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The 28nm technology node is extremely challenging for the production of large chips with very aggressive design rules. For the 1x-Metal layers especially, it has been observed that some design configurations known as hotspots have very limited lithographic process window. They have generally complex bidimensional geometries that make them highly sensitive to any process change and particularly to focus variation or mask error. The first purpose of this study is to present an original methodology to characterize, with good statistics, the hotspots after lithography. The principle is to stack many SEM pictures of the same hotspot, repeated on the wafer, to improve the image signal-to-noise ratio and to average uncontrolled sources of CD variation. With such high quality images it is then possible to extract contours of high accuracy. The second scope of this work is to take advantage of the extracted contours to derive many CD measured preferentially along the hotspot critical sections. The main contributors to CD variation are identified by analyzing the CD of hotspots processed under different experimental conditions like intra-field locations, mask sizing or focus changes.