22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Design and testing of a new type of DBD flow reactors for combinatorial plasma
treatment and deposition at atmospheric pressure
J. Philipp and C.-P. Klages
Institute of Surface Technology (IOT), Technische Universität Braunschweig, Bienroder Weg 54 E, 38108
Braunschweig, Germany
Abstract: Development und experimental testing of a new kind of dielectric barrier
discharge flow reactors applicable for plasma-assisted chemical vapor deposition as well as
surface treatment processes at atmospheric pressure are described. Using a combination of
two upstream modules, a gradient mixer and a homogenizer, the reactor is fed by a laminar
gas flow with an established concentration gradient transversal to the direction of flow.
This new reactor type is tested using PACVD of hexamethyldisiloxane, tetramethylsilane,
and glycidyl methacrylate, plasma (co)polymerization of unsaturated trans-2-hexenal with
4-methylstyrene and plasma (co)polymerization of glycidyl methacrylate with styrene,
respectively.
Keywords:
combinatorial surface technology, gradient surfaces, plasma-assisted
deposition, plasma treatment, high throughput experimentation
1. Introduction
The present contribution reports on the design and
testing of a new kind of reactors for plasma-assisted
chemical vapor deposition (DBD-PACVD) processes or
surface treatment processes based on dielectric barrier
discharges (DBDs) at atmospheric pressure (AP). This
reactor utilizes a new method to prepare “gradient
surfaces,” i.e., surfaces with controlled gradients of
physicochemical properties such as surface free energy,
chemical composition and functional group densities, or in contact with aqueous media - densities of electric
surface charge. The use of gradients makes it possible to
diminish experimental effort and cost of determining
optimum conditions for surface-technological processes.
Another key benefit is that only a small number of
samples is necessary to investigate the effect of
preparation parameters on several surface properties, for
example. A good overview of the scope of surface-bound
gradients is given in the review article of Genzer, which
focuses on gradient structures from soft materials and also
explains gradient attributes and classifications [1].
In principle, the required gradient can be established by
molecular interdiffusion of different gas or vapor species:
A new method of achieving combinatorial area-selective
modifications of polymer surfaces was published recently
by Hinze and coworkers. The polymers were treated after
the principle of atmospheric pressure “plasma printing”
with novel gas-permeable electrodes. These “plasma
stamps” – highly porous metal fiber fleece mats – provide
an exchange of gaseous species from the gas stream with
individual microdischarges sustained in sub-mm-sized
cavities defined by via-holes within an adjacent dielectric
mask layer. Feeding such an electrode with two different
gases from spatially separate locations, a stable
concentration distribution is set up by interdiffusion
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within the fleece, allowing the generation of spot arrays
with controlled gradients of surface properties [2]. This
principle is, however, limited to small gas velocities and
high residence times in the fleece which are required to
establish a concentration gradient across a few
centimeters by diffusive mixing: The molecular interdiffusion coefficients are too small to allow establishing
gradients over a length of roughly 0.1 m at gas speeds of
0.1 to 1 m/s, which are typically used in DBD-type
PACVD and treatment reactors with lengths and widths in
the order of 0.1 m, because the diffusion length l D within
the residence time τ (l D = (Dτ)1/2) is in the order of a few
mm at the most.
The new PACVD flow reactor for plasma treatment and
surface coating at atmospheric pressure presented here
utilizes a gas flow with a premixed spatial concentration
gradient ∂c i /∂y perpendicular to the flow direction (here
taken as x direction) and to the electric field (z direction)
fed to a channel-like DBD reactor. By combining two
modules – gradient mixer and homogenizer – a linear
concentration gradient can be established and used for
deposition or treatment experiments. The gradient mixer
shown in Figure 1 consists of two halves each providing
manifolds with outlet channels of adjusted cross sections,
allowing a controlled mixing of defined volume flows of
two gases feeding the channel from several equidistant
locations.
