Air-bearing Surface Research Articles

Effects of Oxygen on Smear Formation in Heat Assisted Magnetic Recording System

Heat-assisted magnetic recording (HAMR) is expected to be a realistic next-generation technology for increasing the recording density of hard disks. However, the magnetic layer is heated above the Curie temperature, and, as a result, the heated lubricant is desorbed from the disk by decomposition and evaporation, which causes a problem as it adheres to the air-bearing surface (ABS) as a smear. In this study, pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) analysis was performed in helium and air environments to investigate the decomposition mechanism of perfluoropolyether (PFPE) lubricant D-4OH by heating and in the presence of oxygen. In the helium environment, thermal decomposition of the end groups was confirmed at 350°C and above with a possibility of main chain decomposition at 450°C. In the air environment, decomposition of the end group was confirmed at 250°C and above, and decomposition of the main chain was confirmed at 450°C. Experiments using a pin-on-disk tester were conducted to confirm what happens to the area of smear when a thin film of D-4OH lubricant coated on an actual disk is laser heated. As a result, it was confirmed that the area of smear decreased even at an oxygen concentration of 5%.

Time-resolved imaging of an operating hard-disk-drive write head using nano-beam x-ray magnetic circular dichroism

We studied the dynamic magnetization behavior of a hard-disk-drive (HDD) write head by using nano-beam x-ray magnetic circular dichroism (XMCD). The write head was operated by applying a square-wave write current with a frequency of 211.9 MHz to the coil of the write head, which was in synchronization with the fivefold frequency of the x-ray pulses. Snapshot images of the magnetization on the air-bearing surface of the write head were obtained with spatial and time resolutions of 100 nm and 50 ps, respectively, and temporal changes in the magnetization of the main pole and the shield were observed. When a square-wave write current with a transition time of 0.2 ns was applied, the magnetization reversal of the main pole was completed in 0.7 ns, and this reversal time was determined by the dynamic property of the write head. During the main pole reversal, the shield area showed spatially non-uniform magnetization patterns, and its response was slower than that of the main pole. These results show that the XMCD measurement setup developed in this study is a powerful tool for understanding the dynamic behavior of an HDD write head and contributes to the development of higher capacity HDDs.

Numerical Study of Particle Rebound in the Head–Disk Interface

Particle intrusion into the head–disk interface (HDI) is becoming a concern in hard disk drives. Numerical studies of particle movement in HDI are of great importance for reducing particle-induced damage. In this investigation, we study the particle rebound mechanism in HDI. Not only the particle–slider collision but also particle–particle collision models are developed, which are divided into three types, including the collisions of particle and surface (P–S), particle and edge (P–E), and particle and particle (P–P). The subsequent response of particles after the collision, which includes rebound and adhesion, is calculated considering the collisional energy loss. The proposed adhesion/rebound model can more accurately present the particle trajectories in HDI and accumulation on the air bearing surface (ABS). We find that particles are likely to accumulate on the leading rail surfaces of the slider due to the air flow drag and Saffman lift. Considering the adhesion and rebound, particles with a larger diameter are more likely to fly away from the slider after the collision. The existence of particle rebound, to a certain degree, reduces contamination on ABS.

Effect of Helium-Induced Pressure and Temperature on Lubricant Transfer in Helium–Air-Filled Hard Disk Drives

A numerical model combined an air-bearing model with a molecular dynamics model is used to investigate lubricant transfer at the spacing between slider and disk in helium–air-filled drives. The physical properties of the helium–air gas mixtures are calculated with respect to the content of helium. Then, the pressure and temperature distributions induced by helium at the head/disk interface are obtained based on the air-bearing model. Furthermore, the obtained pressure and temperature are used as the inputs of the molecular dynamics model to study the mechanism of lubricant transfer. The results show that the lubricant volume transferred to the air-bearing surface increases with an increase in pressure and temperature. The volume of lubricant transfer decreases slightly as the helium volume in the gas mixture increases from 0% to 80%, after which, the transferred lubricant greatly decreases.

