Tracer study of diffusion and electromigration in thin tin films

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Abstract
A comprehensive study of lateral self-diffusion and electromigration in evaporated thin Sn films over the temperature range −50 to 198 °C is reported. Sn119m atoms were deposited over a central portion of thin Sn film stripes and the subsequent diffusional spreading and migration of the tracer distribution were evaluated by a high-resolution autoradiographic technique. Activation energies suggestive of grain boundary transport processes were obtained. At low temperatures the ionic drift velocity in grain boundaries Vb is measured to be VbT/j=3.4×10−2 exp[−(8710±360 cal/mol)/RT] cm3 K/A s, while the low-temperature grain boundary diffusivity Db is given by Db=4.9 exp[−(11 700±840 cal/mol)/RT] cm2/s. At higher temperatures (i.e., >80 °C) transport measurements were conducted employing Sn films of two different grain sizes, and apparent drift velocities and diffusivities were obtained. Application of the Hart model enabled the extraction of grain-size independent values of the grain boundary ionic drift velocity and diffusivity given by VbT/j=3.3×10−2 exp[−(8980 cal/mol)/RT] cm3 K/A s and Db=8.3×10−2 exp[−(9790 cal/mol)/RT] cm2/s. The effective valence Z* in the grain boundaries was calculated over the complete temperature range using the Db and VbT/j values obtained in the present investigation. A rapid rise in Z* at low temperatures, similar to that previously reported in Au films, was observed, suggesting that at low temperatures the Z* in the grain boundaries is greater than in the lattice. Electromigration in Sn is anode directed at all temperatures.