Patent ID: 6246477
Filing Date: 2001-06-12
Classification: G01B,G03F

Abstract:
A method for detecting a microscopic feature, comprising:(a) generating a first light beam;(b) transmitting the first light beam through a phase shift plate; wherein the phase shift plate comprises a step that induces a phase change of approximately 180.degree. in the photons of the first beam transmitted on opposite sides of the step; whereby the intensity of the transmitted first beam contains a dark line corresponding to the location of the step;(c) scanning the transmitted first beam with the dark line across the feature, and detecting the intensity of light thereby transmitted through or reflected by the feature;(d) generating a second light beam; wherein the intensity distribution of photons in the second beam is approximately the same as the intensity distribution of photons in the transmitted first beam, but without the dark line of the transmitted first beam;(e) scanning the second beam across the feature, and detecting the intensity of light thereby transmitted through or reflected by the feature;(f) subtracting the detected intensity of the first beam from the detected intensity of the second beam; whereby the difference between the two intensities approximately equals the intensity that would have been detected from a hypothetical light beam whose width corresponded to the width of the dark line in the transmitted first beam; and(g) observing changes in the difference between the two intensities as the first and second beams are scanned across the feature, whereby changes in the difference are an indication of the presence and location of the feature.