Fast Optical Mask Alignment System
Original Publication Date: 1981-Oct-01
Included in the Prior Art Database: 2005-Feb-12
A dark field mask alignment system, in particular for step and repeat projection printing cameras, uses a tilted mirror 4 with an elliptical center hole interposed between mask 2 and telecentric projection objective 5 to image imaging light scattered at the surface of wafer 6 onto an image plane I where it is recorded by means of lens 8 and photomultiplier 9. Light directly reflected at the surface of wafer 6 returns through elliptical hole 4a, without being recorded by detector 9. Scattering elements on the surface of wafer 6, such as alignment marks or edges, thus appear as bright images on a dark background.