Thermo-mechanical behavior of silicon at nanoscale — an in situ investigation
Materials at the micrometer and submicrometer scale exhibit mechanical properties that are substantially different from bulk materials. With the increasing miniaturization of devices, accurate characterization of micro/nanoscale materials and fundamental understanding of their deformation mechanism are essential to ensure their reliability and performance. The application of the micro/nano devices are not limited to room temperature as those small devices are often required to operate in high temperature environment. The mechanical properties of micro- and nano-materials are expected to be highly temperature dependent, even more than those of their bulk counterparts. One of such behavior is size dependent brittle-to-ductile transition (BDT) in single crystal silicon (SCS). Several experimental and computational studies suggest that SCS can plastically deform near or even at room temperature with reduction of sample size. However size dependent BDT in SCS is not conclusive as there are controversial experimental results in the literature, i.e., no plastic deformation until brittle failure of a sample irrespective of the sample size. The foremost reason for the relatively limited available data and little understanding of the mechanisms of size dependent BDT is the lack of comprehensive and robust in situ experimental techniques. In particular, there has been no technique to perform in situ deformation experiments in SEM or TEM while simultaneously controlling both the key parameters influencing BDT, namely temperature and specimen size. In this study we have developed novel methods to explore mechanical and thermo-mechanical behavior of micro/nano scale materials with a special emphasis on in situ study. The in situ material testing offers an attractive feature in studying of micro/nano materials as it provides direct structure-property relationship due to ultra high resolution of Electron Microscopy observations. Using this unique in situ measurement ability, we have unambiguously explored size dependent thermo-mechanical behavior in micro/nano scale SCS samples. Our experimental investigation has revealed single crystal silicon, well known brittle material at bulk scale, can plastically deform at substantially lower temperature than well known bulk BDT temperature. For example, we have observed about 31% reduction in BDT temperature for sample size 0.72μm with respect to its bulk counterpart. The stronger surface effects at the micro/nano scale give raise to this unusual behavior. Also, we have, for the first time, considered size dependent yield strength of silicon samples incorporated with stress gradient and material characteristic of silicon due to strong covalent bond. For theoretical study of the size dependent yield behavior, we have employed an isotropic elastic continuum based model. The model shows that stress concentration due to dislocation pile-up decreases by up to 82% with larger stress gradient in a sample. Also, the model predicts that size dependence becomes more important for materials with large Peierls stress like SCS. We have experimentally confirmed substantial increase in yield strength with sample size using SCS samples.
Electron Microscopy, silicon, size effect, brittle-to-ductile transition, stress gradient
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign