Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1991-10-31
1994-01-04
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
356401, G01N 2186
Patent
active
052763374
ABSTRACT:
A method and apparatus is provided for estimating the centerline of an alignment/overlay measurement target by means of projecting light from a tunable, variable wavelength the illumination source onto the target, then performing optical observation of the measurement mark and providing an output signal representing the quantity measured in the observation, computing from the output signal a criterion of signal asymmetry to provide an output product, and tuning the tunable illumination source as a function of the output product. As a result tuning adjustment of the wavelength of illumination is employed to expose an observed feature, and the illumination source is tuned until the criterion is minimized, thus improving the accuracy of the estimated centerline.
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Allen Stephone B.
International Business Machines - Corporation
Jones II Graham S.
Nelms David C.
Peterson Jr. Charles W.
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