SINTERING OF PRINTED NANOPARTICLE STRUCTURES USING LASER
TREATMENT
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Petri Laakso1, Saara Ruotsalainen1, Eerik Halonen2, Matti Mäntysalo2, Antti Kemppainen3
1
2
VTT Technical Research Centre of Finland, Lappeenranta, 53850, Finland
Tampere University of Technology, Institute of Electronics, Tampere, 33101, Finland
3
VTT Technical Research Center of Finland, Oulu, 90571, Finland
typically consist of silver or gold nanoparticles
(nominal particle diameters 2–50 nm). Particles are
encapsulated with a thin protective shell and
dispersed in a liquid solvent [2, 3]. After printing a
conducting structure can be obtained by sintering, i.e.
by partially melting and fusing the adjacent
nanoparticles together. Due to the nanoscale size of
the particles, the typical sintering temperatures of
100–300 °C are only a fraction of the macroscopic
melting point of the corresponding materials [4]. This
allows the use of paper or plastic substrates. Recent
studies have demonstrated thermally sintered silver
and gold conductors with conductivities approaching
half of the bulk material conductivity [1, 2, 5].
Abstract
Printed intelligence is a promising new technology to
produce low-cost electronics. Non-conductive
circuits can be printed using nanoscale metal particle
inks. Due to the nanoscale size of the particles, the
typical sintering temperatures of 100–300 °C are only
a fraction of the macroscopic melting point of the
corresponding materials, thus allowing the use of
paper or plastic substrates.
Sintering of printed nanoparticle structures using
laser treatment has been investigated at VTT. Laser
sintering can be utilized in manufacturing of printed
conductor structures such as antennas, circuits and
sensors. A drop-on demand printer was used to print
patterns with metallo-organic silver nanoparticles on
a flexible polyimide substrate. Laser sintering was
made with a 940 nm CW fiber coupled diode laser.
Process was optimized using different laser power
levels, line separation and repetition rounds.
Conductivity of laser sintered samples was compared
to conductivity of samples sintered in convection
oven.
Since electronics are printed on plastic or fiber based
substrates and multiple different materials are used to
produce conducting and insulating properties it might
not be possible to use oven heating to sinter the
nanoparticle ink due other printed materials or
substrate may suffer from the heat. Oven sintering is
also time consuming. Of course there are also other
alternative methods to enable sintering such as
electrical sintering [6], UV or microwave [7]
illumination.
Introduction
In this research, we have studied laser sintering with
diode laser at 940nm. The resistivity of ink-jetprinted tracks on polyimide is demonstrated to be
close to bulk silver resistivity, providing same
improvement to tracks that were oven-sintered at
high temperatures. Benefits for laser sintering are the
accurate heat load only where it is needed, the
absorbance to the substrate being really low, the short
processing time and last but not least it is possible to
connect laser to quality monitor to see if the process
works like it should. After all laser sintering seems
really promising technique.
One of the driving forces in printed electronics is the
cheap way to produce large amounts of low cost
electronics. Then solution is the roll-to-roll
production for really high throughput. There are only
few potential methods which are truly capable for
R2R processing. Laser sintering offers possibility to
do R2R sintering. If one wants’ to combine multiple
different printed electronic components one needs a
way to create contacts between the components.
There are several ways to print the contacts but in
this paper we chose the ink jet printing of
nanoparticles.
Metallic nanoparticle inks are used in making of
conductors in printed electronics [1]. These inks
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Experimental
printing. The pattern that was printed consist several
ponsa (shown in figure 3) structures. In all structures
the constant A was 15 mm and the constant B was
100 µm, 200 µm, or 350 µm. The one layer patterns
were printed at the resolution of 600 dpi. The
temperature of the printing plate of the printer was
60 ºC. So the ink dried a bit but did not sinter.
Printing of sintered conductors
The samples for the laser sintering trials were inkjet
printed in Tampere University of Technology. The
used printer was iTi XY2.0 Material Deposition
System, in figure 1 below. The printer has a
repeatability of ± 1 µm and the maximum substrate
size is 305 mm x 305 mm. Generally the printing
speed is somewhere around 100-200 mm/s. The print
head was Spectra SQ-128 (Figure 2), which creates
drops of 10 pl. There are 128 nozzles in the head and
the nozzle spacing is 508 microns and the diameter of
a nozzle 35 µm.
A
B
Fig. 3: Test geometry.
