Abstract:
The present invention discloses a liquid crystal display of the IPS type, which has improved characteristics in terms of stability of liquid crystal element, response time, threshold voltage, and driving voltage due to conferring a pretilt angle to a liquid crystal layer, and method for preparation thereof.

Description:
[0001]    The present invention is directed to a liquid crystal display which produces an image and a process for manufacturing same. 
       FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a liquid crystal display which produces an image and a process for manufacturing same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Liquid crystal displays project images by controlling the light transmission rate of liquid crystals by applied electric fields and they are classified into a vertical electric field-type and a horizontal electric field-type. 
         [0004]    In the horizontal electric field-type liquid crystal display, a horizontal electric field applied between pixel and common electrodes positioned side by side on a lower substrate drives liquid crystals of so-called in-plane switching (IPS) mode displays. This horizontal electric field-type display has the merit of a wide viewing angle due to the rotation of liquid crystal directors on a flat substrate, but it disadvantageously shows a poor transmission rate and a slow response time. 
         [0005]    In the more conventional vertical electric field-type liquid crystal displays, liquid crystals of twisted nematic (TN) mode are driven by a vertical electric field applied between pixel and common electrodes which are located on a lower substrate and an upper substrate, respectively, which face each other. This vertical electric field-type has the merit of a high transmission rate due to a large aperture ratio, the possibility to apply a rubbing-free process and a relatively higher transmittance compared to that of the IPS mode, but it has the drawback of a rather narrow viewing angle. Alternatively to the TN mode, vertical electric field-type liquid crystal displays have been realized in the electrically controlled birefringence (ECB) mode also called vertically aligned nematic (VAN) mode. In these modes the vertically aligned liquid crystals have negative dielectric constant anisotropy, leading to a higher rotational viscosity as compared to liquid crystals having positive dielectric constant anisotropy, which causes a slow response time. Additionally the vertical alignment of the liquid crystals is not easy and can only be achieved by one of several rather complicated processes. 
       DISCLOSURE OF THE INVENTION 
     Technical Object of the Invention 
       [0006]    Accordingly, it is an object of the present invention to provide a liquid crystal display which has a high contrast ratio, a wide viewing angle, a low driving voltage, and a rapid response time. 
       Technical Feature of the Invention  
       [0007]    In accordance with one aspect of the present invention, there is provided a liquid crystal display comprising: a first substrate; a second substrate having a first electrode and a second electrode; and a liquid crystal layer disposed between the first substrate and the second substrate and vertically aligned with respect to the plane of the first substrate and the second substrate, wherein a pretilt angle is formed in said liquid crystal layer. 
         [0008]    In accordance with another aspect of the present invention, there is provided a process for manufacturing a liquid crystal display comprising the steps of:
       introducing a liquid crystal layer comprising one or more photoreactive monomers, preferably one or more photoreactive mesogenic monomers and, most preferably, one or more photoreactive liquid crystal monomers into a cell;   applying a voltage to the cell so that the photoreactive monomer attains a pretilt angle itself or either conveys a pretilt angle to the liquid crystals of the liquid crystal layer; and   irradiating actinic radiation, preferably UV radiation, to the cell to polymerize the photoreactive monomer or monomers.       
 
