A study of the factors influencing the kinetics in Ag/Al bilayer systems

Ag (~200 nm)/Al (~20-30 nm) bilayer structures on silicon-dioxide substrates were annealed at temperatures ranging from 400 to 600 °C for times 15-120 min in a flowing ammonia ambient. Rutherford backscattering spectrometry (RBS) analysis showed that upon annealing the Al reduces the SiO₂ as well as diffusing to the free surface to react with residual oxygen in the ambient to form an Al-oxide passivation layer. The reduction of the SiO₂ results in free O inside the Ag, which is available to react with the diffused Al constituting a competition to the surface reaction. The reaction of the diffused Al with the freed O results in an increased trapping of Al inside the Ag instead of diffusing to the surface. The trapped Al translates into an increased residual aluminum concentration. The larger trapping of Al as a result of the SiO₂ reduction imposes an additional barrier to the transport of Al through the Ag and also an upper limit to the Al-oxide passivation layer thickness. Activation energies of between 0.25 and 0.34 eV were obtained for the two different Al thicknesses (20 and 30 nm) used in this experiment. It is therefore evident that the different Al thicknesses have no significant influence on the activation energies but definitely affect the formation of the Al-oxide passivation as a direct consequence of increased trapping of the aluminum in the Ag. It is therefore, desirable to use Al interlayers of less than 10 nm in order to minimize SiO₂ reduction and reduce the residual Al concentration.