U.S. patent application number 10/198729 was filed with the patent office on 2004-12-30 for electrophoretic display with in-plane switching.
Invention is credited to Chen, David, Chung, Jerry, Liang, Rong-Chang.
Application Number | 20040263946 10/198729 |
Document ID | / |
Family ID | 33543836 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040263946 |
Kind Code |
A9 |
Liang, Rong-Chang ; et
al. |
December 30, 2004 |
Electrophoretic display with in-plane switching
Abstract
The present invention relates to an improved EPD which comprises
the in plane switching mode. More specifically, the EPD of the
present invention comprises isolated cells formed from microcups of
well defined size, shape and aspect ratio and the movement of the
particles in the cells is controlled by the in-plane switching
mode. The EPD of the present invention may be produced in a
continuous manufacturing process, and the display gives improved
color saturation.
Inventors: |
Liang, Rong-Chang;
(Cupertino, CA) ; Chung, Jerry; (Mountain View,
CA) ; Chen, David; (Buena Park, CA) |
Correspondence
Address: |
HOWREY SIMON ARNOLD & WHITE, LLP
C/O M.P. DROSOS, DIRECTOR OF IP ADMINISTRATION
2941 FAIRVIEW PK
BOX 7
FALLS CHURCH
VA
22042
US
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 0035198 A1 |
February 20, 2003 |
|
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Family ID: |
33543836 |
Appl. No.: |
10/198729 |
Filed: |
July 16, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10198729 |
Jul 16, 2002 |
|
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09518488 |
Mar 3, 2000 |
|
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60306312 |
Jul 17, 2001 |
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Current U.S.
Class: |
359/296 ;
204/606; 345/107 |
Current CPC
Class: |
G02F 1/1679 20190101;
G02F 2201/12 20130101; G02F 1/134363 20130101; G02F 1/167
20130101 |
Class at
Publication: |
359/296 ;
345/107; 204/606 |
International
Class: |
G02B 026/00; G09G
003/34; G01R 001/00 |
Claims
What is claimed is:
1. An electrophoretic display comprising isolated electrophoretic
cells formed from microcups of well-defined size, shape and aspect
ratio with an in-plane switching mode.
2. The electrophoretic display of claim 1 having two in-plane
electrodes.
3. The electrophoretic display of claim 1 having one in-plane
electrode.
4. The electrophoretic display of claim 1 wherein the microcups
have a top opening ranging from about 10.sup.2 to about
1.times.10.sup.6 .mu.m.sup.2.
5. The electrophoretic display of claim 4 wherein the microcups
have a top opening ranging from about 10.sup.3 to about
1.times.10.sup.5 .mu.m.sup.2.
6. The electrophoretic display of claim 1 wherein the depth of the
microcups is in the range of about 5 to about 200 microns.
7. The electrophoretic display of claim 6 wherein the depth of the
microcups is in the range of about 20 to about 100 microns.
8. The electrophoretic display of claim 1 wherein the opening to
the total area ratio is in the range of from about 0.2 to about
0.95.
9. An electrophoretic display having isolated cells formed from
microcups of well-defined size, shape and aspect ratio, said cells
comprising a transparent viewing layer on one side of the display;
a layer having in-plane electrodes on the opposite side of the
display; and said cells are filled with a dielectric solvent or
solvent mixture having charged particles dispersed therein.
10. The electrophoretic display of claim 9 further comprising a
separate background layer.
11. The electrophoretic display of claim 9 wherein said transparent
viewing layer is colorless.
12. The electrophoretic display of claim 11 wherein said separate
background layer is on top of the layer having in-plane
electrodes.
13. The electrophoretic display of claim 11 wherein said separate
background layer is underneath the layer having in-plane
electrodes.
14. The electrophoretic display of claim 11 wherein said layer
having in-plane electrodes serves as the background layer and the
in-plane electrodes may be white or colored.
15. The electrophoretic display of claim 9 wherein the display is a
monochrome display.
16. The electrophoretic display of claim 15 wherein said dielectric
solvent is clear and colorless.
17. The electrophoretic display of claim 16 wherein all said cells
have particles of the white color and the same background
color.
18. The electrophoretic display of claim 17 wherein said background
color is black, red, green, blue, yellow, cyan or magenta.
19. The electrophoretic display of claim 16 wherein all said cells
have particles of the same color and the white background
color.
20. The electrophoretic display of claim 19 wherein said particles
are black, red, green, blue, yellow, cyan or magenta.
21. The electrophoretic display of claim 16 wherein said individual
cells have particles of mixed colors and the same background
color.
22. The electrophoretic display of claim 21 wherein the mixed
colors are two or more colors selected from a group consisting of
black, white, red, green, blue, yellow, cyan and magenta.
23. The electrophoretic display of claim 22 wherein said background
color is selected from a group consisting of black, white, red,
green, blue, yellow, cyan and magenta.
24. The electrophoretic display of claim 9 wherein said display is
a multiple color display.
25. The electrophoretic display of claim 24 wherein said cells have
particles of the white color and different background colors.
26. The electrophoretic display of claim 24 wherein said cells have
particles of the black color and different background colors.
27. The electrophoretic display of claim 24 wherein said cells have
particles of different colors and a white background.
28. The electrophoretic display of claim 24 wherein said cells have
particles of different colors and a black background.
29. The electrophoretic display of claim 9 wherein said transparent
viewing layer is colored or by adding a color filter.
30. The electrophoretic display of claim 29 wherein said separate
background layer is on top of the layer having in-plane
electrodes.
31. The electrophoretic display of claim 29 wherein said separate
background layer is underneath the layer having in-plane
electrodes.
32. The electrophoretic display of claim 29 wherein said layer
having in-plane electrodes serves as the background layer and the
in-plane electrodes may be white or colored.
33. The electrophoretic display of claim 29 wherein all said cells
have white particles and black background.
