Construction and characterization of microplasma jet for thin film
deposition on inner surface of tubes.
Ramasamy Pothiraja, Nikita Bibinov and Peter Awakowicz
Institute for Electrical Engineering and Plasma Technology, Ruhr-Universität Bochum, 44801 Bochum, Germany
Abstract: Microplasma jet for the generation of pulsed long filamentary
discharge at atmospheric pressure has been devised in order to modify inner
surface properties of tubes. Long plasma filament is generated inside the quartz
tube along with precursor, in a way that precursor molecules decompose to
produce active particles for polymerization at the close vicinity of tube surface.
By this way, uniform film has been deposited for more than 100 mm length of
the tube. For the optimization of deposition condition, plasma and precursordissociation parameters including reduced electric field, electron velocity
distribution function, electron density and rate constant for the different reactions
at different places along the axis the tube, are determined using optical emission
spectroscopy, micro-photography, current-voltage measurement and numerical
simulations. Chemical kinetics for precursor dissociation and follow up chemical
reaction for film deposition process has been simulated on the basis of
experimentally obtained plasma parameters. Simulated results have been
correlated with nature of the deposited film, which has been characterized using
FTIR-ATR, LSM, SEM, EDX, XPS, XRD, etc.
Keywords: tube-coating, amorphous carbon film, plasma characterization
1. Introduction
Plasma enhanced coating of film on the inner surface
of three-dimensional objects at atmospheric
pressure, and understanding of deposition
mechanism at this condition are some of the
challenging tasks in modern plasma science and
technology. Films with various specific properties
(hardness, density, hydrophilicity, etc.) are required
for various specific applications. Chemically active
species like atomic and molecular radicals, electron,
photons and ions, are the main factors in deciding
these properties. The quality of plasma enhanced
deposited films depends on participation of positive
ions in deposition process. These ions are
accelerated in the electric field, bombard deposited
layer, and cause removal of loosely held constituent
of deposited film. At present, good quality films are
deposited mainly in low pressure conditions for
various applications, because of the availability of
high energetic ions in the substrate region.
Film deposition using vacuum system is
expensive. In addition to this, some objects to be
coated cannot be placed in a vacuum chamber
because of big size or sensitivity of the material. In
this case, high pressure plasma deposition can be
used. However, at atmospheric conditions, because
of high collisions frequently of active species with
atoms and molecules of the working gas, electron
temperature (Te= 1-3 eV) and fractional ionization
(ni/ng) are usually low; hence deposition process
occurs mainly through flux of chemically active
neutral species to substrate. These neutral active
species undergo many chemical reactions including
recombination reaction, polymerization reaction, etc.
during transport to the substrate, which will lead to
the growth of clusters like macromolecules or even
nano-sized particles. At these conditions, deposited
films are rough and friable. Therefore, for deposition
of good quality films at atmospheric pressure plasma
conditions, active plasma with moderate ions energy
in the substrate region is highly desired.
We have been working in the field of plasma
enhanced film coating on complex geometries for
various specific applications [1]. Recently, we have
constructed a plasma source operating at
atmospheric pressure for film deposition on inner
surface of the tube [2]. In this plasma source, film
deposition is carried out with the aid of ion
bombardments. The configuration of our plasma
source, plasma characterization, models used for
plasma characterization, film deposition on inner
surface of tubes, and characterization of deposited
films are discussed shortly in the following section.
Details of the results will be discussed during the
oral presentation.
2. Experiments and models
Experimental setup consists of a driven electrode
placed coaxially inside a quartz tube. The grounded
electrode is placed coaxially on the outer surface of
the tube, and far away from the driven electrode.
The inner surface of the tube between these
electrodes is the region of our interest, where film is
deposited by igniting plasma in this region in a
working gas containing a precursor. In the present
studies, methane is used as a precursor, and argon is
used as a working gas. The details of experimental
setup are published previously [2].
Optical emission spectroscopy (OES),
voltage-current measurements, microphotography
and numerical simulations are used for the
characterization of plasma conditions. Various
surface analytical techniques like, FTIR-ATR, UVVis absorption, SEM, 3D-LSM, XPS and EDX are
used to characterize the deposited film. Chemical
kinetics is simulated on the basis of determined
plasma parameters, and correlated with determined
film properties.
The rotational temperature of diatomic
molecules is considered as the gas temperature, since
the rotational and the translational degrees of
freedom have equal temperatures because of very
fast rotational relaxation at atmospheric pressure.
Since the spectral resolution of our echelle
spectrometer is not high enough to determine the
intensities of the separate rotational lines in the
emission spectrum of nitrogen molecules and CN
radical, the rotational temperature is determined by a
fitting procedure. For this purpose, we calculate the
intensity distribution in the emission of N2(C–B, 0–
0) as well as for CN(B–X, 0–0) for different values
of rotational temperature using the program code
developed for this purpose [3] and LIFBASE [4]. By
comparing the measured emission spectra with the
calculated
spectra
for
various
rotational
temperatures, we determine the actual rotational
temperature of nitrogen molecule and CN radical
with a standard deviation of ±30 K.
The relative intensity of N2(C–B, 0–0) with
respect to the intensity of N2+(B–X, 0–0) is used for
the determination of electron velocity distribution
function (EVDF) by including the contribution of
argon metastables for the excitation of nitrogen
molecules. For this purpose, the relative intensity of
N2(C–B, 0–0) with respect to the intensity of N2+(B–
X, 0–0) is simulated for various EVDFs for our
experimental conditions by numerical solution of the
Boltzmann equation in local approximation and
varied electric field applying the program code
“EEDF” developed by Napartovich et al [5]. Finally,
by comparing the experimentally determined relative
intensities (
) with the simulated
relative intensities (
) for various
EVDFs, the actual EVDF is determined. By
multiplying the normalized EVDF with the known
collisional cross section exc(E) (in cm2) for various
processses, we calculate the rate constants k
(cm3·s−1) for electron impact excitation of various
processes.
