Full text of DOI:10.1557/mrs2001.127

www.mrs.org/publications/bulletin
will become the technique that will enable
full-color, high-resolution, polymer LED
displays.21 It has not been applied to or-
ganic transistors yet, presumably because
its resolution capability is considered to be
insufficient for the definition of practical
channel lengths. The resolution of ink-jet
printing is limited to 20–50 ꢁm by statistical
variations of the flight direction of droplets
and their spreading on the substrate. We
show that ink-jet printing allows the fabri-
cation of complete TFT circuits, including
via-hole interconnects, by successive
solution-deposition and printing steps.
High-Resolution
Ink-Jet Printing
of All-Polymer
Ink-Jet Printing Combined with
Transistor Circuits
Surface Free-Energy Patterning
Our approach to overcoming the resolu-
tion limitations of ink-jet printing is to
confine spreading of water-based,
conducting-polymer ink droplets on a
hydrophilic substrate with a pattern of nar-
row hydrophobic surface regions that define
the critical device dimension (Figure 1a).
When water-based ink-jet droplets of the
conducting polymer poly(3,4-ethylenedio-
xythiophene) (PEDOT) doped with poly-
styrene sulfonic acid (PEDOT/
PSS) aredeposited into the hydrophilic sur-
face regions at a distance Rs from the hy-
drophobic barrier, the droplets spread until
they hit the repelling barrier. Using a piezo-
electric ink-jet head, it is possible in this way
to print two parallel source/drain electrode
H. Sirringhaus, T. Kawase, and R.H. Friend
Introduction
Impressive advances in vapor-phase
in which charge-carrier mobilities of
ꢀ1
deposition and photolithographic pattern-
ing techniques have been fueling the sili-
con microelectronics revolution over the
last 40 years. However, for many interest-
ing classes of materials, including bio-
logical materials or functional synthetic
polymers, vacuum deposition and photo-
lithography are not the techniques of
choice for producing ordered structures
and devices. Many of these materials self-
assemble into well-ordered microstructures
when deposited from solution,1 and pat-
terning may be more readily achieved by
solution-based selective deposition and
direct-printing techniques. It is appealing
to consider novel ways of manufacturing
functional circuits and devices based on
techniques that are similar to printing
visual information onto paper.
Microfabrication by solution self-assembly
and direct printing is particularly attractive
for thin-film transistor (TFT) circuits based
entirely on organic polymers, in which
semiconducting conjugated polymers are
used for the active layers, conducting
polymers are used for electrodes and inter-
connects, and conventional insulating poly-
mers are used for the dielectric and isolation
layers. These are of interest for applications
such as active-matrix-display addressing2
or logic circuits in active identification
tags and labels.3 Over the last 3–4 years,
rapid advances in performance have been
reported on the materials front since semi-
conducting conjugated polymers with im-
proved structural regularity and chemical
purity have become available. By making
use of supramolecular self-organization
mechanisms, it is possible to deposit or-
dered thin polymer films from solution
10ꢀ2–10 cm2/V s can be achieved.4–6
These values are approaching those of
thin-film amorphous silicon (0.1–1 cm2/V s)
and small-molecule organic field-effect
transistors7–9 (0.1–5 cm2/V s).
All-polymer TFT circuits consisting of a
few hundred transistors were first fabri-
cated by a group at Philips Research Lab-
oratories, The Netherlands, who used a
more conventional photolithographic ap-
proach for patterning with 1-ꢁm resolu-
tion.10 Circuits of similar complexity and
impressive performance have also been
demonstrated using small organic mole-
cules deposited at the last stage of an
otherwise conventional silicon process.11,12
Several nonconventional, direct-printing
approaches have been proposed. Screen
printing has been used to pattern source/
drain and gate electrodes of conducting
inks with channel lengths of 100 ꢁm.13
Soft lithographic techniques based on
poly(dimethylsiloxane) (PDMS) stamps
have been used to selectively deposit self-
assembled monolayers onto thin films of
gold that can be used as etch masks for the
etching of gold source/drain electrodes.14
Using PDMS stamps as phase-shift masks15
in a photolithographic process, gold source/
drain electrodes have been defined with
channel lengths of only 0.1 ꢁm. Selective
solution-deposition of polymer electrodes
has been achieved by micromolding in
capillaries (MIMIC).16,17
lines, with
a separation of only
5 ꢁm, that are accurately aligned with
the hydrophobic barrier.18 No PEDOT depo-
sition occurs on top of the barrier
regions, even if the width of the barrier is
much smaller than the droplet diameter. The
second unconfined boundary of the printed
lines is much more irregular, with a typical
statistical roughness of the order of 10 ꢁm (Fig-
ure 1b). It is apparent that this would have led
to electrical shorts between the source and
drain electrodes had we attempted to define
such a short channel without a confinement
structure. At present, the resolution of the ink
confinement process is limited only by our
photolithography setup. Atomic force
microscopy (AFM) images such as Figure 1c
suggest that even higher resolution may be
possible. Note that the boundaries of
printed PEDOT lines are accurately pinned
at the edges of the hydrophobic barrier.
