Flying Height Tester Research Articles

Effect of Flying Pitch and Roll Angles on Lubricant Transfer Between Disk and Slider

In this paper, the effect of flying pitch angle and flying roll angle on lubricant transfer from the disk to the slider was experimentally studied. A flying height tester, which can measure flying pitch angle and flying roll angle of a slider, was used to study lubricant transfer from the disk to the slider without head/disk contacts. Transparent glass disks with a thin layer of lubricant were used. A charge-couple device camera was used to observe in situ, the lubricant transfer and lubricant flow on the slider surface, while time-of-flight secondary ion mass spectrometry was used to identify lubricant distribution on the slider surface after the test. The experimental results showed that lubricant flow follows the slider skew direction, i.e., lubricant flow goes toward the outer trailing edge (TE) of the slider when it flies at the inner diameter, and toward the inner TE of the slider when it flies at the outer diameter. In addition, flying roll angle plays a more dominant role than flying pitch angle on lubricant transfer from the disk to the slider.

Improvement of measurement sensitivity near contact in intensity-interferometry flying height testers

In the hard disk drive, the spacing between the read/write head and the magnetic disk or flying height has been greatly reduced to a few nanometers to achieve ultrahigh-density magnetic storage. At flying heights below 10 nm, intensity-interferometry flying height tester, the major technique for flying height measurement suffers from low measurement sensitivity. This paper reports a simple and inexpensive method to improve the sensitivity of intensity-interferometry method for measuring flying heights down to contact by using a glass disk coated with multiple layers of silicon and diamond-like carbon (DLC) films. The optimum film thicknesses were selected in order to improve the measurement sensitivity by theoretically analyzing the light interference at the head-disk interface. The improved sensitivity was confirmed in experiments performed in a flying height tester. It was found that the measurement sensitivity at very low flying heights (0---20 nm) was improved by 85 % using a glass disk coated with four layers: Si1 of 1 nm, DLC1 of 55 nm, Si2 of 3 nm and DLC2 of 25 nm. The proposed method not only significantly improves the sensitivity, but it also considerably increases the durability of glass disks promising the application of the intensity-interferometry flying height tester for measuring flying heights down to contact.

Investigation of wear resistance and lifetime of diamond-like carbon (DLC) coated glass disk in flying height measurement process

Flying height has been greatly reduced to less than 10 nm to achieve high-density magnetic storage. This leads to significant disk wear especially, glass disks used in flying height measurement process. This paper reports the utilization of diamond-like carbon (DLC) overcoat to increase the wear resistance and lifetime of commercial glass disks in a flying height tester. Wear resistance of the DLC coated glass disks was investigated in wear test using a triboindentor. The results showed significant wear resistance improvement of the coated disk where the wear depth reduced from 62.2 nm on an uncoated disk to 5–7 nm on 15-nm thick DLC coated disks. Furthermore, lifetime measurement of the coated disk has been performed in a flying height tester. As a result, lifetime of the coated disk has been drastically improved by more than 30 times in comparison to an uncoated disk.

Flying-Height Measurement With a Symmetrical Common-Path Heterodyne Interferometry Method

To enhance the magnetic storage capacity of hard disk drives, the areal density of recording has to be increased continuously. But as the areal density of recording increases to over 100 Gb/in2, the head-disk physical gap, also called flying height (FH), decreases to less than 10 nm. Measurement of FH becomes a significant challenge. In this paper, we propose a symmetrical common-path heterodyne interferometry method to measure FH, utilizing a transverse mode Zeeman laser and a high-speed phase measurement technology, which has the sampling frequency of 100 kHz. We have developed a prototype FH tester based on the method. To effectively compensate influences of the tilt of the rotating disk, vibration of the optical system, and other environmental disturbances on measurement accuracy, we introduced a tri-spot distribution compensation method. Theoretical analysis indicates that the resolution of the FH measurement method reaches 0.1 nm. We carried out a series of experiments of FH measurement with this system, and the results indicate that the measurement system is sensitive enough to FH variation and can measure FH with a high precision and high response. We compared the experimental results with the predictions of FH by numerical simulation of thin-film gas lubrication of sliders, with good agreement.

LOW FLYING HEIGHT MEASUREMENT WITH MULTI-WAVELENGTH INTERFEROMETRY

A method for measurement of ultra-low flying height in head-disk spacing is described. Three different wavelengths are selected out from white light by filters to measure the spacing simultaneously.Besides solving the ambiguity problem,a more reliable result is achieved by using weighted average of measurement results from three different wavelengths,where the weight is dependent upon spacing.Fringe-bunching correction algorithm(FBC)and spot-tilling technique are adopted to suppress calibration and random errors.Moreover,incident bandwidth correction(IBC) method is introduced to compensate the error caused by low monochromaticity of incident light. Based on dynamic flying height tester(DFHTⅡ),with the redesigned of photo-electric conversion and signal acquirement module,an instrument has been developed.And comparing the experimental data from the instrument with those from a KLA-FHT D6,the discrepancy is less than 5%.It indicates that the instrument is suitable to perform ultra-low flying height measurement and satisfies the requirement of magnetic heads manufacturing.

