1
Study of Facing Target Sputtered Diamond-like Carbon
Overcoats for Hard Disk Drive Media
H.L. Seet1*, K.K. Ng1, X.Y. Chen1, P. Yang2, L. Shen3, R. Ji1, H.X. Ng1, C.B. Lim1
1Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, 5 Engineering Drive 1,
Singapore 117608
2Singapore Synchrotron Light Source (SSLS), National University of Singapore, 5 Research Link, Singapore
117603
3Institute of Materials Research and Engineering, (A*STAR) Agency for Science, Technology and Research, 3
Research Link, Singapore 117602
*Corresponding author: H.L. Seet (e-mail: [email protected]; phone +65-68746934)
Abstract:
The demand for higher areal density in the hard disk drive industry has fuelled extensive
research efforts and focuses on magnetic spacing reduction. In the head-disk interface
arena, one of the key focuses is to reduce the carbon overcoat thickness without
compromising the overcoat protection performance. Thus, in the search for alternative
methods to reduce the carbon overcoat thickness, the facing targets sputtering (FTS)
process for diamond-like carbon deposition has been investigated. The resulting properties
have been presented in this paper, with comparison to conventional diamond-like carbon
(DLC) layers by other processes such as chemical vapour deposition and reactive sputtering
with nitrogen. X-ray reflectometry results showed that facing target sputtered DLC samples
displayed significantly higher density, at 2.87 g/cm3, as compared to hydrogenated and
nitrogenated DLC samples. This was attributed to the higher sp3 content, as obtained by X-
ray photoelectron spectroscopy measurements. As a result of the high sp3 content, hardness
of the FTS deposited samples was higher than that of the hydrogenated and nitrogenated
DLC samples. In addition, the surface energy of FTS samples was observed to be
comparable, but lower, than that of nitrogenated DLC samples through contact angle
measurements. Clearances comparable to that of conventional DLC samples were achieved
and the sample disks were flyable. Wear performance tests also revealed more wear
resistance for the FTS deposited DLC samples, but also higher head wear.
Keywords: Facing target sputtering; diamond-like carbon; overcoat; tribology
2
1. Introduction
To increase areal density of disk drives, magnetic spacing reduction has always been one
of the key research focuses in the hard disk drives industry. In the head-disk interface (HDI)
arena, efforts on overcoat or lubricant thickness reduction have been ever-present in the
quest for spacing reduction. This approach is very challenging, given the already minute
magnitude of the carbon overcoat and lubricant layer thickness. In recent years, there have
been explorations on alternative carbon deposition processes [1, 2] for thinner media
overcoat. For years, there have been studies on filtered cathodic vacuum arc process for
media overcoat due to its reportedly high sp3 content, hardness and density [3-6].
However, despite its promising properties, this process is plagued by issues such as high
particle count, due to its arcing nature, relatively low deposition rate and its incompatibility
with current mass manufacturing tools. As a result, the FCVA process is still confined, in hard
disk drives (HDD) industry, to the deposition of the DLC layer as a slider overcoat.
Facing target sputtering (FTS) process was first introduced by M. Naoe et al. [7, 8] in
1978, where they reported the deposition of magnetic metal films using two facing targets
and a perpendicular magnetic field (to the target surface). A patent on a FTS device was filed
in 1991 by Pioneer Electronic Corporation, Japan [9]. The first report on properties of FTS
deposited carbon was seen in 1997 [10] where K. Noda et al. reported on the Raman
spectroscopy results and surface morphology of FTS deposited carbon and demonstrated
that FTS process for carbon can take place at argon gas pressure of as low as 0.2mTorr and
can avoid plasma damage. J.R. Shi et al. [11] also reported on the beneficial effect of
decreasing Ar pressure on sp3 content and hardness. However, if nitrogen flow rate was
increased during the FTS deposition of a-C:Nx [12-16], decrease in sp3 fraction, hardness,
modulus and corrosion performance of the studied layer was subsequently observed. In
addition, low friction coefficient and good wear resistance for FTS samples were also
observed [15]. Poh et al., have reported the use of hybrid-FTS configuration in Circulus M12
production tool. Improved performance of hybrid-FTS over conventional magnetron
sputtering has been reported [16]. However, FTS is expected to be much better than hybrid
FTS in terms of the sp3 content and hardness and such carbon has not been studied in a
production tool.
In this paper, we evaluate and investigate the suitability of production tool FTS process
on the development of ultra-thin carbon layers for HDD media overcoat. The properties of
FTS deposited diamond-like carbon (DLC) were characterized and the observed results were
investigated in relation to the sp2/sp3 content. In addition, the surface energy, the flyability
performance and wear tests of the FTS deposited DLC was also evaluated.
