Abstract:
In a liquid crystal display device, a TFT substrate retains a contact hole that is formed in a passivation film, and is for connecting the TFT and the pixel electrode. And an opposite substrate retains a columnar spacer for ensuring a cell gap, and a columnar projection for misalignment prevention formed at a position corresponding to a position of the contact hole. Accordingly, misalignment between the TFT substrate and the opposite substrate is suppressed, occurrence of uneven brightness due to misalignment between pixel regions and a bright spot caused by shaving of an orientation film is prevented, and a favorable image is obtained.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP2011-159795 filed on Jul. 21, 2011, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a liquid crystal display device, and in particular relates to a liquid crystal display device in which misalignment between upper and lower substrates is suppressed. 
         [0004]    2. Description of the Related Art 
         [0005]    A liquid crystal display device has a configuration that a thin-film transistor (TFT) substrate in which a pixel electrode, a TFT and the like are formed in a matrix, and an opposite substrate in which a color filter and the like are formed at a position corresponding to that of the pixel electrode of the TFT substrate are arranged with a predetermined gap therebetween, and liquid crystal is retained in the gap between the TFT substrate and the opposite substrate. To regulate the gap between the TFT substrate and the opposite substrate to be constant, a columnar spacer is interposed between the substrates. 
         [0006]    In a liquid crystal display device, when a temperature change occurs between upper and lower substrates due to a use environment condition or lighting of a backlight, because rates of thermal expansion of the upper and lower substrates are different, misalignment of one substrate to another substrate in a surface direction occurs, and a display failure occurs due to occurrence of uneven brightness caused by misalignment between a pixel region of the upper substrate and a pixel region of the lower substrate and a bright spot caused by shaving of an orientation film associated with the misalignment in a surface direction. In particular, when a screen size is large or thickness of polarizing plates laminated on the upper and lower substrates are different, misalignment between the upper and lower substrates in a surface direction becomes more significant. 
         [0007]    About regulation of an interval between substrates and prevention of misalignment between upper and lower substrates, 
         [0008]    Japanese Patent Application Laid-Open Publication No. 2003-131238 discloses that columnar spacers having different height are formed on one substrate, reduction of a frictional resistance between the substrate and an opposite substrate is attempted with a taller spacer, and a cell gap between the substrates is finally ensured with a shorter spacer. 
         [0009]    Japanese Patent Application Laid-Open Publication No. 2003-84290 discloses a liquid crystal display device in which a columnar spacer for maintaining a substrate gap is arranged in a pixel electrode at a contact portion that supplies electrical signals to the pixel electrode to realize stable control of a panel gap without impairing display quality and an aperture ratio of pixels. 
         [0010]    Also, Japanese Patent Application Laid-Open Publication No. 2003-5190 discloses that to suppress a variation in inter-substrate positions and a cell gap caused by a positional variation of a columnar spacer due to application of an external force, a top part of the columnar spacer formed fixedly at an inner surface of one of a pair of substrates is positioned at a concave part of a multilayer structure film of another substrate. 
       SUMMARY OF THE INVENTION 
       [0011]    Japanese Patent Application Laid-Open Publication No. 2003-131238 has a problem in that although a frictional resistance between substrates is reduced so that misalignment between the substrates that has occurred can be easily fixed by bringing only the taller columnar spacer among the columnar spacers having different height into contact with the opposite substrate, occurrence of misalignment between the substrates itself cannot be suppressed. 
         [0012]    Japanese Patent Application Laid-Open Publication No. 2003-84290 has a problem in that to form the columnar spacer to match the contact portion, it is necessary to make an area of a hole bottom part larger than an area of a spacer top part taking into account a positional accuracy of spacer formation (alignment margin), and expansion of the area of the contact portion lowers the pixel aperture ratio and transmittance. 
         [0013]    Japanese Patent Application Laid-Open Publication No. 2003-5190 has a problem in that the concave part of the opposite substrate occupies a significantly large region as compared with the spacer formed between various wirings, and does not have a step such as a contact portion and lacks a positional accuracy; therefore, misalignment between the substrates itself cannot be suppressed. 
