Statistical analysis of the influence of thinning processes on the strength of silicon

Silicon wafer thinning is one of the key enabling techniques in 3D-IC structures. However, the mechanical integrity of Si can be degraded by different thinning steps. In this paper, the strength of Si strips thinned by rough grinding (RG), fine grinding (FG), plasma etching (PE), and chemical-mechanical polishing (CMP) was assessed by 4-point bending. The results were plotted in Weibull statistics and studied with fracture analysis. RG strips show a bimodal Weibull distribution, which is associated with the orientation of the grinding lines. Grinding lines perpendicular to the tensile stress initiate cracks propagating along {110} planes, and lines parallel to the stress result in fracture along {111} planes. FG, PE, and CMP all increase the fracture stress of Si compare with RG. The Weibull plot indicates that PE can enhance part of the FG samples to the level of CMP samples. However, for samples with large defects introduced by FG, there is no improvement after PE. © 2009 Materials Research Society.