Application of mark diagnostics to overlay metrology

A suit of novel diagnostics and culling software for the BioRad Quaestor optical metrology system has been developed and used to improve the overlay (O/L) metrology. It is based on verification of the a priori constraints in images of the target and bullet levels of the O/L measurement marks. The software makes use of the base Quaestor edge detection and matching algorithms to produce two centerlines per level per axis mark detection, rather than one. This enables computation of measures of redundancy or symmetry in images of O/L measurement marks. Such measures of uncertainty of centerline estimation, rather than the values of measured O/L, are the basis for data culling against the user- defined tolerances. Having passed the culling, average centerline is used in the estimate of O/L. The new Quaestor software is shown to result in significant reduction of uncertainty of resulting measurements. Metrology available with the new software is compared to conventional and illustrated with examples. Based on the diagnostics generated by the tool, modifications of O/L mark design, placement, and process of mark formation are pursued with the objective of reducing the net uncertainty of O/L measurements.