Patent Description:
The term "electro-optic", as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence, or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in <CIT>; <CIT>; <CIT>; <CIT><CIT>; <CIT>; <CIT>;<CIT>; and<CIT> (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.

Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example <NPL>; and <NPL>). See also <NPL>. Nanochromic films of this type are also described, for example, in <CIT>; <CIT>; and <CIT>. This type of medium is also typically bistable.

Another type of electro-optic display is an electro-wetting display developed by Philips and described in <NPL>). It is shown in <CIT> that such electro-wetting displays can be made bistable.

One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.

Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC and related companies describe various technologies used in encapsulated and microcell electrophoretic and other electro-optic media. Encapsulated electrophoretic media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. The technologies described in these patents and applications include:.

Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned <CIT>. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word "printing" is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See <CIT>); and other similar techniques. ) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays of the present invention.

An electro-optic display normally comprises a layer of electro-optic material and at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electro-optic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electro-optic layer comprises an electrode, the layer on the opposed side of the electro-optic layer typically being a protective layer intended to prevent the movable electrode damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium-tin-oxide (ITO) or a similar conductive coating (which acts as one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display usable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide. ) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques can be used with other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal member medium may be laminated to a backplane in substantially the same manner as an encapsulated electrophoretic medium.

Electro-optic displays, including electrophoretic displays, can be costly; for example, the cost of the color LCD found in a portable computer is typically a substantial fraction of the entire cost of the computer. As the use of such displays spreads to devices, such as cellular telephones and personal digital assistants (PDA's), much less costly than portable computers, there is great pressure to reduce the costs of such displays. The ability to form layers of electrophoretic media by printing techniques on flexible substrates, as discussed above, opens up the possibility of reducing the cost of electrophoretic components of displays by using mass production techniques such as roll-to-roll coating using commercial equipment used for the production of coated papers, polymeric films and similar media. However, the ability to utilize roll-to-roll coating for the purpose of mass-production of electro-optic displays having relatively large dimensions is limited due to the need for precise alignment of the front plane laminate and backplane.

<CIT> (published as <CIT>) describes providing an electrical connection between the backplane and the front electrode of an electro-optic display by: forming a front plane laminate comprising, in order, a light-transmissive electrically-conductive layer, a layer of electro-optic material, and a layer of lamination adhesive; forming an aperture through all three layers of the front plane laminate; and introducing a flowable, electrically-conductive material into the aperture, the flowable, electrically-conductive material being in electrical contact with the light-transmissive electrically-conductive layer and extending through the adhesive layer.

<CIT> describes a backplane for an electrophoretic display, the backplane having a plurality of layers including a base film layer, an interconnect layer, a foil layer and a display film layer. The foil layer includes at least one laser-formed gap, with the gap being defined in the foil layer after the foil layer has been applied to one of the other layers. The interconnect layer is a printed interconnect layer or a second foil layer.

<CIT> describes a front plane laminate useful in the manufacture of electro-optic displays comprising, in order, a light-transmissive electrically-conductive layer, a layer of an electro-optic medium in electrical contact with the electrically-conductive layer, an adhesive layer and a release sheet. This front plane laminate can be prepared as a continuous web, cut to size, the release sheet removed and the laminate laminated to a backplane to form a display. Methods for providing conductive vias through the electro-optic medium and for testing the front plane laminate are also described.

Thus, there is a need for improved mass production techniques associated with the manufacture of laminated electro-optic displays.

The present invention, provides a method of forming an electro-optic display in accordance with the appended claims.

These and other aspects of the present invention will be apparent in view of the following description.

The drawing Figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations.

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details.

