Kinetics and Characterization of Hydrogenated Carbon with Ruthenium
Interlayer
Jianhui Wang, Sam Zhang, Huili Wang, Khaiern Wong, Quan Zhou,
Yongzhong Zhou
PII:
DOI:
Reference:
S0040-6090(09)00508-2
doi: 10.1016/j.tsf.2009.03.160
TSF 25893
To appear in:
Thin Solid Films
Please cite this article as: Jianhui Wang, Sam Zhang, Huili Wang, Khaiern Wong, Quan
Zhou, Yongzhong Zhou, Kinetics and Characterization of Hydrogenated Carbon with
Ruthenium Interlayer, Thin Solid Films (2009), doi: 10.1016/j.tsf.2009.03.160
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Kinetics and Characterization of Hydrogenated Carbon with Ruthenium Interlayer
Jianhui Wang 1, 2, Sam Zhang 1, Huili Wang 1, Khaiern Wong 2, Quan Zhou 2, Yongzhong
Zhou 2
School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang
Seagate Technology International, Recording Media Operations, 16 Woodlands Loop, Singapore
738340
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Abstract
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Avenue, Singapore 639798
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Amorphous hydrogenated carbon films were prepared by hot filament Plasma Enhanced Chemical
Vapor Deposition using acetylene as precursor. NiP-coated Al substrate was sputtered with thin
layer of ruthenium film prior to carbon deposition for its impact on carbon film growth. Film
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thickness measured and fitted using X-ray Reflectometry showed carbon film growth is an
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exothermic reaction with apparent activation energy of 914.6Jmol-1 in the temperature range studied.
Carbon film density and surface roughness changed with substrate temperature, more profound
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when substrate is pre-heated at 300oC and above, in sync with abrupt ID/IG increase at temperature
higher than 300oC. Substrate temperature higher than 200oC and below 300oC is preferred for hard
and dense structure. Higher deposition rate and porosity were observed at higher C2H2 flow rate due
to more H incorporation. Film surface morphology studied by Atomic Force Microscope showed
Root Mean Square roughness increased at higher C2H2 flow rate due to less energetic ion peening
effect. Atomic bonding structure with sp2 and sp3 contents in the films deposited at varied
temperature and C2H2 flow was analyzed using XPS.
Key words: DLC, Ruthenium interlayer, AFM, XRR, Raman spectroscopy, XPS
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1. Introduction
The use of diamond-like carbon in recording media of hard disk as overcoat material has been
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established for decades due to its exceptional HDI (head-disk interface) performance. As the typical
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film thickness for carbon overcoat has been dropped to a few nanometers, it faces more challenges
in terms of mechanical and anti-corrosion properties while not to compromise electrical
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performance of the recording media. Various metal interlayer materials, eg., Cr, Ni, Ti have been
investigated to improve tribological properties of ultrathin Diamond-like Carbon (DLC) films [1-3].
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Carbon diffusion into magnetic layer has been observed both in recording media and head film
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stack [4-6]. In our previous work [7], ultra-thin ruthenium (Ru) diffusion barrier has been used as an
interlayer to prevent inter-diffusion between carbon and magnetic films. Ruthenium is a transition
metal of exceedingly inactive properties and of very high density. In recent years, Ru has received
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much attention as diffusion barrier for copper metallization in semiconductor device fabrication [8].
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Introduction of ruthenium interlayer in hard disc application, dead layer formation due to carbon
diffusion is reduced [7]. This paper studies the deposition kinetics and properties of carbon
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deposited on ruthenium film.
2. Experimental details
The Ru films were deposited on NiP coated aluminum substrates by dc magnetron sputtering using
a commercial sputter system MDP-250B from Inte-Vac. The sputtering target used is of 99.98% of
purity. The base pressure of the chamber was about 5×10−7 mTorr. Carbon films were prepared in a
hot filament CVD system using acetylene (C2H2) as precursor after Ru deposition without vacuum
break. Gaseous C2H2 is ionized by hot electrons emitted from a filament, and the carbon containing
ionic species are repelled to substrate which is negatively biased. The carbon film thickness was
controlled at 120 Å for all samples through deposition time adjustment by fixing anode voltage
(60V), filament current (0.8A), and substrate bias (-120V). Substrate was pre-heated to desired
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temperature in vacuum through thermal radiation from a lamp heater, and temperature was
monitored by an IR sensor detecting on substrate surface.
