Abstract:
An improved method of forming a display is described. The method describes depositing droplets of a display fluid into a sealing solution. The sealing solution seals the droplets of display fluid such that each droplet forms a miniature cell of display fluid. Electrodes address charged particles in the miniature cells to form viewable images for use in a display.

Description:
BACKGROUND 
     In recent years, reflective display technologies that are thin, light and flexible have been developed. One type of display that has those characteristics is electrophoretic displays. 
     Many electrophoretic displays include micro-cells filled with an electrophoretic ink. The micro-cells help reduce agglomeration and settling of the ink particles. Electrodes on either side of each microcell apply an electric field to the electrophoretic ink. The electric field moves charged particles in the ink. By controlling the electric field applied to the microcells, the movement of charged particles can be adjusted to form a display image. 
     During fabrication of electrophoretic displays, several techniques have been used to form the displays and seal the micro-cells. Example techniques used to make electrophoretic displays are described in United States patent application number U.S. 2002/0196525 A1 entitled “Process for Imagewise Opening and Filling Color Display Components and Color Displays Manufactured Thereof” by Xianhai Chen et al. as well as, PCT application number WO 01/67170 entitled “Electrophoretic Display” by Rong-Chang Liang et al. Both Patent applications are hereby incorporated by reference. 
     One method of sealing the micro-cells after deposition of the electrophoretic ink involves laminating a cover sheet onto the cells. However, the sealing method described usually uses adhesives or polymers. The adhesives used to seal the ink can displace the ink in the microcells during the sealing process. The displacement of the ink by the adhesive can significantly degrade display performance or reduce fabrication yield 
     Thus an improved method of forming and sealing cells in a display is needed. 
     SUMMARY 
     A method of sealing cells in a display is described. The method includes the operation of depositing a sealing solution. A droplet of display fluid is ejected into the sealing solution such that the droplet of display fluid is immersed in the sealing solution. At least a portion of the sealing solution solidifies to form a solid portion such that the solid portion seals the droplet of display fluid to prevent intermixing of the droplet of display fluid with other droplets of display fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side cross sectional view of a two particle electrophoretic display. 
         FIG. 2-6  show a cross sectional side view of a display at various stages of fabrication. 
         FIG. 7  shows a two dimensional top view of a display during fabrication. 
         FIG. 8-10  shows the forming of sealed fluid droplets. 
         FIG. 11  shows a simple display using the sealed fluid droplets formed in  FIGS. 8-10 . 
         FIG. 12  shows a side cross sectional view of a high fill factor display using the sealed fluid droplets of  FIG. 10  and walls or pillars to support a top cover. 
         FIG. 13  shows a top view of the high fill factor display structure of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     A method of fabricating a display is described. The display includes a number of cells or chambers. Independently addressable pixels in the display change states according to an applied electric field. Each pixel may cover one cell, several cells or a fraction of a cell. A typical example of such a display is an electrophoretic display. 
       FIG. 1  shows a cross sectional view of an electrophoretic display  100 . A plurality of cells  104 ,  108 ,  112  are formed over a substrate  116 . Cell walls  120 ,  124 ,  128  separate cells  104 ,  108 ,  112 . Cell walls also support transparent cover layer  132 . In one design, substrate  116  includes electronics such that substrate  116  forms the display “backplane”. Typically both substrate  116  and transparent cover layer  132  may be formed from glass, although a variety of media may be used. When fabricating flexible, paper-like displays, substrate  116  and cover layer  132  may be manufactured from flexible materials such as polymeric materials (e.g. Mylar™ from DuPont Corporation). 
     A deposition mechanism, such as an ink jet printer, deposits a display fluid such as an electrophoretic ink  136  inside each cell. In general, a display fluid may be any liquid that changes, at least to an observer viewing the droplet from at least one direction, either color, or light transmissivity depending on an applied electric field. Examples of display fluids include inks suitable for electrophoretic displays and “inks” for liquid crystal displays. For convenience, the specification will describe the display fluid as an electrophoretic ink, however, it should be understood that other fluidic materials that change states according to an applied electric field may also be used. 
