1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to a multi-domain LCD.
2. Description of the Related Art
Applying a voltage to a liquid crystal display (LCD) changes the alignment of liquid crystal molecules, and then the resulting optical characteristics, such as double refraction, optical rotation, dichromatism, optical confusion and optical scattering cause display variations. Compared with the electric-optical materials used in other optical devices, liquid crystal molecules can distribute substantial variations in optical characteristics with low voltage and low power consumption, and without further treatments. In addition, LCD has the advantages of a thin profile and a lightweight. Therefore, as we know, LCDs play an important role in the flat display market.
Display modes in LCDs differ with each other according to different types of liquid crystal molecules utilized therein. One mode, electrically controlled birefringence (ECB), employs an applied electric field to control the multi-refraction characteristics of the liquid crystal molecules. For example, nematic liquid crystal molecules having a negative anisotropy of dielectric constant are utilized together with a vertical alignment layer. When the applied voltage exceeds a threshold voltage, the liquid crystal molecules that are originally aligned perpendicular to the vertical alignment layer will rotate to an angle corresponding to the applied electric field. As well, to further control the alignment of the liquid crystal molecules, an alignment-control structure is fabricated to increase the number of alignment domain in each pixel area. It is thus possible to reach the goals of wide view angle and high contrast.
Referring to FIG. 1 and FIG. 2, FIG. 2 is a top view of the alignment-control structure in a conventional LCD cell 10, and FIG. 1 is a schematic cross-sectional diagram along line I-Ixe2x80x2 of FIG. 2. As shown in FIG. 1, the LCD cell 10 comprises an upper glass substrate 12, a lower glass substrate 14, and a liquid crystal layer 16 with a negative anisotropy of dielectric constant filling in the space between the two glass substrates 12 and 14. Two electrodes 18 and 22 and two vertical alignment layers 20 and 24 are respectively formed on the inner surface of the glass substrates 12 and 14. Two polarizers 26 and 28 are respectively formed on the outer surface of the glass substrates 12 and 14. In general, the upper glass substrate 12 serves as a color filter substrate. The lower glass substrate 14 serves as a thin film transistor (TFT) substrate where a plurality of TFTs and active matrix drive circuits are formed. The electrode 22 on the lower glass substrate 14 serves as a pixel electrode 22. Furthermore, the LCD cell 10 has a plurality of first strip-shaped protrusions 30 and second strip-shaped protrusions 32 respectively formed above the electrodes 18 and 22 to serve as the alignment-control structures.
As shown in FIG. 2, two transversely-extending gate lines 36 and two lengthwise-extending signal lines 38 define a pixel area. A TFT structure 39 is formed near the intersection of the gate line 36 and the signal line 38, and the pixel electrode 22 is formed on the pixel area. On the upper glass substrate 12, the first strip-shaped protrusions 30 extend transversely and pass through the gate lines 36. On the lower glass substrate 14, the second strip-shaped protrusion 32 extends transversely and passes through the center of the pixel electrode 22. In this way, the first protrusions 30 and the second protrusions 32 extend in parallel to each other and are arranged alternately.
FIGS. 3 and 4 are schematic diagrams showing the variation in alignment of the liquid crystal molecules. As shown in FIG. 3, when no voltage is applied, the liquid crystal layer 16 having a negative anisotropy of dielectric constant is arranged between the vertical alignment layers 20 and 24, all the liquid crystal molecules in the liquid crystal layer 16 are aligned perpendicular to the vertical alignment layers 20 and 24, respectively. For example, the liquid crystal molecules 16A are aligned perpendicular to the glass substrates 12 and 14. The liquid crystal molecules 16B above the protrusions 30 and 32 are vertical to the perpendicular alignment layers 20 and 24 above the protrusions 30 and 32, so that the liquid crystal molecules 16B above the protrusions 30 and 32 are positioned at an angle to the glass substrates 12 and 14.
As shown in FIG. 4, after the voltage is applied to the LCD cell 10, the liquid crystal molecules will rotate perpendicular to the electric field. The arrows show the rotating directions of the liquid crystal molecules. For example, the liquid crystal molecules 16B1, aligned from the upper right toward the lower left in the beginning, and will be rotated in a clockwise direction after the voltage is applied so that the liquid crystal molecules 16B1 fall in the direction parallel to the vertical alignment layers 20 and 24. Consequently, the adjacent liquid crystal molecules 16A1 will rotate in a clockwise direction according to the behavior of the liquid crystal molecules 16B1. In a similar manner, the liquid crystal molecules 16B2 are positioned from the upper left toward the lower right in the beginning, and then fall in the direction parallel to the vertical alignment layers 20 and 24 and rotate in a counterclockwise direction while the voltage is applied. As a result, the adjacent liquid crystal molecules 16A2 will rotate in a counterclockwise direction according to the behavior of the liquid crystal molecules 16B2.
FIG. 5 is a top view showing the alignment domain of the liquid crystal layer 16. After a voltage is applied, these liquid crystal molecules will rotate to parallel to the direction of electric field. Upon application of voltage to the LCD cell 10, part of the liquid crystal molecules 16A1 and 16B1 rotate in a clockwise direction and another part of liquid crystal molecules 16A2 and 16B2 rotates in a counterclockwise direction. Accordingly, two alignment domains are formed both sides of the second protrusion 32 in a pixel area. The arrows show the alignment direction.
However, since the alignment-control structure only forms two domains in a pixel area, this cannot satisfy the requirements of a wide view angle and a high contrast in the LCD. Also, the alignment-control structure is the strip-shaped protrusions 30 and 32, so the aperture ratio of the LCD is reduce, resulting in decreased brightness and lower contrast. Thus, a multi-domain LCD solving the aforementioned problems is called for.
The present invention provides rectangular protrusions in one pixel area to serve as the alignment-control structure.
The LCD includes an upper substrate having an upper electrode on the inner surface of the upper substrate, a lower substrate having a lower electrode on the inner surface of the lower substrate, and a liquid crystal layer with a negative anisotropy of dielectric constant filling the space between the upper substrate and the lower substrate. An electric field is formed between the upper electrode and the lower electrode. A plurality of first alignment-control structures are arranged on the inner surface of the upper substrate, an upper concave is formed between two adjacent first alignment-control structures, and an upper inclined plane is formed between the upper concave and the first alignment-control structure. A plurality of second alignment-control structures are arranged on the inner surface of the lower substrate, a lower concave is formed between two adjacent second alignment-control structures, and a lower inclined plane is formed between the lower concave and the second alignment-control structure. The first alignment-control structure is positioned above the lower concave, and the second alignment-control structure is positioned under the upper concave. The first alignment-control structure and the corresponding lower concave have a first substantially equal size. The second alignment-control structure and the corresponding upper concave have a second substantially equal size. The upper inclined plane is close to and faces the lower inclined plane.
Accordingly, it is an object of the invention to provide the alignment-control structures to achieve more than two domains in one pixel area.
It is another object of the invention to satisfy the requirements of a wide view angle and a high contrast
Yet another object of the invention is to improve the brightness and contrast so as to assure the display quality of the multi-domain LCD.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.