       Effect of the Invention 
       [0012]    As described above, the liquid crystal display according to one embodiment of the present invention provides a wide viewing angle and a high contrast ratio which correspond to the merits of a horizontal electric field-type liquid crystal display and also has the advantage of being realizable by a rubbing-free process. 
         [0013]    In addition, the liquid crystal display according to one embodiment of the present invention is capable of lowering the driving and threshold voltages. 
         [0014]    Further, the liquid crystal display according to one embodiment of the present invention exhibits a rapid response time which makes it possible to view the projected images in a natural way. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show: 
           [0016]      FIG. 1   a : a cross-sectional view of the liquid crystal display according to a first embodiment of the present invention without any applied voltage; 
           [0017]      FIG. 1   b : a cross-sectional view of the liquid crystal display according to a first embodiment of the present invention when a voltage is applied thereto; 
           [0018]      FIG. 2 : a flow diagram of the process for manufacturing the liquid crystal displays shown in  FIGS. 1   a  and  2   b  according to another embodiment of the present invention; 
           [0019]      FIG. 3 : a cross-sectional view of the liquid crystal display according to a first embodiment of the present invention, obtained by the method for introducing a pretilt angle inducing part according to another embodiment; 
           [0020]      FIGS. 4   a  and  4   b : cross-sectional views of the liquid crystal displays according to a second embodiment of the present invention, wherein a voltage is not applied and wherein a voltage is applied, respectively; 
           [0021]      FIG. 5 : a cross-sectional view of the liquid crystal display according to a second embodiment of the present invention, obtained by the method for preparing a pretilt angle inducing part according to another embodiment; 
           [0022]      FIGS. 6   a  and  6   b : cross-sectional views of the liquid crystal displays according to a third embodiment of the present invention, wherein a voltage is not applied and wherein a voltage is applied, respectively; and 
           [0023]      FIG. 7 : a cross-sectional view of the liquid crystal display according to a third embodiment of the present invention, obtained by the method for preparing a pretilt angle inducing part according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Some embodiments of the present invention will be described in detail using appropriate exemplary drawings. Reference signs are added to each of components of the drawings, and it should be noted that the same component is denoted by the same sign whenever possible in other drawings. Also, in explaining an embodiment of the present invention, if a detailed description on a related known constitution or function clouds the gist of the present invention, the detailed description is omitted. 
         [0025]    In addition, the terms “first,” “second,” “A,” “B,” “(a)” and “(b)” may be used in explaining components of an embodiment of the present invention. These terms are used only for distinguishing one component from another component, and they do not limit the essences, turns, or orders of corresponding components. It should be understood that when a component is “connected,” “combined,” or “accessed” to another component, the component may be directly connected, combined, or accessed thereto, and an additional component may be inserted between the two components. 
         [0026]    The present invention provides a liquid crystal display having low driving and threshold voltages, and a rapid response time by way of mixing photoreactive monomers, preferably photoreactive liquid crystal monomers and liquid crystal material having positive dielectric constant anisotropy in a specific ratio, introducing the resulting mixture into a unit cell, applying a horizontal electric field, and irradiating a UV ray to the cell such that the liquid crystal molecules form a pretilt angle even at the stage when a voltage is not applied. 
         [0027]    More particularly, the present invention is characterized by vertically aligning liquid crystals using a horizontal electric field as a driving voltage (not horizontally aligning them). Such liquid crystal display driven by a horizontal electric field, vertical alignment of liquid crystals having positive dielectric constant anisotropy is characterized by a rotational viscosity lower than the vertical-class mode using liquid crystals having negative dielectric constant anisotropy, thereby exhibiting a rapid response time. 
         [0028]    In order to generate a high transmission rate even when driven with a horizontal electric field, the distance between the electrodes should be sufficiently long, which requires a high driving voltage. Therefore, the present invention guides the liquid crystals to orient to a specific direction by using photoreactive monomers, preferably photoreactive liquid crystal monomers to maintain a regular alignment even at the stage when a voltage is not applied, which results in high contrast ratio. Further, the liquid crystals so aligned make it possible to lower the driving voltage and the threshold voltage required in forming the electric field. 
         [0029]    The liquid crystal display and the preparation method thereof according to embodiments of the present invention will be described in detail using specific drawings as follows. When a voltage is not applied, a cross-sectional view of the liquid crystal display according to one embodiment of the present invention is shown in  FIG. 1   a ; and when a voltage is applied, a cross-sectional view of the liquid crystal display, in  FIG. 1   b.    
