Thermal Flying Height Control Slider Research Articles

Investigations of Adhesion under Different Slider-Lube/Disk Contact States at the Head–Disk Interface

Adhesion is the key factor influencing the failure of the hard disk drive operating under ultra-low flying height. In order to mitigate the negative effects of adhesion at the head–disk interface (HDI) and promote further development of the thermal flying height control (TFC) technology, an adhesive contact model based on the Lifshitz theory accounting for the thermal protrusion (TP) geometry of TFC slider, the layered structures of the head and disk, and the operation states of the slider was proposed to investigate the static contact characteristics at the HDI. The simulation results demonstrated the undesirable unstable regions during the transitions between different operation states and the necessity of applying TFC technology. The reduction in the head–media spacing (HMS) was found to be achieved by properly increasing the TP height, decreasing the thickness of the lubricant layer or the thickness of the diamond–like carbon (DLC) layer during the flying state or the TP–lube contact state. At the TP–DLC contact regime, the attractive interaction was stronger than other states, and the strong repulsive interaction made the HMS difficult to be further reduced through the increase in the TP height or the decrease in the lubricant thickness.

Investigation of Bias Voltage and Relative Humidity on Wear and Pit Formation at the Head–Disk Interface

Wear and pit formation of a thermal-flying height-control (TFC) slider was studied as a function of bias voltage and environmental relative humidity (RH; 5%–85%). Burnishing tests were performed by applying an “over-push” heater power to the heater element, causing slider–disk contacts. The experimental results showed that both bias voltage and RH have a large effect on the power-to-contact change and the wear of the slider. Atomic force microscopy (AFM) was used to study the wear and pit formation as a function of bias voltage and RH. The wear of the slider surface was found to be less at the positive disk bias voltage compared with the negative disk bias voltage at all RH conditions. Pit formation was observed on the slider in the vicinity of the read–write transducer area when a positive bias voltage was applied to the disk at high RH conditions (85%). Pit formation was not observed at low RH.

Numerical Simulation of Thermal Fly-Height Control Slider Touchdown Test for Achieving Ideal Subnanometer Clearance Head Slider Touchdown Behaviors

Understanding the dynamics and instability of a thermal fly-height control (TFC) head slider close to and at touchdown (TD) on a magnetic recording disk is essential for developing a head slider with a clearance of ~ 0.5 nm to enhance the recording density of hard disk drives. This paper presents a theoretical simulation of a practical TD test, performed by continuously increasing the head protrusion according to the TFC power, and demonstrates ideal TD behaviors to be established in design guidelines for an effective head slider–suspension–disk system. The effects of the air-bearing stiffness and magnitude of disk waviness (DW) on the TD behaviors and their repeatability were extensively investigated. Comparison with the experimental TD behaviors of effective head sliders demonstrated that the different TD behaviors at different radial disk positions could be explained by the numerical TD simulation of the DW-excited response of a single-degree-of-freedom slider model. Possible reasons for the discrepancies between the experimental and simulation results in the light contact state were analyzed in detail, and a method of designing the head–disk interface conditions to achieve a head spacing of ~ 0.5 nm was developed. In addition, the deviation from the ideal TD behavior according to the DW magnitude was examined. The TD behavior of a head slider on a disk with a perfectly bonded lubricant was theoretically clarified, and the potential for realizing a head slider with reduced clearance was assessed.

Theoretical Simulation of Touchdown Dynamics of Thermal Fly-Height Control Slider

Theoretical Simulation of Touchdown Dynamics of Thermal Fly-Height Control Slider

Effect of Head–Disk Interface Biasing and Relative Humidity on Wear of Thermal Flying Height Control Sliders

In this investigation, we examine wear of thermal flying height control sliders as a function of DC bias voltage across the head–disk interface, relative humidity, and lubricant type. Wear tests were conducted using two different experimental setups, a load/unload tester, and a spin-stand tester with relative humidity control. Head wear was determined by measuring the change in the heater touch-down power (ΔTDP) before and after wear testing. After wear testing, selected recording heads were examined using atomic force microscopy and time of flight secondary ion mass spectrometry to investigate wear and deposit formation in the read/write region. Our results show that the polarity of the head–disk interface bias voltage and the relative humidity influence deposit formation and wear in the read/write region of recording heads.

