Abstract:
The present invention relates to a liquid crystal display that maintains a more uniform cell gap and improves adherence between two display panels by improving adhesion between substrates and their bead spacers, as well as a manufacturing method thereof. An exemplary liquid crystal display includes: a first substrate and a second substrate facing each other; a bead spacer comprising a plurality of beads and a first adhesive coupling the beads to the first substrate; a second adhesive corresponding to the bead spacer and disposed on the second substrate so as to contact the bead spacer; and a liquid crystal layer disposed between the first substrate and the second substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to, and the benefit of, Korean Patent Application No. 10-2011-0022387 filed in the Korean Intellectual Property Office on Mar. 14, 2011, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to a liquid crystal display and a manufacturing method thereof. More particularly, the present invention relates to a liquid crystal display that more uniformly maintains a cell gap and improves adherence between two display panels, as well as manufacturing methods thereof. 
         [0004]    (b) Description of the Related Art 
         [0005]    A liquid crystal display (hereinafter referred to as an LCD) is one of the most widely used types of flat panel displays. An LCD typically includes two display panels provided with electric field generating electrodes, such as pixel electrodes and a common electrode, and a liquid crystal layer interposed between the two display panels. In the LCD, voltages are applied to the electric field generating electrodes to generate an electric field in the liquid crystal layer. Due to the generated electric field, liquid crystal molecules of the liquid crystal layer are aligned, which controls polarization of incident light, thereby displaying images. 
         [0006]    In a liquid crystal display, an upper panel and a lower panel are typically combined by a sealant, which is disposed on an edge circumference of the upper panel and the lower panel and which encloses the liquid crystal layer. Also, an interval between two display panels, that is, a cell gap, may be relatively uniformly maintained by a spacer disposed between the upper panel and the lower panel. 
         [0007]    Here, the spacer may be classified as bead spacers or column spacers. In general, the bead spacer has a spherical shape and is irregularly dispersed on a substrate, and the column spacer is formed to have a predetermined pattern through a photolithography process. 
         [0008]    The bead spacer may be formed without a photolithography process, such that the manufacturing process is simple compared to the column spacer and its manufacturing cost is low. However, its adhesion with substrates is relatively weak, such that it is more difficult to maintain a uniform cell gap when an external force is applied to the liquid crystal display. 
         [0009]    A recent trend has seen efforts directed toward development of a flexible display device that may be flexibly bent and rolled. When the flexible display device is manufactured using bead spacers and the substrate is bent, the cell gap is not uniformly maintained, such that a luminance difference in a region is generated. Also, such bending may move the bead spacer, such that an alignment layer disposed on the substrate may be damaged. 
         [0010]    Further, when combining the upper panel and the lower panel by only the sealant formed at the edge circumference of the substrate, there is no member fixing the two display panels at the center of the substrate, resulting in a weak bond between the substrates. 
         [0011]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a liquid crystal display that more uniformly maintains a cell gap even when a substrate is bent, by improving adhesion between a bead spacer and the substrate. Also provided is a manufacturing method thereof. 
         [0013]    Also described is a liquid crystal display improving adherence between two display panels by using the bead spacer to fix two display panels, and a manufacturing method thereof. 
         [0014]    A liquid crystal display according to the present invention includes: a first substrate and a second substrate facing each other; a bead spacer comprising a plurality of beads and a first adhesive coupling the beads to the first substrate; a second adhesive corresponding to the bead spacer and disposed on the second substrate so as to contact the bead spacer; and a liquid crystal layer disposed between the first substrate and the second substrate. 
         [0015]    The first adhesive and the second adhesive may be made of the same material. 
         [0016]    The first adhesive and the second adhesive may be a thermosetting adhesive. 
         [0017]    The first adhesive and the second adhesive may be made of a material that does not react with the liquid crystal layer. 
         [0018]    The first adhesive and the second adhesive may be made of a material including an epoxy group with a content of metal ions of less than about 10 ppb. 
         [0019]    The first substrate and the second substrate may be flexible substrates. 
         [0020]    The liquid crystal display may further comprise color filters and a light blocking member positioned between the color filters, where the bead spacer is positioned on the light blocking member. 
