Abstract:
A multi-domain liquid crystal display includes a first and a second substrates, and a liquid crystal layer is interposed between the first and the second substrates. A first common electrode is formed on an entire surface of the first substrate. A first dielectric layer is formed on the second substrate and covers first signal lines, and a second dielectric layer is formed on the first dielectric layer and covers second signal lines. A plurality of pixel electrodes are formed on the second dielectric layer, and a plurality of second common electrodes are formed on the second substrate, where a voltage difference existing between the second common electrodes and the pixel electrode produces fringe fields.

Description:
BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The invention relates to a multi-domain liquid crystal display, particular to a multi-domain liquid crystal display where fringe fields are produced to regulate the orientation of liquid crystal molecules. 
   (b) Description of the Related Art 
   Typically, the display contrast ratio and response time offered by a VA (vertically-aligned) mode liquid crystal display, which uses negative liquid crystal materials and vertical alignment films, are better than a TN (twisted-nematic) mode LCD, since liquid crystal molecules are aligned in a vertical direction with zero voltage is applied. Also, it is known the viewing angle performance of a VA mode LCD is improved by setting the orientation directions of the liquid crystal molecules inside each pixel to a plurality of mutually different directions; that is, forming multiple independent domains in the liquid crystal display. 
     FIG. 1A  shows a schematic diagram illustrating a conventional design of a multi-domain vertically-aligned liquid crystal display (MVA LCD). Referring to  FIG. 1A , a top substrate  102  and a bottom substrate  104  are both provided with protrusions  106  having different inclined surfaces and covered by vertical alignment films  108 . Hence, the liquid crystal molecules  112  near the inclined surfaces orientate vertically to the inclined surfaces to have different degrees of pre-tilt angles. In case the pre-tilt liquid crystal molecules exist, surrounding liquid crystal molecules  112  are tilted in the directions of the pre-tilt liquid crystal molecules  112  when a voltage is applied. Thus, multiple domains each having individual orientation direction of liquid crystal molecules  112  are formed. Besides, the domain-regulating structure for providing inclined surfaces includes, but is not limited to, the protrusions  106 , and other structure such as a via structure  116  shown in  FIG. 1B  may also be used. 
   However, when one compares the optical path of light l 1  and that of light l 2  shown both in  FIGS. 1A and 1B , it is clearly found the pre-tilt liquid crystal molecules through which the light l 2  passes under a field-off state may result in a non-zero phase difference (.nd.0) to cause light leakage. Accordingly, additional compensation films must be provided to eliminate the light leakage. 
     FIG. 2  shows a schematic diagram illustrating another conventional design of an MVA LCD. Referring to  FIG. 2 , the transparent electrode  204  on the substrate  202  is provided with slits  206 . Because of the fringe fields produced at edges of transparent electrode  204  and at each slit  206 , the liquid crystal molecules  208  are tilted toward the center of each slit  206  to result in a multi-domain LCD cell. However, the strength of the fringe fields generated by the formation of the slits  206  is often insufficient, particularly when the widths and the intervals of the slits  206  are not optimized. Besides, since the azimuth in which the liquid crystal molecules tilt due to fringe fields includes all directions of 360 degrees, a disclination region  210  often appears beyond the slits  206  or between two adjacent slits  206  to result in a reduced light transmittance. 
   BRIEF SUMMARY OF THE INVENTION 
   Hence, an object of the invention is to provide a multi-domain liquid crystal display that allows for solving the problems of conventional designs as mentioned above. 
