Kinetics model for the self-encapsulation of Ag/Al bilayers

A model is proposed to describe the temperature dependence of the aluminum oxynitride (Al_xO_yN_z) diffusion barrier formation during a silver self-encapsulation process. These barrier layers form in the temperature range of 500-725 °C during anneals of the Ag/Al bilayers on oxidized Si substrates in an ammonia ambient. Experimental results show that temperature bas a significant effect on the kinetics of this process. In this investigation, the diffusion of Al atoms through the Ag layers during self-encapsulation process is modeled using an analytical solution to a modified diffusion equation. This model shows that higher anneal temperatures will minimize the retardation effect by i) reducing the chemical affinity between Al and Ag atoms, and ii) allowing more Al atoms to surmount the interfacial energy barrier between the metal layer (Ag) and the newly formed Al_xO_yN_z diffusion barriers. The theoretical predictions on the amount of segregated Al atom correlate well with experimental results from Rutherford backscattering spectrometry. This model in addition confirms the self-passivation characteristics of Al_xO_yN_z diffusion barriers formed by Ag/Al bilayers annealed between 500-725 °C.