The first used silver nanoparticle ink in printing was
Ink 1 whose curing conditions in convection oven are
100-350 °C for 1-60 minutes. However there were
some problems with the jetting of the ink and hence
the imprint was not as good as desired. Some results
were got but many of the lines were broken. Broken
wires were neglected in sintering.
The second set of trials was made with Ink 2 that was
also a silver nanoparticle ink. Metal content was
about two times higher than in Ink 1 and therefore it
offers better conductivity. The advantage of it was
also that it is jetting better with our print heads.
Normal sintering conditions of the ink are 210220 °C for 60 minutes. The imprint was better with
this ink and the results were also easier to get.
Laser sintering
Fig. 1: iTi XY2.0 material deposition system
The laser sintering experiments were performed at
VTT in Lappeenranta. The laser system (fig. 4) used
in the experiments was a Laserline LDF400-200 fiber
coupled diode laser, where the laser beam was guided
via an Ø400 µm optical fiber to a scanhead. The
diode laser is operated at 940 ± 10 nm wavelength
and the focal length used was 163 mm resulting an
Ø1.0 mm focal spot on the work piece. The beam
profile used was top hat beam. Sintering tests were
done at the focal point and in some tests the beam
was defocused -32 mm to achieve ca. Ø6 mm spot to
work piece. Beam profiles in the focal spot and
defocused beam profile are presented in the figure 5.
In the sintering tests the film was placed on the top of
two glass plates so that the pattern to be sintered was
hanging on free space. Samples were sintered from
the printed side of the film.
Fig. 2: Spectra print head
The chosen substrate was Kapton® (Polyimide).
There was not made other surface treatments to the
substrate than cleaning with isopropanol before
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Laser Sintering Process
When thinking about the laser sintering process one
would need a smooth heat input with minimum
amount of differences and small absorbance to the
base material. Laser has this advantage to bring very
localized heat input [8, 9, 10, 11, 12]. Smooth input
comes from the beam spatial profile and top hat mode
then would be preferred. Diode lasers offer normally
this top hat mode and the chosen 940nm does not
have too high absorbance to the used PI substrate.
Also pulsing of the laser is not preferred and that is
why continuous wave laser is good alternative.
If high average powers are used the intensity might
get quite high. This can be adjusted so that the beam
size is the same as the width of the conductor and
then sintering can be made only with one pass. If it is
not possible to bring enough energy to the material
with one pass, it is possible to heat up material
gradually with several passes. For example if ink
contains too much solvent and the vaporization of
solvent interferes with the sintering process, then
multiple passes is the right approach. Another
method is to use line wise method so that beam is
scanned back and forth over the whole area to be
sintered. This way the sintering has to be even faster
due the sintering has to happen within less than 1 ms
at a time and the process window is a lot smaller due
really fast process. In this line wise method sintered
lines can be overlapped so that conductors are heated
gradually and also cooled gradually. Laser moving
speed has to be some meters per second and then
material does not have time to cool down too much
between passes. Still it is common for all strategies to
heat up the material quickly and locally to enable the
sintering, low process times and minimal heat effect
to the substrate.
Fig. 4: Laser sintering setup.
Fig. 5: Beam profiles a) focal spot (beam Ø 1 mm) b)
defocused -32 mm (beam Ø 6 mm).
Samples were sintered by scanning multiple vertical
lines across the ponsa pattern, line spacing was
0.2mm (figure 6). Tests were done in focal plane and
also in defocus.
5 mm
0,2 mm
Results and Discussion
In sintering tests two different inks were tested and
test geometry was like what is shown in Figure 3.
Three different line thicknesses were printed to see
what the effect of line width is.
20 mm
Fig. 6: Sintering path when laser beam was used in
the focal spot.
First sintering tests were made with Ink 1 printed
geometries using hatch technique. Laser speed was
1000 mm/s and line to line distance 0.2 mm. Beam
size was 1 mm in diameter. After series of pre tests it
was found out that the optimal range could be
between 20 – 50 W average powers. From table 1.
one can see the effect of average power to sheet
resistance.
Sintering experiments were performed using different
laser powers and number of sintering times to found
out the optimal sintering parameters. In all tests the
scanning speed was 1000 mm/s. When the focused
laser beam was used laser powers were between 20
and 50 W. When defocused laser beam was used
laser powers were between 30 and 197 W. Some tests
were done also by sintering samples 1, 2, 5 and 10
times. In these tests used laser powers were 50 or 80
W. These tests were done in defocus.
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Table 1 Sheet resistance values with different average
powers.
