Abstract:
A display in which images can be selectively presented to a viewer, is formed by providing a transparent substrate; forming a transparent, electrically conductive coating over the transparent substrate; forming a light modulating layer including liquid crystal material in a polymer binder over the transparent, electrically conductive layer; forming by directly depositing conductive material in an image wise pattern over the light modulating layer to provide viewable and conductive images, the light modulating layer being effective in a first condition to prevent the viewing of the viewable and conductive images and in a second condition to permit the viewing of the viewable and conductive images. Electrical connection is made so that an electrical field can be applied across selected ones of such viewable and conductive images and the transparent, electrically conductive layer to cause the light modulating layer underlying the selected ones of the viewable and conductive images to change from the first condition to the second condition so as to present such viewable and conductive images for viewing to the viewer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 09/105,507 filed Jan. 26, 1998 now U.S. Pat. No. 6,010,839 and commonly-assigned U.S. patent application Ser. No. 09/146,656 filed Sep. 3, 1998, now U.S. Pat. No. 6,236,442, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The field of invention pertains to image displays that can selectively transmit or reflect actinic light. 
     BACKGROUND OF THE INVENTION 
     Currently, images can be displayed on sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Sheet materials can carry magnetically written areas carrying ticketing or financial information, however magnetically written data is not visible. 
     A structure is disclosed in PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages” which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple electronically written display sheets into a “book”, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page. The device described requires “. . . transparent conducting polymers . . .” formed over the light modulating material. Formation of transparent conductors of the required conductivity require complex vacuum sputtering and photo-lithographic processes. 
     Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is pressed onto the liquid crystal material. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. The display ceases to present an image when de-energized. The Taliq Company supplied products formed using the two sheet method. Offsetting the two sheets permitted connection to conductive traces on the two sheets. 
     The prior art typically requires multiple, separate layers to build up the display. The electrical traces and transparent conductive layers are typically formed through repeated vacuum deposition and photolithography of materials on the substrate. These processes are expensive and require long processing times on capital intensive equipment. 
     The prior art discloses isolating each conductor on separate sides of the display, and connecting the traces to drive electronics using solder connections, wire bonds or pressure contact. Such connections do require that both sets of traces be exposed on a surface for the connection process. The uniform, multi-layer structure prevents connection to the inner conductive layer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a display which has opaque conductive images formed in an effective manner with a minimum number of steps. 
     This object can be achieved by a method of forming a display in which images can be selectively presented to a viewer, comprising the steps of: 
     (a) providing a transparent substrate; 
     (b) forming a transparent, electrically conductive coating over the transparent substrate; 
     (c) forming a light modulating layer including liquid crystal material in a polymer binder over the transparent, electrically conductive layer; 
     (d) forming by directly depositing opaque conductive material in an image wise pattern over the light modulating layer in the form of viewable and conductive images, the light modulating layer being effective in a first condition to prevent the viewing of the viewable and conductive images and in a second condition to permit the viewing of the viewable and conductive images; and 
     (e) providing electrical connections so that an electrical field can be applied across selected ones of such viewable and conductive images and the transparent, electrically conductive layer to cause the light modulating layer underlying the selected ones of the viewable and conductive images to change from the first condition to the second condition so as to present such viewable and conductive images for viewing to the viewer. 
