NEXAFS study on the local structures of DLC thin
films formed by Ar cluster ion beam assisted deposition
Kazuhiro Kanda*, Teruyuki Kitagawa†, Yutaka Shimizugawa*, Harushige
Tsubakino†, Isao Yamada*, and Shinji Matsui*
*Laboratory of Advanced Science and Technology for Industry,
Himeji Institute of Technology, 3-1-2 Kouto, Kamigori, Hyogo 678-1205, Japan
†
Graduate School of Engineering,
Himeji Institute of Technology, 2167 Shosya, Himeji-shi, Hyogo 671-2201, Japan
Abstract. Near-edge X-ray absorption fine structure (NEXAFS) spectra were measured for the optimization of
synthesis conditions on the production of diamond-like carbon (DLC) thin films by the Ar gas cluster ion beam
(GCIB) assisted deposition of fullerene. The sp2 contents of DLC films were estimated from the analysis of the
peak corresponding to the transition of the excitation electron from a carbon 1s orbital to a π* orbital in the
NEXAFS spectrum of the carbon K-edge over the excitation energy range 275-320 eV. Substrate temperature
and Ar cluster ion acceleration voltage in the synthesis conditions of DLC films were optimized to make the
sp2 content minimum.
INTRODUCTION
use at next generation technology. Our group
developed the novel synthesis method of DLC
thin films using gas cluster ion beams [1,2]. The
DLC films fabricated by this method exhibit good
properties of hardness, adhesion and wear
resistance for comparison with the DLC films
formed by other methods. For the optimization of
deposition conditions for the development of
high-quality DLC films, a rapid and reliable
evaluation method was demanded. In the previous
work, we measured the NEXAFS spectra of
carbon K-edge of some DLC films using
synchrotron radiation [3]. The sp2 content of DLC
film produced by the GCIB method was found to
be lower than those by other methods. In addition,
the hardness of DLC film measured with a
nano-indentation technique was strongly related
to the sp2 contents estimated from NEXAFS
spectrum. Therefore, the NEXAFS measurement
was effective in evaluating DLC films. In the
present study, the substrate temperature
Recently, DLC thin films have been
extensively investigated because they are
applicable to various industrial fields due to their
similar properties to diamond, like hardness, low
friction coefficient and chemical inertness.
Mechanical and electronic properties of carbon
material are related to the coordination of the
carbon atoms in films. The DLC films with a
high content of sp3 hybridized carbon, that is a
low content of sp2 hybridized carbon, have been
used by taking advantage of their high hardness.
Therefore, the sp2 content is the most important
information for understanding the properties of
the DLC film.
DLC films have been dominantly produced by
vapor phase methods, such as RF plasma method,
ECR plasma method, ion plating method and so
on. However, material properties of DLC films
produced by these methods were not sufficient to
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759
dependence and the acceleration voltage
dependence in the NEXAFS spectra were
measured in order to optimize the conditions for
forming the DLC film.
For the measurement of the acceleration
voltage dependence, the DLC films were formed
GCIB assisted deposition at Ar cluster ion
acceleration voltage ranging from 3 to 9 kV at
intervals of 2 kV. Substrate was not heated during
deposition.
The NEXAFS measurements were carried out
at the BL8B1 stage by using the 0.75 GeV
electron storage ring of UVSOR at the Institute
for Molecular Science [4]. The experimental
apparatus and procedures employed in the present
study were identical to those in our previous study
[3]. The synchrotron radiation dispersed by a
constant-deviation
constant-length
spherical
grating monochromator was perpendicularly
irradiated to the surface of sample film. The
NEXAFS C K-edge spectra were observed in the
energy range of 275-320 eV with the total
electron yield mode. Typical resolution was 0.5
eV FWHM. The measured signals were
normalized by the spectrum from a gold thin film
in order to compensate for the energy-dependent
intensity of photon flux from the monochromator.
EXPERIMENT
The details of synthesis method of DLC films
by Ar GCIB-assisted deposition of fullerene have
been described in the previous paper [1,2]. In
brief, a neutral Ar cluster beam was formed by
adiabatic expansion of gases through a small
nozzle into high vacuum. After ionization by
electron bombardment from a tungsten filament
with ionization voltage of 150 V, the Ar cluster
ion beam was irradiated to the substrate.
Monomer ions and small-size cluster ions were
eliminated by the electrical fields of an ionizer
and retarding potential technique in order to avoid
the damage by these high-speed particles. The
current density of the Ar cluster ion beam was
more than 3 µA/cm2 for substrates of 90 mm2.