         [0014]    An object of the present invention is to provide a liquid crystal display device that suppresses misalignment between upper and lower substrates, and prevents occurrence of uneven brightness due to misalignment between pixel regions and a bright spot caused by shaving of an orientation film to obtain a favorable image. 
         [0015]    In order to address the above-described problems, a liquid crystal display device according to the present invention includes a TFT substrate in which a pixel electrode, a TFT and the like are formed in a matrix; an opposite substrate in which a color filter and the like are formed at a position corresponding to a position of the pixel electrode and that is arranged with a predetermined gap with the TFT substrate; and a liquid crystal retained in the gap; wherein the TFT substrate retains a contact hole that is formed in a passivation film, and is for connecting the TFT and the pixel electrode; and the opposite substrate retains a columnar spacer for ensuring a cell gap, and a columnar projection for misalignment prevention formed at a position corresponding to a position of the contact hole. 
         [0016]    In the liquid crystal display device according to the present invention, the passivation film may include an organic passivation film. 
         [0017]    Also, in the liquid crystal display device according to the present invention, the passivation film may be comprised only of an inorganic passivation film. 
         [0018]    Also, in the liquid crystal display device according to the present invention, a difference Δh between height h 1  of the columnar projection and height h 2  of the columnar spacer with reference to the opposite substrate may be equal to or less than depth z of the contact hole. 
         [0000]      Δ h=h 1 −h 2 ≦z  
 
         [0019]    It is of note that when there is a difference v between distance from the TFT substrate to a top surface of the contact hole and distance from the TFT substrate to a surface that contacts the columnar spacer, h 1 −(h 2 +v)≦z has to be satisfied. 
         [0020]    Also, in the liquid crystal display device according to the present invention, height h 1  of the columnar projection may be larger than a cell gap d, and may be smaller than a sum of the cell gap d and depth z of the contact hole. 
         [0000]        d&lt;h 1 ≦d+z    
         [0021]    Also, in the liquid crystal display device according to the present invention, the columnar projection may abut on a bottom part of the contact hole when an excessive force is applied thereto. 
         [0022]    Also, in the liquid crystal display device according to the present invention, the contact hole may have an inclining part diameter of which is larger at an upper part. 
         [0023]    Also, in the liquid crystal display device according to the present invention, a diameter of a top part of the columnar projection may be smaller than a diameter of an upper part of the contact hole. 
         [0024]    Also, in the liquid crystal display device according to the present invention, a cross-sectional area of the columnar projection may be smaller than a cross-sectional area of the columnar spacer. 
         [0025]    In the liquid crystal display device according to the present invention, density of the columnar projection may be higher at a peripheral area of a liquid crystal panel than at a center region. 
         [0026]    In the liquid crystal display device according to the present invention, the columnar spacer may be arranged substantially uniformly over an entire screen of the liquid crystal panel. 
         [0027]    In the liquid crystal display device according to the present invention, an interval between the columnar projections may be larger than an interval between the columnar spacers. 
         [0028]    Also, in the liquid crystal display device according to the present invention, the number of the columnar projection is less than the number of the columnar spacer. 
         [0029]    Also, in the liquid crystal display device according to the present invention, the number of the columnar projection may be less than the number of the columnar spacer by an order of magnitude or more. 
         [0030]    The liquid crystal display device according to the present invention is an in-plane switching (IPS), twisted nematic (TN) or vertical alignment (VA) liquid crystal display device. 