The high-resolution, surface free-energy
pattern is prefabricated on the substrate prior
to any of the TFT deposition and printing
steps. A broad range of techniques may read-
ily be used, since at this stage the substrate
does not yet contain any of the radiation-
sensitive or chemically sensitive active lay-
ers of the devices. In our first
demonstration, the barrier was fabri-
cated from a thin layer of hydrophobic
In this article, we discuss novel pattern-
ing approaches that are based on ink-jet
printing.18 Ink-jet printing has emerged
as an attractive patterning technique for
conjugated polymers in light-emitting
diodes (LEDs),19,20 and it appears that it
MRS BULLETIN/JULY 2001
539

High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits
polyimide (PI) polymer patterned by
photolithography and etched in an O2
plasma to expose regions of the bare, hy-
drophilic glass substrate. The PI film is
only of the order of 500 Å thick. Since the
spreading droplets in the liquid state have
a much larger thickness of a few micro-
meters, the ink confinement must be
caused mainly by the surface free-energy
contrast and does not require a topo-
graphic profile of the hydrophobic barrier.
Indeed, we have recently achieved similar
ink confinement by soft lithography using
self-assembled monolayers of hydropho-
bic alkyltrichlorosilanes selectively de-
posited onto a hydrophilic substrate by a
PDMS stamp.
After ink-jet printing of source/drain
electrodes, TFT devices are fabricated in a
top-gate configuration by spin-coating a
continuous film of the active semiconduct-
ing polymer (Figure 2a). We have fabricated
devices with a range of self-organizing
conjugated polymers, including regio-
regular poly(3-hexylthiophene) (P3HT)5
and liquid-crystalline polyfluorene-based
block copolymers, such as poly(9,9-
dioctylfluorene-co-bithiophene) (F8T2).6
Next,
a dielectric polymer, such as
poly(vinylphenol), is spin-coated on top
from a solvent that does not dissolve or
swell the underlying semiconducting
polymer. Finally, a PEDOT/PSS gate elec-
trode line overlapping the channel is ink-
jet printed (Figure 2b). All ink-jet printing
steps are performed under atmospheric
conditions.
In this device configuration, the ink con-
finement barrier can be used to achieve a
dual result. It not only provides the high-
resolution definition of the channel, but
it also acts as an aligning template for
the deposition of the self-organizing semi-
conducting layer, which is crucial for
achieving high field-effect mobilities. Rigid-
rod polymers such as F8T2 exhibit nematic
liquid-crystalline phases at elevated tem-
perature (TLC ꢀ 265ꢂC for F8T2). They can
be aligned into monodomains on top of
a suitable alignment layer, such as an
aligned layer of PI, by annealing into the
liquid-crystalline phase and subsequent
quenching.6 By using mechanically rubbed
PI ink confinement barriers, we have fab-
ricated devices with uniaxial alignment of
polymer chains in the channel of the TFT.
The alignment is evident in optical mi-
croscopy images observed under crossed
polarizers (inset in Figure 2b). The TFT
channel region appears bright, reflecting
the monodomain alignment of F8T2,
which results in a change of polarization
of the incident light, enabling a fraction of
the light to pass through the second, crossed
polarizer. The source/drain electrode re-
gions where the F8T2 is in a multidomain
configuration on top of PEDOT/PSS ap-
pear dark. The degree of polymer alignment
can be quantified by linearly polarized
optical-absorption spectroscopy (Figure 3a).
