Thermal Asperity Research Articles

Head–disk interface (HDI) degradation risk reduction in hard disk drive (HDD) during thermal asperity (TA) track follow mapping and seeking

Head–disk interface (HDI) degradation risk reduction in hard disk drive (HDD) during thermal asperity (TA) track follow mapping and seeking

2-D LDPC Codes and Joint Detection and Decoding for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) is a promising technology for boosting areal densities (ADs) using sophisticated signal processing algorithms within a systems framework. The read/write channel architectures have to effectively tackle 2-D inter-symbol interference (ISI), 2-D synchronization errors, media and electronic noise sources, as well as thermal asperities resulting in burst erasures. The 1-D low-density parity check (LDPC) codes are well studied to correct large 1-D burst errors/erasures. However, such 1-D LDPC codes are not suitable for correcting 2-D burst errors/erasures due to the 2-D span of errors. In this paper, we propose construction of a native 2-D LDPC code to effectively correct 2-D burst erasures. We also propose a joint detection and decoding engine based on the generalized belief propagation algorithm to simultaneously handle 2-D ISI, as well as correct bit/burst errors for TDMR channels. This paper is novel in two aspects: 1) we propose the construction of native 2-D LDPC codes to correct large 2-D burst erasures and 2) we develop a 2-D joint signal detection-decoder engine that incorporates 2-D ISI constraints, and modulation code constrains along with LDPC decoding. The native 2-D LDPC code can correct >20% more burst erasures compared with the 1-D LDPC code over a 128 $\times $ 256 2-D page of detected bits. Also, the proposed algorithm is observed to achieve a signal-to-noise ratio gain of >0.5 dB in bit error rate performance (translating to 10% increase in ADs around the 1.8 Tb/in2 regime with grain sizes of 9 nm) as compared with a decoupled detector-decoder system configuration over a small 2-D LDPC code of size 16 $\times $ 16. The efficacy of our proposed algorithm and system architecture is evaluated by assessing AD gains via simulations for a TDMR configuration comprising of a 2-D generalized partial response over the Voronoi media model assuming perfect 2-D synchronization.

Modeling of Thermal Sensor Response to Disk Asperities

A finite-element model was implemented to study the transient thermoelastic–plastic contact between a thermal asperity sensor and a thermal asperity on a disk in a hard disk drive. The temperature change of the thermal asperity sensor during contact with an asperity was calculated and converted into resistance change of the sensor using experimental measurements for the relationship between the sensor temperature and the sensor resistance. In addition, the effect of asperity material properties on the resistance change of the sensor was studied. The simulation results show that the resistance change of the sensor is a function of the bias voltage applied to the sensor, the friction coefficient between the sensor and the asperity, the disk velocity, and the contact interference between the asperity and the sensor.

An <italic>In Situ</italic> Measurement Method for Electric Potential at Head-Disk Interface Using a Thermal Asperity Sensor

The electrostatic force at head–disk interface (HDI) becomes increasingly significant when the head–disk clearance is dropping into 1 nm scheme. Therefore, detecting and eliminating the electric potential in the HDI is a critical task for hard disk drives. To that end, this paper proposed an in situ measurement method for electric potential at HDI using a thermal asperity (TA) sensor. When calibrating the electric potential, both dc and ac components of the external voltage were supplied to the disk. As a result, the output of the TA sensor to the first harmonic of the ac component was linearly increasing with the decrease in the dc components. Experimental results show that this method is more efficient and flexible than the traditional methods.

Effects of non-spherical colloidal silica slurry on Al-NiP hard disk substrate CMP application

Spherical and non-spherical colloidal silica size and shape were characterized and its effects on aluminum alloy nickel plated (Al-NiP) hard disk substrate during chemical mechanical polishing (CMP) was investigated. Non-spherical colloidal silica slurry shows significantly higher material removal rate (MRR) with higher coefficient of friction (CoF) when compared to spherical colloidal silica of similar size. CMP evaluations on non-spherical colloidal silica slurry particle size distribution (PSD) reveal that MRR can be further increased by using wider PSD. Conventional slurry for Al-NiP hard disk substrates which use alumina–silica composite slurry induces embedded alumina thermal asperities (TA) defects which can cause reliability failure at product level. CMP comparison between conventional alumina–silica slurry and non-spherical colloidal silica slurry shows substrates polished by using non-spherical colloidal silica slurry have no embedded TA defects, lower surface roughness and lower surface defects.

MoA-1-1 Thermal response of a thermal asperity sensor to disk asperities

MoA-1-1 Thermal response of a thermal asperity sensor to disk asperities

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.

