High-throughput characterization of shape memory thin films using automated temperature-dependent resistance measurement
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0894-LL06-06.1
High-throughput characterization of shape memory thin films using automated temperature-dependent resistance measurements S. Thienhaus1,4, C. Zamponi2, H. Rumpf2, J. Hattrick-Simpers3, I. Takeuchi3, and A. Ludwig1, 4 1 Combinatorial Material Science group, caesar, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany 2 Smart Materials group, caesar, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany 3 Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA 4 Ruhr-University Bochum, Institute of Materials, 44780 Bochum, Germany
ABSTRACT Shape memory alloy (SMA) thin films are used as actuator materials in MEMS due to their unique properties. Binary thin films with a composition close to Ni50Ti50 are wellestablished materials, whereas ternaries like NiTiCu, NiTiPd, NiTiHf are less studied. Furthermore, new alloys are being developed which show a magnetic shape memory effect, e.g. Ni2MnGa. For the optimization of known, and the development of new, SMA thin films, a fast and reliable characterization technology is needed, which rapidly identifies the transformation temperatures (i.e. martensite and austenite start and finish temperatures) for a range of material compositions deposited on a whole wafer. In this paper, automated temperature-dependent resistance measurements are discussed as a means which yields the thermal hysteresis of the investigated thin films. Results of monitoring the uniformity of shape memory film depositions on the wafer level, as well as results on the use of this method as a tool for screening for new SMA films by characterization of materials libraries are reported.
INTRODUCTION Thin films of shape memory alloys (SMA) are useful for applications as actuator materials in microsystem technology (MEMS) due to their one-way, two-way and superelasticity effects [1-3]. The main advantage is their high energy density, leading to high forces and displacements in appropriate actuator designs. In thin films on substrates, usually the two-way behavior is used, where the substrate acts like a spring. The cause of the shape memory effects lies in the thermoelastic, reversible martensitic transformation. The drawback of this thermally induced transformation is the limitation to low frequency applications (< 30 Hz), due to slow cooling [4]. A possible way to overcome this limitation would be magnetic actuation of the shape memory effect [5] in ferromagnetic shape memory alloys (FSMA). This behavior has been found in NiMnGa alloys where magnetic field induced strains can be realized; the effect relies on the magnetic selection of martensite variants [6]. The most promising alloys in conventional SMA are NiTi and NiTi based ternaries such as NiTiCu, NiTiPd, NiTiHf, whereas NiMnGa and FePd are examples for ferromagnetic SMA. For the optimization of such SMA thin films and the development of new SMA thin films, a fast and reliable characterization technology is needed which rapidly yields the transformation temperatures (i.e. martensite and austenite start and finish temperatures, a
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