HK1157450A

HK1157450A - Electro-optic displays

Info

Publication number: HK1157450A
Application number: HK11111475.8A
Authority: HK
Inventors: Richard J. Paolini; Thomas H. Whitesides; Steven J. O'neil; Gregg M. Duthaler
Original assignee: E Ink Corporation
Priority date: 2003-10-24
Filing date: 2011-10-25
Publication date: 2012-06-29

Description

Electro-optic display

The application is a divisional application with the application number of 200480031366.X and the application date of 2004, 10 and 22 months and the same subject

The present invention relates to electro-optic displays and methods and devices for producing such displays. As will be apparent from the following description, some aspects of the invention are limited to electrophoretic displays, while other aspects may use other types of electro-optic displays. More particularly, the present invention relates to (a) electro-optic media and displays having a binder that also acts as a lamination adhesive; (b) a method for forming a flexible display; (c) a color electro-optic display; (d) methods and devices for forming electro-optic displays; and (e) methods for manufacturing hybrid displays formed from materials having different coefficients of thermal expansion.

This application is related to the following patents and published applications, to which the reader is referred for more detailed background:

(a) U.S. patent application publication No.2002/0180688 (see related International application WO 99/53373);

(b) U.S. patent application publication nos. 2004/0136048;

(c) U.S. patent application publication No.2004/0027327 (see related International application WO 03/104884);

(d) U.S. patent application publication No.2004/0155857 (see related International application WO 2004/023195);

(e) U.S. patent nos. 6124851;

(f) U.S. patent nos. 6721083;

(g) U.S. patent nos. 6538801;

(h) U.S. patent nos. 6323989;

(i) U.S. patent nos. 6422687; and

(j) U.S. patent No. 6120588.

In the displays of the invention the electro-optic medium (when not an electrophoretic electro-optic medium) will typically be a solid (such displays may be referred to hereinafter for convenience as "solid electro-optic displays") in the sense that the electro-optic medium has a solid outer surface, although the medium may, and often does, have an internal liquid or gas filled space, and in the sense that the method of assembling the display uses such an electro-optic medium. Thus, the term "solid electro-optic display" includes encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other types of displays described below.

The term "electro-optic" as applied to a material or display is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states which differ in at least one optical property by the application of an electric field to the material from its first display state to its second display state. While the optical property is typically a color that is perceived by the human eye, it may be another optical property, such as light transmission, reflectance, luminescence, or, in the case of displays intended to be machine readable, pseudo-color in the sense of a change in electromagnetic wavelength reflectance outside the visible region.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states which differ in at least one optical property such that, after any given element is driven by an addressing pulse of finite duration, either its first or second display state is assumed, that state, after the addressing pulse has terminated, will last at least several times, for example at least four times, for the minimum duration of the addressing pulse required to change the state of the display element. Some particle-type electrophoretic displays, which may have a gray scale, are stable not only in their extreme black and white states, but also in their intermediate gray states, as is the case with other types of electro-optic displays, as indicated in U.S. patent application publication No. 2002/0180687. Such displays are properly referred to as "multi-stable" rather than bi-stable, although for convenience the term "bi-stable" may be used herein to cover both bi-stable and multi-stable displays.

Several types of electro-optic displays are known. One type of electro-optic display is the rotating bichromal member type, as described in: us patent nos.5808783, 5777782, 5760761, 6054071, 6055091, 6097531, 6128124, 6137467 and 6147791 (although such displays are often referred to as "rotating bicolor bead" displays, the term "rotating bicolor member" being preferred for greater accuracy because in some of the above patents the rotating member is not spherical). Such displays use a large number of small objects (spherical or cylindrical in shape) having two or more segments of different optical properties, and an internal dipole. These objects are suspended within a matrix within a liquid-filled vacuole that is filled with liquid so that the objects rotate at will. An electric field is applied to the display, thereby rotating the objects to different positions and changing what is seen through the viewing surface of a segment of the object, thereby changing the appearance of the display. Such electro-optic media are typically bistable.

Another type of electro-optic display uses an electrochromic medium, such as in the form of a nano-chromonic (nanochromic) thin film, that includes an electrode formed at least in part from a semiconducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, e.g., O' Regan, B., et al, Nature 1991, 353, 737; and Wood, d., Information Display, 18(3), 24 (3 months 2002). See also Bach, U.S. et al, adv.Mater, 2002, 14(11), 845 Nanophosphoric films of this type are also described, for example, in U.S. Pat. No.6301038, International application publication No. WO01/27690, and U.S. patent application 2003/0214695.

Another type of electro-optic display, which has been the subject of considerable research and development for many years, is the particle-type electrophoretic display, in which a plurality of charged particles are passed through a suspension under the influence of an electric field. Electrophoretic displays can be characterized by superior brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared to liquid crystal displays. However, problems associated with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays often settle, resulting in inadequate service life for these displays.

A number of patents and applications, assigned to or in the name of the Massachusetts Institute of Technology (MIT) and EInk companies, have recently been published, which describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of capsules, each capsule itself comprising an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsule itself is fixed within a polymer binder to form an adhesive layer between the two electrodes. Such encapsulation media are described, for example, in U.S. patent nos. 5930026; 5961804, respectively; 6017584, respectively; 6067185, respectively; 6118426, respectively; 6120588, respectively; 6120839, respectively; 6124851, respectively; 6130773, respectively; 6130774, respectively; 6172798, respectively; 6177921, respectively; 6232950, respectively; 6249721, respectively; 6252564, respectively; 6262706, respectively; 6262833, respectively; 6300932, respectively; 6312304, respectively; 6312971, respectively; 6323989, respectively; 6327072, respectively; 6376828, respectively; 6377387; 6392785, respectively; 6392786, respectively; 6413790, respectively; 6422687, respectively; 6445374, respectively; 6445489, respectively; 6459418, respectively; 6473072, respectively; 6480182, respectively; 6498114, respectively; 6504524, respectively; 6506438, respectively; 6512354, respectively; 6515649, respectively; 6518949, respectively; 6521489, respectively; 6531997, respectively; 6535197, respectively; 6538801, respectively; 6545291, respectively; 6580545, respectively; 6639578, respectively; 6652075, respectively; 6657772, respectively; 6664944, respectively; 6680725, respectively; 6683333, respectively; 6704133, respectively; 6710540, respectively; 6721083, respectively; 6727881, respectively; 6738050, respectively; 6750473, respectively; and 6753999; and U.S. patent application publication Nos. 2002/0019081; 2002/0021270, respectively; 2002/0060321, respectively; 2002/0060321, respectively; 2002/0063661, respectively; 2002/0090980, respectively; 2002/0113770, respectively; 2002/0130832, respectively; 2002/0131147, respectively; 2002/0171910, respectively; 2002/0180687, respectively; 2002/0180688, respectively; 2002/0185378, respectively; 2003/0011560, respectively; 2003/0020844, respectively; 2003/0025855, respectively; 2003/0038755, respectively; 2003/0053189, respectively; 2003/0102858, respectively; 2003/0132908, respectively; 2003/0137521, respectively; 2003/0137717, respectively; 2003/0151702, respectively; 2003/0214695, respectively; 2003/0214697, respectively; 2003/0222315, respectively; 2004/0008398, respectively; 2004/0012839, respectively; 2004/0014265, respectively; 2004/0027327, respectively; 2004/0075634, respectively; 2004/0094422, respectively; 2004/0105036, respectively; 2004/0112750, respectively; and 2004/0119681; and international application publication nos. wo 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; w O00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/107315; WO 2004/023195; and WO2004/049045.

Known electrophoretic media, encapsulated and unencapsulated, can be divided into two main types, hereinafter referred to for convenience as "single particles" and "double particles", respectively. A single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, at least one optical characteristic of the suspending medium being different from the optical characteristic of the particle. When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending on the relative potentials of the two electrodes, the medium may exhibit the optical properties of the particle (hereinafter the "front" electrode when the particle is adjacent to an electrode close to the viewer) or the optical properties of the suspension medium (hereinafter the "back" electrode when the particle is adjacent to an electrode far from the viewer, so that the particle is hidden by the suspension medium).

Dual particle media have two different types of particles that differ in at least one optical property, as well as a suspension that may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; such differences in mobility may be in terms of polarity (this type may be referred to hereinafter as "oppositely charged dual particle" medium) and/or quantity. When such a dual particle medium is placed between the pair of electrodes, the medium may exhibit the optical properties of either type of particle, depending on the relative potentials of the two electrodes, although the particular manner of implementation may differ depending on whether the difference in mobility is in terms of polarity or quantity. For ease of illustration, consider an electrophoretic medium in which one type of particles is black and the other type is white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles are negatively charged), these particles will be attracted to the two different electrodes, with the result that if, for example, the front electrode is negative with respect to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, with the result that the medium will appear black to the viewer. Conversely, if the front electrode is positive with respect to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, with the result that the medium will appear white to the viewer.

If the two types of particles have charges of the same polarity but differ in electrophoretic mobility (such media may be referred to hereinafter as "dual-particle of the same polarity" media), the two types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, with the result that the type facing the viewer will differ depending on the electrode to which the particles are attracted. For example, assume that the above example is changed so that the black and white particles are positively charged, but the black particles have higher electrophoretic mobility. If now the front electrode is negative with respect to the back electrode, the black-and-white particles will be attracted to the front electrode, but the black particles, because of their higher mobility, will reach the front electrode first, as a result of which the black particle layer will coat the front electrode and the medium will appear black to the viewer. Conversely, if the front electrode is positive with respect to the rear electrode, the black-and-white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility, will reach the rear electrode first, as a result of which the black particle layer will coat the rear electrode such that the white particle layer faces the viewer away from the rear electrode, as a result of which the medium will appear white to the viewer: such dual particle media require that the suspension be sufficiently transparent so that the white particle layer is readily visible to the viewer away from the rear electrode. Typically, the suspension in such displays is not colored at all, but some of the colorant may be incorporated in order to correct any undesirable color in the white particles seen therethrough.

Single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate between the two extreme optical states already described.

Some of the above-identified patents disclose encapsulated electrophoretic media having three or more different types of particles within each capsule. For the purposes of this application, such multi-particle media are considered to be a subset of dual-particle media.

