Whisker Mitigation Mechanisms in Indium-Doped Tin Thin Films: Role of the Surface

The effect of indium doping on whisker mitigation in tin electroplated on copper has been investigated. Whisker growth studies show that indium addition to Sn effectively mitigates whisker growth. Cross-section images showed predominantly columnar grains in the thin film. X-ray photoelectron spectroscopy studies of the oxide revealed a mixture of In2O3 and SnO2, along with a stoichiometrically defective oxide that can serve as a vacancy source at the metal-oxide interface. In addition, Auger electron spectroscopy showed In segregation at the grain boundaries as well as In enrichment just below the oxide layer. Cyclic voltammetry tests showed that the anodic behavior of the thin films exhibits active/passive transition where the passive current increases significantly upon doping Sn with In. We interpret these surface-related results in terms of a less tenacious oxide that permits reduction of in-plane compressive residual stress, thereby minimizing the driving force for whisker formation and growth.