The reaction between a TiNi shape memory thin film and silicon
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The reaction between a TiNi shape memory thin film and silicon Susanne Stemmera) Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
Gerd Duscher and Christina Scheu Max-Planck-Institut f¨ur Metallforschung, Institut f¨ur Werkstoffwissenschaft, 70174 Stuttgart, Germany
Arthur H. Heuer Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
Manfred R¨uhle Max-Planck-Institut f¨ur Metallforschung, Institut f¨ur Werkstoffwissenschaft, 70174 Stuttgart, Germany (Received 1 July 1996; accepted 23 December 1996)
The reaction between shape-memory TiNi thin films and silicon has been characterized by conventional, analytical, and high-resolution transmission electron microscopy. A reaction layer is formed during the 525 ±C post-deposition crystallization anneal of the sputter-deposited TiNi, and consists of several phases: Ti2 Ni, a nickel silicide, and a ternary titanium nickel silicide. The mechanism for the interlayer formation is discussed.
I. INTRODUCTION
TiNi thin films exhibiting the shape memory effect (SME) are promising candidates for actuators in microelectromechanical systems (MEMS). Recently, several research groups have shown that sputtering is a suitable method to grow TiNi films and that these films show the SME.1–4 The application of TiNi films as microactuators requires good adherence of the TiNi to the substrate, for example, to prevent detachment of the film during mechanical actuation. The most commonly used substrates in MEMS are bare silicon, or oxidized silicon if thermal or electrical insulation between the silicon substrate and the TiNi is necessary. Adhesion of TiNi to oxidized silicon has been studied by Su et al. using a dynamical mechanical measurement,5 and qualitatively for bare and oxidized silicon substrates by Wolf and Heuer4 and by Johnson and Busch6 using tape peel tests. Characteristics of the film/substrate interface, such as the presence of reaction products, might influence the adhesion. Sputtered TiNi films are amorphous if the substrate is not heated intentionally during deposition, and annealing at temperatures between 525 and 550 ±C is necessary to crystallize the films and obtain the SME. Reactions between many binary alloys of near-noble and
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Present address: Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium, [email protected].
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 7, Jul 1997
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refractory metals and silicon have been studied, mainly because of the possible applications of these alloys as diffusion barriers in microelectronic devices (see, e.g., Ref. 7). These reactions commonly result in the development of discrete phases of binary silicides, where a near-noble metal silicide is formed next to the silicon at relatively low temperatures. This “phase separation” is often explained as resulting from the diff
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