22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Infrared gas phase studies on plasma-polymer interaction in high-current
dielectric barrier discharges
Y. Liu1,2, S. Welzel1,3, S.A. Starostin1,2, M.C.M. van de Sanden1,3, J.B. Bouwstra2, R. Engeln3 and H.W. de Vries1,2
1
Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, the Netherlands
2
FUJIFILM Manufacturing Europe B.V., Tilburg, the Netherlands
3
Eindhoven University of Technology, Eindhoven, the Netherlands
Abstract: A high current diffuse dielectric barrier discharge (DBD) was operated in a
roll-to-roll reactor using air-like gas mixture. The exhaust gas from the discharge was
studied using a Fourier-transform infrared spectrometer. Absorption features of H x N y O z ,
CO x , and HCOOH were identified and used to establish production rates for these stable
products. An etching effect of the polymer and increased production rates of CO were
observed.
Keywords: infrared, FTIR, atmospheric pressure, PECVD, plasma measurements
1. General
Atmospheric-pressure dielectric barrier discharge (DBD)
is regarded as a tool for plasma enhanced chemical
vapour deposition (PECVD) of functional thin films
because of its industrial capability of large scale
roll-to-roll processing, low costs and high throughout
[1, 2]. Excellent SiO 2 -like barrier layers on polymeric
substrates such as PEN (Polyethylene Naphthalate) have
been achieved in cost-efficient gas mixture of N 2 /Ar/O 2
in conjunction with organosilicon precursors [3, 4]. The
deposition process has been studied using surface
analytical techniques [3].
A competition between
deposition and etching was concluded from these
experiments [5]. In this contribution quantitative ex-situ
infrared studies of the plasma exhaust gas were carried
out using a high-resolution Fourier-transform infrared
(FTIR) spectrometer. The discharge was operated in
precursor-free gas mixture to particularly focus on the
plasma-polymer interaction. The gas phase composition,
the production rates of stable molecules and the plasmapolymer interaction were analysed.
2. Experimental set-up
A schematic picture of the roll-to-roll reactor and the
extraction device which is connected to the FTIR system
is presented in Fig. 1. The discharge was ignited between
two cylindrical copper electrodes with a radius of 120 mm,
both covered by PEN of 100 µm thickness as dielectrics.
The distance between the two electrodes was 0.5 mm.
The gas mixture was injected via a controlled mixing unit
(from left side of discharge area in Fig. 1) while the
electrodes were rotating in the direction of the gas flow.
The exhaust from the plasma was sampled by 12 pipettes
into a high-resolution FTIR spectrometer (~0.15 cm-1
resolution). To increase sensitivity the spectrometer was
equipped with a multi-pass absorption cell adjusted to 7 m
absorption pass. In this study, the frequency of the
applied voltage was 185 kHz. To reduce power load to
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the polymer the injected power was modulated at 625 Hz
with a duty cycle of 50%. A more detailed description of
the experimental set-up can be found elsewhere [4].
Fig. 1. Experimental setup to generate plasma and carry
out ex-situ FTIR absorption spectra measurement.
3. Results and discussion
A typical survey infrared spectrum of atmospheric
pressure air plasma is presented in Fig. 2. In the range
from 750 to 3000 cm-1, N-related species (NO, NO 2 ,
N 2 O, HNO 2 ), C-related species (CO, CO 2 , HCOOH) and
H 2 O can be clearly identified in the spectrum. Ozone,
which is frequently seen in air-containing DBDs, was not
observed in our experimental conditions.
The O 3
production is very likely ‘poisoned’ as e.g., described in
[1] and further supported by the observed levels of
NO/NO 2 and the absence of N 2 O 5 in the gas mixture.
The concentrations of molecules were estimated
according to calculations based on line parameters given
in the HITRAN database using Q-MACSoft-HT software
[6]. In order to eliminate the effect of gas flow speed, the
molecule production rate X was employed to investigate
the variations of reaction products [7]:
X=η∙Φ·n=η∙
∆𝑉
∆𝑡
·
∆𝑁
∆𝑉
=η∙
∆𝑁
∆𝑡
(1)
1
where η is the mixing ratio (or relative partial pressure),
Φ is the total gas flow (standard liter per minute, slm),
and n is the molecular number density (m-3).
