22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma polymers deposited at atmospheric pressure: influence of process
parameters on film properties
K. Fricke1, H. Gagnon2, P.-L. Girard-Lauriault3, K.-D. Weltmann1 and M.R. Wertheimer2
1
Leibniz Institute for Plasma Science and Technology (INP Greifswald e.V.), Greifswald, Germany
2
Department of Engineering Physics, École Polytechnique de Montréal, Montréal, QC, Canada
3
Plasma Processing Laboratory, Department of Chemical Engineering, McGill University, Montréal, QC, Canada
Abstract: Plasma polymer deposition by atmospheric pressure dielectric barrier discharge
(DBD) using different hydrocarbons has been studied. The discharge was monitored by
electrical measurements, while the chemical compositions of resulting coatings were
analyzed by XPS and total combustion (CHN) analysis. SEM was used to determine the
coatings’ morphology, and profilometry for evaluating deposition rates.
Keywords: DBD, atmospheric pressure, plasma polymers, surface chemistry
1. Introduction
Plasma polymerization has been successfully
demonstrated to be an effective method for generating a
large variety of functional thin films on different substrate
materials.
By appropriate choice of deposition
parameters, the surface properties of plasma polymers can
readily be tailored, which make them very attractive for
biological or biomedical applications. Plasma-coating
technologies operating at atmospheric pressure have
evolved considerably in recent years, based on stringent
demands for maintenance- and vacuum-free operating
conditions; indeed, they have become indispensable
among
key
plasma-based
surface
engineering
technologies [1]. Along with plasma jets, dielectric
barrier discharges (DBD) are now routinely used for
surface modification and thin film deposition [2]. In
particular, the DBD apparatus employed in the present
study has amply demonstrated its utility, both for surface
modification of polymers and for depositing nitrogen-rich
plasma polymer films [3, 4]. Deposition parameters like
process gas, operating pressure, and electrical power, can
directly affect the chemical and physical characteristics of
the gas discharge which subsequently influence the
surface properties of the newly formed plasma polymer
layer. This is of particular importance for biomedical
application, because the film's characteristics directly
impact the extent to which biomolecules interact with its
surface. In this contribution, results are presented
regarding the deposition of hydrocarbon-based films
using the DBD system fed with mixtures of argon (Ar) or
nitrogen (N 2 ) plus different hydrocarbon precursors. The
influence of precursor gas mixture and –flow, as well as
excitation frequency and –voltage have been investigated
in order to clarify the role of the discharge mode on
deposited plasma polymer film compositions and
properties.
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2. Experimental
Fig. 1 depicts the atmospheric pressure DBD apparatus
used for depositing hydrocarbon-based films; it comprises
a grounded, planar aluminium electrode and a cylindrical,
dielectric-coated stainless steel high voltage electrode, as
described in detail elsewhere [3].
Fig. 1. Schematic view of the DBD apparatus.
Electrical characteristics of the gas discharge were
monitored using an oscilloscope (GW Instek
GDS-2204A), a high-voltage probe (Tektronix P6015A),
and a voltage probe (Tektronix P6139B) which was
connected to a 50 Ω resistor for measuring the current.
Data were processed by MATLAB to calculate the
electrical power. Films were deposited from gas flow
rates of 5 slm Ar or 7 slm N 2 mixed with hydrocarbon
flows of 0.01 to 0.4 slm. The gas mixture ratio, X, is the
ratio of hydrocarbon to N 2 or Ar flow rates, given in parts
per thousand (‰). The excitation frequency was varied
from 1 to 11 kHz. Chemical composition of the deposits
was determined with X-ray photoelectron spectroscopy
(XPS) using a VG ESCALAB 3 MkII system with nonmonochromatic Mg-Kα radiation (1253.6 eV). Relative
atomic concentration of detected elements was
determined from XPS survey spectra over the 0 - 1100 eV
binding energy range, using a pass energy of 50 eV. For
quantifying all constituent elements, total combustion
(CHN) analysis was used (EA 1108 CHN, Fisons
1
Instruments). Morphology of films deposited on glass
substrates was investigated by scanning electron
microscopy (JSM-7600F, JEOL, USA). Finally, the
thickness of coatings was measured by means of a surface
profiler (Dektak 150, Veeco).
