2.5 Optical System

There are two categories of optical exposure tools. The two shadow methods of contact and proximity printing are the simplest and least expensive techniques, but they are not suited for industrial high volume production because of their high defect generation. For this reason the image forming technique of projection printing is predominantly used in today's semiconductor manufacturing. Both systems rely on a massively parallel approach, i.e., the entire mask or at least a large portion is imaged at once. A totally different method is the optical scanning beam technique that serially synthesizes the pattern by moving a Gaussian elemental beam spot across the wafer. The basic principle of scanning beams remains the same independent of the physical composition of the beam. More information on scanning methods is given in the context of non-optical lithography in Section 2.7.