1. Field of the Invention
The invention relates to a liquid crystal display (LCD) device and its fabrication method and, more particularly, to an LCD capable of simplifying the fabrication process and improving the production yield by reducing the number of masks.
2. Description of the Related Art
As the consumer's interest in information displays is growing and the demand for portable, i.e., mobile, information devices increases, research and commercialization of light and thin flat panel displays (“FPD”) has increased. Flat panel displays are replacing the Cathode Ray Tube (“CRT”), which is the most common existing display device.
The liquid crystal display (“LCD”) is a FPD device for displaying images by exploiting the optical anisotropy of a liquid crystalline material. LCD devices exhibit excellent resolution and color and picture quality, so they are widely applied for notebook computers, desktop monitors, and the like.
An LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
As switching elements of the LCD, thin film transistors (TFTs) are generally used. As a channel layer of the TFT, an amorphous silicon thin film is used.
The fabrication process of the LCD requires multiple masking processes (namely, photographing processes) to fabricate the array substrate including TFTs, so a method for reducing the number of masking processes is necessary to increase productivity.
The structure of a related art LCD will be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing the related art LCD.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes color filters (C) including multiple sub-color filters 7 implementing red, green and blue colors, black matrices 6 for dividing the sub-color filters 7 and blocking light transmission to the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes multiple gate lines 16 and multiple data lines 17 arranged horizontally and vertically to define multiple pixel regions (P). TFTs, the switching elements, are formed at each crossing of the gate lines 16 and data lines 17, and pixel electrodes 18 formed on each pixel region (P).
The color filter substrate 5 and the array substrate 10 are attached facing each other by a sealant (not shown) formed on an outer edge of an image display region to form a liquid crystal display panel, and the two substrates 5 and 10 are attached by an attachment key (not shown) formed on the color filter substrate 5 or on the array substrate 10.
FIGS. 2A to 2E show sectional views sequentially showing a fabrication process of the related art array substrate of the LCD in FIG. 1.
FIG. 2A shows a gate electrode 21 made of a conductive material that is formed using a photolithography process (a first masking process) on a substrate.
Next, as shown in 2B, a first insulation film 15A, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited on the entire surface of the substrate 10 having the gate electrode 21 formed thereon. The amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned using photolithography (a second masking process) to form an active pattern 24 formed of the amorphous silicon thin film on the gate electrode 21.
In this case, the n+ amorphous silicon thin film pattern 25, which has been patterned in the same form as the active pattern 24, is formed on the active pattern 24.
FIG. 2C shows that a subsequent a conductive metal material is deposited on the entire surface of the substrate 10 and then is selectively patterned using photolithography (a third masking process) to form a source electrode 22 and a drain electrode 23 at an upper portion of the active pattern 24. At this time, a certain portion of the n+ amorphous silicon thin film pattern formed on the active pattern 24 is removed through the third masking process to form an ohmic-contact layer 25′ between the active pattern 24 and the source and drain electrodes 22 and 23.
Subsequently, FIG. 2D shows a second insulation film 15B that is deposited on the entire surface of the substrate 10 with the source electrode 22 and the drain electrode 23 formed thereon, and a portion of the second insulation film 15B is removed using photolithographic processing (a fourth masking process) to form a contact hole 40 exposing a portion of the drain electrode 23.
Finally, as shown in FIG. 2E, a transparent conductive metal material is deposited on the entire surface of the substrate 10 and then is selectively patterned using the photolithographic processing (a fifth masking process) to form a pixel electrode 18 that is electrically connected with the drain electrode 23 via the contact hole 40.
As mentioned above, in fabricating the array substrate that includes the TFTs, a total of at least five photolithographic processes are necessary to pattern the gate electrode, the active pattern, the source and drain electrodes, the contact hole and the pixel electrode.
The photolithography process is a complicated process of transferring a pattern formed on a mask onto the substrate on which a thin film is deposited to form a desired pattern, which includes multiple process steps such as coating a photosensitive solution, exposing, developing, etc. As a result, the multiple photolithographic processes have many problems that degrade the production yield and increases the probability of generating a defective TFT.
In particular, the masks designed for forming the pattern are quite expensive, and as the number of masks applied for the processes increases, the fabrication cost of the LCD proportionally increases.