Abstract:
Novel structural configurations of a TFT-LCD (Thin Film Transistor Liquid Crystal Display) which results in both fast response to input data and provides wide-viewing-angles. The structure of the device is comprised of one pixel electrode layer and two common electrode layers. The structure of the invention can be used with liquid crystal display television (LCD-TV) monitors that require both fast-response as well as wide-viewing-angle. In addition, other liquid crystal technologies which require high speed response would benefit from the TFT-LCD of the present invention.

Description:
This invention claims the benefit of priority to U.S. Provisional Application No. 60/405,999 filed Aug. 26, 2002. 
    
    
     FIELD OF INVENTION 
     This invention relates to displays, in particular to methods and apparatus for providing a TFT-LCD (Thin Film Transistor Liquid Crystal Display) having fast responses to high input data rates and allowing for wide viewing angles for viewers. 
     BACKGROUND AND PRIOR ART 
     Slow response time and poor viewing angle of the conventional TFT-LCD are two of the major limitations for its otherwise potentially unlimited wide range of applications. 
       FIG. 1  shows the structure of a conventional TFT-LCD. A liquid crystal (LC) layer  10  is sandwiched between a top glass substrate  11  and a bottom glass substrate  12 . A thin layer of transparent electrode, indium tin oxide (ITO), is coated on each substrate for the application of an electric field to switch the liquid crystals. Normally, the electrode  13  for the top substrate  11  is a common electrode, which has a constant voltage (e.g. 0V). 0V is used herein to mean low voltage. This common electrode is continuous and extends to all pixels in the whole display and hence is called a common electrode. On the other hand the electrode  14  on the bottom substrate  12  is called the pixel electrode since a transistor assigned to each individual pixel controls it. The voltage applied to the LC  10  is varied through this electrode. The electric field profile E is also shown in  FIG. 1 , when the pixel voltage is &gt;0. As can be seen clearly there is only one type of electric field, a vertical field, for this device. This electric field is used to turn on the device by switching the liquid crystal molecules, which is a fast process. However, when we turn off the device, the pixel voltage is either removed or reduced such that the molecules gradually relax back to a lower state. With only one type of electric field generated, this results in a very slow relaxation and hence slow turn off time, and is a major limitation of liquid crystals for many potential applications today. 
     Discussed below are various relevant prior art references. The references are related to three key concepts used in the present invention: crossed-field effect, fringing field switching and multi-domain technology. 
     The concept of crossed-field effect first appeared in 1975 in an article by D. J. Channin,  Applied Physics Letters&#39; , Vol. 26, No. 11, p. 603 (1975) and in a subsequent article by D. J. Channin and D. E Carlson,  Applied Physics Letters&#39; , Vol. 28, No. 6 (1976). Six years later, an article was published by Akihiko Sugimura et al.,  Proceedings of  14 th    Conference on Solid State Devices , Tokyo (1982) and in 1985 Akihiko Sugimura and Takao Kawamura published another article in,  Japanese Journal of Applied Physics , Vol. 24, No. 8, p. 905 (1985). The liquid crystal displays employing the crossed-field effect have various disadvantages, such as, high voltage requirement, low contrast, more complicated structure, non-uniform transmission, and more complicated driving. Driving refers to the electronic circuits used to supply (or drive) the required voltages (data or common) to the TFT-LCD. Some driving schemes are more complicated, e.g, requiring different types of voltage at different time intervals. In the crossed field effect, it normally requires the control of two types of electric field (both vertical and lateral) using extra electrodes and hence more complicated driving. The crossed-field effect concept has therefore not been used for TFT-LCDs since it tends to require much higher operation voltage, more complicated structure and driving and have lower contrast. The present invention however improves many of the above problems by using different electrode designs; thus, making the crossed-field effect possible for use in TFT-LCDs. Moreover, the use of the crossed-field effect in the present invention also provides the inherent wide-viewing-angle property, which is another very important requirement for TFT-LCD television sets (TVs). 
     Prior art research on fringing-field switching (FFS) has been published by Seung Ho Hong et al.,  Japanese Journal of Applied Physics , “Hybrid Aligned Fringing Field” Vol. 40, p.L272, (2001) and Seung Ho Hong et al.,  Japanese Journal, Applied Physics , Vol 41, pp. 4571-4576 (2001). The present invention adopts a structure that is very similar to the Fringing-Field-Switching FFS mode structure described by Seung Ho Hong et al. This mode was used for generating wide-viewing-angle using in-plane-switching with improved efficiency. By adopting this structure in the present invention, the required voltage can be reduced for generating the lateral or fringing field. The reduction in voltage is possible because the gap between electrodes for fringing field generation is small. Hence, the operating voltage is lowered. Moreover, the FFS structure can provide good uniform vertical field without a dead zone, which is defined as a gap between electrodes without an electric field. In the present invention, the gap between electrodes also has an electric field generated by a bottom substrate electrode configuration that consists of an electrode layer with gaps known as a discontinuous electrode separated from a continuous electrode layer by a electrical insulation layer. The segments of the discontinuous electrode are however all connected to the same transistor within a pixel. The bottom substrate electrode structure is similar to the structure of conventional FFS structure. 
