Ti–Ni–Cu shape memory alloy (SMA) thin films with thicknesses ranging from 150 nm to 1000 nm were deposited by magnetron sputter deposition on planar Si/SiO2 substrates and prestructured silicon-on-insulator nano-hinges with the aim to provide optimal films for (bidirectional) microelectromechanical systems (MEMS) actuators. SMA films exhibited excellent compositional homogeneity with a minimal deviation of approximately 2 at.% across a 100 mm diameter wafer. The films transitioned from compressive residual stress in the as-deposited state to tensile stress after annealing, with tensile stress values decreasing from 294 to 71 MPa with increasing film thickness. This correlated with a monotonic increase of the transformation temperatures. In situ transmission electron microscopy between room temperature and 100°C confirmed the B19 → B2 phase transformation, consistent with X-ray diffraction results, which also revealed minor B19′ and Ti2Ni precipitates; atom probe tomography further verified Ti2Ni formation. An ion beam etching sidewall removal was implemented to eliminate unwanted deposition on nano-hinges, supporting reliable fabrication of bidirectional MEMS actuators.
Akbarnejad et al. (Thu,) studied this question.