Study On Partial Cone Cracks In Nano-grinding Of Single Crystal Silicon

Single crystal silicon is the most widely used substrates in IC manufacturing. The surface quality of silicon wafers directly affect the device functionality, the rate of finished products and the lifetime of IC chips. The continuous increasing of the single crystal silicon diameter and shrinking of the device feature size lay increasingly stringent surface quality requirements for silicon wafers. In recent years, some new processes for silicon wafer machining are being researched. Wafer rotation grinding, as a newly developed nano-grinding method, has been widely used in prime wafer flattening and device wafer thinning due to its low damage, high precision and high efficiency. However, grinding would inevitably bring about surface layer microcrack damage and the damage will severely increase the subsequent polishing procedure, and result in ultra-thin wafer breakage loss during handling, dicing, and package assembly. Although a few researchers have studied the machining induced microcrack damage in single crystal silicon, the microcrack formation mechanism caused by nano-grinding is not yet perfectly clarified. Therefore, it is extremely important to investigate the surface layer microcrack damage in silicon wafer grinding for realizing ultra-smooth and damage-free silicon wafer with high precision and high efficiency.The grinding induced microcrack damage in single crystal silicon was studied by means of experiments and analysis of research background. Partial cone cracks were firstly found to be one of the most important microcrack forms in ultra-precision grinding of silicon wafers. It could be generated even under a depth of cut of several tens of nanometers or under a width of cut of several hundreds of nanometers.The surface partial cone cracks of the single crystal silicon were studied by single grain grinding method. The effect of abrasive grain size and crystal orientation on the partial cone cracks was analyzed. The change rules and the generation mechanism of partial cone cracks of the single crystal silicon induced in single grain grinding are studied. The configurations and distribution of partial cone cracks induced by single grain grinding and true grinding process were also studied. The results show that silicon crystal plane and crystal orientation have not significant effect on the configuration of the partial cone cracks.The transition of partial cone cracks in single crystal silicon with depth of cut from micrometers scale to nanometer scale was investigated by single grain grinding experiments. The effect of grain size and grain depth of cut on the partial cone cracks was analyzed. The critical depth of cut of single diamond grain was obtained as well. The experiment results show that the partial cone cracks may eventually disappear when the grain depth of cut reaches 6nm. This means that the wafer surface could be generated in pure ductile mode and ductile mode grinding is feasible when grain depth of cut is less than 6nm as concerned as far as the partial cone cracks are concerned.The stress distribution in contact zone between single crystal silicon and abrasive grain and the partial cone cracks growing mechanism were studied by a series of single grain grinding experiments on single crystal silicon. The relationship between the depth of cut and the width of cracks and the effect of the depth of cut on partial cone crack extending were analyzed, and the critical depth of cut of single diamond grain was also obtained.The research results have a great significance not only for clarification the partial cone crack morphlogy, its growing mechanism and suppression strategy in ultra-precision grinding of silicon wafers, but also for the determination of the boundary condition for ductile mode grinding.