Near Contact Thermal Flying Height Control in Hard Disk Drives

A modeling and optimization approach is proposed and implemented for minimizing variations in flying height of thermal flying height control (TFC) sliders. The method utilizes the resistive heater element of TFC sliders for adjustment of spacing and the embedded thermal contact sensor (TCS) as a relative measurement for changes in flying height. In the modeling approach, the static and dynamic behavior of the TCS as a function of heater power is characterized. A model of the system is identified from experimental step excitation data using a realization algorithm. In the optimization approach, the optimal feedforward heater input profile is calculated via convex optimization techniques based on the static and dynamic behavior of the system. The modeling and optimization approach was verified experimentally, showing that the proposed approach reduces the difference between the maximum and the minimum value of the TCS measurements by a factor of two, indication a twofold reduction of flying height variations.

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.

Thermo-molecular gas-film lubrication dynamics considering boundary temperature and ambient gas (2-DOF numerical analysis by t-MGL equation)

The static and dynamic flying characteristics of a slider in a hard disk drive have become an important consideration owing to recent increases in recording density. In the present paper, the characteristics of a plane inclined slider (Case 1) and a step slider flying in either air or He (Case 2) over a running boundary wall with local temperature distributions are analyzed using the thermo-molecular gas-film lubrication (t-MGL) equation. For a plane inclined slider (Case 1), the fundamental static and dynamic characteristics are analyzed numerically and are examined through two limiting approximations: the approximation for infinite bearing number and the incompressible short bearing approximation. For a step slider (Case 2), the decreases in the minimum spacing for a slider flying in He are significant because the mean free path of He, ?He, is approximately three times that of air, ?air. The increases in the minimum spacing due to laser heating are negligible in both air and He because the heat spot size is very small. Moreover, the decrease in the minimum spacing produced by thermal deformation (projection height, dmax) by laser heating in the thermal fly-height control slider is reduced by the total additional pressure of (1) MGL pressures produced by the air-film wedge effect, (2) t-MGL pressures produced by the applied temperature distribution, and (3) van der Waals attractive pressure due to the ultra-small spacing. The spacing fluctuation in He caused by a running wavy disk is smaller than that in air, because the inlet-to-outlet spacing ratio (h1/h0) in He is larger than that in air.

Head Wear of Thermal Flying Height Control Sliders as a Function of Bonded Lubricant Ratio, Temperature, and Relative Humidity

Head wear of thermal flying height control sliders is studied experimentally by (a) comparing the touch-down power before and after a wear test consisting of 300 consecutive touch-down cycles, (b) examining scanning electron microscopy (SEM) images, and (c) investigating atomic force microscopy (AFM) measurements of unworn and worn heads. The effect of bonded lubricant ratio, relative humidity, temperature, and heater power on head wear is investigated. The experiments were carried out on a commercial load/unload tester inside an environmental test chamber. We conclude that (a) head wear increases with increasing bonded lubricant ratio, (b) temperature has a minor effect on head wear for the temperature range of 30–50 °C, (c) head wear increases with decreasing relative humidity, and (d) head wear increases with increasing heater power during the wear test. SEM images show wear of the write shield for changes in touch-down power typically larger than 6 mW. AFM measurements show changes in surface roughness of heads with changes in touch-down power as small as 1.3 mW compared to new, unused head. A wear coefficient on the order of 10−11 to 10−13 was estimated.

Effect of Disk Asperity Size on Contact-Induced Head Scratches and Reader Temperature of a Thermal Flying Height Control Slider–Disk Interface

The mechanical and thermal response of a thermal flying height control slider during contacts with a disk asperity is numerically studied. The effect of asperity height and asperity diameter on the scratch dimensions in the read/write shields is investigated, and the maximum temperature at the reader location is determined. The effect of the number of contact cycles is studied.

