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Image Formation With Upper and Lower Secondary Electron Detectors in the Low Voltage Field-Emission SEM

High-resolution secondary electron (SE) imaging was first demonstrated at 100 kV in the STEM a decade ago. High-resolution SE imaging is now routinely obtainable in field-emission SEMs. Although nanometer-scale surface features can be examined at low incident beam voltages we still do not fully understand the factors that affect the contrast of low voltage SE images. At high incident beam voltages, SE1 (SEs generated by the incident probe) and SE2 (SEs generated by backscattered electrons at the sample surface) can be spatially separated. SE1 carries high-resolution detail while SE2 contributes to background. At low incident beam voltages, however, the interaction volume of the incident electrons shrinks rapidly with decreasing incident beam voltage. Thus, both the SE1 and SE2 signals carry high-resolution information. At low incident beam voltages, SE3 (SEs generated by backscattered electrons impinging on the sample chamber, pole pieces and etc.) also carries high-resolution detail and contributes significantly to the collected signal, especially for high atomic number materials and at short working distances.