U.S. patent application number 12/020082 was filed with the patent office on 2008-05-22 for inkjet printing of color filters.
This patent application is currently assigned to Orbotech Ltd.. Invention is credited to David Bochner, Mannie Dorfan, Abraham Gross, Gershon Miller, Ofer Saphier.
Application Number | 20080117247 12/020082 |
Document ID | / |
Family ID | 38873136 |
Filed Date | 2008-05-22 |
United States Patent
Application |
20080117247 |
Kind Code |
A1 |
Miller; Gershon ; et
al. |
May 22, 2008 |
INKJET PRINTING OF COLOR FILTERS
Abstract
A method for printing includes providing a substrate on which a
matrix of color elements is defined, the color elements having
respective center lines. A printhead assembly is positioned over
the substrate. The printhead assembly includes multiple
controllable nozzles. At least one of the substrate and the
printhead assembly is translated so that the printhead assembly
scans over the substrate in a scan direction transverse to the
center lines of the color elements. Droplets of ink are ejected
from the nozzles onto the substrate at selected times while the
printhead assembly scans over the substrate. The times at which to
eject the droplets are selected so as to cause the droplets to land
on the color elements at respective locations, such that respective
locations are displaced from the center lines.
Inventors: |
Miller; Gershon; (Rehovot,
IL) ; Dorfan; Mannie; (Nes Ziona, IL) ; Gross;
Abraham; (Ramat Aviv, IL) ; Saphier; Ofer;
(Rehovot, IL) ; Bochner; David; (Ramat Gan,
IL) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
Orbotech Ltd.
Yavne
IL
81102
|
Family ID: |
38873136 |
Appl. No.: |
12/020082 |
Filed: |
January 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11472551 |
Jun 22, 2006 |
7347530 |
|
|
12020082 |
Jan 25, 2008 |
|
|
|
Current U.S.
Class: |
347/15 |
Current CPC
Class: |
G02B 5/201 20130101;
H01L 51/0005 20130101; H01L 27/322 20130101; G02F 1/133516
20130101 |
Class at
Publication: |
347/015 |
International
Class: |
B41J 2/205 20060101
B41J002/205 |
Claims
1. A method for printing, comprising: providing a substrate on
which a matrix of color elements is defined, the color elements
having respective center lines; positioning a printhead assembly
over the substrate, the printhead assembly comprising multiple
controllable nozzles; translating at least one of the substrate and
the printhead assembly so that the printhead assembly scans over
the substrate in a scan direction transverse to the center lines of
the color elements; ejecting droplets of ink from the nozzles onto
the substrate at selected times while the printhead assembly scans
over the substrate; and selecting the times at which to eject the
droplets so that the droplets are aimed to land on the color
elements at respective locations, to a side of the center
lines.
2. The method according to claim 1, wherein providing the substrate
comprises creating elevated borders on the substrate surrounding
and defining the color elements.
3. The method according to claim 2, wherein ejecting the droplets
comprises directing at least some droplets to contact the
borders.
4. The method according to claim 1, wherein the color elements are
arranged in rows along a direction transverse to the scan
direction, and wherein ejecting the droplets comprises coloring
successive, neighboring rows with different, respective colors in
alternation.
5. The method according to claim 1, wherein ejecting the droplets
comprises actuating the nozzles so as to deposit two or more of the
droplets on each of the color elements during a single pass of the
printhead over the color elements.
6. The method according to claim 5, wherein actuating the nozzles
comprises depositing the two or more of the droplets side by side
along an axis transverse to the scan direction.
7. The method according to claim 5, wherein actuating the nozzles
comprises depositing the two or more of the droplets in succession
along the scan direction.
8. The method according to claim 5, wherein actuating the nozzles
comprises depositing the two or more of the droplets so that the
two or more of the droplets overlap and coalesce within each of the
color elements on the substrate.
9. The method according to claim 5, wherein the printhead assembly
comprises multiple, staggered printheads, each comprising a
respective plurality of the nozzles, and wherein actuating the
nozzles comprises ejecting the two or more of the droplets from
different ones of the printheads.
10. The method according to claim 1, wherein selecting the times
comprises displacing the droplets from the center lines so as to
compensate for motion of the printhead assembly relative to the
substrate.
11. The method according to claim 1, wherein the substrate
comprises a glass, and wherein ejecting the droplets comprises
coloring the color elements so as to serve as a filter overlay for
a flat panel display.
12. The method according to claim 1, wherein the substrate
comprises a glass, and wherein ejecting the droplets comprises
ejecting a polymer operative to selectively emit a colored light so
as to serve as an OLED element.
13. The method according to claim 1, wherein the printhead assembly
comprises multiple printing units, each comprising a respective
plurality of the nozzles, wherein the printing units are
distributed along the printhead assembly, and comprising
associating a respective cleaning unit with each of the printing
units and, after scanning the printhead assembly over the
substrate, cleaning excess ink off each of the printing units using
the respective cleaning unit.
