Growth of Sn Whiskers on Semiconductor and Insulator Surfaces

Studies of tin (Sn) whiskers have emphasized the role of intermetallic compound formation at the tin-substrate interface, which induces a high compressive stress state in the film, thought to be a necessary condition for Sn whisker growth. The goal of this work was to determine if whisker growth is possible from films where intermetallic compounds are largely absent. Thin films (~ 1600 Å) of Sn were sputter-deposited on a variety of semiconductor and insulator surfaces (Si, GaAs, InAs, InP, Ge, glass) which, according to equilibrium phase diagrams, do not form intermetallic compounds with Sn. The films were then incubated under ambient room temperature/humidity conditions for three months. All specimens displayed substantial whiskering, with the largest whisker producers (Si and Ge) producing whisker densities >18X higher (~ 39 × 10³ whiskers/cm²) compared to specimens of Sn sputtered on brass under the same conditions. This shows that Sn whiskers grow readily on materials lacking interfacial intermetallic compounds. Rutherford backscattering spectroscopy (RBS) studies on the resultant films showed a uniform Sn film thickness decrease during the incubation period by ~100 Å, suggesting that the Sn feedstock for whisker growth is derived from long-range Sn migration. No simple correlation due to CTE mismatches was found among the various semiconductor substrates and whisker growth.