Improving fracture properties of MEMS components by surface control

Abstract

This thesis studies the mechanical reliability of nanostructures. The strength statistics of Si nanobeams, their dependence on surface morphology and degradation due to air exposure are characterized and necessary conditions for maximum strength and durability are determined.

Due to their small sizes and use of low defect materials, nanostructures have the potential to be used in applications requiring very high stresses at low failure probabilities. Fracture strength of 190-nm thick Si beams have been shown to be as high as 13 GPa, approximately 30 times higher than the strength of macroscale samples. Testing similarly prepared beams etched with relatively smooth morphologies (0.4 nm rms) we showed that the strengths were further improved to 16 GPa, approaching theoretical strengths predicted by previous atomistic calculations.

To explain this influence, a series of fracture mechanics based Monte Carlo simulations were performed. Chemically modified surfaces of the tested beams were measured, statistically characterized and equivalent surfaces were generated. The surfaces consisted of bunched steps which act as stress concentrators, resulting in very high local stresses and hence enhancing material failure. Simulations of nanobeams processed using two different chemical etchants demonstrate the impact of surface morphology on fracture strengths characterized in terms of the Weibull distribution. It was shown that even a small increase in roughness reduces the strength considerably.

This high strength potential is promising for nanomechanical devices requiring high stress levels. Yet, for practical applications, maintenance of strength throughout the structure's service life may be as important as high initial strengths. Tests performed over a period of three weeks showed that this high strength degrades to 11 GPa when the beams are exposed to air. Coating the sample surfaces with protective methyl monolayers resulted in a 10\% higher initial mean strength, which was maintained throughout the test period under the same environmental conditions as the uncoated samples. Our results show that the strength degradation can be prevented by effective protection of surfaces.

The results of our experiments and simulations suggest that surface control is essential for the improvement and maintenance of high mechanical strengths at nanoscales.