14. A method for printing, comprising: providing a substrate on
which a matrix of color elements is defined; positioning a
printhead assembly over the substrate, the printhead assembly
comprising multiple controllable nozzles; translating at least one
of the substrate and the printhead assembly so that the printhead
assembly scans over the substrate in a scan direction; ejecting
droplets of ink from the nozzles onto the substrate at selected
times while the printhead assembly scans over the substrate so as
to deposit two or more of the droplets on each of the color
elements during a single pass of the printhead over the color
elements; and selecting the times at which to eject the droplets so
as to cause the droplets to land on the color elements so that the
two or more of the droplets overlap and coalesce within each of the
color elements on the substrate.
15. The method according to claim 14, wherein ejecting the droplets
comprises depositing the two or more of the droplets side by side
along an axis transverse to the scan direction.
16. The method according to claim 14, wherein ejecting the droplets
comprises depositing the two or more of the droplets in succession
along the scan direction.
17. The method according to claim 14, wherein the printhead
assembly comprises one or more printing units, each printing unit
comprising multiple, staggered printheads for ejecting the droplets
of ink of a given color, each printhead comprising a respective
plurality of the nozzles, and wherein actuating the nozzles
comprises ejecting the two or more of the droplets from different
ones of the printheads.
18. The method according to claim 17, wherein the nozzles in each
of the multiple, staggered printheads in each printing unit are
displaced by respective offsets in a direction transverse to the
scan direction, wherein the respective offsets do not increase
monotonically across the printing unit.
19. Apparatus for printing on a substrate on which a matrix of
color elements is defined, the color elements having respective
center lines, the apparatus comprising: a printhead assembly, which
is arranged to be positioned over the substrate, the printhead
assembly comprising multiple controllable nozzles; a translation
assembly, which is arranged to translate at least one of the
substrate and the printhead assembly so that the printhead assembly
scans over the substrate in a scan direction transverse to the
center lines of the color elements; and a controller, which is
coupled to drive the printhead assembly to eject droplets of ink
from the nozzles onto the substrate at selected times while the
printhead assembly scans over the substrate, while selecting the
times at which to eject the droplets so as to cause the droplets to
land on the color elements at respective locations, such that
respective locations are displaced from the center lines.
20. The apparatus according to claim 19, wherein the substrate
comprises elevated borders, surrounding and defining the color
elements, and wherein the controller is operative to drive the
printhead assembly so that the ejected droplets contact the
borders.
21. The apparatus according to claim 19, wherein the color elements
are arranged in rows along a direction transverse to the scan
direction, and wherein the controller is operative to drive the
printhead assembly so as to color successive, neighboring rows with
different, respective colors in alternation.
22. The apparatus according to claim 19, wherein the controller is
operative to drive the printhead assembly so that the nozzles
deposit two or more of the droplets on each of the color elements
during a single pass of the printhead over the color elements.
23. The apparatus according to claim 22, wherein the two or more of
the droplets are deposited side by side along an axis transverse to
the scan direction.
24. The apparatus according to claim 22, wherein the two or more of
the droplets are deposited in succession along the scan
direction.
25. The apparatus according to claim 24, wherein the two or more of
the droplets are deposited so as to overlap and coalesce within
each of the color elements on the substrate.
26. The apparatus according to claim 24, wherein the printhead
assembly comprises multiple, staggered printheads, each comprising
a respective plurality of the nozzles, and wherein the two or more
of the droplets are ejected from different ones of the
printheads.
27. The apparatus according to claim 19, wherein the controller is
adapted to select the times so as to compensate for motion of the
printhead assembly relative to the substrate.
28. The apparatus according to claim 19, wherein the substrate
comprises a glass, and wherein the ink is adapted to color the
color elements so as to serve as a filter overlay for a flat panel
display.
29. The apparatus according to claim 19, wherein the substrate
comprises a glass, and wherein the ink comprises a polymer
operative to emit colored light in response to an electrical
signal.
30. The apparatus according to claim 19, wherein the printhead
assembly comprises multiple printing units, each comprising a
respective plurality of the nozzles, wherein the printing units are
distributed along the printhead assembly, and comprising multiple
cleaning units, which are associated respectively with the multiple
printing units and which are operative, after the printhead
assembly has scanned over the substrate, to clean excess ink off
the respective printing units.
