AN INVESTIGATION INTO THE
HEAD-DISK INTERFACE TECHNOLOGY
LEADING TO EXTREMELY SMALL
MECHANICAL HEAD-DISK SPACING

MAN YIJUN NATIONAL UNIVERSITY OF SINGAPORE
2013
AN INVESTIGATION INTO THE
HEAD-DISK INTERFACE TECHNOLOGY
LEADING TO EXTREMELY SMALL
MECHANICAL HEAD-DISK SPACING
ii

Acknowledgements
I would like to express my sincere gratitude to my research supervisor in
National University of Singapore, Professor Lim Seh Chun, for his valuable advice and
guidance, encouragement and support throughout the course of this research. He
provides me timely guidance in spite of his busy schedules and spends a large amount
of time reviewing my papers and dissertation. Working with Professor Lim has been
an invaluable and honorable experience from which I will benefit. I am greatly
indebted to my co-supervisor Associate Professor Liu Bo, who has been very
important in working out my research path and for navigating me through every stage
of my career since I joined his group in Data Storage Institute. His insight, knowledge
and guidance are extremely helpful to me throughout my PhD study. I would also like
to thank Associate Professor Sujeet Kumar Sinha, previously of National University of
Singapore and currently of Indian Institute of Technology Kanpur, for the kind support,
advice and encouragement of helping me completing my study.
I owe my gratitude to all the people who have helped me in various aspects of
this research while working in Data Storage Institute, in particular Dr Ma Yansheng, Dr
Yu Shengkai, Dr Yuan Zhimin, Dr Zhang Mingsheng, Mr Ng Kang Kee for their
invaluable discussion, professional advice and support. Special thanks are also given to
Dr Hu Jiangfeng, Associate Professor Chen Jingsheng and Dr Shi Jianzhong for their
encouragement and assistance throughout my PhD study.
Finally, I am deeply indebted to my parents, my brother and my parents-in-law
for their support and encouragement, and most of all, my wife, Jiarui, and my son, Jun
Cheng for their constant love, patience, and understanding. Without their supports, the
dissertation would not have been completed.
iii

1.4 The Read/Write Process
7

1.5 The Head-Disk Interface (HDI)
11

1.6 Motivation
13

1.7 Objective
14

1.8 Structure of the Thesis
15
Chapter 2
Literature Review
17

2.1 Introduction
17

2.2 Flying Height (FH) Adjustment Technologies
18 2.2.1 Different Approaches
19 2.2.2 Thermal Flying Height Control (TFC) Technology
2.4.2.3 Thermal asperity (TA) technology
31
2.4.2.4 Laser Doppler Vibrometer (LDV) technology
31
2.4.2.5 Read signal technology
33 2.4.3 Short Range Forces and Slider-Lubricant Interaction
33

2.5 Lube-Surfing Recording
37 2.5.1 Introduction
37 2.5.2 The Challenges and Approaches for Lube-Surfing
Recording
40


3.1.1.1 Panda sliders – the non-TFC sliders
57
3.1.1.2 Pemto TFC sliders
60 3.1.2 Hard Disk Media
60

3.2 Methodologies for Slider-Disk Interaction Measurement
62 3.2.1 Acoustic Emission (AE) Testing
62 3.2.2 Laser Doppler Vibrometer (LDV) Measurement
64 3.2.3 The Triple Harmonic Method
67

3.3 Surface Analysis Techniques
72

4.2 Experiments
78 4.2.1 Experimental Setup
78 4.2.2 Test Sliders
79 4.2.3 Test Disks
80 4.2.4 Calibration of Slider’s Flying Height
80

4.3 Results and Discussions
84

4.4 Summary
92
Chapter 5
Study of Slider–Lubricant Interaction with Conductive
Atomic Force Microscopy

93



5.2.2 Experimental Studies Using a Modified OSA
100
5.2.2.1 Experimental setup
100
5.2.2.2 Experimental conditions
101
5.2.2.3 Experimental studies
101

5.3 Results and Discussions
101

5.4 Summary
109
Chapter 6
Study of Slider-Lubricant Interaction with Tribo-Current
111

6.1 Introduction
111

122

7.2 Experimental Procedures
124

7.3 Results and Discussions
126 7.3.1 Calibration of TFC Heating Power with Respect to
Variable Heater Resistance
126 7.3.2 Measuring TFC Thermal Actuation Efficiency with
Triple Harmonic Method
130 7.3.3 Simulating the Slider FH Modulation at Close
Proximity with Sinusoidal Function TFC Driving
Voltage
132 7.3.4 Sensitivity of Electrical Current Method Used as a
Contact Detector
136