Many of the above patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus making a so-called polymer-dispersed electrophoretic display, wherein 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 can be considered as capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, for example, 2002/0131147, supra. Thus, for purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subset of encapsulated electrophoretic media.

One related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and the suspension are not encapsulated in microcapsules, but are held in a plurality of cavities, typically polymer films, formed in a carrier medium. See, for example, international application publication No. wo02/01281, and published US application No.2002/0075556, both assigned to Sipix Imaging, Inc.

Although electrophoretic media are often opaque (because, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in reflection, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, the above-mentioned U.S. patent nos.6130774 and 6172798, and U.S. patent nos. 5872552; 6144361, respectively; 6271823, respectively; 6225971, and 6184856. A dielectrophoretic display, which is similar to an electrophoretic display but which operates in a similar manner by virtue of variations in electric field strength; see U.S. patent No. 4418346. Other types of electro-optic displays may also be capable of operating in a shutter mode.

Encapsulated or microcell electrophoretic displays generally do not suffer from the failure modes of clustering and settling of conventional electrophoretic devices and offer additional advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include all forms of printing and coating including, but not limited to, premetered coating such as sheet die coating, slot or die coating, ramp or cascade coating, curtain coating, roll coating such as knifeover roll coating, reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing methods, electrostatic printing methods, thermal printing methods, ink jet printing methods, and other similar techniques.) thus, the resulting display may be flexible. Further, it is inherently (using a variety of methods) printable display media that the display itself can be inexpensively manufactured.

An electro-optic display typically comprises a layer of electro-optic material and at least two other layers, one of which is an electrode layer, disposed on opposite sides of the electro-optic material. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define pixels of the display. For example, one electrode layer may be patterned as an extended row electrode and the other as an extended column electrode running at right angles to the row electrode, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more typically, one electrode layer is in the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each defining a 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 to the electro-optic layer comprises the electrode, the layers on opposite sides of the electro-optic layer being generally protective layers, which are intended to prevent the movable electrode from damaging the electro-optic layer.

The manufacture of a three-layer electro-optic display typically involves at least one lamination operation. For example, in several of the above-mentioned MIT and E Ink patents and applications, a method of manufacturing an encapsulated electrophoretic display is described in which an encapsulated electrophoretic medium comprising capsules in an adhesive is applied to a flexible substrate comprising indium-tin-oxide (ITO) or similar conductive coating (which serves as one electrode of the final display) applied to a thin plastic film, the capsule/adhesive coating being dried to form an adherent layer of electrophoretic medium which adheres tightly to the substrate. Separately, a backplane is prepared which includes an array of pixel electrodes and appropriate conductor arrangements to connect the pixel electrodes to drive circuitry. To form the final display, a lamination adhesive is used, with the substrate having the capsule/adhesive layer thereon laminated to the back plane. (very similar methods can be used to prepare electrophoretic displays that can be used 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 method, the backplane is itself flexible and can be prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique used by this method for mass production of displays is roll lamination using lamination adhesives. Similar manufacturing processes may be used for other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal member medium may be laminated to the backplane in substantially the same manner as the encapsulated electrophoretic medium.

As described in 2004/0027327 above, many devices for solid electro-optic displays and methods for making such displays have been derived from the technology used in liquid crystal displays (LCD's), which are, of course, also electro-optic displays, despite the use of liquids rather than solid media. For example, solid electro-optic displays may use an active matrix backplane (which comprises an array of transistors or diodes and a corresponding array of pixel electrodes) and a "continuous" front electrode on a transparent substrate (in terms of electrodes extending over a plurality of pixels and typically all of the display), these devices being substantially the same as in LCD's. However, the methods used to assemble LCD's cannot be used with solid electro-optic displays. Assembling the LCD' 8 is typically by forming the back plane and front electrodes on separate glass substrates, then adhesively securing the devices together leaving a small hole between them, placing the resulting assembly under vacuum and immersing the assembly in a liquid crystal bath so that liquid crystal flows through the hole between the back plane and the front electrode. Finally, the liquid crystal is put in place, sealing the aperture to provide the final display.

This LCD assembly process cannot be easily transferred to solid electro-optic displays. Since the electro-optic material is a solid, it must be present between the back plane and the front electrode before the two whole bodies are fixed to each other. Furthermore, the liquid crystal material is simply placed between the front electrode and the back plane without being connected to either, whereas in contrast to liquid crystal materials, solid electro-optic media typically need to be fixed to both, as this is typically easier than the following methods: the dielectric is formed on the back plane containing the circuitry and then the front electrode/electro-optic medium is laminated to the back plane (by covering the entire surface of the electro-optic medium with an adhesive and laminating under heat and pressure and perhaps vacuum).

As described in the above-mentioned us patent No.6312304, the following problems also exist in the manufacture of solid electro-optic displays: optical devices (electro-optic media) and electronic devices (in the back plane) have different performance criteria. For example, optics are desired to optimize reflectivity, contrast ratio, and response time, while electronics are desired to optimize conductivity, voltage-current relationship, and capacitance, or to have memory, logic, or other high-order electronics capabilities. The method of manufacturing an optical device is therefore not necessarily ideal for manufacturing an electronic device and vice versa. For example, a method of manufacturing an electronic device may include processing at high temperatures. The processing temperature may be from about 300 ℃ to about 600 ℃. However, many optical devices subjected to such high temperatures can be detrimental to the optical device by chemically decomposing the electro-optic medium or by causing mechanical damage.

This patent describes a method of manufacturing an electro-optic display comprising providing an adjustment layer (comprising a first substrate and electro-optic material provided adjacent the first substrate), the adjustment layer being capable of changing visual state when an electric field is applied; providing a pixel layer (comprising a second substrate, a plurality of pixel electrodes provided on a front surface of the second substrate, and a plurality of contact pads provided on a rear surface of the second substrate), each pixel electrode being connected to a contact pad by a via through the second substrate; providing a circuit layer (including a third substrate and at least one circuit element); the adjustment layer, the pixel layer and the circuit layer are laminated to form an electro-optic display.

Electro-optic displays are often expensive; for example, the cost of a color LCD in a portable computer typically accounts for a large percentage of the total cost of the computer. Because the use of electro-optic displays, such as cellular telephones and personal digital assistants (PDA's), is far less expensive than portable computers, there is a tremendous pressure to reduce the cost of such displays. As discussed above, the ability to form some solid electro-optic medium layers on flexible substrates by printing techniques opens up the possibility of reducing the cost of display electro-optic devices by using industrial equipment for the production of coated paper, polymer films and similar media using mass production processes such as roll-to-roll coating. However, such devices are expensive and areas currently on the market for electro-optic media may not be adequate for adjusting the dedicated devices, and so it may be necessary to transport the coated media from an industrial coating plant to the plant for final assembly of the electro-optic display without damaging the relatively brittle layer of electro-optic media.

Furthermore, most prior art processes for final lamination of electrophoretic displays are essentially batch processes, where the electro-optic medium, the lamination adhesive and the backplane are only put together just before final assembly, and it is desirable to provide a process that is better suited for mass production.

2004/0027327 above describes a method of assembling a solid electro-optic display (including a particle-type electrophoretic display) that is well suited for mass production. Essentially, this pending application describes a so-called "front plane stack" ("FPL") which in turn comprises a light-transmissive conductive layer; a solid electro-optic medium layer in electrical contact with the conductive layer; an adhesive layer and a release sheet layer. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible (in the sense that the substrate can be manually wound onto a 10 inch (254mm) diameter drum without permanent deformation). In this pending application and herein the term "light transmissive" means that the layer so designated emits sufficient light to enable a viewer, looking through the layer, to observe the change in display state of the electro-optic medium which would normally be viewed through the conductive layer and the adjacent substrate (if present). The substrate will typically be a polymer film, and will generally have a thickness of from about 1 to about 25 mils (25-634 μm), preferably from about 2 to about 10 mils (51-254 μm). The conductive layer is suitably a thin metal layer such as aluminium or ITO or may be a conductive polymer. Aluminum or ITO coated poly (ethylene terephthalate) (PET) film is commercially available, for example "aluminized Mylar" ("Mylar" is a registered trademark) available from e.i. du Pont DE Nemours & Company, Wilmington DE, which industrial material can be used with excellent front plane lamination.

Assembly of an electro-optic display using such a front plane laminate can be carried out by removing the release sheet layer from the front plane laminate and contacting the adhesive layer with the back plane under conditions effective to cause the adhesive layer to adhere to the back plane, thereby securing the adhesive layer, the electro-optic medium layer, and the conductive layer to the back plane. This method is well suited for mass production, as the front plane laminate can be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size required for use with a particular back plane.

2004/0027327 above also describes a method of detecting the electro-optic medium in the front plane laminate before incorporating the front plane laminate into a display. In this inspection method the release sheet has a conductive layer and a voltage sufficient to change the optical state of the electro-optic medium is applied between this conductive layer and the conductive layer on the opposite side of the electro-optic medium. The observation of the electro-optic medium then causes any defects in the display medium, thereby avoiding the cost of pressing a defective layer of electro-optic medium into the display, and scrapping the entire display (rather than just the defective front plane laminate).

2004/0027327 above also describes a second method of inspecting the electro-optic medium in the front plane laminate by placing an electrostatic charge on the release sheet layer, thereby forming an image on the electro-optic medium. This image is then viewed in the same manner as before to detect any defects in the electro-optic medium.

2004/0155857 describes a so-called "double release film", which is essentially a simplified version of the front plane laminate of 2004/0027327. One form of dual release sheet layer comprises a layer of solid electro-optic medium sandwiched between two layers of adhesive, one or both of which are covered by a release sheet layer. Another form of the dual release sheet layer includes a layer of solid electro-optic medium sandwiched between two release sheet layers. Both of these forms of dual release film are intended for use in a method of assembling an electro-optic display from a front plane laminate generally similar to that already described, but comprising two separate laminates; typically, in a first stack the dual release sheet layer is laminated to the front electrode to form a front sub-assembly, which is then laminated to the back plane in a second stack to form the final display.

