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 3 μm 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 (<i>A<sub>p</sub></i>) and 32.6°C (<i>R<sub>p</sub></i>) 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.
This paper describes work on two-way shape memory (TWSM) training of 52at.% Ti--48at.% Ni thin films. The amount of recoverable strain of shape memory alloys (SMA) with TWSM is about 2%. With TWSM, NiTi films will remember different high-temperature and low-temperature shapes. These shapes may be cycled fairly reproducibly by simply changing the temperature. In this work, NiTi films were deposited by RF magnetron sputtering from an NiTi target with atomic composition of 56at.% Ti--44at.% Ni. The atomic composition of the sputtered films was determined to be 52at.% Ti--48.0at.% Ni by electron microprobe. Solution treatment of the as-deposited films was required to crystallize and memorize a high-temperature shape, followed by age treatment to increase the transformation temperatures to above room temperature. The crystal structure of the solution-treated films was determined to be B2. The transformation temperatures of the age-treated films were determined by differential scanning calorimeter to be 311 K (A*) and 307 K (R*). TWSM training was carried out by over deforming the specimen while in the R-phase. Below R<SUB>f</SUB>, a load was applied to the specimen beyond the usual strain limit for completely recoverable shape memory. Then, the load was removed prior to the next heating step, upon which the reverse transformation occurred under zero stress. With similar loads and temperatures, the procedure was then repeated. This paper will present details of the fabrication techniques, measurement results and its application.