Abstract:
Liquid crystal displays and fabrication methods thereof. The liquid crystal display comprises a first substrate with an active matrix of a plurality of pixels. A second substrate is provided opposing the first substrate. A liquid crystal layer is interposed between the first substrate and the second substrate. Each pixel comprises a polymer dispersed liquid crystal layer corresponding to a first liquid crystal region and a non-polymer dispersed liquid crystal layer corresponding to a second liquid crystal region.

Description:
BACKGROUND 
   The invention relates to liquid crystal displays and fabrication methods thereof, and more particularly, to polymer dispersed liquid crystal (PDLC) displays and fabrication methods thereof. 
   Among flat panel displays, liquid crystal displays (LCDs) exhibit characteristics of light weight, low power consumption, and good outdoor reliability, and are therefore widely applied in portable computers, notebooks, mobile phones, and personal digital assistances (PDAs). Conventional twisted nematic mode LCDs are disadvantageous due to their small viewing angle. Some proposed LCDs, such as, in-plane switching mode LCD (IPS-LCD) or multi-domain vertical alignment LCD (MVA-LCD) attempt to improve the viewing characteristics of conventional LCDs. 
   Conventional MVA-LCDs, however, possess less transparency than TN mode LCDs. Moreover, conventional MVA-LCDs image sticking may occur in display over a long period of time. U.S. Pat. No. 6,781,665, the entirety of which is hereby incorporated by reference, discloses a method for fabricating a polymer dispersed liquid crystal (PDLC) display. The polymer dispersed liquid crystal (PDLC) display is formed by mixing liquid crystal and monomer and radiating UV thereon with a bias. Polymers dispersed in liquid crystal can help liquid crystal recovery, thereby reducing imaging sticking. 
     FIG. 1  is a schematic view of a conventional method for fabricating a polymer dispersed liquid crystal display. A liquid crystal display panel is disposed in an ultra-violet light irradiation chamber  32 . A power supply  30  applies a bias between an upper electrode  34  and a lower electrode  36 . A liquid crystal layer  38  between the upper electrode  34  and the lower electrode  36  comprises a mixture of liquid crystal and monomer. When the liquid display panel is irradiated by UV light, the monomer is polymerized to form a continuous network. Moreover, during polymerization, the orientation of the liquid crystal becomes more consistent with the polymer. 
     FIG. 2  is a cross section of a conventional polymer dispersed liquid crystal display. After an irradiating or a heating process, monomers in the liquid crystal layer  38  between upper substrate  10  and lower substrate  20  are polymerized into polymer structures  13 . Polymer structures  13  are formed adjacent to alignment layers  46   a  and  46   b . Polymer structures  13  can aid in liquid crystal recovery, thereby reducing image sticking. 
   U.S. Pat. No. 6,014,194, the entirety of which is hereby incorporated by reference, discloses a polymer dispersed liquid crystal (PDLC) display. Monomers are added into liquid crystal. The mixture of monomers and liquid crystals in each of red, green, and blue color pixels are separately polymerized with UV irradiation under different biases, thereby creating a different polymer structure in each pixel. 
   SUMMARY 
   Accordingly, the invention provides a wide viewing angle liquid crystal display. Each pixel of the wide viewing angle liquid crystal display is divided into muti-region with different polymer density and structure. Each region has different optical characteristics to improve the viewing angle. 
   The invention also provides a liquid crystal display, comprising a first substrate with an active matrix of a plurality of pixels. A second substrate is provided opposing the first substrate. A liquid crystal layer is interposed between the first substrate and the second substrate. Each pixel comprises a polymer dispersed liquid crystal layer corresponding to a first liquid crystal region and a non-polymer or low-density polymer dispersed liquid crystal layer corresponding to a second liquid crystal region. 
