Ink-Jet Printing of PLED Displays

Ink-Jet Printing of PLED Displays
After years of materials, hardware, and process development, the ink-jet printing of polymer
light-emitting-diode displays is on the verge of becoming a practical, commercial reality.
by Jonathan Halls
ELLOWING
the first fortuitous observa-
tion of a faint green glow from a sample of
a polymer material by a team at Cambridge
University in 1989, Cambridge Display
Technology (CDT) was founded in 1992 to
develop this luminescence effect into a commercially viable emissive-display technology.
Over the past 12 years, extensive work has
been done by CDT and other companies to
develop the performance - primarily efficiency, color, and stability - of polymer lightemitting-diode (PLED) technology to a level
at which the requirements of real applications
can be met.
PLEDs are a very versatile technology,
which has the potential to be used for lighting,
simple alphanumeric displays, and full-color
flat-panel displays (FPDs). While the lifetimes of red and green devices have been
acceptable for many applications for a number
of years now, the much shorter lifetime of
blue devices has prohibited their widespread
use until recently. However, with an R&D
program on blue PLEDs delivering substantial
improvements in this area, and with the development of a range of new blue-emitting
Jonathan Halls is Strategic Technology
Planner at Cambridge Display Technology,
Ltd., Bldg. 2020, Cambourne Business Park,
Cambridgeshire CB3 6DW, u.K.; telephone
+44-(0)-1954-713600,fax +44-(0)-1954713620, e-mail:[email protected]. The
author thanks David Albertalli (Litrex Corp.),
10hnAttard (Xaar), Linda Creigh (Spectra,
Inc.), and Martin Fleuster and Nijs van der
Vaart (Philips)for useful discussions and
assistance in the preparation of this article.
10
Infonnation
polymers, full-color PLED displays can now
be used in a range of products. Moreover, the
continual rapid progress in lifetime and stability means that more-challenging applications,
such as TV, will probably be feasible in the
foreseeable future.
However, in commercializing a new display
technology, device performance is only one
part of the story. Unless PLED displays can
be manufactured in a cost-effective way in
volumes high enough to satisfy the vast
appetite of the display market, PLEDs will
never fulfill anything other than niche applications.
One characteristic that is very interesting
from a manufacturing perspective is that
PLED materials are solution-processable,
unlike the vacuum-deposited materials used
to fabricate small-molecule-OLED displays.
This brings with it a new set of challenges
and opportunities. First, the coating and
deposition methods used to deposit solutionprocessable PLEDs are scalable to large substrate sizes. In contrast, the complexity, cost,
relatively low material-utilization efficiency,
and mask-registration issues associated with
vacuum deposition present considerable difficulties in the application of small-molecule-
REDNOZZLE
SINGLE
RGB
PIXEL
DROPLET
DROPLETTRAJECTORY
VECTOR
GREEN
BLUE
RED
Fig. 1: In this illustration of the ink-Jet-printing process as applied to PLED-display fabrication, the red-light-emitting-polymer ink is deposited into the appropriate subpixel wells, following deposition of the blue and green materials. (Courtesy of Spectra, Inc.)
Display 2/05
0362-0972/02/2005-010$1.00
+ .00 @ SID 2005
OLED-display production to large substrates.
However, while vacuum-deposition systems
are common in the display industry, coating
with light-emitting-polymer (LEP) semiconductors uses processes much more familiar to
the graphical printing industry.
This article will explore recent developments in the manufacturing techniques, plant,
and infrastructure that will enable PLEDs to
compete effectively with existing and other
emerging display technologies. It will also try
to predict when we can expect to see the first
full-color PLED-display products on the market using these production technologies.
Existing Commercial
Products
Up to now, the commercialization ofPLEDs
has occurred on a limited scale. The first
volume consumer product that incorporated a
PLED display was the Philips Sensotec
shaver, launched in 2002, which used a small
monochrome orange display with icons, fixed
legends, and a small alphanumeric panel to
indicate battery-charge status and maintenance requirements. This display was manufactured in Philips's PLED-display production plant in Heerlen, The Netherlands, using
a simple spin-coating process to deposit the
LEP and hole-transport layers.
