Chapter15 Oxide Driven Strength Degradation on (111) Silicon Scott J. Grutzik and Alan T. Zehnder Abstract Previous results have suggested that oxidation of nanoscale Si structures is correlated with a reduction in strength. The mechanism by which this occurs is unknown however. It is possible that the process of oxidation causes some irreversible change in the surface of the crystalline Si. It may also be possible that the change in strength is due to the presence of the oxide itself, perhaps changing the atomic or electronic distribution in some way. In this work, we allow a set of nominally identical nanoscale Si beams to oxidize over approximately five weeks while taking periodic fracture strength measurements. The oxide is then removed and strength is measured a final time. We see that after removing the oxide, strength does not recover at all, suggesting that the change is strength is indeed due to a change in the Si surface. Keywords Silicon • Nanoscale strength • Nanoscale testing • Oxide • Atomic force microscope 15.1 Introduction Micro- and nanoelectromechanical systems (MEMS and NEMS) play an increasing role in many fields such as telecommunications, defense, microelectronics, and biomedical devices. These systems can be subjected to sustained loads, oscillating loads, dynamic shock loading, or a combination of all three. The small size of these devices make accurate predictions and measurements of strength and durability difficult. As a result they may be designed with overly large factors of safety with respect to mechanical failure. More efficient design will require accurate knowledge of the stochastic nature of fracture at these length scales. For a variety of reasons, it is difficult to extrapolate macroscale material failure behavior down to the length scales required for MEMS and NEMS design. Components of these systems are often small enough that they contain few or no defects other than those introduced during processing. Also, because of their high surface to volume ratio, surface properties tend to play a larger role than volumetric properties. Nonetheless, it is possible to make some general predictions. From a continuum mechanics perspective, a smoother surface should give rise to a stronger structure. Any pit or ledge on the surface will act as a stress concentrator under loading and act as a potential nucleation point for fracture. It is also expected that if the surface can be maintained in a compressive state, strength should increase. The compressive stress would act to hold the faces of any existing cracks together, making it more difficult for cracks to propagate. Alan et al. [1] have reported results that agree with the first of these expectations. Their results suggest that the surface roughness of nanoscale Si beams strongly affects their strength and that if the surface is smooth enough the strength can be close to the ideal strength derived from atomistic simulations. Alan et al. [2] have also reported results that are somewhat at odds with the second expectation that a compressive surface stress should increase fracture strength. Their data show a decrease in strength with increasing surface oxidation and, that if oxidation is prevented, the initially high strength is maintained. What makes this an unexpected result is that silicon dioxide grown on a Si surface grows in a compressive state. From our continuum mechanics reasoning oxidation should lead to an increase in fracture strength, not a decrease as the data suggests. The work presented here is part of an effort to understand the mechanisms behind how oxidation affects nanoscale strength. S.J. Grutzik • A.T. Zehnder ( ) Field of Theoretical and Applied Mechanics, Cornell University, Ithaca, NY 14853, USA e-mail: atz2@cornell.edu J. Carroll and S. Daly (eds.), Fracture, Fatigue, Failure, and Damage Evolution, Volume 5: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06977-7__15, © The Society for Experimental Mechanics, Inc. 2015 113
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