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: SEET_Hang_Li@dsi.a-star.edu.sg; 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%