Flying Height Measurement Research Articles

Dual track Wallace fly height measurement method to monitor slider motion in 2 dimension at near contact regime

Head media spacing of hard disk drive (HDD) is expected to continue its reduction in order to support areal density growth. At sub-nm clearance between the head and the disk, the intermittent head disk contact and interactions may happen. This does not only cause the fly height modulation but also induces the off-track motions as well. It is desirable to understand the characteristics of the 2D motions in the near contact regime, so as to enable further reduction of the clearance and to improve the reliability of HDD. This paper presents a method to measure instantaneous fly height (FH) motion and cross track motion concurrently by using read back signal from the dual data tracks written at different frequencies. The method is able to separate the FH motion and cross track motion of the head.

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.

In-Situ FH Measurement From Arbitrary Data Pattern

In the hard disk drive (HDD), the Wallace based in-situ flying height (FH) test is widely used for FH measurement since the application of thermal FH control (TFC). The Wallace equation shows the spacing loss of signal amplitude at a particular frequency or wavelength. It has a very simple expression for longitudinal recording and becomes more complex in perpendicular recording with soft magnetic underlayer. In the FH measurement, the signal amplitude at testing frequency has to be in good quality in order to achieve high precision FH measurement. In this work, an integration of power spectral density-based version of the Wallace spacing loss equation is developed. The use of the latter equation enables in-situ FH measurement via the monitoring of relaxed quality (multitone) readback signals over a band of frequency. The new method is able to test the FH change at high precision on AC erased track.

A New Code Pattern of Triple Harmonic Method for Precise In-Situ FH Measurement in Perpendicular Recording

The thermal flying height (FH) control technology had been applied to adjust FH in hard disk drive (HDD) during read/write processes. The readback signal-based in-situ FH testing technology is the feasible way to monitor and control the FH of heads in HDD. For perpendicular recording, the soft magnetic underlayer has the effects on readback signal and change the Wallace equation into a hyperbolic formula with no explicit solution to the spacing change versus the signal amplitude. Fortunately, the hyperbolic formula can be reduced to the traditional Wallace spacing loss equation at short wavelength. In the actual application, there is a need to compare the FH change or modulation at different disk locations and the harmonic ratio method is preferred to cancel media magnetic fluctuations and a certain level of off-track. This work proposed a new code pattern (11110000) to generate three major harmonic signals: the first, the third, and the fourth harmonics. With zero of the second harmonic signal, the harmonic energies are able to concentrate to the third and the fourth harmonics with strong signal intensity. The most important feature is that both the third and the fourth harmonics can meet the short wavelength requirement with enough signal intensity. This allows the new triple harmonic method to measure FH by the ratio of the fourth over the third harmonics. With the both harmonics in the range of short wavelength, it guarantees the accuracy of FH measurement during the touchdown process.

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.

Creation of Data Pattern for High Accuracy Flying Height Measurement

A common technique for measuring flying height is using the ratio of the 1st and the 3rd harmonic of a periodic data pattern. Since the 3rd harmonic is typically smaller than the 1st, and the recording channel usually rolls off at higher harmonics, the magnitude of the 3rd harmonic dominates the signal to noise ratio of the measured flying height. Therefore it is desirable to create a data pattern that has a large 3rd harmonic component for flying height measurement. This paper describes a method to create data patterns with a high frequency component larger than the 1st harmonic. The method multiplies a high frequency signal to a basic periodic signal to create a high frequency component and a low frequency component. The high frequency signal may be the 3rd harmonic and can be larger than the low frequency one. In addition, newly created data pattern was applied to real hard disk drive, and the accuracy of measured flying height was demonstrated in comparison with measured flying height based on conventional data pattern.

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.

Slider Optical Constant Distribution, $n$ and $k$ Correlation, and FH Measurement Accuracy

The slider material is a composition of alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and titanium carbide (TiC) with a big difference in the optical constants. The random grain size, shape, and composition of TiC make the effective optical constant of slider air bearing surface (ABS) in the size of test spot vary largely. The spot-by-spot scanning of optical constant over slider ABS by ellipsometer indicates that the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> correlate each other linearly. As the commercia flying height (FH) testers use the averaged <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> value to calculate FH, there is a concern of FH testing error if the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> value at the testing spot derivates from the average value. This work discovered that the FH testing error is mainly determined by the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> correlation rather than the absolute value of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> dispersion. The laser phase detection method improves the repeatability of FH testing significantly, but the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> correlation is the key to estimate the accuracy of FH measurement.