All of the above methods of assembling solid electro-optic displays leave at least one layer of lamination adhesive between the electro-optic medium and one of the electrodes. This is disadvantageous in that it is generally desirable for electro-optic displays to switch as quickly as possible, and in order to achieve such a quick switching, it is necessary to provide as high an electric field as possible across the electro-optic layer. The presence of the lamination adhesive layer and the electro-optic layer between the electrodes necessarily reduces the effect of the electric field on the electro-optic layer at any given voltage condition between the electrodes, since some voltage drop must occur across the lamination adhesive layer; in fact, the lamination adhesive layer wastes part of the effective voltage.

Although one can compensate for the voltage drop across the adhesive layer by increasing the operating voltage of the display (i.e., the voltage difference between the electrodes), increasing the voltage across the electrodes in this manner is undesirable because it increases the power consumption of the display, and may require the use of more complex and expensive control loops to control the increased voltage involved.

As already mentioned, in encapsulated electrophoretic media, the electrophoretic layers typically comprise an adhesive surrounding the capsules and keeping them in the form of a mechanical adhesion layer. Other forms of solid electro-optic media may comprise similar binders; for example, the substrate of a rotating two-color component display may be considered an adhesive, such as may be an end wall of a microcell display. It has now been found that if the properties of this adhesive and the proportion of adhesive present in the electro-optic layer in at least some cases are carefully selected, the adhesive can also act as a lamination adhesive, thereby removing the need for a separate lamination adhesive layer and thus producing improved electro-optic performance in the final display.

Accordingly, in one aspect the invention provides a solid electro-optic display having a binder that also acts as a lamination adhesive.

A second aspect of the invention relates to a flexible display. Flexible display technology is highly desirable for many display applications. One application where flexibility is a critical factor is where the display is used on a mechanical or electrical sensing device where the response of the sensing device is produced by mechanical deformation, for example, by applying a transformation or by mechanically changing the spacing of capacitors or piezoelectric sensors or other electrical or electronic devices. The compliance and flexibility of the display is critical in these applications; if the display layer is too stiff, more force is required to operate the sensor and the effective sensing resolution of the device is reduced, since more than one sensing element may be operated by pressurization at a given point. One example of an application where the rigidity of the display assembly has proven important is in the telephone keypad where it is desirable to have a display on an array of microswitches that are manipulated by finger pressure. The rigidity of current packaged displays coated on thicker plastic supports and using plastic backplane has been shown to complicate the assembly of these keypads and reduce the tactile feel of the switching operation ("clicking" when switching off).

Thus, in a second aspect, the invention relates to a method of assembling a flexible electro-optic display; these methods use devices somewhat similar to the front plane laminate and dual release film described above.

Furthermore, as already mentioned, a further aspect of the invention relates to a color display. One of the problems associated with many electro-optic displays is the limited range of colors that each pixel of the display can produce. As discussed above, single particle and dual particle type electrophoretic displays typically display only two colors at each pixel, the color of the particles and suspension in single particle displays, and the color of the two types of particles in dual particle displays.

One way to extend the limited range of colors available from conventional electro-optic displays is to place a color filter array over the display pixels. For example, consider the effect of an array of color filters (e.g., red, green, and blue) placed over individual pixels of a display on a display that includes white particles in a black fluid. Moving the white particles close to the viewing surface of a pixel covering a red filter will make that pixel appear red, while moving the white particles of the same pixel close to the rear surface of the display will make that pixel appear black. The main problem with color generation in this way is that pixelation of the color filters limits the brightness of the display. For example, if a red color is desired, pixels covered by a red filter are set to appear red, while pixels covered by green and blue filters are set to appear dark, so only part of the display surface has the desired color and the remainder is dark, thus limiting the brightness of any color obtained. A reflective display that can have three optical states (black, white and color or black, white and transparent) would have significant advantages in terms of image quality, cost and ease of manufacture.

One aspect of the invention relates to the use of a shutter mode electro-optic medium to produce an improved color display.

Further, as already mentioned, further aspects of the invention relate to methods and devices for forming electro-optic displays using a front plane laminate and a dual release film as described above. In a practical industrial high-capacity process, it is currently necessary to connect the FPL or the dual release film to the back plane using a thermal lamination process. The backplane may be of the direct drive segmented type with one or more patterned conductive traces (conductive traces) or may be of the nonlinear circuit type (e.g., active matrix).

During the development of methods for laminating FPLs or dual release films to the back plane of a glass active substrate (thin film transistor arrays, or abbreviated TFT's), a number of problems associated with conventional lamination equipment are encountered. The present invention provides the required and desired variations of conventional tools to facilitate the processing of dual release films on FPL's and glass TFT's. The invention described herein can be used in the design of lamination tools for FPL type or dual release thin film type displays, which also use a plastic or metal foil backplane.

Finally, the invention relates to a method of manufacturing a hybrid display formed of materials having different coefficients of thermal expansion. A display battery. Electro-optic displays may be constructed using two glass plates. The first plate forms a front surface and provides one or more electrodes for addressing the electro-optical medium. The second plate forms the back surface and provides one or more electrodes (and perhaps non-linear elements such as thin film transistors) for addressing the electro-optic medium. Ideally, the materials used to form the front and back plates are similar in certain mechanical properties, such as Coefficient of Thermal Expansion (CTE) and coefficient of relative humidity expansion (CHE). Further, in some instances, it is desirable for the material to have a selected combination of thickness and Young's Modulus (E) in order to meet certain requirements of manufacture.

In other cases, such as when a display is formed using an FPL and glass or similar rigid backplane, the resulting "hybrid" electro-optic display inevitably causes the materials of the "plates" to differ in their mechanical properties, both in front of and behind the display. Such hybrid displays present new challenges in their manufacture. For example, a display constructed using encapsulated electrophoretic FPLs and glass TFT backplane actually has a plastic front plate laminated to the glass backplane with essentially asymmetric stacking of layers of heterogeneous materials. As a result of this configuration, the display exhibits mechanical features not found in conventional glass/glass displays. In particular, the asymmetric configuration results in roll-out (warping) of the composite panel in relation to changes in panel temperature or humidity. The stresses and strains associated with warpage present a significant challenge to the design of such systems. Thus, there is a need for panel processing, materials and construction methods that result in panels with acceptable performance over a wide range of operating conditions, and the present invention seeks to meet these requirements.

Accordingly, in one aspect, the present invention provides a method of manufacturing an encapsulated electrophoretic display, the method comprising:

providing an electrophoretic medium comprising a plurality of discrete droplets in a polymeric binder, each droplet comprising a plurality of charged particles, the charged particles being dispersed in a suspension and being movable through the suspension when an electric field is applied to the suspension;

providing a back plane having at least one electrode; and

the electrophoretic medium is laminated to the back plane at a temperature at which the polymeric binder will flow and the electrophoretic medium is in direct contact with the back plane, thereby causing the polymeric binder to flow and fix the electrophoretic medium to the back plane to form the display.

As will be readily apparent to those skilled in the art of electro-optic display manufacturing, this method differs from the conventional method of laminating an electro-optic medium to a backplane in that no lamination adhesive is required between the electro-optic display and the backplane; in effect, the polymer binder functions as both a binder and a lamination adhesive. Thus, for convenience, this process may be referred to hereinafter as the "adhesive-free" process of the present invention.

In this method, the electrophoretic medium may be of any of the types described above. Thus, for example, the electrophoretic medium may be a conventional encapsulated electrophoretic medium in which each droplet is confined to a wall separate from the polymeric binder (although such a wall may itself be comprised of a polymeric material). Alternatively, the electrophoretic medium may be of the polymer-dispersed type, in which the droplets form a discontinuous phase of a two-phase system and are surrounded by a continuous phase forming a polymeric binder.

In the meta-adhesive method of the invention, the electrophoretic medium may be placed on a light-transmissive substrate such that, after lamination, the electrophoretic medium is sandwiched between the substrate and the back plane. The light-transmissive electrode may be disposed between an electrophoretic medium and a substrate, the electrophoretic medium having a release sheet layer covering a surface of the electrophoretic medium remote from the substrate (i.e., the electrophoretic medium may be incorporated into the front plane stack as described above at 2004/0027327), and the release sheet layer being removed prior to lamination.

In the meta-adhesive method, lamination is performed under temperature conditions sufficient to cause the polymeric binder to flow, such that the binder will flow and fix the electrophoretic medium to the backplane. The temperature used should of course not be so high as to cause unacceptable damage to the electrophoretic medium or any other temperature sensitive devices present. Thus, the adhesive should be selected such that it flows under temperature conditions that allow lamination to occur without damaging the electrophoretic medium or other device. In general, it is desirable to use a polymeric binder that flows at a temperature of no more than 150 ℃ and preferably no more than 100 ℃. As detailed in 2003/0025855 above, selecting an lamination adhesive for an electrophoretic display is complicated by the fact that a number of factors must be considered, including the electrical properties of the adhesive, and the same applies to the polymeric binder which also functions as the lamination adhesive. Thus, for the same reasons as described above in 2003/0025855, it is generally preferred that the polymeric binder used in the non-adhesive process be polyurethane.

For reasons detailed below, the ratio of polymeric binder to droplets in the electrophoretic medium for the non-adhesive method is generally higher than for the prior art methods using an adhesive separate from the binder. In the adhesive-free process, typically the polymeric binder will comprise at least 20 wt% and desirably at least 30 wt% of the electrophoretic medium.

The back plane for the adhesive-free process may be of any type known in the art. For example, the backplane may be of the direct drive type having a plurality of pixel electrodes and conductive tracks by which the potentials on the pixel electrodes can be independently controlled. Alternatively, the backplane may be an active matrix backplane comprising a plurality of pixel electrodes and at least one non-linear element associated with each pixel electrode.

The present invention also provides an electrophoretic medium (which may be referred to hereinafter as the "adhesive-free medium" of the present invention) intended for use in the adhesive-free process as described above. The electrophoretic medium comprises a plurality of discrete droplets of the electrophoretic medium in a polymeric binder, each droplet comprising a plurality of charged particles dispersed in a suspension and capable of moving through the suspension when an electric field is applied to the suspension, wherein the polymeric binder flows at a temperature not exceeding 150 ℃.

In the present adhesive media, desirably the polymeric binder flows at a temperature of no more than about 100 ℃. The electrophoretic medium may be of any of the types described above. Thus, for example, the electrophoretic medium may be a conventional encapsulated electrophoretic medium in which each droplet is confined to a wall separate from the polymeric binder (although such a wall may itself be comprised of a polymeric material). Alternatively, the electrophoretic medium may be of the polymer-dispersed type, in which the droplets form a discontinuous phase of a two-phase system and are surrounded by a continuous phase forming a polymeric binder.