         [0030]    Referring to  FIGS. 1   a  and  1   b , the liquid crystal display ( 100 ) comprises a first substrate ( 110 ) and a second substrate ( 120 ) which face each other, and a liquid crystal layer ( 130 ) which is positioned therebetween. 
         [0031]    The first substrate ( 110 ) is a color substrate comprising a color filter (not shown) for creating full-color images. The color filter in the first substrate ( 110 ) may be formed by various methods including an ink-jet printing or etching technique. 
         [0032]    The second substrate ( 120 ) is a thin film transistor array substrate comprising a thin film transistor array (not shown) as a driver circuit. The thin film transistor array is a switch element for converting liquid crystal cells arranged in a matrix form and signals supplied to the liquid crystal cells. The thin film transistor array comprises thin film transistors, in which the thin film transistors are composed of a gate electrode, a gate insulator, a semiconductor layer, source and drain electrodes, and are preferably formed in a region on one surface of the second substrate, which is outside of the pixel(s), i.e. in the “non-pixel (NP) region” ( 120 ). 
         [0033]    The first substrate ( 110 ), a color substrate, and the second substrate ( 120 ), a thin film transistor array substrate, may comprise a first polarizer ( 140 ) and a second polarizer ( 150 ), respectively, on the opposite surfaces of the liquid crystal layer ( 130 ). The first polarizer ( 140 ) and the second polarizer ( 150 ) function to convert the incidence light which vibrates in various directions to a light which vibrates with one direction, i.e., a polarized light. The first polarizer ( 140 ) and the second polarizer ( 150 ) may be adhered to the first substrate ( 110 ) and the second substrate ( 120 ), respectively, by means of an adhesive, but not limited thereto. Light transmission axes of the first polarizer ( 140 ) and the second polarizer ( 150 ) are orthogonal to each other. 
         [0034]    The first substrate ( 110 ) and the second substrate ( 120 ) comprises the first vertical alignment layer ( 160 ) and the second vertical alignment layer ( 170 ), respectively, in contact with the liquid crystal layer ( 130 ). 
         [0035]    The first substrate ( 110 ) may comprise a common electrode (not shown) and a dielectric layer (also not shown) positioned under the first vertical alignment layer ( 160 ). The common electrode formed on the first substrate ( 110 ) generates an electric field with the common electrode ( 180 ), and the pixel electrode ( 190 ) formed on the second substrate ( 120 ), as described below, functions to rotate the liquid crystal layer ( 130 ). 
         [0036]    The second substrate ( 120 ) comprises two electrodes, i.e., the common electrode ( 180 ) and the pixel electrode ( 190 ). The horizontal electronic field (L) is generated between the common electrode ( 180 ) and the pixel electrode ( 190 ), and the liquid crystal molecules in the liquid crystal layer ( 130 ) align with the horizontal electronic field (L). Further, the pixel electrode ( 190 ) which is electrically connected to the drain electrode in the thin film transistor array is formed at the position corresponding to the pixel region (P). The common electrode ( 180 ) is positioned on one side of the pixel electrode ( 190 ) formed in the pixel region (P) at regular intervals, or optionally at irregular intervals, to form an in-plane electric field. 
         [0037]    The pixel electrode ( 190 ) and the common electrode ( 180 ) comprise a transparent metal layer composed of one metal selected from the group consisting of transparent conductive metals such as indium tin oxide (ITO) and indium zinc oxide (IZO), and a plurality of the pixel electrode ( 190 ), while the common electrodes ( 180 ) are alternately placed thereon (not shown clearly in the figure). As an alternative to transparent conductive materials the electrodes or part of the electrodes may consist of normal (i.e. opaque) metals. Such an embodiment is especially easily realized e.g. for reflective displays. An advantage of the use of metals for the electrodes of for parts of the electrodes is the higher conductivity of metals compared e.g. to ITO. 
         [0038]    Further, both the common electrode ( 180 ) and the pixel electrode ( 190 ) are realized in the form of one layer, but they may be formed in separate layers in a modified embodiment. 
         [0039]    Further, all of the pixel electrodes ( 190 ) may be formed with the source and drain electrode of the thin film transistor in the form of one layer, and the common electrode ( 180 ) may be made of the same material as the gate line. 
         [0040]    The second substrate ( 120 ) may further comprise an active matrix layer (not shown), in addition to the common electrode ( 180 ) and the pixel electrode ( 190 ) formed on the same side. The active matrix may comprise a gate bus line and a data bus line. The region defined by the gate bus line and the data bus line forms one pixel. The common electrode ( 180 ) and the pixel electrode ( 190 ) may be made of the same material as the gate bus line or the data bus line. 
         [0041]    In the liquid crystal display, i.e., in-plane switching mode liquid crystal display ( 100 ) comprising the common electrode ( 180 ) and the pixel electrode ( 190 ) formed on the second substrate ( 120 ), a horizontal electric field (L) is formed between two electrodes ( 180 ,  190 ) to align the liquid crystals with the horizontal electric field (L) which is parallel to the two substrates ( 110 ,  120 ), thereby making the viewing angle of the liquid crystal display wide. 
