The scanning electron microscope (SEM) has unique capabilities for high resolution examination of surface structure and composition. Due to the resolution limits of optical inspection techniques, the semiconductor manufacturing industry has become a rapidly expanding field for SEM applications. As microcircuit groundrules (minimum feature sizes) continue to shrink below one micrometer non-optical measurement methods such as scanning electron microscopy must play an increasingly important role in the inspection of semiconductor device structures at various stages during their fabrication [1,2]. The measurement of structure dimensions such as circuit linewidths (or the spaces between lines) [3] and the measurement of circuit overlay [4] requires a minimum resolution of better than 1/10 groundrule dimensions. In fact, many manufacturing line managers state their resolution requirement as less than 1/20 groundrule dimensions, particularly during the development of a new process. Similarly, it is now apparent from device failure analysis that defects as small as 1/10 groundrule dimensions must also be detected and measured.