Accuracy of overlay measurements: tool and mark asymmetry effects

Definitions of centerline and centerline overlay (O/L) are built on the implicit assumptions of certain symmetries. Verification of these symmetries, and the use of redundancies built into the design of O/L measurement marks, can estimate the relative accuracy of O/L measurements, even in the absence of certified reference materials (standards). Verification carried out as of paired comparison is shown to be very effective; in addition, it constitutes the basis for data culling. Several sources of O/L measurement inaccuracy, associated with O/L measurement tools and with O/L measurement marks, are pointed out and some are illustrated using modeling. A simple measure of tool-related inaccuracy, tool-induced shift (TIS), is proposed and utilized as a performance estimate of O/L measurement tools relative to accuracy. Inaccuracies of O/L measurements, caused by O/L measurement tools and marks, on the order of 100 nm, are observed and their causes identified. Examples are given of significant improvements in accuracy. A roadmap toward accurate O/L measurement is outlined. This approach is also applicable to alignment systems.