Abstract:
A liquid crystal display device including two substrates, with a pixel electrode formed on one substrate and an opposite electrode formed on the other substrate. The device also includes an alignment film formed on the opposite electrode, a protrusion formed between the opposite electrode and the alignment film, spherical spacers, liquid crystal, and an alignment film protrusion formed by the alignment film in an area corresponding to the protrusion, such that the protrusion causes the alignment film protrusion to protrude toward the liquid crystal. The alignment film protrusion is recessed on a side opposite one of the substrates and regulates an alignment direction of the liquid crystal contacting it. Further, the alignment film protrusion includes a recessed portion, within an outer surface thereof, within which at least one spherical spacer is seated, whereby the recessed portion reduces compressive stress exerted on the substrates by the spherical spacer seated therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device used for a display part of an electronic equipment, and particularly to a liquid crystal display device including a vertical alignment type liquid crystal having a negative dielectric anisotropy. 
     2. Description of the Related Art 
     A vertical alignment (VA) mode liquid crystal display device has features of a high contrast ratio and a high speed response characteristic, and in recent years, the VA mode liquid crystal display device has been actively developed. Especially, an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device has features of a wide viewing angle, a high contrast ratio and a high speed response characteristic, and receives attention as a display system most suitable for a flat panel display for a TV receiver.  FIG. 7  shows a sectional structure of one pixel of the MVA mode liquid crystal display device. As shown in  FIG. 7 , the MVA mode liquid crystal display device includes a glass substrate  103  of a TFT substrate  102 , an opposite side glass substrate  105  of an opposite substrate  104  disposed to be opposite to the TFT substrate  102 , and a liquid crystal  112  sealed between both the substrates  102  and  104 . 
     An insulating layer  106  is formed on the glass substrate  103 . Plural gate bus lines and plural drain bus lines (neither of them are shown) intersecting each other through the insulating layer  106  are formed on the glass substrate  103 . A not-shown thin film transistor (TFT) is formed at each of intersection parts of the gate bus lines and the drain bus lines. The gate bus lines and the drain bus lines are insulated from each other through the insulating layer  106 . Besides, the insulating film  106  functions as a gate insulating film of the TFT. A pixel electrode  110  made of indium tin oxide (ITO) and patterned into a specified shape is formed on a final protection film  108 . The pixel electrode  110  is connected to a source electrode of the TFT through a not-shown contact hole formed in the final protection film  108 . A pixel electrode slit  116  as an alignment regulating structure for regulating an alignment direction of a liquid crystal molecule  120  is formed in the pixel electrode  110 . A vertical alignment film  114  for vertically aligning the liquid crystal molecule  120  is formed on the whole surface of the pixel electrode  110  and the pixel electrode slit  116 . 
     On the other hand, a not-shown color filter (CF) layer is formed on the opposite side glass substrate  105 . An opposite electrode  124  made of ITO is formed on the CF layer and the whole surface of the substrate. A linear protrusion  118  as an alignment regulating structure protruding on the opposite electrode  124  is formed on the opposite side glass substrate  105 . Similarly to the pixel electrode slit  116 , the linear protrusion  118  is formed in order to regulate the alignment direction of the liquid crystal molecule  120 . A vertical alignment film  122  is formed on the whole surface of the opposite electrode  124  and covers the linear protrusion  118 . In the MVA mode liquid crystal display device, the linear protrusion  118  and the pixel electrode slit  116  are provided in the pixel, so that the alignment control of the liquid crystal  112  and multi-domain formation are realized. 
     [Patent document 1] JP-A-2001-183637 
     [Patent document 2] JP-A-8-122753 
     As a method of keeping the thickness (cell gap) of the layer of the liquid crystal  112  at a desired length, there is used a method of scattering spherical ball spacers  136  each having a diameter equal to the desired cell gap into the layer of the liquid crystal  112 . The ball spacers  136  are made of plastic material or glass material. The ball spacers  136  are scattered on the TFT substrate  102  or the opposite substrate  104 , and the TFT substrate  102  and the opposite substrate  104  are attached to each other through a seal material formed into a frame shape, so that the liquid crystal  112  is sealed between both the substrates  102  and  104 . 
