Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-09-14
2002-06-11
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06404504
ABSTRACT:
THIS INVENTION relates to a method and apparatus for measuring flying height and more particularly for measuring the clearance of a magnetic head slider above a disk surface by using an optical interference mechanism. This invention also relates to a method and apparatus for measuring the thickness change of a thin film between the slider and disk, the thin film being located on either the air-bearing surface of the slider or on the disk surface.
The magnetic disk drive industry has constantly been attempting to achieve higher recording densities. One of the most effective and critical parameters related to recording density is the flying height of the magnetic head slider above the disk surface. The flying height has been reducing over recent years and is presently down to about 25 nm. There is also a trend towards proximity contact recording and contact recording. In proximity contact recording the flying height is usually less than 25 nm and, for example, would be in the region of 15 nm. For contact recording, the flying height approaches zero, the magnetic recording head of the slider actually making contact with the disk surface. Wear and friction will occur as a result of contact recording and proximity contact recording when the magnetic head slider and the disk surface come into contact with one another. Accordingly, as flying heights are reduced, it becomes important to measure accurately the flying height. However, the measurement of such a small flying height is very difficult.
Wear and friction also occur during the slider take-off and landing process, that is, the contact start-stop process. In fact, the typical failure mechanism for a thin film disk, subjected to contact start-stop by a ceramic slider, is lubricant depletion and degradation, followed by carbon wear. Accordingly, the in-situ measuring and monitoring of lubricant film thickness and lubricant transferring process between the air bearing surface of a slider and a disk surface are also becoming very important, especially for high-end magnetic disk drive design.
Optical interferometry has been applied to measure the flying height of magnetic head sliders for many years. Monochromatic fringe counting techniques, using a white light source and an optical grating, provide an accuracy of 0.15 &mgr;m over the range from 1 to 3 &mgr;m. White light interferometry, provides an accuracy of 50 nm for flying heights below 1 &mgr;m, but at a spacing of less than 150 nm, the colours wash together and cannot be interpreted with reasonable accuracy.
Small spacing measurement techniques based on the photo-electrical conversion of interference intensity have been proposed. One example is a paper “A Visible Laser Interferometer for Air Bearing Separation Measurement to Submicron Accuracy” by A. Nigam (ASME Journal of Tribology Vol.104, PP60-65, January 1982). In the paper, Nigam employs a He-Ne laser as a light source and two photo-detectors. The flying height of the slider was measured with an accuracy of 5 nm and spacing fluctuation resulting from suspension resonance up to 2 kHz range.
Another method to measure slider-disk spacing is disclosed in Ohkubo et al's paper “Accurate Measurement of Gas-lubricated Slider Bearing Separation Using Visible Laser Interferometry” (ASME Journal of Tribology, Vol. 110 PP148-155, January 1988). As described in the paper, the system also uses a He-Ne laser, and two photo-detectors of which one is a reference photo-detector which detects variations in the intensity of the laser source, and the other is a measurement photo-detector used for measuring intensity of the fringes. Consequently, they could successfully measure the static and dynamic flying height of the slider with an accuracy of 1.3 nm on the range of the normalised light intensity range 0.2-0.7, and a frequency range of around of 100 kHz. Although the Ohkubo et al system eliminates some measurement error, it has the following disadvantages: the slider must be landed on the glass disk to determine the fringe order for the spacing calculation; at the points where the light intensity is a minimum or maximum, the slope of the interferometric intensity Vs spacing curve becomes zero. At these points, the noise in electronic intensity measurement causes a large error in spacing measurement relative to the other spacing which are not directly on the fringe maximum or minimum; and the effects of the phase shift of the reflective light from the slider surface on the flying height of the slider are not considered.
Another method to measure slider/disk spacing is disclosed in Muranushi et al's paper “The Ultraviolet Light Interference Method to Measure Slider Flying Heights” (Advances in Information Storage Systems, Vol.5, PP435-445, 1993). The tester uses a Xenon lamp source with a monochrometer which produces monochromatic light having a minimum wavelength of about 200 nm, and photo diode arrays. It can measure a 50 nm flying height within an error of less than 1 nm, and a slider's dynamic motion in the 0-100 kHz frequency range. Measurement of the dynamic motion of the slider is made of the dynamic change in interference light intensity at the middle point between the contiguous fringe-peaks by changing the light wavelength. Whilst the measurement sensitivity of the flying height is very high, so that the accuracy is good, this tester has the following disadvantages: at different flying heights of the slider, different light wavelengths must be used, and the different refraction and extinction coefficients must be determined before measurement; and this method cannot measure the flying height when the flying height moves to zero.
In a method disclosed by Lacey et al (U.S. Pat. No. 5,457,534) an interferometer uses a mercury arc lamp light resource and three detectors with separate wavelengths so that three separate interference fringe signals are generated. The system has the following advantages: at the points where the fringe intensity is a minimum or maximum for one wavelength of light, the slopes of the other two wavelength interferometric intensity Vs spacing curve are still high; and the fringe order is easily determined. The disadvantages are: the optical constant of the slider materials must be measured by an additional ellipsometer; and the sensitivity is quite low when the flying height moves to zero.
The method disclosed by Tadashi Fukuzawa et al (U.S. Pat. No. 5,475,488) measured the flying height by using a white light source and a colour CCD camera. Tentative refraction coefficients and extinction coefficients representative of colours are substituted in theoretical equations expressing the relationship between interference light intensity and flying height, thereby optimising the parameters by non-linear regression. This method need not pre-measure the optical constant of the slider, but still cannot measure the flying height when the flying height becomes less than 25 nm and even to zero.
In summary, using the above light intensity techniques for measuring the flying height, there is a basic limitation: the sensitivity becomes very low when the flying height approaches zero.
In more recent products, ellipsometry is used to measure the flying height of the slider. One example of this method is disclosed in U.S. Pat. No. 5,557,399 to Peter de Groot. Another example is disclosed by Christoher Lacey in Phase Metrics. “Full-Surface Detection of Flying Height with In-Situ N and K Measurement” (1996). The advantages of this type of apparatus are that it is capable of measuring the flying height down to zero with high sensitivity, and measuring the optical constant of the slider simultaneously.
The ellipsometry techniques for measuring flying height are highly accurate but these techniques employ a glass disk instead of a real magnetic recording disk. In reality, the interface between a slider and a real magnetic recording disk is quite different from the interface between a slider and a glass disk.
Many techniques have been used to examine thin films disks for lubricant depletion, accumulation, degradation,
Liu Bo
Low Teck Seng
Zhu Yaolong
Dykema Gossett PLLC
Font Frank G.
Natividad Phil S.
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