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 MAN YIJUN (B. Eng., USTB; M. Eng., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _________________________ Man Yijun 12 July 2013 i 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. ii Table of Contents Declaration i Acknowledgements ii Table of Contents iii Summary viii List of Tables x List of Figures xi List of Abbreviations Chapter Chapter xviii Introduction 1.1 Evolution of Hard Disk Drives (HDDs) 1.2 Areal Density of Magnetic Recording Hard Disk 1.3 HDD Components 1.4 The Read/Write Process 1.5 The Head-Disk Interface (HDI) 11 1.6 Motivation 13 1.7 Objective 14 1.8 Structure of the Thesis 15 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 22 2.3 Lubricants 25 2.4 Contact Recording 28 2.4.1 Important Slider Designs Related to Contact Recording 29 iii 2.4.2 Slider-Disk Contact Detection Technologies 2.4.2.1 Acoustic emission (AE) sensor technology 30 2.4.2.2 Piezoelectric transducer (PZT) sensor technology 30 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 2.6 Current and Future Technologies for Magnetic Recording Chapter 29 43 2.6.1 Perpendicular Magnetic Recording (PMR) 43 2.6.2 Bit Patterned Media Recording (BPMR) 44 2.6.3 Heat Assisted Magnetic Recording (HAMR) 45 2.7 Tribocharging and Tribo-Current 48 Materials and Experimental Methodologies 56 3.1 Sliders and Disks for the Experimental Investigations 56 3.1.1 Sliders 56 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 3.2 Methodologies for Slider-Disk Interaction Measurement 60 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 3.3.1 Conductive Atomic Force Microscopy (C-AFM) 72 3.3.2 Optical Surface Analyzer (OSA) 73 3.4 Experimental Setup 75 iv Chapter Chapter 3.4.1 Optical Surface Analyzer (OSA) Based Setup 75 3.4.2 Spin Stand Based Setup 76 A Study of Slider–Lubricant Interactions with Different Slider Designs 78 4.1 Introduction 78 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 Study of Slider–Lubricant Interaction with Conductive Atomic Force Microscopy 93 5.1 Introduction 93 5.2 Experiments 94 5.2.1 Experimental Studies Using the C-AFM 5.2.1.1 Experimental setup 94 5.2.1.2 Sample preparations 95 5.2.1.3 Experimental studies 96 5.2.1.4 Data acquisition 97 5.2.2 Experimental Studies Using a Modified OSA Chapter 94 100 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 Study of Slider-Lubricant Interaction with Tribo-Current 111 6.1 Introduction 111 6.2 Experiments 112 6.2.1 Experimental Setup 112 v 6.2.2 Methods of Data Acquisition and Analysis Chapter Chapter 114 6.3 Results and Discussions 117 6.4 Summary 120 Parametric Studies of Thermal Flying Height Control Sliders for the Investigation of Slider-Lubricant Interactions in Contact Proximity Regime with Electrical Current 122 7.1 Introduction 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 144 The Applications of Electrical Current as a Contact Detector for the Investigation of Slider-Lubricant Contact 146 8.1 Introduction 146 8.2 Experimental Setup 149 8.3 Results and Discussions 151 8.3.1 Effect of Disk RPM 151 8.3.2 Effect of Mobile Lubricant 153 8.3.3 Investigations of the Second Stable Flying State with Electrical Current 157 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/LubricantContact/Takeoff Processes for the Specific TFC Slider with the Electrical Current Method 169 8.4 Summary 173 vi Chapter Conclusions 176 References 180 List of Publications 207 vii 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 Tbits/in2 areal density while the disk-to-slider lubricant transfer, enhanced by the slider-lubricant interactions within such a small spacing, may lead to lubricant pickup by the slider which can affect the head-disk interface (HDI) stability. The investigations of lubricant transfer by the sliders with different designs show that lubricant transfer is not dependent on the air-bearing pressure but the effective size at the slider’s central trailing pad. Slider design with multi-shallow step and a smaller central trailing pad not only achieves a higher air-bearing stiffness but also reduces the redistribution of lubricant. The physical clearance would be further reduced to sub-nanometer in order to achieve 510 Tbits/in2 areal density. This will result in the inevitable intermittent contact between the slider and the lubricant/disk and require a significant change in the HDI. Thermal flying height control (TFC) technology has successfully brought the slider to fly at an ultra-low spacing, realizing sub-nanometer clearances for specific read/write operations. Based on the TFC technology, lube-surfing recording has been proposed and this may impose a tighter magnetic spacing while sustaining a stable HDI. The electrical/tribo-current generated by the slider-lubricant contact during lubesurfing may be used to detect slider-lubricant contact. The conductive atomic force microscopy is applied to simulate the interactions at HDI while the currents generated during the probe-sample contact are investigated. The critical points, which divide the interactions into non-contact, lubricant-contact viii Man, Y. J, Liu, B., Zhang, M. S and Gonzaga, L. (2009a). “Experimental study of slider-lubricant interaction with different slider designs”, Microsystem technologies, 15(10-11), 1515-1520. Man, Y. J, Liu, B., Ma, Y. S, Sinha, S. K. and Lim, S. C. 