In <FIG>, a schematic cross-section of a front plane laminate ("FPL") <NUM> is provided. The FPL <NUM> is similar to those described in aforementioned <CIT>. The FPL <NUM> comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer <NUM> is applied to a light-transmissive substrate <NUM>, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum <NUM> inches (<NUM>) in diameter, for example, without permanent deformation. The term "light-transmissive" is used herein throughout the specification and claims to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will normally be viewed through the electrically-conductive layer and adjacent substrate (if present); in cases where the electro-optic medium displays a change in reflectivity at non-visible wavelengths, the term "light-transmissive" should of course be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate <NUM> may be manufactured from glass or a polymeric film, for example, and may have a thickness in the range of about <NUM> to about <NUM> mil (<NUM> to <NUM>), preferably about <NUM> to about <NUM> mil (<NUM> to <NUM>). The top conductive layer <NUM> may comprise a thin metal or metal oxide layer of, for example, ITO, or may be a conductive polymer, such as PEDOT.

A coating of electro-optic medium <NUM> is applied over the top conductive layer <NUM>, such that the electro-optic medium <NUM> is in electrical contact with the top conductive layer <NUM>. The electro-optic medium <NUM> may, preferably, be in the form of an opposite charge dual particle encapsulated electrophoretic medium of the type described in <CIT>. The medium may comprise dispersion media encapsulated within a binder. The dispersion media may contain a hydrocarbon-based liquid in which are suspended negatively charged white particles and positively charged black particles. Upon application of an electrical field across the electro-optic medium <NUM>, the white particles may move to the positive electrode and the black particles may move to the negative electrode, for example, so that the electro-optic medium <NUM> appears, to an observer viewing the display through the substrate <NUM>, white or black depending upon whether the top conductive layer <NUM> is positive or negative relative to the backplane at any point within the final display. The electro-optic medium <NUM> may alternatively comprise a plurality of colored particles in addition to black and/or white particles, for example, each color having a respective charge polarity and strength.

A layer of lamination adhesive <NUM> is coated over the electro-optic medium layer <NUM>, and a release layer <NUM> is applied over the adhesive layer <NUM>. The release layer <NUM> may be a PET film, for example, that is approximately <NUM> mil in thickness, which may be provided with any appropriate release coating, for example a silicone coating. The presence of this lamination adhesive layer affects the electro-optic characteristics of the displays. In particular, the electrical conductivity of the lamination adhesive layer affects both the low temperature performance and the resolution of the display. The low temperature performance of the display may be improved by increasing the conductivity of the lamination adhesive layer, for example by doping the layer with tetrabutylammonium hexafluorophosphate or other materials as described in <CIT> and <CIT>. The FPL may optionally include a thin second conductive layer, preferably of aluminum, between the release sheet <NUM> and the laminate adhesive <NUM> that may be removed with the release sheet <NUM>. The second conductive layer may be used for testing of the electro-optic medium.

The FPL may also be provided in other forms, such as a "double release sheet" as described in <CIT> or an "inverted front plane laminate", as described in <CIT>. There are three main categories of backplanes: an active matrix, a passive matrix, and a direct drive backplane. Any type of backplane may be used in the present invention in order to provide a top plane connection within the perimeter of the laminate.

For an active matrix backplane, an array of thin film transistors (TFT) are formed on the surface of a substrate and each transistor acts as a switch for a pixel. The TFT is addressed by a set of narrow multiplexed electrodes (gate lines and source lines). A pixel is addressed by applying voltage to a gate line that switches the TFT on and allows a charge from the source line to flow on to the rear electrode. This sets up a voltage across the pixel and turns it on.

Passive-matrix backplanes use a simple grid to supply the charge to a particular pixel on the display. The grids are formed on top and bottom substrates. One substrate forms the "columns" and the other substrate forms the "rows". The wiring of the column or rows is made from a transparent conductive material, usually indium-tin oxide (ITO). The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row.

Assembly of an electro-optic display using FPL <NUM> may be effected by removing the release sheet <NUM> and contacting the adhesive layer <NUM> with a backplane under conditions effective to cause the adhesive layer <NUM> to adhere to the backplane, thereby securing the adhesive layer <NUM>, layer of electro-optic medium <NUM> and electrically-conductive top layer <NUM> to the backplane, and then cut into pieces of any size needed for use with specific backplanes.

