Abstract:
An in-plane-switching mode liquid crystal display device includes a first ferroelectric liquid crystal layer on a first substrate, a second ferroelectric liquid crystal layer on a second substrate, the first and second substrates being bonded to each other with a space therebetween, and a nematic liquid crystal layer at the space between the first and second ferroelectric liquid crystal layers, the first and second ferroelectric liquid crystal layers including a photo-polymerizational monomer.

Description:
[0001]     The present application claims the benefit of Korean Patent Application No. P2003-99336 filed in Korea on Dec. 29, 2003, which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a display device, and, more particularly, to an in-plane-switching mode liquid crystal display device using a ferroelectric liquid crystal material and a method of fabricating the same.  
         [0004]     2. Discussion of the Related Art  
         [0005]     In general, a liquid crystal display (LCD) device controls an electric field applied to a liquid crystal cell. The controlling of the electric field modulates light incident to the liquid crystal cell, thereby displaying a picture. The liquid crystal display devices may employ a vertical electric field method and a horizontal electric field method to drive the liquid crystal cell.  
         [0006]     In the vertical electric field method, a pixel electrode and a common electrode are formed on an upper substrate and a lower substrate, respectively. Thus, the pixel and common electrodes are vertically opposite to each other, and an electric field is generated vertically across a liquid crystal cell by a voltage difference applied between the pixel and common electrodes. For example, a twisted nematic (TN) mode LCD device generally uses the vertical electric field method. The twisted nematic mode LCD device has a relatively wide aperture ratio. However, since the liquid crystal molecules have different refractive indices, a display picture varies for an observer depending on a viewing angle. Thus, there is a disadvantage that the realization of wide viewing angle is difficult.  
         [0007]     Further, in-plane-switching (IPS) mode LCD devices generally use the horizontal electric field method. In the horizontal electric field method, an electric field is generated between the electrodes formed on the same substrate to drive the liquid crystal cell.  
         [0008]      FIG. 1  is a schematic cross-sectional view of an in-plane-switching mode liquid crystal display device according to the related art. In  FIG. 1 , a liquid crystal display device includes an upper glass substrate  12  and a lower glass substrate  18  with a liquid crystal layer having liquid crystal molecules  14  formed therebetween. A polarizer  11  and an alignment layer  13  are respectively formed on an upper surface and a lower surface of the upper substrate  12 . In addition, an alignment film  17  and a polarizer  19  are respectively formed on an upper surface and a lower surface of the lower substrate  18 . In particular, the axes of the polarizers  11  and  19  cross each other.  
         [0009]     Further, a common electrode  15  and a pixel electrode  16  are formed on the alignment film  17  on the lower substrate  18 . In particular, an electric field  20  is generated along a horizontal direction by a voltage difference applied between the common electrode  15  and the pixel electrode  16 . As a result, the liquid crystal molecules  14  are rotated by the electric field  20 , thereby modulating a polarization component of light transmitting through the liquid crystal layer. For instance, if the polarization component of light transmitting through the liquid crystal layer is changed by 90 degrees, then light passes through the upper polarizer  11 . On the other hand, if the polarization component of light does not change, then light cannot pass thorough the upper polarizer  11 .  
         [0010]     The IPS mode liquid crystal display device according to the related art has a wide viewing angle since a refractive index change of the liquid crystal molecules  14  is not large. However, the electric field applied to the liquid crystal molecules  14  is done with the opaque common and pixel electrodes  15  and  16  on the lower substrate  18 . In particular, because a light switching is not made on the common and pixel electrodes  15  and  16 , the electric field applied to the liquid crystal molecules  14  is bent. Thus, the IPS mode liquid crystal display device according to the related art has a disadvantage of having a low aperture ratio.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, the present invention is directed to an in-plane-switching mode liquid crystal display device and a method of fabricating the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.  
         [0012]     An object of the present invention is to provide a liquid crystal display device of in-plane-switching mode, where it is possible to realize a wide viewing angle without a deterioration of the aperture ratio, and a fabricating method thereof.  
         [0013]     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0014]     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, an in-plane-switching mode liquid crystal display device includes a first ferroelectric liquid crystal layer on a first substrate, a second ferroelectric liquid crystal layer on a second substrate, the first and second substrates being bonded to each other with a space therebetween, and a nematic liquid crystal layer at the space between the first and second ferroelectric liquid crystal layers, the first and second ferroelectric liquid crystal layers including a photo-polymerizational monomer.  
         [0015]     In another aspect, a method of fabricating an in-plane-switching mode liquid crystal display device includes forming a first ferroelectric liquid crystal layer on a first substrate, forming a second ferroelectric liquid crystal layer on a second substrate, the first and second ferroelectric liquid crystal layers including a photo-polymerizational monomer, attaching the first and second substrates to each other with a space therebetween, and forming a nematic liquid crystal layer at the space between the first and second ferroelectric liquid crystal layers.  