Fig. 4. Molecule production rates in air/Ar (15/1.0 slm)
plasmas in contact with substrates of (unprotected) PEN
and PEN foil that was protected by a several hundred nm
thick, carbon-free, SiO 2 layer.
Fig. 2. Identification of stable reaction products in air
plasmas: Transmission spectra a) as measured, and b)
calculated.
The variations of molecule production rates in
N 2 /Ar/O 2 (15/1.0/0.5~2.0 slm) mixture with substrate of
PEN are presented in Fig. 3. The production rates of
main gas phase components in air/Ar (15/1.0 slm)
plasmas are shown in Fig. 4.
trends in Fig. 3 to an O 2 flow of about 4 slm (i.e., 20% in
the mixture as in case of air), the production rates X(NO),
X(NO 2 ) and X(N 2 O) would be ~ 6×1018 s-1, ~ 4×1018 s-1
and ~ 3×1018 s-1, respectively. These extrapolated results
are indeed close to those ones observed experimentally
for (unprotected) PEN substrates in contact with air/Ar
plasmas (Fig. 4).
From Fig. 4, the production rates of CO and HCOOH in
case of PEN substrate are about 4.5×1019 s-1 and
1.0×1018 s-1, respectively. However, as soon as the
polymeric substrate is protected by a thin SiO 2 film of a
few hundred nanometers, the production rate of CO
reduces significantly to below 1×1018 s-1 while HCOOH
can even not be observed. This behaviour that was also
observed in several other precursor-free, air-like gas
mixtures indicates that HCOOH as well as CO are mainly
formed as etch products of the polymer. Moreover,
HCOOH also follows the structure of PEN, as shown in
Fig. 5. Besides, among all components in the gas mixture,
O 2 plays the main role in PEN etching as HCOOH
increases with O 2 flow (Fig. 3).
Fig. 3.
Molecule production rates in N 2 /Ar/O 2
(15/1.0/0.5~2.0 slm) plasmas in contact with (unprotected)
substrate of PEN.
The production rates of NO, NO 2 , N 2 O and HCOOH
clearly increase with the O 2 content in the gas flow while
the CO production remains relatively stable as about
10 times higher level than the N-related species (about
4×1019 s-1, Fig. 3). Typically, the production rates of the
latter species are all in the order of 1018 s-1 and are
characterised by X(NO) > X(NO 2 ) > X(N 2 O) (the
corresponding mixing ratios are usually of the order of
100-1000 ppm). Similar results were also observed in [7]
for a plasma jet using Ar as carrier gas and 0.1%
admixture of air. An extrapolation of the increasing
2
Fig. 5. Structure of PEN (Polyethylene Naphthalate).
According to the discussion above and previous results,
it can be concluded that CO has 3 different sources with
significantly different production rates. The first and
dominant source is the etching of the polymer substrate,
which contributes more than 1×1019 s-1 to the whole
production rate. The plasma-polymer interaction is
apparently also the main source for HCOOH production,
which is usually 1 - 2 orders of magnitude lower than
X(CO). The second source of CO could be the impurities
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from the gas injection system such as precursor pollution
on the surface of plastic tubes from previous deposition
experiments. Such carbon reservoirs typically yield
1×1018 s-1 to 1×1019 s-1 to X(CO) (results are not shown
here).
A third source of CO is inevitable, basic
C-impurities in the entire discharge and measurement
system. This may also include, for example the small
amount of CO 2 from ambient air (400 ppm). The
production rate of such sources is less than 1×1018 s-1.
4. Conclusions
In this study, the infrared absorption features of stable
plasma species in high-current atmospheric pressure
air-like discharge were analysed. N-related species (NO,
NO 2 , N 2 O, HNO 2 ), C-related species (CO, CO 2 ,
HCOOH) and H 2 O were identified. The concentrations
and the sequence of N x O y production rates (X(NO) >
X(NO 2 ) > X(N 2 O)) are in a good agreement with
observations reported by other groups (for other type of
air plasma). HCOOH, in this study, appears to be mainly
from the etching of the polymer substrate, and the O 2
content plays a main role during the etching. Similarly,
CO is produced at high rates as soon as polymer etching
occurs while other CO sources (e.g., impurities) are at
least a factor of 5 less efficient in production.
5. References
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