The electron-microscope image distortion generated by electromagnetic interference (EMI) is an important problem for accurate imaging. Available commercial solutions to this problem utilize complex hardware for EMI detection and compensation.
However, the distortion reduction can be based on the measurement of magnetic field inside the microscope. Because the distortions are visible as deformations of registered images, digital identification of these deformations could be even applied for an automatic compensation.
A method is proposed for measurement of magnetic field inside the scanning electron microscope (SEM), using only its microscopic images. It is an easy in use and fast method that enables calculation of magnetic field with use of electron beam deflection revealed in images. Changing the scanning rate serves for separation of nonstationary distortions from stationary specimen shapes. The method is applicable for periodic and constant magnetic fields, nevertheless the field is uniform or nonuniform. In the proposed approach the field-generated electron-beam deflection is separated from image deformations generated by induction of additional currents in electronic circuits of SEM. This separation is based on measurements of image distortions at various distances between the final aperture of electron column and the specimen (i.e. working distances).
The results were validated with a magnetic-field meter placed inside the microscope chamber, although this additional measurement has many limitations avoided in the proposed method. Another validation is the identity of results obtained for different electron beam energies.
The impact of the magnetic field on various parts of SEM was measured and the parts of microscope unit most sensitive to the field were determined. Comparison of the results obtained for the field generated inside and outside the microscope chamber allowed to determine the microscope shielding efficiency.