7.4 Summary

8.3.4 Estimation of the Possible Region of Stable Surfing
State for a Specific TFC Slider
163 8.3.5 Understanding of the Touchdown/Lubricant-
Contact/Takeoff Processes for the Specific TFC
Slider with the Electrical Current Method
169

8.4 Summary
173
vii

Chapter 9
Conclusions
176
References

180
List of Publications
207
viii

Summary
In order to keep increasing the recording density in magnetic hard disk drives,
it is necessary to reduce the physical clearance between the read/write head and disk.
State-of-the-art slider’s flying height is approaching 3.5 nm in order to achieve 1
Tbits/in
2

The in-depth investigations of lubricant-contact by the electrical current
method with a modulated driving voltage applying to the specific TFC slider are
performed. The results suggested that the driving voltage not only produces a localized
protrusion but also dissipates electrical charges to the slider body. The slider is
capacitively coupled with the disk via the TFC heating element. The capacitive current
thus produced dominates the measured current during non-contact, and it mixes with
tribo-current generated during lubricant-contact.
After careful calibrations of the TFC power with respect to the input voltage
and the thermal actuation efficiency, the lube-surfing state during touchdown-takeoff
processes is studied. The proposed current method has a sensitivity which is
comparable to that of the LDV method and it can be used to estimate the touchdown
power and the possible region of stable surfing state. The measured current may be
used not only to accurately detect lubricant-contact but also as a feedback signal for
the fine tuning of slider’s flying status if the electrical charge can be accurately
controlled by further modification of the TFC slider.

x

List of Tables
Table 3.1
Parameters of Panda series sliders.
57
Table 4.1
Parameters of slider’s central trailing pad.
79
Table 4.2
Critical RPM of the slider flying over 3.5 nm at the radius
of 1.2 inch.
83
Table 4.3

2
Figure 1.3
Evolution of areal density. CGR = compound growth rate.
5
Figure 1.4
Structure and components within a hard disk drive.
6
Figure 1.5
Schematic principle of magnetic recording.
7
Figure 1.6
Merged magnetic read/write head in which the second
magnetic shield also functions as one pole of the inductive
write head.
9
Figure 1.7
Schematic diagram of a GMR head structure.
10
Figure 1.8
Schematic view of the head-disk interface.
11
Figure 2.1
Cross-section and schematic of TFC head structure.
24
Figure 2.2
Lubricant thickness vs. pull-rate for Z-Dol (MW 4000)
with concentration of 0.1 wt%.
26
Figure 2.3
Schematic illustration of the targeted lube-surfing recording

Figure 3.5
Principle of acoustic emission measurement.
62
xii

Figure 3.6
PICO HF-1.2 AE sensor.
63
Figure 3.7
Basic components of a laser Doppler vibrometer.
65
Figure 3.8
Polytec OFV-534 LDV operated on the modular OFV-5000
vibrometer controller.
66
Figure 3.9
Spectrum power of the harmonics for the data pattern of (a)
all “1” pattern and (b) “111100” pattern.
71
Figure 3.10
Schematic diagram of conductive AFM.
72
Figure 3.11
DI3100 SPM system (left) and C-AFM sensor (right).
73
Figure 3.12
Optical layout of the optical surface analyzer.
74
Figure 3.13
Candela 5100 OSA integrated with a VENA CSS-L/UL

82
Figure 4.6
Spindle motor profile and slider touchdown test. It is
observed that the contact happened at 5.99 seconds, or 0.99
seconds after the speed of spindle motor is reduced from
15,000 RPM. Therefore, the avalanche point or the RPM
corresponding to the first contact could be calculated as
14,010.
83
Figure 4.7
OSA image of the testing region after 6 minutes track
flying by Panda III slider.
84
xiii

Figure 4.8
Average lubricant distributions in radial direction of (a)
free lubricant region and (b) bonded lubricant region.
85
Figure 4.9
Schematic diagram of lubricant region under the flying
slider.
86
Figure 4.10
The relative amount of lubricant (a) removed from the disk
surface with thick lubricant, (b) transferred onto the disk
surface with thin lubricant and (c) transferred to slider by
Panda II, Panda III and Panda IV sliders.
87
Figure 4.11