         [0042]    The liquid crystal layer ( 130 ) is formed by mixing a liquid crystal material ( 132 ) and a photoreactive liquid crystal monomer ( 134 ) but the mixing method is not limited to a specific mixing process. 
         [0043]    The liquid crystal materials ( 132 ) are liquid crystals whose primary dielectric constant has positive anisotropy to provide fast response time. For example, the liquid crystal material ( 132 ) may be one or more material selected from the group consisting of MJ951160, MJ00435, etc., but any liquid crystal, whose primary dielectric constant has positive anisotropy, can be used without limitation. 
         [0044]    The liquid crystal molecules ( 132 ) are located between the first substrate ( 110 ) and the second substrate ( 120 ) which are parallel and face each other. The liquid crystal molecules ( 132 ) are vertically aligned between the first substrate ( 110 ) and the second substrate ( 120 ). When a voltage is not applied (off state), the liquid crystal molecules ( 132 ) of the liquid crystal layer ( 130 ) are vertically aligned between two substrates ( 110 ,  120 ), as shown in  FIG. 1(   a ). 
         [0045]    When a voltage is applied (on state), the horizontal electric field (L) is generated between the common electrode ( 180 ) and the pixel electrode ( 190 ) and the liquid crystal molecules ( 132 ) of the liquid crystal layer ( 130 ) align themselves with the horizontal electric field (L), as shown in  FIG. 1(   b ). 
         [0046]    The photoreactive liquid crystal monomers ( 134 ) are mixed with the liquid crystal molecules ( 132 ) and polymerized at a position adjacent to the first substrate ( 110 ) and the second substrate ( 120 ), or at a region apart from them. The photoreactive liquid crystal monomers ( 134 ), which are mixed with the liquid crystal molecules ( 132 ) and polymerized, are introduced to the region adjacent or near to the first substrate ( 110 ) and the second substrate ( 120 ), and the polymerized material is aligned at a pretilt angle at the off state. Such pretilt angle of the polymer of the liquid crystal monomers ( 134 ) and the liquid crystal molecules ( 132 ), is greater than 0° but less than 90°, particularly greater than 80° but less than 90°, more particularly greater than 85° but less than 90°, with respect to the parallel substrates ( 110  or  120 ). If the pretilt angle of said polymer is too small (the liquid crystal lies down), a primary dark state cannot be maintained completely to cause a photo leakage. And if an unnecessarily large voltage is applied, the pretilt angle of the reactive liquid crystal monomers ( 134 ) associated with liquid crystal molecules ( 132 ) increases to cause a photo leakage. 
         [0047]    For the off state, the photoreactive liquid crystal monomers ( 134 ) mixed with the liquid crystal molecules ( 132 ) and polymerized generate a pretilt angle as shown in  FIG. 1  a. When an appropriate voltage is applied, a horizontal electric field (L) is generated between the common electrode ( 180 ) and the pixel electrode ( 190 ) and the photoreactive liquid crystal monomers ( 134 ) coupled with the liquid crystal molecules ( 132 ) align themselves with the horizontal electric field (L). 
         [0048]    The photoreactive liquid crystal monomer ( 134 ) is one or more materials selected from the group consisting of RM257 (Formula 1) and EHA (Formula 2), but are not limited thereto. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0049]    The photoreactive liquid crystal monomer ( 134 ) is a liquid crystal material having a terminal group which is polymerizable by the action of a UV-sensitive photo initiator. The photoreactive liquid crystal monomer is a monomer of liquid crystal phase which comprises a mesogen group having liquid crystallinity and a photo-polymerizable terminal group, and can be polymerized by using a UV sensitive photo initiator. An examples of a suitable photo initiator is IRgGCURE®651. The polymerizable compounds, which form the precursor of the polymer may also comprise so called “cross linkers”, an example of which is 1,1,1-trimethylolpropane-triacrylate. 
         [0050]    The depth and density of the layer which is prepared by mixing and polymerization of photoreactive liquid crystal monomers ( 134 ) and liquid crystal material ( 132 ) depend on the kind of liquid crystal material ( 132 ), the intensity of the applied voltage, and the desired response time. For example, the higher response time, the larger depth and density of the layer which is prepared by mixing and polymerizing the photoreactive liquid crystal monomers ( 134 ) and liquid crystal material ( 132 ). 
         [0051]    The liquid crystal display ( 100 ), when exposed to a horizontal electric field by the vertically aligned liquid crystal having a positive anisotropy of dielectric constant, has a lower rotational viscosity and shows a faster response time compared to a vertically aligned liquid crystal display having a negative anisotropy of dielectric constant. However, it requires a longer distance between the electrodes ( 110 ,  120 ) and a higher driving voltage to obtain a high transmittance between the electrodes ( 110 ,  120 ) because it is driven in a horizontal electric field. 
