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

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.

Atomistic modeling of nanowires, small-scale fatigue damage in cast magnesium, and materials for MEMS

“…Prior to presenting the results of thermal holding studies, we fnst discuss the initial thermo-mechanical cycle of the AdSi microcantilevers (Figure 3.3) [44,45,521. After release at room temperature, the beams have an initial curvature denoted by "As Released" in Figure 3.3.…”

“…The relatively flat curvature-temperature relationship continues until a maximum temperature, T, , , is reached and beam cooling is initiated. The cooling response of the beams follows a linear thermo-elastic loading line dictated by the thermal expansion coefficient mismatch between the Au and Si [44,45]. Cooling is continued until a specified holding temperature, Thold, is reached.…”

“…Experimental data for the first thermal cycle of the AdSi microcantilevers is shown in Figure 3.4 [45]. In Figure 3.4 the solid dots represent incremental in-situ measurements during heating and cooling.…”

“…Owing to the comparable layer thicknesses, the distribution and magnitude of stresses in the microcantilever's metallic layer is much different than in thin films on thick substrates [44]. For example, the Si layer in the microcantilevers carries a finite fraction of the stresses resulting in stresses as much as 70 % lower in the metallic layer for an equivalent temperature change [44,45]. It is also important to recognize that bimaterial microcantilevers in MEMS often cannot be thermally annealed to high temperatures typically employed in microelectronic thin film studies, T >> 200 "C. High temperature exposure greatly alters the microcantilever initial position and can render it dysfunctional for sensing and actuation.…”

Nanolaminates with Novel Properties Fabricated Using Atomic Layer Deposition Techniques

“…6 [ Figure 10 superlattice at the top of the list _ with the best reflectivity ever 0. 4 reported for x-ray reflectivity rr n=3 for 1.54 A.…”