     For example, when the ball spacers  136  are scattered on the opposite substrate  104 , there is a case where the ball spacer  136  is disposed on the linear protrusion  118 . The cell gap at the position where the linear protrusion  118  is formed is narrow as compared with the other position. Thus, as shown in  FIG. 7 , the cell gap at the linear protrusion  118  is narrower than the diameter of the ball spacer  136 . Since the ball spacer  136  is made of relatively hard material as compared with the vertical alignment films  114  and  122 , when the opposite substrate  104  and the TFT substrate  102  are attached to each other, there is a case where the vertical alignment films  114  and  122  on the linear protrusion  118  and on the opposite side thereof are damaged by the pressure of the ball spacer  136 . 
     At this state, the alignment control of the liquid crystal molecule  120  at the position becomes impossible, light leak occurs at the linear protrusion  118  and in the vicinity thereof, and a poor display occurs in the MVA mode liquid crystal display device. In general, in order to realize excellent alignment of the liquid crystal molecule  120 , it is necessary that the width of the linear protrusion  118  is about 10 μm. This corresponds to several percents of the area of the pixel region. For example, in a 15-inch MVA mode liquid crystal display device with a resolution of XGA (extended Graphics Array), in the case where the linear protrusion  118  is formed to have a width of 15 μm, and 100 ball spacers per 1 mm 2  are scattered, there is a case where about 7 ball spacers per 1 mm 2  are scattered on the linear protrusion  118 . When the vertical alignment films  114  and  122  are damaged by the ball spacer  136 , the light leak occurring on the display screen becomes very noticeable. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide a liquid crystal display device in which light leak due to damage of an alignment film is prevented and an excellent display characteristic can be obtained. 
     The above object can be achieved by a liquid crystal display device including a pair of substrates disposed to be opposite to each other, spherical spacers scattered between the pair of substrates, a liquid crystal sealed between the pair of substrates, a protrusion as an alignment regulating structure protruding from one of the pair of substrates and for regulating an alignment direction of the liquid crystal, and a recessed part formed in the other of the pair of substrates to be opposite to the protrusion. 
     According to the invention, the liquid crystal display device which prevents the light leak due to the damage of the alignment film and has the excellent display characteristic can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a rough structure of a liquid crystal display device according to a first embodiment of the invention; 
         FIGS. 2A and 2B  are views showing a structure of one pixel of the liquid crystal display device according to the first embodiment of the invention; 
         FIG. 3  is a view showing a sectional structure of one pixel of a liquid crystal display device according to a second embodiment of the invention; 
         FIG. 4  is a view showing a sectional structure of one pixel of a liquid crystal display device according to a third embodiment of the invention; 
         FIG. 5  is a view showing a sectional structure of one pixel of a liquid crystal display device according to a fourth embodiment of the invention; 
         FIG. 6  is a view showing results of a tap test of a liquid crystal display device according to a fifth embodiment of the invention; and 
         FIG. 7  is a view showing a sectional structure of one pixel of a conventional liquid crystal display device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Embodiment 
     A liquid crystal display device according to a first embodiment of the invention will be described with reference to  FIGS. 1 to 2B .  FIG. 1  shows a rough structure of the liquid crystal display device according to this embodiment.  FIGS. 2A and 2B  show a rough structure of one pixel of the MVA mode liquid crystal display device according to this embodiment.  FIG. 2A  shows the structure of the one pixel when a display screen of the liquid crystal display device is viewed in the direction of a normal line, and  FIG. 2B  shows a sectional structure taken along line A-A of  FIG. 2A . 
     As shown in  FIGS. 1 to 2B , the MVA mode liquid crystal display device includes a liquid crystal display panel having such a structure that a TFT substrate  2  on which a pixel electrode  10 , a TFT  11  and the like are formed for each pixel region is made to be opposite to and to be attached to an opposite substrate  4  on which a CF layer (not shown) and the like are formed, and a liquid crystal  12  having a negative dielectric anisotropy is sealed between them. Vertical alignment films  14  and  22  for aligning liquid crystal molecules  20  in, for example, the direction vertical to the substrate surface are formed on the opposing surfaces of both the substrates  2  and  4 . 
