THERMODYNAMIC LIMITATIONS ON THE RESOLUTION OBTAINABLE WITH METAL REPLICAS

The major factor limiting resolution of metal-shadowed surfaces for el ectron and scanning tunnelling microscopy is the granularity of the me tal film, This granularity had been believed to result from a recrysta llization of the evaporated film, and hence could be limited by use of higher melting point materials for replication, or inhibited by addin g carbon or other impurities to the film, However, evaporated and sput tered films of amorphous metal alloys that do not crystallize also sho w a granularity that decreases with increasing alloy melting point, A simple thermodynamic analysis shows that the granularity results from a dewetting of the typically low surface energy sample by the high sur face energy metal film, similar to the beading up of drops of spilled mercury, The metal granularity and the resulting resolution of the met al-coated surface is proportional to the mobility of the metal on the surface after evaporation, which is related to the difference in tempe rature between the melting point of the metal and the sample surface t emperature.