Etching behaviors of tunneling magnetoresistive (TMR) materials by ion beam etching system

In data storage manufacturing, ion etching process is commonly chosen for the preparation of surface patterns, for example, under the read-write head of the hard disk drive (HDD) to form an air bearing surface (ABS). This read-write head consists of a multilayer thin film of different materials to form a tunneling magnetoresistive (TMR) device. In this work, we applied the Monte Carlo-based simulation package to calculate the etching yield of major materials in the HDD head’s structure. The plasma characteristic in the industrial-size ion beam etching (IBE) system has been studied by the special plasma diagnostic probes. Effects of input parameters in IBE system such as incident beam angle, ion beam current and gas flow in the system were considered. Plasma characteristic can be obtained from plasma I-V curve characterization. The sputtering yields of materials were found maximum when an incident angle is about 70° to the normal surface. The floating potential of plasma in the system with the ion/electron compensation from PBN is calculated.

Thermo-hydrodynamic analysis of large-eccentricity hydrodynamic bearings with texture on journal surface

To improve the performance of a heavy-loaded and high-speed hydrodynamic bearing, partial texture is designed on the journal surface. A three-dimensional thermo-hydrodynamic analysis model is developed based on the computational fluid dynamics method. Thermal conduction of bearing bush and journal, heat convection on the air-bearing surface, viscosity–temperature effect, and cavitation effect are taken into account. Performance of textured and un-textured high-speed journal bearing under different eccentricity ratios and different external loads is compared. The results indicated that partial texture on journal surface can play a positive role for large eccentricity ratio conditions in this study, and the textured bearing has a higher load carrying capacity, a lower maximum oil pressure, and a lower oil temperature rise than the un-textured bearing based on an optimal design of texture.

Magnetic Force Microscopy Characterization of Perpendicular Magnetic Writers with Wrap-around Shield

Magnetic force microscopy (MFM) was employed to characterize the magnetic field distribution of two different designs of perpendicular magnetic writers with fully wrap-around shield (WAS). Typical magnetic write heads, located on a slider at the so-called air bearing surface (ABS), are composed of a magnetic write pole and a magnetic return pole. The write pole has a much small cross section area at the ABS than the return pole which is connected to magnetic shield. The write pole and the shield are separated by a non-magnetic gap layer. The shield structure is designed in such a way that it can confine the magnetic fields without significantly degrading field strength. In this work, MFM imaging was used to investigate the relationship between the gap structure and magnetic field strength/gradient. Magnetic field distributions of two different head designs were compared and discussed in detail. Experimental results revealed that the magnetic field intensity is linearly proportional to pole dimensions, especially the pole width. The field gradient, however, is dependent on not only spacing gap between shields and the write pole, but also the pole widths and thicknesses.

Analysis of the Loading Process of a Slider With Reflowed Lubricant in a Hard Disk Drive

As the recording density of hard disk drives has increased exponentially recently, the load/unload process can be affected severely by the nanolevel changes in the air-bearing surface (ABS) induced by the reflow behavior of perfluoropolyether molecules. In this paper, clean-up time and reflow time were first measured by using thermal flying controller measurements to check whether reflowed lubricant can affect loading performance. The measured clean-up time and reflow time were 120 s and 40 min, respectively. Because the clean-up time is so much longer than the loading process, almost all the reflowed lubricant remained on the bottom of the ABS during the loading process. Next, the lubricant behavior at rest was modeled and simulated theoretically by using a diffusion process. At rest, the time-dependent profiles of the reflowed lubricant were investigated. In addition, the ABS was modified with reflowed profiles at various time intervals and loading process was carried out with the modified ABS for various parameters such as reflow time and loading velocity. As a result, the slider–disk contact distinctly occurred during the loading process in the case with reflowed lubricant profiles. The simulation results were verified with the experimental results. Consequently, the effects of reflowed lubricant should be considered when designing the loading process of a slider to improve head–disk interface reliability.