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Film thickness and film density were estimated from X-ray reflectivity curves. Measurements were
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carried out using a PANalytical X’Pert X-ray Reflectometry (XRR) system. Surface morphology of
the films was examined by a Digital Instrument (Dimension 3000) atomic force microscopy (AFM).
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Three-dimensional imaging was done and root mean square roughness (RMS or Rq) was measured
based on a 1×1 µm area scanned at tapping mode. Structure of the carbon films was determined
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using a Raman spectroscopy (Renishaw System) excited with He–Ne laser operated at wavelength
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of 633 nm. The peak deconvolution was performed using a Gauss-Lorentz distribution function, the
fitted curves having a Chi-square less than 1.2 to ensure convergence. X-ray Photoelectron
Spectroscopy (XPS) system using a Kratos-Axis spectrometer with monochromatic AlKα (1486.71
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eV) X-ray radiation (15 kV and 10 mA) and hemispherical electron energy analyzer was employed
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3. Results and Discussion
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to characterize the bonding characteristics of carbon films.
3.1Temperature effect
The range for the substrate temperatures was evaluated up to 350oC beyond which the structure and
composition of C:H films are believed to significantly change [9]. The natural logarithm of film
deposition rate with C2H2 CVD process as a function of inverse Kelvin temperature is shown in Fig.
1. It was observed that the deposition rate decreased with increasing temperature. The trend is
similar to that reported by Kessels and etc. [10]. For plasma deposition of hydrocarbon film, there
are three general stages during the process, i.e. the reactions in the plasma (gas molecule
dissociation, ionization etc.), the plasma-surface interaction and the subsurface reactions in the film
[11]. With C2H2 involved as precursor, it is assumed that C2H and H are the major radicals in the
plasma contributing to film growth [10]. It is known that dissociation reaction of C2H2 gas is
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exothermic and thus reversely affected by temperature. At higher substrate temperature amount of
C2H immediately above the substrate surface, which is product of C2H2 dissociation, is reduced.
Furthermore, the growth rate decreases with increasing substrate temperature due to a less efficient
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incorporation of neutral radicals based on the adsorbed layer model [12]. The etching rate through
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erosion by atomic hydrogen, which increases with increased substrate temperature, as well plays an
important role affecting the temperature dependent film growth rate observed in Fig. 1. Substrate
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temperature impact on film growth rate can be further understood by calculating the activation
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energy of the process concerned. The apparent activation energy (Ea) of 914.6Jmol-1 was derived
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from the Arrhenius plot in Fig. 1 based on Arrhenius equation below.
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where k is the carbon film deposition rate, A is the pre-exponential factor, R is the gas constant, and
T is Kelvin temperature. The Ea value obtained here is considerably lower than the results reported
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by others [13]. It might be attributed to differences in the precursor selection, filament temperature
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and the range of surface temperatures applied in our studies. It showed that the hydrocarbon film
deposited on Ru-coated surface in our study is weakly activated by heat.
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3
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ln(deposition rate)
y = -0.11x + 22.745
1000/T (1/K)
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Fig. 1 Arrhenius plot of deposition rate vs. 1000/T for carbon films deposited by hot filament
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PECVD with -120V of substrate bias
Hydrogenated carbon films with Ru interlayer were analyzed with XRR. A three-layer model
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(C/Ru/NiP) was used to fit the data. Dependence of film density on substrate temperature is shown
in Fig. 2. The density of a-C:H film is an indication of the network coordination, and whereby the
mechanical performance. The film density is also a good indicator of sp3 fraction in hydrogen-free
carbon films [11]. However, in the case of a-C:H film, it is much more complicated due to hydrogen
incorporation. As CHx groups in the film occupy a lot of space, they reduce the film density. The
substrate temperature appears to have a similar effect on a-C:H as bias effect. The reduction in H
content is believed to be the primary effect of substrate heat-up which is to drive off hydrogen as
temperature is increased, thus causing an increase in density. Reduction in film thickness was
observed by Katsuhiro for post argon annealing of hydrogenated DLC films [14]. Film density
increase was accompanied with loss of H2 as indicated in the weakened C–H absorption strength for
films annealed at higher temperatures in his study.
