Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Magnetic or pressure excitation
Reexamination Certificate
1998-09-03
2002-07-30
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Magnetic or pressure excitation
C349S143000, C345S087000
Reexamination Certificate
active
06426782
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device for use in television sets, personal computers, word processors, office automation (OA) equipments or the like, and also relates to a method for driving the same.
2. Description of the Related Art
Since liquid crystal display devices are thin and light, they are used in, for example, television sets, personal computers, word processors and OA equipment. Many of such liquid crystal display devices utilize the fact that liquid crystal molecules have an anisotropy of refractive index and an anisotropy of dielectric constant. In such liquid crystal display devices, a voltage is applied across a liquid crystal layer, whereby optical modulation is conducted by an electric field produced by the voltage.
Since liquid crystal display devices are thin and light, they are used in, for example, television sets, personal computers, word processors and OA equipment. Many of such liquid crystal display devices utilize the fact that liquid crystal molecules have an anisotropy of refractive index and an anisotropy of dielectric constant. In such liquid crystal display devices, a voltage is applied across a liquid crystal layer, whereby optical modulation is conducted by an electric field produced by the voltage.
In such a liquid crystal display device, gate lines and source lines are arranged in a matrix, and a pixel electrode and a thin film transistor are formed at each of the regions surrounded by the gate lines and the source lines. Thus, a voltage across each pixel electrode is controlled by a corresponding thin film transistor. Such a voltage application method will be described later in detail.
Hereinafter, a conventional liquid crystal display device will be described.
FIG. 15
is a cross sectional view schematically showing the conventional liquid crystal display device.
FIG. 16
is a plan view of a matrix substrate used in the conventional liquid crystal display device.
FIG. 17
is a cross sectional view taken along line
17
—
17
′ of FIG.
16
.
As shown in
FIG. 15
, the liquid crystal display device includes a matrix substrate
41
, a counter substrate
42
facing the matrix substrate
41
, and a liquid crystal layer
43
interposed therebetween. The liquid crystal layer
43
is formed by introducing a liquid crystal material into the gap between the matrix substrate
41
and the counter substrate
42
.
In the matrix substrate
41
, source lines
44
, gate lines
45
, thin film transistors
46
and pixel electrodes
47
are formed on a transparent substrate, as shown in FIG.
16
. The transparent substrate is formed from glass or the like. The source lines
44
and the gate lines
45
are arranged in a matrix. A voltage for each pixel electrode
47
is applied from a corresponding source line
44
via a corresponding thin film transistor
46
.
In the counter substrate
42
(not shown in FIG.
16
), a light-shielding film (not shown), a color filter (not shown) and a planar counter-electrode (not shown) are formed on a transparent substrate. The transparent substrate is formed from glass or the like. The light-shielding film has openings corresponding to the pixel electrodes
47
.
FIG. 17
shows a cross-sectional structure of the thin film transistor
46
. A semiconductor layer
50
is formed on a gate electrode
48
with an insulating film
49
interposed therebetween. The gate electrode
48
extends from a corresponding gate line
45
, as shown in
FIG. 16. A
source electrode
51
and a drain electrode
52
are formed thereon so as to be spaced apart from each other. The source electrode
51
extends from a corresponding source line
44
, and is electrically connected to the source line
44
. The drain electrode
52
is electrically connected to the pixel electrode
47
.
As described above, many of the liquid crystal display devices which are now widely used utilize the liquid crystal's anisotropy of dielectric constant.
A liquid crystal display device using a magnetic field is proposed in Japanese Laid-open Publication No. 7-64118. The liquid crystal has also an anisotropy of magnetic susceptibility. This liquid crystal display device utilizes such an anisotropy of magnetic susceptibility. As shown in
FIG. 18
, this liquid crystal display device includes a pair of substrates
54
and a liquid crystal layer
55
interposed therebetween, wherein one of the pair of substrates
54
has a ferromagnetic element
53
including portions
53
a
and
53
b
. The region of the liquid crystal layer
55
which is interposed between the portions
53
a
and
53
b
is controlled by changing a magnetization of the ferromagnetic-element
53
by an external means
56
for applying a magnetic field.
A magnetic energy density fm of the liquid crystal molecules present in the magnetic field is generally given by the following expression:
fm
=−1/2
&khgr;⊥H
2
−1/2&Dgr;&khgr;(
n·H
)
2
where &Dgr;&khgr;=&khgr;∥−&khgr;⊥: anisotropy of magnetic susceptibility;
&khgr;∥: magnetic susceptibility in an alignment direction;
&khgr;⊥: magnetic susceptibility in the direction perpendicular to the alignment direction; and
n: alignment direction of the liquid crystal molecules.
In the case where a magnetic field is applied to the liquid crystal molecules having a positive anisotropy &Dgr;&khgr;, a moment is generated so that a magnetic energy is minimized. In other words, the liquid crystal molecules are aligned parallel to the direction of the magnetic field. In the case where a magnetic field is applied to the liquid crystal molecules having a negative anisotropy &Dgr;&khgr;, a moment is generated so that a magnetic energy is minimized. In other words, the liquid crystal molecules are aligned perpendicular to the direction of the magnetic field.
It is understood from the foregoing that the alignment of the liquid crystal molecules can be controlled not only by the electric field but also by the magnetic field.
A conventional liquid crystal display device using an electric field utilizes thin film transistors for applying a signal voltage corresponding to a pixel to a corresponding pixel electrode.
Stable characteristics of the thin film transistors can be obtained by accurately aligning the respective patterns of the gate electrodes, semiconductor layer, source electrodes and drain electrodes with respect to each other. More specifically, a current flowing between the source and drain electrodes of each thin film transistor is proportional to a signal voltage applied to the source electrode, and substantially inversely-proportional to the distance between the source and drain electrodes. Moreover, a parasitic capacitance substantially proportional to the overlapping width of the gate electrode with each of the source and drain electrodes is produced in the thin film transistor. A potential at each pixel is determined by the current and parasitic capacitance as described above.
The distance between the source and drain electrodes is generally designed to about 10 &mgr;m, and the overlapping width of the gate electrode with each of the source and drain electrodes is generally designed in the range of about 1 &mgr;m to about 2 &mgr;m. Furthermore, an accuracy of about 1 &mgr;m or less is required with respect to the line width and the overlapping width. Therefore, highly accurate photolithography technology is conventionally used for the exposure step. In other words, the thin film transistors are produced using an high-performance exposure apparatus including a projection lens system, whereby the accuracy of about 1 &mgr;m or less is satisfied.
Moreover, the semiconductor layer for the thin film transistors is generally formed from amorphous silicon (a-Si). In order to form a high-quality a-Si film, a PE-CVD (plasma enhanced chemical vapor deposition) apparatus must be used.
Such a liquid crystal display device has a high display quality. However, an expensive production apparatus is required because a highly accurate photolithogra
Nguyen Dung
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Sikes William L.
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