Abstract:
A color electrophoretic display including a plurality of cells each containing electrophoretic particles. Each of the cells in the plurality is capable of displaying at least one of three selected primary colors, when the particles in the cell are moved from a first rest position to a second display position on the cell. An electrode is coupled to each of the cells and is operative when biased to move the particles from the first rest position to the second display position thereby displaying primary colors in the second display position and causing the display to provide full color capability according to particle position in the cells.

Description:
FIELD OF THE INVENTION 
   This invention relates to electrophoretic image displays and, more particularly, to a multi-color electrophoretic image display. 
   BACKGROUND OF THE INVENTION 
   The electrophoretic effect is well known in the art as evidenced by the many patents and articles which describe this effect. In essence, the electrophoretic effect operates on the principle that when certain particles are electrically charged to a particular polarity, the charged particles will migrate away from a surface charged to the same polarity as the particles and toward a surface charged to a polarity which opposite to that of the charged particles. For example, particles which are positively charged will migrate away from a positively charged surface and towards a negatively charged surface. 
   Display devices which utilize the electrophoretic effect are commonly known as electrophoretic image displays (EPIDs). EPIDs are very well known in the art. The following patents issued to Frank J. Disanto and Denis A. Krusos, and assigned to Copytele, Inc., the assignee herein, are illustrative of such EPIDs. 
   U.S. Pat. No. 4,655,897 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, describes an electrophoretic display apparatus comprising an XY matrix of grid and cathode lines, an anode electrode spaced from the grid and cathode matrix, and an electrophoretic dispersion. The patent describes techniques for making such displays as well as suitable dispersions for use with such displays. 
   U.S. Pat. No. 4,732,830 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, describes methods for making electrophoretic displays as well as describing display construction and operation. 
   U.S. Pat. No. 4,742,345 entitled ELECTROPHORETIC DISPLAY PANEL APPARATUS AND METHODS THEREFOR, describes improved electrophoretic display panels exhibiting improved alignment and contrast with circuitry for implementing the same as well as methods for providing such a panel. 
   U.S. Pat. No. 4,746,917 entitled METHOD AND APPARATUS FOR OPERATING ELECTROPHORETIC DISPLAYS BETWEEN A DISPLAY AND A NON-DISPLAY MODE, describes various biasing techniques for operating electrophoretic displays to provide writing, erasing as well as operating the display during a display and non-display mode. 
   U.S. Pat. No. 4,772,820 entitled MONOLITHIC FLAT PANEL DISPLAY APPARATUS, describes methods and apparatus for fabricating flat panel displays employing electrophoretic principles to enable such displays to be biased and driven by additional circuitry. 
   The electrophoretic fluids used in EPIDs typically comprise white, light, or dark colored dielectric particles which are suspended in an optically contrasting fluid medium which is either clear or dark-colored, depending upon the color of the particles. See U.S. Pat. No. 5,360,689 entitled COLORED POLYMERIC DIELECTRIC PARTICLES AND METHOD OF MANUFACTURE, issued to Hou et al., which describes black electrophoretic and light-colored electrophoretic particles formed from crystalline polymer particles using a dispersion polymerization technique. In accordance with the electrophoretic effect described above, the electrophoretic particles in the suspension liquids are caused to selectively migrate to, and impinge upon, a transparent screen electrode, thereby displacing the fluid medium from the screen and creating the desired image. 
   EPIDs have many advantages over other types of flat panel displays. One advantage is that EPIDs use materials which are relatively inexpensive and thus, makes them less costly to manufacture. Another advantage of EPIDs is that the image formed on the screen remains even when power is removed. When the electrophoretic particles or dye particles are caused to move to form an image, the image will not erase and remains on the display even upon removing of power. Hence the image must be erased in the same manner as it was created, by application to the device of an electric field of opposite polarity. Thus, EPIDs have a built-in memory in the sense that the images created by the displays do not have to be refreshed such as those images produced by CRT&#39;s and other types of displays. 
   One drawback associated with most prior art electrophoretic displays is that they are monochromatic. This drawback severely limits the number of applications where EPIDs can be employed. Accordingly, there is a need for a multi-color EPID which is capable of reliable operation and which is economical to fabricate. 