The dichroic ratio between light polarized
parallel and perpendicular to the rubbing
direction is 6–8, from which a lower-limit
estimate for the structural order parame-
ter P2 ꢃ 0.7 can be obtained.6 Printed TFTs
have been fabricated with the direction of
current transport parallel and perpendicu-
lar to the alignment direction of polymer
chains. Due to fast intrachain transport
along the polymer chain, enhanced field-
Figure 1. (a) Schematic diagram of
high-resolution ink-jet printing onto a
prepatterned substrate. (b) Optical
microscopy image of ink-jet printed
poly(3,4-ethylenedioxythiophene)
(PEDOT) source/drain electrodes
confined by a hydrophobic barrier of
polyimide (PI) (L ꢀ 5 ꢁm). (c) Tapping-
mode atomic force micrograph showing
accurate pinning of ink-jet printed
PEDOT/PSS (PSS is polystyrene
sulfonic acid) source and drain
Figure 2. (a) Schematic diagram
ꢀ2
effect mobility of 1 ꢄ 10 cm2/V s to
of top-gate ink-jet printed thin-film
transistor (TFT) with a semiconducting
layer of poly(9,9-dioctylfluorene-co-
bithiophene) (F8T2) (S ꢀ source,
D ꢀ drain, G ꢀ gate). (b) Optical
micrograph of an ink-jet printed TFT.
The inset shows an enlargement of the
channel region seen under crossed
polarizers such that the TFT channel
(L ꢀ 5 ꢁm) appears bright due to the
uniaxial, monodomain alignment of the
F8T2 polymer on top of rubbed PI.
2 ꢄ 10ꢀ2 cm2 V s are achieved for current
flow parallel to the polymer chains,
whereas mobilities in the perpendicular
direction are lower by a factor of 5–7
(Figure 3b).
Figure 4 shows the output and transfer
characteristics of an ink-jet printed, all-
polymer F8T2 TFT. The device exhibits a
high on–off current ratio of the order of
105, as measured between 0 V and ꢀ20 V,
electrodes at the boundary of a
hydrophobic PI barrier with L ꢀ 5 ꢁm.
540
MRS BULLETIN/JULY 2001

High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits
tor circuits, it is necessary to develop other
circuit components, which ideally should
be fabricated by ink-jet technology as well.
Resistors can be printed by using different
compositions of PEDOT/PSS ink because
the resistance depends very sensitively on
the ratio of PEDOT to PSS. A particularly
important component for logic circuits
are via-hole interconnects, which provide
electrical connections between electrodes
and/or interconnects in different layers.
We have developed an efficient ink-jet
process for via-hole fabrication. To fabri-
cate a via hole through a dielectric layer
such as PVP, we ink-jet deposit a sequence
of droplets of a good solvent, which lo-
cally dissolve the dielectric polymer. Re-
markably, upon drying of the solvent, the
polymer redeposits on the side walls of
the region defined by the printed droplet,
but not in its center (Figure 5a). This is
repeated several times until the surface
of the underlying layer is exposed, which
also provides an automatic “etch stop.”
Via holes are then filled with PEDOT/PSS
during the subsequent gate-electrode
printing step. The mechanism for via-hole
formation is believed to be similar to that
which results in the familiar “coffee-stain”
effect, which occurs in drying droplets
when the contact line is pinned. To com-
pensate for an enhanced evaporation rate
near the contact line, a radial liquid flow is
established that transports any dissolved
material to the edges of the droplet. In our
via-hole formation process, the dissolved
material only redeposits at the side walls,
such that the underlying layer is exposed
in the center of the via hole. A more de-
tailed description of this process is given
elsewhere.22
Figure 3. (a) Optical-absorption spectra of a completed TFT device taken in the PEDOT-free
substrate regions with incident light polarized linearly parallel (||) and perpendicular (ꢀ) to
the direction of alignment of the F8T2 polymer. (b) Saturated transfer characteristics of
ink-jet printed F8T2 TFTs fabricated on the same substrate on which the absorption spectra
in (a) were taken. The channels were oriented such that current transport is parallel (||) and
perpendicular (ꢀ) to the chain alignment direction (L ꢀ 5 ꢁm).
