22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Production of SiO x (CH) nanoparticles with tuneable properties using gas
aggregation cluster source
A. Shelemin, A. Shukurov, O. Kylián, J. Kousal, D. Slavínská and H. Biederman
Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics,
V Holesovickach 2, 18000, Prague, Czech Republic
Abstract: PECVD of HMDSO was performed using gas aggregation source to fabricate
SiO x (CH) nano-particles of tuneable size and chemical composition. The influence of the
discharge power, pressure and oxygen admixture on the morphology and chemical structure
of the particles was investigated. Appropriate conditions were found to produce the
particles ranging from plasma polymeric to nearly silicon dioxide.
Keywords: gas aggregation cluster source, HMDSO, nanoparticles.
1. Introduction
A number of techniques have been developed for
production of various nano-particles (NPs) [1-3]. Among
them, gas aggregation cluster sources (GAS) have been
found to be very effective and versatile tool for NPs
fabrication[4].
These sources utilize the effect of
condensation of super-saturated vapors of a material in a
cool inert gas with subsequent dragging of thus-formed
nano-particles by a flow of the buffer gas through an
orifice to a high vacuum deposition chamber. GAS
systems differ in a manner in which the material is
atomized (evaporation, sputtering) as well as in the
number of differential pumping stages and options for
mass/charge separation. Nevertheless, it has been already
shown that even basic configurations of GAS can be
successfully used for production of various metal/metal
oxide and plasma polymer nanoparticles [5-10].
Hexamethyldisiloxane (HMDSO) has been intensively
studied as a precursor for PECVD of thin films.
However, the number of publications devoted to the
formation of NPs from HMDSO is limited [11-15]. This
study is therefore related to the detailed investigation of
the fabrication of HMDSO NPs by means of gas
aggregation cluster source. Special attention was paid to
the issue of how external parameters of deposition
(power, pressure, gas composition) influence the chemical
composition and the size distribution of the NPs.
2. Experimental
The scheme of the experimental setup is shown in
Fig. 1. The gas aggregation cluster source consisted of a
stainless steel cylinder chamber ended by a 3 mm long
nozzle of 1.5 mm in diameter. The walls of the
aggregation chamber were cooled by water. The GAS
was connected to the main deposition chamber which was
pumped down by diffusion and rotary pumps. The RF
(13.56 MHz), water cooled, planar magnetron equipped
with a 1 mm thick graphite target was used as a power
source for PECVD. The distance between the target and
the exit orifice was 100 mm. Various Ar/HMDSO/O 2
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mixtures fed into the gas aggregation chamber were used
as a working gas.
Fig. 1. The scheme of the experimental setup.
The fraction of each component was adjusted by
changing the flow rate of the corresponding gas, except
for Ar which flow was kept constant at 2 sccm. All
experiments were performed at inside GAS pressure in
the range of 44 - 120 Pa and power in the range of
20 - 80 W. The deposition rate of NPs was monitored
using quartz crystal microbalance positioned in the main
deposition chamber at the distance of 25 cm from the exit
orifice of GAS. The morphology of produced NPs was
studied by scanning electron microscopy (Tescan Mira 3).
The chemical composition was analysed by means of XPS
(Phobios 100, Specs) and FTIR (Bruker Equinox 55). For
XPS measurements, NPs were deposited onto PTFE foils.
In the case of FTIR, the silicon wafer overcoated by thin
film of gold was used as a substrate.
3. Results and discussions
3.1. Ar/HMDSO mixture
The first step was to find optimal experimental
conditions. It was observed that relatively small pressure
(44 Pa) and power (30W) were able provide the stable and
reproducible deposition of NPs from the Ar/HMDSO
(10:1) mixture.
In the following experiments, the
influence of power and pressure on the morphology of the
1
NPs was explored. It was found that the increase of power
from 30 W to 80 W led to the decrease of the mean NP
size from 190 nm to 76 nm (Fig. 2). More complex
situation was observed with different pressures in the
aggregation chamber. The elevation of the pressure from
44 to 92 Pa resulted in the increase of the mean NP size
from 190 to 296 nm. Further raising the pressure to
120 Pa subsequently caused the decrease of the size to
234 nm.