Study of adhered particle’s secondary migration on the slider air bearing surface

An entrapped particle on the slider’s air bearing damages the surface of the slider or the disk. The study of particle movement on and the particle adhesion mechanism onto the slider surface is critical to reduce entrapped particle-induced damage. In this paper, we proposed a dynamical model of the adhesive particle’s secondary migration on the slider surface. The model predicts whether the adhered particle will remain static by the action of the aerodynamic and adhesion forces. There are three migration movement styles which are rolling, sliding and lifting away from the original adhered location. Further, particle rolling migration behaviors on the slider air bearing surface (ABS) are analyzed considering the effects of flow shear rate, particle diameter and properties. Finally, the particle migration trajectory and velocity with time on the investigated slider are presented. It was found that most particles that adhere to the slider surface remain static for the investigated slider; pure rolling motion on the slider ABS is the main migration styles. The particle with a larger diameter and air shear rate speeds up to a higher velocity, and it takes more time to speed up to the constant velocity. A particle’s secondary migration motion can be observed for the particle that adheres to the slider recession region and trailing pad. Then the particle enters airflow blowing out of or rolls away from the head-disk interface.

Modeling, design and analysis of a micro flying slider in near-field optical storage system

A micro flying slider is developed to satisfy the demands for large capacity, high data density, and fast transfer rate of mass storage systems. The air-bearing surface of the micro flying slider is designed based on micro fluid theory of hydrodynamics to obtain positive and negative pressures, thereby enhancing flying height stability. The lubrication model is investigated based on the generalized Reynolds equation. The finite volume method is utilized to solve the revised Reynolds equation that governs the air-bearing pressure and flying system behavior. The dynamic model for the slider–disk interface of the micro flying slider is developed, and the air-bearing system is modeled as a 3-degrees-of-freedom, lumped-parameter model to simulate flying characteristics. The air-bearing pressures with flying attitudes in the loading/unloading processes are characterized and calculated using the proposed model, with flying height and pitch angle as the key variables. The sample data obtained via accurate triggering and superposed averaging are processed to acquire modal frequencies and damping ratios based on the parameter identification method of modal analysis technique. System experiments are presented to illustrate the effectiveness of the proposed micro flying slider and its dynamic model for flying-height control. Theoretical analysis and experimental results show that a stable storage working process can be obtained at 1600 rpm and a pitch angle of 1.5°. Moreover, the micro flying slider exhibits satisfactory flying stability, with a near-field working distance of less than 75 nm. Good correlation between the experimental and simulation results is achieved.

A Practical Low-Cost Machine Vision Sensor System for Defect Classification on Air Bearing Surfaces

A Practical Low-Cost Machine Vision Sensor System for Defect Classification on Air Bearing Surfaces

An Area-Based Prior Value Method for Detection of Micro Contamination in Hard Disk Drives

This paper presents a new area-based prior value technique for the improvement of automatic visual inspection in hard disk drive manufacturing. Micro-contaminations are detected on the air-bearing surface of the head gimbal assembly. The new area-based prior value technique uses the locations of contaminations that appear in the inspection area. The experimental results validate the efficiency of the new detection method on low-resolution images, as the proposed method yielded 93.1% accuracy.

Air bearing surface recessed steep wall via optical proximity correction mask and fluorine-based plasma etching

