Focus error correction method and apparatus

摘要

Scanning method and apparatus for maintaining a desired position of an article are presented. The apparatus comprises an illumination unit, a focusing optics and a focus detection unit. The illumination unit generates an incident light beam for illuminating an elongated region of the article and producing light returned from the illuminated region. The focusing optics focuses the incident beam onto a focal plane and collects at least a portion of the returned light. The focus detection unit comprises an imaging optics and a detector having a sensing surface responsive to light impinging thereon for generating data representative thereof. The imaging optics is capable of forming first and second images on the sensing surface. The first and second images are formed by first and second spatially separated substantially identical light components of the collected returned light, respectively, which propagate symmetrically relative to an optical axis of the focusing optics. A position of the first image relative to the second image is indicative of the position of the article relative to the focal plane.