   The invention further provides a method for fabricating a liquid crystal display. A first substrate is provided with an active matrix of a plurality of pixels. A second substrate is provided opposing the first substrate. A liquid crystal layer is interposed between the first substrate and the second substrate. Each pixel comprises a polymer dispersed liquid crystal layer corresponding to a first liquid crystal region and a non-polymer or low-density polymer dispersed liquid crystal layer corresponding to a second liquid crystal region. 
   The invention further provides a method for fabricating a liquid crystal display. A first substrate is provided with an active matrix of a plurality of pixels. A second substrate is provided opposing the first substrate. A liquid crystal layer with monomers is interposed between the first substrate and the second substrate. The monomers are selectively polymerized in the liquid crystal layer such that each pixel comprises a polymer dispersed liquid crystal layer corresponding to a first liquid crystal region and a non-polymer or low-density polymer dispersed liquid crystal layer corresponding to a second liquid crystal region. 

   
     DESCRIPTION OF THE DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of a conventional method for fabricating a polymer dispersed liquid crystal display; 
       FIG. 2  is a cross section of a conventional polymer dispersed liquid crystal display; 
       FIGS. 3A-3D  are cross sections of an embodiment of a method of fabricating a polymer dispersed liquid crystal display; 
       FIGS. 4A-4D  are cross sections showing various embodiments of polymer dispersed liquid crystal (PDLC) displays; and 
       FIGS. 5A-5D  are schematic views showing various embodiments of the lithographic masks. 
   

   DETAILED DESCRIPTION 
     FIGS. 3A-3D  are cross sections of an embodiment of a method of fabricating a polymer dispersed liquid crystal display, in which a liquid crystal display comprises a first substrate  110  with an active matrix array pixel driven, and a second substrate  120  opposing the first substrate  110 . A liquid crystal layer  130  is interposed between the first substrate  110  and the second substrate  120 . A first electrode  112  and a second electrode  122  are separately disposed on the inner surface of the first substrate  110  and the second substrate  120 . The liquid crystal layer  130  comprises liquid crystal molecules  132 , monomers  134  and inducers (not shown) to improve polymerization. Monomers  130  may preferably comprise diacrylate, monoacrylate, or other monomers with a double bond. After irradiation, the monomers are decomposed into free radicals and react with each other. An initiator can be optionally added to improve polymerization. 
   Referring to  FIG. 3B , a liquid crystal display with monomer dispersed therein is disposed in an apparatus installed by UV irradiation. A power supply  230  biases a first and a second electrode of the liquid crystal display. Each pixel of the liquid crystal can be divided into at least one LC region  152 . UV light can irradiate predetermined region and be optionally biased between the first and the second electrodes  112  and  122 . A polymer dispersed liquid crystal display is therefore formed to aid in liquid crystal recovery and reduce imaging sticking and improve the color performance in wide viewing angle. 
   Selective UV irradiation is performed by lithography forming a patterned mask  150  on the first substrate  110  or the second substrate  120 . An opening is formed at the predetermined first LC region  152  on the second substrate  120 . The liquid crystal configuration is then irradiated by UV polymerizing monomers  134  at the first LC region  152 . 
   Referring to  FIG. 3C , a network  136  of monomer  134  is formed, when the liquid crystal layer  130  is irradiated by UV light. After the monomers  134  are polymerized, the orientation of the liquid crystal molecules  132  becomes more consistent. 
   In another embodiment, the light source can optionally be focused and directly irradiated at predetermined region  152 . An optional bias can be applied between the first and the second electrodes to polymerize the monomer  132 , thereby eliminating lithographic masking steps. For example, a laser beam is directly focused on the predetermined region. The unselected region does not irradiate. Alternatively, the unselected region is determined between two laser beams. Interference between the two laser beams creates energy distribution, thereby creating polymer density distribution in the liquid crystal layer. In another embodiment, monomer  134  can alternatively comprise thermally polymeric material. By locally heating the predetermined region, monomer in the liquid crystal layer can be thermally polymerized. In another embodiment, a power supply biases different voltage in different region of one pixel in the UV irradiation process. So the structures of polymer in different regions are different. 
