Patent ID: 7732765

Claim:
A scanning electron microscope comprising: an electron lens for converging an electron beam emitted from an electron source onto a specimen; a pair of first and second detectors and a third detector for detecting secondary signals from said specimen when irradiated with said electron beam, said first and second detectors being provided to be spaced from each other by a predetermined spacing; a storage unit for generating and storing first, second and third pieces of secondary signal data from the secondary signals detected by said first, second and third detectors at respective focuses, said focuses of said electron beam to said specimen are changed a plurality of times; a data calculation unit for performing combining operation to combine said first, second and third pieces of secondary signal data at the respective focuses; and a focus adjustment unit for calculating an evaluation value based on an intensity signal obtained by subjecting said combined secondary signal data at the focuses to predetermined data conversion, and adjusting a focus of said electron beam to said specimen using the focuses calculated based on said evaluation values for the focuses, wherein: said combining operation is carried out by combining said first, second and third pieces of secondary signal data at the focus of said electron beam according to a preset equation, and said equation is an equation expressed by αSE+βL+γR, wherein α, β and γ denote coefficients respectively, and L, R and SE denote first, second and third secondary signal data respectively.