Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NOVEL ADDRESSING SCHEMES FOR ELECTROPHORETIC DISPLAYS
Field of the Invention
The present invention relates to addressing apparatus and methods for
electronic displays
and, in particular, to addressing apparatus and methods for encapsulated
electrophoretic displays.
Cross-Reference to Related Applications
This application is a continuation-in-part of U.S.S.N. 08/504,896 filed July
20, 1995,
U.S.S.N. 08/983,404 filed July I9, 1997, and U.S.S.N. 08/935,800 filed
September 23, 1997, the
contents of all of which are incorporated herein by reference. This
application claims priority to
U.S.S.N. 60/057,133 filed August 28, 1997, U.S.S.N. 60/057,716, filed August
28, 1997,
U.S.S.N. 60/057,122, filed August 28, 1997, U.S.S.N. 60/057,798, filed August
28, 1997,
U.S.S.N. 60/057,799 filed August 28, 1997, U.S.S.N. 60/057,163 filed August
28, 1997,
U.S.S.N. 60/057,118, filed August 28, 1997, U.S.S.N. 60/059,358, filed
September I9, 1997,
U.S.S.N. 60/059,543 filed September 19, 1997, U.S.S.N. 60/065,529, filed
November 11, 1997,
U.S.S.N. 60/065,630 filed November 18, 1997, U.S.S.N. 601065,605 filed
November 18, 1997,
U.S.S.N. 60/066,147, filed November 19, 1997, U.S.S.N, 60/066,245, filed
November 20, 1997,
U.S.S.N. 60/066,246, filed November 20, 1997, U.S.S.N. 60/066,115 filed
November 21, 1997,
U.S.S.N. 60/066,334 filed November 21, 1997, U.S.S.N. 60/066,418 filed
November 24, 1997,
U.S.S.N. 60/070,940 filed January 9, 1998, U.S.S.N. 60/071,371 filed January
15, 1998,
U.S.S.N. 60/072,390 filed January 9, 1998, U.S.S.N. 60/070,939 filed January
9, 1998, U.S.S.N.
601070,935 filed January 9, 1998, U.S.S.N. 60/074,454, filed February 12,
1998, U.S.S.N.
60/076,955 filed March 5, 1998, U.S.S.N. 60/076,959 filed March 5, 1998,
U.S.S.N. 601076,957
filed March 5, 1998, U.S.S.N. 60/076,956 filed March 5, 1998, U.S.S.N.
60/076,978 filed
March 5, 1998, U.S.S.N. 60/078,363 filed March 18, 1998, U.S.S.N. 60/081,374
filed April 10,
1998, U.S.S.N. 60/081,362 filed April 10, 1998, U.S.S.N. 60/083,252 filed
April 27, 1998,
U.S.S.N. 60/085,096 filed May 12, 1998, U.S.S.N. 60/090,223 filed June 22,
1998, U.S.S.N.
60/090,222 filed June 22, 1998, U.S.S.N. 60/090,232 filed June 22, 1998,
U.S.S.N. 60/092,046
filed July 8, 1998, U.S.S.N. 60/092,050 filed July 8, 1998, U.S.S.N.
60/092,742 filed July 14,
1998, and U.S.S.N. 60/093,689 filed July 22, 1998, the contents of all of
which are incorporated
herein by reference.
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Background of the Invention
Traditionally, electronic displays such as liquid crystal displays have been
made by
sandwiching an optoelectrically active material between two pieces of glass.
In many cases each
piece of glass has an etched, clear electrode structure formed using indium
tin oxide. A first
electrode structure controls all the segments of the display that may be
addressed, that is, changed
from one visual state to another. A second electrode, sometimes called a
counter electrode,
addresses all display segments as one large electrode, and is generally
designed not to overlap any
of the rear electrode wire connections that are not desired in the final
image. Alternatively, the
second electrode is also patterned to control specific segments of the
displays. In these displays,
1 o unaddressed areas of the display have a defined appearance.
Electrophoretic display media, generally characterized by the movement of
particles
through an applied electric field, are highly reflective, can be made
bistable, and consume very
little power. Encapsulated electrophoretic displays also enable the display to
be printed. These
properties allow encapsulated electrophoretic display media to be used in many
applications for
15 which traditional electronic displays are not suitable, such as flexible
displays. The electro-optical
properties of encapsulated displays allow, and in some cases require, novel
schemes or
configurations to be used to address the displays.
Summary of the Invention
An object of the invention is to provide a highly-flexible, reflective display
which can be
20 manufactured easily, consumes little (or no in the case of bistable
displays) power, and can,
therefore, be incorporated into a variety of applications. The invention
features a printable display
comprising an encapsulated electrophoretic display medium. The resulting
display is flexible.
Since the display media can be printed, the display itself can be made
inexpensively.
An encapsulated electrophoretic display can be constructed so that the optical
state of the
25 display is stable for some length of time. When the display has two states
which are stable in this
manner, the display is said to be bistable. If more than two states of the
display are stable, then
the display can be said to be multistable. For the purpose of this invention,
the term bistable will
be used to indicate a display in which any optical state remains fixed once
the addressing voltage
is removed. The definition of a bistable state depends on the application for
the display. A
30 slowly-decaying optical state can be effectively bistable if the optical
state is substantially
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unchanged over the required viewing time. For example, in a display which is
updated every few
minutes, a display image which is stable for hours or days is effectively
bistable for that
application. In this invention, the term bistable also indicates a display
with an optical state
sufficiently long-lived as to be effectively bistable for the application in
mind. Alternatively, it is
possible to construct encapsulated electrophoretic displays in which the image
decays quickly
once the addressing voltage to the display is removed (i.e., the display is
not bistable or
multistable). As will be described, in some applications it is advantageous to
use an encapsulated
electrophoretic display which is not bistable. Whether or not an encapsulated
electrophoretic
display is bistable, and its degree of bistability, can be controlled through
appropriate chemical
modification of the electrophoretic particles, the suspending fluid, the
capsule, and binder
materials.
An encapsulated electrophoretic display may take many forms. The display may
comprise
capsules dispersed in a binder. The capsules may be of any size or shape. The
capsules may, for
example, be spherical and may have diameters in the millimeter range or the
micron range, but is
preferably from ten to a few hundred microns. The capsules may be formed by an
encapsulation
technique, as described below. Particles may be encapsulated in the capsules.
The particles may
be two or more different types of particles. The particles may be colored,
luminescent, light-
absorbing or transparent, for example. The particles may include neat
pigments, dyed (faked)
pigments or pigment/polymer composites, for example. The display may further
comprise a
suspending fluid in which the particles are dispersed.