   According to the invention, a multi-domain liquid crystal display includes a first and a second substrates, and a liquid crystal layer having negative dielectric anisotropy is interposed between the first and the second substrates. A first common electrode is formed on an entire surface of the first substrate, and a plurality of first and second signal lines are provided on the second substrate, where two adjacent first signal lines are intersected with two adjacent second signal lines to define a pixel region. A plurality of switching devices are provided in the vicinity of intersections of the first and second signal lines. A first dielectric layer is formed on the second substrate and covers the first signal lines, and a second dielectric layer is formed on the first dielectric layer and covers the second signal lines. A plurality of pixel electrodes are formed on the second dielectric layer, and a plurality of second common electrodes are formed on the second substrate, where a voltage difference existing between the second common electrodes and the pixel electrode produces fringe fields. Further, each second common electrode may include multiple sections that define at least one enclosed region, with each enclosed region overlapping with the pixel electrode to regulate the orientation of liquid crystal molecules. 
   Through the design of the invention, a multi-domain profile of a liquid crystal cell is formed by means of common electrode sections defined from a Metal 1, Metal 2, or Metal 3 layer, which are formed accompanied by typical TFT fabrication processes to produce fringe fields. Thus, compared with the conventional design where a protrusion or a via structure is used to tilt liquid crystal molecules, the residue phase difference is eliminated to avoid light leakage. Further, compared with another conventional design where slits are formed to produce fringe fields, the biased electrode allows for stronger field strength to tilt liquid crystal molecules so as to reduce the areas of a disclination region and further increase the light-transmittance of an LCD. 
   Also, according to the invention, the Metal 1, Metal 2, or Metal 3 layer may be patterned to form the common electrode sections as well as a reflective layer under the same fabrication process, and thus the fabrication of a transflective multi-domain liquid crystal display is simplified. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a schematic diagram illustrating a conventional design of a multi-domain vertically-aligned liquid crystal display. 
       FIG. 1B  shows a schematic diagram illustrating another conventional design of a multi-domain vertically-aligned liquid crystal display. 
       FIG. 2  shows a schematic diagram illustrating another conventional design of a multi-domain vertically-aligned liquid crystal display. 
       FIG. 3  shows a cross-section illustrating a multi-domain liquid crystal display according to an embodiment of the invention. 
       FIG. 4  shows a plan view observed from the normal direction of an array substrate according to an embodiment of the invention. 
       FIG. 5A  shows a cross-sectional view taken along line A-A′ in  FIG. 4 , and  FIG. 5B  shows a cross-sectional view taken along line B-B′ in  FIG. 4 . 
       FIGS. 6A and 6B  show schematic diagrams illustrating the operation principle according to the invention. 
       FIG. 7  shows a simulation diagram illustrating the distribution of tilted liquid crystal molecules. 
       FIG. 8  shows a plan view illustrating the distribution of the common electrode sections according to another embodiment of the invention. 
       FIG. 9  shows a plan view illustrating the distribution of the common electrode sections according to another embodiment of the invention. 
       FIG. 10  shows a plan view illustrating the distribution of the common electrode sections according to another embodiment of the invention. 
       FIG. 11  shows a plan view illustrating the distribution of the common electrode sections according to another embodiment of the invention. 
       FIG. 12  shows a plan view illustrating a transflective pixel structure according to an embodiment of the invention. 
       FIG. 13  shows a cross-sectional view taken along line C-C′ in  FIG. 12 . 
       FIG. 14  shows a plan view illustrating a transflective pixel structure according to another embodiment of the invention. 
       FIG. 15  shows a cross-sectional view illustrating another embodiment of the invention. 
       FIG. 16  shows a cross-sectional view illustrating another embodiment of the invention. 
       FIG. 17  shows a plan view illustrating a transflective pixel structure according to another embodiment of the invention, and  FIG. 18  shows a cross-sectional view taken along line D-D′ in  FIG. 17 . 
       FIG. 19  shows a cross-sectional view illustrating another embodiment of the invention. 
       FIG. 20  shows a cross sectional-view illustrating the interconnection between two adjacent pixels according to the embodiment shown in  FIG. 19 . 
       FIG. 21  shows a plan view illustrating a transflective pixel structure according to another embodiment of the invention, and  FIG. 22  shows a cross-sectional view taken along line E-E′ in  FIG. 21 . 
