Abstract:
A liquid crystal display of the present invention comprises a first substrate, a second substrate and a liquid crystal layer injected there between. The first substrate has first base plate, a gate line formed on the first base plate, a data line formed over the gate line, a switching element electrically connected to the gate line and the data line, a pixel electrode electrically connected to the switching element, an element dividing LC domains formed on the pixel electrode, first alignment layer formed on the domain dividing element, The second substrate comprises second base plate, a common electrode formed on the second base plate, a first protrusion and a second protrusion formed on the common electrode. The first protrusion and the second protrusion have different heights. A second alignment layer is formed on the protrusions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit and priority of Korean Patent Application No. 2004-22054 filed on Mar. 31, 2004, the contents of which are herein incorporated by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a flat panel display and a method of manufacturing the flat panel display. More particularly, the present invention relates to a flat panel display that is effective in high contrast ratio characteristics.  
         [0004]     2. Description of the Related Art  
         [0005]     A liquid crystal display (LCD) realizes various images by virtue of the optical and dielectric anisotropy of the liquid crystal material. In LCDs, the quality of images depends on directions where it is viewed from.  
         [0006]     To overcome the problem that image quality of LCD depends on the viewing direction, a vertically aligned (VA) LCD is strongly recommended these days.  
         [0007]     In a VA LCD, liquid crystal molecules, which are between an upper substrate and a lower substrate, are aligned vertically against the surface of the substrates when electric field is not applied. When electric field is applied between the two substrates, the molecules of the liquid crystal (LC) rotate to the direction parallel with the surface of the two substrates.  
         [0008]     To reduce the direction dependency of the images and to improve the quality of the image in VA LCD, a multi-domain technology is used. The multi-domain technology is to control the direction of molecules of the LC independently from one domain to another. Openings in electrodes or protrusions on electrodes are used to form multi-domains in VA LCDs. Some LCDs use one of the protrusions and the openings. Some LCDs use both of them.  
         [0009]     The protrusions used to form multi-domains are made in the following process.  
         [0010]     First, a photosensitive material is coated on a substrate. The photosensitive layer is exposed to an appropriate light through a mask that has a certain pattern.  
         [0011]     The photosensitive layer is developed to form the above pattern.  
         [0012]     The pattern of photosensitive layer becomes protrusions that form multi-domains.  
         [0013]     In this case, the LC molecules on the protrusion is not aligned vertically but inclined to one side.  
         [0014]     VA LCD is usually normally black mode which shows black state when electric field is not applied. In order to achieve a normally black mode, the LC molecule should align vertically when electric field is not applied. If some of the LC molecules are not aligned vertically, light will leak in the black state. This degrades the display quality.  
       SUMMARY OF THE INVENTION  
       [0015]     An embodiment of the present invention provides an LCD that is effective in reducing the brightness in black state. The reduced brightness in black state will increase the contrast ratio (CR) which represents the ratio of the white luminance to the black luminance.  
         [0016]     An embodiment of the present invention provides a method for manufacturing the above described LCD.  
         [0017]     An embodiment of the present invention provides an LCD that comprises a substrate, an electrode formed on the substrate, a first protrusion and a second protrusion formed on the electrode, and an alignment layer formed on the first protrusion and on the second protrusion.  
         [0018]     The first protrusion and the second protrusion may be formed in parallel.  
         [0019]     The first protrusion and the second protrusion may extend in two or more directions.  
         [0020]     The second protrusion may be formed on both side of the first protrusion and may be lower than the first protrusion.  
         [0021]     An LCD of an embodiment of the present invention may be manufactured by forming an electrode on a substrate, forming a first protrusion and a second protrusion on the electrode with different heights, and forming an alignment layer on the protrusions.  
         [0022]     The protrusion may be formed by coating a photo sensitive layer on the electrode, and exposing the photo sensitive layer to an appropriate light wave through a photo mask. The mask has two kinds of patterns for protrusions. One is wider than the other. The wider pattern may be for a higher protrusion. The narrower pattern may be for a lower protrusion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a brief drawing of an embodiment of the present invention.  
         [0024]      FIG. 2  is a cross-sectional view showing protrusions and photo mask to form the protrusions in the prior art.  
