Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-07-24
2003-07-08
Adams, Russell (Department: 2851)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S493000, C356S369000
Reexamination Certificate
active
06590667
ABSTRACT:
FIELD OF THE INVENTION
The invention disclosed is a method and an apparatus for measuring distance between a slider and a transparent disk with sub-nanometer resolution, particularly in nanometer flying height measurement of a read-write head on a glide disk by applying ellipsometry.
BACKGROUND OF THE INVENTION
According to the prior art, the methods for measuring thickness of an air film, e.g. flying height of a slider, are generally classified into a) capacitance type, b) light interferometry, and c) ellipsometry with fixed incident angle. The capacitance type is used to measure flying information of a slider. Three unshielded capacitance probes are mounted on the slider for monitoring the roll angle, the pitch angle, and the flying height of the slider. However, the measured signal intensity is inversely proportional to the flying height of the slider. Besides, the slider for measuring the flying height by using this method must be made of special materials, e.g. ceramics, and the cost is high. Therefore, the capacitance type is nowadays out of date. Generally, the flying height measured by using this method is limited ranging from 1000 nm to 5000 nm.
Owing to different light beam sources, different methods, e.g. laser beam interferometry or white light interferometry, for measuring the flying height of a slider by applying interferometry are developed. The method for measuring the flying height of a slider by applying laser beam interferometry is published by Best et. al. on pp. 1017-1018, No. 5, Vol. MAG-22, IEEE Transaction On Magnetics in 1986. The monochrome is used as the light source. The phase difference of the incident light toward the slider and the reflected light from the slider is 180 degrees. Because of interference of the incident light and the reflected light, the flying height of the slider can be obtained according to the counted changes of the interference fringes. However, the accuracy of the counted changes of the interference fringes is not high. Moreover, the slider is assumed being made of dielectric material. Therefore, the absorption coefficient of the slider is neglected, and the neglected absorption coefficient thus results in great deviation of the measured flying height of the slider. With respect to the white light interferometry, the interference fringes are analyzed by using a spectrum meter. The two wavelengths of the corresponding maximum light intensity and the corresponding weakest light intensity are obtained for determining the flying height of a slider. The flying height measured by using this method is limited ranging from 127 nm to 750 nm. Once the distance between the slider and the disk is less than 127 nm, there are no apparent peaks and valleys of the interfering light intensity for the interference fringes to determine the flying height of the slider. As for the dynamic interferometry developed by the Phase Metrics Company, the change of the flying height is more than a quarter of the wavelength for obtaining the curves of the corresponding movements for the continuous changes of the maximum light intensity and the weakest light intensity. The curves are used for calibration before the flying height is measured. For example, Ohkuboy developed a system for measuring the flying height of a slider by using the He—Ne laser as the light source. The fringe orders are obtained according to the variations of the interfering light intensities while the slider lands on the disk for being used for calibration before measuring the flying height. In 1992, C. Lacey disclosed to use the mercury arc lamp, which primarily incident the light beam with wavelengths of 436 nm, 548 nm and 580 nm, as the interfering light source. The calibrating curve is obtained while the magnetic head is unloaded from the disk by the rotating arm.
Generally, the laser Doppler Vibrometer/Interferometry (LDV/I) is used to determine the dynamic actions of the suspension systems for a slider of a hard disk. T. C. McMillan and F. E. Talke uses three wavelengths laser beam as the interfering light source in 1994 for measuring the flying height of a slider (pp. 1017-1018, No. 5, Vol. MAG-22, IEEE Transaction On Magnetics). The intensities of the interference fringes are determined by the method of interpolation, where the maximum light intensity and the obtained weakest light intensity are obtained first. The flying height of a slider less than 100 nm is measured by phase demodulation. The method for measuring the flying height of a slider by applying laser beam division interferometry is published by C. K. Lee and T. W. Wu on pp. 1675-1680, No. 9, Vol. 33, AIAA Journal in 1995. One laser beam is projected onto the back of the slider to determine the dynamic characteristic thereof, while the other is projected onto the surface of the disk to determine the dynamic contact point in real time to modify deviation for obtaining higher accuracy. M. Staudenmann, M. J. Donovan and D. B. Bogy disclosed a method for measuring the flying height of a slider on pp. 4173-4175, No. 6, Vol. 30, IEEE Transaction in 1998. The laser beam is projected onto the back of the slider. By comparing the incident laser beam with the reflected laser beam from the slider, the velocity of the slider is obtained by frequency demodulation, and the movement of the slider is obtained by phase demodulation. However, according to this method, the magnetic head must be landed on the disk or a laser beam must be divided onto the surface of the disk for being used as reference light beam for the contact point. Beside, because the obtained flying height is the distance between the slider and the glide disk, deviation of the flying height rises owing to the dynamic actions of the slider and the distance variation.
In recent years, the flying height tends to be much lower because of increase of the coding density of a hard disk. For the present, the flying height is less than 25 nm, therefore more accurate measurement is expectable required. Phase Matrics Company and Zygo Company both disclosed that the flying height of a slider and the optical properties of the surface of the slider can be obtained by applying ellipsometry. C. Lacey assumes that the complex refraction index all over the slider are all the same, therefore the flying height and the complex refraction index values of the slider can be obtained by using a imaging ellipsometer. In his setup, a charge-coupled device (CCD) is used, and the ellipsometric information of multiple points can be obtained without further unloading the slider to vary its flying height. Therefore, the flying height of the slider can be obtained according to the pitch, roll, crown, cross-crown and twist parameters. The essential assumption in the method is that the slider is made of pure substrate regardless of thin films. However, it's not the case. Besides, the variation of the complex refraction index of the slider surface is not negligible for measuring the flying height. In 1996, Peter de Groot discloses how to obtain the complex refraction index of the magnetic by applying ellipsometry with fixed incident angle (U.S. Pat. No. 5,557,399). Please refer to
FIG. 1
, a schematic diagram of the conventional system for the flying height measurement, which is modeled as an air film
302
, of a slider
301
by applying ellipsometry with fixed incident angle. In each measurement of the flying height of the slider, the slider
301
is loaded first, and moved from the distance over one wavelength to the distance less than one wavelength. The vertical and horizontal light intensities and the maximum and minimum values of the phase are measured for the first step, therefore the flying height d and the complex refraction indexes n, k values of the slider can be solved through the ellipsometry.
From 1995, it's well known that the magneto-impedance read-write head (MR head) is generally used as the read-write head of a hard disk. For improving read-write performance and increasing storing density, the flying height of the magneto-impedance read-write head (MR head) is generally
Lee Chih-Kung
Lee Jau-Hu
Lee Shu-Sheng
Shiue Shuen-Chen
Wu Jiun-Yan
Michael Best & Friedrich LLC
National Science Council
Sever Andrew
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