         [0021]    A method for manufacturing a liquid crystal display according to the present invention includes: (a) forming a bead spacer on the first substrate, the bead spacer comprising a plurality of beads mixed with a first adhesive; (b) forming a second adhesive on the second substrate at a position corresponding to the bead spacer; (c) forming a liquid crystal layer between the first substrate and the second substrate, and combining the first substrate and the second substrate; and (d) hardening the bead spacer and the second adhesive. 
         [0022]    In (a) and (b), the bead spacer and the second adhesive may each be formed by a printing method. 
         [0023]    The first adhesive and the second adhesive may be made of the same material. 
         [0024]    The first adhesive and the second adhesive may be a thermosetting adhesive. 
         [0025]    The first adhesive and the second adhesive are made of a material that may not react with the liquid crystal layer. 
         [0026]    The first adhesive and the second adhesive may be made of a material including an epoxy group with an amount of metal ions of less than about 10 ppb. 
         [0027]    The first substrate and the second substrate may be flexible substrates. 
         [0028]    In the above, (a) may further comprise forming the bead spacer on a light blocking member formed on the first substrate. 
         [0029]    The liquid crystal display according to the present invention may more uniformly maintain its liquid crystal cell gap even if the substrate is bent. This is accomplished by improving the adhesion between the substrate and the bead spacer. This improved adhesion may help to prevent damage to the alignment layer. 
         [0030]    The bead spacer is used to fix two display panels such that adherence between the two display panels may be improved. 
         [0031]    The spacer is formed without a photolithography process, such that the manufacturing process may be simplified and a developing solution is not used. This results in a more environmentally friendly fabrication process. 
         [0032]    The bead spacer is formed by a printing method, such that the bead spacer may be formed at a regular and desired position. In detail, the bead spacer is positioned to correspond to a respective light blocking member, so that light leakage may be reduced and aperture ratio may be increased. 
         [0033]    Also, the bead spacer has improved compression characteristics such that the drip margin of the liquid crystal may be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a cross-sectional view of a liquid crystal display according to an exemplary embodiment of the present invention. 
           [0035]      FIG. 2  is a view showing a bead spacer formed on the first substrate of a liquid crystal display according to an exemplary embodiment of the present invention. 
           [0036]      FIG. 3  is a view showing the second adhesive formed on the second substrate of a liquid crystal display according to an exemplary embodiment of the present invention. 
           [0037]      FIG. 4  to  FIG. 6  are cross-sectional views showing a manufacturing method of a liquid crystal display according to an exemplary embodiment of the present invention. 
           [0038]      FIG. 7  and  FIG. 8  are graphs showing a change of a cell gap according a liquid crystal amount in a liquid crystal display of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0039]    The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art will realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
         [0040]    In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
         [0041]    Firstly, a liquid crystal display according to an exemplary embodiment of the present invention will be described with reference to accompanying drawings. 
         [0042]      FIG. 1  is a cross-sectional view of a liquid crystal display according to an exemplary embodiment of the present invention.  FIG. 2  is a view showing a bead spacer disposed on the first substrate of a liquid crystal display according to an exemplary embodiment of the present invention, and  FIG. 3  is a view showing the second adhesive disposed on the second substrate of a liquid crystal display according to an exemplary embodiment of the present invention. 
         [0043]    A liquid crystal display according to an exemplary embodiment of the present invention includes a first substrate  110  and a second substrate  210  facing each other, a bead spacer  320  disposed on the first substrate  110 , a second adhesive  324  disposed on the second substrate  210 , and a liquid crystal layer  3  disposed between the first substrate  110  and the second substrate  210 . 
         [0044]    The first substrate  110  and the second substrate  210  may each be made of a flexible material such as a plastic. When the first substrate  110  and the second substrate  210  are made of the flexible material, they may be used in a flexible display device that may be bent or rolled. That is, “flexible” substrates  110 ,  210  are sufficiently flexible to allow the resulting display device to be bent or rolled. 
         [0045]    A thin film transistor and a pixel electrode  191  connected thereto are disposed on the first substrate  110 . The thin film transistor includes a gate electrode  124  disposed on the first substrate  110 , a gate insulating layer  140  disposed on the gate electrode  124 , a semiconductor layer  151  disposed on the gate insulating layer  140 , a source electrode  173  and a drain electrode  175  separated from each other on the semiconductor layer  151 , and a passivation layer  180  disposed on both the source electrode  173  and the drain electrode  175  and including a contact hole exposing a portion of the drain electrode  175 . The pixel electrode  191  is connected to the drain electrode  175  through the contact hole. 
