Investigation of Crack Initiation in Glass Substrate by Residual Stress Analysis

To investigate the mechanism of crack initiation in the glass substrate, residual stress in the substrate was analyzed by the finite element method (FEM). A Si wafer that has the same mechanical properties of glass was prepared as the substrate. The thin Cu film layer was fabricated on the Si wafer by plating. The warpage of the wafer in the heat treatment process was investigated by laser displacement measurement. Residual stress was calculated by the obtained warpage value using the Stoney formula. It was found that the warpage becomes zero and the stress-free state occurs around at 150°C. On the basis of the FEM analysis result from 150°C to the room temperature in the cooling process, it was found that analyzed warpage values are almost analogous to measured ones. Also, it was confirmed that equivalent stress in the wafer in the vicinity of the Cu/Si interface increases and the stress-concentrated area expands there with increasing the thickness of the Cu film. The results indicates that crack initiation easily occurs in the glass substrate when the thickness of the Cu film increases. Moreover, from the results of the FEM analysis for the warpage of the wafer in the heating process, it was found that the analyzed warpage is good accordance with the measured one from room temperature to around 150°C.