     The disclosed structure has the advantage of directly forming opaque electrically conductive material in an image wise pattern thereby reducing the number of process steps that are required for transparent conductive images. The opaque, electrically conductive material can be a conductive ink deposited using screen printing. Printing processes are widely available, and simple and effective means to provide custom display. Displays in accordance with the present invention can be in the form of display sheets that can be made using conventional coating technology. A single large area of sheet material can be coated and formed into different types of displays by printing opaque, electrically conductive indicia onto the coated sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a sectional view of an unprocessed sheet used to form a display in accordance with the present invention; 
     FIG. 1B is a sectional view of the sheet of FIG. 1A after directly forming opaque conductive material in an image wise pattern; 
     FIG. 1C is a sectional view of the sheet of FIG. 1B connected to a circuit board to form the display in accordance with the present invention; 
     FIG. 2A is a view of the optical characteristics of a chiral nematic material in a planar state reflecting light; 
     FIG. 2B is a view of the optical characteristics of a chiral nematic material in a focal-conic light transmitting state; 
     FIG. 3 is a sectional view of a domain containing chiral nematic liquid crystal material; 
     FIG. 4 is a sectional view showing light striking segments of a the display; 
     FIG. 5 is a schematic of electrical drive circuitry for the present invention; 
     FIG. 6A is a top view of the circuit board of FIG. 1C that is part of the display; 
     FIG. 6B is a top view with cut away sections of the processed sheet of FIG. 1B that is part of the display; 
     FIG. 7A is a top view of the completed display in an inactivated condition; 
     FIG. 7B is a top view of the completed display with an activated segment; 
     FIGS. 8A-8C are sectional views of the steps in processing a sheet in accordance with the present invention; and 
     FIGS. 9A-9C are top views of the steps in processing a sheet in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A sectional view of an unprocessed sheet  10  used in the invention is shown in FIG.  1 A. The sheet  10  will be processed to form a display  5  in accordance with the present invention. The sheet  10  includes a substrate  12 . Substrate  12  can be made of a transparent polymeric material, such as Kodak Estar film base formed of polyester plastic, and have a thickness of between 20 and 200 microns. In an exemplary embodiment, substrate  12  can be a 80 micron thick sheet of polyester film base. Other polymers, such as transparent polycarbonate, can also be used. An optically transparent, electrically conductive layer  13  is formed over the substrate  12 . The transparent, electrically conductive layer  13  can be formed of tin-oxide or Indium-Tin-Oxide (ITO), with ITO being the preferred material. Typically, the transparent, electrically conductive layer  13  is sputtered onto the substrate  12  to a resistance of less than 250 ohms per square. 
     A light modulating layer  30  is formed over the transparent, electrically conductive layer  13 . Light modulating layer  30  can be a polymer encapsulated conventional liquid crystal dispersed in a polymeric matrix. The liquid crystal can be a chiral doped nematic liquid crystal, also known as cholesteric liquid crystal, such as those disclosed in U.S. Pat. No. 5,695,682. Application of fields of various intensity and duration can change the state of chiral doped nematic materials from a reflective to a transmissive state. These materials have the advantage of maintaining a given state indefinitely after the field is removed. Other light reflecting, electrically modulated materials can also be coated such as a micro-encapsulated electrophoretic material. The light modulating layer  30  is effective in two conditions, which will be described in more detail below. 
     FIG. 2A, and FIG. 2B show states of cholesteric liquid crystals. In FIG. 2A, a high voltage field has been applied and quickly switched to zero potential, which causes the liquid crystal molecules to become planar liquid crystals  20 . Incident light  55  can consist of red, green and blue fractions of white light. The pitch of the molecules can be adjusted to create a Bragg diffraction of reflected light  18  comprised of light of a given color and polarity. In this example, the chirality of planar liquid crystals  20  is adjusted to reflect green light. 
     In FIG. 2B, application of a lower voltage field has caused molecules of the chiral nematic material to form tilted cells that are known as the focal conic liquid crystals  22 . The lower voltage field can progressively drive the molecules of the cholesteric material towards a transparent state. A light absorber  24  can be positioned on the side opposing the incident light. In the fully evolved focal-conic state light is transmitted completely an incident light  55  becomes absorbed light  64 . The progressive evolution to a completely focal-conic state causes a viewer to perceive green light that transitions to black as the cholesteric material changes from a planar to a fully evolved focal-conic state. The transition to the light transmitting state is progressive, and varying the low voltage time permits a variable level of reflection. These variable levels can be mapped out to corresponding gray levels, and when the field is removed, light modulating layer  11  will maintain a given optical state indefinitely. The states are more fully discussed in U.S. Pat. No. 5,437,811. 
     FIG. 3 is a cross section through a domain  26  containing a cholesteric material. Domain  26  is a spherical domain about 10 microns in diameter, and cholesteric material anchors on the surface of the domain. Because the surface of domain is spherical, incident light  55  from any angle of observation is reflected. The result is that these polymer dispersed (cholesteric) liquid crystals (PDChLC) have good off-axis reflectivity. 
     In an experiment, E.M Industries cholesteric material BL-118 was dispersed in deionized photographic gelatin. The liquid crystal material was dispersed at an 8% concentration in a 5% deionized gelatin solution. The mixture was homogenized to create 2-10 micron domains of the liquid crystal in aqueous suspension. The material was coated on a photographic film coating machine to provide a 9 micron thick polymerically dispersed cholesteric coating. Other organic binders such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO) could have been used. Such compounds are also machine coatable on equipment associated with photographic films. 