         [0031]    According to an aspect of the present invention, a cell gap can be ensured surely due to the columnar spacer. Also, misalignment between upper and lower substrates is suppressed by inserting and anchoring the columnar projection for misalignment prevention to the contact hole to prevent occurrence of uneven brightness due to misalignment between pixel regions and a bright spot caused by shaving of an orientation film. Accordingly, a favorable image can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1A  is a plane view showing an IPS liquid crystal display device according to a first embodiment of the present invention; 
           [0033]      FIG. 1B  is a cross-sectional view of a portion indicated with a line A-B in  FIG. 1A ; 
           [0034]      FIG. 2  is a diagram showing an arrangement of a columnar spacer in an entire liquid crystal panel; 
           [0035]      FIG. 3  is a diagram showing an IPS liquid crystal display device according to a second embodiment of the present invention; 
           [0036]      FIG. 4A  is a plane view showing a TN or VA liquid crystal display device according to a third embodiment of the present invention; and 
           [0037]      FIG. 4B  is a cross-sectional view of a portion indicated with a line A-B in  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    Embodiments of the present invention are explained with reference to the drawings. In each drawing, identical components are given with identical numbers, and explanation thereof is not repeated. 
       First Embodiment 
       [0039]    A liquid crystal display device according to a first embodiment of the present invention is shown in  FIGS. 1A and 1B . In the first embodiment, the present invention is applied to an IPS liquid crystal display device which is provided with a contact hole of an organic passivation film (an organic PAS film). 
         [0040]      FIG. 1A  is a plane view showing a part of the IPS liquid crystal display panel, and  FIG. 1B  is a cross-sectional view of a portion indicated with a line A-B in  FIG. 1A . 
         [0041]    Putting it simply, in the IPS liquid crystal display device, a comb-shaped opposite electrode  110  is formed on a pixel electrode  108  sandwiching an insulating film  109  therebetween, and an image is formed by rotating liquid crystal molecules  301  by voltage between the opposite electrode  110  and the pixel electrode  108 , and controlling transmittance of light of a liquid crystal layer  300  for each pixel. 
         [0042]    In  FIG. 1A , a scanning line  120  and a signal line  121  are wired in a matrix on a TFT substrate  100 . The pixel electrode  108  is arranged in a region surrounded by the scanning line  120  and the signal line  121 , and a TFT is formed at a crossing part of the scanning line  120  and the signal line  121 . The scanning line  120  is connected to a gate electrode of the TFT, and the signal line  121  is connected to a drain electrode of the TFT. 
         [0043]    Hereinafter, a structure in  FIG. 1B  is explained in detail. The scanning line  120  and the gate electrode  101  are formed on the TFT substrate  100  formed with glass. 
         [0044]    A gate insulating film  102  is formed covering the scanning line  120  and the gate electrode  101 . A semiconductor layer (not shown in the drawings) is formed on the gate insulating film  102  at a position facing the gate electrode  101 . The semiconductor layer forms a channel part of the TFT, and a source electrode  104  and a drain electrode  105  are formed on the semiconductor layer sandwiching the channel part. A motion image signal line doubles as the drain electrode  105 , and the source electrode  104  is connected with the pixel electrode  108 . The source electrode  104  and the drain electrode  105  are formed simultaneously on a same layer. 
         [0045]    The semiconductor layer, the gate electrode  101 , the source electrode  104  and the drain electrode  105  configure the TFT. 
         [0046]    An inorganic passivation film  106  is formed covering the TFT. The inorganic passivation film  106  protects in particular the channel part of the TFT from impurities. An organic passivation film  107  is formed on the inorganic passivation film  106 . The organic passivation film  107  plays a role of protecting the TFT and flattening a surface thereof, and thus is formed thick. Photosensitive acrylic resin, silicon resin, polyimide resin and the like are used for the organic passivation film  107 . A contact hole ill is formed in the organic passivation film  107  at a part that connects the pixel electrode  108  and the source electrode  104 . 
         [0047]    The pixel electrode  108  is formed on the organic passivation film  107 . The pixel electrode  108  is formed by sputtering indium tin oxide (ITO), which is a transparent conductive film, over an entire display area, and patterning ITO for each pixel region. The contact hole  111  connects the pixel electrode  108  and the source electrode  104 . The source electrode  104  that extends from the TFT and the pixel electrode  108  are electrically connected at the contact hole  111 , and motion image signals are supplied to the pixel electrode  108 . 