         [0046]    A color filter  230  is disposed on the pixel electrode  191 , and a light blocking member  220  is disposed between adjacent color filters  230 . The light blocking member  220  is disposed to at least approximately correspond to the thin film transistor. 
         [0047]    A common electrode  270  is disposed on substantially the entire surface of the second substrate  210 . 
         [0048]    As shown in  FIG. 2 , the bead spacer  320  includes beads  320   a  and a first adhesive  320   b  that are mixed and formed on the first substrate  110 . The first adhesive  320   b  substantially encloses the beads  320   a  (or at least encompasses an area between the beads  320   a  and the corresponding underlying surface), thereby adhering the beads  320   a  to the first substrate  110 . 
         [0049]    The bead spacer  320  may be disposed on the light blocking member  220  in a single layer, i.e. not stacked upon one another, and/or all at substantially the same elevation. The bead spacer  320  is disposed to correspond to the light blocking member  220  such that light leakage may be reduced and an aperture ratio may be increased as compared to the case of bead spacers  320  that are disposed in the pixel area. 
         [0050]    As shown in  FIG. 3 , the second adhesive  324  is disposed on the common electrode  270  of the second substrate  210 . The second adhesive  324  is disposed to correspond to the bead spacer  320 , such that the bead spacer  320  is adhered to the second substrate  210 . 
         [0051]    The first adhesive  320   b  and the second adhesive  324  may be made of the same material. For example, they may be made of a thermosetting adhesive or a UV hard setting adhesive. It is preferable that the second adhesive  324  is made of a thermosetting adhesive when the first adhesive  320   b  is made of a thermosetting adhesive, and that the second adhesive  324  is made of a UV hard setting adhesive when the first adhesive  320   b  is made of a UV hard setting adhesive. In the case that the first adhesive  320   b  is a thermosetting adhesive and the second adhesive  324  is made of a UV hard setting adhesive, when heat is applied to adhere the bead spacer  320  to the second substrate  210 , only the first adhesive  320   b  is reacted, such that adhesion does not progress normally. In contrast, when UV is irradiated, only the second adhesive  324  is reacted such that adhesion also does not progress normally. Accordingly, the first adhesive  320   b  and the second adhesive  324  may preferably be made of the same material. 
         [0052]    The first adhesive  320   b  and the second adhesive  324  are preferably formed of a thermosetting adhesive rather than a UV hard setting adhesive. When the bead spacer  320  is disposed to correspond to the light blocking member  220 , if the first adhesive  320   b  is made of a UV hard setting adhesive, when UV light is irradiated upon the first substrate  110 , the UV is blocked by the light blocking member  220  such that UV light does not reach the first adhesive  320   b  and adhesion does not progress in normal fashion, i.e. the adhesive does not adequately set. In contrast, if the first adhesive  230   b  is made of a thermosetting adhesive, when heat is applied outside the first substrate  110 , the heat is not influenced by the light blocking member  220  and is transmitted to the first adhesive  320   b  such that the adhesive sets and adhesion progresses well. 
         [0053]    The first adhesive  320   b  and the second adhesive  324  may be made of a material that does not react with the liquid crystal layer  3 . The first adhesive  320   b  and the second adhesive  324  are disposed between the first substrate  110  and the second substrate  210  along with the liquid crystal layer  3  such that the first adhesive  320   b  and the second adhesive  324  may contact the liquid crystal layer  3 . Thus, if the first adhesive  320   b  and the second adhesive  324  are made of a material that reacts with the liquid crystal layer  3 , the characteristics of the liquid crystal layer  3  may be altered for the worse. 
         [0054]    Both the first adhesive  320   b  and the second adhesive  324  preferably do not include metal ions, so as not to react with the liquid crystal layer  3 . For example, the first adhesive  320   b  and the second adhesive  324  may be made of a material of an epoxy group having an amount of metal ions of less than about 10 ppb (parts per billion). 
         [0055]    Next, a manufacturing method of a liquid crystal display according to an exemplary embodiment of the present invention will be described with reference to the accompanying drawings. 
         [0056]      FIG. 4  to  FIG. 6  are cross-sectional views showing a manufacturing method of a liquid crystal display according to an exemplary embodiment of the present invention. 
         [0057]    First, as shown in  FIG. 4 , an inkjet head  410  is aligned over the first substrate  110  of the flexible material, and a material of which the beads  320   a  and the first adhesive  320   b  are to be made of is dripped onto the first substrate  110  through a nozzle  420  connected to the inkjet head  410 , thus forming bead spacers  320 . 