Abstract Etched steep wall is very difficult to achieve by fluorine-based Reactive Ion Etching (RIE) process on Air Bearing Surface (ABS) made of alumina titanium carbide (Al 2 O 3 -TiC or AlTiC). The ideal wall angle of a fly height target for read/write slider head would be perfectly vertical, however the fabricated patterns normally ends up in slope etched wall of AlTiC. Also, shallow etched ABS walls cause great variation in fly height between inner and outer radii of recording disk when performing data reading or writing. It is necessary to develop etching method that results in steep and clean etched wall of AlTiC substrate. In this study, the effect of photoresist mask angle prior to RIE process on degree of slope etched wall was explored. The goal was to have the etched angle steeper than 40° and re-deposition free etched wall. Sloped wall variation in photoresist was accomplished using optical proximity correction (OPC) mask. Due to technical difficulty in photoresist patterning, the angles of patterned photoresist achieved were between 30°–65° and 70°–90°. It was found that with photoresist's angles between 30°–65°, the etched wall angles of AlTiC were less than 40° and no re-deposition of by-product was observed. With the resist's angle between 70°–90°, the etched wall angles exceed 40° but the built-up byproduct deposition was observed. It is suggested that the photoresist's angles of 65°–70°could provide desired etched angle and clean etched wall. Further development of photoresist patterning technique is required to produce photoresist wall angles of 65°–70°.

A Nanoscale Plasma Etching Process for Pole Tip Recession of Perpendicular Recording Magnetic Head

The pole tip of perpendicular recording head is constructed in a stacked structure with materials of NiCoFe, NiFe, Al 2 O 3 and AlTiC. The surfaces of different materials are set at different heights below the air-bearing surface of slider. This paper presented a plasma dry etching process for Pole Tip Recession (PTR) based on an ion beam etching system. Ar and O 2 mixed plasma at small incident angles have a high removal rate to the nonmagnetic material. It was utilised to etch the reference surface until it reaches the MT value. Low-energy Ar plasma at a small incident angle removes materials with selective ratios of 1 : 1.6 : 2.5 : 2.9 (AlTiC/Al 2 O 3 /NiCoFe/NiFe). It was selected to form the PTR. High-energy Ar plasma at a large incident angle exhibits almost same removal rates for all materials. It was adopted to make overall removal while keeping the recessed profile. An atomic force microscope (AFM) was used for measuring the recessed heights of pole tip and the MT value of the base surface. A transmission electronic microscopy (TEM) was chosen to examine the thickness of subsurface damage. A batch of production showed that the recessed heights can be successfully nanofabricated with the three-step plasma etching process. DOI: http://dx.doi.org/10.5755/j01.ms.22.2.12953

Finite element analysis simulations of thermomechanical head-disk interface contact in thermal flying-height control slider design

In order to achieve higher recording density in hard disk drive (HDD), thermal flying-height control (TFC) slider design has been used to reduce the physical spacing between the read/write elements in the slider and the rotating disk surface. During the read/write operation in hard disk drives, the intermittent contact at the head-disk interface (HDI) directly affects its performance and reliability. In this study, the thermomechanical micro-contact between a TFC slider and a disk defect was systematically investigated through finite element analysis (FEA) modeling and simulations. The thermal actuation technology in a TFC slider was incorporated into the FEA model to enable the thermal protrusion of read/write sensors outward the air bearing surface (ABS). In order to obtain the scientific relationship between the thermal protrusion and the resulting HDI contact behavior, parametric FEA simulations were carried out based on the 23-factorial design of experiment (DOE), where the surface temperature rise and the residual deformation on the TFC slider surface were used as the output parameters. From the statistical data analysis, it could be found that the larger thermal protrusion resulted in higher temperature rise and more residual deformation.

Lubricant Dewetting on the Slider’s Air-Bearing Surface in Hard Disk Drives

In current hard disk drives, the minimum air-bearing clearance is of the order of 1 nm during the read/write process. At this ultra-low spacing, lubricant from the disk often transfers to the slider’s air-bearing surface imposing a significant degradation of its performance. It is necessary to make accurate predictions of the lubricant’s response at the head–disk interface in order to engineer reliable hard disk drives. In this article, we perform numerical simulations to investigate the dewetting behavior of some perfluoropolyether lubricant films used in hard disk drives. We model the lubricant flow on the slider surface using a governing equation based on classical lubrication theory. We consider a disjoining pressure that approximates the properties of a ZTMD lubricant and compare the results with those obtained using a purely van der Waals disjoining pressure. We study the spreading of a lubricant film on a slider both at rest and while flying over a spinning disk. The effect of surface tension, air shear stress, and substrate roughness on the dewetting behavior of the film is also investigated.