The
hereby
generated
step-shaped
concentration profile is then smoothed in a homogenizer
consisting of a packed bed of glass spheres by utilizing
the well-established principle of mechanical mixing in
porous media, which is often described in literature as
dispersion in packed beds [3, 4].
1
with a total gas flow of 3 slm. A defined bypassed part of
this argon flow was bubbled through a flask containing
the liquid precursor of interest.
For some additional experiments the excitation voltage
was set to 4 kV + 0.1 kV and 5 kV + 0.1 kV, respectively,
the treatment time was increased to 300 s and the total
argon flow was varied in a range from 1.5 slm to 6 slm.
Fig. 2. Setup of the DBD reactor (top view), schematics
not to scale
In order to evaluate the potential of this new PACVD
flow reactor, polypropylene (PP) foils – placed within the
plasma zone – were plasma-coated from a dielectric
barrier discharge (DBD) which was fed with the obtained
gas gradient (see Figure 2). The following kinds of
PACVD processes have been investigated so far: (1)
plasma
polymerization
of
hexamethyldisiloxane
(HMDSO) from HMDSO/Ar mixtures and plasma
polymerization of tetramethylsilane (TMS) from TMS/Ar
mixtures, (2) deposition of aldehyde-group-containing
plasma-polymer films from trans-2-hexenal/Ar mixtures,
(3) generation of epoxy-group-containing coatings from
glycidyl methacrylate (GMA)/Ar mixtures and (4) plasma
(co)polymerization of trans-2-hexenal with 4-methylstyrene and glycidyl methacrylate with styrene in Ar,
respectively. Characterization of the plasma treated PP
foils was usually carried out by means of FTIR-ATR
spectroscopy.
2. Deposition Experiments
To provide the necessary power for maintaining the
discharge during film deposition process a high voltage
generator was used in continuous wave (cw) or (for
GMA) pulsed mode with a pulse/pause-ratio of 1/3, 1/9
and 1/19 ms/ms, respectively. An excitation voltage of
3 kV + 0.1 kV (amplitude) with a frequency of 87 kHz
was applied. For all experiments presented here the
treatment time was 60 s. Argon was used as a carrier gas
2
3.1 Plasma Polymerization from HMDSO/Ar
The first experiments with the new reactor were mostly
focused on the effect of HMDSO concentration
c(HMDSO) on thickness profiles and film structures
along the gas flow (x) direction. For concentrations
beyond 130 ppm an almost oil-like film of
poly(dimethylsiloxane)-like material was deposited on the
first millimeters (at 175 ppm) and centimeters (at 215
ppm), respectively, of the coated area. A very interesting
fact is that powder deposition occurred with high rate in a
narrow strip directly behind the discharge zone for
HMDSO concentrations > 35 ppm.
1600
Thickness d / nm
Fig. 1. Picture of the assembled gradient mixer with
schematically shown gas inlets and the specific
conductance ratios per channel.
3. Results and Discussion
To demonstrate here the potential of the new reactor
exemplary the main results from deposition experiments
using plasma polymerization of HMDSO, TMS and GMA
are given in the following. Deposition experiments with
HMDSO and TMS were done at cw mode. For deposition
experiments from GMA/Ar mixtures the generator was
used in pulsed mode. The results from a deposition
process with a pulse/pause-ratio of 1 ms / 9 ms are shown
in Figure 6. Furthermore the excitation voltage was set to
4 kV + 0.1 kV for the experiments with TMS/Ar and
GMA/Ar, the results of which are presented below.
