22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Amino-functionalized surfaces regarding initial growth conditions in plasma
polymerization
M. Vandenbossche, M.I. Butron Garcia, U. Schütz and D. Hegemann
Empa, Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland
Abstract: As the deposition of plasma polymers is both influenced by gas phase and
surface processes, it can be expected that the substrate material affects the initial growth
conditions. Thus, this study was carried out on ultrathin amino-functional plasma polymers
to determine the constitution of the initial and steady-state film. The initial film growth
appears to be more cross-linked due to ion bombardment leading to a more stable film.
Keywords: plasma polymer, ion bombardment, cross-linking, deposition rate
1. Introduction
The deposition of plasma polymer films (PPF) enables
the functionalization of different material surfaces
showing minimized aging effects due to the incorporation
of functional groups within a cross-linked network.
Typically, plasma polymers are deposited in a film
thickness range starting from tens of nanometers to
micrometers. Nevertheless, it is of high interest to
achieve the minimum film thickness for functional plasma
polymers for various reasons: i) reduced processing time,
use of materials and costs, ii) reduced interference with
the signal in SPR sensors, iii) functionalized metal
surfaces enabling a low contact resistance, iv) maintained
mechanical properties of very soft substrates such as
scaffolds for tissue engineering, and v) maintained
degradation properties of biodegradable substrates.
While the deposition rate for plasma polymers is
constant in steady-state conditions, i.e., when the
substrate is covered by a closed layer of the plasma
polymer, the initial growth conditions might be different
[1]. Depending on the number of available reaction sites
at a surface the sticking probability for incident filmforming species from the gas phase is affected during the
initial stage of film growth resulting in a different
deposition rate. Most of all, noble (inert) metal surfaces
reveal a delayed surface coverage (due to dewetting) and
thus reduced deposition rates beside low adhesion [2-4].
Furthermore, the chemistry of the deposited plasma
polymer might be affected during the initial growth due to
bond formation with substrate atoms and inter-mixing
(interphase formation) [5, 6]. In addition, it has been
observed that plasma polymers that are dissolvable in
aqueous environments such as amino-rich a-C:H:N films
leave a few nanometer thick stable remnant film on the
substrate [7, 8]. It has thus been speculated that an
enhanced ion bombardment during the initial film growth
conditions yield stronger cross-linking and reduced
functional group density as compared to subsequent
steady-state conditions (Fig. 1). More permanent aminofunctional plasma polymers can well be obtained using
sufficient ion bombardment during film growth [9, 10].
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This study thus aims in further clarifying early stages of
plasma polymerization by deposition of ultrathin aminofunctional films.
Fig. 1. Representation of a plasma polymer film growing
on a Si substrate. A first highly cross-linked film with a
thickness of around 5 nm is formed, while less crosslinking occurs during steady-state growth.
2. Experimental
A capacitively coupled discharge with 13.56 MHz
radiofrequency excitation was used for the deposition of
The
a-C:H:N films from NH 3 /C 2 H 4 mixtures.
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 the RF generator. As deposition conditions, NH 3 and
C 2 H 4 flow rates of 7 (or 8) sccm (gas ratio 1:1), a
pressure of 10 Pa, and a power range of 10 - 150 W were
selected. Glass slides and Si wafers were used as
substrates in this study, which were pre-treated in an Ar
plasma (10 min, 50 W, 10 Pa) enabling constant initial
growth conditions.
A V/I probe was used for the measurement of the
electrical conditions (voltage and power absorption) and
microwave interferometry (MWI) to observe electron
densities. Mean ion energies and ion flux can thus be
calculated [11]. 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 times
ion flux) per deposition rate [9]. The deposition rate, in
turn, is proportional to the flux of film-forming species
and their sticking probability [7].
1
The mass of the glass slides was measured before and
after the treatment using a balance (Mettler Toledo XS204
deltarange). Film thickness was measured at the surface
of the Si wafers by profilometry (profilometer Vecco
Dektak 150). To examine the chemical composition, XPS
(PHI 5600 spectrometer, Physical Electronics, USA) was
used with non-monochromatized Mg-Kα radiation
(1253.6 eV) and a take-off angle of 45°. Quantification
of the amount of primary amino groups at the surface of
the a-C:H:N films was carried out at the day of the
treatment (T 0 ) and the day after (T 24h ). Therefore,
derivatization
with
4-(trifluoromethyl)benzaldehyde
(TFBA, Aldrich, 98%) vapour at 45°C during 3 h was
performed (Fig. 2).
