Shape memory alloys (SMAs) are materials that, after being severely deformed, can return to their original shape upon heating. These materials possess a number of desirable properties, namely, high power to weight (or force to volume) ratio, thus the ability to induce large transformation stress and strain upon heating/cooling, pseudoelasticity (or superelasticity), high damping capacity, good chemical resistance and biocompatibility, etc. These unique features have attracted much attention to the potential applications of SMAs as smart (or intelligent) and functional materials. More recently, thin film SMAs have been recognized as a new type of promising and high-performance material for microelectromechanical system (MEMS) and biological applications.

Among these SMA films, TiNi based films are the most promising ones. They are typically prepared by a sputtering method. Other technologies, e.g., laser ablation, ion beam deposition, arc plasma ion plating, plasma spray and flash evaporation, have also been reported in the literature, but with some intrinsic problems. It is well known that the transformation temperatures, shape memory behaviors and superelasticity of the sputtered TiNi films are sensitive to metallurgical factors (alloy composition, contamination, thermomechanical treatment, annealing and aging processes, etc.), sputtering conditions (co-sputtering with multi-targets, target power, gas pressure, target-to-substrate distance, deposition temperature, substrate bias, etc.), and the application conditions (loading conditions, ambient temperature and environment, heat dissipation, heating/cooling rate, strain rate, etc.).

The main advantages for MEMS applications of TiNi thin film include high power density, large displacement and actuation force, low operation voltage, etc. The work output per unit volume of thin film SMA exceeds that of all other microactuation materials and mechanisms.