Measurement of contamination rate and stage drift in scanning electron microscopes

Abstract

Dimensional measurements with Scanning Electron Microscopes (SEMs) are done under the presumption that the sample and its location do not change during the measurements. This is never the case, because the stage and the primary electron beam always move slightly, and the sample changes due to contamination and charging. Digital time-lapse scanning electron microscopy is a useful method to investigate the extent and effects of contamination and stage drift. Implemented in hardware and software, this technique involves capture and storage of a sequence of still images. It is then possible to apply image processing or analysis to individual frames, or to view them in rapid sequence to create a motion picture of how the sample changes. The effects of drift and contamination are generally cumulative; their contributions to inaccuracy and imprecision increase with increasing measurement time. It is important to understand this in order to strike the right compromise with other factors that favor longer measurement times, such as improving signal-to-noise ratio by averaging several images or line scans. This paper describes a method to measure rates of contamination and drift of SEM samples, providing information necessary to optimize the accuracy and precision of line width measurements.