USE OF A GENERAL IMAGING MODEL TO ACHIEVE PREDICTIVE AUTOFOCUS IN THESCANNING ELECTRON-MICROSCOPE

This work outlines the development of a general imaging model for use in autofocus, astigmatism correction, and resolution analysis. The mod el is based on the modulation transfer function of the system in the p resence of aberrations, in particular, defocus. The signals used are r elated to the ratios of the Fourier transforms of images captured unde r different operating conditions. Methods are developed for working wi th these signals in a consistent manner. The model described is then a pplied to the problem of autofocus. A fairly general autofocus algorit hm is presented and results given which reflect the predictive propert ies of this model. The imaging system used for the generation of resul ts was a scanning electron microscope (SEM), although the conclusions should be valid across a far wider range of instruments. It is, howeve r, the specific requirements of the SEM that make the generalisation p resented here particularly useful. We have, therefore, confined our in vestigation to SEM.