The electron density (ne, cm-3) is determined
using the following equation (2) from the measured
absolute intensity of N2(C-B, 0-0) emission (
,
-3 -1
-3
phot·cm ·s ), nitrogen density ([N2], cm ), the
electron impact excitation rate constant for N2(C-B,
0-0) emission (
, cm3·s-1), contribution of
excitation of N2(C) by collision with argon
metastables (
) [2], contributions of the
quenching of N2(C) by argon (
), the plasma
3
volume (Vp, cm ), value of fraction of time in which
plasma is active (tf), and the geometrical conditions
during the measurement of emission spectrum (gf).
N
g
From known densities of electron, argon and
methane as well as the rate constants for various
processes, the most probable reaction among others
under our experimental conditions are calculated. On
this basis, chemical kinetics is simulated.
grounded region during the discharge with very less
intensity of emission compared to the region in
between the electrodes. Investigation of this process
is under progress.
12
5x10
The reduced electric field is higher near the
spike of the powered electrode (about 2000 Td) than
near the grounded electrode (about 1100 Td). This is
because of high electric field near the spike of the
anode. The difference in the reduced electric field
between the regions close to the powered and
grounded electrodes influences EVDF at their
respective regions as shown in figure 1.
EVDF, eV
-3/2
10
-3
The pulsed positive filamentary discharge is ignited
in the mixture of argon, methane and nitrogen gases.
The long filament of plasma generated along the axis
of the tube during this discharge has a diameter of
about 600 µm. The duration of the discharge is about
160 ns. The profile and position of filaments in the
tube are changed with respect to time, but with the
frequency of several Hz. Because of this fact and the
pulse frequency is of 22 kHz, about 2000 filaments
in series have the same profile and position. Gas
temperature in plasma is about 1000 K. Even though
it is high, duration of discharge is short. Hence the
stationary temperature is in the range of 350 to 400
K.
Electron density (cm )
3. Results and discussion
12
4x10
12
3x10
12
2x10
12
1x10
0
35
70
105
140
Distance from spike (mm)
Figure 2. Variation of electron density along the axis of tube.
Film deposition experiments are carried out
in argon and methane gas mixture without nitrogen.
Deposited film is amorphous as indicated by XRD.
Three dimensional laser scanning microscopic
analysis indicates that most part of film has smooth
surface (figure 3).
-2
10
-3
10
-4
10
-5
2010 Td at 5 mm
1100 Td at 135 mm
Figure 3. 3D laser scanning microscopic image of film surface.
0
70
140
210
280
350
E, eV
Figure 1. EVDFs near driven electrode and grounded electrode.
The electron density is almost constant in
most part of the region in between the electrodes
(figure 2). The filament propagates far outside of the
FTIR-ATR analysis shows that film is
composed of hydrogenated sp3 carbon and nonhydrogenated sp2 carbon (figure 4) [6].
Transmittance (%)
with less quantity of hydrogenated sp3 carbon. There
are also some traces of oxygen and nitrogen
elements in the film.
100
Acknowledgement
1375
99
1455
2870
1651
2954
2926
98
3000
2000
Wavenumber (cm-1)
1000
Figure 4. FTIR-ATR spectrum of deposited film.
EDX and XPS analysis show that film is
composed of mostly carbon with some traces of
nitrogen and oxygen elements. The film is
transparent in visible region, and stable against
brushing with water, isopropanol and soap solution.
Since the reduced electric field is high, at
this condition, argon ion formation reaction is more
probable than the argon metastable formation
reaction. Following this, argon ion transfers its
charge to methane since the cross section for this
reaction is large. So formed hydrocarbon ions on
further reaction with methane and after-glow
electrons lead to the formation several radicals
including C, CH, C2, etc. Presence of species like C,
CH and C2 is also observed from the emission
spectrum.
4. Summary
A plasma source has been constructed for film
coating on inner surface of tubes at atmospheric
pressure. Film coating was carried out by igniting
positive leader discharge inside tube between
electrodes in argon methane gas mixture. Plasma is
characterized with the aid of OES, current voltage
measurements, microphotography and numerical
simulations. It shows that electric field is high
between electrodes. It is about 1000-2000 Td.
Electron density between electrodes is in the order of
1012. Film deposited at this condition is
characterized using various surface analytical
techniques. Analysis of surface analytical data gives
the conclusion that the deposited film is amorphous;
composed of mostly non-hydrogenated sp2 carbon
This work is supported by the „Deutsche
Forschungsgemeinschaft‟ (DFG) within the frame of
the research group „FOR11 3 - Physics of
Microplasmas‟. We thank Dr. Kirill Yusenko for
XRD spectra.
References
[1] Deilmann M, Halfmann H, Steves S, Bibinov N
and Awakowicz P 2009 Plasma Process.
Polym. 6 S695
[2] Pothiraja R, Bibinov N and Awakowicz P 2010
J. Phys. D: Appl. Phys. 43 495201
[3] Bibinov N K, Fateev A A and Wiesemann K
2001 J. Phys. D: Appl. Phys. 34 1819
[4] Luque J and Crosley D R 1999 "LIFBASE:
Database and spectral simulation (version
1.5)", SRI International Report MP 99-009
[5] Stefanović I, Bibinov N K, Deryugin A A,
Vinogradov I P, Napartovich A P and
Wiesemann K 2001 Plasma Sources Sci.
Technol. 10 406
[6] Pothiraja R, Shanmugan S, Walawalkar M G,
Nethaji M, Butcher R J and Murugavel R 2008
Eur. J. Inorg. Chem. 1834