     The present invention however has at least three important differences from the conventional FFS structure. First, the present invention has two common electrodes; whereas, the conventional FFS structure has only one common electrode of low voltage only. A recently reported FFS mode also used two common electrodes; however, in this case both common electrodes are of low voltage, e.g. 0V. In contrast, in the present invention, one common electrode is high voltage and one common electrode is low voltage. Second, the liquid crystal (LC) mode is different. Conventional FFS uses parallel alignment with in-plane-switching whereas the recently reported FFS mode with two common electrodes uses Hybrid-Aligned-Nematic (HAN). The present invention can use any liquid crystal mode and the wide-viewing-angle generation mechanism is also different compared with the FFS Prior Arts. Third, all prior art FFS structures have slow response time since they are not using the crossed-field effect and the turn-off process relies on natural relaxation of the LC molecules and is slow. 
     Further, prior art references relate to multi-domain technology LCDs. The present invention adopts a mechanism of forming wide-viewing-angle known as multi-domain. The present invention however has important differences from all the prior art using this technology since our invention uses FFS structure for generating the fringing field whereas other prior art references mainly use protrusions for generating multi-domains. See A. Takeda et al.,  SID &#39; 98, “MVA, Multi-Domain Vertical Alignment” p. 1077 (1998). An inter-digital structure for generating the fringing field to cause multi-domain was discussed by K. H. Kim et al.,  SID &#39; 98 p. 1085 (1998). Moreover, the present invention can use many different liquid crystal modes compared with mainly Vertical Alignment (VA) mode used in the prior art. 
     Thus, there is a need for improvement in today&#39;s thin-film transistor liquid crystal display (TFT-LCD) technology. It is desirable for crossed-field effect structures to have low operation voltage, high contrast, simple driving and easy fabrication. Faster response is desired for conventional structures using FFS or multi-domain LCDs. 
     The present invention affords a substantial improvement in the production and performance of TFT-LCDs. Different LC modes can be applied to this structure. Different LC modes can lead to different light efficiency, response time and viewing angle. The choice of the LC mode depends on the type of application. 
     SUMMARY OF THE INVENTION 
     The first objective of the present invention is to provide configurations and methods for using TFT-LCD (Thin Film Transistor Liquid Crystal Display) having fast response to high input data rate. 
     The second objective of the present invention is to provide configurations and methods for using TFT-LCD (Thin Film Transistor Liquid Crystal Display) having two common electrode, one of a lower voltage, e.g., 0V and one of a higher voltage, e.g., 5V and one pixel electrode so that both vertical and non-vertical fields can be generated to switch the liquid crystals at high rate. 
     The third objective of the present invention is to provide configurations and methods such that the crossed-field effect requires less voltage compared to conventional crossed-field devices and therefore can be applied to TFT-LCD (Thin Film Transistor Liquid Crystal Displays). 
     The fourth objective of the present invention is to provide configurations and methods such that the crossed-field effect allows simple driving scheme for use in TFT-LCD (Thin Film Transistor Liquid Crystal Display). 
     The fifth objective of the present invention is to provide configurations and methods such that the crossed-field effect has high contrast capability and simple fabrication process for the TFT-LCD (Thin Film Transistor Liquid Crystal Display) 
     The sixth objective of the present invention is to provide configurations and methods for using TFT-LCD (Thin Film Transistor Liquid Crystal Display) having wide viewing angles for viewers. 
     Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated, schematically in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a conventional prior art TFT-LCD structure with one common electrode. 
         FIG. 2  shows a preferred embodiment of the novel TFT-LCD structure with two common electrodes and one pixel electrode layer. 
         FIG. 3  shows the TFT-LCD structure of  FIG. 2  with uniform vertical field generated when the pixel voltage is 0V (dark state). 
         FIG. 4  shows the TFT-LCD structure of  FIG. 2  with a new electric field pattern generated when the pixel voltage is 5V (bright state). 
         FIG. 5  shows a second preferred embodiment of the novel TFT-LCD structure. 
         FIG. 6  shows the TFT-LCD structure of  FIG. 5  with power being applied to different electrode layers. 