The mean size of Ar cluster ions was estimated to
be ≈2000 atoms/cluster using retarding potential.
Fullerene consisting mainly of C60 was
evaporated to the substrate from a heated crucible.
The use of fullerene as the carbon source has the
following advantages: hydrogen free, easy to treat,
easy to obtain and relatively cheap. The ratio of
fullerene to Ar cluster ion was 1.5. The typical
growth rate of the DLC film was ≈3 nm/min. Film
thickness of produced DLC film was 300 nm. The
pressure in the deposition chamber was kept
under 1×10-3 Pa during deposition.
The substrate silicon was heated using a
mounted heater. The temperature of substrate was
monitored with a thermocouple. The uncertainty
of temperature was estimated ±5 C°. The DLC
films were deposited at four kinds of temperature,
room temperature, 100 C°, 200 C° and 250 C°.
On the production of DLC films at 100 C°, 200
C° and 250 C°, the substrate was kept at each
temperature during deposition. The production of
DLC film at room temperature meant the
deposition without heating. The substrate
temperature was raised from ≈20 C° to ≈60 C° in
the deposition by radiant heat from a tungsten
filament. The Ar cluster ion acceleration voltage
was kept at 5 kV.
RESULT AND DISCUSSION
Normalized absorption [arb.units]
Figure 1 shows NEXAFS carbon K-edge
spectra of the DLC thin films produced by the
GCIB assisted deposition at the various substrate
temperatures. The spectrum of DLC thin film
produced by RF plasma method is also depicted
for comparison. The NEXAFS spectra of various
carbon materials have been investigated
RF plasma
250℃
200℃
100℃
Room Temp.
275
280
285
290
295
300
305
310
315
Photon energy [eV]
FIGURE 1. NEXAFS C K-edge spectra of the
DLC films formed by GCIB-assisted deposition at
the various substrate temperatures.
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previously [5-7]. A pre-edge resonance at 285.3
eV is due to transitions from the C 1s orbital to
the unoccupied π* orbitals principally originating
from sp2 (C=C) sites, including the contribution
of sp (C≡C) sites if present. This peak is not
almost visible in the spectrum of diamond,
because diamond consists of only carbon atoms in
the sp3 (C-C) sites. The broad bands observed in
the 288-310 eV range are assignable to the results
of overlapping C 1s → σ* transitions at sp, sp2
and sp3 sites of the DLC film [8].
The peak positions and spectral shapes in the
NEXAFS spectra of all DLC films in Fig. 1
almost resembled each other. For comparison of
NEXAFS spectrum of the DLC film formed by
RF method with spectra of the DLC films
produced by the GCIB assisted deposition, the
peak intensity of C 1s → π* transition in the
former was larger than those in the latter. This
indicated that the sp2 content of the DLC film
formed by RF plasma was larger than those of the
DLC films produced by the GCIB assisted
deposition as reported in ref. [3]. For comparison
between NEXAFS spectra of the DLC films
produced by the GCIB assisted deposition, the
width of C 1s → π* resonance increased with the
substrate temperature. The broadening of C 1s →
π* resonance was considered that sp2 bonded
carbon atoms existed in the various local
structures, which have different chemical shifts.
The peak intensity of C 1s → π* resonance also
increased with the substrate temperature.
The estimation procedure of sp2 content from
NEXAFS was described in the previous paper [3].
The amount of sp2 bonded carbon atoms can be
extracted by normalizing the area of the resonance
corresponding to 1s→π* transitions at 285.3 eV
with the area of a large section of the spectrum.
The relative sp2 content was determined by
comparing this ratio with the standard ratio
obtained in the same manner for a reference
material. In this study, the DLC thin film formed
by the RF plasma method was assumed as a
tentative reference material. The error range of
the determined sp2 content was estimated to be
less than 10 %.
Figure 2 shows the substrate temperature
dependence of the relative sp2 content of the DLC
thin films formed by the GCIB-assisted
Relative sp 2 content
deposition. The DLC film deposited at room
temperature was plotted at 40 °C. The relative sp2
content increased with the substrate temperature
from 0.68 at room temperature to 1.17 at 250 °C.
Then, the DLC film with the lowest sp2 content
was formed at room temperature. As a result, the
substrate is necessary to keep at low temperature
to the produce the high quality DLC thin films,
because raise of substrate temperature causes the
enhancement of the contents of sp2 bonded carbon
in the DLC thin film.
1
0.5
0
0
50
100
150
200
250
Substrate temperature (℃)
FIGURE 2.
temperature.