The adhesive-free medium of the present invention may be used in conjunction with a light-transmissive substrate covering one surface of the medium, optionally in conjunction with a light-transmissive electrode disposed between the electrophoretic medium and the substrate. The electrophoretic medium may have a release sheet layer that covers the surface of the electrophoretic medium remote from the substrate (i.e., the electrophoretic medium may be incorporated into a front plane stack, as described above at 2004/0027327).

For the reasons stated above, in an adhesive-free medium, typically the polymeric binder will comprise at least about 20 wt% and desirably at least about 30 wt% of the electrophoretic medium, and the polymeric binder may comprise polyurethane.

The present invention also provides a film for use in the manufacture of a display, the film comprising, in order:

a light-transmissive conductive layer;

an electrophoretic medium comprising a plurality of discrete droplets of the electrophoretic medium in a polymeric binder, each droplet comprising a plurality of charged particles, the charged particles being dispersed in a suspension and being movable through the suspension when an electric field is applied to the suspension, the polymeric binder flowing at a temperature not exceeding 150 ℃; and

a release sheet in contact with the polymeric binder.

This film is actually a front plane laminate as described above at 2004/0027327, modified to replace the electrophoretic medium and the laminate adhesive layer of the original front plane laminate with an electrophoretic medium having a binder (which may also function as a laminate adhesive according to the invention).

The present invention also provides a film for use in the manufacture of a display, the film comprising:

a layer of electrophoretic medium comprising a plurality of discrete droplets of the electrophoretic medium in a polymeric binder, each droplet comprising a plurality of charged particles, the charged particles being dispersed in a suspension and being movable through the suspension when an electric field is applied to the suspension, the polymeric binder flowing at a temperature not exceeding 150 ℃, the layer having first and second surfaces on opposite sides thereof;

a first release sheet layer covering the first surface of the electrophoretic medium layer; and

and the second release sheet layer covers the second surface of the electrophoretic medium layer.

This film is actually a dual release sheet layer as described above at 2004/0027327 modified to replace the electrophoretic medium and the lamination adhesive layer of the original front plane lamination with an electrophoretic medium having a binder (which may also function as a lamination adhesive according to the present invention).

In another aspect, the invention is directed to a method of forming a subassembly for an electro-optic display, the method comprising:

depositing an electro-optic medium layer on the first release sheet layer;

depositing a laminate adhesive layer on the second release sheet layer; and

thereafter, the electro-optic medium on the first release sheet layer is contacted with the lamination adhesive on the second release sheet layer under conditions effective to cause the lamination adhesive to adhere to the electro-optic medium, thereby forming a subassembly comprising the lamination adhesive and the electro-optic medium sandwiched between the two release sheet layers. This method, which is primarily although not exclusively intended for assembling flexible displays, may for convenience hereinafter be referred to as the "flexible sub-assembly process" of the invention. This method may further comprise removing the first release sheet layer from the subassembly and laminating the electro-optic medium to a back plane comprising at least one electrode. The method may further comprise laminating a lamination adhesive layer to the back plane prior to laminating the electro-optic medium thereto.

In another aspect, the invention provides an apparatus for displaying a color image, the apparatus comprising an electro-optic display having a plurality of pixels, wherein each pixel is independently settable to a light-transmissive optical state or a substantially opaque optical state, and light emitting means arranged to flash separate pulses of at least two different color lights onto one surface of the electro-optic display. In a further aspect, the present invention provides apparatus for producing pulses of light of different colours, the apparatus comprising a light source and a filtering assembly arranged to receive light from the light source, the filtering assembly comprising:

a first electro-optic layer having a light transmissive state and a colored state having a first optical characteristic;

a first electrode arranged to apply an electric field to the first electro-optic layer, the electric field being capable of switching the first electro-optic layer between its light-transmissive state and its colored state;

a second electro-optic layer having a light-transmissive state and a colored state having a second optical characteristic different from the first optical characteristic; and

a second electrode arranged to apply an electric field to the second electro-optical layer, the electric field being capable of switching the second electro-optical layer between its transmissive state and its colored state.

In another aspect, the present invention provides a first method for manufacturing a hybrid display, the first method comprising:

(a) providing a front plane stack comprising an electro-optic layer and a substrate, the front plane stack having a first Coefficient of Thermal Expansion (CTE);

(b) fabricating an electro-optic display by laminating a front plane laminate to a back plane, the back plane comprising at least one electrode, the back plane having a second CTE;

(c) heating the display to a temperature above a threshold temperature, thereby producing a heated display having a curvature; and

(d) the curvature is sufficiently reduced by gradually cooling to ambient temperature to relieve the structural stresses caused by any differential expansion of the front plane laminate and the back plane.

In another aspect, the present invention provides a second method for manufacturing a hybrid display, the second method comprising:

(a) adhering a front plane stack to a back plane, the front plane stack comprising a first material having a first Coefficient of Thermal Expansion (CTE) and the back plane comprising a second material having a second CTE, thereby fabricating a hybrid display having a first curvature

(b) The curvature of the hybrid display is reduced by forcing the display to temporarily assume a second curvature relative to the first curvature.

In another aspect, the present invention provides a third method for manufacturing a hybrid display, the third method comprising:

(a) providing a front plane laminate comprising a first material having a first Coefficient of Thermal Expansion (CTE);

(b) adhering a back plane to the front plane laminate, the back plane comprising a second material having a second CTE; and

(c) the hybrid display is fabricated by adhering a third panel to the back plane, the third panel comprising a material different from the second material such that the overall curvature of the hybrid panel is substantially reduced compared to a display consisting of only the front plane stack and the back plane, but not the third panel.

Finally, the present invention provides a fourth method for manufacturing a hybrid display, the fourth method comprising:

(a) adjusting a front plane stack to a first temperature, the front plane stack comprising a first material having a first Coefficient of Thermal Expansion (CTE);

(b) adjusting a back plane to a second temperature, the back plane comprising a second material having a second CTE; and

(c) the temperature conditioned front plane laminate is adhered to the temperature conditioned back plane to make a hybrid display.

In each of the above methods for manufacturing a hybrid display, the front plane stack may comprise an electrophoretic layer, which may be of any of the types described above.

Figure 1 of the accompanying drawings is a schematic cross-sectional view through a prior art front plane laminate as described above at 2004/0027327, but with the release layer removed in preparation for lamination to the back plane.

Figure 2 is a schematic cross-sectional view showing a prior art display resulting from laminating the front plane laminate shown in figure 1 to a back plane containing pixel electrodes.

Fig. 3 is a schematic cross-sectional view similar to fig. 1 through a front plane laminate of the present invention, except with the release layer removed in preparation for lamination to the back plane.

Figure 4 is a schematic cross-sectional view similar to figure 2 showing a display resulting from laminating the front plane laminate shown in figure 1 to a back plane containing pixel electrodes. Fig. 5 through 9 are schematic side views illustrating various steps in the flexible sub-assembly process of the present invention.

Fig. 10 is a schematic side view of an apparatus for displaying a color image of the present invention.

Fig. 11 is a schematic side view of an apparatus for producing pulses of light of different colors in accordance with the present invention.

Fig. 12 is a schematic cross-sectional view through a display cell of a hybrid electrophoretic display that may be fabricated in accordance with the present invention. Fig. 13A and 13B are schematic side views illustrating a method that may be used to mitigate warping in a display such as that shown in fig. 12.

FIG. 14 is a schematic side view showing a further method that may be used to prevent warping in a display such as that shown in FIG. 12.

FIG. 15 illustrates a method of manufacturing a hybrid display in which temperature adjustment is used to control any display warp tendencies. Fig. 16 is a greatly enlarged cross-sectional view as compared to fig. 12, showing one possible edge seal configuration that may be used in the display of fig. 12.

As already noted, the present invention has several different aspects relating to electro-optic displays, and methods and devices for producing such displays. These various aspects will be described in detail primarily below with the understanding that a single display, method or device may use more than one aspect of the present invention. For example, different methods of the present invention for manufacturing a hybrid display can be performed using the front plane laminate of the present invention.

Adhesive-free method, medium, front plane laminate and dual release film

All of the above aspects of the present invention are combined in that they all relate to the elimination of lamination adhesives commonly used between an encapsulated electrophoretic medium and at least one other device of an electrophoretic display; the other devices are typically back-plane, but in some cases the invention may allow for the elimination of a lamination adhesive between the electrophoretic medium and the front substrate, which provides the viewing surface through which the viewer intends to view the display. As already mentioned, the elimination of lamination adhesives is carried out by using, in the electrophoretic medium, adhesives that can flow under the temperature conditions used for lamination, and therefore in fact also act as lamination adhesives. The concept of flow is complex but in this context it refers to a polymeric material that has passed through a transition from an elastic to a plastic or viscous state.

The choice of binder for the adhesive-free medium and method of the invention must of course be taken into account not only its flow temperature but also its compatibility with another electrophoretic medium device and the requirements for driving the medium, including in particular the resistivity of the binder. Desirably, the adhesive flows at a temperature of no more than 150 ℃, and preferably at a temperature of no more than 100 ℃. Generally, the preferred type of adhesive is polyurethane; it has been found that certain polyurethanes can meet these preferred flow temperatures while still being compatible with all other devices commonly used in electrophoretic media.

Furthermore, in selecting the binder for use in the meta-adhesive media and methods of the present invention, attention should be paid not only to the type of binder used but also to the proportion of binder. As discussed in several of the above-mentioned E Ink and MIT patents and applications, encapsulated electrophoretic media prepared by applying a mixture of capsules and binder onto a substrate or polymer-dispersed type of media tend to have a non-planar surface because the individual capsules or droplets form "bumps" on the surface of the dried and/or cured electrophoretic media, and even more so when (as the case may be) the media consists essentially of a single layer of capsules or droplets. In conventional methods using a stack adhesive, the stack adhesive not only serves to adhere the electrophoretic medium to the backplane or other device, but also planarizes the initially non-planar surface of the electrophoretic medium, thereby avoiding various problems (e.g., formation of voids and irregular reaction of the electrophoretic medium to the applied electric field) that would otherwise occur if the non-planar surface of the electrophoretic medium were laminated to the planar surface of the backplane or other device.

