On the lower thickness boundary of sputtered TiNi films for shape memory application

In this paper, we study the effects of film thickness on phase transformation of constrained Ti_50.2Ni_49.8 films deposited onto silicon substrates. When the film is too thin, surface oxide and interfacial diffusion layers exert dominant constraining effect that renders high residual stress and low recovery capabilities in the film. The surface oxide and inter-diffusion layer restricts the phase transformation, alters stoichiometry of the remaining TiNi film, and reduces volume of the material available for phase transformation. As a result, a lower thickness boundary (about 100 nm) exists for TiNi films to remain "shape remembering". Results indicate that a maximum recovery stress and actuation speed can be realized with a thickness of about 800 nm. Based on the curvature measurement results, the relationship between the relative stress and strain of the films (during phase transformation with change of temperature) can be derived.