       FIG. 1  shows a two particle electrophoretic ink  136 . In the illustrated embodiment, electrophoretic ink  136  includes a first set of charged particles  148  of a first color and a second set of oppositely charged particles  152  of a second color. The charged particles are suspended or move through a clear carrier liquid  156 . In an alternate embodiment, the electrophoretic ink may be a one charge particle ink. A one charge particle ink typically uses a single set of charged particles that move through a color dyed liquid carrier. 
     An electric field generated by electrodes  160 ,  164  moves the charged particles though the liquid. Many different conducting materials may be used to form electrodes  160 ,  164 , however, making top electrode  164  transparent improves display image quality. In one embodiment, top electrode  164  is formed from a transparent conductive material such as ITO (indium tin oxide). Bottom electrode  160  may be coupled to a variety of circuits well known in the art, including active matrix pixel array structures. The circuits control the electrode voltages and thus the electric field applied to the ink particles. 
     Cell walls  120 ,  124 ,  128  prevent movement of charged particles between adjacent cells. Such movement can cause particle agglomeration and particle settling, such as when the display is held upright. The cell wall height typically ranges from about 5 microns to 200 microns. A variety of techniques including etching, printing, molding and photolithography, may be used to form the cell walls. In one embodiment, each cell within the cell walls represents one pixel in a display. However, in alternate embodiments, each pixel may correspond to several cells or portions of a cell. In still another embodiment, cell walls are completely eliminated, instead, electrodes&#39;  160 ,  164  shape and size define the display pixels and control pixel and image formation. 
     Although cell walls are not necessary to form a display image problems arise when barriers to electrophoretic ink flow across a display surface are completely removed. For example, when barriers are removed, bending of the display can cause excessive charged particle movement thereby reducing the display image quality. Furthermore, electrophoretic ink flow between cells can create particle gradients that result in display and image deterioration. Thus when cell walls are eliminated, other barriers to electrophoretic ink flow should be substituted. One substitution that will be described in detail is to use a droplet seal itself as the barrier. Such displays will be described in the description accompanying  FIGS. 8-13 . 
       FIG. 2  though  FIG. 6  shows one method of forming the cell structure of  FIG. 1 .  FIG. 2  shows cell walls  204 ,  208 ,  212  formed over a substrate  200 . The cell walls may be formed by various techniques including printing, etching, photolithographic patterning and/or molding techniques. Recent developments have enabled the patterning of electronic circuits using printing techniques, including the printing of transistors such as organic transistors. When printed transistors are used in a printed display backplane, printing the walls enables fabrication of the display entirely using printing techniques. The cell walls may be printed using waxes or photocurable inks. Using printing techniques for the display medium also enables accurate registration between the backplane and the display medium. 
     A sealing solution  220  is deposited in the cells formed by the cell walls. The sealing solution may be deposited using a variety of techniques including doctorblading, dipcoating, curtain coating, spray coating, dispensing or printing the solution into the cells. In one embodiment, the sealing solution consists typically of a polymer dissolved in a solvent. A specific example is a fluorocarbon solution such as Cytop CTX-809A from Asahi Chemicals dissolved in a fluoro-solvent such as a Cytop solvent CT-SOLV180 including Perfluorotrialkylamine, also from Asahi Chemicals, in a ratio of 1 volume part Cytop polymer to 3 volume parts of solvent. Using a solvent that evaporates results in a thin film that eventually seals the droplets. 
     Not all sealing solutions contain a solvent. For example, other fluorocarbon polymers such as two-component Fluorothane™ by Cytonix and UV-curable FluorN™, also manufactured by Cytonix may be used for sealing solution  220  without a solvent. In the case of UV-curable materials, UV radiation causes cross linking of the molecules to convert the sealing solution from a liquid to a solid and sealing the droplet of display liquid. 