31. Apparatus for printing on a substrate on which a matrix of
color elements is defined, the apparatus comprising: a printhead
assembly, which is arranged to be positioned over the substrate,
the printhead assembly comprising multiple controllable nozzles; a
translation assembly, which is arranged to translate at least one
of the substrate and the printhead assembly so that the printhead
assembly scans over the substrate in a scan direction; and a
controller, which is coupled to drive the printhead assembly to
eject droplets of ink from the nozzles onto the substrate at
selected times while the printhead assembly scans over the
substrate so as to deposit two or more of the droplets on each of
the color elements during a single pass of the printhead over the
color elements, while selecting the times at which to eject the
droplets so as to cause the droplets to land on the color elements
so that the two or more of the droplets overlap and coalesce within
each of the color elements on the substrate.
32. The apparatus according to claim 31, wherein the two or more of
the droplets are deposited side by side along an axis transverse to
the scan direction.
33. The apparatus according to claim 31, wherein the two or more of
the droplets are deposited in succession along the scan
direction.
34. The apparatus according to claim 31, wherein the printhead
assembly comprises one or more printing units, each printing unit
comprising multiple, staggered printheads for ejecting the droplets
of ink of a given color, each printhead comprising a respective
plurality of the nozzles, and wherein the two or more of the
droplets are ejected from different ones of the printheads.
35. The apparatus according to claim 34, wherein the nozzles in
each of the multiple, staggered printheads in each printing unit
are displaced by respective offsets in a direction transverse to
the scan direction, wherein the respective offsets do not increase
monotonically across the printing unit.
Description
[0001] This is a continuation of Ser. No. 11/472,551 filed Jun. 22,
2006, the entire disclosure of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to inkjet printing,
and specifically to production of flat panel displays using inkjet
technology.
BACKGROUND OF THE INVENTION
[0003] To produce a color flat panel display, a matrix of
light-modulating elements, such as a liquid crystal display (LCD),
is overlaid by a corresponding matrix of color elements. Each color
element filters the light that passes through the corresponding
light-modulating element and thus enables the display to present
color images.
[0004] U.S. Pat. No. 6,645,029, whose disclosure is incorporated
herein by reference, describes a method and apparatus for producing
color filters by ejecting ink onto a substrate from a plurality of
nozzles in an inkjet head. The nozzles are arranged in a first
direction, and the inkjet head is scanned relative to the substrate
in a second, perpendicular direction. Thus, filter elements
adjacent in the second direction may be colored with different
colors. Ejection timings for the nozzles are adjusted so that the
ink dots land along the center line of each of the filter elements
in the first direction. This technique is said to reduce the
occurrence of color mixing.
SUMMARY OF THE INVENTION
[0005] Embodiments of the present invention provide methods and
apparatus for printing matrices of color elements. These methods
and apparatus are useful particularly in producing filter overlays,
OLED elements and the like for color flat panel displays, but they
may similarly be used in other large-scale inkjet printing
applications.
[0006] In disclosed embodiments, a printhead assembly comprising
multiple nozzles is scanned relative to a substrate in a scan
direction generally transverse to color elements to be formed, and
droplets of ink are ejected from the nozzles at selected times
during the scan. The times at which to eject the droplets are
selected and controlled so as to cause the droplets to land at
respective locations on designated color elements, such that
respective locations are displaced from the center lines of the
color elements. This displacement is useful in ensuring that the
ink spreads evenly over the color element by compensating for
factors such as the fluid dynamics of the ejected droplets, as well
as electrostatic and hydrostatic interaction between droplets and
ink that has already been deposited on the substrate.
[0007] As used in this disclosure, the term "ink" includes without
limitation inks employed in the production of color filters as well
as polymer materials, such as polymer materials responsive to
electric signals to emit light as are employed in OLED type
displays.
[0008] There is therefore provided, in accordance with an
embodiment of the present invention, a method for printing,
including:
[0009] providing a substrate on which a matrix of color elements is
defined, the color elements having respective center lines;
[0010] positioning a printhead assembly over the substrate, the
printhead assembly including multiple controllable nozzles;
[0011] translating at least one of the substrate and the printhead
assembly so that the printhead assembly scans over the substrate in
a scan direction transverse to the center lines of the color
elements;
[0012] ejecting droplets of ink from the nozzles onto the substrate
at selected times while the printhead assembly scans over the
substrate; and
[0013] selecting the times at which to eject the droplets so as to
cause the droplets to land on the color elements at respective
locations, such that respective locations are displaced from the
center lines.
[0014] In a disclosed embodiment, providing the substrate includes
creating elevated borders on the substrate surrounding and defining
the color elements, and ejecting the droplets includes directing
the droplets to contact the borders.
[0015] Typically, the color elements are arranged in rows along a
direction transverse to the scan direction, and ejecting the
droplets includes coloring successive, neighboring rows with
different, respective colors in alternation.
