(a) Field of the Invention
The present invention relates to a liquid crystal display (hereinafter referred to as an LCD). In particular, to an in-plane switching (hereinafter referred to as IPS) LCD.
(b) Description of the Related Art
A liquid crystal display in twisted-nematic (TN) mode is a commonly used LCD. The TN mode LCD includes a first substrate having a plurality of pixel electrodes and a second substrate having a common electrode, and a liquid crystal material therebetween. The direction of the liquid crystal molecules is twisted from one substrate to the other substrate, and the electric field between the two substrates due to the applied voltage causes the variation of the direction.
However, the TN mode LCD has a narrow viewing angle which depends on the viewing direction. In order to obtain a wide viewing angle, in-plane switching LCDs are suggested.
In the IPS LCD, a plurality of pixel electrodes and a plurality of common electrodes are formed in one substrate. In the other substrates, color filters and a black matrix are formed, and the black matrix serves as a light shield of the region where the liquid crystal direction cannot be controlled by the ordinary electric field. An example of an IPS LCD is disclosed in European Patent Application No. 93307154.0.
A conventional IPS LCD is described with reference to FIGS. 1-3.
FIG. 1 is a layout view of a conventional IPS LCD, and its sectional views cut along the lines II-II' and III-III' are shown in FIGS. 2 and 3, respectively. FIG. 1 includes a layout of electrodes and wiring of a first substrate as well as of a black matrix of a second substrate.
First, the first substrate having the electrodes and the wiring are described with reference to FIGS. 1-3. Here, a pixel region is the region surrounded by two adjacent gate lines and two adjacent data lines.
A gate line 110 and a pair of common electrode lines 120 is formed in the transverse direction on a transparent insulating substrate 100. Three common electrodes 121, 122 and 123 aligned in the longitudinal direction in a pixel are formed in the substrate 100, and they are connected to the common electrode lines 120. The reference numeral 124 represents a common electrode in the adjacent pixel.
The gate line 110, the common electrode lines 120 and the common electrodes 121, 122 and 123 are covered with a gate insulting layer 130. A data line 140 and a pixel electrode pattern 143, 144, 145 and 146 are formed on the gate insulating layer 130. The data line 140 extends in the longitudinal direction, and there exists a gap between the data line 140 and the adjacent common electrodes 123 and 124. Two pixel electrodes 145 and 146 of the pixel electrode pattern are parallel to the common electrodes 121, 122 and 123, and each pixel electrode 145 or 146 is arranged between the two of the common electrodes 121, 122 and 123. A pair of connecting members 144 and 143 of the pixel electrode pattern are parallel to the common electrode lines 120 and connected to the pixel electrodes 145 and 146, and the connecting members 143 and 144 overlap the common electrode lines 120, thereby forming storage capacitors. Each connecting member 143 or 144 has a width smaller than those of the common electrode lines 120, and thus one border line of each common electrode line 120 towards the common electrodes 121, 122 and 123 is exposed.
When considering the first substrate only, that is, not considering the second substrate, the effective display area is the total area of the aperture regions S surrounded by the common electrodes 121, 122 and 123, the common electrode lines 120, and the pixel electrodes 145 and 146.
Next, the black matrix pattern of the second substrate is described with reference to FIGS. 1-3.
A black matrix pattern 103 is arranged such that it completely shields the common electrode lines 120, the connecting members 143 and 144, the gate line 110 and the data line 140. The longitudinal portion of the black matrix 103 fully shields the data line 140, and partially shields the common electrodes 121 and 123 adjacent to the data line 140, considering the process margin during assembling the two substrates. The transverse portion of the black matrix 103 fully shields the gate line 110 and the common electrode line 120, and partially shields portions of the common electrodes 121, 122 and 123 and the pixel electrodes 145 and 146 near the common electrode lines 120. It is because, considering process margin during assembling the two substrates, the black matrix 103 should shield the disturbed area where the liquid crystal direction is disturbed due to the vertical electric field between the exposed portion of the common electrode line 120 and the pixel electrode pattern 143, 144, 145 and 146.
As a result, the black matrix pattern 103 intrudes the regions S and thus the net effective display area becomes smaller than the total area of the regions S
Accordingly, the conventional IPS LCD has a small aperture ratio, since all electrodes are arranged in one substrate and the width of the black matrix is large.
Furthermore, the black matrix 103 of the conventional IPS LCD is often grounded to discharge electrostatic charges. In this case, the potential difference between the black matrix 103 of the second substrate and the electrodes of the first substrate disturbs the liquid crystal direction, thereby causing the light leakage.
The end portions of the gate line 110 and the data line 140 are exposed and form pads for receiving the external signal. Since aluminum used for the gate line and tantalum used for the data line is apt to be oxidized when exposed to air, the ohmic contact of the pad may be easily deteriorated. In order to solve this problem, a layer made of materials which are not easily oxidized such as ITO (indium tin oxide) may be formed on the pads. However, since an additional mask for forming the layers is necessary, the product cost may increase.