Figure 4. (a) Output and (b) transfer characteristics of an ink-jet printed, all-polymer F8T2
TFT (L ꢀ 5 ꢁm). The nonlinearities in the output characteristics at small source/drain
voltages are caused by source/drain contact resistance effects. Subsequent measurements
under N2 atmosphere with increasing (ꢁ) and decreasing (ꢂ) source/drain and gate
voltage, respectively, demonstrate negligible hysterisis.
Combining the different components de-
scribed, we have fabricated simple logic
transistor circuits by ink-jet printing. In-
verters are the basic building blocks of
a logic circuit. They can be implemented
with two p-type transistors in either an
enhancement-load or a depletion-load con-
figuration23 requiring via-hole interconnec-
tions in both cases. The enhancement-load
configuration, in which the drain and gate
of the load transistor are connected to-
gether through a via hole, is appropriate
for normally off transistors such as ink-jet
printed F8T2 devices. Alternatively, in-
verters with a printed resistor as the load
element have been used. In both configu-
rations, clean inverter action is observed
for switching of the output between ꢀ20 V
(“high”) and ꢀ0 V (“low”) (Figure 5b).
The hysteresis of the inverter characteris-
tics is small, reflecting the stability of the
transistor threshold voltage. An important
requirement for switching a large number
of subsequent stages in a more complex
and good threshold voltage stability. The
hysteresis between characteristics measured
with increasing and decreasing source/
drain and gate voltage, respectively, is
negligible. F8T2 has good stability against
chemical doping by environmental oxy-
gen or residual impurities such as mobile
sulfonic acid in the PEDOT/PSS ink. In
general, the performance of ink-jet printed,
all-polymer F8T2 TFTs is by no means in-
ferior to that of control devices fabricated
in a conventional way with photolitho-
graphically defined gold electrodes. This
shows that through careful choice of the
sequence of solvents/polymers to avoid
dissolution and swelling of underlying
layers, our printing process maintains the
critical integrity and sharpness required of
the different polymer–polymer interfaces
in a multilayer TFT device. Note that due
to the low bulk conductivity of F8T2, no
patterning of the semiconducting layer
is required.
In principle, even higher field-effect
mobilities of up to 0.1 cm2/V s can be
achieved with P3HT, which forms a mi-
crocrystalline lamellar structure with effi-
cient interchain transport in the plane of
the film.4,5 However, P3HT has low stabil-
ity against doping under atmospheric con-
ditions. Since the ink-jet printing steps are
performed in air, printed P3HT devices
show high bulk conductivity and low
on–off current ratios. Note that a reductive
de-doping step, which would be neces-
sary to restore P3HT to a low-conductivity
state, is not possible in the all-polymer
TFT configuration, since the de-doping
would adversely affect the conductivity of
the PEDOT/PSS electrodes.
In order to use ink-jet printed polymer
TFTs in more complex integrated transis-
MRS BULLETIN/JULY 2001
541

High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits
logic circuit is that the voltage gain
dVout/dVin of the individual inverter stage
is larger than 1. For the devices shown in
Figure 5b, the voltage gain is ꢀ2. Higher
gains of 5–7 have been obtained in devices
with larger load resistance at the expense
of a somewhat reduced switching speed.
The switching performance of the in-
verter stages was tested by applying a
square wave input voltage and measuring
the output voltage with an active probe
with a load capacitance of ꢀ20 pF. The
probe capacitance is equivalent to a fan-out
of 4–5, that is, the source/drain-to-gate
overlap capacitance of a discrete transis-
tor, as used for the input transistor in the
inverters, is 4–5 pF. The ink-jet printed
inverters can be switched at frequencies
up to a few hundred hertz, as shown in
Figure 5c for the resistor-load device of
Figure 5b. Further improvements are ex-
pected from increasing polymer mobility
and PEDOT conductivity, and from reduc-
ing channel length and overlap capacitance.
