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One of many challenges the process or metrology engineers face is incorrect flagging on the process control chart. It could either be a result of an un-optimized recipe that measures the wrong feature (a space instead of line) or a feature placement error due to tool limitation. This can be a costly problem in the fab where processes are put on hold, feedback loops are corrupted and backlogs are built up unnecessarily. Often many hours must be spent by operators re-inspecting lots and process or metrology engineers re-qualifying the recipes. Most CD SEMs use algorithms employing pattern matching and contrast difference to differentiate between line and space. However, the shrinking node requirement, limited contrast between line and space images and the 1:1 line and space ratios have revealed limitations of these algorithms.
Recently, a metrology solution using beam tilt on the KLA-Tencor ECD-2 has been developed to tackle the problem of dense line or dense trench array measurements. This methodology is a line vs. space detection mechanism that precedes the metrology measurement. This application is quick and oblivious to low contrast differences between line/space and to systematic errors that occur with narrow feature positioning. Whether or not the specified feature type for measurement is centered in the field of view, it always detects the requested feature type and sets the metrology measurement to be made accordingly and therefore significantly reducing false negatives and false positives. This application also allows for greater tolerance in recipe setup and placement error, and thus lightens the burden of recipe creation on the novice user. This technique can also be incorporated into waferless design based metrology where limited prior knowledge of the wafer is one of the requirements.
Future potential of this metrology solution will also be discussed. This could include detecting undercut situations and possibly even correcting bottom CD based on undercut angular detection.
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