-Supplementary materialControlling wettability in paper by atmospheric-pressure
microplasma processes to be used in µPAD fabrication
Lars Hecht1), Jens Philipp2), Kai Mattern1), Andreas Dietzel1), Claus-Peter Klages2,*)
1) Technische Universität Braunschweig, Institute of Microtechnology (IMT), Alte Salzdahlumer Str.
203, D-38124 Braunschweig
2) Technische Universität Braunschweig, Institute of Surface Technology (IOT), Bienroder Weg 54 E,
D-38108 Braunschweig
*)
c-p.klages@tu-braunschweig.de Phone: +49 531 2155 510 Fax: +49 531 2155 900,
Online Resource 2
1. Plasma etching - parameter determination
In order to define an optimal set of parameters for plasma etching multiple process variables
were varied and their influence on the etching process investigated. The reactor setup
described in Section 2.1 “Setups used for microplasma etching and deposition” was used for
these experiments. All experiments varied one variable while keeping the others constant. In
this part of the supplementary material a few examples of substrates modified by varying
parameters will be shown in order to facilitate an understanding of their influence on the
fabrication process.
The wetting experiments for the characterization of the hydrophilic pattern generated by the
plasma etching were carried out by applying 25 µl test liquid (fountain pen ink Pelikan
Königsblau diluted in DI water with a ratio of 1:8) along the two main channels of the combstructures. In order to visualize the invisible structures the substrate was wetted with DIwater which was subsequently evaporated prior to the application of the test liquid.
The samples were characterized using visual comparison of the wetting behaviour and edge
definition. It should be noted that the samples are not drenched in liquid and that the inkdroplets are applied manually. This may lead to some inhomogeneity in the distribution of the
visualized hydrophilic pattern if the etching process was not sufficient for the complete
hydropilization of the desired areas.
Whatman Filter Paper Grade 6 and Chromatography paper CHR1 were used as substrates
for the experiments. Both types of paper exhibited a similar suitability for the process.
Furthermore, the AKD-concentration of the solution used for the complete hydrophobization
of the substrates was varied in between 1.06 g/l and 1.2 g/l. Neither the substrate type nor
the AKD-concentration turned out to have a significant influence on the etching process if
varied in these ranges.
Gas composition
The composition of the plasma gas was varied by adjusting the ratio of He to N2 in the gas
flow applied to the porous ground electrode. The volume flow ratio was varied from 1:24 (flow
of He: flow of N2) to 1:1. Furthermore the use of pure nitrogen was investigated (See
Table 1).
Table 1 Plasma etching results at varying gas compositions
Top
Bottom
Parameters 0,9 kV peak voltage
Comments
0,9 kV peak voltage
0,9 kV peak voltage
Pure N2
1:2 He to N2-Ratio
1:1 He to N2-Ratio
45 s treatment time
45 s treatment time
45 s treatment time
1,06 g AKD/l
1,06 g AKD/l
1,06 g AKD/l
Etch-rate not sufficient Good results; clear edges No
further
for hydrophilization
on both sides of the improvement
substrate
See note below
For the first experiments, with pure nitrogen as the plasma gas, a coarser porous metal plate
electrode was used as the ground electrode. For most of the subsequent investigations this
electrode was changed to the highly porous metal plate made from sintered metal fibers
(stainless steel 1.4404, fiber diameter between 22 and 27 µm, open porosity about 85 %)
described in chapter 2.1. An optical comparison of the structure of both electrodes is shown
in Fig. 1. All samples treated with the first electrode exhibit a larger form deviation, which
explains the bad edge definition of the pure nitrogen-sample in Table 1 but not the
insufficient hydrophilization.
Fig. 1 Micrographs of the surface of both porous ground electrodes. Left: Coarse electrode
used for the experiments with pure nitrogen as the plasma gas. Right: optimized electrode
with a fiber diameter between 22 and 27 µm and an open porosity of about 85 %. Both
electrodes are made from stainless steel 1.4404 and were manufactured by Fraunhofer
IFAM, Dresden, Germany
Samples treated with a ratio of He to N2 of 1:1 to 1:2 exhibited a superior etch definition and
hydrophilization than lower ratios or pure nitrogen. A ratio of 1:2 is selected for all further
experiments.
Treatment time
Table 1 Plasma etching results after varying treatment times
Top
Bottom
Parameters 0,9 kV peak voltage
Comments
0,9 kV peak voltage
0,9 kV peak voltage
1:2 He to N2-Ratio
1:2 He to N2-Ratio
1:2 He to N2-Ratio
30 s treatment time
45 s treatment time
60 s treatment time
1,14 g AKD/l
1,14 g AKD/l
1,14 g AKD/l
Treatment
sufficient;
definition
time
not Sufficient edge definition No
further
bad
edge and flow rates
improvement
The treatment time was varied from 10 to 120 s after visible plasma ignition. Results are
visualized in Table 2: After a threshold of about 45 s no further improvement can be
observed. Longer treatment times increase the likelihood of thermal damage to the substrate.
A treatment time of 45 s is sufficient for the etching process.
Peak Voltage
As mentioned in Chapter 3.1 the reactor setup has a large influence on the process. A stable
plasma operation under the conditions of these experiments was possible with peak voltages
ranging from 0.8 to 1.2 kV. Substrates after the modification with different peak voltages are
shown in Table 3. Higher voltages generated structures with an improved edge definition as
well as longer flow distances but are also prone to thermal damage in the form of small
punctures.
A peak voltage of 0.9 kV is considered to be a reasonable compromise between good edge
definition and a low likelihood of thermal damage to the substrate.
Table 3 Plasma etching results at different peak voltages
Top
Bottom
Parameters 0,8 kV peak voltage
0,9 kV peak voltage
1,0 kV peak voltage
1:2 He to N2-Ratio
1:2 He to N2-Ratio
1:2 He to N2-Ratio
30 s treatment time
45 s treatment time
60 s treatment time
1,14 g AKD/
Comments
Treatment
sufficient;
definition
time
not Sufficient edge definition No
further
bad
edge and flow rates
improvement; higher
potential for thermal
damage
Results
Within the ranges of experimental parameters good results were achieved when using the
following parameters:






Substrate: Whatman Chromatography paper CHR 1
Hydrophobization with a 1.06 g/l AKD-solution in heptane
He to N2 ratio: 1:2
Gas flow: 9.5 l/min
Peak voltage: 1.1 kV
Treatment time: 45 s.