On the size-dependent elastic behavior of silicon nanocantilevers

Previous experimental and theoretical investigations have reported that the mechanical properties of silicon nanocantilevers, in particular the effective Young's Modulus, deviate from their bulk properties. This size-dependence has been attributed to surface effects. However, differences in experimental and computational investigations imply that there could be other influences besides surface effects. In this paper, we try to determine to what extent the surface effects, like surface stress, surface elasticity, surface contaminatiion and native oxide layers influence the effective Young's Modulus of silicon nanocantilevers. Si films are thinned down to 14 nm on silicon on insulator (SOI) wafers. Si cantilevers are fabricated and the effective Young's Modulus is measured with the pull-in method, recently developed by the authors. The comparison between theoretical models and experimental measurements demonstrates that, although the surface effects influence the effective Young's Modulus of silicon to some extent, they alone are insufficient to explain why the effective Young's Modulus decreases somewhat earlier. For Si films thinner than 50 nm, it is observed that the density of fabricated induced defects abruptly rises when the SOI device layer becomes thinner. A potential explanation is found in the appearance of pinholes. These pinholes, which were created during the thinning process and are made visible with HF etching, could influence the experimentally obtained effective Young's Modulus of ultra-thin Si cantilevers. Keywords: nanocantilever, size-dependence, silicon on insulator, surface stress, pull-in voltage, pinholes.