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Air bearing dynamic stability on bit patterned media disks
Abstract
Bit Patterned media (BPM) recording is one of the potential technologies to be used in future disk drives in order to increase the areal density to 5 Tbit/in2. But one of the main obstacles for BPM is to achieve dynamic stability of the air bearing slider at the head-disk interface (HDI). In this paper we first use a direct simulation method to check the accuracy of our previously developed Homogenization Reynolds equation solution. After confirming the accuracy it is then implemented to study the slider’s flying attitude on BPM disks. Then we investigate the system’s parameters using a system identification method by simultaneously solving the equations of motion of the slider and the Homogenization Reynolds equation. We observe that the first pitch mode frequency of the air bearing increases with increase of pattern groove area ratio and pattern height. And the stiffness decreases when the pattern groove area ratio or pattern height increases. We conclude that a partially planarized BPM is preferred in order to maintain the dynamic stability of the HDI.
... In this way, the final size of a magnetic bit is reduced compared with the current PMR media so the storage density can increase. However, there are still some challenges for BPM, such as the large volume fabrication of the patterned media disks, the head flying stability on the patterned media [2], the control of defects on the media [3], etc. ...
Bit patterned media (BPM) is expected to enable the magnetic storage density in hard disk drives (HDDs) beyond 1 Tb/in2. BPM uses isolated magnetic islands to record the data information. However, the large volume fabrication of those patterned media disks at an affordable cost is a challenge for this new technology. A master template is the first step for patterned media fabrication. Using nano-imprint technology, the master template can be replicated to tens of thousands of pattern disks. A rotary electron beam lithography machine or plasmonic nanolithography machine is recommended to assist in the fabrication of the master template. In both systems, a high resolution encoder system for positioning in the rotary lithography machine is necessary. In this paper, a magnetic rotary encoder system is introduced. The encoder system can be operated at several thousand revolution per minute (RPM). The scale pitch is 90 nm which is one to two orders smaller than current optical encoders. The resolution is about 2.8 million counts per revolution (CPR). A flying magnetic head is used to retrieve the readback signal from the magnetic encoders. A field programmable gate array (FPGA) is implemented to finish the high speed signal processing and provide a digital format encoder signal to trigger the lithography machine at a rate of several Mega Hertz.
In this work, the interaction stresses (i.e., pressures and shear stresses) between a half-space comprising a uniform material and a half-space comprising one-dimensional material distributions in the in-plane direction were theoretically derived. The interaction stress was derived from the Lennard–Jones potential as a vector for the (0, 0, 1) surface using two different material distribution patterns. The first pattern was a periodic distribution of materials (Pattern 1) and the second was a distribution of two materials with a single interface (Pattern 2). The interaction stresses for Pattern 1 were derived based on a Fourier series, while those for Pattern 2 were derived as elementary functions. The pressures possessed non-fluctuation terms and fluctuation terms, while the shear stresses possessed fluctuation terms only. The basic characteristics of these interaction stresses for parallel planes were quantitatively clarified by presenting the distributions and vector diagrams of the interaction stresses.
A method has been established for calculating the interaction pressures between a half-space consisting of a uniform material and a half-space with a two-dimensional (2D) periodic material distribution in the in-plane directions. The periodic material distribution is expanded as a complex Fourier series and the interaction pressures are derived from the Lennard-Jones potential, which includes both attractive and repulsive terms. The interaction pressures for a general 2D distribution were derived, which is applicable to any periodic distributions of many materials. The equations derived have two contributions: (1) the conventional interaction pressures between half-spaces made of uniform materials and (2) the spatial fluctuation originating from the spatially periodic material distribution. As a typical example of the material distributions, a 2D periodic distribution of two materials is introduced to demonstrate the methods. We present calculation results for parallel and inclined planes, a comparison between 2D and one-dimensional analyses, and the effects of the refractive indices and the duty ratios.
The van der Waals (vdW) dispersion pressures between a half-space consisting of a uniform material and a half-space with one-dimensional material distribution in the in-plane direction have been theoretically derived. Two patterns of material distribution were considered here, i.e., a periodic distribution of materials (Pattern 1) and a distribution of two materials with single interface (Pattern 2). The vdW pressure for Pattern 1 was derived based on a Fourier series, while the vdW pressure for Pattern 2 was derived as elementary functions. The basic characteristics of these vdW pressures were clarified.
