U.S. patent application number 09/742168 was filed with the patent office on 2001-06-28 for unit pixel of cmos image sensor with capacitor coupled photodiode.
Invention is credited to Han, Jin-Su, Oh, Hoon-Sang.
Application Number | 20010005018 09/742168 |
Document ID | / |
Family ID | 19631164 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005018 |
Kind Code |
A1 |
Oh, Hoon-Sang ; et
al. |
June 28, 2001 |
Unit pixel of CMOS image sensor with capacitor coupled
photodiode
Abstract
A unit pixel in a CMOS image sensor, which enhances a
capacitance of a photodiode to reduce noises and increase the
maximum output signal of the image sensor, is provided. To achieve
this, the CMOS image sensor includes a photodiode aligned with an
edge of an insulating film for separating elements and formed by
doping impurities to a semiconductor layer by an ion implantation;
and a capacitor formed along with interface between the photodiode
and the insulating film on plan and formed by layering a bottom
electrode, a dielectric and an upper electrode contacted with the
photodiode.
Inventors: |
Oh, Hoon-Sang; (Ichon-shi,
KR) ; Han, Jin-Su; (Ichon-shi, KR) |
Correspondence
Address: |
TOWNSEND and TOWNSEND and CREW LLP
8th Floor
Two Embarcadero Center
San Francisco
CA
94111-3834
US
|
Family ID: |
19631164 |
Appl. No.: |
09/742168 |
Filed: |
December 19, 2000 |
Current U.S.
Class: |
257/59 |
Current CPC
Class: |
H01L 27/14601
20130101 |
Class at
Publication: |
257/59 |
International
Class: |
H01L 029/04; H01L
031/036; H01L 031/20; H01L 031/0376 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
KR |
1999-63845 |
Claims
What is claimed is:
1. A complementary metal oxide semiconductor (CMOS) image sensor,
comprising: a photodiode aligned with an edge of a insulating film
for separating elements and formed by doping impurities to a
semiconductor layer by an ion implantation; and a capacitor formed
along with interface between the photodiode and the insulating film
on plan and formed by layering a bottom electrode, a dielectric and
an upper electrode contacted with the photodiode.
2. The CMOS image sensor as recited in claim 1, wherein the bottom
and upper electrodes include a doped poly-silicon,
respectively.
3. The CMOS image sensor as recited in claim 1, wherein the
dielectric includes one of an oxide, an oxide/nitride/oxide (ONO)
film and Ta.sub.2O.sub.5.
4. The CMOS image sensor as recited in claim 1, wherein the
photodiode includes one of a PN junction, a PNP and NPN junctions.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a CMOS image sensor; and,
more particularly, to a unit pixel in a CMOS image sensor with a
capacitor coupled photodiode to enhance its capacitance.
DESCRIPTION OF THE PRIOR ART
[0002] A complementary metal oxide semiconductor(CMOS) image sensor
is a device that converts an optical image to an electrical signal
using a CMOS manufacturing technology, which employs a switching
scheme of MOS transistor for transportation of photo-electric
charge from photodiode to output node as well as detection of
output signal at the output node. In contrast with a charge coupled
device (CCD) image sensor currently available in the marketplace,
the CMOS image sensor has the following merits; its driving scheme
is simplified and various scanning schemes may be implemented; it
allows a signal processing circuit to be integrated on a single
chip thereby minimize products; it employs therein an
interchangeable CMOS technology to be able to lower production
costs and low power consumptions.
[0003] Referring to FIG. 1, there is shown a connection diagram of
a unit pixel in a conventional CMOS image sensor, which has been
proposed by the applicant. In FIG. 1, a unit pixel in CMOS image
sensor includes one photodiode (PD) and four NMOS transistors. The
four transistors include a transfer transistor (TX) for
transferring photo electric charges generated from the photodiode
to a floating sensing node, a reset transistor (RX) for discharging
the charges stored in the floating sensing node to detect
subsequent signals, a drive transistor (DX) acting as a source
follower, and a select transistor (SX) acting as a switching to an
addressing.
