Disclosed is a method for determining an overlay error between at least two layers in a multiple layer sample. An imaging optical system is used to measure a plurality of measured optical signals from a plurality of periodic targets on the sample. The targets each have a first structure in a first layer...http://www.google.de/patents/US20080024766?utm_source=gb-gplus-sharePatent US20080024766 - APPARATUS AND METHODS FOR DETECTING OVERLAY ERRORS USING SCATTEROMETRY
