Residual Stress of TiNi Shape Memory Alloy Thin Films with (111) Single-crystal Silicon Wafer

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 tensile residual stress decreases from 106 to 37 MPa with increasing annealing temperature from 723 to 923 K. However, it increases again above 1023 K. While increasing the film thickness from 2.4 to 6.5μm, the residual stress reduces from 323 to 80 MPa. A lot of beautiful sunflower-like 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 is the main factor resulting in tensile stress and the formation of martensite phase will release a part of tensile stress.