     Deionized photographic gel is important in providing a binder having low ionic content. The presence of ions in such a binder hinders the development of an electrical field across the dispersed liquid crystal material. Additionally, ions in the binder can migrate in the presence of an electrical field, chemically damaging the light modulating layer  30 . The liquid crystal and gelatin emulsion can be coated to a thickness of between 5 and 30 microns to optimize light modulating of light modulating layer  30 . The coating thickness, size of the liquid crystal bubbles and concentration of the bubbles of liquid crystal materials is designed to maximize electrical switching of the material and optimize the optical properties of the material in both the reflective or transmissive state. 
     FIG. 1B is a sectional view through the sheet  10  after being directly printed with an opaque conductive ink to form conductive image areas  16 . In an experiment, Electrodag 423SS screen printable opaque electrical conductive material from Acheson Corporation was screen printed to form opaque conductive areas  16 . The material is finely divided graphite particles in a thermoplastic resin. The material was not heated, only air dried to form a coating between 25 and 75 microns thick. Each opaque conductive area  16  was separated from adjacent conductive areas by non-conductive areas  18 . Nonconductive areas  18  are typically 50-100 microns wide. Conductive ink can be applied to non-indicia areas so that the coating covers light modulating material with the exception of fine traces that are non-conductive areas  18 . The effective sheet conductivity of the opaque conductive areas  16  was less than 250 ohms per square. Opaque conductive areas  16  were opaque and highly light absorbing, typically having an optical density of greater than 2.0 D to present black images. The light absorbing property of the opaque conductive areas  16  in the experiment was adequate to serve as light absorber  24  for the cholesteric liquid crystal material. Numerous other techniques will suggest themselves to those skilled in the art. For example, in order to directly deposit conductive inks in an image-wise pattern, screen printing has been found to be highly effective. Alternatively, ink jet printing techniques can be used to form the opaque conductive images. Another technique would be to use off-set printing techniques to directly apply opaque conductive inks in a pattern on sheet  10 . The direct depositing of opaque conductive materials in an image wise patterns means that a single step can be used to provide such images. An advantage of this arrangement is a reduction and simplification of process steps to form such opaque conductive images. 
     Experimental sheet  10  was tested by applying an alternating 90-volt field at 1 kilohertz frequency for 25 milliseconds to each opaque conductive area  16  while transparent, electrically conductive layer  13  was grounded. Light modulating material  30  over each segment was driven into a reflective state. A second alternating 40-volt field at 1 kilohertz frequency for 100 milliseconds to each opaque conductive area  16  while transparent, electrically conductive layer  13  was grounded. Light modulating material  30  over each opaque conductive area  16  became nearly transparent. The experiment proved that conductive inks could be applied to polymerically dispersed cholesteric liquid crystal material to create a display sheet  10 . 
     FIG. 1C is a section view of processed sheet  10  connected to circuit board  40  to form a display  5 . Circuit board  40  has a set of traces  45  that are connected to opaque conductive areas  16  through contact pads  47 . Patternable conductive layer  14  can be soft, in which case, connection between circuit board  40  and opaque conductive areas  16  must be done without damage. In one case, conductive adhesive is provided between circuit board  40  and opaque conductive areas  16  to form a connection between circuit board  40  and opaque conductive areas  16 . 
     A power pin  50  formed in the printed circuit board  40  contacts transparent, electrically conductive layer  13  through power connection area  52 . It should be noted that in power connection area  52 , light modulating layer  30  over power connection area  52  to facilitate direct electrical connection to the transparent, electrically conductive layer  13 . Such removal can be accomplished by chemical etching. Alternatively, power pin  50  can have a sharpened point that pierces through light modulating material  30  to contact transparent, electrically conductive layer  13 . Power connection area  52  can be one or more than one area that permit electrical connection to transparent, electrically conductive layer  13 . 
     FIG.  4 . is a drawing of the sheet  10  in both reflective and transparent states. In the central area of the drawing, light modulating material  30  has been cleared. Incident light on that area becomes absorbed light  64 . On the left and right sides of sheet  10 , light modulating material  30  is in a reflective state and portions of incident light  55  be come reflected light  60 . 
