Oxidation-Induced Growth of Sn Whiskers in a Pure Oxygen Gas Environment

Several studies have demonstrated that tin (Sn) whisker growth is enhanced by exposure to atmospheric and high relative humidity conditions. The presumption is that diffusion of oxygen into the Sn thin film results in the lattice expansion of metal oxide phases which place the film under compressive stress. The increased stress is subsequently relieved in part by growth of Sn whiskers. In order to clarify the role of oxygen on whisker production, we exposed sputtered thin films (~ 1400 A) of Sn on brass to a high pressure (1 atm) of pure 99.999% oxygen in a highly controlled, high vacuum environment. A dramatic increase (~ 9X, ~ 16x10³ whiskers/cm²) in whisker growth occurred for the pure 02 exposed samples compared to similar ambient room temperature/humidity incubated samples. X-ray photoelectron spectroscopy (XPS) analysis of the resultant oxidized surface showed that the pure O₂ exposed samples contained a higher fraction of SnO/SnO₂ compared to atmospheric-exposed Sn on brass.