More recently, Philips launched the Magic
Mirror 639 mobile-telephone handset with
a monochrome passive-matrix secondary
display behind a semi-transparent mirror that
forms part of the exterior case of the clamshell
telephone. Philips reports that the telephone
has been selling extremely well.
OSRAM Opto Semiconductors is currently
shipping a range of small monochrome (predominantly green, orange, and yellow) PLED
displays manufactured at its facility in
Penang, Malaysia. The displays are being
sold to OEM manufacturers for inclusion in a
range of electronic devices from point-of-sale
displays to high-end audio-mixing desks.
Delta Electronics and RiTdisplay (both
located in Taiwan) are also manufacturing a
range of simple passive-matrix alphanumeric
and graphic displays. What all of these displays have in common is that they are
monochrome, manufactured by a simple spincoating process, and are generally from 1 to 2
in. on the diagonal.
Full-Color Ink-Jet Printing
In order to make a full-color PLED FPD, an
array of individual red, green, and blue PLED
Fig. 2: This 13-in. ink-jet-printed PLED display was demonstrated by Philips at SID 2004 in
May. (Courtesy of Royal Philips Electronics.)
subpixels must be fabricated on the display
backplane. This requires red, green, and blue
polymers to be deposited on specific areas of
the substrate. A simple blanket deposition
technique such as spin-coating is clearly not
appropriate for such displays, unless a patterning technique is used to remove polymer from
where it is not needed after depositing each
color material. Subtractive patterning, such as
the photolithographic techniques common in
the silicon-semiconductor industry, is feasible
with some LEP materials, but the harsh processing conditions, extra cost, and complexity
of such a multi-stage approach usually rules it
out as a practical manufacturing process.
The manufacturing process that shows the
most promise for full-color PLED displays is
ink-jet printing, which allows a controlled
number of drops of the polymer solution (or
"ink") to be deposited at specified locations
on the display substrate (Fig. 1). As in other
LEP-coating techniques, the solvent evaporates from the deposited polymer solution,
leaving behind a thin LEP film.
Seiko-Epson was the first to demonstrate a
prototype full-color PLED display using inkjet printing, and currently holds the record for
the largest OLED display, a 40-in.-diagonal
panel. Many other companies, including
Toshiba, Samsung, Philips (Fig. 2), and CDT,
have also demonstrated PLED prototype dis-
plays fabricated by ink-jet printing in a range
of sizes and resolutions. Although they are in
an advanced state of development in R&D
labs the world over, full-color PLED displays
produced by this technique are not yet commercial products. The question is, when will
ink-jet-printed LEP materials and ink-jetprinting tools and processes be sufficiently
mature for full-color PLED displays to be
used in commercial products?
So what are the challenges? Isn't ink-jet
printing a mature technology today? In fact,
ink-jet printers are now very familiar as office
and home printers. They offer high resolution
and multi-ink photographic-quality image
reproduction at an extremely low cost. This
being the case, it may seem surprising that inkjet printing has yet to be used in the production
of commercial displays. However, when this
printing technology is applied to PLED FPD
production, the challenges are considerable.
First, the LEP inks must be 0) formulated
so that they jet well, (2) form-flat uniform
films, (3) stable in solution so that they do not
block the very small printhead nozzles, and
(4) as good as (or preferably better than) spincoated devices.
Second, the correct volume of ink, typically
a few tens of picoliters (pL), must be deposited at the exact location on the substrate without overflowing and contaminating neighborInformation Display 2/05 11
PLED displays
information base that is very relevant to
PLED FPD printing.