         FIG. 7  shows a third embodiment using a resistive film with two common electrode. 
         FIG. 8  shows a fourth embodiment using a dielectric layer with the novel configuration of  FIG. 2 . 
         FIG. 9  shows the non-vertical electric field pattern generated with the novel structures. 
         FIG. 10  is a front view of the TFT-LCD structure shown in  FIG. 2 . 
         FIG. 11  is a front view of the TFT-LCD structure shown in  FIG. 5 . 
         FIG. 12  is a front view of the TFT-LCD structure shown in  FIG. 7 . 
         FIG. 13  is a front view of the TFT-LCD structure shown in  FIG. 9 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the disclosed embodiments of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     It should be noted that the present invention comprises a first substrate, a second substrate, a liquid crystal between the first and second substrates, a means for generating an electric field between electrode layers adjacent to both the first and second substrates. The unique features of the present invention are in the arrangement of electrode layers which are now described in detail. 
     There are common electrodes, which can be high voltage or low voltage, continuous or discontinuous, and the voltage applied does not depend on the input data during the operation of the TFT-LCD. There are pixel electrodes that can be continuous or discontinuous, and are the electrodes to which the voltage applied depends on the input data. A discontinuous electrode includes two or more adjacent finger-like extensions that are separated by a distance and connected together at one end. One common electrode can be located in the lower substrate or second substrate; in addition, a third electrode layer in the lower substrate can be discontinuous by design and collectively referred to as one layer, using one numerical reference in all figures presented herein. For example,  FIG. 4  and  FIG. 10  show a discontinuous common electrode layer  23  having three finger-like extensions separated by a distance and connected together at one end. 
     It was not obvious that the combination of two common electrodes with unequal voltages and one pixel electrode would provide a TFT-LCD having fast response and wide viewing angle when varying voltage is applied to the pixel electrode. The fast response is achieved when both the turn on and turn off modes of the device are driven by an electric field; with an electric field the LC molecules can both align and relax very quickly. 
       FIG. 2  shows cross sectional view of a novel design of the TFT-LCD structure of the present invention and Figure LO shows a front view of the structure shown in  FIG. 2 . A major novel feature of this design is that instead of just having one common electrode, there arc two common electrodes  21 ,  23 , one of a lower voltage, e.g. 0V and one of a higher voltage, e.g. 5V. In  FIG. 3 , the first common electrode layer  21  in the top substrate  22  has a constant high voltage of 5V whereas the second common electrode layer  23  in the bottom substrate  24  has a lower voltage of 0V. Common electrode  23  is separated from the pixel electrode  25  by a passivation layer  26 , which is an electrical insulation layer. When a low voltage of 0V is applied to pixel electrode  25 , a uniform vertical field  30  is generated as shown in  FIG. 3 . This uniform vertical field generated when the pixel voltage is 0V usually leads to a dark state and has very fast switching since it is electric-field driven. This is similar to the fast switching produced by the vertical Field generated in the conventional TFT-LCD devices. 
     In  FIG. 4 , when the pixel voltage is 5V there is a bright state. Voltage of 5V is applied to pixel electrode  25 ; a new electric field pattern  40  is established quickly due to the fringing field shown in  FIG. 4 . The continuous common electrode  21  in the top substrate has a voltage of 5V, while the discontinuous common electrode layer  23  in the bottom substrate has a voltage of 0V and the pixel electrode  25  has a voltage of 5V, as mentioned earlier. This leads to a new liquid crystal alignment state with different optical transmission, usually a bright state. The switching speed to this new state is also fast since it is driven by the electric field. Therefore, this new structure of TFT-LCD design leads to both fast turn-on and turn-off speeds since they both are electric-field driven. 
     EXAMPLE 1 
     Lower Voltage for One Common Electrode 
     The voltage for common electrode  21  in  FIG. 2  is 5V; this voltage can be made lower in order to reduce the vertical electric field strength and hence strengthen the lateral field. This can help improve the light efficiency since the lateral field becomes stronger and more molecules are switched to the bright state. This will however increase the corresponding response time for the bright-to-dark state due to the formation of a weaker vertical field. The remaining voltage readings are for common electrode  23  on bottom substrate  24 , V=0V; for pixel electrode  25  V=0 to 5V. Common electrode  23  is electrically insulated from the pixel electrode  25  by a passivation layer  26 . 