where ꢆLV is the surface tension of the liq-
uid, ꢇ is the viscosity, and s is a character-
istic length scale of molecular dimension
(s ꢈꢈ R) in the vicinity of the contact line,
over which slippage occurs at the bound-
ary between the liquid droplet and the
solid substrate.26 When the droplets hit the
barrier with a dynamic contact angle of
ꢅ(Rs) ꢈ ꢅp, they will at first spread a small
distance into the hydrophobic region, but
will immediately be repelled by the un-
compensated Young’s force per unit length
of contact line
Fs ꢀ ꢆSL ꢉ ꢆLV cos ꢅ(Rs) ꢀ ꢆSV
ꢀ ꢆLV (cos ꢅ(Rs) ꢀ cos ꢅp),
(2)
where ꢆSV and ꢆSL are the interfacial ten-
sions at the barrier–air and barrier–liquid
interfaces. The non-laminar fluid-dynamics
processes during repulsion of the droplets
are complex. However, for a qualitative
discussion, it is instructive to compare
the magnitude of the initial uncompen-
sated Young’s force with the inertial mo-
mentum of the moving contact line that is
impinging on the barrier per unit time,
Fm ꢀ ꢊhv2, where ꢊ is the density of the
liquid, h ꢀ 4/3 R03/R2 is a characteristic
thickness of the spreading droplet, and
v is the contact line velocity. The Reynolds
number Re ꢀ ꢊhv/ꢇ is estimated to be
of the order of 50–100.
Discussion
Regarding the mechanism of surface
free-energy assisted, high-resolution ink-
jet printing, it is interesting to consider
which physical processes may be the reso-
lution limiting factors. The spreading of
ink-jet droplets on a partially wetting sub-
strate is determined by the initial kinetic
energy of the spherical droplets (radius R0)
impinging on the substrate with velocity
v0, and the free-energy lowering that the
initially spherical droplets can achieve by
wetting the substrate and adopting a
spherical cap shape with final radius Rg
and contact angle ꢅg. The contact angles of
water droplets on bare hydrophilic glass
and hydrophobic PI are ꢅg ꢀ 20–25ꢂ and
ꢅp ꢀ 70–80ꢂ, respectively. For typical ink-
jet conditions used in our experiments, the
free-energy change is about a factor of 5
larger than the initial kinetic energy, such
that the viscous flow during spreading is
governed by surface and interface tensions
rather than by the initial kinetic energy.24
Under these conditions, the spreading
droplet may be assumed to have a spheri-
cal cap shape with a base radius R continu-
ously increasing toward Rg, and a dynamic
contact angle ꢅ decreasing toward ꢅg. If
the fluid is incompressible, and evapora-
tion can be neglected on the time scale of
spreading, there is a one-to-one relation-
ship between ꢅ and R. In the later stages
of spreading, the velocity of the contact
line v(R) can be shown to be related to ꢅ
and R by25
Figure 6 shows the calculated ratio
Fm/Fs as a function of the ratio Rs/Rg,
where Rg is the maximum possible print-
Figure 5. (a) Optical microscopy
image of two ink-jet printed via holes
connecting drain and gate electrodes of
the load TFT in an enhancement-load
ink-jet printed inverter. (b) Static
characteristics of an enhancement-load
inverter (L ꢀ 5 ꢁm, width of the input
switching transistor WI ꢀ 2150 ꢁm, width
of the load transistor WL ꢀ 450 ꢁm),
and a resistance-load inverter
(L ꢀ 5 ꢁm, WI ꢀ 2150 ꢁm,
RL ꢀ 47 Mꢋ), measured with
increasing (solid line) and decreasing
(dashed line) input voltage Vin
(VDD ꢀ 20 V). (c) Dynamic-switching
characteristics of the resistance-load
inverter in (b) (Vin ꢀ 250 Hz). The
spikes of the output voltage after abrupt
switching of the input voltage are
induced by direct capacitive coupling on
the TFT substrate between input and
output electrodes. The open circles
show the square wave input signal, vin,
while the filled circles show the inverted
output signal.
Figure 6. Plot of the calculated ratio
of the inertial force Fm of the moving
contact line impinging on the
hydrophobic barrier and the repelling
uncompensated Young force Fs (left
axis) and of the contact line velocity v
(right axis) as a function of the printing
distance Rs from the barrier (normalized
to the equilibrium droplet radius on the
hydrophilic surface Rg ꢀ 47.5 ꢁm).
ꢆLV ꢀ 70.3 mN/m, ꢇ ꢀ 3.4 mPa s,
s ꢀ 1 ꢁm, and R0 ꢀ 19.5 ꢁm.
