Back to EveryPatent.com
| United States Patent |
6,126,768
|
|
Lovas
, et al.
|
October 3, 2000
|
Stress-free liquid crystal cell assembly
Abstract
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, includes the steps of disposing a
plurality of spacers in the area of the sealing member between the
substrates, positioning a frame on an outside surface of each substrate,
the frame being aligned with the sealing member, and applying pressure to
each frame so that a region corresponding to a display area is
substantially pressure-free and a uniform cell gap is obtained.
| Inventors:
|
Lovas; Istvan (Mahopac, NY);
Lu; Minhua (Mohegan Lake, NY);
Yang; Kei-Hsiung (Katonah, NY)
|
| Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
| Appl. No.:
|
098146 |
| Filed:
|
June 16, 1998 |
| Intern'l Class: |
B32B 031/00 |
| Field of Search: |
156/99,581,285
|
References Cited
U.S. Patent Documents
| 5442470 | Aug., 1995 | Hashimoto | 359/83.
|
| 5499127 | Mar., 1996 | Tsubota et al. | 359/80.
|
| 5568297 | Oct., 1996 | Tsubota et al. | 359/80.
|
| 5629787 | May., 1997 | Tsubota et al. | 349/153.
|
| 5677749 | Oct., 1997 | Tsubota et al. | 349/160.
|
Primary Examiner: Lorin; Francis J.
Attorney, Agent or Firm: Shofi; David M.
Claims
We claim:
1. 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 comprising the steps of:
disposing a plurality of spacers in the area of the sealing member between
the substrates;
positioning a frame on an outside surface of each substrate, the frame
being aligned with the sealing member; and
applying pressure to each frame so that a region corresponding to a display
area is substantially pressure-free and a uniform cell gap is obtained.
2. The method of claim 1 wherein the frame has a shape of the sealing
member of the liquid crystal display device.
3. The method of claim 1 wherein the frame is made of rubber, plastic,
metal and composite, or any type of material which can serve as pressure
transfer media.
4. The method of claim 1 wherein the frame has a solid structure to fulfill
the requirement of transferring a desired pressure.
5. The method of claim 1 pressure applying step comprises the step of
applying pressure by means of a mechanical, compressed gas or vacuum
device.
6. The method of claim 1 wherein the disposing step comprises the step of
disposing a plurality of sealing spacers in the area of the sealing member
between the substrates, the method further comprising the step of
providing a plurality of display spacers in the display area.
7. The method of claim 6 wherein the sealing spacers are higher density
spacers and the display spacers are lower density spacers.
8. The method of claim 1 wherein the spacers are formed as balls or posts.
9. The method of claim 1 wherein the spacers are manufactured by
photolithographic means.
10. The method of claim 1 wherein one of the pair of substrates comprises
active matrix elements such as thin film transistors made of crystalline
silicon, polycrystalline silicon, or amorphous silicon.
11. The method of claim 1 wherein the liquid crystal display device is a
passive-matrix-addressed LCD.
12. The method of claim 1 wherein the substrates are made of silicon,
glass, quartz, plastic, a dielectric or a metal.
13. The method of claim 1 wherein the liquid crystal display device is a
single-cell assembly or a multi-cell assembly where large substrates
containing a plurality of liquid crystal cells are used.
14. The method of claim 1 wherein the sealing member comprises thermal, UV
or UV thermal sealants.
Description
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 .mu.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 .mu.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 the substrates;
positioning a frame on an outside surface of each substrate, the frame
being aligned with the sealing member; and applying pressure to each frame
so that a region corresponding to a display area is substantially
pressure-free and a uniform cell gap is obtained.
Preferably, the frame has a shape of the sealing member of the liquid
crystal display device and is made of rubber, plastic, metal and
composite, or any type of material which can serve as pressure transfer
media. The frame preferably has a solid structure to fulfill the
requirement of transferring a desired pressure.
Pressure can be applied by means of a mechanical, compressed gas or vacuum
device.
Preferably, a plurality of sealing spacers are disposed in the area of the
sealing member between the substrates and a plurality of display spacers
are disposed in the display area. The sealing spacers are preferably
higher density spacers and the display spacers are preferably lower
density spacers. The spacers are preferably formed as balls or posts and
are manufactured by photolithographic means.
One of the substrates of the liquid crystal display device preferably
includes active matrix elements such as thin film transistors made of
crystalline silicon, polycrystalline silicon, or amorphous silicon.
Alternatively, the liquid crystal display device can be a
passive-matrix-addressed LCD. The substrates are, preferably, made of
silicon, glass, quartz, plastic, a dielectric or a metal.
The liquid crystal display device is either a single-cell assembly or a
multi-cell assembly where large substrates containing a plurality of
liquid crystal cells are used.
The sealing member can include thermal, UV or UV thermal sealants.