   Referring to  FIG. 3D , when driving voltage V is applied to the liquid crystal layer, liquid crystal molecules are rotated in the direction of the applied field. Liquid crystal molecules at the polymer dispersed region, i.e., at the predetermined region are rotated different from those at the non-polymer dispersed region. When a light source  160 , such as a back light, passes through the liquid crystal display the polymer dispersed region  152  and the non-polymer dispersed region  154  possess different transparencies, thereby creating different optical characteristics  160 ′ and  160 ″ to widen the viewing angle. 
     FIGS. 4A-4D  are cross sections showing various embodiments of polymer dispersed liquid crystal (PDLC) displays. A PDLC region comprises a polymer structure and liquid crystal molecules. Referring to  FIG. 4A , polymer structure  136 ′ is dispersed between the first and the second substrates  110  and  120 . Liquid crystal molecules  132 ′ in the first LC region  152  are enclosed in the polymer structure  136 ′. Referring to  FIG. 4B , polymer structures  124  and  136  are formed on an inner surface of the second substrate  120  at the first LC region  152 . The polymer structures  124  and  136  are formed at the interface between the liquid crystal layer  130  and the second substrate  120 . The polymer structure  136  has one end fixed to the polymer structure  124  and the other end extending into the liquid crystal layer  130 . Referring to  FIG. 4C , polymer structures  114  and  136  are formed on an inner surface of the first substrate  110  at the first LC region  152 . The polymer structures  124  and  136  are formed at the interface between the liquid crystal layer  130  and the first substrate  110 . The polymer structure  136  has one end fixed to the polymer structure  114  and the other end extending into the liquid crystal layer  130 . Referring to  FIG. 4D , polymer structures  114 ,  124 , and  136  are formed on an inner surface of the first substrate  110  and an inner surface of the second substrate  120  at the first LC region  152 . The polymer structures  114 ,  124 , and  136  are formed at the interface between the liquid crystal layer  130  and the first substrate  110  and the interface between the liquid crystal layer  130  and the second substrate  120 . The polymer structure  136  has one end fixed to the polymer structure  124  and the other end extending into the liquid crystal layer  130 . The polymer structure  136  has one end fixed to the polymer structure  114  and the other end extending into the liquid crystal layer  130 . 
     FIGS. 5A-5D  are schematic views showing various embodiments of the lithographic masks. Each mask can be used in lithographic steps in  FIG. 3B . Patterns on the mask corresponds to pixel regions between scanning line and data line of the liquid crystal display, and are divided into a transparent region  152  and opaque region  154 . 
   Referring to  FIG. 5A , the opaque region  154  of the mask  152   a  comprises a triangular region at the center of each pixel region. The triangular region comprises a long edge parallel to the longitudinal side of each pixel region. Referring to  FIG. 5B , the opaque region  154  of the mask  152   b  comprises two opposing right angle triangular region formed at corners of each pixel regions. The right angle edges of the right angle triangular regions are parallel to the sides of each pixel region. Referring to  FIG. 5C , the opaque region  154  of the mask  152   c  comprises two opposing trapezoid regions formed at corners of each pixel region. The right angle edges of the trapezoid regions are parallel to the sides of each pixel region. Referring to  FIG. 5D , the opaque region  154  of the mask  152   d  comprises two opposite right angle triangle regions formed at the center of each pixel region. The right angle edges of the right angle triangular regions are parallel to the sides of each pixel region. 
   After UV light irradiation, the liquid crystal layer between the first and the second electrodes is polymerized. Monomers are polymerized into a continuous polymer network. The orientation of the liquid crystal molecules become more consistent in the polymer dispersed region. The unirradiated region does not polymerize, thereby different optical characteristics are created from the polymer dispersed region to widen the viewing angle. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.