Study of Flux Ratio of C60 to Ar Cluster Ion for Hard DLC
Film deposition
K. Miyauchi1), T. Kitagawa1, 2), N. Toyoda1), K. Kanda1), S. Matsui1) and I.
Yamada1, 3)
1) Lab. of adv. science and tech. for industry, Himeji Iinst. of Tech., Ako-gun, Hyogo, 2) Nomura Plating Co., Ltd.,
Nishiyodogawa, Osaka, JAPAN, 3) Collaborative research center for cluster ion beam process technology
Abstract. To study the influence of the flux ratio of C60 molecule to Ar cluster ion on DLC film characteristics, DLC
films deposited under various flux ratios were characterized with Raman spectrometry and Near Edge X-ray Absorption
Fine Structure (NEXAFS). From results of these measurements, hard DLC films were deposited when the flux ratio of
C60 to Ar cluster ion was between 0.7 and 4. Furthermore the DLC film with constant sp2 content was obtained in the
range of the ratio from 0.7 to 4, which contents are lower values than that of conventional films such as RF plasma. DLC
films deposited under the ratio from 1 to 4 had hardness from 40 to 45GPa. It was shown that DLC films with stable
properties of low sp2 content and high hardness were formed even when the fluxes were varied from 1 to 4 during
deposition. It was indicated that this process was useful in the view of industrial application.
spectrometry and NEXAFS measurement using
synchrotron radiation were employed. With Raman
spectrometry, wide range of flux ratio dependence
study was carried out. As Raman spectrometry was not
sensitive to a slight difference of carbon film
properties, NEXAFS spectra were obtained for the
range of flux ratios of C60 to Ar cluster ions in DLC
condition. The stability of DLC deposition was
discussed in the point of this flux ratio.
INTRODUCTION
We’ve been working to develop Diamond-Like
Carbon (DLC) film formation with Ar cluster ion
assisted deposition to obtain very hard films at low
temperature. Although the energy per atom of Ar
cluster ion is several eV/atom, the energy deposition
near target surface realizes to form carbon films with
low fraction of sp2 bonding at low temperature [1]. In
the Gas Cluster Ion Beam (GCIB) assist DLC
deposition method, C60 molecules are evaporated from
a crucible as a carbon source and Ar cluster ions
bombard them on a target simultaneously. It is
important to determine the optimum flux ratio of
incoming C60 molecules and Ar cluster ions to deposit
hard DLC films. From previous study, the ratio of C60
molecules to Ar cluster ions was 2 when the
acceleration energy of Ar cluster ion was 7keV. Under
this condition, DLC films had hardness of 42GPa,
lower sp2 content than that formed by Chemical Vapor
Deposition (CVD), surface average roughness below
0.5 nm, friction coefficient below 0.1 against the
material of alumina [1,2].
EXPERIMENT
DLC deposition system with Ar cluster ion assisted
deposition was constructed based on an ion beam
assisted deposition (IBAD). In this system, C60
molecules were evaporated from a crucible as a carbon
source and Ar cluster ions bombarded a target
simultaneously with acceleration energy up to 9keV.
In this study, the acceleration energy of Ar cluster ions
was fixed at 7keV. Ar cluster ion beams were formed
by ionization of neutral Ar cluster beam generated by
supersonic expansion of Ar through a small nozzle into
high vacuum [3]. A source gas pressure was 0.4MPa.
The ionization electron energy and current were
To investigate this flux ratio dependence on DLC
film properties more precisely, both Raman
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
719
12
Acceleration energy: 7keV
10
1000 1200 1400 1600
-1
Raman Shift [cm ]
8
(b)
6
4
2
0
1
10
100
C60 molecule/Ar cluster ion
(a)
Raman Intensity [arb.units]
Deposition film growth rate[nm/min]
The bonding structure of DLC film was evaluated
with Raman spectroscopy and Near Edge X-ray
Absorption Fine Structure (NEXAFS) using
synchrotron radiation. In Raman spectra measurements,
a wavelength of an excitation laser was 532 nm
(Tokyo Instruments, PLM-3). NEXAFS measurements
of carbon K-edge were performed using synchrotron
radiation of soft X-ray under the range of the photon
energy from 275 to 318eV. The absorption of the Xray was measured with total electron yield mode at
0.5eV FWHM resolution. The measurements were
carried out at the BL8B1 station of UVSOR in the
Institute for Molecular Science [4]. Hardness of DLC
film was measured by Nano-indentation technique
(Tribo-Scope, Hysitiron Inc.). The values of the
hardness were average ones from 5 times
measurements. The indentation depths were shallow
enough not to be affected by hardness of Si substrates.
Moreover the average surface roughness (Ra) of the
film was measured with Atomic Force Microscope
with scanning area of 10µm square (AFM, JSPM-4200,
JEOL).
Raman Intensity [arb.units]
formed. Figure 1(c) shows a Raman spectrum of
carbon films deposited with the ratio above 10. This
spectrum is identified as C60. In this region, DLC was
not deposited due to the shortage of Ar cluster ion flux.
In the case of the ratio below 0.5, no films were
deposited on the substrate. From these results, it was
demonstrated that DLC film was formed under flux
ratios of C60 molecules to Ar cluster ions from 0.7 to 4
at Ar cluster ion acceleration energy of 7keV.
150eV and 160mA, respectively. The ion current
density was 3µA/cm2 at acceleration energy of 7keV.
