Two-way Actuation of Bi-layer Cantilever of Nickel Titanium and Silicon Nitride Thin Films by Shape Memory Effect and Stress Relaxation

摘要

This study analyses the two-way actuation of a bi-layer cantilever of nickel titanium (NiTi) and silicon nitride thin films. The cantilever will curl on low temperature and uncurl on high temperature. The curling mechanism results from the stress relaxation of the NiTi film and the uncurling from the shape memory effect. A NiTi film with thickness of about 3μm was deposited on a silicon substrate coated with a low-stress silicon nitride film with thickness of about 0.6 μm. The NiTi film was heat treated to recrystallise and memorise a flat shape. Over the heat treatment, residual stress built up in the NiTi film. The residual stress was measured to be around 400-800 MPa tensile by the wafer curvature method (Stoney's equation). The transformation temperatures of the NiTi film were measured to be about 36.3°C (A_p) and 32.6°C(R_p) by differential scanning calorimeter. The bi-layer cantilever was released from the silicon substrate by anisotropic wet etching (TMAH). Below R-phase finish temperature (<30°C) the shape memory effect was inactive and the NiTi film relaxed from the residual stress, which caused the cantilever to curl up. Above the austenite finish temperature (>50°C), the NiTi film uncurled toward its memorised shape because of the shape memory effect. Therefore, by cycling the temperature high and low, the cantilever uncurled and curled.