       FIG. 23  shows a plan view illustrating another embodiment of the invention. 
       FIG. 24  shows a plan view illustrating another embodiment of the invention. 
       FIG. 25  shows a plan view illustrating another embodiment of the invention. 
       FIG. 26  shows a plan view illustrating another embodiment of the invention. 
       FIG. 27  shows a schematic diagram illustrating another embodiment of a multi-domain LCD according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a cross-section illustrating a multi-domain liquid crystal display according to an embodiment of the invention. Referring to  FIG. 3 , the multi-domain liquid crystal display  10  includes a color filter substrate  12  and an array substrate  14 , with a liquid crystal layer  16  having negative dielectric anisotropy interposed between them where the liquid crystal molecules are vertically-aligned without being applied with voltage. Further, an additive of chiral dopant may be added to the liquid crystal layer  16  to adjust the twist pitch to a desired value so as to reduce the areas of a disclination region. In the array substrate  14 , a switching device  20  such as a thin film transistor (TFT), a pixel electrode  22 , and a first alignment layer  24  are formed on a transparent substrate  18 . Further, in the color filter substrate  12 , a color filter  28 , a black matrix layer  30 , a common electrode  32 , and a second alignment layer  34  are formed on a transparent substrate  26 . 
   Note that, as used in this description and in the appended claims, the meaning of the phrase “layer A is formed or provided on layer B” is not limited to a direct contact between the layer A and the layer B. For instance, in an embodiment where laminates are interposed between the upper layer A and the lower layer B is encompassed within the scope of the phrase “layer A is formed or provided on layer B”. 
     FIG. 4  shows a plan view observed from the normal direction of an array substrate according to an embodiment of the invention.  FIG. 5A  shows a cross-sectional view taken along line A-A′ in  FIG. 4 , and  FIG. 5B  shows a cross-sectional view taken along line B-B′ in  FIG. 4 . 
   Referring to  FIG. 4 , a plurality of gate lines  44  are arranged extending in the lateral direction of a pixel  40 , and a plurality of data lines  46  are arranged extending in the lengthwise direction of the pixel  40 , with each two gate lines  44  intersected with each two data lines  46  to define a pixel region on the array substrate  14 . A pixel electrode  48  made of transparent conductive films is spread on each pixel region, and the transparent conductive films may be made from indium tin oxide (ITO) or indium zinc oxide (IZO). A switching device such as an amorphous silicon thin film transistor (a-Si TFT)  42  is formed in the vicinity of each intersection of the gate lines  44  and the data lines  46 . 
   Referring to  FIG. 5A , a Metal 1 layer M 1  made from Cr, Ta, or Al/Mo metallic films is deposited on the transparent substrate  18  and patterned to define the gate lines  44  and the gate  42   g  of the a-Si TFT  42 . A dielectric gate insulation layer  52  is formed overlying the Metal 1 layer M 1 . For example, the gate insulation layer  52  may be formed by depositing silicon nitride (SiNx) on the Metal 1 layer M 1  through chemical vapor deposition. A channel region  42   c  (pure amorphous silicon (a-Si:H)), an ohmic contact layer  42   e  (doped amorphous silicon (n+ a-Si:H)) and a Metal 2 layer M 2  are formed on the gate insulation layer  52 . Specifically, the Metal 2 layer M 2  made from Al/Cr, Al/Ti, Ti, or Mo/Al/Mo metallic films is sputtered on the gate insulation layer  52  and patterned to define the source  42   s  and the drain  42   d  of the a-Si TFT  42  and the data lines  46 . The source  42   s  and the drain  42   d  of the a-Si TFT  42  are provided at two sides of the channel region  42   c.    
