Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-02-02
2003-01-07
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S141000, C349S155000, C349S156000
Reexamination Certificate
active
06504591
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an in-plane switching (In-plane switching: IPS) system liquid crystal display apparatus in which the electric field is applied to a liquid crystal layer in substantially parallel direction against a substrate and a liquid crystal display apparatus is operated, in particular relates to a large size panel use structure having a good productivity of an alignment control layer.
BACKGROUND TECHNIQUE
A strong need for a large size, a wide viewing angle and a multi-color performance of a screen size of a liquid crystal display apparatus is heightened. As to a viewing angle characteristic necessary for the large size, many definitions exists which are as a range in which a half-tone level is not reversed, a range in which a brightness, a contrast ratio, and a color tone are not varied, etc.. In the multi-color display, it is necessary to heighten a respective color re-production and further it is necessary to improve drastically the viewing angle characteristic according to the above stated definitions. As the liquid crystal display being compatible with the wide viewing angle and the large screen, it has proposed a combination system (IPS-TFT-LCD) in which IPS liquid crystal and a thin layer transistor (Thin layer transistor TFT) (Oota et al, Proceeding of the Fifteenth International Display Research Conference (Asia Display '95; p.707) and a practical use of a monitor system in which a display screen size has diagonal 13.3 inches (corresponded to 15 type CRT) has started (Kondo et al, SID '96 Digest No.8.1).
However, to realize the practical use of IPS-TFT-LCD corresponded to a screen larger than more than 17 type which will be a main current in CRT after this, it is necessary to develop a large size panel use new structure and process. In the conventional technique, it does not refer to a method for adding a homogeneous alignment characteristic in a panel having a stepped difference structure which is a subject mater of the present invention.
In IPS-TFT-LCD, it accompanies the difficulty of an alignment processing. A margin of this is narrow remarkably in comparison with a conventional type TN (Twisted Nematic) system, in particular in normally open type TN system (at a low voltage side a bright display, at a high voltage side a dark display). The reasons for the narrow margin are three points of following items (1)-(3).
(1) Stepped Difference Structure
In IPS-TFT-LCD, in principle it is necessary to arrange many long and narrow electrodes (at the case may be, it is called as an inter digital electrode) having several microns order degree.
Accordingly, a fine stepped difference structure is formed. A largeness of the stepped difference is determined by a thickness of the electrodes and shapes of various kinds of layers which are formed on the electrodes, in ordinary it has more than 0.1 micron. At the most upper layer of these layers, a high molecular layer such as polyimide is formed as an alignment control layer (it is called as an alignment layer). In the conventional mass-production technique, a surface of this alignment control layer is carried out according to a rubbing processing and a liquid crystal alignment ability is added. On the other hand, the rubbing use cloth is constituted by binding narrow fibers having a thickness of 10-30 microns degree, substantially since every one narrow fiber gives the sharing force in a predetermined direction to a local portion of the alignment layer, a processing for adding the liquid crystal alignment ability is carried out. As the fibers, there is an extremely narrow fiber having several microns degree, however as the rubbing use fiber, however since the rigidity for adding the some degree friction force is required, it has not used practically. Since an electrode interval in IPS system is 10-30 micron degree same to that of a diameter of the above stated fiber, the rubbing at a vicinity of the stepped difference is not carried out fully, as a result the alignment is disturbed easily. The disturbance in the alignment causes a lowering in a picture quality such as a rise of a black level and a lowering in the contrast ratio according to the above stated rise, and a non-homogeneity in the brightness.
As a method for solving the above stated problems, it has proposed a method for flattening a surface of the alignment layer, however the complete flattening manner invites following the side effects and this method is not used practically. A first is a problem which is caused by a phenomenon in which a spacer for controlling a liquid crystal layer at constant is moved easily. By the move of the spacer, a spacer distribution becomes up homogeneously and a liquid crystal layer thickness becomes non-homogeneously, accordingly the non-homogeneity of the brightness is invited. Further, during the spacer move the surface of the alignment layer is hurt and it causes a light leakage. From these points, it is necessary to have some degree stepped difference. Further, to solve the problem about the spacer move, it is desirable to form the stepped difference on one of a pair of substrates, in a case where TFT side is flattened and the stepped difference is tried to form to an opposite substrate side, it is necessary to pay an attention. In IPS system, to apply the effective in-plane switching, it is necessary to form, many thin and narrow shape inter digital electrodes, as a result many stepped differences are formed on the TFT side substrate. To dissolve the stepped differences it is effective to coat thick an organic high molecular layer, however when the thick insulation layer is formed on the electrode, it invites a lowering of the effective voltage which applies to the liquid crystal. As a result, a threshold voltage becomes high and it causes a problem in which since it is necessary to use driver having a high pressure withstand a consumption electric power becomes high. It is realistic to form the insulation layer having a proper thick to the TFT side substrate and to remain a proper stepped difference. As shown in
FIG. 1
, it is necessary to form a ratio a/d between the liquid crystal layer thickness d and the stepped difference a at least more than 0.02. In particular, for example, to drive the large size panel having diagonal size more than 18 inches, to restrain a deterioration of the voltage waveform from a driver LSI, it is necessary to lower the resistance value of the electrode, accordingly the electrode is necessarily to be formed thick. In the large scale panel, the stepped difference remains necessarily.
(2) Alignment Angle
In IPS-TFT-LCD, it is necessary to establish in principle an initial alignment direction to arrange a direction to which the electrode is extended, or by shifting with more than a constant angle from a vertical direction of the above stated direction. Herein, the electrode indicates a signal wiring electrode, a common electrode and a pixel electrode in a pixel. To regulate the initial alignment direction by the rubbing method, as stated in above, it is necessary to rub to a predetermined angle direction by the fibers having about 10-30 microns degree, by the stepped difference between the wiring being extended to the constant direction such as the signal wiring electrode, the common electrode in the pixel, and the pixel electrode in the pixel and the end portions thereof, it causes a problem in which the fibers are drawn into to the stepped difference direction from the design angle.
(3) Profound Degree of the Black Level
As one of the characteristics of IPS-TFT-LCD, it can enumerate that the profound of the black level (the black display) is good. Accordingly, the disturbance in the alignment comes to the fore in comparison with other systems.
In the conventional normally open type TN system, the dark level is obtained by the condition to which the high voltage is applied. In this case, under the high voltage almost liquid crystal molecular are all present to the electric field direction which is a vertical one direction to the substrate face, with the re
Kondo Katsumi
Miwa Takao
Tomioka Yasushi
Yoneya Makoto
Chung David
Hitachi , Ltd.
Parker Kenneth
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