Litrex Corp., followed by Spectra, Inc., has
developed custom "drive per nozzle" (DPN)
electronics, which allows the piezo drive
waveform sent to each of the 128 nozzles to
Fig. 3: The SX128 printhead from Spectra is currently used by several companiesfor printing
PLED displays. (Courtesy of Spectra, Inc.)
ing pixels of a different color. Variations in
ink-drop volume may be perceived as brightness variations across the PLED display.
A typical office printer has a pixel density
of up to 4800 dpi - equivalentto a localpositional accuracy of about 5 11m. In principle,
this accuracy is more than adequate for a fullcolor PLED display, which is likely to have a
subpixel pitch greater than 50 11m. However,
the challenge in printing displays is to have
the printhead nozzles in register with the
"wells" on the substrate that define the subpixels. This requires an absolute positional
accuracy of a few micrometers to be maintained over the entire display surface. For
a standard graphical printer, this absolute
positional accuracy is neither required nor
feasible.
But in the printing of PLED displays, it is
not sufficient for a printer to print with high
absolute positional accuracy; it must do so at
high speed, printing a specified number of
drops in each of a million (approximately)
red, green, and blue subpixels in as short a
time as possible. Any errors, such as a misplaced set of drops or an unfilled subpixel,
will result in the rejection of the entire display.
Ink-Jet-Printed PLED FPDs
An ink-jet printer used in FPD production
does not differ substantially from a standard
graphical printer. The two main components
of a PLED printer are the printhead and the
motorized precision X-Y stage which moves
the printhead relative to the display substrate
(or vice versa). Spectra, Inc., offers an SXl28
printhead which has 128 individual nozzles in
12
Information Display 2/05
a row spaced 508 11mapart (Fig. 3). It is currently in use by several system integrators who
are developing PLED manufacturing lines.
Each nozzle is coupled to an ink-containing
cavity with a piezoelectric driver attached to
one of the chamber walls. Applying an electrical pulse to the piezo transducer produces a
shock wave in the cavity that ejects a controlled volume of ink from the nozzle. Each
of the 128 nozzles, which are formed as perforations in the nozzle plate, can be activated
and deactivated individually, and can fire several thousand drops per second, with each drop
having a volume of about 10 pL. Printing into
subpixels that are closer together than the linear spacing of the nozzles is achieved by
rotating the head relative to the rows of pixels.
Advances
in Printhead
Technology
The key development areas for ink-jet printheads are increased drop-placement and volume accuracy. Drop-placement accuracy is a
function of the position of the nozzle relative
to the ink "wells" on the substrate and the
deviation of the drop trajectory from the normal to the nozzle plate. In order to better control the volume of LEP-ink solution deposited
in each subpixel well, printhead developers
are working to reduce the minimum drop volume, which effectively quantizes the volume
of ink deposited. The averaging that results
from printing a greater number of smaller
drops into a subpixel also allows for a moreuniform volume of material to be deposited in
each cell. The graphical printing industry's
increasing demands for improved gray scale
and variable drop control have created a large
be individually tuned to allow tight control of
drop volume and velocity from jet to jet. This
not only increases display uniformity, but also
allows smaller drops to be printed reliably; the
Spectra SXl28 can print drop volumes down
to 5 pL using DPN-optimized control. DPN
and other ink-jet-printing control implementations are enabled by the widespread commercial availability of op-amps with extremely
short rise times (80 V/sec is necessary for
DPN control) that were previously available
only in custom ICs. The rapid development
of drop-visualization systems has enabled
fully automatic tuning of each nozzle through
optical image analysis of ink drops under
strobe illumination. This allows the velocity
and volume dispensed from each nozzle to be
measured and adjusted through modification
of the piezo drive waveform.
Spectra's SXl28 printhead is widely used
in the industry in both pilot-production facilities
and R&D labs. Its successor, the SX2, is in an
advanced stage of development and is expected
to be launched commercially in early 2005.
While the SXl28 has a gold-plated electroformed nickel nozzle plate, the SX2 nozzle
plate is fabricated in silicon by microelectromechanical-system (MEMS) machining.
