective vertical CCDs
15
is transferred therein toward a horizontal CCD
12
in the active area step by step using a driving signal. The signal charge transferred to horizontal CCD
12
is then transferred in the horizontal direction toward output section
13
for detection of a two-dimensional image.
FIG. 2
is a top plane view of one of the pixels in a typical image sensor of the simultaneous reading type, and
FIG. 2A
is a cross-sectional view of the pixel taken along line A—A in FIG.
2
. The pixel comprises photodiode
14
, vertical CCD
15
, transfer gate area
16
and element isolation region
17
. Vertical CCD
15
comprises a channel and four associated vertical transfer electrodes
18
to
21
. At least one of the four vertical transfer electrodes
18
to
21
, for example, vertical transfer electrode
22
has an additional function as a reading electrode for reading the signal charge from photodiode
14
to vertical CCD
15
.
Vertical transfer electrodes
18
to
21
are formed by three layers of polysilicon. The three-layer polysilicon films are consecutively referred to as a first layer polysilicon film, a second layer polysilicon film, and a third layer polysilicon film from the bottom to the top in this text. Vertical transfer electrode
18
formed by the first layer polysilicon film and shown by a dotted line extends in the horizontal direction as viewed in the drawing. Namely, vertical transfer electrode
18
extends in the imaging region across element isolation region
17
which separates adjacent photodiodes in the vertical direction. Vertical transfer electrodes
19
and
20
formed by the second layer polysilicon film extend in the horizontal direction in the imaging region, overlying the side-wall of vertical transfer electrode
18
. Vertical transfer electrode
21
formed by the third layer polysilicon film overlies a portion of the vertical CCD channel not covered by vertical transfer electrodes
18
,
19
and
20
, and extends in the horizontal direction in the imaging region, overlying vertical transfer electrode
18
.
As shown in
FIG. 2A
, a P-well
24
is formed on the main surface of an N-type silicon substrate
23
, and an N-type buried layer
25
is formed thereon. Vertical transfer electrode
18
is formed thereon with an intervention of a first isolation film
26
, vertical transfer electrodes
19
and
20
are formed thereon with an intervention of a second insulation film
26
, and vertical transfer electrode
21
is formed thereon with an intervention of a third isolation film
26
. These vertical electrodes formed by three layer polysilicon partly overlap one another.
FIG. 3
is a longitudinal-sectional view of horizontal CCD
12
taken along the charge transfer direction. In horizontal CCD
12
, P-well
24
is formed on the main surface of N-type silicon substrate
23
, and N-type buried layer
25
constituting a transfer channel and including N
−
regions
27
is formed thereon. A horizontal accumulation electrode
28
made of the first layer polysilicon film and a horizontal barrier electrode
29
made of the second layer polysilicon film are consecutively overlaid thereon with an intervention of insulation films
26
. Horizontal accumulation electrode
28
a
and a corresponding horizontal barrier electrode
29
a
are electrically connected together, and a horizontal accumulation electrode
28
b
and a corresponding horizontal barrier electrode
29
b
are electrically connected together. These electrodes of horizontal CCD
12
are driven by a two-phase driving signal including a pair of horizontal transfer pulse trains &phgr;H
1
and &phgr;H
2
. N
−
-type buried layer
27
underlying horizontal barrier electrodes
29
a
and
29
b
have a lower impurity concentration and a higher electric potential for the signal charge than N-type buried layer
25
underlying horizontal accumulation electrodes
28
a
and
28
b.
FIG. 4
is a top plan view of a boundary between the imaging region
11
and horizontal CCD
15
, showing conventional technique of a connection between the vertical CCD and the horizontal CCD. The structure shown therein is disclosed in, for instance, JP-B-4(1992)-19752. The following description is made based on a exemplified structure wherein a final electrode
30
of vertical CCD is implemented by the first layer polysilicon film or first vertical electrode. Horizontal barrier electrode
29
a
is formed by the second layer polysilicon film to partly overlap final vertical transfer electrode
30
. Although horizontal barrier electrode
29
a
extends toward vertical CCD
15
to overlie the side-wall of final vertical transfer electrode
30
formed on the channel of vertical CCD
15
, adjacent horizontal barrier electrodes
29
a
corresponding to adjacent vertical CCDs
15
are separated from each other in element isolation region
17
. At side-wall
31
of horizontal accumulation electrode
28
a
, as shown in
FIG. 4
, adjacent horizontal barrier electrodes
29
a
and
29
b
receiving different pulses are disposed with a distance of about 1 micrometer therebetween.
FIG. 5
is a perspective view of the vicinity of horizontal accumulation electrode
28
a
, as viewed in the direction of arrow “P” in
FIG. 4
for showing the side-wall
31
of horizontal accumulation electrode
28
a
. The structure of
FIG. 4
is obtained as follows.
A first layer polysilicon film is deposited and patterned for obtaining horizontal accumulation electrodes
28
, followe
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