Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Plasma excitation
Reexamination Certificate
1997-12-12
2002-02-05
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Plasma excitation
C349S084000, C349S178000
Reexamination Certificate
active
06344883
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. More specifically, the present invention relates to a liquid crystal display device having wide viewing angle characteristics and a method for producing the same.
2. Description of the Related Art
In the past, a liquid crystal display device (hereinafter, also referred to as an “LCD”) in a twisted nematic (TN) mode has been known. The liquid crystal display device in a TN mode has poor viewing angle characteristics (i.e., a narrow viewing angle). As shown in
FIG. 30A
, when TN-LCD
200
is in a gray-scale display, liquid crystal molecules
202
are tilted in one direction. As a result, in the case where TN-LCD
200
is observed in viewing angle directions A and B as shown in
FIG. 30A
, apparent light transmittance varies depending upon the direction. Accordingly, the display quality (e.g., contrast ratio) of TN-LCD
200
greatly depends upon the viewing angle.
In order to improve the viewing angle characteristics of a liquid crystal display device by controlling the alignment state of liquid crystal molecules, it is required to align liquid crystal molecules in at least two directions in each pixel. Examples of such liquid crystal display devices includes those in an axially symmetric aligned microcell (ASM) mode in which liquid crystal molecules are axis-symmetrically aligned in each pixel. Referring to
FIG. 30B
, for example, when a liquid crystal display device
210
in an ASM mode in which a liquid crystal region
214
is surrounded by a polymer region
212
is in gray scales, liquid crystal molecules are aligned in two different directions. In the case where the liquid crystal display device
210
is observed in viewing angle directions represented by arrows A and B, apparent light transmittance is averaged. As a result, the light transmittance in the viewing angle directions A and B becomes substantially equal, whereby viewing angle characteristics are improved compared with those in a TN mode.
Examples of liquid crystal display devices in a mode having improved viewing angle characteristics (hereinafter, referred to as a “wide viewing angle mode”) including an ASM mode will be described below.
(1) There is a technique for electrically controlling a transparent state or an opaque state by utilizing birefringence of a liquid crystal material in a liquid crystal display device which has polymer walls in a liquid crystal cell without polarizing plates and which does not require any alignment treatment. According to this technique, the ordinary index of liquid crystal molecules is matched with the refractive index of a supporting medium. Under the application of a voltage, the liquid crystal molecules are aligned, whereby a transparent state is displayed. When no voltage is being applied, the alignment of the liquid crystal molecules is disturbed, whereby a light scattering state is displayed.
For example, Japanese National Phase PCT Laid-open Publication No. 61-502128 discloses a technique for mixing liquid crystal with a photocurable or thermosetting resin, curing the resin to phase-separate liquid crystal from the resin, thereby forming liquid crystal droplets in the resin. Furthermore, Japanese Laid-open Publication Nos. 4-338923 and 4-212928 disclose a liquid crystal display device in a wide viewing angle mode obtained by combining the device disclosed in Japanese National Phase PCT Laid-open Publication No. 61-502128 with polarizing plates in such a manner that polarization axes are orthogonal to each other.
(2) As a technique for improving viewing angle characteristics of a non-scattering type liquid crystal cell using polarizing plates, Japanese Laid-open Publication No. 5-27242 discloses a technique for producing a composite material containing liquid crystal and a polymer material from a mixture of liquid crystal and a photocurable resin by phase separation. According to this technique, the liquid crystal molecules in liquid crystal domains are randomly aligned by generated polymers, the liquid crystal molecules rise in different directions in each domain under the application of a voltage. Therefore, the apparent light transmittance observed in each direction becomes substantially equal (because retardation d·&Dgr;n is averaged, where d is a thickness of a liquid crystal layer and &Dgr;n is birefringence of a liquid crystal material), so that the viewing angle characteristics in gray scales are improved.
(3) Recently, in Japanese Laid-open Publication No. 7-120728, the inventors of the present invention have proposed a liquid crystal display device in which liquid crystal molecules are omnidirectionally aligned (e.g., in a spiral state) in pixel regions by controlling light using a photomask or the like during photopolymerization. This device uses a technique of axis-symmetrically aligning liquid crystal molecules by utilizing phase separation from a mixture of liquid crystal and a photocurable resin. The liquid crystal molecules are axis-symmetrically aligned when no voltage is being applied, and come closer to homeotropic alignment (alignment vertical to the substrates) under the application of a voltage, whereby the viewing angle characteristics are remarkably improved. This technique is a p-type display mode using a p-type liquid crystal material (i.e., a material with a positive dielectric anisotropy &Dgr;&egr;).
As an example of a method for producing a device as described above, Japanese Laid-open Publication No. 8-95012 discloses a method for forming lattice-shaped polymer walls having a thickness smaller than the cell thickness in each pixel region, injecting a mixture of liquid crystal and a photocurable resin into the cell thus produced, and axis-symmetrically aligning liquid crystal molecules by utilizing two-phase regions in which a liquid crystal phase and a uniform phase exist. This production method does not use alignment films.
(4) Furthermore, Japanese Laid-open Publication No. 6-308496 discloses a liquid crystal display device in a wide viewing angle mode including an axis-symmetrical alignment film made of a crystalline polymer with a spherulite structure on the surface of a substrate.
(5) Japanese Laid-open Publication No. 6-194655 discloses a technique for coating an alignment film on a substrate and aligning liquid crystal molecules in a random direction without performing an alignment treatment such as rubbing.
There are techniques for dividing pixels into a plurality of regions and aligning liquid crystal molecules in each region in such a manner that the viewing angle characteristics in each region compensate for each other. Examples of the method will be described below.
(6) Japanese Laid-open Publication No. 63-106624 discloses a method for dividing each pixel into regions and performing an alignment treatment such as rubbing so that the rubbing directions in the respective regions become different.
FIGS. 31 and 32
show a liquid crystal display device obtained by the above method, having wide viewing angle characteristics and being capable of obtaining a display with a satisfactory contrast.
FIG. 31
is a schematic plan view of the liquid crystal display device, and
FIG. 32
is a cross-sectional view taken along the E-E′ line in FIG.
31
.
A pixel electrode (transparent electrode)
520
provided on each pixel, an alignment film
510
, and a thin film transistor
513
driving the pixel electrode
520
are provided on one glass substrate
522
of the liquid crystal display device. A counter electrode (transparent electrode)
519
and an alignment film
509
are provided on the other glass substrate
521
. The alignment films
509
and
510
are made of polyimide. A pixel B defined by the opposing transparent electrodes
519
and
520
is a square of 200 &mgr;m, for example, and a plurality of pixels B are arranged in a matrix. A band-shaped spacer
523
made of polyimide is provided in a center portion of the pixel electrodes
520
, as a result of which each pixel B is div
Adachi Takako
Kozaki Shuichi
Kume Yasuhiro
Kurihara Takashi
Shimoshikiryo Fumikazu
Nguyen Dung
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Sikes William L.
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