Iterative Detection-Decoding of Interleaved Hermitian Codes for High Density Storage Devices

Traditionally, Reed-Solomon (RS) codes have been employed in magnetic data storage devices due to their effectiveness in correcting random errors and burst errors caused by thermal asperities and inter-symbol interference (ISI). However, as storage densities increase the effect of ISI becomes more severe and much longer RS codes are needed, but this requires significantly increasing the size of the finite field. A possible replacement for RS codes are the one-point Hermitian codes, which are a class of algebraic-geometric (AG) code that have larger block sizes and minimum Hamming distances over the same finite field. In this paper, we present a novel iterative soft detection-decoding algorithm for interleaved Hermitian codes. The soft decoding employs a joint adaptive belief propagation (ABP) algorithm and Koetter-Vardy (KV) list decoding algorithm. It is combined with a maximum a posteriori (MAP) partial response (PR) equalizer and likelihoods from the output of the KV or the ABP algorithm are fed back to the equalizer. The proposed scheme's iterative detection-decoding behavior will be analyzed by utilizing the Extrinsic Information Transfer (ExIT) chart. Our simulation results demonstrate the performance gains achieved by iterations and Hermitian codes' performance advantage over RS codes.

An Improved TA Suppression Method for Coded PR Channels

Thermal asperity (TA) resulting from the collision between the slider and the asperity on a magnetic medium during read process can deteriorate the performance of hard disk drives (HDDs). Without TA detection and correction algorithms, the system performance can be unacceptable, depending on how severe the TA is. This paper presents an improved TA suppression method for coded partial response (PR) channels, which consists of two channels running in parallel. Specifically, one channel is matched to the target H(D), while the other is matched to the target H(D)G(D), where G(D) = 1 – D2 is a bandpass filter and D is a delay operator. The soft-output Viterbi algorithm (SOVA) detector in the H(D) channel yields the high-quality soft information in absence of the TA, while that in the G(D)H(D) channel produces the high-quality soft information in presence of the TA. Then, the overall soft information chosen from these two detectors, depending on if a TA is detected or not, is sent to the decoder according to the turbo equalization principle. Experimental results show that the proposed method performs better than the conventional and the previously proposed ones, when operating at high signal-to-noise ratio (SNR) region where a practical HDD works.

Reducing the Thermal Asperity Effect in Perpendicular Magnetic Recording System

In this paper, we propose a new method to estimate and reduce effectively thermal asperity (TA) in the perpendicular recording channels with state trellis. The proposed method comprises of recursively applying a previous noise estimate to a calculation of a current noise estimate to eliminate the effects of TA from the equalized signal. With partial response a maximum-likelihood detector base on partial-response (PR) target with DC-full component can improve the system performance by more than 50% when TA occurs. Unlike previous studies of TA cancellation or suppression, this method is different because we dont use PR targets with non-dc components and high pass filter.

An investigation of thermal asperity sensors during contact with disk asperities

In this study, a thermo-mechanical finite element model was developed for the response of a thermal asperity sensor sliding against a disk asperity. The temperature change of the thermal asperity sensor due to frictional heating was determined. The effect of disk asperity material properties and contact conditions is investigated.

Tribological Degradation of Head–Disk Interface in Hard Disk Drives Under Accelerated Wear Condition

To further increase the areal storage density of hard disk drive, one solution is to reduce the spacing between the magnetic head and disk to sub-1-nm regime. Such ultra-low spacing introduces great challenges to tribological reliability in the head-disk interface (HDI). Therefore, it is necessary to understand the characteristics and mechanisms of HDI degradation in the tribological condition. This paper investigates the degradation behaviors of HDI based on an accelerated wear experiment. Acoustic emission and thermal asperity sensors were used simultaneously to monitor the touch down dynamics before and after each cycle of the overdrive test to characterize the HDI degradation behaviors. Through analyzing the evolutions of HDI dynamic characteristics, the degradation mechanisms, including wear mechanism shift and HDI instability propagation were figured out.

Thermal Asperity Sensor Application to Hard Disk Drive Operational Shock

Thermal asperity (TA) sensor is used to investigate head disk interface (HDI) dynamics during operational shock. It is revealed that the output of TA sensor as a function of shock acceleration and back-off level. Air bearing modes being excited during operational shock indicates head-disk contact happens. The mechanisms and exciting sources of head-disk contact were explored by analyzing the TA sensor signal. Both experiment and simulation results show that motor base mode and head stack assembly (HSA) bending mode are the two most important contributions to head-disk contact. From this study, TA sensor is found to be useful for in situ monitoring of operational shock dynamics and, thus providing the strategies to improve shock-resistant performance.

<i>In Situ</i> Spacing Fluctuation Monitoring of Hard Disk Drive with Thermal Asperity Sensors

This paper presents an investigation of spacing fluctuation characterization (SFC) of Hard Disk Drive (HDD) using thermal asperity (TA) sensor. Firstly, the spacing fluctuation monitored by TA sensor is correlated with that monitored by a Laser Doppler Vibrometer (LDV). Secondly, a series of experiments are conducted to study the factors affecting the spacing fluctuation, including disk distortion, head crown sensitivity, ZH sensitivity. Good disk assembly and small crown sensitivity and ZH sensitivity will be helpful to reduce spacing fluctuation.

