Patent Publication Number: US-2005128390-A1

Title: Transflective fringe field switching liquid crystal display

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to liquid crystal displays, and especially to a transflective fringe field switching liquid crystal display (FFS LCD).  
      2. Description of the Prior Art  
      Recently, liquid crystal displays have become widely used in computer and communication products such as notebooks, cell phones and personal digital assistants. This is largely due to the thinness, lightness, and low power consumption of liquid crystal displays. Usually a liquid crystal display needs a planar light source, such as a backlight module, to display images. The backlight module is the main power consuming component of the liquid crystal display. In order to reduce power consumption, reflective type liquid crystal displays have been developed. A reflective liquid crystal display uses natural light beams to provide a planar light source. However, conventional reflective liquid crystal displays have some limitations; for example, a long response time and a narrow view angle.  
      To resolve the above-mentioned problems, a reflective fringe field switching liquid crystal display (FFS LCD) is described in U.S. Pat. No. 6,583,842 issued on Jun. 24, 2003. As represented in  FIG. 4 , the FFS LCD  1  includes a first substrate  10 , a second substrate  12 , and a liquid crystal layer  11  interposed between the substrates  10 ,  12 .  
      The first substrate  10  comprises a first glass sheet  101  and a first alignment film  102 . The first alignment film  102  is adhered on one surface (not labeled) of the first glass sheet  101 , the surface facing the liquid crystal layer  11 .  
      The second substrate  12  comprises a second glass sheet  121 , a common electrode  122 , an insulating layer  123 , a plurality of pixel electrodes  124 , and a second alignment film  125 . The second glass sheet  121 , the common electrode  122 , and the insulating layer  123  are stacked from bottom to top in the order. The pixel electrodes  124  are formed on the insulating layer  123 , and are spaced apart from and parallel to each other. The common electrode  122  is uniformly formed on the second glass sheet  121 , and is made of a high reflectivity metal such as aluminum. Therefore, the common electrode  122  functions as both an electrically conductive electrode and a reflector.  
      The reflective FFS LCD  1  can efficiently use natural light beams, due to the reflection of the common electrode  122 . Thus power consumption is reduced. Also, the common electrode  122  and the pixel electrodes  124  are both formed on the second substrate  12 , which provides a dense fringe electric field parallel to the second substrate  12 . The fringe electric field yields a fast response time and a wide view angle.  
      However, when the ambient environment is dark, the reflection of ambient light by the common electrode  122  is limited. The visibility of the reflective FFS LCD display  1  is poor. Conversely, a transmission type liquid crystal display is disadvantageous when the ambient environment is bright.  
      An improved liquid crystal display which overcomes the above-mentioned problems and shortcomings is desired.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a liquid crystal display which can be used not only in a bright environment but also in a dark environment, and which has a wide viewing angle.  
      To achieve the above object, a liquid crystal display of the present invention comprises: a first substrate; a second substrate; a liquid crystal layer between the first substrate and the second substrate; and a plurality of pixel regions each defined by respective pixel electrodes and a common electrode, for application of a voltage to the liquid crystal layer and formation of a fringe electric field at each pixel region. Each pixel region includes a transmissive region and a reflective region. The liquid crystal display can effectively use light beams from the outside environment and from a backlight module. Therefore the liquid crystal display can be used not only in bright conditions, but also in dark conditions.  
      Other objects, advantages and novel features of the present invention will be apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic, cross-sectional view of one sub-pixel area of a transflective FFS LCD according to a first embodiment of the present invention;  
       FIG. 2  is a schematic, cross-sectional view of one sub-pixel area of a transflective FFS LCD according to a second embodiment of the present invention;  
       FIG. 3  is a schematic, cross-sectional view of one sub-pixel area of a transflective FFS LCD according to a third embodiment of the present invention; and  
       FIG. 4  is a schematic, isometric view of one sub-pixel area of a conventional FFS LCD. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
       FIG. 1  is a view of one sub-pixel area of a transflective FFS LCD  3  according to the first embodiment of the present invention. The transflective FFS LCD  3  includes a first substrate  30 , a second substrate  32 , a liquid crystal layer  31 , and a backlight module (not shown). The liquid crystal layer  31  is interposed between the first substrate  30  and the second substrate  32 , and the backlight module is located below the second substrate  32 .  
