The relationships between sputter deposition conditions, grain size, and phase transformation temperatures in NiTi thin films

Sputter-deposited NiTi films on full 100 mm wafers exhibit different phases, depending on the wafer region. Comprehensive characterization and analysis show that, unlike the common assumption, this effect is not related to the Ti/Ni stoichiometry. Instead, we show that the different phases are related to differences in grain size. Our results indicate the existence of a critical grain size of approximately 50-100 nm, below which a significant inhibition of the martensitic transformation occurs. Further, we study the effect of deposition conditions and show that a desired uniform martensitic film can be achieved by decreasing the impact energy of the deposited atoms.