22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Influence of intermediate layer type and thickness on barrier properties of
multilayer PECVD barrier coatings on PET
D. Kirchheim1, K. Bahroun1, H. Behm1, M. Jaritz1, F. Mitschker2, P. Awakowicz2, R. Dahlmann1 and Ch. Hopmann1
1
2
Institute of Plastics Processing, RWTH Aachen University, Pontstrasse 49, DE-52062 Aachen, Germany
Institute for Electrical Engineering and Plasma Technology, Ruhr-University Bochum, Universitätsstrasse 150,
DE-44801 Bochum, Germany
Abstract: Changes in surface nano-morphology and thickness of intermediate layers have
a strong impact on the barrier properties of subsequent SiO x -like coatings on plastics. Even
a complete failure of the barrier is observed. Therefore, when depositing multilayer barrier
coatings, the influence of adhesion promoting SiOCH intermediate layers has to be
considered. Morphological investigations as well as oxygen permeation for several
multilayer setups on polyethylene terephthalate are presented and discussed.
Keywords: surfaces, PECVD coatings, interfaces, thin films, SiO x , PET
1. Introduction
Due to the highly cross-linked structure of plasma
enhanced chemical vapour deposited (PECVD) SiO x coatings, one of the most prominent challenge is the
elasticity of the coatings. This makes it very difficult to
provide access to flexible plastics applications with the
need of high barrier properties such as solar cells and
organic light emitting diodes [1]. A second challenge to
overcome is the limitation of barrier properties with
increasing layer thickness due to pinholes or cracks
subsequent to an imperfect coating process [2].
A common approach proposed in literature is the use of
multilayer (ML) systems with a variable number of dyads
consisting of a thin inorganic barrier layer and an organic
interlayer. The primary idea behind these ML-barriers is
to insert organic layers in order to prevent a connection
between micro-sized defects leading to a propagation of
pinholes through the ML. The influence of a variation in
intermediate layer recipe, stacking order and thickness
using dyad setups on the oxygen barrier properties of ML
PECVD coated polyethylene terephthalate (PET) have to
be taken into account. Therefore, morphological structure
as well as OTR of these ML is discussed based on [3]. In
order to further approach the objective, the influence of
intermediate layer thickness is investigated.
2. Materials and methods
Substrate
The dyad structures investigated are deposited on a
Hostaphan RD 23 from Mitsubishi Polyester Film GmbH,
Wiesbaden, Germany, a coextruded 23 µm thick biaxially
oriented polyethylene terephthalate (BOPET).
PECVD setup
The microwave plasma (2.45 GHz) is driven in pulsed
mode for deposition of 30 nm thick SiO x -like barrier
layers
from
a
hexamethyldisiloxane/oxygen
(HMDSO/O 2 ) mixture at 4 Pa.
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The 130 nm thick SiOCH layers are developed to
enhance adhesion of SiO x barrier coatings on polymer
substrates and are therefore ideal for the formation of a
dyad coating together with the described inorganic barrier
layer. The CCP (13.56 MHz) is driven in constant power
mode to obtain a bias voltage of approximately -150 V.
Subsequently, barrier layers will be referred to as (B),
intermediate layers deposited in MW plasma as (IMW)
and in CCP as (ICCP).
Morphology and layer thickness
Morphological analyses of all films are performed using
a laser scanning microscope (Keyence VK-X210 from
Keyence Deutschland GmbH, Neu-Isenburg, Germany, in
the following always imaging an area of 96×72 µm²) and
an atomic force microscope (AFM, Dimension 3100 from
Veeco Instruments Inc., Plainview, NY, USA) operated in
tapping mode.
Layer thickness is determined by cross-sectional
imaging of very thick coated samples. The samples are
obtained by means of ultra-microtome preparation of gold
sputtered and resin embedded samples, using a TEM
(Zeiss EM 910, Carl Zeiss AG, Oberkochen, Germany).
Thickness of thin PET-IMW-B systems is determined
coating silicon wafer and using an ellipsometer (J.A.
Woollam Co. Inc., Lincoln, NE, USA). Calculations are
based on a SiO 2 -modell. Statistical verification is carried
out using a profilometer (Dektak 3M, Veeco, Plainview,
NY, USA).
Oxygen transmission rate
The oxygen transmission rate (OTR) is measured using
a Mocon Inc., Minneapolis, MN, USA, Ox-Tran 2/61 and
a Systech Instruments (UK) Ltd, Thame, UK, M8001,
which both fulfil the requirements of DIN 53380-3 and
ASTM D3985-81. Tests are carried out at 23 °C and 0 %
relative humidity.