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1.83
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1.81
1.79
1.77
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Density (g/cm3)
1.85
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1.75
100
200
300
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Temperature ( C)
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Fig. 2 Film density as deposited at various substrate temperature
The film roughness varies with substrate temperature, as measured by AFM and shown in Fig. 3. At
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higher substrate temperature rougher film structure with larger cluster size was obtained. It is
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believed that surface diffusion occurs more easily than bulk diffusion, since the activation energy is
lower. Surface diffusion can be promoted either by the energy associated with arrival of impinging
ions which remain within the surface layers or high substrate temperature [15]. Diffusion in the
surface layers of α-C:H films tends to initiate clusters with highly sp2 concentrated structure, i.e.
graphite phase at higher substrate temperature. Such clusters therefore form preferentially on the
surface and produce roughening effect. The observed effect on surface roughness is consistent with
results in literatures [16] [17].
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RMS Roughness, nm
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200
400
600
800
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Temperature, K
temperatures
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Fig. 3 Surface roughness and AFM micrographs of carbon films deposited at varied substrate
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In the amorphous carbon films, the intensity ratio of the D-peak (corresponding to disordered phase)
with respective to G-peak (corresponding to graphite) in a Raman spectrum (ID/IG) is used to
determine the degree of graphitization. The ID/IG ratio and G-peak positions were summarized in
Fig. 4. The ID/IG ratio and G-peak wavenumber increased with deposition temperature. Increase in
the ID/IG ratio indicates the increase of number of sp2 clusters and its size enlargement in DLC films
[18]. The G-peak position in the Raman spectrum provides the information about the sp2/ sp3 ratio
[19] [20]. The G-peak position for DLC films in the study traveled to a higher wavenumber with
increased temperature, meaning an increase in the sp2 bonding fraction in the structure. Therefore,
the Raman spectrum showed that higher thermal energy caused the DLC structure to be transformed
into the graphite structure. Increased temperature also led to release of hydrogen contents in DLC
films and to a transition from sp3 to sp2/ sp3 carbon hybridization. Some tetrahedral bonds in the
films have been broken and transformed to trigonal structure. This development also causes
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increase in the number of sp2 clusters (or graphitization). The result is shown in Fig. 4, in sync with
the surface roughening effect shown in Fig. 3. Therefore, proper selection of energy range and
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200
300
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0.1
400
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1500
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ID/IG Ratio
0.4
1600
1450
500
600
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G-peak position (cm )
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substrate temperature may minimize the graphitization and preserve sp3 carbon bonding structure.
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700
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Temperature (K)
temperature.
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Fig. 4 ID/IG ratio and G-peak positions extracted from Raman data as a function of substrate
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300 C
o
Intensity (A.U.)
180 C
o
60 C
2
sp C
3
sp C
282
283
284
C-O
285
286
287
288
Binding energy (eV)
Fig. 5 XPS spectra of hydrogenated DLC films deposited at 60oC, 180oC and 300oC
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The carbon films were characterized by XPS for sp3 and sp2 bonds. C1s spectra of the films
prepared at different temperatures are shown in Fig. 5. The XPS core level spectra were analyzed by
non-linear least square fitting performed after a Shirley background subtraction. The peak position
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in the C1s envelope shifts toward lower binding energy with increase in deposition temperature,
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corresponding to the shift of binding energy from 285.1eV to 284.9eV and then 284.4eV when
temperature changed from 60oC to 180oC and 300oC. The core level C1s XPS spectra of the a-C:H
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films in Fig. 5 can be resolved into two components. The first component is centered at
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approximately 284.4eV corresponding to sp2 C-C bonds, and the other is at 285.8 which is meant
sp3 C-C bonds. The temperature dependent sp2 and sp3 structure evolution observed is consistent
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with the Raman analysis demonstrated in Fig. 4. Higher deposition temperature resulted in an
increase in the graphitic fraction in the film and the clustering of the sp2 bonded carbon. This
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indicates the phase transition from diamond-like to graphite-like structure as a function of
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3.2 C2H2 gas flow effect
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temperature.
