Practical procedure for coma-free alignment using caustic figure.
The practical procedure for coma-free alignment using a single defocused transmission electron microscopy (TEM) image is presented. Caustic figures observed in the defocused TEM image of a focused probe are utilized. Coma-free alignment can be carried out by coinciding a bright-field spot with the center of a caustic curve as observed in an underfocus TEM image. With this method, beam tilt misalignment is reduced to the sub-mrad order (e.g. 0.3mrad for 300kV FEG-TEM). This can be done without intentional beam tilting, an amorphous specimen, high-resolution TEM images, or fast Fourier transform for diffractogram or cross-correlation, which are used in previous methods. Residual coma aberration is detected using the multiple Bragg images of a known crystal. Similarity between the present coma-free alignment and well-known STEM alignment using shadow image is discussed.