A SEM-based system for photomask placement metrology

Abstract

Mask metrology has long been separated into critical dimension (CD) vs. pattern placement (Registration) in terms of both the parametric definitions as well as measurement techniques applied. The combined effect of measured CD and placement errors on mask-to-mask overlay (OL) is hard to model let alone calculate in definitive terms. As device size continues to shrink, novel lithography solutions being considered for 45nm technology node and beyond such as double exposure and patterning techniques are projected to tighten the overlay requirement much faster than originally anticipated 1,2 Electron optics is generally the preferred solution for small feature size in-die sampling by virtue of its high image resolution, measurement precision, low cross-field distortion and absence of tool induced shift. In this paper we propose to examine and identify the key elements of a new approach in applying electron optics to a mask metrology system that combines CD and pattern placement. We will then present the results from our experiments with a prototype wide field scanning electron microscope (WFSEM) using reticle with optical proximity correction (OPC) features.