22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Film stress of amorphous hydrogenated carbon on biaxially oriented
polyethylene terephthalate
M. Brochhagen1, H. Bahre1, H. Behm3, D. Grochla2, M. Böke1, R. Dahlmann3, C. Hopmann3, A. Ludwig2 and
J. Winter1
1
Institute for Experimental Physics II: Application Oriented Plasma Physics, Ruhr-Universität Bochum,
Universitaetsstraße 150, 44801 Bochum, Germany
2
Institute for Materials, Chair for MEMS Materials, Ruhr-Universität Bochum, Universitätsstraße 150, 44801
Bochum, Germany
3
Institute of Plastics Processing, RWTH Aachen University, Pontstraße 49, 52062 Aachen, Germany
Abstract: Amorphous hydrogenated carbon (a-C:H) films can improve mechanical
properties of polymers. Especially to get a flexible barrier, the stress of the involved
materials is important. PET and silicon show a different stress behavior. A possible reason
for this is a change inside the structure of the PET sample due to thermal loads in surfacenear-regions during the coating process.
Keywords: a-C:H, flexible thin films, film stress, polyethylene terephthalate
1. Introduction
Amorphous hydrogenated carbon (a-C:H) films are of
high interest, because they can improve the gas barrier of
polymers. One of the important properties of an a-C:H
coating is his compressive stress, which has beneficial as
well as unwanted effects.
The stress can cause
deformation of the bulk material or de-lamination of the
film. The mechanical stability can be improved and it is
possible to reduce cracking due to elongation, as the
compressive stress can compensate externally applied
tensile strain. Ozkaya et al. made the observation, that a
PET coating between stainless steel and an a-C:H layer
can influence the stress. This leads to the fact that the
coating cracks at 8% instead of 3% without PET [7]. The
authors had the opinion, that the residual compressive
stress must have been lowered by the introduction of the
PET layer, because the a-C:H films were deposited under
the same conditions. To determine the residual stress of a
well-defined a-C:H film on PET, the change in radius of
curvature on PET substrates has to be measured and out
of it the stress can be calculated with the Stoney equation.
Furthermore, comparisons with values from a microcantilever array, designed for stress measurements, are
given as well as explanations for the deviation of the
compressive stress of PET.
2. Materials and methods
2.1. Principle of stress measurements
In this work, two different substrates were used to
measure residual stress:
- PET-cantilevers with a dimension of 2 cm × 2 cm ×
500 μm;
- Si-micro-cantilever array, which is in the following
referred to as Si-chip.
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For the Si-chip, each cantilever is measured
individually using a Digital Holographic Microscope
(DHM). The complete measurement process is described
in detail elsewhere [4, 6]. For the PET-cantilevers, the
radius of curvature was measured with a Dektak 6M
Stylus Profiler (Veeco Germany, Mannheim). The film
stress was calculated out of the measured radii of
curvature and the correlation between the film stress and
the curvature is described by a modified Stoney equation
[1].
Sensor fabrication
The stress sensors, as shown in Fig. 1, were developed
by Ludwig et al. [5]. They have 8 cantilevers with
different lengths fabricated out of a 4-inch Si wafer.
Fig. 1. Photograph of an Si-chip with 8 micro-cantilevers.
2.2. Polymer Material
The a-C:H thin film was deposited on PET-cantilevers
out of Hostaphan RN from Mitsubishi Polyester Films
Wiesbaden, Germany. It is a 500 μm thick, co-extruded,
biaxially oriented PET (BOPET) substrate.
2.3. Plasma Chamber
The plasma chamber used in this study is an ICP reactor
with an inner diameter of 600 mm and a height of
1
350 mm driven at 13.56 MHz, equipped with a probetransfer-system, which is described in more detail
elsewhere [2].
3. Results
3.1. Residual Stress
Applied bias voltages influence as well the hardness of
the coating as the stress characteristic. To get more
information about this influences, the residual stress was
measured as a function of the bias voltage for the different
substrates. Fig. 2 shows the stress for a-C:H coatings on
Si-chips, Si-cantilevers and PET-cantilevers. By adding a
bias voltage it changes from tensile stress to compressive
stress. The residual compressive stress on a-C:H films
can be described by the Davis model [3] (red solid line in
Fig. 2), which explains the built up of the compressive
stress by two contrary processes. High energetic ions
increase the compressive stress by subplantation, lower
energetic ions just heat the film and decrease the stress.
during its manufacturing process. Influences of heat,
heating the sample over the glass temperature of PET, can
rearrange this orientation and compress the material. If
the heating process is limited to a surface-near-region, a
gradient can develop inside the material and cause a
negative curvature as shown in Fig. 3. To mimic the
thermal loads within the coating process, an experiment
was made, where the radius of curvature was investigated.
