Image interpretation of metal shadowed specimens with the aid of Monte Carlo simulation of the shadowing process

Rotary shadowing is investigated by means of an adapted shadowing model, which has been used for relief reconstruction of unidirectionally shadowed specimens. The metal layer geometry is calculated for a simple geometric object. In order to elucidate the metal accumulation process on the surface, a Monte Carlo simulation of shadowing is applied to synthetic reliefs and to experimental relief reconstruction data of a paracrystalline membrane. For rotary shadowed specimens, no linear relationship between local metal thickness and surface height can be established as in the unidirectional case. Low spatial frequencies of the surface relief contribute very little to the information content of the observed metal film. Contrast produced by rotary shadowed films is lower compared to unidirectionally shadowed ones. The main effects are due to the steep slopes which give rise to the considerable amount of metal viewed in the normal projection, and due to the shadow casts. A further contrast enhancing effect is self-shadowing, an artifact introduced primarily by rotating the specimen during evaporation. The Monte Carlo simulation shows that low elevation angles favor self-shadowing effects. This results in a pronounced granularity that is caused by the random incidence of the atoms and the accumulation at previously deposited atoms or grown clusters. On real surfaces, small decorating clusters can be enlarged by self-shadowing. This explains the increased decoration tendency of rotary shadowed specimens. The Monte Carlo method can be used as a means to differentiate shadowing information from these effects.