Mode I and mixed mode fracture of polysilicon for MEMS

An experimental study was carried out to investigate the local and effective fracture behaviour of polycrystalline silicon for microelectromechanical systems (MEMS). The apparent mode I critical stress intensity factor was determined from MEMS-scale tension specimens containing atomically sharp edge pre-cracks, while local deformation fields were recorded near the crack tip, with high resolution by the in situ Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method previously developed by this group. The effective mode I critical stress intensity factor varied in the range 0.843-1.225 MPa root m. This distribution of values was attributed to local (in grain) cleavage anisotropy and to enhanced grain boundary toughening. The same sources resulted in very different local and macroscopic (apparent) stress intensity factors, which, combined with the small grain size of polysilicon (0.3 mu m,) were the reason for subcritical crack growth that was evidenced experimentally by AFM topographic and AFM/DIC displacement measurements. The effect of local in-grain anisotropy and granular inhomogeneity was stronger under mixed mode loading of edge cracks inclined at angles up to 55 degrees with respect to the applied far-field load. The K(I) -K(II) locus was characterized by scatter in the K(Ic) values but on average it followed the curves calculated by the maximum tensile stress and the maximum energy release rate criteria calculated assuming isotropy.