Abstract:
A reflective memory display is disclosed which includes: a flexible polymeric substrate; a patterned first conductor disposed over the substrate; and a layer including a polymeric dispersed cholesteric liquid crystal material coated over the first patterned conductor. The memory display further includes a patterned second conductor disposed over the polymeric dispersed cholesteric liquid crystal; and control means for repeatedly applying selective unipolar voltages across particular portions of the patterned first and second conductors to cause electric fields which are applied to portions of the polymeric dispersed cholesteric liquid crystal layer to change its reflectance thereby presenting a series of different reflectance images.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned U.S. patent application Ser. No. 09/379,776, filed Aug. 24, 1999 now, U.S. Pat. No. 6,394,870 by Dwight J. Petruchik et al., the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the electrical drive for reflective memory displays. 
     BACKGROUND OF THE INVENTION 
     Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible. 
     Current flat panel displays use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952, a set of electrical traces is sputtered in pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive, and currently are limited to applications having long lifetimes. 
     Fabrication of flexible, electronically written display sheets using liquid crystals materials is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin 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 bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent. 
     U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. The structure has the capacity of maintaining one of the given states in the absence of an electric field. 
     In “Liquid Crystal Dispersions”, World Science, Singapore, 1995, page 408, Paul Drzaic discusses the electrical drive of cholesteric liquid crystal displays. Drzaic also states on page 29 that “The use of gelatin, however, creates a material that is too conductive for practical use in electrically addressed PDLC systems.”. Drzaic further states “ . . . actual displays require AC signals to prevent electrochemical degradation.” Subsequent patents follow Drzaic&#39;s assumptions. Later patents such as U.S. Pat. Nos. 5,251,048 and 5,644,330 and 5,748,277 all require AC fields having a net zero unipolar field for matrix cholesteric liquid crystal displays to prevent ionic destruction of the display. The cited patents have display structures formed using expensive display structures and processes applicable to long-life situations which require AC drive schemes. 
     The drive schemes require that each element be written using alternating electrical fields that provide a net zero field across the display to prevent ionic migration. AC drives require large numbers of power supplies and large numbers of switching elements per line. 
     The development of simple processes to manufacture inexpensive flexible displays will generate new markets of inexpensive, short lived displays that will benefit from a lower cost electrical drive circuitry. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide low cost memory displays generated using coated polymeric dispersed cholesteric liquid crystals on flexible substrates. 
     It is another object of the present invention to provide a simpler, lower cost method of driving coated polymer dispersed cholesteric materials on flexible substrates. 
     These objects are achieved by a reflective memory display, comprising: 
     a) a flexible polymeric substrate; 
     b) a patterned first conductor disposed over the substrate; 
     c) a layer including a polymeric dispersed cholesteric liquid crystal material coated over the first patterned conductor; 
     d) a patterned second conductor disposed over the polymeric dispersed cholesteric liquid crystal; and 
     e) control means for repeatedly applying selective unipolar voltages across particular portions of the patterned first and second conductors to cause electric fields which are applied to portions of the polymeric dispersed cholesteric liquid crystal layer to change its reflectance thereby presenting a series of different reflectance images. 
     The invention reduces the number of voltages required to drive such a display as well as reducing the number of voltage switching elements. 
     It is a feature of the present invention that it eliminates the need to work with glass substrates with associated complex manufacturing techniques. The present invention by making use of flexible polymeric substrates permits the use of unipolar fields to change reflectance. A unipolar drive reduces the associated cost of the drive circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric partial view of a cholesteric liquid crystal display made in accordance with the present invention; 
     FIG. 2 is an assembly diagram of the display in FIG. 1 being attached to a card; 
     FIG. 3A is a schematic sectional view of a chiral nematic material in a planar state reflecting light; 
     FIG. 3B is a schematic sectional view of a chiral nematic material in a focal-conic state transmitting light; 
     FIG. 4 is a plot of the response of a first polymer dispersed cholesteric material to a pulsed electrical field with a first set of imposed voltages; 
     FIG. 5 is a schematic representation of a matrix array of cholesteric liquid crystal elements; 
     FIG. 6 is an electrical schematic of prior art used to drive the display of FIG. 1; 
     FIG. 7 is a diagram of the waveform generated by prior art when driving the display of FIG. 1; 
     FIG. 8 is an electrical schematic of a second embodiment of the current invention used to drive the display of FIG. 1; 
     FIG. 9 is a diagram of the waveform generated by the second embodiment of the current invention to drive the display of FIG. 1; 
     FIG. 10 is a plot of the response of a second polymer dispersed cholesteric material to a pulsed electrical field with a second set of imposed voltages; 
     FIG. 11 is an electrical schematic of a first embodiment of the current invention used to drive the display of FIG. 1; and 
     FIG. 12 is a diagram of the waveform generated by the first embodiment of the current invention to drive the display of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is an isometric partial view of a new structure for a display  10  made in accordance with the invention. Display  10  includes a flexible substrate  15 , which is a thin transparent polymeric material, such as Kodak Estar film base formed of polyester plastic that has a thickness of between 20 and 200 microns. In an exemplary embodiment, substrate  15  can be a 125 micron thick sheet of polyester film base. Other polymers, such as transparent polycarbonate, can also be used. 
