Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-02-14
2004-08-24
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S037000, C349S128000, C349S129000, C345S087000
Reexamination Certificate
active
06781655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to liquid crystal displays, particularly, active-matrix liquid crystal displays for use in electronic devices such as color image projectors, which operate with line inversion drives and column inversion drives.
2. Description of the Related Art
Conventionally, liquid crystal displays of the projection type (namely, liquid crystal display projectors) use active-matrix liquid crystal displays as their liquid crystal light valves, which act as light modulators. The active-matrix liquid crystal display contains an active-matrix substrate that fabricates signal lines electrodes, and switching elements for pixels, and its opposite substrate having common electrodes, wherein these substrates are arranged opposite to each other and are separated from each other with a prescribed gap via a seal material. A liquid crystal is held in such a gap between the substrates of the active-matrix liquid crystal display. A large number of ‘pixel electrodes’ are arranged on the prescribed display area of the display, and they are respectively encompassed by data lines and scan lines. Hence, the pixel electrodes are arranged in a matrix form on the screen of the display.
The recent mainstream technology for active-matrix liquid crystal displays is called a ‘Twisted Nematic’ (simply abbreviated in ‘TN’) mode. This is because the liquid crystal displays of the TN mode provide high brightness, high contrast, and relatively high-speed response, while they can be driven with relatively low voltages and are easy to control in gradations. That is, the liquid crystal displays of the TN mode provide various characteristics, which are essential for the existing displays, with a good balance. The TN mode employs the prescribed structure in which liquid-crystal molecules are twisted in their long-axis directions between the active-matrix substrate and its opposite substrate. Generally speaking, the twisted-nematic liquid crystal display (or ‘TNLCD’) uses a liquid crystal whose twisted-nematic molecules align on a helical axis in the absence of an electric field, twisting polarized light up to 90°.
An alignment direction for aligning liquid-crystal molecules is regulated by surface conditions of the substrates. That is, the liquid-crystal molecules cannot all always be aligned in the prescribed direction by simply aligning them in parallel to the screen surface of the liquid crystal display because they have a certain degree of freedom with respect to the alignment direction. One method for ensuring an alignment of liquid-crystal molecules in the specific direction is to physically control them in their long-axis directions by providing surfaces of the substrates with coating materials or channels directing an alignment in a specific direction. Specifically, the surface of the substrate is coated with a polyimide resin having a specific orientation to form an orientation film thereon. The orientation can be further enhanced by forming scratches extending in the specific direction on the surface of the orientation film. In addition, there are also provided several measures in orientation processes to provide a specific orientation to the orientation film formed on the surface of the substrate. For example, a so-called rubbing method is used to rub the orientation film with the cloth wound about a roll, or a slanted deposition (or slanted evaporation) method is used to deposit an inorganic material in a slanted direction to form an orientation film.
More specific descriptions will be made with respect to a typical example of the active-matrix liquid crystal display that uses thin-film transistors (namely ‘TFT’) as switching elements for pixels. That is, the active-matrix liquid crystal display is composed of an active-matrix substrate for fabricating scan lines, data lines, pixel electrodes and thin-film transistors, and its opposite substrate having common electrodes, wherein a liquid crystal layer is narrowly held in a gap between these substrates that are arranged opposite to each other and are separated from each other via a seal material.
On a front surface of the active-matrix substrate directly facing with the liquid crystal layer, a large number of data lines and scan lines are wired to intersect with each other in grid patterns in connection with thin-film transistors, so that each thin-film transistor is arranged in proximity to a point of intersection between each data line and each scan line. In addition, pixel electrodes are connected to the data lines and scan lines by means of the thin-film transistors respectively. One pixel is defined as a region that contains each one pixel electrode as well as its related data line, scan line, and thin-film transistor. Thus, the active-matrix liquid crystal display can display images of dots by activating respective pixels that are arranged in a matrix form.
Orientation films urging liquid-crystal molecules to prescribed orientation states in a non-power mode (or a power-off mode) where no voltage is applied between the substrates are respectively formed on surfaces of the active-matrix substrate and its opposite substrate sandwiching the liquid crystal layer. Conventionally, the orientation films are composed of orientational high molecular materials such as polyimide, so that organic orientation films whose surfaces are subjected to rubbing processes are widely used. In the rubbing process, the prescribed rubbing cloth is used to rub the surface of the film in a certain direction.
In the active-matrix substrate, regions forming data lines, scan lines, and thin-film transistors have a greater number of layers compared to regions forming pixel electrodes. That is, peripheral portions of pixels containing data lines, scan lines, and thin-film transistors are increased in height compared to center portions of pixels. This causes differences in height being formed between the peripheral portions and center portions of the pixels on the active-matrix substrate.
Recently, liquid crystal displays are manufactured with very fine structures by decreasing dimensions of pixels. In the rubbing process of the orientation film, the rubbing cloth does not make good contact with the differences and their neighboring areas on the active-matrix substrate. Thus, it is very difficult to perform the rubbing process completely on the entire surface area of the orientation film.
If the rubbing process is made incomplete with respect to boundary areas corresponding to neighboring areas of differences formed between peripheral portions and center portions of pixels, defectiveness may occur in these areas of the orientation film. In a non-power mode, liquid-crystal molecules will not be sufficiently regulated by the orientation film in proximity to the aforementioned boundary areas. This may cause orientation failures in which liquid-crystal molecules become unstable in orientation due to various factors. That is, a so-called ‘reverse tilt domain’ (i.e., a region in which liquid-crystal molecules have different directions in building up or tilting) is caused to occur at the boundary areas between the peripheral portions and center portions of the pixels. This may cause display failures such as leakage of light.
In the slanted deposition method, deposition may not be completely performed around peripheral portions of pixels because of shadows of the differences. For this reason, as the display is manufactured with a very fine structure, it may cause a noticeable increase for orientation-incomplete areas in which the orientation process was not performed completely with respect to the orientation film formed on the surface of the active-matrix substrate. In the orientation-incomplete areas substantially corresponding to the peripheral portions of pixels, liquid-crystal molecules are not sufficiently regulated in accordance with the prescribed orientation, so they become unstable in orientation due to various factors. In short, the substrate of the active-matrix liquid crystal display must withstand an orientati
Tanaka Takaaki
Yamazaki Yasushi
Chowdhury Tarifur R.
Oliff & Berridg,e PLC
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