Consideration and compensation for precision optical flying height measurement

It is a big challenge to determine ultra-low slider flying height accurately. The intensity interferometry flying height testing method is widely used to determine slider flying height. However, the intrinsic measurement errors of the method are becoming non-negligible with the decrease in slider flying height. Strategies have to be developed to minimize the errors. To measure flying height with a normal incidence optical flying height tester, a calibration process is required to determine several constants used in flying height calculation. In practice, the calibration is usually done simply by retracting the slider from the disk and measuring the intensity minima and maxima of the interferogram during the retracting process. It has been demonstrated that the single most important source of error in the flying height measurement is associated with errors in the determination of the intensity maxima and minima. In this work, the effects of optical filter, the responding frequency of photodetector, and the lack of the first order intensity minimum on the determination of the intensity maxima and minima are studied. Methodologies to minimize the errors in flying height measurement caused by the above factors are developed.

Fabrication and experimental study of Al2O3-TiC sliders with piezoelectric nanoactuators for flying height control

A gap flying height (FH) of less than 5 nm between the read/write element and the surface of the disk is required for ultrahigh density recording. A stable and constant FH must also be sustained in the presence of altitude and temperature changes and manufacturing tolerance. A FH adjustment or controlled slider that is capable of adjusting its gap FH has been proposed previously. In this paper we demonstrate an inexpensive and low-temperature approach for integrating piezoelectric materials in the fabrication of current small form-factor Al2O3-TiC sliders. A bulk PZT sheet is bonded onto the back of row-bars and the sliders are separated by a standard dicing process. It requires no deep reactive-ion etching (DRIE) or high temperature processes and is suitable for mass production. A conventional design and a new special air bearing surface (ABS) design have been fabricated and tested by an optical profiler and a FH tester. A nonflying actuation stroke of 0.6–0.8 nm/V has been observed. The FH measurements showed that the ABS plays a key role in increasing the actuation efficiency, which also agrees well with the numerical analysis.

Slider-disk interaction and its effect on the flying performance of slider

This paper reports a novel optical head-disk interaction tester setup and experimental investigations into the effects of slider-disk interaction in nanometer spaced head-disk interface. The setup revealed quantitatively the lubricant induced change of slider-disk spacing before the slider enters its fully flying status. The setup was also applied to investigate the slider sinking process which is caused by the meniscus force when the slider starts to rest on disk surface. Results shows that such a setup has the advantage of quantitative characterization of slider-disk interface compared with acoustic emission method. Furthermore, the results also demonstrate its advantages in reflecting the actual interaction between slider, lube, and disk surface, compared with the current optical flying height testers.

Determining the optical constants of read-write sliders during flying-height testing

Flying-height testers for rigid disk drives employ a transparent glass substrate in place of the magnetic disk and use optical interferometry to measure the flight properties of the read-write slider. Because of the material phase change on reflection, the effective optical constants n and k of the slider play an important role in the measurement. We describe an instrument that determines the optical constants simultaneously with flying height, using polarization interferometry. This in situ analysis of n and k obviates the need for independent ellipsometry, while avoiding the problematic retract calibration characteristic of traditional flying-height test equipment. The rms uncertainty for n and k are 0.04, resulting in height uncertainties that range from 3 nm for 250-nm flying heights down to 0.5 nm at contact. We verify these results by use of a variety of experimental techniques on both laboratory samples and actual read-write sliders.

In-situ n & k phase compensation in an interferometric flying height tester

Conventional multiwavelength flying height testers rely upon an external ellipsometer for measurement of the slider's n and k. These values are used to correct the phase offset of the interferometric signal. Typically n and k measurements obtained from a sampling of 20 to 30 sliders are averaged and the results are used indefinitely in production testing. This approach to n and k phase correction may be adequate for high flying heights but it becomes less attractive as flying heights decrease and the correction becomes a large fraction of the nominal. In the new method described here phase offset is measured when a standard retract or rpm calibration is performed. Using the calibration data, an estimate is obtained of the absolute reflectivity of the slider. Next the value of the real part n of the complex index of the slider is inferred from an independent correlation of n to the slider reflectivity. Given these two numbers-the slider reflectivity R and the real part n of the index-the phase correction for flying height is calculated using a well-known relationship giving phase shift on reflection as a function of n and R.

Flying height measurement using frustrated total reflection: determination of the reflectivities by the least-squares method

A new method for determining reflectivities in flying height measurement of a slider using frustrated total reflection (FTR) sensor is evaluated. The method is based on the fact that the reflectivity of FTR has different dependence on flying height for P-polarized light and S-polarized light. The flying height of a slider on a glass disk was measured both by an FTIP sensor using this method and by a traditional flying height tester. The results agree with each other with an error of less than 3 nm.

