22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma post-treatment of Cu/a-C:H nanocomposite thin films
J. Hanuš, T. Steinhartová, O. Kylián and H. Biederman
Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics,
V Holešovičkách 2, CZ-18000, Prague 8, Czech Republic
Abstract: Cu / a-C:H nanocomposite coatings were prepared by means of Gas
Aggregation Source (GAS) and PECVD. Amount of Cu NPs in the film was controlled by
the DC magnetron current used for Cu NPs production. Increase of Cu surface
concentration was achieved by reactive ion post-etching of the films. Washing in the water
showed good stability of the nanocomposite coatings.
Keywords: nanocomposite films, PECVD, magnetron sputtering
1. Introduction
Hard plasma polymer amorphous hydrocarbon coatings
(a-C:H) are widely used in many applications in
machinery and automotive industry for more than
4 decades. It is mainly due to their valuable tribological
properties and hardness. Moreover, a-C:H films exhibits
also excellent barrier properties [1], they are not toxic and
in general transparent in the visible range of light, which
makes that highly interesting for
food packaging
industry.
Incorporation of metallic nanoparticles (NP) into a-C:H
films enables production of coatings with antibacterial
properties or other interesting functionalities. Such
nanocomposite films are usually prepared by PECVD
combined with physical vapour deposition (PVD) or
magnetron sputtering of metals.
In this case the
nanoparticles are formed inside the film by the
coalescence of the metal atoms [2]. Limitation of this
approach is that it does not enable independent control of
matrix properties and amount and size of NPs. Especially
in case of hard a-C:H films the diffusion of metal atoms is
limited which leads to the restriction of the size of
metallic inclusions.
In the presented work is described an alternative
technique for metal / a-C:H film deposition based on the
deposition of a-C:H matrix by PECVD combined with
simultaneous deposition of Cu nanoparticles produced by
the Haberland type Gas Aggregation Source (GAS) [3].
In addition, possibility to enhance the concentration of the
metal on the surface of the film by plasma post-treatment
of such nanocomposite coatings is investigated in detail.
2. Experimental
Experiments were performed in the set-up depicted on
Fig. 1. Cu nanoparticles were prepared by means of Gas
Aggregation Source of our own construction. The GAS
consists of water cooled aggregation chamber 100 mm in
diameter. It is terminated by the cone with an orifice
2 mm in diameter. DC planar magnetron was used for
sputtering of the copper target. The magnetron was
powered by the DC power supply operated in the constant
current regime. The GAS was connected to the main
P-II-7-7
deposition vacuum chamber in the way that the beam of
the Cu NPs was normal to the substrate. More detail
about this type of the GAS can be found in [4, 5].
The substrates were placed on the planar RF electrode
powered by RF generator Dressler Cesar 133. The
distance between the exit orifice of the GAS and the
electrode was 23 cm. The a-C:H matrix was deposited in
a mixture of Ar and n-hexane 4:1 at the pressure 1.5 Pa
and at the RF power 75 W. Optical properties of the films
were investigated by means of UV-Vis spectrophotometry
in transmission mode (Hitachi, U-2900). Chemical
composition of the coatings was measured by means of
XPS using Al Kα X-ray source (Specs XR-50) and
hemispherical energy analyzer (Specs Phoibos 100).
Fig. 1. Schematic drawing of the experimental set-up: 1.
DC magnetron for NPs production; 2. RF electrode with
substrate holder for a-C:H deposition; 3. monomer inlet;
4. Ar inlet; 5. vacuum gauge; 6. orifice; 7. aggregation
chamber with water cooling. Substrates are placed on the
RF electrode.
3. Results and discussion
The experimental conditions for Cu NPs production
were optimized to reach stable and reproducible
deposition rate with narrow size distribution of the Cu
NPs with the mean size 16 nm. More specific, the
pressure in the GAS was set to 40 Pa, aggregation length
was 5 cm and the magnetron current was varied from
50 mA to 300 mA. In this way it is possible to control the
amount of produced nanoparticles that was found to be
directly connected to the magnetron current. Deposition
1
70
65
60
transmitance [%]
55
50
45
40
50 mA
75 mA
100 mA
150 mA
200 mA
300 mA
35
30
25
20
15
10
350
400
450
500
550
600
650
700
750
800
850
900
surface concentration significantly increased as can be
seen in Fig. 3.
before plasma treatment
N2 plasma treated
20
18
N2 plasma treated, washed in water
16
Cu concetration [at%]
conditions for a-C:H matrix were set as described in the
experimental part. However, it was found that in case of
continuous RF discharge almost no Cu NPs reached the
substrate, which is most likely caused by DC negative
self-bias on the substrate holder that repulses all the
arriving nanoparticles as they get negatively charged in
the RF plasma. Because of this the RF discharge was
operated in a pulsed regime with frequency 1 Hz and duty
cycle 50 %. Deposition rate of the matrix at those
conditions was ~ 8 nm / min.
