Liquid crystal display and fabrication method thereof

Details Liquid crystal display and fabrication method thereof Liquid crystal display and fabrication method thereof

2001-04-23

2004-12-14

Kim, Robert H. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S156000, C349S141000, C349S043000, C349S106000

Reexamination Certificate

active

06831726

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display and, particularly, to a liquid crystal display panel of the In-Plane Switching (IPS) system and a fabrication method of the same liquid crystal display panel.
2. Description of the Prior Art
With the increased demand of liquid crystal display, the demand of reduction of fabrication cost of the liquid crystal display panel is also requested. In order to reduce the fabrication cost of a pair of glass substrates used in the liquid crystal display panel, it is usual to fabricate a large size strip-shaped glass substrate and a plurality of liquid crystal display panels are cut apart from the large size strip-shaped glass substrate.
Since the liquid crystal display panel is usually rectangular, the cutting direction of the large size strip-shaped glass substrate (referred to as “raw glass substrate”, hereinafter) with which the largest number of glass substrates for liquid crystal panel can be cut apart from the raw glass substrate is primarily determined by the size of the raw glass substrate. For example, it is possible to cut apart six panel substrates each for a 14.1 inches liquid crystal display panel from a 600 mm×720 mm raw glass substrate. That is, by cutting the raw glass substrate along a shorter side thereof such that longer sides of the respective panel substrates become in parallel, two columns each including three panel substrates can be cut out from the raw glass substrate.
Since the respective raw glass substrates are continuously fabricated as long glass belts having a predetermined width, it is unavoidable that the surface condition and/or thickness of the raw glass substrate has the direction dependency in a width direction and a lengthwise direction thereof the raw glass substrate. Particularly, since a surface of a raw glass substrate fabricated by the slot downdraw process becomes in contact with a slit of a fabrication device, fine line-shaped irregularity extending along a lengthwise direction tends to occur in the surface thereof. Therefore, when the raw glass substrate is to be used for substrates of the liquid crystal display panel, the surface of the raw glass substrate has to be polished to remove the irregularity. However, it is difficult to remove the direction dependency of thickness throughout the whole raw glass substrate even if the fine irregularity can be removed by polishing.
On the other hand, although such fine line-shaped irregularity hardly occurs in a surface of a raw glass substrate fabricated by the fusion process, the latter process is the continuous fabrication method and, therefore, the thickness of the raw glass substrate varies in a direction perpendicular to a pulling direction thereof. Consequently, the direction dependency of the surface condition and the thickness of the raw glass substrate is unavoidable.
Therefore, when a liquid crystal display panel is to be constructed by arranging a TFT (Thin Film Transistor) substrate constructed by forming pixel electrodes and switching elements such as TFT's on one of the rectangular substrates cut apart from the continuously fabricated raw glass substrate and a CF (Color Filter) substrate constructed by forming a color filter (CF) on another of the rectangular substrates in an opposing relation through a liquid crystal layer, spacer members and a sealing member, the direction dependencies of the opposing glass substrates are in the same direction.
When a longer side direction of a TFT substrate
21
is the same as a drawing direction of the raw glass substrate during a fabrication of the raw glass substrate as shown in, for example, in
FIG. 1A
, there is the direction dependency of thickness of the TFT substrate
21
in the shorter side direction thereof as exaggeratedly shown by a cross section thereof in FIG.
1
B. Since the shorter side direction of a CF substrate
22
is the same as the drawing direction of the raw glass substrate similarly as shown in
FIG. 2A
, there is the direction dependency of thickness of the CF substrate
22
in the shorter side direction thereof as exaggeratedly shown by a cross section thereof in FIG.
2
B.
Therefore, when the liquid crystal display panel
23
is constructed by arranging the pair of such rectangular glass substrates in the opposing relation such that the longer sides thereof become in parallel, thicker portions
24
and
34
of the TFT and CF substrates may be substantially overlapped and thinner portions
25
and
35
thereof are also overlapped, as shown in FIG.
3
A and
FIG. 3B
showing a cross section thereof. In such case, a distance
28
between an area
26
in which the thicker portions are overlapped and an area
27
in which the thinner portions are overlapped substantially becomes a half of a distance between adjacent areas of the overlapped thicker portions.
In such case, a larger pressure than a pressure applied to the overlapped thinner portions is applied to the overlapped thicker portions in the pressing step for adhering the TFT substrate to the CF substrate under pressure of the fabrication process of the liquid crystal display panel
23
. That is, a difference in pressure between the area
26
and the area
27
is provided within the liquid crystal display panel
23
. In the area in which the larger pressure is applied, the spacers and the CF layer may be deformed more. The deformation includes elastic deformation and plastic deformation. Although the elastic deformation is removed when the pressure thereto is removed, the plastic deformation cannot be removed even when the pressure is removed. Therefore, in a portion having large plastic deformation, that is, the area
26
in which the thicker portions are overlapped, a gap between the TFT substrate
21
and the CF substrate
22
becomes small and, in the area
27
in which the thinner portions are overlapped, the gap becomes large. In other words, the variation of the thickness of the substrate produces a variation of pressure and hence a variation of the gap between the TFT substrate and the CF substrate. Since light transmittance of liquid crystal depends upon the gap between the TFT substrate
21
and the CF substrate
22
, the variation of gap causes a variation of display. As such, a small variation of the display cell gap is unavoidable throughout the panel.
In a conventional liquid crystal display panel employing TN (Twisted Nematic) mode liquid crystal utilizing rotary polarization, the influence of small variation of the display cell gap on the variation of light transmittance of liquid crystal is small. However, since a liquid crystal display panel using IPS mode or VA (Vertical Alignment) mode, which is a wide view angle mode, utilizes the birefringence effect of liquid crystal, the influence of the variation of cell gap on the light transmittance of liquid crystal is large compared with the conventional liquid crystal display panel employing TN mode. Further, although there is a tendency of reducing the cell gap in order for increase of response speed of display, the reduction of cell gap makes light transmittance of liquid crystal sensitive to the variation of gap.
Therefore, the variation of display caused by variation of cell gap, which caused no problem in the past, becomes a problem recently. That is, it has been requested to control the display cell gap of the liquid crystal display panel uniform.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a substrate arranging structure of, for example, a liquid crystal display device, for reducing the irregularity of display, which is caused by thickness distribution or irregularity distribution of opposing substrates used in the liquid crystal display device, and a fabrication method of the substrate arranging structure.
The present invention is featured by that a direction of thickness distribution or irregularity distribution of a TFT substrate becomes orthogonal to a direction of thickness or irregularity of a CF substrate when the substrates are laid one on t

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