When a lamination adhesive is eliminated in accordance with the present invention to avoid such problems, it is highly desirable that the flowable adhesive not only replaces the adhesive function of the previously used lamination adhesive, but also replaces its planarization function, and to enable the flowable adhesive to do so, it has been found that it is generally desirable to use a higher proportion of adhesive in the adhesive-free electrophoretic media of the present invention than is typically used in the prior art electrophoretic media (which are intended to be used with lamination adhesives). Consider, for example, an idealized encapsulated electrophoretic medium comprising a single layer of hexagonal closely packed spherical capsules against a substrate. The volume fraction of the capsules in the layer is about 60.5%, the remaining 39.5 vol% of the layer being occupied by the binder. This implies that a bladder to adhesive volume ratio of about 3: 2 will be sufficient to enable the adhesive to fill all the spaces between the bladders, thus planarizing the bladder layer. Further consideration of this idealized system suggests that a slightly larger ratio of adhesive (e.g., 1: 1 bladder to adhesive volume ratio) is desirable to ensure that excess adhesive covers the bladder layer, thus reducing the chance that the bladder wall may be damaged and perhaps burst during the lamination process.

However, as described in several of the above-mentioned E Ink and MIT patents and applications (see, in particular, U.S. patent nos.6067185 and 6392785), the encapsulated electrophoretic media actually fabricated differ greatly from an idealized model in that the initially spherical capsules flatten into oblate ellipsoids as the layer of electrophoretic media shrinks during drying or curing, and in at least some cases, because the shrinkage continues, the oblate ellipsoids contact each other and form planar contact regions that extend sufficiently perpendicular to the thickness of the layers that the final capsules have a substantially prismatic shape, ideally hexagonal cylinders. Similar effects are observed with polymer-dispersed electrophoretic media. In contrast to closely packed spherical capsules, oblate ellipsoidal and prismatic shaped capsules represent a significantly greater proportion of the electrophoretic medium, and therefore require less binder in the former case. In addition, the above discussion has focused on volume ratios, most binders tend to be slightly denser than most electrophoretic media, which are mostly composed of low density aliphatic hydrocarbon suspensions, and thus the weight ratio of the binder can be slightly higher than the volume ratio. The optimum proportion of binder for any particular medium is best determined empirically, but as a general guideline it can be stated that the polymeric binder should typically comprise at least 20% of the electrophoretic medium, desirably at least 30% by weight (the ratio being of course calculated on the weight of the substantially dry capsules and on the basis of the binder solid phase material (solid basis), as the binder is typically added as a latex). Typically, the optimum ratio will be from 2 to 3 parts by weight of capsules per part by weight of polymeric binder. The use of large excess amounts of binder should be avoided because such excess amounts tend to "dilute" the capsules beyond the point of their good packing, thereby degrading the electro-optic properties of the media.

The lamination step of the meta-adhesive process of the present invention may be performed using any technique known in the art. Thus, for example, lamination can be carried out using a roll-to-roll process, with the front plane laminate of the present invention (with the release sheet layer peeled therefrom) and the rolled back plane formed on the flexible substrate passing between heated rolls of the laminator.

The front plane laminate and dual release film of the present invention may include any of the optional features described above in 2004/0027327 and 2004/0155857. Thus, for example, the front plane stack may have conductive paths and contact pads as depicted at 2004/0027327. The release sheet layer of the front plane laminate may have a conductive layer to facilitate detection of the front plane laminate in the manner described above.

The front plane lamination of the present invention not only eliminates the layers of the prior art FPL (i.e., the lamination adhesive layer) but also simplifies the overall assembly process. In the prior art method described in 2004/0027327 above, the FPL is typically formed by coating a bladder/adhesive slurry onto a substrate comprising a polymer film containing an indium-tin-oxide (ITO) layer, the slurry being coated onto the ITO-covered surface of the film. The resulting bladder-coated film is then subjected to a first lamination in which a laminate adhesive layer is laminated to the exposed surface of the bladder/adhesive layer, and then a release sheet layer is applied to form the FPL. When the FPL is to be assembled into a display, the release sheet layer is removed and a second lamination is performed to secure the lamination adhesive to the backplane, thus forming the final display. The invention enables the first lamination to be rejected, thus simplifying the overall process of producing the display.

This aspect of the invention will now be illustrated with reference to figures 1-4 of the accompanying drawings. As already mentioned, fig. 1 shows a prior art front plane laminate as described above at 2004/0027327, but with the release layer removed in preparation for lamination to the back plane. As shown in fig. 1, the front plane stack comprises a front substrate 100, formed of a polymer thin film, an indium oxy-oxide containing layer 102 (the thickness of layer 102 is greatly exaggerated compared to the thickness of substrate 100 for ease of illustration), which will form the common front electrode of the final display. The front plane laminate further comprises an electrophoretic layer comprising capsules 104 in an adhesive 106, and a laminate adhesive layer 108.

Figure 2 shows the structure resulting from laminating the front plane laminate of figure 1 to a back plane 110 containing pixel electrodes (not shown). It will be seen that in the stack of figure 2, two electrophoretic layers 104/106 and an adhesive layer 108 are present between the planar electrode layer 102 and the pixel electrode.

Figure 3 shows a front plane laminate of the present invention with the release layer removed in preparation for lamination to the back plane. The front plane laminate of fig. 3 is generally similar to the front plane laminate of fig. 1, but with a thermally flowable adhesive 106' and no adhesive layer.

Finally, fig. 4 shows the structure resulting from the lamination of the front plane laminate of fig. 3 to the back plane 110. It will be seen that in the stack of figure 4 there is only an electrophoretic layer 104/106 between the front plane electrode layer 102 and the pixel electrode. Because the adhesive layer between the electrodes is eliminated, the stack of fig. 4 will typically convert substantially faster than the stack of fig. 2 at any given operating voltage. The following examples are now given, by way of illustration only, to illustrate preferred embodiments of the present invention.

Examples

Two-particle, opposite polarity electrophoretic capsules containing polymer-coated titanium dioxide and carbon black particles in an aliphatic hydrocarbon suspension and having gel/arabic capsule walls were prepared essentially as described above in example 30 of 2002/0185378. These capsules were then mixed with a conventional polyurethane latex adhesive in a 1: 1 weight ratio (capsule/adhesive solids), the result was slot coated onto a 5mil (127 μm) poly (ethylene terephthalate) (PET) film coated with ITO on one surface, and cured to produce a final capsule/adhesive layer comprising essentially a monolayer of capsules and having a thickness of 15-30 μm, essentially as described in this example 30. The resulting capsule-coated film is essentially an FPL of the invention, except that it has no release sheet, which is not necessary because the coated film is used immediately, as described below.

Using a hot roll laminator, the capsule coated film was then laminated to the back plane comprising a polymer film covered with a graphite layer, the electrophoretic layer was contacted with the graphite layer, and the resulting structure was cut to a size of 2 inch squares (51mm squares) to produce a pilot single pixel display that exhibited satisfactory electro-optic performance.

Further similar experiments have shown that satisfactory electro-optic performance can be achieved at lower adhesive to capsule weight ratios of about 1: 2 to 1: 3.

Flexible sub-assembly method

The flexible sub-assembly method of the present invention allows the assembly of extremely flexible and compliant displays to be well suited for applications such as those described above where flexibility is the most important element. A preferred sub-assembly method will now be described with reference to figures 5 to 9 of the accompanying drawings.

The preferred method starts with a first release sheet 500 (fig. 5). A layer of electro-optic medium 502 is coated or otherwise deposited on the first release sheet layer 500. Separately, a layer of lamination adhesive 504 is formed on a second release sheet layer 506 and laminated to the electro-optic medium 502 such that the lamination adhesive 504 adheres to the electro-optic medium 502.

In a further operation, as shown in FIG. 6, a back plane 508 is formed on a third release sheet layer 510 by screen printing or a similar deposition method (see the above-mentioned E Ink/MIT patent and application, a suitable method for forming such a back plane). Separately, a layer of laminate adhesive 512 is formed on the fourth release sheet layer 514 and laminated to the back plane 508 such that the laminate adhesive 512 adheres to the back plane 508.

In the next step of the method, the fourth release sheet layer 514 is removed from the structure shown in FIG. 6, thereby exposing the lamination adhesive 512, and the first sheet 500 is removed from the structure shown in FIG. 5, thereby exposing the electro-optic medium 502. The two resulting structures are then laminated together with a lamination adhesive 512 in contact with the electro-optic medium 502, thus forming the multi-layer structure shown in fig. 7.

In a further operation, another layer of laminate adhesive 516 is coated on the fifth release sheet layer 518. The third release sheet layer 510 is peeled away from the structure shown in figure 7 and the laminate adhesive 516 and fifth release sheet layer 518 are laminated to their back planar surfaces to produce the structure shown in figure 8.

The next step in the process is to affix the structure of figure 8 to a substrate, such as a pressure sensitive switch or sensing device, which has been covered with a thin layer of insulating material to insulate the electro-optic display device of the structure of figure 8. This lamination is effected by peeling the fifth release sheet layer 518 from the fig. 8 structure and laminating the lamination adhesive 516 so that it is exposed to the substrate 520 (fig. 9). Alternatively, the layer of laminating adhesive (which is in contact with the sensing or switching device) may be formed of a highly insulating pressure sensitive adhesive material, in which case the layer of insulating material may be eliminated. Finally, second release sheet layer 506 is removed to expose lamination adhesive 504, and lamination adhesive 504 is then laminated to top planar electrode 522, which top planar electrode 522 will typically be supported on a front substrate such as a polymer film (which serves as a protective layer to protect the final electro-optic display). Alternatively, the top planar electrode 522 may be formed by a coating process, such as by depositing a conductive polymer on the lamination adhesive 504.

Different alternatives can be used following the same basic patterning. For example, the electro-optic medium may be coated directly onto the conductive support in the first step of the method, thereby completely eliminating the removal of the second release sheet layer and the lamination or formation of the top planar electrode. This process results in a structure similar to that of fig. 9, but without the topmost lamination adhesive layer. Alternatively, rather than peeling the first release sheet layer to expose the electro-optic medium, the second release sheet layer may be peeled off to provide a two-layer coating with an exposed adhesive layer. This two-layer coating can then be laminated to the conductive layer on a thin plastic support. In this regard, the first release sheet may be removed and the last step of the process performed as previously described. Other assembly variations are also contemplated and thus a variety of different electro-optic display structures and devices may be constructed in a variety of ways using conventional methods.

