Thermomechanical response of bare and Al 2 O 3 -nanocoated Au/Si bilayer beams for microelectromechanical systems
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Steven M. George Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
Brian A. Corff Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309 (Received 10 December 2002; accepted 9 April 2003)
We present results on the thermomechanical behavior of bare and nanocoated gold/polysilicon (Au/Si) bilayer cantilever beams for microelectromechanical system applications. The cantilever beams have comparable thicknesses of the Au and Si layers and thus experience significant out-of-plane curvature due to a temperature change. The experiments focus on the inelastic behavior of the bilayer beams due to thermal holding and thermal cycling. In uncoated Au/Si beams, thermal holding directly after release or thermal cycling both lead to a curvature decrease as a function of time or cycle number, respectively. The drop in curvature during thermal cycling or thermal holding in uncoated beams was not accompanied by a change in the slope of the thermoelastic curvature–temperature relationship. The absolute change in curvature depends on the temperature and the holding time. When holding or cycling to a temperature of 175 °C, the curvature change in uncoated beams is minimal for hold times up to 4500 min or 15,000 cycles. When holding or cycling to temperatures of 200 or 225 °C, the curvature in uncoated beams drops by a factor of three for hold times up to 4500 min or 15,000 cycles. The surface structure induced by long-term holding of uncoated beams shows grooving at the grain boundaries while the surface structure induced by cycling of uncoated beams shows consolidation of the grain boundaries. The Au/Si beams with a conformal 40-nm atomic layer deposition Al2O3 coating show a considerably different response compared to identical Au/Si bare beams subjected to the same thermal histories. The coating completely suppresses decreases in curvature when the beams are held at 225 °C for 4500 min. On the contrary, the coating does not always suppress thermal ratcheting when the beam is cycled from a low temperature to 225 °C. In the coated beams, the drop in curvature due to thermal cycling was accompanied by a change in the thermoelastic slope of the curvature–temperature relationship. Negligible microstructural changes were detected on the Al2O3-coated Au surface after holding or cycling. The results are discussed in light of potential deformation mechanisms and a simple analysis linking the mismatch strain between the layers to the curvature in the beams. I. INTRODUCTION
Multilayer material systems are a critical component in existing and emerging microelectromechanical systems (MEMS). Example MEMS applications using multilayer material systems include bimorph switches,1 optical mirrors,2 and sensors/actuators.3–5 Although multilayer materials abound in MEMS, the inelastic deformation of these material systems, where the film and the substrate have a comparable thickness, has not been examined in great detail. In many MEMS applications, the materials will be subjected
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