Wire sawing of silicon wafers

Particle-based simulations for your applications

Introduction

One condition for the cost-neutral establishment of photovoltaic power supplies is the achievement of grid parity, i.e. the cost for generating the electricity should be the same as when using conventional energy sources. Cutting the monocrystalline or polycrystalline silicon ingots into wafers accounts for between one-quarter and one-third of the total production costs of solar cells.

Multi-wire saws are used for this cutting process in industry. A steel wire is guided over cylindrical spools whose indentations guarantee a constant spacing of the wires. To saw through the silicon ingot, it is pressed against the parallel wires, which are wetted with an abrasive slurry. The slurry typically consists of polyethylene glycol and angular SiC grains.

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The task

Reducing the wastage due to sawing (kerf loss) would result in a great potential for savings because about half of the silicon is cut away. The kerf loss is primarily determined by the diameter of the wire and the size distribution of the SiC grains in the slurry, but also by the feed rate and the tension of the wire.

The sawing efficiency describes the ratio of the quantity of sawn silicon to the energy input. This ratio should of course be as high as possible. However, the sawing efficiency is also a function of the aforementioned process parameters and the material properties

The challenge is therefore to optimize the sawing efficiency at the same time as minimizing the kerf loss.

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Results

Particle-based simulation can supply new insights into the wire sawing process which are very difficult to obtain through experiments. In doing so, both the hydrodynamics of the carrier fluid and also the dynamics of the SiC grains in the kerf are taken explicitly into account [Bie08a].

Particle modeling was used successfully for the wire sawing process within the scope of the KerfLoss (ref. No. 0327601E) project of the Federal Ministry for the Environment, Nature Conservation & Nuclear Safety. The appearance of various contact regimes, i.e. the number of grain layers between wire and bottom of cut, and hence the associated force exerted on the ingot were able to be determined in relation to the wire tension and the feed rate.

The next step will involve the implementation of the cutting model in order to be able to achieve a concrete optimization between kerf loss and sawing efficiency.

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