         [0052]    As shown in  FIG. 1   a , when a voltage is not applied to the electrodes, the liquid crystal molecules ( 132 ) are vertically aligned to both substrates and thereby the light passed through the second polarizer ( 150 ) is absorbed to the first polarizer ( 140 ) without phase difference to make a dark state, wherein the pretilt angle generated by the photoreactive liquid crystal monomers ( 134 ) does not affect the dark state. 
         [0053]    As shown in  FIG. 1   b , when the voltage is applied to the common electrode ( 180 ) and the pixel electrode ( 190 ), the resulting horizontal electric field creates phase retardation of the liquid crystal layer ( 130 ) to make the image bright. 
         [0054]    Therefore, the liquid crystal molecules of the liquid crystal display ( 100 ) of the present invention maintain a specific arrangement even at the state of off-state and have a high contrast ratio because the liquid crystal layer ( 130 ) is guided towards a certain direction by using the photoreactive liquid crystal monomers ( 134 ) polymerized with the liquid crystal molecules ( 132 ). Further, the deviation of the liquid crystal director is low, and the problems related to the driving voltage and the threshold voltage for generating the required electric field can be solved. 
         [0055]    Alternatively, to lower the driving voltage or to increase the response time as described above, an inclined structure must be formed on the second substrate ( 120 ) to form a pretilt angle. However, this method requires an additional process for manufacturing the inclined structure on the second substrate ( 120 ). In contrast, the liquid crystal display ( 100 ) of the present invention can form a pretilt angle easily by using the photoreactive liquid crystal monomers ( 134 ) of the liquid crystal layer ( 130 ) without any separate process for generating a pretilt angle. 
         [0056]      FIG. 2  is a flow diagram showing the process of preparing the liquid crystal display according to another embodiment.  FIG. 3  is a sectional view of the liquid crystal display prepared by the method according to the first embodiment. 
         [0057]    Referring to  FIG. 2 , a method for forming a pretilt angel of a photoreactive liquid crystal monomer of the liquid crystal display according to the other embodiment ( 200 ) comprises the steps of: introducing a liquid crystal layer mixed with a photoreactive liquid crystal monomer into a cell (S 210 ); applying a voltage thereto to form a constant pretilt angle on the photoreactive liquid crystal monomers (S 220 ); and irradiating a UV ray to polymerize the photoreactive liquid crystal monomers (S 230 ). 
         [0058]    First, in the step of introducing a liquid crystal layer mixed with photoreactive liquid crystal monomers into a cell (S 210 ), the liquid crystal may be of initial positive dielectric anisotropy as described above for fast response time, and it may be one or more selected from the group consisting of MJ951160, MJ00435, and others. In addition, the photoreactive liquid crystal monomer is one or more selected from the group consisting of RM257 (formula I), EHA (formula II), and others. 
         [0059]    Referring to  FIG. 2  and  FIG. 3(A) , the liquid crystal layer ( 130 ) comprises liquid crystal molecules ( 132 ) and photoreactive liquid crystal monomers ( 134 ) uniformly mixed. The optimal mixing ratio may be chosen by way of various embodiments so as to obtain a constant response time and contrast ratio, but if the concentration of the photoreactive liquid crystal monomer ( 134 ) is too high, the resulting liquid crystal layer may disturb the course of light or lead to light leakage. 
         [0060]    In the step of applying a voltage to form a pretilt angle for the photoreactive liquid crystal monomers (S 220 ), both the photoreactive liquid crystal monomers and liquid crystals form a stable pretilt angle in the direction of constant electric field, and the pretilt angle may be from 0° to less than 90°, preferably from 80° to less than 90°, and more preferably from 85° to less than 90°. 
         [0061]    Referring to  FIG. 2  and  FIG. 3(B) , when an electric field is formed by applying a voltage, the liquid crystal molecules ( 132 ) and the photoreactive liquid crystal monomers ( 134 ) become aligned to the applied voltage at a constant tilted angle. If the pretilt angle formed by liquid crystal monomers near the substrate is too small or large, light leakage may occur. Accordingly, the applied voltage is preferably the threshold voltage. 
         [0062]    Referring to  FIG. 2  and  FIG. 3(C) , in the step of irradiating a UV light for polymerizing the photoreactive liquid crystal monomer (S 230 ), the photoreactive liquid crystal monomers migrate towards both substrates of a high anchoring energy, and are “hardened” (i.e. polymerized) to obtain polymers having a constant pretilt angle. Thus, it is possible to maintain a specific arrangement even at an off-state stage and in turn to obtain a high contrast ratio and fast response time, through guiding the liquid crystal layer ( 130 ) to a constant direction using the polymerized photoreactive liquid crystal monomer ( 134 ). However, when the dose of UV irradiation is too high, the polymeric network is not formed uniformly and a large polymeric network is formed due to agglomeration, which may result in light leakage. Accordingly, the UV irradiation may be typically carried out for 180 minutes or less and at an irradiation dose of about 50˜300 J, but not limited thereto, and to attain the desired pretilt angle, the irradiation dose and time may be appropriately adjusted. 
