Abstract:
A method of reducing a fringe field effect in an LCD and related structure. The LCD includes a plurality of pixels thereon. The method includes forming a bump on at least a side of each pixel for controlling inclined directions of liquid crystal molecules in a liquid crystal cell, and forming a concave in each pixel for fixing a position of a reverse domain due to different inclined directions. The method and related structure is able to reduce the fringe field effect in a VAN LCOS display or a high resolution LCD. Consequently, the brightness uniformity and contrast ratio are improved while areas with poor display effect are reduced.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of reducing a fringe field effect in an LCD and related structure, and more particularly, to a method and related structure adapted in pixels of a VAN (vertical aligned nematic) LCOS display for improving the brightness uniformity and contrast ratio.  
           [0003]    2. Description of the Prior Art  
           [0004]    Currently, LCD projection is prevailing over all other digital projection technologies. However, several problems still remain to be overcome. One problem is that the limited aperture ratio and low light usage make the brightness insufficient. The other is the heat dissipation difficulty. Since the LCD projector utilizes high luminant halogen filament bulbs which generate heat when illuminating, the heat dissipation problem reduces the lifetime of the halogen filament bulbs. Therefore, the LCOS (liquid crystal on silicon) projector, such as a VAN (vertical aligned nematic) LCOS display, has been highly developed since it adopts standard semiconductor processes and has the advantages of high resolution and high aperture ratio.  
           [0005]    [0005]FIG. 1 is a schematic diagram of a conventional VAN LCOS display. As shown in FIG. 1, the conventional VAN LCOS display includes a silicon substrate  10 , an insulating layer  12  positioned on the silicon substrate  10 , a passivation layer  14  positioned above the insulating layer  12 , two metal layers  16  and  18 , and an aluminum reflective layer  20 . The metal layer  16  is connected to switch components (not shown), the metal layer  18  is connected to a barrier layer (not shown), and the aluminum reflective layer  20  is used to reflect light beams. The conventional VAN LCOS display further includes liquid crystal molecules  22 , two alignment layers  24 , an ITO electrode  26 , and a glass substrate  28 . The liquid crystal molecules  22  are positioned above the passivation layer  14  and in between the two alignment layers  24 , the ITO electrode  26  is positioned on the alignment layer  24 , and the glass substrate  28  is positioned on the ITO electrode  26 .  
           [0006]    [0006]FIG. 2 is a schematic diagram illustrating a fringe field effect. As shown in FIG. 2, when the distance between pixels in an LCD panel becomes closer, the diffraction effect and the fringe field effect may occur. The diffraction effect results from the pixel electrodes which function as a raster. The fringe field has an extent  32  proportional to a cell gap  30  of the LCD panel. In other words, the larger the cell gap  30 , the broader the extent  32 .  
           [0007]    As technologies progress, LCDs with high resolutions have become standard. However, the fringe field effect is not desirable when pursuing high resolution. Therefore, a method of reducing and controlling the fringe field effect for improving the brightness uniformity and contrast ratio is highly needed.  
         SUMMARY OF INVENTION  
         [0008]    It is a primary objective of the present invention to provide a method of reducing a fringe field effect in an LCD and related structure, particularly in the situation when the distance between two adjacent pixels is less than twice of the cell gap.  
           [0009]    According to the claimed invention, a method of reducing a fringe field effect in an LCD and related structure are disclosed. The LCD includes a substrate having a plurality of pixels arranged in arrays, and each pixel corresponds to a liquid crystal cell. The method includes forming a bump on at least a side of each pixel for controlling inclined directions of liquid crystal molecules in a liquid crystal cell, and forming a concave in each pixel for fixing a position of a reverse domain due to the different inclined directions of the liquid crystal molecules.  
           [0010]    The structure includes a first substrate having a pixel defined thereon, a liquid crystal cell, at least a bump, at least a concave, and a second substrate. The first substrate has a bottom layer thereunder. The liquid crystal cell has a plurality of liquid crystal molecules positioned above the first substrate. The at least bump is positioned on the first substrate and on at least two opposites of the pixel for controlling inclined directions of the liquid crystal molecules. The concave is positioned on the first substrate for fixing a position of a reverse domain due to different inclined directions of the liquid crystal molecules above the concave. The second substrate is positioned above the liquid crystal cell.  
           [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic diagram of a conventional VAN LCOS display.  
         [0013]    [0013]FIG. 2 is a schematic diagram illustrating a fringe field effect.  
         [0014]    [0014]FIG. 3 is a schematic diagram of a VAN LCOS display in a power-off situation according to the present invention.  
         [0015]    [0015]FIG. 4 is a schematic diagram of the VAN LCOS display shown in FIG. 3 in a power-on situation.  
         [0016]    [0016]FIG. 5 is a schematic diagram of the bumps and the concave shown in FIG. 3.  
         [0017]    [0017]FIG. 6 is a top view of the VAN LCOS display in a power-off situation.  
         [0018]    [0018]FIG. 7 is a top view of the VAN LCOS display shown in FIG. 6 in a power-on situation.  