Study of Head-Disk Interface Characterization Using Touchdown Sensor and Electromagnetic Signal in Hard Disk Drives

With the introduction of thermal fly-height control sliders, the local head-disk clearance can be reduced to a range from $\sim10$ nm to contact. This actively controlled touchdown (TD) has been used as a way to study the head–disk interface (HDI), the characteristics of which greatly affect the areal density and reliability of drives. In this paper, a high spatial resolution TD study is performed at the drive level using TD sensor signals and head-media spacing signals. Different flying stages, including passive flying, TD transition, and over push modulation, are identified from the experimental results. A correlation between the aforementioned signals at the passive flying stage is found to represent the disk surface property. The passive flying stage and over push modulation stage of TD are further analyzed in both the time and frequency domains. By establishing a correlation between the fly height and the temperature response, we can provide insight into the heat transfer model for the HDI.

Investigation of Nanoscale Heat Transfer and Flying Characteristics in Thermal Flying Height Control Sliders Considering Near-Field Radiation

Nanoscale heat transfer at the head/disk interface is investigated for thermal flying height control sliders in hard disk drives. Near-field radiation effects due to evanescent photo tunneling are considered in addition to thermal conduction. It is found that the maximum value of the heat flux due to near-field thermal radiation is comparable to that from heat conduction. Near-field thermal radiation is shown to cause a decrease in the thermal protrusion, resulting in a larger flying height than predicted for the case that near-field radiation is neglected. Results from this study are useful for the design of high-performance disk drives.

Analysis of thermal flying-height control slider in a hard disk drive under humid air condition

Thermal flying-height control (TFC) technology is now widely applied in the hard disk drive (HDD) industry, because it enables consistent read/write spacing, increases storage density and improves HDD reliability. In a TFC slider, the air bearing pressure at the transducer area can be raised to ten or fifteen times the ambient pressure and it can decrease the air bearing pressure. This paper investigates the effect of humid air condition in TFC slider. It is found that because of the high pressure in transducer area of slider, the water molecules are condensed and the air bearing pressure is changes. Therefore, the slider cannot maintain the flying attitude and the flying-height is changed. Moreover, the change of air pressure causes the change of heat transfer coefficient between slider and air bearing. Consequently, it causes a change of the performance in TFC slider. And in higher humidity and temperature, the effect is intensified.

Accurate detection of light contact of thermal fly-height control sliders using advanced singular value decomposition

The dynamic contact and nonlinear dynamics of TFC slider has been widely characterized by harmonics in FFT of vibrational signal. The accurate determination of the harmonics have been challenging as the small peaks of the harmonics in FFT are frequently hidden by varied deterministic vibrations and noise. In this paper, advanced singular value decomposition method is employed to enhance the accuracy of the detection of the multi-harmonics. The track matrix of attractor of TFC slider vibration time series in light contact with disk is reconstructed in phase space. By using singular value decomposition theory, the singular spectrum from reconstructed attractor track matrix is estimated in order to increase the signal-to-noise ratio of the LDV response signal, which is further used for the accurate detection of uncertain, abrupt information and for early slider-disk light contact detection. This approach enables signal noise attenuation, which is critical to the accurate detection of weak harmonics, and enables the accurate extraction of light contact information, particularly for the transient abrupt cases, as well as improves the accuracy of harmonics-based system identification.

Numerical investigation of thermal effects on a HAMR head-disk interface

Heat-assisted magnetic recording (HAMR) is one of prospective high density recording technologies in current hard disk industry. In this paper, we incorporate HAMR optical structures into a finite element model of a thermal flying height control slider and study thermal effects of these structures on the HAMR head-disk interface. Our focus will be on the slider flying height (FH) changes due to the heat loss in the waveguide and near field transducer (NFT). According to our results, we find that large heat dissipation in NFT alone could affect the slider's FH due to the additional thermal protrusion induced around the writer. The heat dissipation in the waveguide could also influence on FH drop. In addition, the media back heating effect on the slider temperature and thermal deformation is also analysed in this paper. The numerical result shows that the respective thermal deformation caused by media back heating is about 0.09 nm with 72 nm of heat spot size. This amount could be significant when the slider's FH is below 1 nm. Therefore, it is very important to improve the efficiency of laser optical delivery system and reduce thermal effects on a HAMR head-disk interface.