96
Figure 5.4
Typical topography (a) and current distribution (b) images
of a lubricated disk simultaneously measured on the same
area of 1 × 1 µm
2
with C-AFM.
97
Figure 5.5
(a) Typical images measured at the status of solid-contact
and lubricant-contact, and (b) typical images at the moment
the probe nearly separating with the lubricant and then
scanning with non-contact.
98
Figure 5.6
Schematic diagram of the current as a function of applied
voltage/force at the status of solid-contact, lubricant-contact
and non-contact. 99
xiv

Figure 5.7
The modified OSA which is a commercial Candela 5100
with a VENA load/unload system attached (a), and the
schematic diagram of the experimental setup (b).
100
Figure 5.8
The measured current as a function of the applied voltage

of the spindle or the linear velocity of the flying slider.
109
Figure 6.1
Schematic diagrams of a rapidly-moving TFC slider body
at the flying status of non-contact, lubricant-contact and
solid-contact, respectively.
112
Figure 6.2
Photograph (a) and schematic (b) of the experimental setup
for tribo-current measurements. The slider mount is
electrically isolated from the rest of the system.
113
Figure 6.3
A typical result of the tests. The tribo-current increases
sequentially with the increase of TFC driving voltages from
2.2 to 3.0 V. The flying time of the TFC slider
corresponding to each voltage or position of the TFC
protrusion is about 510 s. It can be observed that the tribo-
current at each position of the TFC protrusion is rather
stable and its value is independent of the flying time.
115
Figure 6.4
A sudden transition of the tribo-current can be observed
when the TFC driving voltage is increased to ~3.9 V, by
which the lube-contact can be differentiated clearly from
the solid-contact.
115
Figure 6.5
OSA images of 2.5-in disk and zoom-in image of the flying
track immediately after sudden transition. Scratches can be

e
) measured from oscilloscope when the TFC
slider is disconnected to voltage source.
126
Figure 7.3
Schematic diagram of effective voltage (V
e
) or the real
input voltage to the heater element and the corresponding
applied voltage (V
a
) measured from oscilloscope when the
TFC slider is connected to the voltage source. The V
a
can
be measured at both static and dynamic states.
127
Figure 7.4
The effective voltage (V
e
) measured with oscilloscope
when the TFC slider is disconnected to the voltage source
and the applied voltage (V
a
) to the heater at both static and
dynamic statuses when the TFC slider is connected to the
voltage source.
128
Figure 7.5
The TFC heating power as a function of TFC input voltage

the plates and d is the distance between the plates.
134
Figure 7.9
The simulated current with respect to the slider FH
variation which is purposely designed with a sinusoidal
modulation to overlap on the original slider FH (5 nm) as a
function of time. The frequency and the amplitude of the
modulation are 1 Hz and 2.5 nm, respectively.
135
Figure 7.10
The simulated current with respect to the slider FH
variation which is purposely designed with a sinusoidal
modulation to overlap on the original slider FH (3 nm) as a
function of time. The frequency and the amplitude of the
modulation are 1 Hz and 2.5 nm, respectively.
136
Figure 7.11
The TFC heating power with respect to the TFC input
voltage as a function of time. It can be found that the
variation of the TFC heating power has the same frequency
and phase as those of the input voltage.
138
Figure 7.12
The slider FH modulation with respect to the TFC heating
power as a function of time. It can be found that the slider
FH modulation has a 180° phase shift relative to the TFC
heating power, and the modulation curve is not exactly
symmetrical to the initial FH. The knee points of FH
variation relative to the initial FH is about 3.4 nm in
positive and  1.86 nm in negative which means the

heating power. The electrical current measured from the
disk without mobile lubricant is clearly higher than that
with mobile lubricant, suggesting that mobile layer of
lubricant reduced the head-disk contact intensity.
155
Figure 8.4
The measured electrical current with respect to the TFC
power, slider FH modulation and LDV signal as a function
of testing time under different V
dc
.
159
Figure 8.5
Schematic diagram on the correlation of capacitor current,
slider-lubricant spacing and the inverse of the spacing.
Once (1/d
3
 1/d
2
) (t
3
 t
2
) = (1/d
2
 1/d
1
) / (t
2
 t

power. Referring to the thickness of mobile lubricant and
the maximum TFC heating power for surfing of slider in
the mobile lubricant, the optimized region of stable surfing
state for the specific TFC slider is estimated and
highlighted in the figure.
169
Figure 8.9
Schematic diagram of typical stages during light
touchdown, surfing state and takeoff processes with respect
to LDV signal, electrical current signal and slider FH as a
function of time.
170

xviii

List of Abbreviations
ABS
air-bearing surface
AFM
atomic force microscopy
AE
acoustic emission
BPMR
bit patterned media recording
C-AFM
conductive atomic force microscopy
CSS
contact start stop
DLC
diamond-like carbon

