Evolution of microstructure in nanocrystalline Mo-Cu thin films during thermal annealing

The evolution of microstructure in Mo-Cu thin films during annealing has been investigated by in situ sheet resistance measurements, ex situ x-ray diffraction, and in situ hot-stage as well as conventional transmission electron microscopy. Mo-Cu thin films, deposited on various glass substrates by magnetron sputtering at ∼30 °C, were supersaturated solid solutions of Cu in Mo with a nanocrystalline microstructure. The as-deposited films had large compressive residual stresses owing to the low homologous deposition temperature and low Ar pressure during deposition. Annealing results showed two distinct sets of microstructural changes occurring in the temperature ranges between ∼300 and 500 °C, and ∼525 and 810 °C. In the lower-temperature range, anisotropic growth of nanocrystallites was accompanied by stress relaxation without any observable phase separation. At temperatures greater than ∼525 °C, the metastable solid solution collapsed and Cu precipitated at the grain boundaries. Increasing temperature resulted in the coarsening of Cu precipitates and simultaneous growth of Mo grains. At temperatures greater than ∼700 °C, phase separation and grain growth approached completion.