Micro- to Nanostructured Devices for the Characterization of Scaling Effects in Shape-Memory Thin Films

Two microfabricated devices designed as test platforms for the investigation of scaling effects in micro- to nanosized substrate-attached shape-memory alloy (SMA) thin films as well as freestanding (suspended) thin-film microbridges are presented. These micromachined test platforms allow for simultaneous nanomechanical, electrical, and thermal tests on thin-film microbridges and can be seen as a basis for nanoscale SMA thin-film applications. The functionality of these devices is demonstrated for Ti₅₂Ni₃₂Cu₁₆ thin films as active material. The martensitic phase-transition temperatures for the thin films as substrate-attached or suspended microstructures as well as the dependence on the lateral dimensions were examined. It was found that decreasing the bridge width from 4 to 1 μm leads to a substantial and asymmetrical decrease of the phase-transition temperatures: 20% [austenite finish temperature (A_f) and martensite start temperature (M_s)] and 80% [austenite start temperature (A_s) and martensite finish temperature (M_f)]. Furthermore, it was found that detaching the thin film from its substrate also leads to a decrease of the transition temperatures. Finally, it is shown that shape-memory thin-film nanowires can be fabricated and characterized using the proposed devices.