         [0063]    In the liquid crystal display ( 100 ) prepared above, when a voltage is not applied to the electrodes, the liquid crystal molecules are vertically aligned with respect to the first and second substrates, and as the consequence, the light passed through the second polarizer ( 150 ) is absorbed by the first polarizer ( 140 ) to create a dark state, wherein the pretilt angle generated by the photoreactive liquid crystal monomers has little effect on the dark state (see  FIG. 1   a ). 
         [0064]    In addition, when a state of brightness is accomplished by supplying a power to a common electrode ( 180 ) and a pixel electrode ( 190 ) (see  FIG. 1   b ), an electric field in the horizontal direction is created by the supplied power and the phase retardation of said liquid crystal mixture leads to a state of brightness. 
         [0065]      FIG. 4   a  and  FIG. 4   b  are sectional views of the liquid crystal display according to the second embodiment when voltage is applied or not applied, respectively. 
         [0066]    Referring to  FIGS. 4   a  and  4   b , the liquid crystal display ( 200 ) according to the second embodiment comprises a first substrate ( 210 ) and the second substrate ( 220 ) which are aligned parallel with each other, and a liquid crystal layer ( 230 ) which is positioned between the first substrate ( 210 ) and the second substrate ( 220 ), wherein the first substrate ( 210 ) and the second substrate ( 220 ) respectively comprise a first vertical alignment layer ( 260 ) and a second vertical alignment layer ( 270 ) toward the liquid crystal layer ( 230 ), and the second substrate ( 220 ) contains two common electrode ( 280 , also referred to “the first pixel electrode”) and pixel electrode ( 290 , also referred to “the second pixel electrode”). This display is identical to the liquid crystal display ( 100 ) according to the first embodiment described by reference to  FIGS. 1   a  and  1   b , and the aforementioned explanation can therefore be used here. 
         [0067]    The liquid crystal layer ( 230 ) is identical to that of the liquid crystal display ( 100 ) according to the above-mentioned first example, wherein the liquid crystal material ( 232 ) having a positive dielectric anisotropy are mixed with polymers of photoreactive liquid crystal monomers ( 234 ) which are present adjacent to or at a fixed distance from the first substrate ( 210 ) and the second substrate ( 220 ) and as a mixture with the liquid crystal material ( 232 ). 
         [0068]    Meanwhile, the liquid crystal display ( 200 ) according to the second example has two electrodes ( 280 ,  290 ) as well as the other common electrode ( 284 ) on the second substrate ( 220 ). This common electrode ( 284 ) is formed at the lower part of two electrodes ( 280 ,  290 ) between the second vertical alignment layer ( 270 ) and the second substrate ( 220 ). Further, a dielectric layer ( 282 ) is formed between two electrodes ( 280 ,  290 ) and the other common electrode ( 284 ). 
         [0069]    The first and second pixel electrodes ( 280 ,  290 ) on the second substrate ( 220 ) may be driven by a second transistor (not shown) and may be driven by a first transistor to become a pixel electrode and common electrode. 
         [0070]    The other common electrode ( 284 ) may be formed into the transparent metal layer made of transparent conductive metal oxides such as indium-tin-oxide (ITO) or indium zinc oxide ( 120 ). 
         [0071]    The dielectric layer ( 282 ) provides an insulating function, and may be formed using one or more selected from the group consisting of photopolymer resin, thermosetting resin, polyamic acid, and other organic resins (epoxy resin, acrylic resin or fluorine resin, etc.); SiO, SiO 2 , or SiN. 
         [0072]    Referring to  FIG. 4   a , the state of darkness is achieved when the light passes through the second polarizer ( 250 ), without phase retardation, is absorbed by the second polarizer ( 240 ), since the liquid crystal molecules are arranged vertically with respect to both substrates due to no voltage applied. 
         [0073]    Further, referring to  FIG. 4   b , if the first pixel electrode ( 280 ), the second pixel electrode ( 290 ), and the other common electrode ( 284 ) are supplied with power, the liquid crystal layer ( 230 ) with positive dielectric anisotropy is driven by the resulting horizontal electric field (L) and fringe field (X) which are formed around the first pixel electrode ( 280 ), the second pixel electrode ( 290 ), the dielectric layer ( 282 ), and the other common electrode ( 284 ). At this time, a state of brightness is achieved by the occurrence of phase retardation of the liquid crystal layer ( 230 ) with positive dielectric anisotropy by the influence of the horizontal electric field (L) and fringe field (X). 
         [0074]    The method for giving the photoreactive liquid crystal monomers a pretilt angle in the LCD ( 200 ) according to the second example is the same as described previously using  FIGS. 2 and 3 : it comprises the steps of introducing a liquid crystal layer mixed with photoreactive liquid crystal monomers into a cell (S 210 ), giving the photoreactive liquid crystal monomers a uniform pretilt angle by applying voltage (S 220 ), polymerizing the photoreactive liquid crystal monomers by applying ultraviolet (UV) lay (S 230 ). 
         [0075]    Unlike the LCD ( 100 ) according to the first example, as described above, the LCD ( 200 ) according to the second example comprises the first pixel electrode ( 280 ) and the second pixel electrode ( 290 ) as well as another common electrode ( 284 ) and an additional dielectric layer ( 282 ). Therefore, the step of giving photoreactive liquid crystal monomers a uniform pretilt angle by applying voltage (S 220 ) is different in that a uniform pretilt angle is conferred to the photoreactive liquid crystal monomers by the action of the horizontal electric field (L) as well as by fringe field (X) on applying an appropriate voltage using a voltage-applying device. 
