22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Hydrophobic and release films from HMDSO– a parametric study for using
atmospheric pressure plasma processes
A. Laukart, M. Thomas, U. Schwarting and C.-P. Klages
Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany
Abstract: We present recent results from a study regarding the oxygen free and oxygen
controlled deposition of organosilicon thin films from HMDSO (hexamethyldisiloxane).
Main emphasis was set on the effect of HMDSO concentration and CO 2 concentration in
the process gas during deposition. Process parameters for polymer-like, hydrophobic films
with release properties as well as silica-like, hydrophilic films were found.
Keywords: HMDSO, release film, hydrophobic, atmospheric pressure plasma, PACVD
1. Introduction
Industrial interest in using plasma assisted chemical
vapour deposition (PACVD) processes at atmospheric
pressure to deposit thin films from Hexamethyldisiloxane
(HMDSO) is rising steadily. These processes do not
require vacuum equipment nor are evacuation processes
needed and are therefore in a number of applications
considered a potentially economically attractive
alternative to low pressure plasma processes.
Chemical composition and topology of organosilicon
thin films deposited from HMDSO can be adjusted for a
wide range of applications. Films optimized with respect
to release or adhesion properties, barrier properties,
corrosion resistance or electrical isolation can be
deposited under ambient pressure [1, 2 and 3].
This study focused on the role of the HMDSO and CO2
concentration in atmospheric pressure PACVD processes
using dielectric barrier discharges (DBD) to generate the
plasma. The deposited films were analysed by attenuated
total reflection Fourier-transform infrared spectroscopy
(ATR-FTIR), X-ray photoelectron spectroscopy (XPS)
and dynamic water contact angle measurement (dWCA).
Release properties were tested using a 90° tape test.
2. Experimental Section
Sheets of 200 µm thick polypropylene (PP) were used
as substrates due to their low FTIR band overlap with
organosilicon thin films. The substrates were placed on a
moveable silicone covered metal table directly beneath
one gas shower head (Fig. 1). The environment of the
substrate was purged for two minutes either with nitrogen
or argon to maintain a process gas atmosphere with an
oxygen concentration not exceeding 50 ppm. The process
gas flow, consisting of 5 slm argon with small amounts of
HMDSO and CO 2 (Table 1) was activated when the
purging procedure was started.
Towards the end of the purging procedure the power
supply was switched on and the continuous sinusoidal
wave peak-to-peak voltage at 22 kHz was adjusted to
3 kV above the ignition threshold. Voltage was measured
by an oscilloscope (LeCroy). The PACVD experiment
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was then carried out with the substrate table pulling the
polymer sheet through the two plasma zones ten times
forth and ten times back at a velocity of 10 mm/s. During
this procedure the substrate did not leave the purged zone
(Fig. 1).
Fig. 1. Scheme of the experimental setup for the
deposition of HMDSO thin films.
After the process, the substrates were purged for
another two minutes to suppress possible oxidation or
hydrolysis reactions of reactive groups of the freshly
deposited films due to the presence of water or oxygen
impurities.
Table 1. Investigated process parameters and factorial
levels.
Factor
Parameter
Unit
Level -1
-5
Level 0
-4
Level +1
A
concentration HMDSO
-
10
10
10-3
B
concentration CO 2
%
0
1
10
Factor
Parameter
Unit
Level I
Level II
C
shower gas type
-
Ar
N2
The samples were characterised by ATR-FTIR to
determine film thickness, deposition rate and oxygen to
silicon ratio (O/Si ratio).
To calculate the film thickness t from ATR-FTIR
spectra a method was used that is based on damping of
1
𝑡=
𝑑𝑝
2
𝐼
∙ 𝑙𝑙 � 𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢�
𝐼𝑐𝑐𝑐𝑐𝑐𝑐
The deposition rate was calculated using a resulting
total deposition time of 48 s for a total of 20 passes.
The O/Si ratio was calculated according to Schäfer
from the ATR-FTIR peak position of the Si-O-Si I band
at approx. 1040 cm-1 to 1070 cm-1 [5].
The carbon content of the samples in at% was
determined by the XPS-calibrated ratio R of integrated
intensities of ATR-FTIR absorption bands according to
equation 2.
(2)
𝑅=
𝐼𝑆𝑆−𝑂−𝑆𝑆 𝐼 + 𝐼𝑆𝑆−𝑂−𝑆𝑆 𝐼𝐼 + 𝐼𝑆𝑆−𝑂−𝑆𝑆 𝐼𝐼𝐼 + 𝐼𝑆𝑆−𝐶𝐶𝐶
3. Results and Discussion
HMDSO concentration has the highest effect on the
chemical composition of the deposited thin films. The
higher the HMDSO concentration, the higher the carbon
content and the lower the oxygen content (Fig. 2). Thus
the O/Si ratio is never falling below approx. 1.4, albeit the
carbon content increases.
