Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-09-21
2002-10-15
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000, C349S156000
Reexamination Certificate
active
06466296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with a large screen having wide viewing angle characteristics
2. Description of the Related Art
In the prior art, there has been a display device utilizing electro-optical effects, such as a twisted nematic (TN) or super twisted nematic (STN) type liquid crystal display device. Now technology has been vigorously studied such that the viewing angle of such a liquid crystal display device may be enhanced.
An example of the technology for enhancing the viewing angle is seen in a TN type liquid crystal display device in which liquid crystal molecules are axially symmetrically aligned in each of liquid crystal regions separated by a wall of polymer (protrusion-like structures), namely, a so-called Axially symmetrically aligned Microcell (ASM) mode liquid crystal display device, as disclosed in Japanese Laid-Open Publication Nos. 6-301015 and 7-120728. A liquid crystal region surrounded by the polymer wall typically corresponds to a pixel region. In the ASM mode liquid crystal display device, liquid crystal molecules are axially symmetrically aligned, so that an observer recognizes less variation in contrast in any viewing directions; that is, wide viewing angle characteristics are obtained.
A production method of such an ASM mode liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 7-120728. In accordance with the method disclosed in the publication, a protrusion-like structure is formed on a substrate in a grid pattern so that liquid crystal molecules are axially symmetrically aligned by the interaction between the protrusion-like structure and the liquid crystal molecules. Japanese Laid-Open Publication No. 10-133206 discloses another ASM mode liquid crystal display device in which the axially symmetrical alignment is achieved by the combination of a liquid crystal material of negative dielectric anisotropy (N-type liquid crystal material) and a vertical alignment layer.
A plasma addressed liquid crystal display device has potential as a large size liquid crystal display device and thus has been vigorously developed. An example of the plasma addressed liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 1-217396. The plasma addressed liquid crystal display device includes a substrate, a thin dielectric sheet, ribs disposed between the substrate and the dielectric sheet, and a discharge channel (plasma channel) in the shape of a line surrounded by the substrate, the dielectric sheet and the ribs. The state of plasma discharge is changed by switching a voltage applied to noble gas filled in the discharge channel using an anode electrode and a cathode electrode. A liquid crystal layer is driven by a voltage applied between the discharge channel and a counter electrode, via the dielectric sheet.
The device disclosed in Japanese Laid-Open Publication No. 1-217396 is of the TN mode and therefore has a problem with its viewing angle characteristics. In order to solve this problem, Japanese Laid-Open publication Nos. 9-197384 and 10-186331 each disclose a plasma addressed liquid crystal display device of the above-described ASM mode.
However, the present inventors have found that the conventional ASM mode liquid crystal display device and the conventional ASM mode plasma addressed liquid crystal display device have the following problems (1) and (2).
(1) Reduction in Transmittance
FIG. 1
illustrates a part of the above-described ASM mode liquid crystal display device including a protrusion-like structure
96
. A color filter layer
92
, an overcoat layer
93
and a transparent electrode
94
are formed on a glass plate
90
. The protrusion-like structure
96
and a column-like protrusion
98
are formed in a liquid crystal layer
95
which is disposed over those layers and the electrode. A counter substrate
99
is disposed on the column-like protrusion
98
. When the protrusion-like structure
96
is black, i.e., light-blocking, and formed within a pixel region, an aperture ratio is significantly reduced and thus transmittance is decreased. When the protrusion-like structure
96
is transparent, the above-described problem does not arise. Nevertheless, the above ASM mode liquid crystal display device still has a disadvantage of a reduction in the aperture ratio, which will be described below with reference to FIG.
2
. As schematically illustrated in
FIG. 2
, the liquid crystal layer
95
has a thickness of d
1
between the transparent electrode
94
and the counter substrate
99
, but a thickness of d
2
directly above the protrusion-like structure
96
is smaller than d
1
. A portion of the liquid crystal layer
95
having the smaller thickness of d
2
does not sufficiently contribute to display. In this case, when an N-type liquid crystal material is used, the state of display appears the same as when the reduction in the aperture ratio decreases the transmittance. When a P-type liquid crystal material (i.e., a liquid crystal material of positive dielectric anisotropy) is used, the contrast of display is reduced. This occurs for the following reason. The liquid crystal display device is designed using as a reference the thickness of d
1
across the region having no protrusion-like structure
96
. In the case where the thickness of d
2
across the liquid crystal layer
95
directly above the protrusion-like structure
96
largely differs from d
1
, the retardation of such a region of the liquid crystal layer is deviated from the designed value, thus reducing the amount of light contributing to display.
(2) Slow Response Speed for a Gray Scale Image
In a conventional plasma addressed liquid crystal display device, a voltage is applied across a liquid crystal layer and a thin dielectric sheet (e.g., a glass sheet about 50 &mgr;m thick). The voltage applied across the liquid crystal layer largely depends on the thickness of the liquid crystal layer. When the plasma addressed liquid crystal display device is of the ASM mode in which the above-described protrusion-like structure is used, a voltage applied across the liquid crystal layer directly above the protrusion-like structure is not sufficient since the thickness of such a region of the liquid crystal layer is thinner than the thickness of a region having no protrusion-like structure. Therefore, such a portion of the liquid crystal layer has a significantly slow response speed, reducing the entire response speed in displaying a gray scale image.
SUMMARY OF THE INVENTION
A liquid crystal display device according to the present invention includes a first substrate, a second substrate disposed facing the first substrate, a liquid crystal layer having liquid crystal molecules, interposed between the first and second substrates, and a groove structure having a plurality of grooves provided on the first substrate. The liquid crystal layer includes a plurality of liquid crystal regions defined by the plurality of grooves.
In one embodiment of the invention, the liquid crystal molecules in at least one of the plurality of liquid crystal regions are axially symmetrically aligned.
In one embodiment of the invention, the liquid crystal display device further includes a vertical alignment layer provided on a surface of at least one of the first and second substrates facing the liquid crystal layer. The liquid crystal layer includes a liquid crystal material of negative dielectric anisotropy.
In one embodiment of the invention, a side wall of the plurality of grooves is sloped with respect to a substrate surface.
In one embodiment of the invention, a slope angle of the sloped side wall is in the range of about 5° to about 70°.
In one embodiment of the invention, a transparent electrode is provided between the liquid crystal layer and the groove structure.
In one embodiment of the invention, at least two of the liquid crystal regions correspond to a pixel region, and at least one of the grooves is provided in the pix
Kurihara Takashi
Yamada Nobuaki
Schechter Andrew
Sharp Kabushiki Kaisha
Ton Toan
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