Patent ID: 6984823

Claim:
An electron microscope comprising: an electron optical system for conducting an electron beam to a sample; a focus control part for controlling a focus position of said electron optical system with respect to said sample; an image detecting part for detecting an image caused by said electron beam aimed at said sample; first operating means for spatial frequency changing each of first and second images, which are detected by said image detecting part at two focus positions of a first focus position and a second focus position which is a focus shifted from said first focus position and which is realized by changing a predetermined physical quantity by a known quantity, to obtain first and second transformed images to mutually divide said first and second transformed images on a spatial frequency plane to derive a measured divided quantity indicative of divided results as a function of a spatial frequency; divided quantity data previously prepared as a function of focus positions and spatial frequencies as a set of theoretical divided quantities, said theoretical divided quantities being obtained by deriving an image transfer function with respect to said electron optical system as a function of focus positions and spatial frequencies, substituting said two physical quantities, which are shifted from each other by said known quantity, for said image transfer function to obtain first and second transfer function values to mutually divide said first and second transfer function values on a spatial frequency plane; and second operating means for making a reference to said divided quantity data to derive said theoretical divided quantities, wherein functional characteristics with respect to a spatial frequency correlate to said measured divided quantity, and for deriving a focus position corresponding to the derived theoretical divided quantity as said first focus position, wherein the focus position of said electron optical system is controlled by said focus control part on the basis of the operated results of said second operating means.