Functionalized Thin-Film Shape Memory Alloys for Novel MEMS Applications

Abstract

Nickel-titanium (NiTi) shape memory alloy (SMA) films are already implemented into microelectromechanical system (MEMS) devices such as sensors, actuators, and implantable medical devices. In this thesis, I used DC magnetron sputter deposition to study the influence of film composition, microstructure, and annealing conditions on the stability of the phase transformation for the NiTi-based SMA thin films TiNiCu, TiNiCuCo, and TiNiHf. SMAs are a type of smart material that can undergo stress or temperature-induced solid-to-solid phase transformation between two different crystalline phases. In NiTi-based SMAs, the two phases are known as martensite with a monoclinic crystalline structure and austenite with a cubic crystal structure. The temperature-induced phase transformations can be used to switch between the martensite and austenite phases, and thus switch between two sets of material properties in the SMA. For example, in NiTi-based SMAs the Young’s modulus, electrical resistivity, and coefficient of thermal expansion of the austenite phase are typically 2X larger than that of the martensite phase. The transformation temperatures, recovery strains, enthalpy of transformation, and fatigue properties of NiTi SMAs can be tuned by alloying NiTi with other elements like copper (Cu), cobalt (Co), and hafnium (Hf). For example, certain compositions of sputtered TiNiCu and TiNiCuCo are known to be ultra-low fatigue SMAs, able to reversibly undergo the phase transformation for 10+ million cycles without degradation in the mechanical or thermal properties. The primary focus of this thesis was the integration of these sputtered NiTi-based SMA thin-films into the following four novel MEMS devices: 1) TiNiCu for magneoelectric sensors, 2) TiNiHf for bistable actuators, 3) TiNiCuCo for stretchable electronics and 4) thin-film SMA stretchable auxetic structures for wearable and implantable medical devices.

The shape memory effect was observed in TiNiCu and TiNiHf films when the film thickness and lateral dimensions are downscaled to micro and nano dimensions. In the research publication “Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys.”, I showed the reproducibility of the thermal-induced phase transformation of Ti50Ni35Cu15 is attractive for integration into MEMS devices that require a high cycle lifetime. I showed how the SMA’s phase transformation can be used to tune the resonant of bending cantilever-type sensors like magnetoelectric sensors. I also demonstrated excellent thin-film piezoelectric and shape memory alloy properties for 2 μm AlN/ 5 μm TiNiCu films composites deposited onto silicon substrates. The large work densities and high strength-to-weight ratio offered by SMAs are attractive for the development of micro and nano actuators. The thermal induced phase transformation between martensite and austenite is also used to develop bi-directional micro-actuators with TiNiHf/Si and TiNiHf/SiO2/Si composites. In another research publication, “TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation”, I demonstrated the influence of film thickness and substrate on the phase transformation properties of TiNiHf thin-films. Ti40.4Ni48Hf11.6 films with thicknesses as low as 220 nm on SiO2/Si substrates can undergo the phase transformation with high transformation temperatures (As > 100 °C) and a wide thermal hysteresis (ΔT > 50 °C). In this publication, we explain how the wide hysteresis and high transformation temperature obtained in TiNiHf films can be used to develop micro and nano-scale bistable actuators based on PMMA/TiNiHf/Si composites.

Even though thin-film NiTi-based SMAs are known to reversibly recover superelastic strains of up to 8%, surprisingly, they have not yet been exploited in the growing field of stretchable electronics. In the technical article “Thin-Film Superelastic Alloys for Stretchable Electronics” I demonstrate the first experimental and numerical studies of freestanding thin-film superelastic TiNiCuCo structured into a serpentine geometry for use as stretchable electrical interconnects. Fabricated electropolished serpentine structures were demonstrated to have low fatigue after cycling external strains between 30% - 50% for 100,000 cycles. The electrical resistivity of the austenite phase of a Ti53.3Ni30.9Cu12.9Co2.9 thin-film at room temperature was measured to be 5.43 × 10-7 Ω m, which is larger than reported measurements for copper thin-films (1.87 × 10-8 Ω m). Expanding upon this work, in the conference proceedings paper “Auxetic Superelastic TiNiCuCo Sputtered Thin-Films for Stretchable Electronics”, I present a new platform for functionalized wearable electronics and implantable medical devices based on superelastic thin-film SMA substrates structured into novel stretchable auxetic geometries. Since thin-film SMAs are conductive, the structured substrate itself could serve as the current collector for such stretchable and flexible devices, or a more conductive electrode can be deposited on top of the stretchable auxetic SMA substrate. Overall, the results discussed in this doctoral thesis look to the future of harnessing the functional properties of thin-film sputtered SMAs for novel uses in next-generation MEMS devices.