UV-Vis spectra of nanocomposite films deposited for
10 min are shown in Fig. 2. As can be seen, anomalous
absorption peak with the maxima ~ 590 nm that is
characteristic for Cu NPs was observed in the produced
nanocomposites. [6]. The intensity of this absorption
peak increases as expected with increasing magnetron
current used for Cu NPs deposition that is consistent with
increasing amount of Cu NPs incorporated into the
growing a-C:H matrix. However, XPS analyses of those
coatings showed that the amount of copper at the surface
is much lower than what was expected form the UV-Vis
spectra: it varied from 0.3 % at 50 mA up to 3.2 % at
300 mA. Taking into account that XPS is surface
sensitive method i.e., the information depth of XPS is
only several nanometers this suggests that the NPs are
embedded in the a-C:H matrix. So they are not accessible
to outer environment which might be problem for many
applications where ion release is required.
14
12
10
8
6
4
2
0
50
100
150
200
250
300
magnetron current [mA]
Fig. 3. Cu surface concentration measured by XPS.
Black squares is the Cu concentration measured before
plasma treatment, red dots after N 2 plasma etching and
blue trianglesafter N 2 etching and washing in H 2 O for
5 min in ultrasonic cleaner.
Stability of the coatings in the aqueous environment is
crucial for many applications. Because it was etched
about 16 nm of the matrix which is the mean size of the
Cu NPs some of the NPs are after etching only weakly
attached to the film. To check the stability of the films in
water and also to remove weakly attached NPs the
nanocomposite films were after plasma etching washed
for 5 min in deionized water in ultrasonic cleaner. It was
found that the copper concentration on the surface
decreased. However, it is still significantly higher than
before plasma post-treatment as can be seen in the Fig. 3.
The total composition of the final coatings i.e., after
nitrogen etching and water cleaning is summarized in
Table 1.
Table 1. Chemical composition of the Cu / a-C:H
nanocomposite films after N2 plasma post-treatment and
water cleaning estimated by the XPS.
wavelength [nm]
Fig. 2. UV-Vis spectra of Cu / a-C:H nanocomposite
films in dependence on DC magnetron current. Pressure
in the GAS was 40 Pa, in the main chamber 1.5 Pa, Ar /
n-hexane = 4/1, RF power 75 W and duty cycle 50 % with
frequency 1 Hz.
One possibility how to remove the polymeric matrix
and thus uncover the NPs is to employ reactive ion
etching. Nitrogen was used in our case as an etching gas
instead of oxygen which is typically used because in this
case Cu NPs does not oxidize. The nitrogen pressure was
set to 1.2 Pa. The samples were placed on the RF
electrode and the applied power was 75 W (continuous
mode). Etching rate of the pure matrix was at these
conditions ~ 4 nm/min. After the 4 min etching, the Cu
2
current
[mA]
XPS
C[at%]
O[at%]
N [at%]
Cu[at%]
50
77.0
12.9
7.7
2.4
75
74.9
12.9
8.5
3.7
100
74.7
10.0
9.3
6.0
150
71.2
11.3
10.1
7.5
200
66.0
13.9
9.9
10.2
300
60.1
17.0
10.7
12.2
4. Conclusions
It was shown that Gas Aggregation Source combined
with PECVD can be used for deposition of metal / a-C:H
P-II-7-7
nanocomposite coatings. The amount of nanoparticles
can be adjusted independently on the matrix properties by
the changes of the operational parameters of the GAS for
instance magnetron current as shown in this study.
Surface concentration of the metal can be significantly
increased by the reactive ion etching. Some of the
nanoparticles are after etching only weakly attached to the
surface. However, most of the NPs remained on the
surface even after sonication of the film in water.
5. Acknowledgements
This research has been supported by the Czech Science
Foundation through the Project 13-09853S.
6. References
[1] O. Polonskyi, O. Kylian, M. Petr, A. Choukourov,
J. Hanus and H. Biederman. Thin Solid Films, 540,
65 (2013)
[2] H. Biederman. Plasma Polymer Films. (London:
Imperial College Press) (2004)
[3] H. Haberland, M. Karrais, M. Mall and Y. Thurner.
J. Vac. Sci. Technol. A, 10, 3266 (1992)
[4] M. Drábik, A. Choukourov, A. Artemenko,
J. Kousal, O. Polonskyi, P. Solař, O. Kylián,
J.Matoušek, J. Pešička, I. Matolínová, D. Slavínská
and H. Biederman. Plasma Process. Polym., 8, 640
(2011)
[5] M. Drabik, A. Choukourov, A. Artemenko,
O. Polonskyi, O. Kylián, J. Kousal, L.Nichtova,
V. Cimrova, D. Slavinska and H. Biederman.
J. Phys. Chem. C, 115, 20937 (2011)
[6] O. Kylian, J. Kratochvil, J. Hanus, O. Polonskyi,
P. Solar and H. Biederman. Thin Solid Films, 550,
46 (2014)
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