Measurement of focus and spherical aberration of an electron microscope objective lens

The displacement between a bright field image and its corresponding dark field image is a measure of both spherical aberration and defocus of the electron microscope objective lens. By measuring the displacements of two image pairs corresponding to two different lattice planes, it is possible to calculate the spherical aberration of a magnetic lens to within ±3% if wave length and magnification are well known. The comparison of experimental results obtained by this technique with the value obtained from a conventional spherical aberration calculation using the distribution of magnetic field over the objective lens shows remarkable accordance. The instrumental astigmatism must be compensated, voltage axis alignment must be carefully made, and one must use a spatially coherent beam for this measurement. The dark field/bright field technique is generally less accurate than the optical diffraction method for focus assessment; however, the optical diffraction method is applicable only to a thin, weak phase object such as carbon film, whereas the displacement method can be used on any substrate. This displacement technique is independent of any phase shift on scattering; thus a combination of the displacement technique with the diffractogram method, which is dependent on the phase shift for scattering, can be used to evaluate the phase shift of the diffracted wave relative to the undiffracted wave.