     EXAMPLE 2 
     Higher Voltage to Bottom Electrode 
     In  FIG. 2 , common electrode  21  has higher voltage; whereas, common electrode  23  has lower voltage and in principle these two electrodes can be interchanged. This interchange is shown in  FIG. 5  which is a cross section view of the TFT-LCD structure and  FIG. 11  which shows a front view of the structure shown in  FIG. 5 . In  FIG. 5 , the first common electrode  51  on top substrate  52  has a lower voltage (0V); whereas the second common electrode layer  53  in the bottom substrate  54  has a high voltage (5V). This alternative design may lead to a less uniform vertical field because of the slightly higher potential difference that is caused by a passivation layer  56 . In  FIG. 5 , a high electric field is emitted from pixel electrode  55  and hence a higher electric field is established across the passivation layer  56  than when the electric field is emitted from the top electrode  51 . It should be noted, that the terminology “passivation layer” in the description of the present invention, is commonly known as an insulation layer. However, the potential difference that is established between pixel electrode  55  and second common electrode layer  53  can principle be reduced by altering the voltage to second common electrode  53  or voltage to the pixel electrode  55  in order to compensate for the voltage drop. 
     EXAMPLE 3 
     Common Electrode and Pixel Electrode Interchanged 
     A first common electrode layer  60  with 5V is in top substrate  61  as shown in  FIG. 6 . The bottom substrate  63  supports a pixel electrode layer  62  with 0 to 5V and a second common electrode layer  64  with 0V. The pixel electrode layer  62  is electrically insulated from the common electrode  64  by a passivation layer  65 . The positions of common electrode  64  and the pixel electrode  62  in this configuration are interchanged compared with the configuration shown in  FIG. 2 . The choice of the configuration depends on the fabrication process capability and the optimized electrode width and gap. 
     EXAMPLE 4 
     Use of Resistive Film 
     In order to extend the distance of the lateral field, a resistive film  70  can be used to connect the pixel electrode and second common electrode in the bottom substrate as shown in  FIG. 7 . A potential gradient is developed between the pixel electrode  72  and second common electrode  71  across the resistive film when the pixel voltage is high. A lateral field is established between the pixel electrode  72  and the second common electrode  71  to switch the LC molecules during the bright state. The first common electrode  74  in the upper substrate has a high voltage of e.g. 5V. This voltage can, however, be reduced to e.g. 2V in order to increase the lateral field strength. On the other hand, when the voltage of pixel electrode  72  is the same as the voltage in common electrode  71 , then there will be no potential difference across the resistive film and a constant uniform potential appears across the film, due to conductive electrons.  FIG. 7  illustrates that there can be a horizontal electric field generated between pixel electrode  72  and second common electrode  71  which results in a longer lateral fringing field and higher efficiency for the bright state. 
     EXAMPLE 5 
     Use of Dielectric Layer 
     As shown in  FIG. 8 , when common electrode layer  80  of the top substrate  81  is 0V, the use of a dielectric layer  82  between common electrode layer  80  and LC layer  83  can increase the lateral field strength in the upper section of the LC cell since the 0V in top substrate is now further away from the bottom electric field. The dielectric layer  82  is adjacent to the common electrode layer  80 . The dielectric layer functions to keep the cell gap small and help make the lateral fringing field stronger because the upper electrode is figuratively further away. The fringing field formed between common electrode layer  84  and pixel electrode layer  85  becomes stronger, thus improving the light efficiency. 
     EXAMPLE 6 
     Natural Wide-Viewing-Angle Formation 
       FIG. 9  is a cross sectional view showing that the fringing field leads to the formation of multi-domains which are symmetrical about the mid-plane of the gap  90 ,  91  between the second common electrode layer  92 .  FIG. 13  is a front view of the of the structure shown in  FIG. 9 . This multi-domain formation will lead to wide viewing angle in two directions, left-right or up-down. It can lead to wide-viewing-angle in all four directions by adopting a zig-zig eletrode structure known as the Multi-domain Vertical Alignment (MVA).  FIG. 9  shows the natural formation of multi-domains due to the symmetrical fringing pattern when the first common electrode layer  93  in the top substrate  94  has 5V and the second discontinuous common electrode layer  92  in the bottom substrate  95  has 0V and the voltage in the pixel electrode  96  is 5V.  FIG. 9  is the same configuration as  FIG. 4 , with the added illustration of how the fringing field allows different poses for the LC molecule resulting in a naturally wide viewing angle. 
     The detailed description, examples and simulation results of the invention provide a means for advancing the knowledge and development of thin film transistor liquid crystal display technology. The novel features of this invention, include, but are not limited to, employing crossed-field effect in TFT-LCD; combining the crossed-field effect with wide-viewing-angle for faster response and wider viewing angle; using a two common-electrode structure of both high and low voltages; using a novel structure for generating crossed-field effect; using a novel structure for generating multi-domain LCDs. 
     While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.