A new mechanism for Hillock formation over electrodeposited thin tin film

Tin film about one micron thick was electroplated over a silicon wafer pre-coated with a layer of Cr and another layer of Ni by evaporation. A special sample holder was designed to apply compressive stresses in the electroplated tin film. After incubation in a vacuum oven at 160degC for 7 days, huge hillocks about 10-30 mum diameter and 30-150 mum length grew with a density about 7 per square mm. On the top of the hillocks there appeared a polycrystalline layer similar to the tin film. FIB technique was used to reveal the inner microstructure of these huge hillocks. They are single crystals of Sn with [001] direction perpendicular to the film. The flow path of tin atoms appears to originate from the stressed Sn/Ni interface moving along the interface radially toward the root of the hillock.