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S. Miyagawa
al. Frontier Carbon Technology
New
Diamondetand
Vol. 16, No. 1 2006
MYU Tokyo
33
NDFCT 502
Electrically Conductive Diamond-Like Carbon
Coatings Prepared by Plasma-Based Ion
Implantation with Bipolar Pulses
Soji Miyagawa*, Setsuo Nakao, Junho Choi,
Masami Ikeyama and Yoshiko Miyagawa
National Institute of Advanced Industrial Science and Technology (AIST),
Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
(Received 14 October 2005; accepted 5 December 2005)
Key words:
DLC, PBII, conductivity, pulse
An electrically conductive diamond-like carbon (DLC) coating technique has been
developed by plasma-based ion implantation with bipolar pulses. Energetic electrons and
ions, which were induced by positive and negative pulses, respectively, were bombarded
alternately on a substrate in C7H8 plasma. As a result, the resistivity of DLC film decreased
with increasing ion energy under electron bombardment, and reached 1 m1cm at –20 kV.
The hardness of the film was 5.4 GPa, and the hydrogen atomic concentration was 10%.
TEM observation showed that the electrically conductive DLC films are composed of
clusters of graphitelike aggregates. The technique can be applied for conductive DLC
coatings on a three-dimensional substrate with high adhesion strength.
1.
Introduction
DLC coatings are of technological interest for properties such as high hardness, low
friction coefficients, and chemical inertness with corrosive resistance. DLC films are
generally characterized by high electrical resistivity spanning a large range of values, from
102 to 1016 1cm, depending on the deposition technique and conditions.(1,2) The electrical
resistivity of DLC films can be reduced by several orders of magnitude through incorporation of metals or nitrogen in the films.(3,4) Recently, superhard conductive carbon
nanocrystallite films of 50 m1cm prepared by ECR sputtering have been reported.(5)
Graphite has high electrical conductivity (53 +1cm on basal plane(5)) and possesses high
chemical inertness against corrosion. However, as regards the mechanical properties of
graphite films, the hardness and the adhesion strength to the substrate, such as stainless steel,
are a serious hindrance for practical uses. Therefore, a carbon coating technique with high
electrical conductivity, high hardness, and high adhesion strength to the substrate has been
desired in many industries.
*
Corresponding author: e-mail: [email protected]
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New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006)
Various deposition methods for DLC coatings have been reported, for instance, ion beam
deposition, CVD, cathodic arc, and plasma-based ion implantation/deposition (PBII/D).
The PBII/D technique has the advantages of high adhesion strength coating because of an
intermixing layer formed by energetic ion implantation, and of conformal coating on threedimensional substrates.(6) Recently, bipolar high-voltage pulses have been applied for the
PBII technique,(7) in which pulsed glow discharge plasma is produced around the substrate
by a positive pulse, and the ions are implanted/deposited on the substrate covered with
hydrocarbon radicals using the subsequent negative high-voltage pulse. The benefits of this
technique include heating of the substrate surface by electron bombardment during the
deposition, in addition to the conformal coating on a large substrate and the simplicity of the
design. In this paper, an electrically conductive DLC coating technique with the bipolar
PBII/D is presented.
2.
Experimentals
DLC coating experiments were performed using PBII/D equipment at AIST-Chubu.
The substrate was held at the center of the cylindrical vacuum chamber (640 mm in diameter,
700 mm in length), and a bipolar pulse voltage was applied to the substrate through a feedthrough, in which a thermocouple was embedded for monitoring the substrate temperature.
The positive pulse worked both for the generation of pulsed glow discharge plasma around
the substrate and for thermal annealing during DLC deposition by energetic electron
bombardments. Target temperature was monitored with a thermocouple thermometer
insulated from the grounded chamber. Three types of substrate were used in the experiments. Silica glass plates with a 0.2 mm thickness were used for measuring the electrical
conductivity. Silicon wafers and stainless steel (SUS304) plates were used for film
characterization and for practical interests, respectively. More details about the experimental setup and plasma characteristics have been described in ref. 7. The DLC coating process
consists of Ar plasma sputter-cleaning of the substrate surface, a carbon ion implantation
using CH4 plasma for increasing the adhesion strength of DLC to the substrate, and then a
DLC deposition in C7H8 plasma. Positive and negative pulse voltages were varied within
+2 to +5 kV and –5 to –20 kV, respectively, and their pulse frequencies were 3 to 5 kHz. The
typical operating pressure was approximately 3×10–2 Pa.