[0016] In disclosed embodiments, ejecting the droplets includes
actuating the nozzles so as to deposit two or more of the droplets
on each of the color elements during a single pass of the printhead
over the color elements. The nozzles may be actuated to deposit the
two or more of the droplets side by side along an axis transverse
to the scan direction or, alternatively or additionally, in
succession along the scan direction. Typically, actuating the
nozzles includes depositing the two or more of the droplets so that
the two or more of the droplets overlap and coalesce within each of
the color elements on the substrate. In some embodiments, the
printhead assembly includes multiple, staggered printheads, each
including a respective plurality of the nozzles, and actuating the
nozzles includes ejecting the two or more of the droplets from
different ones of the printheads.
[0017] Typically, selecting the times includes displacing the
droplets from the center lines so as to compensate for motion of
the printhead assembly relative to the substrate. Additionally or
alternatively, selecting the times includes determining the
respective locations responsively to a repulsion between the
droplets and at least one of the ink already deposited on the
substrate and the ink in other droplets.
[0018] In a disclosed embodiment, the substrate includes a glass,
and wherein ejecting the droplets includes coloring the color
elements so as to serve as a filter overlay for a flat panel
display. Additionally and alternatively, ejecting the droplets
includes ejecting a polymer ink that is responsive to electrical
signals to selectively emit colored light.
[0019] The printhead assembly may include multiple printing units,
each including a respective plurality of the nozzles, wherein the
printing units are distributed along the printhead assembly, and
the method may include associating a respective cleaning unit with
each of the printing units and, after scanning the printhead
assembly over the substrate, cleaning excess ink off each of the
printing units using the respective cleaning unit.
[0020] There is also provided, in accordance with an embodiment of
the present invention, a method for printing, including:
[0021] providing a substrate on which a matrix of color elements is
defined;
[0022] positioning a printhead assembly over the substrate, the
printhead assembly including multiple controllable nozzles;
[0023] translating at least one of the substrate and the printhead
assembly so that the printhead assembly scans over the substrate in
a scan direction;
[0024] ejecting droplets of ink from the nozzles onto the substrate
at selected times while the printhead assembly scans over the
substrate so as to deposit two or more of the droplets on each of
the color elements during a single pass of the printhead over the
color elements; and
[0025] selecting the times at which to eject the droplets so as to
cause the droplets to land on the color elements so that the two or
more of the droplets overlap and coalesce within each of the color
elements on the substrate.
[0026] Typically, the two or more of the droplets are deposited
within a time limit that is determined responsively to a drying
rate of the ink. In disclosed embodiments, the time limit is no
greater than 200 ms, and may be no greater than 20 ms.
[0027] In a disclosed embodiment, the printhead assembly includes
one or more printing units, each printing unit including multiple,
staggered printheads for ejecting the droplets of ink of a given
color, each printhead including a respective plurality of the
nozzles, and actuating the nozzles includes ejecting the two or
more of the droplets from different ones of the printheads.
Typically, the nozzles in each of the multiple, staggered
printheads in each printing unit are displaced by respective
offsets in a direction transverse to the scan direction, and in one
embodiment, the respective offsets do not increase monotonically
across the printing unit.
[0028] There is additionally provided, in accordance with an
embodiment of the present invention, apparatus for printing on a
substrate on which a matrix of color elements is defined, the color
elements having respective center lines, the apparatus
including:
[0029] a printhead assembly, which is arranged to be positioned
over the substrate, the printhead assembly including multiple
controllable nozzles;
[0030] a translation assembly, which is arranged to translate at
least one of the substrate and the printhead assembly so that the
printhead assembly scans over the substrate in a scan direction
transverse to the center lines of the color elements; and
[0031] a controller, which is coupled to drive the printhead
assembly to eject droplets of ink from the nozzles onto the
substrate at selected times while the printhead assembly scans over
the substrate, while selecting the times at which to eject the
droplets so as to cause the droplets to land on the color elements
at respective locations, such that respective locations are
displaced from the center lines.
