22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Growth of functional plasma polymers influenced by reactor geometry in
capacitively coupled discharges
D. Hegemann1, U. Schütz1, D. Lohmann1,2 and M. Drabik1
1
2
Empa, Swiss Federal Laboratories for Materials Science and Technology, St.Gallen, Switzerland
Ruhr University Bochum, Institute for Electrical Engineering and Plasma Technology, Bochum, Germany
Abstract: The deposition of functional plasma polymers such as a-C:H:O films is mainly
influenced by fragmentation of the parent molecules in the gas phase as well as by the
energetic conditions during film growth at the surface. The control of both deposition
conditions enables permanent functional plasma polymer films within different reactor
geometries (symmetric vs. asymmetric at driven electrode and at grounded electrode).
Keywords: Plasma polymer, functional groups, ion bombardment, cross-linking, aging.
1. Introduction
Capacitively coupled discharges, e.g. by using a plane
parallel electrode set-up, are of high interest (also for
industry), since they allow uniform treatment of largearea surfaces such as wafers and web materials.
Difficulties related to this discharge type, however,
involve the actual reactor geometry (degree of
asymmetry), power coupling, gas flow pattern, plasma
expansion, and substrate location. A recently conducted
round-robin study thus revealed a poor comparability of
functional plasma polymer films (using acrylic acid
discharges) deposited within different reactor systems [1].
Functional plasma polymers, mainly comprising
oxygen- or nitrogen-containing groups, are of increasing
importance for biomedical applications [2], but also as
adhesion-promoting layers [3]. Therefore, their stability
in ambient air and in aqueous environments is crucial for
many applications. Enhanced stability can be achieved by
cross-linking of the plasma polymer film which requires
sufficient energy during deposition for bond opening.
Hence, the energetic conditions during film growth need
to be controlled which are strongly related to the reactor
geometry in a capacitively coupled plasma (CCP). To
gain a better control over gas phase and surface processes,
i.e. to control both radical and ion fluxes, e.g., dualfrequency operation is investigated [4]. For simplicity,
however, a better understanding of plasma polymer
deposition in radiofrequency-driven (RF, 13.56 MHz)
CCP reactors is still required.
For this purpose, the deposition of a-C:H:O films in
CO 2 /C 2 H 4 discharges is performed within a symmetric
and a slightly asymmetric reactor set-up, enabling three
different geometries (symmetric, driven and grounded
electrode). While the plasma chemical reaction pathway
is maintained, the influence of the different growth
conditions at the surface can be examined. Both the
conditions in the symmetric as well as in the asymmetric
set-up at the RF electrode were found to be useful for the
deposition of functional plasma polymer films.
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2. Experimental
The cylindrical, symmetric plasma reactor consists of
two plane parallel electrodes separated by a glass ring.
The upper electrode contains a gas shower, while the
chamber is pumped through the lower electrode which is
coupled to a RF generator (Fig. 1a). For the asymmetric
set-up, a slightly smaller electrode is mounted inside the
same plasma chamber adjacent to the gas shower at the
top plate (Fig. 1b). As deposition conditions, CO 2 and
C 2 H 4 flow rates of 8 and 4 sccm, respectively (gas ratio
2:1), a working gas pressure of 10 Pa, and a power range
of 10-250 W were selected.
Fig. 1. Schematic drawing of the used reactor geometries,
a) symmetric set-up, b) asymmetric set-up.
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A V/I probe (mounted as close as possible to the driven
electrode) was used for the measurement of the electrical
conditions (voltage and power absorption) and microwave
interferometry (MWI) to measure the electron densities.
Mean ion energies and ion flux can thus be calculated [5].
As a measure of the deposited energy (per atom), the
energy density during film growth can be calculated from
the energy flux (mean ion energy multiplied by ion flux)
per deposition rate and, likewise, the momentum transfer
using the square root of the ion energies [6, 7].
electrode, since the deposition area is smaller (490 cm2
vs. 690 cm2) [8].
3. Results and discussion
Mass deposition rates of the CO 2 /C 2 H 4 -derived plasma
polymers at three different positions (symmetric at bottom
RF electrode, asymmetric at top RF electrode and at
bottom grounded electrode) have been measured with
varying power input. Using an Arrhenius-like plot (with
power input per monomer flow rate, W/F m , instead of
temperature), a possible activation barrier for the plasma
chemical reaction pathway can be examined (Fig. 2).
Fig. 3. Velocity field of the gas flow pattern in the a)
symmetric and b) asymmetric set-up (calculated using
COMSOL Multiphysics®). The (inner) dashed line
indicates the expansion of the plasma zone.
Fig. 2. Arrhenius-like plot of mass deposition rate (per
monomer flow rate) vs. the inverse energy input using the
nominal power input.
For all three configurations an Arrhenius regime as
indicated by straight lines in Fig. 2 can be identified,
however, with different slopes. In order to relate the
reaction parameter W/F m to the energy invested per
molecule in the gas phase, power absorption (85% for
symmetric and 45% for asymmetric set-up), plasma
expansion and gas flow pattern have to be taken into
account. While there is a well-defined vertical gas flow
in the symmetric set-up (Fig. 3a), the gas inlet is
constrained by the RF electrode in the asymmetric set-up
influencing the residence time in the plasma zone
(Fig. 3b). Moreover, the plasma is more intense in front
of the driven electrode. Considering these effects, the
power input per monomer flow rate can be related to the
plasma zone, (W/F m | pl ) [8]. As it can be seen from Fig. 4,
all deposition rates follow the same slope indicative of the
same plasma chemical reaction pathway. The deposition
rate is higher for the deposition at the asymmetric RF
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Fig. 4. Arrhenius-like plot of mass deposition rate (per
monomer flow rate) vs. the inverse energy input related to
the plasma zone.