Flying Height Measurement on Al2O3 Film of a Magnetic Slider

Flying height has been successfully measured on the Al2O3film of a magnetic slider by using a specially developed ellipsometer, a reflectometer, and an intensity based interferometric flying height tester. The accuracy of the measured flying height values has been verified with a profilometer. It is found that, under certain conditions, the flying height value generated by the currently used methodology in the hard disk drive industry is misleading in the sense that it can be significantly different from both the true gap and the minimum flying heights. Therefore, it is proposed that the flying height measurement methodology reported in this study be used by the hard disk drive industry to correctly measure the gap and minimum flying heights.

Numerical Investigation of tri-pad sliders and comparison to other slider designs

The flying behavior of tri-pad sliders was investigated using a finite element based simulator, and the numerical results were compared with experimental measurements using a commercially available flying height tester. Sensitivity analysis and Monte Carlo calculations were performed to investigate the sensitivity of the tri-pad slider to design and manufacturing tolerances. A comparison of the sensitivity of the tri-pad slider design with a standard two rail slider was performed. In addition, the air bearing stiffness of the tri-pad slider was determined and compared with the air bearing stiffness of typical two rail and sub-ambient pressure sliders.

Correction of Slider-Disk Spacing Measuraed with White Light Interference.

The white light interference method is widely used to measure the flying height of a magnetic head slider. This method is very convenient for measuring flying height but presents difficulty when measuring an absolute and precise value of very low flying heights. We therefore selected two types of white light flying testers to compare with a monochromatic UV light interference flying height tester in measured flying heights. We found that white light testers are subject to measurement errors that vary with the flying height.

Head-disk interaction of proximity sliders studied by the acoustic emission probe, the dynamic flying height tester, and the laser Doppler vibrometer

The dynamic behavior of the head-disk interaction of two types of proximity recording sliders, a catamaran tripad and a negative pressure shaped-rail (NPS) tripad, were investigated using a combination of friction and acoustic emission (AE) measurement on a contact start/stop (CSS) tester, dynamic flying height measurement, and laser Doppler vibrometry (LDV). Interesting differences and sensitivity in the head-disk contact behavior of these two types of proximity sliders were observed for different (a) rotational speeds of the disk and radial positions of the heads on the disk, and (b) z-heights. FFT analysis applied to the unfiltered AE signal, dynamic flying height data and LDV displacement signal revealed vibration resonances of the slider body, the suspension, the air-bearing, and the disk.

A flat-head/bump-disk apparatus for evaluating the accuracy of a flying height tester

A flat-head/bump-disk apparatus, consisting of a 70% catamaran head made of Al/sub 2/O/sub 3/-TiC material without leading taper, and a BK-7 borosilicate crown glass disk, has been built to investigate the measurement accuracy of a flying height tester. Compared with existing flying height tester calibration standards, this apparatus allows one to estimate the amount of elastic deformation occurring at the bump/disk interface, relax the strict requirements of head rail and disk surface flatness, and eliminate manual load/unload operations. With this apparatus, good agreements have been obtained between profilometer measured bump heights and flying height tester measured head/disk separations under 60 nm.

A glass disk distortion effect in an optical flying height tester

A flying height tester has been developed for measuring low disk-to-media gap and high disk rotation speeds. The tester can obtain two-dimensional measurement data for a flying head, using an adaptive detection algorithm and signals from a color television camera. A depolarizing unit is used to suppress measurement errors caused by the photoelastic effect of the glass disk at high rotation speeds.

Flying height measurement metrology for ultra-low spacing in rigid magnetic recording

Flying height measurement metrology is presented for ultra-low spacing in rigid magnetic recording. The issues related to the intensity interferometric flying height measurement method are studied. This includes the flying height measurement shift induced by slider material, the dependence of slider optical constants on measurement angle of incidence and location, flying height measurement repeatability, flying height tester calibration, and the effect of the glass disk.

Error analysis of a multiwavelength dynamic flying height tester

We discuss several different sources of error which are relevant to flying height measurements done with the Phase Metrics dynamic flying height tester and with other commercial interferometric testers. It is argued that although the phase shift that takes place upon reflection on the slider material is indeed an important systematic correction, its role in the repeatibility, reproducibility and accuracy of the measurements is secondary when compared with calibration errors.

Comparison of numerical and experimental flying characteristics of tri-pad sliders

The flying characteristics of a tri-pad slider are calculated numerically and compared with measurements using a commercially available, interferometry based flying height tester. The effect of crown, camber and edge blends on both rails and rear pad is investigated. The simulations indicate that the flying characteristics depend strongly on the edge blend of the rear pad. A sensitivity analysis using a Monte Carlo technique in conjunction with a finite element based air bearing solver is performed to determine the variation in flying height due to manufacturing tolerances.