Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-08-17
2002-12-31
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000
Reexamination Certificate
active
06501524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to liquid crystal display devices, and more particularly to a homeotropic alignment type liquid crystal display device which is suited for use in electronic equipments such as television sets, personal computers, monitoring units and projection type display units (projectors).
2. Description of the Related Art
As thin film transistor (TFT) type liquid crystal display (LCD) devices, there twisted nematic (TN) type LCD devices which employ a normally white mode.
FIGS. 1A and 1B
are diagrams for explaining the operating principle of a TN type LCD panel structure. As shown in
FIGS. 1A and 1B
, alignment layers are provided on corresponding transparent electrodes
12
and
13
which are formed on corresponding glass substrates, with a difference of 90° in alignment directions, so as to sandwich TN liquid crystals therebetween. The liquid crystals contacting the alignment layer are arranged in the alignment direction of the alignment layer due to the nature of the liquid crystals. Hence, as shown in
FIG. 1A
, the liquid crystals are aligned such that the direction of the liquid crystal molecules is twisted by 90°. A pair of polarizing plates
11
and
15
are respectively arranged on respective sides of the transparent electrodes
12
and
13
, in parallel with the alignment directions of the alignment layers.
When a non-polarized light
10
becomes incident to the LCD panel structure described above, the light passing through the polarizing plate
11
becomes a linearly polarized light before reaching the liquid crystals. Since the liquid crystal molecules are aligned with the 90° twist, the incident light is also twisted by 90° when passing through the liquid crystal molecules. As a result, the incident light passes through the lower polarizing plate
15
. This state is referred to as a bright state.
Next, when a voltage is applied on the liquid crystal molecules by applying a voltage across the transparent electrodes
12
and
13
as shown in
FIG. 1B
, the liquid crystal molecules stand and the above described twist is eliminated. But since the alignment restricting force is strong at the alignment layer surface, the alignment direction of the liquid crystal molecules is still along the alignment layer. In such a state, the liquid crystal molecules are isotropic with respect to the passing light, and the polarizing direction of the linearly polarized light incident to the liquid crystals will not rotate. Accordingly, the linearly polarized light passing through the upper polarizing plate
11
cannot pass through the lower polarizing plate
15
. This state is referred to as a dark state. Thereafter, when the voltage is no longer applied across the transparent electrodes
12
and
13
, the liquid crystals return to the bright state due to the alignment restricting force.
On the other hand, as TFT type LCD devices, there are vertically aligned type LCD devices which employ a normally black mode.
FIGS. 2A and 2B
are diagrams for explaining the operating principle of a VA type LCD panel structure. As shown in
FIG. 2A
, alignment layers are provided on corresponding transparent electrodes
22
and
23
which are formed on corresponding glass substrates, with a difference of 180° in alignment directions, so as to sandwich negative type liquid crystals (liquid crystals having negative dielectric constant anisotropy) therebetween. A pair of polarizing plates
21
and
25
are respectively arranged on respective sides of the transparent electrodes
22
and
23
, with a 45° difference to the alignment directions of the alignment layers. Axes of polarization of the two polarizing plates
21
and
25
are perpendicular to each other. As show in
FIG. 2A
, when no voltage is applied across the transparent electrodes
22
and
23
and thus no voltage is applied on the liquid crystal molecules, the liquid crystal molecules stand. However, because the alignment restricting force is strong at the alignment layer surface, the alignment direction of the liquid crystal molecules remain generally along the alignment layer (or slightly inclined to the alignment layer in some cases). In such a state, the liquid crystal molecules are isotropic with respect to the passing light. For this reason, when a non-polarized light
20
becomes incident to the LCD panel structure, the light passing through the polarizing plate
21
becomes a linearly polarized light before reaching the liquid crystals, but no change occurs in the polarized state of this linearly polarized light. Hence, the linearly polarized light passing through the upper polarizing plate
21
cannot pass through the lower polarizing plate
25
, and thereby resulting in a dark state.
In addition, when a voltage is applied on the liquid crystals by applying a voltage across the transparent electrodes
22
and
23
as shown in
FIG. 2B
, the liquid crystals contacting the alignment layer are arranged along the alignment direction of the alignment layer due to the nature of the liquid crystals. Moreover, the liquid crystal molecules which are aligned cause other liquid crystal molecules to align thereto, and as a result, the liquid crystal molecules as a whole become aligned in one direction, that is, in an approximately horizontal direction with respect to the electrode surface. When the non-polarized light
20
becomes incident to the LCD panel structure, the light passing through the polarizing plate
21
becomes a linearly polarized light before reaching the liquid crystals. The light incident to the liquid crystal molecules can pass through the lower polarizing plate
25
due to a birefringence of the liquid crystals changing the polarized state, since a major axis direction of the liquid crystal molecules and the polarizing direction form an angle of 45° thereby resulting in a bright state. Thereafter, when the voltage is no longer applied across the transparent electrodes
22
and
23
, the liquid crystals return to the dark state because the liquid crystal molecules become approximately vertically aligned (form an approximate homeotropic alignment) with respect to the electrode surface (substrate surface) due to the alignment restricting force.
Compared to the TN type LCD device, the VA type LCD device has a high display contrast, a high response speed, and satisfactory viewing angle characteristics with respect to white and black displays.
Conventionally, various alignment processes have been proposed to align the liquid crystals of the LCD panel depending on the alignment layer. However, because of the weak alignment restricting force, the alignment was easily disturbed due to effects of an electrical field and the like, and there was a problem in that a disclination is easily generated. The disclination refers to a part where the alignment of the liquid crystals becomes discontinuous. In addition, when a rubbing process is carried out to align the liquid crystals, the alignment restricting force becomes relatively strong, but unevenness is easily generated in the alignment, and it was difficult to stably align the liquid crystals.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful liquid crystal display device in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a liquid crystal display device which can suppress the disclination and stably align the liquid crystals.
Still another object of the present invention is to provide a homeotropic alignment type liquid crystal display device comprising first and second substrates confronting each other, first and second bus lines arranged in mutually perpendicular directions on the second substrate, liquid crystals provided between the first and second substrates, and first, second and third projecting structures restricting alignment of the liquid crystals, where the first projecting structures have a sloping surface which is inclined with respect to the first
Gotoh Takeshi
Kobayashi Tetsuya
Koike Yoshio
Mayama Takatoshi
Nakanishi Yohei
Fujitsu Limited
Greer Burns & Crain Ltd.
Nguyen Hoan
Sikes William L.
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