35 ppm
70 ppm
105 ppm
1400
800
130 ppm
175 ppm
215 ppm
600
Discharge zone
400
200
0
0
2
4
6
8
10
Distance from plasma zone edge x / cm
Fig. 3. Total estimated thickness trends in the direction of
flow on PP foil coated with plasma polymerized HMDSO
To evaluate thickness trends in x direction d(x) FTIRATR spectra (diamond, 52°) were measured at different
positions of a rectangular array on the plasma treated
substrate in steps of 0.25, 5, or 10 mm, respectively. The
thickness trends shown in Figure 3 were calculated for
HMDSO concentrations of 35, 70, 105, 130, 175 and
215 ppm, respectively, using a formula based on the
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Thickness d / nm
500
70 ppm
105 ppm
130 ppm
400
300
process using N 2 as process gas is not completely
controlled yet, e.g. the discharge appeared optically
homogeneous only on the first centimeters (∆x ~ 1-2 cm)
of the plasma zone. After this “controlled discharge area”
a very strong filamentary discharge occurred which could
not provide a homogeneous coating of the PP foil. Also
the edge regions around 35 and 215 ppm, respectively,
had a filamentary character. However, first results
displayed in Figure 4 already indicate a deposition
behavior which is expected from a simple kinetic model
in which HMDSO is mainly ionized and dissociated by
direct electron collisions, assuming rate constants
independent from c(HMDSO).
These preliminary data indicate a scaling of the d(x)
curves roughly proportional to the monomer
concentration as one would expect it for a generation rate
of growth species proportional to c(HMDSO).
3.2 Plasma Polymerization from TMS/Ar
The main results from deposition experiments with
TMS/Ar mixtures are presented in the following. It
appears to us that TMS/Ar might be the more simple
system, compared to HMDSO/Ar, because no oil-like
films are deposited at the beginning of the plasma zone –
even for high TMS concentrations. As it was reported for
HMDSO, powder deposition was noticed in a small stripe
directly behind the plasma zone with a powder layer
thickness about four times as large as the maximum
thickness within the plasma region.
1300
Thickness d / nm
damping of a substrate absorption peak by the deposited
film. (This method was also used to estimate the thickness
trends from the TMS/Ar and GMA/Ar deposition
experiments.) Figure 3 includes data point in the region
were an oily film was deposited. Here the evaluation
method gives too small results because the liquid plasma
polymer is squeezed out of the ATR contact zone.
The above mentioned powder deposition after the
discharge zone is recognized very clearly by a sharp
increase of the thickness directly after the discharge zone.
Thickness in the powder zone may be larger by a factor of
two, compared with largest thickness in the plasma zone.
Also interesting is the fact, that the strong powder
deposition mainly takes place over only a very small
width (∆x ~ 5 mm). These observations suggest that the
transport of nanoparticles, formed in the plasma zone, to
the substrate surfaces is controlled by electrostatic effects,
i.e., a rapid change of the charge on the particles or/and
the surface upon leaving the plasma zone.
Within the discharge zone the thickness d(x) falls, after
an initial rapid rise and passing a maximum, to an almost
constant level of about 50 nm. The maximum shifts in gas
outlet direction with an increase of the HMDSO
concentration beyond ~130 ppm. In contrast to
expectations based on a simple kinetic model in which
HMDSO is mainly ionized and dissociated by direct
electron collisions, approximately the same deposition
rate around 550 nm + 25 nm per minute is observed for
the first 5 to 10 mm - without a strong influence of
HMDSO concentration (excluding highest and lower
concentrations, 35, 70 and 215 ppm). (The low-thickness
plateau from 0 to 20 mm for a HMDSO concentration of
215 ppm indicates the oil-like PDMS film mentioned
above.) A very similar behavior was also found in more
recent experiments with TMS/Ar (compare 3.2, Figure 5).
115 ppm
235 ppm
360 ppm
1200
400
Discharge zone
300
200
100
0
0
4
6
8
10
Fig. 5. Total estimated thickness trends in the direction of
flow on PP foil coated with plasma polymerized TMS
100
0,2
0,4
0,6
0,8
1,0
Distance from plasma zone edge x / cm
Fig. 4. First estimated thickness trends within the
“controlled discharge area” on PP foil coated with plasma
polymerized HMDSO in N 2 atmosphere
The very first results from an additional experiment
with HMDSO plasma polymerized in nitrogen indicated a
completely different deposition behavior, which should be
mentioned at this point. Unfortunately the deposition
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2
Distance from plasma zone x / cm
200
0
0,0
445 ppm
600 ppm
735 ppm
Directly at the beginning of the deposition zone a
thickness of 225 nm + 25 nm is measured almost
unaffected by the TMS concentration varying between
116 and 735 ppm. Similar to the results obtained with
HMDSO/Ar a deposition rate limitation occurs, in this
case at about 325 nm + 40 nm. For c(TMS) = 735 ppm
the thickness trend appears to develop a plateau at about
325 nm. In both cases the generation rate of “growth
species” derived from HMDSO and TMS, respectively,
by dissociation and ionization processes is virtually independent of the monomer concentration.