A generalized Reynolds-type lubrication equation valid for arbitrary Knudsen numbers, defined as the ratio of the molecular mean free path to the film thickness, is derived from a linearized Boltzmann equation by semi-numerically calculating the flow rates of fundamental flows in the lubrication film: Poiseuille flow, Couette flow, and thermal creep flow. Numerical analysis of the equation for high Knudsen numbers reveals three principal results. First, Burgdorfer's modified Reynolds equation featuring the first-order velocity slip boundary condition overestimates load carrying capacities, while the approximation equation including both the first- and second-order velocity slip boundary condition underestimates them. Second, since the flow rate of the Couette flow, which is independent of Knudsen numbers, becomes dominant as the bearing number increases, all the lubrication equation results tend toward the same asymptotic value for an infinite bearing number.
Bit patterned media (BPM) recording is one of the promising techniques for future disk drives in order to increase the areal
density above 1Tbit/in2. However, the BPM can change the topography of the disk surface and thus have an effect on the flying characteristics of
the air bearing sliders. So achieving a stable flying attitude at the hard disk interface (HDI) becomes one of the main considerations
for BPM. In this paper, we apply three methods (complete homogenization, Taylor expansion homogenization and averaging) to
solve this BPM problem and finally choose the Taylor expansion homogenization method to investigate the slider’s flying attitude
on partially planarized patterned media as well as at transitions over different pattern types such as might occur at boundaries
between data and servo sections.
Patterned media are being considered as a promising approach for achieving storage densities beyond 1Tb/in2 on hard disks. Patterned media reduce cross talk of adjacent bits and improve thermal stability of the media. In this investigation,
the flying characteristics of femto-type sliders over bit patterned media (BPM) are investigated. The discrete bits of the
disk are modeled as isolated conical protrusions on the disk surface.
It is anticipated that a radical shift in design paradigm will be required in order to enable perpendicular magnetic recording (PMR) areal densities beyond 1 Tb/in<sup>2</sup> . The shift in question will involve the use of advanced magnetic recording assist technologies such as patterned media, heat-assisted magnetic recording (HAMR), or microwave-assisted magnetic recording (MAMR). This paper is concerned with one of the challenges associated with the practical implementation of patterned media technology, namely, the impact of non-planarity, and shifts in pattern orientation (at data-servo transitions, in particular) on the dynamic fly performance of air bearing sliders. The nature of the latter challenge becomes apparent when considering that key to realizing recording areal densities exceeding 1 Tb/in<sup>2</sup> will be air bearing sliders with superior dynamic fly stability at sub-2 nm clearances. Of the two types of patterned media technologies in development in the disk drive industry, namely, discrete track media (DTM) and bit patterned media (BPM), the former was chosen as a case study in this work.
We apply the dynamic simulation and modal analysis method to analyze the dynamic properties of slider-air bearings. First, the theoretical background and proposed methods are described. Then, five basic types, one of which is first proposed in this paper, of the air bearing surfaces (ABS) are briefly discussed. The dynamic properties of the sliders are investigated, and compared with each other. It is found that a negative pressure slider has the highest stiffness and lowest damping, the TPC and two newly proposed sliders demonstrate higher damping. Finally, the general ABS design problem is briefly discussed. A new advanced slider is designed, analyzed, and compared with the other sliders. The air bearing of the new slider design has larger stiffness and the highest damping of those studied.
This paper proposes the use of a Poiseuille flow rate database for rapid calculation of a generalized lubrication equation for high Knudsen number gas films. The database is created by numerical calculations based on the linearized Boltzmann equation. The proposed interpolation method is verified to reduce calculation time to several tenths of that required to perform rigorous calculations with the same accuracy.
The dynamic characteristics of a slider flying over various servo patterns on a discrete track medium surface were investigated.
