Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Optical excitation
Reexamination Certificate
1998-07-29
2002-07-23
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Optical excitation
C349S038000, C349S113000, C349S110000
Reexamination Certificate
active
06424388
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to display devices and more particularly to a spatial light modulator array on a semiconductor substrate with appropriate control circuits for controlling respective pixels.
BACKGROUND OF THE INVENTION
Spatial light modulator (SLM) arrays, also referred to as light-valve arrays are used in projection displays, optical interconnects, holographic storage, and other applications where light is modulated spatially and temporally in response to an array of data. There are a number of advantages to Si based SLM's. With a Si based reflective SLM, high optical throughput can be achieved even with small pixels because the address lines and pixel storage capacitors do not block the light as in a transmissive display where the light passes through the substrate. With single crystal Si transistors, the transistor switching speeds are faster than with amorphous-Si or polycrystalline Si transistors now used for flat panel displays and would permit displays with a higher frame rate and higher information content. Additionally, even older Si chip manufacturing facilities can support much finer feature sizes on a SLM than are available with current flat panel manufacturing technology. Minimizing the feature size and hence the pixel size is desirable since the size of the other optical components for a display scale with the size of the SLM.
In U.S. Pat. No. 4,999,619 by T. S. Te Velde which issued Mar. 12, 1991, an electro-optic display device is described having a layer of liquid crystalline material between a first transparent supporting plate having at least one transparent first control electrode and a second supporting plate and at least one semiconductor body having one or more switching elements for driving a picture element matrix arranged in rows and columns and having picture electrodes which can be electrically driven separately. The second supporting plate may be provided with an additional reflective layer (dielectric mirror) which covers the picture elements and (possible) intermediate parts of the semiconductor material. The intermediate semiconductor material is then still better shielded from incident light.
In a publication by J. Glueck et al., entitled “Improvement in light efficiency of a-Si:H TFT-addressed reflective &lgr;/4-HAN-Mode light valves for Color TV projection”, p. 299, SID 1993 Digest, a large optical active area is achieved by placing the TFT's, the storage capacitors, and the row and column lines underneath an array of reflective Aluminum pixel electrodes to provide a-Si:H TFT-addressed light valve with a resolution of 400 by 200 pel, a pixel size of 50 &mgr;m by 55 &mgr;m and an aperture ratio of 84%.
In a publication by A. O'Hara et al., entitled “Mirror quality and efficiency improvements of reflective spatial light modulators by the use of dielectric coatings and chemical-mechanical polishing” Applied Optics, 32,5549 (1993), the mirror quality for a spatial light modulator was improved either by coating the mirror on top with silicon dioxide or by chemical-mechanical polishing silicon dioxide below before depositing the aluminum to form the mirror.
With flat panel displays, plastic spheres a few microns in size may be randomly dispersed in the liquid crystal device gap to serve as spacers. For the very small pixel size being used (17 µm on a side) this is not practical as a single spacer ball could block 4% of the area of a mirror which is significant since with a 4-bit grey scale the least significant bit corresponds to 6% and any clustering of spheres could cause more severe mirror shadowing.
The use of rigid SiO
2
spacers built on the front glass has been reported by J. Glueck et al., “Color-TV projection with fast-switching reflective HAN mode light valves”, p. 277, SID 1992 Digest and J. Glueck et al., p. 299, SID 1993 Digest, cited above. U.S. Pat. No. 4,999,619 by T. S. Te Velde cited above, describes spacers or posts formed from a layer of magnesium oxide by photolithography and etching.
SUMMARY OF THE INVENTION
In accordance with the present invention, a spatial light modulator array for modulating light and its fabrication is provided comprising a plurality of liquid crystal devices positioned over respective mirrors on a dielectric layer on a semiconductor substrate, a plurality of electrical circuits formed in the semiconductor substrate coupled to the liquid crystal devices, respectively, for modulating light passing there through, and a reflector/absorber layer positioned and patterned with respect to the mirrors. The reflector/absorber layer provides optical shielding for the plurality of electrical circuits from ambient and the impinging light, the reflector/absorber layer having an edge overlapping an edge of the mirror to form an overlapping region to attenuate the ambient and impinging light from passing into the semiconductor substrate by a factor of about 100,000 or higher.
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Colgan Evan George
Harper James McKell Edwin
Kaufman Frank Benjamin
Manny Margaret Paggi
Melcher Robert Lee
International Business Machines - Corporation
Morris Daniel P
Ton Toan
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