A FLEXURE-BASED ROLL-TO-ROLL MACHINE FOR FABRICATING
FLEXIBLE PHOTONIC DEVICES
Xi Zhou, Huihua Xu, Ni Zhao, and Shih-Chi Chen
Department of Mechanical and Automation Engineering
The Chinese University of Hong Kong
Shatin, N.T., Hong Kong SAR, China
In this paper, we present the design and
characterization of a flexure-based roll-to-roll
(R2R) printing system as well as the adaptation
of the Microcontact Printing (MCP) process to
the R2R platform for fabricating flexible photonic
devices. For the first time in the world, we have
demonstrated R2R printed 300/600nm optical
gratings and various gold and silver electrodes
for optoelectronic devices over a 4” PET web.
High-resolution R2R printing was enabled by the
fully automated flexure-based R2R machine with
multi-axis misalignment correction capability.
ULTRA PRECISE R2R PRINTING
Compatibility with R2R processes is the main
driving force for flexible electronics and organic
photovoltaic devices. However there is a
common
misperception
that
this
high
throughput, low-cost process produces devices
of lower resolution and quality. This will not be
true if contact printing techniques such as
Microcontact Printing [1] are implemented on
properly designed R2R platforms. In fact, the
printing resolution may even be better than
lithographic processes as contact printing is not
limited by diffraction. However, this has never
been realized as current state-of-the-art R2R
systems, e.g. gravure printing and flexographic
printing, only have ~15 micron print resolution.
This is because conventional bearings are used
in these systems to guide the printing motion
and lack the required repeatability and precision
that lead to non-uniform pressure distribution
and pattern distortions. To utilize a R2R process
for precision printing, we must address the
machine’s repeatability and accuracy issue. A
R2R machine must possess nanometer
repeatability in order to create patterns with
submicron resolution.
MECHANICAL CHARACTERIZATION
As shown in Figure 1, we have developed a
flexure-based R2R printing system
for
submicron resolution printing [2]. Nanometer
repeatability and multi-axis error correction
capabilities are achieved through the flexurebased positioning stage which holds the print
roller via two air bearings.
PID control is
implemented in all sub-modules of the R2R
system, including the positioning stage, the web
tension controller, and the web guide system [2].
FIGURE 1. Illustration of misalignments between
impression roller and print roller
Figure 2A presents the open-loop and closedloop printing force data over one revolution of
the print roller with a target printing force of 15N.
As expected, the open-loop results exhibited
significant force variation due to print roller
eccentricity which cannot be avoided at
submicron level. Figure 2B shows the zoom-in
view of the closed-loop results subtracting the
target force (15N), which indicate printing force
was precisely controlled within 0.05N in most
regions and satisfied the functional requirements
for submicron printing using MCP.
A
results present for the first time the high-quality
submicron patterns over a large area produced
by a R2R system.
1
2
3
4
B
FIGURE 2. Open-loop and closed-loop printing
force data
A
B
FIGURE 4. R2R printed samples on PET web
(A); SEM images of printed gold (1 & 2) and
silver (3&4) electrodes; scale bars in 1-4 are 350
microns
STAMP PREPARATION
To adapt the MCP process for R2R operation,
we replaced the conventional rigid substrate with
a 4” wide metal coated PET roll. For stamp
preparation, PDMS stamps are first fabricated
by standard MCP procedures [1], following
which the stamp is bonded to a glass cylinder
that is then securely mounted to an airexpandable motor-driven print roller shaft.
Figure 3 shows an image of the glass cylinder
with the bonded PDMS stamp. We have also
developed new gold and silver etching recipes
that are compatible with the fast R2R process
(etch time < 10 ms).
FIGURE 5. R2R printed sample of optical
grating (gold lines); the line width in the SEM
image is 600 nanometers; scale bar = 6 micron
FIGURE 3. PDMS stamp securely bonded to a
glass cylinder by oxygen plasma treatment
DEVICE FABRICATION
Figure 4 and 5 show various metal patterns
printed by the R2R system on a 4” PET web.
Figure 4A shows an image of flexible gold
electrodes around 2” x 2” in size (cut from a 4”
web). Figure 4B shows four SEM images of both
gold and silver square/hexagonal grids with line
widths ranging from 20-50 microns. Figure 5
shows the SEM image of a R2R patterned
optical grating with a line width of 600
nanometers, printed on the 4” web. These
CONCLUSION
We have developed a flexure-guided R2R
machine
and
achieved
nanometer-level
positioning resolution and uniform force control
(within 0.05N) over a 4” web. By adapting MCP
to the R2R system, we have, for the first time,
scaled up the MCP process over a large area (4”
PET web) and achieved 100 nanometer print
resolution. The printed metal electrodes were
used to fabricate various photonic devices
including an organic photovoltaic cell and photodetector. More machine characterization and
experimental results and fabricated devices will
be presented in the extended abstract.
REFERENCES
[1] Xia Y, Whitesides GM. Soft Lithography.
Annual Review of Materials Science. 1998;
28(0): 153-184.
[2] Zhou X, Cheng J, Zhao N, and Chen S. A
Flexure-Based High-Throughput Roll-toRoll Printing System. Proc. of the Annual
Meeting of the ASPE, Saint Paul, MN, USA,
Oct. 2013, pp.353-57