st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Elongation tolerant Multilayer Gas Barrier Coatings for PET films
H. Bahre1, K. Bahroun2, H. Behm2, M. Böke1, R. Dahlmann1, T. Schlebrowski1, J. Winter1
2
1
Institute for experimental physics II, Ruhr-Universität Bochum, Bochum, Germany
Institute of Plastics Processing (IKV) at RWTH Aachen University, Aachen, Germany
Abstract:
Polymers gain more and more interest in packaging applications, with increasing demands towards their gas barrier properties.
In addition, their tolerance to mechanical stress remains to be improved, especially for glass like SiOx coatings. Alternative
approaches are organic coatings and the use of multilayer. However, many of these systems are hardly understood. Here, a
multilayer system based on films deposited from C2H2 as well as SiH4 is evaluated to assess their applicability as barrier layers.
Inorganic a-Si:H layers alternate with organic a-C:H layers. A capacitively coupled plasma discharge (CCP) driven at
13.56 MHz is used for deposition. We found that the hardness as well as the induced compressive stress in a-C:H coatings are
limiting factors in the gas barrier properties in multilayer films. However, soft a-C:H layers deposited at low self-bias voltages
and high pressures are tolerant to up to 6 % strain before first cracks occur.
Keywords: capacitively coupled plasma, a-C:H, DLC, a-Si:H, gas barrier, multilayer, PET
1. Introduction
Polymers progressively substitute glass due to their
combination of favourable properties such as low weight,
transparency and toughness. This combination makes
them suitable for many packaging applications [1].
Nonetheless, raw plastics do not possess sufficient gas
barrier functionality. Gas barrier against oxygen and
water vapour avoids degradation of packaged goods and
is of paramount importance. Thin gas barrier films can be
deposited by plasma processes using very few material at
low cost, causing less damage to the environment
[2, 3, 4]. However, current gas barriers are very brittle and
have almost no tolerance to elongation.
2. Experimental Details
Deposition Chamber and Processes
Both amorphous hydrogenated carbon (a-C:H) as well
as amorphous hydrogenated silicon (a-Si:H) films were
deposited with a capacitively coupled plasma (CCP)
reactor driven at 13.56 MHz. The lower electrode has a
diameter of 100 mm and a distance to the upper electrode
of 30 mm. A negative self-bias potential builds up at the
lower, powered electrode, which holds the samples. It is
measured at a low pass filter within a matching network
connected to the electrode. This self-bias potential
directly influences the energy of the particles which hit
the substrate [5].
The substrates were introduced with a transfer rod
through a pre-evacuated load lock chamber. Gas is
injected into the chamber through a ring-shaped shower
head. We deposited a-Si:H layers from ultra-high purity
silane (diluted in 10 % argon) and a-C:H layers from
acetylene. The layer thickness was determined with
silicon wafers that were coated along with the samples
and then measured with an ellipsometer (J. A. Woollam
Co., Lincoln, USA) at an angle of 75° and wavelengths
between 245 and 999 nm.
Forming tolerance testing
To evaluate strain-tolerance properties of coated films, a
motorized microstraining device (Kammrath & Weiß
GmbH, Dortmund, Germany), equipped with a load cell
and an inductive displacement sensor, was utilized.
Coated films with a size of 40 mm × 8 mm were fixed on
the micro-straining device, which was then used to impart
strain to the film. In this arrangement, the films were
strained uniaxially. Under steady-state strain conditions,
which were stepwise increased, observations were made
using a laser scanning microscope (Keyence VK-X210
from Keyence Deutschland GmbH, Neu-Isenburg,
Germany). All measurements were performed imaging an
area of 96 μm × 72 μm. The images obtained were
visually examined towards crack formation and possible
delamination indicating a poor layer adhesion.
Polymer Material
The polymer film used in this study is Hostaphan
RD 23 from Mitsubishi Polyester Films Wiesbaden,
Germany. It is a 23 µm thick, coextruded, biaxially
oriented polyethylene terephthalate (BOPET) film with
two different surface topographies. While one side is
standard Hostaphan PET film with a surface roughness of
RRMS = 3 nm, the functional side is extremely smooth,
with a surface roughness of RRMS = 0.7 nm. The
roughness is given as an average for a 5 × 5 µm2 area.
This difference in roughness is caused by anti-block
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
particles which are not present on the functional side. For
this study, only the smooth side was coated.
The film is taken from a roll and inserted directly into
the chamber without any rinsing or ultrasonic cleaning to
avoid treatment effects that do not originate from the
plasma.
