Credit: © 2006 AIP

Nanoscale structures, such as those used for nanoelectromechanical systems, are expected to have enhanced strength compared with their larger-scale counterparts. This is because they are less likely to support defects where a fracture can propagate. The effect of surface roughness, however, is a different issue. Tuncay Alan and colleagues1 at Cornell University in the US show that a fourfold increase in surface roughness reduces the critical pressure for fracture by 20% in silicon nanobeams.

The 5-micrometre-long silicon nanobeams were lithographically defined to less than 200 nm thick and 500 nm wide. Chemical etching determined the nanobeam surface morphology, which was quantified with atomic force microscope imaging. The roughness was characterized by an average deviation from a perfectly flat surface, with 1.5 nm and 0.4 nm average deviations corresponding to ‘rough’ and ‘smooth’ surfaces, respectively. To measure beam strength, pressure from a calibrated atomic force microscope cantilever tip was applied to the beam centre up to the breaking point. The beams consistently broke at the centre and along the same crystallographic cleavage plane.

Significantly, from the large number of beams studied, each with the same nominal roughness, a wide distribution of fracture strengths was observed. These results underline the sensitivity of structural defects to small changes in processing conditions.