         [0025]      FIG. 3  is a cross-sectional view showing a structure of dual pattern protrusion and a photo mask to form the dual pattern protrusion of an embodiment of the present invention on the upper substrate.  
         [0026]      FIG. 4  is a cross-sectional view showing a structure of a dual pattern protrusion and photo mask to form the dual pattern protrusion of an embodiment of the present invention on the lower substrate.  
         [0027]      FIG. 5  is a graph comparing the contrast ratio (CR) of the LCDs made by an embodiment of the present invention and by the prior art.  
         [0028]      FIG. 6  is a plan view showing dual pattern protrusions of an embodiment of the present invention.  
         [0029]      FIG. 7  is a plan view showing a protrusion pattern formed on the electrode of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0030]     It should be understood that the exemplary embodiments of the present invention described below may be varied in many different ways without departing from the inventive principles disclosed herein, and the scope of the present invention is therefore not limited to these particular following embodiments. Rather, these embodiments are provided so that this disclosure will be through and complete, and will fully convey the concept of the invention to those skilled in the art by way of example and not of limitation.  
         [0031]     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.  
         [0032]     In the drawings, thicknesses of the layers are exaggerated for ease of recognition.  
         [0033]      FIG. 1  shows an LCD of an exemplary embodiment comprising a lower substrate  100 , a upper substrate  200 , and an LC layer  3  injected between the upper substrate and lower substrate.  
         [0034]     The lower substrate  100  comprises a gate line  121 , a data line  171  that crosses the gate line a thin film transistor (TFT) Q electrically coupled to the gate line and the data line, and a pixel electrode  19  electrically coupled to the TFT.  
         [0035]     The upper substrate  200  comprises, a color filter  230  corresponding to the pixel electrode  19 , and a common electrode  210  is deposited on the whole display area of the upper substrate  200  and receives common voltage.  
         [0036]     The LC layer  3  may have negative dielectric anisotropy and may be vertically aligned to the surfaces of the upper substrate and the lower substrate.  
         [0037]     LC molecules in the LC layer  3  tend to be aligned in a vertical direction to the electric field when the electric field is applied according to the voltage differences between the pixel electrode  19  and the common electrode  210 .  
         [0038]     The pixel electrode  19  on the lower substrate  100 , the common electrode  210  on the upper substrate  200  and the LC layer form a capacitor (C LC ).  
         [0039]     The pixel electrode  19  forms storage capacitor (C ST ) with another signal line (not shown) with a certain voltage applied thereon. The certain voltage may be same to the common voltage. The pixel electrode may also form a storage capacitor with previous gate line and insulation layer formed between the pixel electrode and the gate signal line.  
         [0040]     Color filter  230  may be formed on the lower substrate  100 . In such a case, it can be above or below the pixel electrode.  
         [0041]      FIG. 3  is a cross-sectional view showing a dual pattern protrusion and a photo mask to form the dual pattern protrusion as an embodiment of the present invention.  
         [0042]      FIG. 3  shows a lower substrate  200 , an insulation layer  205 , an electrode  210 , a first protrusion  221  and a second protrusion  222 , and an alignment layer  220 . First protrusion  221  is higher than the second protrusion  222 . The second protrusion  222  is formed at both side of the first protrusion  221 . The first protrusion  221  is between about 4 μm and 6 μm wide, and the second protrusion  222  is from about 1.5 μm to about 2.5 μm wide. The first protrusion is separated from the second protrusion about 5 μm to about 6 μm when measured from center to center. The alignment layer  220  may be vertically aligning material. The insulation layer  205  may be an overcoat layer. The insulation layer may be omitted.  
         [0043]      FIG. 3  shows a mask  20  having a transparent substrate  25 , the first pattern  21  and the second pattern  22 .  
         [0044]     Herein the first pattern  21  faces the first protrusion  221 , and the second pattern  22  faces the second protrusion  222 .  
         [0045]     The first pattern  21  may be wider than the second pattern  22 .  
         [0046]     The first pattern  21  is from about 4 μm to about 6 μm wide and the second pattern  22  is from about 1.5 μm to about 2.5 μm wide. The first pattern  21  and the second pattern  22  are separated apart from about 1.5 μm to about 2.5 μm.  
         [0047]     The process forming protrusions on the common electrode is described below.  