         [0048]    An inorganic passivation film  109  is formed covering the pixel electrode  108 . Thereafter, ITO to be the opposite electrode  110  is formed on the inorganic passivation film  109  by sputtering. The sputtered ITO is patterned to form the opposite electrode  110 . 
         [0049]    As shown in  FIG. 1A , the opposite electrode  110  is a comb-shaped electrode whose both ends are closed. Slits  112  are formed between comb teeth. The planar pixel electrode  108  is formed below the opposite electrode  110 . When motion image signals are supplied to the pixel electrode  108 , the liquid crystal molecules  301  are rotated by an electric line of force generated between the pixel electrode  108  and the opposite electrode  110  through the slits  112 . Thereby, an image can be formed by controlling light that passes through the liquid crystal layer  300 . 
         [0050]    Constant voltage is applied to the opposite electrode  110 , and voltage due to the motion image signals is applied to the pixel electrode  108 . When voltage is applied to the pixel electrode  108 , an electric line of force occurs, the liquid crystal molecules  301  are rotated and directed toward the direction of the electric line of force, and transmission of light from a backlight is controlled. An image is formed because transmission of light from the backlight is controlled for each pixel. 
         [0051]    In an example of  FIGS. 1A and 1B , the pixel electrode  108  formed planarly is arranged on the organic passivation film  107 , and the comb electrode  110  is arranged on the inorganic passivation film  109 . However, on the contrary, the opposite electrode  110  formed planarly may be arranged on the organic passivation film  107 , and the comb-shaped pixel electrode  108  may be arranged on the inorganic passivation film  109  in another case. 
         [0052]    An orientation film  113  for orienting the liquid crystal molecules  301  is formed on the opposite electrode  110 . 
         [0053]    In  FIG. 1B , an opposite substrate  200  is installed sandwiching the liquid crystal layer  300 . Color filters  201  are formed on an inner side of the opposite substrate  200 . Red, green and blue color filters are formed as the color filters  201  for each pixel, and thus a color image is formed. A light-shielding black matrix  202  is formed between the color filters  201  to improve contrast of an image. The light-shielding black matrix  202  also plays a role as a light-shielding film of the TFT, and prevents photocurrent from flowing through the TFT. 
         [0054]    An overcoat film  203  is formed covering the color filters  201  and the light-shielding black matrix  202 . Because the color filters  201  and the light-shielding black matrix  202  have irregular surfaces, the overcoat film  203  flattens the surfaces. 
         [0055]    The orientation film  113  for deciding an initial orientation of liquid crystal is formed on the overcoat film  203 . 
         [0056]    In the present embodiment, as a characteristic configuration, a columnar projection  210  for misalignment prevention and a columnar spacer  220  for ensuring a cell gap are provided on the overcoat film  203  of the opposite substrate  200 . 
         [0057]    The columnar projection  210  is provided at a position corresponding to that of the contact hole  111  of the TFT substrate  100 , and is inserted to the contact hole  111  when the TFT substrate  100  and the opposite substrate  200  are assembled. As shown in  FIG. 1B , the contact hole  111  retains an inclining part such that a diameter of an upper part becomes larger than that of a bottom part, and the columnar projection  210  is inserted to the contact hole  111  by being guided by the inclining part. Thereby, by anchoring the columnar projection  210  with the contact hole  111 , misalignment between the TFT substrate  100  and the opposite substrate  200  can be suppressed substantially completely. 
         [0058]    The diameter of the top part of the columnar projection  210  is smaller than that of the contact hole ill, the columnar projection  210  can easily slide into the contact hole  111  when a load is applied at the time of panel assembly (ODF), and can easily be deformed when the columnar projection  210  hits against the inclining part of the contact hole ill, and the positional likelihood of the columnar projection  210  with the contact hole  111  can be enhanced. 