Characterization of Laser-Induced Protrusion in HAMR by the Burnish Method

Heat-assisted magnetic recording (HAMR) has become one of the leading approaches to increase the areal density in hard disk drives to several Tb/in2. Laser-induced protrusion significantly impacts the HAMR heads’ performance and lifetime. Nonetheless, the pointy local protrusion amount is hard to accurately quantify by touchdown sensors. Based on the previous indirect thermal-mechanical studies with different sensors and techniques, a new methodology is developed to measure the laser-induced protrusion by burnishing the head during writing with laser powered ON and by utilizing atomic force microscopy scans. This burnish method requires the air-bearing surface (ABS) to physically contact the rotating disk without an excessive overdrive, which relies on the precise settings of the touchdown sensors and the detection methods. The time dependence of the burnish behavior is studied. The results show that its time constant is much longer than the pure thermal and mechanical time constants. The repeatability of the experiment is validated. The local protrusion induced by the laser is in the range of 2–5 nm based on current head design. Accordingly, the detailed studies about heater power effects and protrusion amount as a function of laser power have been conducted. In addition, the impact of varying degrees of stiffness of the ABS is discussed.

Decomposition of Perfluoropolyether Lubricant on Air Bearing Surfaces during Heat-Assisted Magnetic Recording

The thermal decomposition of a thin film of D-4OH lubricant under heating conditions on air-bearing surfaces, such as those found in magnetic recording devices, was investigated using time-of-flight secondary ion mass spectroscopy. The substrates used in this experiment were those of actual magnetic head sliders with diamond-like carbon (DLC) thin-film coatings. Two kinds of surfaces were prepared: an undisturbed DLC thin-film surface and an Al2O3 surface exposed by sputtering of the DLC thin film. Changes in the positive mass spectra as a result of heating confirmed that thermal decomposition of the D-4OH lubricant thin-film occurred, and that it was catalytically accelerated by Al2O3 at heating temperatures below 150 °C. This observation contrasts with the fact that the D-4OH lubricant thin film on the undisturbed DLC thin-film surface did not decompose.

Repeatable and Non-repeatable FH Modulation Study Near Head Contact

The lower head–media spacing (HMS) is always expected for achieving better recoding performance at high linear density. Besides the efforts to reduce the overcoat thickness of head and media, the low flying height (FH) and smaller FH modulation (FHM) are necessary to ensure the low HMS and good interface reliability. In the current HDD, the FH change is measured from the readback signal by the Wallace spacing loss equation. The real-time dynamic FH signals consist of repeatable and non-repeatable FH changes, or FHM. The repeatable FHM (RFHM) correlates with the disk morphology variations. It usually has the variation of transition parameter $a$ inside. Because the change of $a$ is the noise of the RFHM, it has to be very careful for the compensation of the RFHM by thermal FH control. Otherwise, it may cause unexpected head–disk contact. The non-RFHM (NRFHM) measurement is able to remove the media-related noise, which has a good testing accuracy of around 1 A. This paper studies both RFHM and NRFHM characteristics of four types of sliders with different air-bearing surface designs.

A survey of non-prehensile pneumatic manipulation surfaces: principles, models and control

Many manipulation systems using air flow have been proposed for object handling in a non-prehensile way and without solid-to-solid contact. Potential applications include high-speed transport of fragile and clean products and high-resolution positioning of thin delicate objects. This paper discusses a comprehensive survey of state-of-the-art pneumatic manipulation from the macro scale to the micro scale. The working principles and actuation methods of previously developed air-bearing surfaces, ultra-sonic bearing surfaces, air-flow manipulators, air-film manipulators, and tilted air-jet manipulators are reviewed with a particular emphasis on the modeling and the control issues. The performance of the previously developed devices are compared quantitatively and open problems in pneumatic manipulation are discussed.