Miniaturized Shape Memory (SMA) Bimorph Actuators with Polymer Layers

Shape-memory alloy (SMA) actuators based on NiTi are promising candidates for miniaturized sensors and actuators in MEMS applications. Thermal processing constraints currently limit SMA actuator integration onto soft polymer substrates. Historically, NiTi crystallization requires sputter or anneal temperatures of 450°C or more so there is a desire to obtain shape memory effects at lower processing temperatures.

To motivate the mating of polymer and crystallized NiTi SMA layers, we developed and carried out the microfabrication of a simple, yet novel, SMA bimorph actuator. By following crystallized NiTi SMA film with a 1 micron photosensitive polymer layer, we created a bimorph with a large coefficient of thermal expansion (CTE) mismatch (>40 ppm/°C) allowing significant yet predictable curvature upon release. An analytical strain model predicted the radius of curvature to within 10% and resulted in a measured radius of curvature down to 50 µm for an actuator that folded flat upon actuation.

The full benefits of combining SMA materials with polymers can only be realized without the constraint of high temperature crystallization coming before polymer deposition. To this end, we carried out experiments to investigate the crystallization of amorphous NiTi using a Novacentrix pulse forge additive manufacturing tool with (>10,000 W/cm²) microsecond bursts of light. We tested experimental stacks of sputtered, amorphous submicron thick films of NiTi on 1.2 µm films of polyimide. XRD analysis indicates that the pulse forge method may be a novel technique to crystallize shape memory materials while limiting thermal exposure of adjacent polymer layers.