34. The electrophoretic display of claim 33 wherein all said cells
have the transparent viewing layer of the same color.
35. The electrophoretic display of claim 33 wherein said cells have
the transparent viewing layers of different colors.
36. A process for the manufacture of an electrophoretic display
having isolated cells formed from microcups having well defined
size, shape and aspect ratio, said process comprises a) forming a
layer by coating a radiation curable material over a transparent
insulator substrate; b) forming microcups on the radiation curable
material by microembossing or imagewise exposure to radiation; c)
filling the microcups with an electrophoretic composition; d)
sealing the microcups; e) laminating the sealed microcups with a
substrate containing in-plane electrodes.
37. The process for the manufacture of an electrophoretic display
of claim 36 with substrate containing in-plane electrodes and an
adhesive.
38. A process for the manufacture of an electrophoretic display
having isolated cells formed from microcups having well defined
size, shape and aspect ratio, said process comprises a) forming a
layer by coating a radiation curable material over a substrate
containing in-plane electrodes; b) forming microcups on the
radiation curable material by microembossing or imagewise exposure
to radiation; c) filling the microcups with an electrophoretic
composition; d) sealing the microcups; e) laminating the sealed
microcups with a transparent insulator substrate.
39. The process for the manufacture of an electrophoretic display
of claim 38 with a transparent insulator substrate and an
adhesive.
40. The process of claim 36 wherein the microcups have a top
opening ranging from about 10.sup.2 to about 1.times.10.sup.6
.mu.m.sup.2.
41. The process of claim 38 wherein the microcups have a top
opening ranging from about 10.sup.2 to about 1.times.10.sup.6
.mu.m.sup.2.
42. The process of claim 36 wherein said transparent insulator
substrate is colorless.
43. The process of claim 42 wherein all said cells have the same
background color and are filled with an electrophoretic composition
comprising charged particles of the same color dispersed in a clear
colorless dielectric solvent.
44. The process of claim 42 wherein said cells have different
background colors and are filled with an electrophoretic
composition comprising charged particles of the same color
dispersed in a clear colorless dielectric solvent.
45. The process of claim 42 wherein said cells have the same
background color and are filled with an electrophoretic composition
comprising charged particles of the different colors dispersed in a
clear colorless dielectric solvent.
46. The process of claim 38 wherein said transparent insulator
substrate is colorless.
47. The process of claim 46 wherein all said cells have the same
background color and are filled with an electrophoretic composition
comprising charged particles of the same color dispersed in a clear
colorless dielectric solvent.
48. The process of claim 46 wherein said cells have different
background colors and are filled with an electrophoretic
composition comprising charged particles of the same color
dispersed in a clear colorless dielectric solvent.
49. The process of claim 46 wherein said cells have the same
background color and are filled with an electrophoretic composition
comprising charged particles of different colors dispersed in a
clear colorless dielectric solvent.
50. The process of claim 36 wherein said transparent insulator
substrate is colored.
51. The process of claim 50 wherein said cells have the black
background and are filled with an electrophoretic composition
comprising charged white particles dispersed in a clear colorless
dielectric solvent.
52. The process of claim 51 wherein all cells have the transparent
insulator substrate of the same color.
53. The process of claim 51 wherein said cells have the transparent
insulator substrate of different colors.
54. The process of claim 38 wherein said transparent insulator
substrate is colored.
55. The process of claim 54 wherein said cells have the black
background and are filled with an electrophoretic composition
comprising charged white particles dispersed in a clear colorless
dielectric solvent.
56. The process of claim 55 wherein all cells have the transparent
insulator substrate of the same color.
57. The process of claim 55 wherein said cells have the transparent
insulator substrate of different colors.
58. The electrophoretic display of claim 9 wherein a substrate
containing an array of thin film transistors is used as the layer
of in-plane electrodes.
Description
BACKGROUND
[0001] The electrophoretic display (EPD) is a non-emissive device
based on the electrophoresis phenomenon influencing charged pigment
particles suspended in a colored dielectric solvent. This general
type of display was first proposed in 1969. An EPD typically
comprises a pair of opposed, spaced-apart plate-like electrodes,
with spacers predetermining a certain distance between the
electrodes. At least one of the electrodes, typically on the
viewing side, is transparent. For the passive type of EPDs, row and
column electrodes on the top (the viewing side) and bottom plates
respectively are needed to drive the displays. In contrast, an
array of thin film transistors (TFT) on the bottom plate and a
common, non-patterned transparent conductor plate on the top
viewing substrate are required for the active type EPDs. An
electrophoretic fluid composed of a colored dielectric solvent and
charged pigment particles dispersed therein is enclosed between the
two electrodes.
[0002] When a voltage difference is imposed between the two
electrodes, the pigment particles migrate by attraction to the
plate of polarity opposite that of the pigment particles. Thus, the
color showing at the transparent plate, determined by selectively
charging the plates, can be either the color of the solvent or the
color of the pigment particles. Reversal of plate polarity will
cause the particles to migrate back to the opposite plate, thereby
reversing the color. Intermediate color density (or shades of gray)
due to intermediate pigment density at the transparent plate may be
obtained by controlling the plate charge through a range of
voltages. No backlight is needed in this type of reflective EPD
displays.
[0003] A transmissive EPD is disclosed in U.S. Pat. No. 6,184,856
in which a backlight, color filters, and substrates with two
transparent electrodes are used. The electrophoretic cells serve as
a light valve. In the collected state, the particles are positioned
to minimize the coverage of the horizontal area of the cell and
allow the backlight to pass through the cell. In the distributed
state, the particles are positioned to cover the horizontal area of
the pixel and scatter or absorb the backlight. However, the
backlight and color filter used in this device consume a great deal
of power and are not desirable for hand-held devices such as PDAs
(personal digital assistants) and e-books.
