Modeling Stresses of Contacts in Wiresaw Slicing of Polycrystalline and Crystalline Ingots: Application to Silicon Wafer Production

Abstract

Wiresaw is a cost-effective technology with high surface quality for slicing large diameter silicon wafers. Though wiresaws have been deployed to cut polycrystalline and single crystal silicon ingot since early 1990s, very little is known about the fundamental cutting process. We investigate this manufacturing process and propose a contact stress model of wiresaw slicing which illustrates the interactions among the wire, ingot, and abrasives (e.g., SiC) carried by the slurry. Stresses created by wiresaw slicing silicon wafers are analyzed in this paper. During the cutting process, the wire moves at high speed (5–15 m/s) with respect to the silicon ingot. The abrasives in the slurry are lose third-body particles caught between the wire and ingot at the contact surface. The forces applied by the wire carry the abrasive particles and cause them to roll on the surface and at the same time to be constrained to indent the surface. Such rolling-indenting interactions result in the formation of isolated chips and surface cracks. The cracks and discontinuity on the surface also cause high stress concentration. As a result the material is cut and removed. The stress fields of a single circular cone of the abrasive particle indenting on 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 functions of normalized geometric parameters. The results show that the maximum normal stress occurs at the indentation point while the maximum shear stress (σzx) 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 explaining 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.