Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-18
2004-06-15
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06750934
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display of an active matrix type, and more particularly to an active-matrix liquid crystal display of a multi-domain type with favorable viewing angle characteristics.
2. Description of the Prior Arts
Active-matrix liquid crystal displays which employ TN (twisted nematic) type liquid crystal have been widely used conventionally. An active-matrix liquid crystal display using the TN type liquid crystal comprises: a TFT (thin film transistor) substrate provided with a TFT for driving and a pixel electrode for each pixel; an opposite substrate having an opposite electrode thereon and disposed opposite to the TFT substrate; a color filter; and a polarizer. Liquid crystal is filled between the TFT substrate and the opposite substrate. In this liquid crystal display, when no voltage is applied between the pixel electrode and the opposite electrode, the TN liquid crystal molecules are aligned in parallel with the surface of the TFT substrate at that surface, and the alignment direction varies along the direction perpendicular to the substrate, resulting in twisted alignment of the liquid crystal. On the other hand, when a voltage is applied, the liquid crystal molecules between the substrates rise to change the polarization state in the liquid crystal layer. While the liquid crystal display using the TN type liquid crystal realizes selective display by utilizing the aforementioned change in the polarization state in the liquid crystal layer, visibility depends on the relationship between the alignment direction of the liquid crystal molecules and the position of a viewer, thereby presenting a problem of a small range of viewing angles for obtaining optimal visibility and resulting in insufficient viewing angle characteristics.
To overcome the problem, liquid crystal of a VA (vertical aligned) type has been proposed and put to practical use instead of the TN type liquid crystal for improving the insufficient viewing angle characteristics of the TN type liquid crystal display. A VA type liquid crystal display comprises a liquid crystal cell formed by liquid crystal in homeotropic (vertical) alignment with negative dielectric constant anisotropy between a TFT substrate and an opposite substrate, in which the liquid crystal molecules stand upright on the substrate when no voltage is applied, and when a voltage is applied, the liquid crystal molecules are laid in the substrate plane direction by an oblique electric field produced between a pixel electrode and an opposite electrode, thereby performing display. In this case, an area for one pixel is divided into a plurality of divided areas, and the directions in which the liquid crystal molecules are laid are varied among the divided areas to average the ways the display is viewed on the whole, which results in a wide viewing angle and favorable viewing angle characteristics. The divided area is also referred to as an alignment area. Such a liquid crystal display is referred to as a VA type multi-domain liquid crystal display.
While one pixel area is divided into a plurality of divided areas in accordance with directions in which the liquid crystal molecules are laid as described above in the VA type multi-domain liquid crystal display, it is important to provide stable dividing (partitioning) positions, that is, positions of boundaries between alignment areas. The dividing positions are boundaries of divided areas and the position in which so-called disclination occurs, and if division is made at an unexpected position, display quality is deteriorated.
As an attempt to stabilize the positions at which each pixel is divided into divided areas in the VA type multi-domain liquid crystal display, Japanese Patent Laid-open Publication No. 7-311383 (JP, A, 0731183) discloses a display which comprises an alignment control bank portion for each of a TFT substrate and an opposite substrate such that the positions at which the alignment control bank portions are provided are stabilized as dividing positions.
FIG. 1
is a sectional view showing an example of a configuration of a VA type multi-domain liquid crystal display including an alignment control bank portion for each of a TFT substrate and an opposite substrate.
FIG. 2
is a plan view schematically showing the arrangement of projections serving as the alignment control bank portions in the liquid crystal display shown in FIG.
1
.
In the VA type multi-domain liquid crystal display shown in
FIG. 1
, liquid crystal layer
83
including a liquid crystal material with negative dielectric constant anisotropy is provided between TFT substrate
81
and opposite substrate
82
disposed opposite to each other. TFT substrate
81
comprises, on transparent support member
86
, TFTs (thin film transistors)
87
for respective pixels and pixel electrodes
88
each electrically connected to a source electrode of corresponding TFT
87
. On each pixel electrode
88
, linear projections
89
each having a generally triangular cross section are provided as the alignment control bank portions. On the other hand, opposite substrate
82
has a structure in which color filter
92
and opposite electrode
93
are stacked in this order on transparent support member
91
. On opposite electrode
93
, linear projections
94
each having a generally triangular cross section are provided as the alignment control bank portions. As shown in
FIG. 2
, linear projections
89
,
94
extend alternately in zigzag shape on TFT substrate
81
and opposite substrate
82
, respectively.
Liquid crystal molecules
95
tend to be aligned perpendicularly to the surface of pixel electrode
88
and the surface of opposite electrode
93
when no voltage is applied. However, since the surfaces of projections
89
and
94
are inclined to the surfaces of pixel electrode
88
and opposite electrode
93
, liquid crystal molecules
95
tend to be aligned in a direction slightly deviated from the direction perpendicular to the surfaces of pixel electrode
88
and opposite electrode
93
. This affects the entire liquid crystal layer
83
, and as a result, liquid crystal molecules
95
are aligned in a direction slightly deviated from the perpendicular direction even when no voltage is applied, as shown in FIG.
1
. When a voltage is applied between pixel electrode
88
and opposite electrode
93
, the alignment of liquid crystal molecules
95
is changed such that they are inclined in a direction deviated further from the perpendicular direction. The direction of the inclination when no voltage is applied depends on the positional relationships between liquid crystal molecules
95
and linear projections
89
,
94
as shown in FIG.
2
.
As a result, projections
89
and
94
determine the dividing position between divided areas A and B to stabilize the dividing position in this VA type multi-domain liquid crystal display.
In addition, Japanese Patent Laid-open Publication No. 8-76125 (JP, A, 08076125) proposes a display in which an opposite electrode itself is divided to stabilize dividing positions instead of using the aforementioned alignment control bank portions (i.e., linear projections). In a VA type liquid crystal display using a liquid crystal material with negative dielectric constant anisotropy, when a voltage is applied, the liquid crystal molecules tend to be aligned in the direction perpendicular to the electrical field. As shown in
FIG. 3
, division of opposite electrode
93
by slit
96
and pixel electrode
88
already provided for each pixel produce wraparound electric fields as shown in arrows in
FIG. 3
between pixel electrode
88
and opposite electrode
93
when a voltage is applied. In the area in which the wraparound electric field is produced, i.e. near the ends of pixel electrode
88
and near the ends of opposite electrode
93
, liquid crystal molecules
95
tend to be aligned perpendicularly to the direction of the wraparound electric field when a voltage is applied. For this reason, liquid crystal molecules
95
Hirai Yoshihiko
Ishii Toshiya
Kawada Kiyomi
Matsuyama Hiroaki
Okamoto Mamoru
Hayes & Soloway P.C.
Kim Robert H.
NEC LCD Technologies Ltd.
Rude Timothy L
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