(a) Field of the Invention
The present invention relates to a lateral-electric-field-mode liquid crystal display (LCD) device and, more particularly, to a LCD device wherein liquid crystal (LC) molecules in a LC layer are rotated in a plane parallel to both the surface of the substrates sandwiching therebetween the LC layer.
(b) Description of the Related Art
Active-matrix (AM) LCD devices are known in the art wherein active elements, such as thin-film-transistors (TFTs), are used as switching devices for controlling respective pixels. The AM LCD device has a high-definition image quality, and is widely used as a monitor of a desktop computer, etc. Typical operational modes of the LCD device include: a twisted-nematic (TN) mode in which the LC molecules (or directors) of the LC layer are rotated in a plane perpendicular to the surface of both the substrates; and a lateral-electric-field mode, such as in-plane-switching (IPS) mode, in which the LC molecules are rotated in a plane parallel to the surface of both the substrates.
In each pixel of the IPS-mode LCD device, the pixel electrode and common electrode extend parallel to each other on a glass substrate (or transparent substrate) configuring a TFT panel or active-matrix panel. Upon display of an image, a drive voltage is applied between the pixel electrode and the common electrode, to generate a lateral electric field parallel to the substrate surface. The lateral electric field rotates the directors of the LC layer to control the amount of light passed by the LC layer. Due to this rotational direction of the directors, the IPS-mode LCD device is substantially free from the problem that the relationship between the amount of the transmitted light and the applied voltage is different between the direction normal to the substrate surface and the alignment direction of the directors of the LC. Thus, the IPS-mode LCD device has a higher image quality in a wider viewing angle.
Generally, the IPS-mode LCD device has the configuration wherein a homogeneously-aligned LC layer is sandwiched between a pair of transparent substrates to form a LC element, which is in turn sandwiched between a pair of polarizing films. The polarizing films have polarization axes (such as optical transmission axes or optical absorption axes) extending perpendicular to each other. In the IPS-mode LCD device, a typical configuration is such that the polarization axis of one of the polarizing films is set substantially parallel to the alignment direction of the LC molecules, whereby absence of an applied voltage provides a dark state and presence of the applied voltage rotates the LCD molecules to provide a bright state. This configuration allows the brightness of the LCD device to be stable upon display of the dark state.
Although the IPS-mode LCD device can realize a higher viewing angle characteristic, there is a problem that a coloring phenomenon occurs upon display of a bright state as viewed in a slanted direction. This results from the fact that both the pixel electrode and common electrode are substantially linear to thereby rotate the LC molecules in a single rotational direction. Patent Publication JP-1997-311334A proposes a solution of this problem, wherein both the pixel electrode and common electrode are provided with a bent having a shape of dogleg.
FIG. 9 shows the structure of the pixel of the conventional IPS-mode LCD device in a top plan view, whereas FIG. 10 shows a sectional view thereof taken along line X-X in FIG. 9. The LCD device, generally designated at numeral 200, includes a first transparent substrate (glass substrate) 222 on which TFTs are formed, a second transparent substrate 213 on which color filters are formed, and a LC layer 218 sandwiched between these substrates. On the first transparent substrate 222, gate electrodes 201 and data lines 202 extend perpendicularly to one another, and TFTs 204 are disposed at respective intersections.
The TFTs are controlled for ON or OFF depending on the potential of the gate electrode 201 underlying the gate insulation film 221. The source of TFTs 204 is connected to a data line 202 via a source electrode 203, and the drain thereof is connected to a drain electrode 205. The drain electrode 205 is made of a metallic film underlying a planarization film (overcoat) 220, and connected to the pixel electrode 206 configured as a transparent electrode on the overcoat 220 via a contact hole. The common electrode 207 includes a lead portion or trunk line made of a metallic film underlying the overcoat 220, and a transparent portion disposed within the pixel and connected to the trunk line via a contact hole.
A pair of shield lines 208 made of a transparent film overlie both the edge portions of the data line 202 for shielding the LC layer 218 against the electric field generated by the data line 20. The shield lines 208 are connected to the trunk line of the common electrode 207 underlying the overcoat 220 via a contact hole. In each pixel, the pixel electrode 206 and common electrode 207 have a dogleg bent at the substantially center of the electrodes, and extend parallel to each other at each of the linear portions of the electrodes. On the second transparent substrate 213, there are formed a black matrix film 215 for defining the pixel area, color layers 214 for representing RGB colors, and an overcoat 216 covering these underlying layers. In addition, a conductive film 212 is also provided on the surface of the second transparent substrate 213 far from the LC layer 218, for reducing the vertical electric field within the LC layer 218.
Alignment films 219 and 217 are formed by coating on the surface of the first and second transparent substrates near the LC layer, for determining the initial orientation of the LC molecules. The LC layer 218 is homogeneously aligned in the direction 210 parallel to the extending direction of the data lines 202. The polarizing films 211 and 223 bonded onto the outer surface of the first and second transparent substrates 222 and 213, respectively. The polarization axes of these polarizing films 211 and 223 intersect substantially at right angles, and one of the polarization axes is parallel to the initial orientation of the LC molecules in the LC layer 218. In operation of the LCD device, a data signal is provided to the pixel electrode 206 to apply a lateral electric field to the LC layer between the pixel electrode 206 and the common electrode 207, whereby the directors of the LC layer are rotated to display an image on the screen.
In the LCD device 200 of FIG. 9, when a drive voltage is applied between the pixel electrode 206 and the common electrode 207, the two linear portions provided at both sides of the dogleg bent generate an electric field in different directions, whereby the LC directors (LC molecules) are rotated in different directions. In the example of FIG. 9, the LC directors 209 in the first region shown at the top side of the drawing are rotated in the counter-clockwise direction, whereas the LC directors 209 in the second region shown at the bottom side of the drawing are rotated in the clockwise direction, as shown in the drawing. In this case, the LCD device 200, upon displaying a bright state, assumes different colors as viewed in a slanted direction, whereby both the first and second regions compensate each other to reduce the coloring of the LCD device.
The inventor analyzed problems of the conventional IPS-mode LCD device as described above. The dogleg bent formed in the pixel electrode 206 and common electrode 207 causes that the extending direction of the electrodes is not parallel to the orientation of the LC directors, whereby the rubbing treatment of the alignment film is not parallel to the extending direction of the electrodes. The step difference formed on the surface of the alignment film by the presence and absence of the electrodes may incur an obstacle against a smooth rubbing treatment, to cause an uneven surface of the alignment film. The uneven surface reduces the contrast ratio of the LCD device. In addition, the uneven surface may incur a dead space between the pattern of the TFT substrate and the counter substrate, thereby reducing the effective aperture ratio of each pixel. Furthermore, a deviation in the angle between the optical axis of the polarizing films and the extending direction of the electrodes causes an optical diffraction at the edge of the electrodes, deviates the direction of polarization to generate leakage light, and thus reduces the contrast ratio.