3. Results and Discussion
Deposition rates, R, were determined with the help of a
surface profiler. Fig. 2 depicts R as a function of gas
mixture ratio, X, for different hydrocarbon precursors,
prepared in nitrogen plasma. The precursors were chosen
according to their different carbon-to-hydrogen ratio,
namely acetylene (C 2 H 2 ) > ethylene (C 2 H 4 ) > ethane
(C 2 H 6 ) > methane (CH 4 ).
polymerization process: The influence of excitation
frequency, f, and of absorbed electrical power, P el , on
deposition rate, R, is plotted in Fig. 3 for the case of a
given C 2 H 2 /Ar mixture. R increased from 3 to 46 nm × s-1
as f was raised from 1 to 10 kHz. While the correlation
between f and R was linear, that between R and P el clearly
was not.
Fig. 3. Deposition rate of plasma polymer films from
C 2 H 2 /Ar plasma, as a function of frequency and electrical
power (deposition parameters: X: 40‰, 6 kV pp ).
Fig. 2. Deposition rate as a function of gas mixture ratio
for different hydrocarbon precursors (deposition
parameters: 7 slm N 2 , 11 kHz, 17 kV pp ).
As noted from the plotted results, an increase of the
deposition rate with increasing gas mixture ratio was
observed for all coatings under study. Among the
different hydrocarbons investigated, acetylene showed the
highest susceptibility towards plasma polymerization. As
confirmed by the literature, the presence of unsaturated
carbon bonds leads to high deposition rates [5, 6]. Hence,
in this work, R values obtained for C 2 H 2 , which contains
a triple bond, are found to be much higher compared with
C 2 H 4 , C 2 H 6 , and CH 4 , which all display considerably
lower values. However, it should be mentioned that in the
case of C 2 H 2 with X > 7‰, powder formation was
observed on the substrate.
For the case of films deposited from argon plasma, the
gas mixture ratio was a particularly decisive process
parameter. More specifically, high X values were needed
for film deposition. This can be explained in terms of a
competition between deposition and etching processes,
which was very clearly observed for the case of low X.
Therefore, a uniform plasma polymer film was deposited
only for X > 20‰.
Besides the type of hydrocarbon monomer and the
carrier gas, several other factors affect the plasma
2
The elemental surface composition of the deposits was
determined by XPS combined with CHN analysis.
Selective results for plasma polymer films derived from
acetylene in argon and nitrogen plasma, respectively, are
summarized in Table 1.
Table 1. Elemental composition (in at.%) of different
coatings determined by XPS and combustion analysis
(CA) (deposition parameters: a) 5 slm Ar, X: 40‰, 2 kHz,
6 kV pp ; b) 7 slm N 2 , X: 7 ‰, 11 kHz, 17 kV pp ).
C 2 H 2 /Ar
C 2 H 2 /N 2
XPS
C
O
93.1 6.9
57.8 4.5
N
37.7
CA
C
47.2
32.7
H
52.8
43.6
N
23.7
A considerable amount of bonded oxygen was detected
on the C 2 H 2 /Ar coating. This surface-near oxygen is
inherently due to post-reaction exposure to ambient air,
and resulting oxidation of residual free radicals.
Furthermore, Kobayashi et al. pointed out that such
reactions with atmospheric oxygen are more extensive for
acetylene- and ethylene-derived plasma polymers, more
susceptible on account of their unsaturation than single
bonds in alkane-based plasma polymers [6]. In C 2 H 2 /N 2
plasma, the reaction of excited nitrogen species with the
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hydrocarbon leads to the formation of plasmapolymerized C:H:N films that contain a high
concentration of chemically bonded N. Accompanying
combustion analyses revealed hydrogen contents of
53 at.% and 44 at.%, respectively, for argon- and
nitrogen-based deposits, confirming their “polymer-like”
nature. A further interesting observation which can be
made from Table 1 is that the C 2 H 2 /Ar coating has almost
the same C/H ratio as the monomer. However, as
expected, the N concentration of the C 2 H 2 /N 2 coating is
much lower than the value determined by XPS, of course
on account of the hydrogen content that must be
considered here [4].