[0004] Besides the normal top/bottom electrode switching mode of
EPDs, reflective "in-plane" switching EPDs have been disclosed in
E. Kishi, et al., "5.1: development of In-Plane EPD", Canon
Research Center, SID 00 Digest, pages 24-27 (2000) and Sally A.
Swanson, et al., "5.2: High Performance Electrophoretic Displays",
IBM Almaden Research Center, SID 00 Digest, pages 29-31, (2000).
However, only monochrome in-plane switching EPDs are disclosed in
these references. To prepare a multicolor display, either color
filters or isolated color pixels or cell structures are needed for
color separation and rendition. Color filter is typically expensive
and not power-efficient. On the other hand, the preparation of
isolated pixels or cells for color separation and rendering in the
in-plane switching mode has not been taught previously.
[0005] EPDs of different pixel or cell structures have been
reported in prior art, for example, the partition-type EPD (M. A.
Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8,
pp 1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos.
5,961,804 and 5,930,026), and each of these has its own problems as
noted below.
[0006] In a partition-type EPD, there are partitions between the
two electrodes for dividing the space into smaller cells in order
to prevent undesired movements of the particles such as
sedimentation. However, difficulties are encountered in the
formation of the partitions, the process of filling the display
with the fluid, enclosing the fluid in the display, and keeping the
suspensions of different colors separated from each other.
[0007] The microencapsulated EPD has a substantially two
dimensional arrangement of microcapsules each having therein an
electrophoretic composition of a dielectric fluid and a dispersion
of charged pigment particles that visually contrast with the
dielectric solvent. The microcapsules are typically prepared in an
aqueous solution and, to achieve a useful contrast ratio, their
mean particle size is relatively large (50-150 microns). The large
microcapsule size results in a poor scratch resistance and a slow
response time for a given voltage because a large gap between the
two opposite electrodes is required for large capsules. Also, the
hydrophilic shell of microcapsules prepared in an aqueous solution
typically results in sensitivity to high moisture and temperature
conditions. If the microcapsules are embedded in a large quantity
of a polymer matrix to obviate these shortcomings, the use of the
matrix results in an even slower response time and/or a lower
contrast ratio. To improve the switching rate, a charge-controlling
agent is often needed in this type of EPDs. However, the
microencapsulation process in an aqueous solution imposes a
limitation on the type of charge-controlling agents that can be
used. Other drawbacks associated with the microcapsule system
include poor resolution and poor addressability for color
applications.
[0008] An improved EPD technology was recently disclosed in
co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3,
2000 (corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed
on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000
(corresponding to WO02/01281) and U.S. Ser. No. 09/784,972, filed
on Feb. 15, 2001, all of which are incorporated herein by
reference. The improved EPD comprises closed isolated cells formed
from microcups of well-defined shape, size and aspect ratio and
filled with charged pigment particles dispersed in a dielectric
solvent. The electrophoretic fluid is isolated and sealed in each
microcup.
[0009] The microcup structure, in fact, enables a format flexible,
efficient roll-to-roll continuous manufacturing process for the
preparation of EPDs. The displays can be prepared on a continuous
web of conductor film such as ITO/PET by, for example, (1) coating
a radiation curable composition onto the ITO/PET film, (2) making
the microcup structure by a microembossing or photolithographic
method, (3) filling the electrophoretic fluid and sealing the
microcups, (4) laminating the sealed microcups with the other
conductor film, and (5) slicing and cutting the display to a
desirable size or format for assembling.
[0010] One advantage of this EPD design is that the microcup wall
is in fact a built-in spacer to keep the top and bottom substrates
apart at a fixed distance. The mechanical properties and structural
integrity of microcup displays are significantly better than any
prior art displays including those manufactured by using spacer
particles. In addition, displays involving microcups have desirable
mechanical properties including reliable display performance when
the display is bent, rolled, or under compression pressure from,
for example, a touch screen application. The use of the microcup
technology also eliminates the need of an edge seal adhesive which
would limit and predefine the size of the display panel and confine
the display fluid inside a predefined area. The display fluid in a
conventional display prepared by the edge sealing adhesive method
will leak out completely if the display is cut in any way, or if a
hole is drilled through the display. The damaged display will be no
longer functional. In contrast, the display fluid in the display
prepared by the microcup technology is enclosed and isolated in
each cell. The microcup display may be cut to almost any dimension
without the risk of damaging the display performance due to the
loss of display fluids in the active areas. In other words, the
microcup structure enables a format flexible display manufacturing
process, wherein the process produces a continuous output of
displays in a large sheet format which can be sliced and diced to
any desired format. The isolated microcup or cell structure is
particularly important, when cells are filled with fluids of
different specific properties such as colors and switching rates.
Without the microcup structure, it will be very difficult to
prevent the fluids in adjacent areas from intermixing, or being
subject to cross-talk during operation.
[0011] Multi-color displays may thus be manufactured by using a
spatially adjacent array of small pixels formed of microcups filled
with dyes of different colors (e.g., red, green or blue). However
there is a major deficiency in this type of system with the
traditional top/bottom electrode switching mode. The white light
reflected from the "turned-off" colored pixels greatly reduces the
color saturation of the "turned-on" colors. More details in this
regard are given in the following "Detailed Description"
section.
[0012] While this deficiency may be remedied by an overlaid shutter
device such as a polymer dispersed liquid crystal to switch each
pixel to the black color, the disadvantage of this approach is the
high cost of the overlaid device and the complicated driving
circuit design.
[0013] Thus, there is still a need for an EPD with improved
properties that can also be prepared in an efficient manner.
