In IC manufacturing industry, scanning electron microscopes (SEM) are widely used as an inspection tool for detecting delects on wafers. The image quality of an SEM inspection tool is thus very important to ensure a reliable and consistent evaluation. The image quality, inter alia, is determined by stage positioning errors of a system. Horizontal errors will lead to image shift and/or image blur whereas vertical errors result in defocus. Thus the positioning errors are quite relevant for SEM imaging, which makes the system much more sensitive to any such errors and disturbances. For the focus performance of e-beams irradiated onto die wafer, the vertical positioning error is relevant. Especially, in a multi beam inspection (MBI) tool, multiple electron-beams (e-beams) simultaneously scan multiple small, adjacent, square areas. The combination of these small square areas leads to a larger total field of view (FoV).
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