     In  FIG. 3 , a print head, such as print head  316 , deposits colored ink into each cell  304 ,  308 ,  312  containing sealing solution  220 . Print head  316  ejects ink droplets typically using ink jet or quill-pin techniques. The print head could include a single or multiple ejectors and each cell may be filled with a single drop or with multiple small drops. A variety of droplet ejection technologies may be used, including but not limited to piezo-electric, thermal, and acoustic inkjet. In a color display, different colored droplets are deposited in adjacent cells. In three adjacent cells of a RGB style display, one cell may be filled with red ink, a second with green ink and a third adjacent cell with blue ink. In more accurate color systems, additional colors such as cyan, magenta and yellow may be added for achieving a display with a wider color gamut. In known text regions, a black/white pixel may be added (e.g. RGBW) to achieve a better display white state. The color that may be used and the possible distribution include a variety of geometric patterns which are known in the art of display technologies. 
     Ink chemistry is selected such that once each ink droplet  324 ,  328 ,  332  enters sealing solution  220 , it becomes immersed into the sealing solution. In the illustrated embodiment of  FIG. 3 , the ink droplet  324  does not wet the underlying substrate. Instead, the droplet  324 ,  328 ,  332  remains suspended by or encapsulated in the sealing solution. In an alternate embodiment, shown in  FIG. 4  (shown is the state in which at least most of the solvent has evaporated), the droplets sink to the bottom and wet substrate  200  surface by displacing sealing solution  220 . In such a “tenting effect”, the ink is sometimes denser than the sealing solution  200 . However in other cases, such as when Cytop or Isopar based inks are used, the ink may be less dense than the sealing solution and the immersion into the sealing solution results at least partially from surface tension forces. For purposes of this patent, both embodiments shall be considered sealed by sealing solution, whether the sealing solution fully encapsulates the droplet or whether the sealing solution seals the droplet in conjunction with other surfaces. As used herein, “encapsulated” means completely surrounds. As used herein, “sealing” merely means to create a container, with or without the cooperation of adjacent surfaces, that prevents the sealed liquid from exiting the container. 
     The operation of immersing the ink droplet may result from gravity, or surface tension or a combination of the two. Thus the ink and sealing solution should be chosen such that surface tension effects and relative densities cause sealing solution to cover the ink droplet. One example of a suitable ink is an ink based on isoparaffinic solvents such as Isopar™ from Exxon Corporation. Other inks that also are suitable include hydrocarbon-based inks and silicone-oil inks. Typical charged particles in the ink may include titanium dioxide mixed with polymers to produce white, carbon based particles mixed with polymer to produce black. A detailed description of other traditional compounds for electrophoretic inks is provided in U.S. Pat. No. 6,017,584 entitled “Multi-color electrophoretic displays and materials for making the same” by Albert et al. which is hereby incorporated by reference. An example of a sealing solution is a fluorocarbon solution such as Cytop in solvent although other sealing materials may also be used. Ink and sealing material are selected such that both don&#39;t ‘substantially’ intermix. (of course most materials are miscible on the molecular level to a certain degree). When a fluorocarbon sealing solution is used, the thin film is typically a fluorocarbon film. 
     In the example of  FIG. 4 , after the solvent carrying the sealing polymer evaporates, a thin solid layer, or thin sealing film  404  remains sealing ink droplets  324 ,  328 ,  332 . The thin sealing layer may be a fluorocarbon film such as Cytop. As previously described, the sealing film may completely surround the droplet or may seal the droplet in conjunction with the substrate and/or other cell walls. The height of each droplet is typically between 5 and 200 microns while the thickness of the film is less, typically between 100 nm and 5 microns. The actual thicknesses may vary widely, and depends heavily on the polymer concentration in the sealing solution. The tolerance for film thickness also depends on the droplet size, for smaller ink drops thinner films are preferred; for larger ink drops thicker films may be suitable. As used herein, “thin” means the film layer is less than the height of the ink droplet. 