The lamination of the FPL to the backplane may advantageously be carried out by vacuum lamination. Vacuum lamination is effective in expelling air from between the two materials being laminated, thus avoiding unwanted air bubbles in the final display; such air bubbles may introduce undesirable artifacts in the images produced on the display. However, vacuum lamination of the two parts of an electro-optic display in this manner imposes stringent requirements upon the lamination adhesive used, especially in the case of a display using an encapsulated electrophoretic medium. The lamination adhesive should have sufficient adhesive strength to bind the electro-optic layer to the backplane, and in the case of an encapsulated electrophoretic medium, the adhesive should also have sufficient adhesive strength to mechanically hold the capsules together. The adhesive is preferably chemically compatible with all the other materials in the display. If the electro-optic display is to be of a flexible type, the adhesive should have sufficient flexibility not to introduce defects into the display when the display is flexed. The lamination adhesive should have adequate flow properties at the lamination temperature to ensure high quality lamination. Furthermore, the lamination temperature is preferably as low as possible. An example of a useful lamination adhesive that may be incorporated in the various embodiments of the present invention an aqueous polyurethane dispersion known as a "TMXDI/PPO" dispersion, as described in <CIT>.

In the present invention, an electro-optic display is made according to a process in which an FPL is laminated to a backplane after a conductive material is applied to either a conductor located on the backplane or the adhesive layer of the FPL.

For example, referring to <FIG>, a conductive material <NUM> may first be applied to one of a plurality of conductors <NUM>, <NUM> located on a surface of a substrate <NUM> forming the backplane. The backplane may subsequently be laminated to an FPL comprising, in order, a light-transmissive substrate <NUM> (such as PET), a light-transmissive electrically conductive layer <NUM> (such as ITO), a layer of electro-optic media <NUM>, and a layer of lamination adhesive <NUM>. The lamination step is performed such that the conductive material <NUM> penetrates through the layer of electro-optic media <NUM> to provide an electrical connection between the conductor <NUM> and the electrically conductive layer <NUM>, thereby forming a TPC.

The conductive material <NUM> contacts the electrically conductive layer <NUM> and conductor <NUM> after lamination. However, if the conductivity of the conductive material <NUM> is sufficiently high, a conductive material <NUM> in close proximity to, but not contacting, one or both of the conductive layer <NUM> and conductor <NUM> may still provide a TPC.

As noted above, the conductive material <NUM> may alternatively be applied to the layer of lamination adhesive <NUM> prior to the lamination step. However, it is preferred to apply the conductive material <NUM> to the conductor <NUM> located on the backplane to avoid potential misalignment of the TPC after lamination.

The conductive material may comprises various materials known to those of skill in the art. For example, the conductive material may comprise conductive particles of carbon or non-reactive metals, such as gold, and/or anisotropic epoxy conductors. The conductive particles preferably have a mean particle diameter of <NUM> microns. Anisotropic epoxy materials may preferably be cast into specific shapes prior to application to the backplane or FPL prior to lamination. The shapes may be designed to facilitate penetration through the layer of electro-optic media. During high temperature lamination, the epoxy may maintain enough rigidity to displace ink and adhesive, but deform as a lamination roller presses the FPL and backplane together.

Claim 1:
A method of forming an electro-optic display (<NUM>) comprising:
forming a front plane laminate comprising, in this order, a first substrate (<NUM>), a first conductive layer (<NUM>), a layer of electro-optic medium (<NUM>), and an adhesive (<NUM>), the first substrate (<NUM>) and first conductive layer (<NUM>) being transparent;
providing a backplane (<NUM>) comprising a conductor (<NUM>, <NUM>) located on a surface of the backplane (<NUM>); and
applying a conductive material (<NUM>) to at least one of the conductor (<NUM>) and the adhesive (<NUM>);
characterised in that the method includes:
laminating the front plane laminate to the backplane (<NUM>) such that the conductive material (<NUM>) penetrates the layer of electro-optic medium (<NUM>) to provide an electrical connection between the first conductive layer (<NUM>) and the conductor (<NUM>), wherein the conductive material (<NUM>) contacts the first conductive layer (<NUM>) only after the laminating step.