   SUMMARY OF THE INVENTION 
   A color electrophoretic display comprising a plurality of cells each containing electrophoretic particles. Each of the cells in the plurality is capable of displaying at least one of three selected primary colors, when the particles in the cell are moved from a first rest position to a second display position on the cell. An electrode is coupled to each of the cells and is operative when biased to move the particles from the first rest position to the second display position thereby displaying primary colors in the second display position and causing the display to provide full color capability according to particle position in the cells. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings, wherein: 
       FIG. 1A  is an exploded perspective view of a multi-color electrophoretic image display (EPID) according to a first embodiment of the invention; 
       FIG. 1B  is a front elevational view of the multi-color EPID illustrated in  FIG. 1A ; 
       FIG. 2A  is an elevational view of an anode used in the EPID of the invention illustrating anode lines formed on the inner surface of the anode; 
       FIG. 2B  is an elevational view of the anode illustrating a color filter array formed on the outer surface of the anode; 
       FIG. 2C  is an elevational view of the anode illustrating an alternate color filter array design formed on the outer surface of the anode; 
       FIG. 3A  is an elevational view of a cathode used in the EPID of the invention illustrating a two dimensional array of cells formed on the inner surface of the cathode; 
       FIG. 3B  is a perspective view of a segment of the cathode illustrating an integrated circuit for driving the pixel cells formed on the outer surface of the cathode; 
       FIG. 4  is a cross-sectional view through the EPID of the first embodiment of the invention; 
       FIGS. 5A and 5B  are cross-sectional views through the EPID of the first embodiment of the invention illustrating the operation thereof; 
       FIG. 6  is an elevational view illustrating the cathode of an EPID according to a second embodiment of the invention; 
       FIG. 7A  is a cross-sectional view illustrating an EPID according to a third embodiment of the invention; 
       FIG. 7B  is an elevational view illustrating the cathode of the EPID of  FIG. 7A ; 
       FIG. 7C  is an enlarged view of the cathode shown in  FIG. 7B ; 
       FIG. 8A  is a front elevational view of an EPID according to a fourth embodiment of the invention; 
       FIG. 8B  is a side elevational view of the EPID of the fourth embodiment of the invention; and 
       FIG. 8C  is an exploded view of the EPID of the fourth embodiment of the invention. 
   

   It should be understood that the drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B  collectively show a multi-color electrophoretic image display (EPID)  10  according to a first embodiment of the invention. The EPID  10  comprises a pair of parallel electrodes  11 ,  12  sealingly assembled together with spacers  13  to form a liquid and gas sealed enclosure having a small space S between the electrodes  11 ,  12  ( FIG. 4 ), and an electrophoretic fluid  14  filling the space S between the electrodes. The electrophoretic fluid  14  is conventional, comprising a dielectric liquid of a dark color, such as a blue or red, having suspended therein millions of polymer/pigment composite dielectric particles  16  (electrophoretic particles  16 ) of a light color, such as white or yellow, which can be charged in accordance with known techniques. The EPID  10  is typically rectangular in shape, although other geometrical configurations can be employed as well. 
   Electrode  11 , referred to hereinafter as anode  11 , is constructed from a generally planar sheet of transparent plastic or glass. As shown in  FIG. 2A , the anode  11  includes parallel rows of electrically conductive, transparent electrode or anode lines  18  on an inner surface  17  thereof. The anode lines  18  are typically fabricated by depositing a thin (about 300 Angstroms in thickness) transparent layer of conductive material, such as indium-tin-oxide (ITO), on the inner surface of the sheet and selectively etching the layer to form the anode lines. This can be accomplished using conventional thin-film deposition and etching techniques. 
   As shown in  FIG. 2B , a multi-color light filter array  20  is provided on an outer surface  19  of the anode  11 . The light filter array  20  can include a two dimensional array of red, blue, and green colored filters  21 . The filters  21  are typically fabricated using conventional printing or lamination techniques. Alternatively as shown in  FIG. 2C , the light filter array  21  can be constructed as alternating rows of red, blue, and green colored filters  22 . The filters  21  are typically colored plastic presenting primary colors red, green and blue. As is well known, such colors can provide all colors of the spectrum, as in conventional color displays. 
   Electrode  12 , referred to hereinafter as cathode  12 , is constructed from a generally planar sheet of plastic or glass. As shown in  FIG. 3A , the inner surface  23  of the cathode  12  defines a two dimensional array  24  of cells  25  which resembles an egg-crate structure. Each cell  25  of the array includes one or more side walls  26  (four side walls  26  are illustrated in the embodiment of  FIG. 3A ) which project generally perpendicularly from the inner surface  23  of the cathode  12 . The floor of each cell includes an electrode pad  27  formed by a coating of an electrically conductive material such as ITO. The electrode pads  27  can be deposited using conventional semiconductor deposition techniques. 
   As shown in the cross-sectional view of  FIG. 4 , each cell  25  of the array  24  is filled with a portion of the electrophoretic fluid  14  and a corresponding portion of the electrophoretic particles  16  dispersed therein, and is operative as one pixel cell for imaging. The cells  25  tend to isolate the electrophoretic particles  16  from each other, therefore, significantly improving the electrical, colloidal, operational, and life-time stability of the EPID  10 . Moreover, the cells  25  can be easily dimensioned to provide hundreds of pixels per inch, thereby enabling one to obtain extremely fine resolution, hence creating high resolution display capabilities which exceed the resolution of present commercially available display. 
   As shown in  FIG. 3B , an integrated circuit  30  for driving the pixel cells  25  is formed on an outer surface  29  of the cathode  12 . The drive circuit  30  is conventional in design and operation and includes a plurality of diode or transistor amplifiers  31  which are interconnected by electrically conductive lines  32  made for example from ITO. The drive circuit  30  can be fabricated on the outer surface  29  of the cathode  12  using well known integrated circuit manufacturing techniques. 