The deposition rate derived from the film thickness measured by XRR and deposition time set for
each sample varied with C2H2 flow rate as given in Fig. 6. Deposition rate increased almost linearly
when C2H2 flow rate increased from 20 to 45sccm, while it started to saturate when the flow rate
was at 50sccm beyond which it was not studied due to MFC (Mass Flow Controller) capacity
constraint. The results suggested that the DLC films formed in the process studied were chemically
restricted by the supply of ion species decomposed from C2H2 gas. When C2H2 flow rate is high
enough, the mean free path of the particles in the chamber becomes shorter. Ionic species which
contribute to film deposition might be blocked and/or recombined to neutral C2H2, resulting in a
saturated or lower deposition rate. Similar trend was reported by Miyakawa and etc who studied the
dependence of DLC deposition rate on C2H2 flow rate by means of filtered arc deposition technique
[21]. A reduced mass density of DLC films from 1.89 to 1.44g/cm3 measured by XRR was also
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noticed as C2H2 flow rate increased from 20 to 50 sccm. The result could be both explained by
energetic impingement of ionic particles and H incorporation into the films. Increased gas flow
raises the chance of collisions between atoms and ions in plasma, which results in more ion
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scattering and broadens the ion energy distribution. It leads to the loss of the effective energy of the
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impinging ions on the growing films. Increased C2H2 flow implies a reduction in the plasma density
and an increase of H incorporation in the films as well. Rossnagel et al found film density varied
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inversely with gas pressure. The variation can be attributed to the ion fraction being higher at low
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gas pressures [22]. The reduction of C2H2 gas flow to certain level favors the formation of high
density and low-hydrogen content DLC films without trade-off of deposition rate. The range within
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30-40sccm will be advised for the optimal conditions to achieve high density without compromising
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manufacturability for the setup in our study.
2.5
1.80
2.0
1.60
1.5
1.40
1.0
1.20
10
20
30
40
50
C2H2 gas flow (sccm)
Mass density (g/cm^3)
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2.00
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Deposition rate (nm/s)
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Fig. 6 Deposition rate and film density dependence on C2H2 gas flow
The dependence of sp2 and sp3 contents on C2H2 flow rate was studied. Similarly, intensity ratio of
the D and G peaks (ID/IG) and G-peak positions extracted from Raman spectra were plotted as a
function of C2H2 gas flow shown in Fig. 7. The G-peak positions and the ID/IG ratio showed a clear
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minimum point, and varied with the C2H2 flow. As the C2H2 gas flow increased from 20 to 40sccm
(first stage), the G peak and ID/IG ratio decreased. As it increases further beyond 40sccm (second
stage), the trend reversed. It has been reported that the position of the G peak and ID/IG intensity
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ratio are important factors determining the film structure. The increase of G shift and ID/IG ratio
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when C2H2 flow rate is higher than 40sccm reveals the decrease of sp3 fraction and increase of sp2
fraction. For mechanical property, the hardness of the films which is corresponding to ID/IG ratio is
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expected to decrease when C2H2 is at the range higher than 40sccm. Studies by Wei have suggested
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a correlated trend when carbon film thickness was varied [23]. The phenomenon in Fig. 7 can be
explained by the role of hydrogen in the films. It is generally accepted that hydrogen in a-C:H was
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to promote sp3 bonding, but that too high a H content produces many = CH2 groups and a poorly
cross-linked network which is on the other hand affected by ion bombardment [ 24 ].
