22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Scaling of the atmospheric pressure DBD assisted deposition process for gas
diffusion barrier film
S.A. Starostin1,2, A. Meshkova1,2, F.M. Elam1,2, M.C.M. van de Sanden1,3, J.B. Bouwstra2 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 BV, P.O. Box, 5000 LJ, Tilburg, the Netherlands
3
Eindhoven University of Technology, Applied Physics, P.O. Box 513, 5600 MB Eindhoven, the Netherlands
Abstract: The silica-like moisture barrier films were roll-to-roll deposited on polymer web
by atmospheric pressure dielectric barrier discharge in N 2 , O 2 gas mixture with the addition
of tetraethyl-orthosilicate. It is discussed how the scaling of the moisture barrier film
throughput can be achieved by controlling the non-uniform deposition rate profile and by
application of a bi-layer architecture.
Keywords: dielectric barrier discharge, gas diffusion barrier films, AP-PECVD
1. Introduction
Recently in [1] it was reported that excellent silica-like
moisture barrier films (WVTR ~ 10-3 g m2 day-1 at 40 °C,
90% RH) can be produced at atmospheric pressure by
industrially compatible roll-to-roll PECVD process. In
this work the diffuse high current dielectric barrier
discharge (DBD) between cylindrical electrodes was
employed as a source of non-thermal plasma. It was
demonstrated that film composition, morphology and
moisture permeation properties have strong nonlinear
relation to dynamic deposition rate and specific energy
delivered per precursor molecule in plasma. It was also
shown that good quality gas diffusion barrier films are
synthesized at relatively low dynamic deposition rates and
high specific energies (in order of few keV per deposited
silicon atom).
From the practical point of view it is important to
investigate the possibility to scale the reactor throughput
in order to increase the production of high quality barrier
film. Considering that the energy delivered per precursor
molecule should remain constant (in analogy to the
similarity defined by the Yasuda composite power
parameter [2]), increase in throughput means increase in
reactive gas flow rates with proportional increase in
power dissipated in the plasma.
However, (in
experiment) this straightforward approach is difficult to
apply due to several limitations such as the dependency of
the DBD regime on the dissipated power density, thermal
sensitivity of the polymeric substrates and a limited length
of the reactive plasma zone.
At the same time both the dynamic deposition rate and
specific energy spent per precursor molecule, discussed in
[1], represent the values averaged over PECVD reactor.
While the local deposition rate as well as the local plasma
chemical kinetics significantly varies along the gas flow
due to the precursor depletion.
This contribution will discuss the qualitative
understanding of the local kinetics, resulting in dense or
porous film growth, which can help to scale–up and
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optimize the AP-PECVD reactor throughput.
Fig. 1. Characteristic deposition rate profile in a
precursor deficient regime as a function of downstream
position.
2. Experimental
A detailed description of the experimental setup has
already been given elsewhere [7]. The DBD was ignited
between two metal rotary drum electrodes with a radius of
120 mm, covered by polyethylene-2,6-naphthalate (PEN)
foil of 100 µm thickness. The purpose of the foil was
twofold: First, it served as the dielectric layer in the DBD
arrangement and, second, it provided the substrate for the
deposition process. The effective discharge width was
150 mm. The gaseous gap was 0.6 mm and the
characteristic plasma expansion length along the gas flow
was about 20 mm. The operational frequency was tuned
in the interval of 185-205 kHz. The voltage amplitude
1
applied between the electrodes was 2-3 kV leading to a
specific dissipated discharge power of 20 W∙cm-2.
Tetraethyl orthosilicate (TEOS), and oxygen were used
as reactive gases and were diluted in nitrogen as carrier
gas for the AP-PECVD process.
The thickness of the deposited films was determined by
variable angle spectroscopic ellipsometry. The deposition
rate profiles can be derived from the spatially resolved
thickness measurements of the samples that were
deposited on the substrate under static conditions (zero
web speed). Water vapour transmission rate (WVTR)
was measured using Technolox Deltaperm instrument for
the conditions of 40 °C, 90% RH. The WVTR value of
the pristine PEN substrate is 1.7 g∙m-2∙day-1.
Fig. 3. Deposition rate profile inside PECVD reactor for
two different values of carrying gas flow.
Fig. 2. Schematic depth structure of the permeation
barrier film. Left: gradient film resulted by the nonuniform deposition rate profile. Right: bi-layer structure
with porous buffer layer and dense sealing layer.
