Fabrication of Nano-structures of NiTi Shape Memory Films and Investigations on Size dependence of their Mechanical Properties

摘要

A proper understanding of material behavior with decrease in the sample size especially at nano-scale has received significant importance in realizing MEMS devices. Ti-rich NiTi films were deposited by sputtering a NiTi alloy target (45:55) on (100) silicon substrates. Gallium ion (Ga+) Focused Ion Beam (FIB) technique has been used to create nano-structures of different dimensions and aspect ratios in these films. The influence of FIB process parameters such as ion current and etching time on etching rate and etch depth to generate various features at nano-scale is reported. The etched depths were found to be directly proportional to the ion current. The etching rate for NiTi film was 25 nm/minute approximately for a beam current of 0.35 nA. Scanning electron microscopy has been used to examine the surface morphology of etched areas of nano-pillars, pyramids and beams in different aspect ratios. Nanoindenter (Agilent Technologies G-200) has been used to investigate the young's modulus and hardness of these nanostructures and the phase transformation has been studied. In this paper, we observed that the two phases responsible for shape memory effect are more stable in nano-pillars compared to the corresponding values in bulk. Size effect on feature shape, size and aspect ratio on recovery stress values are also studied.