Several factors affecting the surface morphology, transformation characteristics and shape recovery of co-sputtered NiTi-based shape memory alloy thin films were investigated. It is found that the Ar gas pressure, substrate orientation, deposition temperature and annealing condition all affect the surface morphology. In particular, when under high Argon gas pressure, the deposited film contains surface cracks, while the film is smooth and dense if a 2.3mTorr Argon gas pressure was used during deposition. The film deposited at 450°C on a (100) Si substrate has a rough surface associated with a large distribution of island sizes. Post-annealing treatment leads to a more homogeneous distribution of island dimensions and a smoother surface. On the other hand, the films deposited at 450°C on a (111) Si substrate or on a SiO<sub>2</sub> buffer layer have more homogeneous island sizes. It is also found that both the substrate orientation and the SiO<sub>2</sub> buffer layer dramatically affect the transformation behavior and shape recovery magnitude. Film deposited on (100) Si has a higher recovery strain than that deposited on (111) Si under the same stress. The oxygen in SiO<sub>2 </sub>buffer layer may have deteriorated the deposited film, which results in a very low shape recovery strain. Interfacial stress between the substrate and the thin films is found to lower the transformation hysteresis.
Shape memory alloys (SMAs) offer a unique combination of novel properties, such as shape memory effect, super- elasticity, biocompatibility and high damping capacity, and thin film SMAs have the potential to become a primary actuating mechanism for micro-actuators. In this study, TiNiCu films were successfully prepared by mix sputtering of a Ti<SUB>55</SUB>Ni<SUB>45</SUB> target with a separated Cu target. Crystalline structure, residual stress and phase transformation properties of the TiNiCu films were investigated using X-ray diffraction (XRD), differential scanning calorimeter (DSC), and curvature measurement methods. Effects of the processing parameters on the film composition, phase transformation and shape-memory effects were analyzed. Effects of the processing parameters on the film composition, phase transformation and shape-memory effects were analyzed. Results showed that films prepared at high Ar gas pressure exhibited a columnar structure, while films deposited at a low Ar gas pressure showed smooth and featureless structure. Chemical composition of TiNiCu thin films was dependent on the DC power of copper target. DSC, XRD and curvature measurement revealed clearly the martensitic transformation of the deposited TiNiCu films. When the freestanding film was heated and cooled, a two-way shape memory effect can be clearly observed.
Thin film shape-memory alloys have been recognized as a promising and high performance material in the field of microelectromechanical systems applications. In this investigation, TiNi films were prepared by sputtering Ti and Ni target in argon gas using a magnetron sputtering system. Chemical composition, crystallography, microstructure and phase transformation behaviors of the deposited TiNi film were studied. Differential scanning calorimeter results showed that a two-stage transformation occurs in a sequence of monoclinic martensitic phase to rhombohedral phase, then to B2 phase upon heating, and vice versa on cooling. X-ray diffraction analysis also revealed the crystalline structure changes with the change of the temperatures. Nano- indentation reveals the elastic modulus of the film is about 5.11 GPa and the film intrinsic hardness is 2.84 +/- 0.5 GPa. By depositing TiNi films on the bulk micromachined Si cantilever structures, we obtained micro-grippers exhibiting a good shape-memory effect.