       FIG. 5  shows strengthening material  504  deposited over the sealed ink droplets. Strengthening material  504  may be deposited by doctorblading, dip-coating, curtain-coating, spray-coating, printing or a host of other deposition techniques. Strengthening material  504  provides mechanical strength to the display and may be optically transparent, particularly if the display is to be viewed from the top. Polymers including elastomeric polymers serve as excellent strengthening materials. 
       FIG. 6  shows a cover layer  604  deposited over strengthening material  504 . Cover layer  604  may be formed using laminating, printing or other coating techniques. In an electrophoretic display, cover layer  604  also may include a counter electrode used to generate the electric field that moves the charged particles in ink droplets  324 ,  328 ,  332 . The counter electrode may be made from a transparent material to allow viewing of the image formed by the charged particles in droplets  324 ,  328 ,  332 . Examples of transparent coatings that could be formed with a printing technique, or general liquid coating technique include, but are not limited to the clear conductive ink Electrodag PF-427 from Acheson Colloids Company, Michigan and, the carbon nanotube Invisicon™ inks from Eikos of Franklin, Mass. As previously described, the counter electrode  604  could also be a laminated sheet of ITO coated Mylar.) 
       FIG. 7  shows a top view of a two dimensional array of cells  704 ,  708 ,  712  in a display system. Backplane  716  includes driver electronics that control the state of pixels in each cell. In one embodiment, an active matrix circuit drives each pixel, although passive matrix circuits as well as other control circuitry may also be used. 
     The structures of  FIGS. 1-7  include cell walls formed prior to ink droplet deposition. These cell walls separate droplets and provide a barrier to ink fluid flow across a display surface. However, when the sealing film has sufficient strength, the sealing film can itself, with or without the cooperation of a strengthening material, serve as a barrier or “wall” that controls display fluid movement.  FIGS. 8-13  shows examples of a display that relies on the sealing material as the primary barrier to lateral movement across the display surface and operations used to form such a display. 
       FIG. 8-10  shows the operations used to form a low fill factor display structure that relies primarily on the sealing layer to prevent lateral display fluid movement. In  FIG. 8 , sealing solution  804  is deposited on a substrate  808 . An ink droplet  812  is ejected into sealing solution  804 . In one example, the sealing solution is a solution of a fluorocarbon polymer, although other materials may be used. The ink droplet is typically a hydrocarbon, such as Isopar from Exxon, that includes charged colored particles although other ink materials may be used. 
       FIG. 9  shows ink droplet  812  immersed in (surrounded by) sealing solution  804 . Over time, a carrier such as a solvent in sealing solution  804  evaporates leaving a thin solid portion or thin film that seals ink droplet  812 .  FIG. 10  shows ink droplet  812  sealed by thin film  1004 . 
       FIG. 11  shows the individual droplets of  FIG. 8-10  used in a simple display device.  FIG. 11  provides a cross sectional side view of a display made from a plurality of display ink droplets  1104 ,  1108 ,  1112  distributed over a first electrode  1114 . Electrode  1114  is formed over substrate  1116 . A sealing film  1120  seals each droplet. The sealing film may or may not be continuous across the surface of the display. Strengthening material  1124 , which may be a flexible or elastomeric material, deposited around sealing film  1120  reinforces the sealing film  1120  and can also enhance the overall structural strength of display  1100 . 
     A transparent conductive layer  1128  such as ITO on Mylar™ deposited (laminated in the case of ITO on Mylar) over strengthening material  1124  serves as a second electrode. By connecting a power source across first electrode  1114  and transparent conductive layer  1128 , a controllable electric field can be generated across ink droplets  1104 ,  1108 ,  1112 . 
