Abstract
Scatterometry-based critical dimension (CD), also known as optical CD (OCD), significantly matches CD scanning electron microscopy in accuracy and precision, in addition to offering superior full-profile reconstruction. OCD, however, is computationally intensive. We construct an extremely fast screening tool that determines whether a sample should or should not proceed to subsequent manufacturing steps. To this end, we examine the diffraction signals of the grating in order to determine whether a sample is in or out of its specification limits. This allows us to allocate traditional metrology resources only for samples that show unusual behavior. Support vector machines (SVMs) are trained to classify each incoming sample as in-spec or out-of-spec. The constructed classifier is applied to gratings exposed with a focus-exposure matrix for a rectangular silicon-bottom anti-reflective coating-photoresist stack, which include erroneous samples with under–over exposure, necking, and bridging problems. The misclassification rates as well as false and missed alarm rates are analyzed. Results show that our prototype screening system has misclassification errors on the order of 5% to 10%, while the computation time is on the order of one vector dot product.
