Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-02-28
2003-07-22
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S141000
Reexamination Certificate
active
06597414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Liquid-Crystal Display (LCD) device and more particularly, to an LCD device using the horizontal electric-field running approximately parallel to a pair of glass substrates in a liquid crystal layer, which prevents the display quality from degrading due to static electricity charged on one or both of the substrates and so on.
2. Description of the Prior Art
In recent years, LCD devices have been extensively used as display terminals for electronic equipment, such as personal computers and word processors. This is because LCD devices have advantages that they dissipate low electric power, they are compact and light-weight, and they are operable at a low supply voltage.
LCD devices have various driving methods such as the static driving, multiplex driving, and active matrix driving methods, all of which have been used in practice. Nowadays, the active matrix driving method has formed its main stream.
LCDs using the active matrix driving method, i.e., active-matrix LCDs, are divided into two groups by the difference in electrode structure; the vertical electric-field and horizontal electric-field types.
With the vertical electric-field type LCDs, a liquid crystal is confined in the space between first and second glass substrates coupled together to be opposite and parallel. Display electrodes for forming pixels and Thin-Film Transistors (TFTs) for selecting the pixels to be driven are formed on the first substrate. A common electrode is formed on the second substrate to be opposite to the display electrodes. A driving voltage is selectively applied across desired ones of the display electrodes and the common electrode, thereby applying a vertical electric-field to the liquid crystal. Thus, the orientation of the molecules of the liquid crystal is changed continuously from the horizontal direction to the vertical one, thereby controlling or modulating the transmitted light through the liquid crystal layer.
The vertical electric-field type LCDs have the following disadvantage.
Specifically, when a user look at the display area of the LCD device of this type obliquely, he tends to see sudden change in brightness or inversion of light and shade. In other words, the LCDs of this type have a bad viewing-angle characteristic. This is because the molecules of the liquid crystal that have been aligned vertically with respect to the substrates have different apparent lengths for the user.
On the other hand, with the horizontal electric-field type LCDs, a liquid crystal is confined in the space between first and second glass substrates coupled together to be opposite and parallel. This structure is the same as that of the vertical electric-field type ones. Unlike the vertical electric-field type ones, display electrodes for forming pixels, TFTs for selecting the pixels to be driven, and reference electrodes are formed on the first substrate. No electrodes are formed on the second substrate. A driving voltage is selectively applied across desired ones of the display electrodes and those of the reference electrodes, thereby applying a horizontal electric-field to the liquid crystal. Thus, the orientation of the molecules of the liquid crystal is changed continuously from the vertical direction to the horizontal one, thereby controlling or modulating the transmitted light through the liquid crystal layer.
With the horizontal electric-field type LCD devices, unlike the vertical electric-field type ones, the above-described disadvantage of the bad viewing-angle characteristic can be improved. This is because the molecules of the liquid crystal that have been aligned horizontally with respect to the substrates have a small difference in apparent length. However, the LCD devices of this type might have a problem that the display quality tends to degrade. For example, if the LCD device is of the normally black mode, the display area tends to be partially or entirely turned white. If the LCD device is of the normally white mode, it tends to be partially or entirely turned black. The problem is caused by the following reason.
Specifically, the second substrate (to which no electric-field is directly applied) might be electrified due to static electricity or some unintended electric-field, which may be applied from the outside of the LCD device. In this case, electric charges are stored on the second substrate and therefore, they create unwanted vertical electric-field with respect to the first and second substrates in the liquid crystal layer. Thus, the orientation of the molecules of the liquid crystal tends to be changed toward the vertical direction due to the unwanted vertical electric-field, resulting in degradation of the display quality.
To solve the above-described problem of display quality degradation in the horizontal electric-field type LCD devices, various solutions have been developed and reported. An example of the solutions is disclosed in the Japanese Non-Examined Patent Publication No. 9-105918 published in April 1997.
FIGS. 1 and 2
show the structures of the prior-art horizontal electric-field type LCD devices disclosed in the Publication No. 9-105918.
In
FIG. 1
, a glass substrate
101
and another glass substrate
102
are coupled together to be opposite and parallel, forming a space therebetween. The space is sealed by a sealing member
109
located along the whole periphery of the substrate
101
. Liquid crystal LC is confined in the sealed space, forming a liquid-crystal layer. Although not shown in
FIG. 1
, display electrodes, reference electrodes, and TFTs are formed on the inner surface of the substrate
102
.
A transparent conductive layer
103
is additionally formed on the outer surface of the substrate
101
. A metal frame
104
, which surrounds entirely the substrate
101
as a casing, is mechanically and electrically connected to the conductive layer
103
at the periphery of the substrate
101
by way of a conductive rubber layer
105
. Thus, even if electric charges are stored on the substrate
101
, they are quickly discharged toward outside through the conductive layer
103
, the conductive rubber layer
105
, and the frame
104
, thereby preventing the display quality from degrading due to any unwanted vertical electric-field existing in the liquid crystal LC.
The structure of the prior-art LCD device shown in
FIG. 2
is the same as that shown in
FIG. 1
, except that the transparent conductive layer
103
is electrically connected to a grounding terminal
106
formed on the inner surface of the substrate
102
by way of a cable
107
.
The above-identified Publication No. 9-105981 further discloses that a conductive paste or a conductive metal tape may be used instead of the cable
107
. The conductive metal tape is formed by a metal tape and an adherent material containing a conductive substance.
The prior-art LCD devices shown in
FIGS. 1 and 2
have problems explained below.
With the prior-art LCD device shown in
FIG. 1
, the conductive rubber layer
105
is used to electrically connect the conductive layer
103
to the metal frame
104
and therefore, the thickness of the LCD device becomes larger. In recent years, there has been the strong need that the thickness of LCD devices is set to be possibly smaller. From this point of view, the structure of
FIG. 1
is not preferred.
Also, when the metal frame
104
is connected to the substrate
101
, the rubber layer
105
needs to be strongly pressed onto the substrate
101
in order to stabilize the contact resistance of the rubber layer
105
. At this stage, a counter force to the pressing force is applied to the frame
104
and as a result, some deformation tends to occur in the frame
104
. Thus, the pressing force is unable to be set to be as high as desired.
Furthermore, according to the result of the inventor's test, the following problem was found.
Specifically, when a pressure equal to or greater than a specific value is applied to the substrate
101
in the adhesion process of the frame
104
, there arise
McGinn & Gibb PLLC
NEC LCD Technologies Ltd.
Nguyen Dung
Ton Toan
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