Residual stress of TiNi shape memory alloy thin films on silicon substrate

As micro-actuator materials, TiNi shape memory alloy thin films with substrate are used more and more in MEMS field. The residual stress in Ti-rich TiNi thin films with silicon substrate prepared by the magnetron-sputtering technique is measured by both X-ray glancing and contour method. The influence of crystallization annealing temperature and film thickness on the residual stress is tested. It is shown that the tension residual stress decreases from 106 to 37MPa with increasing annealing temperature from 723 to 923K, however, it increases again above 1023K. While increasing the film thickness from 2.4 to 6.5 mum, the residual stress reduces from 323 to 80Mpa. A lot of beautiful sunflower structures in crystal grains are observed by OM and TEM. The origin of the residual stress and its affecting factors are discussed. The thermal stress during cooling after annealing are the main factors resulting in tensile stress and the formation of martensite phase will release a part of tensile stress.