22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Development of atmospheric pressure plasma polymerized nitrogen-rich
(PPC:H:N) thin films
K. Fricke, M. Levien, K.-D. Weltmann and M. Polak
Leibniz Institute for Plasma Science and Technology (INP Greifswald e.V.), Greifswald, Germany
Abstract: Polymer thin films, rich in functional moieties, were deposited from different
nitrogen (N 2 ) and ethylene (C 2 H 4 ) gas mixtures using non-thermal atmospheric pressure jet
plasmas. The obtained coatings were characterized by profilometry, X-ray photoelectron
spectroscopy (XPS), and atomic force microscopy (AFM). Results revealed dense and uniform coatings with good stability in water.
Keywords: atmospheric pressure, plasma jet, coating, surface analysis
1. Introduction
Adjustable surface properties are essential for materials
which are in contact with biological systems. Biological
response depends on a large variety of parameters with
the surface chemistry being considered as one of the most
determining factor. In order to introduce a desired surface
chemistry
and
morphology
plasma-deposited
functionalized organic layers are favoured for providing
biocompatible surfaces. Especially, nitrogen-rich films
are of interest in cell culture and tissue engineering.
Atmospheric pressure plasma treatments with nitrogencontaining gases are successfully applied for surface
functionalization and thin film deposition of cellcontacting materials [1-3]. In this study a non-thermal
atmospheric pressure plasma jet was used for thin film
deposition. This plasma jet allows gentle and local
surface treatment of microstructures, including cavities,
and complex three dimensional shapes. Gas mixtures of
nitrogen and hydrogenous gases (e.g., acetylene and
ethylene) enable the generation of nitrogen-functionalities
into plasma-polymerized films [4-5]. Therefore, the
objective of this paper was the development and
characterization of nitrogen-bearing plasma polymerized
hydrocarbon coatings (PPC:H:N) intended for biological
applications.
2. Experimental
The atmospheric pressure plasma jet used in this study
consists of two ring electrodes around a quartz capillary
(Fig. 1.). The upper electrode is capacitively coupled to
the RF power generator (27.12 MHz) through a matching
network while the bottom electrode is connected to the
ground potential [6]. A ceramic capillary is placed in the
centre of the quartz capillary which enabled the
introduction of N 2 and C 2 H 4 in the discharge region. The
process gas Argon (Ar) is flowing in the quartz capillary
with a flow rate of 5 slm. Unless otherwise stated,
coatings were prepared from mixtures of 500 sccm N 2
and 50 sccm C 2 H 4 at 30 W. The applied process
parameters ensure surface temperatures below 100 °C
P-III-6-17
(Fig. 1) which allows the treatment of heat-sensitive
materials, too.
Fig. 1. Left: Scheme of the plasma jet (flow of Ar
through outer capillary, flow of C 2 H 4 /N 2 gas mixtures
through inner capillary). Right: top) thermographic
measurement of specimen during plasma treatment;
bottom) PPC:H:N deposition pattern on silicon wafer
after 180 s.
Elemental compositions and chemical binding
properties of deposited coatings were analysed by XPS
(Axis Ultra, DLD detector, Kratos) using a
monochromatic Al Kα source at 1486.6 eV (150 W).
Chemical binding components were calculated by peak fit
procedures of high-resolution C 1s spectra using
Gaussian-Lorentzian distribution, Shirley baseline, and a
FWHM of maximal 1.3 eV. For the determination of
primary amino group density chemical derivatization with
4-trifluoromethyl-benzaldehyde (TFBA) in a saturated
gas phase at 40°C for 2 h, accompanied by XPS
quantification, was implemented. Surface topography
1
was examined with a scanning probe microscope diCP-II
(Veeco, USA) in the non-contact mode.
Five AFM-images with a scanning region of
10 x 10 µm2 were recorded on each sample to evaluate the
averaged roughness R a of PPC:H:N coatings (SPMLab
Ver. 6.0.2.,Veeco). Surface profiles were measured by
using a surface profiler (Dektak 3ST, Veeco).
3. Results and Discussion
Process parameters such as RF power, jet-nozzle to
substrate distance, and gas flow rate affect the coating
thickness and hence, the deposition rate. For instance,
film thickness was found to be dependent on precursor
flow rate. Fig. 2 shows the evolution of the coating
thickness as a function of C 2 H 4 flow rate.
Fig. 2. Coating thickness as a function of C 2 H 4 flow rate
(n = 8, deposition time: 10 min).
The results revealed an increase in film thickness with
rising precursor flow rate.
Deposition rates were
calculated by measuring surface profiles of the deposits
by profilometry. The ring shaped deposition pattern
results in a parabolic profile peaking at the centre.
Deduced from the surface profiles, deposition rates of
0.2 nm s-1 for a precursor flow rate of 10 sccm up to
3.8 nm s-1 for 50 sccm C 2 H 4 were obtained.
