1. Field of the Invention
The present invention relates to a method of fabricating a liquid crystal display (LCD), and more particularly, to a method of forming an alignment layer in an LCD to improve the image quality of the LCD.
2. Description of the Related Art
In general, cathode ray tube (CRT) display devices have been the most widely used display devices for displaying image data on a screen, but CRTs have many inconveniences due to their large volume and weight. With the development of electronic industries, display devices have been used in televisions and monitors for personal computers, notebook computers, wireless terminals, vehicle instrument panels, electronic display boards, and the like. Also, due to the development of information communication technology, the transmission of large capacity image information is possible. Therefore, the importance of a next generation display device for processing and displaying the large capacity image information has gradually increased.
Such next generation display devices are required to realize light weight, slim profile, high brightness, large screen size, low power consumption, and low price. Among such next generation display devices, liquid crystal display (LCD) devices have drawn much attraction. The LCD exhibits a better resolution than other flat displays and has a rapid response time, which is comparable to that of the CRT, in implementing a moving picture.
Recently, an active matrix LCD (AM-LCD) in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are arranged in a matrix is most widely used. FIG. 1 is an exploded perspective view of a related art AM-LCD.
As shown in FIG. 1, a related art color LCD includes a top substrate 105 having a black matrix 106, a color filter 108 having sub color filters (R, G, B), a transparent common electrode 118 on the color filter 108, and a bottom substrate 122 having a pixel region P, a pixel electrode 117 and a switching element T formed on the pixel region P. Liquid crystal is injected between the top substrate 105 and the bottom substrate 122. For convenience of explanation, the top substrate 105 is illustrated upside down.
The bottom substrate 122 is also called an array substrate. In the bottom substrate 122, TFTs T acting as switching elements are arranged in a matrix. Gate lines 113 and data lines 115 are intersected with the TFTs T. Also, the pixel region P is a region defined by intersections of the gate lines 113 and the data lines 115. The pixel electrodes 117 formed on the pixel regions P are formed of transparent conductive metal having a relatively excellent transmittance, such as indium-tin-oxide (ITO). Alignment layers are formed on the uppermost layers of the top and bottom substrates 105, 122 and determine an initial alignment direction and pretilted angle of the liquid crystal.
In such an LCD, liquid crystal molecules of the liquid crystal layer 114 disposed on the pixel electrode 117 is aligned by a signal applied from the TFT, and controls an amount of light passing through the liquid crystal layer 114 according to the alignment degree of the liquid crystal layer 114. Here, the physical properties of the liquid crystal are changed depending on the molecular arrangement, and therefore, there is difference in the response to an external force, such as an electric field.
Because of these characteristics of the liquid crystal, the arrangement control of the liquid crystal molecules is an essential technology in the construction of the LCD as well as the physical properties of the liquid crystal. Specifically, a rubbing process allowing the liquid crystal molecules to be uniformly aligned in a constant direction is an important factor in determining the normal driving of the LCD and the uniform characteristic of the LCD.
The related art alignment layer forming process for determining the initial alignment direction of the liquid crystal molecules will be described below in more detail.
First, a polymer thin film is deposited, and liquid crystal molecules are aligned in a constant direction. Generally, a polyimide-based organic material is deposited and is subjected to a rubbing process to form the alignment layer. According to the rubbing method, a polyimide-based organic material is coated on a substrate and a solvent is dried at a temperature of 60-80° C. Then, the material is aligned and hardened at a temperature of 80-200° C., thereby forming a polyimide alignment layer. The alignment layer is rubbed in a constant direction using a rubbing cloth such as velvet. Through these processes, the alignment direction is formed. The rubbing method is easy and well suited for mass production. Also, a stable alignment can be achieved.
However, the rubbing process may cause a defect when the alignment layer is rubbed using a roller wrapped with a poor rubbing cloth. That is, since the rubbing process is performed through a direct contact between the rubbing cloth and the alignment layer, liquid crystal cells may be contaminated due to particles. Also, TFTs may be damaged due to an electrostatic discharge, an additional cleaning process may be required after the rubbing process, and the liquid crystal molecules may be non-uniformly aligned in a wide-screen LCD. Consequently, the production yield of the LCD may be degraded. Specifically, as the size of the substrate becomes larger, rubbing defects, such as vertical line, scratch and blemish (mura), are more likely to occur, thereby degrading the yield of the LCD.
FIG. 2A is a photograph of a vertical line in the alignment layer of the related art LCD, and FIG. 2B is a sectional view of a scratch in the alignment layer of the related art LCD. As illustrated in FIGS. 2A and 2B, light leakage occurs because an alignment degree of the liquid crystal is disordered due to the rubbing defects, such as a vertical line or scratch in the alignment layer. Also, a black luminance and contrast ratio increase, thereby degrading the image quality of the LCD.