     FIG. 5 shows the external drive circuitry for changing the state of light modulating material  30 . Display drive  70  is connected to a set of switching elements D 1  through Dn, with one switching element per opaque conductive area  16 . Power supply  72  can provide either 120 volts (high) or 40 volts (low) to display  5 . The voltage is applied as a one kilohertz alternating voltage. The voltage from power supply  72  is connected to the transparent, electrically conductive layer  13  through power pin  50 . Display drive  70  grounds all opaque conductive areas  16  and uses high/lower power signal  74  to apply a high, 120 volts filed across light modulating material  30  for approximately 120 milliseconds. This writes all conductive areas into the reflective state. Display drive  70  then grounds those opaque conductive areas  16  that should be in the transparent, dark state and uses high/lower power signal  74  to apply a low, 40 volt filed across light modulating material  30  for 120 milliseconds to clear those areas that are to be dark. 
     Thus, the light modulating layer  30  is driven to two effective conditions. In a first condition, the light modulating layer  30  presents a dark, light absorbing state over opaque conductive areas  16 , and in a second condition, the light modulating layer  30  presents a light, light reflective surface over opaque conductive areas  16 . 
     FIGS. 6A and 6B show top views of each of the two parts of display  5 . FIG. 6A shows a circuit board  40 , which has circuit board traces  45  running from a position under each opaque conductive area  16  to connection areas  52 . Contact pads  47  are located under each opaque conductive area  16  to provide connection to each conductive trace  45  on sheet  10 . FIG. 6B is a top view of sheet  10  with light modulating material  30  sectioned away to show opaque conductive areas  16 . Non-conductive areas  18  serve to electrically isolate opaque conductive areas  16 . Power pin  50  on circuit board  40  contacts a connection area  52  transparent, electrically conductive layer  13 . Multiple power pins  50  connected to multiple connection areas  52  could be used to ensure uniform electrical field across the transparent, electrically conductive layer  13 . 
     Top views of display  5  are shown in FIGS. 7A and 7B. Sheet  10  from FIG. 6B has been placed over circuit board  40  from FIG.  6 A. Circuit board  40  has a series of circuit board traces  45  that provide interconnection the drive circuit in FIG.  5 . FIG. 7A shows all opaque conductive areas  16  in light modulating layer  30  have been written into a reflective state. FIG. 7B shows a single opaque conductive area  16  has been grounded during the low voltage phase of display writing. Light modulating material  30  in that segment has become transparent, and the area over that opaque conductive areas  16  appears dark due to the light absorbing properties of opaque conductive area  16   
     FIG.  8 A through FIG. 8C are sectional views and FIGS. 9A through 9C are corresponding front views of steps for printing on a sheet  10  in accordance with the present invention. In this arrangement, the circuit board  40  is replaced with a multi layer conductive structure on sheet  10 . 
     FIGS. 8A and 9A show the completed sheet  10  of FIG. 1B of the earlier embodiment, having opaque conductive areas  16  and non-conductive areas  18  to delineate opaque conductive areas  16  to isolate the opaque conductive areas  16  from adjacent opaque conductive areas  16 . In FIGS. 8B and 9B, a non-conductive printed polymer, which is a dielectric, has been applied to sheet  10  to create an insulator  80  over opaque conductive areas  16  and non conductive areas  18 . Openings or holes  82  are provided in insulator  80  over each opaque conductive area  16 . In FIG.  8 C and FIG. 9C a second conductive material  84  has been applied over insulator  80  create traces  45  that connect through the holes  82  to opaque conductive areas  16  and out to the edge of sheet  10  to power connection areas  52 . The resulting sheet  10  does not need circuit board  40  and associated interconnect structures thereon. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       5  display 
       10  sheet 
       12  substrate 
       13  transparent, electrically conductive layer 
       14  patternable conductive layer 
       16  opaque conductive areas 
       18  non-conductive areas 
       20  planar liquid crystal 
       22  focal-conic liquid crystal 
       24  light absorber 
       26  domain 
       30  light modulating layer 
       40  circuit board 
       45  traces 
       47  contact pad 
       50  power pin 
       52  power connection area 
       55  incident light 
       60  reflected light 
       64  absorbed light 
       70  display drive 
       72  power supply 
       74  high/low power signal 
       80  insulator 
       82  Holes 
       84  Second conductive material 
     D 1 , D 2 , . . . , Dn Driver  1  through driver n