The comparison of Fig. 2 and Fig. 3 in the region of
high HMDSO concentrations of 10-3 shows a smaller but
significant effect of CO 2 in the process gas on the carbon
concentration in the deposited films: The lower the CO 2
concentration, the higher the carbon content in the film.
The effect of the two concentrations on the thin film
composition can be easily understood qualitatively: If a
low HMDSO concentration is used during the process, the
plasma energy dose per molecule of HMDSO is higher.
This presumably leads to a higher degree of HMDSO
c_HMDSO = 1E-3
c_HMDSO = 1E-4
c_HMDSO = 1E-5
1.9
1.7
1.5
𝐼𝑆𝑆−𝐶𝐶𝐶
The XPS calibration is only valid in the range between
6 at% and 30 at% of carbon. Concentrations above 30 at%
were determined by extrapolation and not yet validated.
The advancing water contact angle was measured using
a dosing rate of 0.06 µl/s and an elliptical fit for the
contour of the drop. The receding water contact angle was
measured using a dosing rate of 0.1 µl/s and a leaning
tangent.
All samples were tested for release properties by a tape
test at 90° on a universal testing machine using a
Tesa 4333 adhesive tape that was mounted for 48 h and
then pulled off at a velocity of 120 mm/min. On the
samples with the best release properties a tape was
mounted and pulled off for five times. If significant
amounts of film material were transferred to the adhesive
side, the tape would not achieve its original adhesive
force on a reference substrate, which was tested
thereafter.
2
2.1
1.3
0
20
40
60
carbon concentration / at%
80
Fig. 2. Plot of O/Si ratio of all films against their carbon
concentration. The form of the markers indicates the
HMDSO concentration used during processing.
2.1
O/Si ratio
(1)
decomposition and a higher loss of methyl groups.
Additional CO 2 will enhance this process by releasing
oxygen which reacts with the alkyl groups and creates
fugitive molecules. If a high HMDSO concentration is
used, the decomposition of a single molecule is less
efficient and most of the alkyl groups remain in the
deposited film. If just counting for O, Si and C atoms, the
carbon content in the deposited film can exceed approx.
60 %, which corresponds to the carbon content in the
HMDSO monomer.
O/Si ratio
substrate absorption bands due to the absorption of the
film. Equation 1 derives from Tompkins [4]; d p is the
penetration depth. To determine the intensities I uncoated and
I coated of the uncoated and coated substrate, the substrate
band between 1415 cm-1 and 1485 cm-1 was integrated.
c_CO2 = 10 %
c_CO2 = 1 %
c_CO2 = 0 %
1.9
1.7
1.5
1.3
0
20
40
60
carbon concentration / at%
80
Fig. 3. Plot of O/Si ratio of HMDSO films against their
carbon concentration. The shading of the markers
indicates the CO 2 concentration used during processing.
Due to their high amount of non-polar moieties, thin
films with high carbon content are expected to be
hydrophobic. Films with low carbon content are expected
to be opposite in nature – an O/Si ratio of approx. 2.0
suggests that the hydrophilicity is comparable to that of
glass.
These assumptions were confirmed by water contact
angle measurements: Fig. 4 and Fig. 5 are showing the
receding and advancing water contact angle of the
deposited films.
Silica like films (group A, Fig. 4), deposited with low a
low HMDSO concentration and a high CO 2
concentration, exhibit advancing water contact angles of
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approx. 60° and receding contact angles of less than 20°,
which are comparable to glass surfaces. If the CO 2
concentration is reduced to zero or the HMDSO
concentration is increased to 10-4, the advancing angle is
increases little to approx. 70° to 80°, but the receding
contact angle reaches angles of approx. 60° (group B,
Fig. 4). This decrease in contact angle hysteresis is
presumably caused by a decrease in polar surface groups,
as the effect of polar interactions on the receding contact
angle is higher than on the advancing contact angle.
approx. 110° with a much lower, partly not measurable
hysteresis, due to a very low amount of polar surface
groups. The three single results that were not grouped
seem to be outliers; it was not possible for us to reproduce
them.
From the contact angle measurement results one can
assume that groups C1 and C2 have fair release
properties. Fig. 6 and Fig. 7 show the results of the tape
test and the deposition rate for each film.
B
C2
C1
A
0
20
40 60 80 100 120 140 160
advancing WCA / °
Fig. 4. Plot of receding against advancing water contact
angle. The form of the markers indicates the HMDSO
concentration used during processing. The dotted line is a
guide for the eye for a contact angle hysteresis of zero.
The groups A, B, C1 and C2 are explained in the text.
receding WCA / °
line force / N cm-1
c_HMDSO = 1E-3
c_HMDSO = 1E-4
c_HMDSO = 1E-5
160
140
120
100
80
60
40
20
0
c_CO2 = 10 %
c_CO2 = 1 %
c_CO2 = 0 %
c_HMDSO = 1E-3
c_HMDSO = 1E-4
c_HMDSO = 1E-5
1.2
0.9
0.6
C1
0.3
C2
0.0
0
5
10
15
deposition rate / nm s-1
20
Fig. 6. Plot of line force from tape testing against the
deposition rate. The form of the markers indicates the
HMDSO concentration used during processing. The
group C2 is explained in the text.