Taking this MEMS technology a step further, the next generation of printheads - the
M-class - will have a printhead module
(cavities and ink channels in addition to the
nozzle plate) entirely fabricated by MEMS
machining and the subsequent lamination of
two silicon wafers and a piezoelectric-crystal
wafer. This printhead module will have 304
individually addressable nozzles with a spacing of about 280 11m,allowing printing at a
native resolution of 180 dpi. It will be capable of high drop-firing frequencies; 40 kHz is
being achieved routinely in prototypes, and
higher-frequency operation is expected as
development continues.
The use of silicon and the MEMS process
for the SX2 nozzle plates in Spectra's new
generations of printhead modules offers many
more benefits the previous metal technology.
Silicon nozzles are an improvement over
electroformed nickel nozzles. They reduce
the number of jet-trajectory and nozzle-
placement errors, allowing far more precise
and consistent deposition. Silicon nozzle
plates can also be fabricated to a greater thickness, allowing far-better rigidity and dimensional stability, resulting in improved nozzle
positioning and jetting accuracy. The thin
electroformed nozzle plates are susceptible to
permanent bowing, which brings the row of
nozzles out of alignment.
The current technology has other weaknesses as well. The thin gold coating of the
SX128 printhead provides chemical resistance
to the solvents used to dissolve LEP materials
and to the highly acidic PEDOT:PSS - the
hole-transport material commonly used in
PLED displays. The electroformed nozzle
plate is typically 25 flm in thickness, and its
soft gold coating is easily damaged by abrasion during maintenance. A thicker electroformed layer would prevent acceptable drop
formation and compromise the directional
accuracy of the ejected drops. If the gold
layer is defective, the nozzle plate would be
damaged by the acidic inks, and jetting performance would be severely compromised,
requiring replacement of the entire printhead.
Spectra, Inc., is just one of the companies
working to deliver next-generation printheads.
Konica has developed a printhead capable of
dispensing drops in volumes down to 7 pL, a
figure that can be reduced to 3 pL by utilizing
DPN nozzle tuning. Xaar (Cambridge, U.K.)
is developing a 1O00-nozzle printhead based
on their proprietary "Side Shooter" technology.
Each nozzle will be individually tunable and
capable of variable-drop-size dispensing,
based on a 3-pL sub-drop volume with eight
or more levels of gray scale. The nozzle plate
will have a parylene coating to protect it from
chemical corrosion, and precision construction
will deliver a drop-landing accuracy of less than
5 flill at a l-mm substrate-printhead spacing.
The printhead will be specially designed to
cope with the relatively high viscosity of LEP
inks and will be capable of firing up to 8000
full drops per second from each nozzle at a
velocity of more than 6 m/sec. Xaar will also
introduce the continuous recycling of ink
through the channels, manifolds, and reservoirs,
allowing removal of air bubbles and debris that
would otherwise block nozzles or stop them
from firing. The printhead is projected to be
available to system integrators by Q4 '05.
Advances in Ink-Jet Printers
A PLED-display printer integrates the print-
head described above with the substrate- and
head-positioning systems, ink supply, and
other services necessary to deposit the conjugated-polymer inks into the individual subpixel wells that form the red, green, and blue
subpixels and do so quickly and consistently.
Litrex Corp. (Pleasanton, California), which
manufactures printers specially designed for
printing PLED displays, currently produces
machines capable of printing onto Gen 2
substrates (370 x 470 mm). Litrex has shipped
more than 30 of these l40P and l42P printers
(Fig. 4) to display manufacturers and R&D
institutions. The printers position the printhead relative to the substrate by moving it
along one axis and the substrate chuck along
the perpendicular axis. Litrex has also shipped
20 Gen 1 machines, which are called the 80L.
The LCD industry has witnessed a rapid
progression in substrate size, with a number of
Gen 5, 6, and 7 plants coming on line in 2004.
