Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-03-09
2004-03-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S143000, C349S123000
Reexamination Certificate
active
06704084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid-crystal display, particularly to a liquid-crystal display in which a potential of an alignment film is controlled.
2. Discussion of Related Art
Liquid-crystal displays of various modes have been developed so far. For example, there are TN (Twisted Nematic) mode and VA (Vertical Alignment) mode. The TN mode seals a nematic liquid crystal having a positive dielectric anisotropy between two glass substrates and twists an arrangement of liquid crystals by using an alignment film formed on both the substrates. By applying an electric field to the liquid crystals, the arrangement of the liquid crystals is controlled in the vertical direction to perform screen display by using polarization of transmitted light. The VA mode arranges liquid crystals having negative dielectric anisotropy in a direction vertical to a glass substrate. Transmission of light is controlled by applying an electric field to the liquid crystals and thereby tilting the direction of the liquid crystals.
In addition to liquid-crystal displays having the above modes, a liquid-crystal display is known which has a mode referred to as IPS (In Plane Switching) developed to improve a viewing angle. The liquid-crystal display controls transmission of light by operating a direction of a liquid crystal in a plane parallel with a glass substrate and has a structure in which electrodes for applying an electric field to liquid crystals are formed on the same substrate.
FIG. 7
shows a conventional IPS-mode TFT liquid-crystal display, which is an illustration showing a configuration of one of sub-pixel portions arranged like a matrix in a display pixel area on a TFT array substrate. In
FIG. 7
, reference numeral
702
denotes a plurality of pairs of common wirings extending in one direction in parallel with each other and
703
denotes gate wirings extending in one direction in parallel with each other and arranged in parallel with the common wirings.
Reference numeral
704
denotes a plurality of source wirings extending in one direction in parallel with each other and arranged in a direction almost orthogonal to the common wiring
702
and the gate wiring
703
. Reference numeral
705
denotes a sub-pixel portion that is enclosed by the common wiring
702
, gate wiring
703
, and source wiring
704
. Reference numeral
706
denotes a TFT serving as a switching device. The TFT
706
is constituted by arranging a source electrode
707
extending along the gate wiring
703
from the source wiring
704
and a drain electrode
708
in parallel with each other at both the sides of a constant channel
709
. The gate wiring
703
is formed below the channel
709
formed of a semiconductor layer through an insulating film (not illustrated).
Two common electrodes
710
extend downward in
FIG. 7
from the common wiring
702
along two source wirings
704
while the drain electrode
708
of the TFT
706
is connected to a pixel electrode
711
. The pixel electrode
711
extends upward in parallel with these two common electrodes
710
between two common electrodes
710
. Moreover, a plurality of pairs of parallel electrodes are formed in one pixel and contrast of pixels is produced by controlling an orientation of liquid crystals in accordance with the intensity of an electric field between these electrodes to provide a display screen. Some of electric charges supplied to the pixel electrode
711
from the source wiring
704
through the TFT
706
are held by a storage capacitance
712
.
Though not shown in
FIG. 7
, it is needless to say that similarly to an ordinary TFT liquid-crystal display an alignment film is formed on a TFT array substrate having the above configuration and surfaces of counter substrates arranged in parallel with each other separately from the array substrate by a predetermined gap and the gap between these two substrates is filled with liquid crystal. Moreover, the portion shown in
FIG. 7
shows a pixel portion of each of R, G, and B constituting one pixel when performing color displaying.
FIG. 8
is a sectional view showing a configuration of a liquid-crystal cell of a conventional IPS-mode TFT liquid-crystal display. Reference numeral
801
denotes a liquid-crystal cell having a function for displaying video information on a screen as the information of light transmittance of the pixel
705
by electrooptically converting an electrical signal including the video information input from a driving circuit. Reference numeral
802
denotes a TFT array substrate. A signal input from a driving circuit is distributed to the pixel electrode
711
through the gate wiring
703
, source wiring
704
, and TFT
706
. The sub-pixels
705
are arranged in a display pixel area
817
like a matrix. A common potential is distributed to the common electrode
710
in the pixel
705
through a common-potential supply wiring
819
and the common wiring
702
.
A gate insulating film
804
and a passivation film
805
are formed on the TFT array substrate
802
. Reference numeral
803
denotes a counter substrate. The following are formed on the counter substrate
803
: a black matrix
807
for shading a boundary portion of the pixel
705
and a display-screen circumferential area
818
, a color filter
808
for dividing light into three primary colors of R, G, and B, and a protective film
809
. The TFT array substrate
802
and the counter substrate
803
are arranged in parallel with each other while keeping a predetermined gap between them and liquid crystal
810
is sealed between them. An alignment film
806
for determining the initial orientation of the liquid crystal
810
is formed on faced surfaces of two substrates. Moreover, a polarization film
811
is formed on the outside surfaces of two substrates.
Because potentials different from each other are supplied to the common electrode
710
and the pixel electrode
711
, an electric field is generated between the two electrodes, the electric field works on the liquid crystal
810
whose initial orientation is determined by the alignment film
806
in a direction different from the direction of the electric field and orientations of the liquid crystal are changed. In this case, the way of change of orientations of liquid crystal depends on the intensity of an electric field. When polarized light obtained after the light emitted from the backlight
812
passes through the polarization film
811
, passes through the layer of the liquid crystal
810
, intensities of the light can be changed due to orientation change of liquid crystal. Therefore, it is possible to change intensities of the light emitted from the other polarization film
811
. Thereby, it is possible to change the information of an electrical signal to the information of intensity of light.
In the case of the conventional IPS-mode TFT liquid-crystal display shown in
FIGS. 7 and 8
, an alignment film is electrically floated as an insulating film is present between two electrodes and the alignment film in the display pixel area on the TFT array substrate. The two electrodes are for supplying an electric field to liquid crystal, that is, a pixel electrode and a common electrode. The alignment film is for providing initial orientation for liquid-crystal molecules. Moreover, because only materials having a high electric resistance are formed on a counter substrate, an alignment film formed on the counter substrate is also electrically floated. In the case of the above electrically insulated alignment film, there is a deviation in the direction of an electric field passing through an alignment film. Thereby, impurity ions corresponding to the direction of the electric field are easily collected on the film. Because ions are originally easily adsorbed by an alignment film, a screen display trouble such as an after-image or image-sticking occurs due to collected impurity ions.
Moreover, in a circumferential area nearby a display pixel area, particularly an area for extending a gate wiring to a driving circuit, the gate-w
Kimura Shin-ichi
Kodate Manabu
Kusafuka Kaoru
Shimizu Hidehisa
Chowdhury Tarifur R.
Qi Mike
Trepp Robert M.
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