Tailor-made plasma-polymerized tetravinylsilane
L. Hoferek, R. Trivedi, S. Kontarova, V. Cech
Institute of Materials Chemistry, Brno University of Technology
Purkynova 118, CZ-612 00 Brno, Czech Republic
Abstract: Plasma polymer films of tetravinylsilane were deposited on silicon
wafers using an RF glow discharge at a range of RF power (10-70 W).
Spectroscopic techniques revealed that physico-chemical properties of plasma
polymer depend on the RF power used if the flow rate was constant. An
organic/inorganic character (C/Si ratio) of films and a content of vinyl groups
could be controlled by the RF power. The refractive index of films increased with
power from 1.7 (10 W) to 2.1 (70 W) at a wavelength of 633 nm and the
extinction coefficient rose sharply with the power 0.18 (10 W)–0.30 (70 W) at a
wavelength of 250 nm. The Young’s modulus and hardness of plasma polymer
could be varied from 12 GPa (10 W) to 81 GPa (70 W) and from 0.84 to 8.8 GPa,
respectively.
Keywords: PECVD, tetravinylsilane, thin films
1. Introduction
Plasma-polymerized organosilicones constitute a
class of materials with a rich and varied scientific
background [1,2]. This class of materials possesses a
special characteristic, which distinguishes it from
other plasma polymers – the ability to vary and
control the degree of its organic/inorganic character
(i.e., the carbon content) and the polymer crosslinking by the appropriate choice of fabrication
variables [3]. This allows one to control many
physicochemical properties over wide ranges
resulting in an extraordinary potential for useful
applications, which are only now beginning to be
tapped. Such films can find many applications as
low-k dielectrics [4], gas barrier coatings [5],
corrosion protection [6], or functional interlayers in
polymer composites [7]. Surface properties of
plasma-polymerized tetravinylsilane (pp-TVS)
films were analyzed in our previous study [8]. The
current work is aimed at physicochemical properties
of plasma-polymerized tetravinylsilane films that
can be controlled by RF power very effectively.
2. Experimental details
Tetravinylsilane, Si–(CH=CH2)4 (TVS, purity 97%,
Sigma Aldrich), was used as the monomer for the
thin film deposition on infrared-transparent silicon
wafers ((100), 0.8×10×10 mm3, ON Semiconductor).
Argon gas (99.999%) was employed for the cleaning
procedure before plasma polymerization, Ar-plasma
pretreatment of silicon wafers, and post-deposition
treatment of samples.
The pp-TVS films were deposited on polished
silicon wafers by PECVD employing an RF (13.56
MHz) capacitive coupling system with plan-parallel
electrodes [9]. The vacuum system was evacuated to
a basic pressure of 1×10-5 Pa. The substrates were
pretreated with argon plasma (10 sccm, 5.0 Pa, 5 W)
for 10 min to improve film adhesion. Pp-TVS films
were prepared at a mass flow rate of 3.8 sccm
(3.0 Pa) and powers set at 10, 20, 25, 50, and 70 W.
Mass spectroscopy (Process Gas Analyser, HPR-30,
Hiden) was used to analyze neutral plasma products
that were pumped from the plasma reactor. A batch
of six samples was deposited simultaneously using a
special bottom electrode enabling loading of up to
six substrates under vacuum. When the deposition
process was completed, the whole apparatus was
flushed with argon gas (10 sccm, 5.0 Pa); after 60
min the chamber was evacuated to a basic pressure
of 1×10-5 Pa, and after a further 12 h the prepared
specimens were characterized by ellipsometry and
then removed to the load-lock chamber, which was
flooded with air to atmospheric pressure. The
specimens were conveyed from the chamber into a
desiccator to avoid contamination before subsequent
measurements. Six identical specimens were
prepared in one deposition cycle.
film thickness [11] using the Oliver-Pharr method
[12].
The bulk elemental composition (Si, C, O, and H) of
the thin films was studied by conventional and
resonant Rutherford Backscattering Spectrometry
(RBS) and Elastic Recoil Detection Analysis
(ERDA) methods using a Van de Graaf generator
with a linear electrostatic accelerator.
The deposited films were analyzed by various
spectroscopic techniques (RBS, ERDA, and FTIR)
in order to compare their elemental composition and
chemical structure. The atomic concentrations (Si,
C, O, and H) determined by RBS and ERDA are
plotted in Fig. 1 as a function of RF power used for
deposition of pp-TVS film. The elemental
composition of films prepared at a lower power (1020 W) was influenced by post-deposition oxidation.
The films deposited at a higher power (25-70 W)
exhibited an increase of carbon concentration at an
expense of hydrogen concentration.
The near-surface mechanical properties of the ppTVS films were investigated using a 2D TriboScope
(Hysitron) attached to an NTegra Prima Scanning
Probe Microscope (NT-MDT) enabling in situ
topography analysis. A Berkovich tip with a radius
of curvature of about 50 nm was used. The Young’s
modulus and hardness of film were determined from
unload-displacement curves measured at 10% of the
Atomic concentration [at.%]
A phase-modulated spectroscopic ellipsometer
UVISEL (Jobin-Yvon) was employed to determine
the film thickness and optical properties of the ppTVS films. The measurement range was 250 – 830
nm with a step of 2 nm; the angle of incidence was
about 70° and the spot size (100×300) μm2; the
integration time was set at 200 ms. The dispersion
dependence of the dielectric function was fitted
using the five-parametric Tauc-Lorentz formula,
which has been derived for the parameterization of
the opto-electronic response of amorphous
dielectrics [10].
