Atomic scale analysis of phase formation and diffusion kinetics in Ag/Al multilayer thin films

Abstract

Thin films generally exhibit unusual kinetics leading to chemical reactions far from equilibrium conditions. Binary metallic multilayer thin films with miscible elements show some similar behaviors with respect to interdiffusion and phase formation mechanisms. Interfacial density, lattice defects, internal stresses, layer morphologies and deposition conditions strongly control the mass transport between the individual layers. In the present work, Ag/Al multilayer thin films are used as a simple model system, in which the effects of the sputtering power and the bilayer period thickness on the interdiffusion and film reactions are investigated. Multilayers deposited by DC magnetron sputtering undergo calorimetric and microstructural analyses. In particular, atom probe tomography is extensively used to provide quantitative information on concentration gradients, grain boundary segregations, and reaction mechanisms. The magnitude of interdiffusion was found to be inversely proportional to the period thickness for the films deposited under the same conditions, and was reduced using low sputtering power. Both the local segregation at grain boundaries as well as pronounced non-equilibrium supersaturation effects play crucial roles during the early stages of the film reactions. For multilayers with small periods of 10 nm supersaturation of the Al layers with Ag precedes the polymorphic nucleation and growth of the hcp γ-Ag2Al phase. In larger periods the γ phase formation is triggered at junctions between grain boundaries and layers interfaces, where the pathway to heterogeneous nucleation is local supersaturation. Other Ag-rich phases also form as intermediate phases due to asymmetric diffusion rates of parent phases in the γ phase during annealing.