     As shown in  FIG. 1 , the TFT substrate  2  is provided with a gate bus line drive circuit  80  on which a driver IC for driving plural gate bus lines  7  (see  FIG. 2A ) is mounted, and a drain bus line drive circuit  82  on which a driver IC for driving plural drain bus lines  9  (see  FIG. 2A ) is mounted. Both the drive circuits  80  and  82  output a scanning signal and a data signal to a specified gate bus line or drain bus line on the basis of a specified signal outputted from a control circuit  84 . 
     A polarizing plate  86  is attached to a surface of the TFT substrate  2  at the opposite side to the element formation surface thereof. A backlight unit  88  constructed of, for example, a linear primary light source and a planar optical waveguide plate is disposed at the far side of the polarizing plate  86  with respect to the TFT substrate  2 . On the other hand, a polarizing plate  87  is attached to a surface of the opposite substrate  4  at the opposite side to the resin CF layer formation surface thereof. 
     As shown in  FIG. 2B , the MVA mode liquid crystal display device includes a glass substrate  3  used for the TFT substrate  2 , an opposite side glass substrate  5  used for the opposite substrate  4  disposed to be opposite to the TFT substrate  2 , and the liquid crystal  12  sealed between both the substrates  2  and  4 . In order to keep a cell gap at a specified length, plural spherical ball spacers (spherical spacers)  36  are scattered in the liquid crystal  12 . Each of the ball spacers has a diameter almost equal to a desired cell gap. 
     As shown in  FIGS. 2A and 2B , a CF layer (not shown) is formed on the opposite side glass substrate  5 . An opposite electrode  24  made of ITO is formed on the CF layer and the whole surface of the substrate. A linear protrusion  18  as an alignment regulating structure protruding on the opposite electrode  24  is formed on the opposite side glass substrate  5 . The linear protrusion  18  is formed in order to regulate the alignment direction of the liquid crystal molecule  20 . The length (height) of the linear protrusion  18  from the opposite electrode  24  to its top is formed to be, for example, 1.5 μm. The vertical alignment film  22  for vertically aligning the liquid crystal molecule  20  is formed on the whole surface of the opposite electrode  24  and covers the linear protrusion  18 . 
     On the other hand, the TFT substrate  2  includes the plural gate bus lines  7  extending in the horizontal direction of  FIG. 2A  on the glass substrate  3 . An insulating layer  6  is formed on the gate bus lines  7 . The plural drain bus lines  9  extending in the vertical direction of  FIG. 2A  are formed to intersect the gate bus lines  7  through the insulating layer  6 . The TFT  11  is formed at each of intersection parts of the gate bus lines  7  and the drain bus lines  9 . The insulating layer  6  between the gate bus line  7  and a drain electrode  11   a  or a source electrode  11   b  functions as a gate insulating film of the TFT  11 . A final protection film  8  is formed on the insulating layer  6  and covers the drain bus lines  9 . 
     The pixel electrode  10  patterned into a specified shape and made of ITO is formed in each of pixel regions surrounded by the gate bus lines  7  and the drain bus lines  9  on the final protection film  8 . The pixel electrode  10  is connected to the source electrode  11   b  of the TFT  11  through a contact hole  13  formed in the final protection film  8 . A storage capacitor bus line  15  extending in parallel to the gate bus line  7  is formed to cross almost the center of the pixel region. A storage capacitor electrode (intermediate electrode)  17  is formed on the storage capacitor bus line  15  through the insulating film  6  for each of the pixel regions. 
     A pixel electrode slit  16  with an electrode cutout structure is formed as an alignment regulating structure in the pixel electrode  10 . Similarly to the linear protrusion  18 , the pixel electrode slit  16  is formed in order to regulate the alignment direction of the liquid crystal molecule  20 . The vertical alignment film  14  for vertically aligning the liquid crystal molecule  20  is formed on the whole surface of the pixel electrode  10  and the pixel electrode slit  16 . 
     The TFT substrate  2  includes a recessed part  26  formed at a position opposite to the linear protrusion  18 . The recessed part  26  is formed by removing the insulating layer  6  and the final protection film  8 . The recessed part  26  is formed along the linear protrusion  18 . The depth of the recessed part  26  is formed to be, for example, 1 μm. 