         [0019]    [0019]FIG. 8 is a top view of another VAN LCOS display in a power-off situation.  
         [0020]    [0020]FIG. 9 is a top view of the VAN LCOS display shown in FIG. 8 in a power-on situation.  
         [0021]    [0021]FIG. 10 is a schematic diagram illustrating a reverse domain.  
         [0022]    [0022]FIG. 11 is a cell gap vs. reverse domain chart.  
         [0023]    [0023]FIG. 12 is a chart illustrating relations between the height of bump and response time.  
         [0024]    [0024]FIG. 13 is a schematic diagram of a VAN LCOS display when the frame-plus-bias driving method is adopted.  
         [0025]    [0025]FIG. 14 is a schematic diagram of another VAN LCOS display when the frame-plus-bias driving method is adopted. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIG. 3 is a schematic diagram of a VAN LCOS display  34  in a power-off situation according to the present invention. As shown in FIG. 3, the VAN LCOS display  34  includes a first substrate  36  having a bottom layer  38 , a liquid crystal cell  40  having a plurality of vertically aligned liquid crystal molecules positioned above the substrate  36 , a second substrate  42  positioned above the liquid crystal cell  40 , two bumps  44  and  46  positioned on two opposite sides of a pixel of the first substrate  36 , and a concave  48  positioned on the first substrate  36  between the bumps  44  and  46 . The bumps  44  and  46  are for controlling inclined directions of the liquid crystal molecules of the liquid crystal cell  40 , and the material of the bumps  44  and  46  include silicon oxide, silicon nitride, and other inorganic materials so as to reduce the fringe field effect. The concave  48  is used to fix a position of a black line due to different inclined directions of the liquid crystal molecules in the liquid crystal cell  40 . It is to be noted that the concave  48  can be alternatively positioned anywhere between the two bumps  44  and  46 . Preferably, the concave  48  is positioned halfway between the bumps  44  and  46 . In such a case, the liquid crystal molecules alongside the concave  48  are arranged symmetrically, and thereby reveal identical optical characteristics. In addition, if a frame-plus-bias inverse driving method is applied, two electrodes  50  and  52  corresponding to the bumps  44  and  46  must be installed on the bottom layer  38 . In such case, the bumps  44  and  46  can generate an electric field so as to control the inclined directions of the liquid crystal molecules in the liquid crystal cell  40 .  
         [0027]    [0027]FIG. 4 is a schematic diagram of the VAN LCOS display  34  shown in FIG. 3 in a power-on situation. As shown in FIG. 4, while power is provided, the bumps  44  and  46  enable the liquid crystal molecules in the liquid crystal cell  40  to incline from the bumps  44  and  46  to the concave  48  such that a reverse domain is formed. The reverse domain causes the brightness to be uneven, and therefore the purpose of the concave  48  is to fix the position of the reverse domain right above the concave  48 .  
         [0028]    [0028]FIG. 5 is a schematic diagram of the bumps  44  and  46 , and the concave  48  shown in FIG. 3. As shown in FIG. 5, the liquid crystal cell  40  has a cell gap d, the bumps  44  and  46  have a same height h1, and the concave  48  has a depth h2. The bumps  44  and  46 , and the concave  48  are used to control the inclined directions of the liquid crystal molecules in the liquid crystal cell  40 . By way of adjusting the relations among the cell gap d, the height h1, and the depth h2, the brightness uniformity and the contrast ratio are improved while the area having a poor display effect is reduced. According to the present invention, the height h1 of the bumps  44  and  46 , the depth h2 of the concave  48 , and the cell gap d of the liquid crystal cell  40  are consistent with the following relations.  
         {fraction (1/15)} ≦h 1/ d≦ 1  (EQ-1)  
         {fraction (1/50)} ≦h 2/ d≦ ⅓  (EQ-2)  
         [0029]    According to EQ-1 and EQ-2, proper values of h1 and h2 can be obtained. The bumps  44  and  46  have a height h1 ranging from 0.3 μm to 3 μm, and a width ranging from 0.3 μm to 20 μm. The concave  48  has a depth ranging from 0.05 μm to 3 μm, and a width ranging from 0.05 μm to 20 μm.  
         [0030]    [0030]FIG. 6 is a top view of the VAN LCOS display in a power-off situation. As shown in FIG. 6, the VAN LCOS display includes two bar bumps  54  and  56 , and a bar concave  60 . The bar bumps  54  and  56  have a bar-shaped structure and are positioned at two opposite sides of a pixel for controlling the inclined directions of liquid crystal molecules  58 . The bar concave  58  also has a bar-shaped structure in parallel with the bar bumps  54  and  56 , and is positioned between the bar bumps  54  and  56  for fixing a disclination line  62  generated when the liquid crystal molecules  58  are inclined.  