At enhanced energy input, the deposition rates start to
deviate from the Arrhenius line, which has been ascribed
to a change in the chemical reaction pathway, i.e.,
increasing formation of radicals [9]. In the symmetric and
the RF electrode case, the deposition rates are even
decreasing showing dominant ion-induced effects.
Beyond the Arrhenius regime, the incorporation of
oxygen-containing groups into the a-C:H:O films has
been found to be reduced ([O]/[C] ratio decreases from
0.3 to 0.2) [6].
Furthermore, film densities have been calculated from
mass deposition rates and film thicknesses (measured
using profilometry). Film density is closely related to
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cross-linking and to the density of functional (polar)
groups in the a-C:H:O plasma polymers. Fig. 5 shows the
dependence of film densities on the momentum transfer
during film growth (which is transferred by the interaction
with energetic particles from the plasma) for the three
different plasma configurations. In addition, static contact
angles have been measured (with water) right after
deposition and after one month aging in air (23 °C, 65%
relative humidity).
Fig. 5. Film density of a-C:H:O plasma polymers vs.
momentum transfer during film growth. The change in
contact angles is given after aging for 1 month in air.
For enhanced ion bombardment, the film densities were
found to increase linearly with momentum transfer at all
three configurations, which agrees well with earlier
findings [6]. Hence, the densification is directly related to
the actual growth conditions at the substrate surface
independent of the reactor geometry. Most of all, the
deposition at the RF electrode, both in symmetric and
asymmetric set-up, enables a broad variation of deposition
conditions.
At the grounded electrode, on the other hand, the flux
of film-forming species increases in the same degree as
the flux of energetic particles resulting in an almost
constant momentum transfer. Nevertheless, a slight
increase in film density (with W/F m ) can be noticed. An
increase in film density at constant momentum transfer
indicates increasing importance of fragmentation in the
gas phase [7]. Hence, gas phase reactions mainly
determine the plasma polymer growth when deposited at
the grounded electrode. The control over the film
properties, however, is low in this case.
Moreover, a-C:H:O films deposited in this regime
(corresponding to a deposited energy below 6 eV per
atom) were found to be partly soluble in aqueous
environments and showed strong aging effects. With
respect to stability, moderate ion bombardment is thus
required to obtain sufficient cross-linking and stable
contact angles around 54°. This result agrees well with
recent findings for CO 2 /C 3 H 6 O-derived plasma polymers
showing stable contact angles of around 55° (at [O]/[C]
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ratio of 0.2) thanks to a low amount of rather instable
COOR groups [10]. Most of all, the suitability of the aC:H:O films as bioactive surfaces has been demonstrated
[11].
Further improvements can be achieved by
consequently increasing the ether/hydroxyl part at the
expense of ester/carboxyl groups, e.g. by using
H 2 O/C 2 H 2 discharges [12].
In terms of power input, a power around 70 W resulted
in the deposition of stable a-C:H:O films in the symmetric
set-up, while 250 W was required in the asymmetric setup (at fixed gas flow rates) thanks to the superior power
coupling in the symmetric reactor. Both energy inputs
fall into the regime where the deposition rates starts to
deviate from the Arrhenius regime accompanied by a
reduced incorporation of oxygen ([O]/[C] ratio of around
0.2). It is noteworthy that the highest deposition rates
(around 10 nm·min-1 for the used conditions) can be
achieved for such coatings which can be enhanced further
by using higher gas flow rates (and in the same degree
higher power input). A too strong ion bombardment can
then be balanced by increasing the pressure.
4. Conclusions
The deposition of CO 2 /C 2 H 4 -derived a-C:H:O films
within three different plasma geometries (symmetric
CCP, asymmetric CCP at RF electrode and grounded
electrode) has been investigated at comparable gas phase
conditions, i.e., within the same range of energy invested
per particle (W/F m | pl ). Differences in film properties
(such as film density and functional group density) are
thus related to the actual growth conditions at the
substrate surface. As a measure for the densification, the
momentum transfer during film growth has been
determined showing a linear relationship (above a certain
threshold energy). While the conditions for deposition on
grounded electrode in the asymmetric set-up have been
found to be rather invariable resulting in a poor film
quality, deposition on the RF electrode and within the
symmetric set-up enables a broad variation (and thus
optimization) of film properties. Non-aging functional
plasma polymer films with a static contact angle around
54° have been obtained with moderate ion bombardment.
The observed stability of the CO 2 /C 2 H 4 -derived aC:H:O films deposited at enhanced ion bombardment
agrees well with findings for different starting monomers
thanks to a reduction of rather instable COOR groups.
Hence, the deposition of permanent functional plasma
polymer films is possible using simple gases within
different reactor geometries at acceptable deposition rates
which also allows further up-scaling. The wettability,
however, is limited. To enhance wettability, i.e. to obtain
more polar groups at the surface, different concepts such
as vertical gradient layers need to be followed.
In any case, deeper insights into plasma polymerization
processes are gained by the investigation of the flux of
film-forming species (via deposition rates) and the flux of
energetic particles, also enabling the comparison of
different reactor geometries and transfer to industry. For
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example, the developed highly permanent a-C:H:O films
are nowadays in use for blood filtration.
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