Before getting into imaging theory and practice, I would like to present exposure systems so that readers are better able to comprehend the abstract concepts in Chapters 3 and 4.
An exposure system replicates the mask image to an aerial image in order to expose the photoresist layer on a wafer. After development, this resist image is used as an etch, implant, plating, or lift-off mask for pattern transfer to a thin-film layer on the wafer. There are different systems that can perform the replication function. When the feature size was in the 2- to 5-μm regime, and the budget for semiconductor manufacturing was low, the aerial image in the 10- to 20-μm proximity of the mask sufficiently produced a useable resist image, maintaining feature size and placement control. Below 2 μm, infidelity of the aerial image, defect generation, and alignment viewing difficulties drove the bulk of exposure systems to 1X full-wafer projection printing. The field size of full-wafer systems grew with wafers from 50 to 125 mm in diameter. The ever-increasing wafer size and reducing feature size - together with the associated requirement in feature-size control and overlay accuracy - drove exposure systems to reduction step-and-repeat. Further increases in field size and a reduction in feature size, as well as difficulties in containing the 1X mask making within the total feature size control and overlay accuracy budgets, made it necessary to use reduction step-and-scan systems.
This chapter describes proximity and projection-printing systems, full-wafer, step-and-repeat, and step-and-scan systems. The significance of reduction imaging is also analyzed.