Lithography is the process of defining structures on the wafer according to a specified pattern or mask level. A radiation sensitive material commonly called resist is used to record the pattern. The resist may be positive or negative if it is removed or remains after the development of the irradiated regions. The development may either be carried out with wet chemical etching, by dry plasma etching or by conversion to volatile compounds through the exposure radiation itself. The exposure radiation may be in the form of visible, deep ultraviolet or X-ray photons, or electron or ion beams of particles and can be made by a parallel process such as contact or projection printing from a mask or by serially scanning of one or more beams. Among all these technologies optical lithography holds the leading position in today's semiconductor technology mainly because of its high wafer throughput [Sch97a].