22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Nano-granular titanium films overcoated-impregnated with C:H plasma
polymer
A. Shukurov, I. Melnichuk, A. Shelemin, P. Solař, J. Hanuš, D. Slavínská and H. Biederman
Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics, Prague,
Czech Republic
Abstract: Nano-granular films consisting of 60 nm Ti nano-particles were fabricated by a
magnetron-based gas aggregation cluster source. The films were overcoated by soft C:H
plasma polymer which was found to impregnate the voids between the particles during the
early stages of the deposition. After filling the entire free space, plasma polymer
contributed to the increase of the film thickness approaching the self-affine growth.
Keywords: Ti nano-particles, C:H plasma polymer, nanocomposite, self-affine surfaces
1. Introduction
Nanoparticles (NPs) are of extreme importance in
various fields of science and technology. Plasma-based
methods including those using Gas Aggregation Sources
(GAS) have proven to be very effective for fabrication of
NPs and for their deposition onto solid supports [1, 2]. In
many cases however, NPs arrive onto the surface in a socalled soft-landing regime. This results in poor adhesion
of NPs to the surface which is then given predominantly
by weak van der Waals interactions. Loose anchoring of
NPs may deteriorate the functionality of nano-coatings
and a number of possible solutions to this problem were
suggested involving combining the deposition of NPs
together with the deposition of embedding matrices of
plasma polymers [3]. The deposition can be performed
simultaneously (which is technically more complicated)
or sequentially when the film of NPs produced in the first
step is overcoated by the film of plasma polymer in the
second step. In the case of such sequential deposition, the
question arises as to what extent the plasma polymer
penetrates the underlying layers of NPs and what is the
mechanism of such growth. This question is a particular
instance of a more generic issue of polymerization within
disordered and porous medium, a problem which is
presently far from complete understanding by scientists.
2. Experimental
In the first step, beams of Ti NPs were produced by a
gas aggregation particle source (GAS) which utilized DC
magnetron sputtering of a titanium target (Fig. 1). The
particles with average size of 60 nm were fabricated
under the Ar pressure of 28 Pa, flow rate of 1.6 sccm and
magnetron current of 300 mA. The beams were directed
onto silicon substrates and a series of 12 min depositions
was performed to produce a set of identical granular films
consisting of randomly packed Ti NPs.
In the second step, auxiliary RF plasma was ignited in
plan-parallel configuration such that the Ti NPs samples
and reference flat silicon substrates rested on the
grounded substrate holder opposing the live 13.56 MHz
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electrode with a graphite target. An Ar/n-hexane (1:1)
mixture under the total pressure of 3 Pa, the flow rate of
11.5 sccm and the power of 100 W were used. The
deposition time of hydrocarbon plasma polymer (ppC:H)
varied from 27 sec to 18 min.
Fig. 1. The scheme of the experimental arrangement.
Step 1: deposition of Ti NPs using the GAS; step 2:
overcoating of Ti NPs by thin films of C:H plasma
polymer.
After the depositions, the samples were analysed by
AFM (Ntegra Prima, NT-MDT, semi-contact mode) and
SEM (Tescan Mira III, no additional conductive overlayer
1
was applied). Reference samples of ppC:H on flat silicon
were analysed by ellipsometry (Woollam M-2000 DI),
nanoindentation (Hysitron TS-75 combined with the
Ntegra Prima AFM, NT-MDT) and RBS/ERDA (a Van
de Graaf accelerator with alpha-particles at a kinetic
energy of 2.68 MeV) to determine their thickness,
Young’s modulus and chemical composition.
3. Results and Discussion
The deposition of ppC:H proceeded with about
10 nm/min rate as it was determined by ellipsometry for
the films on flat Si substrates. The films consisted of
56% of C and 44% of H, and their Young’s modulus was
E = 5 GPa. Thus, these films can be attributed to soft
plasma hydrocarbon polymers as opposed to hard
amorphous hydrogenated carbon (a-C:H, typical
hydrogen concentration H < 30%, typical E > 300 GPa
[4]).
When deposited over the films of Ti NPs, ppC:H
induces non-trivial changes in thickness. Fig. 2 shows the
representative SEM images taken on the cross-sections of
the Ti NPs films overcoated with ppC:H for different
periods of time.
involve penetration of radicals from plasma into the inner
pores and growth of the plasma polymer within the nanogranular Ti film. The impregnation proceeds without
noticeable increase of the film thickness until the free
volume is filled, at least to the largest extent. After that,
the C:H plasma polymer starts to grow on top of thus
formed nanocomposite coating increasing the overall
thickness.
Further information can be obtained from the statistical
analysis of the AFM data. Fig. 3 shows the AFM images
of the ppC:H films deposited over the Ti NPs for different
periods of time analogous to those shown in the SEM
images of Fig. 2.
Fig. 2. The SEM cross-section images of Ti NPs
overcoated with ppC:H of different thickness: a) 12 nm
(27 s time), b) 72 nm (270 s), and c) 190 nm (1080 s).
Apparently, the thickness does not change substantially
during the initial stages of the deposition up to at least
270 s time. Taking into account the diameter of Ti NPs
(60 nm), their surface density (1730 µm-2), and assuming
their packing density to be between random loose packing
(55%) and random close packing (64%) [5] one can
calculate the volume of voids per unit surface area
(72x106 nm3µm-2) and the effective thickness of ppC:H
required to fill this volume on the flat substrate (72 nm).
The thickness of 72 nm ppC:H corresponds to the
deposition time of 270 s which is in good agreement with
the SEM results. Hence, the early stages of the deposition
2
Fig. 3. The AFM top-view images of Ti NPs overcoated
with ppC:H of different thickness: a) 12 nm (27 s time),
b) 72 nm (270 s), and c) 190 nm (1080 s).
