22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Complementary study of gas dynamics simulation and deposition in a dielectric
barrier discharge
P. Jelínek1, A. Obrusník1, A. Manahkov2 and L. Zajíčková1,2
1
Department of Physical Electronics, Masaryk University, Brno, Czech Republic
Plasma Technologies at CEITEC, Masaryk University, Brno, Czech Republic
2
Abstract: This contribution presents a complementary numerical and experimental study
of a dielectric barrier discharge operating at the atmospheric pressure. With the help of
numerical simulations, the gas flow in the discharge chamber is studied for multiple inlet
geometries. Subsequent deposition experiments confirm that more uniform gas flow
improves the homogeneity of the deposited layer notably.
Keywords:
dynamics
dielectric barrier discharge, deposition amine-rich films, simulation, gas
1. Introduction
The atmospheric pressure dielectric barrier discharges
(DBD) are sources of non-equilibrium plasma in which
at least one dielectric layer is placed between
the electrodes [1]. Dielectric barrier discharges were
originally used mainly for ozone production [2] because
their tendency to form filaments did not permit
applications in the field of material science.
However, in the past two decades, numerous
experimental works dealing with the filamentary nature of
the plasma (inspired by original work of Okazaki
et al. [3]) were published and it was shown that it is
possible to adjust the experimental conditions so that
the filament formation is suppressed and the discharge
switches from filamentary regime to diffuse regime (also
called homogeneous regime or atmospheric pressure glow
discharge (APGD)) [4, 5].
Rapid development in understanding this type
of discharge, especially the transition between its regimes
[6] in the past years has lead to a rapid increase in the
number of applications. It is possible to obtain
homogenous DBD in many gases or gas mixtures. Apart
from discharges in He, Ne or N 2 , it is possible to achieve
this regime in a mixture of argon and acetylene.
Nowadays, DBD is no longer used only for ozone
production but also for deposition of thin films [7] and
for plasma processing of soft materials. Although it is
often not possible to reach or sustain the conditions
required for the truly homogeneous regime, it is usually
possible to find conditions in which the discharge
is macroscopically homogeneous [8] which is sufficient
for most applications. For the deposition of thin films
using DBD, it is usually very important to achieve not
only discharge homogeneity but also the uniformity of
the precursor gas delivery to the deposition region. If the
gas velocity shows substantial spatial variation
the precursor consumption rate remains constant, it is
difficult to adjust other experimental parameters in such a
way that the resulting films are homogenous.
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The aim of this work was to redesign the geometry
of the gas inlet to a DBD reactor in order to achieve more
uniform gas velocity above the substrate. The optimised
inlet should improve the homogeneity of deposited layers.
To that end, we utilized numerical simulations of the gas
dynamics. Results of these numerical simulations were
directly applied in designing a novel gas inlet for the
deposition reactor.
The model is implemented using the COMSOL
Multiphysics platform, in particular its Computational
Fluid Dynamics module [9]. The differential equations are
discretized using the finite element method and solved
in full three-dimensional geometry. Four different
geometries were analysed and compared in the numerical
study and the most promising geometry was tested
afterwards in real deposition experiments where maleic
anhydride and acetylene (C 2 H 2 ) were co-polymerized
in macroscopically homogeneous DBD plasma in argon
atmosphere.
2. Methods
The original gas supply geometry consisted of an inlet
tube and a buffer chamber with a narrow exit slit (fig. 1)
and it was designed purely empirically. The idea was
to make the gas velocity uniform in the direction parallel
to the slit (y-direction) while the uniformity in the xdirection is achieved by periodic movement of the gas
supply.
1
We used the numerical model for a parametric study
with respect to the flow rate. The flow rate of argon was
changed from 100 sccm to 2000 sccm and the uniformity
of the gas flow under the slit was observed in order to
achieve best results in the given configuration.
Fig. 1 Original geometry of the gas supply.
Although the original gas supply shown in figure 1
provided much faster deposition rates and quality
compared to a pipe-like gas supply, the thickness of the
layer varied strongly along the y axis. This was caused by
the fact that the precursor gas was not evenly distributed.
The gas velocity varied a lot in the active plasma region
which resulted in a strong variation in the residence time
of the precursor and, therefore, inhomogeneous
deposition.
To further improve the gas supply, qualitative
considerations were no longer sufficient and it was
necessary to employ a numerical model. The model
describes the gas flow in full-three dimensional geometry
since we do not want to limit its applicability to axiallysymmetrical designs. In some of the geometries,
especially the ones involving narrow slits, the Reynolds
number can be very high (up to 15 000) and, therefore, we
have to take turbulence into account in our simulation.
This is achieved by solving the so-called ReynoldsAveraged
Navier-Stokes
(RANS)
equations
complemented by the k-ε turbulence model [10].
This turbulence model is capable of describing only the
time-averaged effect of the turbulence on the gas flow,
because it is not dealing with individual eddies and
vortices. Both the RANS equation and the turbulence
model are included in COMSOL's Computational Fluid
Dynamics module [9]. The gas dynamics equations are
solved at the atmospheric pressure and the standard
temperature.
The model is constrained by several boundary
conditions (BCs). At most boundaries, the wall BC is
imposed, setting the velocity to zero. The boundary
condition for gas inlet is set on beginning of the gas
supply pipe. In order to make the inlet BC consistent with
the wall BCs, the velocity constraint is applied in the
integral form.
Finally, at boundaries which are far enough from the
inlet, the Dirichlet boundary condition for pressure is
imposed, setting p = 1 atm. The aforementioned
differential equations were discretised using the finite
element method. Tetrahedral mesh was used for the
discretization, consisting of more than 400 000 elements.