3. Results and discussion
The precursor depletion via dissociation in the plasma
along the gas flow as well as the drift-diffusion transport
of the fragments results in non-uniform deposition rate
profile [3]. The characteristic shape of such profile is
shown in Fig. 1. On the qualitative level one can expect
the properties of the film deposited at different locations
will vary due to the different deposition rates along the
gas flow. It was discussed in the literature [4] that in the
PECVD process using TEOS precursor low deposition
rates lead to denser silica-like films. This takes place due
to the finite rates of heterogeneous reactions at the
interface between the growing film and plasma, like a
cross-linking reaction of two adjacent Si-OH groups with
release of water and formation of a Si-O-Si bridge. If the
deposition rate is too fast, silanol groups will have no
time to react and will be buried in the film bulk, forming a
porous structure [4]. Therefore, the deposition rate profile
as shown in Fig. 1, with the substrate moving in the same
direction as the gas flow, will result in a film with depth
gradient properties, i.e., the porosity of the film reduces
from the bottom to top (see also Fig. 2). It can also be
argued that for a single layer moisture barriers with a
thickness of about 100 nm, such as studied in [1], only the
top layer may have good barrier properties.
From the discussion above, the increase in reactor
throughput for the dense permeation barrier film may not
require a proportional rise of the local deposition rate (as
this can lead to a more porous film), but flattening and
elongation of the deposition rate distribution. However,
this is difficult to achieve if the plasma length is limited
and the electrodes have a curvilinear geometry.
2
An alternative approach for reactor throughput scaling
is to produce a bi-layer structure that consists of rapidly
deposited porous buffer layer and a thin dense capping
layer with gas diffusion barrier properties (see Fig. 2).
Effectively such bi-layer structure replaces the gradient
layer shown in Fig. 2, offering more possibilities for
optimization. Specifically the bi-layer architecture of the
barrier film allows to significantly increase the effective
energy spent per precursor molecule and hence to
improve the quality of the silica network without
sacrificing web speed of the roll-to-roll process. This
gain in throughput comes from the fact that although the
film thickness is inversely proportional to the web speed,
already a very thin capping layer (~10 nm) is sufficient to
reach 10-3 g m-2 day-1 WVTR range. It can be speculated
that the capping layer effectively seals the porous
structure of the buffer layer but at the same time the
buffer layer protects the polymeric substrate from plasmaetching during deposition of the dense film. Moreover,
optimization of a dense capping layer can be done by
tailoring deposition rate profile via change in carrying gas
flow rate.
Increase in gas flow rate (this means increase in drift
transport of precursor and fragments) will flatten and
elongate the local deposition rate profile and, according to
the considerations above, lead to a denser film with better
barrier properties.
The results of experimental studies are presented in
Figs. 3 and 4. The bi-layers studied in Figs. 3 and 4
consist of a 70 nm silica-like buffer layer produced at
efficient energy of 1 keV dissipated in plasma per TEOS
molecule (according to [1] such a layer has no moisture
barrier properties). In turn, the buffer layer is coated with
a 10 nm thick dense layer deposited at various N 2 flow
rates. The specific discharge energy spent for this
capping layer was about ~20 keV per TEOS molecule.
The deposition rate profiles for dense layers at different
gas flow are shown in Fig. 3. In Fig. 4 one can see that
threefold change in N 2 flow results in more than one
order of magnitude barrier film improvement. It is
important to note that throughput in the case of bilayer
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barrier structure can increase up to 4-5 times in
comparison to a single layer film with the similar barrier
performance using the same DBD electrode
configuration.
Fig. 4. Water vapour transmission rate trough bi-layer
barrier as a function of the carrying gas flow during
deposition of the capping layer.
4. Conclusion
The scaling of the AP PECVD reactor throughput for
gas diffusion layer production strongly depends on the
non-uniform deposition rate profile along the gas flow,
caused by the downstream precursor depletion.
Governing the deposition rate profile shape as well as
application of a bi-layer architecture can increase the
throughput up to several times, without change in the
reactor configuration.
5. Acknowledgment
The presented study was performed in the frame of
LIFE12 ENV_NL_000718 “Green plasma process
technology for manufacturing of flexible electronics”
project.
6. References
[1] S.A. Starostin, M. Creatore, J.B. Bouwstra,
M.C.M. van de Sanden and H.W. de Vries. Plasma
Process. Polymers, DOI: 10.1002/ppap.201400194
(2014)
[2] H.K. Yasuda. Plasma Polymerization. (Orlando:
Academic Press Inc.) (1985)
[3] I. Enache, H. Caquineau, N. Gherardi, T. Paulmier,
L. Maechler and F. Massines. Plasma Process.
Polymers, 4, 806 (2007)
[4] S.C. Deshmukh and E.S. Aydil. J. Vac. Sci.
Technol., 13, 2355 (1995)
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