50 W
40 W
30 W
20 W
100 µm
0,16
0,18
0,29
0,52
200 µm
0,17
0,27
0,31
0,59
350 µm
0,16
0,19
0,28
0,38
From these results can be seen that the higher the
average power was the lower the resistance was. If
higher than 50 W average power was used,
nanoparticle ink starts really easily to burn and
conductivity is not as good anymore. Bulk resistance
of silver is 1.6 µOhm·cm. If accurate layer thickness
of tested samples would be known, the bulk
resistance of the sintered samples could be
calculated. Layer thickness is around 1-2 µm which
would mean almost the same bulk resistance as with
the bulk silver.
Fig. 7: Ink 2 sintered with 40W in focus with hatch
method.
Because as high intensities could not be used as with
Ink 1, the laser beam was decided to be enlarged to
make intensity lower and to give process more time.
Sintering seemed to work better even if sintering was
done multiple times. Again after short pre tests it was
found out that right parameter window was between
80 and 197 W with the 6 mm beam used. From table
2 one can see how conductivity changes when
average power is changed. Less than 80 W average
power sintering does not seem to work.
Laser sintering is quite sensitive if any dust particles
etc. are on the substrate during sintering. These
impurities would absorb laser light more than the ink
and this is why a serious damage may occur to the
surface of ink or substrate.
Table. 2 Different average powers and corresponding
sheet resistance values of 100 µm wide sintered
lines.
Average power
Resistance
[W]
[Ohm]
After laser sintering and measuring of samples they
where put to convection oven to sinter the materials
thoroughly. Small increase in conduction was noticed
but this still needs further testing to get reliable data.
Anyway this indicates that laser sintering did not
sinter the nanoparticle ink fully. When ink 1 was only
oven sintered 60 minutes at 160°C, the average sheet
resistance of 12 samples after sintering was 0.46
ohm. This is a bit worse than what was obtained with
laser but due PET substrate oven temperature was not
as high as would be needed to sinter ink fully. This
value for oven sintering is also only couple times
higher value than with bulk silver which is good
result.
Due nanoparticle ink 1 was not so easy to print it was
decided to change the ink to Ink 2 to get better print
quality. With the new ink it was immediately noticed
that this ink does not allow as high laser intensity as
the ink 1. In figure 7 we can see the damage to
sintered Ink 2 due too high intensity.
197
14,3
180
16,8
160
17,1
140
20,3
120
25,3
100
24,9
90
33,5
80
148
70
N/A
With multiple scanning technique was tried to see if
quasi-simultaneous sintering could be done. From
Figure 8 we can see that laser powers below sintering
threshold with one pass does sinter the material by
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One issue in both tested inks was that solvent from
the ink evaporates when sintering. It can be seen after
process with microscope that evaporated ink spreads
on both sides of the conductor which might cause
problems in the next steps of whole production
process. In figure 7 one can also see this evaporated
residue near the sintered conductor.
multiple passes. This makes process also more gentle
for the ink. Of course this kind of operation takes
more time but also it would enable more solvent
containing inks to be sintered. It is notable that if ink
is already once sintered it does not sinter more with
multiple passes with the same laser power and speed.
This might be also due after first sintering the surface
of line changes color to human eye from light blue to
silver like outlook. This silver surface will most
probably reflect near infrared light really well which
in fact lets lower amount of light to the process itself
and may explain why multiple passes does not help if
first pass sinters the conductor. Well a way around
this might be the increase in power in consecutive
passes but this may lead to substrate damage and that
is why it was not tested. With ten passes and high
average power material may suffer due high heat
input (see figure 8 lowest right and the red arrow).
50 W x1 N/A
80 W x1 ~ 0,11
50 W x5 ~ 0,42
80 W x5 ~ 0,08
Another issue seen as a problem in laser sintering is
the thickness variation of printed ink. In figure 9 we
can see a picture about differences in printing quality
and thickness before laser sintering. At some points
thickness of the printed ink is so thin that one can
even see through.
Fig. 9: After printing, thickness differences of ink.
50 W x10 ~ 0,34
If one can see through the printed layer it is clear that
the absorbance of laser light into this printed area has
to be different than places where thickness is so thick
that one cannot see through the printed conductor. In
figure 10 we can see the thickness variation after
sintering. Picture is not exactly at the same position
than in figure 9 but thickness variation is similar. It is
obvious that ink is only partially sintered. Sintered
material have silver outlook and blue parts are
unsintered. It is certain that laser has passes all areas
due multiple similar tests made.
