Effects of Stress Induced by the Underlying Layers on the Micro-Structures and Characteristic Phase Transformation Temperatures of Sputtered TiNi Thin Films

The micro-structures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multilayer substrates have been studied. Differences in the micro-structures, such as the lattice constants and the relative concentrations of TiNi, Ti₂Ni and TiNi₃ phases, were observed (1) among the free-standing and the attached films, (2) among the films attached on different underlying multi-layers and (3) among films with different relative orders of aging and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both substrate- and process-induced stress significantly affect the micro-structures, hence the phase transformation characteristics, of the resulting shape-memory alloy thin films.