Dynamics of Thin Film NiTi Cantilevers on Si
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Dynamics of Thin Film NiTi Cantilevers on Si A.Peter Jardine and Peter G. Mercado Dept. of Materials Science and Engineering S.U.N.Y. at Stony Brook Stony Brook, NY 11794-2275
Abstract Although the thermo-mechanical properties of NiTi are well known for bulk material, its deposition and utilization as a thin film are still in their earliest stages of research. The deposition of thin-films of Shape Memory Effect NiTi onto Si(l 00) wafers offers several advantages over bulk NiTi, including fast response times and comparatively large transformation forces. This has made it a promising candidate material as micro-actuators for Micro-Electro-Mechanical (MEMS) systems as well as for strain measurements. The cycling time for actuation was measured for a 20 jim free standing NiTi thin film cantilever. It was demonstrated that cycling frequencies of up to 50 Hz are achievable.
Introduction The Shape Memory Effect is associated with dramatic, temperature dependent mechanical properties, resulting from a first order martensitic phase transformation from a high symmetry, high temperature austenitic phase to a low symmetry, low temperature martensitic phase. The low temperature phase is easily twinned into many crystallographic twin variants, with one crystallographic twin growing at the expense of the others with an applied stress. On heating, the twins revert to their original configuration before transformation into the
austenitic phase. Macroscopically, the martensitic material is easily deformed through the growth of these twins, which can accommodate seemingly plastic true strains (of up to 8% for NiTi), which are recovered upon heating through its transformation into its high temperature phase and shape, hence the term the Shape Memory Effect. Deposition of micron thin layers of SME NiTi by either DC or RF Magnetron sputtering onto a variety of substrates allow for applications as microdevices. NiTi is easily deposited onto Si wafers and lithographic patterning technologies currently available can be used to make specific geometries. There are significant advantages in using NiTi as a thin-film MEMS actuator. The material is in the form of a thin sheet, which provides a small thermal mass to heat and cool as well as a very good coupling to Si ( which is a good thermal conductor). NiTi has transformation temperatures which are highly dependent on stoichiometry, ranging from about 150 TCto cryogenic temperatures. It is thus possible, with control of the deposition, to produce films which possess transformation temperatures in the range of several tens of degrees above room temperature, which will likely be a standard operating temperature of most actuator devices although it is implicit that with deposition control, lower or higher transformation temperatures are attainable. Issues which result from the use of developing NiTi actuators in micro-machining are largely material related. First, how chemically reactive is the Si surface to the amorphous NiTi and will adverse chemical reactions affect the final microstructure
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