The PET-cantilever, sticking on the electrode, which was
used for the coating, was heated up with a hot plate for
30 seconds for different temperatures, which is drawn in
Fig. 4a. After this process the radius of curvature was
measured with a profilometer (Fig. 4b). Fig. 4c shows,
that the radius for room temperature (Ref) and 60 °C is
nearly the same with a positive curvature, whereas for
100 °C a positive and a negative curvature can be
observed. For 110 °C the direction of the curvature
turned completely into negative.
Fig. 3. Schematic of the orientation within a biaxially
oriented PET film: a) During processing, the orientation
of polymeric chains increases. b) If the temperature is
increased, the orientation is released again, and a sheet of
PET shrinks. c) A temperature gradient can induce a
deformation by partial shrinking of the substrate.
Fig. 2. Residual stress of a-C:H films deposited at bias
voltages up to -250 V on Si-cantilevers, Si-chips and
PET-cantilevers. A compressive stress model by Davis
was fitted to the Si-chip data.
The result of this experiment is, that the stress of the
PET film differs significant from the stress of the Si
samples. Whereas the stress from the Si samples can be
reduced to nearly zero by the bias voltage, the PET film
shows a remarkable compressive stress. To understand
this differences, other influences have to be taken into
account like thermal influences.
3.2. BOPET film
One assumption is, that the thermally induced stress is
not as important as the internal stress, especially because
the temperature during the coating-process is near the
room temperature. In addition the thermal expansion
coefficient is approximately the same for DLC coatings
and silicon.
Depending on the coating and the polymer thermal
loads cause changes during the coating-process can be
more important. The PET film is a biaxially oriented
film, because it has been stretched in two dimensions
2
Fig. 4. a) a PET-cantilever is heated on a hot plate for
30s, with the electrode set on top. b) Afterward, the
radius of curvature is measured. Representative profiles
are shown in c).
To connect a measured radius of curvature with the
upcoming stress, the Stoney formula was used to calculate
the stress with the assumption of a 100 nm coating. The
measured curvatures (a) and the calculated stresses (b) are
displayed in Fig. 5 as a function of the temperature of the
hot plate. Until 100 °C no significant change was
observed and only residual stress is shown. For 100 °C,
two different radii are measured and two different values
for the stress result out of this. When the temperature was
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increased above 100 °C only one curvature left and tensile
stress remains with an order of magnitude of two GPa.
This is high enough to compensate the compressive stress
from the coating.
4. Conclusions
It was shown, that for a-C:H coatings the bias voltage
has an influence to the kind of upcoming stress on Si
substrates. For PET samples the stress level differs
compared to Si- samples. This can be caused by changes
in the structure of the PET film during the heating process
near the surface due to temperatures above the glass level.
5. Acknowledgement
This project is supported by DFG (German Research
Foundation) within the framework of the Collaborative
Research Center SFB-TR 87 and the Research
Department ‘Plasmas with Complex Interactions’ at RuhrUniversity Bochum.
Fig. 5. a) Measured radii as a function of the substrate
temperature. At 100 °C, a double curve and thus two
radii could be measured. b) The calculated stress using
the Stoney equation while assuming a 100 nm film has
been deposited.
Unfortunately, the temperature gradient inside the PET
is not measurable. To estimate it a COMSOL model was
made to simulate the hot-plate-experiment. Fig. 6 shows
the simulated temperature as a function of the position of
the cantilever and the electrode. The temperature gradient
inside the PET-cantilever is very high, because PET has a
low heat capacitance and a low heat conductivity. The
surface of the cantilever facing the cold electrode remains
nearly at room temperature during the heating-process
and only a part of the PET material is heated above the
glass temperature, were the biaxial orientation probably
changes (grey area in Fig. 6).
6. References
[1] G.A. Abbas, P. Papakonstantinou, T.I.T. Okpalugo,
J.A. McLaughlin, J. Filik and E. Harkin-Jones. Thin
Solid Films, 482, 201-206 (2005)
[2] H. Bahre, K. Bahroun, H. Behm, S. Steves,
P. Awakowicz, M. Böke, et al. J. Phys. D: Appl.
Phys., 46, 84012 (2013)
[3] C.A. Davis. Thin Solid Films, 226, 30-34 (1993)
[4] H. Dong. Surf. Coatings Technol., 111, 29-40
(1999)
[5] F.D. Egitto and L.J. Matienzo. IBM J. Res. Dev.,
38, 423-439 (1994)
[6] L. Gerke, J.C. Schauer, M. Pohl and J. Winter. Surf.
Coatings Technol., 203, 3214-3218 (2009)
[7] B. Ozkaya, H. Bahre, M. Böke, D. Höwer, S. Reese,
J. Winter and G. Grundmeier. Surf. Coatings
Technol., 244, 173-179. (2014)
Fig. 6. Time-dependent simulation of the temperature
within the PET-cantilever and the electrode.
Therefore the heating processes can influence the inner
orientation of PET films and is the explanation for the
differences in the stress level which were shown in Fig. 2.
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