Low distortion reflection electron microscopy for surface studies

Owing to its surface sensitivity, reflection electron microscopy (REM) is widely used to study surface structure and surface dynamic processes. However, because of the glancing conditions, REM images are severely foreshortened in one direction by a factor varying between 1/40 and 1/70. If this foreshortening does not really affect the study of roughly one-dimensional objects as monoatomic steps on a surface, it is dramatic when considering the shape of two-dimensional objects (as 2D islands) or even 3D objects (as growth or evaporation spirals) at the surface. In these cases, the distorted images may be numerically corrected, but the finite size of the pixels limits the final resolution. In this paper we show that a simple modification inside the column of the microscope (based on a controlled tilt of the screen, on the use of an intensity magnification and on the large depth of focus available in electron microscopes) allows a true correction of the distortion. We thus are able to obtain low distorted REM images. The results are illustrated by some images of the Si(0 0 1) and Si(1 1 1) surfaces.