Relative sp2 content vs substrate
Normalized absorption [a.u.]
In the Ar GCIB-assisted fullerene deposition,
the acceleration voltage of Ar cluster ion beam,
that is collision energy between the Ar cluster ion
and the substrate, is the important parameter of
the formation of the DLC film. In the case that the
acceleration voltage was not sufficiently high, the
R F plasm a
9kV G C IB
7kV G C IB
5kV G C IB
3kV G C IB
275
280
285
290
295
300
305
310
315
Photon energy [eV]
FIGURE 3. NEXAFS C K-edge spectra of the DLC
films formed by GCIB-assisted deposition at the
various Ar ion acceleration voltages.
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growth rate of the DLC film was reduced by the
decrease of cluster ion beam current, and the
structure of fullerene could not be destroyed well
due to the shortage of collision energy in the
reaction region. On the other hand, in the case that
the acceleration voltage was significantly high,
the DLC film was damaged by high-speed small
particles in the cluster ion beam. Figure 3 shows
the NEXAFS spectra of the DLC thin films
formed by GCIB-assisted deposition at various
acceleration voltages. The intensity of 1s→π*
peak take a minimum at the acceleration voltage
of 5 kV.
The acceleration voltage dependence of the
relative sp2 contents estimated by the above
procedure was shown in Figure 4. The DLC film
with the lowest sp2 content was formed by Ar
GCIB-assisted deposition at 5 kV acceleration
voltage in this voltage region. Therefore, the
acceleration voltage of 5 kV is suitable to form
the DLC films with a high content of sp3
hybridized carbon by the GCIB-assisted
deposition.
produce the high-quality DLC film with a high
content of sp3 hybridized carbon by Ar
GCIB-assisted deposition of fullerene, the
substrate temperature is demanded to keep at
sufficient low temperature, and Ar ion cluster
acceleration voltage is needed to maintain at 5 kV
during deposition.
ACKNOWLEDGEMENTS
The authors thank Mr. K. Miyauchi for his
experimental assistance. This work was supported
by New Energy and Industrial Technology
Development Organization (NEDO) and the Joint
Studies Program of the Institute for Molecular
Science.
REFERENCES
1. Yamada,
I.,
Kitagawa,
T.,
Matsuo,
J.,
and
Kirkpatrick, A., Mass. Char. Trans. Inorg. Mater.
957 (2000).
2. Kitagawa, T., Yamada, I., Toyoda, N., Tsubakino, H.,
Matsuo, J., Takaoka, and G., Kirkpatrick, A., Nuc.
Ins. Meth. B, submitted.
Integral of π* peak [arb. units]
normalized by RF plasma DLC
Figure 3: 1.0
NEXAFS C K-edge spectra of the DLC
3. Kanda,
0.9 by GCIB-assisted deposition at the
films formed
various Ar0.8
ion acceleration voltages.
K.,
Kitagawa,
T.,
Shimizugawa,
Y.,
Haruyama, Y., Matsui, S., Terasawa, M., Tsubakino,
H., Yamada, I., Gejo, T., and Kamada, M., Jpn. J.
0.7
Appl. Phys. 41, 4295 (2002).
0.6
0.5
4. Hiraya. A., Nakamura, E., Hasumoto, M., Kinoshita,
0.4
T., Sakai, K., Ishiguro, E., and Watanabe, M., Rev.
0.3
Sci. Instrum. 66, 2104 (1995).
0.2
5. Batson, P.E., Phys. Rev. B 48, 2608 (1993).
0.1
6. Jaouen, M., Tourillon, G., Delafond, J., Junqua, N.,
3
4
5
6
7
8
9
and Hug, G., Diamond Relat. Mater. 4, 200 (1995).
Ar cluster ion acceleration voltage [keV]
7.
Lenardi, C., Piseri, P., Briois, V., Bottani, C.E., Li
Bassi A., and Milani, P., J. Appl. Phys. 85, 7159
FIGURE 4. Relative sp2 content vs acceleration
(1999).
voltage.
8.
Morar, J.F., Himpsel, F.J., Hollinger, G., Hughes,
G., and Lordan, J.L., Phys. Rev. Lett. 54, 1960
SUMMARY
(1985).
The substrate temperature dependence and
acceleration voltage dependence of the local
structure in the DLC films formed by Ar
GCIB-assisted fullerene deposition method were
investigated by the NEXAFS measurement of
carbon K-edge using synchrotron radiation. The
sp2 content of DLC film was found to increase
with the substrate temperature. In order to
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