The investigation shows that the air-flow field is disturbed and causes flying amplitude during the transition from grooved
longitudinal discrete tracks to a transversal and near-random pattern in the servo field. The effects of the parameters that
define the servo pattern including land-area ratio of burst patterns, groove depth, servo-pattern frequency, and the lengths
of synchronization (sync), servo address mark (SAM), padding (PAD) blocks, and burst pattern type—on the flying height responses
were evaluated. The evaluation results indicate that the flying height amplitude depends on servo-pattern frequency, burst
land-area ratio, groove depth, and the lengths of the sync, SAM, and PAD block and burst pattern type. Amplitude of a slider
flying over a servo pattern therefore can be reduced by optimizing the servo-pattern design from the viewpoint of these parameters.
Moreover, flying-height responses over servo patterns with thermal flying height control (TFC) power were studied and compared
with those without TFC power.
KeywordsAir bearing design–Discrete track medium–Servo pattern–Discrete track recording–Flying height–Flying height amplitude–Air bearing slider–Hard disk drive
Future disk drives may have discretized tracks or bit patterns for increased thermal stability and higher areal density. This affects the surface topography, resulting in different flying characteristics of the currently used air bearing slider. The actual flying height drop versus the pattern density and pattern depth has not been measured yet. However, it has been recently simulated. We have made flyable patterned disks with angular sections having various topographies. This allowed the measurement of flying height modulation and clearance drop within one revolution in different sections of the disks. Slider-to-disk clearance changes were measured using pulsed thermal protrusion. Furthermore, we investigated the slider dynamics and disk-lubricant changes during the transition from a flat zone to a patterned zone. This is of relevance since the servo sectors have different patterns compared to data zones on the disk leading to dynamic excitations and flying height modulation. These effects may still be detectable even after disk planarization since they are periodic and may excite the air bearing resonance. Based on our experimental observation, we have established an empirical equation that predicts slider clearance loss versus pattern density. The clearance change was measured when the slider air bearing was transitioning from a large scale pattern (in micrometers) to a small scale pattern (in nanometers), indicating increased lift with an increased number of walls/holes-even at a similar nominal pattern density. Functional drives were built that contained discrete track media with imprinted servo sectors.
A system identification method using modal analysis is proposed to
analyze the dynamic characteristics of slider-air bearings in hard disk
drives. A procedure for estimating the modal parameters, as well as the
stiffness and damping matrices, is presented. The
“Nutcracker” slider used as an example is analyzed, and the
results are verified. The differences of the stiffness matrices
calculated through different methods are found and explained. The
results of the example show the assumptions of the method are satisfied
in a wide amplitude range. Therefore the technique provides a powerful
tool for head-medium interface analysis and experiment
In patterned media recordings such as discrete track and bit-patterned media (BPM) recordings, the presence of pattern structures on the media surface causes the slider air bearing problems more complicated to be analyzed and studied. Using the 3-D direct simulation Monte Carlo (DSMC) method, the plain slider surface and bit/discrete track-patterned disk surface are designed for case studies in this paper. This paper reports the local bearing pressure variations acted on the slider surface due to the air bearing effects of pattern structures on the disk surface using the DSMC method. From the results obtained under the steady-state condition, it is observed that the bearing pressure profiles will be smoother and the bearing forces will approach to a stable value when the pattern pitch values are smaller than 0.4 μm. This critical pitch value is close to 0.3 μm that we reported previously based on modified Reynolds equations. We also investigate the effects of the pattern depth and total recess area ratio on the air bearing characteristics in BPM recording. Furthermore, in order to understand the effect of bit-pattern movements, we simulate the motion of bit-pattern structures by using four typical moving bit-pattern layouts. As of the outcomes, we can conclude that this effect could be negligible if the bit size is in several tens of nanometer scale.
Numerical simulations of slider dynamics over patterned media with servo zones
- L Bogy
- Db
L, Bogy DB (2011b) Numerical simulations of slider dynamics over patterned media with servo zones, IEEE International Magnetics Conference, April 25–29.
A modal analysis method for slider air bearing in hard disk drives
- Q H Zeng
- D B Bogy
- QH Zeng
Head-disk-suspension dynamics
- Y Hu
Air bearing slider dynamics and stability in hard disk drives
- V Gupta