Fabrication and Composition Control of NiTi Shape Memory Thin Films for Microactuators
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Fabrication and Composition Control of NiTi Shape Memory Thin Films for Microactuators David J. Getchel and Richard N. Savage Materials Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407, U.S.A. ABSTRACT Microactuators fabricated with NiTi thin films take advantage of the shape memory effect’s large energy density (~5-10 joules/cm3) and high strain recovery (~8%). Microelectromechanical Systems (MEMS) designed with these actuators can serve as biosensors, micro-fluidic pumps or optical switches. However, the fundamental mechanical properties of these shape memory NiTi films have not been fully characterized with micro-scale test structures. Equiatomic NiTi thin films were deposited by co-sputtering NiTi and Ti targets with the intension of fabricating such test structures. Dual cathodes allowed direct control of the film composition by adjusting the Ti cathode power. Energy Dispersive Spectroscopy (EDS) quantified the film composition relative to pure standards. A thin (~50 nm) chromium film on a pure silicon substrate created excellent film adhesion. Oxidized Si wafers did not bond with the Cr and NiTi films. This deposition method enabled control of film composition and the necessary adhesion. INTRODUCTION The shape memory effect (SME) of NiTi outperforms traditional electrostatic and thermal MEMS actuators when large range of motion and high actuation forces are desired [1]. NiTi is a challenging actuator material to use because the shape memory behavior is sensitive to deposition parameters and post deposition annealing [2]. The realization of NiTi MEMS actuators requires performance data from test structures similar in size to current MEMS devices. Previous tensile and diaphragm NiTi testing provide valuable film properties, but not on the micro-scale [3, 4]. Smaller test structures may expose how factors such as surface oxides or compositional variation in thickness [5] affect the mechanical and shape memory properties. Fabricating and testing a series of micro-scale NiTi test structures to examine how deposition parameters affect actuator performance motivated this study. An ideal MEMS test structure would use scanning probe microscopy (SPM) instruments to deflect miniature thin film cantilevers. Nanoindenters are the ideal instruments for measuring force vs. displacement data from cantilevers because they are capable of measuring both tip displacement and tip force continuously during measurements [6]. Triangular cantilevers with optimized strain profiles [7] and pure tensile fixed beams [8] have also been fabricated for nanoindenter testing. A profilometer is a less expensive alternative with comparable displacement measurement precision, and is capable of measuring stylus displacement at fixed tip forces. Force displacement measurements of Ni cantilevers over silicon trenches have already been taken with a profilometer [9]. NiTi cantilevers have been made and tested with tiny weights [10], but they were made with very thick rolled sheet (50-95 microns). Bendi
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