2. Experimental Details
Sample deposition was carried out in the sputtering chambers of Intevac 200 Lean
system. The studied sample structures were of glass substrate/NiTa-36 nm/DLC layers. FTS
3
samples were deposited using Intevac confined dense plasma (CDP) source using the
following deposition parameters: a) sputtering power = 2 kW, b) Ar gas flow rate = 15 sccm;
c) pass-by speed of 54 mm/s, d) no substrate bias. The FTS process involves 2 parallel
rectangular shaped carbon (99.999%) targets (with complementing magnets behind), with
the substrate perpendicular to the target plane. The close-loop magnetic and electric field
confines the electrons and the plasma density between targets increases due to the high
electron concentration. The increased plasma density allows for the deposition process to
take place at a low working pressure. The working pressure was observed to be 147 mPa. a-
C:Hx layers were deposited using chemical vapour deposition process, with the following
deposition parameters: a) anode voltage = 60 V; b) emission current = 0.6 A; Bias voltage =
120 V; Ar gas flow rate = 2 sccm; C2H2 gas flow rate = 24 sccm while a-C:Nx layer are
deposited using DC reactive sputtering process, with the following deposition parameters:
a) sputtering power 0.5 kW; b) Ar gas flow rate = 40 sccm; c)N2 gas flow rate = 20 sccm.
The deposited layer thicknesses were measured and monitored using both transmission
electron microscopy (TEM) and XPS depth profile. (Figure 1) FEI Tecnai X-TWIN TEM system
was operated at 200 kV to view cross section of the sample after focus ion beam cut. XPS
measurement was conducted on a PHI Quantera SXM Scanning X-ray Microprobe with a
monochromatic Al Kα source. The system was operated at 15KeV, 40W, 45° take-off angle,
55eV pass-energy with 0.1eV energy gap and 200µm size of beam. XPS depth profiles were
performed with the ion energy of the Ar+ sputter gun at 500eV and the sputtering area of
2mm × 2mm. The density of the DLC films was measured by high resolution x-ray
reflectometry (HR-XRR) at grazing incidence in the X-ray demonstration and development
(XD) beamline at Singapore synchrotron light source (SSLS). The diffractometer is the Huber
4-circle system 90000-0216/0, with high-precision 0.0001 step size for omega and two-
theta circles. The storage ring, Helios 2, was running at 700 MeV, typically stored electron
beam current of 300 mA. X-ray beam was conditioned by a Si (111) channel-cut
monochromator (CCM) and toroidal focusing mirror, blocked to be 0.9 mm high in vertical
direction and 3.0 mm wide in horizontal direction by a slit system. Such set-up yielded X-ray
beam with about 0.006° in vertical divergence. The detector slit was adjusted to be 1.00 mm
high to ensure recording of all reflected photons. The typical counting time was 5 seconds
for every step and step size 0.01° in 2-theta. Diffuse scattering (background) of off-set scans
were also measured at 2-theta off-set angle of 0.20° in the range of above measurement.
The pure reflectivity was then obtained by subtracting the diffuse scattering from the raw
data. The simulations were done using simulating software M805 and LEPTOS 1.07 release
2004 (Bruker). The nano-indentation tests were performed using a MTS nano-indenter XP
system with the dynamic contact module, which offers high sensitivity of contact stiffness
measurements from the tip-material interaction. A Berkovich indenter tip (with radius less
than 20 nm) was normally used to indent coatings to a specified maximum depth (in this
case, 50 nm). The load was applied at a constant strain rate of 0.05 s-1. Modulus (E) and
4
hardness (H) were then derived from the contact stiffness (s), based on equations (1) and
(2):
𝐸𝑟 = 1
𝛽
√𝜋
2
1
√𝐴𝑐𝑠 (1)
𝐻 =𝑃
𝐴𝑐 (2)
where Er is reduced modulus calculated from equation (3), Ac is the contact area, is a
geometrical constant taken as 0.75 for Berkovich indenter, P is the instantaneous load
incurred along loading. The subscript i denotes the property of the indenter. υ is Poisson’s
ratio. Ei =1140 GPa and υi = 0.07 for the diamond indenter.
1
𝐸𝑟=
1−𝜐𝑖2
𝐸𝑖+
1−𝜐2
𝐸 (3)
The average E and H values should be taken at displacements (from surface) less than 5-10%
of studied film thickness, in order to obtain values that are free of substrate effect. For the
density and hardness measurements, the thickness of the DLC films was fixed at 20 nm and
50 nm thick respectively.
