Thermal Asperity Research Articles

Thermal asperity of TMR heads for removable disk drives

The thermal asperity of the tunneling magnetoresistive (TMR) head was studied using Zip-type flexible media, and compared with that of the anisotropic magnetoresistive (AMR) head. The examined head had a shielded structure with the TMR element close to the air bearing surface. Nevertheless, it generated relatively small thermal asperity even when the mechanical spacing between the head and medium was less than 20 nm, at which large and frequent thermal asperity was observed when the AMR head was used. Such relatively small thermal asperity of the TMR head Is attributed mainly to the small low-temperature coefficient of the electrical resistivity of the TMR element.

Read channel issues in perpendicular magnetic recording

Perpendicular recording is an alternative to longitudinal recording that shows promise in mitigating the super-paramagnetic limit. This channel passes DC and thus presents challenges to the read channel design. Compounding the problem, the MR head signal is susceptible to thermal asperities which manifest as low-frequency disturbances. We compare the performance of a DC-coupled PRML channel architecture which uses a non-DC-free target with a first-order DC loop to remove low-frequency disturbances to that of an alternate architecture that uses a DC-free target. We show through analysis and simulation results that the former architecture degrades with increasing latency of the DC loop and performs worse than the DC-free target for large latencies. In addition, the detection performance with DC-free target can be improved by the use of a block code with single parity bit.

Head and media design considerations for reducing thermal asperity

Recent use of magnetoresistive (MR)/giant-magnetoresistive (GMR) heads in disk drives requires tighter control on particle contamination that may generate thermal asperities at the head/disk interface. In this study, the effects of slider air bearing surface (ABS) design and media on TA reduction capability are investigated. The motion of particles at the head/media interface is simulated numerically. Drive level TA tests are performed using a particle injection chamber. It is observed that a new ABS design, that has an aerodynamic U-shaped rail and a central airflow passage, is beneficial in reducing the particle contamination on the slider. Scratch-resistance of the media surface is correlated to TA reduction capability of the media; more scratch-resistant media produced less TA. On the other hand, the adhesion properties of the media have minor effects on TA reduction, as the particles that usually generate TA in a drive are too small to spin off the media.

A method for reducing the effects of thermal asperities

A new method of combating the effects of thermal asperities (TAs) in disk drives is proposed. In the classic method, changing a parameter in the equalizer modifies the duration of a TA but no attempt is made to modify the detector that follows the equalizer or to improve its performance. The proposed method involves modification of both the equalizer and the detector. Specifically, two channels are run in parallel. One channel is matched to the partial response polynomial P(D), whereas the other is matched to the partial response polynomial (1-D)P(D). P(D) is the target polynomial of the existing magnetic recording channel. The Viterbi detector in the (1-D)P(D) channel has better decoded bit error rate (BER) during a TA, and the Viterbi detector in the P(D) channel has better BER in the absence of a TA. The overall decoded bit stream is selected from these two Viterbi detectors in accordance as to whether a TA has been detected. The performance of the proposed method was studied via simulation for the magnetic recording channel with the target polynomial P(D)=(1-D)(1+D)/sup 2/ (EPR4). The proposed system was found to be superior to existing methods in the presence of TAs, while having the same performance in the presence of additive white Gaussian noise (AWGN).

Concatenated code system design for storage channels

A number of papers have been published on the concatenation of an outer code with a partial response (PR) channel, where the outer code is a turbo code, a convolutional code, or a low-density parity-check code. This paper deals with the second case, assuming EPR4 and modified extended EPR4 (MEEPR4) partial response (PR) targets. The goals in this work include (1) the joint optimization of interleaver and precoder for a fixed outer convolutional code and PR target, (2) the choice of optimal code rate for both PR targets assuming a Lorentzian model, and (3) an assessment of the performance of these codes in the presence of thermal asperities. We introduce mathematical and algorithmic tools for accomplishing these goals and present simulation results that support our approach. Among the positive results is the ability to lower the well-known error rate floor of these concatenated schemes for arbitrary PR channels.