SUMMARY OF THE INVENTION
[0014] The present invention relates to an improved EPD which
comprises the in plane switching mode for image formation. More
specifically, the EPD of the present invention comprises isolated
cells formed from microcups of well defined size, shape and aspect
ratio and the movement of the particles in the cells is controlled
by the in-plane switching mode. The EPDs of the present invention
may be produced in a continuous roll-to-roll manufacturing process,
and the resultant displays have improved color saturation and
contrast ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] It is noted that all figures are shown as schematic and are
not to scale.
[0016] FIG. 1 illustrates the common deficiency of the traditional
EPD with only the top/bottom switching mode.
[0017] FIG. 2 illustrates a typical electrophoretic cell of the
present invention and the general locations of the in-plane
switching electrodes.
[0018] FIGS. 3A and 3B illustrate the monochrome display of the
present invention.
[0019] FIGS. 4A-4D illustrate the various multiple color scenarios
of the present invention.
[0020] FIGS. 5A and 5B illustrate the manufacture of microcups
involving imagewise photolithographic exposure through a
photomask.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Unless defined otherwise in this specification, all
technical terms are used herein according to their conventional
definitions as they are commonly used and understood by those of
ordinary skill in the art. The terms "cell", "microcup",
"well-defined", "aspect ratio", and "imagewise exposure" are as
defined in the co-pending applications identified above.
[0022] The term "isolated" refers to the electrophoretic cells
which are individually sealed and the fluid in the cells may not be
transferred from one cell to the other cells.
[0023] I. The Disadvantages of Electrophoretic Display with the
Traditional Top/Bottom Switching
[0024] The EPD of FIG. 1 has conventional top/bottom electrode
switching mode. The cells are filled with a suspension in which
white charged particles are dispersed in a colored (red, green and
blue) dielectric solvent. All three cells in FIG. 1 are shown
charged with a voltage difference between the top and bottom
electrodes (not shown). In the green and blue cells, the white
particles migrate to the top viewing electrode which is
transparent, and as a result, the color of the particles (i.e.,
white) is reflected to the viewer through the transparent conductor
film in the two cells. In the red cell, the white particles migrate
to the bottom of the cell, and the color of the solvent (i.e., red)
is seen through the top transparent conductor film. In FIG. 1, the
white light reflected from the green and blue cells dramatically
reduces the saturation and contrast ratio of the red color.
[0025] In addition to the above mentioned problem, low solubility
and poor fastness of the dyes in dielectric solvents of very low
polarity, such as perfluoro and hydrocarbon solvents, have been a
challenge for achieving high contrast ratio in the top/down type of
EPDs.
[0026] II. Electrophoretic Display of the Present Invention
[0027] FIG. 2 illustrates a typical electrophoretic cell of the
present invention. The cell (20) comprises a top layer (21) and a
bottom layer (22). The bottom layer has the in-plane switching
electrodes (23) and (24) and the background layer (25). There is a
common electrode (29) between the two in-plane electrodes separated
by gaps (30). Alternatively, the bottom layer may have only one
in-plane switching electrode, and one common electrode with a gap
in between. Another alternative is where the background layer (25)
is on top of the electrodes in the bottom layer (not shown). The
in-plane electrode layer may also serve as the background layer and
in this case the in-plane electrode(s) may be white or colored.
[0028] Typically, the cells of FIG. 2 are filled with a dispersion
of colored particles (31) in a clear dielectric solvent (32). The
particles may be white, black or colored (i.e., red, green or
blue). The background layer (25) may be colorless, white, black or
colored. The filled cells are subsequently sealed with a sealing
layer (26). The top layer (21) having a transparent insulator layer
(27) and preferably an adhesive layer (28) is then laminated over
the sealed cells.
[0029] Preferably the microcup array is prepared in an up-side-down
manner. In this scenario, the microcup array is prepared on the top
transparent insulator substrate by either microembossing or
photolithography as disclosed in the co-pending patent
applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000
(corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed on
Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000
(corresponding to WO02/01281) and U.S. Ser. No. 09/784,972, filed
on Feb. 15, 2001, all of which are incorporated herein by
reference. The microcups are filled with the electrophoretic fluid
and sealed subsequently with a sealing layer. The bottom layer
which contains the patterned electrodes and preferably an adhesive
layer is then laminated over the sealed microcup. The color
background may be added by painting, printing, coating or
laminating a color layer to the bottom electrode substrate.
[0030] One of the advantages of the in-plane switching mode is the
possibility of making the microcups on a clear plastic insulator
substrate. This eliminates the risk of breaking the brittle
conductor electrode such as ITO/PET during the microembossing and
other web handling steps. The patterned in-plane conductor film
will only be used at the last step for lamination onto the filled
and sealed microcups to complete the making of the display
panel.
[0031] (1) Reflective Monochrome Display
[0032] In the cell as shown in FIG. 3A, white particles are
dispersed in a clear, colorless dielectric solvent. The background
of all cells is of the same color (black, blue, cyan, red, magenta,
etc.). When there is a voltage difference between the common (not
shown) and the two in-plane switching electrodes (not shown), the
white particles migrate to the sides of the cells, resulting in the
color of the background being seen through the top transparent
opening. When there is no voltage difference between the common and
the two in-plane electrodes, the white particles are distributed in
the dielectric solvent and as a result, the color of the particles
(i.e., white) is seen through the top transparent insulator
layer.
[0033] Alternatively, as shown in FIG. 3B, particles of the same
color are dispersed in a clear, colorless dielectric solvent in all
cells and the background of the cells is white. When there is a
voltage difference between the common (not shown) and the two
in-plane switching electrodes (not shown), the colored particles
migrate to the sides of the cells, resulting in the color of the
background (i.e., white) being seen through the top transparent
opening. When there is no voltage difference between the two
in-plane electrodes, the colored particles are distributed in the
dielectric solvent and as a result, the color of the particles is
seen through the top transparent layer.
[0034] (2) Reflective Multiple Color Display
[0035] FIGS. 4A-4D illustrate the multiple color displays of the
present invention.
