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The Changing Role of Metrology and Inspection in Lithography Metrology and inspection have been called “non-valueadded” operations in the past, but it is clear that any fab that does not use these tools effectively will see significant losses in yield, bin-sort performance, scrap, and rework. Inspection and metrology’s role has become one of “non-value-subtracted,” as well as one of real value added. Like defect inspection, metrology is a strategic weapon for competitiveness, and for process capability and viability. Especially in lithography, where the small and shrinking process window creates challenges in manufacturability, a fab must view metrology as part of the overall lithography system.1 Metrology provides the visibility to understand where the optimum process lies, and where it is moving. Inspection and measurement provide information that helps engineering to map out the process’s response surface, and also provides confidence in the shape of that surface. This provides the context for yield learning, which is defined as yield improvement rate, or the change in average yield over a period of time, typically one month. (This may also be applied to rework or scrap.) To positively and predictably impact yield, the process engineer must understand the components which contribute to yield loss. The best practices among fabs include a prioritization of the yield detractors, and then focussing on improving or fixing the top detractors. It is only by reducing the impact of each 1 Ashkenaz, Scott, editorial in YMS Summer 2000, Vol 2, Issue 3
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Fall 2001
Yield Management Solutions
(and moving it to the end of the Pareto chart) that predictable progress can be made. In the case of metrology, it is also essential to define a budget. For example, many fabs are struggling to fit their process into a 40 nm or 50 nm overlay budget, without a full understanding of each component of the budget. As described above, with the budget defined (along with the proper way to statistically combine the components), it then becomes easy and obvious where to focus improvement resources. This issue of the Yield Management Solutions magazine illustrates many of the ways that inspection and metrology are used in lithography and other process modules to bring the process into focus, and to assist in improving it, with great economic benefit for the fab. Overlay is proving to be one of the major challenges for 0.13 µm design rules (not to mention for 0.18 µm). KLA-Tencor, working with several fabs, has identified the impact of improper sampling for overlay estimation. In some cases, common (improper) sampling across a wafer can consume 25 nm out of a 40 nm budget, and can cause several percentage points of yield loss! However, by doing a proper systematic analysis, this sampling bias can be reduced below 10 nm or less. Defect issues in the litho cell are well beyond the point where manual inspection can provide useful information. While defect reduction methodologies are well established in all other process modules in the fab, lithography has lagged in best practices for defect yield learning;