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Applications of shape memory alloys: advantages, disadvantages, and limitations

Titanium-nickel (TiNi) based shape memory alloys (SMAs) are used in a wide range of applications. They are especially practical as thin film actuators because of the large work output per unit of actuator mass and ability for rapid thermal cycling due to large surface to volume ratio. Sputter deposited thin TiNi film has been developed for use in miniature actuators for microvalves, microrelays, optical switches and also for building small implantable medical devices. Chemical composition of the deposited SMA must be held within close limits and for the film to have shape memory properties a crystallization anneal is required. To avoid flaws in film quality the surface on which SMA is deposited has to meet certain criteria. Basic MEMS processes (photolithography and chemical etching) are used for device fabrication. Although TiNi is resistant to most chemicals, some acids used in MEMS can damage it. Thus, selection of processes and reagents compatible with TiNi requires care and experimentation. This paper discusses some applications of SMA thin films along with experience gained in bringing device s to production readiness. It illustrates simple design rules for incorporating shape memory microactuators in MEMS devices and describes some of the pitfalls to be avoided.