1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) and a fabrication method thereof.
2. Discussion of the Related Art
As the active matrix LCD (AM-LCD) has rapidly developed in its performance, such an AM-LCD is being widely used in a flat panel television (TV), a portable computer, a large information monitor and the like.
A twisted nematic mode (TN)-LCD is generally known as one type of AM-LCDs. In the TN-mode, an electrode is formed on each of two substrates, liquid crystal directors are arranged in such a way that they are twisted by 90°, and then a voltage is applied to the electrodes to drive the liquid crystal directors.
However, the TN-LCD is fundamentally problematic in that its viewing angle is narrow and a slow response characteristic is shown in its gray scale operation.
Accordingly, research has been performed to solve the aforementioned problems of the TN-LCD.
As a result of the research, an in-plane switching mode (IPS mode) LCD has been developed. In the IPS-mode LCD, two electrodes are formed on one substrate and an electrical field is generated in a horizontal direction with respect to the substrate by applying a voltage between the electrodes, thereby twisting liquid crystal directors in a plane parallel to an alignment layer.
The related art IPS-mode LCD will now be described with reference to FIGS. 1 through 3.
FIG. 1 is a plan view of the IPS-mode LCD, FIG. 2 is a view illustrating a voltage distribution in the IPS-mode LCD, FIG. 3A is a plan view of the IPS-mode LCD when a voltage is not applied, and FIG. 3B is a plan view of the IPS-mode LCD to when a voltage is applied.
The IPS-mode LCD is constructed to include first and second substrates facing each other and a liquid crystal layer formed therebetween, a black matrix for preventing light leakage is formed on the first substrate, a RGB color filter layer for reproducing a color is formed on the black matrix, and an over coat layer is formed for protecting the RGB color filter layer.
In FIG. 1, the second substrate includes thereon gate lines 110 and data lines 111 crossing each other to define pixel regions. Each pixel region includes a common line 112 arranged to be parallel to the gate lines 110, a thin film transistor (TFT) arranged near a crossing of the gate line 110 and the data line 111, common electrodes 113 extended from the common line 112 and parallel to the data lines 111, and pixel electrodes 114 connected to the TFT and arranged between and parallel to the common electrodes 113. Also, storage electrodes 115 are extended from the pixel electrodes 114 and are formed on the gate lines 110, and a passivation layer (not shown) is formed on a front surface including the data lines 111 to protect patterns on the second substrate.
A gate-line layer and a data-line layer are isolated from each other by a gate insulation layer (not shown) having an excellent insulation property and interposed therebetween. Also, the common line 112 and the common electrodes 113 are formed together with the gate lines 110, and the pixel electrodes 114 are formed together with the data lines 111. Here, the gate lines 110 and the data lines 111 are made of a low-resistance metal.
Also, the common electrodes 113 and the pixel electrodes 114 may either be formed on different planes between which an insulation layer is interposed, as above, or may also be formed on the same plane unless an electrical short occurs between the common electrodes 113 and the pixel electrodes 114.
The TFT is constructed to include a gate electrode 110A extended from the gate line 110, a gate insulation layer, an active layer 119 having a channel region, and a source electrode 111A and a drain electrode 111B extended from the data line 111.
In the above-described IPS-MODE LCD, if a voltage of 5V is applied to the common electrodes 113 and a voltage of 0V is applied to the pixel electrodes 114, then equipotential surfaces are horizontally distributed over the electrodes 113 and 114 and equipotential surfaces are nearly vertically distributed between the electrodes 113 and 114 as shown in FIG. 2.
Accordingly, because an electric field direction is perpendicular to an equipotential surface, a horizontal electric field rather than a vertical electrical field is formed between the electrodes 113 and 114, a vertical electric field is formed over the electrodes 113 and 114, and horizontal and vertical electric fields are formed around edges of the electrodes 113 and 114.
By using such electric fields, the IPS-MODE LCD adjusts an arrangement of a liquid crystal.
If a sufficient voltage is applied to liquid crystal molecules 135 initially aligned in a direction identical to that of a transmittance axis of a polarizer as shown in FIG. 3A, then major axes of the liquid crystal molecules 135 are arranged in parallel to the resulting electric field as shown in FIG. 3B.
Here, if a dielectric anisotropy f the liquid crystal is negative, then shorter axes of the liquid crystal molecules are arranged in parallel to the resulting electric field.
Specifically, if first and second polarizers respectively attached on outer surfaces of the first and second substrates are arranged in such a way that their transmittance axes are perpendicular to each other, and if an alignment layer formed on the first substrate is rubbed parallel to a transmittance axis of one of the two polarizers, then the liquid crystal becomes a normally black (NB) mode.
That is, if a voltage is not applied to a corresponding element, then the liquid crystal molecules 135 are arranged as shown in FIG. 3A, thereby expressing a black state. Otherwise, if a voltage is applied to a corresponding element, then the liquid crystal molecules 135 are arranged parallel to the resulting electric field as shown in FIG. 3B, thereby expressing a white state.
In FIGS. 3A and 3B, reference numerals 113 and 114 indicate a common electrode and a pixel electrode, respectively.
However, the above related art IPS-MODE LCD has slow response characteristics and low light transmittance.