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Improving fracture properties of MEMS components by surface control

Author
Tuncay, Alan
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.
Sponsorship
Cornell Center for Materials Research (CCMR), a Materials Research Science and Engineering Center of the National Science Foundation (DMR-0520404)
Date Issued
2006-09-01Subject
fracture; mems; surface
Type
dissertation or thesis