2
3. Results
The numerical model provided multiple applicationrelevant findings. Most importantly, it helped to explain
why the homogeneity issues appeared when using the
original gas supply. It is apparent (see cross section
of simulation results on Figure 2) that there are three
maxima in magnitude of the velocity along the narrow
exit slit. The strongest maximum is in the middle of the
slit and its origin is obvious, because it is just underneath
of inlet tube, but there are also two maxima at ends of this
Fig. 2 Simulated gas flow velocity in the DBD reactor
(with original geometry of the gas inlet) for the Ar flow
rate of 3 slm.
slit which result from very complex re-circulation patterns
in the buffer chamber. This three maxima profile leads to
inhomogeneous layer deposition, because of different
residence time of gas mixture above the substrate. This
complex gas velocity profile flowing through exit slit also
leads to complex flow patterns, such as vortices, directly
above the substrate which can also have strong influence
on the homogeneity of deposited layers.
In order to minimize the complex flow patterns in the
buffer chamber, it was necessary to redesign the geometry
of the gas inlet. The first attempt was a “trapezoid”
geometry shown in Figure 3. In this case it was assumed
that it is possible to reduce the complex flow pattern in
the buffer chamber by enlarging the outlet and by
changing the angle of the walls of the buffer chamber.
The “trapezoid” design is the only one which does not
include a narrow-slit outlet. However, removing the slit
made the impact of recirculation even more pronounced
because the vortices were in direct contact with the
substrate.
The second design which was considered is referred to
as the “inverse trapezoid”. This design also contains the
narrow slit (similar to the original design) but the width of
the buffer chamber decreases gradually from the gas
supply tube towards the slit.
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The best simulation results were obtained for the socalled longslit design which replaces the buffer chamber
with a long narrow rectangular duct. This design leads to
very uniform velocity profiles but it also has to be pointed
out that the characteristic velocity magnitude at a given
flow rate is approx. 4 times lower compared to the
original one.
In order to compare the four different geometries
quantitatively, the gas velocity parallel to the substrate
was evaluated. Figure 4 shows the x component of the
velocity at the distance of 3 mm from the exit slit, 0.5 mm
above the substrate. The aim is to achieve constant
velocity along the exit slit.
It is apparent from figure 4 that the original design was
good in comparison with trapezoid or inverse trapezoid
designs. In the case of the trapezoid design, negative
parallel velocity is observed which is a result of backflow
and is very unsuitable for the depositions.
Clearly the best velocity profile is achieved for the
longslit design. In this case, the parallel velocity along the
exit slit is nearly constant which should ensure uniform
layer thickness in the y direction. Therefore, the longslit
design was tested experimentally. The depositions were
carried out at the atmospheric pressure in a DBD reactor
powered by a high voltage (6.6 kHz) supplied with a
tunable generator providing 12 W power input. Copolymered layers were deposited in nearly homogeneous
mode of DBD from the mixture of Maleic anhydride and
C 2 H 2 diluted in argon.
The depositions were relatively large-scale (3-inch
silicon wafers) and the spatial uniformity of the resulting
thin film in the y direction is very good. In future, these
large-scale depositions will be used in combination with
the numerical model for estimating the consumption rate
of the precursor.
Fig. 3. Trapezoid geometry of the gas inlet.
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Fig. 4. Comparing the uniformity of the gas velocity
above the substrate for the four different geometries and
the flow rate of 3 slm. The exit slit extends from y = 3 cm
to y = 7 cm
4. References
[1] M. Ondrášková, J. Ráheľ, A. Zahoranová, R. Tino and
M. Černák, Plasma Chem. Plasma P. 28 203-211 (2008)
[2] U. Kogelschatz, B. Eliasson and W. Egli, J. Phys. IV
07 (2007) doi: 10.1051/jp4:1997405
[3] S. Okazaki, M. Kogoma, H. Uchimaya, in Proc. of the
3rd Int. Symp. High Pressure Low Temperature Plasma
Chemistry (Hakone III), (1991), p. 101
[4] D. Trunec, A. Brablec and J., J. Phys. D: Appl. Phys. 34,
1697 (2001)
[5] N. Gherardi, G. Gouda, E. Gat, A. Ricard and F.
Massines, Plasma Sources Sci. Technol. 9 340 (2000)
[6] R Brandenburg et al 2009 J. Phys. D: Appl.
Phys. 42 085208 doi:10.1088/0022-3727/42/8/085208
[7] D. Trunec, L. Zajíčková, V. Buršíková et al. J. Phys.
D: Appl. Phys. 43 (2010)
8] J. Ráheľ, M.Šíra, P. Sťahel, D. Trunec Contributions
to Plasma Physics, 47 (1-2), pp. 34-39. (2007)
[9] CFD Module User’s Guide, COMSOL AB,
Stockholm, 2011. Electronic document
[10] L. Ignat, D. Pelletier and F. Ilinca, Comput. Methods
Appl. Mech, Engrg. 189 1119-1139 (2000)
4. Acknowledgements
This work was supported by the project CEITEC Central
European
Institute
of
Technology
(CZ.1.05/1.1.00/02.0068) from European Regional
Development Fund. K.P.A and S.P. would like to thank
the U.S. Department of Energy Office of Science, Office
of Basic Energy Sciences under Award Number DEFC02-04ER15533
Adam Obrusník is a Brno Ph.D. Talent Scholarship
holder – funded by the Brno City Municipality.
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