These vast fabrication facilities - Gen 7 glass
has a size of 1.8 x 2.1 m - come with multibillion-dollar price tags, but they ultimately
reduce production costs through economies
of scale. Demand for larger PLED-display
screens and reduced costs will ultimately
drive a similar increase in substrate sizes for
PLED-display manufacture, and Litrex Corp.
is developing its M-series Gen 4-7 printers to
deliver this capability in 2005 (Fig. 5).
Most current ink-jet-printing machines
being used to print PLED displays have a single printhead which performs multiple passes
over the substrate to fill all of the subpixel
wells. The complexities associated with
changing inks, and the need to flush the ink
system and printhead carefully when inks are
changed over, generally require the use of a
separate printer that is set up for each LEP-ink
Fig. 4: This l40P ink-jet printer from Litrex is being used by Philips and OTB/lnnoled in
PLED-display production lines currently under development. (Courtesy of Litrex Corp.)
Information Display 2/05 13
PLED displays
Substrate
Load! Unload Robot
Substrate
Alignment
Optics
204M x 204M Glass Substrate
Cleaning Solvent Module
Vacuum Chuck with Lifter Pins
Ink Supply Modules
Substrate
Variable Pitch Inkjet Array
Drop Inspection
Load! Unload Robot
Module
Inkjet Maintenance
Module
System Controller!
Power Module
Fig. 5: This drawing illustrates the Litrex Corp. Gen 7 ink-jet-printing system. (Courtesy of Litrex Corp.)
color and an additional printer to deposit the
hole-transport material.
The key to reduced takt time - the average
time it takes for a production process to produce one unit - and optimum equipment
usage is the development of printers with
multiple printheads that can print red, green,
and blue LEP inks in as few passes of the
printhead over the substrate as possible.
Litrex Corp. proposes to equip its next generation of printers with 4-24 printheads, and it is
developing integrated printhead and electronic
control systems that will allow automated
submicron alignment of multiple printheads
and automatic generation of complex interleaved print patterns. The M-series printhead
modules will be interchangeable, allowing
off-line set up, testing, and nozzle-tuning
before use. The printer downtime during
printhead-module replacement will be less
than 2 minutes.
The accuracy of the printer's X -Y positioning equipment plays a key role in determining
the precision of ink-drop placement. Precision stage mapping is vital to ensure correct
registration of the nozzles with the subpixel
14
Information Display 2/05
wells, and this is a well-developed technology
for the FPD inspection-tool industry. A total
positioning-system deviation of less than
5 J.UIlis required for high-resolution display
printing, and next-generation printers promise
to deliver this precision. But attaining this
degree of precision is difficult when large tool
sizes and multi-printhead payloads are
required, and it is driving a move to double
overhead rails to support and position the
printhead arrays.
The precision must be maintained while the
printhead and substrate are accelerated and
decelerated rapidly relative to each other.
This rapid change in movement can also disturb the ink in the piezo pumping chambers
through movement of the printhead and inksupply tubes that connect the nozzles to the
main supply reservoirs. These movements
can cause nozzles to stop firing and require
them to be re-primed, with the associated
equipment downtime.
CDT and Litrex Corp. have developed a
control system based on a local pressurecontrolled ink reservoir in close proximity to the
printhead nozzles, which ensures that the printer
can continue to function effectively after rapid
movement. This meniscus-control fluiddelivery system also provides better control
over jetting at higher frequencies, and printing
at 15 kHz (15,000 drops per second per nozzle)
has been demonstrated using this technique.
PLEDs
in Production
So far, we have looked at the currently available technologies for PLED-display production by ink-jet printing of the LEP inks and
hole-transport materials as well as the developments that will allow PLED displays to be
made in high volumes with high yields.
In addition to these very specific PLED
tools and processes, the complete PLEDdisplay fabrication process naturally involves
substrate patterning, cleaning, and surface
treatment prior to LEP printing and subsequent cathode deposition and encapsulation.