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.

3811 斜め入射光学系を用いたHDDスライダ浮上量の高精度測定技術に関する検討(S54 情報機器コンピュータメカニクス,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

3811 斜め入射光学系を用いたHDDスライダ浮上量の高精度測定技術に関する検討(S54 情報機器コンピュータメカニクス,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

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.

On-Spot (n,k) Compensation by CCD for Precision Optical Flying Height Measurement

On-Spot (n,k) Compensation by CCD for Precision Optical Flying Height Measurement

Flying Height Measurement of Magnetic Disk Using Double Common-path Heterodyne Interferometer

The magnetic storage capacity depends significantly on the area density, which is close related to the flying-height (FH) of magnetic head. In this paper a double common-path heterodyne interferometer is proposed to measure the FH. The resolution of FH measurement reaches 0.1nm by means of phase measurement method. The influence of vibration of magnetic disk and work table is considered in the configuration design so as to reduce the system error. The experimental results show that the error compensation is better than 10nm when the vibration of disk is 1.2μm.

Consideration and compensation of calibration falloff for optical flying height measurement

In sub-10 nm low flying height (FH) region, system calibration is a significant challenge to achieve precise FH measurement. Unloading calibration mechanism is utilized for every FH measurement using three-wavelength interferometry. Our experiment has shown that the cutoff frequency of photodetectors and the bandwidth of optical filters induce calibration falloff during the calibration process. As a result, the FH measured is underestimated, i.e., the FH measured is lower than its true value. In this paper, mathematical models are proposed to eliminate the side effects due to the bandwidth of optical filters and the cutoff frequency of photodetectors. Results indicate that the proposed compensation schemes are effective in terms of improving the calibration accuracy.

Thermal sensor for in situ flying height measurements of optical flying heads

The application of flying head technology in optical recording promises a considerable increase in recording density. It may be used for both far field and near field recording. For dynamic in situ flying height and flight attitude measurements, a prototype of a thermal test head has been developed. It takes advantage of the heat transfer in sub-micrometer air bearings as well as its dependence from the air gap width. Such a test head has a slider with a similar air bearing surface as the recording head, but with thermo-resistive sensors that are energized by Joule's heat and embedded at each of the four corners. The paper describes the head design and the fabrication technology and provides first experimental test results as well.

2310 磁気ディスク用ヘッドの浮上測定におけるキャリブレーションの影響(要旨講演,情報機器コンピュータメカニクス2)

2310 磁気ディスク用ヘッドの浮上測定におけるキャリブレーションの影響(要旨講演,情報機器コンピュータメカニクス2)

A gap capacitance method for slider flying height measurement in near-field optical disk drives

In order to overcome the diffraction limit of conventional optical disk drives, and substantially increase data storage capacity and density, near-field optical disk drives remain to be realized. The slider of a flying pickup head in a near-field optical disk drive has to fly at a stable spacing above the disk surface. To sense the slider flying height, a gap capacitance method is developed in this study to measure capacitance variation between the pickup head and disk surface. The capacitance varying with the flying height is modulated by a Colpitts oscillator. Subsequent demodulation accounts for height variation of the flying pickup head. Measurement results of this method are verified by using a laser Doppler interferometer.

Precision in situ flying height measurement during track seeking process with intersymbol interference compensation

In order to use the harmonic ratio to measure the flying height with wide usable bands and high signal to noise ratio for ultrahigh density magnetic recording systems, the harmonic selection channel should be made narrow. But intersymbol interference (ISI) could be, therefore, introduced when this setup is used in the track seeking process. As a result, the measurement accuracy deteriorates significantly. Due to the band-limited nature of the harmonic selection channel, a linear equalizer based on the minimum mean-square error algorithm can be adopted to compensate for ISI and the accuracy achievable is 99%.

The effect of slider surface texture on flyability and lubricant migration under near contact conditions

Magnetron sputtering and ion beam sputtering were used to texture the air-bearing surface of magnetic recording sliders. Flying height measurements and Laser-Doppler interferometry were used to compare the ‘flyability’ of textured and untextured sliders. Lubricant redistribution on the disk surface caused by slider/disk interactions was investigated using scanning ellipsometry (Surface Reflectance Analyzer (SRA)). The results show that slider surface texture causes only small changes in the flying height of sliders but reduces slider in-plane and out-of-plane vibrations. Textured sliders were found to cause less lubricant depletion on the disk surface than untextured sliders under ‘near contact’ conditions.