     First patterned conductors  20  are formed over substrate  15 . First patterned conductors  20  can be tin-oxide or indium-tin-oxide (ITO), with ITO being the preferred material. Typically the material of first patterned conductors  20  is sputtered as a layer over substrate  15  having a resistance of less than 250 ohms per square. The layer is then patterned to form first patterned conductors  20  in any well known manner. Alternatively, first patterned conductors  20  can be an opaque electrical conductor material such as copper, aluminum or nickel. If first patterned conductors  20  are opaque metal, the metal can be a metal oxide to create light absorbing first patterned conductors  20 . First patterned conductors  20  are formed in the conductive layer by conventional lithographic or laser etching means. 
     A polymer dispersed cholesteric layer  30  overlays first patterned conductors  20 . Polymer dispersed cholesteric layer  30  includes a polymeric dispersed cholesteric liquid crystal material, such as those disclosed in U.S. Pat. No. 5,695,682, the disclosure of which is incorporated by reference. Application of electrical fields of various intensity and duration can drive a chiral nematic material (cholesteric) into a reflective to a transmissive state or an intermediate state. These materials have the advantage of maintaining a given state indefinitely after the field is removed. Cholesteric liquid crystal materials can be Merck BL112, BL118 or BL126, available from EM Industries of Hawthorne, N.Y. 
     In the preferred embodiment, polymer dispersed cholesteric layer  30  is E.M Industries cholesteric material BL-118 dispersed in deionized photographic gelatin. The liquid crystal material is dispersed at 8% concentration in a 5% deionized gelatin aqueous solution. The mixture is dispersed to create 10 micron diameter domains of the liquid crystal in aqueous suspension. The material is coated over a patterned ITO polyester sheet to provide a 9 micron thick polymer dispersed cholesteric coating. Other organic binders such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO) can be used. Such compounds are machine coatable on equipment associated with photographic films. 
     Second patterned conductors  40  overlay of polymer dispersed cholesteric layer  30 . Second patterned conductors  40  should have sufficient conductivity to carry a field across light modulating material  30 . Second patterned conductors  40  can be formed in a vacuum environment using materials such as aluminum, tin, silver, platinum, carbon, tungsten, molybdenum, tin or indium or combinations thereof. The second patterned conductors  40  are as shown in the form of a deposited layer. Oxides of said metals can be used to darken second patterned conductors  40 . The metal material can be excited by energy from resistance heating, cathodic arc, electron beam, sputtering or magnetron excitation. Tin-oxide or indium-tin oxide coatings permit second patterned conductors  40  to be transparent. 
     In a preferred embodiment, second patterned conductors  40  are printed conductive ink such as Electrodag 423SS screen printable electrical conductive material from Acheson Corporation. Such printed materials are finely divided graphite particles in a thermoplastic resin. In the preferred embodiment, second patterned conductors  40  are formed using the printed inks to produce a low cost display. The use of a flexible support for substrate  15 , laser etching to form first patterned conductors  20 , machine coating polymer dispersed cholesteric layer  30  and printing second patterned conductors  40  permits the fabrication of very low cost memory displays. Small displays formed using these methods can be used as electronically re-writable tags for inexpensive, limited rewrite applications. 
