Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

Lubricant transfer caused by breakage of liquid meniscus bridge

In recent magnetic storage systems, the spacing between the flying head and the disk has been decreased remarkably to less than 5?nm in order to realize ultra-high density recording. Lubricant on the disk is picked up by the flying head slider as a result of intermittent contact between the slider and the disk, or due to condensation of the lubricant vapor. In the present study, the basic characteristics of lubricant transfer (lubricant pick-up) caused by the breakage of a liquid meniscus bridge are investigated experimentally and theoretically. An experimental method by which to measure the volume fraction of the lubricant pick-up has been established. The theoretical results obtained from a simple model proposed by the authors showed good agreement with the experimental results. The validity of the theoretical model was verified based on the experimental results.     

Frictional heating effect on thermal protrusion in head disk interface

The frictional heating effect on the clearance loss in the reader and writer elements of hard disk drive was investigated by conducting a coupled simulation between the air bearing dynamics and the head thermal-mechanics using a frictional heating model. The frictional heating power was obtained through experimentation. In this experiment, the frictional heating power was quantified based on the giant magneto-resistance, which varies with the frictional heating, and the frictional heating induced temperature rise was 15 °C. By matching it with the simulated relationship between the temperature rise and frictional heating power, we found that the frictional heating power was about 3 mW. The simulation results at this power indicated when a touchdown (TD) occurs on the wrap-around shield (WAS), the frictional heating can cause a larger additional protrusion on the writer element, which overestimates the reader topography loss to 0.32 nm, and when the TD point is on leading pole (LP), the frictional heating can cause a larger additional protrusion on the reader element, which underestimates the reader topography loss to 0.2 nm. However, the amount of protrusion caused by frictional heating in the reader and writer elements when TD point in on the WAS and LP is almost the same at only a small difference of 0.052 nm. Finally, we analyzed the frictional heating effect on the clearance in the reader and writer elements. The clearance in the pullback state is not equal to the spacing caused by the TFC pullback power, but the sum of the clearance change caused by the TFC pullback power and the clearance change caused by the frictional heating power, and the clearance change in the reader and writer elements caused by frictional heating are different. Therefore, it is very important to account for the frictional heating effect when designing the reader/writer clearance.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Dew point and relative humidity measurement using a quartz resonant sensor

A new dew point measurement device for humidity measurement in high temperature environment using a quartz crystal sensor was proposed. Combined with Peltier module and quartz crystal, active condensation occurs in the quartz surface to change the mass on the surface of quartz crystal, and use the shift of its resonant frequency identify the time of condensation. This quartz sensor does not require any absorbent material, and it is directly stuck on the Peltier element. The sensor system can also be achieved relative humidity measurement based on dew point and ambient-temperature measuring. It can operate in the range of dew point temperature from 50 to ?30 °C, and in the range of relative humidity from 1 to 90 % RH. The measured dew points values and relative humidity values showed very good agreement with reference values and were within ±0.3 °C, 1 % RH, respectively over the whole temperature range.     

A flyability and durability dilemma in hard disk drives

A magnetic hard disk drive lubrication dilemma was demonstrated through the touch down, wearability and burnish rate tests. It was found that attempts to improve durability and flyability through changes in lubricant film thicknesses, the addition of additives, changes in lubricant molecular weight and irradiating lubricant with deep UV rays (185 nm) have been futile with loss in either flyability or durability of magnetic hard disk. Three key types of head disk interface testing methods were introduced and the results from each parameter change were shown. The touch down test was used to check the fly height of the magnetic heads. The wearability test was used to check the wear resistance of the magnetic hard disks when in contact with the magnetic heads and the burnish rate test was employed to determine the amount of wear of the magnetic heads when in contact with magnetic hard disks. These three techniques may be used for the feasibility study for any newly designed lubricant or technique to reduce the spacing between the magnetic head and magnetic disk. We demonstrate the capability of the three techniques to discriminate different process treatments. The experiments were conducted in a class 100 cleanroom.     