         [0076]      FIG. 5  is cross-sectional views of the LCD of the second embodiment according to different stages for preparing the pretilt angle induced part. 
         [0077]      FIGS. 5(A)  to (D) show the method for giving a pretilt angle to the photoreactive liquid crystal monomers in the LCD ( 200 ) according to the second example, in the same manner as in  FIGS. 3(A)  to (D). As shown in  FIGS. 5(B)  and (C), the method comprises applying a voltage by a voltage-applying device and generating a horizontal electric field (L) by applying UV, and also comprises giving photoreactive liquid crystal monomers a uniform pretilt angle by the action of a fringe field (X). 
         [0078]      FIGS. 6   a  and  6   b  are cross-sectional drawings of the LCD according to a third embodiment when applied voltage is on and off, respectively. 
         [0079]    Referring to  FIGS. 6   a  and  6   b , an LCD ( 300 ) according to the third embodiment is the same as the LCD ( 100 ) according to the first embodiment as well as the LCD ( 200 ) according to the second embodiment in that it comprises the first board ( 310 ) and the second board ( 320 ) which face each other and a liquid crystal layer ( 330 ) disposed between the first board ( 310 ) and the second board ( 320 ). The first board ( 310 ) and the second board ( 320 ) comprise the first vertical alignment layer ( 360 ) and the second vertical alignment layer ( 370 ) vertically aligned with respect to the direction of the liquid crystal layer ( 330 ), and the second board ( 320 ) comprises two common electrodes ( 380 ) and a pixel electrode ( 390 ). 
         [0080]    The LCD ( 300 ) according to the third embodiment is different from the LCD ( 100 ) according to the first embodiment or the LCD ( 200 ) according to the second embodiment in that it comprises an additional common electrode ( 384 ) between the first vertical alignment layer ( 360 ) and the first board ( 310 ) and a dielectric layer ( 382 ) between the first board and the additional common electrode. 
         [0081]    The upper board may be prepared by forming the additional common electrode ( 384 ) on the first board ( 310 ), a dielectric layer ( 282 ) on the common electrode ( 383 ), and the first vertical alignment layer ( 360 ) on the dielectric layer ( 282 ), sequentially. 
         [0082]    Referring to  FIG. 6   a , as the liquid crystal molecules ( 332 ) are arranged vertically with respect to the planes of both boards ( 310 ,  320 ) due to lack of applied voltage, the light which passes through the second polarizing plate ( 350 ) does not suffer a phase retardation and it is absorbed by the second polarizing plate ( 340 ) so that it becomes dark. 
         [0083]    Moreover, referring to  FIG. 6   b , when a voltage is applied to the common electrode ( 380 ), the pixel electrode ( 390 ), and additional common electrode ( 384 ), the liquid crystal layer ( 330 ) having positive dielectric constant anisotropy is driven by an oblique electric field (Y) and a horizontal electric field (L) formed around the common electrode ( 380 ), the pixel electrode ( 390 ), the dielectric layer ( 382 ), and the additional common electrode ( 384 ). At this time, the oblique electric field (Y) and the horizontal electric field (L) induce a phase retardation in the liquid crystal layer ( 230 ) having positive dielectric constant anisotropy so that it becomes bright. 
         [0084]    The LCD ( 300 ) according to the third embodiment has advantages in that a disclination region is not generated between the electrodes and the response time becomes fast. 
         [0085]      FIG. 7  is cross-sectional drawings for the LCD of the third embodiment according to different stages for preparing the pretilt angle induced part. 
         [0086]    As shown in  FIGS. 7(A)  to (D), the method for giving the photoreactive liquid crystal monomers a pretilt angle in LCD ( 300 ) according to the third embodiment is the same as in the LCDs ( 100 ,  200 ) according to the first and the second embodiments, except for giving the photoreactive liquid crystal monomers a uniform pretilt angle by the horizontal electric field (L) as well as by the oblique electric field (Y) on applying a voltage using a voltage-applying device in the step of giving the photoreactive liquid crystal monomers a uniform pretilt angle. 
         [0087]    In other words, as shown in  FIGS. 7(B)  and (C), the method comprises applying a voltage by a voltage-applying device and generating a horizontal electric field (L) by applying UV and it also comprises giving the photoreactive liquid crystal monomers a uniform pretilt angle by the action of the oblique electric field (Y). 
         [0088]    Hereinafter described are comparative examples which measure the variation of the transparency of the LCD ( 100 ) according to the first embodiment as a function of the pretilt angle as well as the voltage applied to the photoreactive liquid crystal monomers. It is obvious that the results are also applied to the LCDs ( 200 ,  300 ) according to the second and the third embodiments. 
         [0089]    An LCD of the conditions of Table 1, e.g., an electrode width of 3 μm, an electrode distance of 10 μm, and a cell gap of 3.5 μm, was prepared, and the photoreactive liquid crystal monomers were examined with regard to an applied voltage as well as the transparency depending on the pretilt angle. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Electrode width (μm) = w 
                 3 
               