     FIG. 2 an assembly diagram of display  10  in FIG. 1 being attached to a card  12 . Card  12  can be a transparent sheet, approximately 1 millimeter in thickness which has information printed on one surface. A non-printed area  13  provides a clear window for viewing the contents of display  10 , which has been bonded to the opposite side of card  12 . Display  10  in this example has a transparent substrate  15 , and is inverted from the position shown in FIG. 1 during the attachment process. Information written to display  10  is seen through non-printed area  13 , through card  12  and through transparent substrate  15 . Card  12  attached to display  10  according to the current invention can be inserted into a writer (not shown) and contacts  14  can be connected during the insertion process to second patterned conductors  40  on display  10  to update information on display  10 . Display  10  can be used a financial transaction (credit/debit) card typically requiring less than 10,000 updated images. 
     FIG.  3 A and FIG. 3B show two stable states of cholesteric liquid crystals. In FIG. 3A, a high voltage field has been applied and quickly switched to zero potential, which converts cholesteric liquid crystal to planar state  22 . Incident light  26  striking cholesteric liquid crystal in planar state  22  is reflected to create a bright image. In FIG. 3B, application of a lower voltage field leaves cholesteric liquid crystal in a transparent focal conic state  24 . Incident light  26  striking a cholesteric liquid crystal in focal conic state  24  is transmitted. Second patterned conductors  40  can be black which will absorb incident light  26  to create a dark image when the liquid crystal material is in focal conic state  24 . As a result, a viewer perceives a bright or dark image depending on if the cholesteric material is in planar state  22  or focal conic state  24  respectively. 
     FIG. 4 is a plot of the response of a cholesteric material to a pulsed electrical field. Such curves can be found in U.S. Pat. Nos. 5,453,863 and 5,695,682 and are also recited in the above-cited Drzaic reference. For a given pulse time, typically between 5 and 200 milliseconds, a pulse at a given voltage can change the optical state of a cholesteric liquid crystal. Voltage below disturbance voltage V 1  can be applied without changing the state of the cholesteric material. A higher voltage pulse at a focal-conic voltage V 3  will force a cholesteric material into the focal conic (FC) or transparent state. A voltage pulse at planar voltage V 4  will force the cholesteric material into the planar (P) or reflective state. The curve characteristic of cholesteric liquid crystal permits passive matrix writing of cholesteric displays. The prior art written for cholesteric displays covers displays built using expensive conventional flat panel display processes. Consequently, current state of the art requires AC voltage drive schemes for cholesteric displays to prevent ionic damage. Those bipolar drives require at least two voltages and two separate semiconductor switching elements for each drive line. 
     FIG. 5 is a schematic representation of a matrix array of cholesteric liquid crystal elements according to prior art. Row voltage Vr is set midway between V 3  and V 4  on a selected row while the remaining rows are set to a ground voltage. A positive or negative column voltage Vc is set across all columns to offset Vr to either focal-conic voltage V 3  or planar voltage V 4 , depending on the desired final state of a row of pixels. The positive and negative column voltages Vr-V 3  and V 4 -Vr are less than disturbance voltage V 1  so that rows at ground potential experience voltages less than disturbance voltage V 1  and are not changed. The material characteristic permits sequential row writing. 
     FIG. 6 is an electrical schematic of typical prior art used to drive the display of FIG. 1 based on the teaching in U.S. Pat. No. 5,644,330. Four power supplies  50  are needed to supply +Vc, −Vc, ground, +Vr and −Vr. Using conventional drive chip architecture, data is shifted serially through the chip and stored in latches  60 . Each of 6 voltages (+Vr, −Vr, 0row, +Vc, −Vc, Ground) must have a switch  70  to drive a matrix cholesteric display. More complex switching schemes have been proposed which have an higher number of power supply and switching elements, such as U.S. Pat. No. 5,748,277. 
     FIG. 7 is a diagram of the waveform generated by prior art when driving the display  10  of FIG.  1 . An bipolar row voltage Vr can be applied to a selected row, while a bipolar column voltage Vc is applied either in phase or out of phase with the row voltage Vr. If the AC voltages are out of phase, the pixel will experience alternating bipolar high pixel voltage Vp, corresponding to V 4  and be written into the planar state. If the two voltages are in of phase, then a pixel experiences lower alternating bipolar pixel voltage Vp, corresponding to V 3  and be written into the focal conic state FC. Columns held at a ground state ( 0 ) will experience a bipolar alternating column voltage Vc as an alternating AC field equivalent to half the voltage difference between V 4  and V 3 . That voltage must be less than disturbance voltage V 1 . The variation in phase creates a net DC filed across the cholesteric display material. 
