1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD and its fabrication method with enhanced reliability.
2. Discussion of the Related Art
Generally, an LCD has a lower substrate having a thin film transistor (TFT) and a pixel electrode, an upper substrate having a color filter for displaying color and a common electrode, and a liquid crystal filled between the lower and upper substrates. Here, glass or crystalline is generally used as the material for the substrates.
A crystal substrate facilitates the use of a high temperature process, but is rarely used to fabricate a large-sized display. Also, a crystal substrate is too expensive.
A glass substrate, however, is relatively inexpensive, but cannot be used in a high temperature process, and has a reduced reliability from stress. That is, the stress on the glass substrate is different when depositing a chrome layer on the glass substrate as opposed to depositing an amorphous silicon layer on the glass substrate.
FIGS. 1A and 1B illustrate a stress characteristics when chrome is generally deposited on a glass substrate. FIGS. 2A and 2B illustrate stress characteristics when amorphous silicon is generally deposited on the glass substrate.
Referring to FIG. 1A, when a chrome layer 2 is deposited on the glass substrate 1, the substrate takes on a tensile stress. Thus, the glass substrate 1 is bent upward, as shown in FIG. 1B.
Referring to FIG. 2A, when an amorphous silicon layer 3 is deposited on the glass substrate 1, the glass substrate 1 is bent downward, as shown in FIG. 2B.
Accordingly, when a data line is formed of chrome in the LCD, the glass substrate is bent by the tensile stress as shown in FIG. 1B, but tends to return to its original state. Hence, a weak portion in the data line may become open.
The conventional LCD is described below with reference to the attached drawings.
Generally, an upper substrate of the LCD includes an active display area where a TFT and pixel electrode are arranged, a gate line pad for applying a driving signal to a TFT gate electrode of the active display area, and a data line pad for applying a data signal to a TFT source electrode. The TFT and pixel electrode of the active display area is illustrated in FIG. 3. In particular, the TFT is formed at the intersection of the gate line and data line, where the gate line is connected to a gate electrode, the data line is connected to a source electrode, and the drain electrode is formed to be connected to an ITO pixel electrode.
Here, for the active area of the TFT, the amorphous silicon layer is deposited between the gate electrode and source/drain electrode, and the active area is formed under the data line and is wider than the data line.
A method for forming the conventional LCD is described below with reference to FIGS. 4A-4E, which are cross-sectional views taken along the line I--I' in FIG. 3.
As illustrated in FIG. 4A, a metal such as aluminum (Al) is deposited on a glass substrate 1, and then selectively etched to form a gate line including a gate electrode 4. As illustrated in FIG. 4B, a gate insulating layer 6 (such as a silicon nitride layer), an amorphous silicon layer 7, and a high concentration n-type amorphous silicon layer 8 are sequentially deposited on the overall surface of the glass substrate 1 including the gate line. As illustrated in FIG. 4C, the amorphous silicon layer 7 and high concentration n-type amorphous silicon layer 8 are selectively removed except on the active area and the data line region of TFT.
As illustrated in FIG. 4D, a chrome layer 9 is deposited on the overall surface of the substrate, and then selectively removed except on the high concentration n-type amorphous silicon layer 8 of the data line region and on both sides of the amorphous silicon layer 7 and high concentration n-type amorphous silicon layer 8 of the active area to form the data line and source/drain electrode of the TFT. The exposed center portion of the high concentration n-type amorphous silicon layer 8 is removed using the chrome layer of the source/drain electrode as a mask. Here, the data line and source electrode are integrally formed (not shown).
As illustrated in FIG. 4E, a passivation layer 10 is formed on the overall surface of the substrate having the chrome layer 9. Then, the passivation layer on the chrome layer 9 of the drain electrode is selectively removed to form a contact hole. A transparent conductive layer is deposited on the overall surface and then selectively etched to remain only on the pixel area to form a transparent electrode 11. The transparent electrode 11 of the pixel area is electrically connected to the chrome layer 9 of the drain electrode through the contact hole.
However, the conventional LCD has problems as follows.
As the chrome layer is deposited in a state where the amorphous silicon layer and high concentration n-type amorphous silicon layer are formed on the active area and data line area, (with the remaining portion removed) the glass substrate takes on a tensile stress and becomes bent. Moreover, because the glass substrate also has a tendency to return to its original (unbent) state, the data line becomes open. Therefore, the reliability of the LCD is decreased.