Obviously, the morphology of the surface continuously
changes by increasing the size of the particles being
enveloped with the ppC:H. No threshold between the
early and the late stages of the deposition can be observed
visually. However, the temporal dependence of the RMS
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roughness of the surface, w, shown in Fig. 4 demonstrates
that in the early growth, the roughness does not change
and stays at the value of the uncoated Ti NP film (24.2
nm). This regime is characterized by the growth exponent
β = 0 derived from the dependence w ∼ tβ, and it is
consistent with continuous filling of inner voids of nanogranular Ti with ppCH. After reaching about the
complete filling (deposition time of 270 s), the ppC:H
film proceeds growing in normal direction which is
accompanied with a slight decrease of roughness
(β = -0.16 ± 0.03). The negative value of β indicates that
the plasma polymer tends to fill the valleys between the
surface topographical features thus leading to overall
smoothening of the surface roughness.
120
26
β=0
110
RMS roughness, nm
90
β = -0.16
80
22
70
20
60
1/z = 0.16
50
correlation length, nm
100
24
lateral direction, and the negative sign stresses the trend
of smoothening of RMS roughness with time in this late
growth regime.
The HH correlation functions shown in Fig. 5a allow
for the calculation of the roughness exponent by taking a
linear regression in the small shift distance, r, region in
accordance with the relationship HH(r)∼r2αloc. Here, the
subscript of α loc stands for ‘local’ and it emphasizes the
fact that the roughness exponent is calculated for the
length scales much smaller than the correlation length.
The values of α loc for different deposition time are shown
in Fig. 5b. Obviously, local roughness exponents are not
constant in the early growth but they continuously
increase starting from α loc = 0.78. It is also worth noting
that α loc ≠ α (here, α bears the meaning of the global
roughness exponent), which means that the surface in this
transient regime exhibit the multi-affine character and
different length scales of the surface scale differently. At
longer depositions, the values of α loc approach and
become equal to 1.0. Equality α loc = |α| reveals that all
length scales at the surface are equivalent in terms of
scaling; i. e. the surface becomes self-affine.
18
RMS roughness characterizes only vertical fluctuations
of the surface and does not account for topographical
changes in lateral direction. To make the surface
characterization more detailed, height-height (HH)
correlation functions were plotted and the correlation
length, ξ, was determined as the value of the shift distance
at which the HH functions decrease by 1/e of their plateau
value (Fig. 5a). The temporal variation of the surface
correlation length is characterized by the dynamic
exponent, z, so that ξ∼ t1/z. Fig. 4 demonstrates that the
correlation length gradually increases with time from the
values slightly above 50 nm (which is consistent with the
Ti NP size) to approximately 100 nm. The linear fit of
the power law dependence gives the value
1/z = 0.16 ± 0.02. Thus, in contrast to the behaviour of
the RMS roughness, the topographical features broaden in
lateral dimension from the very beginning of the
deposition.
With β and 1/z determined, we are able to calculate the
roughness exponent α = β x z which interrelates the
dynamics of growth at vertical and lateral scales. For
t < 270 s, α = 0 and it reflects the fact that vertical
roughness does not changes in this regime. For t > 270 s,
α = -1.0 ± 0.3. Equality to unity emphasizes the equal
rates of topographical changes in the vertical and in the
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HH correlαtion, nm
Fig. 4. The RMS roughness and the correlation length
calculated from the AFM images of Ti NPs overcoated
with ppCH.
2
1000
time, s
100
27 s
10
270 s
1080 s
1
1
10
100
1000
r, nm
b)
1.0
αloc
100
α)
1000
40
0.9
0.8
0
200
400
600
800
1000
1200
time, s
Fig. 5. The height-height correlation functions (a) and
the local roughness exponents (b) derived thereof for
Ti NPs overcoated with ppC:H.
3
4. Conclusions
Soft hydrocarbon plasma polymer was deposited over
nano-granular films consisting of 60 nm Ti NPs. Two
regimes of the deposition were identified. In the first,
transient regime, the plasma polymer impregnates the
nano-granular films by filling the voids between the
particles. The topographical features expand in lateral
dimension but the RMS roughness does not change which
results in zero growth and roughness exponents. The
local and the global roughness exponents are not equal
evidencing that the surface is spatially multi-affine. In the
second, late growth regime, the inner voids become
almost completely filled and the plasma polymer grows
on top of the films.
The growth proceeds with
preferential filling of the valleys thus leading to
smoothening of the surface. The decrease of roughness is
manifested in negative growth and global roughness
exponents. Equality between the absolute values of the
local and global roughness exponent in this case
evidences about self-affine growth dynamics.
4
5. Acknowledgements
This research has been supported by the Czech Science
Foundation through the Project 13-09853S.
6. References
[1] O. Kylián, A. Choukourov and H. Biederman. Thin
Solid Films, 548, 1 (2013)
[2] M. Drabik, A. Choukourov, A. Artemenko,
O. Polonskyi, O. Kylian, J. Kousal, L. Nichtova,
V. Cimrova, D. Slavinska and H. Biederman.
J. Phys. Chem. C, 115, 20937 (2011)
[3] O. Polonskyi, P. Solař, O. Kylián, M. Drábik,
A. Artemenko, J. Kousal, J. Hanuš, J. Pešička,
I. Matolínová, E. Kolíbalová, D. Slavínská and
H. Biederman. Thin Solid Films, 520, 4155-4162
(2012)
[4] J. Robertson. Mater. Sci. Engng. R Reports, 37, 129
(2002)
[5] P. Wang, C. Song, Y. Jin and H. A. Makse. Phys. A
Stat. Mech. Its Appl., 390, 427 (2011)
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