22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Time-resolved optical emission spectroscopy of dusty low-pressure RF plasmas
with pulsed injection of hexamethyldisiloxane
V. Roy Garofano1, L. Stafford1, B. Despax2, R. Clergereaux2 and K. Makasheva2
1
2
Département de physique, Université de Montréal, Montréal, Québec H3C 3J7, Canada
LAPLACE (Laboratoire Plasma et Conversion d’Energie), Université de Toulouse, CNRS, UPS, INPT, 118 route de
Narbonne, F-31062 Toulouse 09, France
Abstract: This work focuses on the low-frequency cyclic evolution of the electron energy
and electron density caused by repetitive formation and loss dynamics of dust inside argon
RF plasmas with pulse injection of hexamethyldisiloxane (HMDSO). We observe a rise in
two temperatures characterizing different parts of the EEDF when dust occupation
increases.
Keywords: Dusty plasmas, cyclic evolution, electron temperature and density.
1. Introduction
Historically, the presence of nanoparticles in reactive
plasmas was undesired since they would contaminate the
coating during PECVD. Active research was therefore
carried out to eliminate these dust particles, but during the
process many applications were found for such powderscontaining plasmas. The physics driving dusty plasmas
based on silane, methane, and acetylene precursors is
rather well-documented [1]. In those cases, the growth of
powders occurs through successive steps of gas phase
nucleation induced by the formation of negative ions,
followed by a more rapid growth by coagulation of
clusters, and then continued growth by surface deposition
of neutral dissociation fragments with associated buildup
of negative charges. These phenomena induce a complex
dynamic of formation and disappearance of powders,
which strongly depends on the geometry of the discharge,
and that leads to oscillations of the plasma parameters.
In this work, we investigate the time evolution of the
plasma parameters in dusty plasmas based on pulse
injection of hexamethyldisiloxane (HMDSO). As shown
below, such pulse injection of the precursors allows for
cyclic formation and loss of nanoparticles and thus for
detailed, fundamental studies on the role of these
nanoparticles on the plasma characteristics.
2. Experimental set-up
The plasma investigated in this work is produced
between a smaller, RF driven, silver target and a bigger
substrate holder. This reactor was designed for the growth
of nanocomposite thin films based on silver nanoparticles
embedded into an organosilicon matrix [2]. As described
in Fig. 1, both electrodes are on the same axis and are
separated by 3.5 cm. Using an injection ring situated
around the target, we were also able to introduce a small
amount of hexamethyldisiloxane (HMDSO).
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Fig. 1. (top) Side view of the experimental setup.
(bottom) Top view showing how scattering measurements
were made using an He-Hg lamp.
In this study, HMDSO injection was pulsed with
injection time t on and period T (T = t on + t off ) fixed to 5 s.
The presence of nanoparticles was analyzed by measuring
the intensity of scattered light coming from an Hg lamp,
as shown in Fig. 2. Effects of precursor injection time
(t on ) on the dust formation have shown faster formation of
dusts for longer injection times and shorter period of the
formation/disappearance dust cycles when t on increases
[see, ref. 3]. From Fig. 1 it is clear that at least two cyclic
evolutions of different time scales occur. This works
focuses only on the low-frequency (about 200s) cyclic
formation/disappearance of the dust population inside the
RF discharge.
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Fig. 2. Time evolution of the injection of the HMDSO
precursor (dash-dot) for a duty cycle of 0.56 (t on = 2.8 s)
and the intensity of the Hg line at 546 nm (solid line).
3. Experimental results and discussion
Optical emission spectra were recorded in the dust
cloud. A first wavelength interval containing close Ar
emission lines around 600 nm emanating from Ar excited
states close to the ionization level was used to construct a
Boltzmann diagram. An excitation temperature could then
be calculated assuming an excitation/de-excitation
equilibrium between these Ar excited states. By
introducing a trace of rare gases (20% Xe, 20% Kr, 20%
Ar, and 40% Ne) in the nominally pure Ar plasma, tracerare gases optical emission spectroscopy measurements
could be performed [4]. The amount of this rare gases
mixture was sufficiently small to ensure minimal
perturbation of the plasma characteristics (less than 5%).
In this context, a second set of lines coming from the 2p x
(in Paschen's notation) levels of Ar, Kr and Xe in the 750900 nm wavelength range were recorded (Ne emission at
585.2 nm could not be observed over the range of
conditions investigated). The measured emission
intensities of Ar, Kr, and Xe were then compared to the
predictions of a collisional-radiate model (see, ref. [4] for
details), with the electron temperature (assuming
Maxwellian electron energy distribution function
(EEDF)) as the only adjustable parameter.
Fig. 3 shows the results for both temperatures,
superposed to an emission line coming from Ar 603.2 nm
and Ar 750.4 nm. It clearly appears that both temperatures
follow a similar cyclic behaviour than those of the
emission lines, with a period of around 175s. We
observed a rise in both temperatures when dust
occupation increases, followed by a decrease when the
dust is lost. While nanoparticle formation and lost
dynamics are not yet understood for HMDSO-containing
plasmas, such behaviors of the electron energy and
electron density are in good agreement with our
knowledge of well-documented dusty plasmas, in which
the formation of negative ions followed by electron
attachment on nanoparticle's surfaces are critical
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phenomena driving dust particles growth. We have to
note that the quite low values obtained for the
temperatures do not reflect the mean temperature of all
electrons in the plasma. It can be expected that the
excitation temperature determined from high-energy Ar
states characterizes the low-energy part of the EEDF,
while the electron temperature obtained from the TRGOES analysis characterizes the tail of the EEDF. Indeed,
over the range of experimental conditions studied, all 2p x
states of Ar, Kr, and Xe giving rise to the emission lines
between 750 and 900 nm were found to be mostly
populated by electron-impact excitation on ground state
Ar, Kr, and Xe atoms. Therefore, only electrons with
energy above 9 eV can contribute to the measured
emission lines, and thus to the electron temperature
deduced from such TRG-OES measurements. Our results
point out that both the low-energy portion and the highenergy portions of the EEDF become modified in
presence of nanoparticles.
Fig. 3. Low-frequency cyclic evolution of (a) the argon
line at 603.2 nm and the excitation temperature and (b)
the argon line at 750.4 nm and the electron temperature
Electron density was also estimated using the Arrhenius
formula. As shown in Fig. 3, the opposite trend was
observed for the electron density, compared to both
electron temperatures. The former decreases by two
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orders of magnitude when dusts are at their maximum
occupation of the discharge, which is coherent with the
idea that powders behave as sinks for the electrons and
ions.
Fig. 4. Low-frequency cyclic evolution of the electron
temperature and density
4. Conclusion
Although the growth mechanisms of HMDSO-based
dusty plasmas remain uncertain, our present observations
allow us to believe they very might be similar to those of
well understood cases. Indeed, the decrease in two
temperatures corresponding to the low and high energy
parts of the EEDF, simultaneous to the increase in
electron density, when dusts occupy the discharge is
consistent with the idea that nanoparticles, when
embedded inside the plasma, trap electrons and ions in
large numbers.
5. References
[1] Y. Watanabe, J. Phys. D. Appl. Phys. 39, R329 (2006)
[2] B. Despax and P. Raynaud, Plasma Process. Polym. 4,
127 (2007)
[3] B. Despax, K. Makasheva, and H. Caquineau, J. Appl.
Phys. 112, 093302 (2012)
[4] V.M. Donnelly, J. Phys. D. Appl. Phys. 37, R217
(2004)
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