The successful construction of an encapsulated electrophoretic display
requires the proper
interaction of several different types of materials and processes, such as a
polymeric binder and,
optionally, a capsule membrane. These materials must be chemically compatible
with the
electrophoretic particles and fluid, as well as with each other. The capsule
materials may engage
in useful surface interactions with the electrophoretic particles, or may act
as a chemical or
physical boundary between the fluid and the binder.
In some cases, the encapsulation step of the process is not necessary, and the
electrophoretic fluid may be directly dispersed or emulsified into the binder
(or a precursor to the
binder materials) and an effective "polymer-dispersed electrophoretic display"
constructed. In
such displays, voids created in the binder may be referred to as capsules or
microcapsules even
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though no capsule membrane is present. The binder dispersed electrophoretic
display may be of
the emulsion or phase separation type.
Throughout the specification, reference will be made to printing or printed.
As used
throughout the specification, printing is intended to include all forms of
printing and coating,
including: premetered coatings such as patch die coating, slot or extrusion
coating, slide or
cascade coating, and curtain coating; roll coating such as knife over roll
coating, forward and
reverse roll coating; gravure coating; dip coating; spray coating; meniscus
coating; spin coating;
brush coating; air knife coating; silk screen printing processes;
electrostatic printing processes;
thermal printing processes; and other similar techniques. A "printed element"
refers to an element
formed using any one of the above techniques.
This invention provides novel methods and apparatus for controlling and
addressing
particle-based displays. Additionally, the invention discloses applications of
these methods and
materials on flexible substrates, which are useful in large-area, low cost, or
high-durability
applications.
In one aspect, the present invention relates to an encapsulated
electrophoretic display.
The display includes a substrate and at least one capsule containing a highly-
resistive fluid and a
plurality of particles. The display also includes at least two electrodes
disposed adjacent the
capsule, a potential difference between the electrodes causing some of the
particles to migrate
toward at least one of the two electrodes. This causes the capsule to change
optical properties.
In another aspect, the present invention relates to a colored electrophoretic
display. The
electrophoretic display includes a substrate and at least one capsule
containing a highly-resistive
fluid and a plurality of particles. The display also includes colored
electrodes. Potential
differences are applied to the electrodes in order to control the particles
and present a colored
display to a viewer.
In yet another aspect, the present invention relates to an electrostatically
addressable
display comprising a substrate, an encapsulated electrophoretic display
adjacent the substrate, and
an optional dielectric sheet adjacent the electrophoretic display. Application
of an electrostatic
charge to the dielectric sheet or display material modulates the appearance of
the electrophoretic
display.
3o In still another aspect, the present invention relates to an
electrostatically addressable
encapsulated display comprising a film and a pair of electrodes. The film
includes at least one
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capsule containing an electrophoretic suspension. The pair of electrodes is
attached to either side
of the film. Application of an electrostatic charge to the film modulates the
optical properties.
In still another aspect, the present invention relates to an electrophoretic
display that
comprises a conductive substrate, and at least one capsule printed on such
substrate. Application
of an electrostatic charge to the capsule modulates the optical properties of
the display.
In still another aspect the present invention relates to a method for matrix
addressing an
encapsulated display. The method includes the step of providing three or more
electrodes for
each display cell and applying a sequence of potentials to the electrodes to
control movement of
particles within each cell.
In yet another aspect, the present invention relates to a matrix addressed
electrophoretic
display. The display includes a capsule containing charged particles and three
or more electrodes
disposed adjacent the capsule. A sequence of voltage potentials is applied to
the three or more
electrodes causing the charged particles to migrate within the capsule
responsive to the sequence
of voltage potentials.
In still another aspect, the present invention relates to a rear electrode
structure for
electrically addressable displays. The structure includes a substrate, a first
electrode disposed on
a first side of the substrate, and a conductor disposed on a second side of
the substrate. The
substrate defines at least one conductive via in electrical communication with
both the first
electrode and the conductor.
Brief Description of the Drawings
The invention is pointed out with particularity in the appended claims. The
advantages of
the invention described above, together with further advantages, may be better
understood by
referring to the following description taken in conjunction with the
accompanying drawings. In
the drawings, like reference characters generally refer to the same parts
throughout the different
views. Also, the drawings are not necessarily to scale, emphasis instead
generally being placed
upon illustrating the principles of the invention.
FIG. lA is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display in which the smaller electrode has been
placed at a voltage
relative to the large electrode causing the particles to migrate to the
smaller electrode.
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FIG. 1B is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display in which the larger electrode has been
placed at a voltage
relative to the smaller electrode causing the particles to migrate to the
larger electrode.
FIG. 1C is a diagrammatic top-down view of one embodiment of a rear-addressing
electrode structure.
FIG. 2A is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure having a retroreflective layer associated with the larger electrode
in which the smaller
electrode has been placed at a voltage relative to the large electrode causing
the particles to
migrate to the smaller electrode.
FIG. 2B is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure having a retroreflective layer associated with the larger electrode
in which the larger
electrode has been placed at a voltage relative to the smaller electrode
causing the particles to
migrate to the larger electrode.
FIG. 2C is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure having a retroreflective layer disposed below the larger electrode
in which the smaller
electrode has been placed at a voltage relative to the large electrode causing
the particles to
migrate to the smaller electrode.
FIG. 2D is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure having a retroreflective layer disposed below the larger electrode
in which the larger
2o electrode has been placed at a voltage relative to the smaller electrode
causing the particles to
migrate to the larger electrode.
FIG. 3A is a diagrammatic side view of an embodiment of an addressing
structure in
which a direct-current electric field has been applied to the capsule causing
the particles to
migrate to the smaller electrode.
FIG. 3B is a diagrammatic side view of an embodiment of an addressing
structure in which
an alternating-current electric field has been applied to the capsule causing
the particles to
disperse into the capsule.
FIG. 3C is a diagrammatic side view of an embodiment of an addressing
structure having
transparent electrodes, in which a direct-current electric field has been
applied to the capsule
causing the particles to migrate to the smaller electrode.
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FIG. 3D is a diagrammatic side view of an embodiment of an addressing
structure having
transparent electrodes, in which an alternating-current electric field has
been applied to the
capsule causing the particles to disperse into the capsule.
FIG. 4A is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display in which multiple smaller electrodes
have been placed at a
voltage relative to multiple larger electrodes, causing the particles to
migrate to the smaller
electrodes.
FIG. 4B is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display in which multiple larger electrodes
have been placed at a
voltage relative to multiple smaller electrodes, causing the particles to
migrate to the larger
electrodes.
FIG. SA is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display having colored electrodes and a white
electrode, in which the
colored electrodes have been placed at a voltage relative to the white
electrode causing the
particles to migrate to the colored electrodes.
FIG. 5B is a diagrammatic side view of an embodiment of a rear-addressing
electrode
structure for a particle-based display having colored electrodes and a white
electrode, in which the
white electrode has been placed at a voltage relative to the colored
electrodes causing the
particles to migrate to the white electrode.