[0032] There is further provided, in accordance with an embodiment
of the present invention, apparatus for printing on a substrate on
which a matrix of color elements is defined, the apparatus
including:
[0033] a printhead assembly, which is arranged to be positioned
over the substrate, the printhead assembly including multiple
controllable nozzles;
[0034] a translation assembly, which is arranged to translate at
least one of the substrate and the printhead assembly so that the
printhead assembly scans over the substrate in a scan direction;
and
[0035] a controller, which is coupled to drive the printhead
assembly to eject droplets of ink from the nozzles onto the
substrate at selected times while the printhead assembly scans over
the substrate so as to deposit two or more of the droplets on each
of the color elements during a single pass of the printhead over
the color elements, while selecting the times at which to eject the
droplets so as to cause the droplets to land on the color elements
so that the two or more of the droplets overlap and coalesce within
each of the color elements on the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
[0037] FIG. 1 is a schematic, pictorial illustration showing
apparatus for printing a matrix of color elements, in accordance
with an embodiment of the present invention;
[0038] FIG. 2 is a schematic bottom view of a printhead assembly,
in accordance with an embodiment of the present invention;
[0039] FIG. 3 is a schematic detail view of a matrix of color
elements, which is printed in accordance with an embodiment of the
present invention;
[0040] FIG. 4A is a schematic detail view of a single color element
showing deposition of ink droplets thereon, in accordance with an
embodiment of the present invention;
[0041] FIG. 4B is a schematic bottom view of a printhead assembly,
in accordance with another embodiment of the present invention;
[0042] FIGS. 5A-5C are schematic, sectional views of a pair of
color elements during successive stages in the deposition of ink
droplets on one of the elements, in accordance with an embodiment
of the present invention;
[0043] FIG. 6 is a schematic detail view of a matrix of color
elements showing locations for deposition of ink droplets thereon,
in accordance with an embodiment of the present invention; and
[0044] FIG. 7 is a schematic detail view of a matrix of color
elements showing locations for deposition of ink droplets thereon,
in accordance with another embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0045] FIG. 1 is a schematic, pictorial illustration showing
apparatus 20 for printing a matrix of color elements on a substrate
22, in accordance with an embodiment of the present invention. A
printhead assembly 26, comprising multiple printing units 28, is
positioned over substrate 22. Each printing unit comprises multiple
inkjet nozzles, which are configured to eject inks, such as colored
inks, onto substrate 22 to form color elements 23, as shown in the
figures that follow and described in detail hereinbelow. Once the
printing process has been completed, the substrate may be used, for
example, as a color filter overlay on a conventional flat panel
display or as an OLED type flat panel display.
[0046] A translation assembly causes printhead assembly 26 to scan
over substrate 22 in a scan direction, which is taken arbitrarily
to be the Y-direction. In this embodiment, the translation assembly
comprises a translation stage 24, such as an air table, on which
substrate 22 is mounted. An exemplary air float conveyor system of
this sort is described in U.S. Pat. No. 6,810,297, whose disclosure
is incorporated herein by reference. (The air float may also be
used to heat substrate 22, in order to speed the drying of the ink
and improve adhesion on the substrate.) Optionally, the translation
stage is also configured to translate the substrate in the
X-direction, so that different parts of the substrate may be
covered by the inkjet nozzles during successive Y-direction scans.
Alternatively or additionally, the translation assembly may move
the printhead assembly. Mechanical elements not essential to an
understanding of the present invention, such as components of the
translation assembly and components used in mounting and supplying
inks to the printhead assembly and in transporting substrate 22,
are omitted from the figures for the sake of simplicity.
[0047] A control console 30 controls the scan of printhead assembly
26 over substrate 22 and drives the ejection of ink droplets from
the printing unit nozzles. The ejection timing is precisely
controlled so that the droplets land at the desired location on
each color element 23. Methods used for this purpose are described
further hereinbelow. Optionally, system 20 comprises an optical
inspection unit (not shown), such as a miniature digital camera,
for checking the accuracy of alignment of the ink droplets
deposited on the substrate. For example, the control console may
command the printhead assembly to print a line of test spots at one
or more predetermined target locations, typically near the edge of
the substrate. If the optical inspection unit detects a deviation
between the actual locations of the spots and the target locations,
console 30 may adjust the timing and/or other parameters of ink
ejection in order to correct the deviation. The inventors have
developed techniques, as explained further hereinbelow, that
promote self-alignment of the multiple droplets that are deposited
in each color element 23. When these techniques are used, the
inventors have found in many cases that measurement of and
correction for global average deviations, such as average shift,
scaling and rotation of ink ejection by printhead units 28, provide
acceptable printing results over the entire substrate, without the
need for local measurement and correction of printing of individual
color elements or groups of color elements. Alternatively,
localized measurement and correction of deviations may be used when
necessary.
[0048] FIG. 2 is a schematic bottom view of printhead assembly 26,
showing the side of the assembly that faces substrate 22, in
accordance with an embodiment of the present invention. Assembly 26
comprises multiple printing units 28, each comprising multiple
printheads 32, with ink ejection nozzles 34. In a typical
embodiment, each printing unit comprises ten printheads, which are
stacked side-by-side in the scan (Y) direction as shown in the
figure. Printheads 32 in each printing unit are staggered, as shown
in the figure, so that nozzles 34 are mutually offset in the
transverse (X) direction. As a result, the effective pitch
(spot-to-spot distance) of the printhead unit in the transverse
direction is very fine and may be made smaller than the actual
droplet diameter. The ink may then be printed on substrate 22 in a
dense swath of overlapping spots, as described further hereinbelow.
(The actual number of printheads in each unit and the X-direction
offset may be determined according to the spot size, the desired
degree of overlap, the sequence of droplet ejection, and the
spacing between nozzles 34. An alternative arrangement of the
printheads is shown in FIG. 4B and described hereinbelow.)
