Thermo-mechanical evolution of multilayer thin films: Part I. Mechanical behavior of Au/Cr/Si microcantilevers

MEMS microcantilever test structures were utilized to examine the microstructural evolution of Au/Cr/Si thin films subject to annealing. Curvature evolution of the micron-sized structures was measured in response to anneals at various times and temperatures. Particular emphasis was placed on the accelerated annealing condition of 225 °C for 24 h. The thermo-mechanical response of the microcantilevers consisted of both linear-elastic and inelastic regimes. The temperature at which the thermo-mechanical profile deviates from linear thermo-elasticity is influenced by the stress, curvature and/or the microstructure of the specimens. Stress analysis suggests that microstructural evolution, not plastic yielding, controls the inelastic portion of the thermo-mechanical profile. Maximum stress increases of 146.3 and 202.9 MPa (i.e. 500% relative to the as-deposited state) were observed in the gold layer of the microcantilevers of different silicon thickness, as the result of the inelastic strain at elevated temperature. Increasingly greater curvature change is observed for specimens as annealing temperature is increased up to 150 °C, whereas the magnitude of curvature change is diminished as annealing temperature is increased above 150 °C. A complex curvature evolution is observed at 225 °C over a 24-h timeframe. Curvature evolution during isothermal hold occurs in response to the development of intrinsic stress within the metals. Use of a nitrogen atmosphere or nano-thickness alumina surface coatings was seen to alter the stability of the curvature evolution at 225 °C. The critical thickness for a protective alumina passivation occurs between 6.5 and 32.5 nm. Thermo-mechanical behavior is discussed here, while the corresponding microstructural evolution is discussed in the second part of this paper. © 2006 Elsevier B.V. All rights reserved.

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Author Ken Gall Thomas Lord Department of Mechanical Engineering and Materia ...