         [0048]     Light shielding layer is formed on a substrate like a glass plate to block light leakage between pixel electrodes. Then the color filter is formed on the light shielding layer. Overcoat layer may be formed on the color filter. Next, a common electrode  210  is formed on the overcoat layer  205 . Then, a photo sensitive material is coated on the common electrode. After that, the photo sensitive material is exposed to a light wave through the photo mask described in  FIG. 3 . The first protrusion  221  is formed on the position corresponding to the first pattern  21 . The second protrusion  222  is formed on the position corresponding to the second pattern  22 . An alignment layer  220  like polyimide resin is coated on the first protrusion and the second protrusion.  
         [0049]      FIG. 4  shows that a similar process can be applied to the lower substrate to form protrusions.  
         [0050]     As shown in  FIG. 3  and  FIG. 4  the width of the first pattern and the second pattern on the mask is different from each other. Even though a pattern on the mask shield light from reaching to the shade area, light is diffracted to the shade area. A wider light shielding pattern on a mask may block light from reaching the center area of the shadow. The less light reaches the photosensitive layer the thicker pattern is formed.  
         [0051]     Because protrusions are formed by using the photo mask of an embodiment of the present invention which has the first pattern  21  and the second pattern  22  in the  FIG. 3 , and the alignment layer smooth the slopes of the protrusions, the whole area of the protrusions changes gently. When electric field is not applied to the LC layer, the LC molecules in the vicinity of the protrusions  221  and  222  are aligned almost vertically to the substrate surface.  
         [0052]     Accordingly, when protrusions are used as a means for forming domains in a normally black mode LCD, the light leakage in the vicinity of the protrusions is improved.  
         [0053]     Luminance of the black and white and the contrast ratios are shown in the Table 1.  
         [0054]      FIG. 5  is a graph comparing the contrast ratio (CR) of the LCDs made by the present invention and by the prior art. The Table 1 shows the result when the first pattern  21  in the mask (CD, critical dimension) is 5 μm wide, the second pattern  22  in the mask (CD, critical dimension) is 2 μm wide, the distance from the first pattern  21  to the second pattern  22  is 2 μm, and the CD of the prior art (single pattern protrusion) is 14 μm.  
                                                                                       TABLE 1                                       Application   Non-application           of Overcoat layer   of Overcoat                B   W   CR   B   W   CR                        Dual Pattern   0.58   446.4   764.4   0.56   451.5   803.4       Single pattern   0.72   424.4   586.2   1.10   418.3   380.6                    
         [0055]     Table 1 and  FIG. 5  shows that LCDs with dual pattern protrusions show much better black luminance and contrast ratio than the LCDs with single pattern protrusion regardless of the overcoat layer. The mask for dual pattern protrusions has two kinds of patterns whose width is different from each other as shown in  FIG. 3  and  FIG. 4 .  
         [0056]      FIG. 2  shows a single pattern protrusion and its forming process that has been conventionally used. An electrode  310  is formed on a substrate  300 . A single pattern protrusion  331  is formed on the electrode  310  by using a photo mask  30 . The photo mask  30  comprises a transparent plate  35  and an opaque pattern  31 . The opaque pattern corresponds to the protrusion pattern  331 . In this example the protrusion  331  is made of positive photo resist. An alignment layer  320  is formed later on the electrode  310  and the protrusion  331 .  
         [0057]      FIG. 6  is a plan view showing dual pattern protrusions made by an embodiment of the present invention.  
         [0058]     A first protrusion  221  and a second protrusion  222  extend parallel with each other. The second protrusion  222  can be either at both sides of the first protrusion  221  or at one side of the first protrusion  221 .  
         [0059]      FIG. 7  is a plan view showing a protrusion pattern formed on the electrode of an embodiment of the present invention.  
         [0060]     As shown in the  FIG. 1  and the  FIG. 7 , an embodiment of the present invention has an open portion of the pixel electrode  191  on a lower substrate  100  and protrusions  223  and  224  on the common electrode  210  of the upper substrate  200 . The protrusions  223  and  224  have different heights.  
         [0061]     The protrusions  223  and  224  have a bending portion and a vertical portion connected to the both sides of the bending portion.  
         [0062]     The bending portion has two inclined lines that meet each other at almost a right angle.  