FIG. 6 is a diagrammatic side view of an embodiment of a color display element
having
red, green, and blue particles of different electrophoretic mobilities.
FIGs. 7A-7B depict the steps taken to address the display of FIG. 6 to display
red.
FIGs. 8A-8D depict the steps taken to address the display of FIG. 6 to display
blue.
FIGS. 9A-9C depict the steps taken to address the display of FIG. 6 to display
green.
FIG. 10 is a perspective embodiment of a rear electrode structure for
addressing a seven
segment display.
FIG. 11 is a perspective embodiment of a rear electrode structure for
addressing a three by
three matrix display element.
FIG. 12 is a cross-sectional view of a printed circuit board used as a rear
electrode
addressing structure.
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FIG. 13 is a cross-sectional view of a dielectric sheet used as a rear
electrode addressing
structure.
FIG. 14 is a cross-sectional view of a rear electrode addressing structure
that is formed by
printing.
FIG. 15 is a perspective view of an embodiment of a control grid addressing
structure.
FIG. 16 is an embodiment of an electrophoretic display that can be addressed
using a
stylus.
Detailed Description of the Invention
An electronic ink is an optoelectronically active material which comprises at
least two
9 o phases: an electrophoretic contrast media phase and a coatinglbinding
phase. The electrophoretic
phase comprises, in some embodiments, a single species of electrophoretic
particles dispersed in a
clear or dyed medium, or more than one species of electrophoretic particles
having distinct
physical and electrical characteristics dispersed in a clear or dyed medium.
In some embodiments
the electrophoretic phase is encapsulated, that is, there is a capsule wall
phase between the two
15 phases. The coating/6inding phase includes, in one embodiment, a polymer
matrix that surrounds
the electrophoretic phase. In this embodiment, the polymer in the polymeric
binder is capable of
being dried, crosslinked, or otherwise cured as in traditional inks, and
therefore a printing process
can be used to deposit the electronic ink onto a substrate. An electronic ink
is capable of being
printed by several different processes, depending on the mechanical properties
of the specific ink
20 employed. For example, the fragility or viscosity of a particular ink may
result in a different
process selection. A very viscous ink would not be well-suited to deposition
by an inkjet printing
process, while a fragile ink might not be used in a knife over roll coating
process.
The optical quality of an electronic ink is quite distinct from other
electronic display
materials. The most notable difference is that the electronic ink provides a
high degree of both
25 reflectance and contrast because it is pigment based (as are ordinary
printing inks). The light
scattered from the electronic ink comes from a very thin layer of pigment
close to the top of the
viewing surface. In this respect it resembles an ordinary, printed image.
Also, electronic ink is
easily viewed from a wide range of viewing angles in the same manner as a
printed page, and such
ink approximates a Lambertian contrast curve more closely than any other
electronic display
30 material. Since electronic ink can be printed, it can be included on the
same surface with any
other printed material, including traditional inks. Electronic ink can be made
optically stable in all
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display configurations, that is, the ink can be set to a persistent optical
state. Fabrication of a
display by printing an electronic ink is particularly useful in low power
applications because of this
stability.
Electronic ink displays are novel in that they can be addressed by DC voltages
and draw
very little current. As such, the conductive leads and electrodes used to
deliver the voltage to
electronic ink displays can be of relatively high resistivity. The ability to
use resistive conductors
substantially widens the number and type of materials that can be used as
conductors in electronic
ink displays. In particular, the use of costly vacuum-sputtered indium tin
oxide (ITO) conductors,
a standard material in liquid crystal devices, is not required. Aside from
cost savings, the
1 o replacement of ITO with other materials can provide benefits in
appearance, processing
capabilities (printed conductors), flexibility, and durability. Additionally,
the printed electrodes
are in contact only with a solid binder, not with a fluid layer (like liquid
crystals). This means that
some conductive materials, which would otherwise dissolve or be degraded by
contact with liquid
crystals, can be used in an electronic ink application. These include opaque
metallic inks for the
15 rear electrode (e.g., silver and graphite inks), as well as conductive
transparent inks for either
substrate. These conductive coatings include semiconducting colloids, examples
of which are
indium tin oxide and antimony-doped tin oxide. Organic conductors (polymeric
conductors and
molecular organic conductors) also may be used. Polymers include, but are not
limited to,
polyaniline and derivatives, polythiophene and derivatives, poly3,4-
ethylenedioxythiophene
20 (PEDOT) and derivatives, polypyrrole and derivatives, and
polyphenylenevinylene (PPV) and
derivatives. Organic molecular conductors include, but are not limited to,
derivatives of
naphthalene, phthalocyanine, and pentacene. Polymer layers can be made thinner
and more
transparent than with traditional displays because conductivity requirements
are not as stringent.
As an example, there are a class of materials called electroconductive powders
which are
25 also useful as coatable transparent conductors in electronic ink displays.
One example is Zelec
ECP electroconductive powders from DuPont Chemical Co. of Wilmington,
Delaware.
Referring now to FIGs. lA and 1B, an addressing scheme for controlling
particle-based
displays is shown in which electrodes are disposed on only one side of a
display, allowing the
display to be rear-addressed. Utilizing only one side of the display for
electrodes simplifies
30 fabrication of displays. For example, if the electrodes are disposed on
only the rear side of a
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display, both of the electrodes can be fabricated using opaque materials,
because the electrodes do
not need to be transparent.
FIG. lA depicts a single capsule 20 of an encapsulated display media. In brief
overview,
the embodiment depicted in FIG. lA includes a capsule 20 containing at least
one particle 50
dispersed in a suspending fluid 25. The capsule 20 is addressed by a first
electrode 30 and a
second electrode 40. The first electrode 30 is smaller than the second
electrode 40. The first
electrode 30 and the second electrode 40 may be set to voltage potentials
which affect the
position of the particles 50 in the capsule 20.
The particles 50 represent 0.1% to 20% of the volume enclosed by the capsule
20. In
1 o some embodiments the particles 50 represent 2.5% to 17.5% of the volume
enclosed by capsule
20. In preferred embodiments, the particles 50 represent 5% to 15% of the
volume enclosed by
the capsule 20. In more preferred embodiments the particles 50 represent 9% to
11% of the
volume defined by the capsule 20. In general, the volume percentage of the
capsule 20 that the
particles SO represent should be selected so that the particles 50 expose most
of the second, larger
electrode 40 when positioned over the first, smaller electrode 30. As
described in detail below,
the particles 50 may be colored any one of a number of colors. The particles
50 may be either
positively charged or negatively charged.
