Two major trends can be discerned. First, the requirement of 0.25 μm imaging over fields larger than a square inch forces the adoption of step and scan technologies as the cost and size of full field lenses grow noncompetitive. Second, in order to reduce the overall cost of ownership of the photolithography process, the industry is adopting mix-and-match strategies using high NA steppers to print critical mask layers and high speed, low NA, wide field steppers to print non-critical layers.
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William H. Arnold, Gary C. Escher, "Wafer steppers for the 64-M and 256-Mbit memory generations," Proc. SPIE 10273, 64-to 256-Megabit Reticle Generation: Technology Requirements and Approaches: A Critical Review, 1027305 (1 January 1994);