               
                 Electrode distance (μm) = l 
                 10 
               
               
                 Cell gap (μm) = d 
                 3.5 
               
               
                 dΔn (μm) 
                 0.42 
               
               
                 Rotational viscosity (mPa/s) 
                 147 
               
             
          
           
               
                 LC 
                 Δn 
                 0.12 
               
               
                   
                 Δε 
                 7.4 
               
               
                   
                 K1 
                 11.7 
               
               
                   
                 K2 
                 5.1 
               
               
                   
                 K3 
                 16.1 
               
               
                   
               
             
          
         
       
     
         [0090]    Table 2 shows the results of measuring the applied voltage and the transparency of a LCD measured under the same condition specified in Table 1 depending on the pretilt angle. In Table 2, V 10  (V) means the threshold voltage; V 10  (%), the percent decrease in the threshold voltage at a pretilt angle 90°; V 100  (V), the voltage (driving voltage) at the maximum transmission rate; and V 100  (V), the decrease percent of the driving voltage at a pretilt angle 90°. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Pre-tilt (°) 
                 V10 (V) 
                 V10 (%) 
                 V100 (V) 
                 V100 (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 90 
                 7.7 
                 0 
                 16.8 
                 0 
               
               
                   
                 89 
                 7.5 
                 −2.59 
                 16.6 
                 −1.19 
               
               
                   
                 88 
                 7.4 
                 −3.89 
                 16.4 
                 −2.38 
               
               
                   
                 87 
                 7.3 
                 −5.19 
                 16.2 
                 −3.57 
               
               
                   
                 86 
                 7.1 
                 −7.79 
                 16 
                 −4.76 
               
               
                   
                 85 
                 6.9 
                 −10.39 
                 15.8 
                 −5.95 
               
               
                   
                   
               
             
          
         
       
     
         [0091]    Referring to Table 2, the pretilt angle of 90°, i.e., in case of photoreactive liquid crystal monomers in the liquid crystal layer are not given with a pretilt angle, leads an applied voltage of 7.7V, while a pretilt angle of 89° to 85° leads an applied voltage of 7.5V to 6.9V. In other words, it is found that as the pretilt angle, the angle from vertical alignment, increases, the applied voltage decreases. 
         [0092]    Meanwhile, under the same applied voltage, it is found that the relative response time becomes shorten. 
         [0093]    Meanwhile, LCD according to the embodiments above can solve the problems of disclination due to unstable alignment and slack of response time. 
         [0094]    The term “comprise”, “consist of” or “have” as used herein means that a component may be inherent, unless explicitly described otherwise, and thus, it should be construed that the relevant subject may further include other components, without excluding them. All technical and scientific terms as used herein have the same meanings as understood by those skilled in the art, unless defined otherwise. The general terms such as those defined in a dictionary should be interpreted as a contextual meaning used in the relevant art, unless clearly defined otherwise, and should not be interpreted as an ideal or excessively formal meaning. 
         [0095]    While the invention has been described with respect to the above specification, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art, which also fall within the scope of the invention. Thus, the above-described embodiments are intended to illustrate the present invention without limiting the scope of the invention, and the scope of the present invention is not limited by the embodiments. The scope of the present invention should be construed by the following claims, and all features within an equivalent scope of the present invention will be intended to be included in the appended claims. 
       EXPLANATION OF THE REFERENCE NUMBERS 
       [0096]      110 ,  210 ,  310 : the first substrate 
         [0097]      120 ,  220 ,  320 : the second substrate 
         [0098]      130 ,  230 ,  330 : liquid crystal layer 
         [0099]      132 ,  232 ,  332 : liquid crystal molecule 
         [0100]      134 ,  234 ,  334 : photoreactive liquid crystal monomer 
         [0101]      140 ,  240 ,  340 : upper polarizer 
         [0102]      150 ,  250 ,  350 : lower polarizer 
         [0103]      160 ,  260 ,  360 : upper alignment layer 
         [0104]      170 ,  270 ,  370 : lower alignment layer 
         [0105]      180 ,  280 ,  380 : common electrode 
         [0106]      282 ,  382 : common electrode 
         [0107]      284 ,  384 : dielectric layer 
         [0108]      190 ,  290 ,  390 : pixel electrode