Head-disc Interface Research Articles

Modeling of formation and breaking of lubricant bridge in the head–disk interface by molecular dynamic simulation

The technology of heat-assisted magnetic recording (HAMR) has improved the storage density of hard disc drives. The PFPE molecules of lubricant layer adhered on the disc can transfer from the lubricant layer and form the lubricant bridge which can deteriorate the stability of read/write process. In this paper, the formation and breaking of lubricant bridge at the head–disc interface (HDI) affects HAMR stability and deserves to be investigated. Using molecular dynamic simulation, a full-atom model was built to evaluate the behaviour of the lubricant bridge. Moreover, the effects of lubricant temperature, heating-up time, disc rotation speed and bearing pressure on the HDI were studied. It has been found that the amount of transferring atoms sharply increased when the lubricant temperature was above 700 K. The loss rate of lubricant layer decreased gradually during the heating process and it took about 2.2 ns for the remaining lubricant to reach stability. Furthermore, transferring PFPE molecules can form the lubricant bridge. A shorter heating-up time makes the lubricant bridge thicker and more robust. And the duration of lubricant bridge is notably affected by heating-up time, rotation speed and bearing pressure. A shorter heating-up time leads to a longer duration of lubricant bridge, whereas a higher rotation speed and bearing pressure reduces the duration of lubricant bridge.

Drive level acoustic defectoscopy for head disc interface (HDI) integration and manufacturing

A short product development cycle for modern hard disc drive (HDD) depends entirely on early defect characterization. However, deviations from desired HDI mechanical performance are not always manifested in electromagnetic read back or servo control signals. The best non-destructive tool to monitor HDI dynamics at the HDD level is a passive acoustic technique that tracks equilibrium of the head gimbal assembly and air bearing (AB) modes. This technique is capable of active head protrusion detection as well as detection of embedded particles. The drawback of the passive acoustic monitoring technique at drive level is that a noisy HDD environment makes the detectability of a useful signal challenging. The proposed HDD level in situ passive HDI acoustic monitoring technique consists of external AE sensors driven by adequate electronics. These are enhanced by advanced signal processing routines that include adaptive discrete wavelet transforms. The system is tuned to monitor acoustic HDI signatures during drive spin-up/spin-down cycles as well as during passive and active height across the data zones. In addition, integrated acoustic energy metrics can be used in AB design evaluation stages. In this work examples of HDD level passive acoustic monitoring applications for HDI development, and manufacturing defect tracking, are presented and supported by failure analysis results.

Passive acoustic monitoring of head disc interface interactions

A hard disc drive (HDD) recording head thermal protrusion is monitored by the passive acoustic characterization technique where adaptive discrete wavelet (ADW) filtering has been introduced to declare the contact. A phenomenological model is built to demonstrate detectability of the passive acoustic monitoring. The model is based on the mechanical impedance approach where impedances of head disc interface (HDI) and acoustic emission (AE) sensor are compared for matching over air bearing/head gimble assembly frequency bandwidth. A synthesized HDI response signal derived from the HDI mechanical impedance function is compared to the real AE signal obtained during the thermal protrusion based contact detection. A methodology of HDD level AE signal characterization presented in this work consists of the ADW filtering technique where the fifth order DB7 wavelet base function is used in AE signal decomposition. The signal decomposition order is selected by the AE signal entropy minimization.

Ultra sensitive in-situ acoustic characterization system for HDD head disc interface defectoscopy

An ultra sensitive drive level acoustic characterization system has been developed for in-situ Head Disc Interface defectoscopy. Multimode acoustic emission (AE) sensor installed on the drive cover is designed for tracking air bearing (AB) and slider modes. Monitored modal changes at the AB and slider bandwidth correlate to the weak head disc interface (HDI) interactions such as lubricant modulation and particles induced defects. Two or three orders of magnitude increase in sensitivity can be achieved by a combination of advanced sensors, data acquisition hardware and digital signal processing algorithms. Continuous and Discrete Wavelet Transform and Joint Time-Frequency analyses are implemented for the AB modal data mining process. Performance of the newly developed technology is demonstrated on a normally operating hard disc drive (HDD).

Accelerated wear testing of head-disc interfaces

To better assess wear characteristics of head and media combinations under conditions that approximate drive operation, we describe an accelerated wear test in which the z-height is increased so that the trailing edge of the air bearing surface makes contact. Using this test, we find that the wear rate for a particular head-media combination decreases as time 1/2 and we document the effect of media wear on the head wear rate. These experimental findings are combined with numerical modeling of head-disc interface wear that correlates well with our results and predicts a shift in wear mechanisms as surface deformation shifts from dominantly plastic to elastic.

Towards 100 Gb/in<sup>2</sup> MO storage using a blue laser, high-NA far-field optics and domain-exoansion media

A Magneto-Optical recording technology is described for storage densities of 50-100 Gb/in2 and data rates of 100-200 Mb/s. A sliding optical head which forms a key component for this recording technology has been developed. The head incorporates a thin-film coil for high data rate Magnetic Field Modulation recording and a dual lens objective with a Numerical Aperture of 0.85. To limit head-disc interface problems due to heating by the laser a cover layer of a few tens of μm is applied to the disc. Cover layer requirements are discussed and MFM recording results obtained with a NA=0.6 version of the sliding head and TbFeCo media with a thin cover layer are presented.