         [0059]    The columnar spacer  220  is provided at a position such that the columnar spacer  220  overlaps the light-shielding black matrix  202  of the opposite substrate  200  and overlaps for example the gate wiring  120  at a position corresponding to a portion other than the contact hole  111  of the TFT substrate  100 , and when the TFT substrate  100  and the opposite substrate  200  are assembled, the columnar spacer  220  abuts on the orientation film  113  to ensure the cell gap. To play a role as a spacer, a cross-sectional area of the columnar spacer  220  is larger than a cross-sectional area of the columnar projection  210 . 
         [0060]    Preferably, the columnar projection  210  does not contact the bottom part of the contact hole  111  in a normal state. 
         [0061]    Therefore, the columnar projection  210  (h 1 ) is larger than the cell gap (d) and is smaller than a sum of the cell gap (d) and depth (z) of the contact hole  111 . 
         [0000]        d&lt;h 1 ≦d+z    
         [0062]    Also, a difference (Δh) between height (h 1 ) of the columnar projection  210  and height (h 2 ) of the columnar spacer  220  is equal to or less than the depth (z) of the contact hole  111 . 
         [0000]        Δh=h 1 −h   2≦z    
         [0063]    It is of note that there is a case that a step part so-called pedestal is provided on the TFT substrate corresponding to the columnar spacer  220 . In this case, there may be a difference v between distance from the TFT substrate  100  to a top surface of the contact hole  111 , and distance from the TFT substrate  100  to a surface corresponding to the columnar spacer  220 , that is, a surface that contacts the columnar spacer  220 . In this case, h 1 −(h 2 +v)≦z has to be satisfied. 
         [0064]    When an excessive load is applied to the substrates, the top part of the columnar projection  210  abuts on the bottom part of the contact hole  111 , and functions as a spacer. 
         [0065]      FIG. 2  shows an arrangement of the columnar projection  210  and the columnar spacer  220  in the entire liquid crystal panel. 
         [0066]    As shown in  FIG. 2 , density of the columnar projection  210  that functions as a misalignment prevention stopper is higher at a peripheral area than at a center area. Intra-surface misalignment due to a temperature change is caused because a warp is more significant at the peripheral part of a screen than at the vicinity of the center, and by increasing the density of the columnar projection  210  at the peripheral part, misalignment can be suppressed effectively. In contrast, the columnar spacer  220  for ensuring the cell gap is distributed substantially uniformly throughout the entire liquid crystal panel. Thereby, a cell gap interval can be ensured effectively throughout the entire liquid crystal panel. 
         [0067]    The number of the columnar projection  210  is smaller than the number of the columnar spacer  220  by an order of magnitude or more, for example. Intervals between the columnar projections  210  are larger than intervals of the columnar spacer  220 . 
       Second Embodiment 
       [0068]      FIG. 3  shows a liquid crystal display device according to a second embodiment of the present invention. In the second embodiment, the present invention is applied to an IPS liquid crystal display device provided with the contact hole  111  only of an inorganic passivation film (an inorganic PAS film). 
         [0069]    While in the first embodiment, the inorganic passivation film  106 , the organic passivation film  107  and the inorganic passivation film  109  are provided between the TFT and the pixel electrode  108  or the opposite electrode  110 , only the inorganic passivation films  106 ,  109  are provided without providing the organic passivation film  107  in the present embodiment. Depth of the contact hole  111  is smaller by the thickness of the organic passivation film  107 . 
         [0070]    The tall columnar projection  210  and the short columnar spacer  220  are provided on the overcoat film  203  of the opposite substrate  200 , and the columnar projection  210  is inserted to the contact hole  111  when the TFT substrate  100  and the opposite substrate  200  are assembled. Also, when the TFT substrate  100  and the opposite substrate  200  are assembled, the columnar spacer  220  abuts on the orientation film  113  to ensure the cell gap. 
         [0071]    In the present embodiment, when a diameter of the top part of the columnar projection  210  is not sufficiently smaller compared to a hole diameter of the contact hole  111 , the columnar projection  210  is deformed and fixed due to a load at the time of panel assembly (ODF). 