[0036] In FIG. 4A, the cells are filled with a colorless dielectric
solvent with white charged particles dispersed therein, and have
different background colors (i.e., red, green or blue). When there
is a voltage difference between the in-plane electrodes (not
shown), the white particles migrate to either side of the cell, the
color of the background (i.e., red, green or blue) is seen from the
top transparent opening. When there is no voltage difference
between the in-plane electrodes, the particles are distributed in
the dielectric solvent resulting in the white color (i.e., the
color of the particles) being seen from the top transparent
opening.
[0037] In FIG. 4B, the cells are filled with a colorless dielectric
solvent with black particles dispersed therein, and have different
background colors (i.e., red, green or blue). When there is a
voltage difference between the in-plane electrodes (not shown), the
particles migrate to either side of the cell, the color of the
background (i.e., red, green or blue) is seen from the top
transparent opening. When there is no voltage difference between
the in-plane electrodes, the particles are distributed in the
dielectric solvent, resulting in the black color (i.e., the color
of the particles) being seen from the top transparent opening.
[0038] FIG. 4C shows the cells filled with a colorless dielectric
solvent with particles of different colors (i.e., red, green or
blue) dispersed therein. The background of the cells is black. When
there is a voltage difference between the in-plane electrodes (not
shown), the colored charged particles migrate to either side of the
cell, the color of the background (i.e., black) is seen from the
top transparent opening. When there is no voltage difference
between the in-plane electrodes, the colored particles are
distributed in the dielectric solvent, resulting in the color of
the particles (i.e., red, green or blue) being seen from the top
transparent opening. In this design, the black state is of high
quality.
[0039] In FIG. 4D, the cells are filled with a colorless dielectric
solvent with particles of different colors (red, green or blue)
dispersed therein. The background of the cells is white. When there
is a voltage difference between the in-plane electrodes (not
shown), the particles migrate to either side of the cell, the color
of the background (i.e., white) is seen from the top transparent
opening, resulting in a high quality white state. When there is no
voltage difference between the in-plane electrodes, the particles
are distributed in the dielectric solvent, resulting in the color
of the particles (i.e., red, green or blue) being seen from the top
transparent opening.
[0040] As shown in FIGS. 4A-4D, the in-plane switching mode allows
the particles to move in the planar (left/right) direction, and
different color combinations of particles, background, and fluid,
wherein each is individually white, black, red, green or blue, can
be used to generate various multi-color EPDs.
[0041] In addition, the particles in the dielectric solvent may be
of mixed colors and the cells have the same background color.
[0042] In an alternative reflective display of the present
invention, the top transparent viewing layer of the display may be
colored or by adding a color filter. In this case, the cells are
filled with an electrophoretic composition comprising white charged
particles in a clear colorless or colored dielectric solvent and
the background of the cells may be black. In a monochrome display,
the transparent viewing layer on each pixel is of the same color
(such as black, red, green, blue, yellow, cyan, magenta, etc.). In
a multiple color display, the transparent viewing layers may be of
different colors.
[0043] III. Preparation of Microcup Array of the Present
Invention
[0044] The microcups generally may be manufactured by
microembossing or photolithography as disclosed in U.S. patent
application Ser. Nos. U.S. Ser. No. 09/518,488 filed Mar. 3, 2000
(corresponding to WO01/67170) and U.S. Ser. No. 09/784,972 filed on
Feb. 15, 2001.
[0045] III(a) Preparation of the Microcup Array by Microembossing
Preparation of the Male Mold
[0046] The male mold may be prepared by any appropriate method,
such as a diamond turn process or a photoresist process followed by
either etching or electroplating. A master template for the male
mold may be manufactured by any appropriate method, such as
electroplating. With electroplating, a glass base is sputtered with
a thin layer (typically 3000 .ANG.) of a seed metal such as chrome
inconel. It is then coated with a layer of photoresist and exposed
to radiation, such as ultraviolet (UV). A mask is placed between
the UV and the layer of photoresist. The exposed areas of the
photoresist become hardened. The unexposed areas are then removed
by washing them with an appropriate solvent. The remaining hardened
photoresist is dried and sputtered again with a thin layer of seed
metal. The master is then ready for electroforming. A typical
material used for electroforming is nickel cobalt. Alternatively,
the master can be made of nickel by electroforming or electroless
nickel deposition as described in "Continuous manufacturing of thin
cover sheet optical media", SPIE Proc. Vol. 1663, pp. 324 (1992).
The floor of the mold is typically between about 50 to 400 microns
thick. The master can also be made using other microengineering
techniques including e-beam writing, dry etching, chemical etching,
laser writing or laser interference as described in "Replication
techniques for micro-optics", SPIE Proc. Vol. 3099, pp. 76-82
(1997). Alternatively, the mold can be made by photomachining using
plastics, ceramics or metals.
[0047] The male mold thus prepared typically has protrusions
between about 1 to 500 microns, preferably between about 2 to 100
microns, and most preferably about 4 to 50 microns. The male mold
may be in the form of a belt, a roller, or a sheet. For continuous
manufacturing, the belt type of mold is preferred. Prior to
applying a UV curable resin composition, the mold may be treated
with a mold release to aid in the demolding process.
[0048] Microcups may be formed either in a batchwise process or in
a continuous roll-to-roll process as described in U.S. Ser. No.
09/784,972 filed on Feb. 15, 2001.