100
Silicon
Oxygen
Carbon
Hydrogen
90
80
70
60
50
40
30
20
10
0
10 W
20 W
25 W
50 W
70 W
RF power [W]
Figure 1. Elemental composition of pp-TVS films as a function
of RF power.
8
O/Si
C/Si
H/C
7
Element ratio [a.u.]
Infrared measurements in the wavenumber range of
400 to 4000 cm-1 were made using a Nicolet Impact
400
Fourier
transform
infrared
(FTIR)
spectrophotometer in an H2O-purged environment.
Transmission spectra were obtained on films
deposited on infrared-transparent silicon wafers. An
absorption subtraction technique was used to remove
the spectral features of Si wafer, and background
correction was carried out before each measurement
to avoid any contribution from the atmosphere. The
spectral resolution was 2 cm-1. Approximately 256
scans were recorded to achieve a reasonable signalto-noise ratio.
3. Results and discussion
6
5
4
3
2
1
0
10 W
20 W
25 W
50 W
70 W
RF power [W]
Figure 2. Element ratio in pp-TVS films as a function of RF
power.
The organic/inorganic character (C/Si ratio) of
plasma polymer varied widely, from 4.6 to 7.3 with
enhanced power (Fig. 2).
Infrared spectra of films deposited at different power
are given in Fig. 3. The assignment of IR absorption
bands was summarized in Table 1.
JK M
N
L
D
A B
E
H I
FG
0.40
2.2
70 W
50 W
25 W
20 W
10 W
10 W
2.1
20 W
0.35
0.30
50 W
70 W
4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumber [cm ]
Figure 3. Infrared spectra of pp-TVS films prepared at different
RF power.
2.0
0.25
0.20
1.9
0.15
1.8
Extinction coefficient
25 W
Refractive index
Absorbance [a.u.]
C
Optical properties of plasma polymer films were
analyzed by spectroscopic ellipsometry in the range
250-830 nm. The refractive index was ranging from
1.7 (10 W) to 2.1 (70 W) for a wavelength of 633
nm, the extinction coefficient was 0.18 (10 W)–0.30
(70 W) for a wavelength of 250 nm (Fig. 4), and the
band gap can be controlled by RF power in range 2.0
(10 W)–1.2 eV (70 W) (Fig. 5).
0.10
Wave number
[cm-1]
Assignment
A
3650-3200
O-H stretching
B
3312
C=C stretching in vinyl
C
3000 - 2800
CH2, CH3 stretching
D
2122
Si-H stretching
E
1714
C=O stretching
F
1591
C=C stretching in vinyl
G
1461
CH2 scissoring
H
1412
CH2 deformation in vinyl
I
1255
CH2 wagging in Si-CH2-R
J
1100-1000
Si-O-C, Si-O-Si stretching
K
1015
=CH wagging in vinyl
L
959
=CH2 wagging in vinyl
M
845
Si-H bending
N
732
Si-C stretching
0.05
1.7
0.00
1.6
400
600
800
400
600
800
Wavelength [nm]
Figure 4. Dispersion dependences of refractive index and
extinction coefficient for pp-TVS films deposited at different
power.
2.2
2.0
Band gap [eV]
Absorption
band
1.8
1.6
1.4
1.2
1.0
0
10
20
30
40
50
60
70
RF power [W]
Table 1. Assignment of IR absorption bands.
Figure 5. Band gap in pp-TVS films as a function of RF power.
The Young’s modulus and hardness of plasma
polymer could be varied from 12 GPa (10 W) to 81
GPa (70 W) and from 0.84 to 8.8 GPa, respectively,
as
was
determined
from nanoindentation
measurements (Fig. 6).
18
Young's modulus
70
16
Hardness
14
60
12
50
10
40
8
30
6
20
4
Hardness [GPa]
Young's modulus [GPa]
80
2
10
10
20
30
40
50
60
70
RF power [W]
Figure 6. Power dependence of mechanical properties for ppTVS films.
4. Conclusion
An increased RF power (10-70 W) influenced the
elemental composition and chemical structure of ppTVS films, where C/Si ratio rose (4.6-7.3), and a
content of vinyl groups decreased. A higher crosslinking of plasma-polymer network with enhanced
power influenced optical and mechanical properties
of pp-TVS films. The films deposited at a lower
power (10-20 W) were strongly influenced by postdeposition oxidation resulted in a modification of
optical properties.
The RF power can be characterized as an effective
tool for tailoring of plasma polymer films according
to their applications.
Acknowledgements: This work was supported in part
by the Academy of Sciences of the Czech Republic,
grant no. KAN101120701, the Czech Ministry of
Education, grant no. ME09061, and the Czech
Science Foundation, grant no. P106/11/0738.
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