The sp2/sp3 content of the DLC films was characterized using X-ray photoelectron
spectroscopy (XPS). XPS C1s peaks were de-convoluted by five Gaussian distributions
corresponding to carbide (283.5±0.2 eV), C-C sp2 (284.4±0.2 eV), C-C sp3 (285.2±0.2 eV), C-
N/C-O (286.6±0.2 eV) and C=O (288.3±0.2 eV) respectively. The contact angle of the DLC
samples was characterized using goniometer and the surface energy calculated based on
the geometric equations. The surface roughness of the samples was obtained using atomic
force microscopy machine.
3. Results and Discussions
3.1. Materials Properties Evaluation
The hardness of the 50 nm thick FTS deposited samples was measured using nano-
indentation and the results were compared to that of a-C:Hx and a-C:Nx samples. Nano-
indentation tests are one of the typical tests on DLC layers to determine the hardness of the
layer [17]. Figure 2a shows the hardness trend of the DLC samples with the displacement
from the surface while figure 2b shows the modulus trends. For all the measurements, as
the indentation depth reaches ~5 nm, substrate effects sets in and the overall values
reduces with increasing displacement into surface. From Figure 2, it can be observed that
FTS deposited samples displayed the highest hardness at 31.5GPa (at 5-7 nm) and modulus
at 496 GPa (2-4 nm) as compared to 26GPa and 336GPa for a-C:Hx samples, and 10.5 GPa
and 151 GPa for a-C:Nx samples for the same indentation depth range.
The density of the FTS deposited samples was also investigated using XRR and
compared to DLC samples from the other processes. XRR has also been used by other
5
researchers on DLC density measurements [18]. Figure 3 shows the XRR spectra of the DLC
samples for 2-Theta values from 0.2 to 12. As observed from the zoom-in plot (and
indicated by an arrow) for the XRR spectra for 2-Theta values from 0.2 to 1, the critical
angle of the XRR spectra, corresponding to DLC layer, was observed to be the highest for
FTS deposited samples, followed by the a-C:Hx samples and a-C:Nx samples. This strongly
indicates that the density of the FTS samples is higher than that of the other two samples as
the critical angle are apparently connected with the densities of the layers [19]. After curve
fittings, density of the FTS deposited sample was found to be 2.87 g/cm3 while density of a-
C:Hx and a-C:Nx was found to be 2.08 g/cm3 and 1.95 g/cm3 respectively.
The relatively high hardness and density values of FTS deposited samples can be
attributed to the resulting sp3 content. From XPS measurements, the FTS deposited samples
were found to possess sp3 content of up to 55 % as compared to a-C:Hx of up to 42 %. Due
to high concentration of plasma sufficiently confined by crossed electric and magnetic fields
with region of target, the deposition process in FTS process could be carried out at relatively
low gas pressures. This allows for the carbon atoms or ions to reach the substrate at a
higher energy (as compared to other processes) and thus higher sp3 content layer, since the
mean free path is longer during travel (due to lower gas pressure) and collision with Ar
atoms is less likely [11]. The high plasma confinement also leads to higher probability of
ionized species during the process.
Figure 4 shows the contact angle measurements (by goniometer) of the different
DLC samples by different testing liquids (DI water, ethylene glycol and diiodomethane) while
table 1 shows the surface energy values of different DLC samples, calculated using
geometric equation. It was observed that the FTS deposited samples possessed surface
energy of 73.71 mN/m, which is lower than a-C:Nx but higher than a-C:Hx. While the
dispersive share of the surface energy stems from Van der Waals forces, the polar
component of surface energy arises from the dipole interactions which are much stronger in
nature [20]. The presence of CN bonds in nitrogenated carbon contributed to the greater
polar energy. The dispersive component of both the nitrogenated carbon and FTS carbon
appeared to be similar, despite the density of the FTS carbon being higher than the
nitrogenated carbon. This may be attributed to the difference in the surface conditions
arising from the two types of carbon systems.
