Optical alignment optimizations for reducing wafer-induced shift

Detecting the position of wafers after chemical mechanical polishing (CMP) is a critical issue in current and forthcoming IC manufacturing. A wafer alignment system must be highly accurate for all processes. To satisfy such requirements, we have studied and analyzed factors that have made alignment difficult. From the results of the studies, we have developed new optical alignment optimizations that improve the accuracy of FIA (wafer alignment sensor of Nikon's exposure system) and examined them. The approaches are optimizing the focus position based on new classification of measurement errors, developing an advanced algorithm for position determination, and selecting a suitable mark design. The new classification method classifies measurement errors into errors caused by light amplitude errors and errors caused by phase errors. In the experiment, we have fabricated special wafers that make it possible to evaluate the influence of CMP processes on the alignment accuracy. The simulation and experimental results show that overlay error decreases markedly with the new alignment optimizations. FIA with these new optimizations will be highly accurate and suitable alignment sensor for CMP and other processes of future-generation LSI production.