This work outlines the development of a general imaging model for use in autofocus, astigmatism correction, and resolution analysis. The model is based on the modulation transfer function of the system in the presence of aberrations, in particular, defocus.
The signals used are related to the ratios of the Fourier transforms of images captured under different operating conditions. Methods are developed for working with these signals in a consistent manner.
The model described is then applied to the problem of autofocus. A fairly general autofocus algorithm is presented and results given which reflect the predictive properties of this model.
The imaging system used for the generation of results was a scanning electron microscope (SEM), although the conclusions should be valid across a far wider range of instruments. It is, however, the specific requirements of the SEM that make the generalisation presented here particularly useful. We have, therefore, confined our investigation to SEM.