Electron beam deposition and characterization of thin film Ti-Ni for shape memory applications

摘要

Thin film of Ti-Ni alloy has a potential to perform the microactuation functions required in the microelectromechanical system (MEMS). It is essential, however, to have good uniformity in both chemical composition and thickness to realize its full potential as an active component of MEMS devices. Electron beam evaporation technique was employed in this study to fabricate the thin films of Ti-Ni alloy on different substrates. The targets used for the evaporation were first prepared by electron beam melting. The uniformity of composition and microstructure of the thin films were characterized by electron probe microanalysis (EPMA), Auger electron spectroscopy (AES), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The mechanical property of the thin films was evaluated by the nano-indentation test. The martensitic transformation temperature was measured by differential scanning calorimetry (DSC). It is confirmed that the chemical composition of deposited thin films is identical to that of the target materials. Furthermore, results from depth profiling of the chemical composition variation reveal that the electron beam evaporation process yields better compositional homogeneity than other conventional methods such as sputtering and thermal evaporation. Microstructural observation by TEM shows that nanometer size precipitates are preferentially distributed along the grain boundaries of a few micron size grains. The hardness and elastic modulus of thin films decreases with an increase in Ti contents.