         [0072]    In the present embodiment, the contact hole  111  plays a role of a pedestal having a concave shape corresponding to a shape of the columnar projection  210 ; thereby, a frictional resistance between the upper and lower substrates becomes significantly large, and an effect of suppressing misalignment between the upper and lower substrates can be enhanced. 
       Third Embodiment 
       [0073]      FIGS. 4A and 4B  show a liquid crystal display device according to a third embodiment of the present invention. In the third embodiment, the present invention is applied to a vertical electric field TN or VA liquid crystal display device provided with the contact hole  111  of a high transmittance pixel (using an organic PAS film). 
         [0074]      FIG. 4A  is a plane view showing a part of the VA (TN) liquid crystal display panel, and  FIG. 4B  is a cross-sectional view of a portion indicated with a line A-B in  FIG. 4A . 
         [0075]    In the vertical electric field liquid crystal display device, the pixel electrode  108  is arranged on the TFT substrate  100 , and a common electrode  204  is arranged on the opposite substrate  200 . By applying or not applying voltage to the pixel electrode  108 , an array state of the liquid crystal molecules  301  of the liquid crystal layer  300  is changed to control transmission of light. 
         [0076]    In the TN system, at zero electric field when voltage is not applied to the pixel electrode  108 , the liquid crystal molecules  301  are arrayed in a horizontal direction with respect to both substrates to transmit light, and when voltage is started to be applied to the pixel electrode  108 , the liquid crystal molecules  301  rise vertically to block light. 
         [0077]    In the VA system, the liquid crystal layer  300  with negative dielectric anisotropy is arranged. At zero electric field when voltage is not applied to the pixel electrode  108 , the liquid crystal molecules  301  are arrayed in a vertical direction to block light. With an electric field when voltage is applied to the pixel electrode  108 , an electric field occurs between the pixel electrode  108  and the common electrode  204 , and the liquid crystal molecules  301  incline in a horizontal direction to transmit light. 
         [0078]    In the present embodiment also, the organic passivation film  107  on the TFT substrate  100  retains the contact hole  111  having the inclining part. Also, the tall columnar projection  210  and the short columnar spacer  220  are provided on the overcoat film  203  of the opposite substrate  200 . The columnar projection  210  is provided at a position corresponding to that of the contact hole  111  of the TFT substrate  100 , and when the TFT substrate  100  and the opposite substrate  200  are assembled, the columnar projection  210  is inserted to the contact hole  111 . The columnar spacer  220  is provided at a position such that the columnar spacer  220  overlaps the light-shielding black matrix  202  of the opposite substrate  200 , and overlaps for example the gate wiring  120  at a position corresponding to a portion other than the contact hole  111  of the TFT substrate  100 , and when the TFT substrate  100  and the opposite substrate  200  are assembled, the columnar spacer  220  abuts on the orientation film  113  to ensure the cell gap. An action of the columnar projection  210  and the columnar spacer  220  is similar to that in the first embodiment. 
         [0079]    Naturally, other than the above-described liquid crystal display devices, a configuration with the plane electrode and the comb-shaped electrode disclosed in  FIGS. 1A and 1B  may be a configuration of a pair of the comb-shaped electrodes. Also, not being limited to a method of driving the liquid crystal molecules oriented in a direction parallel with the liquid crystal substrate by using an electric field that is generated by a pair of electrodes formed on the TFT substrate and is parallel with the substrate, a method of driving the liquid crystal molecules oriented vertical to the substrate may be adopted. 
       INDUSTRIAL APPLICABILITY  
       [0080]    The present invention can prevent occurrence of uneven brightness due to misalignment and a pressing load of upper and lower substrates of a liquid crystal display device. The present invention can be used for an IPS, VA, TN or other liquid crystal display. In particular, a more significant effect is achieved when the present invention is applied to a liquid crystal display with a large screen and a liquid crystal display using a phase difference polarizer for viewing angle compensation.