The particles 50 are dispersed in a dispersing fluid 25. The dispersing fluid
25 should have
a low dielectric constant. The fluid 25 may be clear, or substantially clear,
so that the fluid 25
2o does not inhibit viewing the particles 50 and the electrodes 30, 40 from
position 10. In other
embodiments, the fluid 25 is dyed. In some embodiments the dispersing fluid 25
has a specific
gravity matched to the density of the particles 50. These embodiments can
provide a bistable
display media, because the particles 50 do not tend to move in certain
compositions absent an
electric field applied via the electrodes 30, 40.
The electrodes 30, 40 should be sized and positioned appropriately so that
together they
address the entire capsule 20. There may be exactly one pair of electrodes 30,
40 per capsule 20,
multiple pairs of electrodes 30, 40 per capsule 20, or a single pair of
electrodes 30, 40 may span
multiple capsules 20. In the embodiment shown in FIGS. lA and 1B, the capsule
20 has a
flattened, rectangular shape. In these embodiments, the electrodes 30, 40
should address most, or
3o all, of the flattened surface area adjacent the electrodes 30, 40. The
smaller electrode 30 is at
most one-half the size of the larger electrode 40. In preferred embodiments
the smaller electrode
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is one-quarter the size of the larger electrode 40; in more preferred
embodiments the smaller
electrode 30 is one-eighth the size of the larger electrode 40. In even more
preferred
embodiments, the smaller electrode 30 is one-sixteenth the size of the larger
electrode 40. It
should be noted that reference to "smaller" in connection with the electrode
30 means that the
electrode 30 addresses a smaller amount of the surface area of the capsule 20,
not necessarily that
the electrode 30 is physically smaller than the larger electrode 40. For
example, multiple capsules
20 may be positioned such that less of each capsule 20 is addressed by the
"smaller" electrode 30,
even though both electrodes 30, 40 are equal in size. It should also be noted
that, as shown in
FIG. 1 C, electrode 3 0 may address only a small corner of a rectangular
capsule 20 (shown in
phantom view in FIG. 1C), requiring the larger electrode 40 to surround the
smaller electrode 30
on two sides in order to properly address the capsule 20. Selection of the
percentage volume of
the particles 50 and the electrodes 30, 40 in this manner allow the
encapsulated display media to
be addressed as described below.
Electrodes may be fabricated from any material capable of conducting
electricity so that
electrode 30, 40 may apply an electric field to the capsule 20. As noted
above, the rear-addressed
embodiments depicted in FIGs. lA and 1B allow the electrodes 30, 40 to be
fabricated from
opaque materials such as solder paste, copper, copper-clad polyimide, graphite
inks, silver inks
and other metal-containing conductive inks. Alternatively, electrodes may be
fabricated using
transparent materials such as indium tin oxide and conductive polymers such as
polyaniiine or
2o polythiopenes. Electrodes 30, 40 may be provided with contrasting optical
properties. In some
embodiments, one of the electrodes has an optical property complementary to
optical properties
of the particles 50.
In one embodiment, the capsule 20 contains positively charged black particles
50, and a
substantially clear suspending fluid 25. The first, smaller electrode 30 is
colored black, and is
smaller than the second electrode 40, which is colored white or is highly
reflective. When the
smaller, black electrode 30 is placed at a negative voltage potential relative
to larger, white
electrode 40, the positively-charged particles 50 migrate to the smaller,
black electrode 30. The
effect to a viewer of the capsule 20 located at position 10 is a mixture of
the larger, white
electrode 40 and the smaller, black electrode 30, creating an effect which is
largely white.
3o Referring to FIG. 1B, when the smaller, black electrode 30 is placed at a
positive voltage
potential relative to the larger, white electrode 40, particles 50 migrate to
the larger, white
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electrode 40 and the viewer is presented a mixture of the black particles 50
covering the larger,
white electrode 40 and the smaller, black electrode 30, creating an effect
which is largely black.
In this manner the capsule 20 may be addressed to display either a white
visual state or a black
visual state.
Other two-color schemes are easily provided by varying the color of the
smaller electrode
30 and the particles 50 or by varying the color of the larger electrode 40.
For example, varying
the color of the larger electrode 40 allows fabrication of a rear-addressed,
two-color display
having black as one of the colors. Alternatively, varying the color of the
smaller electrode 30 and
the particles 50 allow a rear-addressed two-color system to be fabricated
having white as one of
the colors. Further, it is contemplated that the particles 50 and the smaller
electrode 30 can be
. different colors. In these embodiments, a two-color display may be
fabricated having a second
color that is different from the color of the smaller electrode 30 and the
particles 50. For
example, a rear-addressed, orange-white display may be fabricated by providing
blue particles 50,
a red, smaller electrode 30, and a white (or highly reflective) larger
electrode 40. In general, the
optical properties of the electrodes 30, 40 and the particles 50 can be
independently selected to
provide desired display characteristics. In some embodiments the optical
properties of the
dispersing fluid 25 may also be varied, e.g, the fluid 25 may be dyed.
In other embodiments the larger electrode 40 may be reflective instead of
white. In these
embodiments, when the particles 50 are moved to the smaller electrode 30,
light reflects offthe
2o reflective surface 60 associated with the larger electrode 40 and the
capsule 20 appears light in
color, e.g. white (see FIG. 2A). When the particles 50 are moved to the larger
electrode 40, the
reflecting surface 60 is obscured and the capsule 20 appears dark (see FIG.
2B) because light is
absorbed by the particles 50 before reaching the reflecting surface 60. The
reflecting surface 60
for the larger electrode 40 may possess retroflective properties, specuiar
reflection properties,
diffuse reflective properties or gain reflection properties. In certain
embodiments, the reflective
surface 60 reflects light with a Lambertian distribution The surface 60 may be
provided as a
plurality of glass spheres disposed on the electrode 40, a diffractive
reflecting layer such as a
holographically formed reflector, a surface patterned to totally internally
reflect incident light, a
brightness-enhancing film, a diffuse reflecting layer, an embossed plastic or
metal film, or any
other known reflecting surface. The reflecting surface 60 may be provided as a
separate layer
laminated onto the larger electrode 40 or the reflecting surface 60 may be
provided as a unitary
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part of the larger electrode 40. In the embodiments depicted by FIGs. 2C and
2D, the reflecting
surface may be disposed below the electrodes 30, 40 vis-a-vis the viewpoint
10. In these
embodiments, electrode 30 should be transparent so that light may be reflected
by surface 60. In
other embodiments, proper switching of the particles may be accomplished with
a combination of
alternating-current (AC) and direct-current (DC) electric fields and described
below in connection
with FIGS. 3A-3D.