         [0063]     The vertical portion makes an obtuse angle with the bending portion and almost corresponds to the edge of the pixel electrode  19 .  
         [0064]     The bending portion of the bigger protrusion  223  traverses slantingly the whole width of the pixel electrode  19 . The bending portion of the smaller protrusion  224  traverses slantingly half of the whole width of the pixel electrode  19 .  
         [0065]     In the horizontal direction of the pixel electrode  19 , the bigger protrusion  223  and the smaller protrusion  224  are disposed alternately. The bigger protrusion  223  and the smaller protrusion  224  are parallel with each other. There is an open portion  191  between the bigger is protrusion  223  and the smaller protrusion  224 . The open portion  191  is parallel to the bigger protrusion  223  and the smaller protrusion  224 . In the  FIG. 7 , the bigger protrusion  223  is connected to the smaller protrusion  224  of the adjacent pixel electrode  19 , but they may be separated.  
         [0066]     The opening of the pixel electrode  191  may be substituted by a protrusion.  
         [0067]     The LC molecules of both side of the openings  191  and the protrusions  223  and  224  slant the opposite directions to each other.  
         [0068]     Because the openings  191  and the protrusions  223  and  224  have a bending portion bent almost 90 degree, the LC molecules on the portion of a pixel electrode slant four directions which reduce the viewing dependency of the quality of an LCD.  
         [0069]     The light shielding pattern and the color filter may be formed on the lower substrate. In such a case, they may locate above or below the pixel electrode. The overcoat may be omitted.  
         [0070]     According to an embodiment of the present invention, the dual pattern protrusion may have dual height. The wider pattern may be higher than the narrower pattern. The dual height protrusion may be formed on the pixel electrode in the same way as formed on the common electrode. The dual height protrusion may be formed on both substrates or on one of the two substrates. In this case, the electrode on the other substrate may have an opening.  
         [0071]     The dual height protrusion smoothes the slope of the whole protrusion and can improve the quality of the black state and the contrast ratio of an LCD.  
         [0072]     An exemplary embodiment of the present invention also shows that the dual pattern protrusion may be a triplet or more pattern protrusion. The dual height protrusion may have more than two sub-protrusions with heights different from each other.  
         [0073]     Here are some examples of the experiment using embodiments of the present invention. We coated photo resist (PR) PC459 produced by Japan Synthetic Rubber (JSR) on an electrode formed on a glass substrate with 850 rpm for 20 seconds. Then the PR was dried with a 90° C. hot plate for 10 seconds in proximity, after that the PR was dried for 120 seconds on the plate. After the PR was exposed to a Hg light with about 420 nm ˜500 nm wave length range through a photo mask, it was developed with 2.38% tetra methyl ammonium hydroxide (TMAH; N(CH 3 ) 4 OH) solution. The second pattern  22  of  FIG. 3  was 2 μm wide. The first pattern was split to 5 μm, 7.5 μm, and 10 μm wide each. The distance between the first pattern and the second pattern was split to 1.3 μm, 1.5 μm, and 2.0 μm each. Some prior art also were tested with single pattern, and it was split to 8 μm, 11 μm, and 14 μm each. The first protrusion  221  in  FIG. 3  was about 1.1 μm thick, and the second protrusion  222  in  FIG. 3  was about 0.4 μm. All of the displays of dual pattern protrusions showed better CR than any of the displays of single pattern protrusions. When the first pattern is equal to or closer than 1.3 μm to the second pattern on the mask, the first protrusion may reach to the second protrusion on the substrate. Even the first protrusion and the second protrusion reach each other, CR of the display is better than prior arts. If the viscosity of an alignment layer is low, reaching the first protrusion to the second protrusion may be better. In this case, the dual pattern protrusions look like a single pattern protrusion. If the viscosity of an alignment layer is big enough, the alignment layer can smooth the slop of the dual pattern protrusion. The second pattern on the mask may be half tone pattern instead of a slit pattern. The half tone pattern transmits light less than the first pattern on the mask.  
         [0074]     Though the embodiments of the present disclosure shows only positive type PR, a negative type PR can be used.  
         [0075]     This invention has been described with reference to the exemplary embodiments. It is evident, however, that many alternative modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the present invention embraces all such alternative modifications and variations as fall within the spirit and scope of the appended claims.