Stress-free liquid crystal cell assembly

Details Stress-free liquid crystal cell assembly Stress-free liquid crystal cell assembly

1998-06-16

2001-09-18

Lorin, Francis J. (Department: 1775)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S581000, C156S285000

Reexamination Certificate

active

06290793

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to liquid crystal displays. Specifically, the present invention is directed to an apparatus and method of liquid crystal cell assembly having no spacers in the display area and virtually no stress across the cell.
BACKGROUND OF THE INVENTION
Conventionally, the dimensions of liquid crystal display (“LCD”) cell gaps are controlled by spacers or spacer posts. Cell gap uniformity is determined by the size, density and distribution of the spacers. Spacers ensure that the LCD panels have correct cell gaps and support substrates, thereby preventing them from collapsing onto each other. However, liquid crystal molecules adjacent to the spacers in the display area are distorted from the corresponding bulk orientations. As a result, there are light leakages at the spacer locations. The problem is more pronounced in projection displays where miniature displays are viewed at ten or hundred times magnification. There, spacers appear as defective spots, degrading contrast ratio and image quality.
One approach to addressing this problem has been to place the spacers out of the display or viewing area, i.e. in the peripheral area and in between the pixel gap. For direct view displays, spacers can be hidden within the inter-pixel gap which, measuring about 10 &mgr;m, is larger than the cell gap. Therefore, the whole panel can be supported evenly. For projection displays, however, the inter-pixel gap is only about 1 &mgr;m or less. In this case, the spacers are visible even when placed at the corner of the pixel. Therefore, the spacers must be limited to the peripheral area in projection display devices to eliminate the spacer visibility problem.
FIG. 1
is an illustration of a conventional prior art LCD cell assembly method. Typically, during LCD panel assembly, an external force or pressure is applied over the entirety of one or both of the substrates by means of mechanical, compressed air or vacuum to press the substrates so that they both contact the spacers. In
FIG. 1
, external force is applied to a rigid plate
100
and a flat platform
110
which are disposed against the outside surfaces of two substrates
120
and
130
, respectively. As a result of the pressure, the substrates
120
,
130
both are made to contact the spacers
140
. After the pressure is released, the panels either maintain a rather uniform cell gap or bowl up due to built-in stress. In the case where the panels bowl up, the cell gap will be corrected by another press during the end seal process after the injection of liquid crystal.
If, in this arrangement, the spacers
140
were eliminated from the viewing area to improve viewing quality, there would be no support to withstand the external pressure. For displays which have smaller than a half inch diagonal, the rigidity of the substrates
120
,
130
might be enough to hold the gap without collapsing. However, for displays larger than a half inch, the two substrates will collapse onto each other. As a result, the cell gap would not be uniform and viewing quality would diminish.
One attempt to address this problem is disclosed in U.S. Pat. No. 5,499,127 issued Mar. 12, 1996 to Tsubota et al. In Tsubota et al., the invention of which is shown in
FIG. 2
, spacers
240
are located in a curing resin that forms the sealing member of the LCD devices. The two substrates
220
,
230
(one coated with the sealing member) are initially pressed together with pressing plates
200
,
210
until the spacers
240
are against each substrate, thereby forming a panel of the devices (not shown). Next, an elastic sheet
250
of glass fiber is disposed on the top substrate
220
adjacent the pressing plate
200
. Portions of this elastic sheet
250
is cut out so that the remaining sheet is positioned over and in between the various sealing members
240
. Upon a second pressing, the top substrate
220
is deformed such that the gap between the substrates
220
,
230
is narrow at and outside the sealing member and is wide inside the sealing member. Thereafter, the sealing member is heat cured to reduce the built-in stress of the glue and a liquid crystal injection apparatus is used to restore some cell gap uniformity.
The method disclosed in Tsubota et al. has the following problems. First, the use of the single elastic sheet creates stress on the LCD device such that LCD cells are produced with 3-5 fringes associated with the Newton rings which are caused by the interference of the light from the two inner surfaces of the liquid crystal cell cavity when the cell gap is not uniform. In other words, cell gap distortion occurs and uniformity suffers. Second, the stress created by the disclosed method cannot be completely eliminated even after aneals before and after subsequent liquid crystal injection and end sealing. To improve the cell gap uniformity, the reduction of built-in stress is essential. Sealant breakage is very possible under this stress level. Third, since the substrates are not 100% rigid and flat, the bottom substrate
230
disposed directly against the pressing plate
210
is made to conform to the surface of that plate and away from that of the upper substrate
220
. This results in a non-uniform cell gap. Finally, particles trapped between the bottom substrate
230
and the adjacently disposed plate
210
will distort the display.
It is an object of the present invention to provide an apparatus and method for assembling LCD devices which do not suffer from the problems listed hereinabove.
SUMMARY OF THE INVENTION
The LCD cell assembly apparatus and method of the present invention achieves cell gap uniformity while creating substantially zero stress and containing no spacers in the display area. Two frames are used to direct pressure to the sealant area only. With the novel two frame method, a uniform cell gap is created with little or no stress to the LCD panel. Experiments have shown that, in 70% of the cells assembled, no fringes are produced using the present invention. The other 30% have, at most, one fringe. The low stress and uniform cell gap resulting from the present invention relax the requirements for liquid crystal injection and end seal procedures found in the prior art. Furthermore, LCD devices assembled using the present invention have better long term reliability in terms of cell gap uniformity and peripheral seal quality. The two frame method of the present invention ensures that the substrates will not contact directly with the pressure application plates. Therefore, the substrates can conform to each other either in a concave or convex relationship such that the cell gap is uniform even though the shapes of the substrates have a certain curvature. Because the pressure in the present invention is applied only to the framed area, the pressure on the rest of the display areas is substantially zero and the problems caused by smoothness, flatness or rigidity of the substrates and pressure plates are avoided.
In another aspect of the present invention, two frames of the apparatus of the present invention can be positioned relative to each other according to the type and size of the displays in order to optimize cell gap uniformity. Because neither substrate is directly in contact with a pressure plate, there is no concern for non-uniformity due to trapped particles.
Finally, because the present invention results in a uniform cell gap and substantially zero stress, the glue used for the sealant can be fully cured by UV irradiation only without the need for heating.
This spacerless cell assembly method of the present invention can be applied to LCD cells with one or both of the substrates made of the following materials: Si-wafer, glass, quartz, and plastic material.
Specifically, the present invention provides a method of assembling a liquid crystal display device including a pair of substrates combined together with a sealing member and a liquid crystal interposed between the substrates, the method including the steps of disposing a plurality of spacers in the area of the sealing member between th

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