Θ CuAl₂ Precipitate Coarsening in Al-2% Cu Thin Films

Abstract

Θ phase CuAl₂ precipitate size evolution during coarsening at 310°C in 0.5 μm thick Al-2% (wt) Cu thin films was characterized by transmission electron microscopy. Films were sputter deposited onto oxidized Si substrates by standard techniques. The coarsening process preferred the growth of blocky Θ morphologies at Al triple points. Coarsening was via solute Cu diffusion along Al grain boundaries during annealing. The average Θ size dependence on annealing time (t) is approximately (t)^1/4 in general agreement with models for particle coarsening along grain boundaries. Concurrent Al grain growth was shown to initially enhance the Θ coarsening rate above (t)^1/4 behavior. This boundary coarsening process leads to a grain size dependence of the coarsening rate which has been observed in related and other previous work in thin films. These results are shown to be relevant for effects produced during accelerated electromigration testing, such as previous ‘curious’ θ morphologies at triple points observed by others, the enhanced flux of Cu during testing, and possible mechanisms affecting electromigration failure processes.