      The first substrate  30  comprises a first glass sheet  301 , and a first alignment film  302  covering the first glass sheet  301  and facing the liquid crystal layer  31 . The second substrate  32  comprises a second glass sheet  321 , a common electrode  350 , an insulating layer  323 , a plurality of pixel electrodes  324 , and a second alignment film  325 . The second glass sheet  321 , the common electrode  350 , the insulating layer  323 , and the pixel electrodes  324  are stacked from bottom to top in the order. A plurality of pixel regions is defined by the pixel electrodes  324  and the common electrode  350 . Each of the pixel regions includes a transmissive region T and a reflective region R. Light reflected in the reflective region R and light transmitted through the transmissive region T is utilized in displaying an image.  
      The pixel electrodes  324  are transparent strip electrodes, and are spaced apart from and parallel to each other. The common electrode  350  has a transmissive area  351  according to the corresponding transmissive region T, and a reflective area  353  according to the corresponding reflective region R. The common electrode  350  is made of an aluminum film, and a transmission ratio of the aluminum film depends on a thickness thereof. When the thickness is equal to 100 nanometers, the transmission ratio is 1%. If the thickness is decreased, the transmission ratio increases proportionately, and vice versa. Therefore, the thickness of the reflective area  353  is defined as being more than 100 nanometers, and the thickness of the transmissive area  351  is defined as being less than 100 nanometers. Accordingly, the reflective area  353  can reflect natural light beams from the outside environment, and light beams  33  from the backlight module can pass through the transmissive area  351 . In other words, the transflective FFS LCD  3  can be used in a dark environment and also in a bright environment.  
      The transmissive area  351  of the aluminum film has a higher impedance than the reflective area  353 , because the thickness of the transmissive area  351  is less than that of the reflective area  353 . In order to decrease the impedance of the transmissive area  351 , an indium tin oxide (ITO) film (not shown) is attached to one surface thereof.  
       FIG. 2  is a view of one sub-pixel area of a transflective FFS LCD  4  according to the second embodiment of the present invention. Unlike the transflective FFS LCD  3 , the transflective FFS LCD  4  has a common electrode  450 , and the common electrode  450  includes a reflective area  453  and a transmissive area  451 . The reflective area  453  is made of a metal film; for example, an aluminum film. A thickness of the aluminum film is more than 100 nanometers. The transmissive area  451  is made of an indium tin oxide (ITO) film or an indium zinc oxide (IZO) film. Both the indium tin oxide film and the indium zinc oxide film are transparent.  
       FIG. 3  is a view of one sub-pixel area of a transflective FFS LCD  5  according to the third embodiment of the present invention. A common electrode  550  includes a transparent conductive film  552 , and a transflective film  551  covering the transparent conductive film  552 . The transflective film  551  has a multi-layer construction, and commonly comprises seven to nine layers. In particular, the transflective film  551  comprises a plurality of layers of different transparent materials stacked one on the other in alternate fashion. The layers are typically indium tin oxide (ITO) films and indium zinc oxide (IZO) films. The refractive ratio and thickness of each of the layers can be configured according to need, and the number of layers can also be configured according to need. In this way, the transflective film  551  having a desired transmission ratio and a desired reflective ratio can be obtained.  
      The transflective FFS LCDs  3 ,  4 ,  5  can effectively use light beams from the outside environment and from respective backlight modules. Therefore the transflective FFS LCDs  3 ,  4 ,  5  can be used not only in bright conditions, but also in dark conditions. In addition, in a further embodiment, the common electrode and the pixel electrodes can be used to form a transflective element. That is, the common electrode is a transparent film, and the pixel electrodes are reflective elements. In particular, the common electrode can be an indium tin oxide film or an indium zinc oxide film. The pixel electrodes can be made of a metal, such as aluminum.  
      While the present invention has been described with reference to particular embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Therefore, various modifications of the described embodiments can be made by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.