Size dependent fracture strength and cracking mechanisms in freestanding polycrystalline silicon films with nanoscale thickness

The size dependent fracture strength and fracture mechanisms of polycrystalline silicon films are investigated by analyzing a wide range of specimen lengths and widths with thicknesses equal to 240 and 40 nm. An on-chip method is used to deform the films and to determine the strength. The strength increases with decreasing external surface area. The thinnest films exhibit the lowest strength and a weaker size effect. The crack path changes from transgranular to intergranular with decreasing thickness. These results are related to differences in microstructure and surface roughness characteristics of the films as controlled by the fabrication process and thickness.