Seeing the forest for the trees: a new approach to CD control

Abstract

Critical dimension (CD) control in advanced semiconductor manufacturing has driven the in-line implementation of complex, high-resolution metrology systems to monitor minimum product dimensions. Yet the high-resolution approach is inconsistent with in-line CD control objectives: resolution does not translate to precision or accuracy, weak response to process parameters inhibits feedback control, sparse sampling precludes partitioning contributors to CD variation, and extendibility to future product generations is in question. We show that superior, affordable and extendable CD control is achieved by optical critical dimension (OCD) measurement of pattern arrays ('forests') whose individual features ('trees') need not be resolved by the metrology tool. The array dimension, averaged over multiple features, responds to CD change about a target value calibrated to the desired device dimension. Response sensitivity greater than that of the minimum dimension can improve the signal-to-noise for feedback control as well as the precision-to-tolerance for product dispositioning. Relatively low-cost and high-speed metrology enables increased product sampling. We describe our application of OCD metrology to 0.25 micrometer CMOS products.