         [0016]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0018]      FIG. 1  is a schematic cross-sectional view of an in-plane switching mode liquid crystal display device according to the related art;  
         [0019]      FIGS. 2A  to  2 D are schematic cross-sectional views illustrating a method of fabricating a liquid crystal display according to an embodiment of the present invention; and  
         [0020]      FIG. 3  is a schematic perspective view illustrating the movement of a ferroelectric liquid crystal and a nematic liquid crystal in the liquid crystal display according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings.  FIGS. 2A  to  2 D are schematic cross-sectional views illustrating a method of fabricating a liquid crystal display according to an embodiment of the present invention. In  FIG. 2A , an upper electrode  22  and an upper alignment film  23  are formed on an upper substrate  21 . In addition, a lower electrode  32  and a lower alignment film  33  are formed on a lower substrate  31 . The upper and lower substrates  21  and  31  may be formed of a transparent glass material. The upper and lower electrodes  22  and  32  may be formed of a transparent conductive material, such as indium-tin-oxide (ITO).  
         [0022]     Further, the upper and lower alignment films  23  and  33  may be formed of an organic material, such as polyimide. The upper and lower alignment films  23  and  33  also may be rubbed to set a pre-tilt angle of ferroelectric liquid crystal molecules  24  and  34  (shown in  FIG. 2B ) that are subsequently formed therebetween. The polarizer (not shown) crossing a light transmission axis is formed onto the light incident plane of the lower substrate  31  and the light out-coming plane of the upper substrate  21 .  
         [0023]     As shown in  FIG. 2B , a slight amount of photo-polymerizational monomers  25  and  35  are added into ferroelectric liquid crystal molecules  24  and  34  during a chiral smectic C* phase of forming the ferroelectric liquid crystal molecules  24  and  34 . For instance, the ferroelectric liquid crystal molecules  24  and  34  and the photo-polymerizational monomers  25  and  35  may have a composition ratio as shown in Table 1 below.  
                           TABLE 1                                       Ferroelectric liquid crystal   95 wt %˜99 wt %           Photo-polymerizational monomer   1 wt %˜5 wt %                      
 
         [0024]     In addition, the ferroelectric liquid crystal molecules  24  and  34  may include any known ferroelectric liquid crystal material, and the photo-polymerizational monomers  25  and  35  may include any known nematic photo-polymerizational monomer. In particular, the photo-polymerizational monomers  25  and  35  may be uniformly mixed with the ferroelectric liquid crystal molecules  24  and  34 , respectively, and then the mixture may be evenly spread on the alignment films  23  and  33 , respectively. The ferroelectric liquid crystal molecules  24  and  34  may be formed in a nematic system.  
         [0025]     Further, the mixture of the ferroelectric liquid crystal molecules  24  and  34  and the photo-polymerizational monomers  25  and  35  may be exposed to a medium of which the electrical negativity is high. Alternatively, an electric field or a magnetic field may be applied to the mixture of the ferroelectric liquid crystal molecules  24  and  34  and the photo-polymerizational monomers  25  and  35 . Thus, the ferroelectric liquid crystal molecules  24  and  34  may be aligned along the spontaneous polarization direction, shown as the arrows in  FIG. 2B . For instance, the mixture may be exposed to an atmosphere of water (H 2 O) or oxygen (O 2 ) as a medium with a high polarity.  
         [0026]     When exposing to the medium with a high polarity, the ferroelectric liquid crystal molecules  24  and  34  change from an isotropic phase to a smectic A phase, a chiral smectic C* phase and a chiral nematic N* phase due to the transition temperature. In addition, the spontaneous polarization of the ferroelectric liquid crystal molecules  24  and  34  faces toward the medium. On the contrary, when exposing to a medium with a low polarity, such as nitrogen (N 2 ) or air, the spontaneous polarization of the ferroelectric liquid crystal molecules  24  and  34  faces toward the opposite of the medium. Thus, a temperature treatment is carried out to make phase transition from the smectic A phase or the chiral nematic N* phase to the chiral smectic C* phase.  
         [0027]     Further, when applying an electric field or a magnetic field to the ferroelectric liquid crystal molecules  24  and  34  under the transition temperature, the ferroelectric liquid crystal molecules  24  and  34  change from the isotropic phase to the smectic A phase, the chiral smectic C* phase and the chiral nematic N* phase. The spontaneous polarization of the ferroelectric liquid crystal molecules  24  and  34  is aligned in parallel to the electric field or the magnetic field.  
