Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-07-12
2001-11-20
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S487000
Reexamination Certificate
active
06320665
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for the dynamic measurement of micro and macro features using an acousto optic scanning means.
BACKGROUND OF THE INVENTION
Non-contact vibration measurement using laser doppler vibrometry is a well-established technique. The laser doppler vibrometry method uses an interferometer to measure the doppler frequency shift induced by the vibration of the object. In order to measure the vibration of the object, the measurement beam strikes the object to be measured and interferes with a reference beam. The resulting frequency shift induced in the interference beam is the vibration of the target surface. The measured vibration signal is the average vibration of the object over the entire beam diameter of the measurement beam striking the object.
U.S. Pat. No. 4,554,836 (Rudd), U.S. Pat. No. 5,394,233 (Wang), U.S. Pat. No. 5,838,439 (Zang et al.), U.S. Pat. No. 5,883,715 (Steinlechner et al.), etc., describes single point vibration measurement using laser doppler vibrometry. Some applications require scanning the beam over the object surface in order to measure the dynamic parameter of the object over an area rather than at a single point for better analysis. Moving the object stage in the X-axis and Y-axis and keeping the beam at a fixed point will allow measurement of the vibration over an area. But such a means for scanning the beam over the object is limited by the accuracy of the moving stages, and in most application it is not possible to move the object. Moreover, the motion of the stage creates vibration and increases the noise. In which case, the object of measurement should be fixed, and the beam needs to scan from one point to the next.
Laser doppler vibrometer can also be used to measure the flying height, i.e., the distance between the slider head and the disk surface in a hard disk. The flying height is a critical parameter and needs to be measured accurately in order to assure optimal performance. Methods such as capacitance, monochromatic interferometry, and white light interferometry are other methods which can also be applied to the measurement of flying height. All the interferometric measurement techniques involve measurement of flying height at a single point or by scanning the beam by moving the hard disk in the X-axis and the Y-axis. This process will not lead to accurate measurement of flying height due to the error induced by the mechanical movement of the hard disk. Also, the measured flying height is the average of the overall flying height of the area of the measurement spot, which is rather large in all these system. Therefore, the smaller the spot size the more accurate is the measured flying height.
SUMMARY OF INVENTION
The first preferred embodiment of the present invention is a method and apparatus for the measurement of dynamic parameters of micro and macro features. The disclosed invention includes a beam spatial filter to filter the laser beam profile. The filtered beam passes through a diaphragm or a slot to further improving the beam quality. This process of beam filtering will achieve a small and uniform profile spot.
The disclosed invention consists of non-mechanical scanning means using an acousto optic deflector. Acousto optic deflectors for X- and Y-axes scanning are placed in both the measurement and the reference path of the laser beam, thereby scanning the reference and the measurement beams simultaneously. The two X-axis and Y-axis acousto optic deflectors in the reference and the measurement path are driven by the common driver for X-axis and Y-axis, respectively. The acousto optic deflectors in combination with the scanning lens, scan the laser beam along the surface of the target along the X-Y plane. The position of the optical components, acousto optic deflectors and scanning lens are such that the scanning measurement and reference beam interfere automatically at all the scanning points, and they are focused on to a stationary photodetector. Further modification of the first preferred embodiment of the present invention includes using common beam splitters for producing the measurement and the reference interference signal.
The second embodiment of the present invention is the measurement of flying height. The reference beam is made to scan the disk surface, and the measurement beam is made to scan the slider head surface. Thus, the relative height between the disk surface and the slider head, i.e., the flying height, is measured. Further modification of the present invention includes use of common beam splitters for producing the measurement and the reference interference signals as in the previous embodiment.
The third embodiment of the present invention includes the modification of the previous embodiment by using two scanning lenses, one for the scanning beam on the slider surface, and the other for the scanning beam on the disk surface. This will achieve the same spot size on both the slider head and the disk surface, which is not possible by using a common scanning lens for both beams. This process will lead to more accurate results. A spherical concave reflector can be used to reflect the reference scanning beam in the same path as the input scanning beam.
REFERENCES:
patent: 4298283 (1981-11-01), Makosch et al.
patent: 4554836 (1985-11-01), Rudd
patent: 4762417 (1988-08-01), Wu et al.
patent: 5394233 (1995-02-01), Wang
patent: 5481360 (1996-01-01), Fugita
patent: 5694216 (1997-12-01), Riza
patent: 5838439 (1998-11-01), Zang et al.
patent: 5883715 (1999-03-01), Steinlechner et al.
Ngoi Bryan Kok Ann
Venkatakrishnan Krishnan
Burns Doane , Swecker, Mathis LLP
Turner Samuel A.
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