Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-06-21
2003-12-16
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06665077
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to method and apparatus for performing measurement/testing of the flying height of read-write head sliders utilized in disk-type data/information recording, storage, and retrieval systems. More particularly, the present invention relates to method and apparatus for performing flying height measurement/testing with increased sensitivity at very low. flying heights on the order of 5&mgr; inches or less, e.g., 1&mgr; inch or less.
BACKGROUND OF THE INVENTION
Thin film magnetic and magneto-optical (“MO”) recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (“CSS”) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping.
It is considered desirable during reading and recording operations, and for obtainment of high areal recording densities, to maintain the transducer head as close to the associated recording surface as is possible, i.e., to minimize the “flying height” of the head slider. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk surface to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion.
As should be evident from the above, an experimental method for verification and testing of the fly height of the head slider during both the design and production phases of read-write heads for rotating disk magnetic and MO storage media is necessary. At present, three (3) wavelength interferometry, as for example, disclosed in U.S. Pat. No. 5,280,340 to C. Lacey, the entire disclosure of which is incorporated herein by reference, is the most commonly employed technique for direct measurement of fly heights. According to this technique, a test apparatus (e.g., such as manufactured by Phase Metrics, Inc., San Diego, Calif.) is utilized which comprises an optically flat, very smooth, light transparent (e.g., glass), rotating disk, typically coated on a first (e.g., lower or back side) with a very thin (e.g., 0.5-1.0 nm) layer of a perfluoropolyether lubricant, and a means for controllably positioning a head slider at a very small spacing (i.e., flying height or air gap) from the first surface of the disk. As shown in
FIG. 1
, white light emanating from a suitable source impinges the second (i.e., upper or front side) surface of the disk (illustratively at substantially normal incidence) and is transmitted through the disk. A first portion of the transmitted incident light travels through the air gap d between the first surface of the disk and the head slider, and reflected thereat back through the disk for ultimate receipt by a suitable detector positioned above the second (front side) surface of the disk; whereas a second portion of the transmitted incident light is reflected at the first (back side) surface of the disk back through the disk for ultimate receipt by the detector. The first and second portions of the transmitted light reflected from the head slider surface and from the first (back side) surface of the disk, respectively, are both constructively and destructively combined by interference in the space before the detector to yield a detector output which produces an intensity vs. wavelength pattern, depending upon the spacing (flying height) between the glass disk and the head slider (the preceding assumes that any portion of the incident light reflected from the second or front side surface of the glass disk is small and that any interference effect resulting therefrom is very small due to the thickness of the disk being much greater than the flying height d).
More specifically, and with reference to
FIG. 2
, the total reflected light intensity vs. wavelength resulting from the constructive and destructive interference of the first and second portions of the reflected incident light is modulated at a specific air gap or flying height d to produce a generally sinusoidally-shaped intensity vs. wavelength pattern having spaced-apart maxima and minima, and is compared with a calibration curve to determine the actual flying height. If incident light of a particular wavelength is utilized for the measurement, a half-cycle of the reflected intensity modulation corresponds to a change in the air gap or flying height d equal to one quarter (¼) of the particular incident wavelength. For example, for yellow/green incident light of 560 nm wavelength, the wavelength spacing between adjacent peaks of reflected light intensity of the intensity vs. wavelength pattern corresponds to a change in air gap or flying height of about 140 nm.
However, unlike typical interferometric measurements, the distances or spacings to be measured in air gap or flying height applications are much less than the wavelengths of the light utilized for the measurement, typically on the order of 25 nm or less; consequently, only a small portion of the peak-to-peak reflected light intensity vs. wavelength modulation pattern can be utilized for flying height measurement. Therefore, in order to maximize the sensitivity of the measurement, it is advantageous for the air gaps or flying heights corresponding to the relatively slowly changing reflected light intensities at the maximum and minimum of the modulation pattern of the reflected light intensity to be far from the air gap or flying height region of interest, where the reflected light intensity is desired to change rapidly with change in air gap or flying height. Further, in order to maximize measurement sensitivity at spacings of 25 nm or less, it is considered essential that overall intensity losses arising from the disk substrate due to, inter alia, internal reflection and absorption within the glass disk and external reflection and scattering therefrom, be minimized.
As indicated above, the continuing requirement for decreased flying heights for obtaining increased areal recording densities of magnetic recording media has necessitated continuing improvements in the sensitivity and accuracy of flying height measurements at very low head-to-disk spacings. However, accurate determination of flying heights below about 25 nm utilizing lubricated optical glass disks, as described supra, have become ever more problematic. While the sources or origins of the difficulties are several and varied, they are, in essence, dominated by the fact that the glass material utilized for the disk is a poor tribological surface for interaction with head sliders which are typically provided at their
Kuo David Shiao-Min
Pitchford Thomas Roy
Stirniman Michael Joseph
Artman Thomas
Bruce David V.
McDermott & Will & Emery
Seagate Technology LLC
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