ABSTRACT
This chapter discusses alignment in an exposure tool and its net result, overlay. The content should be sufficient to make understanding the details and issues related to alignment and overlay in particular tools relatively straightforward. The purpose of alignment is to ensure that the layer currently being patterned is perfectly positioned with respect to any previously patterned layers, within the tolerances required by the technology. The alignment process requires, in general, both the translational and rotational positioning of the wafer and/or the projected image as well as some distortion of the image to match the actual shape of the patterns already present. The alignment marks are specially designed targets provided by the scanner tool supplier and are normally specific to each supplier. In the through-the-lens approach, the alignment sensor looks through the same, or mostly the same, optics that are used to project the aerial image onto the wafer.