3. Results & Discussion
Barrier properties depend crucially on the thickness of
the deposited layer. Usually, a critical thickness in the
range of tens of nm is necessary for a closed layer and gas
barrier properties [7]. Similarly, Figure 1 illustrates that
our a-Si:H and a-C:H layers possess an oxygen gas barrier
at 10 nm and 17 nm respectively. Compared to the
reference with no layer deposited, the barrier is improved
by a factor of 27. Moreover, increasing the thickness of
the a-Si:H layer further improves the OTR to 1.5 cm³/m²
day. However, the oxygen transmission of the 85 nm aC:H layer increases as shown in Figure 1. This means that
increasing the layer thickness can have a detrimental
effect on the oxygen gas barrier. Similarly, the OTR of
the double layer, which consists of 10 nm a-Si:H and
17 nm a-C:H, is higher than each layer alone.
Micro-straining tests showed that both pressure and
applied substrate bias determine the initial cracking of the
coatings, as shown in Figure 2. The effect of the bias is
illustrated in c) and d). We observed that for low (-60 to
-100 V) bias voltages, the tolerance to strain is improved
and the layers do not crack until 5.5 % (Figure 2d)). But
self-bias voltages between -200 V and -500 V result in
initial cracks at 1.5 % to 2 % strain.
Figure 2: Microcracking patterns of a-C:H layers for different
plasma parameters. a) and b) show layers deposited at 1 and
5 Pa respectively. While a) is already cracked in both the direction of strain (horizontal) and perpendicular to it, b) is
completely intact. Figure c) also shows cracks, as well as
meander-like structures, whereas d), deposited at low bias, is
free of defects.
Figure 1: Oxygen transmission rate (OTR) of different coat-ings
deposited directly on the PET substrate: The thin (10 nm) a-Si:H
coating shows a good barrier performance, as well as the 17 nm
a-C:H coating. However, a combination of these two in a double
layer has a decreased barrier performance.
A possible explanation for this behavior is the residual
stress of the coatings. a-C:H layers are known to possess
residual stress in the range of GPa, depending strongly on
the deposition parameters [8]. Self-bias voltages in the
range of -100 V to -300 V create hard, diamond like
carbon layers with high compressive stress [9].
Impinging, energetic ions densifying the deposited layer
are considered to be the origin of this stress. Soft, polymer
like layers form at low bias-voltages or even ground
potential [10].
a-C:H coatings deposited at low pressure and -100 V
bias show cracks both in the direction of strain
(horizontal) and perpendicular to it at 4.5 %. On the
contrary, the a-C:H layer deposited at 5 Pa is free of
cracks at the same elongation. In addition to the cracks
due to the strain, the a-C:H layer deposited at -200 V selfbias also shows meander-like structures, which are also
visible in the pictures without strain (not shown here).
These meander-like structures occur at high self-bias
voltages and are possibly due to the compressive stress in
the layer.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
4. Conclusion
We deposited different gas barrier layers. a-Si:H layers
were found to work as oxygen gas barrier for very thin
and (10 nm) intermediate (40 nm) thicknesses. In contrast,
hard a-C:H layers only had a considerable gas barrier for
thin (17 nm) coatings due their compressive stress.
By varying the a-C:H deposition parameters, they were
found to have lower maximum strains when deposited at
parameters that are considered to produce hard coatings.
Therefore, a combination of soft a-C:H layers with a-Si:H
layers is a promising approach and should be further
investigated.
Figure 3: The maximum strain before first cracks occur
decreases by a factor of two for self-bias voltages below -100 V.
Figure 3 and Figure 4 show an analysis of the pictures
taken from the strain tests. We label the strain at which
the first cracks are visible in the micrographs maximum
strain. This maximum strain decreases by a factor of two
at bias voltages below -100 V. This is probably due to
compressive stress that builds up in the a-C:H layer.
Meander-like structures as shown in Figure 2 c) appear in
all layers deposited at higher bias voltages, and the films
are curved after they are released from the sample holder.
This means that hard a-C:H layers deposited on PET have
a much lower tolerance to strain than soft a-C:H layers.
Figure 4: The maximum strain before first cracks occur vs the
pressure during the plasma deposition. Higher pressures increase
the maximum strain.
For the pressure variation shown in Figure 4, the
maximum strain doubles for higher pressures from 3.5 %
at 1 Pa up to 6 % at 4 Pa. Layers deposited at higher
pressures have seen lower ion energies, since ions collide
more often while travelling through the plasma sheath. In
addition, the fragmentation of C2H2 is lower at higher
pressures for the same applied power, which means that
larger molecules at lower energies are implemented into
the a-C:H layer.
5. Acknowledgements
This project is supported by DFG (German Research
Foundation) within the framework of the Special Research Field SFB-TR 87 and the Research Department
‘Plasmas with Complex Interactions’ at Ruhr-Universität
Bochum.
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