This work studies the consequences of alloying copper with 2 at % magnesium on the transport of copper through the SiO2 dielectric when subjected to bias temperature aging (BTA). Metal-oxide semiconductor structures, that were thermally annealed at different temperature, were BTA under an electric field of 2.35 MV/cm at varying temperatures and for varying periods of time. The resulting capacitance–voltage (C–V) and I–V curves indicate that the samples thermally annealed above 300 °C form a passivating film between the oxide-metal interface and the metal surface. This oxide film is assumed to cause an inhibition of the diffusion or drifting of copper into the SiO2 and is responsible for the observed stability in the C–V and I–V curves. © 1997 American Vacuum Society.