80 W x10 ~ 0,11
Fig. 8: Multiple pass tests with consecutive sheet
resistance.
When ink 2 was only oven sintered 60 minutes at
260°C the average sheet resistance of 12 samples
after sintering was 0.05 ohm. This is again a bit better
than what was obtained with laser. Again
measurement was done in different place so
confirmation is needed. This value also is really close
to bulk silver value.
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temperatures in process. Camera was operated at 500
frames per second. From the measurements it was
easy to see how heat diffuses to PI substrate from the
conductor after laser has passed by. Due the surface
change from light blue outlook to silver outlook may
explain why thermal camera does not see the actual
temperature from sintered conductor. Also silver has
really good heat conducting properties and that is
why temperatures are roughly measured from PI and
silver interface (figure 12).
Fig. 10: After sintering, thin sections unsintered.
By doing multiple passes one can sinter a bit better
the thinner areas also but it still does not help
completely. In figure 11 we can see on the left with
80W and 1 pass sintered pattern, which has
unsintered areas on the edges. With the same power
if the sintering was done five times edges are almost
fully sintered. Ten times would not anymore make
the situation any better.
Fig. 12: Thermal camera image just after laser has
passed by with Ink 1.
When comparing different average power results
from 20 to 50 Watts, the 50 W average power seems
to have a bit smaller temperature after laser has
passed by than 40 W. This is maybe because surface
has melted due laser heat and that is why it is seen
cooler by the thermal camera (Fig 13).
Fig. 11: On left 1x and on right 5x sintered.
If these kinds of samples are processed after laser
sintering in convection oven one hour at 220°C
everything is sintered. So we can note that too thin
printed layer is a challenge for the laser and also big
thickness differences within printed layers might also
be problematic for CW diode laser sintering. At high
powers even the thickest prints might be left partially
unsintered if optical penetration depth and heat
diffusion is a lot smaller than print thickness.
Normally then printing is done layer by layer to
achieve high thicknesses. Due at high average powers
the substrate starts to deform we decided to analyze
the absorbance of PI which was measured with
spectrometer to be around 5 %, reflectance around 15
% and transmission 80 %. So according to this diode
wavelength is a good choice.
Fig. 13: Measured temperatures with different
average powers with Ink 1.
Conclusions
Laser sintering of nanoparticle inks seems to be
promising technique if substrate cannot be put to an
oven or R2R production would be needed. Choosing
the right ink for the process and keeping the substrate
clean are the key factors to successful operation.
Continuous CW diode laser seems to be a good
choice for sintering due no problems with ablation
was not seen. Of course too high intensity will
To find out what kind of temperatures are in the
process during sintering, we used high speed thermal
camera CEDIP Titanium 560BB to see roughly the
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[10] Ko S.; Pan H.; Grigoropoulos C.; Luscombe C.;
Frechet J.; Poulikakos D.: All-inkjet-printed flexible
electronics fabrication on a polymer substrate by lowtemperature high-resolution selective laser sintering
of metal nanoparticles, Nanotechnology 18 (2007)
345202.
vaporize the ink. Since samples were a bit pre dried
next step might be sintering of these conductors right
after printing when they contain much more solvent.
This high amount of solvent might be a problem if
the same laser parameters would be tested.
Laser sintering seems to be really sensitive if printed
layer is too thin or there is too much variation in
thickness. Resistance values are already quite close to
what bulk silver has so this is good thing, but still
process has to be improved and testing is to be done
also in real Roll to Roll environment in the near
future.
[11] Ko S.; Pan H.; Grigoropoulos C.; Luscombe C.;
Frechet J.; Poulikakos D.: Air stable high resolution
organic transistors by selective laser sintering of inkjet printed metal nanoparticles, Applied Physics
Letters 90, 141103, 2007.
[12] Auyeung, R.; Kim H, Mathews, S.; Piqué, A.:
Laser Direct-Write of Metallic Nanoparticle Inks,
JLMN-Journal of Laser Micro/Nanoengineering Vol.
2, No. 1, 2007.
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Meet the Author
Petri Laakso works as research scientist at VTT and
has worked with several different fields of laser
materials processing such as cutting, welding of
polymers, laser ablation with ultra fast lasers and
systems integration.
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