The effects of slider design on thermal asperity rejection capability

Particle contamination has been an ongoing problem affecting the reliability of the magnetic hard disk drives. Especially the recent use of MR head requires much tighter control of particle contamination due to thermal asperity (TA) phenomenon. In this study, the effects of slider air bearing surface design on TA reduction capability were investigated by manufacturing two types of sliders. Numerical methods were used to simulate the motion of particles in the head/media interface. Experiments were conducted to verify the results predicted by the numerical simulation. Drives were built and exposed to particle contamination using a particle injection chamber, which turned out to be a very simple and reliable particle generation method over conventional aerosol technique. Then the number of TA generated in the drives was recorded and compared. Also the contacts between slider and particles were investigated by acoustic emission study. It was found that a new ABS design, which has aerodynamic U-shaped rail and central flow passage, was beneficial in reducing the particle contamination on the slider.

Slider dynamics during continuous contact with textured and smooth disks in ultra low flying height

Dynamics of a nano-slider during continuous contact with textured and smooth disks are studied using laser Doppler vibrometer, friction sensor, acoustic emission (AE) transducer and read back signal from a magneto-resistive head on the slider. The flying height of the slider is controlled from flying to contact at the same velocity by changing the environmental pressure. When the slider flies over the textured disk, contact occurs at flying height of 14 nm, and friction and AE signal increase gradually with decreasing flying height. The increase in the amplitude of the slider vibration is within a few nanometers from flying to contact. When the slider flies over the smooth disk, contact occurs at much lower flying height of 6 nm. However, friction, AE signal and the amplitude of the vibration show a steep increase upon contact. Both pitch and roll vibration modes of the slider air bearing are strongly excited with amplitude of about 10 nm. The pitch vibration results in the slider impacting the disk surface severely. As a result, the disk vibrates at the same frequency. In addition, thermal asperities with extraordinarily high amplitude are observed associated with the impacts induced by the pitch vibration.

The Effects of Corrosion on MR Disk

With use of MR heads, drives will be increasingly sensitive to corrosion and contamination. Particularly, heating of MR sensor due to contact with corrosion products and other contaminants on the disk causes a thermal asperity. This paper focuses on investigation of the effect of corrosion on occurrence of thermal asperities. Several factors which influence corrosion in high temperature and humidity were investigated using ion chromatography and TOF-SIMS. It is found that corrosion causes significant migration of cobalt and nickel compounds from the disk. Higher cobalt migration causes more thermal asperities, but nickel migration does not. Hydrogenated carbon overcoat showed the least cobalt migration among different overcoat layers. However, the most effective way to reduce cobalt migration is the application of Cr layer between carbon overcoat and magnetic layer. Nickel in NiP layer can migrate onto the disk surface when carbon overcoat is thin and water contact angle is low. Final manuscript approved January 29

Thermal Asperities Sensitivity to Particles: Methodology and Test Results

Thermal asperities (TA’s) are tribological events that cause repercussions for giant magnetoresistance (GMR) and MR heads in the hard disk drive industry. A TA is a read signal spike caused by sensor temperature rise due to contact with disk asperities or contaminant particles. TA events may cause GMR and MR heads to temporarily lose their reading capability, and may potentially damage the transducer. It is difficult to completely avoid particle contamination in hard drive applications. Hence it is necessary to design heads/media with a minimum TA sensitivity to particles. A test method for TA sensitivity to particles is needed. This work developed a test method for TA sensitivities to particles. The test system includes a CSS tester with TA detection capability, a chamber to contain the head/disk interface, a particle atomizer, and a particle counter. Aluminum silicate particles used in the test have sizes ranging from 0.2 to 1.0 μm. Particles are injected into the chamber during head scan for TA’s from ID to OD with adjustable air-borne concentrations in the chamber from 10×106 to 30×106particles/m3. TA counts of 30 scans are averaged to obtain reliable TA sensitivity data. Media with different lubricant thickness, different carbon overcoats, and different lubricant types are tested with this method. The results indicated that this methodology can effectively differentiate TA sensitivity to particles for the media studied.