The construction techniques illustrated herein specifically provide for the preparation of at least two novel structures comprising electro-optic display devices directly adhered to mechanical sensors. The difference between the two new structures is that one, the top of the display device, is protected by a protective sheet (typically a plastic film), while the second (where the top planar electrode is used as a conductive polymer material, such as PBDOT) has no further protective layer. The preferred configuration depends on the application and the desired durability of the device. In the case of mechanical sensors activated by buttons, the button face in contact with the surface of the electro-optical device should be made smooth and/or slightly compliant, and therefore, a protective plastic layer may not be required. If actuation of the sensor occurs through the use of a finger or stylus or other sharp object (as in touch screen applications), a tip protective layer would likely be required for durability. In either case, the durability of the device will be improved if the electro-optic medium is a polymer dispersed electrophoretic medium.

Other methods of applying the electro-optic medium may also be used. In particular, electrodeposition of an electrophoretic capsule and adhesive onto a patterned backplane (see international application PCT/US2004/009421) would be a particularly suitable method for bonding electro-optic media, and would eliminate some of the lamination/delamination steps of the above process. The present technology will also be able to enhance the display function using multiple spot colors other than black or white.

It will be appreciated from the above that the flexible sub-assembly method of the present invention can provide an electro-optic display that is directly coupled to a mechanical sensor and a general method of assembling such a device using a series of lamination steps. The direct coupling between the display and the sensor eliminates the need for at least one relatively rigid support sheet, improving the resolution and tactile feel of the coupling device. One such device that may be implemented by the present invention is a touch screen using an encapsulated electrophoretic display; the other is a telephone keypad with switchable button indicators. This is one use of an extremely powerful construction technique that can take advantage of the durability and flexibility of the encapsulated electrophoretic medium.

Apparatus for displaying color images and generating light pulses

As already mentioned, the invention provides a device for displaying a color image, and a device for generating pulses of light of different colors, the latter device being intended to be used as a sub-assembly in a device for displaying color images.

As mentioned above, encapsulated electrophoretic media and similar electro-optic media have been exemplified as being suitable for color filter arrays. However, there are several fundamental challenges associated with integrating color filter arrays, electrophoretic media, or other electro-optic media with electronic drivers. A key issue associated with such display designs relates to the use of sub-pixels to form the pixels of the display. In this configuration, the sub-pixels (e.g., red R, green G, and blue B) must be individually addressed in order to trick the human eye into seeing the spectrum of the primary color. In other words, the smallest addressable element displays only a single color and its hue, i.e., a sub-pixel cannot display the full spectrum of visible light. In a preferred embodiment, the smallest addressable element of the display can display each primary color (R, G and B in our example). The device of the present invention solves this problem using a louvered version of the electro-optic medium and a field sequential operation technique (i.e., a technique in which "sub-images" representing the individual colour channels of the overall image are separated in time rather than space, but the separation is in such a way that the overall colour image is seen by the observer's eyes).

There are two methods of using field sequential addressing to eliminate the problems described above. First, with the apparatus for displaying color images of the present invention, a color sequential backlight (backlight) (e.g., a backlight manufactured by LumiLEDs Corporation, San Jose, CA) may be used in the display, and an electro-optical sensor of "shutter mode" is used. The field sequential backlight flashes the primary colors of the display, such as red, green and blue, periodically, in synchronization with the shutter speed of the optical sensor. Full color images can be presented to the viewer by converting the optical sensor from clear to opaque using appropriate spatiotemporal control methods. Such a device is illustrated in a very schematic way in fig. 10 of the accompanying drawings. Fig. 10 shows an electro-optic display comprising a substrate 1000 comprising a plurality of pixel electrodes (not shown), a layer 1002 of an electro-optic medium, for example an encapsulated electrophoretic medium, and a continuous front electrode 1004. The display has a field sequential backlight 1006 that flashes red, green and blue, which is synchronized with the shutter speed of the electro-optic medium layer 1002. The sensor and backlight should switch quickly enough to allow the human eye to temporally integrate the colors emitted by the display.

Secondly, using stacked electro-optic films, a color sequential backlight (i.e., an apparatus of the present invention for generating pulses of light of different colors) can be constructed. These stacked electro-optic films, which are intended for use with "monochrome" dimmers (which may be of the electro-optic type as shown in fig. 10), only require continuous electrodes, so they will have a low production cost. Such a device is illustrated in a very schematic way in fig. 11 of the accompanying drawings. FIG. 11 shows a stacked color sequential back reflector comprising a blue shutter mode electro-optic medium layer 1100 (e.g., an encapsulated electrophoretic medium) having continuous electrodes 1102 and 1104, a green shutter mode electro-optic medium layer 1106 having continuous electrodes 1108 and 1110, and a red shutter mode electro-optic medium layer 1112 having continuous electrodes 1114 and 1116. (it will be apparent to those skilled in the art that by providing a voltage source capable of generating multiple drive voltages, each of the adjacent electrode pairs 1104/1108 and 1110/1114 can be eliminated.) in a full color optical sensor such as that shown in FIG. 11, the electro-optic medium films should be stacked to optimize their performance. For example, if the transparent conductor used for the film is particularly absorptive of a portion of the visible spectrum, that color should exhibit a higher chroma in the stack than the rest. A shutter mode optical sensor, or any other optical sensor (liquid crystal, suspended particle display, cholesteric liquid crystal, bistable nematic liquid crystal, etc.) can be used with the device as a monochromatic optical sensor.

Although fig. 11 illustrates the use of three stacked films, which is the most common configuration (red/green/blue or yellow/cyan/magenta), the present invention is not limited to the use of three stacked films; in some cases, it may be possible to produce a beneficial color image by carefully selecting the color range, using only two stacked films, or using more stacked films to improve the color gamut of the final image. Likewise, in the apparatus of the invention for displaying a color image, the light emitting means will typically be arranged to flash separate pulses of three different colors, although a smaller or larger number of colors may be used.

This aspect of the invention enables the use of shutter mode electro-optic media, particularly encapsulated electrophoretic media, to produce full colour displays. A device of the type shown in fig. 10 allows for very high color saturation and brightness, thus providing color sequential LED backlights with realistic color gamut performance and light output. A stacked electrophoretic shutter mode back side device of the type shown in fig. 11 is a low power, potentially low cost design for the device that generates the light pulses.

Method and device for forming electro-optic display

As already mentioned, during the development of a process for laminating FPL's to the back plane of a glass active substrate (usually carrying thin film transistor arrays, or abbreviated TFT's), a number of problems are encountered in connection with conventional lamination equipment. The present invention provides the required and desired variations of conventional tools to facilitate lamination of FPL's on glass TFT's. These variations can also be used to design a lamination tool for FPL type displays using plastic or metal foil backplane. The variants can be suitably divided into three main areas, namely the temperature control of the stack, the stacking of the FPL with respect to the back plane and the design of the stage where the lamination is performed, which are discussed separately below.

Temperature control of a laminate

Conventional polarizer laminators useful in Liquid Crystal Display (LCD) manufacturing are basically suitable for laminating FPL's to a backplane, provided that the conventional machines are modified to include a system for heating the parts to be laminated. There are preferred methods in heating the parts to be laminated, however, some are not readily apparent.

Most desirably, the heat is applied in a manner that raises the laminate bonding temperature sufficiently to cause plastic flow in the laminate adhesive, which is typically polyurethane. There are many metrics that can be used to describe plastic flow, which are well known to those skilled in the art. For the present purpose, it can be stated that the bonding layer temperature should be equal to or higher than the temperature at which the volume and shear modulus of the lamination adhesive are equal (hereinafter referred to as the intersection point). This temperature level can be achieved simply by heating the back plane to a temperature much higher than the intersection point and coating the FPL under pressure (applied by an unheated roller or mandrel).

In the improved method, the roll or mandrel may also be heated so that finer control of the temperature of the adhesive layer may be achieved. In another variation, the rollers, the backplane and the FPL themselves may be heated for a higher degree of control. In this last variation, preheating of the FPL provides a significant improvement in throughput because the adhesive is pre-softened before entering the area where lamination occurs. In all embodiments, thermally conductive plates (copper, aluminum, etc.) may be used to improve the thermal uniformity of the selected heating element.

Stack controlConventional polarizer laminators used in the LCD industry can place plastic films on glass substrates very precisely (± 0.2mm- ± 0.3mm stacking accuracy is common to state-of-the-art machines). When considering thermal lamination of FPL's, however, it was found that the lamination temperature could not be so high that the FPL tended to slip during the lamination process. There are four main parameters that affect FPL slip:

1. selection of lamination adhesive materials;

2. the thickness of the laminated adhesive;

3. lamination temperature, and

4. the force exerted on the FPL during lamination.

For example, for the currently preferred polyurethane laminate adhesive to be used at 18 μm thickness in an FPL, it is necessary to control the glass backplane to a temperature of less than 85 ℃ to prevent significant slippage of the FPL on the glass during lamination (assuming normal tension on the FPL during lamination and the use of unheated rollers). For a 15 μm thick adhesive, the temperature is less than 95 ℃, and slip is acceptable (for normal tension on the FPL during the process). Therefore, the four parameters described above should be controlled to avoid significant slippage of the FPL during lamination. Desirably, the slip is less than 1mm, more desirably less than 0.5mm, and most desirably less than 0.3 mm.

Workbench design

The present invention provides an improvement in two FPL support tables for an FPL lamination tool. The first improvement is the thermal control of the support table. This temperature control enables preheating of the FPL and thus the adhesive on the FPL, so that the lamination speed is greatly increased. Heating may be accomplished by conduction, convection, or radiation heating. The heating of the FPL should of course not be so intense as to damage the FPL.

Second, the table should be smooth so that it does not scratch the plastic surface of the FPL. The table may be coated or constructed with polytetrafluoroethylene (e.g., "TEFLON", a registered trademark sold by e.i. du Pont de nemours & co., Wilmington, Delaware, usa) or some similar soft, non-scratching plastic material. Alternatively, the table may be constructed of metal and coated or anodized with a non-scratching surface. Porous stone vacuum tables are available, but these tables can increase the tendency of the tool to scratch the FPL.