In still other embodiments, the rear-addressed display previously discussed
can be
configured to transition between largely transmissive and largely opaque modes
of operation
(referred to hereafter as "shutter mode"). Referring back to FIGS. 1 A and 1
B, in these
embodiments the capsule 20 contains at least one positively-charged particle
50 dispersed in a
substantially clear dispersing fluid 25. The larger electrode 40 is
transparent and the smaller
electrode 30 is opaque. When the smaller, opaque electrode 30 is placed at a
negative voltage
potential relative to the larger, transmissive electrode 40, the particles 50
migrate to the smaller,
opaque electrode 30. The effect to a viewer of the capsule 20 located at
position 10 is a mixture
of the larger, transparent electrode 40 and the smaller, opaque electrode 30,
creating an effect
which is largely transparent. Referring to FIG. 1B, when the smaller, opaque
electrode 30 is
placed at a positive voltage potential relative to the larger, transparent
electrode 40, particles 50
migrate to the second electrode 40 and the viewer is presented a mixture of
the opaque particles
50 covering the larger, transparent electrode 40 and the smaller, opaque
electrode 30, creating an
effect which is largely opaque. In this manner, a display formed using the
capsules depicted in
FIGS. lA and 18 may be switched between transmissive and opaque modes. Such a
display can
be used to construct a window that can be rendered opaque. Although FIGS. lA-
2D depict a pair
of electrodes associated with each capsule 20, it should be understood that
each pair of electrodes
may be associated with more than one capsule 20.
A similar technique may be used in connection with the embodiment of FIGs. 3A,
3B, 3C,
and 3D. Refernng to FIG. 3A, a capsule 20 contains at least one dark or black
particle 50
dispersed in a substantially clear dispersing fluid 25. A smaller, opaque
electrode 30 and a larger,
transparent electrode 40 apply both direct-current (DC) electric fields and
alternating-current
(AC) fields to the capsule 20. A DC field can be applied to the capsule 20 to
cause the particles
50 to migrate towards the smaller electrode 30. For example, if the particles
50 axe positively
charged, the smaller electrode is placed a voltage that is more negative than
the larger electrode
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40. Although FIGS. 3A-3D depict only one capsule per electrode pair, multiple
capsules may be
addressed using the same electrode pair.
The smaller electrode 30 is at most one-half the size of the larger electrode
40. In
preferred embodiments the smaller electrode is one-quarter the size of the
larger electrode 40; in
more preferred embodiments the smaller electrode 30 is one-eighth the size of
the larger electrode
40. In even more preferred embodiments, the smaller electrode 30 is one-
sixteenth the size of the
larger electrode 40.
Causing the particles SO to migrate to the smaller electrode 30, as depicted
in FIG. 3A,
allows incident light to pass through the larger, transparent electrode 40 and
be reflected by a
reflecting surface 60. In shutter mode, the reflecting surface 60 is replaced
by a translucent layer,
a transparent layer, or a layer is not provided at all, and incident light is
allowed to pass through
the capsule 20, i.e. the capsule 20 is transmissive.
Referring now to FIG. 3B, the particles 50 are dispersed into the capsule 20
by applying
an AC field to the capsule 20 via the electrodes 30, 40. The particles 50,
dispersed into the
capsule 20 by the AC field, block incident light from passing through the
capsule 20, causing it to
appear dark at the viewpoint 10. The embodiment depicted in FIGs. 3A-3B may be
used in
shutter mode by not providing the reflecting surface 60 and instead providing
a translucent layer,
a transparent layer, or no layer at all. In shutter mode, application of an AC
electric field causes
the capsule 20 to appear opaque. The transparency of a shutter mode display
formed by the
apparatus depicted in FIGS. 3A-3D may be controlled by the number of capsules
addressed using
DC fields and AC fields. For example, a display in which every other capsule
20 is addressed
using an AC field would appear fifty percent transmissive.
FIGs. 3C and 3D depict an embodiment ofthe electrode structure described above
in
which electrodes 30, 40 are on "top" of the capsule 20, that is, the
electrodes 30, 40 are between
the viewpoint 10 and the capsule 20. In these embodiments, both electrodes 30,
40 should be
transparent. Transparent polymers can be fabricated using conductive polymers,
such as
polyaniline, polythiophenes, or indium tin oxide. These materials may be made
soluble so that
electrodes can be fabricated using coating techniques such as spin coating,
spray coating,
meniscus coating, printing techniques, forward and reverse roll coating and
the like. In these
3o embodiments, light passes through the electrodes 30, 40 and is either
absorbed by the particles 50,
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reflected by retroreflecting layer 60 (when provided), or transmitted
throughout the capsule 20
(when retroreflecting layer 60 is not provided).
The addressing structure depicted in FIGs. 3A-3D may be used with
electrophoretic
display media and encapsulated electrophoretic display media. FIGs. 3A-3D
depict embodiments
in which electrode 30, 40 are statically attached to the display media. In
certain embodiments, the
particles 50 exhibit bistability, that is, they are substantially motionless
in the absence of a electric
field. In these embodiments, the electrodes 30, 40 may be provided as part of
a "stylus" or other
device which is scanned over the material to address each capsule or cluster
of capsules. This
mode of addressing particle-based displays will be described in more detail
below in connection
1 o with FIG. 16.
Referring now to FIGS. 4A and 4B, a capsule'20 of a electronically addressable
media is
illustrated in which the technique illustrated above is used with multiple
rear-addressing
electrodes. The capsule 20 contains at least one particle 50 dispersed in a
clear suspending fluid
25. The capsule 20 is addressed by multiple smaller electrodes 30 and multiple
larger electrodes
15 40. In these embodiments, the smaller electrodes 30 should be selected to
collectively be at most
one-half the size of the larger electrodes 40. In further embodiments, the
smaller electrodes 30
are collectively one-fourth the size of the larger electrodes 40. In further
embodiments the
smaller electrodes 30 are collectively one-eighth the size of the larger
electrodes 40. In preferred
embodiments, the smaller electrodes 30 are collectively one-sixteenth the size
of the larger
20 electrodes. Each electrode 30 may be provided as separate electrodes that
are controlled in
parallel to control the display. For example, each separate electrode may be
substantially
simultaneously set to the same voltage as all other electrodes of that size.
Alternatively, the
electrodes 30, 40 may be interdigitated to provide the embodiment shown in
FIGS. 4A and 4B.
Operation of the rear-addressing electrode structure depicted in FIGS. 4A and
4B is
25 similar to that described above. For example, the capsule 20 may contain
positively charged,
black particles 50 dispersed in a substantially clear suspending fluid 25. The
smaller electrodes 30
are colored black and the larger electrodes 40 are colored white or are highly
reflective.
Referring to FIG. 4A, the smaller electrodes 30 are placed at a negative
potential relative to the
larger electrodes 40, causing particles 50 migrate within the capsule to the
smaller electrodes 30
3o and the capsule 20 appears to the viewpoint 10 as a mix of the larger,
white electrodes 40 and the
smaller, black electrodes 30, creating an effect which is largely white.
