This chapter is concerned with masks for optical lithography. Photomasks used in present-day semiconductor manufacturing are reduction reticles, where the pattern is formed in a chromium layer over a fused silica substrate. The pattern represents one level of an integrated circuit (IC) design. An optical stepper forms a four to five demagnified image of the reticle on the wafer. The reticle usually contains one or more identical circuit patterns plus wafer process test patterns and marks for aligning the reticle in the optical stepper. Typically, 20 to 25 different mask levels are required for a complete IC device.
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