Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-12-03
2002-08-13
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000
Reexamination Certificate
active
06433852
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device of the active matrix type in which the axis of liquid crystal molecules is controlled by applying an electric field substantially in parallel with a substrate inside of a surface of the display panel, thereby providing a wide viewing angle and a high image quality simultaneously. This invention also relates to a liquid crystal display device having spacers of a novel constitution which assure a constant distance between a pair of substrates between which a liquid crystal composition is provided.
Liquid crystal display devices have been widely used as display devices for notebook type computers and computer monitors which are capable of producing a color display of high definition.
Such liquid crystal display devices are roughly classified into two systems. In the liquid crystal display device of one system (a simple matrix type liquid crystal device), a liquid crystal composition is substantially sandwiched between opposing faces of at least two substrates with at least one substrate being made of transparent glass, thus constituting a so-called liquid crystal panel, and a voltage is selectively applied to various pixel forming electrodes formed on the substrate of the liquid crystal panel so as to turn on or off given pixels. In the liquid crystal display device of the other system (active matrix liquid crystal display device), the above-mentioned various electrodes are provided with active elements for selecting pixels and given pixels are turned on or off by selecting these active elements.
The active matrix liquid crystal display device is represented by a liquid crystal display device which uses thin film transistors (TFT) as active elements. The liquid crystal display devices using the thin film transistors have been widely used as display terminal monitors of OA equipment, since the devices are thin and light-weight and exhibit a high image quality equivalent to that of cathode ray tubes.
In view of the difference in the method of driving the liquid crystal, the display system of the liquid crystal display devices is roughly classified into two types. In one display system, the liquid crystal composition is sandwiched by two substrates constituted by transparent electrodes and is driven by a voltage applied to the transparent electrodes, and light incident upon the liquid crystal composition layer after being transmitted through the transparent electrodes is modulated to provide the display. Most of the products that are now available have adopted this system.
Another one is a system in which the liquid crystal composition is driven by an electric field which is nearly parallel with the surface of a substrate, being generated between two electrodes that are formed on the same substrate, and light incident upon the liquid crystal composition layer through a gap between the two electrodes is modulated to provide a display. This system is characterized by its remarkably wide viewing angle and is an extremely prominent system in connection with the active matrix liquid crystal display device. Features of the latter system have been disclosed in, for example, Japanese Publication of the Translation of International Patent Application No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-open No. 160878/1994. Hereinafter, the liquid crystal display device of this system will be referred to as a transverse electric field liquid crystal display device.
FIG. 17
is a cross sectional view of an essential part of a display panel showing how an electric field is generated by the transverse electric field liquid crystal display device. In this liquid crystal display device, a video signal line DL, a counter electrode CT and a pixel electrode Px are formed on one substrate SUB
1
. The liquid crystal display device includes an orientation control layer ORI
1
, which is formed on the interface between a protective film PSV, formed on upper layers of the video signal line DL, the counter electrode CT and the pixel electrode PX, and a layer of a liquid crystal composition LC. On the other substrate SUB
2
, there are color filters FIL, defined by a black matrix BM, an overcoat film OC, which is formed such that it covers upper layers of these color filters FIL and the black matrix BM and is formed so as to prevent the constituting members of color filters and the black matrix from affecting the liquid crystal composition (hereinafter called liquid crystal simply), and an orientation control layer ORI
2
which is formed on an interface between the overcoat layer OC and the layer of the liquid crystal LC.
Insulation films GI and AOF are formed on the one substrate SUB
1
, a video signal line DL is made of conductive films d
1
and d
2
, a counter electrode CT is made of a conductive film g
1
, and the pixel electrode PX is made of a conductive film g
2
.
The distance between the pair of substrates SUB
1
, SUB
2
(the thickness of the layer of the liquid crystal: cell gap) is, in general, set to a given value by arranging spherical spacers (not shown in drawings) in a distributed manner between both substrates. Although polarizing plates are provided on the outer surfaces of the substrate SUB
1
and the substrate SUB
2
, they are omitted from the drawings.
Although no relevancy is found with the transverse electric field liquid crystal display device, Japanese Patent Laid-open No. 73088/1997 discloses the use of conical spacers which are fixedly formed on a protective film of a color filter substrate in place of such spherical spacers, or conical spacers, which are fixedly formed by laminating color filters.
In
FIG. 17
, the orientation direction of molecules of the liquid crystal LC is controlled in response to the electric field, which is generated between the pixel electrode PX and the counter electrode CT nearly in parallel with the substrates so as to provide, an image display. Here, however, an electric field which does not contribute to the display is generated between the video signal, line DL and the counter electrode CT. In case the gap between these electrodes is too narrow, the intensity of the electric field between the video signal line DL and the counter electrode CT is strengthened, and, hence, the liquid crystal LC is driven and an undesirable light is transmitted through the liquid crystal.
Although the black matrix BM is positioned in a region defined between these electrodes, in case the optical density of the black matrix BM is low, the above mentioned transmitting light cannot be shielded completely, and, hence, a leakage of light occurs. This leakage of light gives an adverse influence on the quality of the display, such as the lowering of the contrast or the occurrence of crosstalk. To solve such problems, it becomes necessary to increase the distance between the above-mentioned electrodes or to increase the optical density of the black matrix BM. Although
FIG. 17
shows the case where the video signal line DL and the counter electrode CT are arranged close to each other, similar problems also occur in case the video signal line DL and the picture electrode PX are arranged close to each other.
FIG. 18
is a schematic cross sectional view showing an example of an arrangement of electrodes which form one pixel of a liquid crystal panel constituting a conventional transverse electric field liquid crystal display device. On an active matrix substrate SUB
1
, there is a pixel electrode PX, counter electrodes CT
1
, CT
2
, a drain electrode SD
2
, an insulation film PSV, an orientation film ORI
1
and the like, while on a color filter substrate SUB
2
, there are a black matrix BM, color filters FIL(R), FIL(G), FIL(B) (not shown in drawings), an overcoat film OC, an orientation film ORI
2
and the like. A liquid crystal LC is filled in a gap defined between opposing faces of both substrates. In the drawing, POL
1
, POL
2
denote polarizing plates, E denotes an electric field for selecting a pixel, a
Asuma Hiroaki
Matsuyama Shigeru
Oh-e Masahito
Sonoda Hidehiro
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