Grains, Growth, and Grooving

We report the in situ investigation of grain growth and grain boundary migration, performed with a variable-temperature scanning tunneling microscope (STM) on a polycrystalline gold film. Atomic step resolution allowed us to identify the individual grains and, thus, also the grain boundaries. Our special, thermal-drift-compensated STM design made it possible to follow the same sample area over large temperature intervals. In this way, we have directly observed grain boundary migration and grain growth. In a first quantitative analysis we correlate the observed, unexpected changes in surface roughness with the evolution of the grain and grain boundary configuration. Atomic step resolution on all grains allows us to correlate changes in the surface roughness with changes in the film structure. We have investigated the structure of polycrys- talline gold films and followed their evolution during in situ heat treatments on a sufficiently small scale to distinguish the mechanisms at work. We observe a strong initial decrease in film roughness, followed by a slow increase at later stages. This roughness evolution is ac- companied by changes in both the grain orientations and the average grain size. We find that precisely these changes are responsible for the roughness evolution.