On the Thermal Behavior of Giant Magnetoresistance Heads

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat can cause head/disk contacts, resulting in thermal asperities and pole tip damage. The damage at the head–disk interface due to protruded pole tips and alumina overcoat may degrade the drive mechanical performance when flying height is below 10 nm. In this study the change in pole tin recession (PTR) with temperature and current in the writer coil, are measured using an optical profiler and an atomic force microscope for heads having a stack design with single and dual layers of writer coils. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with a single layer of writer coils exhibit significantly lower thermal PTR than those with dual layers of coils. The ABS profiles at elevated temperature generated using the finite element modeling of the differential thermal expansion of various layers in the head stack are in close agreement with the measured profiles. The thermal PTR and alumina overcoat protrusion can be reduced by optimizing the thermal expansion coefficient of the alumina basecoat and overcoat, the height of the head stack, and by replacing alumina by SiO2 and SiC.

Head design considerations for lower thermal pole tip recession and alumina overcoat protrusion

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat result increased thermal asperities and can interfere with the higher disc asperities and be damaged due to high contact. The damage at the head-disk interface due to protruded pole tips and alumina overcoat may become a major roadblock in the drive mechanical performance when flying height is below 10 nm. In this study, the thermal PTR defined as change in PTR with temperature is measured using an optical profiler and an AFM for heads having stack design with single and dual layers of writer coil. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with single layer of writer coil exhibit significantly lower thermal PTR than those with dual layers of coils.

Anti-thermal asperity head: design and performance analysis

As technology moves to 40–100 Gb/in 2 areal densities, the head-disk spacing will be reduced to a level of 5–10 nm. As a result, the head-disk contact/impact and impact-induced degradation of readback signal are becoming serious concerns. A read/write head structure design is described and discussed in this work, aiming to support high read/write resolution, eliminate the concern on read–write alignment, and reduce the effect of such thermal asperity significantly.

Particle Generation Trend with Variation of Rest Time and Seek Mode in Hard Disk Drive Operation

Particles existing in a hard disk drive are known as a major source of TA(thermal asperity). Researchers have investigated how particles induce the TA phenomena, but have not verified yet the reason why and how particles are generated in a HDD. The objective of this study is to investigate why and how particles are generated, and in what condition, the largest number of particles is generated. The number of particles generated in a HDD was measured over the landing zone after various rest times of slider and during various motions and positions of slider. It is found that the large number of particles was generated when the HDD was turned on after a long rest time of slider and that a few of particles were continuously generated when the slider flied over the disk surface. It is thought that the number of particles generated in a HDD was related to the rest time of slider because the rest time of slider increased stiction, and that there were intermittent contacts between the slider and the disk surface when the slider flied over the disk surface.

Analysis of a biphase-based servo format for hard-disk drives

Biphase modulation in an embedded-servo format for hard-disk drives is investigated. It is shown that for biphase, at the low linear densities typical of servo information, near-maximum-likelihood performance can be attained by a simple bit detector consisting of a full-response linear equalizer and a binary slicer. Compared to the commonly used method of dibit coding, a signal-to-noise ratio gain of some 4 dB is achieved. The same equalizer may be used as the basis for near-maximum-likelihood position error signal amplitude estimation and timing recovery. Simulations of a practical servo demodulator based on a fifth-order analog filter show that at typical linear densities, this ideal performance is closely approached. The equalizer has a band-pass character and yields excellent suppression of the effects of thermal asperities and magneto-resistive head asymmetry.