Substrate was kept at room temperature during
depositions. The flux of C60 molecules was obtained
from a deposition rate measured with a quartz crystal
monitor. Also the flux of Ar cluster ions was measured
by an ion current density at a target. The thickness of
carbon films was 0.3 µm. The DLC formation using
Ar cluster ion beam assisted deposition was also
described in detail on the literature [1,2].
1000 1200 1400 1600
-1
Raman Shift [cm ]
(c)
FIGURE1.
(a) C60 molecule/Ar cluster ion ratio
dependence on deposited film growth rate with Ar cluster
ion energy of 7keV, (b). Raman spectrum of carbon film
formed under the condition of C60 molecule/Ar cluster ion
ratio between 0.7 and 4, (c). Raman spectrum of carbon film
formed under the condition of C60 molecule/Ar cluster ion
ratio
As it was difficult to discuss the difference of DLC
films deposited under flux ratio between 0.7 and 4
with Raman spectrometry, NEXAFS measurements
were performed for these carbon films. The sp2 content,
which represented graphitic carbon contents, was
usually employed to identify DLC films from
NEXAFS spectra [6]. Figure 2 shows NEXAFS
spectra of DLC films formed by Ar GCIB assisted
deposition under narrow flux ratios. NEXAFS
spectrum of a DLC film deposited by RF plasma
method was also shown in figure 2 as a standard. A
pre-edge resonance at 285.3eV is identified with
transitions of the inner shell electron from C 1s to
unoccupied π* orbital principally originating from sp2
(C=C) hybridized orbital [7]. This peak does not exist
in the diamond spectrum [8], because the diamond
consists of only sp3 (C-C) orbital. Therefore the
intensity of the peak at 285.3eV represented as an
index of sp2 content. By taking integrals of the peak at
285.3eV, contents of sp2 bonding or graphitic carbon
contents could be compared.
RESULT AND DISCUSSION
The flux ratio of C60 molecules to Ar cluster ion
was studied with Raman spectrometry, in order to
investigate films deposited with the wide range ratio.
Figure 1(a) shows the flux ratio dependence on
deposited film growth rate at Ar cluster ion
acceleration energy of 7keV. The deposited film
growth rate was decreased with various fluxes though
evaporation rate of C60 was constant of 15nm/min,
because sputtering of Ar cluster ion to the carbon films
occurred. In Figure 1(a), three areas are drawn and
deposited films in each area are different. Figure 1(b)
shows a Raman spectrum of a carbon film formed in
region of the ratio among 0.7 and 4. This broad
spectrum resembled to a DLC film deposited with RF
plasma and various CVD deposited DLC films [5].
Therefore, in this region, mainly DLC films were
720
Normalized absorption [arb.units]
DLC films deposited under various fluxes ratios.
When the flux ratio was among 1 and 4, hardness was
in the range from 40 to 45Gpa. It was shown that DLC
films with the stable properties of low sp2 content and
high hardness were formed even if the fluxes were
varied from 1 to 4 during deposition. The average
surface roughness (Ra) of the film was measured by
AFM. Various Ra of the DLC films formed under the
ratios from 0.7 to 4 were in the range from 0.3 to
0.6nm. Therefore, it figured out that Ra values were
independent on the ratio, and were enough lower.
RF plasma
ratio:4
ratio:2
ratio:1
ratio:0.7
Acceleration energy: 7keV
275 280 285 290 295 300 305 310 315
50
Photon energy [eV]
45
40
Hardness [GPa]
FIGURE2. Carbon K-edge NEXAFS spectra of DLC films
formed by Ar GCIB assisted deposition and by RF plasma
method
Figure 3 shows integrals of this peak in NEXAFS
spectra shown in figure 2. The integral of sp2 contents
were normalized with that of RF plasma DLC.
Compared to DLC films with RF plasma deposition,
the peak intensities of DLC film deposited under
various flux ratios were approximately 60 to 70%. It
meant that low-sp2 contents or high-sp3 contents in the
DLC film was realized with Ar cluster ion assisted
deposition. However, there was no remarkable
dependence on the flux ratio. From these results, it
could be said that stability of DLC film deposition
process with constant low-sp2 contents was very high
against the flux ratio of C60 to Ar cluster ions.
Integral of π* peak
normalized by RF plasma DLC
25
20
15
5
0
0
Acceleration energy: 7keV
1
2
3
4
C60 molecule / Ar cluster ion ratio
FIGURE4. C60 molecule / Ar cluster ion ratio dependence
on the hardness of DLC films with Ar cluster ion energy of
7keV
SUMMARY
0.9
0.8
In order to investigate the influence of various
fluxes conditions of C60 molecule to Ar cluster ion on
DLC film characteristics, DLC films were deposited
under the fluxes from 0.7 to 4 with Ar cluster ion
acceleration energy of 7keV at room temperature.
DLC films with stable properties of low sp2 content,
high hardness and smooth surface were obtained with
the ratio from 1 to 4. Therefore the DLC deposition
with GCIB technique had the wide process window. It
was indicated that GCIB process for DLC deposition
was useful in the view of industrial application.
0.7
0.6
0.5
0.4
0.3
0.1
0
30
10
1.0
0.2
35
Acceleration energy: 7keV
1
2
3
4
C60 molecule / Ar cluster ion ratio
FIGURE3. C60 molecule/Ar cluster ion ratio dependence on
sp2 contents of DLC films with Ar cluster ion energy of
7keV
ACKNOWLEDGMENTS
This work is supported by New Energy and
Industrial Technology Development Organization
(NEDO).
In the viewpoint of hardness of film, the deposition
process was also very stable. In Figure4, hardness of
721
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