Fig. 3. SEM images of nanoparticles prepared at different
O 2 /HMDSO ratios: a) 1:1; b) 2.5:1; c) 3.5:1; d) 4.5:1.
Fig. 2. The SEM image and distribution of sizes of
clusters fabricated at power of the discharge: a) 30 W; b)
80 W. Other experimental parameters were kept constant.
3.2 Ar/HMDSO/O 2 mixture
The technology of the fabrication of SiO x thin films
from HMDSO is very well known [16]. It is based on the
addition of oxygen into the plasma during the formation
of the coatings. The similar approach was adopted in this
study for the preparation of SiO x NPs. The power and the
composition of the Ar/HMDSO mixture were kept
constant whereas the percentage of oxygen was gradually
increased. Addition of O 2 from 0.1 to 0.5 sccm was
found to significantly reduce the deposition rate from 22
to 9 nm/min. Deposition time was adjusted in order to
achieve comparable NPs surface density (coverage) for all
samples.
The evolution of morphology of the NPs fabricated
with advancing addition of oxygen is depicted in Fig. 3.
The films deposited with the smallest concentration of O 2
consisted of two groups of the NPs with the mean size of
180 nm and 35 nm, respectively. The 2.5-fold increase of
the O 2 /HMDSO ratio led to the reduction of the size of
both generations of the NPs (Fig. 3b). On the other hand,
the coatings fabricated at higher concentrations of oxygen
consisted of a single group of the NPs. Their mean size
increased from 149 to 179 nm with increasing of the
O 2 /HMDSO ratio from 3.5:1 to 4.5:1.
2
The above described changes in the size distribution of
produced NPs are accompanied with the characteristic
alteration of the chemical composition of the NPs.
Fig. 4 shows the elemental content obtained from the
XPS measurements. For the Ar/HMDSO mixture without
oxygen, the NPs are composed of SiO x CH plasma
polymer with high contribution from the organic phase.
The addition of O 2 leads to the gradual growth of the
silicon and oxygen content, while the concentration of
carbon rapidly decreases.
Fig. 4. Chemical composition of NPs in dependence on
the O 2 /HMDSO ratio as determined by XPS.
At the highest O 2 /HMDSO ratio, the composition of the
NPs can be described by the formula SiO 1.8 with very
small contamination from carbon. The analysis of the
high resolution Si 2p spectra showed that its position
shifted from 102.0 eV (without O 2 ) to 103.7 eV
(O 2 /HMDSO – 4.5:1). This shift supports the findings of
the elemental composition and confirms the chemical
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transformation of the NPs from the organic to
predominantly inorganic state.
These observations were further confirmed by FT-IR
analysis of deposited samples presented in Fig. 5. For the
samples prepared without O 2 , the most intense absorption
bands were observed for the Si-C (800 cm-1, 842 cm-1),
the Si-O (1000 - 1100 cm-1) and the CH 3 -CH x groups
(1408 cm-1, 2900 - 2960 cm-1). The addition of O 2
resulted in the significant attenuation and even
disappearance of the signal from the organic components,
and provided the spectrum consisting almost exclusively
of the Si-O absorption band.
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Fig. 5. The FTIR spectra of NPs deposited from various
O 2 /HMDSO ratios.
4. Conclusions
It was shown that the gas aggregation source can be
used for the production of nanoparticles from HMDSO.
The morphology of the NPs deposited in the Ar/HMDSO
mixture is strongly dependent on the deposition
parameters. The mean size of NPs was found to increase
with decreasing power and increasing pressure in the
aggregation chamber. Introduction of oxygen into the
working gas mixture dramatically changed the chemical
composition of the NPs: with increased fraction of oxygen
the inorganic character of produced NPs increased. In
addition, at high concentration of O 2 NPs possessed
chemical composition close to SiO 2 .
5. Acknowledgments
This research has been supported by the Czech Science
Foundation through the Project 13-09853S.
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