The present invention also provides the following further improvements for laminating the FPL to the backplane:

providing the ability to easily adjust the starting position of the FPL contacted by the roller. The ability to easily adjust the roll drop to the starting position of the FPL is provided. (in conventional LCD-type devices, rollers always fall off the edges of the film, which is undesirable for FPL lamination. simple adjustment of such machines enables the starting position and length of the FPL to be changed, rather than the starting position of the rollers; it is desirable to provide such adjustment.)

Heating the rollers is provided to produce better temperature uniformity and control, except for the possibility of temperature drift of the laminate adhesive layer during operation.

Variations of the thin film stage include providing vacuum holes to best match the finished shape of the FPL; a heating station (which may have a secondary effect on ensuring a uniform stack temperature, although roller heating is more important); a flat surface is provided on the table to minimize damage to the FPL.

Manufacturing hybrid displays

As already mentioned, the present invention provides several methods of manufacturing a hybrid display. This aspect of the invention relates to design methodologies that enable the fabrication of hybrid displays, i.e., displays constructed using front and back surfaces composed of heterogeneous materials. This aspect of the invention is applicable to various types of electro-optic displays.

As already noted, manufacturing a hybrid display is extremely complex, since problems arise that are almost not present in conventional display cell manufacturing. The present invention provides a method and apparatus for manufacturing a hybrid display and includes the following:

1. panel curvature resulting from CTE and CHE mismatch is often noted and stress relief or reduction methods and devices are used to correct the curvature;

2. carefully controlling environmental conditions during the manufacturing process; and/or

3. A balanced zero stress curvature is created that matches the curvature required for the product housing or structure.

In one embodiment of the invention, the bend reduction process includes a creep mechanism. Heating the hybrid display to a temperature above the threshold. The panel temperature is then gradually reduced, in some cases over a longer period of time, to relieve structural stresses caused by differential expansion between the display panels.

In another embodiment, the curvature reduction method reduces the curvature of the hybrid display by forcing the display to temporarily assume an opposite curvature and "snapping" the display back. For example, the display may be pressed between a weight and a curved surface to forcibly compensate for its innate curvature.

In a further embodiment, a third layer is adhered to the back of the back panel to compensate for physical differences between the panel stack and the back panel that would otherwise result in a greater degree of curvature. The additional third layer may be different from the back panel in many of the following physical properties: CTE, CHE, young's modulus, and thickness.

In yet another embodiment, the temperature of the front and back panels is adjusted to be different from each other such that the curvature is reduced (if not eliminated) when they are adhered to form a hybrid display. At least one of the panels may be a web moved by a roller.

According to this aspect of the invention, other aspects of the manufacturing process are also improved to reduce the risk of curvature of the display, for example, during edge sealing and structuring processes.

Fig. 12 illustrates an electrophoretic display cell of the type shown in fig. 20 and manufactured using the FPL process described above at 2004/0027327 (generally designated 1210). The display unit 1210 may be flexible, i.e., bendable or rollable without permanent deformation. In this configuration, the front plane 1212 of the display 1210 is plastic and the back plane 1214 is formed of glass and has an array of TFTs. There may be a further protective layer 1215 on the front plane 1212. The protective layer 1215 can be a uv resistant protective layer or a barrier to oxygen or moisture ingress into the display 1210. Alternatively, protective layer 1215 can provide additional impact resistance or can enhance certain optical effects, for example, with an anti-reflective coating.

An electrophoretic medium layer 1216 is located between front plane 1212 and back plane 1214. The electrophoretic layer 1216 may include one or more capsules 1218 in an adhesive 1220 (the electrophoretic particles are omitted from fig. 12 for clarity). Conductive layer 1222 serves as a common front electrode for display 1210 and is disposed between front plane 1212 and electrophoretic layer 1216. In one embodiment, conductive layer 1222 comprises a thin, light transmissive, conductive material, such as ITO, alumina, or a conductive polymer. A circuit board 1224 is schematically shown connected to the back plane 1214 for addressing the electrophoretic layer 1216 and the conductive layer 1222.

An adhesive layer 1226 may be disposed between the electrophoretic layer 1216 and the backplane 1214. Further layers and features may be present in display 1210 for various functions, such as barrier films (to further protect against external contaminants such as moisture), contact pads (for detecting addressing electrophoretic layer 1216), adhesion promoter layers, and the like. Some of those embodiments are described in more detail in 2004/0027327 and 2004/0155857, above. Seal 1230 may surround one or more edges of display 1210. In one embodiment, as already described, the layer between and including the front plane 1212 and the adhesive layer 1226 is first fabricated as an FPL1232 and then laminated to the back plane 1214. Protective layer 1215 may be considered to be a portion of the FPL if it is attached to the front plane 1212 before the FPL is laminated to the back plane 1214. For simplicity in illustration, FPL1232 is shown in the figures to include protective layer 1215. When the fabrication of the FPL1232 is complete, a release sheet layer (not shown) is temporarily attached to the adhesive layer 1226 opposite the electrophoretic layer 1216. The release sheet layer is removed prior to laminating the FPL1232 to the back plane 1214.

Examples of materials that may be used to make the front plane 1212 and/or the protective layer 15 include thermally stable poly (ethylene terephthalate) (e.g., Melinex grade 504 from Dupont teijin films, Wilmington, Delaware, usa) and high performance borosilicate glass (1737 from Corning Incorporated, Corning, New York, usa). In other embodiments, the PET film may be replaced with polyethylene naphthalate (PEN), Polyethersulfone (PES), or other optically transparent or near-transparent films may be constructed without departing from the scope of the present invention.

Other display unit structures similar to those shown in fig. 1 may be constructed without departing from the scope of the invention. Other embodiments include encapsulated electrophoretic and similar electro-optic display units that use:

1. a metal foil backplane, the surface of which contains one or more active or passive electronic circuits or devices,

2. a plastic film back plane, the surface of which contains one or more active or passive electronic circuits or devices,

an FPL comprising a rigid or flexible color filter array on the viewer-side surface of an electrophoretic medium and a display, or

4. A plastic or glass substrate with active or passive electronic circuits or devices and a color filter array (in either case, the front plane stack effectively becomes the back plane stack).

Those skilled in the art of electronic display design and integration can readily recognize other display configurations in which the principles of the present invention may be applied without departing from the scope of the invention.

In the display unit cell shown in fig. 12, there is a significant difference in mechanical properties between the materials making up the various layers of the display 1210, particularly between the FPL1232 and the back plane 1214. For example, thermally stable PET materials useful for making FPL1232 typically have a CTE of about 18 ppm/deg.C and a CHE of about 7 ppm/% relative humidity (relative humidity), while the glass making up the back plane 1214 has a CTE of about 3.76 ppm/deg.C and a CHE of about 0 ppm/% relative humidity. Due to these differences in properties, display unit 1210 exhibits very irregular mechanical properties. For example, when subjected to heat or moisture, the display panel 1210 will bend downward (in the side view of fig. 12, "frown"), and upon cooling or drying, the display panel 1210 will bend upward (in fig. 12, "smile"). This property is clearly undesirable in the final product. Moreover, the normal lamination process requires high temperatures to connect the FPL1232 to the glass/TFT back plane 1214. Therefore, without special measures, panel bending almost inevitably occurs during the manufacturing of the hybrid display.

To eliminate or reduce the bowing/warping phenomena, several methods and related devices are provided to eliminate or minimize panel bowing during and/or after manufacturing. In order to practice the invention, individual methods or combinations of these methods may be used.

Referring again to fig. 12, in a first approach, some or all of the display panel 10 is subjected to thermal cycling to reduce the stresses associated with CTE and CHE mismatch at high temperatures used for lamination operations in panel fabrication. In one embodiment, some or all of the display panel is heated in an oven or heater through a threshold temperature (e.g., 50 ℃ or 60 ℃) for a first period of time (e.g., 6-10 hours), after which the temperature ramp decreases for a second period of time and perhaps longer. The cooling/annealing process causes the polymer binder 1216, which bonds the encapsulated electrophoretic material 1218 together, and the stack adhesive, which adheres the encapsulated electrophoretic material 1218 to a substrate layer (e.g., conductive layer 1222), to release intrinsic stress through a creep mechanism. It is important that the display unit 1210 be generated at an appropriate time and temperature to adequately moderate the system to a desired state. In one embodiment, the temperature is gradually decreased (e.g., 1-2 degrees/hour) to ambient temperature over the second period of time. The cooling rate need not be constant and can vary.

FIG. 13A illustrates a second approach to reducing warpage, a "spring back" mechanism, which may be coupled with a creep mechanism as described above. The curved portion of display 1210 is temporarily placed on a curved correction surface having a curvature that is opposite to the curvature of display 1210. The curvature of the corrective surface may be smooth. In one embodiment, the corrective surface simply constitutes the protrusion 1340 on the flat surface. The display unit 1210 is secured against a correction surface 1340, for example, by a weight 1342 on the other side, perhaps under high temperature conditions, for example, 60 ℃. As shown in fig. 13B, the display unit 1210 is forced to assume a curvature defined by the protrusion 1340. After a suitable time (which may range from minutes to days), the quantity 1342 is removed, or the display panel 1210 is otherwise released. The display panel 1210 then springs back, in some cases, gradually to the desired shape, i.e., flat or with a prescribed curvature. The rebound process can also be temperature controlled, for example, the temperature is gradually reduced from a previously high temperature to ambient temperature.

FIG. 14 illustrates a further method for preventing warping in a hybrid display as shown in FIG. 12. In this approach, one or more layers 1446 of one or more different materials than back plane 1214 are attached to the back side of back plane 1214 and laminated with the rest of display 1210, including FPL1232, as a final product. The materials in the additional layer 1446 are selected to have a particular CTE, CHE, young's modulus, or thickness such that display 1210 exhibits little or no warping during lamination and subsequent processing. Due to the additional layer 1446, the display 10 exhibits mainly axial expansion and contraction without much warping, since it is mechanically a symmetric structure. The additional layer 1446 compensates for differences in CTE, CHE, young's modulus, or thickness between the FPL1232 and the back plane 1214 that would otherwise result in warpage. In one embodiment, the additional layer 1446 is composed of a material used to fabricate FPL1232, such as thermally stable PET, PEN, or PEg.