         [0058]    A plurality of nozzles  420  may be formed at the lower surface of the inkjet head  410 . Also, the inkjet head  410  is connected to a transferring unit (not shown) to be moved such that the material of which the bead  320   a  and the first adhesive  320   b  are made may be deposited in multiple regions of the first substrate  110 . 
         [0059]    A thin film transistor and a pixel electrode  191  connected thereto are formed on the first substrate  110 . The thin film transistor includes a gate electrode  124  formed on the first substrate  110 , a gate insulating layer  140  formed on the gate electrode  124 , a semiconductor layer  151  formed on the gate insulating layer  140 , a source electrode  173  and a drain electrode  175  separated from each other formed on the semiconductor layer  151 , and a passivation layer  180  formed on both the source electrode  173  and the drain electrode  175  and including a contact hole exposing a portion of the drain electrode  175 . The pixel electrode  191  is connected to the drain electrode  175  through the contact hole. 
         [0060]    A color filter  230  is formed on the pixel electrode  191 , and a light blocking member  220  is formed between adjacent color filters  230 . The light blocking member  220  is disposed to correspond to (e.g., at least partially cover) the thin film transistor. 
         [0061]    The material of which the bead  320   a  and the first adhesive  320   b  are mixed may be dripped to be disposed on the light blocking member  220 . By disposing the bead spacer  320  to correspond to the light blocking member  220 , light leakage may be reduced and the aperture ratio may be increased compared with the case in which the bead spacers  320  are disposed in the pixel area. 
         [0062]    Next, as shown in  FIG. 5 , the inkjet head  410  is aligned on the flexible second substrate  210 , and the second adhesive  324  is dripped on the second substrate  210  through the nozzle  420  connected to the inkjet head  410 . 
         [0063]    A common electrode  270  is formed on substantially the entire surface of the second substrate  210 , and the second adhesive  324  is formed on the common electrode  270  of the second substrate  210 . The second adhesive  324  is disposed at a position allowing contact with the bead spacer  320  when combined with the first substrate  110  in a subsequent process. 
         [0064]    Here, the second adhesive  324  is dripped in a plurality of regions on the second substrate  210  through a plurality of nozzles  420  while moving the inkjet head  410 . 
         [0065]    The first adhesive  320   b  and the second adhesive  324  may be made of the same material. For example, they may be made of a thermosetting adhesive or a UV hard setting adhesive. As described above, the first adhesive  320   b  and the second adhesive  324  are preferably formed of the thermosetting adhesive rather than the UV hard setting adhesive. 
         [0066]    The first adhesive  320   b  and the second adhesive  324  may be made of a material that does not react with the liquid crystal layer (i.e., layer  3  of  FIG. 3 ). In detail, the first adhesive  320   b  and the second adhesive  324  may be made of a material from an epoxy group having an amount of metal ions less than about 10 ppb (parts per billion). 
         [0067]    Next, as shown in  FIG. 6 , a liquid crystal layer  3  is formed between the first substrate  110  and the second substrate  210 , and the first substrate  110  and the second substrate  210  are combined. For example, after the liquid crystal material is dripped onto the first substrate  110  or the second substrate  210 , the first substrate  110  and the second substrate  210  may be combined. Alternatively, after the first substrate  110  and the second substrate  210  are combined, the liquid crystal material may be injected between the first substrate  110  and the second substrate  210 . 
         [0068]    When the first adhesive  320   b  and the second adhesive  324  are made of the thermosetting adhesive, heat  500  is then applied outside the first substrate  110  and the second substrate  210  to harden the bead spacer  320  and the second adhesive  324 . 
         [0069]    The bead spacer  320  is fixed to the first substrate  110  by the first adhesive  320   b , which has been formed to enclose the beads  320   a . The bead spacer  320  is also fixed to the second substrate  210  by the second adhesive  324 , which has been formed at a position corresponding to the bead spacer  320 . 
         [0070]    That is, the bead spacer  320  is attached to both the first substrate  110  and the second substrate  210 , so that it is fixed in place. Accordingly, even if the first substrate  110  and the second substrate  210  are bent, the cell gap may be maintained substantially uniform, thus improving the adherence characteristics of the first substrate  110  and the second substrate  210 . 
         [0071]    The bead spacer  320  and the second adhesive  324  are formed by a particular printing method, being dripped onto the first substrate  110  and the second substrate  210  through the nozzle  420  connected to the inkjet head  410 . However, the present invention is not limited thereto, and the bead spacer  320  and the second adhesive  324  may be formed on the first substrate  110  and the second substrate  210  by using various other methods. 