F
F
F
F
F
F
Fig. 3. Arrhenius-like plot of mass deposition rate (per
monomer flow rate) versus the inverse energy input
related to the plasma zone. Data indicated by an asterisk
were used for further investigations.
NH2
H
W/F m . The gas phase processes are thus also independent
of the used substrate.
N
O
Fig. 2. Formation of an imine group after derivatization
of primary amino groups with TFBA.
Data treatment and peak-fitting procedures were
performed using CasaXPS software. Obtained spectra
were rescaled by shifting the spectra relative to the
aliphatic carbon at 285 eV. Thanks to the derivatization,
a simple determination of the elemental composition in
the film is necessary to know the primary amino
concentration [12] using the following calculations:
[NH 2 ] b / [C] b = [F] a / (3*[C] a – 8*[F] a )
(1)
[NH 2 ] b / [N] b = [F] a / (3*[N] a )
(2)
Furthermore, the deposited mass and the actual film
thickness was used to calculate the film density of the aC:H:N plasma polymers. In addition, the energetic
conditions during film growth (in steady-state conditions)
have been analysed using the energy density, ε surf ,
available during film growth for the Arrhenius regime
(Fig. 4). This deposited energy gives the amount of
energy available per deposited atom (C and N, in this
case) resulting from the kinetic energy of the incident ions
yielding bond formation.
with [NH 2 ] b , [C] b and [N] b the concentration of amino
groups, carbon and nitrogen before derivatization and
[F] a , [C] a and [N] a after derivatization.
3. Results and discussion
Mass deposition rates of the NH 3 /C 2 H 4 -derived plasma
polymers have been measured for a broad range of the
reaction parameter, W/F m (power input per monomer flow
rate). Considering the effective power absorbed in the
plasma and the expansion of the plasma zone between the
plane parallel electrodes, the external parameters can be
related to the energy invested per molecule in the gas
phase, (W/F m | pl ), which governs the creation of
film-forming species in the gas phase [1]. Using an
Arrhenius-like plot, a possible activation barrier (with
activation energy E a ) for the plasma chemical reaction
pathway can then be examined (Fig. 3).
It appears that the deposition (in steady-state
conditions) can well be fitted by an Arrhenius behaviour
over a broad parameter range around E a as indicated by
the straight line in Fig. 1. For this range a steady plasmachemical reaction pathway can be assumed, for which the
flux of film-forming species is enhanced with increasing
2
Fig. 4. Film density of a-C:H:N plasma polymers versus
energy density during film growth. Data indicated by an
asterisk are the same as indicated in Fig. 1.
With increasing bond formation and abstraction of
hydrogen, an increased film density of the a-C:H:N films
can be found that causes a reduction in functional group
density such as primary amino groups (NH 2 ) [9]. From
these data two deposition conditions (with 20 and 50 W
power input) were selected for further investigations.
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Different treatments were thus considered in order to
deposit thin coatings of a-C:H:N onto Si wafers. To
obtain a thickness of about 5 nm, a treatment time of 40 s
at 50 W and 60 s at 20 W was selected. To obtain
coatings of around 50 nm, 330 s at 50 W and 510 s at
20 W was used. The amount of amino groups was then
quantified by XPS using TFBA. In addition thicker films
of around 100 nm were considered.
Results obtained from the XPS analyses are given in
Table 1. First of all, the thin layer PPFs prepared at 20 W
were not stable enough during the selected derivatization
conditions, and consequently, as it was very difficult to
quantify the amount of amino groups in these films, the
results obtained are not further discussed. Nevertheless,
somewhat higher stability has been observed for the 5 nm
film compared to the thicker one.
Table 1. Quantification by XPS of the amount of primary
amino groups in the a-C:H:N polymer plasma film.