   As shown in the cross-sectional view of  FIG. 4 , an electrically conductive through-hole or via  33 , extends through the cathode and electrically connects the electrode pad  27  of each cell  25  to one of the wires  32  of the drive circuit  30  formed on the outer surface  29  of the cathode  12 , thereby permitting each cell  25  to be electrically driven. As one of ordinary skill in the art will recognize, by applying proper biasing potentials on the respective amplifiers  31 , a biasing potential is created between the anode and cathode  11 ,  12  which will cause the electrophoretic particles  16  in any cell  25  to move between the anode and the cathode  11 ,  12  in accordance with the electrophoretic effect. For example, if the electrophoretic particles  16  are initially disposed in their associated cells  25  of the cathode  12  (adjacent from corresponding positions on the anode lines  18 ) attracted there by their charge, which is opposite to the applied voltage, reversal of the sign of the applied voltage will cause these particles  16  to move to their corresponding positions on the anode lines  18  of the anode  11 . If the electrophoretic particles  16  are initially disposed on the anode lines  18  of the anode  11  (adjacent their associated cells  25 ) attracted there by their charge, which is opposite to the applied voltage, reversal of the sign of the applied voltage will cause these particles  16  to move to their associated cells  25  of the cathode  12 . 
   As shown in  FIGS. 5A and 5B , when the electrophoretic particles  16  within each cell  25  are electrically driven to a corresponding position on the anode lines  18  of the anode  11  where they remain, the particles  16  on the anode  11  generate a reflective surface thereunder that reflects incoming light passing through each cell&#39;s  25  respective color filter  21  to produce red, blue, and green light. By combining the appropriate number of cells  25  producing red, blue, and green light, a multi-color image can be produced including multi-color alpha numeric characters or graphics, such as television pictures. 
   Referring again to  FIG. 4 , the spacers  13  are sealed to the inner surfaces of the anode and cathode  11 ,  12  around the perimeter of the display using conventional sealing methods. The spacers  13  have a thickness T which is at least 1 mil thicker than the height. H of the cell walls  26  which creates a gap G between the inner surface  17  of the anode  11  and the free edges of the cell walls  26 . This gap G permits the electrophoretic fluid  14  to flow into and fill up each cell  25  of the cathode  12  when the EPID  10  is filled with the fluid  14 . 
   In a second embodiment of the EPID of the invention, the inner surface  23  of the cathode  12 , as shown in  FIG. 6 , defines parallel rows  51  of elongated cells instead of an egg-crate structure as in the first embodiment. Each elongated cell  51  operates as a line or row pixel. The light filter array (not shown) used in this embodiment can be constructed as described in  FIG. 2C  with alternating lines of red, blue, and green colored filters, each of which operates as a light filter for a corresponding one of the cells  51 . 
     FIGS. 7A-7C  collectively illustrate the EPID  70  according to a third embodiment of the invention. In this embodiment, the cathode  12  essentially omits the cell side walls which project from the inner surface  23  thereof in the previous embodiments. This allows the drive circuit  30 , TO which includes the electrically conductive metal lines  32  that electrically interconnect the diodes or transistors  31  disposed between the pixels, to be formed on the inner surface  23  of the cathode  12  along with the electrode pads  27  so that the electrically conductive through-holes used in the previous embodiments can be eliminated. Electrically conductive contact pads  71 , which connect the drive diodes or transistors  31  to a driver chip, are also formed on the inner surface  23  of the cathode  12  adjacent opposing edges of the cathode  12 . 
     FIGS. 8A-8C  collectively illustrate an EPID  60  according to a fourth embodiment of the invention. In this embodiment, the EPID  60  is constructed by combining three individual EPIDs  61 ,  62 ,  63  together in a face-to-face manner. The front, middle, and rear EPIDs  61 ,  62 ,  63  can be constructed essentially as described above in the previous embodiments, but without the filter arrays. Each EPID  61 ,  62 ,  63  contains electrophoretic particles preferably of one of the primary colors red, blue, and green. For example, EPID  61  can contain red particles, EPID  62  can contain blue particles, and EPID  63  can contain green particles. Thus, the front EPID  61  displays only red light, the middle EPID  62  displays only blue light, and the rear EPID  63  displays only green light. 
   Further, the cells or pixels  64 ,  65 ,  66  of the EPIDs  61 ,  62 ,  63  are oriented so that the light produced by the cells or pixels  65 ,  66  of the middle and rear EPIDs  62 ,  63  can be view through the front EPID  61 . This can be accomplished, as shown in  FIG. 8C , by spacing apart the cells or pixels  64 ,  65 ,  66  in each of the EPIDs  61 ,  62 ,  63  and by aligning the cells  64 ,  65 ,  66  of the front, middle, and rear EPIDs  61 ,  62 ,  63  in an offset manner. Additionally, both of the electrodes in at least the front and middle EPIDs  61 ,  62 ,  63  are constructed from transparent plastic or glass sheets and employ transparent electrode lines and electrode pads. 
   In operation, the EPID  60  combines the appropriate number of red, blue, and green cells  64 ,  65 ,  66  from the front, middle, and rear EPIDs  61 ,  62 ,  63  to produced a multi-color image which is viewed through the front EPID  61 . 
   While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.