Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-03-19
2001-02-20
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
Reexamination Certificate
active
06191830
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electro-optical display devices, such as active matrix liquid crystal display (AMLCD) devices, and more particularly relates to such devices incorporating storage capacitors.
Reflective high density active matrix liquid crystal displays (AMLCDs) on silicon are being considered for high resolution color projection systems, due to the fact that the matrix structure of row and column electrodes, switches and storage capacitors can all be integrated on the silicon substrate below the reflective pixel electrodes, thus enabling the high pixel density required for such high resolution displays.
For example, the 1280 columns and 1024 rows required for HDTV requires that more than 1.3 million pixels must fit onto an area of about 2 square cm. In order to have a sufficient optical aperture for acceptable brightness and contrast ratio, the pixels must be as large as possible. With the reflective AMLCD structure, inter-pixel spacings of around 1 micron are possible, which provides the pixel area necessary to achieve an optical aperture sufficient for acceptable performance.
A typical defect density in the sub-micron CMOS processing techniques used to fabricate such AMLCDs is around one pixel defect per square centimeter. However, in such high resolution displays, one pixel defect in the entire array is sufficient to result in rejection of the display.
The most common defects are cross-over shorts, pixel switch failure and storage capacitor shorts. For example, capacitor shorts reduce the ability of the pixel to hold its display information during the interval between addressing and readdressing, affecting, for example, grey scale rendition.
In order to overcome such defects, and thereby achieve high display quality with high manufacturing yield, redundant structures have been designed into such AMLCDs, including redundant switches and electrodes.
In such displays, a large area (almost 75% of the pixel area) and a thin dielectric layer (400 nm) are needed for the pixel storage capacitor to provide the required capacitance (10 times the capacitance of the liquid crystal material) to hold the pixel voltage over the frame period. Due to this large area and thin dielectric layer, chances of a short are somewhat higher than for the pixel switches and other circuit elements.
A redundant storage capacitor structure is described in published international patent application WO 91/02372. In that case, the pixel capacitor can be a junction or oxide type capacitor or a combination thereof. Since the combination structure is a multilevel multilayer structure, the fabrication thereof requires multiple processing steps, which increases manufacturing cost and decreases yield.
OBJECTS AND SUMMARY OF THE INVENTION
According to the invention, redundant storage capacitors in electro-optic display devices such as AMLCD devices are achieved without the need to redesign the layout of the device, or to resort to multilevel structures, by simply dividing one electrode of each of the capacitors into two electrodes, resulting in two capacitors in series. With such a structure, if a short occurs in one capacitor, the other capacitor will maintain sufficient pixel voltage until readdressing.
A particular advantage of such a structure is that both capacitors are produced in a single level of the device, so that no additional levels, with their associated mask steps and processing steps, are required.
In accordance with a preferred embodiment of the invention, a triple layer of oxide-nitride-oxide is used as the dielectric for the storage capacitors. This composite dielectric has a higher dielectric constant, better breakdown voltage and higher yield than the conventional single layer dielectric.
The reduced level of defects due to capacitor shorts, resulting from the redundant storage capacitor design of the invention, results in improved grey scale rendition and overall improved display quality.
REFERENCES:
patent: 4639087 (1987-01-01), Cannella
patent: 5042916 (1991-08-01), Ukai et al.
patent: 5126865 (1992-06-01), Sarma
patent: 5151806 (1992-09-01), Kawamoto
patent: 5333004 (1994-07-01), Mourey et al.
patent: 5412493 (1995-05-01), Kunii et al.
patent: 5700699 (1997-12-01), Han et al.
patent: 5886365 (1999-03-01), Kouchi et al.
patent: WO9102372 (1991-02-01), None
Gogna Pawan
Janssen Peter J.
Fox John C.
Parker Kenneth
Philips Electronics North America Corporation
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