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Before discussing modified illumination schemes it is natural to ask: What is unmodified illumination? In the days when device critical dimensions (CDs) were large compared with λ NA , provision for user modification of illumination configuration was unnecessary; the exposure systems did not cater to user adjustment. These tools operated at the same partial coherence factor regardless of the patterns being printed. For imaging at k 1 ≤0.75 , however, adjustment of source configuration depending on mask patterns is essential for successful lithography. Common illumination modes include small, medium, and large partial coherence factors, as well as dipole, quadrupole, and annular illumination. Shown by their effective source diagrams in Fig. 3.1, these schemes are the subject of this chapter.
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