Referring to Fig. 4B, when
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the smaller electrodes 30 are placed at a positive potential relative to the
larger electrodes 40,
particles 50 migrate to the larger electrodes 40 causing the capsule 20 to
display a mix of the
larger, white electrodes 40 occluded by the black particles 50 and the
smaller, black electrodes 30,
creating an effect which is largely black. The techniques described above with
respect to the
embodiments depicted in FIGs. lA and 1B for producing two-color displays work
with equal
effectiveness in connection with these embodiments.
FIGs. 5A and 5B depict an embodiment of a rear-addressing electrode structure
that
creates a reflective color display in a manner similar to halftoning or
pointillism. The capsule 20
contains white particles 55 dispersed in a clear suspending fluid 25.
Electrodes 42, 44, 46, 48 are
colored cyan, magenta, yellow, and white respectively. Referring to FIG 5A,
when the colored
electrodes 42, 44, 46 are placed at a positive potential relative to the white
electrode 48,
negatively-charged particles 55 migrate to these three electrodes, causing the
capsule 20 to
present to the viewpoint 10 a mix of the white particles 55 and the white
electrode 48, creating an
effect which is largely white. Referring to FIG. 5B, when electrodes 42, 44,
46 are placed at a
negative potential relative to electrode 48, particles 55 migrate to the white
electrode 48, and the
eye 10 sees a mix of the white particles 55, the cyan electrode 42, the
magenta electrode 44, and
the yellow electrode 46, creating an effect which is largely black or gray. By
addressing the
electrodes, any color can be produced that is possible with a subtractive
color process. For
example, to cause the capsule 20 to display an orange color to the viewpoint
10, the yellow
electrode 46 and the magenta electrode 42 are set to a voltage potential that
is more positive than
the voltage potential applied by the cyan electrode 42 and the white electrode
48. Further, the
relative intensities of these colors can be controlled by the actual voltage
potentials applied to the
electrodes.
In another embodiment, depicted in FIG. 6, a color display is provided by a
capsule 20 of
size d containing multiple species of particles in a clear, dispersing fluid
25. Each species of
particles has different optical properties and possess different
electrophoretic mobilities (~) from
the other species. In the embodiment depicted in FIG. 6, the capsule 20
contains red particles 52,
blue particles 54, and green particles 56, and
i~~i > i,~ei ~ iu~i
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That is, the magnitude of the electrophoretic mobility of the red particles
52, on average, exceeds
the electrophoretic mobility of the blue particles 54, on average, and the
electrophoretic mobility
of the blue particles 54, on average, exceeds the average electrophoretic
mobility of the green
particles 56. As an example, there may be a species of red particle with a
zeta potential of
100 millivolts (mV), a blue particle with a zeta potential of 60 mV, and a
green particle with a
zeta potential of 20 mV. The capsule 20 is placed between two electrodes 32,
42 that apply an
electric field to the capsule.
FIGs. 7A-7B depict the steps to be taken to address the display shown in FIG.
6 to display
a red color to a viewpoint 10. Refernng to FIG. 7A, all the particles 52, 54,
56 are attracted to
one side of the capsule 20 by applying an electric field in one direction. The
electric field should
be applied to the capsule 20 long enough to attract even the more slowly
moving green particles
56 to the electrode 34. Refernng to FIG. 7B, the electric field is reversed
just long enough to
allow the red particles 52 to migrate towards the electrode 32. The blue
particles 54 and green
particles 56 will also move in the reversed electric field, but they will not
move as fast as the red
particles 52 and thus will be obscured by the red particles 52. The amount of
time for which the
applied electric field must be reversed can be determined from the relative
electrophoretic
mobilities of the particles, the strength of the applied electric field, and
the size of the capsule.
FIGs. 8A-8D depict addressing the display element to a blue state. As shown in
FIG. 8A,
the particles 52, 54, 56 are initially randomly dispersed in the capsule 20.
All the particles 52, 54,
56 are attracted to one side of the capsule 20 by applying an electric field
in one direction (shown
in FIG. 8B). Referring to FIG. 8C, the electric field is reversed just long
enough to allow the red
particles 52 and blue particles 54 to migrate towards the electrode 32. The
amount of time for
which the applied electric field must be reversed can be determined from the
relative
electrophoretic mobilities of the particles, the strength of the applied
electric field, and the size of
the capsule. Referring to FIG. 8D, the electric field is then reversed a
second time and the red
particles 52, moving faster than the blue particles 54, leave the blue
particles 54 exposed to the
viewpoint 10. The amount of time for which the applied electric field must be
reversed can be
determined from the relative electrophoretic mobilities of the particles, the
strength of the applied
electric field, and the size of the capsule.
FIGS. 9A-9C depict the steps to be taken to present a green display to the
viewpoint 10.
As shown in FIG. 9A, the particles 52, 54, 56 are initially distributed
randomly in the capsule 20.
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All the particles 52, 54, 56 are attracted to the side of the capsule 20
proximal the viewpoint 10
by applying an electric field in one direction. The electric field should be
applied to the capsule 20
long enough to attract even the more slowly moving green particles 56 to the
electrode 32. As
shown in FIG. 9C, the electric field is reversed just long enough to allow the
red particles 52 and
the blue particles 54 to nugrate towards the electrode 54, leaving the slowly-
moving green
particles 56 displayed to the viewpoint. The amount of time for which the
applied electric field
must be reversed can be determined from the relative electrophoretic
mobilities of the particles,
the strength of the applied electric field, and the size of the capsule.
In other embodiments, the capsule contains multiple species of particles and a
dyed
dispersing fluid that acts as one of the colors. In still other embodiments,
more than three species
of particles may be provided having additional colors. Although FIGS. 6-9C
depict two
electrodes associated with a single capsule, the electrodes may address
multiple capsules or less
than a full capsule
In FIG. 10, the rear substrate 100 for a seven segment display is shown that
improves on
normal rear electrode structures by providing a means for arbitrarily
connecting to any electrode
section on the rear of the display without the need for conductive trace lines
on the surface of the
patterned substrate or a patterned counter electrode on the front of the
display. Small conductive
vias through the substrate allow connections to the rear electrode structure.
On the back of the
substrate, these vias are connected to a network of conductors. This
conductors can be run so as
2o to provide a simple connection to the entire display. For example, segment
112 is connected by
via 114 through the substrate 116 to conductor 118. A network of conductors
may run multiple
connections (not shown) to edge connector 122. This connector can be built
into the structure of
the conductor such as edge connector 122. Each segment of the rear electrode
can be
individually addressed easily through edge connector 122. A continuous top
electrode can be
used with the substrate 116.
The rear electrode structure depicted in FIG. 10 is useful for any display
media, but is
particularly advantageous for particle-based displays because such displays do
not have a defined
appearance when not addressed. The rear electrode should be completely covered
in an
electrically conducting material with room only to provide necessary
insulation of the various
3o electrodes. This is so that the connections on the rear of the display can
be routed with out
Attorneys Docket No.: INK-035PC
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, , , , , . ..