   A dielectric passivation layer  54  is formed overlying the gate insulation layer  52  and the Metal 2 layer M 2  to cover the source  42   s  and the drain  42   d  of the a-Si TFT  42  and the data lines  46 . The passivation layer  54  may be made from silicon nitride (SiNx), acrylic resin, or polyimide. Then, transparent conductive films made from indium tin oxide (ITO) or indium zinc oxide (IZO) are deposited on the passivation layer  54  and patterned to form the pixel electrode  48 . The gate  42   g , the source  42   s  and the drain  42   d  of the a-Si TFT  42  are respectively connected to the gate lines  44 , the data lines  46 , and the pixel electrode  48 . 
   When the above typical TFT fabrication processes finish, according to the invention, another dielectric layer  56  and a Metal 3 layer M 3  are sequentially formed on the pixel electrode  48 , as shown in  FIG. 5B . The dielectric layer  56  may be made from silicon nitride (SiNx), acrylic resin, or polyimide. The Metal 3 layer M 3  is deposited on the dielectric layer  56  and wired up to the common electrode  32  on the color filer substrate (shown in  FIG. 3 ), so that the Metal 3 layer M 3  is provided with a voltage potential Vcom and thus functions as another common electrode on the array substrate  14 . Further, the Metal 3 layer M 3  may be made of transparent conductive films such as ITO and IZO, or made of metallic conductive films such as Al/Nd or Al/Mo. 
   The distribution of the Metal 3 layer M 3  on the dielectric layer  56  and its accompanying effect are described below. 
   First, as indicated by the hatched lines shown in  FIG. 4 , the Metal 3 layer M 3  includes multiple strip-shaped sections that extend parallel to the gate lines  44  (such as sections M 3   a  and M 3   b ) or parallel to the data lines  46  (such as sections M 3   c  and M 3   d ), and all strip-shaped sections define multiple rectangular enclosed regions  62 , such as three rectangular enclosed regions  62  shown in  FIG. 4 . Since each enclosed region  62  overlaps with the pixel electrode  48 , a voltage difference exists between each common electrode section M 3   a , M 3   b , M 3   c  or M 3   d  and the pixel electrode  48  produces fringe fields. 
   The operation principle about how the Metal 3 layer M 3  (common electrode) induces fringe fields to tilt liquid crystal molecules is described below with reference to  FIGS. 6A and 6B . 
   Referring to  FIG. 6A , when no voltage is applied across a common electrode  32  and the pixel electrode  48 , the liquid crystal molecules  64  with negative dielectric anisotropy are naturally vertically-aligned. Then, when a voltage is applied across the common electrode  32  and the pixel electrode  48  for a period, fringe fields are produced due to the voltage difference between the Metal 3 layer (having a voltage potential Vcom) and the pixel electrode  48  (having a voltage potential Vpixel). Thus, the liquid crystal molecules  64  are directed to a direction perpendicular to the oblique electric field as indicated in  FIG. 6B . In that case, since each enclosed region are defined by four common electrode sections M 3   a , M 3   b , M 3   c , and M 3   d , the orientation of liquid crystal molecules  64  within one pixel is divided into four tilt directions in relation to the four common electrode sections to obtain a four-domain profile of a liquid crystal cell. 
     FIG. 7  shows a simulation diagram illustrating the distribution of tilted liquid crystal molecules. Referring to  FIG. 7 , it can be clearly seen the liquid crystal molecules  64  spread in the two sides of the pixel electrode  48  are slanted toward the center of the pixel electrode  48  due to the voltage difference between the Metal 3 layer M 3  and the pixel electrode  48 . 
   According to this embodiment, a multi-domain profile of a liquid crystal cell is formed by means of common electrode sections of the Metal 3 layer M 3 , which are formed accompanied by typical TFT fabrication processes to produce fringe fields. Thus, compared with the conventional design where a protrusion or a via structure is used to tilt liquid crystal molecules, the residue phase difference is eliminated to avoid light leakage. Further, compared with another conventional design where slits are formed to produce fringe fields, the biased electrode allows for stronger field strength to tilt liquid crystal molecules so as to reduce the areas of a disclination region and further increase the light-transmittance of an LCD. 