Performance Improvement System for Perpendicular Magnetic Recording with Thermal Asperity

In this paper, we propose a new method to estimate and effectively reduce the effect of thermal asperity (TA) in the perpendicular magnetic recording (PMR) channels with the state trellis. The TA is estimated from the state trellis, then its average is used to modify the equalized signal entering the Viterbi detector. For the partial response (PR) targets with DC component, the proposed method with a maximum-likelihood detector can improve the bit error rate performance by more than an order of magnitude when TA occurs and degrades when the giant magneto-resistive (GMR) nonlinearity and base line wander (BLW) effects are present. Unlike the previous studies, this method allows the use of PR targets with DC component under the presence of TA.

Joint TA Suppression and Turbo Equalization for Coded Partial Response Channels

Thermal asperities (TAs) cause a crucial problem in magnetic recording systems because they can cause a burst of errors in data detection process. Without a TA detection and correction algorithm, the system performance can be unacceptable, depending on how severe the TA effect is. This paper proposes an iterative scheme to suppress the TA effect. This scheme jointly performs TA suppression and turbo equalization on the partial-response channels with error-correction codes. At each turbo iteration, an improved readback signal is obtained by subtracting the reconstructed TA signal from the TA-affected readback signal, where the reconstructed TA signal is computed based on a least-squares fitting technique with an aid of soft decisions from previous turbo iterations. Simulation results indicate that the proposed scheme outperforms the conventional receiver with separate TA suppression and turbo equalization, and is only marginally more complex than the conventional receiver.

Effect of Bandpass Filters for TA Suppression in Perpendicular Recording System

Thermal asperities (TAs) cause a crucial problem in magnetic recording systems because they can distort the readback signal to the extent of causing possible sector read failure. This problem becomes even more severe in perpendicular recording channels because these channels contain a d.c. component. This paper presents a novel TA suppression method by use of a bandpass ¯lter to mitigate the TA effect. We also investigate the effect of different bandpass filters for TA suppression. Results indicate that the proposed TA suppression method using a bandpass filter polynomial G(D) = (1+D)2(1-D) yields lower bit-error rate than the existing ones at the expense of increased complexity, and is also robust to large peak TA amplitudes.

Thermal asperity suppression based on least-squares fitting in perpendicular magnetic recording systems

Thermal asperity (TA) causes a major problem in data detection process. Without the TA detection and correction algorithm, the system performance (even with perfect synchronization) can be unacceptable, depending on how severe the TA effect is. This paper proposes a new method to suppress the TA effects in perpendicular magnetic recording channels. The TA detection is a threshold-based approach, while the TA correction is done by averaging the readback signal and applying a least-squares fitting technique to estimate the TA signal. Then, the corrected readback signal is obtained by subtracting the TA-affected readback signal by the reconstructed TA signal. Results indicate that the proposed method performs better than the existing one in terms of bit-error rate and is robust to changes in the peak TA amplitude.

Design and Performance Test of Comb-Shaped Clamp/Spacer for Improvement of Recirculation Filter Efficacy in a Hard Disk Drive

With the recent increase of the rotational speed of hard disk drives, nanoparticle contamination can cause serious problems including thermal asperity. We designed a comb-shaped clamp and spacer to improve the efficacy of the recirculation filter in the operation of the hard disk drive (HDD). We calculated flow characteristics by a commercial computational fluid dynamics (CFD) code for six cases of the comb-shaped clamp and spacer. The calculation results showed that the outward velocity was the highest when the comb-shape angle was 45deg. We measured temporal variations of particle concentration during the operation of the HDD by experiments. Considering the decay rate and the percent clean-up of particle concentration, we found the case of 45deg to be the best design.

Contact Force and Frictional Heating due to “Large” Particles in the Head Disk Interface

Particles in the head disk interface may cause large contact forces acting on the slider as well as thermal asperities in the read/write signal. This is especially true for the close spacing required for 1Tbit∕in.2. In this paper, a three-body contact model is employed to study the effects of a particle entrapped between a slider and a disk. A criterion for determining a particle’s movement pattern is proposed. The study of particles in the head disk interface shows that large particles are likely to slide between the slider and disk interface, and the particles going through the trailing pad of an air bearing slider cause severe contact forces on the slider and generate large heat sources. The frictional heating study shows that the temperature around the magnetoresistive head increases to about 5°C for a single 200nm particle passing through the trailing pad of the slider. The effects of the particle size, disk material, and friction coefficient are also studied. It is found that the disk and slider materials and the frictional coefficient between the materials largely affect the contact force acting on the slider by an entrapped particle as well as the temperature rise at its contact region. It is also found that the friction coefficient largely affects a particle’s movement pattern in the head disk interface.