Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-03-31
2002-01-01
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S139000
Reexamination Certificate
active
06335771
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device incorporating Thin-Film Transistors and other switching elements, for use in display devices such as computer and television devices, and to a method of manufacturing said liquid crystal display device.
BACKGROUND OF THE INVENTION
In the past, liquid crystal display devices of the active matrix type (using Thin-Film Transistors (TFTs), etc.) or simple matrix type (Super-Twisted Nematic (STN), etc.) have been in use. It is well known that in all of these liquid crystal display devices, the light transmittance was dependent on the viewing angle, because the light passed through liquid crystals of different relative alignment states according to the angle from which the screen was viewed. In particular, such screens were difficult to see when viewed from the side. Accordingly, there has been much research seeking to improve these viewing angle characteristics.
FIG. 13
is a plan view showing the structure of one pixel in the active matrix substrate of the liquid crystal display element of Japanese Unexamined Patent Publication No. 5-273569/1993. As shown in
FIG. 13
, on the active matrix substrate, a Thin-Film Transistor
55
is provided adjacent to the intersection of a gate line
52
with a source line
53
.
As shown in
FIG. 14
, the TFT
55
is made up of the following layered on a transparent insulating substrate
51
a
of glass or similar material: a gate electrode
56
which is connected to the gate line
52
, a gate insulating film
57
, and a semiconductor layer
58
, which is made of amorphous silicon and which is layered on the gate electrode
56
. On the semiconductor layer
58
, in sections in order to partially cover the semiconductor layer
58
, are n
+
—Si layers
59
, which serve as ohmic contact layers. On one n
+
—Si layer
59
is a source electrode
60
, which is connected to the source line
53
, and on the other n
+
—Si layer
59
is a drain electrode
61
, which is connected to a pixel electrode
62
.
The pixel electrode
62
is provided in the rectangular area bordered by the gate line
52
and the source line
53
, and distributed on the pixel electrode
62
in islet form are transparent insulating films
63
made of SiN
X
, SiO
2
, or similar material. Each transparent insulating film
63
also serves as a protective film. The active matrix substrate is completed with the covering of TFT
55
, pixel electrode
62
, and transparent insulating films
63
with an alignment film (not shown).
On a transparent insulating substrate
51
b,
provided opposite the active matrix substrate, are layered a counter electrode
65
and an alignment film (not shown), in that order. The liquid crystal display element is completed by filling the space between the two substrates with a liquid crystal
66
.
In a liquid crystal display device of this structure, at a′, where the transparent insulating films
63
do not cover the pixel electrode
62
, the image signal voltage is applied directly to the counter electrode
65
, but at b′, where the transparent insulating films
63
are provided, a divided capacitance voltage is applied, because the image signal voltage is applied through the serial capacitance of the capacitance of the liquid crystal
66
and the capacitance of the transparent insulating film
63
.
In this way, two domains are created within one pixel, each of which applies a different voltage to the liquid crystal
66
, resulting in a different light transmittance of the liquid crystal
66
within each domain. Accordingly, viewing angle characteristics when viewing the screen from the side can be improved. Further, by forming a tapered section c′ around the edges of the transparent insulating films
63
, a clear image without rough edges can be obtained.
However, a disadvantage of the conventional structure outlined above is that the inclusion of the transparent insulating films
63
on the pixel electrode
62
creates a plurality of areas that have uneven surfaces, resulting in disturbance of alignment and deterioration of display characteristics. The thicker transparent the insulating films
63
are made (in order to increase the difference between the voltage applied at area a′ and that applied at area b′), the more likely the disturbance of alignment. Further, another problem with increasing the thickness of the transparent insulating films
63
is that gap control becomes difficult, because the cell gap at area a′ is different from that at area b′.
In order to solve these problems, Japanese Unexamined Patent Publication No. 7-175037/1995, shown in
FIG. 15
, disclosed a liquid crystal display device which would prevent the deterioration of alignment by providing a thicker alignment film
67
on the active matrix substrate side, and by making the interface between the alignment film
67
and the liquid crystal
66
a flat surface.
However, in making the surface of the alignment film
67
flat in order to prevent the disturbance of alignment, a minimum film thickness of approximately 0.5 &mgr;m (=500 nm) is necessary, but the applied voltage must be increased, leading to the problem of increased power consumption. Further, the polyimide generally used for the alignment film
67
is not completely colorless, thus decreasing the display quality.
In general, the active-matrix-type liquid crystal display devices have had a comparatively wide viewing angle with good display quality, but the simple-matrix-type liquid crystal display devices have had a narrower viewing angle. For this reason, users selected the type of liquid crystal display device according to their needs, using the active-matrix-type devices having wider viewing angles, for example, for regular use in the office and elsewhere, or for presentations, while using the simple-matrix-type devices having narrower viewing angles in situations calling for privacy, for example, when preparing documents aboard aircraft or elsewhere in public. However, since there were cases when the same liquid crystal display device was used both in the office and aboard aircraft and elsewhere, purchasing a device with a viewing angle suited for one type of use created inconveniences when using the device in other ways.
Japanese Unexamined Patent Publication No. 6-59287/1994, discloses a liquid crystal display device which can meet both types of needs with one device. This liquid crystal display device, as shown in
FIG. 21
, controls the viewing angle using a TN-type liquid crystal display panel
151
and, for viewing angle control, a guest-host-type liquid crystal panel
152
. In concrete terms, when using the device in the office or for a presentation, no voltage is applied to the guest-host-type liquid crystal panel
152
, thus scattering the light and enabling a wide viewing angle (see FIG.
21
(
a
)); when using the device aboard an aircraft or where viewing of screen images by others is unwanted, a voltage is applied to the guest-host-type liquid crystal panel
152
, allowing the light to be transmitted in one direction only, increasing the parallelism of the backlight, and thereby enabling a narrow viewing angle (see FIG.
21
(
b
)).
However, since this liquid crystal display device uses two liquid crystal panels, the thickness and weight of the device as a whole is increased, as are costs. Further, power to drive the two liquid crystal panels is necessary, as well as power to ensure that the backlight is not dimmed due to passing through the two liquid crystal panels, resulting in the problem of increased power consumption. For these reasons, the liquid crystal display device discussed above could not be used for laptop-type personal computers or other portable information terminals.
SUMMARY OF THE INVENTION
The primary object of this invention is to provide a liquid crystal display device which enables widening of the angle of visibility by effectively forming within a single pixel domains with different viewing angle characteristics, but without giving rise to
Chowdhury Tarifur R.
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Sikes William L.
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