After the deposition, the DC electric resistivity was measured with four-point-probe
equipment (Micro-Swiss) at room temperature, and the microhardness was measured using
a microindentor (Hysitron). The corrosion behavior was assessed by recording potentiostatic
anodic polarization curves in 0.5 M H2SO4 solution. The hydrogen concentration was
determined by ERD using 2.8 MeV He ions with a 75° incident beam angle with respect to
the surface normal. Raman D and G peak intensities and sp2/sp3 ratios were also measured
using a Raman spectroscope (Renishaw, Ar:514.5 nm) and XPS (VG, Al:K_), respectively.
The microstructure of DLC films was observed with a high resolution transmission electron
microscope (HRTEM, JEOL, JEM2010) at an accelerating voltage of 200 kV. DLC film
thickness was measured using a surface profilemeter. Target temperature can be increased
up to 1000°C by increasing the positive pulse voltage and pulse frequency. In this
experiment, most of the depositions were performed in the temperature range of 200°C to
450°C.
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S. Miyagawa et al.
3.
Results and Discussion
Resistivity (1cm)
Figure 1 shows a pulsed negative voltage dependence of the electrical resistivity of DLC
films formed between –5 kV and –20 kV. Pulsed positive voltage (+4 kV), pulse frequency
(3 kHz), deposition time (60 min), and all other parameters were kept constant. The result
shows that the resistivity of DLC films decreases with increasing negative voltage, and
reaches less than 1 mWcm at 20 kV. The target temperature increased with negative pulse
voltage and it was approximately 400°C at 20 kV. Film thickness ranges from 0.6 to 1.1 mm
depending on the negative voltage. Higher negative voltage induces an increase in ion
current on the substrate and in plasma density, resulting in an increase in film thickness,
while sputtering using energetic carbon ions and etching using atomic hydrogen at high
temperature (chemical sputtering) induces a decrease in film thickness. As a result, the
maximum deposition rate was approximately –10 kV. The resistivity can still be further
decreased by increasing the pulsed positive voltage and/or pulse frequency, which led to an
increase in the amount of electron bombardment on the substrates, and the value approached
that of graphite above 800°C.
The resistivity of DLC deposited on a stainless steel plate (resistivity 57 +1cm) under
the same deposition conditions was also measured by the four-point-probe method, and the
resistivity of the stainless steel plate coated with a DLC film of 1.0–0.5 +m thickness was
100–200 +1cm at –20 kV. In this case, an oxide layer of the stainless steel surface was
removed by an Ar plasma cleaning process prior to the DLC deposition. No exfoliation of
the film on the substrate was found even at a thickness more than 1 +m. A carbon
composition gradient formed by CH4 ion implantation in the PBII/D process was effective
in enhancing the adhesion strength of DLC films on the stainless steel substrate.(7,8) On
corrosion behavior, the DLC coated films showed a marked decrease in anodic current
density, which is more than two figures compared with those of the uncoated plate.
Figure 2 shows the pulsed negative voltage dependence of the microhardness of DLC
films. The hardness decreased to 5.4 GPa at –20 kV with increasing negative pulse voltage.
Bias Voltage (kV)
Fig. 1. Pulsed negative voltage dependence of electrical resistivity of DLC films deposited on glass
substrate at positive voltage of 4 kV and pulse frequency of 3 kHz.
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New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006)
Intensity (counts)
Hardness (GPa)
The hardness is lower than that of conventional DLC films (10–20 GPa), but higher than that
of glassy carbon (~3 GPa). The decrease in the hardness is due to the increase in graphite
content, but the decrease seems to be restricted by a blocking of crack propagation by the
randomly oriented graphite cluster, as shown later.