However, representative AFM images and height
profiles of the films deposited on silicon wafers as a
function of treatment time are presented in Fig. 3. After
3 min a smooth and dense film can be observed. Distinct
changes in surface topography appeared after 5 min. The
AFM micrograph displays the formation of randomly
distributed particles of up to 75 nm in height and 600 nm
in width (see height profile in Fig. 3). Further plasma
processing resulted in the formation of clusters with a
typical size between 100 and 1000 nm. Hence, the
number and dimension of spherical particles formed on
the films surface is strongly related to the deposition time.
A consequence of these surface features is the alteration
in surface roughness.
2
Fig. 3. Surface topography of deposited PPC:H:N films
after 3, 5, and 10 min (10 x 10 µm2). White dashed lines
indicate the position where height profiles were extracted.
The calculated averaged roughness of PPC:H:N films
deposited after 3, 5, and 10 min was 2 nm, 15 nm, and
122 nm, respectively. The corresponding surface profiles
yielded film thicknesses of 0.4 µm, 0.7 µm, and 2.3 µm.
In summary, coatings with a thickness above 1 µm were
found to exhibit a strongly featured surface with large
clusters (micrometre sized).
Chemical composition of PPC:H:N coatings was
examined by XPS. The relative atomic concentration
(at.%) of the elements detected is listed in Table 1.
Results revealed a nitrogen-rich carbon film with a small
amount of oxygen. The presence of oxygen on the
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surface is inherently due to operation in ambient air and
post-oxidation of residual free radicals on the surface [7].
(USB) in Milli-Q water for 5 min. Inset figure: C 1s peak
fit of a freshly deposited PPC:H:N film.
Table 1. Elemental surface composition (in at.%) of
PPC:H:N as deposited and after ultrasonic agitation
(USB) in Milli-Q water for 5 min. Amino group density
(NH 2 /C) in % was calculated after derivatization. (n = 5).
In particular, component C1 which is assigned to C–C
and various C–H bonds, C2 corresponding to amines
(C-N), C3 which is attributed to imines/nitriles/hydroxyl
groups (C=N, C≡N, C-O), and C4 indicating the presence
of imide/ketones/aldehydes (N-C=O, C=O). Fig. 4 shows
the relative contributions of each bond type determined
for a freshly deposited film and after 5 min ultrasonic
treatment. It was found that component C1 was increased
while components C3 and C4 were decreased. The
contribution of C-N bonds (C2) remained unchanged.
As deposited
After USB for 5
min
C
73 ± 2
76 ± 1
N
18 ± 2
16 ± 1
O
NH 2 /C
9 ± 1 3.5 ± 0.2
8 ± 1 3.1 ± 0.1
Bonding components in C 1s / %
Furthermore, XPS elemental analyses showed closed
pinhole-free films since no Si signal of the subsurface was
detected in any case. Stability and solubility of PPC:H:N
coating on Si wafer was examined by ultrasonic agitation
in Milli-Q water for 5 min. Corresponding XPS-data are
listed in Table 1. Comparing XPS spectra before and
after ultrasonic bath showed only slight alterations,
namely N and O portions were reduced by 2 at.% and
1 at.%, respectively. The most likely explanation is the
removal of weakly bonded molecules by washing.
Accompanied analysis of surface profile revealed a
thickness of 450 ± 30 nm for the freshly deposited film
and 430 ± 20 nm after ultrasonic treatment. Please note:
the difference of 20 nm is within the error margin of 10%.
Hence, no delamination of the studied coatings was
observed. Also summarized in Table 1 is the relative
concentration of primary amino groups based on
derivatization reactions. According to the decreased
N portion, a slight reduction in primary amino groups
occurred after ultrasonic bath. Additionally, there was
little or no apparent change in the concentrations after
immersion of the coatings for 24 h (data not shown).
High-resolution XPS C1s spectra verify the existence of
different bonding components (inset Fig. 4).
As deposited
60
After USB for 5 min
50
40
30
20
10
0
C1
C2
C3
C4
Fig. 4. Comparison of carbon-based functional chemical
groups determined by peak fitting of high-resolution C 1s
spectra after film deposition and after ultrasonic agitation
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4. Summary
Polymer-like
coatings
were
deposited
from
ethylene/nitrogen mixtures by using an atmospheric
pressure plasma jet. Depending on the precursor flow
rate, deposition rates of up to ~ 4 nm s-1 were achieved.
Surface roughness of the deposits is determined by the
treatment time and thus, film thickness. Prolonged
plasma treatment (> 3 min) led to particle formation on
the surface. The elemental surface composition of
PPC:H:N coatings, analysed by XPS, revealed high
portion of nitrogen as well as small amounts of oxygen.
Water solubility and wash stability was evaluated by
ultrasonic treatment. PPC:H:N films were found to be
largely resistant against agitation.
Overall, neither
hydrolysis (no uptake of oxygen) nor delamination was
observed. In a nutshell, the PPC:H:N coatings presented
here might be highly suitable for biomedical applications.
5. Acknowledgments
This work was supported by the Ministry of Economics
of the Federal State Mecklenburg Vorpommern (grant no.
V-630-00INP-20144/018).
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