Diffusion in metal and insulator thin films studied by optical methods

Diffusion between thin film couples of Au-Sn, Au-A and across an A1₂O₂ barrier in an Au-A1₂O₂-AI sandwich have been studied in real time using optical methods. For the Au-Sn films, the attenuated total reflectance method developed by Loisel and Arakawa was used. For the Au-Al and Au Al1sub>20₃-A1 studies a more versatile technique using ordinary angular reflectivity measurement was developed.

The Au-Sn diffusion coefficients obtained between 25 and 42°C were compared with Dyson's single crystal diffusion coefficients D_a and D_cfor different Sn grain directions. That the measured diffusion rates are close to the D_a values suggests a preferred grain orientation in evaporated Sn films.

The observed diffusion in the thin A1₂0₃ layer did not obey the usual parabolic diffusion law. The diffusion rate is observed to decrease exponentially with the oxide thickness, and thus resembles the diffusion process responsible for A1 oxidation in air. The diffusion mechanism is assumed to be similar to Mott's electron tunneling and ion migration model. The difference in the Fermi levels between Au and A1 establishes a very strong electric field across the oxide layer (about 5x10⁶ V/cm), which drives the A1 ions across the oxide. A decrease in the diffusion rate with time is also observed, from which the change of the work function of the Au-A1 layer may be determined for increasing A1 concentrations.