Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-03-30
2003-01-14
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000, C349S130000, C349S139000
Reexamination Certificate
active
06507382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an active matrix type of liquid-crystal display where thin film transistors (TFTS) arranged in a matrix form are used as switching elements.
2. Description of the Related Art
An active matrix type of TFT liquid-crystal display using TFTs as switching elements has been utilized as a high-quality flat display in a wide variety of applications. In a conventional TFT liquid-crystal display, a liquid-crystal layer is driven by applying an electric field vertical to a substrate between electrodes sandwiching the liquid-crystal layer, typically in a twisted nematic display.
This type of TFT liquid-crystal display, however, has a relatively narrower angle of visibility and a poor contrast, resulting in a whitish screen and a narrow angle of visibility. Thus, there is room for improvement.
JP-As 4-261552 and 6-43461 have suggested techniques for improving visual-angle properties of a liquid-crystal display. In these techniques, a homeotropically oriented liquid-crystal cell is prepared; the cell is sandwiched between two polarizing plates whose polarization axes are perpendicular to each other; a common electrode having an opening is used to generate an oblique electric field in each pixel for dividing each pixel into two or more liquid-crystal domains and thus improving the visual-angle properties. In particular, in JP-A 4-261552, a high contrast can be achieved by controlling a tilting direction of the liquid-crystal during voltage application. In a technique described in JP-A 6-43461, an optical compensator is used as appropriate, to improve visual-angle properties in black. In addition, in JP-A 6-43461, each pixel is divided into two or more domains with an oblique electric field not only in a homeotropically oriented liquid-crystal cell but also in a TN oriented cell, to improve visual-angle properties.
JP-A 5-505247 has suggested an IPS (In-Plane Switching) mode of liquid-crystal display where both of two electrodes are formed on one substrate for allowing a crystal molecule to be rotated while being parallel to the substrate and a voltage is applied between the electrodes to generate an electric field parallel to the substrate. In this type of display, the longitudinal axis of the crystal molecules does not rise in relation to the substrate when applying a voltage. Thus, change in a birefringence of the liquid crystal is small when changing a line of sight, and therefore a wide angle of visibility can be achieved.
There will be described an active matrix type of TFT liquid-crystal display employing an IPS mode where both of two electrodes are formed on one substrate as described above.
FIGS.
5
(
a
)-
5
(
b
) show a conventional TFT liquid employing an IPS mode, where FIG.
5
(
a
) is a cross section taken on line A-A′ in FIG.
5
(
b
).
In FIG.
5
(
a
), a gate electrode
502
made of Cr and a common electrode
503
are formed on a glass substrate
501
, and a gate insulating film
504
made of silicon nitride is formed, covering these electrodes.
On the gate electrode
502
, there is formed a semiconductor film
505
made of amorphous silicon acting as an active layer for a transistor, via the gate insulating film
504
. A drain electrode
506
made of molybdenum and a source electrode
507
are superposed on a part of the pattern of the semiconductor film
505
, and a protective film
508
made of silicon nitride is formed over all of the elements.
In FIG.
5
(
b
), a region for one pixel is disposed between the source electrode
507
and the extracted common electrode
503
.
There is formed an oriented film OR
11
which has been rubbed, on the active matrix substrate in which unit pixels constructed as described above are arranged in a matrix form.
On the other hand, a color filter
532
delimited by a shield
533
, and a protective film
534
is formed over these elements. Again, an oriented film OR
12
which has been rubbed is formed on the protective film
534
.
The glass substrate
501
and the opposed substrate
531
are disposed in a manner that their oriented films OR
11
and OR
12
face to each other, with a space which is filled with a liquid-crystal composition
540
. Polarizing plates are formed on the outer faces of the glass substrate
501
and the opposed substrate
531
. The shield
533
dividing the color filter
532
is formed such that a part of the shield is superposed over a thin film transistor consisting of the semiconductor film
505
.
In a TFT liquid-crystal display constructed as described above, crystal molecules
541
a
are substantially parallel to the extended electrode direction when an electric field is not applied to the crystal composition
540
. Specifically, the liquid-crystal molecules
541
a
are oriented such that the longitudinal (optical axis) direction of the liquid-crystal molecules
541
a
forms an angle of at least 45° to less than 90° to the direction of the electric field generated between the source electrode
507
and the extracted common electrode
503
. The mutually facing glass substrate
501
and opposed substrate
531
are parallel to the liquid-crystal molecules. The dielectric anisotropy of the liquid-crystal molecules
541
a
herein is positive.
On turning on the thin film transistor (TFT) by applying a voltage to the gate electrode
502
, a voltage is applied to the source electrode
507
and then an electric field is induced between the source electrode
507
and the opposed common electrode
503
. The electric field then changes the direction of the liquid-crystal molecules from
541
a
to
541
b
. The liquid-crystal molecules
541
b
are substantially parallel to the direction of the electric field generated between the source electrode
507
and the opposed common electrode
503
.
Since the molecular axis of the liquid-crystal molecules are controlled as described above in an IPS mode, the polarized-light transmitting axis of the polarizing plate
551
may be set at a given angle to change a light transmittance.
As described above, an IPS mode of TFT liquid-crystal display may provide a contrast without a transparent electrode. Furthermore, the longitudinal axis of the liquid-crystal molecules is substantially parallel to the substrate face in the above IPS mode of TFT liquid-crystal display, and therefore, does not rise by voltage application. Thus, in such a display, there may occur a small change in brightness when changing a visual angle, and visual properties may be improved.
Journal of Applied Physics, Vol. 45, No. 12, p.5466 (1974) and JP-A 10-186351 have disclosed, besides the above IPS mode, a mode where a liquid-crystal having a positive dielectric anisotropy is homeotropically oriented to a vertical direction to a substrate and the liquid-crystal molecules are oriented to a direction parallel to the substrate by an electric field parallel to the substrate, where the homeotropically oriented liquid-crystal molecules are divided into two or more regions whose tilting angles are different from each other, due to the direction of the electric field.
However, since a color filter is disposed between a layer on which liquid crystal is placed and an opposed substrate in the above prior art, an electric field generated by applying a voltage between a source electrode and a common electrode affects a color filter, leading to deterioration of display properties for the TFT liquid-crystal display. Pigments in a color filter contain impurities such as sodium ions, causing charging of the color filter when applying an electric field to the filter. The opposed substrate is made of an insulating material. Therefore, when the color is charged, an unwanted electric field may remain in the liquid-crystal under the charged region, which adversely affects the display properties.
To solve the problem, a color filter might be formed on the substrate comprising TFTs (hereinafter, referred to as a “TFT substrate”) instead on the opposed substrate. There are a variety of interconnecting layers on the TFT substrate. Such a configuration might, t
Ihara Hirofumi
Kikkawa Hironori
Matsuyama Hiroaki
Nakata Shinichi
Okamoto Mamoru
Duong Tai
Hayes & Soloway PC
NEC Corporation
Sikes William L.
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