3.2. HDI Performance Evaluation
The head-disk interface (HDI) related tests were performed on the Guzik V2002
spinstand system. Conventional head-disk contact detection methodology based on the
Laser Doppler Vibrometer (LDV) velocity signal [21, 22] were employed for the
determination of the head-disk contact point, or commonly referred as the touchdown
point (TDP). The experimental setup is shown in 5 (a). The LDV was set up to shine a laser
vertically at the trailing edge of a Pemto form factor (1.25 mm (L) x 0.7 mm (W) x 0.23 mm
(H)) thermal fly height control (TFC) slider. Two separate tests were performed to evaluate:
6
1. The head-disk clearance margin; and 2. The durability of the FTS disk samples as
compared to the a-C:Hx (3 nm)/a-C:Nx (1 nm) disk samples. The structure of the disk
samples consist of the DLC overcoat on top of a 36 nm thick NiTa layer, sputtered on glass
disk substrates using Intevac Lean 200 sputtering system. The mean surface roughness Ra of
the FTS deposited samples was found to be 0.125 nm. The disks were dip-coated with Z-
tetraol lubricant. The lubricant was diluted using Vertral-XF solvent to obtain concentration
of 0.03 wt%. The dip-coating process parameters were tuned to provide lubricant thickness
of 1.3 – 1.4 nm and bonding ratio of about 75 % for all samples (see table 2). The bonding
ratio was achieved by exposing the disks under ultra-violet (UV) irradiation after the dip-
coating process [23]. Similar process parameters were used for the post process of both
types of DLC samples.
For the head-disk clearance margin test, the TFC slider head was loaded on the disk
at radius of 23 mm and 0 skew angle. The disk was spinning at 5400 RPM. The initial mean-
plane spacing, or flying height was about 10 nm. The spacing was gradually reduced using
the TFC technology. In this technology, electrical power is supplied to a resistive heater
element embedded at the trailing edge of the slider, to thermally actuate a small region of
the slider head closer to the disk surface. The heater element was driven by the Agilent
33220A function generator. 25 ms DC pulses of incremental voltage were supplied to the
heater element to gradually reduce the flying height, while the root-mean-square (RMS)
values of the LDV velocity signals were closely monitored. The TDP was determined as the
heater power when the RMS value exceeded the threshold. The threshold was set at a value
that is 15% higher than the average RMS values, obtained during stable flying at 10 nm. In
order to get valid results for comparison, the same slider-head was used for both types of
disk samples. The experiments were repeated several times to ensure that the sequence of
testing will not affect the overall results and conclusions.
Figure 6 shows one of the experimental results of the clearance margin test. As the
TFC power was increased to about 88 mW, the LDV RMS ratio increased significantly. It was
found that the TDPs of about 91 mW were obtained for both a-C:Hx/a-C:Nx and FTS
samples. Hence, the experimental results confirmed that the samples with FTS DLC overcoat
have comparable clearance margin to the samples with conventional a-C:Hx/a-C:Nx DLC
overcoat.
The accelerated wear test was conducted to evaluate the durability performance of
the disk samples with FTS DLC overcoat. It is known that the TDP will be increased if a
significant amount of wear occurs at either the head or disk. Such phenomenon is due to
the removal of the asperities during contact, hence allowing more mean-plane spacing
between the head and disk that eventually requires additional thermal actuation to reach
the TDP [24]. In order to differentiate head related wear and disk related wear, the
accelerated wear test procedures were designed as follows [24]: 1) Load head at a reference
track of the disk (see Figure 5 (b)), perform touchdown test to obtain TDP at the reference
7
track; 2) Shift the head to a test track (see Figure 5(b)), which is near the reference track,
and perform touchdown test to obtain TDP at the test track; 3) Perform accelerated wear
test at the test track by applying a heater power of TDP + 40 mW for 30 minutes; 4) Perform
touchdown test at the test track to obtain TDP after the wear test. The difference in TDP
before and after the wear test is attributed to the wear contributed by both head and disk.
It is to be noted that the observed disk wear depth includes the effects of the lubricant
depletion; 5) Shift the head back to the reference track and perform touchdown test to
obtain TDP at the reference track after the accelerated wear test. The difference in the TDP
before and after the wear test at the reference track is attributed to the wear related to
head only, as the surface condition of the disk was unchanged.
The results of the accelerated wear tests are shown in Figure 7. The wear depth was
estimated by a separate set of experiment based on the Wallace spacing loss equation [25],
which provides the relationship of the heater power change versus relative mean-plane
spacing change. It was noted that the FTS samples had significantly lesser wear on the disk.
The wear depth of the FTS sample was about 1.7 Å less than the a-C:Hx/a-C:Nx sample,
which confirms that the FTS DLC is a harder film. With a harder DLC overcoat on the disk, it
is found that the head wear of the FTS sample was more severe. From Figure 7, it was found
that the FTS samples produced wear depth of about 1 Å deeper than the a-C:Hx/a-C:Nx
samples.
As a conclusion, the clearance margin test confirmed the flyability performance of
the FTS samples as comparable to the conventional DLC samples, provided the lubricant
properties are tuned to be similar to the conventional a-C:Hx/a-C:Nx samples. The
accelerated wear test concluded that the FTS samples produced less disk wear, with the
trade-off of more severe head wear. However, with the load/unload and TFC technologies
of current HDD systems, the amount of head wear under such accelerated wear condition
might not be a gating issue, as compared to the gain in DLC thickness reduction that allows
for further areal density growth.
