Creating sharp images on a transmission electron microscope involves extremely precise setting of the electromagnetic focusing elements in the system. In the context of focus control it is useful to distinguish coarse and fine grain setting regimes, as these have different requirements. Coarse grain focus setting by electromagnetic lenses is difficult, mainly due to hysteresis caused by the iron constituents of the lens. This chapter describes an approach to control and optimize the coarse-grain setting of focus, including an explicit trade-off analysis of speed versus accuracy. In fine-grain setting the challenges reside mainly in image-based multi-parameter optimization of image sharpness, which is influenced by both lens and stigmator settings. This chapter also describes methods to optimize for focus and astigmatism, and their application in measurement procedures.
KeywordsDefocus control Image sharpness Electromagnetic lens Hysteresis Trade-off analysis Astigmatism Sharpness function Multi-parameter optimization Nelder-Mead simplex method
The author likes to acknowledge Rob van Vucht and Seyno Sluyterman of FEI for their support.
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