General Considerations on Optics Relevant to Lithography

Optics for photolithography, in Microlithography Science and Technology, J.R. Sheats and B.W. Smith, Eds., p. 183, Marcel Dekker, New York (1998). 

In a projection lithographic exposure tool, when hght is incident on a mask, it is diffracted toward the objective lens in the projection system. The hght propagation inside the tool determines how the tool ultimately performs, depending on the coherence of the hght that illuminates the mask. 

It should he noted that Fraunhofer diffraction represents spatial frequency spectrum or pattern detail. It is essentially the Fourier transform of the mask 

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