   Referring again to  FIG. 4 , though each pixel region is divided into three enclosed regions  62  each surrounded by four common electrode sections M 3   a -M 3   d , this division is not limited. In an alternate embodiment, each pixel region may be divided by the Metal 3 layer M 3  into two enclosed regions  62 , as shown in  FIG. 8 . Alternatively, each pixel region may be divided into four or six enclosed regions  62  arranged in two columns, as shown in  FIG. 9  and  FIG. 10 . Though the response time of liquid crystal molecules is reduced as the number of the enclosed regions  62  in each pixel region is increased, such division is not limited and is determined according to the actual demand. 
   Further, the relative positions of the Metal 3 layer M 3  and the pixel electrode  48  are not limited as long as sufficient field strength is provided. In one embodiment, the periphery portions of the Metal 3 layer M 3  are outside the projection of the pixel electrode  48 , as shown in  FIG. 4 . In an alternate embodiment, the periphery portions of the pixel electrode  48  are outside the projection of the Metal 3 layer M 3 , as shown in  FIG. 11 . 
   Moreover, according to the design of the invention, the overlapped portions between the Metal 3 layer M 3  and the pixel electrode  48  also form a storage capacitor Cst, with the dielectric layer  56  interposed between them. 
     FIG. 12  shows a plan view illustrating a transflective pixel structure  60  according to another embodiment of the invention, and  FIG. 13  is a cross-sectional view taken along line C-C′ in  FIG. 12 . According to this embodiment, the Metal 3 layer M 3  that cooperates with the pixel electrode  48  to produce fringe fields is made of metallic materials having high reflectivity. As shown in  FIG. 12  and  FIG. 13 , the reflective Metal 3 layer M 3  is patterned to form multiple common electrode sections used to produce fringe fields and a reflective layer  58 . The reflective layer  58 , which constitutes the reflective region of the transflective pixel structure  60 , is surrounded by the common electrode sections and maintains a gap apart from them. Certainly, the area of the reflective layer  58  may be arbitrary selected depending on any factor such as environmental brightness. For example, as shown in  FIG. 14 , in case the Metal 3 layer M 3  divides a pixel region into three rectangular enclosed regions  62 , the reflective layer  58  may spread within only one enclosed region  62  when the area of the transmissive region is required to be larger than that of the reflective region. In comparison, when the area of the reflective region is required to be larger than that of the transmissive region, the metal layer  3  may spread within two enclosed regions  62 . Hence, according to the invention, since the Metal 3 layer M 3  are patterned to form both the common electrode sections M 3   a -M 3   d  and the reflective layer  58  under the same fabrication process, the fabrication of a transflective liquid crystal display is simplified. 
     FIG. 15  shows a cross-sectional view illustrating another embodiment of the invention. Referring to  FIG. 15 , during the fabrication processes of a multi-domain LCD, a flattened dielectric layer  66  is additionally formed on the passivation layer  54 , and the pixel electrode  48  is formed on the flattened dielectric layer  66 . Hence, the formation level of the pixel electrode  48  is raised to allow for more spread areas and thus to improve the aperture ratio of a multi-domain LCD. 
     FIG. 16  shows a cross-sectional view illustrating another embodiment of the invention. Referring to  FIG. 16 , a Metal 2 layer M 2  is deposited on the gate insulation layer  52 , and the Metal 2 layer M 2  is patterned to define data lines  46  and a common electrode  68 . The common electrode  68  may include multiple sections having a distribution identical to the Metal 3 layer M 3  shown in  FIG. 4  to produce fringe fields. Also, the overlapped portions between the common electrode  68  and the pixel electrode  48  also form a storage capacitor Cst. 
