A study of feed-forward strategies for overlay control in lithography processes using CGS technology

In advanced lithography, controlling the overlay budget is one of the most critical requirements. For device nodes at 10nm and below, there are many process-based sources of surface displacement that contribute to the lithography overlay budget that are independent of the lithography process, i.e. etch, anneal, CMP, etc. By developing an understanding of how process-induced surface displacement impacts overlay, displacement information can be fed-forward to the lithography tool for overlay correction during the lithography process. This paper describes the use of displacement measurement technology, the Coherent Gradient Sensing (CGS) interferometer, to characterize the effects of process-induced overlay error on the overlay budget through several processes in a typical semiconductor process flow. The CGS technique facilitates the generation of high-density displacement maps (>3 million points on 300mm wafer) such that the stresses induced die-by-die and process-by-process can be tracked in detail. Case studies are presented that summarize the use of the CGS data to reveal correlations between displacement variation and overlay variation, and demonstrate how feed forward can be applied for controlling overlay error.