     As shown in  FIG. 2B , the recessed part  26  is provided at the position of the TFT substrate  2  opposite to the linear protrusion  18 , so that the cell gap at the position where the linear protrusion  18  is formed becomes large as compared with a cell gap at the same position in the case where the recessed part  26  is not provided as in the related art. The cell gap at the position where the linear protrusion  18  is formed becomes almost equal to that at the other position. Thus, when the ball spacers  36  are scattered on the opposite substrate  4 , even if the ball spacer  36  is disposed on the linear protrusion  18 , it is possible to relieve compressive stress exerted on the vertical alignment films  14  and  22  from the ball spacer  36  on the linear protrusion  18  when the opposite substrate  4  and the TFT substrate  2  are attached to each other. By this, the vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof are not damaged. Accordingly, since the light leak due to poor alignment of the liquid crystal molecule  20  does not occur at the linear protrusion  18  and in the vicinity thereof, the contrast ratio can be improved. 
     As described above, according to this embodiment, the MVA mode liquid crystal display device includes the recessed part  26  at the position of the TFT substrate  2  opposite to the linear protrusion  18 . By this, it is possible to relieve the compressive stress exerted on the vertical alignment films  14  and  22  from the ball spacer  36  on the linear protrusion  18  when both the substrates  2  and  4  are attached to each other. Thus, the vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof can be prevented from being damaged, and the light leak at the linear protrusion  18  and in the vicinity thereof does not occur. By this, the contrast ratio is improved, and the MVA mode liquid crystal display device having the excellent display characteristic can be realized. 
     Second Embodiment 
     A liquid crystal display device according to a second embodiment of the invention will be described with reference to  FIG. 3 .  FIG. 3  shows a sectional structure of one pixel of the MVA mode liquid crystal display device according to this embodiment. As shown in  FIG. 3 , the MVA mode liquid crystal display device according to this embodiment is characterized in that circular polarizing plates  30  and  34  are disposed on the opposite sides of opposing surfaces of a TFT substrate  2  and an opposite substrate  4  having the same pixel structure as the former embodiment. 
     As shown in  FIG. 3 , the circular polarizing plate  30  is disposed on the side of the TFT substrate  2 , and the circular polarizing plate  34  is disposed on the side of the opposite substrate  4 . The circular polarizing plates  30  and  34  are disposed in crossed Nicols at both sides of a liquid crystal  12 . The circular polarizing plate  30  includes a ¼ wavelength plate  28  and a polarizing plate  86  disposed in sequence from the side of the TFT substrate  2 . The ¼ wavelength plate  28  and the polarizing plate  86  are disposed so that an angle between an optical axis (delay phase axis) of the ¼ wavelength plate  28  and an absorption axis of the polarizing plate  86  becomes about 45°. The circular polarizing plate  34  includes a ¼ wavelength plate  32  and a polarizing plate  87  disposed in sequence from the side of the opposite substrate  4 . The ¼ wavelength plate  32  and the polarizing plate  87  are disposed so that an angle between an optical axis of the ¼ wavelength plate  32  and an absorption axis of the polarizing plate  87  becomes about 45°. The optical axes of both the ¼ wavelength plates  28  and  32  are almost perpendicular to each other. 
     In the case where the circular polarizing plates  30  and  34  are used for the MVA mode liquid crystal display device, since the transmissivity of light does not depend on the tilt direction of a liquid crystal molecule  20 , as compared with the case where only the polarizing plates  86  and  87  are used, the transmissivity is improved. On the other hand, the light leak occurring due to poor alignment of the liquid crystal molecule  20  caused by few scratches of the vertical alignment films  14  and  22  becomes noticeable. However, in this embodiment, a recessed part  26  is provided at a position opposite to a linear protrusion  18 , so that the vertical alignment films  14  and  22  are not damaged. Thus, the circular polarizing plates  30  and  34  can be used without fear of the light leak due to the damage of the vertical alignment films  14  and  22 , and the transmissivity of the MVA mode liquid crystal display device can be improved. 