ꢀ1
ꢆL
R
s
v ꢀ
ln
ꢃ
ꢅ3
ꢀ
ꢅg3ꢄ,
(1)
ꢁ ꢂ
9
ꢇ
542
MRS BULLETIN/JULY 2001

High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits
ꢃ
D.D.C. Bradley, Appl. Phys. Lett. 77 (2000) p. 406.
7. H. Klauk, D.J. Gundlach, J.A. Nichols, and
T.N. Jackson, IEEE Trans. Electron Devices 46
(1999) p. 1258.
8. H.E. Katz, A.J. Lovinger, J. Johnson, C. Kloc,
T. Siegrist, W. Li, Y.-Y. Lin, and A. Dodabalapur,
Nature 404 (2000) p. 478.
ing distance from the hydrophobic barrier
corresponding to the equilibrium final ra-
dius of the spherical cap on the hydro-
philic surface (Rg ꢀ 47.5 ꢁm). The simple
model suggests that for a broad range of
printing distances Rs (Figure 1a), the re-
pelling Young’s force is orders of magni-
tude larger than the inertial forces. This
may explain why we have observed the
confinement process to be robust with re-
spect to variations of the droplet volume
or the distance Rs, which are caused by ex-
ternal perturbations of the droplet flight
direction or changing ejection conditions
at the nozzle. No evidence for accidental
shorts between source and drain electrodes
was found. The simple model also sug-
gests that it may be possible to achieve sig-
nificantly higher resolution by this method
of ink confinement. Experiments to inves-
tigate resolution limits are under way.
Ink-jet printing provides accurate regis-
tration over large areas because the ink-jet
head can be aligned locally with respect
to a previously deposited pattern. This
local registration capability, which can be
automated, is particularly important for
flexible substrates that inevitably distort
between subsequent patterning steps.
9. J.H. Schön, S. Berg, Ch. Kloc, and B. Batlogg,
Science 287 (2000) p. 1022.
ꢃ
Application-specific or even end-user-
10. G.H. Gelinck, T.C.T. Geuns, and D.M. de
Leeuw, Appl. Phys. Lett. 77 (2000) p. 1487.
11. B. Crone, A. Dodabalapur, Y.-Y. Lin, R.W.
Filas, Z. Bao, A. LaDuca, R. Sarpeshkar, H.E.
Katz, and W. Li, Nature 403 (2000) p. 521.
12. C.D. Sheraw, J.A. Nichols, D.J. Gundlach,
J.R. Huang, C.C. Kuo, H. Klauk, T.N. Jackson,
M.G. Kane, J. Campi, F.P. Cuomo, and B.K.
Greening, in Proc. 2000 Int. Electron Devices
Meeting (IEDM) (Institute of Electrical and Elec-
tronics Engineers, Piscataway, NJ, 2000) p. 619.
13. Z. Bao, Y. Feng, A. Dodabalapur, V.R. Raju,
and A.J. Lovinger, Chem. Mater. 9 (1997) p. 1299.
14. J.A. Rogers, Z. Bao, A. Dodabalapur, and
A. Makhija, IEEE Electron Device Lett. 21 (2000)
p. 100.
15. J.A. Rogers, A. Dodabalapur, Z. Bao, and
H.E. Katz, Appl. Phys. Lett. 75 (1999) p. 1010.
16. J.A. Rogers, Z. Bao, and V.R. Raju, Appl.
Phys. Lett. 72 (1998) p. 2716.
17. W.S. Beh, I.T. Kim, D. Qin, Y. Xia, and
G.M. Whitesides, Adv. Mater. 11 (1999) p. 1038.
18. H. Sirringhaus, T. Kawase, R.H. Friend,
T. Shimoda, M. Inbasekaran, W. Wu, and
E.P. Woo, Science 290 (2000) p. 2123.
19. T.R. Hebner, C.C. Wu, D. Marcy, M.H. Lu,
and J.C. Sturm, Appl. Phys. Lett. 72 (1998) p. 519.
20. Y. Yang, S.-C. Chang, J. Bharathan, and
J. Liu, J. Mater. Sci.: Mater. Electron. 11 (2000)
p. 89.
