 Overlay control has become even more critical now because the combination of increasing pattern density and innovative techniques such as double patterning and 193 nm immersion lithography creates a novel set of pattern-based yield challenges at the 45 nm technology node and below.This combination causes error budgets to shrink below 30 percent of design rules, where existing overlay metrology solutions cannot meet total measurement uncertainty (TMU) requirements.Overlay metrology solutions with both higher measurement accuracy/precision and process robustness are key factors when addressing increasingly tighter overlay budgets.