Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-06-29
2002-01-15
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06339462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. In particular, the present invention relates to a liquid crystal display device having liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions separated by a polymer wall, and a method for producing the same.
2. Description of the Related Art
Conventionally, TN (twisted nematic)-type liquid crystal devices or STN (super twisted nematic)-type liquid crystal devices have been used as a display device employing electrooptic effects. Technologies to enlarge a viewing angle have actively been developed.
As one of the technologies for enlarging the viewing angle which have been developed, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a liquid crystal display device having liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions separated by a polymer wall. Such a device is commonly referred to as an ASM (axially symmetrically aligned microcell) mode liquid crystal display device. The liquid crystal regions substantially surrounded by the polymer wall are typically formed pixel by pixel. In an ASM mode liquid crystal display device, liquid crystal molecules are axially symmetrically aligned, and thus observers experience less variations in the contrast, irrespective of a viewing direction in which the observers view the display. In other words, such a device has a wide viewing angle characteristic.
An ASM mode liquid crystal display device disclosed in the above-mentioned publications is fabricated by polymerization-induced phase separation of a mixture containing a polymerizable material and a liquid crystal material.
A method for producing a conventional ASM mode liquid crystal display device will be described with reference to
FIGS. 10A through 10I
. First, a glass base plate
908
(shown in
FIG. 10A
) is provided with a color filter and electrodes formed on one side (upper surface) thereof. For simplicity, the color filter and the electrodes formed on the upper surface of the glass base plate
908
are not shown. A process of forming the color filter will be described later.
Then, as shown in
FIG. 10B
, a polymer wall
917
for axially symmetrically aligning liquid crystal molecules is formed, for example, in lattice on the surface of the glass base plate
908
, where the electrodes and the color filter are formed. The polymer wall
917
is formed in lattice by spin-coating the glass base plate
908
with a photosensitive resin material, and then performing exposure and development using a photomask having a predetermined pattern. The photosensitive resin material may be either a negative type or a positive type. Alternatively, the polymer wall can be formed by employing a resin material with no photosensitivity, although a separate step of forming a resist layer must be added.
As shown in
FIG. 10C
, column-like projections
920
are discretely patterned on a portion of an upper surface of the polymer wall
917
thus formed. The column-like projections
920
are formed in a discrete manner by patterning a photosensitive resin material on a portion of an upper surface of the polymer wall
917
, and by performing proximity exposure and development.
As shown in
FIG. 10D
, the surface of the glass base plate
908
is coated with a vertical alignment material
921
such as polyimide or the like so as to cover the polymer wall
917
and the column-like projections
920
. Thus, a substrate is formed. Likewise, as shown in
FIGS. 10E and 10P
, a counter glass base plate
902
is also coated with the vertical alignment material
921
so as to cover an electrode (not shown) formed thereon, thereby forming a counter substrate.
As shown in
FIG. 10G
, the two resultant substrates are attached together in such a way that the surfaces having electrodes are facing inward. In this manner, a liquid crystal cell is formed. A gap between the two substrates (i.e., a thickness of a liquid crystal layer described later; referred to as a “cell gap” is defined by the sum of the heights of the polymer wall
917
and the column-like projections
920
.
As shown in
FIG. 10H
, a liquid crystal material is injected into a gap in the liquid crystal cell thus obtained by a vacuum injection method or the like, thereby forming a liquid crystal layer
916
. The liquid crystal layer
916
is divided into a plurality of liquid crystal regions
915
(only one is shown in
FIG. 10I
) by the polymer wall
917
. As shown in
FIG. 10I
, liquid crystal molecules in the liquid crystal region
915
are controlled to be axially symmetrically aligned with respect to an axis
918
(shown by the dotted line) which is perpendicular to both the glass base plates
908
and
902
. The liquid crystal molecules are thus controlled by, for example, applying a voltage between a pair of electrodes respectively provided on the glass base plates
908
and
902
and facing each other.
A cross section of a color filter is shown in
FIG. 11. A
black matrix (BM)
510
and a color resin layer
512
including a red color (R) pattern, a green color (G) pattern, and a blue color (B) pattern are formed on a glass base plate
508
. The red, green, and blue color patterns each correspond to a pixel. The black matrix
510
blocks light passing through a gap between the color patterns. An overcoat (OC) layer
514
formed of an acrylic resin, an epoxy resin or the like is provided on the black matrix
510
and the color resin layer
512
to a thickness of about 0.5 &mgr;m to about 2.0 &mgr;m so as to improve the smoothness and the like of the surface of the color filter. On top of the overcoat layer
514
, a transparent signal electrode
516
formed of an indium tin oxide (ITO) layer is further provided. The black matrix
510
is generally made of a metal chromium layer having a thickness of about 100 nm to about 150 nm. As the color resin layer
512
, a resin material colored by a dye or pigment is used. Generally, the thickness of the color resin layer
512
is about 1 &mgr;m to about 3 &mgr;m.
The color resin layer
512
is formed by patterning a photosensitive color resin layer formed on the glass base plate
508
by photolithography. For example, by forming, exposing and developing red, green, and blue photosensitive color resin layers (i.e., each step is repeated three times in total), the color resin layer
512
including red, green, and blue patterns can be fabricated. The photosensitive color resin layers can be formed by applying a liquid photosensitive color resin material (diluted with a solvent) on a base plate by a spin-coating method or the like, or by transferring a photosensitive color resin material in the form of a dry film onto a plate. By fabricating the above-described ASM mode liquid crystal display device with such a color filter, a color liquid display device having a wide viewing angle characteristic can be obtained.
However, when the above-described ASM mode liquid crystal display device and the method for producing such a device are applied to a large liquid crystal display device having high resolution display characteristics, the following problems described with reference to
FIGS. 12A through 12D
will arise.
As shown enlarged in
FIGS. 12A through 12D
, the polymer wall
917
and the column-like projections
920
are formed in such a way that their side surfaces are inclined (i.e., tapered) with respect to the base plate
908
. Such an inclination is inevitable in proximity exposure usually employed in photolithography performed for a large base plate, by which a photomask and a base plate are exposed in the state of being proximate to each other. The reason is that a proximity gap (i.e., a gap between large base plate and a correspondingly large photomask) cannot be extremely reduced. In the case where the proximity gap is extremely reduced, the base plate and the mask are occasionally in contact with each other due to a warp or a flexion of the base plate and the mask.
Endo Kazuyuki
Kishimoto Katsuhiko
Kondo Nobuhiro
Nixon & Vanderhye P.C.
Schechter Andrew
Sharp Kabushiki Kaisha
Ton Toan
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