21. T. Shimoda, S. Kanbe, H. Kobayashi, S. Seki,
H. Kiguti, I. Yudasak, M. Kimura, S. Miyashita,
R.H. Friend, J.H. Burroughes, and C.R. Towns,
in Proc. Society for Information Display, Vol. 99
(Society for Information Display, San Jose, CA,
1999) p. 376.
specific circuits can be defined by simple
ink-jet printing of a network of intercon-
nections and via holes on a prefabricated
array of transistor gates.
ꢃ
Compared with other liquid-phase pat-
terning techniques, such as simple dip-
coating of a substrate containing a surface
free-energy pattern, ink-jet printing allows
precise local control of deposited droplet
volume and drying time to form patterns
with arbitrary shapes and thicknesses. In
dip-coating, the patterns are defined by
equilibrium configurations of the liquid
and suffer from problems such as bulge
formation, capillary breakup, or a sensi-
tive dependence of film thickness on the
shape and size of the pattern.27
Conclusions
We have shown that solution self-
assembly and direct ink-jet printing tech-
niques allow the controlled fabrication of
high-mobility, short-channel (5-ꢁm) poly-
mer transistors and complete transistor
circuits, including via-hole interconnects.
The device performance of printed poly-
mer thin-film transistors with mobilities
Acknowledgments
We acknowledge Dr. T. Shimoda and
Mr. S. Nebashi of Seiko-Epson Corp. for
support of the ink-jet printing project;
Dr. M. Inbasekaran, Dr. W. Wu, Dr. E.P.
Woo, and Dr. J. O’Brien of Dow Chemical
Corp. for supplying the F8T2 polymer;
Bayer Chemical Corp. for supplying the
PEDOT/PSS; and Dr. C. Newsome and
M. Banach for valuable contributions. The
work was supported by the Epson Cam-
bridge Laboratory and the Royal Society.
ꢀ2
of up to 2 ꢄ 10 cm2/V s and on–off
current-switching ratios of 105 is believed
to be adequate for practical applications in
active-matrix-display addressing or logic
circuits in identification tags consisting of
a few hundred transistors. Ink-jet printing
has several advantageous attributes, some
of which are particularly relevant if one
envisions continuous, reel-to-reel manu-
facturing of cheap integrated circuits on
flexible substrates:
References
1. J.S. Moore, MRS Bull. 25 (4) (2000) p. 26.
2. H. Sirringhaus, N. Tessler, and R.H. Friend,
Science 280 (1998) p. 1741.
22. T. Kawase, H. Sirringhaus, and R.H. Friend,
in preparation (2001).
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Ink-jet printing is a noncontact printing
23. T.A. de Massa and Z. Ciccone, Digital
Integrated Circuits (Wiley, New York, 1996).
24. T. Kawase, H. Sirringhaus, R.H. Friend,
and T. Shimoda, Proc. 2000 Int. Electron Devices
Meeting (IEDM) (Institute of Electrical and Elec-
tronics Engineers, Piscataway, NJ, 2000) p. 623.
25. M.J. de Ruijter, J. de Coninck, T.D. Blake,
A. Clarke, and A. Rankin, Langmuir 13 (1997)
p. 7293.
technique that allows low levels of particle
defects and printing without abrasion and
is capable of high-throughput, particu-
larly if a large number of nozzles are used
in parallel. It is environmentally friendly,
as it uses only a minimum amount of
polymer materials and solvents. It also
allows simultaneous printing of different
materials, which can be delivered from
multiple nozzles.
3. D.M. de Leeuw, Phys. World 12 (3) (1999)
p. 31.
4. Z. Bao, A. Dodabalapur, and A.J. Lovinger,
Appl. Phys. Lett. 69 (1996) p. 4108.
5. H. Sirringhaus, P.J. Brown, R.H. Friend, M.M.
Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss,
A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P.
Herwig, and D.M. de Leeuw, Nature 401 (1999)
p. 685.
6. H. Sirringhaus, R.J. Wilson, R.H. Friend,
M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, and
26. R.G. Cox, J. Fluid Mech. 168 (1986) p. 169.
27. A. Darhuber, S.M. Troian, S. Miller, and
S. Wagner, J. Appl. Phys. 87 (2000) p. 7768.
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