LCD having electrode(s) outside display area which adsorb...

Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal

Reexamination Certificate

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C349S034000, C349S192000

Reexamination Certificate

active

06803976

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device for use as a display section of a notebook personal computer, a portable terminal apparatus, and the like.
2. Description of the Related Art
FIG. 9
is a circuit diagram illustrating a configuration of a conventional liquid crystal display device
10
.
The liquid crystal display device
10
includes a plurality of switching elements (TFTs
2
in this example) which are arranged in a matrix pattern on an active matrix substrate (TFT substrate). The liquid crystal display device
10
also includes gate signal lines
3
for supplying gate signals for driving the TFTs
2
and source signal lines
4
for supplying display signals (source signals) to the TFTs
2
. The gate signal lines
3
and the source signal lines
4
are arranged so as to cross each other. The gate electrode of each TFT
2
is electrically connected to the gate signal line
3
, the source electrode of each TFT
2
is electrically connected to the source signal line
4
, and the drain electrode of each TFT
2
is connected to a pixel electrode
1
and to one electrode of an auxiliary capacitor (Cs)
5
. The other electrode of the auxiliary capacitor (Cs)
5
is connected to a common line
6
. The TFT substrate opposes a counter substrate (color filter (CF) substrate) with a liquid crystal layer being interposed therebetween.
The liquid crystal display device
10
is driven, for example, by scanning the gate signal lines
3
upwardly or downwardly to turn ON the TFTs
2
along each gate signal line
3
. A source signal is applied to each pixel (across the liquid crystal layer in that pixel) so as to charge the liquid crystal layer and the auxiliary capacitor
5
of that pixel to the potential of the source signal, whereby the potential of the liquid crystal layer in each pixel is kept constant after the TFT
2
is turned OFF until the pixel is scanned in the next sequence. Thus, an image is displayed on the liquid crystal display device
10
.
When the liquid crystal material of the liquid crystal display device
10
is contaminated with an ionic impurity, some current is conducted through the liquid crystal layer before the next sequence so as to reduce the potential which has been applied across the liquid crystal layer. In such a case, a normal display cannot be maintained.
Such an ionic impurity may be any organic and inorganic impurity, e.g., Na
+
, Ca
2+
, Cu
2+
, Cl

, OH

, COOH

, or the like. Such an ionic impurity may easily be introduced into the liquid crystal material during the production process of the liquid crystal display device.
In recent years, liquid crystal display devices have been used in portable terminal apparatuses. Therefore, attempts have been made in the art to reduce the power consumption of the liquid crystal display devices so that the portable terminal apparatuses can be used outdoor for a long period of time. Accordingly, it has been necessary to develop a liquid crystal material which can be driven with a low voltage. However, the capability of being driven with a low voltage means that the liquid crystal material has a large dielectric anisotropy, which in turn means that the liquid crystal material itself has a potential. Such a liquid crystal material itself is likely to attract an ionic substance, thereby increasing the probability that the liquid crystal material may be contaminated during the production process of the liquid crystal display device.
It is well known in the art that increasing the auxiliary capacitance Cs is effective to address these problems. However, increasing the auxiliary capacitance Cs has a problem of reducing the aperture area of each pixel. Then, in order to achieve a display brightness of a liquid crystal display device which is equivalent to that of other conventional liquid crystal display devices, it is necessary to increase the illuminance of the back light, which is the light source of the liquid crystal display device. However, the power consumption of a back light typically accounts for about ⅔ of the total power consumption of the liquid crystal display device. Therefore, the power consumption of the liquid crystal display device as a whole cannot be reduced in this way.
These problems have been addressed in the art by, for example, Japanese Laid-Open Publication Nos. 4-125617, 4-295824, 6-289408 and 8-201830, which disclose methods in which the surrounding region of the display pixel area is provided with an electrode pattern. An electric signal having a DC component is externally applied to the electrode pattern to adsorb the ionic impurity which has been introduced into the liquid crystal layer onto the electrode pattern, so as to maintain the purity of the liquid crystal layer in the display pixel area.
However, such conventional methods in which an electrode pattern is provided in the surrounding region of the display pixel area have the following problems.
In Japanese Laid-Open Publication No. 4-125617, an ion adsorption electrode pattern is provided on an active matrix substrate having TFTs provided thereon, while the display electrode on a CF substrate is not provided in a position opposing the electrode pattern.
However, the interval between a region of the display pixel area in which the ion adsorption electrode pattern is provided and a region in which a sealing material is provided is as small as about 1 mm to 3 mm. In order to ensure that the display electrode on the CF substrate does not oppose the ion adsorption electrode pattern, this may be too small for methods which are typically employed in the prior art, i.e., methods in which display electrodes are patterned while directly masking the display electrode portions with a metal mask during the display electrode formation. Thus, it is necessary to pattern the display electrodes on the CF substrate with a photolithography technique, thereby increasing the number of production steps.
Moreover, when such an electrode pattern is provided on a typical liquid crystal display device, an interlayer insulating film is employed to electrically isolate the electrode pattern from the source or gate signal lines which cross the electrode pattern. However, an inorganic film of silicon nitride (SiN), or the like, which is typically employed for the interlayer insulating film is deposited by a CVD (chemical vapor deposition) method, and has a thickness of several hundreds of nanometers and a dielectric constant as high as 8. Therefore, depending upon the potential to be applied to the electrode pattern, the obtained display may be substantially affected by the capacitance at the intersection between the electrode pattern and the signal lines.
In addition, according to the drawings of Japanese Laid-Open Publication No. 4-125617, a protective film is provided on the electrode pattern. When a TFT production process is considered, the protective film needs to be deposited separately, thereby further increasing the number of production steps.
Japanese Laid-Open Publication No. 4-295824 discloses an arrangement in which an ion adsorption electrode pattern is provided between a display region and a sealing material. This conventional technique is directed primarily to duty drive type liquid crystal display devices. Therefore, the electrode pattern can be provided only in a direction parallel to segment lines and in a direction parallel to common lines. Signals are input to the segment lines and the common lines individually. However, in a liquid crystal display device with TFTs, in order to input signals other than the counter potential to the CF substrate, which corresponds to the substrate on which the common lines are provided, it is necessary to pattern the display electrodes on the CF substrate by a photolithography technique as in Japanese Laid-Open Publication No. 4 -125617.
Japanese Laid-Open Publication No. 8-201830 discloses a similar arrangement for liquid crystal display devices with TFTs. This arrangement also has a proble

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