Ellipsometric measurement accuracy of ultrathin lubricant thickness on magnetic head slider

The quantitative analysis of lubricant transferred from disk to slider is important in understanding the interaction in head-disk interface and designing a stable head-disk system. When applying ellipsometric technology to determine the lubricant thickness on slider, the measurement accuracy is of concern due to the location-to-location variations of slider optical constants. This paper carried out a systematic and quantitative study on how the variations of slider optical constants affect the measurement accuracy of lubricant thickness. In this study, the distribution of slider optical constant was obtained; a differential method was used to calculate the uncertainty in lubricant thickness and the calculated results were experimentally verified. The results show that for the state-of-art sliders, the uncertainty in lubricant thickness is about 20?% for below 2?nm thicknesses and less than 15?% for around 3?nm thicknesses when measured at 632.8?nm wavelength. The results of this study might be also useful for the other optical instruments used to determine the amount of the transferred lubricant.     

Quantitative study of lubricant puddling at the head/disk interface

 This paper presents a new methodology to quantify the micro-level lubricant accumulation (lubricant puddling) on a magnetic recording hard disk for flyability testing. After flying over the disk for certain period of time, the head was parked on the disk for certain time duration to allow the lubricant picked up on the head to flow to the disk surface. Using the optical surface analyzer with proper calibration and background removal, the volume of lubricant in a lubricant puddle on the disk has been measured accurately. The effect of various factors, including lubricant type, thickness, the way of head parking, and head parking duration, on lubricant puddling has been investigated. This method is a useful tool for flyability study and other studies that need to quantify the volume of localized lubricant accumulation on the disk. Received: 5 July 2001/Accepted: 11 December 2001     

Ultraviolet irradiation of lubricant and additives thin films to reduce wear to the magnetic head

This paper examined the effects of using 1 nm thickness lubricant thin film combined with additives and deep ultraviolet (UV) irradiation at 185 nm wavelength on the magnetic hard disk to the wear of the magnetic head during contact. Different types and amount of additives were added into the lubricant thin film either with or without deep UV irradiation. A test involved burnishing the magnetic head on the lubricated magnetic hard disk was conducted. The experiment was conducted in a class 100 cleanroom. Contrary to previous studies, the addition of additives into the lubricant film did not lead to a decrease in the amount of wear to the magnetic head. Without deep UV irradiation, the lubricant film combined with additives causes more wear to the magnetic head. The effects of using different percentages of cyclotriphosphazene based additives in perfluoropolyether lubricant were also discussed in this paper. We conclude that deep UV irradiation needed if additives were added when the total lubricant thin film thickness is at 1 nm or below.

     

Adaptive grid generation technique of sub-5?nm flying height air bearing slider with clearance discontinuities

An adaptive grid generation technique including three kinds of weight functions related to the maximum pressure gradient is proposed to simulate the pressure distribution of a sub-5?nm flying height air bearing slider with clearance discontinuities in the interface of the head and the disk in hard disk drives. Considering the clearance discontinuities of a slider with complex geometrical shape, we have defined a discontinuous factor to describe the mass flux crossing the discontinuous boundaries. The effect of different parameters in the weight functions on the node distribution of a typical slider is investigated. The pressure profile of a slider with sub-5?nm flying height is obtained based on the grid distribution calculated from the weight functions. The computational efficiency for simulating the pressure distributions is compared for different kinds of weight functions.     

Effect of thermal pole tip protrusion and disk roughness on slider disk contacts

Ultra-high areal density for hard disk drives requires a stable head disk interface at a flying height lower than 8 nm. At such a low flying height, small flying height variations may cause slider/disk contacts. Slider/disk contacts can also occur when a write-current is applied to the write coil since the flying height between slider and disk can be affected by the thermal expansion of the pole tip. In this paper, we investigate the vibration characteristics of sliders during thermally induced contacts using laser Doppler vibrometry. We perform a parametric study of contact events using disks with different surface roughness and lubricant thicknesses, and analyze the slider motion statistically. For a given write current, we observe that the slider vibrations increase with disk roughness and lubricant thickness.     

Rates of change of total ozone column and surface relative humidity: seasonal variations over Dum Dum (22° 38′ N, 88° 26′ E)

This article presents a study of the seasonal variability of the rate of change of total ozone column (TOC) and the rate of change of surface relative humidity (RH) over Dum Dum (22° 38′ N, 88° 26′ E) during different seasons of the period 1997–2005. The rate of formation of TOC was a maximum during the pre-monsoon and winter seasons and the rate of decrease of TOC was a maximum during the monsoon and post-monsoon seasons throughout the period of study. The rate of increase of surface RH was a maximum in the pre-monsoon season and the rate decreased during the other seasons. At the monsoon and post-monsoon seasons the rate of decrease of TOC increased with the rate of decrease of surface RH.     

