Strength and fracture of Si micropillars: A new scanning electron microscopy-based micro-compression test
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L. Barbieri Advanced Photonics Laboratory, Swiss Federal Institute of Technology EPFL, 1015 Lausanne, Switzerland
J. Michler EMPA Thun, Swiss Federal Institute for Materials Testing and Research, Laboratory for Materials Technology, 3602 Thun, Switzerland (Received 14 March 2006; accepted 18 December 2006)
A novel method for in situ scanning electron microscope (SEM) micro-compression tests is presented. The direct SEM observation during the instrumented compression testing allows for very efficient positioning and assessment of the failure mechanism. Compression tests on micromachined Si pillars with volumes down to 2 m3 are performed inside the SEM, and the results demonstrate the potential of the method. In situ observation shows that small diameter pillars tend to buckle while larger ones tend to crack before failure. Compressive strength increases with decreasing pillar diameter and reaches almost 9 GPa for submicrometer diameter pillars. This result is in agreement with earlier bending experiments on Si. Difficulties associated with precise strain measurements are discussed. I. INTRODUCTION
With the ongoing miniaturization of electronic components and also, increasingly, mechanical components, the investigation of size effects in physical and mechanical properties becomes more important. Generally, size effects are expected as soon as dimensional or microstructural length scales of the microcomponent or material investigated become comparable with the length scale of the deformation or failure mechanism. This length scale may be in the range of micrometers for plasticity mechanisms, millimeters for fracture mechanisms, or below a nanometer for elastic properties.1 We can generally distinguish between the microstructural length scale (e.g., grain size) and the dimensional length scale (e.g., wire diameter).2 Here we limit ourselves to the study of dimensional constraints. The mechanical properties of single-crystalline silicon as a function of specimen size have been investigated by a number of researchers. Many experiments have been performed on sub-millimeter-sized specimens.3–7 Only a few experiments have reported on size effects in the micrometer and submicrometer regimes.8 With brittle failure dominating the behavior of single-crystalline silicon, statistical means are often necessary to describe ma-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0140 1004 J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 16 Mar 2015
terial strength. Weibull statistics are most commonly used in such cases.9 Thus, a suitable testing method needs to have a sufficiently high throughput that allows the testing of a large number of specimens for statistical analysis. All of the articles mentioned so far have used either bending or tensile tests for the determination of the mechanical properties. No results on the compressive behavior of submicrometer and micrometer-sized silicon specimens are available in literature. This is in c
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