Stochastic printing failures in extreme ultraviolet lithography

Stochastic effects in lithography are known to be the cause of critical dimension (CD) variability, quantified, e.g., by linewidth roughness or local CD uniformity, but—when the CDs are small—they can also lead to printing failures, and hence have a significant impact on yield. We will focus on these printing failures and review three important topics in this relatively new field. First, we will look at how these failures can be detected and show how either CD-SEM, e-beam inspection, and optical defect inspection can play a useful role. Next, we consider the important question of how the stochastic failure probability (which we quantify by a metric we call “Not OK,” abbreviated NOK) depends on the lithography- and process-settings, a question that is of obvious importance when looking for conditions that will minimize the failure probability and hence maximize the yield. Finally, we will discuss to what extent we can fit experimentally observed failure probabilities, to empirical models. Such models can play a vital role in future OPC-verification flows, where screening full mask layouts for “stochastic hot spots” is expected to become a mandatory step.