The historical change of mechanical strength of silicon wafer during the large scale integration(LSI) fabrication process has been studied. It was confirmed that the major factor which determines the strength of the bulk crystal is the bulk microdefect (BMD) density in the metal oxide semiconductor Field effect transistor fabrication process with local oxidation of silicon (LOCOS) isolation. Unfortunately, the LOGOS fabrication process increases the BMD density and reduces the wafer strength. Further, this study revealed that the ion implantation process induces the most serious degradation of the mechanical strength in the surface region of the Si substrate in the LSI fabrication processes. Whether or not this deterioration in strength is recovered depends on the status of ion implantation and the method of subsequent annealing. The deterioration of strength was caused by ion implantation due to the effect of interstitial silicon atoms and their clusters in a low dose situation (<10(14) cm(-2)), and the amorphous layer or surface roughness in a high dose condition (>10(15) cm(-2)). In order to reduce the crystal defects or wafer-slips, attention should be paid to the procedural steps which cause reduction in the mechanical strength of silicon crystals while designing the LSI fabrication process.