Electron microscope and method for controlling focus position thereof

Abstract

There are provided an electron microscope capable of carrying out focusing and astigmatism correction without depending on characteristics of a sample, and a method for controlling its focus position. The electron microscope according to the present invention comprises: an electron optical system (); a focus control part (); an image detecting part (); a first operating part () for mutually dividing first and second transformed images () and (), which are obtained by carrying out the fast Fourier transform of first and second images () and () detected at two focus positions of a first focus position (f) and a second focus position (f) shifted from the first focus position by a known focus shifted quantity Δf, to obtain a measured divided quantity Rexp; divided quantity data () previously prepared and stored as a function of focus positions and spatial frequencies as a set of theoretical divided quantities, the theoretical divided quantities being obtained by substituting the two focus positions shifted by the focus shifted quantity Δf for an image transfer function (r,f) to obtain first and second transfer function values K(r;f) and (r;f+Δf) to mutually divide the first and second transfer function values K(r;f) and (r;f+Δf) on a spatial frequency plane; and a second operating part () for making a reference to the divided quantity data () to derive a theoretical divided quantity K(r;f) correlating to the measured divided quantity Rexp, and for deriving a focus position fcorresponding to the derived theoretical divided quantity K(r;f)/K(r;f+Δf) as a first focus position f