The coatings’ morphology was examined by SEM,
representative micrographs of films deposited from
C 2 H 2 /Ar and C 2 H 2 /N 2 mixtures being shown in Fig. 4.
structure of the deposits was investigated. The main
results can be summarized as follows:
1. R was found to vary systematically with X, increasing
roughly three-fold when X increased from about 1 to
14‰.
2. R increased by a factor of 15 with rising frequency,
from 1 to 10 kHz.
3. Based on XPS and CA analyses, films deposited from
C 2 H 2 /Ar mixtures showed sizeable amounts of bonded
oxygen, attributed to post-reaction oxidation, while
C 2 H 2 /N 2 -based deposits contained high concentrations
of bonded nitrogen.
4. SEM images showed partly dense film structures, but
with differing surface-near porosity depending on the
hydrocarbon precursor. In the case of nitrogen-based
films, the porosity depended greatly on the choice of
hydrocarbon.
5. All of these plasma polymer films exhibited a
cauliflower-like surface morphology.
5. Acknowledgements
The authors are grateful for financial support from the
Natural Sciences and Engineering Research Council of
Canada (NSERC) and from INP Greifswald e.V. They
thank P. Plamondon for support with SEM measurements
and Y. Leblanc for his skilled technical support.
Fig. 4. SEM images of coatings deposited on glass
substrates. Insets are top views (deposition parameters:
a) 5 slm Ar, X: 40‰, 2 kHz, 6 kV pp ; b) 7 slm N 2 , X: 7‰,
11 kHz, 17 kV pp ).
Coatings produced from C 2 H 2 /Ar plasma manifested a
compact morphology with spheroidal features at the top.
On the contrary, their N-containing counterparts from
C 2 H 2 /N 2 plasma were found to exhibit a rather porous
structure, wherein the SEM image reveals a tens of
nanometers thin dense sub-layer, with columnar structures
growing on top of it.
Additionally, the various
hydrocarbons used here led to remarkable differences in
the morphology of their N 2 plasma-based coatings (data
not shown). Whereas the C 2 H 2 plasma polymer was
porous, others deposited from C 2 H 4 and C 2 H 6 mixtures
were found to be dense, comparable to the C 2 H 2 /Ar film
shown above. However, all of these plasma-polymerized
films exhibited the same cauliflower-like surface
morphology (see top-view images in Fig. 4).
6. References
[1] G. Da Ponte, E. Sardella, F. Fanelli, R. d'Agostino
and P. Favia. Eur. Phys. J. Appl. Phys., 56, 2 (2011)
[2] F. Massines, C. Sarra-Bournet, F. Fanelli, N. Naude
and N. Gherardi. Plasma Process. Polymers, 9,
11-12 (2012)
[3] S. Guimond, I. Radu, G. Czeremuszkin, D. Carlsson
and M.R. Wertheimer. Plasmas Polymers, 7, 1
(2002
[4] P.L. Girard-Lauriault, P. Desjardins, W.E.S. Unger,
A. Lippitz and M.R. Wertheimer. Plasma Process.
Polymers, 5, 7 (2008)
[5] I. Retzko, J.F. Friedrich, A. Lippitz and
W.E.S. Unger. J. Electron Spectrosc. Relat.
Phenom., 121, 1-3 (2001)
[6] H. Kobayashi, A.T. Bell and M. Shen.
Macromolecules, 7, 3 (1974)
4. Summary
In this study, plasma polymer films were deposited at
atmospheric pressure from hydrocarbon mixtures with
argon or nitrogen, using a dielectric barrier discharge.
The influence of various process parameters, namely
excitation frequency, f, gas mixture ratio, X, and type of
hydrocarbon, on deposition rate, R, composition and
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