Process Modeling of a Wire Saw Operation
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Multicrystalline (MC) silicon solar cells are manufactured from bread-loaf sized ingots of solar-grade silicon. These ingots are sliced by a multi-wire saw mechanism consisting of a single thin and extremely long stainless steel wire wound on constant-pitch wire grooves. The wire is wound over each groove to create a web consisting of 500-700 parallel wires. The wire is kept at a constant tension using feedback control and the wire speeds typically are 10-15 m/s. A high speed nozzle directs an aqueous slurry of oil and SiC particles to the top of the wire array and the crystal silicon ingot is pushed upwards against the wire array during the cut. In a typical wire saw system MC ingots are sliced with an area of 100x100 square mm and the latest wire saw systems can achieve thicknesses down to 300 microns. What makes this a challenging simulation problem is the wide range of timescales that characterize the overall cutting process. The slowest dynamics are associated with the evolution of the cut, which is described by a spatially dependent differential equation in time and in which the cutting rate is modeled much in the same manner as the Chemical Mechanical Planarization (CMP) process. Cutting rate is a direct function of the distance between the wire and ingot surface. Because the wire dynamics are orders of magnitude faster than cut evolution, the wire deflection is modeled by a static circular beam. The goal of this modeling work is to understand the physical mechanisms that limit how thin the wafers can be cut and to determine the sensitivity of cutting time and cutting rate based on process operating conditions.