Raman spectra of the DLC films deposited with negative pulse voltages of –5 kV and –20
kV are shown in Fig. 3. The spectra consist mainly of two peaks, a G peak around 1570 cm–1
and a D peak around 1350 cm–1. The ratio of the D to G peak intensities I(D)/I(G) ratio was
calculated by fitting Gaussians to estimate the degree of ordering of sp2-hybridized carbon
atoms. I(D)/I(G) ratio increased continuously with negative bias voltage from 2.4 at –5 kV
to 3.4 at –20 kV, and G peak position also increased from 1575 cm–1 to 1594 cm–1, as shown
in Fig. 4. It is well known that an increase in the amount of sp2-bonded atomic sites in
amorphous carbon results in an increase in I(D)/I(G) ratio and a shift in G band position to
a higher wave number.(2) Therefore, this provides a confirmation of the promotion of sp2-
Bias Voltage (kV)
Raman Shift (cm–1)
I (D)/I (G)
G Position (cm–1)
Fig. 2 (left). Pulse negative voltage dependence of microhardness of electrically conductive DLC
films deposited on Si substrate. The applied load was 100 mgf.
Fig. 3 (right). Raman spectra of DLC films deposited at different pulsed negative voltages, –5 kV
and –20 kV.
Bias Voltage (kV)
Fig. 4. Negative voltage dependences of I(D)/I(G) ratio and G position of Raman spectra of DLC
films.
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S. Miyagawa et al.
bonding with increasing ion energy. XPS spectra were also measured for the film formed
under the conditions of +4 kV and –20 kV, as shown in Fig. 5. The spectrum has been
deconvoluted into three binding energies representing bonding states, and sp2/sp3 ratio can
be estimated to be 0.8. Figure 6 shows the negative voltage dependence of the hydrogen
concentration obtained from the ERD spectrum of DLC films. The hydrogen concentration
decreases from 24% at –5 kV to 10% at –20 kV.
Figure 7 shows an example of an HRTEM image of the DLC film obtained under the
deposition conditions (+4 kV, –20 kV). The figure shows many fringe patterns, and the
fringe spacing is approximately 0.34 nm, which corresponds to the lattice spacing of the
graphite (002) plane. The direction of the lattice is oriented randomly and the film consists
of densely distributed graphite clusters.
12000
8000
Hydrogen Fraction (at%)
Intensity (counts)
10000
6000
4000
2000
0
280
282 284 286 288
Binding Energy (eV)
290
Bias Voltage (kV)
Fig. 5 (left). C1s XPS spectra of DLC films (+4 kV, –20 kV).
Fig. 6 (right). Hydrogen concentration of DLC films measured by ERD method.
Fig. 7. HRTEM bright-field image of DLC films deposited at –20 kV.
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New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006)
The deposition takes place due to repeat sticking of neutral radicals and bombardment
of energetic ions and electrons on the film surface. The energy deposition on the
subplantation layer by energetic ions and electron bombardment leads to considerable local
heating and hydrogen desorption. It is known that ion energy plays a critical role in the
deposition, and the ion energy used in the deposition is much larger than the energy, around
100 eV per C ion, at which the highest fraction of sp 3 is formed. (2) Energetic ion
bombardments result in an atomic redistribution of mobile hydrogen due to displacement of
hydrogen from CH bonds, and the displaced hydrogen can then diffuse toward the surface
under the electron bombardment and recombine with other hydrogen to form H2 molecules,
which are desorbed from the film surface. This enhances the graphitization of growing DLC
films. The surface topography of the DLC films prepared even at –20 kV was optically flat.
This may be ascribed to the removal of dissociated carbon atoms by atomic hydrogen in
plasma, because of predominant chemical sputtering (etching) in the temperature range.(9)
The electrical conductivity of the DLC films at room temperature could be modeled as
taking place by thermally activated conduction along linkages or chains of sp2 carbon atoms
with a variable range and variable orientation hopping.(10,11)
4.
Summary
Electrically conductive DLC coatings on a three-dimensional substrate were performed
using PBII/D with bipolar pulses. The resistivity of the film decreased with the pulsed
negative voltage and reached 1 m1cm at –20 kV. It has been shown by Raman spectroscopy
and by TEM observation that DLC films are composed of clusters of graphitelike aggregates.
The electrically conductive DLC coated stainless steel can be used as an electrode in
electrochemical studies, such as those involving water treatment and fuel batteries.
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