     As stated above, in the MVA mode liquid crystal display device according to this embodiment, since the transmissivity is improved, the display characteristic can be improved. Besides, in the case where the brightness of the display screen of the liquid crystal display device of this embodiment and that of the liquid crystal display device of the former embodiment are made identical to each other, since the transmissivity of the liquid crystal display device of this embodiment is high, the surface brightness of a backlight unit can be lowered. Thus, consumed electric power of the backlight unit can be reduced. Accordingly, in the liquid crystal display device of this embodiment, the consumption electric power can be reduced more than the liquid crystal display device of the former embodiment. 
     The circular polarizing plates  30  and  34  can also be applied to MVA mode liquid crystal display devices of third to fifth embodiments described later. 
     Third Embodiment 
     A liquid crystal display device according to a third embodiment of the invention will be described with reference to  FIG. 4 .  FIG. 4  shows a sectional structure of one pixel of the MVA mode liquid crystal display device according to this embodiment. As shown in  FIG. 4 , the MVA mode liquid crystal display device according to this embodiment is characterized in that the width of a linear protrusion  18  is formed to be shorter than the diameter of a ball spacer  36 . 
     When the width of the linear protrusion  18  in the direction (horizontal direction in the drawing) parallel to a resin CF layer formation surface of an opposite substrate  4  is made smaller than the diameter of the ball spacer  36 , in the case where the ball spacer  36  is scattered on the linear protrusion  18 , the ball spacer  36  comes in contact with the linear protrusion  18  at a point or a line. Thus, as shown in  FIG. 4 , when a TFT substrate  2  and the opposite substrate  4  are attached to each other, the ball spacer  36  easily rolls down from the linear protrusion  18 , and moves from the first scattered position. By this, since the ball spacer  36  is not sandwiched between the linear protrusion  18  and the TFT substrate  2 , vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof can be prevented from being damaged. Besides, when the width of the linear protrusion  18  is made short, the ratio of the area of the linear protrusion  18  occupying a pixel region becomes small. Accordingly, a probability that the ball spacer  36  is scattered on the linear protrusion  18  is decreased, and the vertical alignment films  14  and  22  can be prevented from being damaged. 
     As described above, according to this embodiment, in the MVA mode liquid crystal display device, the width of the linear protrusion  18  is made shorter than the diameter of the ball spacer  36 , so that the vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof are not damaged. Accordingly, the light leak at the linear protrusion  18  and in the vicinity thereof does not occur. By this, the contrast ratio is improved, and the MVA mode liquid crystal display device having the excellent display characteristic can be realized. 
     Fourth Embodiment 
     A liquid crystal display device according to a fourth embodiment of the invention will be described with reference to  FIG. 5 .  FIG. 5  shows a sectional structure of one pixel of the MVA mode liquid crystal display device according to this embodiment. As shown in  FIG. 5 , the MVA mode liquid crystal display device according to this embodiment is characterized by including a linear protrusion  18  formed to be recessed on the side opposite to a TFT substrate  2 . 
     The linear protrusion  18  on the opposite side to the TFT substrate  2  is formed to be recessed, so that a cell gap at a position where the linear protrusion  18  is disposed becomes large as compared with a cell gap at the same position in the case where the top of the linear protrusion  18  is not formed to be recessed as in the related art. Thus, similarly to the first embodiment, even if a ball spacer  36  is scattered on the linear protrusion  18 , it is possible to reduce compressive stress exerted on vertical alignment films  14  and  22  from the ball spacer  36  on the linear protrusion  18  when an opposite substrate  4  and the TFT substrate  2  are attached to each other. By this, the vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof are not damaged. 
     As described above, according to this embodiment, in the MVA mode liquid crystal display device, the linear protrusion  18  on the opposite side to the TFT substrate  2  is formed to be recessed, so that it is possible to release the compressive stress exerted on the vertical alignment films  14  and  22  from the ball spacer  36  when both the substrates  2  and  4  are attached to each other. Thus, the vertical alignment films  14  and  22  on the linear protrusion  18  and on the opposite side thereof can be prevented from being damaged, and the light leak at the linear protrusion  18  and in the vicinity thereof does not occur. By this, the contrast ratio is improved and the MVA mode liquid crystal display device having the excellent display characteristic can be realized. 
