Tailoring Functional Properties of Ti–Ni–Cu Shape Memory Alloy Thin Films for MEMS Actuators

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.