These front- and back-end processes are well
established in the semiconductor and LCDpanel industries, and existing toolsets are
readily adapted for PLED-display production.
With ink-jet-printing tools already available
for printing onto Gen 2 substrates, and with
In addition to Philips and Innoled, OSRAM
Opto Semiconductors, Delta Optronics, and
others are also setting up full-color-PLEDdisplay manufacturing facilities, while several
other companies are establishing advanced
research activities in full-color-PLED-display
production. Both Casio and Seiko-Epson
have announced that they intend to begin
manufacturing large-screen - greater than 40
in. - PLED-TV screens in 2007, which will
Etch
require large-format printers capable of delivering a high volume of displays.
Aligning, load lock
Fluid caddy
Fig. 6: This illustration shows the InnolediOTB/CDTfully integrated in-line PLED-displayfabrication system. (Courtesy OTB Engineering.)
parallel advances in LEP-ink fonnulation to
enable LEP-semiconductor films to be
deposited uniformly and reliably, display
manufacturers are getting closer to exploiting
this new production technology. But how
close are they?
Since mid-20m, the PolyLED team at
Philips has been building a full-color-PLEDdisplay production facility in Heerlen, The
Netherlands, where the monochrome displays
for the Philips Sensotec shaver are manufactured using spin-coating techniques. Philips
has integrated four Litrex Corp. 140P printers
equipped with SX128 printheads, which
deposit the PEDOT:PSS hole-transport layer,
followed by red- green-, and blue-LEP inks.
Robotic handling equipment transports substrates between the four printers and to adjacent ovens that allow intennediate baking of
layers when required. An integrated cathodedeposition-and-encapsulation
tool completes
the fabrication line.
The production line is configured to fabricate displays on a 14-in. substrate. Each substrate contains several displays, which are
separated after fabrication. The fab line produces 15 completed 14-in. panels every hour,
which is equivalent to a takt time of 4 minutes. The actual printing of the LEP-ink
materials is not the rate-detennining step in
the process; handling and substrate alignment
take a greater proportion of the time. Operation of the facility is completely automatic,
with the system performing automated print-
head maintenance and inspection to minimize
engineer intervention.
During 2004, Philips worked extensively to
establish the process settings and ensure that
the system works reliably and produces highquality PLED displays with a high yield. The
company has developed a 1.I-in. full-color
passive-matrix PLED display and plans to
advance to the next stage of their roadmap and
develop higher-quality larger-fonnat activematrix PLED displays at the Heerlen facility.
Philips is not alone in building full-colorPLED-display production facilities, although
it is likely to be one of the first to ship
displays to customers. OTB Engineering
(Eindhoven, The Netherlands) is currently
working with CDT to develop the first fully
integrated and automated in-line PLEDdisplay production facility that will be available to display companies wishing to establish
their own PLED-display production capability.
The first system is being built for Innoled,
a subsidiary of Eastgate Technology. The fab
line integrates substrate patterning and pretreatment, Litrex Corp. 140P ink-jet printers,
fast cathode deposition, and thin-film encapsulation processes, and will allow significant
cost reductions to be made through fast substrate throughput with minimum intervention
(Fig. 6). The facility does not require a cleanroom because each tool is enclosed in a clean-
Conclusion
The unique partnership between the graphical
printing industries and elements of the moreconventional display and semiconductor
industries has permitted the development of
an efficient and fast manufacturing process
for fabricating PLED displays from solutionprocessable LEP semiconductors. The first
commercial products from Gen 2 manufacturing facilities are expected in early 2005, and
construction of larger Gen 4 and 7 production
facilities are expected to begin in mid-2005.
This very new manufacturing technique has
rapidly reached maturity.
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air cabinet, and the facility will be capable of
fabricating 30 PLED-display substrates per
hour, equivalent to a 2-minute takt time.
Information Display 2/05 15