3
We think that this surprising result can be related to a
predominance of Penning ionization of HMDSO over
ionization by electron collision, suggested also by a
strong decrease of breakdown voltage of Ar upon
additions of a few ppm of HMDSO or TMS. The limitation of deposition rates at values of about 550 nm/min
(HMDSO/Ar) or 325 nm/min (TMS/Ar) may be due to a
limitation of total charges which can be transferred to the
dielectrics within one half cycle of the DBD. More details
will be reported in a separate publication [5].
3.3 Plasma Polymerization from GMA/Ar
The thickness trends d(x) obtained from deposition
experiments with GMA/Ar mixtures are shown in
Figure 6. In contrast to the typical appearance of d(x)
dependencies observed in the examples described above,
the maximum of the film thickness is located close to the
beginning of the plasma zone and is nearly proportional to
the calculated GMA concentrations, ranging from 150 to
960 ppm. Thicknesses decrease in the direction of flow
and eventually reach a virtually constant level of about
15 nm + 5 nm.
Thickness d / nm
300
150 ppm
360 ppm
470 pm
250
trans-2-hexenal, respectively, were investigated. The first
results from experiments with HMDSO, TMS and GMA,
respectively, in argon shown here indicate the usefulness
of this new type of DBD reactors for PACVD processes
and demonstrate the utility of such a combinatorial
method for plasma-based surface science and technology,
for example for investigations of (co)deposition behavior
of different precursors.
5. References
[1] J. Genzer, Annu. Rev. Mater. Res. 42, 435 (2012).
[2] A. Hinze, A. Marchesseault, S. Büttgenbach,
M. Thomas, C.-P. Klages in: M. Thomas,
K.L. Mittal, (Eds.), Atmospheric Pressure Plasma
Treatment of Polymers, Scrivener Publishing LLC,
Massachusetts 2013.
[3] K. Jousten (Ed.), Wutz Handbuch VakuumtechnikTheorie und Praxis 9. Auflage, Vieweg & Sohn
Verlag, GWV Fachverlage GmbH, Wiesbaden, 2006.
[4] J. M. P. Q. Delgado, Heat Mass Transfer 42, 279
(2006).
[5] J. Philipp, C.-P. Klages et al., to be published.
[6] C.-P. Klages, K. Höpfner, N. Kläke, R. Thyen,
Plasmas and Polymers 5, 79 (2000).
580 ppm
780 ppm
960 ppm
200
150
100
50
0
0
1
2
3
4
5
Distance from plasma zone x / cm
Fig. 6. Thickness trends d(x) within the first 5 cm in the
direction of flow on PP foil coated with plasma
polymerized GMA
We have reasons to believe that this deposition
behavior is related to a polymerization process dominated
by reactions between radical centers on the surface and
intact GMA monomer molecules, in agreement with
previous studies [6].
4. Conclusions
The technical realization of a DBD reactor fed with a
premixed precursor gas concentration gradient transverse
to the direction of flow was successfully implemented.
The concentration gradient was established using a
gradient mixer consisting of a combination of two
manifolds with outlet channels of adjusted cross sections
together with a packed bed of glass beads, acting as a
homogenizer. By feeding a gas flow with such a gradient
to the reactor, DBD plasma deposition processes with the
precursors HMDSO, TMS, GMA, 4-methylstyrene, and
4
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