The Spin-Off of Particles on a Magnetic Disk

A spin-off ratio is used in this study to assess the repelling capability of a disk surface to thermal asperity for MR drives. A test methodology is described and a simple model is developed. Three types of spherical particles with different size and material properties are used in this study to evaluate the spin-off ratios of disks from various manufacturers and with different lubricant thickness and texture roughness. It is found that the dominant factors determining whether a particle is spun off a disk surface are the stiction force and the centrifugal force. Particle size and material properties, disk spinning speed, disk radius, surface tension and contact angle have significant effects on the spin-off ratio, while lubricant thickness and disk surface roughness have little effect. The manufacturing process and material used to produce a disk also affect how well the disk behaves when thermal asperity is a concern. [S0742-4787(00)03901-1]

Thermal asperity trends

A thermo-mechanical model that predicts the changes in thermal asperities as a function of increasing areal density is described. Conceptually, the problem is divided into two portions: the collision and relative motion of an asperity along the slider, and the subsequent diffusion of thermal energy into the head in the vicinity of the read element. The former is treated using a quasi-static spring model for the various mechanical degrees of freedom; the later with a scaling model for the heat transfer. The thermal model was verified by using a focused fast laser pulse to simulate a thermal transient at the read element. The predictions of the full model were also compared with measured thermal asperities that were produced on a spin-stand. The results support one of the model's main predictions: a significant increase in the amplitude of worst case thermal asperity events can be expected in the next few years.

Magnetoresistive heads for helical-scan tape systems

In this paper, the helical-scan tape systems using the shielded MR heads will be discussed. We studied the issues related to the head/tape contact including the thermal asperity and the head wear. The study suggests that these issues can be resolved to the allowable level in the practical system. In this study, we have successfully demonstrated a high recording density using MR heads in these systems.

A 450-Mb/s analog front end for PRML read channels

The high user densities and data rates supported by today's hard-disk drives (HDD) demand complex read channel ICs. High-performance analog front-end (AFE) circuits provide automatic gain control (AGC), programmable band limiting and pulse shaping prior to signal sampling and further processing of the data in the digital domain. A 450 Mb/s analog front-end, integrated into a 16/17 code rate EPR4 read channel, contains an AGC loop, which includes a programmable gain stage (PGA), an exponential variable-gain amplifier (VGA), a 7/sup th/-order 120 MHz lowpass filter (LPF), an active dc offset cancellation circuit, and digital feedback. Utilizing multilevel qualification and a variable loop time constant, the AGC acquires a 12 dB gain change within 5 data bytes. Thermal asperity (TA) and amplitude asymmetry compensation make the analog front-end ideally suited for magnetoresistive (MR) head-based applications. Implemented in 5V/3.3V dual voltage 0.35 /spl mu/ BiCMOS, the complete circuit occupies 2.29 mm/sup 3/ and dissipates 232 mW.

Electrostatic popping in AMR and GMR heads

Dielectric breakdown of insulating layers between the MRE (magneto resistive element) and the conducting shields generates another type of base line shift or popping noise in MR heads. Pulse shapes are similar to those of thermal asperities, but they can be bipolar and can occur without a disk. Measurements illustrate the difficulties of characterizing these pulses. A physical model is proposed to explain the behavior, and a simple SPICE simulation shows similar pulse behavior.

Chemical Integration and Contamination Control In Hard Disk Drive Manufacturing

As we approach the turn of the century, the demand for storage is rapidly increasing, fueled by multimedia and its associated applications. Hard disk drive (HDD) manufacturers continue to respond by chasing the areal density curve to provide higher capacity and higher performance disk drives. The required technology changes are expected to aggravate performance and reliability problems, such as stiction, associated with organic contamination; thermal asperities, associated with particulate contamination; and corrosion, associated with ionic contamination. Anticipation and proactive resolution of chemical integration and contamination control problems are key to the successful development and manufacturing of advanced HDDs. In this paper, types and sources of contamination that can impact HDD performance are described; various contamination-related problems are reviewed; and a methodology by which successful chemical integration in the hard disk drive business can be accomplished is outlined.