Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-11-23
2001-04-17
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S234000, C257S294000, C257S432000, C257S435000, C257S440000
Reexamination Certificate
active
06218692
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to the field of solid state photosensors, and in particular to Active Pixel Sensors (APS).
BACKGROUND OF THE INVENTION
APS are solid state imagers where each pixel contains a photosensing means, charge to voltage conversion means, reset means and all or part of an amplifier. APS devices are typically operated in a manner wherein each line, or row, of the imager is integrated, read out and then reset at a different time interval. Therefore, in reading out the entire image, each line has captured the scene at a different point in time. Since illumination conditions vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the image. This limits the usefulness of APS devices in applications where high quality motion or still images are required.
Additionally, the charge to voltage conversion region, and other active transistor regions (i.e. other than the photosensing region), of APS devices are not shielded from the scene illumination. As a result, free electrons will be generated in those regions. These electrons are not effectively confined to the pixel in which they were generated, and can diffuse into adjacent pixels. This causes a degradation of the spatial accuracy of the image signal, and the modulation transfer function (MTF) of the image sensor. This is especially problematic in color image sensors where this pixel cross-talk leads to color mixing, adversely affecting the color balance of the image.
Since APS devices are typically fabricated in CMOS foundries, they do not incorporate color filter arrays (CFA) or micro-lens arrays (&mgr;Lens), and the shape and size of the photosensing area has not been optimized for incorporation of CFA and &mgr;Lens. A reason for this is that prior art active pixel sensors are typically monochrome. For most imaging applications it is desirable to have a color sensor. Even if CFA and &mgr;Lens were placed on prior art APS devices, the resulting images would have poor color MTF due to cross-talk and photosensing area that has not been optimized.
To solve the above discussed problems, it is desirable to perform integration at the same point and interval of time for every pixel, and subsequently transfer this charge to a storage region in each pixel that is shielded from the scene illumination. This is referred to as frame integration. It is also desirable to have all regions except for the photodetector effectively shielded from the scene illumination to improve the MTF. It is further desirable to provide blooming control during integration, and more importantly for frame integration, during storage and read out of the device. Finally, it is desirable to incorporate CFA and &mgr;Lens, and design the photosensing region to enable effective use of CFA and &mgr;Lens. These and other issues are solved by the teachings of the present invention.
SUMMARY OF THE INVENTION
A new APS device pixel architecture, and method of operation, has been devised that uses simultaneous integration of the scene illumination at each pixel, and subsequent simultaneous transfer of signal electrons at each pixel to a light shielded charge storage region. It comprises an alternative charge integration and transfer scheme, and pixel architecture that enable contemporaneous electronic shuttering and image capture and storage of all pixels on the sensor. This is provided by the present invention active pixel sensor comprising a semiconductor substrate having a plurality of pixels formed on the substrate such that at least one of the pixels has: a photodetector region upon which incident light will form photoelectrons to be collected as a signal charge, a color filter above the photodetector region, a light shield over at least the charge storage region having an aperture above the photodetector region, a means for transferring the signal charge from the photodetector region to a charge storage region, a sense node that is an input to an amplifier, the sense node being operatively connected to the signal storage region.
In this invention the shape of the photodetector is designed with dimensional symmetry so that a &mgr;Lens could be used to maximize the effective fill factor, and provide high quality color images.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
Advantageous Effect of the Invention
Reset and integration at each pixel simultaneously once per frame eliminates artifacts caused by changing scene illumination or scene motion. Light shielding of the pixel including the charge storage region allows this operation.
Creation of an aperture within the light shield at least over the photodetector region, improves device MTF by preventing generation of photoelectrons in regions outside of the photodetector of a given pixel from being collected in the photodetector or charge storage region of adjacent pixels. Inclusion of a photodetector lateral or vertical overflow drain (LOD or VOD), provides blooming control during storage and read out. Inclusion of CFA and microlenses and an appropriately designed photodetector region provides high quality color images.
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Guidash Robert M.
Lee Paul P.
Lee Teh-Hsuang
Eastman Kodak Company
Ngo Ngan V.
Watkins Peyton C.
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