Special Topics in Scanning Electron Microscopy

From the basic description in Chapter 4, the SEM image formation process can be summarized as a geometric mapping of information collected when the beam is sequentially addressed to an x–y pattern of specific locations on the specimen. When we are interested in studying the fine-scale details of a specimen, we must understand the factors that influence SEM image resolution. We can define the limit of resolution as the minimum spacing at which two features of the specimen can be recognized as distinct and separate. Such a definition may seem straightforward, but actually applying it to a real situation becomes complicated because we must consider issues beyond the obvious problem of adjusting the beam diameter to the scale of the features of interest. The visibility of a feature must be established before we can consider any issues concerning the spatial scale. For a feature to be visible above the surrounding general background we must first satisfy the conditions contained within the threshold equation (4.26). For a specified beam current, pixel dwell time, and detector efficiency, the threshold equation defines the threshold contrast, the minimum level of contrast (C = ΔS/S_max) that the feature must produce relative to the background to be visible in an image presented to the viewer with appropriate image processing.