- ~, .. ... .. ..
- 19-
concern for affecting the appearance of the display. Having a mostly
continuous rear electrode
pattern assures that the display material is shielded from the rear electrode
wire routing.
In FIG. 11, a 3x3 matrix is shown. Here, matrix segment 124 on a first side of
substrate
116 is connected by via 114 to conductor 118 on a second side of substrate
116. The conductors
18 run to an edge and terminate in a edge connector 122. Although the display
element of FIG.
11 shows square segments 124, the segments may be shaped or sized to form a
predefined
display pattern.
In FIG. 12, a printed circuit board 138 is used as the rear electrode
structure. The front of
the printed circuit board 138 has copper pads 132 etched in the desired shape.
There are plated
vias 114 connecting these electrode pads to an etched wire structure 136 on
the rear of the printed
circuit board 138. The wires 136 can be run to one side or the rear of the
printed circuit board
138 and a connection can be made using a standard connector such as a surface
mount connector
or using a flex connector and anisotropic glue (not shown). Vias may be filled
with a conductive
substance, such as solder or conductive epoxy, or an insulating substance,
such as epoxy.
Alternatively, a flex circuit such a copper-clad polyimide may be used for the
rear
electrode structure of FIG. 10. Printed circuit board 138 may be made of
polyimide, which acts
both as the flex connector and as the substrate for the electrode structure.
Rather than copper
pads 132, electrodes (not shown) may be etched into the copper covering the
polyimide printed
circuit board 138. The plated through vias 114 connect the electrodes etched
onto the substrate
the rear of the printed.circuit board 138, which may have an etched conductor
network thereon
(the etched conductor network is similar to the etched wire structure 136).
In FIG. 13, a thin dielectric sheet 150, such as polyester, polyimide, or
glass can be used
to make a rear electrode structure. Holes 152 are punched, drilled, abraded,
or melted through
the sheet where conductive paths are desired. The front electrode 1 S4 is made
of conductive ink
printed using any technique described above. The holes should be sized and the
ink should be
selected to have a viscosity so that the ink fills the holes. When the back
structure 156 is printed,
again using conductive ink, the holes are again filled. By this method, the
connection between
the front and back of the substrate is made automatically.
In FIG. 14, the rear electrode structure can be made entirely of printed
layers. A
conductive layer 166 can be printed onto the back of a display comprised of a
clear, front
AMEP~DED SHEET
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electrode 168 and a printable display material 170. A clear electrode may be
fabricated from
indium tin oxide or conductive polymers such as polyanilines and
polythiophenes. A dielectric
coating 176 can be printed leaving areas for vias. Then, the back layer of
conductive ink 178 can
be printed. If necessary, an additional layer of conductive ink can be used
before the final ink
structure is printed to fill in the holes.
This technique for printing displays can be used to build the rear electrode
structure on a
display or to construct two separate layers that are laminated together to
form the display. For
example an electronically active ink may be printed on an indium tin oxide
electrode. Separately,
a rear electrode structure as described above can be printed on a suitable
substrate, such as
plastic, polymer films, or glass. The electrode structure and the display
element can be laminated
to form a display.
Referring now to FIG. 1 S, a threshold may be introduced into an
electrophoretic display
cell by the introduction of a third electrode. One side of the cell is a
continuous, transparent
electrode 200 (anode). On the other side of the cell, the transparent
electrode is patterned into a
set of isolated column electrode strips 210. An insulator 212 covers the
column electrodes 210,
and an electrode layer on top of the insulator is divided into a set of
isolated row electrode strips
230, which are oriented orthogonal to the column electrodes 210. The row
electrodes 230 are
patterned into a dense array of holes, or a grid, beneath which the exposed
insulator 212 has been
removed, forming a multiplicity of physical and potential wells.
A positively charged particle 50 is loaded into the potential wells by
applying a positive
potential (e.g. 30V) to all the column electrodes 210 while keeping the row
electrodes 230 at a
less positive potential (e.g. 15V) and the anode 200 at zero volts, The
particle 50 may be a
conformable capsule that situates itself into the physical wells of the
control grid. The control
grid itself may have a rectangular cross-section, or the grid structure may be
triangular in profile.
It can also be a diil'erent shape which encourages the microcapsules to
situate in the grid, for
example, hemispherical.
The anode 200 is then reset to a positive potential (e.g. SOV). The particle
will remain in
the potential wells due to the potential difference in the potential wells:
this is called the Hold
condition. To address a display element the potential on the column electrode
associated with
that element is reduced, e.g. by a factor of two, and the potential on the row
electrode associated
with that element is made equal to or greater than the potential on the column
electrode. The
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particles in this element will then be transported by the electric field due
to the positive voltage on
the anode 200. The potential difference between row and column electrodes for
the remaining
display elements is now less than half of that in the normal Hold condition.
The geometry of the
potential well structure and voltage levels are chosen such that this also
constitutes a Hold
condition, i.e., no particles will leave these other display elements and
hence there will be no half
select problems. This addressing method can select and write any desired
element in a matrix
without affecting the pigment in any other display element. A control
electrode device can be
operated such that the anode electrode side of the cell is viewed.
The control grid may be manufactured through any of the processes known in the
art, or
1 o by several novel processes described herein. That is, according to
traditional practices, the
control grid may be constructed with one or more steps of photolithography and
subsequent
etching, or the control grid may be fabricated with a mask and a
"sandblasting" technique.
In another embodiment, the control grid is fabricated by an embossing
technique on a
plastic substrate. The grid electrodes may be deposited by vacuum deposition
or sputtering,
15 either before or after the embossing step. In another embodiment, the
electrodes are printed onto
the grid structure after it is formed, the electrodes consisting of some kind
of printable conductive
material which need not be clear (e.g. a metal or carbon-doped polymer, an
intrinsically
conducting polymer, etc.).
In a preferred embodiment, the control grid is fabricated with a series of
printing steps.
2o The grid structure is built up in a series of one or more printed layers
after the cathode has been
deposited, and the grid electrode is printed onto the grid structure. There
may be additional
insulator on top of the grid electrode, and there may be multiple grid
electrodes separated by
insulator in the grid structure. The grid electrode may not occupy the entire
width of the grid
structure, and may only occupy a central region of the structure, in order to
stay within
25 reproducible tolerances. In another embodiment, the control grid is
fabricated by photoetching
away a glass, such as a photostructural glass.
In an encapsulated electrophoretic image display, an electrophoretic
suspension, such as
the ones described previously, is placed inside discrete compartments that are
dispersed in a
polymer matrix. This resulting material is highly susceptible to an electric
field across the
3o thickness of the film. Such a field is normally applied using electrodes
attached to either side of
the material. However, as described above in connection with FIGS. 3A-3D, some
display media
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may be addressed by writing electrostatic charge onto one side of the display
material. The other
side normally has a clear or opaque electrode. For example, a sheet of
encapsulated
electrophoretic display media can be addressed with a head providing DC
voltages.