[0004] The transfer transistor (TX) and the reset transistor (RX)
are made of an native NMOS transistor having extremely low
threshold voltage nearly equal to zero volt to prevent the
degradation of charge transfer efficiency, which may be incurred by
the loss of electric charges associated with the voltage drop
resulting from the positive threshold voltage of the transistor.
Provided between an output OUT of the unit pixel and a ground GND
is a load transistor VB for biasing. The photodiode PD and a
substrate of the floating sensing node are grounded. In Fig. "CF"
represents a capacitance of the floating sensing node and "CP"
represents a capacitance of a pinned photodiode.
[0005] Referring to FIG. 2, there is shown a sectional view of a
unit pixel in the conventional CMOS image sensor, which has been
proposed by the applicant. In FIG. 2, a p.sup.+-silicon substrate,
a p-epitaxial layer, a p-well, a field oxide film, a gate oxide
film, a gate electrode, an N-diffusion region, P.degree. diffusion
region, N.sup.+-floating diffusion region and an oxide spacer film
are depicted by reference numerals 1 to 10, respectively. Referring
to FIG. 2, the photodiode PD includes a PNP junction structure,
which is formed by the sequential ion implantation of N-type and
P-type dopants, or vice versa, into the P-epitaxial layer 2, the
N.sup.--diffusion region 7 and the P.degree. diffusion region 8,
wherein the capacitance CP of the photodiode PD is formed by the
N.sup.--diffusion region 7 and the capacitance CF of the floating
sensing node is formed by the floating diffusion region FD, i.e., a
portion at which one end of the transfer transistor (TX) and one
end of the reset transistor (RX) are commonly coupled.
[0006] The unit pixel in the image sensor fabricated thus is
operated as follows. First, the reset transistor (RX), the transfer
transistor (TX) and the select transistor (SX) are turned on to
reset the unit pixel. As such, the N.sup.--diffusion region 7
gradually begins to be depleted and is completely depleted. In this
situation, the capacitance CP entails a carrier charging up to a
pinning voltage on the completed depletion, and the capacitance CF
of the floating sensing node entails a carrier charging up to a
supply voltage VDD. Thereafter, the transfer transistor (TX) is
turned off, the select transistor (SX) is turned on and then the
reset transistor (RX) is turned off. In this situation, an output
voltage V1 is generated from the output terminal OUT and stored in
a buffer (not shown). Next, the transfer transistor (TX) is turned
on to move carriers in the capacitance CP which has been changed
according to a light intensity, to the capacitance CF. After that,
an output voltage V2 is generated from the terminal OUT and an
analog data for V1-V2 is converted to a digital data. Thus, one
operation period for the unit pixel is terminated.
[0007] As mentioned above, each unit pixel constituting the CMOS
image sensor includes the photodiode in which electrons are
generated by light incident externally and stored therein, and a
circuit for receiving the electrons stored in the photodiode and
converting the same to electrical output signals (voltages or
currents). In this case, since a maximum output signal is directly
proportional to the number of electrons to be extracted from the
photodiode, the maximum output signal increases with increased
electron acceptability, i.e., an increased capacitance, of the
photodiode. In general, a silicon based photodiode is fabricated
using a PN junction, and a duplex structure such as PNP or NPN
junctions, wherein the electrons containing performance (electron
capacity) of these junctions depend on the doping level of silicon
substrate itself and the concentration profile of the dopants
implanted into the substrate to form the junction. In case the
photodiode does not have sufficient capacitance, a certain portion
of the photo-electrons which cannot be stored in the photodiode may
leak out toward the substrate thereby decrease the maximum output
signal and entails an additional problem such as noise generation
by the leaked electrons.
[0008] Therefore, a sufficient capacitance of the photodiode is of
importance. Unfortunately, however, since the capacitance of the
photodiode strongly depends on the doping profile of the photodiode
which is determined by the implantation conditions of N-type and
P-type dopants, it is very difficult to adjust the capacitance of
the photodiode at its option.
