Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-07-13
2002-02-12
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000
Reexamination Certificate
active
06346978
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates generally to liquid crystal display (LCD) panels for projection display and more particularly to a substrate structure and method of fabrication for forming high-quality thin film transistor (TFT) controlled pixel electrodes for use in LCDs.
Flat panel displays employ liquid crystal material sandwiched between parallel panels of light-transmissive material. The panels are usually made of quartz, glass, plastic, or the like. One panel has an array of pixels formed on its surface. Each pixel on the panel includes a light-transmissive pixel electrode controlled by a switching transistor. In active matrix displays the transistor, generally a thin film transistor (TFT), is operatively connected by thin metal lines on the panel to driver circuitry which selectively energizes the pixels. In active matrix displays each pixel (i.e., a pixel electrode controlled by a TFT) is addressed and controlled individually.
Light directed through the LCD passes through a first polarizing filter applied to one of the two parallel panels. The other panel has a second polarizing filter oriented in a different direction from the first. The liquid crystal material, which fills the volume between the panels, contains molecules which rotate the incident light in a well-defined manner when the adjacent pixel electrode is energized. Typically, the LCD is configured so that a particular pixel, when turned on, rotates the polarized light as it passes through the liquid crystal material, causing it to pass through the second polarizing filter. When the pixel is turned off, the polarized light is not rotated and thus will not pass through the second polarizer. An array of pixels turned on or off in a predetermined order and pattern produces images. By using multiple color filters, and given a sufficient density of pixels, full-color images are produced on the LCD screen. In projection-type LCDs light is directed through the LCD and is projected onto a screen.
Forming LCDs is a manufacturing challenge because a high density of pixels must be formed on a large area of transparent or translucent flat panel material. The pixels (TFTs and pixel electrodes) are fabricated in a layer of silicon applied to a quartz, glass, or another substrate (referred to herein after as the “transparent substrate”). The form of silicon that is easiest to apply to transparent substrates is uncrystallized amorphous silicon, which is widely used in LCD panels. Amorphous silicon yields poor TFT performance because of low electron mobility, but it is adequate for active matrix pixel control in most applications. TFTs formed in amorphous silicon lack the frequency response for display driver circuitry, however. The driver circuitry must be fabricated separately, usually as integrated circuits formed in single-crystal silicon. The separately-formed drivers must then be connected to the LCD, increasing manufacturing costs.
Polycrystalline silicon can be formed on a transparent substrate, as an alternative to amorphous silicon, by partially crystallizing deposited amorphous silicon through heating. Polycrystalline silicon (also known as polysilicon) yields higher-quality TFTs, but the high process temperatures required to crystallize amorphous silicon presents major difficulties, particularly for glass and plastic substrates. Heat sufficient to produce crystallized silicon on a transparent substrate can damage the substrate. Therefore, LCD manufacturers still use amorphous silicon on glass panels for the formation of the TFT pixel arrays used in LCDs. The driver circuitry, which requires a higher frequency response and better performance, is fabricated separately and connected to the panel around its periphery.
Projection-type LCDs, wherein light is directed through the LCD for projection onto a screen at a distance from the LCD, have heretofore been fabricated like direct-view LCDs. A circuit panel, with a plurality of TFT-controlled pixel electrodes formed in amorphous or partially crystallized silicon, is formed on glass or a similar transparent substrate. Liquid crystal material fills the void between the circuit panel and a second panel. Individual pixel electrodes on the circuit panel control whether or not light passes through the LCD and onto a projection screen. Suitable color filters are used to produce full-color images.
The poor performance of TFTs formed in amorphous or partially crystallized silicon is an ongoing problem for LCD manufacturers. As compared with TFTs formed in single crystal silicon (such as in IC chips), TFTs formed on transparent substrates have substantially lower electron mobility and higher leakage currents. If the TFTs used in LCD pixel arrays could be fabricated in single-crystal silicon, the result would be improved frequency response and sharper images. Single crystal silicon would also allow for reduced manufacturing costs because the fast logic required in LCD driver circuits could be integrated into the display panel. Heretofore, the only way to provide single-crystal silicon TFTs on glass is to adhere a layer of silicon, formed separately, to a glass substrate. That solution presents adhesion problems due to differing heat expansion coefficients, particularly during fabrication processing.
It would be advantageous to fabricate LCDs for projection display using single-crystal silicon strongly adhering to, and preferably integrated with, a transparent supporting layer.
It would also be advantageous to form LCD pixel arrays for projection display on a substrate which can be processed, without damage, at temperatures higher than the melting point of glass or plastic.
It would also be advantageous to have a new LCD processing system which employs silicon wafer integrated circuit processing methodologies to form high-quality TFTs and driver circuits in single-crystal silicon.
Accordingly, a liquid crystal display (LCD) array substrate is provided for use in projection-type LCDs. The LCD array substrate comprises a portion or segment of a silicon on insulator (SOI) wafer which is processed to include a first layer of substantially all single-crystal silicon on a first side of the substrate, and an insulating layer beneath the first layer. The LCD array substrate further comprises a plurality of pixel structures formed on the top single-crystal silicon layer. Each pixel structure includes a pixel electrode which, when used in a LCD, controls light transmissivity through a subregion of the LCD. Light directed to pass through the insulating layer and the pixel electrodes is, in a completed LCD structure, controlled (i.e., permitted to pass through or not pass through the LCD) by the pixel structures on the LCD array substrate.
In a preferred embodiment of the invention the SOI wafer further includes areas of a second layer of silicon on a second side of the substrate, which is on the other side of (i.e., opposite) the insulating layer from the first layer. The SOI wafer from which the LCD array substrate is made generally includes at least three layers, a top or first layer of single-crystal silicon, an intermediate layer formed of insulating material, and a bottom or second silicon layer also generally formed of single-crystal silicon. Portions of the second layer, beneath the pixel structures, have been removed to form openings in the second layer. The portions which remain cover parts of the insulating layer which extend generally around the periphery of the segment of the SOI wafer. Thus, the areas beneath the pixel structures are free of the silicon of the second layer.
In a preferred embodiment of the invention, the LCD array substrate forms part of a complete LCD array for projection display. The LCD array substrate incorporates a pixel array, which includes a plurality of thin film transistors (TFTs), wherein each TFT controls a pixel electrode. Conductors formed on the LCD array substrate provide operative connections between each TFT and a pixel controller, which is preferably an active matrix control system, the controller also being for
Hsu Sheng Teng
Shroyer Jon Allen
Krieger Scott C.
Rabdau Mathew D.
Ripma David C.
Schechter Andrew
Ton Toan
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