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(2011). “Characterization of slider-lubricant interaction with tribo-current,” Microsystem technologies, 17(5-7), 1003–1007. Man, Y. J., Liu, B., Ma, Y. S., Sinha, S. K. and Lim, S. C. (2009). “Slider-lubricant interaction and corresponding tribocurrent,” IEEE transactions on magnetics, 45(11), 5069-5072. Man, Y. J., Liu, B., Zhang, M. S. and Gonzaga, L. (2009). “Experimental study of slider-lubricant interaction with different slider designs,” Microsystem technologies, 15(10-11), 1515-1520. Publications in Conferences Man, Y. J., Liu, B., Ng, K. K., Yu, S. K., Sinha, S. K. and Lim, S. C. (2011). “Investigation of possible head-disk spacing at light lube-contact,” The 21st ASME International Conference on Information Storage & Processing Systems (ISPS 2011), June 13-14, Santa Clara, CA, USA. Man, Y. J., Liu, B., Sinha, S. K. and Lim, S. C. (2010). “Tribo-current investigations at different head-disk spacing,” invited talk, Asia-Pacific Magnetic Recording Conference (APMRC 2010), November 10-12, Singapore. Man, Y. J., Liu, B., Hu, S. B., Ye, K. D., Ma, Y. S., Sinha, S. K. and Lim, S. C. (2010). “Characterization of slider-lubricant interaction with tribo-current,” The 20th ASME Annual Conference on Information Storage & Processing Systems (ISPS 2010), June 14-15, Santa Clara, CA, USA (top university paper award). 207 Man, Y. J., Liu, B., Ng, K. K., Yu, S. K., Sinha, S. K. and Lim, S. C. (2014). “Investigations of light contact and lube-surfing state with electrical current,” IEEE International Magnetics Conference (INTERMAG Europe 2014), May 4-8, Dresden, Germany. Liu, B., Zhang, M. S., Yu, S. K., Hua, W., Gonzaga, L., Ma, Y. S., Man, Y. J. and Zhou, W. D. (2008). “Surfing Recording and Feasibility Exploration,” invited talk, The 19th Magnetic Recording Conference (TMRC 2008), July 29-31, Singapore. Top University Paper Award 208 [...]... film on the head surface and the thickness of the carbon and lubricant overcoats on the disk surface During operation of a HDD, a nearly constant FH between the slider and the disk is maintained If the FH is much larger than the sum of the lubricant thickness and the carbon overcoat, the effect of the carbon overcoat thickness and lubricant thickness can be ignored In present day HDDs, for which the magnetic... From left to right, the conductive probe is firstly engaged on the sample disk under the setpoint of 0.5 V The voltage thereafter is increased to 1 V immediately after the engagement of the probe Then the voltage is gradually reduced with a step of 0.1 V which leads to the separation of the probe from disk, the scanning in the lubricant and the separation of the probe from the lubricant in the final... lubricant-contact is still lacking This has been the motivation behind this research project 1.7 Objective The objective of the research reported in this thesis is to investigate the interactions between the slider and the lubricant, in particular the slider-lubricant interaction within the extremely small mechanical slider -disk spacing The interactions 14 between the slider-lubricant and the disk will... while short-range interactions within such a small mechanical spacing will be increased to a level that may degrade the HDI performance Lubricant transfer from the disk to the slider occurs through evaporation/condensation mechanisms even for a non-contacting interface and is enhanced by slider-lubricant interactions during contact, may lead to lubricant pickup by the slider and may cause lubricant redistribution... will result in the inevitable intermittent contact between the slider and the lubricant layer or the surface of the disk A 13 significant change in HDI may in fact be required to meet future slider -disk spacing needs Thermal flying height control (TFC) technology has successfully brought the slider to fly at an ultra-low spacing above the disk surface and realized sub-nanometer clearances for specific... disk drive (Wood, 2009) disks which are rotated by a spindle motor at a substantially constant high speed and accessed by an array of read/write heads which store data on tracks defined on the disk surfaces The read/write element is positioned at the trailing edge of the slider and the slider body is attached to the suspension by the gimbal spring The suspension positions the slider body onto the disk, ... lubricant redistribution This can affect the stability of the HDI Thus, the identification and characterization of lubricant transfer between disk and slider with different slider ABS designs are very important to achieve small physical spacing and to realize the areal density goal of 1 Tbits/in2 The slider -disk spacing would be further reduced to sub-nanometer in order to achieve 510 Tbits/in2 areal... transferred to slider by Panda II, Panda III and Panda IV sliders 87 Figure 4.11 The effects of maximum air-bearing pressure (P) and area (A) of slider’s central trailing pad on the amount of lubricant transferred 88 Figure 4.12 A simplified model of lubricant transfer in the head- disk interface (a), and geometry of head- disk interface when the pitch angle is counted (b) 89 Figure 5.1 Optical surface analyzer... tribocharging The resulting electrostatic potential can further destabilize the slider’s flyability proving detrimental to HDI performance On the other hand, tribocharging and tribocharginginduced tribo-current can be used as means to signal the onset of slider -disk contact or even be used as indicators of the onset of lesser contact such as slider-lubricant contact The ability to know when the slider comes into. .. and thus, the reading of stored information The GMR sensor is much more sensitive in detecting a change of the magnetic field than a MR sensor 1.5 The Head- Disk Interface (HDI) One of the biggest challenges for HDD manufacturer, from a tribological point of view, is to maintain a very small spacing between the read/write head and the disk (Talke, 1995) State of the art HDDs operate at a head- disk separation, . 2013 AN INVESTIGATION INTO THE HEAD-DISK INTERFACE TECHNOLOGY LEADING TO EXTREMELY SMALL MECHANICAL HEAD-DISK SPACING MAN YIJUN (B. Eng., USTB; M. Eng., NUS) A THESIS. AN INVESTIGATION INTO THE HEAD-DISK INTERFACE TECHNOLOGY LEADING TO EXTREMELY SMALL MECHANICAL HEAD-DISK SPACING MAN YIJUN NATIONAL. leads to the separation of the probe from disk, the scanning in the lubricant and the separation of the probe from the lubricant in the final. 96 Figure 5.4 Typical topography (a) and current