The present invention also provides an apparatus for assembling a liquid
crystal display device including a pair of substrates and a sealing member
located on an inside surface of the substrates for bonding the substrates,
the sealing member having at least one spacer for determining a cell gap
between the substrates, the apparatus including two frames, each disposed
on an outside surface of one of the substrates, and means for applying a
pressure to the frames such that stress on the liquid crystal device in a
region corresponding to the display area is substantially zero and the
cell gap is uniform.
The frames preferably have a shape of the sealing member and is made of
rubber, plastic, metal and composite, or any type of material which can
serve as pressure transfer media. The frame preferably has a solid
structure to fulfill the requirement of transferring a desired pressure.
The pressure applying means can include a mechanical, compressed gas or
vacuum device.
The apparatus can further include a plurality of display spacers in the
display area. The at least one spacer in the sealing member is preferably
a higher density spacer and the display spacers are preferably lower
density spacers. The spacers are balls or posts and are preferably
manufactured by photolithographic means.
One of the substrates of the liquid crystal display device preferably
includes active matrix elements such as thin film transistors made of
crystalline silicon, polycrystalline silicon, or amorphous silicon.
Alternatively, the liquid crystal display device can be a
passive-matrix-addressed LCD. The substrates are, preferably, made of
silicon, glass, quartz, plastic, a dielectric or a metal. The substrates
are preferably made of silicon, glass, quartz, plastic, a dielectric or a
metal.
The liquid crystal display device is either a single-cell assembly or a
multi-cell assembly where large substrates containing a plurality of
liquid crystal cells are used.
The sealing member preferably comprises thermal, UV or UV thermal sealants.
BRIEF DESCRIPTION OF THE DRAWING
Further objects, features, and advantages of the present invention will
become apparent from a consideration of the following detailed description
of the invention when read in conjunction with the drawing figures in
which:
FIG. 1 depicts an LCD cell assembly method of the prior art;
FIG. 2 depicts another LCD cell assembly method of the prior art; and
FIG. 3 depicts the LCD cell assembly method of the present invention.
DETAILED DESCRIPTION
FIG. 3 shows the stress-free, spacerless LCD cell assembly method of the
present invention. Two pressure plates 300, 310 are used to assemble the
cells. As described hereinabove, the pressure can be achieved by
mechanical, compressed air or vacuum means. Pressure plate 300 should be
UV transparent to permit curing of the sealant (if UV-curable sealant is
used). Pressure plate 310 can be made of an opaque material. Spacers 320
are disposed in the peripheral regions of the cell only between the two
substrates 330, 340 (only one cell is depicted in FIG. 3). The spacers 320
can be formed as balls or posts. Substrates 330, 340 can be formed of
silicon, glass, plastic, dielectric or metal. The spacers 320 can be made
on the substrates 330, 340 by means of photolithography or can be mixed in
the sealant and dispensed onto the substrates 330, 340. Two frames 350,
which have the shape of the peripheral sealant, are placed on both sides
of the panel (on the outside of each substrate) such that they are aligned
with the sealant pattern (while only the sides of each frame is depicted
in FIG. 3, four sides actually exist if the sealant is in the shape of a
rectangle). When pressure is applied by the pressure plates 300, 310, the
frames 350 transfer the pressure to the regions of the cell which have
spacers 320. As a result, the array areas are pressure-free and the
substrates 330, 340 will not collapse onto each other. That is, since the
pressure is only applied to where the cell has supports, there is no
stress built into the display area which does not have spacers to separate
the two substrates. Consequently, the cell gap remains uniform and
unchanged even after the pressure is removed. Because, for liquid crystal
on silicon displays, different mechanical properties exist between silicon
and glass, built-in stress may cause a long-term stability problem. This
assembly method eliminates this problem by eliminating the built-in
stress.
The frames can be made of rubber, plastic, metal and composite or any type
of material which can serve as a satisfactory pressure transfer medium.
Preferably, its structure is solid.
This assembly method can be used on LCD displays with a 0.5" to 3.0"
diagonal. The cell gap uniformity of the resulting cells are within one
fringe or better. This cell assembly method can be used to assemble LC
cells with either thermal, UV or UV thermal glue as peripheral sealant.
The pressure application mechanism can be either mechanical, compressed
gas or vacuum.
Depending on the materials used in the cell, the frames and pressure
transfer mechanisms can be selected accordingly to satisfy the different
requirements. For example, UV transparent material must be used for at
least one piece of frame and pressure transfer plate if a UV or UV thermal
sealant is used in LC cell.
Furthermore, the frames can be positioned relative to each other, depending
on the substrate characteristics, to optimize the cell gap uniformity.
While the present invention has been described with respect to preferred
embodiments, numerous modifications, changes, and improvements will occur
to those skilled in the art without departing from the spirit and scope of
the invention. For instance, it may be effective to provide a certain
number of spacers to the array or display area to optimize cell gap
uniformity under certain conditions and with certain materials. In this
case, the spacers located in the sealant (peripheral) region preferably
are of higher density than the spacers in the array region.
Top