The importance of beam alignment and crystal tilt in high resolution electron microscopy

The relative influences of crystal tilt and beam alignment on high-resolution electron-microscopic imaging have been investigated. With the use of contrast transfer theory in generalised dimensionless form, the major effect of slight beam misalignment has been shown to be the introduction of an antisymmetric phase shift in the diffracted beams so that the presence of any such misalignment cannot be detected by the standard diagnostic tool of high-resolution electron microscopy, namely the optical diffractogram. Specific image simulations, at 100 and 500 keV, for materials of both small and large unit cells (SnO₂ and Ti₂Nb₁₀O₂₉ respectively) show, however, that even slight beam tilt can have a marked effect on the images of crystalline materials, causing considerable spurious detail and a loss of expected symmetry. The various options for ensuring accurate beam and crystal alignment are briefly reviewed, and some aspects of the alignment problems are demonstrated using some recent experimental images recorded at 500 kV.