Studies of failure mechanisms for head-disc interface using in situ instrumentation

The failure mechanism for a head-disc interface has been studied using in situ multi-instrumentation. Combined with ex situ atomic force microscopy observations, the sequence of events for the failure of a head-disc interface is proposed as depletion of the lubricant on laser bumps, uniform wear of the carbon coating on laser bumps, failure of the carbon coating on laser bumps and abrasive wear of the underlying substrate.

Investigation of head-disc dynamics with laser pattern simulated disc microwaviness

As the fly height requirement for ultrahigh density magnetic recording becomes more stringent, the control of head media spacing (HMS) modulation becomes extremely critical not only for the reliability of the head-disc interface but also for magnetic read/write performance. Disc topographical features with lateral dimensions ranging from 10% to 80% of the slider length are known as and these features can have a profound influence on HMS modulation. In this work, we demonstrate that disc microwaviness can be simulated by laser texture patterns. The controllability of such simulated microwaviness, in terms of frequency, amplitude and aspect ratio, makes this technique particularly powerful to gain fundamental understanding of the effect of disc microwaviness on the HMS modulation. It was found that when the laser pattern frequency intercepts the air bearing frequency, the resulting resonance induces severe HMS modulation. The response is a linear function of head-disc spacing, and the effect of spacing modulation becomes much more pronounced at higher laser bump heights.

Fly/stiction: mechanical instability of a head-disc interface

The fly/stiction phenomenon is described in detail. Unlike the normal stiction values, fly/stiction values exhibit a unique bi-/tri-modal distribution. Fundamentally, the fly/stiction phenomenon is related to the mechanical instability of a head-disc interface, triggered by the excessive accumulation of liquids at the interface. An energy barrier model is presented to explain various observed aspects of the fly/stiction phenomenon, and to provide directions to reduce the risk of fly/stiction failure.

Head-disc dynamics of low resonance laser textures-a spectrogram analysis

Spectrogram analysis provides a systematic way to study the head-disc interface dynamics and demonstrates how the frequency spectrum varies during the head take-off or landing. The regular pattern laser texture has strong input excitations, which includes not only the fundamental frequency of bump pitch density but also the higher order harmonics. When the excitation frequencies intercept the system resonance frequencies, such as the slider/sensor body natural frequencies, the sensor output is amplified, which causes a false reading in the glide testing and glide avalanche measurement. Spiral-line laser texture solves the problem of resonance by completely removing the source of excitation.

Stiction model for a head-disc interface: experimental

Experiments have been conducted to assess the theoretical predictions of a stiction model for a head-disc interface recently proposed. One of the key predictions of this model is that the relationship between stiction and the real area of contact is non-monotonic, and that minimum stiction should occur at an optimal value of the real area of contact. The regular textures of different asperity densities, generated by the laser texturing technique, were used in this study. The single-crater type of laser bumps could be accurately simulated by either spherical bumps with an effective radius of curvature, or cylindrical bumps with an effective Young's modulus. The stiction measurement results were found to be in excellent agreement with the theoretical prediction. The comparison between the model predictions and the experimental results were made for the stiction vs. asperity height relationships for different size laser bumps, at different spacings, and at different lubrication levels. In all cases, good agreements between the model and the experimental data have been confirmed. In this study, it has also been found that a bonded Zdol lubricant film still behaves like a liquid in terms of its ability to form menisci around contacting points.

Mechanical integration of high recording density drives

Novel methodology has been developed to evaluate the long term reliability of head-disc interface and to estimate drive lifetimes. The tests are based on stressed low flying or gliding as good start-stop result is only necessary but not a sufficient condition for long term reliability. Stresses are introduced by operating at low pressures at several stress levels with track following and seeking. Lifetimes are monitored by head-disc contacts with piezoelectric sensors, magnetic signals and error rates. Lifetimes determined in stressed conditions are extrapolated to normal operating conditions. Magnetic recording drives having zone textured media and operating at average flying height of 25 nm at the inside diameter have estimated average lifetimes of 3 years with standard deviation of 1.3 years in continuous operation.

Detection window and lineal density in digital recording systems

A theoretical model has been established describing the required detection window for a given bit error rate (BER) as a function of lineal transition density (FCI). The model was developed utilizing an extension of a quasi-optimum matched filter analysis for noise spectral density based on media or white noise-dominant recording channels. The relationship between the minimum achievable degradation in RMS timing error vs. the relative separation or pulse slimming factor is used to calculate the minimum detection window at a given error rate as a function of lineal transition density. In addition, calculations of required data detection window versus lineal data density, BER versus lineal data density, and BER versus normalized data detection window are presented. Comparison of the theoretical calculations with experimental data for a practical head-disc interface exhibit remarkable agreement near the region of operation. The model has been demonstrated to be applicable to various encoding schemes and is useful to evaluate detection window margins and to gain insight to system operating characteristics.