INSIC
information storage industry consortium
LDV
laser doppler vibrometer
xix

LMR
longitudinal magnetic recording
L/UL
load-unload
LZT
laser zone texture
MEMS
micro-electromechanical systems
MR
magneto-resistive
MW
molecular weight
OSA
optical surface analyzer
PFPE
perfluoropolyether
PMR
perpendicular magnetic recording
PZT
piezoelectric transducer
RAMAC
random access method of accounting and control
RH
relative humidity


Chapter 1
Introduction
1.1 Evolution of Hard Disk Drives (HDDs)
In 1952, Reynold Johnson of IBM was asked to start a new research team to
develop a better technology for fast access to large volumes of data. It was decided
early on to use inductive magnetic recording as the base technology because it was a
proven technology with the magnetic tapes and drums. The open question was what
configuration the new device should be for achieving fast random access at low cost.
In the end, a new, flat platter design, as first reported in 1952 by Jacob Rabinow
(Rabinow, 1952) was chosen over a simpler cylinder concept. In his famous article,
Rabinow dealt with “The Notched-Disk Memory” and triggered the invention of what
is known today as the computer hard disk drive (HDD). Johnson accurately foresaw its
better potential for future improvements, and successfully demonstrated the first disk
drive systemRandom Access Method of Accounting and Control (RAMAC) in 1955
(Stevens, 1981). Fig. 1.1
IBM RAMAC 305 computer system (Hoagland, 2011).

2

The prototype was so successful that in 1956 it was marketed as RAMAC 305,
as shown in Figure 1.1, the first commercial computer with a magnetic HDD. The one-
ton, double-freezer-size disk drive, named as RAMAC 350, consisted of fifty 24-inch
diameter aluminum disks mounted on a common shaft. The shaft was driven by an AC
motor spinning at 1200 rotations per minute (RPM). The disks were coated on both
sides with a magnetic iron oxide material, so there were 100 recording surfaces. The
whole disk stack was served by two read/write heads shuttling up and down the disk

systems. In 1980, Seagate revolutionized the HDD industry by introducing the ST506,
a 5 ¼-inch form factor (the physical size and shape of a device) HDD for the nascent
personal computer (PC) market (Kryder, 2006). Eventually, the PC HDD market far
exceeded the enterprise storage market in terms of volume shipment. Until very
recently several 2 terabytes (TB) HDDs were available on the market, like Seagate
Barracuda LP and Western Digital (WD) RE4 etc. Among them, WD RE4 has four
thin-film 3.5-inch platters which translate to a density of approximately 400 Gbits/in²
and 8 ceramic sliders with dual stage actuator technology. In fact, this drive is not the
newest and the best, it just was randomly chosen to make one point  there has been a
huge progress in the field of HDD technology in the ~57 years, and the rate of this
progress is just increasing year after year.
4

1.2 Areal Density of Magnetic Recording Hard Disk
A recent study forecasted explosive growth of the digital universe from 130
exabytes (EB, 13010
18
bytes) in 2005 to 40 zettabytes (ZB, 4010
21
bytes), or 40
trillion gigabytes (GB) in 2020. From now until 2020, the digital universe will about
double every two years (Gantz and Reinsel, 2012). It is thus vitally important to ensure
the continued rapid increases in capacity of the ubiquitous HDD that provides the
foundation for this digital universe. The biggest lever for higher HDD capacities is to
increase the areal density, which is the number of bits that can be recorded per square
inch. For a given disk diameter, this parameter determines the amount of data that can
be stored on each platter. This, in turn, dictates the total storage capacity of a HDD
given the number of platters it contains. Even though there are many other contributing
factors, ultimately, this is the single most important parameter that governs the cost per
megabyte (MB) of a HDD. It is the incredible and consistent rapid growth rate of areal