[0009] FIG. 3A is a top view of the conventional unit pixel, which
depicts a substantial size of the photodiode and its status,
wherein only the photodiode and a gate of the transfer transistor
(TX) and the floating sensing node (FD) are schematically shown.
FIG. 3B is a sectional view of the conventional unit pixel as taken
along the line A-A' of FIG. 3A.
SUMMARY OF THE INVENTION
[0010] It is, therefore, a primary object of the present invention
to provide a unit pixel in a CMOS image sensor, which is capable of
enhancing the capacitance of photodiode to reduce noises and
increasing the maximum output signal of the image sensor.
[0011] In accordance with a preferred embodiment of the present
invention, there is provided a CMOS image sensor, comprising: a
photodiode aligned with an edge of an insulating film for
separating elements and formed by doping impurities into a
semiconductor layer by an ion implantation; and a capacitor formed
along with interface between the photodiode and the insulating film
on plan and formed by layering a bottom electrode, a dielectric and
an upper electrode contacted with the photodiode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and other objects and features of the present
invention will become apparent from the following description of
the preferred embodiments given in conjunction with the
accompanying drawings, in which:
[0013] FIG. 1 shows a connection diagram of a unit pixel in a
conventional CMOS image sensor;
[0014] FIG. 2 is a sectional view of a unit pixel in the
conventional CMOS image sensor;
[0015] FIGS. 3A and 3B are a top view and a sectional view of the
conventional unit pixel which depicts a substantial size of the
photodiode and its status; and
[0016] FIGS. 4A and 4B are a top view and a cross sectional view of
a unit pixel in a CMOS image sensor in accordance with a preferred
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] FIGS. 4A and 4B are a top view and a cross sectional view of
a unit pixel in a CMOS image sensor in accordance with a preferred
embodiment of the present invention.
[0018] As is apparent from FIGS. 4A and 4B, in the unit pixel in
the image sensor of the present invention, a capacitor Cap is
coupled with a photodiode.
[0019] Specifically, a silicon substrate is provided with a
insulating film for separating elements (FOX) for defining an
active region and a photodiode with aligned to an edge of the
insulating film (FOX) and an edge of one side in gate of a transfer
transistor (TX).
[0020] A floating diffusion region (FD) is formed on the substrate
at the other side of the gate of the transfer transistor (TX).
[0021] Doping impurities to a semiconductor layer by an ion
implantation forms the photodiode, which may be a PN photodiode, a
PNP or NPN photodiode.
[0022] The capacitor Cap is formed by layering a bottom electrode,
a dielectric and an upper electrode in this order along with
boundary of the insulating film and the photodiode on plan, to
thereby increase the surface area of the capacitor to insure a
capacitance thereof at maximum.
[0023] The bottom electrode of the capacitor is contacted with the
photodiode with an intervened insulating film. The bottom and upper
electrodes are made of doped poly-silicons. A silicon oxide film
may be used as the dielectric, or an insulating film with
oxide/nitride/oxide (ONO) structure or a material with a high
dielectric constant such as Ta.sub.2O.sub.5 may be used as the
dielectric to increase the capacitance still more.
[0024] In the following, the operation of such structure will be
described in brief.
[0025] In case the photodiode generates electrons by light incident
externally, the application of a positive voltage to the upper
electrode of the capacitor allows the electrons to be contained in
the capacitor. In other words, in contrast with the case without an
additional capacitor, the present invention allows the photodiode
to contain additional electrons equal to the capacitance of
additional capacitor. After the electrons have been completely
stored in the capacitor, if the electrons begin to be discharged
therefrom, the present invention removes the bias applied to the
upper electrode to allow the electrons to be extracted by an
external circuit.
[0026] As previously mentioned, in contrast with the prior art
photodiode, the present invention can enhance the electron
containing performance (electron capacity) of the photodiode, to
thereby allow the photodiode to charge a substantial amount of
electrons therein, which, in turn, enlarges an output voltage range
of the image sensor resulting in an improved resolution
thereof.
[0027] Although the preferred embodiments of the invention have
been disclosed for illustrative purposes, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
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