Experimental and Simulation Study of Thermal Protrusion-Induced Head-Disk Contact Instabilities in Hard Disk Drives

Stability of the thermal fly height control slider has profound effects on the read-write performance and reliability of the head-disk interface. As magnetic storage technologies evolve aggressively toward higher areal density in hard disk drives, the fly height around the transducer area on the slider has achieved a read-write physical spacing of \({\sim }2\) nm, and is expected to be even lower to meet the magnetic storage road map in the near future. At such a low spacing, instabilities can occur in terms of slider vibration and prominent lubricant transfer from the disk to the slider, resulting in significant interface change that raises reliability concerns. In this paper, we experimentally investigate the thermal protrusion induced slider instability at contact proximities, and perform numerical simulations to explain the measured dynamics at head-disk contacts. The results suggest that as the slider's thermal protrusion area approaches the disk surface, instabilities mainly come from down-track force excitations. These excitations lead to more significant vibrations in the down-track direction and on the slider's leading edge rather than its trailing edge. The behavior of slider vibration at disk proximity is also dependent on the head-gimbal-assembly design.

Investigations of Thermal Asperity Sensors in Thermal Flying-Height Control Sliders

The characteristics of a thermal asperity sensor for detecting slider/disk asperity contact were investigated. A thermomechanical finite element model was used to determine the temperature change of the thermal asperity sensor while sliding against a disk asperity. The effect of sensor size and operational temperature, disk asperity material properties, and contact conditions were investigated.

Nanoscale dynamics of MEMS TFC active slider

With enabling of a thermal actuator, an air bearing slider can fly at sub-nanometer level spacing on a magnetic disk while the recording elements are functioning. At such spacing, the slider stability and head-disk interface reliability remains to be understood. In this study, a novel understanding on dynamics of the MEMS thermal flying-height control (TFC) slider in touchdown process is developed. By using average and variational-iteration methods, closed-form spectrum estimations of slider vibration are derived. The derived formulation offers an insight of the relationship between spectrum and interface parameters. Physics-based simulation is also conducted to quantify the spectrum of slider vibrations as a function of varied interfacial parameters. To further extend the analytical and numerical analysis, the experimental study of a TFC slider while flying on a rotating disk at sub-nanometer spacing are performed. The analysis reveals the dominance of the air bearing force among other interfacial forces at sub-nanometer spacing.

Thermal Protrusion-Induced Air Bearing Slider Instability at Disk Proximity and Light Contact

As the head-disk spacing is locally reduced to a few nanometers by the introduction of thermal fly-height control sliders, the instability of the flying slider at disk proximity and the detection of slider-disk contacts are of fundamental importance. Slider-disk contacts have been reported in several component-level experimental studies by observing an increase in averaged acoustic emission or laser Doppler vibrometry signals. In this paper, we investigate the transient slider dynamics at multiple locations on a slider body to study the instability from the conditions of disk proximity to light contacts. We define the onset of contact using the pattern transitions of acoustic emission signals, and we study separately the instabilities at the slider's trailing edge center (TEC), leading edge center (LEC), and down-track direction (DTD). The results show there is an evolution of slider instability that occurs during the transition from flying to contacts. Slider dynamics modulation in the DTD are more sensitive to the onset of contacts than at the LEC, whereas the dynamics at the TEC show the least instability. The results suggest that the down-track modulation, which happens prior to a severe TEC modulation, provides a preferred early contact detection criterion that can be used to prevent a more engaged slider-disk contact and mechanical wear.

Identification of contact bouncing vibrations using TFC active slider and adaptive fuzzy control for advanced active slider design

The recent advance of magnetic storage technology includes the active thermal fly-height control (TFC) technique that is able to reduce the clearance between read/write heads and disk surface to be of few nanometers. However, it has been estimated that TFC technique alone cannot provide the even smaller clearances necessary to achieve Tbit/in2 recording densities due to the presence of bouncing vibrations and instability. In this work we investigate hybrid active slider technique with both thermal and piezoelectric actuators to achieve extremely high-density recording. The nonlinear system dynamics of thermal actuated slider is established by test-based identification. We propose an adaptive fuzzy control hybrid active slider design in which piezoelectric actuation is added to the existing thermal actuated slider design, so as to fully eliminate the high frequency bouncing vibrations. It can yield more than Tbit/in2 recording density with sub-nanometer level clearance, while using the state-of-the-art and matured manufacturing techniques for active piezoelectric slider and TFC slider.