1.5
line force / N cm-1
receding WCA / °
1.5
160
140
120
100
80
60
40
20
0
c_CO2 = 10 %
c_CO2 = 1 %
c_CO2 = 0 %
1.2
0.9
0.6
0.3
0.0
0
0
20
40 60 80 100 120 140 160
advancing WCA / °
Fig. 5. Plot of receding against advancing water contact
angle. The shading of the markers indicates the CO 2
concentration used during processing. The dotted line is a
guide for the eye for a contact angle hysteresis of zero.
The highly polymer-like films with advancing contact
angles of approx. 100° to 120° can be subdivided into two
groups with respect to the receding contact angle (groups
C1 and C2, Fig. 4). Films deposited with additional CO 2
in the process gas partly exhibit higher advancing contact
angles of up to approx. 120°, but the receding contact
angle ranges between 60° and 80°. This high contact
angle hysteresis is presumably caused by additional polar
surface groups or higher roughness. Films without any
CO 2 in the process gas exhibit contact angles of up to
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5
10
15
deposition rate / nm s-1
20
Fig. 7. Plot of line force from tape testing against the
deposition rate. The shading of the markers indicates the
CO 2 concentration used during processing.
Groups C1 and C2 exhibit the best release properties if
compared to the other groups and the line force of approx.
0.45 N/cm on an uncoated PP substrate. Although all
films that belong to group C2 were deposited with high
HMDSO concentration, the deposition rate of less than
2 nm/s is comparably low. In dependence of the CO 2
concentration, the deposition rate of Group C1 is in the
range of 5 nm/s to 20 nm/s, whereas the low deposition
rate processes reveal the best release functionality.
Groups A and B, which are not indicated in Fig. 6, have
adhesion promoting properties and, due to a lower
HMDSO concentration during the process, a low mean
deposition rate with a maximum of 5 nm/s. For films
3
deposited with low HMDSO concentration, there is a
clear positive dependence of the adhesion promotion
ability from CO 2 concentration.
4. Summary and Outlook
This study focused on the variation of HMDSO and
CO 2 concentration in the process gas using a screening
design of experiments in a wide concentration range. The
chemical composition of the deposited films can be
controlled by controlling the concentrations of HMDSO
and CO 2 in the process gas. The higher the concentration
of HMDSO and the lower the concentration of CO2, the
higher the carbon content in the film. For silica like,
carbon free films opposite parameters have to be chosen.
The wetting properties of the deposited films can be
controlled in a wide range depending on these two
process parameters. Very hydrophilic, silica-like films as
well as polymer-like hydrophobic films can be deposited.
The hysteresis can be controlled by the amount of CO 2 in
the process gas: The lower the CO 2 concentration, the
lower the amount of polar groups at the surface and thus
the higher the receding contact angle and the lower the
hysteresis respectively.
Films with good release properties could be deposited
at low to medium deposition rates of up to 5 nm/s. As thin
films are adequate for transferring a surface property onto
a surface, even the low deposition rate processes could be
interesting for industrial application.
After this parameter screening, we would like to set
main emphasis on the optimization of film properties for
different applications, comprising release films,
hydrophobic films, hydrophilic films and barrier films.
[3] U. Lommatzsch and J. Ihde, Plasma Processes and
Polymers, 6, 10 (2009)
[4] H. G. Tompkins, Applied Spectroscopy, 28, 335
(1974)
[5] J. Schäfer, R. Foest, A. Quade, A. Ohl and K.
Weltmann, Journal of Physics D: Applied Physics, 41, 19
(2008)
5. Acknowledgements
We would like to thank Ms Antje Jung and Mr Jens
Philipp from the Institut für Oberflächentechnik (IOT) of
the Technical University Braunschweig for XPS
measurements and Mr Andreas Pflug from the
Fraunhofer IST Simulation Group for adapting his batch
curve fitting software to the needs of FTIR spectra.
This research was funded by the German Federal
Ministry for Economic Affairs and Energy (BMWi) via
the Industrial Collective Research (IGF) funding scheme
of the German Federation of Industrial Research
Associations AiF e. V. under the code number 17465 N/1.
The project was accompanied by the AiF member
organisation Deutsche Gesellschaft für Galvano- und
Oberflächentechnik e. V. We would also like to thank all
industrial partners for their support.
6. References
[1] D. Merche, N. Vandencasteele and F. Reniers, Thin
Solid Films, 520, 13 (2012)
[2] J. Schäfer, S. Horn, R. Foest, R. Brandenburg, P.
Vasina and K.-D. Weltmann, In: 12th International
Conference on Plasma Surface Engineering PSE
Proceedings, 205, Supplement 2 (2011)
4
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