     In an experiment, gelatin dispersed cholesteric material dispersed and coated to the preferred embodiment was coated over ITO coated flexible substrate  15  to form polymer dispersed cholesteric layer  30 . A one inch square conductive patch was printed over the gelatin dispersed cholesteric material to provide a field across the coating. A 20 millisecond unipolar field was switched across display  10  every 5 seconds to switch the state of the material between the planar and focal conic states. The gelatin dispersed cholesteric material was driven through a limited life test of 10,000 re-writes. The life testing was equivalent to 200 seconds of continuous applied unipolar voltage to display  10 . The test patch operated with no apparent visible degradation throughout the life test. The life test was then extended to 100,000 cycles. The test display  10  continued to perform with little degradation. From this experiment, it was concluded that polymeric dispersed cholesteric materials on flexible substrates  15  with printed conductors can be driven by unipolar fields for at least the limited number of life cycles needed for limited-life display applications. 
     FIG. 8 is a representative plot of the response of a first polymer-dispersed cholesteric material of the preferred embodiment to a pulsed electrical field with a set of imposed voltages according to the present invention. The DC unipolar life test implies that displays can be produced using unipolar fields on cholesteric materials for substantial number of actuations. A first embodiment of this drive scheme for such displays uses a hybrid scheme for materials having the difference between the planar voltage V 4  and focal-conic voltage V 3  greater than disturbance voltage V 1  as shown in FIG. 4. A voltage response curve for such a material has been shown in FIG.  4 . In the hybrid scheme, a high unipolar voltage is used for row voltage Vr, and a lower bipolar voltage is used as a column voltage Vc. The writing fields are applied as a unipolar field, with positive and negative column voltages setting the optical state for succesive rows. The column voltage Vc in either polarity can be as high as the disturbance voltage V 1 . The drive architecture eliminates −Vr, one power supply for the row voltage, and eliminates one switch  60  for the row voltage Vr. 
     FIG. 9 is a schematic of the waveforms generated by the drive shown in FIG.  8 . The column voltage Vc drives cholesteric material to the reflective state in the negative state and to the focal-conic (clear) state with a positive voltage. Both drive states provide a unipolar field of varying intensity across the material in each pixel. The column voltage is bipolar, however the net effect on each pixel is to provide a unipolar field. The hybrid drive structure eliminates one power supply and one switching element for each column. 
     FIG. 10 is the plot of the response of a second polymer-dispersed cholesteric material to a pulsed electrical field with a set of imposed voltages according to the present invention. The second embodiment of this invention requires that the difference in voltage between focal-conic voltage V 3  and planar voltage V 4  be less than disturbance voltage V 1 . Row voltage Vr is set at the potential required to drive cholesteric material into the planar state, planar voltage V 4 . A positive column voltage is applied to offset Vr down to focal-conic voltage V 3 . Switching a ground voltage to a column applies the full column voltage Vc to a pixel and drives the material to the reflective state. 
     FIG. 11 is an electrical schematic of a first embodiment of the current invention used to drive the display  10  of FIG.  1 . The power supply requirement is reduced from four to two, one to provide row voltage Vr and one to provide column voltage Vc. Two switches  70  are required for each row and each column line instead of three. 
     FIG. 12 is a diagram of the waveform generated by the first embodiment of the current invention to drive the display  10  of FIG.  1 . An individual row is selected by row voltage Vr, while the other rows are set to zero potential. If column voltage Vc is at ground ( 0 ), pixel voltage Vp will be equal to planar voltage V 4  and the pixel will be driven into the planar state. If the column voltage Vc is set to disturbance voltage V 1 , the pixel experiences a low field strength equivalent to focal-conic voltage V 3  and the pixel will be driven into the focal-conic (FC) state. The focal conic voltage is less than disturbance voltage V 1  and the unwritten rows held at ground are undisturbed by an applied column voltage Vc. 
     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. 
       10  display 
       12  card 
       13  non-printed area 
       14  connector pins 
       15  substrate 
       20  first patterned conductors 
       22  planar state 
       24  focal conic state 
       26  incident light 
       28  reflected light 
       30  polymer dispersed cholesteric layer 
       35  display drive 
       40  second patterned conductors 
       50  power supply 
       60  registers 
       70  switches 
     FC focal-conic 
     P planar 
     V 1  disturbance voltage 
     V 3  focal-conic voltage 
     V 4  planar voltage 
     Vc column voltage 
     Vr row voltage 
     Vp pixel voltage