Giant pop-ins in nanoindented silicon and germanium caused by lateral cracking
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Giant pop-ins in nanoindented silicon and germanium caused by lateral cracking D.J. Olivera) Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia
B.R. Lawn, R.F. Cook, and M.G. Reitsma Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
J.E. Bradby and J.S. Williams Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia
P. Munroe Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia (Received 16 September 2007; accepted 16 November 2007)
Giant “pop-in” displacements are observed in crystalline silicon and germanium during high-load nanoindentation with a spherical diamond tip. These events are consistent with material removal triggered by lateral cracking during loading, which poses a hazard to microelectromechanical systems (MEMS) operation. We examine the scaling of the pop-in displacements as a function of peak indentation load and demonstrate a correlation with the depth of the plastic contact zone. We argue that giant pop-ins may occur in a broad range of highly brittle materials.
Silicon (Si) and germanium (Ge) are basic Group IV semiconducting materials widely used in electronics, integrated circuitry, and microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices. The two materials have the same crystal structure and are highly brittle with similar mechanical properties, including hardness, H, and fracture toughness, Kc (Table I). They show similar responses on indentation with sharp tips, with well-defined hardness impressions accommodated by punching-in of dislocated shear faults immediately beneath the contact at high stresses.1,2 Some differences are also evident: in Si, deformation is additionally accommodated by a surface-localized pressureinduced phase transformation to a dense, metallic Si-II phase3,4; in Ge, there is evidence of accompanying mechanical twinning within the shear fault zone.5,6 The net result is an approximately hemispherical plastic impression beneath the indent,7 containing nucleation sources for ensuing crack initiation and propagation.8–11
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0070 J. Mater. Res., Vol. 23, No. 2, Feb 2008
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Over the past two decades, much attention has been given to the use of depth-sensing nanoindentation to characterize mechanical properties of a wide range of materials. It is well documented that both hardness and Young’s modulus can be extracted from the indentation force–displacement (P–h) responses.6 More recently, discontinuities in the displacement responses, known as pop-ins, have been studied as markers of abrupt deformation events during indentation. These include the onset of a
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