         [0072]    Nanoimprint lithography, gravure printing, reverse offset printing, offset printing, and microcontact printing are examples of various possible printing methods. 
         [0073]    Nanoimprint lithography is a technique for forming a minute pattern through a precise mold of a nanoscan. In this method, a thermoplastic resin or photo-curable resin is coated on the substrate, and then pressure is applied, similar to stamping a seal, to transcribe the pattern. Gravure printing is a printing method using a printing gravure formed by corroding the surface of a substrate to include an ink pocket made of an anticorrosion film in the substrate by using a gravure screen and a carbon tissue for printing. In offset printing, an ink image is transferred from a printing plate to a rubber blanket, and is then printed. Microcontact printing is a method of forming a pattern by using an elastic polymer stamp. 
         [0074]    In the above-described exemplary embodiment of the present invention, a color filter on array (COA) structure was employed in which the thin film transistor, the pixel electrode  191 , the color filter  230 , and the light blocking member  220  are formed on the first substrate  110 , and the common electrode  270  is formed on the second substrate  210 . However, the present invention is not limited thereto, and various alternative structures may instead be employed, such as a structure in which the color filter  230  and the light blocking member  220  are formed on the second substrate  210  and a structure in which the common electrode  270  is formed on the first substrate  110 . 
         [0075]    Next, an effect in which a drip margin is improved in a liquid crystal display will be described. 
         [0076]      FIG. 7  and  FIG. 8  are graphs showing a change in cell gap as a function of a liquid crystal amount, in a liquid crystal display constructed according to an embodiment of the present invention. 
         [0077]    In the graphs, each horizontal axis represents a liquid crystal amount that is 100% when the liquid crystal completely fills the volume corresponding to a space between two substrates. A number less than 100% corresponds to a situation in which the liquid crystal does not sufficiently fill the space between the two substrates, and a number greater than 100% corresponds to a situation in which liquid crystal overfills the space between the two substrates. 
         [0078]    In the graphs, each vertical axis represents an interval between the two substrates, that is, the cell gap. 
         [0079]    Referring to  FIG. 7 , as the liquid crystal amount is increased, the cell gap is increased. When the liquid crystal amount is insufficiently filled at less than 100% or is excessively at filled more than 100%, deterioration may be generated. Here, it has been found that the range of the liquid crystal amount in which deterioration is not generated is about more than 94% and less than 106%. Accordingly, the drip margin of the liquid crystal is about 12%. 
         [0080]    In contrast, when the bead spacer is not used but the column spacer is used in the liquid crystal display of the same condition, the corresponding range of liquid crystal amount to avoid image deterioration is about more than 100% and less than 106%. Accordingly, the drip margin of the liquid crystal is about 6%. 
         [0081]    In  FIG. 8 , the range of the liquid crystal amount that avoids image deterioration due to insufficient or excessive filling of the liquid crystal is about more than 85% and less than 107%. Accordingly, the drip margin of the liquid crystal is about 22%. 
         [0082]    In contrast, when the bead spacer is not used but the column spacer is used in the liquid crystal display of the same condition, the corresponding range of liquid crystal amount to avoid image deterioration is about more than 100% and less than 107%. Accordingly, the drip margin of the liquid crystal is about 7%. 
         [0083]    From the above, it can be seen that use of bead spacers instead of column spacers increases the drip margin of the liquid crystal from about 6% to 7% to about 12% to 22%. This is believed due to the fact that the bead spacer is made of a material having elasticity, which results in improved compression characteristics for the resulting display. 
         [0084]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
       DESCRIPTION OF SYMBOLS 
       [0085]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  3: liquid crystal layer 
                 110: the first substrate 
               
               
                   
                 124: gate electrode 
                 140: gate insulating layer 
               
               
                   
                 151: semiconductor layer 
                 173: source electrode 
               
               
                   
                 175: drain electrode 
                 180: passivation layer 
               
               
                   
                 191: pixel electrode 
                 210: the second substrate 
               
               
                   
                 220: light blocking member 
                 230: color filter 
               
               
                   
                 270: common electrode 
                 320: bead spacer 
               
               
                   
                 320a: beads 
                 320b: first adhesive 
               
               
                   
                 324: second adhesive