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Dehydrogenation to produce such cross-linked network driven by energetic ions is believed to be
hindered at higher gas flow rate. In our experiment, sp2 fraction organized in aromatic rings
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decreased for the first stage as told by Id/Ig ratio. The down shift of G-peak can be explained by the
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change of the sp2 configuration from mainly rings to short chains (olefins). When C2H2 flow rate
exceeded 40sccm, the film tends to be more polymer-like structure. More free hydrogen atoms are
able to exist as clusters between the planes and in the voids. High hydrogen concentration enhances
the formation of a polymeric CHn (n>1) structure, which changes the bonding constraints of sp3bonded carbon by binding more H carbons. The increase of Id/Ig and shift of G peak position to
higher could be attributed to the simultaneous decrease of C–C sp3 (and/or C=C sp2 ) bonds and
increase of C–H bonds, because the probability of broken bond passivation by hydrogen atom
should be higher for high hydrogen concentration in the films. The disruption of the C-C sp3 and
C=C bonding by atomic hydrogen leads to the disordering of the films [25]. As a result, the
hardness of the DLC film was reduced as the structural integrity of the films was weakened due to
the presence of large amount of H. Hydrogen effusion experiments have shown that H-rich a-C:H
films present an increased density of voids, which will influence the network connectivity and
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hardness [26]. Gas flow below 40sccm is preferred to obtain a film with adequate mechanical
performance in our study.
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1500
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ID/IG Ratio
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0.0
20
30
40
50
C2H2 gas flow (sccm)
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10
1600
1450
G-peak position (cm-1)
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0.8
1400
60
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for carbon deposition
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Fig. 7 ID/IG ratio and G-peak positions extracted from Raman data as a function of C2H2 gas flow
XPS analysis on films deposited at varied C2H2 flow rates as shown in Fig. 8 further confirmed the
observation in Fig. 7. The binding energy of C1s shifted towards higher value from 284.7eV to
284.9eV when C2H2 flow increased from 20 to 40sccm, and then shifted back to 284.6eV as flow
rate further increased to 50sccm. The perceived change illustrated that the sp3 content in the film
bonding structure depends directly on the increasing C2H2 flow. Reduction of sp2 content was
observed when C2H2 increased from 20-40sccm, and seemed to increase or saturate when it
increased further.
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Intensity (A.U.)
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40sccm
3
sp C
35sccm
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sp C
282
283
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284
285
286
20sccm
50sccm
287
288
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Binding energy (eV)
Fig. 8 XPS spectra of hydrogenated DLC films deposited at varied C2H2 flow rate of 20, 35, 40,
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and 50 sccm
The film surface morphology affected by C2H2 gas flow was studied by AFM measurements and
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shown in Fig. 9. It demonstrated that the increase of C2H2 flow from 20 to 45sccm promotes film
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with rougher surface structure. An increase of C2H2 led the DLC films to be grown under less ion
impingement from positively charged ions such as C2H2+. The ion-atom interactions induced by the
impingement of these ions on the growing film surface were complicated and resulted in films
grown with more randomly oriented coarse clusters. The results indicate that the films produced at
gas flow lower than 40sccm are more suitable, because low friction coefficient related to lower
values of roughness is expected to enhance mechanical performance of the coating.
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0.6
0.4
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RMS roughness, nm
0.8
10
20
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30
40
50
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C2H2 gas flow, sccm
Fig. 9 Surface roughness dependence on C2H2 flow rate and AFM micrographs of DLC films
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deposited at C2H2 flow of 20, 35, and 45sccm
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4. Conclusions
In hot filament plasma enhanced chemical vapor deposition, the deposition rate of a-C:H films on
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Ru interlayer is inversely affected by substrate temperature. Apparent activation energy (Ea) of
914.6Jmol-1 for the process was derived from the generated Arrhenius plot. Higher substrate
temperature resulted in denser film structure but rougher surface due to dehydrogenation.
Graphitization of carbon film occurs at increased temperature, thus deposition at lower than 250oC
is preferred. The deposition rate of carbon film is increased as C2H2 flow increases, but the film
becomes more porous due to more H incorporation and less ion impingement, resulting in decreased
hardness. Hydrogenation is believed to be the key affecting bonding structures of carbon films.
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