Conclusions
The results have shown that DLC samples, deposited using the facing targets sputtering
process, possessed the highest density and hardness, as compared to the a-C:Hx and a-C:Nx
samples. This was attributed to the high sp3 content as measured by XPS. HDI tests showed
comparable flyability performance and clearance for the FTS deposited samples. In addition,
wear performance tests revealed better wear resistance for the FTS deposited DLC samples,
as compared to DLC samples by other processes. However, the head wear was also higher.
Acknowledgements
The authors will like to acknowledge the contributions of Intevac, Inc. for providing the CDP
process source as well as technical discussions on the CDP process with Dr. David Brown and
8
Dr. Jun Xie. The authors will also like to acknowledge Serene Ng L.G. for the TEM
measurements. P. Yang is supported by the SSLS via NUS Core Support C-380-003-003-001.
9
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11
List of Tables:
Table 1: Table showing calculated values of surface energy using geometric equation.
Table 2: Lubricant parameters of the FTS and a-C:Hx/a-C:Nx samples
List of Figures:
Figure 1: (a) TEM image of the cross section of FTS deposited samples for thickness
calibration; (b) XPS depth profile of the same samples for etch rate
Figure 2: Plot of (a) hardness; (b) modulus; with displacements from surface for the three
types of DLC samples.
Figure 3: Plot showing XRR spectra of the three different kinds of DLC with zoom in plots
showing the increasing of the critical angle with increasing density, X-ray wavelength
λ=1.540 Å.
Figure 4: Images showing contact angle measurements by goniometer of a-C:Nx, FTS and a-
C:Hx samples by different testing liquids.
Figure 5: (a) Experimental setup for HDI related testing; (b) Reference track and Test track of
the accelerated wear test. For the current test, the reference track was at radius of 24 mm,
while the test track was at radius of 25 mm, both were with zero skew angle.
Figure 6: Experimental results for HDI clearance margin test. It was shown that the clearance
margin was comparable between the FTS and the conventional a-C:Hx/a-C:Nx.
Figure 7: Experimental results of the accelerated wear test. The FTS samples produced less
disk wear, with the trade-off of more severe head wear.
12
Figure 1
0
5000
10000
15000
20000
25000
30000
35000
40000
0 20 40 60 80 100
Inte
nsi
ty o
f C
1s
Etching time (min)
5.6min
7.6min
11.6min
16min(a) (b)
13
Figure 2
0 10 20 30 40 505
10
15
20
25
30
35
Hard
ness (
GP
a)
Displacement into Surface (nm)
a-C:Nx
a-C:Hx
FTS
(a)
0 10 20 30 40 50
100
200
300
400
500
600
700
800
(b)
Modulu
s (
GP
a)
Displacement into Surface (nm)
a-C:Nx
a-C:Hx
FTS
14
Figure 3
0 2 4 6 8 10 121E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
0.3 0.6 0.9
1
FTS measured spectra
a-C:Hx measured spectra
a-C:Nx measured spectra
No
rma
lize
d In
ten
sity
2-Theta (degrees)
FTS measured spectra
a-C:Hx measured spectra
a-C:Nx measured spectra
2-Theta (degrees)
No
rma
lize
d I
nte
nsity
Zoom-in
15
Figure 4
16
Figure 5
LDV
Oscilloscope
Function Generator
Polytec OFV-5000 Controller
Slider
Disk
Spindle
50 kΩ
Laser Beam Vheater
Cartridge
(a) (b)
17
Figure 6
-5%
0%
5%
10%
15%
20%
25%
0 20 40 60 80 100
LDV
Ve
loci
ty R
MS
Rat
io, %
Heater Power, mW
a-C:Hx/a-C:Nx
FTS
18
Figure 7
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Ref Delta (Hd related Wear) Test Delta - Ref Delta (Disk relatedWear)
We
ar D
ep
th, n
m
FTS a-C:Hx+a-C:Nx
19
Table 1
Type of Carbon sd s
p s (mN/m)
a-C:Nx 46.29 30.18 76.47 FTS 45.94 27.78 73.71
a-C:Hx 49.13 15.35 64.48
20
Table 2
Sample Full layer thickness (Å) Std (Å) Bonded layer thickness (Å) Std (Å) Bonding Ratio (%)
FTS 14.16 0.34 10.7 0.74 76%
a-C:Hx/a-C:Nx 12.56 0.37 8.98 0.36 72%