Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-06-10
2001-07-24
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C399S178000, C399S123000
Reexamination Certificate
active
06266122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for manufacturing the same. In particular, the present invention relates to a liquid crystal display device having liquid crystal molecules axially symmetrically aligned within a liquid crystal region partitioned by a polymer wall.
2. Description of the Related Art
Conventionally, a TN (twisted nematic) type or a STN (super-twisted nematic) type using a nematic liquid crystal is used as a display device employing an electro-optic effect. A technique of widening a viewing angle of such a liquid crystal display device has been developed.
As one suggested technique of widening a viewing angle of the TN-type liquid crystal display device, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a so-called ASM (Axially Symmetrically aligned Microcell) mode, in which a liquid crystal display device has liquid crystal molecules axially symmetrically aligned within a liquid crystal region partitioned by polymer walls. The liquid crystal region substantially surrounded by the polymer wall is typically formed for each picture element. In a liquid crystal display device of this mode, the liquid crystal molecules are axially symmetrically aligned, so that there are a few changes in contrast regardless of the direction in which a viewer sees a screen of the liquid crystal display device. Thus, it has a wider viewing angle.
The ASM-mode LCD disclosed by the aforementioned publications is produced by a polymerization induced phase separation of a mixture containing a polymerizable material and a liquid crystal material.
A method for manufacturing the conventional ASM-mode liquid crystal display device will be described with reference to
FIGS. 10A
to
10
I. First, a substrate is prepared by forming a color filter and an electrode on one surface of a glass substrate
908
(FIG.
10
A). Note that for simplicity the electrode and the color filter formed on the glass substrate
908
are not shown. A method for forming the color filter will be described later.
Next, on the surface of the glass substrate
908
where the electrode and the color filter are formed, polymer walls
917
for aligning liquid crystal molecules in axial symmetry is formed in a lattice pattern, for example (FIG.
10
B). After a photosensitive resin material is spin-coated, it is exposed through a photomask having a predetermined pattern and developed. As a result, the polymer walls in a lattice pattern are formed. The photosensitive resin material may be either a negative-type or a positive-type. A step of forming an additional resist film is added, but it can be formed by using a resin material with no photosensitivity.
On a top portion of each of the resultant polymer walls
917
, a pillar-like protrusion
920
is separately formed by patterning (FIG.
10
C). Like the polymer walls
917
, the pillar-like protrusions
920
are also formed by exposing and developing a photosensitive resin material.
The surface of the glass substrate
908
having the polymer walls
917
and the pillar-like protrusions
920
is coated with a vertical alignment agent
921
such as polyimide (FIG.
10
D). Separately, a glass counter substrate
902
with an electrode formed thereon is also coated with a vertical alignment agent
921
(FIGS.
10
E and
10
F).
A liquid crystal cell is formed by attaching the two resultant substrates to each other so that the surfaces having the electrodes face each other (FIG.
10
G). A gap between two substrates (i.e., a cell gap (a thickness of a liquid crystal layer)) is defined as a sum of the polymer wall
917
and the pillar-like protrusion
920
.
A liquid crystal material is injected into a gap of the resultant liquid crystal cell by a vacuum injection method, for example (FIG.
10
H). In the end, by applying a voltage between the opposing electrodes, for example, the crystal molecules within the liquid crystal region
916
are axially symmetrically aligned (FIG.
10
I). The liquid crystal molecules within the liquid crystal region
916
partitioned by the polymer walls
917
are symmetrically aligned with respect to an axis
918
(perpendicular to both substrates) represented by a broken-line in FIG.
10
I.
FIG. 11
is a cross-sectional view illustrating a structure of a conventional color filter. On a glass substrate
508
, a black matrix (BM)
510
for blocking light from being transmitted through gaps between colored portions, and colored resin layers
512
of red, green, and blue (R, G, B) corresponding to respective picture element are formed. An over-coat (OC) layer
514
of acrylic resin, epoxy resin, or the like, with a thickness of about 0.5-2.0 &mgr;m is formed over the BM
510
and the colored resin layer
512
in order to improve smoothness, for example. In addition, an indium tin oxide (ITO) layer
516
, which is a transparent signal electrode, is formed on the OC layer
514
. The BM is generally composed of metal chrome having a thickness of about 100-150 nm. Resin materials colored with dyes or pigments are used for the colored resin layer
512
, and a thickness of this layer is generally about 1-3 &mgr;m.
As a method for forming a color filter, a photosensitive colored resin layer formed on a substrate is patterned by using a photolithography method. For example, a red (R), green (G), and blue (B) color filter can be formed when photosensitive resin layers of respective colors are formed, exposed, and developed (three times in total) by employing the photosensitive resin materials of the respective colors. In order to form a photosensitive colored resin layer, a substrate may be spin-coated with a liquid photosensitive colored resin material (which is diluted with a solvent). Alternatively, a photosensitive colored resin material may be transferred onto the substrate in the form of a dry film. By manufacturing the aforementioned liquid crystal display device of the ASM mode by using the color filter, a color liquid crystal display device having a wide viewing angle characteristic can be obtained.
However, the inventors of the present invention have found the following problems in the conventional ASM-mode liquid crystal display device and the method for manufacturing the same.
In the conventional ASM-mode liquid crystal display device, while it is possible to obtain a wide viewing angle characteristic, the brightness of the display device is reduced because the polymer walls reduce the light transmission. Moreover, the liquid crystal molecules present on the polymer walls in the conventional liquid crystal display device cannot contribute to display of images, so that the transmittance of the liquid crystal display device is reduced. In addition, the axial symmetry alignment of the liquid crystal molecules in the vicinity of the polymer walls is disturbed, thereby causing flickers in images (e.g., light leakage) in a black display.
In addition, when the above-described ASM-mode liquid crystal display device and the method for manufacturing the same are applied to a plasma-addressed liquid crystal display, the following problems arise. In a plasma-addressed liquid crystal display device, a plasma cell portion and a liquid crystal cell portion, which form a switching portion, have different heat history during the manufacturing steps (typically, 500° C. for the plasma cell, and 200° C. for the liquid crystal cell). Accordingly, difference in dimension resulting from heat contraction between both cells also varies. Thus, it is difficult to exactly align plasma electrodes with ITO electrodes. In view of this, a structure which eliminates the need of such a difficult alignment process has been employed (also known as an alignment-free structure). When the plasma-addressed liquid crystal display device and the ASM-mode are combined with the alignment-free structure, polymer walls (conventionally made from a black material) which are often formed within the aperture of each pixel in order that liquid crystal molecules are aligned in axial symmetry. As a r
Hamada Kenji
Imai Masato
Kishimoto Katsuhiko
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Ton Toan
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