Crystallization of nanoscale NiTi alloy thin films using rapid thermal annealing

This work utilizes short time heat treatments of submicrometer-thickness NiTi alloy films fabricated using biased target ion beam deposition and investigates crystallization. Films were fabricated on Si substrates, and thicknesses were about 150 nm, which were much less than conventional thicknesses on the order of micrometers. To understand the composition dependence, Ni concentrations were varied such that alloys ranged from Ti-rich to near-equiatomic. Rapid thermal annealing was used for the heat treatment and temperatures ranged from 465 up to 540 °C for 10 min. X-ray diffraction measurements for each of the NiTi alloy compositions revealed that the crystallization temperature was equivalent (∼490 °C) and the B2 austenitic atomic crystal structure existed. Evolutions of surface morphologies, measured using atomic force microscopy, as a function of heat treatment temperature confirmed the composition independence of the crystallization temperature. To investigate the structure using transmission electron microscopy, 150 nm-thickness films were also deposited on ultrathin SiN substrates and heat treated, which confirmed equiaxed grains existed. Crystallization and annealing heat treatments for nanoscale films can be carried out for time on the order of minutes, which should curtail detrimental diffusion effects known to compromise shape memory behavior.