[0049] In the first step of the microembossing process, a UV
curable resin is first coated on a substrate, preferably a
transparent insulator, by any appropriate means, such as roller
coating, die coating, slot coating, slit coating, doctor blade
coating, and the like. Suitable transparent insulator substrates
include polyethylene terephthalate, polyethylene naphthate,
polyaramid, polyimide, polycycloolefin, polysulfone, epoxy and
their composites. The radiation curable material used is a
thermoplastic or thermoset precursor, such as multifunctional
acrylate or methacrylate, vinylether, epoxide and their oligomers,
polymers and the like. Multifunctional acrylates and their
oligomers are the most preferred. A combination of a
multifunctional epoxide and a multifunctional acrylate is also very
useful to achieve desirable physico-mechanical properties. The UV
curable resin may be degassed prior to dispensing and may
optionally contain a solvent. The solvent, if present, readily
evaporates.
[0050] The radiation curable material coated on the substrate is
embossed by the male mold under pressure. If the male mold is
metallic and opaque, the plastic insulator is typically transparent
to the actinic radiation used to cure the resin. Conversely, the
male mold can be transparent and the plastic insulator can be
opaque to the actinic radiation. The plastic insulator is
preferably transparent because it is typically the viewing side. In
this case, the electrodes can be opaque. Alternatively, the
microembossing can be performed on the substrate containing the
electrodes.
[0051] After exposure to radiation, the radiation curable material
becomes hardened. The male mode is then removed exposing the
microcups formed.
[0052] III(b) Preparation of Microcup Array by Photolithography
[0053] The photolithographic process for preparation of the
microcup array is shown in FIGS. 5A and 5B.
[0054] As shown in FIGS. 5A and 5B, the microcup array (50) may be
prepared by exposure of a radiation curable material (51a), coated
by any known methods onto an insulator substrate base (53), to UV
light (or alternatively other forms of radiation, electron beams
and the like) through a mask (56) to form walls (51b) corresponding
to the image projected through the mask (56).
[0055] In the photomask (56) in FIG. 5A, the dark squares (54)
represent the area opaque to the radiation employed, and the space
(55) between the dark squares represents the radiation-transparent
area. The UV radiates through the opening area (55) onto the
radiation curable material (51a).
[0056] As shown in FIG. 5B, the exposed areas (51b) become hardened
and the unexposed areas (protected by the opaque area (54) of the
mask (56)) are then removed by an appropriate solvent or developer
to form the microcups (57). The solvent or developer is selected
from those commonly used for dissolving or dispersing radiation
curable materials such as methylethylketone, toluene, acetone,
isopropanol or the like.
[0057] Alternatively, the exposure can be done by placing the
photomask underneath the insulator substrate. In this case, the
substrate must be transparent to the radiation wavelength used for
exposure.
[0058] The openings of the microcups prepared according to the
methods described above may be round, square, rectangular,
hexagonal, or any other shape. The partition area between the
openings is preferably kept small in order to achieve a high color
saturation and contrast while maintaining desirable mechanical
properties. Consequently the honeycomb-shaped opening is preferred
over, for example, the circular opening.
[0059] For reflective electrophoretic displays, the dimension of
each individual microcup may be in the range of about 10.sup.2 to
about 1.times.10.sup.6 .mu.m.sup.2, preferably from about 10.sup.3
to about 1.times.10.sup.5 .mu.m.sup.2. The depth of the microcups
is in the range of about 5 to about 200 microns, preferably from
about 20 to about 100 microns. The opening to the total area ratio,
total area being defined as that of one cup including walls
measured from wall centers, is in the range of from about 0.2 to
about 0.95, preferably from about 0.5 to about 0.9. The distances
of the openings usually are in the range of from about 15 to about
450 microns, preferably from about 25 to about 300 microns from
edge to edge of the openings.
[0060] III(c) Sealing of the Microcups
[0061] the microcups are filled with an eletrophoretic fluid, they
are sealed. The critical step of sealing of the microcups may be
accomplished in a number of ways. A preferred approach is to
disperse a UV curable composition into an electrophoretic fluid
comprising charged pigment particles dispersed in a colored
dielectric solvent. The suitable UV curable materials include
acrylates, methacrylates, styrene, alpha-methylstyrene, butadiene,
isoprene, allyacrylate, polyvalent acrylate or methacrylate,
cyanoacrylates, polyvalent vinyl including vinylbenzene,
vinylsilane, vinylether, polyvalent epoxide, polyvalent isocyanate,
polyvalent allyl, and oligomers or polymers containing
crosslinkable functional groups. The UV curable composition is
immiscible with the dielectric solvent and has a specific gravity
lower than that of the electrophoretic fluid, i.e., the combination
of the dielectric solvent and the pigment particles. The two
components, UV curable composition and the electrophoretic fluid,
are thoroughly blended in an in-line mixer and immediately coated
onto the microcups with a precision coating mechanism such as Myrad
bar, gravure, doctor blade, slot coating or slit coating. Excess
fluid is removed by a wiper blade or a similar device. A small
amount of a weak solvent or solvent mixture such as isopropanol or
methanol may be used to clean the residual electrophoretic fluid on
the top surface of the partition walls of the microcups. Volatile
organic solvents may be used to control the viscosity and coverage
of the electrophoretic fluid. The thus-filled microcups are then
dried and the UV curable composition floats to the top of the
electrophoretic fluid. The microcups may be sealed by curing the
supernatant UV curable layer during or after it floats to the top.
UV or other forms of radiation such as visible light, IR and
electron beam may be used to cure the sealing layer and seal the
microcups. Alternatively, heat or moisture may also be employed to
cure the sealing layer and seal the microcups, if heat or moisture
curable compositions are used.
[0062] A preferred group of dielectric solvents exhibiting
desirable density and solubility discrimination against acrylate
monomers and oligomers are halogenated hydrocarbons and their
derivatives. Surfactants may be used to improve the adhesion and
wetting at the interface between the electrophoretic fluid and the
sealing materials. Surfactants include the FC surfactants from 3M
Company, Zonyl fluorosurfactants from DuPont, fluoroacrylates,
fluoromethacrylates, fluoro-substituted long chain alcohols,
perfluoro-substituted long chain carboxylic acids and their
derivatives.
