Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-12-21
2002-12-24
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S292000, C348S308000
Reexamination Certificate
active
06498331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of CMOS imagers, and more particularly to an apparatus and method for achieving uniform low dark current with CMOS photodiodes.
2. Description of the Related Art
Visible imaging systems implemented in CMOS have the potential for significant reductions in cost and power requirements, as compared to traditional CCDs. Due to the advantages offered by CMOS imagers, there has been considerable effort to develop active-pixel sensor (APS) devices. Active-pixel sensors can provide low read noise comparable or superior to scientific grade CCD systems. Prior art active-pixel CMOS imagers, however, suffer from unacceptable levels of “dark current.” Dark current is electronic signal noise present in the pixel outputs that is independent of the image being sensed and that is present even when the pixel array is not being exposed to an image. In other words, dark current is present even when substantially no light is incident upon the image sensing area.
In most CMOS processes, it is generally difficult to reduce the dark current through process mitigation. Even if certain adjustments could be made to the CMOS processes to reduce the dark current for 0.8 &mgr;m or 0.5 &mgr;m design rules, as the designs are migrated to 0.25 &mgr;m or 0.18 &mgr;m CMOS processes, the dark current problems will only increase.
One approach to reduce the dark current is to use a pinned photodiode, as taught by Lee et al. in U.S. Pat. No. 5,625,210, entitled “ACTIVE PIXEL SENSOR INTEGRATED WITH A PINNED PHOTODIODE” issued Apr. 29, 1997 and assigned to the Eastman Kodak Company. The pinned diode approach is undesirable for CMOS applications, however, because the p implant creates a punchthrough barrier. A high bias voltage significantly larger than the standard supply voltage is needed in order to punch through the p device in order to not compromise the dynamic range. Furthermore, without proper process controls, experiments have shown that pinned photodiodes can actually increase the dark current.
Thus, it would be desirable to be able to reduce the dark current associated with CMOS imagers, without relying on process adjustments or pinned photodiodes.
SUMMARY OF THE INVENTION
The present invention is an apparatus and method for achieving uniform low dark currents with CMOS photodiodes. A threshold voltage of a reset FET in an active pixel sensor is set to an appropriate value such that the dark current from a photodiode is actively removed through the reset FET. This reduces the dark current by over 3 orders of magnitude as compared to conventional active pixel sensors, without requiring pinned photodiodes.
According to the present invention, the threshold voltage should be set per the simplified expression:
V
T
Optimum
=
V
x
[
ln
(
μ
W
rst
L
rst
C
ox
V
x
2
2
J
dark
A
det
)
]
-
V
x
where V
x
=nkT/e
n is the ideality of the reset MOSFET (about 1.5 for well-designed mixed-signal CMOS processes), J
dark
is the dark current density for the photodetector, A
det
is the photodetector area, &mgr; is the MOSFET mobility, W
rst
is the width of the reset MOSFET, L
rst
is the length of the reset MOSFET, and C
ox
is the MOSFET capacitance density. For example for 0.5&mgr;m CMOS where we assume a subthreshold ideality of 1.5 along with J
dark
=2 nA/cm
2
, A
det
=8.78×10
−7
cm
2
(28% optical fill factor for 5.6&mgr;m×5.6&mgr;m pixel), electron mobility &mgr;600 cm
2
/V-sec, W
rst
=1.3&mgr;m, L
rst
=m, 1&mgr;m, and C
ox
=4.6×10
−7
F/cm
2
for 90Åoxide thickness, the optimum threshold voltage should be about 0.760V to null the dark current at 295 K.
Thus, by raising the threshold voltage V
t
for the reset transistor from 0.55 to approximately 0.76 volts for a 0.5 &mgr;m process, the typical CMOS photodiode dark current of about 1 nA/cm
2
can be balanced off. This results in a zero net dark current, independent of the integration time. The present invention is similarly applicable to other CMOS processes (0.25 &mgr;m, 0.18 &mgr;m, etc.), and a respective optimized threshold value for these other processes can be determined by using the standard MOSFET electrical parameters associated with each process.
REFERENCES:
patent: 4249122 (1981-02-01), Widlar
patent: 4463383 (1984-07-01), Soneda et al.
patent: 4466018 (1984-08-01), Soneda et al.
patent: 4676761 (1987-06-01), Poujis
patent: 4794247 (1988-12-01), Stineman, Jr.
patent: 5043820 (1991-08-01), Wyles et al.
patent: 5083016 (1992-01-01), Wyles et al.
patent: 5296696 (1994-03-01), Uno
patent: 5345266 (1994-09-01), Denyer
patent: 5471515 (1995-11-01), Fossum et al.
patent: 5541402 (1996-07-01), Ackland et al.
patent: 5576763 (1996-11-01), Ackland et al.
patent: 5587596 (1996-12-01), Chi et al.
patent: 5608243 (1997-03-01), Chi et al.
patent: 5892540 (1999-04-01), Kozlowski et al.
patent: 6242728 (2001-06-01), Merrill et al.
