MODELING STRESSES OF CONTACTS IN WIRE SAW SLICING OF POLYCRYSTALLINE AND CRYSTALLINE INGOTS - APPLICATION TO SILICON-WAFER PRODUCTION

Wire saw slicing is a cost effective technology with high surface qual ity for slicing large diameter silicon wafers. Though wire saws have b een deployed to cut polycrystalline and single crystal silicon ingot s ince the early 1990s, very little is known about the fundamental cutti ng process. We investigate this manufacturing process and propose a co ntact stress model of wire saw slicing that illustrates the interactio ns among the wire, ingot, and abrasives (e.g., SiC) carried by the slu rry. Stresses created by wire saw slicing silicon wafers are analyzed in this paper. During the cutting process, the wire moves at high spee d (5-15 m/s) with respect to the silicon ingot. The abrasives in the s lurry are lose third-body particles caught between the wire and ingot at the contact surface. The forces applied by the wire carry the abras ive particles and cause them to roll on the surface and at the same ti me to be constrained to indent the surface. Such rolling-indenting int eractions result in the formation of isolated chips and surface cracks . The cracks and discontinuity on the surface also cause high stress c oncentration. As a result, the material is cut and removed. The stress fields of a single circular cone of the abrasive particle indenting o n silicon crystal with normal and tangential forces can be calculated and analyzed from the modeling equations and boundary conditions. The stresses are expressed with dimensionless stress measures, as function s of normalized geometric parameters. The results show that the maximu m normal stress occurs at the indentation point, while the maximum she ar stress (sigma(zx)) occurs below the surface of contact, as expected . Such subsurface shear facilitates the peeling effects of the silicon cracks. Both the normal and tangential forces applied at the contacts are incorporated in the model. The model is very effective in explain ing and predicting the behaviors and distributions of stresses during the cutting process, and can be used to determine the optimal geometry of the abrasive particles in the rolling-indenting process.