The martensitic phase transformation in Ti_40.4Ni₄₈Hf_11.6 shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO₂/Si composites. The shape memory properties of magnetron sputtered Ti_40.4Ni₄₈Hf_11.6 films annealed at 635°C – 5 min are influenced by film thickness and the underlying substrate. Decreasing TiNiHf film thickness from 21 μm to 110 nm results in the reduction of all characteristic transformation temperatures until a critical thickness is reached. Particularly, Ti_40.4Ni₄₈Hf_11.6 thin films as low as 220 nm show transformations above room temperature when deposited on SiO₂ buffer layer, which is of great interest in nano-actuation. In comparison, 220 nm films on Si substrates are austenitic at room temperature, and thus not suitable for actuation. Thermal fatigue tests on TiNiHf/SiO₂/Si bimorphs demonstrate better functional fatigue characteristics than freestanding films, with an average reduction of 15°C after 125 cycles, with temperature stabilization subsequently. Experimental bi-directional actuation results are promising in the development of bistable actuators within a PMMA/TiNiHf/Si trimorph composite, whereby the additional PMMA layer undergoes a glass transition at 105°C. With the aid of constitutive modeling, a route is elaborated on how bistable actuation can be achieved at micro- to nanoscales by showing favorable thickness combinations of PMMA/TiNiHf/Si composite. Graphical abstract

We present a novel design approach for the power optimization of cantilever-based shape memory alloy (SMA)/Si bimorph microactuators as well as their microfabrication and in situ characterization. A major concern upon the miniaturization of SMA/Si bimorph microactuators in conventional double-beam cantilever designs is that direct Joule heating generates a large size-dependent temperature gradient along the length of the cantilevers, which significantly enhances the critical electrical power required to complete phase transformation. We demonstrate that this disadvantage can be mitigated by the finite element simulation-assisted design of additional folded beams in the perpendicular direction to the active cantilever beams, resulting in temperature homogenization. This approach is investigated for TiNiHf/Si microactuators with a film thickness ratio of 440 nm/2 µm, cantilever beam length of 75–100 µm and widths of 3–5 µm. Temperature-homogenized SMA/Si microactuators show a reduction in power consumption of up to 48% compared to the conventional double-beam cantilever design.

The high work density and beneficial downscaling of shape memory alloy (SMA) actuation performance provide a basis for the development of actuators and systems at microscales. Here, we report a novel monolithic fabrication approach for the co-integration of SMA and Si microstructures to enable SMA-Si bimorph microactuation. Double-beam cantilevers are chosen for the actuator layout to enable electrothermal actuation by Joule heating. The SMA materials under investigation are NiMnGa and NiTi(Hf) films with tunable phase transformation temperatures. We show that Joule heating of the cantilevers generates increasing temperature gradients for decreasing cantilever size, which hampers actuation performance. In order to cope with this problem, a new method for design optimization is presented based on finite element modeling (FEM) simulations. We demonstrate that temperature homogenization can be achieved by the design of additional folded beams in the perpendicular direction to the active beam cantilevers. Thereby, power consumption can be reduced by more than 35 % and maximum deflection can be increased up to a factor of 2 depending on the cantilever geometry.

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/SiO ₂ 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 Ti ₂ Ni precipitates; atom probe tomography further verified Ti ₂ Ni formation. An ion beam etching sidewall removal was implemented to eliminate unwanted deposition on nano‐hinges, supporting reliable fabrication of bidirectional MEMS actuators.