Printheads 32 may be fixed parallel to the X-direction, since the
configuration shown here obviates the need to angle the printheads
in order to match the pitch of the color elements on the substrate
as in some systems known in the art.
[0049] Because of the stacked and staggered design shown in FIG. 2,
printheads 32 may comprise conventional, off-the-shelf inkjet
printheads. Examples of such printheads include Ricoh E1 Drop on
Demand 96 (Ricoh Printing Systems America, Inc., Simi Valley,
Calif.); Spectra SX-128 (Dimatix Spectra Printing Division,
Lebanon, N.H.); and Xaar XJ500 (Xaar plc, Cambridge, United
Kingdom). In all, printhead assembly 26 may comprise tens of
thousands of nozzles 34, which are individually controlled by
console 30 to eject a "drop on demand" with precisely-controlled
timing and/or drop volume and/or drop velocity. The timing is
controlled in order to compensate, inter alia, for the relative
offsets of printheads 32 in the Y-direction and for the need to
deposit droplets precisely within the desired color elements 23, as
described hereinbelow.
[0050] In the embodiment shown in FIG. 2, printing units 28 are
grouped together in modules, each comprising one printing unit for
each of the primary colors (red, green and blue). As printhead
assembly 26 scans over substrate 22, the nozzles of each color are
actuated at the appropriate times to print the different color
elements. For reasons of cost reduction, packaging, and convenience
in cleaning the printheads, inter alia, the printing units may be
spaced apart along the X-axis as shown in the figure. In this case,
in order to cover the entire surface of the substrate, two or more
Y-direction scan passes are used, with the printhead assembly
offset in the X-direction between successive passes so that on the
second scan the printing units cover gaps left between the printing
units in the first scan. In an embodiment, the printhead assembly
and scan pattern are designed so that one third of the substrate is
covered in each of three successive scan passes in the Y-direction.
For enhanced throughput, the first and third passes may be in one
direction, with the second pass in the opposite direction.
Additionally or alternatively, in successive passes nozzles in the
printhead units may be activated to cover the same areas of the
substrate in order to deposit two or more layers of ink in each
color element. Multi-layer printing in this fashion may enhance the
uniformity of the color elements.
[0051] When printing units 28 are spaced apart as shown in FIG. 2,
cleaning modules 36 may be interleaved between the printing units.
Each cleaning module comprises a wiper blade 38. Typically, the
cleaning modules are activated, after a predetermined number of
scans or in case of erratic print-head operation, as part of a
flushing and cleaning procedure to clean off excess ink that may
have accumulated on the printheads. During this cleaning procedure,
each blade 38 typically slides over the corresponding printing unit
28 in order to wipe the excess ink into a "spittoon" (not shown),
located for example, at the end of stage 24. This sort of cleaning
is useful in maintaining optimal printing resolution, since excess
ink can adversely affect the direction and velocity of droplets
that are ejected from the printheads. The cleaning procedure may be
followed by a printing quality test after each scan or after
completing the printing of each panel, or after several scans or
panels, although this need not be the case.
[0052] FIG. 3 is a simplified schematic view of a matrix of color
elements 40 printed on substrate 22, in accordance with an
embodiment of the present invention. The color elements in this
embodiment are arranged in rows of red, green and blue along the
X-direction, as is common in matrices of color elements that are
used with flat panel displays. More generally, the color elements
may have substantially any suitable polygonal shape, such as
"zigzag" and "boomerang" shapes that are known in the art. The
color elements are separated from their neighbors by borders 42,
which are commonly referred to as a "black matrix." In accordance
with an embodiment, a phantom border, known as a "bank matrix",
that is not opaque, is provided to delineate among color elements.
These borders are deposited on substrate 22 and protrude slightly
above the substrate surface, thus defining recesses into which the
ink is injected by printing apparatus 20. Alternatively, the
apparatus and methods described herein may be used in depositing
color elements that are predefined geometrically in the program of
console 30 without reliance on borders of this sort.
[0053] Typically, the color elements defined by borders 42 are
elongated in the X-direction, transverse to the scan, relative to
their Y-direction dimension. (In other words, the long dimension of
the color elements is aligned with the long dimension of printheads
32 and is transverse to the scan direction.) In embodiments, the
center-to-center spacing between adjacent color elements is between
210 and 600 .mu.m in the X-direction and between 70 and 200 .mu.m
in the Y-direction. Alternatively, apparatus 20 may be adapted to
print matrices of larger or smaller color elements.
[0054] On the basis of FIGS. 2 and 3, it can be seen that as
printhead assembly 26 scans over substrate 22, nozzles 34 in each
printhead 32 in red printing units 28 will be triggered to eject
drops as the printhead passes over the row of red color elements
40. The nozzles in the green and blue printhead units will be
triggered accordingly over their respective rows. As the printhead
then scans over the other rows (of different colors), the nozzles
of the non-corresponding colors are shut off. During this interval,
the ink at each nozzle is replenished, and maintenance procedures
(for example phantom operation of the nozzles) are performed on the
nozzles as necessary.