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3. Results and discussion
Morphological characterization
Very thick intermediate layers are combined with a
standard SiO x barrier layer, in order to enable
investigation of morphological changes induced by
intermediate layers.
Figure 1 shows cross-sectional TEM images of the
structure of deposited single layers and dyads. In all
images the PET substrate is located at the lower edge and
the resin at the upper edge.
Figure 2. Surface structure of the pristine substrate and
coated with different layer types.
Figure 1. TEM images of cross-sections through single
layer coatings (top) and two dyad coatings (bottom) on
PET.
Here, the thickness of a single layer SiO x barrier
coating can be determined to approximate 30 nm, while
both deposited intermediate layers have a thickness of
approximate 130 nm. No delamination is observed, which
indicates a good adhesion to the PET-film. Images of the
dyad systems show some remarkable details. On the
observable scale, layer thickness is not affected by a
subsequent coating process. Additionally, it can be
concluded that the adhesion between ML and substrate as
well as between the individual layers of the ML is
sufficiently high to withstand high mechanical loads
induced during ultra-microtome preparation.
Figure 2 shows LSM height images of the uncoated
substrate and the single layer coatings deposited on the
PET substrate. Samples coated with SiO x barrier layers
and IMW layers tend to develop a surface structure which
can be compared to the structure of the substrate surface
with some visible macro-defects, while ICCP layers seem
to develop a more grain-like surface.
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Figure 3 shows the surface structure of selected dyad
coatings. The distinctions between the different layer
types disappear after deposition of two dyad coatings. The
observed planarization effect on the ICCP layers appears
in the TEM cross-section images as well in the LSM
height images. A smoothing effect from the added oxygen
during deposition of dyad layers, comparable to an
oxygen pre-treatment of polymer surfaces [5], has to be
considered. All coatings seemingly present a surface
structure, which is comparable to the uncoated substrate
surface roughness. Only small differences in the images
can be observed, thus it is impossible to determine the
influence of defects on layer formation induced by
different excitation modes.
Figure 3. Surface structure for selected dyad coatings
(LSM images).
Although no obvious difference in the micro-scale
morphological constitution can be found for the two
different dyad systems investigated, changes on the nanoscale have to be considered.
Therefore, Figure 4 shows AFM height images of
selected samples of coated substrates in a much smaller
scale. As reference value, the surface roughness R a of the
uncoated substrate has been determined to approx.
0.7 nm. Despite their similarity on the micro-scale ICCP
and IMW dyad differ significantly on the nano-scale.
It is discovered, that intermediate layers deposited in a
CCP possess a smooth structure while comparable
coatings deposited in a pulsed microwave plasma develop
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a distinct granular structure (figure 4). Taking the results
of the morphological characterisations and the
experimental setup into account, as well as chemical
analysis presented in [3], the described changes in
morphology on the nano-scale have to be related to the
different excitation types used for deposition of SiOCH
layers.
Figure 4. AFM height images of coated selected samples.
R a values are obtained using a cut-off wavelength of 0.2
µm.
Furthermore, these varieties in layer formation are not
limited just to the formation of the interlayer. It can be
seen on the surface images of barrier layers deposited
subsequent to an intermediate layer (Figure 4, right), that
these pull through to the surface layer. The coating
surfaces with a barrier deposited subsequent and prior to
an ICCP-interlayer show comparable smooth surface
structures. The same observation can be made for the
dyad systems of barrier layers subsequent and prior to
IMW-interlayers, however, the obtained images show a
significantly increase in coarse-grainend structure of the
surfaces. Thus a more granular structure of IMWintermediate layers seems to be likely, which propagates
through a subsequent coating affecting the structure of the
barrier layer itself. The effect of these morphological
differences on oxygen transmission rate has to be
investigated.
Oxygen transmission rate (OTR)
From
the
OTR
of
uncoated
PET
film
OTR = 68.51 ± 1.17 cm³/(m²∙day∙bar), a SiO x coating
improves barrier properties to OTR = 1.43 ± 0.17
cm³/(m²∙day∙bar). Looking at the dyad system (ICCP-B)
barrier functionality is worse than the reference SiO x
layer for one dyad. Performance improves to the level of
the reference barrier coating after deposition of another
dyad layer. (B-ICCP) and (B-IMW) systems seem to form
barrier functionalities in the range of the reference
coating. The barrier performance of the system (B-IMW)
seems to decrease with the number of dyads deposited,
while the system (B-ICCP) shows a slightly enhanced
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performance with a rising number of dyads. These results
correlate nicely with the provided AFM studies on the
formation and propagation of a grossly-grained surface
topography through a subsequent barrier coating. Similar
correlations between the nano-roughness (of the same
substrate) obtained by various oxygen plasma pretreatments of the substrates and barrier properties of
subsequently deposited SiO x coatings have been reported
in [5].