L.J. Kozlowski, et al., “Theoretical basis and experimental confirmation: why a CMOS imager is superior to a CCD,”SPIE, The International Society for Optical Engineering, vol. 3698, pp. 388-396, Apr. 1999.
R.H. Dyck and G.P. Weckler, “Integrated Arrays of Silicon Photodetectors for Image Sensing,”IEEE Transactions on Electron Devices, vol. ED-15, No. 4, pp. 196-201, Apr. 1968.
James D. Plummer and James D. Meindl, “MOS Electronics for a Portable Reading Aid for the Blind,”IEEE Journal of Solid-State Circuits, pp. 111-119, Apr. 1972.
Norio Koike, et al., “MOS Area Sensor: Part I-Design Consideration and Performance of an n-p-n Structure 484×384 Element Color MOS Imager,”IEEE Transactions on Electron Devices, vol. ED-27, No. 8, pp. 1676-1681, Aug. 1980.
Shinya Ohba, et al., “MOS Area Sensor: Part II-Low-Noise MOS Area Sensor with Antiblooming Photodiodes,”IEEE Transactions on Electron Devices, vol. ED-27, No. 8, pp. 1682-1687, Aug. 1980.
EG&G Reticon Sales Catalog,High-Speed Solid State Image Sensor Array, RA2568N, pp. 179-186.
EG&G Reticon Sales Catalog,Solid State Sensor Arrays for Part Nos. RA0100A/RA0128N,pp. 163-170.
EG&G Reticon Sales Catalog,Solid State Image Sensor Array for Part No. RA0256B,pp. 171-178.
Haruhisa Ando, et al., “Design Consideration and Performance of a New MOS Imaging Device,”IEEE Transactions on Electron Devices, vol. ED-32, No. 8, pp. 1484-1489, Aug. 1985.
Nobuyoshi Tanaka et al., “A Novel Bipolar Imaging Device with Self-Noise-Reduction Capability,”IEEE Transactions on Electron Devices, vol. 36, No. 1, pp. 31-38, Jan. 1989.
Weckler, G.P., “Storage Mode Operation of a Phototransistor and its Adaptation to Integrated Arrays for Image Detection,”IEDM, Oct. 1966, (Abstract Only) No known paper available from professional document delivery service.
Nobuyoshi Tanaka, et al., “A Low-Noise Bi-CMOS Linear Image Sensor with Auto-Focusing Function,”IEEE Transactions on Electron Devices, vol. 36, No. 1, pp. 39-45, Jan. 1989.
N. Tanaka, S. Hashimoto, M. Shinohara, S. Sugawa, M. Morishita, S. Matsumora, Y. Nakamura and T. Ohmi, “A 310k Pixel Bipolar Imager (BASIS),”ISSCC, 1989.
Gene P. Wecker, “Charge storage lights the way for solid-state image sensors,”Electronics, pp. 75-78, May 1, 1967.
Gene P. Weckler, “Operation of p-n Junction Photodetectors in a Photon Flux Integrating Mode,”IEEE Journal of Solid-State Circuits, vol. SC-2, No. 3, pp. 65-73, Sep. 1967.
Gene P. Weckler and Rudolph H. Dyck, “Integrated Arrays of Silicon Photodetectors for Image Sensing,”WESCON67 Technical Papers, Western Electronic Show and Convention, Session 13: Solid State Imaging, an Evolving Technology, Aug. 22-25, 1967.
L.J. Kozlowski et al., “Comparison of Passive and Active Pixel Schemes for CMOS Visible Imagers,”SPIE, vol. 3360, Apr. 1998.
Ming-Jer Chen, et al., “Weak Inversion Charge Injection in Analog MOS Switches,”IEEE Journal of Solid-State Circuits, vol. 30, No. 5, pp. 604-606, May 1995.
Peter W. Fry, et al., “Fixed-Pattern Noise in Photomatrices,”IEEE Journal of Solid-State Circuits, vol. SC-5, No. 5, pp. 250-254.
M. DeGr
Kozlowski Lester J.
Mann Richard A.
Akin Gump Strauss Hauer & Feld & LLP
Pictos Technologies, Inc.
Pyo Kevin
Rourk Christopher J.
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