[0055] The droplets from adjacent nozzles in a given printing unit
are typically all deposited in the color element within less than
one second, overlapping in the X-direction. The droplets thus
create a dense swath of liquid ink, which coalesces to create a
uniform fluid layer over the entire color element. In general, the
time between deposition of the droplets in a color element is
limited by the drying rate (i.e., the evaporation rate of the
volatile components) of the ink: For good coalescence, it is
desirable that each successive droplet be deposited before the
adjacent droplet or droplets have dried sufficiently to adhere to
the substrate. Depending on the ink characteristics, it is usually
necessary to deposit adjacent droplets within 200 ms of one
another, and the optimal maximum time between deposition of
adjacent droplets may be no greater than 20 ms. Longer delays in
deposition of adjacent droplets may cause non-uniform distribution
and drying and result in defects. Thus, in embodiments of the
present invention, system 20 is designed and programmed so that the
timing of deposition of the adjacent droplets is determined
responsively to the evaporation rate of the ink droplets on
substrate 22.
[0056] FIG. 4A is a simplified schematic detail view of one color
element 40, showing deposition of droplets 44 of ink within borders
42 of the color element, in accordance with an embodiment of the
present invention. Each droplet 44 typically comprises between 20
and 50 pl (pico-liters) of ink. A 33 pl droplet of ink, for
example, will have dimensions in air (in transit between the nozzle
and the substrate surface) of about 30 .mu.m diameter by 60 .mu.m
height. Assuming that borders 42 define a recess of width (in the
X-direction) 300 .mu.m, for example, approximately nine of these
droplets, spaced 34 .mu.m apart center-to-center along an axis
transverse to the scan direction, will cover the width of the
recess with adequate overlap.
[0057] Timing of droplet ejection by nozzles 34 is controlled so
that in the Y-direction, droplets 44 are intentionally deposited to
one side of the center line of the color element. Specifically, in
this example, the droplets are aimed to land to one side of the
center line and spread across the center line because of the scan
motion. Considerations of importance in determining the landing
point (and hence ejection timing) of the droplets are further
described hereinbelow. If the Y-dimension of the color elements is
sufficiently small, relative to the volume of the droplets, then a
single row of droplets, as shown in FIG. 4A, may be sufficient to
spread over the entire surface of each color element.
Alternatively, elements that are wider in the Y-direction may
require that each nozzle 34 release two or more successive droplets
per color element 40. For example, the inventors have found that a
single row of 33 pl droplets is sufficient to cover color elements
that are up to about 100 .mu.m wide in the Y-direction, whereas two
rows of droplets give better results when the color elements are
about 120 .mu.m wide for example.
[0058] The inventors have also discovered that densely-spaced
droplets, such as those shown in FIG. 4A, tend to align with one
another on the substrate, as long as the adjacent droplets are
deposited within the time limits described above. As a result, in
calibrating and adjusting the ejection timing of the nozzles, it
may be sufficient simply to calibrate the timing of the entire
printhead to compensate for average deviation factors (such as
shift, scaling and rotation), rather than calibrating and adjusting
the timing of each nozzle individually. Alternatively, for very
high precision, a timing correction may be determined and applied
for each nozzle.
[0059] FIG. 4B is a schematic bottom view of one of printing units
28, in accordance with another embodiment of the present invention.
In this embodiment, the printing unit comprises ten printheads 32,
which are staggered in a complex pattern. In this pattern, the
offset of nozzles 34 (relative to the nozzles in the first
printhead) does not increase monotonically across the printing
unit, unlike the simple stagger pattern shown in FIG. 2. The timing
of droplet ejection from each printhead 32 is controlled, relative
to the scan velocity of printhead assembly 26, so that droplets 44
form a single, dense swath of ink, as shown at the bottom of FIG.
4B. The complex stagger pattern is advantageous in ensuring that
the time elapsed between deposition of each droplet and the
deposition of the adjacent droplets (from other printheads) is less
than the time limit imposed by the drying rate of the ink.
[0060] FIGS. 5A-5C are schematic, sectional views of a pair of
color elements 40a, 40b during successive stages in the deposition
of ink droplets on element 40b, in accordance with an embodiment of
the present invention. These figures show a cross-section in the
Y-Z plane, with the scan (in the Y-direction) moving to the
right.
[0061] In preparation for printing the color elements, borders 42
are deposited on a base layer 50, which typically comprises glass.
The borders typically comprise a suitable polymer containing opaque
black pigment, such as PSK.TM.2000 black matrix resin, distributed
by Brewer Science (Rolla, Mo.), which is deposited on the base
layer to a thickness between about 1.2 and 1.4 .mu.m.