Figure 5. Measured OTR data plotted as a function of
type and number of dyads.
Moreover, the poor barrier performance of a first
(ICCP-B) dyad is enhanced by a decade when deposited
an additional time. This leads to the assumption, that the
interaction between the different layers is not only limited
towards the SiO x coating. It seems that deposition of an
ICCP layer on top of a preceding SiO x coating leads to a
surface morphology which allows a subsequent SiO x
coating to preserve barrier functionality. This is not the
case for the analysed IMW layers.
Up to this point, the presented layers are exceptionally
thick which needs to be taken into account, too. In order
to further investigate the influence of intermediate layers,
measurements of OTR at varying layer thicknesses have
been carried out. For the purpose of noticeable
differences, barrier layer thickness has been set to approx.
18 nm, so that both increase and decrease in OTR can be
investigated.
Figure 6. Measured OTR data plotted as a function of
combined layer thickness of (IMW-B) systems on PET.
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Figure 6 shows the OTR as a function of the combined
layer thickness. As a reference, the spot at approximate
18 nm shows the barrier coated PET sample. The
following data shows the system (IMW-B), where only
the thickness of IMW has been varied.
A very thin layer of IMW (figure 6, less than 14 nm)
applied before the barrier decreases the OTR. After
roughly 20 nm of IMW coating, the OTR slowly increases
and probably loses all of its barrier properties for thicker
IMW layers. One possible explanation is for thin IMW
coatings to be oxidized by the following barrier
depositing process, thus creating a slightly thicker barrier
coating. Whether it is a barrier coating or not, this could
imply, that up to a certain amount there still is no real
IMW coating underneath the barrier coating.
4. Conclusions
Both in excitation and in layer thickness different
HMDSO-based layers and dyad structures are deposited
on PET in order to determine the influence of thickness
and type of layers on surface morphology and oxygen
barrier properties of ML coatings containing SiO x barrier
layers. Dyad layers consist of SiO x barrier layers and
SiOCH intermediate layers deposited in pulsed
microwave plasma (IMW) as well as SiOCH intermediate
layers deposited in CCP (ICCP).
Characterization of the morphological structure of the
coatings in micrometre-scale shows no significant
influence of the different setups. Nevertheless, changes in
barrier performance from barrier properties in the range
and below the reference barrier layer to a failure of the
barrier coating are observed. These observations seem to
be attributed to changes in the nano-structure of the
intermediate layers affecting the morphological structure
of a subsequent barrier layer (see AFM measurements).
This leads to the assumption that properties of SiO x
barrier layers are strongly influenced by the substrate and
preceding coatings. Therefore two considerations seem
inevitable. If an adhesion promoting layer has to be used,
its influence on following coatings should be well known.
Since their influence both depends on substrate and
excitation type, intermediate layers used in ML coatings
have to be carefully selected according to their impact on
barrier layer formation. According to the research at hand,
the deposition of SiOCH intermediate layers using pulsed
microwave plasmas should be avoided, unless average
layer thickness is kept below 14 nm.
All in all, it seems IMW coatings beneath barrier
coatings need to be further investigated in terms of barrier
properties.
6. References
[1] Y. Leterrier. Durability of nanosized oxygen-barrier
coatings on polymers. Progress in Materials Science, 48
1–55 (2003)
[2] A. Morlier, S. Cros, J.-P. Garandet and N. Alberola.
Gas barrier properties of solution processed composite
multilayer structures for organic solar cells encapsulation.
Solar Energy Materials and Solar Cells, 115 93–9 (2013)
[3] K. Bahroun, H. Behm, F. Mitschker, P. Awakowicz,
R. Dahlmann and Ch. Hopmann. Influence of layer type
and order on barrier properties of multilayer PECVD
barrier coatings. J. Phys. D: Appl. Phys., 47 015201
(2014)
[4] M.R. Alexander, F.R. Jones and R.D. Short. RadioFrequency Hexamethyldisiloxane Plasma Deposition: A
Comparison of Plasma- and Deposit-Chemistry. Plasmas
and Polymers, 2 277–300 (1997)
[5] H. Bahre, K. Bahroun, H. Behm, S. Steves, P.
Awakowicz, M, Böke, Ch. Hopmann and J. Winter.
Surface pre-treatment for barrier coatings on polyethylene
terephthalate. J. Phys. D: Appl. Phys., 46 084012 (2013)
5. Acknowledgement
The depicted research has been funded by the Deutsche
Forschungsgemeinschaft (DFG) as part of the
Collaborative Research Centre SFB-TR 87. We would
like to extend our thanks to the DFG.
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