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...http://www.google.es/patents/US5276337?utm_source=gb-gplus-sharePatente US5276337 - Accuracy of alignment and O/L measurement systems by means of tunable source and handling of signal
Accuracy of alignment and O/L measurement systems by means of tunable source ...