Alternatively, borders of different types and thickness may be
used, for example non-opaque "bank matrix" borders, or the color
elements may be printed without pre-deposited borders, as noted
above.
[0062] In the example shown in FIGS. 5A-5C, it is assumed that
color element 40a has already been printed, and is thus filled with
ink 52 that has dried. As seen in FIGS. 5A-5C, the formation of ink
52 includes at least two coats of ink resulting the ink being at a
height that is above borders 42. The ink may comprise, for example,
a Brewer Science color filter resin, similar in composition to
photoresist materials known in the art. The resin is typically
modified by addition of a volatile additive, such as BYK.RTM.-358 N
acrylic additive (BYK Chemie, Wesel, Germany) for improved
leveling. Alternatively, the ink may comprise, for example, a
polymer resin that emits light in response to electrical signals as
used form OLED elements.
[0063] Optionally, for enhanced adhesion and homogeneity of the
ink, an intermediate layer (not shown) is deposited on base layer
50 and borders 42 before printing of the color elements. The
inventors have found, however, that when the techniques described
herein are used, the intermediate layer is not required. Rather, as
long as the ink droplets come into contact with border 42 all
around the edges of the color element, the borders act as a sort of
"tent peg," drawing the ink into the interface between the borders
and the base layer and thus ensuring homogeneous ink coverage. For
this reason, it is desirable that the ink droplets slightly overlap
the borders, as shown in FIGS. 4A, 4B and 5B.
[0064] The width of color elements 40a, 40b in the scan direction
in FIGS. 5A-5C is such that two rows of droplets 44 are required in
order to cover the entire width, as shown in the figure. Typically,
the ejection timing of nozzles 34 is controlled so that the
droplets are deposited to either side of a transverse (X-direction)
center line 54 of element 40b, as shown in FIG. 5A.
[0065] The inventors have found, furthermore, that there is a
repulsive effect between ink 52 that has previously been deposited
in one color element (such as element 40a in these figures) and
fresh ink deposited in the neighboring color element (element 40b).
Thus, droplets 44 are not deposited symmetrically about center line
54, but are rather displaced laterally, as shown in FIG. 5A, in
order to compensate for the repulsive effect. The relative spacing
between droplets 44 may also be adjusted, typically by decreasing
the time between ejection of successive droplets, in order to
compensate for mutual repulsion between the droplets.
[0066] Immediately following deposition of droplets 44 on base
layer 50, the droplets spread laterally, as shown in FIG. 5B, and
contact borders 42 on all sides of color element 40b. This
spreading stage is typically completed within about 0.1 sec of
droplet deposition. Droplets 44 subsequently dry to form an ink
layer 56 that is approximately uniform in thickness, as shown in
FIG. 5C. The ink volume contracts due to evaporation of volatile
components of the ink during drying. As noted above, for enhanced
uniformity, an additional ink layer may subsequently be printed
over layer 56.
[0067] Thus, to summarize, the ejection timing of nozzles 34 is
adjusted so as to take the following factors into account:
[0068] Elongation of the droplet in the scan direction due to
length of ejected drop and motion of printhead assembly 26 relative
to substrate 22.
[0069] Repulsive forces due to ink already deposited in neighboring
color elements (or alternatively, the absence of such forces when
neighboring elements have not yet been printed).
[0070] Geometry of borders 42.
[0071] Optimal drop positions under given conditions of color
element size and status of neighboring color elements may be
computed from first principles or determined empirically based on
trial deposition under different conditions.
[0072] FIGS. 6 and 7 are schematic detail views of matrices of
color elements 40, showing locations for deposition of ink droplets
44 thereon by nozzles 34, based on the principles outlined above,
in accordance with an embodiment of the present invention. FIG. 6
shows color elements 40c, 40d, 40e and 40f, in which a single
droplet 44 is deposited in the Y direction during scan of nozzles
34 over each color element. FIG. 7 shows color elements 40g, 40h,
40j and 40k, on which the respective nozzles deposit two successive
droplets 44 in the Y direction on each color element during each
scan. The location of the droplets in each case can be seen to
depend on the presence or absence of ink 52 in color elements 40 to
either side of the present row. In FIG. 7, the mutual proximity of
the two droplets in each color element is also adjusted. The
droplets are aimed for deposit off center, taking into account
various forces acting on the droplets.
[0073] Although the embodiments described above relate specifically
to printing of color filters for display applications, the
principles of the present invention may similarly be used in other
large-scale inkjet printing applications. It will thus be
appreciated that the embodiments described above are cited by way
of example, and that the present invention is not limited to what
has been particularly shown and described hereinabove. Rather, the
scope of the present invention includes both combinations and
subcombinations of the various features described hereinabove, as
well as variations and modifications thereof which would occur to
persons skilled in the art upon reading the foregoing description
and which are not disclosed in the prior art.
* * * * *