Defocus and twofold astigmatism correction in HAADF-STEM.

A new simultaneous autofocus and twofold astigmatism correction method is proposed for High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). The method makes use of a modification of image variance, which has already been used before as an image quality measure for different types of microscopy, but its use is often justified on heuristic grounds. In this paper we show numerically that the variance reaches its maximum at Scherzer defocus and zero astigmatism. In order to find this maximum a simultaneous optimization of three parameters (focus, x- and y-stigmators) is necessary. This is implemented and tested on a FEI Tecnai F20. It successfully finds the optimal defocus and astigmatism with time and accuracy, compared to a human operator.

Evaluating sharpness functions for automated scanning electron microscopy.

Fast and reliable autofocus techniques are an important topic for automated scanning electron microscopy. In this paper, different autofocus techniques are discussed and applied to a variety of experimental through-focus series of scanning electron microscopy images with different geometries. The procedure of quality evaluation is described, and for a variety of scanning electron microscope samples it is demonstrated that techniques based on image derivatives and Fourier transforms are in general better than statistical, intensity and histogram-based techniques. Further, it is shown that varying of an extra parameter can dramatically increase quality of an autofocus technique.