Crystallization of sputtered NiTi films

Sputtered, crystalline thin films of nickel-titanium (NiTi) can display both superelastic properties and the shape memory effect, either of which may be used in films for MEMS sensors and actuators. However, direct deposition of crystalline NiTi films requires high deposition temperature and cooldown can lead to catastrophic delamination from extrinsic residual stress. To avoid delamination, especially for thick films (>5 micrometers) the amorphous form of NiTi can be sputter deposited at a low temperature, patterned and etched, released, and then crystallized to develop the proper microstructure. Here we report some results from a study of the crystallization process using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The minimum temperature for complete crystallization in a reasonable length of time was found to be about 400degreesC. Crystalline grains always nucleated first at the surface, rapidly grew laterally until impingement, and then continued to grow inward to form columnar grains as the parent amorphous phase was consumed. Surface roughness delayed the onset of surface nucleation. For very smooth surfaces, crystallization nucleated quickly, but after lateral impingement, growth inward was apparently more sluggish. Multiple DSC exotherms observed in some cases suggest that additional nucleation events may have occurred in the interior of the films.