W
nm
%C
%O
%N
%F
%Si
NH2/C
NH2/N
20
100
75.2
6.7
18.1
0
0
-
-
50
50
74.2
9.4
18.1
0
0
-
-
50
100
76.4
6.9
16.7
0
0
-
-
50
100
75.8
7.3
12.7
4.1
0
2.1%
10.8%
50
5
71.4
9.7
15.2
1.7
2.0
0.9%
3.8%
For the 50 W conditions, the N/C ratio is the same
whatever is the thickness of the film, showing that almost
the same amount of nitrogen is introduced in the film.
From the derivatized samples, it becomes apparent that
the first stage of film growth results in a reduced amount
of primary amino groups, i.e., around 1% of [NH 2 ]/[C]
compared to around 2% for steady-state conditions
(100 nm film). In addition, it can be concluded from the
XPS results that the first stage of the film growth yields
stronger cross-linking as 96.2% of nitrogen may be
implied in the cross-linking for the 5 nm film instead of
89.2% for the steady-state conditions.
Furthermore, the XPS analysis on the ultrathin sample
detected a low percentage of silicon from the substrate,
i.e., 2.0%, which agrees well with a film thickness of
≤ 5 nm. Hence, the initial deposition rate might indeed be
reduced. In addition, the ultrathin a-C:H:N film deposited
at 50 W was found to show the highest stability, as the
same NH 2 /C ratio, i.e., 1%, is obtained after one day,
indicating a higher cross-linking degree.
Note that for the thicker coating (100 nm) a different
derivatization procedure has been followed [9]. It was
indeed observed recently that some nitrogen-containing
oligomers can be leached from the surface of the Si wafer
during the derivatization process [8] as this step implies
the formation of water molecules. Thus, further studies
will be carried out in order to optimize this process and
ensure a better quantification of the primary amine groups
in the film.
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4. Conclusions
Investigating the initial film growth conditions for
NH 3 /C 2 H 4 -derived plasma polymers, it can be proven
that the deposited energy during film growth indeed
scales inversely proportional with deposition rate, i.e., the
flux of film-forming species multiplied by their sticking
probability. With otherwise constant conditions in the gas
phase and for the ion bombardment, a reduced sticking
probability in the initial growth phase results in enhanced
ion bombardment for the deposited atoms and thus higher
cross-linking. The latter is accompanied by a higher film
stability, however, at the expense of the functional group
density. These effects have to be taken into account when,
for example, functionalizing the Au surface of SPR
sensors.
5. References
[1] D. Hegemann. J. Phys. D: Appl. Phys., 46, 205204
(2013)
[2] D. Käfer, L. Ruppel and G. Witte. Phys. Rev. B, 75,
086309 (2007)
[3] K. Vasilev, A. Michelmore, P. Martinek, J. Chan,
V. Sah, H.J. Griesser and R.D. Short. Plasma
Process. Polymers, 7, 824 (2010)
[4] Y. Li, B.W. Muir, C.D. Easton, L. Thomsen,
D.R. Nisbet and J.S. Forsythe. Appl. Surf. Sci., 288,
288 (2014)
[5] G. Grundmeier and M. Stratmann. Thin Solid
Films, 352, 119 (1999)
[6] G. Dennler, A. Houdayer, P. Raynaud, I. Seguy,
Y. Segui and M.R. Wertheimer. Plasmas Polymers,
8, 43 (2003)
[7] D. Hegemann, B. Hanselmann, N. Blanchard and
M. Amberg. Contrib. Plasma Phys., 54, 162 (2014)
[8] D. Hegemann.
Thin Solid Films, DOI:
10.1016/j.tsf.2014.11.078 (2014)
[9] D. Hegemann, E. Körner, N. Blanchard, M. Drabik
and S. Guimond. Appl. Phys. Lett., 101, 211603
(2012)
[10] D. Hegemann, B. Hanselmann, S. Guimond,
G. Fortunato, M.N. Giraud and A.G. Guex. Surf.
Coatings Technol., 255, 90 (2014)
[11] J. Trieschmann and D. Hegemann. J. Phys. D:
Appl. Phys., 44, 475201 (2011)
[12] M.F.S. Dubreuil, E.M. Bongaers and P.F.A. Lens.
Surf. Coatings Technol., 206, 1439 (2011)
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