     Fifth Embodiment 
     A liquid crystal display device according to a fifth embodiment of the invention will be described with reference to  FIG. 6 . The MVA mode liquid crystal display device according to this embodiment is characterized by including ball spacers made of relatively soft material as compared with vertical alignment films  14  and  22 . 
       FIG. 6  shows experimental results on light leak at a linear protrusion and in the vicinity thereof when three kinds of ball spacers different in hardness are used. A sectional structure of one pixel of the MVA mode liquid crystal display device used for the experiment is the same as the conventional MVA mode liquid crystal display device shown in  FIG. 7 , and is fabricated in a manner described below. First, a TFT substrate on which a vertical alignment film RN1663 made by Nissan Chemical Industries, Ltd. is formed to have a thickness of 100 nm is formed. Next, three kinds of ball spacers different in hardness and having a diameter of 4.5 μm are dry scattered uniformly on the TFT substrate surface at a density of 130±30 spacers/mm 2 . The 10% K values of the three kinds of ball spacers are 3.92 mPa, 4.70 mPa and 5.98 mPa. Next, the TFT substrate on which the ball spacers are scattered is bonded to an opposite substrate on which a vertical alignment film RN1663 made by Nissan Chemical Industries, Ltd. is formed to have a thickness of 100 nm. Next, an n-type liquid crystal MJ961213 made by Merck Ltd. is vacuum injected, and an injection port is sealed after the end of the injection. In this way, a liquid crystal display panel is completed. 
     Two liquid crystal display panels are fabricated for each of the three kinds of ball spacers, and a tap test of the liquid crystal display panels is performed. As shown in  FIG. 6 , in the liquid crystal display panels (sample No. 1 and No. 2) using the ball spacers with the 10% K value of 3.92 mPa, the light leak is observed in both cases before and after the tap test. In the liquid crystal display panels (sample No. 3 and No. 4) using the ball spacers with the 10% K value of 4.70 mPa, the light leak is not observed before the tap test, and on the other hand, after the tap test, although the light leak is slightly observed in the sample No. 3, the light leak is not observed in the sample No. 4. In the liquid crystal display panels (sample No. 5 and No. 6) using the ball spacers with the 10% K value of 5.98 mPa, the light leak is not observed in any case before and after the tap test. 
     As stated above, as the K value of the ball spacer becomes small, the light leak hardly occurs when the TFT substrate and the opposite substrate are attached to each other (before the tap test), or even if a shock is given by tapping the display surface of the liquid crystal display panel (after the tap test). Accordingly, the ball spacer made of relatively soft material as compared with the vertical alignment films  14  and  22  and having a small K value has a remarkable effect to prevent the occurrence of the light leak due to the damage of the vertical alignment film. Especially, in the liquid crystal display panel using the ball spacer with the 10% K value of 4.70 mPa or less, the effect of preventing the occurrence of the light leak is high. 
     As described above, in the MVA mode liquid crystal display device of this embodiment, the ball spacer made of relatively soft material as compared with the vertical alignment films  14  and  22  and having a small K value is used, so that the vertical alignment films can be prevented from being damaged, and the light leak can be prevented from occurring. By this, in the MVA mode liquid crystal display device of this embodiment, the same effect as the former embodiment can be obtained. 
     The present invention is not limited to the above embodiments, and can be variously modified. 
     Although the MVA mode liquid crystal display device according to the fifth embodiment has the same pixel structure as the conventional liquid crystal display device, the same pixel structure as the liquid crystal display device of the first to fourth embodiments may be adopted. Also in this case, the same effect as the above embodiment can be obtained. 
     Besides, although the TFT substrate  2  of the MVA mode liquid crystal display device according to the third and fourth embodiments has the same structure as the TFT substrate  102  of the conventional liquid crystal display device, the invention is not limited to this. For example, like the TFT substrate  2  of the liquid crystal display device according to the first and second embodiments, the TFT substrate  2  may have the recessed part  26  at the position opposite to the linear protrusion  18 . Also in this case, the same effect as the above embodiment can be obtained.