Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-09-20
2002-04-02
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S291000
Reexamination Certificate
active
06365926
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to CMOS active pixel photodetectors and more particularly to an improved CMOS active pixel photodetector array which includes scavenging diodes between adjacent photodetectors, which prevents cross-talk between adjacent photodetectors, and prevents diffusion of thermally generated electrons from isolation regions into the photodetectors.
BACKGROUND OF THE INVENTION
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use complementary metal oxide semiconductors (CMOS) active pixel sensor (APS) cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1
(from U.S. Pat. No. 5,970,316, incorporated herein by reference) is a schematic diagram that illustrates a conventional CMOS active pixel sensor cell
10
. As shown in
FIG. 1
, the cell
10
includes a photodiode
12
, a reset transistor
14
whose source is connected to photodiode
12
, a source-follower transistor
16
whose gate is connected to photodiode
12
, and a row-select transistor
18
whose drain is connected in series to the source of source-follower transistor
16
.
Operation of active pixel sensor cell
10
is performed in three steps: a reset step, where cell
10
is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
As shown in
FIG. 1
, during the reset step, the gate of reset transistor
14
is briefly pulsed with a reset voltage V
R
(5 volts). The reset voltage V
R
turns on reset transistor
14
which pulls up the voltage on photodiode
12
and the gate of source-follower transistor
16
to an initial reset voltage. The initial reset voltage placed on the gate of source-follower transistor
16
also defines an initial intermediate voltage on the source of source-follower transistor
16
which is one threshold voltage drop less than the initial transfer voltage. Immediately after the gate of reset transistor
14
has been pulsed, the gate of row-select transistor
18
is pulsed with a row-select voltage V
RS
. The row-select voltage V
RS
on the gate of row-select transistor
18
causes the initial intermediate voltage on the source of source-follower transistor
16
to appear on the source of row-select transistor
18
as an initial integration voltage which, in turn, is read out and stored by an imaging system.
During integration, light energy, in the form of photons, strikes photodiode
12
, thereby creating a number of electron-hole pairs. Photodiode
12
is designed to limit recombination between the newly formed electron-hole pairs. As a result, the photo-generated holes are attracted to the ground terminal of photodiode
12
, while the photo-generated electrons are attracted to the positive terminal of photodiode
12
where each additional electron reduces the voltage on photodiode
12
. Thus, at the end of the integration step, the potential on photodiode
12
and the gate of source-follower transistor
16
will have been reduced to a final integration voltage where the amount of the reduction represents the intensity of the received light energy. As above, the final integration voltage on the gate of source-follower transistor
16
defines a final intermediate voltage on the source of source-follower transistor
16
.
Following the image integration period, the final intermediate voltage on the source of source-follower transistor
16
is then read out as a final integration voltage by again pulsing the gate of row-select transistor
18
with the row-select voltage V
RS
. As a result, a collected voltage which represents the total charge collected by cell
10
can be determined by subtracting the final integration voltage from the initial reset voltage.
FIG. 2
is a schematic top view of an active pixel
10
. The cell
10
, is preferably formed upon a p-type substrate. The photodiode
12
preferably comprises a lightly doped n-type region within the p-type substrate. In
FIG. 2
, the gate of the row select transistor
18
is shown as item
23
. Similarly, the gate of the reset transistor
14
is shown as the reset gate
26
. The gate of the source-follower transistor
16
is shown as item
20
. The source (voltage supply, VDD) for the cell is shown as item
21
.
FIGS. 3 and 4
illustrate an array of such cells
10
. More specifically,
FIG. 4
is a top view similar to FIG.
2
and
FIG. 3
is a cross-sectional view along line A-A′. Such an array suffers from the disadvantage of color cross-talk. More specifically, as shown in
FIG. 3
, photoelectrons
30
produced by photons which naturally land between the cells
10
in the isolation region
31
have a tendency to migrate toward adjacent photodetectors
12
. In the case of a color image sensor, where a color filter array pattern is used in conjunction with the pixel array, photons which have passed through a first pixel with a first color can impinge on the isolation region in between the first and second pixels and produce photoelectrons which end up in the photodetector of the second adjacent pixel. Since the signal integrated in the second adjacent pixel is ideally desired to represent the amount of photons passing through a second color filter above that pixel, any photoelectrons that reach the second photodetector that were created by photons passing through the first color filter cause color mixing or color cross-talk. Accurate representation of color image requires minimal color mixing. If substantial color mixing or cross-talk occurs, this will produce a hue shift in the resulting processed color image, and degrade color fidelity.
Color cross-talk or color mixing can also occur if photoelectrons bloom from the first pixel and flow over the isolation region into the second adjacent pixel. This can occur if the incident light of the first pixel is bright enough to completely fill the first photodetector, so that excess photoelectrons are created. These excess photoelectrons can then spill or bloom from the first pixel, traverse the isolation region
31
and be collected in the photodetector of the second adjacent pixel or pixels. This produces photoelectrons generated by photons associated with a first color being collected in a pixel or photodetector associated with a second color.
The direction of this cross-talk or color mixing will depend on the exact physical structure and layout of the pixel. In the case of the pixel array shown in
FIG. 4
, the mixing or cross-talk in the vertical direction is low due to the large space between photodiodes, and the effective drain of photoelectrons through the n+-substrate diode of the voltage supply
21
. To the contrary, cross-talk in the horizontal direction is high due to the lack of a light shield to keep photoelectrons away from the pixel boundaries and the small isolation area/space
31
between adjacent photodiodes.
Isolation regions
31
are also typically areas that have a high thermal generation rate of electrons, typically referred to as dark current. These thermally generated electrons can diffuse or migrate from the isolation regions into adjacent photodetectors. These dark current electrons are then mixed with and read out with the photo-generated electrons. Since the thermally generated electrons are not associated with the incident optical signal, they are referred to as noise electrons, and degrade the signal to noise ratio performance of the pixel and image sensor.
Therefore, there is a need for a structure which removes photo-generated or thermally generated electrons which are created in the isolation area
31
, before they can migrate to the photodetectors
12
.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a camera/imaging system having a co
Eastman Kodak Company
Ngo Ngan V.
Watkins Peyton C.
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