         [0031]    [0031]FIG. 7 is a top view of the VAN LCOS display shown in FIG. 6 in a power-on situation. As shown in FIG. 7, the liquid crystal molecules  58  are inclined from the bar bumps  54  and  56  toward to the bar concave  60  when power is provided. The disclination line  62  is a black line generated by the liquid crystal molecules  58  above the bar concave  60  or by the liquid crystal molecules  58  close to the bar concave  60 . Consequently, the position of the disclination line  62  is decided by the position of the bar concave  60 . FIG. 8 is a top view of another VAN LCOS display in a power-off situation. As shown in FIG. 8, the VAN LCOS display includes a circular bump  64  around a pixel for controlling inclined directions of liquid crystal molecules  66 , and a concave  68  positioned within the circular bump  64  for fixing a black dot generated while the molecules  68  are inclined.  
         [0032]    [0032]FIG. 9 is a top view of the VAN LCOS display shown in FIG. 8 in a power-on situation. As shown in FIG. 9, the liquid crystal molecules  66  are inclined from the circular bump  64  toward the concave  68 . The concave  68  is used to fix the black dot resulting from the liquid crystal molecules  66  above the concave  68  or the liquid crystal molecules  66  close to the concave  68 . Similarly, the position of the black dot is decided by the position of the concave  68 . Preferably, the concave  68  is positioned at a symmetrical center of the circular bump  64 . Accordingly, the liquid crystal molecules  66  are arranged symmetrically, and render the VAN LCOS display a better display effect.  
         [0033]    [0033]FIG. 10 is a schematic diagram illustrating a reverse domain. As shown in FIG. 10, while the inclined directions of liquid crystal molecules  70  are inconsistent, a reverse domain  72  occurs. The reverse domain  72  causes the brightness to be uneven. Therefore, a concave is adopted to fix the position of a black line or a black dot resulting from the reverse domain  72 .  
         [0034]    [0034]FIG. 11 is a cell gap vs. reverse domain chart, where different curves are obtained in different phase differences (Δnd). As shown in FIG. 11, a curve  74  is obtained when Δnd equals 270 nm, a curve  76  is obtained when Δnd equals 300 nm, and a curve  78  is obtained when Δnd equals 330 nm. At a fixed cell gap, the variation of phase difference is weakly correlated with the extent of reverse domain. However, at a fixed phase difference, the variation of liquid crystal cell gap is strongly correlated with the extent of reverse domain. As a result, the correlation between the liquid crystal cell gap and the extent of reverse domain is more evident than the correlation between the phase difference and the extent of reverse domain.  
         [0035]    The VAN LCOS display of the present invention is free to adopt different driving methods such as dot inversion, frame inversion, and frame-plus-bias inversion, and none of these methods has a light leakage problem in the dark state. Since there are no light leakage problems, the contrast ratio, which is a brightness ratio of a luminous state to that of a dark state, is high. However, the dot inversion method has the disadvantage of uneven brightness that causes a large extent of reverse domain. Therefore, the frame-plus-bias inversion driving method is preferred.  
         [0036]    [0036]FIG. 12 is a chart illustrating relations between the height of bump and response time, where the phase difference (Δnd=275 nm) and the liquid crystal cell gap (d=2.0 μm) are both fixed. As shown in FIG. 12, a curve  80  is obtained when the depth of the concave is 0 μm, a curve  82  is obtained when the depth of the concave is 0.1 μm, and a curve  84  is obtained when the depth of the concave is 0.5 μm. While the bump and the concave are absent, different modes of LCDs, such as ECB mode, TN mode, INV-TN mode, etc., suffer from the uneven brightness problem due to the fringe field effect. The presence of the bump can reduce the fringe field effect, and the presence of the concave can fix the position of the disclination line. In other words, the reverse domain is formed right above the concave. It is worth noting that the phase difference is selected according to different modes of LCDs to enhance the function of the bump and the concave. Preferably, the phase difference is between 150 nm to 410 nm.  
         [0037]    [0037]FIG. 13 is a schematic diagram of a VAN LCOS display when the frame-plus-bias driving method is applied, where FIG. 13A shows a power-off situation, and FIG. 13B shows a power-on situation. As shown in FIG. 13A, a plurality of bumps  90  are formed between adjacent pixels on a substrate  86  which has a bottom layer  88  thereunder. A plurality of concaves  92  are then formed on the substrate  86  between two adjacent bumps  90 . Electrodes  94  corresponding to the bumps  90  are formed on the bottom layer  88  to generate an electric field. As shown in FIG. 13B, when power is provided, only liquid crystal molecules  96  positioned above the bumps  90  are not influenced by the electric field. The rest of the liquid crystal molecules  96  are inclined from the bumps  90  toward the concaves  92 , and thereby form reverse domains positioned above each concave  92 .  
         [0038]    [0038]FIG. 14 is a schematic diagram of another VAN LCOS display when the frame-plus-bias driving method is adopted where FIG. 14A shows a power-off situation, and FIG. 14B shows a power-on situation. The differences between this VAN LCOS display and the VAN LCOS display shown in FIG. 13 is that an electrode layer  98  replaces the electrodes  94  to generate an electric field.  
         [0039]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.