     FIG. 17  shows a plan view illustrating a transflective pixel structure according to another embodiment of the invention, and  FIG. 18  shows a cross-sectional view taken along line D-D′ in  FIG. 17 . As shown in both  FIG. 17  and  FIG. 18 , in this embodiment, the Metal 2 layer is made of metallic materials having high reflectivity and patterned to define both the common electrode  68  and a reflective layer  58 , with the reflective layer  58  maintaining a gap apart from the common electrode  68  and constituting the reflective region of a transflective liquid crystal display. 
     FIG. 19  shows a cross-sectional view illustrating another embodiment of the invention. Referring to  FIG. 19 , a Metal 1 layer M 1  is deposited on a transparent substrate  18  and patterned to define the gate lines  44  (not shown) and a common electrode  72 . The common electrode  72  may include multiple sections having a distribution identical to the Metal  3  layer M 3  shown in  FIG. 4  to produce fringe fields. Also, the overlapped portions between the common electrode  72  and the pixel electrode  48  form a storage capacitor Cst.  FIG. 20  shows a cross sectional-view illustrating the interconnection between two adjacent pixels according to the embodiment shown in  FIG. 19 . Referring to  FIG. 20 , a portion of the gate insulation layer  52  on the common electrode  72  in a first pixel is removed to form a fist contact hole  70 , and a portion of the gate insulation layer  52  on the common electrode  72 ′ in a second pixel adjacent to the first pixel is removed to form another contact hole  70 ′. The contact holes  70  and  70 ′ are connected with each other through a patterned Metal 2 layer M 2 . 
     FIG. 21  shows a plan view illustrating a transflective pixel structure according to another embodiment of the invention, and  FIG. 22  shows a cross-sectional view taken along line E-E′ in  FIG. 21 . In this embodiment, the Metal  1  layer is made of metallic materials having high reflectivity and patterned to define both the common electrode  72  and a reflective layer  58 , with the reflective layer  58  maintaining a gap apart from the common electrode  72  and constituting the reflective region of a transflective liquid crystal display. 
     FIG. 23  shows a plan view illustrating another embodiment of the invention. Referring to  FIG. 23 , except a Metal  1  layer is patterned to define multiple common electrode sections to produce fringe fields, the pixel electrode  48  is also provided with slits  74  to enhance the field strength for tilting the liquid crystal molecules within selected regions so as to further reduce the areas of a disclination region. Certainly, the common electrode sections may be formed from a Metal  2  layer under the condition that the slits  74  are provided. 
   Further, the shape and location of the slits are not limited. Preferably, the slits  74  may be strip-shaped and substantially parallel to the common electrode sections. For example, as shown in  FIG. 24 , the pixel electrode  48  may be provided with both slits  74   a  overlapping the common electrode sections and slits  74   b  not overlapping the common electrode sections. Moreover, under the condition the common electrode sections together with the slits are both formed to produce fringe fields, the distribution of the common electrode sections is also not limited. For example, each pixel region may be divided into two or three enclosed regions  62  as shown in  FIGS. 24 and 25 , or alternatively, each pixel region may be divided into six enclosed regions  62  arranged in two columns, as shown in  FIG. 26 . 
   Besides, referring to  FIG. 27 , a polarizer  76   a  is positioned next to the transparent substrate  26  and opposite to the liquid crystal layer, and a polarizer  76   b  is positioned next to the transparent substrate  18  and opposite to the liquid crystal layer. A pair of quarter wavelength plates  78   a  and  78   b  are respectively provided between the transparent substrate  26  and the polarizer  76   a  and between the transparent substrate  18  and the polarizer  76   b , so that a linear polarized liquid crystal cell is transformed into a circular polarized liquid crystal cell to improve the light transmittance of a multi-domain LCD. 
   While the invention has been described by way of examples and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. For example, the gate lines  44  may be defined from the Metal 2 layer, and the data lines  46  may be defined from the Metal 1 layer. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.