In another implementation, the encapsulated electrophoretic suspension can be
printed
onto an area of a conductive material such as a printed silver or graphite
ink, aluminized mylar, or
any other conductive surface. This surface which constitutes one electrode of
the display can be
set at ground or high voltage. An electrostatic head consisting of many
electrodes can be passed
over the capsules to addressing them. Alternatively, a stylus can be used to
address the
encapsulated electrophoretic suspension.
In another implementation, an electrostatic write head is passed over the
surface of the
material. This allows very high resolution addressing. Since encapsulated
electrophoretic
material can be placed on plastic, it is flexible. This allows the material to
be passed through
normal paper handling equipment. Such a system works much like a photocopier,
but with no
consumables. The sheet of display material passes through the machine and an
electrostatic or
electrophotographic head addresses the sheet of material.
In another implementation, electrical charge is built up on the surface of the
encapsulated
display material or on a dielectric sheet through frictional or triboelectric
charging. The charge
can built up using an electrode that is later removed. In another
implementation, charge is built
up on the surface of the encapsulated display by using a sheet of
piezoelectric material.
FIG. 16 shows an electrostatically written display. Stylus 300 is connected to
a positive
or negative voltage. The head of the stylus 300 can be insulated to protect
the user. Dielectric
layer 302 can be, for example, a dielectric coating or a film of polymer. In
other embodiments,
dielectric layer 302 is not provided and the stylus 300 contacts the
encapsulated electrophoretic
display 304 directly. Substrate 306 is coated with a clear conductive coating
such as ITO coated
polyester. The conductive coating is connected to ground. The display 304 may
be viewed from
either side.
Microencapsulated displays offer a useful means of creating electronic
displays, many of
which can be coated or printed. There are many versions of microencapsulated
displays, including
microencapsulated electrophoretic displays. These displays can be made to be
highly reflective,
3o bistable, and low power.
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To obtain high resolution displays, it is useful to use some external
addressing means with
the microencapsulated material. This invention describes useful combinations
of addressing means
with microencapsulated electrophoretic materials in order to obtain high
resolution displays.
One method of addressing liquid crystal displays is the use of silicon-based
thin film
transistors to form an addressing backplane for the liquid crystal. For liquid
crystal displays, these
thin film transistors are typically deposited on glass, and are typically made
from amorphous
silicon or polysilicon. Other electronic circuits (such as drive electronics
or logic) are sometimes
integrated into the periphery of the display. An emerging field is the
deposition of amorphous or
polysilicon devices onto flexible substrates such as metal foils or plastic
films.
The addressing electronic backplane could incorporate diodes as the nonlinear
element,
rather than transistors. Diode-based active matrix arrays have been
demonstrated as being
compatible with liquid crystal displays to form high resolution devices.
There are also examples of crystalline silicon transistors being used on glass
substrates.
Crystalline silicon possesses very high mobilities, and thus can be used to
make high performance
devices. Presently, the most straightforward way of constructing crystalline
silicon devices is on a
silicon wafer. For use in many types of liquid crystal displays, the
crystalline silicon circuit is
constructed on a silicon wafer, and then transferred to a glass substrate by a
"liftoff' process.
Alternatively, the silicon transistors can be formed on a silicon wafer,
removed via a liftoff
process, and then deposited on a flexible substrate such as plastic, metal
foil, or paper. As another
implementation, the silicon could be formed on a different substrate that is
able to tolerate high
temperatures (such as glass or metal foils), lifted off, and transferred to a
flexible substrate. As yet
another implementation, the silicon transistors are formed on a silicon wafer,
which is then used in
whole or in part as one of the substrates for the display.
The use of silicon-based circuits with liquid crystals is the basis of a large
industry.
Nevertheless, these display possess serious drawbacks. Liquid crystal displays
are inefficient with
light, so that most liquid crystal displays require some sort of backlighting.
Reflective liquid
crystal displays can be constructed, but are typically very dim, due to the
presence of polarizers.
Most liquid crystal devices require precise spacing of the cell gap, so that
they are not very
compatible with flexible substrates. Most liquid crystal displays require a
"rubbing" process to
align the liquid crystals, which is both difficult to control and has the
potential for damaging the
TFT array.
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The combination of these thin film transistors with microencapsulated
electrophoretic
displays should be even more advantageous than with liquid crystal displays.
Thin film transistor
arrays similar to those used with liquid crystals could also be used with the
microencapsulated
display medium. As noted above, liquid crystal arrays typically requires a
"rubbing" process to
align the liquid crystals, which can cause either mechanical or static
electrical damage to the
transistor array. No such rubbing is needed for microencapsulated displays,
improving yields and
simplifying the construction process.
Microencapsulated electrophoretic displays can be highly reflective. This
provides an
advantage in high-resolution displays, as a backlight is not required for good
visibility. Also, a
high-resolution display can be built on opaque substrates, which opens up a
range of new
materials for the deposition of thin film transistor arrays.
Moreover, the encapsulated electrophoretic display is highly compatible with
flexible
substrates. This enables high-resolution TFT displays in which the transistors
are deposited on
flexible substrates like flexible glass, plastics, or metal foils. The
flexible substrate used with any
type of thin film transistor or other nonlinear element need not be a single
sheet of glass, plastic,
metal foil, though. Instead, it could be constructed of paper. Alternatively,
it could be constructed
of a woven material. Alternatively, it could be a composite or layered
combination of these
materials.
As in liquid crystal displays, external logic or drive circuitry can be built
on the same
substrate as the thin film transistor switches.
In another implementation, the addressing electronic backplane could
incorporate diodes
as the nonlinear element, rather than transistors.
In another implementation, it is possible to form transistors on a silicon
wafer, dice the
transistors, and place them in a large area array to form a large, TFT-
addressed display medium.
One example of this concept is to form mechanical impressions in the receiving
substrate, and
then cover the substrate with a slurry or other form of the transistors. With
agitation, the
transistors will fall into the impressions, where they can be bonded and
incorporated into the
device circuitry. The receiving substrate could be glass, plastic, or other
nonconductive material.
In this way, the economy of creating transistors using standard processing
methods can be used to
create large-area displays without the need for large area silicon processing
equipment.
CA 02300827 2000-02-18
WO 99!10768 PCTIUS98/17735
-25-
While the examples described here are listed using encapsulated
electrophoretic displays,
there are other particle-based display media which should also work as well,
including
encapsulated suspended particles and rotating ball displays.
While the invention has been particularly shown and described with reference
to specific
preferred embodiments, it should be understood by those skilled in the art
that various changes in
form and detail may be made therein without departing from the spirit and
scope of the invention
as defined by the appended claims.
