Monitoring system of effective exposure dose and focus

摘要

We developed a process monitoring system that calculates the effective dose and focus of device wafers using an overlay metrology tool. The effective dose is monitored by measuring the overall width of the fine line-and-space (LS) patterns, the duty ratio of which gradually changes. The effective focus is monitored by measuring the line-end-shortening of the printed line patterns. We used newly designed focus-monitor marks along with conventional LS marks for line-end-shortening. The new marks, which can be measured as an overlay, showed better reproducibility than conventional marks. We calculated the focus shifts caused by variations in the layer structure of device wafers by measuring shots that had been intentionally defocused. Using the defocused shots, we were able to improve the accuracy of our focus-shift calculations. The focus monitor displayed various properties depending on the measurement methods, the design of the marks, and device-layer conditions. Therefore, the mark design must be optimized to each device layer. We demonstrated the accuracy of this monitoring system by applying it to the various layers in a 65-nm-node Cu/low-k interconnect process.