Crystallization and microstructure evolution of nanoscale NiTi thin films prepared by biased target ion beam deposition

NiTi alloy thin films of nanoscale thickness were fabricated using a novel technique known as biased target ion beam deposition (BTIBD). Ni-poor/Ti-rich, near equiatomic NiTi, and Ni-rich film composition ranges were investigated in the as-deposited condition. Heat treatment was necessary to crystallize the otherwise amorphous as-deposited films. Crystallization and microstructure evolution were contrasted with those for nanoscale thickness films fabricated using the more common magnetron sputtering technique. For each composition range, the as-deposited magnetron sputtered films exhibit a columnar-void morphology. In situ transmission electron microscopy heating results show that crystallization requires the morphology to merge, which produces small grains. Larger grains are formed in crystallized BTIBD films, which are attributed to increased adatom mobility facilitated by independent control of low energy ions. This work postulates that enhanced mobility in BTIBD eliminates the columnar-void morphology and reduces crystallization energy barrier that restricts grain size during heat treatment.