[0063] Alternatively, the electrophoretic fluid and the sealing
precursor may be coated sequentially into the microcups to prevent
intermixing, if the sealing precursor is at least partially
compatible with the dielectric solvent. Thus, the sealing of the
microcups may be accomplished by overcoating a thin layer of
sealing material which is hardenable by radiation, heat, moisture
or interfacial reactions on the surface of the filled microcups.
Volatile organic solvents may be used to adjust the viscosity and
the thickness of the coatings. When a volatile solvent is used in
the overcoat, it is preferred that it is immiscible with the
dielectric solvent to reduce the degree of intermixing between the
sealing layer and the electrophoretic fluid. To further reduce the
degree of intermixing, it is highly desirable that the specific
gravity of the overcoating is significantly lower than that of the
electrophoretic fluid. In the copending patent application, U.S.
Ser. No. 09/874,391, filed Jun. 4, 2001, thermoplastic elastomers
have been disclosed as the preferred sealing material.
[0064] Examples of useful thermoplastic elastomers include ABA, and
(AB)n type of di-block, tri-block, and multi-block copolymers
wherein A is styrene, .alpha.-methylstyrene, ethylene, propylene or
norbonene; B is butadiene, isoprene, ethylene, propylene, butylene,
dimethylsiloxane or propylene sulfide; and A and B cannot be the
same in the formula. The number, n, is .gtoreq.1, preferably 1-10.
Particularly useful are di-block or tri-block copolymers of styrene
or .alpha.-methylstyrene such as SB (poly(styrene-b-butadiene)),
SBS (poly(styrene-bbutadiene-b-styrene- )), SIS
(poly(styrene-b-isoprene-b-styrene)), SEBS (poly(styrene-b-ethylen-
e/butylenes-b-styrene)) poly(styrene-b-dimethylsiloxane-b-styrene),
poly((.alpha.-methylstyrene-b-isoprene),
poly(.alpha.-methylstyrene-b-iso- prene-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-propylene
sulfide-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-dimethylsi-
loxane-b-.alpha.-methylstyrene).
[0065] Alternatively, interfacial polymerization followed by UV
curing has been found very beneficial to the sealing process.
Intermixing between the electrophoretic layer and the overcoat is
significantly suppressed by the formation of a thin barrier layer
at the interface by interfacial polymerization. The sealing is then
completed by a post curing step, preferably by UV radiation. The
two-step overcoating process is particularly useful when the dye
used is at least partially soluble in the thermoset precursor.
[0066] III(d) Lamination of the Microcups
[0067] The sealed microcups are then laminated with a top layer
comprising a patterned in plane conductor film and preferably an
adhesive layer. Suitable adhesive materials include acrylic and
rubber types of pressure sensitive adhesives, UV curable adhesives
containing for example, multifunctional acrylates, epoxides, or
vinylethers, and moisture or heat curable adhesives such as epoxy,
polyurethane, and cyanoacrylate.
[0068] The cells prepared from the methods of Sections
III(a)-III(d) may be used in an up-side-down manner with the
transparent viewing layer at the top and the layer with the
in-plane electrodes at the bottom.
[0069] III(e) Alternative Methods
[0070] Alternatively, in the microembossing process, the UV curable
resin is dispensed over the male mold by any appropriate means,
such as coating, dipping, pouring and the like. The dispenser may
be moving or stationary. A patterned in-plane conductor film on a
plastic substrate such as polyethylene terephthalate, polyethylene
naphthate, polyaramid, polyimide, polycycloolefin, polysulfone,
epoxy and their composites is then overlaid on the UV curable
resin. Pressure may be applied to ensure proper bonding between the
resin and the plastic substrate and to control the thickness of the
floor of the microcups. If the male mold is metallic and opaque,
the plastic substrate is typically transparent to the actinic
radiation used to cure the resin. Conversely, the male mold can be
transparent and the plastic substrate can be opaque to the actinic
radiation.
[0071] After exposure to UV radiation, the UV curable resin becomes
hardened, and the male mode may then be removed. The microcup
arrays formed are filled and sealed as described above. The sealed
microcups are then laminated with a transparent insulator layer
preferably using an adhesive.
[0072] Although less preferred, the photolithographic exposure may
also be performed on the substrate having the in-plane electrodes.
A radiation curable material is coated on the patterned conductor
film. The microcups are formed by exposure of the radiation curable
material to radiation through a photomask as shown in FIG. 5 and
described in Section III(b) above.
[0073] The microcups thus prepared are then filled and sealed as
described above and laminated with a transparent insulator layer,
preferably with an adhesive.
[0074] In any of the methods for the preparation of the microcups
disclosed in this section, a substrate containing an array of thin
film transistors (TFT) may be used as the bottom in-plane electrode
layer and in this case the TFT layer also provides an active
driving mechanism.
[0075] IV. Preparation of the Suspensions
[0076] The suspensions filled in the microcups comprise a
dielectric solvent with charged pigment particles dispersed therein
and the particles migrate under the influence of an electric field.
The suspensions may optionally contain additional colorants which
do not migrate in the electric field. The dispersion may be
prepared according to methods well known in the art, such as U.S.