Fig. 15 illustrates a further method for controlling warpage by adjusting (i.e., heating or cooling) the FPL1232 back plane 1214 to different temperatures before, during, and/or after the lamination process. In fig. 15, a flex-on-flex (flex) structure or a roll-to-roll lamination process on the flex is shown as an example of this method. The web of FPL1232 with its release sheet layer 1233 stripped off is laminated to a web of back plane 1214 formed on a flexible substrate. The web of the back plane 1214 may use transistors formed of a polymer semiconductor, as described in some of the above-mentioned E Ink and MIT patents and published applications. This roll-to-roll lamination may be performed with the two webs passing through a nip between counter rolls 1548 and 1550 maintained at different temperatures. The FPL1232 and back plane 1214 are heated or cooled to different average temperatures by passing rapidly through rollers 1548 and 1550, respectively, causing differential expansion of the FPL1232 and back plane 1214 prior to and/or during lamination. After the roll-to-roll lamination process, the combined "display" web is cut to produce individual display units 1210. The differential expansion is set in a controlled manner to achieve the desired overall change in curvature of the final display unit 1210.

Other lamination processes may also be used. For example, in a "sheet-to-sheet" process, wherein individual FPL slices are laminated to individual back planes, each FPL sheet can be preheated or cooled to a temperature different from the back plane. Other suitable lamination processes include a "roll-to-roll" process in which a continuous roll of FPL stripped of any release sheet layer is laminated to a back plane arranged in a plurality of fixtures.

Other steps in the display manufacturing process, including heating, may also be improved by the present invention, as heating potentially causes panel warping. Because the encapsulated electrophoretic media are typically manufactured using aqueous coating techniques, the materials are inherently water-absorbent to some extent. However, the electrical properties of the material may change disadvantageously as water is absorbed into the system. To ensure reasonably uniform operation over a wide range of environmental conditions, the display must be sealed to prevent the ingress of moisture, for example, by the edge sealing and front barrier film techniques disclosed in 2004/0027327 above. In some sealing methods, heat is used and may contribute to panel warpage.

In one particular edge seal configuration illustrated in fig. 16, sealant 1652 is drawn into a thin cavity or gap around the perimeter of display unit 1210. The cavity top surface 1654 is formed by a protrusion of the front protective sheet 1215 on the FPL1232 support and the cavity bottom surface 1656 is formed by the front surface of the back plane 1214. In one form of the display, the display panel 1210 is sealed using a liquid adhesive that is cured by ultraviolet radiation and/or heat. Liquid sealant 1652 containing an adhesive can be drawn into the cavity by surface tension effects, but these effects are slowed by the viscosity of the adhesive. For high productivity, it is desirable to optimize the suction rate. To this end, the following aspects of the sealing method can be drawn or improved:

1. increasing the panel temperature reduces the adhesive viscosity;

2. increasing the temperature of the sealant dispenser head reduces the adhesive viscosity;

3. depositing a bead of adhesive large enough to completely fill the cavity and leave a chamfer of adhesive just outside the cavity;

4. an optimized cavity height (which is typically 100-300 μm);

5. ensuring that the adhesive substantially finishes all of the cavity-forming material;

6. making cavities of uniform width around the periphery of the panel, particularly at the corners;

7. using a blending system with XY (in the plane of the panel) drive control and Z (height above the panel) correction;

8. a deployment system that utilizes a deployment system with xy θ (θ dimension increases rotational capability, e.g., deployment needle bends at right angles near the tip) drive control and Z correction; and/or 9. a fitting system that can track edge features through machine vision and appropriate software algorithms is utilized.

The edge seal may be formed by a proper combination of the capillary suction process and the chamber design, material selection and process control system. For example, a thin chamber having a nominal dimension of 1.5mm wide by 0.22mm high may be treated with a UV curable adhesive (e.g., Nagase Chemtex corporation XNR-5516 model, Nagase)&Co., ltd., tokyo, japan). When the panel is held at an elevated temperature (e.g., 40-70 ℃), rapid imbibition can be achieved to reduce the viscosity of the adhesive. It is also preferable to control the temperature of the panel during the ultraviolet irradiation. For example, when the UV dosage is 40mW/cm²The panel may be controlled at 40 ℃ at 300-.

One of the most critical aspects of the sealing method described above involves heating the panel to reduce the adhesive viscosity (which can cause stress to develop in the display material and cause warping). To minimize or reduce this warping effect, according to one method of the present invention, the heating is limited to the perimeter of the panel 1210 to which the sealant 1652 is applied. This heating may be by radiation, conduction or convection means. If bulk heat must be applied, one of the other bend moderation methods of the present invention may be used, preferably before the adhesive is cured.

In some embodiments, display panel 1210 will be secured in a cabinet or other structure within the final product to ensure that it presents a flat or uniformly curved surface to a viewer. After the edge seal has been formed to conform the panel to the chassis or structure, bending of the display panel 1210 is undesirable because it adds additional stress to the laminate and edge seal. According to another aspect of the invention, the display panel is manufactured to have a "zero stress" equilibrium curvature that matches or substantially matches the desired curvature of the final structured product over the common operating temperature and relative humidity ranges. In one embodiment, the "zero stress" equilibrium curvature is set at or near the midpoint of the operating environment range.

For example, typical operating conditions for most electro-optic displays are about 0 ℃ to 40 ℃ and 10% to 90% relative humidity, but in some cases, operation is carried out over a broader range of temperatures and humidities. In addition, testing electro-optic displays typically includes thermal shock testing, which subjects the display to cycles of exposure to very low and very high temperatures (e.g., -30 ℃ to +80 ℃). For this wide temperature range, it is desirable to minimize the stress to which the panel will be subjected by placing the equilibrium curvature of the panel near the midpoint of the ambient range (e.g., temperatures of about 20-25℃. and 40-60% relative humidity).

One method of manufacturing a display panel with stress-free curvature is to select front and back panel materials with matching mechanical properties such as CTE and CHE. This design methodology can be used for display structures other than the plastic on glass structures discussed above, e.g., metal or plastic film back planes, Color Filter Array (CFA) on front planes, CFA on back planes, and so on. For example, for a stainless steel foil back plane, the CTE is about 17 ppm/deg.C and the CHE is about 0 ppm/% relative humidity. In this case, the bow is significantly reduced compared to the case of plastic on glass, but it is possible that non-zero or partial bow reduction techniques as described above may be used. For plastic backplane displays, it is desirable to use the same FPL and backplane materials. If this is not possible and a CTE or CHE mismatch occurs, the same bend reduction technique as described above can be used.

In displays incorporating Color Filter Arrays (CFAs), the challenges described herein combine the requirement for alignment of CFA sub-pixels with addressing elements on the backplane with accuracy. For a glass CFA on the back plane of a metal or plastic film, the CTE/CHE mismatch will result in the opposite performance to that described in the above example; heating or humidification results in a "smile" and cooling or drying results in a "frown". Despite this opposite performance, the same bend mitigation or reduction process as described above may be used to reduce or eliminate bends in the system. However, in this configuration, edge seals are preferably used to lock the FPL in a calibrated state prior to performing the bend relaxation process. If the edge seal is too compliant, differential expansion and contraction of the front and back planes will cause undesirable color shifts in the temperature dependent display.

In a preferred embodiment, a convoluted CFA structure is fabricated in which a flexible CFA is attached to a metal foil base plate having a CTE closely matched to the flexible CFA. For example, a flexible CFA can be constructed on thermally stable PET (CTE of about 18 ppm/c, CHE of about 7 ppm/% relative humidity) and a back plane can be constructed on stainless steel foil (CTE of about 17 ppm/c, CHE of about 0 ppm/% relative humidity). The front PET film may be insulated from moisture using a front barrier film. Film options include SiO₂，SO_xSiO, ITO or other transparent ceramic barrier films, or polymeric materials such as aclar (Honeywell International Corporation, Morristown, new jersey, usa). The use of this spacer minimizes PET expansion and contraction due to CHE action. With this type of assembly, the flexible active matrix backplane can be precisely aligned (to an accuracy in the range of several microns) under tensioned environmental conditions on a relatively large sheet (many tens of centimeters) of high resolution convoluted CFA (with sub-pixel spacing of hundreds of pixels per inch). Of course, reduced resolution embodiments can be readily constructed by those skilled in the art.

In another embodiment, the color pattern of the CFA is constructed directly below or directly above the TFTs on a single glass substrate and the display is viewed through the TFTs and color filters. Advantageously, this configuration does not require any alignment.

However, in a preferred embodiment, the electro-optic layer is attached to the TFT/CFA using a "dual release film" as described above and in 2004/0155857 above. According to this method, an electro-optic layer or film is applied to a release material and then transferred from the release material to the TFT/CFA. The dual release method uses a thin film of adhesive to attach the front surface of the electro-optic layer to the TFT/CFA and a second layer of laminating adhesive to attach the resulting structure to the backplane. The dual release approach provides two fundamental advantages, namely that the electro-optic layer front surface provides better optical properties than another approach in which the lamination adhesive faces the viewer, and because only a thin adhesive is present on the side closest to the TFT, the resolution of the display is improved due to reduced cross-talk between pixels.

Except as specifically noted above, for the preferred materials and preferred methods of forming electrophoretic and other electro-optic media used in the present invention, as well as for the formation of such media, similar prior art techniques are the same, and for an in-depth discussion of such preferred materials and methods used in the fabrication of encapsulated electrophoretic media, the reader is referred to the above-mentioned E Ink and MIT patents and applications.

Claims

1. A method for forming a sub-assembly for an electro-optic display, the method characterized by:

depositing a layer of electro-optic medium (502) on a first release sheet layer (500);

depositing a layer of lamination adhesive (504) on a second release sheet layer (506); and

thereafter, the electro-optic medium (502) on the first release sheet layer (500) is brought into contact with the lamination adhesive (504) on the second release sheet layer (506) under conditions effective to cause the lamination adhesive (504) to adhere to the electro-optic medium (502), thereby forming a subassembly comprising the lamination adhesive (504) and the electro-optic medium (502) sandwiched between the two release sheet layers (500, 506).