         [0028]     As a result, by exposing the mixture to the medium with an electrical polarity or by applying an electric field or a magnetic field to the mixture, the ferroelectric liquid crystal molecules  24  formed on the upper substrate  21  may have the spontaneous polarization direction facing the opposite direction of the upper substrate  21  and the ferroelectric liquid crystal molecules  34  formed on the lower substrate  31  may have the spontaneous polarization direction facing toward the lower substrate  31 .  
         [0029]     As shown in  FIG. 2C , a photo-polymerization of the photo-polymerizational monomers  25  and  35  may be induced by illuminating ultraviolet ray (not shown) on the mixture. In particular, the photo-polymerizational monomers  25  and  35  may have a bridge bond generated by the photo-polymerization to form a polymer network. As a result, the ferroelectric liquid crystal molecules  24  and  34  have the spontaneous polarization direction sustained uniformly and their initial alignments stabilized.  
         [0030]     In particular, a polymer stabilized FLC(PSFLC) alignment film may be formed on the substrates  21  and  31 , thereby enabling the alignment state to be stabilized. Further, since a small amount of the photo-polymerizational monomers  25  and  35  is added, the extent of the bridge bond of the ferroelectric liquid crystal molecules  24  and  34  allows the ferroelectric liquid crystal molecules  24  and  34  to be rotated.  
         [0031]     As shown in  FIG. 2D , the upper and lower substrates  21  and  31  are bonded facing each other with a predetermined cell gap therebetween by a sealant (not shown) at a periphery of the substrates  21  and  31 . Further, a nematic liquid crystal material  30  is formed at the cell gap between the upper and lower substrates  21  and  31 . In particular, the ferroelectric liquid crystal layers containing the ferroelectric liquid crystal molecules  24  and  34  are not mixed with the nematic liquid crystal material  30 . Thus, a phase separation is formed at an interface between the nematic liquid crystal material  30  and the ferroelectric liquid crystal layers. The nematic liquid crystal material  30  may be of a positive type or a negative type liquid crystal material.  
         [0032]      FIG. 3  is a schematic perspective view illustrating the movement of a ferroelectric liquid crystal and a nematic liquid crystal in the liquid crystal display according to an embodiment of the present invention. As shown in  FIG. 3 , when a voltage difference is applied to the upper and lower electrodes  22  and  32  (shown in  FIG. 2D ), a nematic liquid crystal molecule  30  is driven in plane, thereby modulating light transmitted therethrough.  
         [0033]     In addition, a ferroelectric liquid crystal molecule  34  rotates along a virtual cone, is driven in plane, and induces the in-plane-drive of the nematic liquid crystal molecule  30  adjacent thereto. In particular, when an electric field is applied to the ferroelectric liquid crystal molecule  34 , the ferroelectric liquid crystal molecule  34  has a permanent polarization, i.e., spontaneous polarization. Thus, the interaction of the electric field and the spontaneous polarization like an interaction of magnets causes the ferroelectric liquid crystal molecule  34  to rapidly rotate.  
         [0034]     As a result, the liquid crystal display device minimizes the deterioration of an aperture ratio by applying the electric field using the vertical electric field method, and realizes a wide viewing angle by the in-plane-driving of the nematic liquid crystal molecule  30 . Further, the ferroelectric liquid crystal molecule  34  causes the nematic liquid crystal molecule  30  to rotate rapidly. Thus, the response speed of the nematic liquid crystal molecule  30  is improved.  
         [0035]     As described above, the liquid crystal display device of the in-plane switching mode and the fabricating method thereof according to an embodiment of the present invention forms a ferroelectric liquid crystal layer on an alignment film in each of the upper and lower substrates and forms a nematic liquid crystal layer between the ferroelectric liquid crystal layers. The liquid crystal display device applies an electric field to the ferroelectric liquid crystal layers and the nematic liquid crystal layer using the vertical electric field method. As a result, the liquid crystal display device of the in-plane switching mode and the fabricating method thereof according to an embodiment of the present invention drives the liquid crystal molecules of the nematic liquid crystal layer in plane by an induction of the ferroelectric liquid crystal layers. Thus, an aperture ratio is increased and a wide viewing angle is achieved.  
         [0036]     Further, the liquid crystal display device of the in-plane switching mode and the fabricating method thereof according to an embodiment of the present invention include inducing a photo-polymerization in the ferroelectric liquid crystal layers. Thus, the ferroelectric liquid crystal layers have the spontaneous polarization direction sustained uniformly and their initial alignments stabilized.  
         [0037]     It will be apparent to those skilled in the art that various modifications and variations can be made in the in-plane-switching mode liquid crystal display device and the method of fabricating the same of the present invention without departing from the sprit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.