     Display  1100  includes a plurality of pixels. Each pixel corresponds to one or more droplets. Each pixel also corresponds to an electrode that can apply an electric field to all droplets in the pixel. In an electrophoretic display, the electric field moves charged particles in display ink droplets such as droplets  1104 ,  1108 ,  1112 . The distribution of charged particles creates an image. A transparent protective layer formed over transparent conductive layer  1128  protects the display. In one embodiment, the transparent conductive layer  1128  and the transparent protective layer are integrated into a single top layer, such as a sheet of ITO coated Mylar. 
     One disadvantage of the structure illustrated in  FIG. 11  is that the illustrated display has a low fill factor. The spacing and curvature of the droplets results in large areas that are not accessible to the charged particles that form an electrophoretic display image. In particular, the space between droplets such as space  1150  and space  1154  do not contribute to image formation. Thus, to improve the fill factor, the droplets should be placed closer together. 
     When droplets are placed very close together, surface tension forces the droplets to agglomerate. Some droplets may coalesce into a single larger droplet, but many of the droplets remain surrounded and separated by a sealing film. As the solvent evaporates, a high density cell structure results. This dense cell structure is ideally suited for use in a high fill factor display. 
       FIG. 12  shows a cross sectional side view of a high fill factor display  1204  formed by placing droplets very close together. In the structure of  FIG. 12 , a number of droplets  1204 ,  1208 ,  1212  are deposited very close together in a sealing solution. As the sealing solution evaporates, the droplets agglomerate resulting in very little spacing between adjacent droplets such as droplet  1204  and  1208 . In the illustrated embodiment, only sealing film  1220  separates adjacent droplets. Thus the sealing film alone prevents display ink flow between adjacent droplets. Thus the sealing film should be of reasonable strength and thickness to withstand the ink charged particle motion. 
     Droplet agglomeration also usually produces less rounded structures compared to the separated droplets shown in the display structure of  FIG. 11 . Pressure from adjacent droplets causes the droplet side walls, such as side wall  1224  of droplet  1204 , to form a larger angle with substrate  1228  surface than would otherwise result. The increased angle is particularly well suited for electrophoretic display structures in which a “squarish” droplet increases display fill factor. 
       FIG. 12  also shows walls  1232  used to provide additional support, both to droplet walls and to support a protective surface including a top electrode (not shown) deposited over the droplets. Pillars may be substituted for walls  1232 ′ when the walls are not needed to confine or support the ink droplets. The walls  1232  or pillars may be formed from a variety of substances, including but not limited to polymer structures. 
       FIG. 13  shows a top view of a display formed by agglomerated droplets such as droplet  1304 ,  1308 ,  1312 . Sealing film  1316  separates adjacent agglomerated display ink droplets. An agglomerated display ink droplet may be completely surrounded on all sides by droplets such that all sides and corners of the droplet are in contact with adjacent droplets in a “honeycomb” style structure. In one embodiment, structural walls  1320  may separate clusters of droplets such that each droplet, such as droplet  1324 , is a mini “droplet cell” inside a larger cell structure  1328  defined by structural walls  1320 . The larger cell structure may include a plurality of droplet cells. 
     Alternatively, when the strength and uniformity provided by structural wall  1320  is unnecessary and high fill factors are the primary concern, the walls  1320  can be replaced by pillars. Pillars, such as pillars  1332 ,  1336  allow space otherwise occupied by display walls to be filled with display ink. Thus, using pillars instead of walls decreases cell symmetry through the display, but increases the percentage of display area occupied by addressable display ink 
     In regions of the display that rely on pillars, the electrode size, shape and position determine the pixel size, shape and position. Ideally, the shape and size of the electrode is larger than the droplet size such that each electrode covers several droplet cells. Allowing each electrode to cover several cells allows the electrode edge which corresponds to a pixel edge to have a high probability of being in close proximity to a droplet edge. Thus this droplet edge serves as a barrier to minimize motion of charged particles between adjacent pixels. 
     The preceding description has described a method of making a display. A number of details have been provided in the description, such as materials used in the inks, walls and sealing material. However, such details are provided as examples to facilitate understanding of the invention. These details should not be used to limit the invention. Instead, the invention should only be limited by the claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.