Atomic transport in amorphous Mo-Cu and Ta-Cu immiscible systems

Abstract

Metastable binary systems with positive enthalpy of mixing have practically no diffusional data in literature. Experimental data on Cu diffusion in amorphous immiscible systems such as Mo-Cu and Ta-Cu are presented in this study. These studies were conducted in sputtered Ni/Mo1−xCux/Ni and Ni/Ta1−xCux/Ni tri-layers, where the nanocrystalline Ni layers acted as effective sinks for Cu atoms located in the amorphous layers. The secondary neutral mass spectrometry (SNMS) technique allowed us to observe the out-diffusion of Cu into the Ni sink layer at temperatures between 100 °C and 250 °C. Additionally, high-resolution transmission electron microscopy (HR-TEM) and energy dispersive spectroscopy (EDX) was used to characterize Mo-Cu and Ta-Cu layers. As a result of a coupled diffusion equation being solved for both amorphous (source) and polycrystalline (sink) layers, we were able to construct best-fit concentration profiles and calculate Cu diffusion parameters for both layers. We assumed bulk diffusion in amorphous films, while transport along moving grain boundaries was assumed in polycrystalline Ni layers. With this model, we were able to replicate the observed profile features at the amorphous/polycrystalline interface. Based on these evaluations, the pre-exponential factors and activation energies are 5 × 10−14 m2/s, 55.3 ± 12 kJ/mol in Mo-Cu and 1.2 × 10−12 m2/s, 76 ± 19 kJ/mol in Ta-Cu systems.