Factors that determine the optimum reduction factor for wafer steppers

Abstract

The optimum reduction factor for stepper lenses is determined by trade-offs among several competing constraints and practical limitations. Lens reduction factor were chosen initially on the basis of several factors, including maximum lens element size, usable reticle field, stepper throughput and reticle glass size. These considerations led to the choice of 5x for the reduction factor initially and 4x for the most recent generation of step-and-scan systems. A large reduction factor is beneficial because it reduces the negative impacts of reticle linewidth variations, reticle registration errors, and reticle defects. This is particularly important for optical lithography processes that operate near the diffraction limit, where the mask error factor can be large. For this reason, as well travel down the roadmap, the 4x reduction factor for critical stepper lenses needs to be reconsidered. Before a decision is made, all consequences of a large reduction factor must be taken in to account. For fixed field sizes, reduction factors have been limited to 4x in order to achieve compatibility between 26 mm X 33 mm field sizes and 6 inch reticles, and the assumption of large die size. The reduction factor of 4x can be reconsidered if prior predictions of large die size are not realized of capability for making 230 mm reticles becomes available. The economics of 230 mm reticles changes favorably when the reduction factor is increased. Large reduction factors have relatively neutral effects on lens cost, but will make fast scanning more difficult. A proposal for a possible new optimum reduction is given from the analysis of these critical factors.