Tribocharge build-up and decay at a slider-disk interface

As the gap between the head and disk decreases and the sensitivity of recording transducers increases, the head and disk are more likely to come in contact, which may damage the recording transducer during start/stop and flying cases. One important effect associated with the intermittent head-disk contacts is the tribocharge/tribocurrent phenomenon. In this study, tribocharge and tribocurrent generation during a pico-slider/disk interaction were measured by using an electrometer. These triboelectrical properties were compared with friction force and acoustic emission signals. The electrical potential difference between the slider and the disk was caused by mechanical interactions between them. The tribocharge was generated during the slider-disk interaction and its saturation charge level was about 1 V. The tribocharge build-up level was independent of the slider-disk interaction time. However, the tribocharge decayed when there was no interaction between the slider and the disk. The decay of tribocharge was inversely proportional to the square root of time. Tribocurrent generation coincided with tribocharge generation. The tribocurrent was also independent of the slider-disk interaction time. The current level measured in this study may not reach the electrostatic discharge (ESD) damage level, but it can induce the dissociation of lubricant.This work was supported by KOSEF (Korea Science and Engineering Foundation; Grant No. R01-2003-000-10142-0) and CISD (Center for Information and Storage Device; Grant No. R11-1997-042-12001-0).     

Thin polymer film based rapid surface acoustic wave humidity sensors

Polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) thin films, deposited on the surface of glass slides, were studied using transmission FTIR spectroscopy upon varying relative humidity (RH) from 2 to 70%. The obtained data revealed fast dynamics of water vapor adsorption-desorption with responses on the order of several seconds. Based on the fast FTIR signal intensity changes versus RH, it was proposed that a similar rapid response can be achieved for PVA and PVP coated SAW devices due to changes in mass-loading and film viscoelastic properties upon absorption of water vapor in the films. Sub-micron thickness films were spin-coated onto the surface of LiNbO₃ SAW substrates. Both PVA and PVP based humidity sensors revealed prompt reversible response to variations in humidity, although PVP-based device demonstrated better sensor parameters with total insertion loss variation of about 50 dB over the studied RH range and response time 1.5 s for the humidity step 5-95% (recovery time - 2.5 s), representing one of the fastest SAW-based humidity sensors reported to date.     

Dynamic conformations of fluorocarbon molecules on magnetic hard disk surfaces with charged sites

To meet the demand of extremely high recording density of magnetic storage device, magnetic head is expected to reduce its flying height to sub 5 nm. Lubricant films in such system become more important and the conformation characteristics of lubricant molecules, which receive attractive forces both from the disk and the head, must be clarified for the stable flying of the head. In this report molecular dynamics simulations are carried out to investigate the conformation of lubricant molecules. The model is composed of solid surface and polar-ended fluorocarbon molecules. The surface has several reactive sites, which interact with polar end groups of fluorocarbon molecules. Varying the number of reactive sites, the processes that the reactive sites attract molecules are simulated. Results from the present simulations indicate that lubricant molecules tend to gathered and piled up. It is difficult to achieve 100% coverage.     

100-nm-Wide track pattern readout using a protruded triangular aperture mounted optical head slider

In this study, we evaluated the read-out signal quality from narrow track patterns, utilizing linearly arranged slender track patterns, while changing the track width from 10 to 0.1 ??m and the pattern density from 200?nm line-and-space (L/S) to 120?nm L/S. To acquire narrow track readout signals, we adjusted the aperture??s in-plane position to cross a linear track at shallow angles of less than 1°, and we could successfully transform directly acquired signals into those of an aperture crossing tracks at a right angles. The results of an experiment utilizing a 10-??m-wide track (which is thought to represent an infinitely wide track) clarified that the stationary field was spread to an approximately 1.2?C1.6?times larger region than the typical aperture size of 330?nm. The results also clarified that the ??field spread?? depended on the pattern density, that is, the case in which polarization direction ?? equals 0 or 45°, and that the field spread increased monotonically as the line or space width became smaller. When the polarization direction equals 90°, the field spread had its local maximum when the line or space width was approximately 150?nm. An approximate prediction of the read-out signal amplitude was based on the rule that the signal amplitude was proportional to the net field spread that passed across the track pattern, and this prediction corresponded well to the experimental results, except when the interaction between the stationary field and the track side walls was not taken into account.     