Pat. Nos. 6,017,584, 5,914,806, 5,573,711, 5,403,518, 5,380,362,
4,680,103, 4,285,801, 4,093,534,4,071,430, and 3,668,106, and as
described in IEEE Trans. Electron Devices, ED-24, 827 (1977), and
J. Appl. Phys. 49(9), 4820 (1978).
[0077] The suspending fluid medium is a dielectric solvent which
preferably has a low viscosity and a dielectric constant in the
range of about 2 to about 30, preferably about 2 to about 15 for
high particle mobility. Examples of suitable dielectric solvents
include hydrocarbons such as decahydronaphthalene (DECALIN),
5-ethylidene-2-norbornene, fatty oils, paraffin oil, aromatic
hydrocarbons such as toluene, xylene, phenylxylylethane,
dodecylbenzene and alkylnaphthalene, halogenated solvents such as
dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluorid- e,
chloropentafluoro-benzene, dichlorononane, pentachlorobenzene, and
perfluoro solvents such as perfluorodecalin, perfluorotoluene,
perfluoroxylene, FC-43, FC-70 and FC-5060 from 3M Company, St. Paul
Minn., low molecular weight fluorine containing polymers such as
poly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,
poly(chlorotrifluoroethylene) such as Halocarbon Oils from
Halocarbon Product Corp., River Edge, N.J., perfluoropolyalkylether
such as Galden , HT-200, and Fluorolink from Ausimont or Krytox
Oils and Greases K-Fluid Series from DuPont, Del. In one preferred
embodiment, poly(chlorotrifluoroethylene) is used as the dielectric
solvent. In another preferred embodiment, poly(perfluoropropylene
oxide) is used as the dielectric solvent.
[0078] The non-migrating fluid colorant may be formed from dyes or
pigments. Nonionic azo and anthraquinone dyes are particularly
useful. Examples of useful dyes include, but are not limited to:
Oil Red EGN, Sudan Red, Sudan Blue, Oil Blue, Macrolex Blue,
Solvent Blue 35, Pylam Spirit Black and Fast Spirit Black (Pylam
Products Co., Arizona), Thermoplastic Black X-70 (BASF),
anthraquinone blue, anthraquinone yellow 114, anthraquinone reds
111 and 135, anthraquinone green 28 and Sudan Black B (Aldrich).
Fluorinated dyes are particularly useful when perfluorinated
solvents are used. In the case of a pigment, the pigment particles
for generating the non-migrating fluid colorant may also be
dispersed in the dielectric solvent and these colored particles are
preferably uncharged. If the pigment particles for generating the
non-migrating fluid colorant are charged, they preferably carry a
charge which is opposite from that of the charged migrating pigment
particles. If both types of pigment particles carry the same
charge, then they should have different charge density or different
electrophoretic mobility. The dye or pigment for generating the
non-migrating fluid colorant must be chemically stable and
compatible with other components in the suspension.
[0079] The charged migrating pigment particles are preferably
white, and may be organic or inorganic pigments, such as
TiO.sub.2.
[0080] If colored migrating particles are used, they may be formed
from phthalocyanine blue, phthalocyanine green, diarylide yellow,
diarylide AAOT yellow, and quinacridone, azo, rhodamine, perylene
pigment series (Sun Chemical), Hansa yellow G particles (Kanto
Chemical), and Carbon Lampblack (Fisher). Submicron particle size
is preferred. These particles should have acceptable optical
characteristics, should not be swollen or softened by the
dielectric solvent, and should be chemically stable. The resulting
suspension must also be stable against sedimentation, creaming or
flocculation under normal operating conditions.
[0081] The migrating pigment particles may exhibit a native charge,
or may be charged explicitly using a charge control agent, or may
acquire a charge when suspended in the dielectric solvent. Suitable
charge control agents are well known in the art; they may be
polymeric or non-polymeric in nature, and may also be ionic or
non-ionic, including ionic surfactants such as Aerosol OT, sodium
dodecylbenzenesulfonate, metal soaps, polybutene succinimide,
maleic anhydride copolymers, vinylpyridine copolymers,
vinylpyrrolidone copolymers (such as Ganex, International Specialty
Products), (meth)acrylic acid copolymers, and
N,N-dimethylaminoethyl (meth)acrylate copolymers. Fluorosurfactants
are particularly useful as charge controlling agents in
perfluorocarbon solvents. These include FC fluorosurfactants such
as FC-170C, FC-171, FC-176, FC430, FC431 and FC-740 from 3M Company
and Zonyl fluorosurfactants such as Zonyl FSA, FSE, FSN, FSN-100,
FSO, FSO-100, FSD and UR from Dupont.
[0082] Suitable charged pigment dispersions may be manufactured by
any of the well-known methods including grinding, milling,
attriting, microfluidizing, and ultrasonic techniques. For example,
pigment particles in the form of a fine powder are added to the
suspending solvent and the resulting mixture is ball milled or
attrited for several hours to break up the highly agglomerated dry
pigment powder into primary particles. Although less preferred, a
dye or pigment for producing the non-migrating fluid colorant may
be added to the suspension during the ball milling process.
[0083] Sedimentation or creaming of the pigment particles may be
eliminated by microencapsulating the particles with suitable
polymers to match the specific gravity to that of the dielectric
solvent. Microencapsulation of the pigment particles may be
accomplished chemically or physically. Typical microencapsulation
processes include interfacial polymerization, in-situ
polymerization, phase separation, coacervation, electrostatic
coating, spray drying, fluidized bed coating and solvent
evaporation.
[0084] For pigment suspensions, there are many possibilities. For a
subtractive color system, the charged TiO.sub.2 particles may be
suspended in a dielectric fluid of cyan, yellow or magenta color.
The cyan, yellow or magenta color may be generated via the use of a
dye or a pigment. For an additive color system, the charged
TiO.sub.2 particles may be suspended in a dielectric solvent of
red, green or blue color generated also via the use of a dye or a
pigment. The red, green, blue color system is preferred for most
applications.
[0085] While the present invention has been described with
reference to the specific embodiments thereof, it should be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted without departing from the
true spirit and scope of the invention. In addition, many
modifications may be made to adapt a particular situation,
materials, compositions, processes, process step or steps, to the
objective, spirit and scope of the present invention. All such
modifications are intended to be within the scope of the claims
appended hereto.
[0086] It is therefore wished that this invention to be defined by
the scope of the appended claims as broadly as the prior art will
permit, and in view of the specification.
* * * * *