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Thermomechanical response of bare and Al2O3-nanocoated Au/Si bilayer beams for microelectromechanical systems

Published online by Cambridge University Press:  31 January 2011

Ken Gall ,
Michael Hulse ,
Martin L. Dunn ,
Dudley Finch ,
Steven M. George  and
Brian A. Corff
Ken Gall
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
Michael Hulse
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
Martin L. Dunn
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
Dudley Finch
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
Steven M. George
Affiliation:
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
Brian A. Corff
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
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Abstract

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.

Type
Articles
Information
Journal of Materials Research , Volume 18 , Issue 7 , July 2003 , pp. 1575 - 1587
Copyright
Copyright © Materials Research Society 2003

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