Experimental study of slider–lubricant interaction with different slider designs

In this study, a new type of femto slider (Panda IV) with improved force arrangement and higher air pressure/higher stiffness at trailing edge of the slider was evaluated and compared experimentally with our previous type of femto sliders (Panda II and III). The evaluations were conducted on the Candela 5100 Optical Surface Analyzer (OSA) with the VENA CSS/load–unload system attached. Disks coated with a layer of 1.2 nm lubricant were specially prepared with a lubricant step for the comparison tests. Flying height (FH) of each slider was calibrated by bump disk with 3.5 nm bump height for maintaining the consistent FH during the tests. The relative amount of lubricant redistribution was analyzed with an OSA immediately after each slider is flown on-track for a period of time. It is found that the amount of lubricant transferred or the slider–lubricant interaction are minimized by reducing the size of slider’s central trailing pad. Finally, possible explanations on the lubricant transfer are discussed.     

用复阻抗法分析纳米钛酸钡湿敏元件感湿机理

用复阻抗方法对硅衬底纳米钛酸钡湿敏元件的感湿机理进行了分析。实验测量了湿敏元件在不同相对湿度下的复阻抗特性曲线及阻抗、电容等参数随频率的变化曲线。分析出相应的等效电路,推导了等效电路有关参数,分析讨论了纳米钛酸钡湿敏元件的感湿机理。低湿时,感湿材料本身颗粒电阻和电容及吸附的少量水分子共同起作用;高湿时,吸附的水分子电离和极化起主要作用。     

Investigation of slider dynamics under electro-static force

 With the continual increase in the areal density of hard disk drives, the head/disk interface engineers are continuously challenged in designing an interface with a lower mechanical clearance between the slider and disk. Although, this has traditionally been achieved by making the disk smoother, some novel ideas have been discussed, where the slider generally flies high, and when necessary, a part of the slider is moved closer to the disk surface. Another method that has been discussed is applying a voltage across the slider and disk to bring the whole slider closer to the disk. However, rapid application of voltages across the interface may lead to undesirable slider oscillations. In this paper, we study the dynamics of the slider when a voltage is applied across the slider and disk. We show that one can easily excite the air-bearing frequency and if the voltage is not well controlled, the slider may also damage the disk. We also identify the most probable mechanisms of current flow and show that the disk lubricant plays a critical role in initiating current flow. Received: 26 June 2002 / Accepted: 9 September 2002     

Using trunk posture to monitor heat strain at work

Abstract

This study aimed to determine if trunk posture during walking is related to increases in rectal temperature (T_re). 24 males treadmill walked in one of four conditions (1): 30?min at 3.0?mph and 0% grade, 20?°C and 50% relative humidity (RH), wearing healthcare worker (HCW) PPE; (2): 30?min at 3.0?mph and 0% grade, 27.5?°C and 60% RH, HCW PPE; (3): 30?min at 3.0?mph and 0% grade, 32.5?°C and 70% RH, HCW PPE; and (4): 40?min at 40% VO₂max, 30?°C and 70% RH, wearing firefighter PPE. Trunk posture (Zephyr BioHarness 3) and T_re were measured continuously. T_re was positively related to trunk posture, controlling for covariates (B?=?3.49, p?<?.001). BMI and age moderated this relationship (T_re×age, B?=?0.76, p?<?.001; T_re*BMI, B?=??1.85, p?<?.001). Trunk posture measurement may be useful in monitoring fall potential and magnitude of heat stress of workers in hot environments.

Practitioner Summary: Occupational hyperthermia increases worker risk for heat illness and injury but is difficult to monitor in the field. This investigation shows that trunk posture is independently and positively related to core temperature. Non-invasive measurement or visual inspection of trunk posture could provide novel insight on individual heat strain level.